The performance and audible noise of any stepper motor system depends on the torque ripple generated by both the motor and the load. The torque ripple is defined by the variation in torque at each microstep. For most stepper motors, the standard sinusoidal microstep indexer is sufficient to achieve acceptable torque ripple and a good performance.

However, for some motor and load torque combinations, altering the current profile can reduce torque ripple, resulting in lower vibration and audible noise. When properly programmed, the customized current waveform ensures equally distanced microstep positions with constant torque and therefore also the best positional accuracy.

For example, in case of permanent magnet motors, variations in torque are more prominent due to larger step angle (3.6° to 18°) than that of hybrid motors (0.9° or 1.8°). Due to fewer number of stator teeths, less amount of flux interacts between the stator teeth and the rotor when the rotor is in between two stator teeths. If the current level is increased at these intermediate positions, the torque ripple will be lower compared to the default sinusoidal indexer.

The DRV8461 features a lookup table for tailoring the microstepping current profile to suit the requirements of a specific motor. The modified current profile is used in place of the default sinusoidal profile by writing '1' to the EN_CUSTOM bit. The frequency of the STEP input in custom microstepping mode should not exceed 300 kHz. The details of the interpolation process is described below -

• The user should program the current (% of TRQ_DAC) corresponding to the first quadrant of coil A current in a 1/8 microstepping setting.
• These current values are stored in CUSTOM_CURRENT1 to CUSTOM_CURRENT8 registers.
• The position for these current values correspond to 11.25°, 22.5°, 33.75°, 45°, 56.25°, 67.5°, 78.75° and 90° electrical angles.
• The current value for 0° position is assumed to be zero.
• The nine current values (including 0% full-scale current) are interpolated to a total of 256 points using a piecewise-linear approach to build the complete current waveform. The interpolated waveform always corresponds to 1/256 microstep, irrespective of the programmed microstepping mode.
• The values for the first quadrant are then mirrored and repeated for the other three quadrants of coil A and again for the four quadrants of coil B current to construct the complete current waveform.

Table 7-15 shows an example of the user inputs.

Figure 7-9 shows the corresponding modified current waveform of coil A for one full electrical angle, compared to waveform generated by sine indexer.

Figure 7-9 Customizable Microstepping