Rover: add MOT_SKID_FRIC to up skid-steer motor out while stopped

APMrover2/AP_MotorsUGV.cpp

@@ -72,6 +72,15 @@ const AP_Param::GroupInfo AP_MotorsUGV::var_info[] = {
     // @User: Standard
     AP_GROUPINFO("THR_MAX", 6, AP_MotorsUGV, _throttle_max, 100),
 
+    // @Param: SKID_FRIC
+    // @DisplayName: Motor skid steering friction compensation
+    // @Description: Motor output for skid steering vehicles will be increased by this percentage to overcome friction when stopped
+    // @Units: %
+    // @Range: 0 100
+    // @Increment: 1
+    // @User: Standard
+    AP_GROUPINFO("SKID_FRIC", 7, AP_MotorsUGV, _skid_friction, 0.0f),
+
     AP_GROUPEND
 };
 
@@ -268,9 +277,10 @@ void AP_MotorsUGV::output_skid_steering(bool armed, float steering, float thrott
 
     // deal with case of turning on the spot
     if (is_zero(throttle_scaled)) {
-        // full possible range is not used to keep response equivalent to non-zero throttle case
-        motor_left += steering_scaled * 0.5f;
-        motor_right -= steering_scaled * 0.5f;
+        // steering output split evenly between left and right motors and compensated for friction
+        const float friction_comp = MAX(0.0f, 1.0f + (_skid_friction * 0.01f));
+        motor_left += steering_scaled * 0.5f * friction_comp;
+        motor_right -= steering_scaled * 0.5f * friction_comp;
     } else {
         // add in steering
         const float dir = is_positive(throttle_scaled) ? 1.0f : -1.0f;

APMrover2/AP_MotorsUGV.h

@@ -99,6 +99,7 @@ protected:
     AP_Int8 _slew_rate; // slew rate expressed as a percentage / second
     AP_Int8 _throttle_min; // throttle minimum percentage
     AP_Int8 _throttle_max; // throttle maximum percentage
+    AP_Float _skid_friction;    // skid steering vehicle motor output compensation for friction while stopped
 
     // internal variables
     float   _steering;  // requested steering as a value from -4500 to +4500