Bipolar stepper motors

Remember from our first column, I mentioned the 4-wire bipolar stepper motors. This motors require a proper of sequencing the direction of the current through two sets of coils. We can do this if we have two H-bridges, one for each coil. Now we are using lots of transistors, eight of them, to control a single motor. Fortunately many H-bridge ICs (such as the L293D) are dual H-bridge chips.

Table 1. The Bipolar Sequence

Step	$C_{11}$	$C_{12}$	$C_{21}$	$C_{22}$
1	-V	+V	-V	+V
2	-V	+V	+V	-V
3	+V	-V	+V	-V
4	+V	-V	-V	+V

So step 1 is the condition where coil 1 has a negative voltage on wire 1 a positive voltage on wire 2, coil 2 has a negative voltage on wire 1 and an positive voltage on wire 2. Step 2 reverses the current in the second coil, leaving the first alone.

Listing 3, bipolar.fth shows the code to drive a bipolar stepper motor through the above sequence.