I am trying to build a self-stabilized model rocket with actuated fins. Suppose the rocket has 4 fins. The diagonal fins must rotate together to control the yaw/pitch motion, and all the fins are tilted to control the roll motion. However, when both the yaw/pitch motion and the roll motion of the rocket need to be stabilized, how should the tilt angle of each fin be determined? I believe this is similar to a V tail, where both the yaw motion and the pitch motion depend on the same pair of control surfaces.
You will need to combine the two inputs somehow.
The simplest method is to simply add the two inputs for each fin together. For instance, let's say your control software commands the yaw fins to deflect 2 degrees to the left, and the roll fins (i.e. all of them) to deflect 3 degrees clockwise. Then, you'll wind up with one yaw fin deflected 5 degrees, and one deflected -1 degrees. This is usually good enough for a simple model rocket.
A more comprehensive solution would require either calculating, or empirically testing, how much force each fin creates at each angle of deflection. Then, you add the desired force for each fin together, and then calculate the desired angle of deflection based on that. This is usually more stable with less chance of oscillation, but does require more work to set up.