Plane: added throttle suppression for quadplanes

this tries to prevent motor start when on ground. Motors instead go
into spin_when_armed state

ArduPlane/quadplane.cpp

@@ -1011,6 +1011,58 @@ void QuadPlane::update(void)
     tiltrotor_update();
 }
 
+/*
+  see if motors should be shutdown. If they should be then change AP_Motors state to 
+  AP_Motors::DESIRED_SHUT_DOWN
+
+  This is a safety check to prevent accidental motor runs on the
+  ground, such as if RC fails and QRTL is started
+ */
+void QuadPlane::check_throttle_suppression(void)
+{
+    // if the motors have been running in the last 2 seconds then
+    // allow them to run now
+    if (AP_HAL::millis() - last_motors_active_ms < 2000) {
+        return;
+    }
+
+    // see if motors are already disabled
+    if (motors->get_desired_spool_state() < AP_Motors::DESIRED_THROTTLE_UNLIMITED) {
+        return;
+    }
+
+    // if the users throttle is above zero then allow motors to run
+    if (plane.channel_throttle->get_control_in() != 0) {
+        return;
+    }
+
+    // if we are in a fixed wing auto throttle mode and we have
+    // unsuppressed the throttle then allow motors to run
+    if (plane.auto_throttle_mode && !plane.throttle_suppressed) {
+        return;
+    }
+
+    // if our vertical velocity is greater than 1m/s then allow motors to run
+    if (fabsf(inertial_nav.get_velocity_z()) > 100) {
+        return;
+    }
+
+    // if we are more than 5m from home altitude then allow motors to run
+    if (plane.relative_ground_altitude(plane.g.rangefinder_landing) > 5) {
+        return;
+    }
+
+    // allow for takeoff
+    if (plane.control_mode == AUTO && plane.mission.get_current_nav_cmd().id == MAV_CMD_NAV_VTOL_TAKEOFF) {
+        return;
+    }
+    
+    // motors should be in the spin when armed state to warn user they could become active
+    motors->set_desired_spool_state(AP_Motors::DESIRED_SPIN_WHEN_ARMED);
+    motors->set_throttle(0);
+    last_motors_active_ms = 0;
+}
+
 /*
   output motors and do any copter needed
  */
@@ -1020,6 +1072,10 @@ void QuadPlane::motors_output(void)
         // output is direct from run_esc_calibration()
         return;
     }
+
+    // see if motors should be shut down
+    check_throttle_suppression();
+    
     motors->output();
     if (motors->armed()) {
         plane.DataFlash.Log_Write_Rate(plane.ahrs, *motors, *attitude_control, *pos_control);
@@ -1029,6 +1085,12 @@ void QuadPlane::motors_output(void)
             attitude_control->control_monitor_log();
         }
     }
+
+    // remember when motors were last active for throttle suppression
+    if (motors->get_throttle() > 0.01f || tilt.motors_active) {
+        last_motors_active_ms = AP_HAL::millis();
+    }
+    
 }
 
 /*

ArduPlane/quadplane.h

@@ -187,6 +187,8 @@ private:
 
     void guided_start(void);
     void guided_update(void);
+
+    void check_throttle_suppression(void);
     
     AP_Int16 transition_time_ms;
 
@@ -262,10 +264,10 @@ private:
     } transition_state;
 
     // true when waiting for pilot throttle
-    bool throttle_wait;
+    bool throttle_wait:1;
 
     // true when quad is assisting a fixed wing mode
-    bool assisted_flight;
+    bool assisted_flight:1;
 
     struct {
         // time when motors reached lower limit
@@ -324,6 +326,9 @@ private:
         bool motors_active:1;
     } tilt;
 
+    // time when motors were last active
+    uint32_t last_motors_active_ms;
+    
     void tiltrotor_slew(float tilt);
     void tiltrotor_update(void);
     void tilt_compensate(float *thrust, uint8_t num_motors);