MultirotorMixer: Apply thrust gain independently from other axes

Align the way motor output is computed with the model used in the mixer generation script.
previous:
`out = (roll_gain*roll + pitch_gain*pitch + yaw_gain*yaw + thrust)*out_gain`
new:
`out = roll_gain*roll + pitch_gain*pitch + yaw_gain*yaw + thrust_gain*thrust`
which is the standard matrix*vector multiplication.

This commit has 0 effect on symmetric platforms (i.e. most) as thrust_gain==1 on these platforms.
On asymmetric platform --where some thrust_gain are lower than 1-- the previous formulation resulted in coupling between thrust and other axes.

fixes #8628

src/lib/mixer/mixer.h

@@ -546,10 +546,10 @@ public:
 	 * Precalculated rotor mix.
 	 */
 	struct Rotor {
-		float	roll_scale;	/**< scales roll for this rotor */
+		float	roll_scale;		/**< scales roll for this rotor */
 		float	pitch_scale;	/**< scales pitch for this rotor */
-		float	yaw_scale;	/**< scales yaw for this rotor */
+		float	yaw_scale;		/**< scales yaw for this rotor */
-		float	out_scale;	/**< scales total out for this rotor */
+		float	thrust_scale;	/**< scales thrust for this rotor */
 	};
 
 	/**

src/lib/mixer/mixer_multirotor.cpp

@@ -187,9 +187,7 @@ MultirotorMixer::mix(float *outputs, unsigned space)
 	for (unsigned i = 0; i < _rotor_count; i++) {
 		float out = roll * _rotors[i].roll_scale +
 			    pitch * _rotors[i].pitch_scale +
-			    thrust;
+			    thrust * _rotors[i].thrust_scale;
-
-		out *= _rotors[i].out_scale;
 
 		/* calculate min and max output values */
 		if (out < min_out) {
@@ -206,6 +204,11 @@ MultirotorMixer::mix(float *outputs, unsigned space)
 	float boost = 0.0f;		// value added to demanded thrust (can also be negative)
 	float roll_pitch_scale = 1.0f;	// scale for demanded roll and pitch
 
+	// Note: thrust boost is computed assuming thrust_gain==1 for all motors.
+	// On asymmetric platforms, some motors have thrust_gain<1,
+	// which may result in motor saturation after thrust boost is applied
+	// TODO: revise the saturation/boosting strategy
+
 	if (min_out < 0.0f && max_out < 1.0f && -min_out <= 1.0f - max_out) {
 		float max_thrust_diff = thrust * thrust_increase_factor - thrust;
 
@@ -262,9 +265,7 @@ MultirotorMixer::mix(float *outputs, unsigned space)
 		float out = (roll * _rotors[i].roll_scale +
 			     pitch * _rotors[i].pitch_scale) * roll_pitch_scale +
 			    yaw * _rotors[i].yaw_scale +
-			    thrust + boost;
+			    (thrust + boost) * _rotors[i].thrust_scale;
-
-		out *= _rotors[i].out_scale;
 
 		// scale yaw if it violates limits. inform about yaw limit reached
 		if (out < 0.0f) {
@@ -300,7 +301,7 @@ MultirotorMixer::mix(float *outputs, unsigned space)
 		outputs[i] = (roll * _rotors[i].roll_scale +
 			      pitch * _rotors[i].pitch_scale) * roll_pitch_scale +
 			     yaw * _rotors[i].yaw_scale +
-			     thrust + boost;
+			     (thrust + boost) * _rotors[i].thrust_scale;
 
 		/*
 			implement simple model for static relationship between applied motor pwm and motor thrust