Phidgets PID
References
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
- A quick and dirty tuning experiment
"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
