Stepper Motor Pull-in Characteristics

This example shows how to parameterize and tune a stepper motor using manufacturer datasheet information and a test harness. The model is parameterized using numerical data extracted from a datasheet. The simulation generates pull-in torque characteristics that you can compare to a manufacturer-provided pull-in curve. To tune the stepper motor model, the example uses a test harness that varies the drive type and load parameters.

Test Harness

The stepper is modeled using the Stepper Motor block from the Simscape™ Electrical™ library. The stepper motor driver is modelled by a current source and a first order filter. The motor is placed in a test harness. The test harness gradually increases the load until slip occurs for each step rate demand tested. Slip detection is implemented in the Slip Detection subsystem. The subsystem contains a Simulink® assertion block that determines the difference between expected and actual rotor angle.

Pull-in Torque Characteristics

The plot shows the pull-in characteristics generated by the test harness simulation. Results are superimposed on the pull-in curve from the manufacturer datasheet. Achieving an exact match for the pull-in characteristics can be challenging because most datasheets do not specify the test conditions. Furthermore, some datasheets do not provide all the numerical values required to model the stepper motor. In this case, the pull-in curve from the simulation is used to determine for representative values that yield an acceptable pull-in curve match.

Pull-in Characteristics Sensitivity to Drive and Load Parameters

In pull-in mode, a stepper motor must start and stop without losing synchronization. Due to the dynamic nature of stepper motors, pull-in torque-speed performance is highly sensitive to stepper motor drive configuration and load parameters.

When modeling or tuning a stepper motor, consider that:

• Increasing the load inertia reduces the stepper motor pull-in torque at higher step rates. Generally, for high step-rate operation, load inertia is less than three times that of rotor inertia. It is a generally-accepted practice to limit the load inertia to less than ten times that of rotor inertia.

• Loads with higher damping component results in better performance at lower step rates because damping helps to overcome the effects of stepper motor resonance. Similarly, internal motor rotor damping can also help to improve the performance at low step rates.

• Stepper motors with low winding resistance are often driven using a constant current drive. To reduce the current rise time, higher than the rated voltage is applied to the motor. The higher supply voltage yields higher pull-in torque capability at higher step rates.

• Pull-in torque curve provided on a manufacturer datasheet is typically given for a particular driver and load configuration (load type, load inertia and load damping). Manufacturers tend to test stepper motors either with a dyno setup or by applying friction to the rotor wheel. The test method is rarely included on the data sheet. Therefore, it is always important to simulate the stepper motor to generate the curve for parameterization validation purposes.

The plot shows how the pull-in torque is sensitive to rotor damping, drive voltage, load inertia, and load type.

For any given stepper motor, the set of model parameter values that matches the pull-in torque characteristics is not typically unique. To ensure the parameterization is representative, it is good practice to also generate and compare the pull-out curve with the datasheet.