Stepping Motors: a guide to theory and practice (4th Edition)
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This book provides an introductory text which will enable the reader to both appreciate the essential characteristics of stepping motor systems and understand how these characteristics are being exploited in the continuing development of new motors, drives and controllers. A basic theoretical approach relating to the more significant aspects of performance is presented, although it is assumed throughout that the reader has no previous experience of electrical machines and is primarily interested in the applications of stepping motors.
Inspec keywords: stepping motors; microprocessor chips; open loop systems; closed loop systems; machine control
Other keywords: closed-loop control; drive circuit; microprocessor-based stepping motor system; load positioning accuracy; bifilar-wound motor; static torque-speed characteristic; open-loop control
Subjects: Small and special purpose electric machines; Microprocessor chips; Microprocessors and microcomputers; Control of electric power systems
- Book DOI: 10.1049/PBCE063E
- Chapter DOI: 10.1049/PBCE063E
- ISBN: 9780852964170
- e-ISBN: 9780863411380
- Page count: 172
- Format: PDF
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Front Matter
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1 Stepping motors
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The essential property of the stepping motor is its ability to translate switched excitation changes into precisely defined increments of rotor position ('steps'). Step ping motors are categorised as doubly salient machines, which means that they have teeth of magnetically permeable material on both the stationary part (the 'stator') and the rotating part (the 'rotor').
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2 Drive circuits
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There is a bewildering number of circuits for switching current between the motor phases, but in this chapter discussion is confined to the basic circuits, since the potential advantages of more advanced drives are examined at a later stage (Chapter 5).
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3 Accurate load positioning: static torque characteristics
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Most stepping motor applications involve accurate positioning of a mechanical load. For example, the position of a print head is defined very precisely by the number of switched excitation changes that have taken place in the controlling motor. External load torques, perhaps caused by friction, give rise to a small error in position when the motor is stationary. The motor must develop enough torque to balance the load torque and the rotor is therefore displaced by a small angle from the expected step position. The resultant 'static position error' depends on the external torque, but is independent of the number of steps previously executed; the position error is noncumulative.
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4 Multi-step operation: torque/speed characteristics
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In this chapter, if a stepping motor is used to change the position of a mechanical load by several steps the system designer needs to know how much torque the motor can produce whilst accelerating, decelerating or running at constant speed. The motor must produce sufficient torque to overcome the load torque and accelerate the load inertia up to the maximum speed. The necessary information is supplied in the form of a graph known as the pull-out torque/speed characteristic showing the maximum torque which the motor can develop at each operating speed. This maximum torque is termed the 'pull-out' torque because if the load torque exceeds this value the rotor is pulled out of synchronism with the magnetic field and the motor stalls. In this case the motor would be able to drive a load torque of 0.2 Nm at all speeds up to 500 steps per second, because the pull-out torque exceeds the load torque over this speed range. However, for a load torque of 0.4 Nm the maximum operating speed would have to be limited to 200 steps per second and there would be additional problems in operating at speeds around 20 and 40 steps per second. For a given load the maximum operating speed is referred to as the 'pull-out' rate, so in this example the pull-out rate for a load of 0.2 Nm is 500 steps per second.
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5 High-speed operation
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In many applications the motor must be able to produce a large pull-out torque over a wide range of stepping rates, so the time taken to position a load is minimised. At high stepping rates each phase is excited for only a short time interval and the build-up time of the phase current is a significant proportion of the excitation interval. When a motor is operating at the highest speeds the current in each phase may not even reach its rated value before the excitation interval finishes and the phase is turned off. In addition the time taken for the phase current to decay becomes important at high speeds, because the phase current continues flowing (through the freewheeling diode) beyond the excitation interval dictated by the drive transistor switch.
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6 Open-loop control
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The initial stages of system design are concerned with steady-state performance; the choice of stepping motor and drive circuit is mainly dictated by the maximum tolerable position error and the maximum required stepping rate. When this task of selection is completed, the designer must consider how the motor and drive are to be controlled and interfaced to the remainder of the system. The open-loop control schemes discussed in this chapter have the merits of simplicity and consequent low cost. Digital phase control signals are generated by the microprocessor and amplified by the drive circuit before being applied to the motor. Although the system illustrated receives its phase control signals from a microprocessor a number of alternatives are presented later in this chapter.
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7 Closed-loop control
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In a closed-loop stepping motor system the rotor position is detected and fed back to the control unit. Each step command is issued only when the motor has responded satisfactorily to the previous command and so there is no possibility of the motor losing synchronism. A schematic closed-loop control is shown. Initially the system is stationary with one or more phases excited. The target position is loaded into the downcounter and a pulsed START signal is applied to the control unit, which imme diately passes a step command to the phase sequence generator. Consequently there is a change in excitation and the motor starts to accelerate at a rate dictated by the load parameters.
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8 Microprocessor-based stepping motor systems
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Stepping motors are often used as output devices for microprocessor-based control systems. The essential feature of these systems is that the microprocessor program produces a 'result' and the stepping motor must then move the load to the position cor responding to this 'result'. In this chapter we shall be considering the ways in which the microprocessor can be involved in control of the stepping motor. In complete contrast is the 'hardware-intensive' system. Here the microprocessor program merely feeds the target position information and a start command to the hardware controller, which generates the phase control signals for the motor drive circuits and a 'finish' signal for the microprocessor when the target is reached.
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9 Appendix: pull-out torque/speed characteristics
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The theory of torque production in the hybrid stepping motor, requires some modification if the motor is bifilar-wound. Each phase is split into two bifilar windings, which have equal numbers of turns and are located on the same stator poles, but are wound in opposite senses.
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Back Matter
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