Class SwerveDriveKinematics

  • public class SwerveDriveKinematics
    extends Object
    Helper class that converts a chassis velocity (dx, dy, and dtheta components) into individual module states (speed and angle).

    The inverse kinematics (converting from a desired chassis velocity to individual module states) uses the relative locations of the modules with respect to the center of rotation. The center of rotation for inverse kinematics is also variable. This means that you can set your set your center of rotation in a corner of the robot to perform special evasion manuevers.

    Forward kinematics (converting an array of module states into the overall chassis motion) is performs the exact opposite of what inverse kinematics does. Since this is an overdetermined system (more equations than variables), we use a least-squares approximation.

    The inverse kinematics: [moduleStates] = [moduleLocations] * [chassisSpeeds] We take the Moore-Penrose pseudoinverse of [moduleLocations] and then multiply by [moduleStates] to get our chassis speeds.

    Forward kinematics is also used for odometry -- determining the position of the robot on the field using encoders and a gyro.

    • Constructor Detail

      • SwerveDriveKinematics

        public SwerveDriveKinematics​(Translation2d... wheelsMeters)
        Constructs a swerve drive kinematics object. This takes in a variable number of wheel locations as Translation2ds. The order in which you pass in the wheel locations is the same order that you will recieve the module states when performing inverse kinematics. It is also expected that you pass in the module states in the same order when calling the forward kinematics methods.
        wheelsMeters - The locations of the wheels relative to the physical center of the robot.
    • Method Detail

      • toSwerveModuleStates

        public SwerveModuleState[] toSwerveModuleStates​(ChassisSpeeds chassisSpeeds,
                                                        Translation2d centerOfRotationMeters)
        Performs inverse kinematics to return the module states from a desired chassis velocity. This method is often used to convert joystick values into module speeds and angles.

        This function also supports variable centers of rotation. During normal operations, the center of rotation is usually the same as the physical center of the robot; therefore, the argument is defaulted to that use case. However, if you wish to change the center of rotation for evasive manuevers, vision alignment, or for any other use case, you can do so.

        chassisSpeeds - The desired chassis speed.
        centerOfRotationMeters - The center of rotation. For example, if you set the center of rotation at one corner of the robot and provide a chassis speed that only has a dtheta component, the robot will rotate around that corner.
        An array containing the module states. Use caution because these module states are not normalized. Sometimes, a user input may cause one of the module speeds to go above the attainable max velocity. Use the normalizeWheelSpeeds function to rectify this issue.
      • toChassisSpeeds

        public ChassisSpeeds toChassisSpeeds​(SwerveModuleState... wheelStates)
        Performs forward kinematics to return the resulting chassis state from the given module states. This method is often used for odometry -- determining the robot's position on the field using data from the real-world speed and angle of each module on the robot.
        wheelStates - The state of the modules (as a SwerveModuleState type) as measured from respective encoders and gyros. The order of the swerve module states should be same as passed into the constructor of this class.
        The resulting chassis speed.
      • normalizeWheelSpeeds

        public static void normalizeWheelSpeeds​(SwerveModuleState[] moduleStates,
                                                double attainableMaxSpeedMetersPerSecond)
        Normalizes the wheel speeds using some max attainable speed. Sometimes, after inverse kinematics, the requested speed from a/several modules may be above the max attainable speed for the driving motor on that module. To fix this issue, one can "normalize" all the wheel speeds to make sure that all requested module speeds are below the absolute threshold, while maintaining the ratio of speeds between modules.
        moduleStates - Reference to array of module states. The array will be mutated with the normalized speeds!
        attainableMaxSpeedMetersPerSecond - The absolute max speed that a module can reach.