Combining Motion Profiling and PID in Command-Based

Nota

For a description of the WPILib PID control features used by these command-based wrappers, see PID Control in WPILib.

Nota

Unlike the earlier version of PIDController, the 2020 ProfiledPIDController class runs synchronously, and is not handled in its own thread. Accordingly, changing its period parameter will not change the actual frequency at which it runs in any of these wrapper classes. Users should never modify the period parameter unless they are certain of what they are doing.

A common FRC® controls solution is to pair a trapezoidal motion profile for setpoint generation with a PID controller for setpoint tracking. To facilitate this, WPILib includes its own ProfiledPIDController class. To further aid teams in integrating this functionality into their robots, the command-based framework contains two convenience wrappers for the ProfiledPIDController class: ProfiledPIDSubsystem, which integrates the controller into a subsystem, and ProfiledPIDCommand, which integrates the controller into a command.

ProfiledPIDSubsystem

Nota

In C++, the ProfiledPIDSubsystem class is templated on the unit type used for distance measurements, which may be angular or linear. The passed-in values must have units consistent with the distance units, or a compile-time error will be thrown. For more information on C++ units, see The C++ Units Library.

The ProfiledPIDSubsystem class (Java, C++) allows users to conveniently create a subsystem with a built-in PIDController. In order to use the ProfiledPIDSubsystem class, users must create a subclass of it.

Creating a ProfiledPIDSubsystem

When subclassing ProfiledPIDSubsystem, users must override two abstract methods to provide functionality that the class will use in its ordinary operation:

getMeasurement()

protected abstract double getMeasurement();

The getMeasurement method returns the current measurement of the process variable. The PIDSubsystem will automatically call this method from its periodic() block, and pass its value to the control loop.

Users should override this method to return whatever sensor reading they wish to use as their process variable measurement.

useOutput()

protected abstract void useOutput(double output, TrapezoidProfile.State setpoint);

The useOutput() method consumes the output of the PID controller, and the current setpoint state (which is often useful for computing a feedforward). The PIDSubsystem will automatically call this method from its periodic() block, and pass it the computed output of the control loop.

Users should override this method to pass the final computed control output to their subsystem’s motors.

Passing In the Controller

Users must also pass in a ProfiledPIDController to the ProfiledPIDSubsystem base class through the superclass constructor call of their subclass. This serves to specify the PID gains, the motion profile constraints, and the period (if the user is using a non-standard main robot loop period).

Additional modifications (e.g. enabling continuous input) can be made to the controller in the constructor body by calling getController().

Using a ProfiledPIDSubsystem

Once an instance of a PIDSubsystem subclass has been created, it can be used by commands through the following methods:

setGoal()

Nota

If you wish to set the goal to a simple distance with an implicit target velocity of zero, an overload of setGoal() exists that takes a single distance value, rather than a full motion profile state.

The setGoal() method can be used to set the setpoint of the PIDSubsystem. The subsystem will automatically track to the setpoint using the defined output:

// The subsystem will track to a goal of 5 meters and velocity of 3 meters per second.
examplePIDSubsystem.setGoal(5, 3);

enable() and disable()

The enable() and disable() methods enable and disable the automatic control of the ProfiledPIDSubsystem. When the subsystem is enabled, it will automatically run the motion profile and the control loop and track to the goal. When it is disabled, no control is performed.

Additionally, the enable() method resets the internal ProfiledPIDController, and the disable() method calls the user-defined useOutput() method with both output and setpoint set to 0.

Full ProfiledPIDSubsystem Example

What does a PIDSubsystem look like when used in practice? The following examples are taken from the ArmBot example project (Java, C++):

 5package edu.wpi.first.wpilibj.examples.armbot.subsystems;
 6
 7import edu.wpi.first.math.controller.ArmFeedforward;
 8import edu.wpi.first.math.trajectory.TrapezoidProfile;
 9import edu.wpi.first.wpilibj.Encoder;
10import edu.wpi.first.wpilibj.controller.ProfiledPIDController;
11import edu.wpi.first.wpilibj.examples.armbot.Constants.ArmConstants;
12import edu.wpi.first.wpilibj.motorcontrol.PWMSparkMax;
13import edu.wpi.first.wpilibj2.command.ProfiledPIDSubsystem;
14
15/** A robot arm subsystem that moves with a motion profile. */
16public class ArmSubsystem extends ProfiledPIDSubsystem {
17  private final PWMSparkMax m_motor = new PWMSparkMax(ArmConstants.kMotorPort);
18  private final Encoder m_encoder =
19      new Encoder(ArmConstants.kEncoderPorts[0], ArmConstants.kEncoderPorts[1]);
20  private final ArmFeedforward m_feedforward =
21      new ArmFeedforward(
22          ArmConstants.kSVolts, ArmConstants.kCosVolts,
23          ArmConstants.kVVoltSecondPerRad, ArmConstants.kAVoltSecondSquaredPerRad);
24
25  /** Create a new ArmSubsystem. */
26  public ArmSubsystem() {
27    super(
28        new ProfiledPIDController(
29            ArmConstants.kP,
30            0,
31            0,
32            new TrapezoidProfile.Constraints(
33                ArmConstants.kMaxVelocityRadPerSecond,
34                ArmConstants.kMaxAccelerationRadPerSecSquared)),
35        0);
36    m_encoder.setDistancePerPulse(ArmConstants.kEncoderDistancePerPulse);
37    // Start arm at rest in neutral position
38    setGoal(ArmConstants.kArmOffsetRads);
39  }
40
41  @Override
42  public void useOutput(double output, TrapezoidProfile.State setpoint) {
43    // Calculate the feedforward from the sepoint
44    double feedforward = m_feedforward.calculate(setpoint.position, setpoint.velocity);
45    // Add the feedforward to the PID output to get the motor output
46    m_motor.setVoltage(output + feedforward);
47  }
48
49  @Override
50  public double getMeasurement() {
51    return m_encoder.getDistance() + ArmConstants.kArmOffsetRads;
52  }
53}

Using a ProfiledPIDSubsystem with commands can be very simple:

63    // Move the arm to 2 radians above horizontal when the 'A' button is pressed.
64    new JoystickButton(m_driverController, Button.kA.value)
65        .whenPressed(() -> {
66          m_robotArm.setGoal(2);
67          m_robotArm.enable();
68        }, m_robotArm);

ProfiledPIDCommand

Nota

In C++, the ProfiledPIDCommand class is templated on the unit type used for distance measurements, which may be angular or linear. The passed-in values must have units consistent with the distance units, or a compile-time error will be thrown. For more information on C++ units, see The C++ Units Library.

The ProfiledPIDCommand class (Java, C++) allows users to easily create commands with a built-in ProfiledPIDController. As with ProfiledPIDSubsystem, users can create a ProfiledPIDCommmand by subclassing the ProfiledPIDCommand class. However, as with many of the other command classes in the command-based library, users may want to save code by defining it inline.

Creating a PIDCommand

A ProfiledPIDCommand can be created two ways - by subclassing the ProfiledPIDCommand class, or by defining the command inline. Both methods ultimately extremely similar, and ultimately the choice of which to use comes down to where the user desires that the relevant code be located.

In either case, a ProfiledPIDCommand is created by passing the necessary parameters to its constructor (if defining a subclass, this can be done with a super() call):

32  /**
33   * Creates a new PIDCommand, which controls the given output with a ProfiledPIDController.
34   * Goal velocity is specified.
35   *
36   * @param controller        the controller that controls the output.
37   * @param measurementSource the measurement of the process variable
38   * @param goalSource        the controller's goal
39   * @param useOutput         the controller's output
40   * @param requirements      the subsystems required by this command
41   */
42  public ProfiledPIDCommand(ProfiledPIDController controller, DoubleSupplier measurementSource,
43                            Supplier<State> goalSource, BiConsumer<Double, State> useOutput,
44                            Subsystem... requirements) {
45    requireNonNullParam(controller, "controller", "SynchronousPIDCommand");
46    requireNonNullParam(measurementSource, "measurementSource", "SynchronousPIDCommand");
47    requireNonNullParam(goalSource, "goalSource", "SynchronousPIDCommand");
48    requireNonNullParam(useOutput, "useOutput", "SynchronousPIDCommand");
49
50    m_controller = controller;
51    m_useOutput = useOutput;
52    m_measurement = measurementSource;
53    m_goal = goalSource;
54    m_requirements.addAll(Set.of(requirements));
55  }

controller

The controller parameter is the ProfiledPIDController object that will be used by the command. By passing this in, users can specify the PID gains, the motion profile constraints, and the period for the controller (if the user is using a nonstandard main robot loop period).

When subclassing ProfiledPIDCommand, additional modifications (e.g. enabling continuous input) can be made to the controller in the constructor body by calling getController().

measurementSource

The measurementSource parameter is a function (usually passed as a lambda) that returns the measurement of the process variable. Passing in the measurementSource function in ProfiledPIDCommand is functionally analogous to overriding the getMeasurement() function in ProfiledPIDSubsystem.

When subclassing ProfiledPIDCommand, advanced users may further modify the measurement supplier by modifying the class’s m_measurement field.

goalSource

The goalSource parameter is a function (usually passed as a lambda) that returns the current goal state for the mechanism. If only a constant goal is needed, an overload exists that takes a constant goal rather than a supplier. Additionally, if goal velocities are desired to be zero, overloads exist that take a constant distance rather than a full profile state.

When subclassing ProfiledPIDCommand, advanced users may further modify the setpoint supplier by modifying the class’s m_goal field.

useOutput

The useOutput parameter is a function (usually passed as a lambda) that consumes the output and setpoint state of the control loop. Passing in the useOutput function in ProfiledPIDCommand is functionally analogous to overriding the useOutput() function in ProfiledPIDSubsystem.

When subclassing ProfiledPIDCommand, advanced users may further modify the output consumer by modifying the class’s m_useOutput field.

requirements

Like all inlineable commands, ProfiledPIDCommand allows the user to specify its subsystem requirements as a constructor parameter.

Full ProfiledPIDCommand Example

What does a ProfiledPIDCommand look like when used in practice? The following examples are from the GyroDriveCommands example project (Java, C++):

 5package edu.wpi.first.wpilibj.examples.gyrodrivecommands.commands;
 6
 7import edu.wpi.first.math.trajectory.TrapezoidProfile;
 8import edu.wpi.first.wpilibj.controller.ProfiledPIDController;
 9import edu.wpi.first.wpilibj.examples.gyrodrivecommands.Constants.DriveConstants;
10import edu.wpi.first.wpilibj.examples.gyrodrivecommands.subsystems.DriveSubsystem;
11import edu.wpi.first.wpilibj2.command.ProfiledPIDCommand;
12
13/** A command that will turn the robot to the specified angle using a motion profile. */
14public class TurnToAngleProfiled extends ProfiledPIDCommand {
15  /**
16   * Turns to robot to the specified angle using a motion profile.
17   *
18   * @param targetAngleDegrees The angle to turn to
19   * @param drive The drive subsystem to use
20   */
21  public TurnToAngleProfiled(double targetAngleDegrees, DriveSubsystem drive) {
22    super(
23        new ProfiledPIDController(
24            DriveConstants.kTurnP,
25            DriveConstants.kTurnI,
26            DriveConstants.kTurnD,
27            new TrapezoidProfile.Constraints(
28                DriveConstants.kMaxTurnRateDegPerS,
29                DriveConstants.kMaxTurnAccelerationDegPerSSquared)),
30        // Close loop on heading
31        drive::getHeading,
32        // Set reference to target
33        targetAngleDegrees,
34        // Pipe output to turn robot
35        (output, setpoint) -> drive.arcadeDrive(0, output),
36        // Require the drive
37        drive);
38
39    // Set the controller to be continuous (because it is an angle controller)
40    getController().enableContinuousInput(-180, 180);
41    // Set the controller tolerance - the delta tolerance ensures the robot is stationary at the
42    // setpoint before it is considered as having reached the reference
43    getController()
44        .setTolerance(DriveConstants.kTurnToleranceDeg, DriveConstants.kTurnRateToleranceDegPerS);
45  }
46
47  @Override
48  public boolean isFinished() {
49    // End when the controller is at the reference.
50    return getController().atGoal();
51  }
52}