Step motors when micro stepped with a sine wave excitation generally do no give a linear response for a variety of mechanical and electrical reasons. The pole shapes may not be ideal, and the magnetic path is affected by saturation. The following graph shows the error response of very good step motor when driven with a sine wave.
The vertical scale represents error in microsteps of 1/256th of a step. The horizontal scale represents four steps across one pole of the motor, while the depth starting at the back represents which of the 50 poles on the motor. The very first pole graph is offset by the hysteresis of the motor. The test setup was done with a Gurly 36,000 count per revolution high resolution encoder, with the motor driven with our ss4544 motor control system.
Creating the Compensation Tables
The motor is actually analyzed at four different current points that span the motors normal operating range. The analysis creates a phase offset that can be used for correction. This phase offset is the average across all 50 poles of the motor. The graph is set up with the phase compensation on the vertical axis in 1/16384th of a step. The horizontal axis represents one pole transition or four steps. The depth axis represents four different current levels. Blue is 100% of the rated motor current, while Yellow is 120% of rated current, Green 80% or rated current, and Red 60% of rated current. You can easily see from the graphs that the non linearity increases with increasing current. This is due to magnetic saturation in the motor.
Microstep Response after Compensation
The compensation tables thus created are then loaded into the ss4544 motion control system which interprets them to create the appropriate drive current waveform. The following graph shows the motor microstep response with the new current drive waveform. Note that the scale on this graph is 5 times finer than the first graph showing the uncompensated motor response. This improvement in positioning accuracy is even more dramatic on other step motors that do not have as good performance as the one analyzed here.
Note: We also have a method of elimination of the hysteresis of the
motor in positioning applications where extreme precision is needed.
This is standard in the ss4544
motor control system. In the ss483
driver it would be a custom version for OEM use.
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