Stepper motor: Stepper motors are used for precise control of position of shaft. The shaft of motor rotates incrementally in equal steps in response to a programmed input pulse train.
To know the final position of the rotor all that is required is to count the no. of pulses fed into the motor stator phase winding.
— The no. of pulses per time unit determine the motor speed.,
— The step angles of the shaft are obtainable typically from 1.8° to 90° depending on the particular motor choice. Thus with a nominal step angle of 1.8° to stream of 1000 pulses with give an angular displacement Of 1800° or five complete revolutions.
— If more than one motor is driven from the same source then they will maintain perfect synchronization.
— Stepper motor can rotate in both directions.
— It can sustain a holding torque at zero speed.
— Stepper motor can be interfaced with digital circuit directly.
— It is used for lo power position control applications.
— For controlling motor feedback is not required but an encoder or position sensor is used to control motor accurately. This reduces feedback control complexity.
— Stepper motor has a lower output and efficiency as compared to other motors.
Types of stepper motor:
(i) Variable reluctance stepper motor. (Four pole stepper motor).
(ii) Permanent magnet stepper motor.
(iii) Hybrid stepper motor.
1.Variable reluctance stepper motor: ‘Magnetic reluctance’ or simply ‘Reluctance’ the analog of electric resistance. Just as current occurs only in a closed loop, so a magnetic flux occurs only around a closed spath, although this path may be more varied than that of current. Figure (a) shows the basic form of the variable reluctance stepper motor. The rotor is made of soft steel and it has four poles, whereas the stator has six poles. When one of the phases, say AA’, is excited due to a DC current passing through the coils around the poles the rotor positions itself to complete the flux path shown in Fig. (a) Note t4iat there is a main flux path through the aligned rotor and stator teeth, with the secondary flux paths occurring as indicated. When rotor and stator teeth are aligned in this manner, the reluctance is minimized and the rotor is at rest in this position. This flux path can be considered to be rather like an elastic thread which always trying to shorten itself. The rotor will move until the rotor and stator poles are lined up. This is termed the position of minimum reluctance. To rotate the motor counter-clockwise, phase AA’ is turned off the and phase BB’ is excited. At that point the main flux path has the form indicated in Figs. (b) and (c). This form of stepper motor generally gives step angles of 7.5° or 15°.
- Permanent magnet stepper motor: The basic method of operation of a permanent magnet type is similar to the variable reluctance type. As illustrated in Fig. there are two coils A and B each of them producing four poles but displaced from each other by half a pole pitch. The rotor is of permanent magnet construction and has four poles, as
- Basic configuration
(b) Beginning of step (C) Completed step
Fig. Variable reluctance stepper motor.
Illustrated. Each pole is wound with field winding, the coils on opposite pairs of poles being in series. Current is supplied from a DC source to the winding through switches. It can be seen in Fig. (a) That the motor is at rest with the poles of the permanent magnet rotor held between the residual poles of the stator. In this position the rotor is locked unless a turning force is applied. If the coils are energized and, in the first pulse the magnetic polarity of the poles of coil A is reversed, the rotor will experience a torque and will rotate counter-clockwise as shown in Fig. (b) the reverse poles are shown as A’.
If coil B poles are now reversed to B’ as shown in Fig. (c) The rotor will again experience a torque, and step round once more until the poles of the rotor are positioned midway between the stator poles. Thus, by switching the currents through the coils the rotor rotates by 45°. If in the first pulse the poles of coil B had been reversed then the motor would have rotated clockwise. With this type of motor, commonly produced step angles are 1.8°, 7.5°, 15°, 34°, 90°.
- Hybrid Stepper Motor: Hybrid stepper motors combine the features of both the variable reluctance and permanent magnet motors, having a permanent magnet engaged in iron caps which are cut to have teeth. The rotor sets itself in the minimum reluctance position in response to a pair of stator coils being energised. Typical step angles are 0.9° and 1.8°. From the descriptions above, it is therefore apparent that the rate at which the pulses are applied determines the motor speed, the total number of pulses determines the angular displacement, and the order of energising the coils in the first instance determines the direction of rotation. It is because of this ease of driving using direct digital control that stepper motors are well suited for use in a computer controlled robot, although the motor does require interfacing with a high current pulse source.
Fig. Permanent magnet stepper motor.