September 1997, Issue 86

PC/104 Quarter:
Motion Control with PC/104

STEPPER BASICS

While servo systems seem to be favored over stepper systems, the latter has recently gained attention through breakthroughs like microstepping and five-phase control.

But since stepper motion operating in open-loop mode is more complex to control and requires more preliminary analysis, servos sometimes end up in applications otherwise ideally suited for stepper systems.

Load inflections, acceleration rates, top velocity, high-velocity torque reduction, zero following error, and other factors all contribute to the success of a stepper application.

STEPPER CONTROLLERS

It's critical to understand both the operational and system-reaction differences among various types of steppers. For example, a chopper drive always causes stepper motors to vibrate when not commanded to move.

In photo-film handling, a vibrating stepper motor directly coupled to a drive roller can cause the film to vibrate, which is not acceptable for positioning, developing, or splicing. A bilevel current-limited, nonchopper-type drive is a better system fit.

If the translator is current limiting via an external resistor, the power-supply voltage may be increased, improving the speed-torque curve. However, consider power supply limitations, external resistor power dissipation, and motor driver constraints.

Two common translators, as defined by current flow, are bipolar and unipolar. Both are available in resistance-limited, bilevel, VSI, and two- and four-quadrant chopper models.

The microstep translator is so-named because the stepper motor's current-generated magnetic field enables the armature to be positioned anywhere between its actual detent positions. The most popular microstep ranges available (at reasonable cost) are 2, 4, 8, 16, 32, 64, and 128 steps per full step.