AP_Parachute: added CHUTE_OPTIONS

allow for servo release forever to cope with high altitude drops where
servo may be frozen

libraries/AP_Parachute/AP_Parachute.cpp

@@ -73,8 +73,14 @@ const AP_Param::GroupInfo AP_Parachute::var_info[] = {
     // @Increment: 1
     // @User: Standard
     AP_GROUPINFO("CRT_SINK", 6, AP_Parachute, _critical_sink, AP_PARACHUTE_CRITICAL_SINK_DEFAULT),
-    
-    
+
+    // @Param: OPTIONS
+    // @DisplayName: Parachute options
+    // @Description: Optional behaviour for parachute
+    // @Bitmask: 0:hold open forever after release
+    // @User: Standard
+    AP_GROUPINFO("OPTIONS", 7, AP_Parachute, _options, 0),
+
     AP_GROUPEND
 };
 
@@ -123,6 +129,8 @@ void AP_Parachute::update()
     uint32_t time_diff = AP_HAL::millis() - _release_time;
     uint32_t delay_ms = _delay_ms<=0 ? 0: (uint32_t)_delay_ms;
 
+    bool hold_forever = (_options.get() & uint32_t(Options::HoldOpen)) != 0;
+
     // check if we should release parachute
     if ((_release_time != 0) && !_release_in_progress) {
         if (time_diff >= delay_ms) {
@@ -136,7 +144,8 @@ void AP_Parachute::update()
             _release_in_progress = true;
             _released = true;
         }
-    } else if ((_release_time == 0) || time_diff >= delay_ms + AP_PARACHUTE_RELEASE_DURATION_MS) {
+    } else if ((_release_time == 0) ||
+               (!hold_forever && time_diff >= delay_ms + AP_PARACHUTE_RELEASE_DURATION_MS)) {
         if (_release_type == AP_PARACHUTE_TRIGGER_TYPE_SERVO) {
             // move servo back to off position
             SRV_Channels::set_output_pwm(SRV_Channel::k_parachute_release, _servo_off_pwm);

libraries/AP_Parachute/AP_Parachute.h

@@ -113,6 +113,12 @@ private:
     bool        _released:1;             // true if the parachute has been released
     bool        _is_flying:1;            // true if the vehicle is flying
     uint32_t    _sink_time_ms;           // system time that the vehicle exceeded critical sink rate
+
+    enum class Options : uint8_t {
+        HoldOpen = (1U<<0),
+    };
+
+    AP_Int32    _options;
 };
 
 namespace AP {