Using Limits to Control Range of Motion

In Part 2: Robot Control the idea of creating a limit switch was introduced using a physical sensor, like a touch sensor. We can make use of our motor's built-in encoder to do something similar. While a physical sensor would be described as a hard limit, using the built-in encoder is called a soft limit.

To set the soft limits we will build off the program created in the last sections (HelloRobot_ArmEncoder)!

@TeleOp

public class HelloRobot_ArmEncoder extends LinearOpMode {
    private DcMotor  arm;

    static final double     COUNTS_PER_MOTOR_REV    = 288; 
    static final double     GEAR_REDUCTION    = 2.7778;   
    static final double     COUNTS_PER_GEAR_REV    = COUNTS_PER_MOTOR_REV * GEAR_REDUCTION;
    static final double     COUNTS_PER_DEGREE    = COUNTS_PER_GEAR_REV/360;
    
    @Override
    public void runOpMode() {
        arm = hardwareMap.get(DcMotor.class, "arm");
        
        
        waitForStart();
    
        while (opModeIsActive()) {
            telemetry.addData("Status", "Running");
            telemetry.update();

        }
    }
}

Creating minPosition and maxPosition

To start, we need to create our upper and lower limits. Create two new variables one called minPosition and one called maxPosition to be added to initialization.

Add both of these to the in the initialization section of the OpMode above the waitForStart(); command.

public void runOpMode() {
        arm = hardwareMap.get(DcMotor.class, "arm");
        
        int minPostion;
        int maxPosition;
        waitForStart();
    

For now we want the minPosition set as our starting position and the maxPosition set to our 90 degree position. Set minPosition equal to 0 and set maxPosition equal to COUNTS_PER_DEGREE times 45.

Remember you need to make a data type conversion!

int minPostion = 0;
int maxPosition = (int)(COUNTS_PER_DEGREE *45);

Adjusting our If/Else Statement

To start, our If/Else Statement will be changed back to a simplified format like we had at the beginning of estimating the position of the arm.

while(opModeIsActive()){
     if(gamepad1.dpad_up){
            arm.setPower(0.5);         
            }
     else if (gamepad1.dpad_down){
            arm.setPower(-0.5); 
            }   
     else { 
            arm.setPower(0); 
            }   
     } 

To set the limit we need to edit our if/else statement to include our limits:

• If up on the Dpad is pressed and the position of the arm is less than the maxPosition, then the arm will move to the maxPosition.

• If down on the Dpad is pressed and the position of the arm is greater than the minPosition then the arm will move towards the minPosition.

while (opModeIsActive()) {
    if (gamepad1.dpad_up && arm.getCurrentPosition() < maxPosition) {
            arm.setPower(0.5);
            } 
    else if (gamepad1.dpad_down && arm.getCurrentPosition() > minPosition) {
            arm.setPower(-0.5);
            } 
    else {
            arm.setPower(0);
            }
    }

The current code configuration will stop the motor at any point that the conditions to power the motor are not met. Depending to factors like the weight of the mechanism and any load that it is bearing, when the motor stops the arm may drop below the maxPosition. Take time to test out the code and confirm that it behaves in the way you expect it to.

Overriding Limits

One of the benefits of having a soft limit is being able to exceed that limit.

Remember that the encoders zero tick position is determined by the position of the arm when the Control Hub powers on! So if we aren't careful to reset the arm before powering on our robot this will effect the arm's range of motion. For instance, if we have to reset the Control Hub while the arm is in the 90 degree position, the 90 degree position will become equal to 0 encoder ticks.

As a back up, we can create an override for the range of motion. There are a few different ways an override can be created, but in our case we are going to use the "A" button and touch sensor to help reset our range.

Adding a Gamepad Override

Start by editing the if/else if statement to add another else if condition. Use the line gamepad1.a as the condition. Add a the line arm.setPower(-0.5); as the action item.

while (opModeIsActive()) {
    if (gamepad1.dpad_up && arm.getCurrentPosition() < maxPosition) {
            arm.setPower(0.5);
            } 
    else if (gamepad1.dpad_down && arm.getCurrentPosition() > minPosition) {
            arm.setPower(-0.5);
            } 
    else if(gamepad1.a){
            arm.setPower(-0.5);
    else {
            arm.setPower(0);
            }
    }

Now that we have this change in place, when the "A" button is pressed the arm will move toward the starting position.

Adding a Touch Sensor Limit

When the arm reaches and presses the touch sensor we want to STOP_AND_RESET_ENCODER.

We can create another if statement that focuses on performing this stop and reset when the Touch Sensor is pressed.

if (touch.isPressed()) {
          arm.setMode(DcMotor.RunMode.STOP_AND_RESET_ENCODER);
        }

So, if the Touch Sensor returns true (or is pressed) the motor run mode STOP_AND_RESET_ENCODER will be activated causing the motor encoder to reset to 0 ticks.

Now that this code is done, try testing it!

@TeleOp

public class HelloRobot_ArmEncoder extends LinearOpMode {
    private DcMotor arm;
    private Servo claw;
    private DcMotor leftmotor;
    private DcMotor rightmotor;
    private TouchSensor touch;
    
    static final double     COUNTS_PER_MOTOR_REV    = 288; 
    static final double     GEAR_REDUCTION    = 2.7778;   
    static final double     COUNTS_PER_GEAR_REV    = COUNTS_PER_MOTOR_REV * GEAR_REDUCTION;
    static final double     COUNTS_PER_DEGREE    = COUNTS_PER_GEAR_REV/360;


    @Override
    public void runOpMode() {
        arm = hardwareMap.get(DcMotor.class, "arm");
        claw = hardwareMap.get(Servo.class, "claw");
        leftmotor = hardwareMap.get(DcMotor.class, "leftmotor");
        rightmotor = hardwareMap.get(DcMotor.class, "rightmotor");
        touch = hardwareMap.get(TouchSensor.class, "touch");
        
        int minPostion = 0;
        int maxPosition = (int)(COUNTS_PER_DEGREE *45);
        
        waitForStart();
            
        // run until the end of the match (driver presses STOP)
        while (opModeIsActive()) {
            if (gamepad1.dpad_up && arm.getCurrentPosition() < maxPosition) {
                arm.setPower(0.5);
                    } 
            else if (gamepad1.dpad_down && arm.getCurrentPosition() > minPosition) {
                arm.setPower(-0.5);
                    } 
            else if (gamepad1.a) {
                arm.setPower(-0.5);
                    } 
            else {
                arm.setPower(0);
                }
            if (touch.isPressed()) {
              arm.setMode(DcMotor.RunMode.STOP_AND_RESET_ENCODER);
                    }
        telemetry.addData("Arm Test", arm.getCurrentPosition());
        telemetry.update(); 
        }
    }
}