quartiq
IIR filter coefficients set incorrectly
Hello,
Thanks for keeping up the development of the stabilizer. I've just tried to set a low-pass filter using your script at https://github.com/quartiq/stabilizer/blob/main/py/stabilizer/iir_coefficients.py, but I think the IIR BA coefficients are returned with the wrong sign. Specifically, lines 59-61 say
# Returns:
# [b0, b1, b2, -a1, -a2] IIR coefficients to be programmed into a
# Stabilizer IIR filter configuration.
However, in line 144, it's actually +a1, not -a1 being returned. Therefore, this implements the wrong filter.
It seems to me that this is a problem for all filters (+a_i returned instead of -a_i), but it seems OK for the PID (I haven't tested yet).
I'm not a 100% sure why -a_i needs to be sent to the stabilizer, but when I change a1 to -a1 in line 144 I do get the expected, correct behaviour - so it's probably just a matter of updating the Python script.
Is there any chance you could release a temporary fix for this coefficient sign issue while presumably a more general rewrite lands?
Further to the above, I think the PID coefficients are also returned with the wrong sign, and the b0 coefficient of the high-pass filter is wrong (it has an extra factor of f0_bar in the numerator).
Furthermore, I'd like to get the PID setpoint (the x offset) working, which it seem to me is not implemented in the this python file - I wonder if there is any description somewhere regarding how that could be done?
Some background on PID/IIRs is here: https://hackmd.io/IACbwcOTSt6Adj3_F9bKuw
A proven implementation is here: https://github.com/quartiq/idsp/blob/main/src/iir/pid.rs
while the underlying Biquad impl that dual-iir uses is this: https://github.com/quartiq/idsp/blob/main/src/iir/biquad.rs
It's on my list to get iir_coefficients back up to speed (i.e. to the state that idsp is in).
Thanks very much! I've already read the PID primer which was very useful. I'm less familiar with Rust, hence the need for the python interface.