Step 3: Creating a Drive Subsystem

Now that our drive is characterized, it is time to start writing our robot code proper. As mentioned before, we will use the command-based framework for our robot code. Accordingly, our first step is to write a suitable drive subsystem class.

The full drive class from the RamseteCommand Example Project (Java, C++) can be seen below. The rest of the article will describe the steps involved in writing this class.

JavaC++ (Header)C++ (Source)

package edu.wpi.first.wpilibj.examples.ramsetecommand.subsystems;

import edu.wpi.first.math.geometry.Pose2d;
import edu.wpi.first.math.kinematics.DifferentialDriveOdometry;
import edu.wpi.first.math.kinematics.DifferentialDriveWheelSpeeds;
import edu.wpi.first.wpilibj.ADXRS450_Gyro;
import edu.wpi.first.wpilibj.Encoder;
import edu.wpi.first.wpilibj.drive.DifferentialDrive;
import edu.wpi.first.wpilibj.examples.ramsetecommand.Constants.DriveConstants;
import edu.wpi.first.wpilibj.interfaces.Gyro;
import edu.wpi.first.wpilibj.motorcontrol.MotorControllerGroup;
import edu.wpi.first.wpilibj.motorcontrol.PWMSparkMax;
import edu.wpi.first.wpilibj2.command.SubsystemBase;

public class DriveSubsystem extends SubsystemBase {
  // The motors on the left side of the drive.
  private final MotorControllerGroup m_leftMotors =
      new MotorControllerGroup(
          new PWMSparkMax(DriveConstants.kLeftMotor1Port),
          new PWMSparkMax(DriveConstants.kLeftMotor2Port));

  // The motors on the right side of the drive.
  private final MotorControllerGroup m_rightMotors =
      new MotorControllerGroup(
          new PWMSparkMax(DriveConstants.kRightMotor1Port),
          new PWMSparkMax(DriveConstants.kRightMotor2Port));

  // The robot's drive
  private final DifferentialDrive m_drive = new DifferentialDrive(m_leftMotors, m_rightMotors);

  // The left-side drive encoder
  private final Encoder m_leftEncoder =
      new Encoder(
          DriveConstants.kLeftEncoderPorts[0],
          DriveConstants.kLeftEncoderPorts[1],
          DriveConstants.kLeftEncoderReversed);

  // The right-side drive encoder
  private final Encoder m_rightEncoder =
      new Encoder(
          DriveConstants.kRightEncoderPorts[0],
          DriveConstants.kRightEncoderPorts[1],
          DriveConstants.kRightEncoderReversed);

  // The gyro sensor
  private final Gyro m_gyro = new ADXRS450_Gyro();

  // Odometry class for tracking robot pose
  private final DifferentialDriveOdometry m_odometry;

  /** Creates a new DriveSubsystem. */
  public DriveSubsystem() {
    // We need to invert one side of the drivetrain so that positive voltages
    // result in both sides moving forward. Depending on how your robot's
    // gearbox is constructed, you might have to invert the left side instead.
    m_rightMotors.setInverted(true);

    // Sets the distance per pulse for the encoders
    m_leftEncoder.setDistancePerPulse(DriveConstants.kEncoderDistancePerPulse);
    m_rightEncoder.setDistancePerPulse(DriveConstants.kEncoderDistancePerPulse);

    resetEncoders();
    m_odometry = new DifferentialDriveOdometry(m_gyro.getRotation2d());
  }

  @Override
  public void periodic() {
    // Update the odometry in the periodic block
    m_odometry.update(
        m_gyro.getRotation2d(), m_leftEncoder.getDistance(), m_rightEncoder.getDistance());
  }

  /**
   * Returns the currently-estimated pose of the robot.
   *
   * @return The pose.
   */
  public Pose2d getPose() {
    return m_odometry.getPoseMeters();
  }

  /**
   * Returns the current wheel speeds of the robot.
   *
   * @return The current wheel speeds.
   */
  public DifferentialDriveWheelSpeeds getWheelSpeeds() {
    return new DifferentialDriveWheelSpeeds(m_leftEncoder.getRate(), m_rightEncoder.getRate());
  }

  /**
   * Resets the odometry to the specified pose.
   *
   * @param pose The pose to which to set the odometry.
   */
  public void resetOdometry(Pose2d pose) {
    resetEncoders();
    m_odometry.resetPosition(pose, m_gyro.getRotation2d());
  }

  /**
   * Drives the robot using arcade controls.
   *
   * @param fwd the commanded forward movement
   * @param rot the commanded rotation
   */
  public void arcadeDrive(double fwd, double rot) {
    m_drive.arcadeDrive(fwd, rot);
  }

  /**
   * Controls the left and right sides of the drive directly with voltages.
   *
   * @param leftVolts the commanded left output
   * @param rightVolts the commanded right output
   */
  public void tankDriveVolts(double leftVolts, double rightVolts) {
    m_leftMotors.setVoltage(leftVolts);
    m_rightMotors.setVoltage(rightVolts);
    m_drive.feed();
  }

  /** Resets the drive encoders to currently read a position of 0. */
  public void resetEncoders() {
    m_leftEncoder.reset();
    m_rightEncoder.reset();
  }

  /**
   * Gets the average distance of the two encoders.
   *
   * @return the average of the two encoder readings
   */
  public double getAverageEncoderDistance() {
    return (m_leftEncoder.getDistance() + m_rightEncoder.getDistance()) / 2.0;
  }

  /**
   * Gets the left drive encoder.
   *
   * @return the left drive encoder
   */
  public Encoder getLeftEncoder() {
    return m_leftEncoder;
  }

  /**
   * Gets the right drive encoder.
   *
   * @return the right drive encoder
   */
  public Encoder getRightEncoder() {
    return m_rightEncoder;
  }

  /**
   * Sets the max output of the drive. Useful for scaling the drive to drive more slowly.
   *
   * @param maxOutput the maximum output to which the drive will be constrained
   */
  public void setMaxOutput(double maxOutput) {
    m_drive.setMaxOutput(maxOutput);
  }

  /** Zeroes the heading of the robot. */
  public void zeroHeading() {
    m_gyro.reset();
  }

  /**
   * Returns the heading of the robot.
   *
   * @return the robot's heading in degrees, from -180 to 180
   */
  public double getHeading() {
    return m_gyro.getRotation2d().getDegrees();
  }

  /**
   * Returns the turn rate of the robot.
   *
   * @return The turn rate of the robot, in degrees per second
   */
  public double getTurnRate() {
    return -m_gyro.getRate();
  }
}

Configuring the Drive Encoders

The drive encoders measure the rotation of the wheels on each side of the drive. To properly configure the encoders, we need to specify two things: the ports the encoders are plugged into, and the distance per encoder pulse. Then, we need to write methods allowing access to the encoder values from code that uses the subsystem.

Encoder Ports

The encoder ports are specified in the encoder’s constructor, like so:

JavaC++ (Source)

  // The left-side drive encoder
  private final Encoder m_leftEncoder =
      new Encoder(
          DriveConstants.kLeftEncoderPorts[0],
          DriveConstants.kLeftEncoderPorts[1],
          DriveConstants.kLeftEncoderReversed);

  // The right-side drive encoder
  private final Encoder m_rightEncoder =
      new Encoder(
          DriveConstants.kRightEncoderPorts[0],
          DriveConstants.kRightEncoderPorts[1],
          DriveConstants.kRightEncoderReversed);

Encoder Distance per Pulse

The distance per pulse is specified by calling the encoder’s setDistancePerPulse method. Note that for the WPILib Encoder class, “pulse” refers to a full encoder cycle (i.e. four edges), and thus will be 1/4 the value that was specified in the SysId config. Remember, as well, that the distance should be measured in meters!

JavaC++ (Source)

    m_leftEncoder.setDistancePerPulse(DriveConstants.kEncoderDistancePerPulse);
    m_rightEncoder.setDistancePerPulse(DriveConstants.kEncoderDistancePerPulse);

Encoder Accessor Method

To access the values measured by the encoders, we include the following method:

Important

The returned velocities must be in meters! Because we configured the distance per pulse on the encoders above, calling getRate() will automatically apply the conversion factor from encoder units to meters. If you are not using WPILib’s Encoder class, you must perform this conversion either through the respective vendor’s API or by manually multiplying by a conversion factor.

JavaC++ (Source)

  /**
   * Returns the current wheel speeds of the robot.
   *
   * @return The current wheel speeds.
   */
  public DifferentialDriveWheelSpeeds getWheelSpeeds() {
    return new DifferentialDriveWheelSpeeds(m_leftEncoder.getRate(), m_rightEncoder.getRate());
  }

We wrap the measured encoder values in a DifferentialDriveWheelSpeeds object for easier integration with the RamseteCommand class later on.

Configuring the Gyroscope

The gyroscope measures the rate of change of the robot’s heading (which can then be integrated to provide a measurement of the robot’s heading relative to when it first turned on). In our example, we use the Analog Devices ADXRS450 FRC Gyro Board, which has been included in the kit of parts for several years:

JavaC++ (Header)

  private final Gyro m_gyro = new ADXRS450_Gyro();

Gyroscope Accessor Method

To access the current heading measured by the gyroscope, we include the following method:

JavaC++ (Source)

  /**
   * Returns the heading of the robot.
   *
   * @return the robot's heading in degrees, from -180 to 180
   */
  public double getHeading() {
    return m_gyro.getRotation2d().getDegrees();
  }

Configuring the Odometry

Now that we have our encoders and gyroscope configured, it is time to set up our drive subsystem to automatically compute its position from the encoder and gyroscope readings.

First, we create a member instance of the DifferentialDriveOdometry class:

JavaC++ (Header)

  // Odometry class for tracking robot pose
  private final DifferentialDriveOdometry m_odometry;

Updating the Odometry

The odometry class must be regularly updated to incorporate new readings from the encoder and gyroscope. We accomplish this inside the subsystem’s periodic method, which is automatically called once per main loop iteration:

JavaC++ (Source)

  @Override
  public void periodic() {
    // Update the odometry in the periodic block
    m_odometry.update(
        m_gyro.getRotation2d(), m_leftEncoder.getDistance(), m_rightEncoder.getDistance());
  }

Odometry Accessor Method

To access the robot’s current computed pose, we include the following method:

JavaC++ (Source)

  /**
   * Returns the currently-estimated pose of the robot.
   *
   * @return The pose.
   */
  public Pose2d getPose() {
    return m_odometry.getPoseMeters();
  }

Voltage-Based Drive Method

Finally, we must include one additional method - a method that allows us to set the voltage to each side of the drive using the setVoltage() method of the MotorController interface. The default WPILib drive class does not include this functionality, so we must write it ourselves:

JavaC++ (Source)

  /**
   * Controls the left and right sides of the drive directly with voltages.
   *
   * @param leftVolts the commanded left output
   * @param rightVolts the commanded right output
   */
  public void tankDriveVolts(double leftVolts, double rightVolts) {
    m_leftMotors.setVoltage(leftVolts);
    m_rightMotors.setVoltage(rightVolts);
    m_drive.feed();
  }

It is very important to use the setVoltage() method rather than the ordinary set() method, as this will automatically compensate for battery “voltage sag” during operation. Since our feedforward voltages are physically-meaningful (as they are based on measured identification data), this is essential to ensuring their accuracy.