Analysis and application of two phase spindle motor driven by sensorless BLDC mode

... ANALYSIS AND APPLICATION OF TWO- PHASE SPINDLE MOTOR DRIVEN BY SENSORLESS BLDC MODE WEI TAILE (B Eng., Huazhong Univ of Sci and Tech.) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING... speed of two- phase spindle motor 44 Figure 2.21 The EM torque of two- phase spindle motor 44 Figure 2.22 The phase back-EMF of two- phase spindle motor 45 Figure 2.23 The phase voltages... Thirdly, the mathematical model of the two- phase spindle motor is presented and analyzed And the optimal commutation angle of the two- phase spindle motor driven by BLDC mode is also given Then

ANALYSIS AND APPLICATION OF TWO-PHASE SPINDLE MOTOR DRIVEN BY SENSORLESS BLDC MODE WEI TAILE NATIONAL UNIVERSITY OF SINGAPORE 2004 ANALYSIS AND APPLICATION OF TWO-PHASE SPINDLE MOTOR DRIVEN BY SENSORLESS BLDC MODE WEI TAILE (B. Eng., Huazhong Univ. of Sci. and Tech.) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2004 Acknowledgement I am sincerely grateful to my supervisor Dr. Bi Chao for his guidance and support through these two years when I was in DSI. His abundant knowledge and unmatched wisdom always amazed me. Without Dr. Bi’s insightful advice and excellent judgment, this thesis would not be possible. I cannot be thankful enough to my co-supervisor Dr. Jiang Quan for his timely help and strong support on my research work. His rigorous attitude to do the research and inspiring thinking to solve problems are invaluable for my future professional career. I would like to express my heartfelt thanks to Mr. Lim Choon Pio for his unselfish assistance and warmhearted help on introducing hardware knowledge to me and giving a necessary direction to my research. Furthermore, I would like to appreciate my fellow graduate students as well as my friends in DSI. They are Lin Song, Wu Dezheng, Huang Ruoyu, Meng Bin, Wu Daowei, and Chen Li. The friendship between us is a big treasure to me. Their hardworking, sharing, and self-motivate affected me a lot. Here, I am especially grateful to Lin Song for treating me to frequent discussions on research problems. Last, but not least, I would like to thank my parents. Without their continuous encourage and support, this work would be simply impossible. i Table of Content Acknowledgement..........................................................................i Table of Content............................................................................ii Summary… ..................................................................................iv Nomenclature ................................................................................v List of Figures ..............................................................................vi List of Tables............................................................................. viii Chapter 1 Introduction ..................................................................1 1.1 Background ................................................................................. 1 1.2 BLDC Motors and Sensorless Drives ......................................... 4 1.3 Starting Capability Problem and Solutions................................. 9 1.4 Organization of Thesis .............................................................. 11 Chapter 2 Two-Phase Spindle Motor Driven by Sensorless BLDC Mode..............................................................13 2.1 Spindle Motor with Two-Phase Structure................................. 13 2.1.1 Three-phase structure................................................................... 13 2.1.2 Two-phase structure..................................................................... 15 2.2 Back-EMF Detection Scheme................................................... 18 2.3 Modeling of Two-phase Spindle Motor.................................... 20 2.4 The Optimal Commutation Angle of Two-Phase Spindle Motor ................................................................................................. 24 2.4.1 Constant current drive mode........................................................ 26 2.4.2 Constant voltage drive mode ....................................................... 29 2.5 Simulation Model and Results .................................................. 36 2.5.1 Simulation model in Matlab......................................................... 36 2.5.2 Simulation results......................................................................... 43 2.6 System Implementation and Experimental Results .................. 49 2.6.1 Two-phase sensorless BLDC motor drive system ....................... 49 2.6.2 Experimental results..................................................................... 54 ii Chapter 3 Analysis of the Starting Capability of Two-Phase Spindle Motor ...........................................................59 3.1 Starting Algorithm .................................................................... 59 3.2 Starting Process ......................................................................... 63 3.3 Starting Capacity of Two-phase Spindle Motor with 4-step Stepping Mode......................................................................... 65 3.4 Starting Capacity of Two-phase Spindle Motor with 8-step Stepping Mode......................................................................... 68 3.5 Comparison of 4-step and 8-step Stepping Mode..................... 71 Chapter 4 Comparing the Starting Capability of Two-Phase Spindle Motor with Three-Phase Spindle Motor......75 4.1 Analysis of the Advantages of Two-phase Spindle Motor in Starting..................................................................................... 75 4.2 Comparison of 4-step and 6-step Stepping Mode..................... 79 4.3 Comparison of 8-step and 12-step Stepping Mode................... 86 Chapter 5 Conclusions and Future Work....................................94 5.1 Conclusions ............................................................................... 94 5.2 Future Work .............................................................................. 97 Reference.....................................................................................99 iii Summary With the fast development of HDD products, which trends to high data recording areal density and fast access data rate, the more accurate and higher speed of the spindle motor is required. Recently, the spindle motor speed in server HDDs has reached 15,000 rpm, and some of them are even higher than 20,000 rpm. One major concern for high-speed three-phase sensorless spindle motor is that its starting ability is poor as the phase back-EMF is too low to be detected at the low speeds. Many methods are being developed to solve this problem and using two-phase EM structure is one of the potential solutions. In this thesis, a sensorless BLDC drive system is presented to drive the two-phase spindle motors used in HDD. The optimal commutating angle based on constant voltage mode is discussed. Driving the motor with the optimal commutating angle, the motor can be driven with high efficiency. The starting capability of two-phase spindle motor is analyzed and compared with the three-phase spindle motor. And based on the conventional stepping drive mode, an improved starting method is introduced, which can enhance the starting capability of the two-phase spindle motor. The performances of the two-phase sensorless BLDC spindle motor have been widely investigated through the simulations and experiments. The results show that the proposed two-phase spindle motor drive has potential to solve the starting problem of the high-speed HDD. iv Nomenclature Back-EMF: back electromotive force BLDC: brushless direct current CPLD: complex programmable logic device DSP: data signal processor EM torque: electromagnetic torque HDD: hard disk drive MOSFET: metal-semiconductor field-effect transistor PID control: proportional-integral-derivative control PM: permanent magnet SFF: smaller form factors VCM: voice coil motor v List of Figures Figure 1.1 The structure of hard disk drive..................................................................... 1 Figure 1.2 The structure of the spindle motor used in HDD. ......................................... 2 Figure 1.3 The typical three-phase BLDC motor control system with a position sensor. ....................................................................................................................... 4 Figure 1.4 The conducting sequence of 6-step commutation. ........................................ 5 Figure 1.5 The typical three-phase BLDC motor control system without position sensors........................................................................................................... 7 Figure 2.1 Three-phase spindle motor with 12-slot/8-pole........................................... 14 Figure 2.2 Two-phase spindle motor with 12-slot/18-pole........................................... 15 Figure 2.3 Two connection ways of the two-phase spindle motor. .............................. 16 Figure 2.4 The conducting sequence of 4-step commutation. ...................................... 17 Figure 2.5 The phase current in phase with the phase back-EMF in BLDC mode. ..... 18 Figure 2.6 The optimal commutation angle of three-phase BLDC spindle motor. ...... 24 Figure 2.7 The optimal commutation angle of two-phase BLDC spindle motor in constant current drive mode........................................................................ 28 Figure 2.8 The torque ripple with commutation angle. ................................................ 34 Figure 2.9 The optimal commutation angle of two-phase BLDC spindle motor in constant voltage drive mode. ...................................................................... 34 Figure 2.10 The model of two-phase motor drive system in Matlab............................ 36 Figure 2.11 The block of “two-phase motor model”. ................................................... 37 Figure 2.12 The sub-block of “voltage equation”......................................................... 38 Figure 2.13 the sub-block of “motion equation”. ......................................................... 38 Figure 2.14 The sub-block of “phase back-EMF”. ....................................................... 39 Figure 2.15 The sub-block “EM torque” ...................................................................... 40 Figure 2.16 The sub-block of “two-phase motor driver”............................................. 41 Figure 2.17 The sub-block “speed control”. ................................................................. 41 Figure 2.18 The sub-block of “switch signals”............................................................. 42 Figure 2.19 The block of “two-phase inverter”. ........................................................... 42 Figure 2.20 The speed of two-phase spindle motor...................................................... 44 Figure 2.21 The EM torque of two-phase spindle motor.............................................. 44 Figure 2.22 The phase back-EMF of two-phase spindle motor.................................... 45 Figure 2.23 The phase voltages of two-phase spindle motor........................................ 46 Figure 2.24 The phase currents of two-phase spindle motor. ....................................... 46 Figure 2.25 The effect of changing commutation angle. .............................................. 48 Figure 2.26 Block diagram of two-phase sensorless BLDC motor drive system......... 49 Figure 2.27 The configuration of back-EMF detector .................................................. 50 Figure 2.28 The block diagram of DSP ADMC401. .................................................... 51 Figure 2.29 Configuration of the two-phase inverter.................................................... 52 Figure 2.30 The switching signals of a BLDC mode.................................................... 52 Figure 2.31 The terminal voltages of two phases. ........................................................ 54 Figure 2.32 The terminal voltages and the phase voltage of one phase. ...................... 55 Figure 2.33 The phase voltages of two phases. ............................................................ 56 Figure 2.34 The phase voltages and currents of one phase........................................... 57 Figure 2.35 The phase back-EMF of each phase. ......................................................... 58 Figure 3.1 The switch sequences for stepping drive mode. .......................................... 61 Figure 3.2 The starting process of two-phase spindle motor. ....................................... 63 Figure 3.3 The stepping time of two-phase spindle motor varies with the initial rotor position by using 4-step stepping mode...................................................... 66 vi Figure 3.4 The commutation sequence of 8-step stepping mode.................................. 68 Figure 3.5 The stepping time of two-phase spindle motor varies with the initial rotor position by using 8-step stepping mode...................................................... 70 Figure 3.6 The starting process of two-phase motor at the most difficult starting initial rotor position............................................................................................... 72 Figure 3.7 The starting processes of two-phase motor at the most easy starting initial rotor positions. ............................................................................................ 73 Figure 4.1 Three-phase inverter and Y-connection spindle motor. .............................. 76 Figure 4.2 Relationship between phase back-EMFs of two-phase and three-phase spindle motors............................................................................................. 76 Figure 4.3 The commutation sequence of 6-step stepping mode.................................. 80 Figure 4.4 The stepping time of three-phase spindle motor varies with the initial rotor position by using 6-step stepping mode...................................................... 81 Figure 4.5 The stepping starting processes with stepping drive mode at the most difficult starting initial rotor position.......................................................... 83 Figure 4.6 The stepping starting processes with stepping drive mode at the most easy starting initial rotor position........................................................................ 84 Figure 4.7 The commutation sequence of 12-step stepping mode................................ 87 Figure 4.8 The stepping time of three-phase spindle motor various with the initial rotor position by using 12-step stepping mode.................................................... 88 Figure 4.9 The stepping starting processes at the most difficult starting initial rotor position........................................................................................................ 90 Figure 4.10 The stepping starting processes at the most easy starting initial rotor position........................................................................................................ 91 vii List of Tables Table 2. 1 Motor parameters in Matlab. ....................................................................... 43 Table 3. 1 Parameters of two-phase spindle motor....................................................... 65 Table 3. 2 The starting capability of the two-phase spindle motor with 4-step stepping mode...................................................................................................................... 67 Table 3. 3 The starting capability of the two-phase spindle motor with 8-step stepping mode...................................................................................................................... 70 Table 4. 1 The starting capability of the two-phase spindle motor with 6-step stepping mode...................................................................................................................... 81 Table 4. 2 The starting capability of the three-phase spindle motor with 12-step stepping mode. ...................................................................................................... 88 Table 4. 3 The starting capability of both spindle motors. ........................................... 92 viii Chapter 1 Introduction 1.1 Background With the rapid development of power semiconductors, microprocessors, logic ICs, the brushless dc (BLDC) motors have been widely used in many applications due to their high efficiency, quiet operation, compactness, reliability, and low maintenance. One of the most important applications is used as spindle motor in hard disk drives (HDD). With the fast progress in computer and information technology, the demands on mass data storage have been increasing rapidly. These impact the developments of HDD technology as it is the most effective mass data storage device now, and is expected to dominate the mass storage market in near future. Base plate Actuator Electronics Figure 1.1 The structure of hard disk drive 1 Figure 1.1 shows the structure of a HDD, which has a spindle motor, read/write heads, electronics circuits, an actuator and a voice-coil motor (VCM), media platters and a HDD base plate. The platters are spun up by the spindle motor after the HDD is powered on. And the heads on the actuator are controlled by VCM to seek the tracks required for reading, or writing. The operating speed of the HDD has been increased in these years in order to reduce the access time in the data read/write. Hence the demand on spindle motors becomes higher. In a HDD, the spindle motor is one of the key components and in many ways it determines the performances of the HDD. Rotor Shell Magnetic Ring Stator Core with Windings Figure 1.2 The structure of the spindle motor used in HDD. 2 Figure 1.2 presents a typical spindle motor used in HDD. Usually, the spindle motors are three-phase motors with concentrated armature winding, outer rotor and surface mounted permanent magnet (PM). This kind of spindle motors has small inductance, weak armature reaction, and back-EMF close to sinusoidal waveform. Without specific indications, the motor discussed in this thesis is this kind of PM spindle motor. Usually, to have a stable spin speed and fast dynamic responses as well as high efficiency, the spindle motor is driven by sensorless BLDC mode. 3 1.2 BLDC Motors and Sensorless Drives BLDC motors are one kind of permanent magnet synchronous motors [1], having permanent magnets on the rotor and employing a dc power supply switched to the stator phase windings of the motor by power electronic devices. And the phase current commutation is determined by the rotor position. The phase current of BLDC motor is synchronized with the phase back-EMF to drive the motor in BLDC mode efficiently. The motors operating in BLDC mode require the rotor position information to provide the proper commutation sequence through the inverter. In many applications, the rotor position information can be obtained by using hall sensors or encoders. A typical three-phase BLDC motor control system with position sensors is shown in Figure 1.3. + T1 D1 T3 D3 T5 D5 A Vdc T2 D2 T4 D4 T6 D6   B C Rotor position sensor _ Switch signals Controller Rotor position signals Figure 1.3 The typical three-phase BLDC motor control system with a position sensor. 4 Generally, most of BLDC motors are three-phase structure, and are controlled through a three-phase inverter with so-called 6-step commutation. The phase conducting sequence is A+B-—A+C-—B+C-—B+A-—C+A-—C+B-, which is shown in Figure 1.4. 2π 3 π 3 T1 T2 T3 T4 T5 T6 0 sequ. α α + 2π α +π 1 2 3 4 5 θ 6 step A+B- A+C- B+C- B+A - C+A- C+B- upper T1 T1 T3 T3 T5 T5 lower T4 T6 T6 T2 T2 T4 Figure 1.4 The conducting sequence of 6-step commutation. The conducting interval for each commutation is 60°. Each conducting stage is called one step. Usually, two phases conduct at any time, leaving the third phase floating. In order to produce efficient driving torque, the inverter should be commutated every 60° so that phase current is aligned with the phase back-EMF. The commutating moment is determined by the rotor position, which can be detected by the position sensors. 5 However, these sensors increase the cost and size of the motor, and a special mechanical arrangement is needed to mount the sensors. These sensors, particularly Hall sensors, are temperature sensitive, which limit the operation of the motor to be below about 75°C [2]. On the other hand, the position sensors increase the system complexity and reduce the system reliability because of the more components and wiring. In some applications, such as in HDDs, it is even impossible to mount any position sensor on the motor. Therefore, the inconveniences brought by the position sensors have motivated the researches in the area of BLDC motor drives with position sensorless control. In the last two decades, several sensorless drive solutions have been reported in the literature and are reviewed in [4][5]. They are based on one of the following techniques. i. Position sensing using phase back-EMF of the motor [6]-[8]. ii. Position information using the stator 3rd harmonic component [9]-[11]. iii. Position sensorless operation based on the detection of the conducting state of free-wheeling diodes [12][13]. iv. Position detection using phase current sensing [14]. Basically, one of the most popular sensorless drive methods is to get indirectly the rotor position through detecting the phase back-EMF of the floating winding. In threephase BLDC motor, only two of three phases are excited at one time, leaving the third winding floating at the same time. In the motor operation, the exciting windings are used to produce the electromagnetic (EM) torque to drive the rotor. And the phase back-EMF in the floating winding is used to establish a switching sequence for 6 commutation of power devices in the three-phase inverter to realize the BLDC sensorless control [15][16]. Figure 1.5 shows a typical three-phase BLDC motor control system based on phase back-EMF detecting. + T1 D1 T3 D3 T5 D5 A Vdc T2 D2 T4 D4 T6 D6 ✁ B C _ Switch signals Controller Zero-crossing signals Phase back-emf detector Figure 1.5 The typical three-phase BLDC motor control system without position sensors. Besides the phase back-EMF detection solution, other sensorless methods are also effective for BLDC motors to eliminate the costly and fragile position sensor in the limited speed ranges. The rotor position can also be determined by the 3rd harmonic voltage component [9][11]. To detect the 3rd harmonic voltage, a three-phase set of resistors is connected across the motor windings. The voltage across the two neutrals determines the 3rd 7 harmonic voltage. This voltage is integrated and input to a zero-crossing detector, and the output of the zero-crossing detector determines the switching sequence for commutation of power devices. The main disadvantage of this method is the relatively low value of the 3rd harmonic voltage at low speeds. This makes the integration difficult. The rotor position information can be determined based on the conducting state of freewheeling diodes in the unexcited phase [12][13]. The inverter gate drive signals are chopped during each 120° operation. The open phase current under chopping operation results from the back-EMF produced in the motor windings. The position information is obtained every 60° by detecting whether the freewheeling diodes are conducting or not. Using this method, the sensing circuit is relatively complicated and low speed operation is still a problem. The exact rotor position signals can be obtained just by detecting and processing the phase current waveforms [14]. Using a signal processing circuit, the phase current signals can be converted into the required rotor position signals. However, the system built by using this method is noise sensitive, and also has the problem in starting and low speed operation. 8 1.3 Starting Capability Problem and Solutions All the sensorless solutions mentioned in last section have the problem in starting and low speed operation. For some applications, such as the spindle motors used in HDD, the starting capability of the motors is very important. In HDD, the most common sensorless drive scheme is based on the method of detecting the phase back-EMF. But it is difficult to start the motor with this method because the back-EMF is proportional to the rotating speed of the motor. Therefore, no rotor position can be detected with sensorless method when the motor is at standstill. And when a BLDC motor operates at the low speed, the phase back-EMF is low, and thus it is difficult to detect the motor position. All these make the motor starting difficult because the BLDC drive mode cannot be used in the starting procedure, and the effective driving torque is thus reduced. This leads to the motor starting capability problem. As the spin speed of HDD spindle motors trends to be higher and higher, one major concern for these high-speed motors is that the starting of the motor is poorer and needs longer time. Therefore, some methods have been developed to solve the starting problem [3][8][12][17]-[22]. One of the major sensorless starting algorithms is to utilize the inductance variations on the relative position of a rotor and stator [17]-[22]. This method can detect the rotor position at standstill by comparing the rise time of the currents due to the inductance variation after a current pulse is injected into all six segments of an EM cycle for threephase motor. Another popular sensorless starting method is so called “align and go” [3][8][12], which is an initial position orientation mode. These starting methods are 9 difficult to be applied to the spindle motors in HDDs as their permanent magnets are surface mounted on the rotor and the variations of the winding inductance are too small. Changing the structure of the motors is another possible approach to improve the starting capability of the motors. Two-phase EM structure is one potential solution. In this thesis, we will study the two-phase spindle motor driven by sensorless BLDC mode and its starting capability. 10 1.4 Organization of Thesis The work of this thesis introduces an effective method to realize the sensorless BLDC control for the two-phase spindle motors. The starting capability of the two-phase spindle motor is studied, and the advantages and disadvantages of the two-phase spindle motor are also discussed. In Chapter 2, firstly, the structure of the two-phase spindle motor is shown. Secondly, the common back-EMF detection scheme is introduced briefly. Thirdly, the mathematical model of the two-phase spindle motor is presented and analyzed. The optimal commutation angle of the two-phase spindle motor driven by BLDC mode is also studied. Then the hardware implementation of two-phase motor drive system is introduced in detail. Finally, the simulation and experiment results are provided and analyzed. These will show how the two-phase spindle motor can operate in sensorless BLDC mode. In Chapter 3, we will analyze the starting capability of the two-phase spindle motor. Where, a starting algorithm will be introduced firstly, and, secondly, the starting process will be shown and analyzed. Thirdly, an improved starting method is presented and compared with the conventional starting method. In Chapter 4, we will compare the starting capabilities between the two-phase motor and the three-phase motor. Firstly, we will theoretically analyze the reason why the two-phase spindle motor has better starting capability than the three-phase motor. Secondly, the starting capability of the three-phase spindle motor will be provided for 11 comparing. Thirdly, the simulation and experimental results will be presented to show the improvement of the starting capability of two-phase spindle motor. Finally, Chapter 5 concludes the thesis, and future research works are also suggested. 12 Chapter 2 Two-Phase Spindle Motor Driven by Sensorless BLDC Mode In this chapter, the two-phase spindle motor drive system is introduced. Firstly, the structure of the two-phase spindle motor is shown. Secondly, the back-EMF detection sensorless control scheme is introduced briefly. Thirdly, the mathematical model of the two-phase spindle motor is presented and analyzed. And the optimal commutation angle of the two-phase spindle motor driven by BLDC mode is also given. Then the hardware implementation of two-phase motor drive system is introduced in detail. Finally, the simulation and experiment results show how the two-phase spindle motor operates in sensorless BLDC mode. 2.1 Spindle Motor with Two-Phase Structure 2.1.1 Three-phase structure The spindle motors in HDDs are usually three-phase structure. Figure 2.1 shows a typical three-phase spindle motor. As mentioned in Section 1.1, the spindle motors in HDDs adopt the concentrated armature windings. In this kind of winding, every 3 slots can form one EM cycle, which means, if the fundamental component of the EM field is used, every 3 slots can realize 2 poles. Therefore, for the spindle motor with 12 stator slots, its magnetic poles are 8. 13 Figure 2.1 Three-phase spindle motor with 12-slot/8-pole. However, for the three-phase spindle motor, its back-EMF dose not contain the 3-time order harmonics with the, i.e., it dose not contain the 3rd, 6th, 9th, … harmonics. We could use the fundamental, or the 2nd order harmonics to build up the airgap magnetic field. In fact, some three-phase spindle motors utilize the 2nd harmonic of the magnetic field of the armature windings as the operation field. For these spindle motors, every 3 slots can form 4 poles. This is why the motors with 9-slot/12-pole structure are also widely used in HDD spindle motors. Increasing the number of poles is helpful to realize accurate speed control. 14 2.1.2 Two-phase structure In order to research the two-phase spindle motors, we built a prototype of the twophase spindle motor with the same stator of a three-phase spindle motor in hand. The stator of the prototype motor is 12-slots. The two-phase motor utilizes every 4 slots to form one EM cycle. If utilizing the fundamental component of the EM field, a 12 stator-slot spindle motor can realize 6 poles. But for the two-phase spindle motors, they do not contain the even harmonics. The EM field produced by the 12 slots armature windings contains rich 3rd harmonics. Therefore, the 3rd harmonic can be used to build up the effective airgap magnetic field. Since every 4 slots can realize 3 pole-pairs, i.e., 6 poles, of the field, a 12 stator slot spindle motor can realize 9 polepairs or 18 poles of the field, which is shown in Figure 2.2. Figure 2.2 Two-phase spindle motor with 12-slot/18-pole. 15 It is well know that the phase windings of a three-phase spindle motor can be “Yconnection” or “∆-connection”. Similarly, the two-phase spindle motor shown in Figure 2.2 also has two connection ways. Figure 2.3 uses the simple symbol to denote its two connection ways. They are called as “ cross-connection” and “ squareconnection”. B A A B Y X X Y cross-connection square-connection Figure 2.3 Two connection ways of the two-phase spindle motor. For the limitation in research time, only the cross-connection of the two-phase spindle motor is studied in this thesis. In the cross-connection, the two-phase windings are quadrature windings in magnetic field coupling. In this thesis, we define that phase BY lags phase AX by 90°. Each phase winding has two conducting states. For example, phase AX has the AX and XA conducting states. Therefore, the two-phase spindle motor usually operates in a 4-step 16 commutation. The phase conducting sequence is AX—BY—XA—YB, which is shown in Figure 2.4. π 2 π 2 AX BY π 2 0 π 2π 3π 2 sequ. 1 2 3 4 step AX BY XA YB θ Figure 2.4 The conducting sequence of 4-step commutation. Therefore, the commutation takes place every 90°, and the conducting interval for each phase is also 90° after each commutation. 17 2.2 Back-EMF Detection Scheme As mentioned in the Section 1.2, the conventional position detection scheme of spindle motors in HDD is based on the back-EMF detection. Therefore, a brief review of the existing back-EMF detections will be given below. The waveform and value of the phase back-EMF induced in the armature coils is determined by the EM structure, the EM materials used in the motor, and the rotor speed. For most of the spindle motors in HDD, it is easy to design such that the phase back-EMF produced has very small harmonics, i.e., the waveform can be considered as sinusoidal. In order to get the optimal control and efficiently generate the EM torque, the current of each phase should be commutated in phase with the phase back-EMF, which is so-called BLDC mode. Figure 2.5 shows the phase current and the phase back-EMF of a winding. The rotor position must be detected first for realizing the current commutation. Phase back-EMF Phase current Silent state Exciting state Silent state Exciting state Silent state Figure 2.5 The phase current in phase with the phase back-EMF in BLDC mode. 18 As shown in Figure 2.5, one operation cycle of each phase is formed by the exciting state and the silent state. The exciting state produces the EM torque to drive the rotor and load. The silent state can be used to detect the phase back-EMF to realize the sensorless rotor position detecting [1][2][23]. For one phase winding, its phase back-EMF crosses zero point two times in one EM cycle. Therefore, in one rotor revolution, one phase back-EMF can generate 2p zerocrossing points, where “p” is the number of the pole-pairs. As the zero-crossing points are determined only by the rotor positions, one phase winding signal can get 2p discrete rotor position information in one rotor revolution. Then, the commutation sequences of the inverter are controlled with these zero-crossing points. In order to drive a spindle motor by BLDC mode, the zero-crossing point signals have to be phase-shifted to the predetermined commutation angle. The above rotor position detection scheme is simple and effective. It has been used for a long time in the PM motor drive [2][3][24][25]. In this thesis, this method is also used to realize the sensorless control of a two-phase BLDC spindle motor. 19 2.3 Modeling of Two-phase Spindle Motor In order to study the performances of two-phase spindle motors, a mathematic model is built. In general, a multi-phase permanent magnet motor can be modeled as [2]: uk = Rk ik + d dt l Lkl il + ek (2.1) where k=1, 2,…, m; l=1, 2,…, m; m is the number of phase; uk is the voltage of phase k; Rk is the resistance of phase k; ik is the current of phase k; Lkl is the mutual inductance between the phase k to the phase l, and when l is equal to k, Lkl is the self inductance of phase k; ek is the back-EMF of phase k. Since one phase winding of a two-phase spindle motor is quadrature to another, therefore, the mature inductances between the two phases are zero. Eq. (2.1) can thus be written as: u AX (θ ) = RAX ⋅ iAX (θ ) + LAX u BY (θ ) = RBY ⋅ iBY (θ ) + LBY di AX (θ ) + eAX (θ ) dt diBY (θ ) dt + eBY (θ ) (2.2) (2.3) where u AX and u BY are the phase voltages; iAX and iBY are the phase currents; RAX and RBY are the phase resistances; LAX and LBY are the self inductances; eAX and eBY are the phase back-EMF; θ indicates the rotor position relative to the d axis. 20 As mentioned in Section 2.1.2, the phase BY lags the phase AX by 90°. And the phase back-EMF of two-phase spindle motor is designed sinusoidal, and therefore, it can be expressed by the peak value and phase angle: eAX (θ ) = Em sin θ = K E ⋅ ωm sin θ eBY (θ ) = eAX θ − π 2 = Em sin θ − where the phase angle − π 2 (2.4) π 2 = − K E ⋅ ωm cos θ (2.5) shows that in two-phase spindle motor, phase BY lags phase AX by 90°. In Eq. (2.4) and (2.5), Em is amplitude of the phase back-EMF and It is proportional to the angular speed of the rotor. And K E is the voltage constant and ωm is the mechanical angular speed of the rotor. The speed has the following relationship with the electrical angle of the rotor: ωm = 1 dθ p p dt (2.6) where p p is the number of the rotor pole-pairs. In general, the equation of a motor motion is: TEM (θ ) = J d ωm + Tl dt (2.7) 21 where TEM is the EM torque produced by spindle motor, J is the system inertia and Tl is the load torque including the friction torque of bearing, the windage torque, and the equivalent torque caused by the core loss. Usually, Tl is set as a constant value when the motor operates at a steady speed. Also the electrical angular speed ω and the actual speed in rpm n can be calculated by the following equations: ω = p p ⋅ ωm n= (2.8) 60ωm 2π (2.9) According to the law of energy conservation, the mechanical output of the motor provided by the EM torque must be equal to the electrical input absorbed from power supply. Therefore, the equation of power is: PEM (θ ) = TEM (θ ) ⋅ ωm = m k =1 ek (θ ) ⋅ ik (θ ) (2.10) Then, from Eq. (2.8) and Eq. (2.10), the EM torque can be calculated as: TEM (θ ) = PEM (θ ) ωm = pp ⋅ m k =1 ek (θ ) ⋅ ik (θ ) ω (2.11) In two-phase spindle motor, the EM torque produced is: 22 TEM (θ ) = p p ⋅ eAX (θ ) ⋅ iAX (θ ) + eBY (θ ) ⋅ iBY (θ ) ω (2.12) The mathematical model of two-phase spindle motor is built up with Eq. (2.2)- Eq. (2.9) and Eq. (2.12). 23 2.4 The Optimal Commutation Angle of Two-Phase Spindle Motor It is mentioned in Section 2.2, in order to operate a spindle motor in BLDC mode, when the zero-crossing points are detected, the zero-crossing signals have to be phaseshifted to the optimal commutation angle. In this way, the BLDC motor can produce the maximum torque with the same effective current. For the BLDC motor, we could use different commutation angles to drive the motor. However, we expect that the commutation angle can be optimal, i.e., it can produce the maximum EM torque with the same drive current or can produce the minimum torque ripple in the motor operation. In three-phase BLDC spindle motor, it is well known that the natural commutation angle is 30° and the conducting interval of each phase is 60°, which is shown in Figure 2.6, which is optimal commutating angle in the cases where the inductance effects can be neglected. Phase back-EMF Phase current π 6 Commutation angle π 2π 3 6 Conducting interval Figure 2.6 The optimal commutation angle of three-phase BLDC spindle motor. 24 Then, what is the optimal commutation angle of two-phase BLDC spindle motor? The following section will analyze the two-phase spindle motor operating in constant current and voltage drive modes first, and then derive the optimal commutation angle from the analysis results. 25 2.4.1 Constant current drive mode For the two-phase spindle motor under constant current drive mode, its phase current can be described by using the following equation: Im , θ ∈ α, α + π 2 iAX (θ ) = − I m , θ ∈ α + π , α + 0, 3π 2 (2.13) others Im , iBY (θ ) = iAX θ − π 2 θ∈ α+ = −Im , θ ∈ α + 0, π 2 , α +π 3π , α + 2π 2 (2.14) others where I m is the constant current value offered by the power supply during the exciting period. In the silent period, the phase current is zero. Substituting Eq. (2.4), Eq. (2.5), Eq. (2.13), Eq. (2.14) into Eq. (2.12), we can get: TEM (θ ) = pp ω ⋅ eAX (θ ) ⋅ iAX (θ ) + eBY (θ ) ⋅ iBY (θ ) 26 pp ω pp = ω pp ω pp ω π ⋅ I m K Eωm sin θ , θ ∈ α, α + ⋅ I m K Eωm ( − cos θ ) , θ∈ α+ ⋅ ( − I m ) K Eωm sin θ , 3π θ ∈ α +π, α + 2 ⋅ ( − I m ) K Eωm ( − cos θ ) , θ∈ α+ π 2 2 , α +π (2.15) 3π , α + 2π 2 From Eq. (2.15), the EM torque TEM is a function of the rotor position, and it varies with the rotor position. When the speed is steady, the average EM torque can be introduced instead of the transient torque. Then the average EM torque TEM of a twophase spindle motor can be expressed as: TEM = 1 2π α + 2π α TEM (θ ) dθ (2.16) Substitution Eq. (2.15) into Eq. (2.16) yields: TEM = = = pp 2πω ⋅4⋅ α+ α π 2 I m K Eωm sin θ dθ K E Im ⋅ 4 ( sin α + cos α ) 2π 2 2 π K E I m sin α + π 4 (2.17) Form Eq. (2.17), it is easy to find out that the maximum TEM happens at: 27 α= π (2.18) 4 Then the maximum TEM is: TEM max = TEM α= π = 4 2 2 π KE Im (2.19) The equations above prove that in constant current drive mode, the optimal commutation angle of two-phase BLDC spindle motor is 45°. Figure 2.7 shows the optimal commutation angle of two-phase BLDC spindle motor in constant current BLDC drive mode. Phase back-EMF Phase current π 4 Commutation angle π π 2 4 Conducting interval Figure 2.7 The optimal commutation angle of two-phase BLDC spindle motor in constant current drive mode. 28 2.4.2 Constant voltage drive mode For the two-phase spindle motor under constant voltage drive mode, the phase voltages can be described by the following equation: θ ∈ α, α + Um , u AX (θ ) = −U m , π 2 θ ∈ α +π, α + 3π 2 (2.20) eAX (θ ) , others Um , u BY (θ ) = u AX θ − π 2 = −U m , θ∈ α+ θ∈ α+ π 2 , α +π 3π , α + 2π 2 (2.21) eBY (θ ) , others where U m is the constant voltage value offered by the power supply. And the silent phase current is zero and the phase voltage is equal to phase back-EMF. For the phase windings of a two-phase spindle motor are designed and made to be symmetrical, it is, RAX = RBY = R (2.22) LAX = LBY = L (2.23) 29 And its self-inductances of the PM surface mounted spindle motor are very small. Neglecting these inductances LAX , LBY and considering Eq. (2.22) and Eq. (2.23), the phase currents can be calculated by transferring Eq. (2.2) and Eq. (2.3) into: iAX (θ ) = iBY (θ ) = u AX (θ ) − eAX (θ ) (2.24) R u BY (θ ) − eBY (θ ) (2.25) R Substituting Eq. (2.20), Eq. (2.4) into Eq. (2.24) and Eq. (2.21), Eq. (2.5) into Eq. (2.25), then we can get the phase currents: iAX (θ ) = U m − K Eωm sin θ , R θ ∈ α, α + −U m − K Eωm sin θ , R θ ∈ α +π, α + 0, others π 2 U m − K Eωm ( − cos θ ) , R iBY (θ ) = iAX θ − π 2 = 3π 2 θ∈ α+ (2.26) π 2 , α +π −U m − K Eωm sin ( − cos θ ) 3π , θ∈ α+ , α + 2π R 2 0, (2.27) others Then Substituting Eq. (2.4), Eq. (2.5), Eq. (2.26), Eq. (2.27) into Eq. (2.12), and considering Eq. (2.8), we can get the EM torque: 30 TEM (θ ) = pp ω 1 ⋅ n=0 iAX θ − nπ nπ ⋅ eAX θ − 2 2 U m − K Eωm sin θ ⋅ K E sin θ , R U m − K Eωm ( − cos θ ) = R θ ∈ α, α + ⋅ K E ( − cos θ ) , −U m − K Eωm sin θ ⋅ K E sin θ , R −U m − K Eωm ( − cos θ ) R θ∈ α+ π 2 π 2 , α +π 3π θ ∈ α +π, α + 2 ⋅ K E ( − cos θ ) , θ ∈ α + (2.28) 3π , α + 2π 2 From Eq. (2.28), the EM torque TEM is also a function of the rotor position, and it is various with the rotor position. Therefore, the average EM torque TEM is also introduced, and after substituting Eq. (2.28) into Eq. (2.16), it can be expressed as: TEM = = 1 ⋅4⋅ 2π α+ π 2 α U m − K Eωm sin θ ⋅ K E sin θ dθ R KE π 4 2U m sin α + − K Eωm (π + 2sin 2α ) 2π R 4 (2.29) Using the average EM torque TEM as the objective function will generate many problems in the analysis. To get a clear optimization result, we used the objection based on the torque ripple analysis to judge the optimization result. The objective function Tripple is defined as: Tripple (α ) = 1 2π α + 2π α TEM (θ ) − TEM 2 dθ (2.30) 31 where TEM (θ ) , TEM are the transient EM torque and the average EM torque respectively. Substituting Eq. (2.28) and Eq. (2.29) into Eq. (2.30) yields: KE Tripple (α ) = R − 2 ⋅ 2 α+ π α 2 2U m π π 2 [(U m − K Eωm sin θ ) sin θ sin α + π 4 + 1 K Eωm (π + 2sin 2α )]2 dθ 2π K E2 = [3K E2ωm2 (π 2 − 4) + 12U m2 (π 2 − 8) − 24K EωmU m (π − 2)( cos α + sin α ) 2 2 24π R (2.31) + 8K EωmU m (π − 6 )( cos3α − sin 3α ) + 24U m2 (π − 4 ) sin 2α + 12 K E2ωm2 cos 4α ] Calculating the derivative of Eq. (2.31), we get: dTripple (α ) dα = 2 K E2 ⋅ ( cos α − sin α ) ⋅ U m − K Eωm ( cos α + sin α ) π 2 R2 ⋅ (π − 4 ) U m ( cos α + sin α ) + 2 K Eωm sin 2α (2.32) For solving the extremum of Eq. (2.31), let Eq. (2.32) equal to zero, and then we can get: ( cos α − sin α ) = 0 U m − K Eωm ( cos α + sin α ) = 0 (π − 4 )U m ( cos α + sin α ) + 2 K Eωm sin 2α (2.33) =0 32 Solving Eq. (2.33), we can obtain the following solutions: α1 = π 4 α2 = − α 3,4 = 3π 4 π 4 α 5,6,7,8 = ± arccos π 4 (2.34) Um 2 K Eωm 2 ( 4 − π ) U m ± 2 (π − 4 ) U m2 + 32 K E2ωm2 2 ± arccos 8 K E ωm For analyzing clearly, the torque ripple with commutation angle in the range (-π, π) is plotted and shown in Figure 2.8. From Figure 2.8, it is obvious that there are totally 8 extremums of the torque ripple in one cycle, which is corresponding to 8 solutions in Eq. (2.34). The commutation angles in the range (0, π 2 ) have only meaning in the two-phase BLDC mode, and Figure 2.8 shows that there are 3 extremums in the range (0, π 2 ), in which one is the minimum, and the other two are the maximum. It is clear that the minimum torque ripple appears at the commutation angle α= π 4 . It means that the torque ripple can get the smallest when the commutation takes place at α= π 4 . Therefore, α= π 4 is the optimal commutation angle in constant voltage drive mode. 33 Tripple − π 3π 4 4 π 2 α Figure 2.8 The torque ripple with commutation angle. Figure 2.9 shows the optimal commutation angle of two-phase BLDC spindle motor in constant voltage drive mode. Phase voltage Phase back-EMF Phase current π 4 Commutation angle π π 2 4 Conducting interval Figure 2.9 The optimal commutation angle of two-phase BLDC spindle motor in constant voltage drive mode. 34 The analysis above shows that, no matter in constant current drive mode or constant voltage drive mode, the optimal commutation angle of two-phase BLDC spindle motor is 45°, at which the spindle motor can show the best performance. 35 2.5 Simulation Model and Results Due to the fact that the most of spindle motors in HDD usually operate in the constant voltage drive mode or constant voltage PWM chopping mode, we have built up such a drive system of the two-phase sensorless BLDC spindle motor in Matlab, and made simulations to test the performances of the model. 2.5.1 Simulation model in Matlab In general, the whole drive system model is made up of two parts. One is the twophase spindle motor, and another is the two-phase driver. The overall diagram of the two-phase motor drive system is shown in Figure 2.10. Figure 2.10 The model of two-phase motor drive system in Matlab. 36 The inputs of block named “the two-phase spindle motor” are the phase voltages offered by the block named “two-phase motor driver”. And when the motor operates in BLDC mode, the motor speed and the phase back-EMF can be detected, and then this information from the two-phase spindle motor is fed back to the two-phase motor driver for close-loop control and sensorless control. The block named “the two-phase spindle motor” describes the equations of two-phase spindle motor introduced in Section 2.3. Figure 2.11 shows the model of two-phase spindle motor in details. Figure 2.11 The block of “two-phase motor model”. 37 The block named “voltage equation” that is shown in Figure 2.12 realizes the equations Eq. (2.2) and Eq. (2.3). In this block, we neglect the mutual inductances since they are much smaller than the self-inductances. Figure 2.12 The sub-block of “voltage equation”. And the block named “motion equation” that is shown in Figure 2.13 can realize the equations Eq. (2.7). Figure 2.13 the sub-block of “motion equation”. 38 From this block, we can calculate the mechanical angle θ m and mechanical speed ωm of the rotor. Then according to the equation Eq. (2.9), we can know the rotor’s speed in rpm. And the mechanical angle of the rotor θ m has the following relationship with θ : θm = θ (2.35) pp And based on the equations Eq. (2.4) and Eq. (2.5), we can use this information to calculate the phase back-EMF in block named “phase back-EMF”, which is shown in Figure 2.14. Figure 2.14 The sub-block of “phase back-EMF”. Then the outputs of the block “phase back-EMF” are inputted to the block “voltage equation” to calculate the phase currents. Besides, the phase back-EMF is also the important information source to detect the rotor position, which is used by the twophase motor driver for sensorless control. The EM torque is calculated in the block named “EM torque”, which is shown in Figure 2.15. 39 Figure 2.15 The sub-block “EM torque” This block is based on the following equation by substituting Eq. (2.4), Eq. (2.5) into Eq. (2.12): TEM (θ ) = p p ⋅ CEωm sin θ ⋅ iAX (θ ) + CEωm ( − cos θ ) ⋅ iBY (θ ) ω = CE sin θ ⋅ iAX (θ ) − cos θ ⋅ iBY (θ ) (2.36) The block named “two-phase motor driver” is the drive part to control the two-phase spindle motor in sensorless BLDC mode. Figure 2.16 shows the two-phase motor driver in details. In this block, the speed calculated by the block “two-phase motor model” is inputted to the block “speed control” to realize the close-loop speed control. Usually, it is realized by a PID controller, and the speed is controlled through adjusting the phase voltages. The following equation presents the adjustable phase voltage calculated by the PID controller according to the speed error. 40 Vdc ( t ) = t d ∆n ( t ) 2π CE K p ⋅ ∆n ( t ) + K i ⋅ ∆n ( s ) ds + K d ⋅ 0 60 dt (2.37) where K p , K i and K d are the proportional coefficient, the integral coefficient, and the derivative coefficient respectively. ∆n is the speed error, which is a time function. And Figure 2.17 shows the speed controller. Figure 2.16 The sub-block of “two-phase motor driver”. Figure 2.17 The sub-block “speed control”. And the phase back-EMF offered by the block “two-phase motor model” is used to detect the rotor position, and it gives the right switch signals into the two-phase 41 inverter to drive the two-phase spindle motor operate properly through the block named “switch signals”, which is shown in Figure 2.18. Figure 2.18 The sub-block of “switch signals”. In this block, the right switch signals are produced to realize BLDC mode by shifting 45° from the zero-crossing points of the phase back-EMF. The switch signals and the adjustable DC voltage are inputted to the block named “two-phase inverter”, which is shown in Figure 2.19. The phase voltages are generated in this block and are offered to drive the two-phase spindle motor. Figure 2.19 The block of “two-phase inverter”. 42 2.5.2 Simulation results The model of two-phase sensorless BLDC motor drive system has built up. We give the measured actual motor parameters into the model, which is presented in Table 2. 1. And then the model is simulated in Matlab. Table 2. 1 Motor parameters in Matlab. System Parameters Symbol Value Phase Resistance R 4.5Ω Phase Inductance L 0.7mH Motor Inertial J 2.2×10-5kg⋅m2 Voltage Constant CE 0.0024V/(rad/s) Load Torque Tl 9.8×10-5N⋅m Rated Speed n 4200rpm Pole-Pair pp 4 Proportional Coefficient Kp 50 Integral Coefficient Ki 3×103 Derivative Coefficient Kd 0 Figure 2.20 shows the speed of rotor. From this figure, the rotor starts up from standstill to a setting speed, and then it starts much faster to the rated speed. This is because the motor is in stepping drive mode at the beginning, and then it is switched into the BLDC drive mode after the amplitude of the phase back EMF is big enough. The starting capability of the two-phase spindle motor will be discussed in the following chapters. After the speed reaches the rated speed, it would stabilize at the rated speed after vibrating for several times due to the closed loop control. 43 Figure 2.20 The speed of two-phase spindle motor. Figure 2.21 The EM torque of two-phase spindle motor. 44 And Figure 2.21 shows the corresponding starting EM torque along with the speed in the motor operation. When the two-phase spindle motor is stable at rated speed, it operates in the BLDC mode. Its phase back-EMF, voltage and current are shown from Figure 2.22- to Figure 2.24. There are three rows in these figures. The first row shows the waveforms of phase AX, the second row shows the waveforms of phase BY, and the third row describes the phase difference between two phases. Form these figures, they all show that the phase BY lags phase AX by 90°. Figure 2.22 The phase back-EMF of two-phase spindle motor. 45 Figure 2.23 The phase voltages of two-phase spindle motor. Figure 2.24 The phase currents of two-phase spindle motor. 46 It has been discussed in Section 2.4.2, in constant voltage drive mode, the phase voltages detected on the phase windings are the constant voltages supplied by DC power when the phase windings are excited, and equal to the phase back-EMF when the phase windings are silent. When the two-phase spindle motor is driven in optimal commutating BLDC mode, the phase voltages are the shapes shown in Figure 2.23, which are also described by the equations Eq. (2.20) and Eq. (2.21). The phase currents generated in the phase windings as shown in Figure 2.24 are corresponding to the shape shown in Figure 2.9. In constant voltage drive mode, the phase currents are not constant when the phases are conducting. They can be inferred from the Eq. (2.24) and Eq. (2.25). It is clear that the commutation is expected at the optimal commutation angle 45°, but if we consider the effect of the inductance, the optimal commutation angle will change. Therefore, how much will the inductance affect the optimal commutation angle is a question. From Figure 2.24, obviously, to generate the same EM torque at a certain speed, the phase current is minimum when the motor is operating at the optimal commutation angle, therefore the copper loss is also minimum. But if the motor is not operating at the optimal commutation angle, the phase current will be increased to generate the same EM torque, and then the copper loss is also increased. Therefore, the copper loss with the same EM torque can be used to measure the effect of the commutation angle. Figure 2.25 shows the effect of changing the commutation angle. 47 From Figure 2.25, considering the inductance, the optimal commutation angle is about 44.4°. If commutation takes place at 45°, the copper loss increases about 0.12%. Therefore, two conclusions can be obtained from the result. One is that the inductance can affect the optimal commutation angle indeed. Another is that the effect is very small as the inductance is weak. Hence, when studying the spindle motor used in HDD, the effect of the inductance can be neglected, and the optimal commutation angle can be regarded as 45° for convenience. Therefore, the conclusions got in Section 2.4 are reasonable. Copper loss / EM torque 63.65 63.6 63.55 63.5 63.45 63.4 63.35 63.3 42.5 43 43.5 44 44.5 45 45.5 46 46.5 Commutation angle (degree) Figure 2.25 The effect of changing commutation angle. 48 2.6 System Implementation and Experimental Results In our research, we have built up a two-phase spindle motor and its sensorless BLDC drive system, and have driven it in sensorless BLDC mode. In this section, we will introduce the implementation of the two-phase sensorless BLDC motor drive system. 2.6.1 Two-phase sensorless BLDC motor drive system Figure 2.26 shows the block diagram of two-phase sensorless BLDC motor drive system. In this motor drive system, a back-EMF zero-crossing detector and a data signal processor (DSP) ADMC401 are used to process the phase back-EMF zerocrossing signal and detect the rotor position, and give the correct switch signals to the two-phase inverter to control the two-phase spindle motor properly. Two-Phase Inverter Vdc Two-Phase Motor Um DC DC DC-link Voltage Regulator Switch Signals Vdc* DSP ADMC401 Position Signals Terminal Voltages Phase Back-EMF Detector Figure 2.26 Block diagram of two-phase sensorless BLDC motor drive system 49 The back-EMF detector in Figure 2.27 is used to detect the terminal voltages of the two-phase spindle motor. In the silent state of the phase windings, these detected voltage signals are just the phase back-EMF signals. Then the detector uses a compact programmable logic device (CPLD) to obtain the zero-crossing signals of phase backEMF from the comparators. The zero-crossing signals are used as the rotor position signals, and are inputted to the DSP for further processing. uA uX uB uY + _ Subtracter + _ uAX + _ Comparative Signals Comparator uBY + _ CPLD Zero Crossing Signals Comparative Signals Figure 2.27 The configuration of back-EMF detector The DSP ADMC401 is used to process the zero-crossing signals from CPLD. For the effects of diodes, there are some noises when one phase is commutated to another phase. These noises will produce false zero-crossing signals, which will lead to the motor operate abnormally. After recognizing the true zero-crossing signals, the DSP shifts the zero-crossing signals by 45° to keep the phase current in phase with phase back-EMF, and exports the switching signals to the two-phase inverter. Then the twophase spindle motor runs in the sensorless BLDC mode. Moreover, the zero-crossing signals can also be utilized to detect the rotor speed, According to the detected speed, 50 the PID controller and DC-link voltage regulator are used to implement the close-loop speed control of the two-phase spindle motor. Figure 2.28 shows the block diagram of DSP ADMC401. Zero Crossing Signals DSP ADMC401 Logic operation Speed Detector True Zero Crossing Signals n - + π 4 Switch Signals Shifter Speed Error PI V dc* Speed Control n* Figure 2.28 The block diagram of DSP ADMC401. Then the voltage adjustment signal Vdc* from DSP is exported to the DC-link voltage regulator LM338 to realize constant voltage drive mode, and offer the two-phase inverter the proper phase voltage to realize the close-loop speed control. The switch signals from DSP are exported to the two-phase inverter to drive the twophase spindle motor in sensorless BLDC mode. Figure 2.29 shows the configuration of the two-phase inverter. Figure 2.30 shows the switching signals exported to the twophase inverter for BLDC mode. 51 Demux Switch Signals SAX SBY SXA SYB + T1 D1 T3 D3 uA Um uX D2 T2 T5 D5 T7 uB D4 T4 T6 D7 uY D6 T8 D8 _ B A X Y Figure 2.29 Configuration of the two-phase inverter. π π 2 2 SAX SBY SXA SYB α 0 α+ π α +π 2 α+ 3π 2 α + 2π sequ. 1 2 3 4 step AX BY XA YB upper T1 T5 T3 T7 lower T4 T8 T2 T6 θ Figure 2.30 The switching signals of a BLDC mode. 52 The two-phase spindle motor has been introduced in the Section 2.1.2. In Figure 2.29, we use cross-connection to demo the two-phase structure. And the terminal voltages of each winding are detected and inputted to the phase back-EMF detector to confirm the rotor position. 53 2.6.2 Experimental results Based on the two-phase sensorless BLDC motor drive system built up, we have driven the two-phase spindle motor in BLDC mode successfully. The following figures show some key operating waveforms of the two-phase sensorless BLDC motor drive system. Figure 2.31 shows the terminal voltages of two phases. And Figure 2.32 shows the terminal voltages and the phase voltage of one phase. uA uX uB uY 5.00V/div Figure 2.31 The terminal voltages of two phases. 54 uA uX uAX 5.00V/div Figure 2.32 The terminal voltages and the phase voltage of one phase. The phase voltages of two phases are shown in Figure 2.33. Phase BY lags phase AX by 90° and the phase detects the phase back-EMF when silent, which is the same as the phase voltages analyzed in Figure 2.23. 55 uAX uBY 5.00V/div Figure 2.33 The phase voltages of two phases. Figure 2.34 shows phase voltages and phase currents of one phase in both experimental and simulation results. It is clear that the commutation happens at about 45° after the zero-crossing point of the phase back-EMF, at which the two-phase spindle motor is operating in optimal commutating sensorless BLDC mode. And the experimental results in Figure 2.34 (a) can match well with the simulation results in Figure 2.34 (b). 56 uAX iAX uAX: 5.00V/div iAX: 0.2A/div (a) Experimental results. (b) Simulation results. Figure 2.34 The phase voltages and currents of one phase. 57 The whole experimental waveform of the phase back-EMF cannot be measured when the motor operates in BLDC mode. However, it can be measured during freewheeling and the phase currents are zero. Figure 2.35 shows the phase back-EMF of each phase during the beginning of the freewheeling. eAX eBY 5.00V/div Figure 2.35 The phase back-EMF of each phase. From this figure, it is obvious that the phase back-EMF of two-phase spindle motor is quite close to sinusoidal, and this characteristic has been explained in Section 1.1. The experimental results match well with the simulation results. 58 Chapter 3 Analysis of the Starting Capability of Two-Phase Spindle Motor In this chapter, we will analyze the starting capability of the two-phase spindle motor. Firstly, the starting algorithm we used will be introduced. Secondly, the starting process is presented and analyzed. And then an improved starting method is presented and compared with the conventional starting method. The results will show the improved starting method has better starting capability. 3.1 Starting Algorithm When a spindle motor operates in BLDC mode, detecting the rotor position is necessary. Since the value of phase back-EMF is proportioned to the rotor speed, the rotor position is undetectable when the motor is at standstill or at low speeds. It means that the motor should be spun up with some methods to a certain speed before the back-EMF is sufficient to be detected to apply a sensorless BLDC starting algorithm. One of the popular sensorless starting algorithms is to utilize the inductance variation with the relative position of a rotor and stator [17]-[22]. This method can detect the rotor position at standstill by comparing the rise time of the currents due to the inductance variation after a current pulse is injected into all six segments of an electrical cycle for three-phase motor. However, the method cannot be used in the spindle motors, as their inductances are almost constant as it was mentioned in Chapter 1. 59 Another popular sensorless starting method is so called “align and go” [3][8][12], which is an initial position orientation mode. This method aligns the rotor to the specified position by energizing any two phases of the motor. If the rotor is not in the desired position, the forcing torque of the excited phases causes it to rotate and stop at the desired position. After energizing two of the three motor phases for enough time to ensure the rotor will lock into the desired position, the rotor is accelerated according to the given firing sequences with decreasing time intervals. It usually incurs a time delay to align the rotor owing to its inertia. Our starting algorithm for two-phase spindle motor is to adopt the stepping drive mode at the standstill or at low speeds. This method is a simple and effective starting method for sensorless BLDC motors. Using this method, the speed is increased step by step at the beginning by providing a rotating stator field with a gradual increasing frequency and voltage profile until sufficient back-EMF can be detected [10][26]. Usually, 4-step commutation can be used in starting process of the two-phase spindle motor. Therefore, the switch sequences of the two-phase inverter in stepping drive mode are AX—BY—XA—YB, which is shown in Figure 3.1. 60 π π 2 2 SAX SBY SXA SYB 0 α α+ π α +π 2 α+ 3π 2 α + 2π sequ. 1 2 3 4 step AX BY XA YB upper T1 T5 T3 T7 lower T4 T8 T2 T6 θ Figure 3.1 The switch sequences for stepping drive mode. The switch sequences of the stepping drive mode in motor starting shown in Figure 3.1 are the same as the switch signals for BLDC mode shown in Figure 2.30. However, they are different. The switch signals for BLDC mode are produced based on the zerocrossing signals of phase back-EMF by close-loop control, and they can make sure that the motor is in the optimal commutating BLDC mode. While these switching sequences for stepping drive mode during the motor starting are set by DSP program in advance. As the rotor initial position is unknown in starting, the commutation angle α in Figure 3.1 may be changed. As it is analyzed in Section 2.4, the commutation angle affects the EM torque, and the commutation angle at the beginning of starting is relative to the initial rotor position. Therefore, the starting capability of two-phase spindle motor is affected by the initial rotor position in starting. 61 In Figure 3.1, the conducting interval is 90°, and the commutation interval is also performed every 90°. Then there are totally 4 steps in one cycle. Therefore, this stepping mode is so-called “4-step stepping mode”. 62 3.2 Starting Process When the two-phase spindle motor starts from standstill to the rated speed, the starting process consists of stepping mode and BLDC mode. For example shown in Figure 3.2, the starting time is from zero to the time when the motor reaches the rated speed. BLDC starting time stepping time starting time Figure 3.2 The starting process of two-phase spindle motor. The starting time is also made of two parts. The first part is the stepping time that the two-phase spindle motor starts from standstill to the setting speed with stepping mode. The second part is the BLDC starting time that the two-phase spindle motor accelerates from the setting speed to the rated speed. The setting speed is the speed with sufficient back-EMF for the two-phase spindle motor to be switched into BLDC 63 mode. As the BLDC mode can generate the maximum torque with the same current, utilizing the BLDC mode earlier can reduce significantly the time consumed in the motor stepping starting, and this is an issue very concerned by HDD industry. In Figure 3.2, the time from zero to about 1 second is the stepping time in this example, and the period from about 1 second to 1.42 second is the BLDC starting time. Obviously, the BLDC starting time is very short, and the speed in BLDC mode accelerates much faster than in stepping mode. And the starting time in BLDC mode is almost the same if the setting speed and the rated speed is unchanged. Therefore, the length of the starting time is determined mainly by the stepping time. If the stepping time is shorter, the motor will be switched to the BLDC mode earlier and reach the rated speed faster, which means the starting capability is better. 64 3.3 Starting Capacity of Two-phase Spindle Motor with 4-step Stepping Mode The starting capacity of a motor is related with many factors, such as inertia, starting current, motor impedance, cogging torque, friction torque, and initial rotor position, including the motor structure that will discuss later. Table 3. 1 Parameters of two-phase spindle motor. System Parameters Symbol Value Phase Resistance R 4.5Ω Phase Inductance L 0.7mH Motor Inertia J 2.2×10-5kg⋅m2 Voltage Constant CE 0.0024V/(rad/s) Load Torque Tl 9.8×10-5N⋅m Rated Speed n 4200rpm Pole-Pair pp 9 Proportional Coefficient Kp 50 Integral Coefficient Ki 3×103 Derivative Coefficient Kd 0 The starting capability is analyzed with the constant voltage drive mode. The parameters of the motor are set as the same as the one used in the experiment, which are listed in Table 3. 1. During the experiments, a disk is mounted on the spindle motor. And we set the constant starting current as the same. Therefore, the starting capacity of the two-phase spindle motor is determined only by the initial rotor position. 65 Since the initial rotor position is unknown, we have to set the initial rotor position in motor starting in advance to find out the relationship between the initial rotor position and the starting capability. Figure 3.3 shows that the stepping time of two-phase spindle motor varies greatly with the different initial rotor positions when the 4-step stepping mode is employed. In this test, the setting speed was set at about 400rpm. After that, the back-EMF is greater than 0.4V, which is sufficient for the two-phase Stepping time (s) spindle motor to be switched into BLDC mode. 2.5 2 Dangerous area 1.5 1 0.5 0 0 50 100 150 200 35 250 8 300 350 Initial rotor position (degree) Figure 3.3 The stepping time of two-phase spindle motor varies with the initial rotor position by using 4-step stepping mode. It is difficult to set the initial rotor position in the experiment as the rotor position sensor can not be used. Therefore, we use the simulation model introduced in Section 2.5.1 to replace the real two-phase spindle motor, since the simulation model can fully simulate the starting performances of the two-phase spindle motor. 66 From Figure 3.3, the stepping time is irregular with the initial rotor position increasing. In order to analyze conveniently, we define “dangerous area” in which the stepping time of the spindle motor exceeds 0.5 second. Therefore, the smaller the dangerous area is, the more easily the spindle motor starts up. We also use the average stepping time to describe the starting capability of the spindle motor. And the longest and shortest stepping time present the poorest and best starting capabilities of the spindle motor respectively. Hence, from Figure 3.3, the starting capability of the two-phase spindle motor with 4-step stepping mode is shown in Table 3. 2. Table 3. 2 The starting capability of the two-phase spindle motor with 4-step stepping mode. Dangerous area About 43° Average stepping time 0.283s Longest stepping time 1.91s at about 230° Shortest stepping time 0.06s at about 10° 67 3.4 Starting Capacity of Two-phase Spindle Motor with 8-step Stepping Mode In 4-step stepping mode, as shown in Figure 3.1, only one phase winding and two MOSFETs work at any time, while another phase and the other six MOSFETs are silent. This mode does not utilize the inverter adequately and effectively. Hence, an improved starting method is introduced in starting process of the two-phase spindle motor. Relative to the 4-step stepping mode, this improved starting method is so-called “8-step stepping mode”, which is shown in Figure 3.4. 3π 4 π 4 SAX SBY SXA SYB 0 sequ. step upper lower α α+ 1 2 π 2 3π α + π α + 2 α + 2π 3 4 5 6 7 8 YB AX AX AX BY BY XA BY XA YB XA YB T1 T1 T1 T3 T3 T3 T7 T5 T5 T5 T4 T4 T4 T6 θ T7 T7 T2 T2 T2 T8 T8 T8 T6 T6 Figure 3.4 The commutation sequence of 8-step stepping mode. 68 As is seen in Figure 3.4, the commutation sequence of the two-phase inverter in 8-step stepping mode is YBAX—AX—AXBY—BY—BYXA—XA—XAYB—YB. In this mode, another 4 steps are added based on the former 4-step stepping mode, and both two phases and four MOSFETs work at the same time in these added 4 steps. The conducting interval is 135°, and the commutation interval of the two-phase inverter is changed to 45°. Because the working period of each phase becomes longer, the efficiency of the two-phase inverter increases and the average EM torque generated in one cycle becomes larger. This is benefit for the motor starting. Similarly, using the same two-phase spindle motor tested with 4-step stepping mode, we also gave the same starting current and installed the same disk on the same motor. We have made the same test for the starting capability of the two-phase spindle motor with 8-step stepping mode by setting different initial rotor position in motor starting. Figure 3.5 shows the stepping time of two-phase spindle motor various with the different initial rotor position by using 8-step stepping mode too. For comparing easily, the setting speed was also set at about 400rpm with the back-EMF at 0.4V in this test. And after that, the two-phase spindle motor was switched to BLDC mode. 69 Stepping time (s) 1.25 1 Dangerous area 0.75 0.5 0.25 0 0 50 100 150 18 250 200 300 350 Initial rotor position (degree) Figure 3.5 The stepping time of two-phase spindle motor varies with the initial rotor position by using 8-step stepping mode. From Figure 3.5, the starting capability of the two-phase spindle motor with 8-step stepping mode can be analyzed and shown in Table 3. 3. Table 3. 3 The starting capability of the two-phase spindle motor with 8-step stepping mode. Dangerous area About 18° Average stepping time 0.151s Longest stepping time 1.13s at about 265° Shortest stepping time 0.03s at about 25° 70 3.5 Comparison of 4-step and 8-step Stepping Mode Comparing Table 3. 2 and Table 3. 3, the average stepping time with 8-step stepping mode is shorter than that with 4-step stepping mode. It means that the 8-step stepping mode has better starting capability than 4-step stepping mode. Even at the most difficult starting rotor position, the longest stepping time with 8-step stepping mode is about 1.13 second. It is nearly 0.78 second less than the longest stepping time with 4step stepping mode, which is 1.91 second. This means the spindle motor in 8-step stepping mode will switch to the BLDC drive faster than in 4-step stepping mode at the most difficult starting rotor position. Figure 3.6 shows the starting process at the most difficult starting rotor position with 4-step and 8-step stepping mode separately. (a) Motor starting process with 4-step stepping mode. 71 (b) Motor starting process with 8-step stepping mode. Figure 3.6 The starting process of two-phase motor at the most difficult starting initial rotor position. And the shortest stepping time with 8-step stepping mode is about 0.03 second, which is almost the same as the shortest stepping time with 4-step stepping mode (0.06 second). This means the two-phase spindle motor can start up very fast at the easiest starting rotor position with both 4-step and 8-step stepping modes. Figure 3.7 shows the starting processes at the most easy starting rotor position with 4-step and 8-step stepping mode separately. 72 (a) Motor starting process with 4-step stepping mode. (b) Motor starting process with 8-step stepping mode. Figure 3.7 The starting processes of two-phase motor at the most easy starting initial rotor positions. 73 Besides the stepping time, the dangerous area of 8-step stepping mode is about 18°. It is much narrower than that of 4-step stepping mode and is about 43°. It means that the two-phase spindle motor has more chances to start up easily and faster with 8-step stepping mode. Generally, the motor starting capability with 8-step stepping mode is superior to that with 4-step stepping mode. And no matter 4-step or 8-step stepping mode, once the motor produces sufficient back-EMF, the sensorless starting algorithm of the BLDC motor is switched to the back-EMF detection method by utilizing the zero crossing points of the phase back-EMF. 74 Chapter 4 Comparing the Starting Capability of Two-Phase Spindle Motor with Three-Phase Spindle Motor In this chapter, we will compare the starting capability of the two-phase with that of the three-phase spindle motor. Firstly, we will analyze theoretically the reason why the two-phase spindle motor has better starting capability than the three-phase spindle motor. Secondly, the starting capability of the three-phase spindle motor will be provided for comparing. And then the results will present how much two-phase spindle motor will improve in motor starting. 4.1 Analysis of the Advantages of Two-phase Spindle Motor in Starting As is mentioned before, two-phase structure has the advantage in motor starting relative to three-phase structure. Why does two-phase structure have the advantage in motor starting? And how much is the advantage? To analyze the reason, Figure 4.1 shows the Y-connection three-phase sensorless BLDC motor and its three-phase inverter. This three-phase structure is most popular used in HDD. 75 + T1 D1 T3 D3 T5 D5 A Vm T2 D2 T4 D4 X D6 T6 B Y Z C _ Figure 4.1 Three-phase inverter and Y-connection spindle motor. Comparing with the two-phase spindle motor, when they are both under constant voltage drive mode, the three-phase spindle motor has two phases exciting at the same time, while in two-phase spindle motor, only one phase is exciting, and another phase is silent at the same time. Figure 4.2 shows the relationship of the phase back-EMFs between two-phase and three-phase spindle motors. A eA A eAX eAB B X eB Figure 4.2 Relationship between phase back-EMFs of two-phase and three-phase spindle motors. 76 From Figure 4.2, with the same DC link voltage, the phase back-EMF generated by the two-phase spindle motor eAX is equal to the line back-EMF generated by the threephase spindle motor eAB at the same speed. That is: eAX = eAB (4.1) For the three-phase motor, the relationship of phase back-EMF eA and line back-EMF eAB is: eA = eB = 1 eAB 3 (4.2) Then from Eq. (4.1) and Eq. (4.2), the relationship of phase back-EMF of two-phase and that of three-phase spindle motor is: eAX = 3eA (4.3) Therefore, Eq. (4.3) shows that the phase back-EMF of two-phase spindle motor is 3 times of the phase back-EMF of three-phase spindle motor at the same speed. In another word, to obtain the same phase back-EMF, the speed of two-phase spindle motor only needs about 1/ 3 times as that of three-phase spindle motor, since the phase back-EMF is proportional to the rotor speed. Since the phase back-EMF detecting method uses phase back-EMF to detect the rotor position, the two-phase structure is very helpful for the motor to switch to the BLDC 77 mode in the lower speed operation, because two-phase spindle motor is able to obtain the phase back-EMF signals earlier or at the lower speed than the three-phase spindle motor. The result shown in Figure 3.2 has explained that the starting time is almost determined by the stepping starting. Hence, reducing the time consumed by the stepping starting, i.e., being switched earlier from the stepping mode into BLDC mode, the stepping starting time can be reduced. As the two-phase motor can be switched to the BLDC mode at lower speed than the three-phase motor, the two-phase spindle motor has better starting capability than the three-phase motor. 78 4.2 Comparison of 4-step and 6-step Stepping Mode In order to make a comparison between three-phase and two-phase spindle motor in starting capability, a comparable three-phase spindle motor was made with the same inertia, the same size, the same load, the same frictional coefficient and the same torque constant as the two-phase spindle motor. And we also gave the same starting current to the three-phase spindle motor to test its starting capability, For the limitations of the experiment condition and the difficulty of setting the initial rotor position in advance, we make this comparison in Matlab. The starting algorithm for the three-phase spindle motor was also the stepping drive mode, which is the same as the two-phase spindle motor. In this method, the 6-step stepping mode is commonly used in starting course of three-phase spindle motor. And the commutation sequence of the 6-step stepping mode is A+B-—A+C-—B+C-—B+A—C+A-—C+B-, which is shown in Figure 4.3. As is seen in Figure 4.3, in 4-step stepping mode, there are two phases and two MOSFETs work at any time, while another phase and the other six MOSFETs are silent. The conducting interval is 120°, and the commutation interval of the three-phase inverter is 60°. 79 2π 3 π 3 T1 T2 T3 T4 T5 T6 0 sequ. α α + 2π α +π 1 2 3 4 5 θ 6 step A+B- A+C- B+C- B+A - C+A- C+B- upper T1 T1 T3 T3 T5 T5 lower T4 T6 T6 T2 T2 T4 Figure 4.3 The commutation sequence of 6-step stepping mode. And also supposed that the three-phase spindle motor was switched to BLDC mode with the phase back-EMF reaching 0.4V, when the speed of three-phase spindle motor was about 700rpm. Figure 4.4 shows that the stepping time of three-phase spindle motor varies with the different initial rotor position by using normal 6-step stepping mode. 80 Stepping time (s) 2.5 2 Dangerous area 1.5 1 0.5 0 0 50 100 150 200 52 250 300 350 Initial rotor position (degree) Figure 4.4 The stepping time of three-phase spindle motor varies with the initial rotor position by using 6-step stepping mode. According to Figure 4.4, the starting capability of the two-phase spindle motor with 6step stepping mode is presented in Table 4. 1. Table 4. 1 The starting capability of the two-phase spindle motor with 6-step stepping mode. Dangerous area About 52° Average stepping time 0.324s Longest stepping time 2.1s at about 260° Shortest stepping time 0.12s at about 5° Comparing Table 3. 2 and Table 4. 1, the average stepping time with 4-step stepping mode is shorter than that with 6-step stepping mode. It means the two-phase spindle 81 motor with 4-step stepping mode has better starting capability than the three-phase spindle motor with 6-step stepping mode. The longest stepping time appears at the most difficult starting initial rotor position, which is about 2.1 second. And the fastest stepping time is about 0.12 second, which is also longer than that of the two-phase spindle motor in 4-step stepping mode. Therefore, it is obvious that the starting capability of the two-phase spindle motor is better than three-phase spindle motor. Figure 4.5 shows the stepping starting processes of both three-phase spindle motor with 6-step stepping mode and two-phase spindle motor with 4-step stepping mode at the most difficult starting initial rotor position. After that, the phase back-EMF reaches 0.4V, which is sufficient for the motor to start up rapidly by with BLDC starting mode. (a) The three-phase spindle motor with 6-step stepping mode. 82 (b) The two-phase spindle motor with 4-step stepping mode. Figure 4.5 The stepping starting processes with stepping drive mode at the most difficult starting initial rotor position. From Figure 4.5, at the most difficult starting initial rotor position, the stepping time of the three-phase spindle motor cost is about 2.1 second when the motor speed is nearly 700rpm, while the stepping time of the two-phase spindle motor is about 1.91 second with the motor speed about 400rpm. Therefore, the two-phase spindle could switch to BLDC mode about 0.2 second earlier than the three-phase spindle motor, when both motor started at the worst situation. Figure 4.6 shows the stepping starting processes of both three-phase spindle motor with 6-step stepping mode and two-phase spindle motor with 4-step stepping mode at the most easy starting initial rotor position. Then, the phase back-EMF reaches 0.4V, which is sufficient for the motor to start up rapidly by switching into BLDC mode. 83 (a) The three-phase spindle motor with 6-step stepping mode. (b) The two-phase spindle motor with 4-step stepping mode. Figure 4.6 The stepping starting processes with stepping drive mode at the most easy starting initial rotor position. 84 From Figure 4.6, at the most easy starting initial rotor position, the stepping time of the three-phase spindle motor is about 0.12 second when the motor speed is nearly 700rpm, while the stepping time of the two-phase spindle motor is about 0.06 second with the motor speed about 400rpm. Therefore, the two-phase spindle can be switched into BLDC mode about 0.06 second earlier than the three-phase spindle motor, when both motor started at the best situation. Besides the stepping time, the dangerous area of the two-phase spindle motor with 4step stepping mode is about 43°. It is less than that of the three-phase spindle motor with 6-step stepping mode, which is about 43°. It means the two-phase spindle motor with 4-step stepping mode has more chances to start up easily and faster than the threephase spindle motor with 6-step stepping mode. From the comparisons above, the two-phase spindle motor with 4-step stepping mode can start up faster at lower speed than the three-phase spindle motor with 6-step stepping mode. Obviously, the two-phase spindle motor has better starting capability than three-phase spindle motor if using 8-step stepping mode. 85 4.3 Comparison of 8-step and 12-step Stepping Mode As the previous section, 4-step stepping mode of two-phase spindle motor proves to have better starting capability than 6-step stepping mode of three-phase spindle motor. Therefore, the improved 8-step stepping mode has the more advantage in motor starting. However, similar to two-phase spindle motor, the three-phase spindle motor also has its improved starting method in stepping mode, which is so-called “12-step stepping mode”, which is shown in Figure 4.7. As is seen in Figure 4.7, the commutation sequence of the three-phase inverter in this stepping mode is A+B-C+—A+B-—A+B-C-—A+C-—A+B+C-—B+C-—A-B+C-—A-B+— A-B+C+—A-C+—A-B-C+—B-C+. Corresponding to the two phases and two MOSFETs conducting in 6-step stepping mode, 12-step stepping mode has another 6 steps. During these added 6 steps, all three phases and three MOSFETs work together. The conducting interval is extended to 150°, and the two-phase inverter commutation interval is changed to 30°. This improved stepping mode of three-phase spindle motor can also increase the exciting period of each phase, and then the utilization of the three-phase inverter and the average starting EM torque becomes higher. This stepping mode is also benefit for the motor starting. Therefore, comparing with the starting capability of the three-phase spindle motor with this 12-step stepping mode, does the two-phase spindle motor with 8-step stepping mode still have the advantage? 86 5π 6 π 6 T1 T2 T3 T4 T5 T6 0 α α +π α + 2π θ sequ. 1 2 3 4 5 6 7 8 9101112 A+ A+ AAAstep A+ B- A+ B- A+ B+ B+ B+ A- B+ A- B- Bupper C+ B- C- C- C- C- C- B+ C+ C+ C+ C+ T1 T1 T1 T1T1 T3 T3 T3 T3 T3 T5 T5 T5 T5 T2 T2 T2 T2 T2 T5 lower T4 T4 T4 T4 T4 T6 T6 T6 T6 T6 Figure 4.7 The commutation sequence of 12-step stepping mode. Using a similar three-phase spindle motor tested with 6-step stepping mode, we also gave the same starting current and installed the same disk on the motor to test the 12step stepping mode. And also supposed that the three-phase spindle motor was switched to BLDC mode at the moment of the phase back-EMF reaching 0.4V, when the speed of three-phase spindle motor is about 700rpm. Figure 4.8 shows the stepping time of the three-phase spindle motor varies with the different initial rotor position in 12-step stepping mode. 87 Stepping time (s) 2.5 2 Dangerous area 1.5 1 0.5 0 0 50 100 150 200  250 300 350 Initial rotor position (degree) Figure 4.8 The stepping time of three-phase spindle motor various with the initial rotor position by using 12-step stepping mode. From Figure 4.8, the starting capability of the three-phase spindle motor with 12-step stepping mode is presented in Table 4. 2. Table 4. 2 The starting capability of the three-phase spindle motor with 12-step stepping mode. Dangerous area About 36° Average stepping time 0.264s Longest stepping time 1.98s at about 265° Shortest stepping time 0.08s at about 10° Comparing Table 3. 3 and Table 4. 2, the average stepping time with 8-step stepping mode is shorter than that with 12-step stepping mode. It means the two-phase spindle 88 motor with 8-step stepping mode has better starting capability than the three-phase spindle motor with 12-step stepping mode. Figure 4.9 shows the stepping starting processes of both three-phase spindle motor with 12-step stepping mode and two-phase spindle motor with 8-step stepping mode at the most difficult starting initial rotor position. After that, the phase back-EMF reaches 0.4V, which is sufficient for the motor to spin up rapidly by being switched to BLDC mode. (a) The three-phase spindle motor with 12-step stepping mode. 89 (b) The two-phase spindle motor with 8-step stepping mode. Figure 4.9 The stepping starting processes at the most difficult starting initial rotor position. From Figure 4.9, at the most difficult starting initial rotor position, the stepping time of the three-phase spindle motor cost is about 1.98 second when the motor speed is nearly 700rpm, while the stepping time of the two-phase spindle motor is about 1.13 second with the motor speed about 400rpm. Therefore, the two-phase spindle could be switched into BLDC mode about 0.85 second earlier than the three-phase spindle motors, when both motor started at the worst situation. Figure 4.10 shows the stepping starting processes of both three-phase spindle motor with 12-step stepping mode and two-phase spindle motor with 8-step stepping mode at the most easy starting initial rotor position. 90 (a) The three-phase spindle motor with 12-step stepping mode. (b) The two-phase spindle motor with 8-step stepping mode. Figure 4.10 The stepping starting processes at the most easy starting initial rotor position. 91 From Figure 4.10, at the most easy starting initial rotor position, the stepping time of the three-phase spindle motor cost is about 0.08 second when the motor speed is nearly 700rpm, while the stepping time of the two-phase spindle motor is about 0.03 second with the motor speed about 400rpm. Therefore, the two-phase spindle motor can be switched into BLDC mode about 0.05 second earlier than the three-phase spindle motor, when both motors starting at the best situation. The dangerous area of the two-phase spindle motor with 8-step stepping mode is about 18°. It is less than that of the three-phase spindle motor with 12-step stepping mode, which is about 36°. It means the two-phase spindle motor with 8-step stepping mode has more chances to start up easily and faster than the three-phase spindle motor with 12-step stepping mode. From the comparisons above, the two-phase spindle motor with 8-step stepping mode can start up faster at lower speed than the three-phase spindle motor with 12-step stepping mode. Table 4. 3 The starting capability of both spindle motors. Dangerous area Average stepping time Longest stepping time Shortest stepping time Two-phase/4-step About 43° 0.283s 1.91s 0.06s Two-phase/8-step About 18° 0.151s 1.13s 0.03s Three-phase/6-step About 52° 0.324s 2.1s 0.12s Three-phase/12-step About 36° 0.264s 1.98s 0.08s 92 Table 4. 3 summarizes the starting capability of the spindle motor mentioned above. Generally, the comparison results show that the two-phase structure has better performances in starting than the three-phase motor. 93 Chapter 5 Conclusions and Future Work 5.1 Conclusions With the progress of the large capacity and high spin speed HDD, the spindle motors used in HDD are facing more and more strict requirements. The starting capability of the spindle motor is a major concern as the HDD is trending to be high spin speed operation. As all the HDD products use three-phase spindle motor, there are many solutions to improve the starting capability with different motor structures. For the commonly used phase back-EMF detection method, it is difficult for threephase spindle motor to detect phase back-EMF at still state and low speeds. No matter which starting algorithm is used, it must start up the motor at first to a certain speed where the phase back-EMF is detectable, and then the sensorless BLDC mode can be used. Driving the motor to the certain speed is a time consuming procedure, especially to the spindle motor with the high rated speed. Therefore, in this thesis, the spindle motor with two-phase EM structure and its drive system are proposed and analyzed to improve the starting capability of the spindle motor. The two-phase structure motor has the inherent advantage in motor starting. As analyzed in Chapter 4, the two-phase spindle motor can be switched into sensorless BLDC mode at the lower speed than the three-phase spindle motor. No matter whichever starting method is used, the two-phase spindle motor can detect sufficient phase back-EMF earlier than the three-phase spindle motor. Therefore, the starting capability of the spindle motor is improved. 94 In our research, a prototype of the two-phase sensorless BLDC spindle motor drive system has been built up. A phase back-EMF detecting circuit is presented to realize the sensorless control. A DSP and a two-phase inverter are used as the control and power units to drive the motor in BLDC mode, and realize the close-loop speed control and constant voltage drive mode. The experiments have been done to test the performances of the two-phase spindle motor driven in sensorless BLDC mode. And its performances are also simulated by using the built-up mathematical mode. The experimental results match well with the simulation results. Both of them prove the effectiveness of the two-phase sensorless BLDC spindle motor drive system built up. The starting capability of the two-phase sensorless BLDC spindle motor is analyzed in this thesis. The 4-step stepping mode is used to start the two-phase spindle motor, and the test results show that the starting capability of the spindle motor depends on the initial starting rotor position. An improved starting method of two-phase spindle motor is also introduced. The testing results show that two-phase spindle motor with 8-step stepping mode can be switched into the BLDC mode faster than that with 4-step stepping mode. This means 8-step stepping mode has better starting capability than 4-step stepping mode for the two phase spindle motor. 95 In this thesis, the comparisons of the starting capability between two-phase and threephase spindle motors are also discussed. The results prove that the two-phase motor has better starting capability. This advantage of two-phase spindle motor offers a method to solve the starting problem caused by the high-speed spindle motor, which is very concerned in the new generation HDD. 96 5.2 Future Work So far, our research has been around the sensorless BLDC control of the two-phase spindle motor. Though the results show that the two-phase BLDC spindle motor has an attractive starting capability, but there are still many researches should be done for applying the technology in HDD products, and following the new requirements of HDD development. The important areas include: i. EM analysis of the two-phase spindle motor. As the restriction in the research time, we had to utilize the stator core of a threephase spindle motor to rebuild a two-phase spindle motor and used it in our research. In this procedure, we did not do EM optimization. The further research is necessary in analyzing the effects of the EM pole number, slot number, tooth width, permanent magnet characteristics to the cogging torque and efficiency of the two phase spindle motor. This is important in realizing a high performance two-phase BLDC system. The starting capability of the two-phase spindle motor can be further improved through EM optimization design. ii. Control system to the two-phase spindle motor with square-connection. The armature windings of the two-phase spindle motor can be connected in crossconnection or square-connection. Different connections make the motor show different performances. How to realize BLDC drive mode to the square-connection spindle motor? What are the advantages and disadvantages of the squareconnection two-phase spindle motors in comparing with the cross-connection ones? Only the further research can find the answers. 97 iii. Effects of the two-phase spindle motor used in the small form factor HDDs. 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Ehsani, “Four Quadrant Sensorless Brushless ECM Drive.” IEEE Applied Power Electronics Conference and Exposition, pp. 305-312, June 1980. 101 [...]... presented and analyzed And the optimal commutation angle of the two- phase spindle motor driven by BLDC mode is also given Then the hardware implementation of two- phase motor drive system is introduced in detail Finally, the simulation and experiment results show how the two- phase spindle motor operates in sensorless BLDC mode 2.1 Spindle Motor with Two- Phase Structure 2.1.1 Three -phase structure The spindle. .. mathematical model of the two- phase spindle motor is presented and analyzed The optimal commutation angle of the two- phase spindle motor driven by BLDC mode is also studied Then the hardware implementation of two- phase motor drive system is introduced in detail Finally, the simulation and experiment results are provided and analyzed These will show how the two- phase spindle motor can operate in sensorless BLDC. .. thesis, and future research works are also suggested 12 Chapter 2 Two- Phase Spindle Motor Driven by Sensorless BLDC Mode In this chapter, the two- phase spindle motor drive system is introduced Firstly, the structure of the two- phase spindle motor is shown Secondly, the back-EMF detection sensorless control scheme is introduced briefly Thirdly, the mathematical model of the two- phase spindle motor is... capability 10 1.4 Organization of Thesis The work of this thesis introduces an effective method to realize the sensorless BLDC control for the two- phase spindle motors The starting capability of the two- phase spindle motor is studied, and the advantages and disadvantages of the two- phase spindle motor are also discussed In Chapter 2, firstly, the structure of the two- phase spindle motor is shown Secondly,...List of Tables Table 2 1 Motor parameters in Matlab 43 Table 3 1 Parameters of two- phase spindle motor 65 Table 3 2 The starting capability of the two- phase spindle motor with 4-step stepping mode 67 Table 3 3 The starting capability of the two- phase spindle motor with 8-step stepping mode 70 Table 4 1 The starting capability of the two- phase spindle motor. .. is simple and effective It has been used for a long time in the PM motor drive [2][3][24][25] In this thesis, this method is also used to realize the sensorless control of a two- phase BLDC spindle motor 19 2.3 Modeling of Two- phase Spindle Motor In order to study the performances of two- phase spindle motors, a mathematic model is built In general, a multi -phase permanent magnet motor can be modeled as... AX by 90° And the phase back-EMF of two- phase spindle motor is designed sinusoidal, and therefore, it can be expressed by the peak value and phase angle: eAX (θ ) = Em sin θ = K E ⋅ ωm sin θ eBY (θ ) = eAX θ − π 2 = Em sin θ − where the phase angle − π 2 (2.4) π 2 = − K E ⋅ ωm cos θ (2.5) shows that in two- phase spindle motor, phase BY lags phase AX by 90° In Eq (2.4) and (2.5), Em is amplitude of. .. number of phase; uk is the voltage of phase k; Rk is the resistance of phase k; ik is the current of phase k; Lkl is the mutual inductance between the phase k to the phase l, and when l is equal to k, Lkl is the self inductance of phase k; ek is the back-EMF of phase k Since one phase winding of a two- phase spindle motor is quadrature to another, therefore, the mature inductances between the two phases... LAX u BY (θ ) = RBY ⋅ iBY (θ ) + LBY di AX (θ ) + eAX (θ ) dt diBY (θ ) dt + eBY (θ ) (2.2) (2.3) where u AX and u BY are the phase voltages; iAX and iBY are the phase currents; RAX and RBY are the phase resistances; LAX and LBY are the self inductances; eAX and eBY are the phase back-EMF; θ indicates the rotor position relative to the d axis 20 As mentioned in Section 2.1.2, the phase BY lags the phase. .. stable spin speed and fast dynamic responses as well as high efficiency, the spindle motor is driven by sensorless BLDC mode 3 1.2 BLDC Motors and Sensorless Drives BLDC motors are one kind of permanent magnet synchronous motors [1], having permanent magnets on the rotor and employing a dc power supply switched to the stator phase windings of the motor by power electronic devices And the phase current