Clarify Documentation of THR_MDL_FAC

The documentation of the thrust model parameter `THR_MDL_FAC` did not
mention both thrust and "PWM" being relative values. Also the use of the
term PWM could be misleading, since the model is applicable to CAN ESCs
as well.

This commit rephrases the user documentation string and a few source
code comments, but no logic changes are made.

Closes #13105

src/lib/mixer/mixer.h

@@ -241,7 +241,7 @@ class Mixer
 	virtual unsigned get_trim(float *trim) = 0;
 
 	/*
-	 * @brief      Sets the thrust factor used to calculate mapping from desired thrust to pwm.
+	 * @brief      Sets the thrust factor used to calculate mapping from desired thrust to motor control signal output.
 	 *
 	 * @param[in]  val   The value
 	 */
@@ -444,7 +444,7 @@ class MixerGroup : public Mixer
 	}
 
 	/**
-	 * @brief      Sets the thrust factor used to calculate mapping from desired thrust to pwm.
+	 * @brief      Sets the thrust factor used to calculate mapping from desired thrust to motor control signal output.
 	 *
 	 * @param[in]  val   The value
 	 */
@@ -708,7 +708,7 @@ class MultirotorMixer : public Mixer
 	}
 
 	/**
-	 * @brief      Sets the thrust factor used to calculate mapping from desired thrust to pwm.
+	 * @brief      Sets the thrust factor used to calculate mapping from desired thrust to motor control signal output.
 	 *
 	 * @param[in]  val   The value
 	 */

src/lib/mixer_module/mixer_module.hpp

@@ -233,7 +233,7 @@ class MixingOutput : public ModuleParams
 	DEFINE_PARAMETERS(
 		(ParamInt<px4::params::MC_AIRMODE>) _param_mc_airmode,   ///< multicopter air-mode
 		(ParamFloat<px4::params::MOT_SLEW_MAX>) _param_mot_slew_max,
-		(ParamFloat<px4::params::THR_MDL_FAC>) _param_thr_mdl_fac, ///< thrust to pwm modelling factor
+		(ParamFloat<px4::params::THR_MDL_FAC>) _param_thr_mdl_fac, ///< thrust to motor control signal modelling factor
 		(ParamInt<px4::params::MOT_ORDERING>) _param_mot_ordering,
 		(ParamInt<px4::params::CBRK_IO_SAFETY>) _param_cbrk_io_safety
 

src/modules/px4iofirmware/protocol.h

@@ -235,7 +235,7 @@ enum {							/* DSM bind states */
 
 #define PX4IO_P_SETUP_MOTOR_SLEW_MAX		24 	/**< max motor slew rate */
 
-#define PX4IO_P_SETUP_THR_MDL_FAC 		25	/**< factor for modelling static pwm output to thrust relationship */
+#define PX4IO_P_SETUP_THR_MDL_FAC 		25	/**< factor for modelling motor control signal output to static thrust relationship */
 
 #define PX4IO_P_SETUP_THERMAL			26	/**< thermal management */
 

src/modules/sensors/motor_params.c

@@ -55,10 +55,14 @@
 PARAM_DEFINE_FLOAT(MOT_SLEW_MAX, 0.0f);
 
 /**
- * Thrust to PWM model parameter
+ * Thrust to motor control signal model parameter
  *
- * Parameter used to model the relationship between static thrust and motor
- * input PWM. Model is: thrust = (1-factor)*PWM + factor * PWM^2
+ * Parameter used to model the nonlinear relationship between
+ * motor control signal (e.g. PWM) and static thrust.
+ *
+ * The model is: rel_thrust = factor * rel_signal^2 + (1-factor) * rel_signal,
+ * where rel_thrust is the normalized thrust between 0 and 1, and
+ * rel_signal is the relative motor control signal between 0 and 1.
  *
  * @min 0.0
  * @max 1.0