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Re: [Paparazzi-devel] Proposed update in the fixedwing pitch control loo

From: Hector Garcia de Marina
Subject: Re: [Paparazzi-devel] Proposed update in the fixedwing pitch control loop
Date: Sat, 17 Sep 2016 08:36:47 +0200

ah btw, 

I have found this change very appropriated too! it is better to employ an analytical _expression_ whenever is possible than a numerical one.

On Sat, Sep 17, 2016 at 8:33 AM, Hector Garcia de Marina <address@hidden> wrote:
Hi Michal,

instead of 

float d_err = h_ctl_ref.pitch_rate - stateGetBodyRates_f()->q;

should not be (pseudocode) the following?

float d_err = h_ctl_ref.pitch_rate - (q * cos(roll) - r * sin(roll) );

Indeed, the first _expression_ is a particular case of the second when the vehicle is flying with roll = 0, e.g. "following a straight line". However, it is also
normal to fly following circles or other cases where the average of the steady-state roll is not zero.

What do you think?

On Sat, Sep 17, 2016 at 3:09 AM, Michal Podhradsky <> wrote:
Dear paparazzi users,

there is a proposed update in the way the fixedwing pitch control loop handles the derivative term. Currently a pseudo-derivative is used (see here) - the pitch rate error is calculated as a difference in the previous and current pitch error.

The proposed change follows a solution already present in the adaptive stabilization (here).
The difference is that now the pitch rate error is calculated using a pitch rate (measured from the gyro) and a rate setpoint (typically zero).

The benefit of this change is that it unifies the control loops, within paparazzi and also with standard control theory notation.

The drawback is that the D gain currently used would have to be updated using this formula (see here for details): D_new = D_old * P_old (the old D gain wouldn't work).

Since this is a change that affects multiple fixedwing airframes I would like to know your opinion first - would you be strongly opposed to the change or are you OK with updating the gains (and possibly retuning the aiframe) if it means unified and standardized control loops?

Thank you for your feedback.

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