Differential Drive Odometry

A user can use the differential drive kinematics classes in order to perform odometry. WPILib contains a DifferentialDriveOdometry class that can be used to track the position of a differential drive robot on the field.

Note

Because this method only uses encoders and a gyro, the estimate of the robot’s position on the field will drift over time, especially as your robot comes into contact with other robots during gameplay. However, odometry is usually very accurate during the autonomous period.

Creating the Odometry Object

The DifferentialDriveOdometry class constructor requires three mandatory arguments and one optional argument. The mandatory arguments are:

The angle reported by your gyroscope (as a Rotation2d)
The initial left and right encoder readings. In Java / Python, these are a number that represents the distance traveled by each side in meters. In C++, the units library must be used to represent your wheel positions.

The optional argument is the starting pose of your robot on the field (as a Pose2d). By default, the robot will start at x = 0, y = 0, theta = 0.

Note

0 degrees / radians represents the robot angle when the robot is facing directly toward your opponent’s alliance station. As your robot turns to the left, your gyroscope angle should increase. The Gyro interface supplies getRotation2d/GetRotation2d that you can use for this purpose. See Coordinate System for more information about the coordinate system.

JAVA

// Creating my odometry object. Here,
// our starting pose is 5 meters along the long end of the field and in the
// center of the field along the short end, facing forward.
DifferentialDriveOdometry m_odometry = new DifferentialDriveOdometry(
  m_gyro.getRotation2d(),
  m_leftEncoder.getDistance(), m_rightEncoder.getDistance(),
  new Pose2d(5.0, 13.5, new Rotation2d()));

C++

// Creating my odometry object. Here,
// our starting pose is 5 meters along the long end of the field and in the
// center of the field along the short end, facing forward.
frc::DifferentialDriveOdometry m_odometry{
  m_gyro.GetRotation2d(),
  units::meter_t{m_leftEncoder.GetDistance()},
  units::meter_t{m_rightEncoder.GetDistance()},
  frc::Pose2d{5_m, 13.5_m, 0_rad}};

PYTHON

from wpimath.kinematics import DifferentialDriveOdometry
from wpimath.geometry import Pose2d
from wpimath.geometry import Rotation2d

# Creating my odometry object. Here,
# our starting pose is 5 meters along the long end of the field and in the
# center of the field along the short end, facing forward.
m_odometry = DifferentialDriveOdometry(
  m_gyro.getRotation2d(),
  m_leftEncoder.getDistance(), m_rightEncoder.getDistance(),
  Pose2d(5.0, 13.5, Rotation2d()))

Updating the Robot Pose

The update method can be used to update the robot’s position on the field. This method must be called periodically, preferably in the periodic() method of a Subsystem. The update method returns the new updated pose of the robot. This method takes in the gyro angle of the robot, along with the left encoder distance and right encoder distance.

Note

If the robot is moving forward in a straight line, both distances (left and right) must be increasing positively – the rate of change must be positive.

JAVA

@Override
public void periodic() {
  // Get the rotation of the robot from the gyro.
  var gyroAngle = m_gyro.getRotation2d();

  // Update the pose
  m_pose = m_odometry.update(gyroAngle,
    m_leftEncoder.getDistance(),
    m_rightEncoder.getDistance());
}

C++

void Periodic() override {
  // Get the rotation of the robot from the gyro.
  frc::Rotation2d gyroAngle = m_gyro.GetRotation2d();

  // Update the pose
  m_pose = m_odometry.Update(gyroAngle,
    units::meter_t{m_leftEncoder.GetDistance()},
    units::meter_t{m_rightEncoder.GetDistance()});
}

PYTHON

def periodic(self):
  # Get the rotation of the robot from the gyro.
  gyroAngle = m_gyro.getRotation2d()

  # Update the pose
  m_pose = m_odometry.update(gyroAngle,
    m_leftEncoder.getDistance(),
    m_rightEncoder.getDistance())

Resetting the Robot Pose

The robot pose can be reset via the resetPosition method. This method accepts four arguments: the current gyro angle, the left and right wheel positions, and the new field-relative pose.

Important

If at any time, you decide to reset your gyroscope or encoders, the resetPosition method MUST be called with the new gyro angle and wheel distances.

Note

A full example of a differential drive robot with odometry is available here: C++ / Java / Python

In addition, the GetPose (C++) / getPoseMeters (Java / Python) methods can be used to retrieve the current robot pose without an update.