Pitch Controller input "AirspeedScaler"?

power_broker · November 3, 2019, 2:55am

I’m trying to recreate ArduPlane’s basic PID flight controller algorithms for my custom Arduio/Teensy plane.

I’ve decided to look into implementing a pitch controller first and then move on to roll and yaw. I was able to find the attached picture of ArduPlane’s pitch controller diagram. I’m able to understand how to implement all of the pieces except for the input “AirspeedScaler”.

My question: Is “AirspeedScaler” a global constant defined somewhere (if so, what is the value?) or is it user defined (if so, what are the value limits)?

I’d look in the source code, but I know from previous experiences that ArduPilot is written/structured in a way that is very difficult for someone new to the codebase like me to understand/find things, lol.

tridge · November 3, 2019, 3:50am

It is calculated in Plane::get_speed_scaler() here:

github.com

ardupilot/ardupilot/blob/master/ArduPlane/Attitude.cpp#L8


      
          #include "Plane.h"
          
          /*
            calculate speed scaling number for control surfaces. This is applied
            to PIDs to change the scaling of the PID with speed. At high speed
            we move the surfaces less, and at low speeds we move them more.
           */
          float Plane::calc_speed_scaler(void)
          {
              float aspeed, speed_scaler;
              if (ahrs.airspeed_estimate(aspeed)) {
                  if (aspeed > auto_state.highest_airspeed && arming.is_armed_and_safety_off()) {
                      auto_state.highest_airspeed = aspeed;
                  }
                  // ensure we have scaling over the full configured airspeed
                  const float airspeed_min = MAX(aparm.airspeed_min, MIN_AIRSPEED_MIN);
                  const float scale_min = MIN(0.5, g.scaling_speed / (2.0 * aparm.airspeed_max));
                  const float scale_max = MAX(2.0, g.scaling_speed / (0.7 * airspeed_min));

It is the scaling factor for surface speed scaling. Control surfaces need to move less at higher speeds.

power_broker · November 3, 2019, 4:19am

Interesting. But what is “g.scaling_speed”? Is that a set constant? Where does it come from/what is it’s value?

tridge · November 3, 2019, 8:43am

That is the mid-point for the scaling. It acts as an overall scaler for the fixed wing surface PIDs. Setting it to the cruise speed of the aircraft allow you to use the default PIDs on a much wider range of aircraft.
Basically if you are flying at airpeed=g.scaling_speed then the scale is 1.0
I usually only recommed that people set this on really fast aircraft (eg. jets)

power_broker · November 3, 2019, 8:22pm

I’m only planning on flying a somewhat slow electric prop plane (maybe 20mph max) - in that case, should my calculation of speed_scaler be:

if (aspeed > 0.0001f) speed_scaler = 1.0 / aspeed;
else speed_scaler = 2.0;

Also, one last question: I tried looking into how ArduPilot converts the controller output into a servo signal (either us pulse length or servo angle), but I couldn’t figure it out. How can I convert the pitch controller output to a servo us pulse length?

power_broker · November 10, 2019, 4:12am

Is the above question too difficult to answer in a forum post? Does anyone know the answer?

I’m sorry, I’m just trying to understand and learn