Page 687 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 673

Code S1 S2 S3 S4 Code S S2 S3 S4
1 1 0 0 1 1 0 0 0 1
2 1 0 1 0 2 1 0 0 0
3 0 1 1 0 3 0 0 1 0
4 0 1 0 1 4 0 1 0 0

Full steps Half steps
FIGURE 8.49: Operating principles of a stepper motor: a unipolar step motor winding with a
unipolar drive model is shown.

Let us consider the switching sequence shown on the left four illustrations at the
bottom of Figure 8.49. In this case, at any given time all of the stator phases are energized.
At all times, each rotor pole is attracted by two winding poles. Following the four switching
sequences, the rotor would take the shown stable positions. The current pattern for these
four discrete switch states are shown in Figure 8.50a. This type of phase current switching,
where both phases are energized, is referred as the “full-step” mode of operation. As
ON/OFF states of switches are changed in the order shown in the figure, the magnetic field
generated by the stator rotates in space. Hence, the permanent magnet (PM) rotor follows it.
Now, let us consider the four sequences of switch states shown on the right hand side
of Figure 8.49. In this case, only one of the stator phases is energized while the other phase