Step 2: Entering the Calculated Constants

Nota

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.

Nota

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

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;

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.

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

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.

Aviso

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.

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

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 shown below should work well for most robots, provided distances have been correctly measured in meters - for more information on tuning these values (if it is required), see Constructing the Ramsete Controller Object.

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;

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;