Add DifferentialDrive voltage constraint (#2075)

wpilibc/src/main/native/cpp/trajectory/constraint/DifferentialDriveVoltageConstraint.cpp

@@ -0,0 +1,79 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "frc/trajectory/constraint/DifferentialDriveVoltageConstraint.h"
+
+#include <algorithm>
+#include <limits>
+
+#include <wpi/MathExtras.h>
+
+using namespace frc;
+
+DifferentialDriveVoltageConstraint::DifferentialDriveVoltageConstraint(
+    SimpleMotorFeedforward<units::meter> feedforward,
+    DifferentialDriveKinematics kinematics, units::volt_t maxVoltage)
+    : m_feedforward(feedforward),
+      m_kinematics(kinematics),
+      m_maxVoltage(maxVoltage) {}
+
+units::meters_per_second_t DifferentialDriveVoltageConstraint::MaxVelocity(
+    const Pose2d& pose, curvature_t curvature,
+    units::meters_per_second_t velocity) {
+  return units::meters_per_second_t(std::numeric_limits<double>::max());
+}
+
+TrajectoryConstraint::MinMax
+DifferentialDriveVoltageConstraint::MinMaxAcceleration(
+    const Pose2d& pose, curvature_t curvature,
+    units::meters_per_second_t speed) {
+  auto wheelSpeeds =
+      m_kinematics.ToWheelSpeeds({speed, 0_mps, speed * curvature});
+
+  auto maxWheelSpeed = std::max(wheelSpeeds.left, wheelSpeeds.right);
+  auto minWheelSpeed = std::min(wheelSpeeds.left, wheelSpeeds.right);
+
+  // Calculate maximum/minimum possible accelerations from motor dynamics
+  // and max/min wheel speeds
+  auto maxWheelAcceleration =
+      m_feedforward.MaxAchievableAcceleration(m_maxVoltage, maxWheelSpeed);
+  auto minWheelAcceleration =
+      m_feedforward.MinAchievableAcceleration(m_maxVoltage, minWheelSpeed);
+
+  // Robot chassis turning on radius = 1/|curvature|.  Outer wheel has radius
+  // increased by half of the trackwidth T.  Inner wheel has radius decreased
+  // by half of the trackwidth.  Achassis / radius = Aouter / (radius + T/2), so
+  // Achassis = Aouter * radius / (radius + T/2) = Aouter / (1 + |curvature|T/2)
+  // Inner wheel is similar.
+
+  // sgn(speed) term added to correctly account for which wheel is on
+  // outside of turn:
+  // If moving forward, max acceleration constraint corresponds to wheel on
+  // outside of turn
+  // If moving backward, max acceleration constraint corresponds to wheel on
+  // inside of turn
+  auto maxChassisAcceleration =
+      maxWheelAcceleration /
+      (1 + m_kinematics.trackWidth * units::math::abs(curvature) *
+               wpi::sgn(speed) / (2_rad));
+  auto minChassisAcceleration =
+      minWheelAcceleration /
+      (1 - m_kinematics.trackWidth * units::math::abs(curvature) *
+               wpi::sgn(speed) / (2_rad));
+
+  // Negate acceleration corresponding to wheel on inside of turn
+  // if center of turn is inside of wheelbase
+  if ((m_kinematics.trackWidth / 2) > 1_rad / units::math::abs(curvature)) {
+    if (speed > 0_mps) {
+      minChassisAcceleration = -minChassisAcceleration;
+    } else {
+      maxChassisAcceleration = -maxChassisAcceleration;
+    }
+  }
+
+  return {minChassisAcceleration, maxChassisAcceleration};
+}