Merge pull request #942 from PX4/mc_mixer_fix

Multirotor mixer: more careful limiting

src/modules/systemlib/mixer/mixer.h

@@ -516,7 +516,7 @@ private:
 	float				_roll_scale;
 	float				_pitch_scale;
 	float				_yaw_scale;
-	float				_deadband;
+	float				_idle_speed;
 
 	unsigned			_rotor_count;
 	const Rotor			*_rotors;

src/modules/systemlib/mixer/mixer_multirotor.cpp

@@ -67,6 +67,11 @@
 namespace
 {
 
+float constrain(float val, float min, float max)
+{
+	return (val < min) ? min : ((val > max) ? max : val);
+}
+
 /*
  * These tables automatically generated by multi_tables - do not edit.
  */
@@ -171,12 +176,12 @@ MultirotorMixer::MultirotorMixer(ControlCallback control_cb,
 				 float roll_scale,
 				 float pitch_scale,
 				 float yaw_scale,
-				 float deadband) :
+				 float idle_speed) :
 	Mixer(control_cb, cb_handle),
 	_roll_scale(roll_scale),
 	_pitch_scale(pitch_scale),
 	_yaw_scale(yaw_scale),
-	_deadband(-1.0f + deadband),	/* shift to output range here to avoid runtime calculation */
+	_idle_speed(-1.0f + idle_speed * 2.0f),	/* shift to output range here to avoid runtime calculation */
 	_rotor_count(_config_rotor_count[geometry]),
 	_rotors(_config_index[geometry])
 {
@@ -276,67 +281,67 @@ MultirotorMixer::from_text(Mixer::ControlCallback control_cb, uintptr_t cb_handl
 unsigned
 MultirotorMixer::mix(float *outputs, unsigned space)
 {
-	float		roll    = get_control(0, 0) * _roll_scale;
+	float		roll    = constrain(get_control(0, 0) * _roll_scale, -1.0f, 1.0f);
 	//lowsyslog("roll: %d, get_control0: %d, %d\n", (int)(roll), (int)(get_control(0, 0)), (int)(_roll_scale));
-	float		pitch   = get_control(0, 1) * _pitch_scale;
+	float		pitch   = constrain(get_control(0, 1) * _pitch_scale, -1.0f, 1.0f);
-	float		yaw     = get_control(0, 2) * _yaw_scale;
+	float		yaw     = constrain(get_control(0, 2) * _yaw_scale, -1.0f, 1.0f);
-	float		thrust  = get_control(0, 3);
+	float		thrust  = constrain(get_control(0, 3), 0.0f, 1.0f);
 	//lowsyslog("thrust: %d, get_control3: %d\n", (int)(thrust), (int)(get_control(0, 3)));
-	float		max     = 0.0f;
+	float		min_out = 0.0f;
-	float		fixup_scale;
+	float		max_out = 0.0f;
+	float		scale_yaw = 1.0f;
 
-	/* use an output factor to prevent too strong control signals at low throttle */
+	/* perform initial mix pass yielding unbounded outputs, ignore yaw */
-	float min_thrust = 0.05f;
-	float max_thrust = 1.0f;
-	float startpoint_full_control = 0.40f;
-	float output_factor;
-
-	/* keep roll, pitch and yaw control to 0 below min thrust */
-	if (thrust <= min_thrust) {
-		output_factor = 0.0f;
-		/* linearly increase the output factor from 0 to 1 between min_thrust and startpoint_full_control */
-
-	} else if (thrust < startpoint_full_control && thrust > min_thrust) {
-		output_factor = (thrust / max_thrust) / (startpoint_full_control - min_thrust);
-		/* and then stay at full control */
-
-	} else {
-		output_factor = max_thrust;
-	}
-
-	roll *= output_factor;
-	pitch *= output_factor;
-	yaw *= output_factor;
-
-
-	/* perform initial mix pass yielding un-bounded outputs */
 	for (unsigned i = 0; i < _rotor_count; i++) {
-		float tmp = roll  * _rotors[i].roll_scale +
+		float out = roll * _rotors[i].roll_scale +
 			    pitch * _rotors[i].pitch_scale +
-			    yaw   * _rotors[i].yaw_scale +
 			    thrust;
 
-		if (tmp > max)
+		/* limit yaw if it causes outputs clipping */
-			max = tmp;
+		if (out >= 0.0f && out < -yaw * _rotors[i].yaw_scale) {
+			yaw = -out / _rotors[i].yaw_scale;
+		}
 
-		outputs[i] = tmp;
+		/* calculate min and max output values */
+		if (out < min_out) {
+			min_out = out;
+		}
+		if (out > max_out) {
+			max_out = out;
+		}
+
+		outputs[i] = out;
 	}
 
-	/* scale values into the -1.0 - 1.0 range */
+	/* scale down roll/pitch controls if some outputs are negative, don't add yaw, keep total thrust */
-	if (max > 1.0f) {
+	if (min_out < 0.0) {
-		fixup_scale = 2.0f / max;
+		float scale_in = thrust / (thrust - min_out);
+
+		/* mix again with adjusted controls */
+		for (unsigned i = 0; i < _rotor_count; i++) {
+			outputs[i] = scale_in * (roll * _rotors[i].roll_scale + pitch * _rotors[i].pitch_scale) + thrust;
+		}
 
 	} else {
-		fixup_scale = 2.0f;
+		/* roll/pitch mixed without limiting, add yaw control */
+		for (unsigned i = 0; i < _rotor_count; i++) {
+			outputs[i] += yaw * _rotors[i].yaw_scale;
+		}
 	}
 
-	for (unsigned i = 0; i < _rotor_count; i++)
+	/* scale down all outputs if some outputs are too large, reduce total thrust */
-		outputs[i] = -1.0f + (outputs[i] * fixup_scale);
+	float scale_out;
+	if (max_out > 1.0f) {
+		scale_out = 1.0f / max_out;
 
-	/* ensure outputs are out of the deadband */
+	} else {
-	for (unsigned i = 0; i < _rotor_count; i++)
+		scale_out = 1.0f;
-		if (outputs[i] < _deadband)
+	}
-			outputs[i] = _deadband;
+
+	/* scale outputs to range _idle_speed..1 */
+	for (unsigned i = 0; i < _rotor_count; i++) {
+		outputs[i] = _idle_speed + (outputs[i] * (1.0f - _idle_speed) * scale_out);
+	}
 
 	return _rotor_count;
 }