Lesson 8.4: Choosing Between Coasting and Stopping with setPower(0.0)

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Technical Context

Precision in autonomous and TeleOp depends on predictable stops. If a programmer relies on mechanical friction alone to stop a robot, the stopping distance will vary based on battery voltage, field tile material, and the robot's momentum at the moment power cuts. Using setPower(0.0) in conjunction with the correct ZeroPowerBehavior ensures the robot stops consistently across different match conditions.

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How Zero Power Changes the Way a Mechanism Settles

setPower(0.0) is a deliberate command that tells the Control Hub to cease applying voltage to the motor port. Its physical effect is entirely determined by the ZeroPowerBehavior established in Lesson 8.3:

ZeroPowerBehavior	Effect of setPower(0.0)
FLOAT (default)	Motor terminals disconnected — robot coasts to a stop via friction
BRAKE	Motor terminals shorted — robot decelerates rapidly and holds position

In a high-speed drivetrain, switching to 0.0 power with FLOAT behavior can cause the robot to over-travel its target by several inches — enough to miss a scoring zone entirely. BRAKE mode reduces this over-travel dramatically, making autonomous navigation far more repeatable.

A common professional pattern is to set BRAKE on all drive motors in init(), then send setPower(0.0) as the last command of every autonomous step to guarantee a clean stop before the next movement begins.

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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="Coast_vs_Stop_Demo")
public class CoastVsStopDemo extends LinearOpMode {

    private DcMotor driveMotor;

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

        // BRAKE ensures setPower(0.0) stops the robot, not just cuts voltage
        driveMotor.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);

        waitForStart();

        // Drive forward for 2 seconds
        driveMotor.setPower(0.6);
        sleep(2000);

        // This stops the motor AND holds position — not a coast
        driveMotor.setPower(0.0);

        telemetry.addData("Status", "Stopped at target position");
        telemetry.update();

        sleep(1000); // Hold stopped state briefly for visibility
    }
}

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Fill-in-the-Blank Practice

1. Sending a power value of __________ is the standard way to stop a motor in the SDK.
2. Relying on __________ alone to stop a robot is inconsistent because it varies with the battery voltage and field surface material.
3. To stop a motor currently running at full reverse speed, you would change its power from -1.0 to __________.

Show answers

1. 0.0
2. mechanical friction (or coasting / FLOAT behavior)
3. 0.0

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Template Challenge

Robot Scenario: You are writing an emergency stop function. If gamepad1.b is pressed, all power to "motor1" and "motor2" must be cut immediately and both motors must resist any further movement.

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.OpMode;
import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
import com.qualcomm.robotcore.hardware.DcMotor;

@TeleOp(name="Emergency_Stop_Challenge")
public class EmergencyStop extends OpMode {

    private DcMotor motor1;
    private DcMotor motor2;

    @Override
    public void init() {
        motor1 = hardwareMap.get(DcMotor.class, "motor1");
        motor2 = hardwareMap.get(DcMotor.class, "motor2");

        // INSERT CODE HERE: Set BRAKE behavior on both motors
    }

    @Override
    public void loop() {
        motor1.setPower(-gamepad1.left_stick_y);
        motor2.setPower(-gamepad1.right_stick_y);

        // INSERT CODE HERE: If gamepad1.b is pressed, set both motors to 0.0
    }
}

Show answer

@Override
public void init() {
    motor1 = hardwareMap.get(DcMotor.class, "motor1");
    motor2 = hardwareMap.get(DcMotor.class, "motor2");

    motor1.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);
    motor2.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);
}

@Override
public void loop() {
    motor1.setPower(-gamepad1.left_stick_y);
    motor2.setPower(-gamepad1.right_stick_y);

    if (gamepad1.b) {
        motor1.setPower(0.0);
        motor2.setPower(0.0);
    }
}

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