Some questions about encoder offset calibration algorithm and correct phase angle

1

Hi,
I studied the encoder offset calibration algorithm recently.

I know the calibration algorithm has 3 steps:

  1. Align encoder angle shadow_count_ to alpha axis.

  2. Accumulate encoder angle and num_steps during turning rotor forward and backward.
    i.e.
    encvaluesum += shadow_count_
    num_steps++

  3. Calculate phase_offset and phase_offset_residual.

config_.phase_offset = encvaluesum / num_steps;
int32_t residual = encvaluesum - ((int64_t)config_.phase_offset * (int64_t)num_steps);
config_.phase_offset_float = (float)residual / (float)num_steps + 0.5f;  // add 0.5 to center-align state to phase

then the electrical angle is corrected in 3 steps:

  1. Calculate corrected encoder angle.
    int32_t corrected_enc = count_in_cpr_ - config_.phase_offset;

  2. Interpolate encoder angle.
    float interpolated_enc = corrected_enc + interpolation_;
    where interpolation_ is 0.0, 0.5 or 1.0 is depended on moving.

  3. Calculate electrical angle.
    float ph = elec_rad_per_enc * (interpolated_enc - config_.phase_offset_float);

My questions are below:

  1. Why just accumulate encoder angle and num_steps?
    And could I treat the encoder angle aligned to the alpha axis as phase_offset?
    Or accumulate the difference between the encoder angle and electrical angle * pole pairs then take average.

  2. Why calculate phase_offset_float?
    And could I ignore it?
    Because I don’t think the residual divided by steps is a large number.

  3. Why add 0.5 to phase_offset_float?
    I can’t understand the meaning of add 0.5 to center-align state to phase.
    Is the 0.5 value for interpolating?

  4. Why interpolate the encoder angle?

There are many questions.
I will vary much appreciate if someone reply me.

2

In general you need to scan backwards and forwards to average out friction and other irregularities in the motor, locking in to a single point, say along alpha, is sometimes not so accurate (depending on motor).

When using hall sensors the space between states is large.

Why add 0.5 to phase_offset_float?

It’s just from how the state alignments are defined, what counts as “0” electrical angle.

Why interpolate the encoder angle?

Hall sensors are coarse.

3

Thanks, your explanations are very clear.
The friction and the hall sensor (low resolution) should be considered.

But, I didn’t understand why the offset is the average of the mechanical angle, not the average of the difference of the mechanical angle between the Electrical angle / pole pairs.
image
where
the red line is the mechanical angle
the green line is the Electrical angle / pole pairs

4

In your plot you put x axis is time. But x axis is electrical angle. So the real plot looks like this:

image

If you average the y coordinate of all the points in blue, you get the red X which is the offset we are looking for.

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