# 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.

  82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143  /** * Use this to pass the autonomous command to the main {@link Robot} class. * * @return the command to run in autonomous */ public Command getAutonomousCommand() { // Create a voltage constraint to ensure we don't accelerate too fast var autoVoltageConstraint = new DifferentialDriveVoltageConstraint( new SimpleMotorFeedforward(DriveConstants.ksVolts, DriveConstants.kvVoltSecondsPerMeter, DriveConstants.kaVoltSecondsSquaredPerMeter), DriveConstants.kDriveKinematics, 10); // Create config for trajectory TrajectoryConfig config = new TrajectoryConfig(AutoConstants.kMaxSpeedMetersPerSecond, AutoConstants.kMaxAccelerationMetersPerSecondSquared) // Add kinematics to ensure max speed is actually obeyed .setKinematics(DriveConstants.kDriveKinematics) // Apply the voltage constraint .addConstraint(autoVoltageConstraint); // An example trajectory to follow. All units in meters. Trajectory exampleTrajectory = TrajectoryGenerator.generateTrajectory( // Start at the origin facing the +X direction new Pose2d(0, 0, new Rotation2d(0)), // Pass through these two interior waypoints, making an 's' curve path List.of( new Translation2d(1, 1), new Translation2d(2, -1) ), // End 3 meters straight ahead of where we started, facing forward new Pose2d(3, 0, new Rotation2d(0)), // Pass config config ); RamseteCommand ramseteCommand = new RamseteCommand( exampleTrajectory, m_robotDrive::getPose, new RamseteController(AutoConstants.kRamseteB, AutoConstants.kRamseteZeta), new SimpleMotorFeedforward(DriveConstants.ksVolts, DriveConstants.kvVoltSecondsPerMeter, DriveConstants.kaVoltSecondsSquaredPerMeter), DriveConstants.kDriveKinematics, m_robotDrive::getWheelSpeeds, new PIDController(DriveConstants.kPDriveVel, 0, 0), new PIDController(DriveConstants.kPDriveVel, 0, 0), // RamseteCommand passes volts to the callback m_robotDrive::tankDriveVolts, m_robotDrive ); // Reset odometry to the starting pose of the trajectory. m_robotDrive.resetOdometry(exampleTrajectory.getInitialPose()); // Run path following command, then stop at the end. return ramseteCommand.andThen(() -> m_robotDrive.tankDriveVolts(0, 0)); } 

## 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:

 89 90 91 92 93 94 95 96  // Create a voltage constraint to ensure we don't accelerate too fast var autoVoltageConstraint = new DifferentialDriveVoltageConstraint( new SimpleMotorFeedforward(DriveConstants.ksVolts, DriveConstants.kvVoltSecondsPerMeter, DriveConstants.kaVoltSecondsSquaredPerMeter), DriveConstants.kDriveKinematics, 10); 

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:

  98 99 100 101 102 103 104 105  // Create config for trajectory TrajectoryConfig config = new TrajectoryConfig(AutoConstants.kMaxSpeedMetersPerSecond, AutoConstants.kMaxAccelerationMetersPerSecondSquared) // Add kinematics to ensure max speed is actually obeyed .setKinematics(DriveConstants.kDriveKinematics) // Apply the voltage constraint .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.

 108 109 110 111 112 113 114 115 116 117 118 119 120  Trajectory exampleTrajectory = TrajectoryGenerator.generateTrajectory( // Start at the origin facing the +X direction new Pose2d(0, 0, new Rotation2d(0)), // Pass through these two interior waypoints, making an 's' curve path List.of( new Translation2d(1, 1), new Translation2d(2, -1) ), // End 3 meters straight ahead of where we started, facing forward new Pose2d(3, 0, new Rotation2d(0)), // Pass config 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.

 137 138  // Reset odometry to the starting pose of the trajectory. m_robotDrive.resetOdometry(exampleTrajectory.getInitialPose()); 

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++)

 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136  RamseteCommand ramseteCommand = new RamseteCommand( exampleTrajectory, m_robotDrive::getPose, new RamseteController(AutoConstants.kRamseteB, AutoConstants.kRamseteZeta), new SimpleMotorFeedforward(DriveConstants.ksVolts, DriveConstants.kvVoltSecondsPerMeter, DriveConstants.kaVoltSecondsSquaredPerMeter), DriveConstants.kDriveKinematics, m_robotDrive::getWheelSpeeds, new PIDController(DriveConstants.kPDriveVel, 0, 0), new PIDController(DriveConstants.kPDriveVel, 0, 0), // RamseteCommand passes volts to the callback m_robotDrive::tankDriveVolts, m_robotDrive ); 

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 characterization 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 characterization 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 characterization 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: