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Lesson 10.5: Challenge — Developing a Precise Drive and Stop System for Linear Navigation

Why Encoder Resets Matter More Than You Think​

Here is a scenario that trips up a lot of teams. The autonomous drives forward 1000 ticks -- works perfectly. Then it is supposed to drive forward another 500 ticks. But you forgot to reset the encoder, so the motor is now at position 1000 and you called setTargetPosition(500). The Hub thinks the target is behind the motor and either drives backward or does nothing at all.

The fix is STOP_AND_RESET_ENCODER. This mode tells the Hub to stop the motor, zero out the tick counter, and hold position until you switch to a movement mode. Running it before every new move segment gives you a clean logical zero to work from every time, and it also means errors from the previous segment -- like being bumped by another robot -- cannot carry forward and compound over multiple steps.

Putting the Full Sequence Together​

A complete encoder-based move has five steps that you will repeat for every segment of an autonomous routine:

  1. Reset the encoder with STOP_AND_RESET_ENCODER
  2. Set the target position with setTargetPosition()
  3. Switch to RUN_TO_POSITION
  4. Apply power with setPower()
  5. Wait for the move to finish with isBusy()

The isBusy() method returns true as long as the Hub is actively driving toward the target. Once the motor arrives and the Hub transitions to holding mode, isBusy() returns false and your while loop exits. This is the correct condition to use -- checking a tick threshold yourself would be less reliable.

One thing to remember: after STOP_AND_RESET_ENCODER, the motor is stopped. You must switch to a movement mode before calling setPower() or nothing will happen.

Annotated Code​

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.LinearOpMode;
import com.qualcomm.robotcore.eventloop.opmode.Autonomous;
import com.qualcomm.robotcore.hardware.DcMotor;

@Autonomous(name="Drive_And_Stop_Demo")
public class DriveAndStopDemo extends LinearOpMode {

    @Override
    public void runOpMode() {
        DcMotor drive = hardwareMap.get(DcMotor.class, "drive");
        drive.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);

        waitForStart();

        // Step 1: Zero out any accumulated ticks from previous runs
        drive.setMode(DcMotor.RunMode.STOP_AND_RESET_ENCODER);

        // Step 2: Define the absolute target from zero
        drive.setTargetPosition(1000);

        // Step 3: Activate position control
        drive.setMode(DcMotor.RunMode.RUN_TO_POSITION);

        // Step 4: Apply power -- the Hub starts moving now
        drive.setPower(0.5);

        // Step 5: Block until the Hub reports arrival
        while (opModeIsActive() && drive.isBusy()) {
            telemetry.addData("Target",   drive.getTargetPosition());
            telemetry.addData("Position", drive.getCurrentPosition());
            telemetry.update();
        }

        drive.setPower(0);
        telemetry.addData("Status", "Move complete");
        telemetry.update();
    }
}

Fill-in-the-Blank Practice​

  1. To zero out accumulated ticks before a new move, you apply the DcMotor.RunMode.__________ mode.
  2. After a reset, the motor must be switched to a movement mode because the reset command also __________ the motor shaft.
  3. The __________ method returns a boolean indicating whether the motor is still actively traveling toward its encoder target.
Show answers
  1. STOP_AND_RESET_ENCODER
  2. stops (halts / disables)
  3. isBusy()

Template Challenge​

Robot Scenario: Create a precise autonomous sequence for a "slider" motor. Reset its encoder, move exactly 500 ticks forward, and wait until the move is complete before stopping. Display the current position during the move.

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.LinearOpMode;
import com.qualcomm.robotcore.eventloop.opmode.Autonomous;
import com.qualcomm.robotcore.hardware.DcMotor;

@Autonomous(name="Precision_Slide_Challenge")
public class PrecisionSlide extends LinearOpMode {

    @Override
    public void runOpMode() {
        DcMotor slider = hardwareMap.get(DcMotor.class, "slider");
        slider.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);

        waitForStart();

        // INSERT CODE HERE: Reset the encoder
        // INSERT CODE HERE: Set target to 500 ticks
        // INSERT CODE HERE: Switch to RUN_TO_POSITION
        // INSERT CODE HERE: Apply 0.5 power

        while (opModeIsActive() && slider.isBusy()) {
            telemetry.addData("Position", slider.getCurrentPosition());
            telemetry.update();
        }

        slider.setPower(0);
        telemetry.addData("Status", "Done");
        telemetry.update();
    }
}
Show answer
slider.setMode(DcMotor.RunMode.STOP_AND_RESET_ENCODER);
slider.setTargetPosition(500);
slider.setMode(DcMotor.RunMode.RUN_TO_POSITION);
slider.setPower(0.5);

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