MA732 - Encoder bandwidth selection

Motor Drivers/Sensors
1

I’m using MA732 for closed loop motor control application where I know maximum angular velocity and maximum angular acceleration of the motor.

Can I calculate what’d be the best encoder bandwidth based upon maximum angular velocity and maximum angular acceleration of the motor?

For example, I have been calculating bandwidth as

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Taking into account your application note “Understanding the MagAlpha Digital Filter: Benefits of the MA732 & MA330 | Article | MPS”
Dynamic lag can be reduced with increased bandwidth which seems to be related to acceleration of motor. AngleLag_Dynamic = a * τ 2

Also, I agree with your application note “Magnetic Angle Sensors: Resolution Explained | Article | MPS” saying,
Sensor time constant should be 10 times smaller than control loop bandwidth.

However, I would like to know if it’s possible to calculate required encoder bandwidth based upon maximum angular velocity and maximum angular acceleration of the motor?

2

Hello Bijendra,

The maximum angular velocity is not involved in the filter bandwidth selection. When the velocity is constant, the sensor output shows a lag with respect to the real magnet angle, but this lag is independent of the filter bandwidth. It only depends on the intrinsic latency of the sensor and the angular velocity.

Moreover, there is no direct relationship between the maximum angular acceleration and the filter bandwidth to choose. As you have already figured out from the article about the filter, these two values are indeed linked, but only to the extent that they determine the lag of the sensor output when there is an acceleration.

As a consequence, you can follow either of these approaches to select the filter bandwidth:

  • Find the control loop bandwidth and select a >10x times larger filter bandwidth. Note that the “10x” is not a strict value, the control system performance might be good enough even if the bandwidth is smaller. This depends on specifications of the system.
  • Determine the maximum lag that is acceptable in your application and calculate the filter bandwidth from this value and the maximum acceleration, using the formula you wrote.

I hope this helps.

Best regards,

Carmine

1 Like
3

Thanks Carmine for helping me understand better with lags and bandwidth selection :smiley: