Introduction to Kinematics and The Chassis Speeds Class¶
What is kinematics?¶
The brand new kinematics suite contains classes for differential drive, swerve drive, and mecanum drive kinematics and odometry. The kinematics classes help convert between a universal
ChassisSpeeds object, containing linear and angular velocities for a robot to usable speeds for each individual type of drivetrain i.e. left and right wheel speeds for a differential drive, four wheel speeds for a mecanum drive, or individual module states (speed and angle) for a swerve drive.
What is odometry?¶
Odometry involves using sensors on the robot to create an estimate of the position of the robot on the field. In FRC, these sensors are typically several encoders (the exact number depends on the drive type) and a gyroscope to measure robot angle. The odometry classes utilize the kinematics classes along with periodic user inputs about speeds (and angles in the case of swerve) to create an estimate of the robot’s location on the field.
The Chassis Speeds Class¶
ChassisSpeeds object is essential to the new WPILib kinematics and odometry suite. The
ChassisSpeeds object represents the speeds of a robot chassis. This struct has three components:
vx: The velocity of the robot in the x (forward) direction.
vy: The velocity of the robot in the y (sideways) direction. (Positive values mean the robot is moving to the left).
omega: The angular velocity of the robot in radians per second.
A non-holonomic drivetrain (i.e. a drivetrain that cannot move sideways, ex: a differential drive) will have a
vy component of zero because of its inability to move sideways.
Constructing a ChassisSpeeds object¶
The constructor for the
ChassisSpeeds object is very straightforward, accepting three arguments for
omega. In Java,
vy must be in meters per second. In C++, the units library may be used to provide a linear velocity using any linear velocity unit.
Creating a ChassisSpeeds Object from Field-Relative Speeds¶
ChassisSpeeds object can also be created from a set of field-relative speeds when the robot angle is given. This converts a set of desired velocities relative to the field (for example, toward the opposite alliance station and toward the right field boundary) to a
ChassisSpeeds object which represents speeds that are relative to the robot frame. This is useful for implementing field-oriented controls for a swerve or mecanum drive robot.
ChassisSpeeds.fromFieldRelativeSpeeds (Java) /
ChassisSpeeds::FromFieldRelativeSpeeds (C++) method can be used to generate the
ChassisSpeeds object from field-relative speeds. This method accepts the
vx (relative to the field),
vy (relative to the field),
omega, and the robot angle.
The angular velocity is not explicitly stated to be “relative to the field” because the angular velocity is the same as measured from a field perspective or a robot perspective.