LTVUnicycleController.h

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// Copyright (c) FIRST and other WPILib contributors.
// Open Source Software; you can modify and/or share it under the terms of
// the WPILib BSD license file in the root directory of this project.
 
#pragma once
 
#include <wpi/SymbolExports.h>
#include <wpi/array.h>
#include <wpi/interpolating_map.h>
 
#include "frc/EigenCore.h"
#include "frc/geometry/Pose2d.h"
#include "frc/kinematics/DifferentialDriveKinematics.h"
#include "frc/trajectory/Trajectory.h"
#include "units/angular_velocity.h"
#include "units/time.h"
#include "units/velocity.h"
 
namespace frc {
 
/**
 * The linear time-varying unicycle controller has a similar form to the LQR,
 * but the model used to compute the controller gain is the nonlinear unicycle
 * model linearized around the drivetrain's current state.
 *
 * This controller is a roughly drop-in replacement for RamseteController with
 * more optimal feedback gains in the "least-squares error" sense.
 *
 * See section 8.9 in Controls Engineering in FRC for a derivation of the
 * control law we used shown in theorem 8.9.1.
 */
class WPILIB_DLLEXPORT LTVUnicycleController {
 public:
  /**
   * Constructs a linear time-varying unicycle controller with default maximum
   * desired error tolerances of (0.0625 m, 0.125 m, 2 rad) and default maximum
   * desired control effort of (1 m/s, 2 rad/s).
   *
   * @param dt Discretization timestep.
   * @param maxVelocity The maximum velocity for the controller gain lookup
   *                    table.
   * @throws std::domain_error if maxVelocity &lt;= 0.
   */
  explicit LTVUnicycleController(
      units::second_t dt, units::meters_per_second_t maxVelocity = 9_mps);
 
  /**
   * Constructs a linear time-varying unicycle controller.
   *
   * See
   * https://docs.wpilib.org/en/stable/docs/software/advanced-controls/state-space/state-space-intro.html#lqr-tuning
   * for how to select the tolerances.
   *
   * @param Qelems The maximum desired error tolerance for each state.
   * @param Relems The maximum desired control effort for each input.
   * @param dt     Discretization timestep.
   * @param maxVelocity The maximum velocity for the controller gain lookup
   *                    table.
   * @throws std::domain_error if maxVelocity <= 0 m/s or >= 15 m/s.
   */
  LTVUnicycleController(const wpi::array<double, 3>& Qelems,
                        const wpi::array<double, 2>& Relems, units::second_t dt,
                        units::meters_per_second_t maxVelocity = 9_mps);
 
  /**
   * Move constructor.
   */
  LTVUnicycleController(LTVUnicycleController&&) = default;
 
  /**
   * Move assignment operator.
   */
  LTVUnicycleController& operator=(LTVUnicycleController&&) = default;
 
  /**
   * Returns true if the pose error is within tolerance of the reference.
   */
  bool AtReference() const;
 
  /**
   * Sets the pose error which is considered tolerable for use with
   * AtReference().
   *
   * @param poseTolerance Pose error which is tolerable.
   */
  void SetTolerance(const Pose2d& poseTolerance);
 
  /**
   * Returns the linear and angular velocity outputs of the LTV controller.
   *
   * The reference pose, linear velocity, and angular velocity should come from
   * a drivetrain trajectory.
   *
   * @param currentPose        The current pose.
   * @param poseRef            The desired pose.
   * @param linearVelocityRef  The desired linear velocity.
   * @param angularVelocityRef The desired angular velocity.
   */
  ChassisSpeeds Calculate(const Pose2d& currentPose, const Pose2d& poseRef,
                          units::meters_per_second_t linearVelocityRef,
                          units::radians_per_second_t angularVelocityRef);
 
  /**
   * Returns the linear and angular velocity outputs of the LTV controller.
   *
   * The reference pose, linear velocity, and angular velocity should come from
   * a drivetrain trajectory.
   *
   * @param currentPose  The current pose.
   * @param desiredState The desired pose, linear velocity, and angular velocity
   *                     from a trajectory.
   */
  ChassisSpeeds Calculate(const Pose2d& currentPose,
                          const Trajectory::State& desiredState);
 
  /**
   * Enables and disables the controller for troubleshooting purposes.
   *
   * @param enabled If the controller is enabled or not.
   */
  void SetEnabled(bool enabled);
 
 private:
  // LUT from drivetrain linear velocity to LQR gain
  wpi::interpolating_map<units::meters_per_second_t, Matrixd<2, 3>> m_table;
 
  Pose2d m_poseError;
  Pose2d m_poseTolerance;
  bool m_enabled = true;
};
 
}  // namespace frc