AP_Motors: Add a seprate spool down time, if it's 0 use spool up time

libraries/AP_Motors/AP_MotorsMulticopter.cpp

@@ -165,7 +165,7 @@ const AP_Param::GroupInfo AP_MotorsMulticopter::var_info[] = {
     // @Param: SPOOL_TIME
     // @DisplayName: Spool up time
     // @Description: Time in seconds to spool up the motors from zero to min throttle. 
-    // @Range: 0 2
+    // @Range: 0.05 2
     // @Units: s
     // @Increment: 0.1
     // @User: Advanced
@@ -222,6 +222,15 @@ const AP_Param::GroupInfo AP_MotorsMulticopter::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO("OPTIONS", 43, AP_MotorsMulticopter, _options, 0),
 
+    // @Param: SPOOL_TIM_DN
+    // @DisplayName: Spool down time
+    // @Description: Time taken to spool down the motors from min to zero throttle. If set to 0 then SPOOL_TIME is used instead.
+    // @Range: 0 2
+    // @Units: s
+    // @Increment: 0.001
+    // @User: Advanced
+    AP_GROUPINFO("SPOOL_TIM_DN", 44, AP_MotorsMulticopter, _spool_down_time, 0),
+
     AP_GROUPEND
 };
 
@@ -477,6 +486,7 @@ void AP_MotorsMulticopter::update_throttle_hover(float dt)
 // run spool logic
 void AP_MotorsMulticopter::output_logic()
 {
+    const constexpr float minimum_spool_time = 0.05f;
     if (armed()) {
         if (_disarm_disable_pwm && (_disarm_safe_timer < _safe_time)) {
             _disarm_safe_timer += _dt;
@@ -493,12 +503,11 @@ void AP_MotorsMulticopter::output_logic()
         _spool_state = SpoolState::SHUT_DOWN;
     }
 
-    if (_spool_up_time < 0.05) {
+    if (_spool_up_time < minimum_spool_time) {
         // prevent float exception
-        _spool_up_time.set(0.05);
+        _spool_up_time.set(minimum_spool_time);
     }
 
-    const float spool_step = _dt / _spool_up_time;
     switch (_spool_state) {
     case SpoolState::SHUT_DOWN:
         // Motors should be stationary.
@@ -539,7 +548,9 @@ void AP_MotorsMulticopter::output_logic()
 
         // set and increment ramp variables
         switch (_spool_desired) {
-        case DesiredSpoolState::SHUT_DOWN:
+        case DesiredSpoolState::SHUT_DOWN: {
+            const float spool_time = _spool_down_time > minimum_spool_time ? _spool_down_time : _spool_up_time;
+            const float spool_step = _dt / spool_time;
             _spin_up_ratio -= spool_step;
             // constrain ramp value and update mode
             if (_spin_up_ratio <= 0.0f) {
@@ -547,8 +558,10 @@ void AP_MotorsMulticopter::output_logic()
                 _spool_state = SpoolState::SHUT_DOWN;
             }
             break;
+        }
 
-        case DesiredSpoolState::THROTTLE_UNLIMITED:
+        case DesiredSpoolState::THROTTLE_UNLIMITED: {
+            const float spool_step = _dt / _spool_up_time;
             _spin_up_ratio += spool_step;
             // constrain ramp value and update mode
             if (_spin_up_ratio >= 1.0f) {
@@ -559,15 +572,19 @@ void AP_MotorsMulticopter::output_logic()
                 }
             }
             break;
-
+        }
-        case DesiredSpoolState::GROUND_IDLE:
+        case DesiredSpoolState::GROUND_IDLE: {
+            const float spool_up_step = _dt / _spool_up_time;
+            const float spool_down_time = _spool_down_time > minimum_spool_time ? _spool_down_time : _spool_up_time;
+            const float spool_down_step = _dt / spool_down_time;
             float spin_up_armed_ratio = 0.0f;
             if (thr_lin.get_spin_min() > 0.0f) {
                 spin_up_armed_ratio = _spin_arm / thr_lin.get_spin_min();
             }
-            _spin_up_ratio += constrain_float(spin_up_armed_ratio - _spin_up_ratio, -spool_step, spool_step);
+            _spin_up_ratio += constrain_float(spin_up_armed_ratio - _spin_up_ratio, -spool_down_step, spool_up_step);
             break;
         }
+        }
         _throttle_thrust_max = 0.0f;
 
         // initialise motor failure variables
@@ -575,7 +592,8 @@ void AP_MotorsMulticopter::output_logic()
         _thrust_boost_ratio = 0.0f;
         break;
     }
-    case SpoolState::SPOOLING_UP:
+    case SpoolState::SPOOLING_UP: {
+        const float spool_step = _dt / _spool_up_time;
         // Maximum throttle should move from minimum to maximum.
         // Servos should exhibit normal flight behavior.
 
@@ -608,8 +626,10 @@ void AP_MotorsMulticopter::output_logic()
         _thrust_boost = false;
         _thrust_boost_ratio = MAX(0.0, _thrust_boost_ratio - spool_step);
         break;
+    }
 
-    case SpoolState::THROTTLE_UNLIMITED:
+    case SpoolState::THROTTLE_UNLIMITED: {
+        const float spool_step = _dt / _spool_up_time;
         // Throttle should exhibit normal flight behavior.
         // Servos should exhibit normal flight behavior.
 
@@ -636,6 +656,7 @@ void AP_MotorsMulticopter::output_logic()
             _thrust_boost_ratio = MAX(0.0, _thrust_boost_ratio - spool_step);
         }
         break;
+    }
 
     case SpoolState::SPOOLING_DOWN:
         // Maximum throttle should move from maximum to minimum.
@@ -656,6 +677,8 @@ void AP_MotorsMulticopter::output_logic()
 
         // set and increment ramp variables
         _spin_up_ratio = 1.0f;
+        const float spool_time = _spool_down_time > minimum_spool_time ? _spool_down_time : _spool_up_time;
+        const float spool_step = _dt / spool_time;
         _throttle_thrust_max -= spool_step;
 
         // constrain ramp value and update mode

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -169,6 +169,7 @@ protected:
 
     // time to spool motors to min throttle
     AP_Float            _spool_up_time;
+    AP_Float            _spool_down_time;
 
     // scaling for booster motor throttle
     AP_Float            _boost_scale;