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.

הערה

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 requires one mandatory argument and one optional argument. The mandatory argument is the angle reported by your gyroscope (as a Rotation2d). 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.

הערה

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. By default, WPILib gyros exhibit the opposite behavior, so you should negate the gyro angle.

חשוב

The encoder positions must be reset to zero before constructing the DifferentialDriveOdometry class.

// 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(
  getGyroHeading(), new Pose2d(5.0, 13.5, new 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.

הערה

The encoder distances in Java must be in meters. In C++, the units library can be used to represent the distance using any linear distance unit. If the robot is moving forward in a straight line, both distances (left and right) must be positive.

@Override
public void periodic() {
// Get my gyro angle. We are negating the value because gyros return positive
// values as the robot turns clockwise. This is not standard convention that is
// used by the WPILib classes.
var gyroAngle = Rotation2d.fromDegrees(-m_gyro.getAngle());

// 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 resetPose method. This method accepts two arguments – the new field-relative pose and the current gyro angle.

חשוב

If at any time, you decide to reset your gyroscope, the resetPose method MUST be called with the new gyro angle. Furthermore, the encoders must also be reset to zero when resetting the pose.

הערה

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

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