Reasons for Microstepping a Stepper Motor (Part Two)

Microstepping can replace a mechanical gearbox in certain applications when you need small relative movements or greater step resolution. Even if you have to use a larger stepper motor, this is often a preferable solution in a number of applications. You need to carefully select the appropriate stepper motor to get the best possible results, and you should also consider developing customized sine/cosine profiles.

You can increase stepper motor position accuracy beyond the manufacturer’s specifications by microstepping. One way to accomplish this is by designing a microprocessor-based microstepping system using the motor at two-phase-on stop positions, which are typically the most accurate rotor stop positions. Use an automatic or manual factory calibration process to store a correction value for every stop position on each motor you use.

You use the correction value to send adjusted full-step positions to the stepper motor. These adjusted positions have slightly different current levels in the windings, which compensates for the deviations of the position at the original stop positions. This type of microstepping technique is ideal when optimal step accuracy is the most important design criteria. When you use this technique, the stepper motor system must use a rotor home position indicator for synchronizing the rotor and the compensation profile.

Although the electronics needed to generate microstepping are more complex than the electronics used in half- and full-stepping, the total system complexity — including gearbox, transmission, and stepper motor — is less complex and costly in most applications. In addition, microstepping can simplify or altogether replace gearboxes and mechanics for damping noise and vibrations. Moreover, stepper motor selection is easier and more flexible.

You can use software and PWM-timers or digital-to-analog converters in the microprocessor as a replacement for external stepper motor controllers in microprocessor-based microstepping applications in order to achieve the lowest possible hardware cost, which is to say you can obtain microstepping hardware for about the same price as half- and full-step systems for comparable stepper motor sizes.

Read Part One

George

George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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