mc pos control: add MPC_THR_CURVE param

The hover thrust param also changed the thrust scaling in Stabilized mode.
However if the hover thrust is very low (e.g. below 20%), the throttle
stick input becomes very distorted.

src/lib/FlightTasks/tasks/ManualStabilized/FlightTaskManualStabilized.cpp

@@ -145,16 +145,22 @@ void FlightTaskManualStabilized::_updateSetpoints()
 
 float FlightTaskManualStabilized::_throttleCurve()
 {
-	/* Scale stick z from [-1,1] to [min thrust, max thrust]
-	 * with hover throttle at 0.5 stick */
+	// Scale stick z from [-1,1] to [min thrust, max thrust]
 	float throttle = -((_sticks(2) - 1.0f) * 0.5f);
 
-	if (throttle < 0.5f) {
-		return (_throttle_hover.get() - _throttle_min_stabilized.get()) / 0.5f * throttle + _throttle_min_stabilized.get();
+	switch (_throttle_curve.get()) {
+	case 1: // no rescaling
+		return throttle;
 
-	} else {
-		return (_throttle_max.get() - _throttle_hover.get()) / 0.5f * (throttle - 1.0f) + _throttle_max.get();
+	default: // 0 or other: rescale to hover throttle at 0.5 stick
+		if (throttle < 0.5f) {
+			return (_throttle_hover.get() - _throttle_min_stabilized.get()) / 0.5f * throttle + _throttle_min_stabilized.get();
+
+		} else {
+			return (_throttle_max.get() - _throttle_hover.get()) / 0.5f * (throttle - 1.0f) + _throttle_max.get();
+		}
 	}
+
 }
 
 bool FlightTaskManualStabilized::update()

src/lib/FlightTasks/tasks/ManualStabilized/FlightTaskManualStabilized.hpp

@@ -77,6 +77,7 @@ private:
 					(ParamFloat<px4::params::MPC_MAN_TILT_MAX>) _tilt_max_man, /**< maximum tilt allowed for manual flight */
 					(ParamFloat<px4::params::MPC_MANTHR_MIN>) _throttle_min_stabilized, /**< minimum throttle for stabilized */
 					(ParamFloat<px4::params::MPC_THR_MAX>) _throttle_max, /**< maximum throttle that always has to be satisfied in flight*/
-					(ParamFloat<px4::params::MPC_THR_HOVER>) _throttle_hover /**< throttle value at which vehicle is at hover equilibrium */
+					(ParamFloat<px4::params::MPC_THR_HOVER>) _throttle_hover, /**< throttle at which vehicle is at hover equilibrium */
+					(ParamInt<px4::params::MPC_THR_CURVE>) _throttle_curve /**< throttle curve behavior */
 				       )
 };

src/modules/mc_pos_control/mc_pos_control_params.c

@@ -71,6 +71,28 @@ PARAM_DEFINE_FLOAT(MPC_THR_MIN, 0.12f);
  */
 PARAM_DEFINE_FLOAT(MPC_THR_HOVER, 0.5f);
 
+/**
+ * Thrust curve in Manual Mode
+ *
+ * This parameter defines how the throttle stick input is mapped to commanded thrust
+ * in Manual/Stabilized flight mode.
+ *
+ * In case the default is used ('Rescale to hover thrust'), the stick input is linearly
+ * rescaled, such that a centered stick corresponds to the hover throttle (see MPC_THR_HOVER).
+ *
+ * Select 'No Rescale' to directly map the stick 1:1 to the output. This can be useful
+ * in case the hover thrust is very low and the default would lead to too much distortion
+ * (e.g. if hover thrust is set to 20%, 80% of the upper thrust range is squeezed into the
+ * upper half of the stick range).
+ *
+ * Note: in case MPC_THR_HOVER is set to 50%, the modes 0 and 1 are the same.
+ *
+ * @value 0 Rescale to hover thrust
+ * @value 1 No Rescale
+ * @group Multicopter Position Control
+ */
+PARAM_DEFINE_INT32(MPC_THR_CURVE, 0);
+
 /**
  * Maximum thrust in auto thrust control
  *