Improving the Accuracy of Microstepping Motors

Built-in technique eliminates zero crossing error

By Design News Staff – Design News

A trial-and-error process is one of the classic ways to reduce the jerking and uneven motion in a stepper motor. This approach counteracts the magnetic pull, the detent torque, of the motor as it is pulled toward its full step “on” position. However, there are trade-offs and a nonoptimal solution can affect other areas of the driver’s performance. A driver with an integrated method to reduce the jerking provides a more consistent and time-saving solution.

In a bipolar stepper motor, one phase has 100 percent current and the other phase has 0 percent current at the zero crossing point. Both phases are offset 90 degrees and have sinusoidal current waveforms. Drivers that output a perfect sinusoidal current waveform do not have perfect accuracy. Altering the current sine waves reduces the “jerk” but the zero crossing points are sensitive to the minimal amount of change and altering the waveform can impact other areas of the driver’s performance.

Spike Stopper
A 1.5-sec view of the zero crossing of a microstepping motor with standard drive circuitry shows the spike that distorts the linear operation of the stepper motor (top image). With the same stepper motor, the R325 driver eliminates the spike, producing a linear output (bottom image).

RMS Technologies developed a driver that specifically addresses zero crossing correction. Operating from +15 to 48V dc, the R325 driver’s phase currents range from 0.3 to 3.0A (peak) with selectable step resolutions including Full, 2A—, 4A—, 8A—, 16A—, 32A—, 64A—, 128A—, and 256A— microsteps. By outputting a predetermined amount of optimal holding and running current into the motor, the R325 overcomes the motor’s detent torque and eliminates the “jerk” at zero crossing.

Using the same stepper motor to perform step-to-step tests, engineers at Lin Engineering compared the R325 to a comparable driver without zero crossing correction. By setting both units to 64 microsteps and aligning the shaft of the motor to an encoder, they graphed the position of the motor over time. The “before” case, without zero crossing correction, showed a spike and distortion of the linearity in a 20-sec operating period. The step motor and R325 driver combination produced an even amount of degrees between each step resulting in a straight line.