Step 4: Creating and Following a Trajectory

With our drive subsystem written, it is now time to generate a trajectory and write an autonomous command to follow it.

As per the standard command-based project structure, we will do this in the getAutonomousCommand method of the RobotContainer class. The full method from the RamseteCommand Example Project (Java, C++) can be seen below. The rest of the article will break down the different parts of the method in more detail.

 74  /**
 75   * Use this to pass the autonomous command to the main {@link Robot} class.
 76   *
 77   * @return the command to run in autonomous
 78   */
 79  public Command getAutonomousCommand() {
 80    // Create a voltage constraint to ensure we don't accelerate too fast
 81    var autoVoltageConstraint =
 82        new DifferentialDriveVoltageConstraint(
 83            new SimpleMotorFeedforward(
 84                DriveConstants.ksVolts,
 85                DriveConstants.kvVoltSecondsPerMeter,
 86                DriveConstants.kaVoltSecondsSquaredPerMeter),
 87            DriveConstants.kDriveKinematics,
 88            10);
 89
 90    // Create config for trajectory
 91    TrajectoryConfig config =
 92        new TrajectoryConfig(
 93                AutoConstants.kMaxSpeedMetersPerSecond,
 94                AutoConstants.kMaxAccelerationMetersPerSecondSquared)
 95            // Add kinematics to ensure max speed is actually obeyed
 96            .setKinematics(DriveConstants.kDriveKinematics)
 97            // Apply the voltage constraint
 98            .addConstraint(autoVoltageConstraint);
 99
100    // An example trajectory to follow. All units in meters.
101    Trajectory exampleTrajectory =
102        TrajectoryGenerator.generateTrajectory(
103            // Start at the origin facing the +X direction
104            new Pose2d(0, 0, new Rotation2d(0)),
105            // Pass through these two interior waypoints, making an 's' curve path
106            List.of(new Translation2d(1, 1), new Translation2d(2, -1)),
107            // End 3 meters straight ahead of where we started, facing forward
108            new Pose2d(3, 0, new Rotation2d(0)),
109            // Pass config
110            config);
111
112    RamseteCommand ramseteCommand =
113        new RamseteCommand(
114            exampleTrajectory,
115            m_robotDrive::getPose,
116            new RamseteController(AutoConstants.kRamseteB, AutoConstants.kRamseteZeta),
117            new SimpleMotorFeedforward(
118                DriveConstants.ksVolts,
119                DriveConstants.kvVoltSecondsPerMeter,
120                DriveConstants.kaVoltSecondsSquaredPerMeter),
121            DriveConstants.kDriveKinematics,
122            m_robotDrive::getWheelSpeeds,
123            new PIDController(DriveConstants.kPDriveVel, 0, 0),
124            new PIDController(DriveConstants.kPDriveVel, 0, 0),
125            // RamseteCommand passes volts to the callback
126            m_robotDrive::tankDriveVolts,
127            m_robotDrive);
128
129    // Reset odometry to the initial pose of the trajectory, run path following
130    // command, then stop at the end.
131    return Commands.runOnce(() -> m_robotDrive.resetOdometry(exampleTrajectory.getInitialPose()))
132        .andThen(ramseteCommand)
133        .andThen(Commands.runOnce(() -> m_robotDrive.tankDriveVolts(0, 0)));
134  }
135}

45frc2::CommandPtr RobotContainer::GetAutonomousCommand() {
46  // Create a voltage constraint to ensure we don't accelerate too fast
47  frc::DifferentialDriveVoltageConstraint autoVoltageConstraint{
48      frc::SimpleMotorFeedforward<units::meters>{
49          DriveConstants::ks, DriveConstants::kv, DriveConstants::ka},
50      DriveConstants::kDriveKinematics, 10_V};
51
52  // Set up config for trajectory
53  frc::TrajectoryConfig config{AutoConstants::kMaxSpeed,
54                               AutoConstants::kMaxAcceleration};
55  // Add kinematics to ensure max speed is actually obeyed
56  config.SetKinematics(DriveConstants::kDriveKinematics);
57  // Apply the voltage constraint
58  config.AddConstraint(autoVoltageConstraint);
59
60  // An example trajectory to follow.  All units in meters.
61  auto exampleTrajectory = frc::TrajectoryGenerator::GenerateTrajectory(
62      // Start at the origin facing the +X direction
63      frc::Pose2d{0_m, 0_m, 0_deg},
64      // Pass through these two interior waypoints, making an 's' curve path
65      {frc::Translation2d{1_m, 1_m}, frc::Translation2d{2_m, -1_m}},
66      // End 3 meters straight ahead of where we started, facing forward
67      frc::Pose2d{3_m, 0_m, 0_deg},
68      // Pass the config
69      config);
70
71  frc2::CommandPtr ramseteCommand{frc2::RamseteCommand(
72      exampleTrajectory, [this] { return m_drive.GetPose(); },
73      frc::RamseteController{AutoConstants::kRamseteB,
74                             AutoConstants::kRamseteZeta},
75      frc::SimpleMotorFeedforward<units::meters>{
76          DriveConstants::ks, DriveConstants::kv, DriveConstants::ka},
77      DriveConstants::kDriveKinematics,
78      [this] { return m_drive.GetWheelSpeeds(); },
79      frc::PIDController{DriveConstants::kPDriveVel, 0, 0},
80      frc::PIDController{DriveConstants::kPDriveVel, 0, 0},
81      [this](auto left, auto right) { m_drive.TankDriveVolts(left, right); },
82      {&m_drive})};
83
84  // Reset odometry to the initial pose of the trajectory, run path following
85  // command, then stop at the end.
86  return frc2::cmd::RunOnce(
87             [this, initialPose = exampleTrajectory.InitialPose()] {
88               m_drive.ResetOdometry(initialPose);
89             },
90             {})
91      .AndThen(std::move(ramseteCommand))
92      .AndThen(
93          frc2::cmd::RunOnce([this] { m_drive.TankDriveVolts(0_V, 0_V); }, {}));
94}

Configuring the Trajectory Constraints

First, we must set some configuration parameters for the trajectory which will ensure that the generated trajectory is followable.

Creating a Voltage Constraint

The first piece of configuration we will need is a voltage constraint. This will ensure that the generated trajectory never commands the robot to go faster than it is capable of achieving with the given voltage supply:

80    // Create a voltage constraint to ensure we don't accelerate too fast
81    var autoVoltageConstraint =
82        new DifferentialDriveVoltageConstraint(
83            new SimpleMotorFeedforward(
84                DriveConstants.ksVolts,
85                DriveConstants.kvVoltSecondsPerMeter,
86                DriveConstants.kaVoltSecondsSquaredPerMeter),
87            DriveConstants.kDriveKinematics,
88            10);

46  // Create a voltage constraint to ensure we don't accelerate too fast
47  frc::DifferentialDriveVoltageConstraint autoVoltageConstraint{
48      frc::SimpleMotorFeedforward<units::meters>{
49          DriveConstants::ks, DriveConstants::kv, DriveConstants::ka},
50      DriveConstants::kDriveKinematics, 10_V};

Notice that we set the maximum voltage to 10V, rather than the nominal battery voltage of 12V. This gives us some “headroom” to deal with “voltage sag” during operation.

Creating the Configuration

Now that we have our voltage constraint, we can create our TrajectoryConfig instance, which wraps together all of our path constraints:

90    // Create config for trajectory
91    TrajectoryConfig config =
92        new TrajectoryConfig(
93                AutoConstants.kMaxSpeedMetersPerSecond,
94                AutoConstants.kMaxAccelerationMetersPerSecondSquared)
95            // Add kinematics to ensure max speed is actually obeyed
96            .setKinematics(DriveConstants.kDriveKinematics)
97            // Apply the voltage constraint
98            .addConstraint(autoVoltageConstraint);

52  // Set up config for trajectory
53  frc::TrajectoryConfig config{AutoConstants::kMaxSpeed,
54                               AutoConstants::kMaxAcceleration};
55  // Add kinematics to ensure max speed is actually obeyed
56  config.SetKinematics(DriveConstants::kDriveKinematics);
57  // Apply the voltage constraint
58  config.AddConstraint(autoVoltageConstraint);

Generating the Trajectory

With our trajectory configuration in hand, we are now ready to generate our trajectory. For this example, we will be generating a “clamped cubic” trajectory - this means we will specify full robot poses at the endpoints, and positions only for interior waypoints (also known as “knot points”). As elsewhere, all distances are in meters.

100    // An example trajectory to follow. All units in meters.
101    Trajectory exampleTrajectory =
102        TrajectoryGenerator.generateTrajectory(
103            // Start at the origin facing the +X direction
104            new Pose2d(0, 0, new Rotation2d(0)),
105            // Pass through these two interior waypoints, making an 's' curve path
106            List.of(new Translation2d(1, 1), new Translation2d(2, -1)),
107            // End 3 meters straight ahead of where we started, facing forward
108            new Pose2d(3, 0, new Rotation2d(0)),
109            // Pass config
110            config);

60  // An example trajectory to follow.  All units in meters.
61  auto exampleTrajectory = frc::TrajectoryGenerator::GenerateTrajectory(
62      // Start at the origin facing the +X direction
63      frc::Pose2d{0_m, 0_m, 0_deg},
64      // Pass through these two interior waypoints, making an 's' curve path
65      {frc::Translation2d{1_m, 1_m}, frc::Translation2d{2_m, -1_m}},
66      // End 3 meters straight ahead of where we started, facing forward
67      frc::Pose2d{3_m, 0_m, 0_deg},
68      // Pass the config
69      config);

Note

Instead of generating the trajectory on the roboRIO as outlined above, one can also import a PathWeaver JSON.

Creating the RamseteCommand

We will first reset our robot’s pose to the starting pose of the trajectory. This ensures that the robot’s location on the coordinate system and the trajectory’s starting position are the same.

129    // Reset odometry to the initial pose of the trajectory, run path following
130    // command, then stop at the end.
131    return Commands.runOnce(() -> m_robotDrive.resetOdometry(exampleTrajectory.getInitialPose()))

84  // Reset odometry to the initial pose of the trajectory, run path following
85  // command, then stop at the end.
86  return frc2::cmd::RunOnce(

It is very important that the initial robot pose match the first pose in the trajectory. For the purposes of our example, the robot will be reliably starting at a position of (0,0) with a heading of 0. In actual use, however, it is probably not desirable to base your coordinate system on the robot position, and so the starting position for both the robot and the trajectory should be set to some other value. If you wish to use a trajectory that has been defined in robot-centric coordinates in such a situation, you can transform it to be relative to the robot’s current pose using the transformBy method (Java, C++). For more information about transforming trajectories, see Transforming Trajectories.

Now that we have a trajectory, we can create a command that, when executed, will follow that trajectory. To do this, we use the RamseteCommand class (Java, C++)

112    RamseteCommand ramseteCommand =
113        new RamseteCommand(
114            exampleTrajectory,
115            m_robotDrive::getPose,
116            new RamseteController(AutoConstants.kRamseteB, AutoConstants.kRamseteZeta),
117            new SimpleMotorFeedforward(
118                DriveConstants.ksVolts,
119                DriveConstants.kvVoltSecondsPerMeter,
120                DriveConstants.kaVoltSecondsSquaredPerMeter),
121            DriveConstants.kDriveKinematics,
122            m_robotDrive::getWheelSpeeds,
123            new PIDController(DriveConstants.kPDriveVel, 0, 0),
124            new PIDController(DriveConstants.kPDriveVel, 0, 0),
125            // RamseteCommand passes volts to the callback
126            m_robotDrive::tankDriveVolts,
127            m_robotDrive);

71  frc2::CommandPtr ramseteCommand{frc2::RamseteCommand(
72      exampleTrajectory, [this] { return m_drive.GetPose(); },
73      frc::RamseteController{AutoConstants::kRamseteB,
74                             AutoConstants::kRamseteZeta},
75      frc::SimpleMotorFeedforward<units::meters>{
76          DriveConstants::ks, DriveConstants::kv, DriveConstants::ka},
77      DriveConstants::kDriveKinematics,
78      [this] { return m_drive.GetWheelSpeeds(); },
79      frc::PIDController{DriveConstants::kPDriveVel, 0, 0},
80      frc::PIDController{DriveConstants::kPDriveVel, 0, 0},
81      [this](auto left, auto right) { m_drive.TankDriveVolts(left, right); },
82      {&m_drive})};

This declaration is fairly substantial, so we’ll go through it argument-by-argument:

  1. The trajectory: This is the trajectory to be followed; accordingly, we pass the command the trajectory we just constructed in our earlier steps.

  2. The pose supplier: This is a method reference (or lambda) to the drive subsystem method that returns the pose. The RAMSETE controller needs the current pose measurement to determine the required wheel outputs.

  3. The RAMSETE controller: This is the RamseteController object (Java, C++) that will perform the path-following computation that translates the current measured pose and trajectory state into a chassis speed setpoint.

  4. The drive feedforward: This is a SimpleMotorFeedforward object (Java, C++) that will automatically perform the correct feedforward calculation with the feedforward gains (kS, kV, and kA) that we obtained from the drive identification tool.

  5. The drive kinematics: This is the DifferentialDriveKinematics object (Java, C++) that we constructed earlier in our constants file, and will be used to convert chassis speeds to wheel speeds.

  6. The wheel speed supplier: This is a method reference (or lambda) to the drive subsystem method that returns the wheel speeds

  7. The left-side PIDController: This is the PIDController object (Java, C++) that will track the left-side wheel speed setpoint, using the P gain that we obtained from the drive identification tool.

  8. The right-side PIDController: This is the PIDController object (Java, C++) that will track the right-side wheel speed setpoint, using the P gain that we obtained from the drive identification tool.

  9. The output consumer: This is a method reference (or lambda) to the drive subsystem method that passes the voltage outputs to the drive motors.

  10. The robot drive: This is the drive subsystem itself, included to ensure the command does not operate on the drive at the same time as any other command that uses the drive.

Finally, note that we append a final “stop” command in sequence after the path-following command, to ensure that the robot stops moving at the end of the trajectory.

Video

If all has gone well, your robot’s autonomous routine should look something like this: