Revision 0:a3b128cdb78b, committed 2011-04-27

Comitter:

shimniok

Date:

Wed Apr 27 18:55:38 2011 +0000

Commit message:

Initial release

Changed in this revision

diff -r 000000000000 -r a3b128cdb78b Steering.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Steering.cpp	Wed Apr 27 18:55:38 2011 +0000
@@ -0,0 +1,42 @@
+#include "Steering.h"
+#include "math.h"
+
+/** create a new steering calculator for a particular vehicle
+ *
+ */
+Steering::Steering(float wheelbase, float track)
+    : _wheelbase(wheelbase)
+    , _track(track)
+    , _intercept(2.0)
+{
+}
+
+void Steering::setIntercept(float intercept)
+{
+    _intercept = intercept;
+}
+
+/** Calculate a steering angle based on relative bearing
+ *
+ */
+float Steering::calcSA(float theta) {
+    float radius;
+    float SA;
+    bool neg = (theta < 0);
+
+    // I haven't had time to work out why the equation is slightly offset such
+    // that negative angle produces slightly less steering angle
+    //
+    if (neg) theta *= -1.0;    
+    
+    // The equation peaks out at 90* so clamp theta artifically to 90, so that
+    // if theta is actually > 90, we select max steering
+    if (theta > 90.0) theta = 90.0;
+    
+    radius = _intercept/(2*sin(angle_radians(theta)));
+    SA = angle_degrees(asin(_wheelbase / (radius - _track/2)));
+
+    if (neg) SA *= -1.0;
+
+    return SA;
+}

diff -r 000000000000 -r a3b128cdb78b Steering.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Steering.h	Wed Apr 27 18:55:38 2011 +0000
@@ -0,0 +1,45 @@
+
+#define PI 3.141592653589
+
+/** A class for managing steering angle calculations based on current and desired heading
+ *  and specified intercept distance along the new path.  
+ *
+ *  See Notebook entry: http://mbed.org/users/shimniok/notebook/smooth-steering-for-rc-car/
+ */
+class Steering
+{
+  public:
+    /** create a new steering calculator
+     *
+     * @param wheelbase vehicle wheelbase
+     * @param track vehicle track width
+     * @param intercept new course intercept distance
+     */
+    Steering(float wheelbase, float track);
+
+    /** set intercept distance
+     * @param intercept distance along new course at which turn arc will intercept
+     */
+    void setIntercept(float intercept);
+
+    /** convert course change to average steering angle
+     * assumes Ackerman steering, with track and wheelbase
+     * and course intercept distance specified.
+     *
+     * See notebook: http://mbed.org/users/shimniok/notebook/smooth-steering-for-rc-car/
+     *
+     * @param theta relative bearing of the new course
+     * @returns steering angle in degrees
+     */ 
+    float calcSA(float theta);
+    
+  private:
+
+    inline static float angle_radians(float deg) {return (PI/180.0)*deg;}
+
+    inline static float angle_degrees(float rad) {return (180/PI)*rad;}
+
+    float _wheelbase;
+    float _track;
+    float _intercept;
+};
\ No newline at end of file