Motors and Drives
and adding the idea of a surfer explains the occasional tendency of the step motor to ‘fall off the train’ and coast rapidly to a stop during acceleration or at high speed. What is happening is that the stepper motor and load cannot keep up with the request for motion and they lose step-lock, also known as synchrony or synchronicity, with the commanded profile.
On-the-fly recovery
Just like a surfer who has been pushed over the top of the wave, once the condition starts, on-the-fly recovery is almost impossible - the profile proceeds even farther forward while the step motor comes to an unceremonious halt. The valley metaphor helps explain another phenomenon, namely that under the influence of an outside force, the step motor does not always settle at the theoretical commanded position. This is because an outside force will tend to push the axis ‘up the hill’ to one side or the other, so the motor won’t quite be at the theoretical location (the exact bottom of the valley) that you expected. Yet another problem that stems from the open
loop ‘settling in the force valley’ aspect of step motor operation is called mid-range instability. The symptom
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Fig. 1. Stepper motors are multi-phase devices, meaning they have two or more motor coils which must be electronically driven to create motion.
of this behaviour is that there is a drop in available torque at a specific frequency located somewhere in the ‘mid-range’ of overall step rate output. The instability is an interaction of the natural
resonance frequency of the motor as it ‘climbs’ and ‘falls back’ around the valley target position (very much like a resonant spring), and the current control circuit. At the instability point, the effective current in each coil may lag excessively, reducing or even reversing damping effects in the system that normally counteract the tendency of the motor to resonate. Motion smoothness and stability is lowest in full step
motors, and best in micro steppers, although even with micro steppers there is still likely to be some cogging. This can be further reduced using a motor with a skewed rotor design or advanced control techniques. This article has explained the basics of stepper motor operation. ●
Gerard Bush is with INMOCO, Daventry, UK.
www.inmoco.co.uk
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