Absolute encoder Offset calibration PMSM

I am trying to find offset of an absolute encoder.My procedure is to inject a small vd and slowly increment the electrical angle.When the electrical angle reaches zero I subtract the encoder angle and the theoritical angle for example if motor has 4 pole pairs 90 degree mech is one electrical so I know 90 mech is the ideal angle.Now if encoder gives me 100 degree or 80 degree 10 is the offset.I calculate the difference between these ideal and encoder values and average the offset.The motor runs but the problem is I get maximum torque at a different offset the calculated offset has some error.Is the procedure correct Should i take into account anything else to calculate the offset precisely.

Answer 1

[Kothuri]

Hi Pratheesh,

I understand that you are trying to determine the offset of an absolute encoder by injecting a small ( V_d ) and incrementally adjusting the electrical angle.

You can follow the below steps for precise calculation of offset:

• Ensure that the encoder's resolution is sufficient to detect small changes in angle accurately.
• Minimize noise in the encoder signal as they could affect the accuracy of the offset calculation.
• Make sure that the mechanical alignment of the encoder with the motor shaft is precise, as any misalignment can introduce errors in the readings.
• When adjusting the electrical angle, make sure that the increments are small enough to capture precise changes in torque and encoder readings.
• Take multiple measurements at each angle and average them to reduce random errors and improve reliability.
• Since you are getting maximum torque at a different offset, this could indicate that the torque-producing current is not perfectly aligned with the rotor field. Make sure that the ( V_d ) and ( V_q ) components are correctly oriented.

You can refer the below documentation links for more info:

https://www.mathworks.com/help/mcb/gs/quadrature-encoder-offset-calibration-pmsm-motor.html

https://www.mathworks.com/help/mcb/gs/algorithm-export-custom-hardware-quadrature-encoder-offset.html