Smooth motion results from presenting the patterns in the proper order. They are tricky to control because they require changing the direction of the current flow through the coils in the proper sequence. We will discuss these motors further when we get to the topic of DC motor control. The internal arrangement of the coils for a unipolar stepper motor. Wires a through d are attached to the positive motor power supply.
Stepper motors vary in the amount of rotation delivered per step.
They can turn as little as 0.72 degree to as much as 90 degrees per step. Many have integral reduction gear trains so that they have even higher angular resolution. The motor shaft will freely rotate when none of the coils are energized, but if the last pattern in a series is maintained, the motor will resist being moved to a different position. Because the motors are open-loop, if you do manage to mechanically overwhelm the motor and turn the shaft to a new position, the motor will not try to restore itself to the old position. The sequences are relatively easy to generate with a couple of TTL or CMOS chips at a much lower cost.