Rover: added BRAKING_SPEEDERR parameter

this controls how much speed error you need before brakes are used

APMrover2/Parameters.h

@@ -46,6 +46,7 @@ public:
 
         // braking
         k_param_braking_percent = 30,
+        k_param_braking_speederr,
 
         // misc2
         k_param_log_bitmask = 40,
@@ -198,6 +199,7 @@ public:
 
     // braking
     AP_Int8     braking_percent;
+    AP_Float    braking_speederr;
 
 	// Telemetry control
 	//

APMrover2/Parameters.pde

@@ -154,6 +154,15 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @User: Standard
 	GSCALAR(braking_percent,    "BRAKING_PERCENT",  0),
 
+    // @Param: BRAKING_SPEEDERR
+    // @DisplayName: Speed error at which to apply braking
+    // @Description: The amount of overspeed error at which to start applying braking
+    // @Units: m/s
+    // @Range: 0 100
+    // @Increment: 1
+    // @User: Standard
+	GSCALAR(braking_speederr,   "BRAKING_SPEEDERR",  3),
+
     // @Param: PIVOT_TURN_ANGLE
     // @DisplayName: Pivot turn angle
     // @Description: Navigation angle threshold in degrees to switch to pivot steering when SKID_STEER_OUT is 1. This allows you to setup a skid steering rover to turn on the spot in auto mode when the angle it needs to turn it greater than this angle. An angle of zero means to disable pivot turning. Note that you will probably also want to set a low value for WP_RADIUS to get neat turns.

APMrover2/Steering.pde

@@ -125,11 +125,15 @@ static void calc_throttle(float target_speed)
         channel_throttle->servo_out = constrain_int16(throttle, g.throttle_min, g.throttle_max);
     }
 
-    if (!in_reverse && g.braking_percent != 0 && groundspeed_error < -g.speed_cruise/2) {
+    if (!in_reverse && g.braking_percent != 0 && groundspeed_error < -g.braking_speederr) {
         // the user has asked to use reverse throttle to brake. Apply
         // it in proportion to the ground speed error, but only when
-        // our ground speed error is more than half our cruise speed
-        float brake_gain = constrain_float(-groundspeed_error / (float)g.speed_cruise, 0, 1);
+        // our ground speed error is more than BRAKING_SPEEDERR.
+        //
+        // We use a linear gain, with 0 gain at a ground speed error
+        // of braking_speederr, and 100% gain when groundspeed_error
+        // is 2*braking_speederr
+        float brake_gain = constrain_float(((-groundspeed_error)-g.braking_speederr)/g.braking_speederr, 0, 1);
         int16_t braking_throttle = g.throttle_max * (g.braking_percent * 0.01f) * brake_gain;
         channel_throttle->servo_out = constrain_int16(-braking_throttle, -g.throttle_max, -g.throttle_min);