AP_MotorsMulticopter: limit hover throttle

libraries/AP_Motors/AP_MotorsMulticopter.cpp

@@ -378,7 +378,8 @@ void AP_MotorsMulticopter::set_throttle_range(int16_t radio_min, int16_t radio_m
 void AP_MotorsMulticopter::update_throttle_hover(float dt)
 {
     if (_throttle_hover_learn != HOVER_LEARN_DISABLED) {
-        _throttle_hover = _throttle_hover + (dt/(dt+AP_MOTORS_THST_HOVER_TC))*(get_throttle()-_throttle_hover);
+        // we have chosen to constrain the hover throttle to be within the range reachable by the third order expo polynomial.
+        _throttle_hover = constrain_float(_throttle_hover + (dt/(dt+AP_MOTORS_THST_HOVER_TC))*(get_throttle()-_throttle_hover), AP_MOTORS_THST_HOVER_MIN, AP_MOTORS_THST_HOVER_MAX);
     }
 }
 

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -17,6 +17,8 @@
 #define AP_MOTORS_THST_EXPO_DEFAULT     0.65f   // set to 0 for linear and 1 for second order approximation
 #define AP_MOTORS_THST_HOVER_DEFAULT    0.5f    // the estimated hover throttle, 0 ~ 1
 #define AP_MOTORS_THST_HOVER_TC         10.0f   // time constant used to update estimated hover throttle, 0 ~ 1
+#define AP_MOTORS_THST_HOVER_MIN        0.125f  // minimum possible hover throttle
+#define AP_MOTORS_THST_HOVER_MAX        0.6875f // maximum possible hover throttle
 #define AP_MOTORS_SPIN_MIN_DEFAULT      0.15f   // throttle out ratio which produces the minimum thrust.  (i.e. 0 ~ 1 ) of the full throttle range
 #define AP_MOTORS_SPIN_MAX_DEFAULT      0.95f   // throttle out ratio which produces the maximum thrust.  (i.e. 0 ~ 1 ) of the full throttle range
 #define AP_MOTORS_SPIN_ARM_DEFAULT      0.10f   // throttle out ratio which produces the armed spin rate.  (i.e. 0 ~ 1 ) of the full throttle range