[wpilib] Add bang-bang controller (#3676)

Co-authored-by: Tyler Veness <calcmogul@gmail.com>

wpimath/src/main/java/edu/wpi/first/math/controller/BangBangController.java

@@ -0,0 +1,159 @@
+// 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.
+
+package edu.wpi.first.math.controller;
+
+import edu.wpi.first.math.MathSharedStore;
+import edu.wpi.first.math.MathUsageId;
+import edu.wpi.first.util.sendable.Sendable;
+import edu.wpi.first.util.sendable.SendableBuilder;
+import edu.wpi.first.util.sendable.SendableRegistry;
+
+/**
+ * Implements a bang-bang controller, which outputs either 0 or 1 depending on whether the
+ * measurement is less than the setpoint. This maximally-aggressive control approach works very well
+ * for velocity control of high-inertia mechanisms, and poorly on most other things.
+ *
+ * <p>Note that this is an *asymmetric* bang-bang controller - it will not exert any control effort
+ * in the reverse direction (e.g. it won't try to slow down an over-speeding shooter wheel). This
+ * asymmetry is *extremely important.* Bang-bang control is extremely simple, but also potentially
+ * hazardous. Always ensure that your motor controllers are set to "coast" before attempting to
+ * control them with a bang-bang controller.
+ */
+public class BangBangController implements Sendable {
+  private static int instances;
+
+  private double m_tolerance;
+
+  private double m_setpoint;
+  private double m_measurement;
+
+  /**
+   * Creates a new bang-bang controller.
+   *
+   * <p>Always ensure that your motor controllers are set to "coast" before attempting to control
+   * them with a bang-bang controller.
+   *
+   * @param tolerance Tolerance for {@link #atSetpoint() atSetpoint}.
+   */
+  public BangBangController(double tolerance) {
+    instances++;
+
+    setTolerance(tolerance);
+
+    SendableRegistry.addLW(this, "BangBangController", instances);
+
+    MathSharedStore.reportUsage(MathUsageId.kController_PIDController2, instances);
+  }
+
+  /**
+   * Creates a new bang-bang controller.
+   *
+   * <p>Always ensure that your motor controllers are set to "coast" before attempting to control
+   * them with a bang-bang controller.
+   */
+  public BangBangController() {
+    this(Double.POSITIVE_INFINITY);
+  }
+
+  /**
+   * Sets the setpoint for the bang-bang controller.
+   *
+   * @param setpoint The desired setpoint.
+   */
+  public void setSetpoint(double setpoint) {
+    m_setpoint = setpoint;
+  }
+
+  /**
+   * Returns the current setpoint of the bang-bang controller.
+   *
+   * @return The current setpoint.
+   */
+  public double getSetpoint() {
+    return m_setpoint;
+  }
+
+  /**
+   * Returns true if the error is within the tolerance of the setpoint.
+   *
+   * @return Whether the error is within the acceptable bounds.
+   */
+  public boolean atSetpoint() {
+    return Math.abs(m_setpoint - m_measurement) < m_tolerance;
+  }
+
+  /**
+   * Sets the error within which atSetpoint will return true.
+   *
+   * @param tolerance Position error which is tolerable.
+   */
+  public void setTolerance(double tolerance) {
+    m_tolerance = tolerance;
+  }
+
+  /**
+   * Returns the current tolerance of the controller.
+   *
+   * @return The current tolerance.
+   */
+  public double getTolerance() {
+    return m_tolerance;
+  }
+
+  /**
+   * Returns the current measurement of the process variable.
+   *
+   * @return The current measurement of the process variable.
+   */
+  public double getMeasurement() {
+    return m_measurement;
+  }
+
+  /**
+   * Returns the current error.
+   *
+   * @return The current error.
+   */
+  public double getError() {
+    return m_setpoint - m_measurement;
+  }
+
+  /**
+   * Returns the calculated control output.
+   *
+   * <p>Always ensure that your motor controllers are set to "coast" before attempting to control
+   * them with a bang-bang controller.
+   *
+   * @param measurement The most recent measurement of the process variable.
+   * @param setpoint The setpoint for the process variable.
+   * @return The calculated motor output (0 or 1).
+   */
+  public double calculate(double measurement, double setpoint) {
+    m_measurement = measurement;
+    m_setpoint = setpoint;
+
+    return measurement < setpoint ? 1 : 0;
+  }
+
+  /**
+   * Returns the calculated control output.
+   *
+   * @param measurement The most recent measurement of the process variable.
+   * @return The calculated motor output (0 or 1).
+   */
+  public double calculate(double measurement) {
+    return calculate(measurement, m_setpoint);
+  }
+
+  @Override
+  public void initSendable(SendableBuilder builder) {
+    builder.setSmartDashboardType("BangBangController");
+    builder.addDoubleProperty("tolerance", this::getTolerance, this::setTolerance);
+    builder.addDoubleProperty("setpoint", this::getSetpoint, this::setSetpoint);
+    builder.addDoubleProperty("measurement", this::getMeasurement, null);
+    builder.addDoubleProperty("error", this::getError, null);
+    builder.addBooleanProperty("atSetpoint", this::atSetpoint, null);
+  }
+}