Latest revision as of 13:10, 20 September 2017

References

• ROS PID
• ROS PID API

ROS PID Code

This is in motor_control_pc_nav.cpp.

Includes and globals

   #include <control_toolbox/pid.h>

   // Motor pid control
   double k_p_ = 1.0;
   double k_i_ = 0.0;
   double k_d_ = 0.0;

   // motor characteristics
   control_toolbox::Pid pid_motor0_;
   control_toolbox::Pid pid_motor1_;
   ros::Time last_time_;
   ros::Time beginning_time_;
 
   // Phidget motor control settings
   double power_wheel0_;
   double power_wheel1_;

PID intialization

   void initMotorPower ()
   {
       // pid related
       pid_motor0_.initPid(k_p_, k_i_, k_d_, 0.0, -0.0);
       pid_motor1_.initPid(k_p_, k_i_, k_d_, 0.0, -0.0);
       last_time_ = ros::Time::now();

       // motor power related
       power_wheel0_ = 0;
       power_wheel1_ = 0;

}

In the odometry callyback function

   void setMotorPower ()
   {
       ...
 
       // Compute motor control power 
       // Motor power computed between -100 and 100 percent of full power
       ros::Time time = ros::Time::now();
       power_wheel0_ = power_wheel0_ + pid_motor0_.computeCommand(
                           wheel0_target_speed - wheel0_speed, time - last_time_);
       if (power_wheel0_ >  100) power_wheel0_ =  100;
       if (power_wheel0_ < -100) power_wheel0_ = -100;
       power_wheel1_ = power_wheel1_ + pid_motor1_.computeCommand(
                            wheel1_target_speed - wheel1_speed, time - last_time_);
       if (power_wheel1_ >  100) power_wheel1_ =  100;
       if (power_wheel1_ < -100) power_wheel1_ = -100;
       last_time_ = time;
 
       // *** apply power to the motors ***
       CPhidgetMotorControl_setVelocity (phid_, 0, power_wheel0_);
       CPhidgetMotorControl_setVelocity (phid_, 1, -power_wheel1_);
 
       ...
   }

PID Calibration

• ROS PID Tuning
  • A quick and dirty tuning experiment
    • Target linear speed 0.2 m/sec, angular 0.2 radians/sec
    • Definite oscillation with P = 60.
    • Set P to 1/2 of that or 30
    • Tried a I of 60
  • See Phidgets PID Tuning for details

• Manual PID Tuning from Wikipedia

"If the system must remain online, one tuning method is to first set K_i and K_d values to zero. Increase the K_p until the output of the loop oscillates, then the K_p should be set to approximately half of that value for a "quarter amplitude decay" type response. Then increase K_i until any offset is corrected in sufficient time for the process. However, too much K_i will cause instability. Finally, increase K_d, if required, until the loop is acceptably quick to reach its reference after a load disturbance. However, too much K_d will cause excessive response and overshoot. A fast PID loop tuning usually overshoots slightly to reach the setpoint more quickly; however, some systems cannot accept overshoot, in which case an over-damped closed-loop system is required, which will require a K_p setting significantly less than half that of the K_p setting that was causing oscillation."

Other PID References

• PID Controller Theory from Wikipedia
• Using PID based Techniques For Competitive Odometry and Dead-Reckoning G.W. Lucas