Handbook of Small Electric Motors MAZ Part 17 ppt

CHAPTER 10 DRIVES AND CONTROLS Chapter Contributors Birch L. Devault Duane C. Hanselman Daniel P. Heckenkamp Dan Jones Douglas W. Jones Ramani Kalpathi Todd L. King Robert M. Setbacken 10.1 10.1 MEASUREMENT SYSTEMS TERMINOLOGY* 10.1.1 Measurement Units The linear unit of length is the meter (m). It is the distance light travels in approxi- mately 1/300,000,000 (1/299,792,458) s. Linear measurement systems commonly define design parameters in units of the micron (µ), or 0.000001 m. 1 µ is equivalent to approximately 0.000040 in. 0.0001 in = 2.54 µ.The angular unit is the radian (rad), which is the angle subtended by an arc whose length is equal to the radius of a circle. This unit of measurement is most commonly used in military applications. The degree (°) is used mostly in commercial applications. Fine angles are represented as both fractions of degrees and as minutes (′) and seconds (″). 1′=1/60°;1″=1/60′. 10.1.2 Accuracy and Resolution Defined Accuracy is the ability to repeatably indicate an exact location, while resolution is the ability to detect motion in finer and finer increments. For a rotary encoder, this is in cycles per revolution (cpr) or pulses per revolution (ppr). For a linear system, * Sections 10.1 to 10.5 contributed by Robert M. Setbacken, Renco Encoders. this is counts per inch, or it is defined in terms of the graduation pitch in microns. Accuracy and resolution are not directly related. Although it is generally true that high accuracy systems usually resolve smaller increments, a measuring device could in principle have very coarse resolution and still be very accurate. 10.1.3 Quadrature In Fig. 10.1, the 90° electrical separation (one-quarter period) between the two sig- nals is referred to as quadrature. Quadrature signals allow the user to know what direction the system is turning, and provide additional resolution by allowing edge counting. 10.1.4 Edge Counting Again referring to Fig. 10.1, it can be seen that within one cycle, there are four edge transitions between the two output signals.This can effectively be used to provide a resolution of 4 times the base resolution. 10.1.5 Direction Sensing Referring to Fig. 10.2, one can see that when B makes a low transition, the value of A is locked into Q. When the system is moving clockwise (CW), Q will be low. When the system is moving counterclockwise (CCW), Q will be high. This scheme can be used within the sensor to provide a pulse output with a high/low direction indicator. 10.2 CHAPTER TEN FIGURE 10.1 Output waveform definitions. 10.1.6 Interpolation or Multiplication Interpolation is the process of dividing an analog signal into phase-shifted copies, which are then recombined to give a higher effective resolution.When the output of a sensor is sinusoidal and there are two outputs in quadrature, the signals can be interpolated. Transistor-transistor logic (TTL) signals can not be interpolated. As a result, interpolation can be used to improve overall accuracy by reducing the error component due to quantization. 10.1.7 Contacting Systems There are various interpretations of what this term means. Linear encoders that use bearings to control the gap between the read head and the scale are called noncon- tacting. Linear encoders that use low-friction coatings on the glass surfaces to float the read head over the scale are contacting. A more explicit definition of contacting sensors includes potentiometers and pin-contact encoders. Although contact meth- ods are still used, and some companies have developed very robust examples, long- term reliability is favoring noncontacting designs. Some applications still find uses for contacting sensors, especially pin-contact encoders. One major example is in the nuclear industry, where pin-contact encoders generally last as long as the measured system itself. Magnetic systems loose magnetization, and optical systems using plas- tic are fogged due to the radiation in these environments, so pin-contact encoders work very well. 10.1.8 Non-contacting Systems These systems generally offer higher reliability, and are typified by the following ● Optical, capacitive, and magnetic encoders ● Brushless resolvers DRIVES AND CONTROLS 10.3 FIGURE 10.2 Quadrature direction encoding. ● Most modular or kit encoders ● Open-frame linear scales. Note that the use of incorporation of bearings into a feedback device does not exclude it from being described as a noncontacting sensor. Make sure you are fully aware of the manufacturing principles when specifying a noncontacting sensor.Truly noncontacting sensors, like modular rotary encoders or brushless resolvers, can still become partially contacting devices if seals are incorporated in the final installation to the application. 10.2 ENVIRONMENTAL STANDARDS 10.2.1 Specification of the Application Environment The end user needs to have some idea of the environment in which the sensor is to be placed. Many times the final installed environment cannot be known. This is generally the case for motor manufacturers that ship to original equipment manu- facturers (OEMs), which then ship products of various types all over the world. In order to address such situations, various standards organizations have developed guidelines which can be used to characterize the applications a device should be able to withstand. In Europe, the International Electrotechnical Commission (IEC) has developed a large suite of specifications covering every imaginable detail. The United States has been relying on military standards (MIL-STDs) when such guidance is required. Finally, the various industries themselves develop de-facto standards through the published specifications for their products. In the United States, the two most widely referenced standards for feedback elements are the following: MIL-STD-810 Environmental test methods MIL-STD-202 Test methods for electronic and electrical component parts Similar standards from the IEC are the following: IEC 68-1 Part 1: General and guidance IEC 68-2-1 Test A: Cold IEC 68-2-2 Test B: Dry heat IEC 68-2-3 Test Ca: Damp heat, steady state IEC 68-2-6 Test Fc and guidance:Vibration (sinusoidal) IEC 68-2-27 Test Ea and guidance: Shock mounting of components, IEC 68-2-47 equipment, and other articles for dynamic tests including shock (Test Ea), bump (Test Eb), vibration (Tests Fc and Fd), steady-state acceleration (Test Ga), and guidance IEC 68-2-48 Guidance on the application of the tests in IEC Publication 68 to simulate the effects of storage IEC 529 Degrees of protection provided by enclosures (IP code) 10.4 CHAPTER TEN IEC 34-5 Classification of degrees of protection provided by enclo- sures of rotating electrical machines When possible, the user should request a test program report for the device being considered. Even if a device is tested, it is important to know what passing the test entails.The IEC uses the following definitions: A No degradation during or after B Degradation during, not after C Loss of function but undamaged; operation restored by reset Standard Atmospheric Conditions IEC specifications for ambient atmospheric conditions are as follows: Temperature Relative humidity Air pressure 23 Ϯ 2°C 45 to 55% 86 to 106 kPa Test Programs. A reasonable test program for sensor design verification should consist of both environmental and mechanical testing. Environmental Testing. This testing should consist of climatic sequencing. The IEC guidelines suggest the following order: 1. Dry heat 2. Damp heat 3. Cold 4. Low air pressure 5. Damp heat, cyclic Not all test programs must include all tests, but the tests included should run in this order.An interval of not more than 3 days is permitted between any of these condi- tionings, except for the interval between the first cycle of the damp heat cyclic con- ditioning and the cold conditioning. For this period, the interval shall not be more than 2 h, including recovery. Suggested severity levels for environmental testing of feedback devices are as follows: ● Dry heat. 1000 h of dry heat at 110 Ϯ 2°C, with relative humidity during the test- ing not exceeding 50 percent. ● Damp heat steady state. 500 h of damp heat at 85 Ϯ 2°C, with relative humidity during the testing at 85 Ϯ 10 percent. ● Cold. 500 h of cold at −30 Ϯ 2°C, with relative humidity during the testing not specified. Mechanical Testing. This testing must provide assurance that the sensor can withstand the effects of storage, transportation, and the final application environ- ment. IEC guidelines provide model environments, such as would be found in ground, air, or space applications. Suggested severity levels for mechanical testing of feed- back devices are as follows: DRIVES AND CONTROLS 10.5 ● Vibration testing. Between 10 and 2000 Hz, with an amplitude of gs above 57 Hz. Below this frequency, the motion will be amplitude limited to approximately 0.030 in maximum, with frequency sweep from low to high and back 10 times at a sweep rate of 1 octave/min.This test should be conducted in the vertical and hor- izontal axes. ● Shock testing. Using a half-sine wave form at 100 g for 6 m, 3 shocks in the posi- tive and negative direction for each axis, for a total of 6 shocks. Responsibility for Test Certifications. If you are involved with the shipment of motion-control products to Europe, the CE mark is now the means through which the European Community will check to see if you have done your homework. Sup- pliers of products which require the CE mark must not only have designed the units using safe practices, used proper design rules, and validated the designs with proper testing, they must also make the design process records available to anyone who needs them within 3 days of a request. 10.2.2 Environmental Protection Sealed. Although there are National Electrical Manufacturers Association (NEMA) specifications for many types of devices and enclosures, the IEC specifica- tions seem to be the most common. Tables 10.1 and 10.2 summarize the Interna- tional Protection (IP) codes. For a complete discussion of these ratings, the specifications IEC 529 and/or IEC 34-5 should be examined. Exposed. Open-frame tachometers, resolvers, and encoders must be protected by the application equipment from environmental concerns. In the servo industry today, three basic technologies are used in the majority of applications.These consist of sensors using either magnetic, inductive, or optical methods. Magnetic sensors are of two types: those using ac technology, such as synchros, inductosyns, and resolvers; and those using permanent-magnet (PM) technology, such as magnetic encoders, Hall devices, and the like. They tend to be used in very low cost, low-accuracy applications, or when the sensor must be run exposed to the elements (e.g., in submerged or high-particulate environments). Inductive transducers, particularly resolvers, are used in extremely rugged envi- ronments where accuracy is not of first importance. Optical encoders are chosen for applications in which accuracy and stability are of primary importance. The cost of an inductive transducer is generally lower than that of an optical one, but the costs equalize or begin to favor the encoder when interface electronics and overall performance issues are directly compared. Today, integrated circuit (IC) technology and application-specific IC (ASIC) integration capabilities are making the inductive interface circuits more simple, robust, and cost-effective, while manu- facturers of optical sensors are using the same methods to lower product part count and overall costs. The Institute for Applied Microelectronics has developed a two-chip set that will implement the entire drive electronics for a brushless dc (BLDC) motor. The chip set will accept sinusoidal commutation signals and incremental encoder and resolver inputs, and has a small-scale integration (SSI) interface for communication with absolute encoders. When components of this capability become available, system cost will depend exclusively on performance requirements. 10.6 CHAPTER TEN The sensor configuration of the motor and sensor package chosen depends ulti- mately on the intended application. Cost is always an important issue,and for BLDC motors, there appear to be five categories of applications. 1. Low-cost motors for basically constant-speed operation. Typical examples are fan motors, fuel pumps, and disk drives. These are very high volume, low-cost applications where tooling of molded magnets and Hall structures can be justi- fied.Alternatively, many are doing away with Hall sensors and going to smart IC controls. Control chips made by Allegro Microsystems, Inc., Hitachi America DRIVES AND CONTROLS 10.7 TABLE 10.1 IP Nomenclature—Degrees of Protection Indicated by the First Characteristic Numeral First characteristic numeral Brief description* Definition 0 Machine nonprotected No special protection. 1 † Machine protected against Accidental or inadvertent contact with or solid objects >50 mm approach to live and moving parts inside the enclosure by a large surface of the human body, such as a hand (but no protec- tion against deliberate access). Ingress of solid objects exceeding 50 mm in diameter. 2 † Machine protected Contact with or approach to live or moving against solid objects parts inside the enclosure by fingers or sim- >12.5 mm ilar objects not exceeding 80 mm in length. Ingress of solid objects exceeding 12 mm in diameter. 3 † Machine protected Contact with or approach to live or moving against solid objects parts inside the enclosure by tools or wires >2.5 mm exceeding 2.5 mm in diameter. Ingress of solid objects exceeding 2.5 mm in diameter. 4 † Machine protected Contact with or approach with live or mov- against solid objects ing parts inside the enclosure by wires or >1 mm strips of thickness greater than 1 mm. Ingress of solid objects exceeding 1 mm in diameter. 5 ‡ Machine dust-protected Contact with or approach to live or moving parts inside the enclosure. Ingress of dust not totally prevented, but dust does not enter in sufficient quantity to interfere with satisfactory operation of the machine. 6 § Machine dust-tight No ingress of dust. * This description should not be used to specify the form of protection. † Machines assigned a first characteristic numeral of 1,2,3, or 4 will exclude both regularly or irregularly shaped solid objects provided that three normally perpendicular dimensions of the object exceed the appro- priate description in the Definition column. ‡ The degree of protection against dust defined by this standard is a general one. When the nature of the dust (dimensions of particles and, their nature; for instance, fibrous particles) is specified, test conditions should be determined by agreement between the manufacturer and the user. § Not specified under IEC 34-5 for rotating machines. Degree of protection of equipment Ltd., Micro Linear Corporation, Signetics Company, Silicon Systems, Inc., and SGS-Thomson Microelectronics, Inc., can provide complete commutation of BLDC motors.Some of these controllers even provide braking and speed control as part of the package, so an external sensor like an encoder or a resolver is not needed for this type of servo application. 2. Traditional BLDC motors with resolver or encoder feedback. These are motors which contain an encoder or a resolver for position feedback and possibly a tach- ometer as well, depending on the control system being implemented. Encoder- based systems also require Hall sensors for commutation. Resolver systems used with a rectangular drive could use Hall sensors as well, but this is usually all that is needed.These types of motors have been the backbone of the BLDC motor indus- try for the past decade and are found in a wide variety of applications. 10.8 CHAPTER TEN TABLE 10.2 IP Nomenclature—Degrees of Protection Indicated by the Second Characteristic Numeral Degree of protection of equipment Second characteristic numeral Brief description* Definition 0 nonprotected No special protection. 1 Machine protected against Dripping water (vertically falling drops) dripping water shall have no harmful effect. 2 Machine protected against Vertically dripping water shall have no dripping water when harmful effect when the machine is tilted tilted up to 15° at any angle up to 15° from its normal posi- tion. 3 Machine protected against Water falling as a spray at an angle up to spraying water 60° from the vertical shall have no harmful effect. 4 Machine protected Water splashing against the machine from against splashing water any direction shall have no harmful effect. 5 Machine protected Water projected by a nozzle against the against water jets machine from any direction shall have no harmful effect. 6 Machine protected Water from heavy seas or water projected against powerful water in powerful jets shall not enter the machine jets in harmful quantities. 7 Machine protected Ingress of water in the machine in a harm- against the effects of ful quantity shall not be possible when the temporary immersion in machine is immersed in water under stated water conditions of pressure and time. 8 Machine protected The machine is suitable for continuous sub- against continuous mersion in water under conditions which submersion shall be specified by the manufacturer. † * This brief description should not be used to specify the form of protection. † Normally, this will mean that the machine is hermetically sealed. However, with certain types of machines it can mean that water can enter but only in such a manner that it produces no harmful effect. Degree of protection of equipment 3. Integrated-sensor motors. These use optical encoders which generate rotor- position as well as incremental-position signals. The rotor-position signals are electrically the same as can be obtained from Hall switches,and they can be used for commutation of two-, three-, or four-pole-pair motors. Integrated-sensor BLDC motors are being used in Japan and the United States to provide high- performance servodrive solutions to cost-critical applications. The encoders are built-in hollow-shaft encoders, and generally come in resolutions up to 13 bits (2 13 = 8192 cpr). 4. High-performance integrated-sensor motors. These are used in systems requir- ing large dynamic range in the speed control (such as z-axis control in a machine tool), very high resolution, or very low speed operation. These are being devel- oped primarily in Europe and are distinguished by sinusoidal rather than TTL output signals. 5. Smart motors. These are high-performance integrated-sensor motors requiring additional capabilities such as absolute positioning, bus interfaces, storage for motor data, temperature monitoring, etc.This is currently a very small portion of the market, but it is definitely growing. The sensors for these motors provide commutation outputs, incremental outputs, and up to 25 bits of absolute-position data, 13 bits per turn with 12-bit turn counting. 10.3 FEEDBACK ELEMENTS 10.3.1 Rotary and Linear Incremental Optical Encoders Optical encoders (Fig. 10.3) can be characterized by the physical measurement prin- ciple they use (diffraction or directed light), their design features, and the protection requirements to which they are built. They range from completely enclosed and sealed units to open-frame kit units.They are typically used in velocity- or position- feedback systems such as those found in tape transport equipment, machine-tool spindle controls, bed positioning equipment, woodworking machines, robots, material-handling equipment, textile machines, plotters, printers, tape drives, and a variety of measuring and testing devices. Commercial encoders are generally defined as being capable of measuring angles of up to 30″. For higher resolutions, an angular measurement device must be used. These devices are capable of measuring angles as fine as 0.000010° (0.036″). There are three categories of encoders from an environmental protection view- point. Sealed encoders are generally protected to the levels of IP 64 or better.These are stand-alone units that have internal bearings and seals and are not intended to allow user access to internal workings. Self-contained encoders are not necessarily dust proof. These have internal bearings and are stand-alone units, but some cus- tomer access may be possible or may even be necessary during installation. Modular encoders are completely open units which rely entirely on the application for pro- tection. These units do not contain bearings. They are sometimes referred to as kit encoders or tach kits. Sealed units are the most expensive, and generally are not well suited for high- speed operation because of the seals. However, these can be very high accuracy, high-resolution devices, capable of resolutions ranging up to 10,000 cpr. Modular encoders are the lowest in cost. These units generally have the best price-to-performance rating, but they require some care on the part of the user as DRIVES AND CONTROLS 10.9 they can be damaged if not installed properly. Modular units are available with res- olutions up to 2500 cpr. Self-contained encoders span the entire performance envelope, at a slightly higher cost than modular devices. The self-contained hollow-shaft encoders are widely used in the drive industry, as they eliminate coupling resonance. Hollow-shaft encoders are also widely used with integrated commutation elec- tronics.This provides a simplified assembly process to the manufacturer by allowing elimination of the Hall board.This approach also simplifies overall alignment. Terms amplitude modulation Using the code wheel and mask as an optical shutter, or to create Moiré patterns to modulate the intensity of light impinging on the pho- todetectors. code wheel A circular disk of transparent material with patterns of transmissive and opaque regions equally spaced about the perimeter. Light shining through the clear regions is passed onto the mask.The spacing on the code wheel defines the line count of the encoder.1024 opaque regions separated by 1024 clear spaces will create a 1024-cpr encoder. disk Another term for code wheel. 10.10 CHAPTER TEN FIGURE 10.3 Rotary optical encoder. [...]... Greek, act of turning; to turn; to twist; action of whirling): The movement of the classical Greek chorus while turning from one side to the other of the orchestra (Webster’s Seventh New Collegiate Dictionary, 1971) Methods of Fabrication A single-turn absolute encoder can generally be produced with up to 14 bits of position information 14 bits results in 16,384 unique positions per revolution of the encoder... termed polystrophic because of this characteristic of multiple bit changes Polystrophism is a problem because in a real-world situation, all these bits will not change simultaneously There will be some slight ambiguity, for however small a time, which will result in the possibility of the encoder generating incorrect outputs All of the problems associated with the manufacture of an accurate incremental... cycles The selection of values for R2 and R3 controls the amount of hysteresis in the circuit, while the values for R1 and R4 control the photodiode output-signal levels Balance Adjustment To develop a 50 percent duty cycle at the output of the comparator, the input offset levels must be identical This will never occur naturally for a number of reasons, but primarily because the amount of light shining... hundreds of meters In their most basic form, they are comprised of a graduated scale, a read head, and mounting hardware The read head contains the illumination source, the scanning reticle, and the signal-conditioning electronics The scale can be made of glass, steel, or plastic Linear encoders are found in a wide variety of applications, and because of this, there is a need for various types of environmental... encoders, but TTL outputs tend to be only of the line-drive type (RS422) Analog outputs are either amplified sine-wave or current outputs of the 11-µA peak-to-peak type Accuracy and Resolution Linear encoders are capable of accurately measuring 1 µm/m, [1 part per million (ppm)] An absolute accuracy of 0.5 µm is readily available as well, but not as common The major source of error in a system using linear... pole pieces added (Fig 10.20) allow gain adjustment The addition of a pole piece allows actuation at a greater distance (D4 instead of D3; D2 instead of D1) Bias magnets can FIGURE 10.16 Sensor output hysteresis curve DRIVES AND CONTROLS FIGURE 10 .17 characteristics 10.31 Digital unipolar Hall sensor operational allow the addition of an offset to the flux curve, allowing the user to fine-tune it Care... (Electro-Craft Handbook, 1980; Ernst, 1989) Error consists of intrinsic instrument errors in the encoder, plus system errors System errors are due to the following causes: q q Hysteresis effects The amount of hysteresis used to control noise will effect overall accuracy, as this changes the switching point of the output in a TTL system and introduces phase lag in an analog system Runout due to eccentricity of. .. minimize the effects of detector changes, light variation, and voltage variation When the sensors drift out of balance with each other, symmetry in the quadrature output will change Interpolation There are many methods of developing higher-resolution TTL outputs by processing the analog sinusoidal signals developed in the measurement system One consists of developing phase-shifted copies of the original... gearing to drive smaller encoders, an additional 12 bits of information can be obtained The multiple encoder outputs must be carefully combined, using overlap bits, to ensure that transition errors will not occur Of course, these devices are complex and require that precision mechanical components work properly They are available from a number of manufacturers Application Considerations Because of large output... Figure 10.6 shows how interpolation of 5× would compare with the original output Interpolation of this type can be used for multiplication up to 25× with reasonable success Higher subdivisions are obtained using digital methods One such DRIVES AND CONTROLS FIGURE 10.6 10 .17 Interpolation of 5× compared to original output method computes the arctangent using the values of the two analog quadrature signals . application of the tests in IEC Publication 68 to simulate the effects of storage IEC 529 Degrees of protection provided by enclosures (IP code) 10.4 CHAPTER TEN IEC 34-5 Classification of degrees of. types of motors have been the backbone of the BLDC motor indus- try for the past decade and are found in a wide variety of applications. 10.8 CHAPTER TEN TABLE 10.2 IP Nomenclature—Degrees of Protection. of BLDC motors. Some of these controllers even provide braking and speed control as part of the package, so an external sensor like an encoder or a resolver is not needed for this type of servo application. 2.