Stepper Motors

Stepper Motors

Stepper motor manufacturers as well as end-users of stepper motors benefit greatly from the ability to measure and analyze the characteristics of a stepper motor/driver.

The measurements below were obtained by connecting an encoder to the shaft of a stepper motor, and monitoring the encoder and stepper-motor indexer signals using the Stepper Motor Dynamics Analyzer. The Stepper Motor Dynamics Analyzer provides a means to quickly:

analyze and compare stepper motors and drivers
measure shaft dynamics and winding currents and voltages
evaluate motor/system performance

These measurements can also be made with the Motion Capture Card using LabVIEW® drivers or the SensorPlot capture and analysis software.

Static Step Accuracy
The angular position accuracy of a stepper motor varies from one step to the next. This inaccuracy is influenced by the construction of the motor, the load it is driving, and the driver attached to motor. The rightmost graph below illustrates the static step error of one particular stepper motor as it is single stepped through one revolution. The graph is plotting the static step error (ideal step position - measured step position) versus the angle of the shaft of the motor.

The graph on the left is showing the min, max and average step response position accuracy measured over one revolution. The arrows indicate the peak to peak static step accuracy at a point where the motor shaft has settled.

Micro-step Accuracy
Micro-stepping is often used to position the shaft of a stepper motor between the full step positions. As illustrated in the left graph below, the shaft of a motor doesn't always follow the ideal micro-step position dictated by the indexer. In this case, 256:1 microstepping was applied to a 1.8° motor over one electrical cycle (4 full steps). The ideal shaft position (indexer pulse), the actual shaft position and the driver currents were measured simultaneously at every micro-step command for 1024 steps. Micro-stepping accuracy is determined by the construction of the motor and the accuracy of the driver. In the right graph below, notice how the driver is unable to generate clean sinusoidal currents near zero. Micro-stepping is rarely more accurate than 1/10th of a step. In the presence of a frictional load, micro-stepping can result in a stick and slide motion.

Velocity Resonance
An unloaded stepper motor (micro-stepped 16:1) was ramped in velocity up to five Revolutions Per Second. At low velocities, resonance caused the velocity of the motor to fluctuate severely. At higher velocities (above the resonance frequency of the motor), the velocity became smoother. Idle detent torque excited the resonance even when micro-stepping was employed.

Step Response
Stepper motors resonate even when stepped very slowly. In this case, the motor was stepped at a rate of one-half step every 200mS. Notice how the motor resonates almost a full step (1.8°) after each half step. The frequency of the decaying resonance can be used to estimate the holding torque of the motor.

Dynamic Position Accuracy
The graph to the right is plotting step position error (dynamic position error) versus velocity. The shaft of a stepper motor will lag behind the ideal step position in the presence of a torque load. In this case, the torque load (friction) increased with velocity causing the shaft to lag more-and-more as the velocity of the motor was increased.

Motor Drive Current
The performance of a stepper motor is strongly dependent upon the driver. As the velocity of a motor increases, the driver's ability to deliver current is impeded by the back EMF of the motor, the non-zero inductance of the motor windings, and the rail voltages applied to the driver. The diminishing current results in a loss of torque and a reduction in dynamic accuracy.

Skip
Stepper motors skip. In the graph to the right, the ideal and measured shaft positions of a 0.45° motor are plotted on the left Y-axis while the difference is plotted on the right. Ideal shaft position is measured by connecting SensorPlot™ to the step command of the stepper-motor indexer. This motor skipped 9°.

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