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I want to control the attitude(roll, pitch, yaw) of a vehicle capable of pitching and rolling. To do this I have created a quaternion PID controller. First I take the current attitude of the vehicle converting it to a quaternion Qc and do the same for the desired attitude with the quaternion Qd. I then calculate the input of my PID controller as Qr = Qc' x Qd. The imaginary parts of the quaternions are then fed as force requests on the roll, pitch, yaw axes of the vehicle. I test on a simulator and the control works but becomes unstable in some cases (request for R: 60 P: 60 Y:60). I also want this to work around singularities (i.e. pitch 90)

Does anyone know why I get this behavior and if so explain (thoroughly) what I'm doing wrong?

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You are using one PID controller to try to drive a quaternion? A quaternion by definition represents three degrees of freedom, roll pitch yaw, and a PID controller is Single Input, Single Output (SISO) controller.

You're trying to mask a Multiple Input, Multiple Output (MIMO) system by hiding your three variables in a quaternion.

I would suggest maybe you consider using three PID controllers. If you're concerned about singularities, try going for an axis-angle configuration, where two controllers drive the azimuth and elevation of the axis and a third drives the rotation.

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  • $\begingroup$ Sorry for the misunderstanding I am already using a PID controller per axis (Roll, Pitch, Yaw) which means 3 controllers in total. $\endgroup$ – MrHat Sep 8 '15 at 9:41
  • $\begingroup$ Nevermind in the end it was just a silly bug to do with the way I was transforming euler angles to quaternions. Thanks for the suggestion though. $\endgroup$ – MrHat Sep 8 '15 at 13:25

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