Stepper Motor - Experimenting with Raspberry Pi - page 125

Half-Step Drive (Mode 2)
Figure 6-8 illustrates the drive waveforms for half-step drive mode. As in full-step mode,
an overlapped drive is applied to the field coils. Unlike full-step mode, the overlap occurs
on the first third and the last third of a given coil's drive. For two-thirds of the waveform,
there is overlapped drive. In the middle third, only one winding is active.

Figure 6-8. Half-step drive

This mode provides increased angular resolution but suffers from having less torque
(about 70% of full-rated torque). 59

Software
To demonstrate stepper-motor driving without getting into a complex assignment, the
program unipolar.c simply positions the shaft of the motor to various hour positions of
the clock, based on single-key commands.
With a pointer attached to the shaft of your motor, press 6, and the motor will point
at 6 o'clock. Press 3, and the motor figures out that it is quickest to step counterclockwise
back to the 3 o'clock position. Press 7, and the motor steps forward to 7 o'clock. All of
this, of course, requires that you point the shaft at 12 o'clock before you begin (the motor
provides no information to the program about where it is currently pointing).
The program presented uses the GPIO pin assignments in Table 6-2 for driving the
stepper motor (your wire colors may differ):

Table 6-2. GPIO Assignments Used by the Program unipolar.c

GPIO GENX P1 Mode Stepper Wire ULN2003A Description

17 GEN0 P1-11 Output Red 1B Field A

24 GEN5 P1-18 White 2B Field B

22 GEN3 P1-15 Brown 3B Field C

23 GEN4 P1-16 Green 4B Field D