- FRC Game Manual
- FRC Game Q&A

# Step 2: Entering the Calculated Constants

Note

In C++, it is important that the feedforward constants be entered as the correct unit type. For more information on C++ units, see The C++ Units Library.

Now that we have our system constants, it is time to place them in our code. The recommended place for this is the `Constants` file of the standard command-based project structure.

The relevant parts of the constants file from the RamseteCommand Example Project (Java, C++) can be seen below.

## Feedforward/Feedback Gains

Firstly, we must enter the feedforward and feedback gains which we obtained from the identification tool.

Note

Feedforward and feedback gains do not, in general, transfer across robots. Do not use the gains from this tutorial for your own robot.

JAVA

    39    // These are example values only - DO NOT USE THESE FOR YOUR OWN ROBOT!
    40    // These characterization values MUST be determined either experimentally or theoretically
    41    // for *your* robot's drive.
    42    // The Robot Characterization Toolsuite provides a convenient tool for obtaining these
    43    // values for your robot.
    44    public static final double ksVolts = 0.22;
    45    public static final double kvVoltSecondsPerMeter = 1.98;
    46    public static final double kaVoltSecondsSquaredPerMeter = 0.2;
    47
    48    // Example value only - as above, this must be tuned for your drive!
    49    public static final double kPDriveVel = 8.5;

C++

    47// These are example values only - DO NOT USE THESE FOR YOUR OWN ROBOT!
    48// These characterization values MUST be determined either experimentally or
    49// theoretically for *your* robot's drive. The Robot Characterization
    50// Toolsuite provides a convenient tool for obtaining these values for your
    51// robot.
    52inline constexpr auto ks = 0.22_V;
    53inline constexpr auto kv = 1.98 * 1_V * 1_s / 1_m;
    54inline constexpr auto ka = 0.2 * 1_V * 1_s * 1_s / 1_m;
    55
    56// Example value only - as above, this must be tuned for your drive!
    57inline constexpr double kPDriveVel = 8.5;

## DifferentialDriveKinematics

Additionally, we must create an instance of the `DifferentialDriveKinematics` class, which allows us to use the trackwidth (i.e. horizontal distance between the wheels) of the robot to convert from chassis speeds to wheel speeds. As elsewhere, we keep our units in meters.

JAVA

    29    public static final double kTrackwidthMeters = 0.69;
    30    public static final DifferentialDriveKinematics kDriveKinematics =
    31        new DifferentialDriveKinematics(kTrackwidthMeters);

C++

    38inline constexpr auto kTrackwidth = 0.69_m;
    39extern const frc::DifferentialDriveKinematics kDriveKinematics;

## Max Trajectory Velocity/Acceleration

We must also decide on a nominal max acceleration and max velocity for the robot during path-following. The maximum velocity value should be set somewhat below the nominal free-speed of the robot. Due to the later use of the `DifferentialDriveVoltageConstraint`, the maximum acceleration value is not extremely crucial.

Warning

Max velocity and acceleration, as defined here, are applied only during trajectory generation. They do not limit the `RamseteCommand` itself, which may give values to the `DriveSubsystem` that can cause the robot to greatly exceed these velocities and accelerations.

JAVA

    57    public static final double kMaxSpeedMetersPerSecond = 3;
    58    public static final double kMaxAccelerationMetersPerSecondSquared = 1;

C++

    61inline constexpr auto kMaxSpeed = 3_mps;
    62inline constexpr auto kMaxAcceleration = 1_mps_sq;

## Ramsete Parameters

Finally, we must include a pair of parameters for the RAMSETE controller. The values `b` and `zeta` shown below should work well for most robots, provided distances have been correctly measured in meters.

Larger values of `b` make convergence more aggressive like a proportional term whereas larger values of `zeta` provide more damping in the response. These controller gains only dictate how the controller will output adjusted velocities. It does NOT affect the actual velocity tracking of the robot. This means that these controller gains are generally robot-agnostic.

Note

Gains of `2.0` and `0.7` for `b` and `zeta` have been tested repeatedly to produce desirable results when all units were in meters. As such, a zero-argument constructor for `RamseteController` exists with gains defaulted to these values.

JAVA

    60    // Reasonable baseline values for a RAMSETE follower in units of meters and seconds
    61    public static final double kRamseteB = 2;
    62    public static final double kRamseteZeta = 0.7;

C++

    64// Reasonable baseline values for a RAMSETE follower in units of meters and
    65// seconds
    66inline constexpr auto kRamseteB = 2.0 * 1_rad * 1_rad / (1_m * 1_m);
    67inline constexpr auto kRamseteZeta = 0.7 / 1_rad;