APM_Control: added a ground vehicle steering controller

this will be used both for the rover code, and for ground steering of
a plane on takeoff

libraries/APM_Control/APM_Control.h

@@ -1,3 +1,4 @@
 #include "AP_RollController.h"
 #include "AP_PitchController.h"
-#include "AP_YawController.h"
+#include "AP_YawController.h"
+#include "AP_SteerController.h"

libraries/APM_Control/AP_SteerController.cpp

@@ -0,0 +1,145 @@
+// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
+/*
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+//	Code by Andrew Tridgell
+//  Based upon the roll controller by Paul Riseborough and Jon Challinger
+//
+
+#include <AP_Math.h>
+#include <AP_HAL.h>
+#include "AP_SteerController.h"
+
+extern const AP_HAL::HAL& hal;
+
+const AP_Param::GroupInfo AP_SteerController::var_info[] PROGMEM = {
+	// @Param: T_CONST
+	// @DisplayName: Steering Time Constant
+	// @Description: This controls the time constant in seconds from demanded to achieved bank angle. A value of 0.5 is a good default and will work with nearly all models. Advanced users may want to reduce this time to obtain a faster response but there is no point setting a time less than the aircraft can achieve.
+	// @Range: 0.4 1.0
+	// @Units: seconds
+	// @Increment: 0.1
+	// @User: Advanced
+	AP_GROUPINFO("TCONST",      0, AP_SteerController, _tau,       0.5f),
+
+	// @Param: P
+	// @DisplayName: Steering turning gain
+	// @Description: The proportional gain for steering. This should be approximately equal to the diameter of the turning circle of the vehicle at low speed and maximum steering angle
+	// @Range: 0.1 10.0
+	// @Increment: 0.1
+	// @User: User
+	AP_GROUPINFO("P",      1, AP_SteerController, _K_P,        1.8f),
+
+	// @Param: I
+	// @DisplayName: Integrator Gain
+	// @Description: This is the gain from the integral of steering angle. Increasing this gain causes the controller to trim out steady offsets due to an out of trim vehicle.
+	// @Range: 0 1.0
+	// @Increment: 0.05
+	// @User: User
+	AP_GROUPINFO("I",        3, AP_SteerController, _K_I,        0.2f),
+
+	// @Param: D
+	// @DisplayName: Damping Gain
+	// @Description: This adjusts the damping of the steering control loop. This gain helps to reduce steering jitter with vibration. It should be increased in 0.01 increments as too high a value can lead to a high frequency steering oscillation that could overstress the vehicle.
+	// @Range: 0 0.1
+	// @Increment: 0.01
+	// @User: User
+	AP_GROUPINFO("D",        4, AP_SteerController, _K_D,        0.02f),
+
+	// @Param: IMAX
+	// @DisplayName: Integrator limit
+	// @Description: This limits the number of degrees of steering in centi-degrees over which the integrator will operate. At the default setting of 1500 centi-degrees, the integrator will be limited to +- 15 degrees of servo travel. The maximum servo deflection is +- 45 centi-degrees, so the default value represents a 1/3rd of the total control throw which is adequate unless the vehicle is severely out of trim.
+	// @Range: 0 4500
+	// @Increment: 1
+	// @User: Advanced
+	AP_GROUPINFO("IMAX",     5, AP_SteerController, _imax,        1500),
+
+	AP_GROUPEND
+};
+
+
+/*
+  internal rate controller, called by attitude and rate controller
+  public functions
+*/
+int32_t AP_SteerController::get_steering_out(float desired_accel)
+{
+	uint32_t tnow = hal.scheduler->millis();
+	uint32_t dt = tnow - _last_t;
+	if (_last_t == 0 || dt > 1000) {
+		dt = 0;
+	}
+	_last_t = tnow;
+
+    float speed = _ahrs.groundspeed();
+    if (speed < 1.0e-6) {
+        // with no speed all we can do is center the steering
+        return 0;
+    }
+
+    // this is a linear approximation of the inverse steering
+    // equation for a ground vehicle. It returns steering as an angle from -45 to 45
+    float scaler = 1.0f / speed;
+
+	// Calculate the steering rate error (deg/sec) and apply gain scaler
+    float desired_rate = desired_accel / speed;
+	float rate_error = (ToDeg(desired_rate) - ToDeg(_ahrs.get_gyro().z)) * scaler;
+	
+	// Calculate equivalent gains so that values for K_P and K_I can be taken across from the old PID law
+    // No conversion is required for K_D
+	float ki_rate = _K_I * _tau * 45.0f;
+	float kp_ff = max((_K_P - _K_I * _tau) * _tau  - _K_D , 0) * 45.0f;
+	float delta_time    = (float)dt * 0.001f;
+	
+	// Multiply roll rate error by _ki_rate and integrate
+	// Don't integrate if in stabilise mode as the integrator will wind up against the pilots inputs
+	if (ki_rate > 0) {
+		// only integrate if gain and time step are positive.
+		if (dt > 0) {
+		    float integrator_delta = rate_error * ki_rate * delta_time;
+			// prevent the integrator from increasing if steering defln demand is above the upper limit
+			if (_last_out < -45) {
+                integrator_delta = max(integrator_delta , 0);
+            } else if (_last_out > 45) {
+                // prevent the integrator from decreasing if steering defln demand is below the lower limit
+                integrator_delta = min(integrator_delta, 0);
+            }
+			_integrator += integrator_delta;
+		}
+	} else {
+		_integrator = 0;
+	}
+	
+    // Scale the integration limit
+    float intLimScaled = _imax * 0.01f / scaler;
+
+    // Constrain the integrator state
+    _integrator = constrain_float(_integrator, -intLimScaled, intLimScaled);
+	
+	// Calculate the demanded control surface deflection
+	// Note the scaler is applied again. We want a 1/speed scaler applied to the feed-forward
+	// path, but want a 1/speed^2 scaler applied to the rate error path. 
+	// This is because acceleration scales with speed^2, but rate scales with speed.
+	_last_out = ( (rate_error * _K_D * 45.0f) + _integrator + (desired_rate * kp_ff) ) * scaler;
+	
+	// Convert to centi-degrees and constrain
+	return constrain_float(_last_out * 100, -4500, 4500);
+}
+
+void AP_SteerController::reset_I()
+{
+	_integrator = 0;
+}
+

libraries/APM_Control/AP_SteerController.h

@@ -0,0 +1,43 @@
+// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
+
+#ifndef __AP_STEER_CONTROLLER_H__
+#define __AP_STEER_CONTROLLER_H__
+
+#include <AP_AHRS.h>
+#include <AP_Common.h>
+#include <AP_SpdHgtControl.h>
+#include <math.h>
+
+class AP_SteerController {
+public:
+	AP_SteerController(AP_AHRS &ahrs) :
+        _ahrs(ahrs)
+    { 
+		AP_Param::setup_object_defaults(this, var_info);
+	}
+
+    /*
+      return a steering servo output from -4500 to 4500 given a
+      desired lateral acceleration rate in m/s/s.
+     */
+	int32_t get_steering_out(float desired_accel);
+
+	void reset_I();
+
+	static const struct AP_Param::GroupInfo var_info[];
+
+private:
+	AP_Float _tau;
+	AP_Float _K_P;
+	AP_Float _K_I;
+	AP_Float _K_D;
+    AP_Int16  _imax;
+	uint32_t _last_t;
+	float _last_out;
+
+	float _integrator;
+
+	AP_AHRS &_ahrs;
+};
+
+#endif // __AP_STEER_CONTROLLER_H__