hệ thống điều khiển động cơ Servo

Chapter Motors & Servo Drives @VLC-2015 3.1 Stepper Motors 3.1.1 Introduction Stepper motors convert electrical energy into discrete mechanical rotation Stepping motors have the following advantages and disadvantages  Advantages:  Full torque when rotation is stopped This aids in maintaining the current position  Precise open-loop positioning and repetition High quality stepping motors have three to five percent precision within a single step  Quick starts, stop, and reverse capability  High reliability because there is no brush or physical contact required for commutation @VLC-2015 3.1.1 Introduction  Disadvantages:  Inherent resonance can cause noise, jerky rotation, and at extreme levels, loss of position  It is possible to lose position control in some situations, because no feedback is natively provided  Power consumption does not decrease to zero, even if load is absent or motor is in stop mode  Stepping motors have low-power density and lower maximum speed compared to brushed and brushless DC motors Typical loaded maximum operating speeds for stepper motors are around 1000 RPM @VLC-2015 3.1.2 Classification  Types of Stepping motors:  Variable reluctance motors  Permanent magnet motors  Hybrid motors  Variable Reluctance (VR) Motors VR stepping motors have three to five windings and a common terminal connection, creating several phases on the stator The rotor is toothed and made of metal, but is not permanently magnetized 4 teeth and independent windings (six phases), creating 30 degree steps @VLC-2015 VR Stepper Motors Operation: The rotation of a VR motor is produced by energizing individual windings When a winding is energized, current flows and magnetic poles are created, which attracts the metal teeth of the rotor The rotor moves one step to align the offset teeth to the energized winding When the phases are turned on sequentially, the rotor rotates continuously @VLC-2015 12 steps per revolution PM Stepper Motors  Permanent Magnet (PM) Motors  A PM stepping motor consists of a stator with windings and a rotor with permanent magnet poles Alternate rotor poles have rectilinear forms parallel to the motor axis  Stepping motors with magnetized rotors provide greater flux and torque rotor pole pairs and than motors with variable reluctance independent stator windings, creating 30  PM motors are subjected to influence from the back-EMF of degree steps the rotor, which limits the @VLC-2015 maximum speed PM Motors Operation: Rotation of a PM stepping motor is produced by energizing individual windings in a positive or negative direction When a winding is energized, a north and south pole are created, depending on the polarity of the current flowing These generated poles attract the permanent poles of the rotor The rotor moves one step to align the offset permanent poles to the corresponding energized windings When the phases are turned on sequentially the rotor is continuously rotated @VLC-2015 @VLC-2015 12 steps per revolution PM Motors Another alternative to rotate a permanent magnet rotor is to energize both windings in each step The vector torque generated by each of the coils is additive; this doubles the current flowing in the motor, and increases the torque 12 steps per revolution Typical PM motors have more poles to create smaller steps To make significantly smaller steps down to one degree or even lower @VLC-2015 Hybrid Stepper Motors  Hybrid Motors  Hybrid stepping motors combine a permanent magnet and a rotor with metal teeth to provide features of the VR and PM motors  Hybrid motors are expensive, but they use smaller steps, then have greater torque, and have greater maximum speeds  Rotation of a hybrid stepping motor is produced with the same control method as a PM motor, by energizing individual windings in a positive or negative direction @VLC-2015 3.1.3 Motors Connection and Wiring  Identify the motor leads The color code of the wires coming out of the motor are not standard; however, using a multimeter/ohmmeter, it is easy to identify the winding ends and center tap  leads: the motor is a bipolar motor If the resistance measured across two terminals is finite, then those are ends of a coil If the multimeter shows an open circuit then the terminals are of different windings 10 @VLC-2015 3.2.3 DC Servo Drive  4X Encoding State Transition 38 @VLC-2015 3.2.3 DC Servo Drive  Measuring Speed and Position  To prepare the variable for fractional operations performed by control algorithms, you need to convert the position counter result into a signed fractional number  The resolution: MAXCOUNT 3600 𝑅𝐸𝑆𝑂𝐿𝑈𝑇𝐼𝑂𝑁 = 𝑀𝐴𝑋𝐼𝑀𝑈𝑀 𝐶𝑂𝑈𝑁𝑇 𝑃𝐸𝑅 𝑅𝐸𝑉𝑂𝐿𝑈𝑇𝐼𝑂𝑁  With this resolution, the position count variable needs to be converted from to (MAXCOUNT-1) to a signed fractional 16-bit value of to 32767 The following formula shows the scaling factor 39 @VLC-2015 𝐶𝑜𝑢𝑛𝑡𝑉𝑎𝑙𝑢𝑒 × 32768 𝐴𝑛𝑔𝑃𝑜𝑠 = 𝑀𝐴𝑋𝐶𝑂𝑈𝑁𝑇 3.2.3 DC Servo Drive  Code example:  Interrupt Period Calculation The velocity calculation is performed in a periodic interrupt This interrupt interval must be less than the minimum time required for a ½ revolution at maximum speed 40 @VLC-2015 𝐼𝑁𝑇𝐸𝑅𝑅𝑈𝑃𝑇𝑃𝐸𝑅𝐼𝑂𝐷 = 60 (s) 2×𝑀𝐴𝑋_𝑆𝑃𝐸𝐸𝐷_𝑅𝑃𝑀 3.2.3 DC Servo Drive  Code example of Speed Calculation: 41 @VLC-2015 3.2.3 DC Servo Drive  PID Controller Block diagram de(t ) u (t )  K p e(t )  K D  K I  e(t )dt dt 𝐾𝑝 𝑒(𝑡) 𝐾𝐷 𝐾𝐼 𝑒 𝑡 𝑑𝑡 Derivative filter 42 @VLC-2015 𝑢(𝑡) 𝑑𝑒(𝑡) 𝑑𝑡 Anti-windup 𝑢(𝑡) 3.2.3 DC Servo Drive  PID Calculations When implementing the PID controller in practice, the input variable (error) is obtained by sampling the plant‟s out put at the sample rate Then, the PID algorithm is also calculated at the same rate uk  u P k  u D k  u I k The sample rate is called sampling time, 𝑇 (𝑠)  P Calculations u P k  K P ek  D Calculations 43 u @VLC-2015 D k ek  ek 1  KD T 3.2.3 DC Servo Drive Filter 𝐸(𝑠) 𝐾𝐷 𝑠 𝑁𝑠 + 𝑈 𝐷 (𝑠) K D sE ( s )  (Ns  1)U D (s ) de(t ) du D (t ) KD N  u D (t ) dt dt ek  ek 1 u D k  u D k 1 KD N  u Dk T T KD N D D u k  u k 1  (ek  ek 1 ) N T N T 44 @VLC-2015 3.2.3 DC Servo Drive  I Calculations There are three methods to calculate the integral term as the following figures We will only a method, the others could be done similarly e(t) e(t) e(t) 𝑒𝑘 𝑒𝑘 𝑒𝑘−1 𝑒𝑘−1 𝑒𝑘 𝑒𝑘−1 t (k-1)T t kT (k-1)T a) 𝑒 𝜏 𝑑𝜏 ≈ 𝑇 45 @VLC-2015 𝑒 𝜏 𝑑𝜏 ≈ 𝑇 𝑒(𝑘 − 1) u I k  u I k 1  K I Tek kT c) 𝑡 𝑡 𝑒(𝑘) (k-1)T kT b) 𝑡 𝑡 t 𝑡 𝑡 𝑒 𝜏 𝑑𝜏 ≈ 𝑇 0 (𝑒 𝑘 + +𝑒(𝑘 − 1)) 3.2.3 DC Servo Drive  Code example: 46 @VLC-2015 3.2.3 DC Servo Drive  Anti-windup PID 𝑒 𝑒𝑑𝑡 𝑢 𝐾𝐼 𝐼 𝑢 𝐾𝑏 𝑢 𝑒𝑟𝑒𝑠𝑒𝑡 t u I (t )    K I e( )  K b ereset ( ) d u 47 I k  u I k 1  K I Tek  K bTe reset k Students use the above equation to write C codes by themselves That is considered as your homework @VLC-2015 3.3 AC Servo Motors Currently, majority of AC servomotors are of the squirrel cage two-phase induction type and utilized in low power applications But recently three phase induction motors has been modified so that they can be used in high power servo applications  Advantages:  Low cost  Higher efficiency and less maintenance  Disadvantages:  Nonlinear characteristics  More difficult to control in positioning applications 48 @VLC-2015 3.3 AC Servo Motors  Introduction to Yaskawa AC servo driver Servopacks are divided into the following two types according to usage:  Speed/Torque Control: SGDA□□□S Type This type uses speed or torque reference input Reference input is by analog voltage  Position Control: SGDA-□□□P Type This type uses position reference input Reference input is by pulse train 49 @VLC-2015 3.3 AC Servo Motors  The most common usage of a Servopack for speed/torque control is shown below: Speed Control The host controller can freely perform the control required for the servo mechanism The Servopack undertakes the speed control loop and subsequent control processing 50 @VLC-2015 3.3 AC Servo Motors Torque Control The host controller outputs a torque reference or speed reference to control the Servopack It also receives a pulse train (position information) from the Servopack and uses it to monitor the position 51 @VLC-2015 3.3 AC Servo Motors  Servopack for position control can be used as below The host controller can send a position reference (pulse train) to the Servopack to perform positioning or interpolation This type of Servopack contains a position control loop 52 For more information about AC servo drivers, students should read manual documents of their manufacturers, such as: Yaskawa, Panasonic, Mitsubishi,… ... step (bipolar) Table for microstep (bipolar) 25 @VLC-2015 3.2 DC Servo Motors 3.2.1 Introduction Permanent Magnet (PM) Brush DC Servo Motor  Advantages: @VLC-2015 26  Smooth rotation at low... s ) Tm s  3.2.3 DC Servo Drive  The structure of a driver Controller Power Amplifier Saturation Encoder Microcontroller 30 @VLC-2015 H Bridge Circuit DC Motor 3.2.3 DC Servo Drive  H Bridge... to the outer casing is very inefficient  Increased installed cost: the installed cost of a DC servo system is higher than that of a stepper due to the requirement for feedback components 3.2.2