AP_MotorsMulticopter: add inverse thrust_to_actuator and thrust curve functions

libraries/AP_Motors/AP_MotorsMulticopter.cpp

@@ -338,20 +338,37 @@ float AP_MotorsMulticopter::get_current_limit_max_throttle()
 // apply_thrust_curve_and_volt_scaling - returns throttle in the range 0 ~ 1
 float AP_MotorsMulticopter::apply_thrust_curve_and_volt_scaling(float thrust) const
 {
-    float throttle_ratio = thrust;
+    float battery_scale = 1.0;
+    if (is_positive(_batt_voltage_filt.get())) {
+        battery_scale = 1.0 / _batt_voltage_filt.get();
+    }
     // apply thrust curve - domain -1.0 to 1.0, range -1.0 to 1.0
     float thrust_curve_expo = constrain_float(_thrust_curve_expo, -1.0f, 1.0f);
     if (is_zero(thrust_curve_expo)) {
         // zero expo means linear, avoid floating point exception for small values
-        return _lift_max * thrust;
+        return _lift_max * thrust * battery_scale;
     }
+    float throttle_ratio = ((thrust_curve_expo - 1.0f) + safe_sqrt((1.0f - thrust_curve_expo) * (1.0f - thrust_curve_expo) + 4.0f * thrust_curve_expo * _lift_max * thrust)) / (2.0f * thrust_curve_expo);
+    return constrain_float(throttle_ratio * battery_scale, 0.0f, 1.0f);
+}
+
+// inverse of above
+float AP_MotorsMulticopter::remove_thrust_curve_and_volt_scaling(float throttle) const
+{
+    float battery_scale = 1.0;
     if (is_positive(_batt_voltage_filt.get())) {
-        throttle_ratio = ((thrust_curve_expo - 1.0f) + safe_sqrt((1.0f - thrust_curve_expo) * (1.0f - thrust_curve_expo) + 4.0f * thrust_curve_expo * _lift_max * thrust)) / (2.0f * thrust_curve_expo * _batt_voltage_filt.get());
-    } else {
-        throttle_ratio = ((thrust_curve_expo - 1.0f) + safe_sqrt((1.0f - thrust_curve_expo) * (1.0f - thrust_curve_expo) + 4.0f * thrust_curve_expo * _lift_max * thrust)) / (2.0f * thrust_curve_expo);
+        battery_scale = 1.0 / _batt_voltage_filt.get();
     }
-
-    return constrain_float(throttle_ratio, 0.0f, 1.0f);
+    // apply thrust curve - domain -1.0 to 1.0, range -1.0 to 1.0
+    float thrust_curve_expo = constrain_float(_thrust_curve_expo, -1.0f, 1.0f);
+    if (is_zero(thrust_curve_expo)) {
+        // zero expo means linear, avoid floating point exception for small values
+        return  throttle / (_lift_max * battery_scale);
+    }
+    float thrust = ((throttle / battery_scale) * (2.0f * thrust_curve_expo)) - (thrust_curve_expo - 1.0f);
+    thrust = (thrust * thrust) - ((1.0f - thrust_curve_expo) * (1.0f - thrust_curve_expo));
+    thrust /=  4.0f * thrust_curve_expo * _lift_max;
+    return constrain_float(thrust, 0.0f, 1.0f);
 }
 
 // update_lift_max from battery voltage - used for voltage compensation
@@ -417,12 +434,19 @@ int16_t AP_MotorsMulticopter::output_to_pwm(float actuator)
 }
 
 // converts desired thrust to linearized actuator output in a range of 0~1
-float AP_MotorsMulticopter::thrust_to_actuator(float thrust_in)
+float AP_MotorsMulticopter::thrust_to_actuator(float thrust_in) const
 {
     thrust_in = constrain_float(thrust_in, 0.0f, 1.0f);
     return _spin_min + (_spin_max - _spin_min) * apply_thrust_curve_and_volt_scaling(thrust_in);
 }
 
+// inverse of above
+float AP_MotorsMulticopter::actuator_to_thrust(float actuator) const
+{
+    actuator = (actuator - _spin_min) /  (_spin_max - _spin_min);
+    return constrain_float(remove_thrust_curve_and_volt_scaling(actuator), 0.0f, 1.0f);
+}
+
 // adds slew rate limiting to actuator output
 void AP_MotorsMulticopter::set_actuator_with_slew(float& actuator_output, float input)
 {

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -83,12 +83,15 @@ public:
     // get minimum or maximum pwm value that can be output to motors
     int16_t             get_pwm_output_min() const;
     int16_t             get_pwm_output_max() const;
-    
+
     // parameter check for MOT_PWM_MIN/MAX, returns true if parameters are valid
     bool check_mot_pwm_params() const;
 
     // converts desired thrust to linearized actuator output in a range of 0~1
-    float               thrust_to_actuator(float thrust_in);
+    float               thrust_to_actuator(float thrust_in) const;
+
+    // inverse of above
+    float               actuator_to_thrust(float actuator) const;
 
     // set thrust compensation callback
     FUNCTOR_TYPEDEF(thrust_compensation_fn_t, void, float *, uint8_t);
@@ -122,6 +125,8 @@ protected:
 
     // apply_thrust_curve_and_volt_scaling - returns throttle in the range 0 ~ 1
     float               apply_thrust_curve_and_volt_scaling(float thrust) const;
+    // inverse of above
+    float               remove_thrust_curve_and_volt_scaling(float throttle) const;
 
     // update_lift_max_from_batt_voltage - used for voltage compensation
     void                update_lift_max_from_batt_voltage();