ATC_STR_RAT Filtering parameters

### ATC_STR_RAT_FILT: Steering control filter frequency

Steering control input filter. Lower values reduce noise but add delay.

Increment Range Units
0.1 0.000 - 100.000 hertz

### ATC_STR_RAT_FLTT: Steering control Target filter frequency in Hz

Target filter frequency in Hz

Increment Range Units
0.1 0.000 - 100.000 hertz

### ATC_STR_RAT_FLTE: Steering control Error filter frequency in Hz

Error filter frequency in Hz

Increment Range Units
0.1 0.000 - 100.000 hertz

### ATC_STR_RAT_FLTD: Steering control Derivative term filter frequency in Hz

Looking at the steering controller filtering parameters, ATC_STR_RAT_FILT says it reduces noise but adds delay - three questions:

Is this a filter on the final steering output?
Is it possible to quantify the delay - is it equal to the filtering period, half the filtering period etc?
Do the other FLTx parameters introduce a similar delay?

I’m seeing a fairly high rate oscillation in the steering in AUTO mode when it’s almost on path, and wondering if this delay is causing it.

What’s also interesting is that I don’t see ATC_STR_RAT_FILT (nor ATC_SPEED_FILT) in the parameter list in the actual autopilot?

Hi @mroberts,

These are filters on the inputs to the steering’s rate controller. So this FF+PID controller converts a desired turn rate into a steering output.

The _FILT parameter description is from 4.0 and we should remove that now that 4.1 has been released.
FLTT filters the target turn rate.
FLTE filters the error (target turn rate - actual turn rate) that is then used for P, I and D terms.
FLTD filters the error that is passed into the D term only. I think this is applied after the FLTE filter mentioned above.

I’m afraid I can’t say what the delay is but it is a very standard low pass filter.

@rmackay9 do you know who wrote the filter code?


Yes, it was @Leonardthall who is our controls maintainer.

1 Like