PIDSubsystems for built-in PID Control

Important

This documentation describes the use of the legacy command-based library. While this documentation has been preserved to help teams that have yet to do so, teams are strongly encouraged to migrate to the new command-based library.

Note

If a mechanism uses a sensor for feedback, then most often a PID controller will be used to control the motor speed or position. Examples of subsystems that might use PID control are: elevators with potentiometers to track the height, shooters with encoders to measure the speed, wrists with potentiometers to measure the joint angle, etc.

There is a PIDController class built into WPILib, but to simplify its use for command based programs there is a PIDSubsystem. A PIDSubsystem is a normal subsystem with the PIDController built in and exposes the required methods for operation.

Controlling the Angle of a Wrist Joint

In this example you can see the basic elements of a PIDSubsystem for the wrist joint:

Java

C++

package org.usfirst.frc.team1.robot.subsystems;
import edu.wpi.first.wpilibj.*;
import edu.wpi.first.wpilibj.command.PIDSubsystem;
import org.usfirst.frc.team1.robot.RobotMap;


public class Wrist extends PIDSubsystem { // This system extends PIDSubsystem

   Victor motor = RobotMap.wristMotor;
   AnalogInput pot = RobotMap.wristPot();

   public Wrist() {
      super("Wrist", 2.0, 0.0, 0.0);// The constructor passes a name for the subsystem and the P, I and D constants that are used when computing the motor output
      setAbsoluteTolerance(0.05);
      getPIDController().setContinuous(false);
   }

   public void initDefaultCommand() {
   }

   protected double returnPIDInput() {
       return pot.getAverageVoltage(); // returns the sensor value that is providing the feedback for the system
   }

   protected void usePIDOutput(double output) {
       motor.pidWrite(output); // this is where the computed output value from the PIDController is applied to the motor
   }
}

#include "subsystems/Wrist.h"

#include <frc/smartdashboard/SmartDashboard.h>

Wrist::Wrist() : frc::PIDSubsystem("Wrist", kP_real, 0.0, 0.0) {
#ifdef SIMULATION  // Check for simulation and update PID values
  GetPIDController()->SetPID(kP_simulation, 0, 0, 0);
#endif
  SetAbsoluteTolerance(2.5);
}

void Wrist::InitDefaultCommand() {}

void Wrist::Log() {
  // frc::SmartDashboard::PutData("Wrist Angle", &m_pot);
}

double Wrist::ReturnPIDInput() { return m_pot.Get(); }

void Wrist::UsePIDOutput(double d) { m_motor.Set(d); }