Programming Limit Switches

Limit switches are often used to control mechanisms on robots. While limit switches are simple to use, they only can sense a single position of a moving part. This makes them ideal for ensuring that movement doesn’t exceed some limit but not so good at controlling the speed of the movement as it approaches the limit. For example, a rotational shoulder joint on a robot arm would best be controlled using a potentiometer or an absolute encoder. A limit switch could make sure that if the potentiometer ever failed, the limit switch would stop the robot from going too far and causing damage.

Limit switches can have “normally open” or “normally closed” outputs. This will control if a high signal means the switch is opened or closed. To learn more about limit switch hardware see this article.

Controlling a Motor with Two Limit Switches

JAVA

  DigitalInput toplimitSwitch = new DigitalInput(0);
  DigitalInput bottomlimitSwitch = new DigitalInput(1);
  PWMVictorSPX motor = new PWMVictorSPX(0);
  Joystick joystick = new Joystick(0);

  @Override
  public void teleopPeriodic() {
    setMotorVelocity(joystick.getRawAxis(2));
  }

  /**
   * Sets the motor velocity based on joystick input while respecting limit switches.
   *
   * @param velocity the desired velocity of the motor, positive for up and negative for down
   */
  public void setMotorVelocity(double velocity) {
    if (velocity > 0) {
      if (toplimitSwitch.get()) {
        // We are going up and top limit is tripped so stop
        motor.setThrottle(0);
      } else {
        // We are going up but top limit is not tripped so go at commanded velocity
        motor.setThrottle(velocity);
      }
    } else {
      if (bottomlimitSwitch.get()) {
        // We are going down and bottom limit is tripped so stop
        motor.setThrottle(0);
      } else {
        // We are going down but bottom limit is not tripped so go at commanded velocity
        motor.setThrottle(velocity);
      }
    }
  }

C++

  wpi::DigitalInput toplimitSwitch{0};
  wpi::DigitalInput bottomlimitSwitch{1};
  wpi::PWMVictorSPX motor{0};
  wpi::Joystick joystick{0};
  void SetMotorVelocity(double velocity) {
    if (velocity > 0) {
      if (toplimitSwitch.Get()) {
        // We are going up and top limit is tripped so stop
        motor.SetThrottle(0);
      } else {
        // We are going up but top limit is not tripped so go at commanded
        // velocity
        motor.SetThrottle(velocity);
      }
    } else {
      if (bottomlimitSwitch.Get()) {
        // We are going down and bottom limit is tripped so stop
        motor.SetThrottle(0);
      } else {
        // We are going down but bottom limit is not tripped so go at commanded
        // velocity
        motor.SetThrottle(velocity);
      }
    }
  }