Estimating the Position of the Arm

For this tutorial, our OpMode is named HelloRobot_ArmEncoder!

Estimating the Position of the Arm using Telemetry

Let's start by creating a simple program for moving our robot's arm. The one below will look very similar to the code created during Part 2: Robot Control!

if(gamepad1.dpad_up){
       arm.setPower(0.2);         
            }
else if (gamepad1.dpad_down){
       arm.setPower(-0.2); 
            }   
else { 
       arm.setPower(0); 
            } 

Adding Telemetry

Within the while loop add the lines telemetry.addData("Arm Test", arm.getCurrentPosition()); and telemetry.update();

while(opModeIsActive){
    if(gamepad1.dpad_up){
       arm.setPower(0.2);         
            }
     else if (gamepad1.dpad_down){
       arm.setPower(-0.2); 
            }   
     else { 
       arm.setPower(0); 
            } 
     telemetry.addData("Arm Test", arm.getCurrentPosition());
     telemetry.update();
          
 }

Finding the Position with Telemetry

Build the OpMode and run it.

Use the gamepad commands to move the arm to the 90 degree position. Once you have the arm properly positioned read the telemetry off the Driver Hub to determine the encoder count relative to the position of the arm.

Remember that the encoder position is set to 0 each time the Control Hub is turned on! This means that if your arm is in a position other than the starting position when the Control Hub is turned on, that position becomes zero instead of the starting position.

Adding RUN_TO_POSITION to the Program

To add the RUN_TO_POSITION code, the if/else statement must first have the following three lines of code need to be added:

arm.setTargetPosition(0);
arm.setMode(DcMotor.RunMode.RUN_TO_POSITION);
arm.setPower(0);

When DpadUp is pressed, the arm should move to the the 90 degree position. WhenDpadDown is pressed the arm should move back to the starting position. To do this set the firstarm.setTargetPosition(0); equal to the number of ticks it took your arm to get to 90 degrees, for this example we will use 83 ticks.

Since we want DpadDown to return the arm to the starting position, keeping the arm.setTargetPosition(0); set to 0 will allow us to accomplish this. Set both arm.setPower(0); equal to 0.5.

if(gamepad1.dpad_up){
     arm.setTargetPosition(83);
     arm.setMode(DcMotor.RunMode.RUN_TO_POSITION);
     arm.setPower(0.5);
            }
else if (gamepad1.dpad_down){
      arm.setTargetPosition(0);
      arm.setMode(DcMotor.RunMode.RUN_TO_POSITION);
      arm.setPower(0.5);
            } 

Despite our power being a positive value for both directions, the arm will move up or down based on the set position!

Testing the Program

If you try running this code you may notice that the arm oscillates around the 90 degree position. When this behavior is present you should also notice the telemetry output for the encoder counts fluctuating.

Recall RUN_TO_POSITION is a Closed Loop Control, which means that if the arm does not perfectly reach the target position, the motor will continue to fluctuate until it does. When motors continue to oscillate and never quite reach the target position this may be a sign that the factors determining tolerances and other aspects of the closed loop are not tuned to this particular motor or mechanism.

There are ways to tune the motor, or ways to have the program exit once the position is reached, but for now we want to focus on working with the arm and expanding on how limits and positions work with regards to the mechanism.