6.1.1 Full Stepping

Since this is not a hardware class, we'll only use bipolar stepper motors in our discussion. There are more unipolar stepper motors, but bipolar stepper motors are more efficient. As a result, most energy-conserving robots use bipolar stepper motors.

Figure 6.1: Schematics of a bipolar stepper motor.

Figure 6.1 shows a bipolar stepper motor in its ``schematic form''. In this diagram, there are two ``phases''. Each phase is, essentially, a set of coils. The coils with terminals ``A'' and ``C'' represent one phase, while the coils with terminals ``B'' and ``D'' represent another phase. The arrow in the diagram represents the ``rotor'', which is a freely rotating permanent magnet. In a real stepper motor, the permanent magnet is connected to the drive axle of a stepper motor.

As current passes through coils, a magnetic field develops. This magnetic field, in return, tries to align the permanent magnet (rotor) in a certain direction. Let's assume that when current passes from ``A'' to ``C'', the magnet arrow points to the north. As we turn off coil A-C and turn on coil B-D (with current flowing from ``B'' to ``D''), the magnet rotates clockwise to point to the east.

Here comes the fun part. If we turn off coil B-D, then turn on coil A-C in reverse (current flowing from ``C'' to ``A''), the magnet rotates clockwise and points south. This is why it is called a ``bipolar'' stepper motor: each terminal can assume two polarities. Not surprisingly, we can also turn off coil A-C, then make current flow from ``D'' to ``B'' so that the magnet rotates and points west.

This completes one cycle. With a two-phase bipolar stepper motor, there are these four steps: A-C, B-D, C-A and D-B.