This kind of motor has four coils which, when energised in the correct sequence, cause the permanent magnet attached to the shaft to rotate.

There are two basic step sequences. After step 4, the sequence is repeated from step 1 again.

Reversing the order of the steps in a sequence will reverse the direction of rotation.

a. Single-Coil Excitation - Each successive coil is energised in turn.

Step	Coil 4	Coil 3	Coil 2	Coil 1
a.1	on	off	off	off
a.2	off	on	off	off
a.3	off	off	on	off
a.4	off	off	off	on

You can see it animated

This sequence produces the smoothest movement and consumes least power.

b. Two-Coil Excitation - Each successive pair of adjacent coils is energised in turn.

Step	Coil 4	Coil 3	Coil 2	Coil 1
b.1	on	on	off	off
b.2	off	on	on	off
b.3	off	off	on	on
b.4	on	off	off	on

You can see it animated

This is not as smooth and uses more power but produces greater torque.

(If this is the sequence generated by the TM100 Disk Drive's Logic PCB, then how come the KP4M4-001 motor takes 100 steps per complete revolution?)

Note:
The excitation of Coil 4 is always the inverse of the excitation of Coil 2.
The excitation of Coil 1 is always the inverse of the excitation of Coil 3.
So, with the right circuit, you can generate this sequence with only two data lines.

Interleaving the two sequences will cause the motor to half-step

Step	Coil 4	Coil 3	Coil 2	Coil 1
a.1	on	off	off	off
b.1	on	on	off	off
a.2	off	on	off	off
b.2	off	on	on	off
a.3	off	off	on	off
b.3	off	off	on	on
a.4	off	off	off	on
b.4	on	off	off	on

You can see it animated

This gives twice as many stationary positions between steps. In the case of the KP4M4-001, 200 steps per complete revolution - 1.8° per step.

Note:
The numbering of the coils as above is purely arbitrary. I have used this ordering scheme because I think it most clearly matches the bit patterns required to generate the stepping sequences.

last updated: 1-Jul-97 Ian Harries <ih@doc.ic.ac.uk>