TABLE OF CONTENTS Most stepping motor applications involve accurate positioning of a mechanical load. For example, the position of a print head is defined very precisely by the number of switched excitation changes that have taken place in the controlling motor. External load torques, perhaps caused by friction, give rise to a small error in position when the motor is stationary. The motor must develop enough torque to balance the load torque and the rotor is therefore displaced by a small angle from the expected step position. The resultant static position error depends on the external torque, but is independent of the number of steps previously executed; the position error is noncumulative.
The maximum allowable positional error under static conditions often dictates the choice of motor, so this chapter deals with the relationship between the static position error and the parameters of the motor, drive and load. In many cases the static error can be reduced if several phases of the stepping motor are excited at the same time, so the potential benefits of multi-phase excitation are examined with reference to both hybrid and variable-reluctance stepping motors. An alternative method of minimising static error is to connect the motor to the load by a gear or, if linear load positioning is required, by a leadscrew, so the effects of these mechanical connections are also investigated.
Manufacturers generally supply information about the torque-producing capability of a stepping motor in the form of a graph known as...
