Lesson 8.5: Challenge — Build a Two-Way Linear Slide with Software Limits

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Technical Context

Linear slides are prone to mechanical failure if driven past their physical limits — either bottoming out at the base or over-extending at the top. Using a DigitalChannel touch sensor as a software limiter prevents the motor from applying power in a direction that would cause hardware damage, protecting the motor from stall current and the mechanism from structural failure.

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How to Keep a Linear Slide Safe in Both Directions

This challenge requires combining concepts from Units 7 and 8. You must:

1. Map both a DcMotor and a DigitalChannel in init()
2. Read the touch sensor state each loop cycle
3. Apply conditional logic that overrides driver input when the sensor is triggered

A critical hardware nuance: the REV Touch Sensor uses active-low logic. getState() returns true when the sensor is not pressed and false when it is pressed. This means you must use the logical NOT operator (!) to invert the signal for intuitive use — boolean isPressed = !limitSwitch.getState().

The limiter logic then becomes: if the driver is commanding the slide downward (negative power) AND the bottom limit is pressed, override the power to 0.0. Movement upward is still permitted because the limiter only blocks the dangerous direction.

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Annotated Code

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.LinearOpMode;
import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
import com.qualcomm.robotcore.hardware.DcMotor;
import com.qualcomm.robotcore.hardware.DigitalChannel;

@TeleOp(name="Safe_Slide_Demo")
public class SafeSlideDemo extends LinearOpMode {

    @Override
    public void runOpMode() {
        DcMotor       liftMotor   = hardwareMap.get(DcMotor.class, "lift_motor");
        DigitalChannel limitSwitch = hardwareMap.get(DigitalChannel.class, "limit_bottom");

        limitSwitch.setMode(DigitalChannel.Mode.INPUT);
        liftMotor.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);

        waitForStart();

        while (opModeIsActive()) {
            double power = -gamepad1.left_stick_y;

            // Active-low: false = pressed, so invert for intuitive logic
            boolean isAtBottom = !limitSwitch.getState();

            // Block downward movement if the bottom limit is triggered
            if (isAtBottom && power < 0) {
                power = 0.0;
                telemetry.addData("Safety", "BOTTOM LIMIT REACHED");
            }

            liftMotor.setPower(power);
            telemetry.addData("Slide Power",   power);
            telemetry.addData("At Bottom",     isAtBottom);
            telemetry.update();
        }
    }
}

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Fill-in-the-Blank Practice

1. A software limiter uses __________ operators and sensor data to prevent mechanical binding at the physical end of travel.
2. If a touch sensor returns false when physically pressed (active-low logic), you must use the __________ operator in Java to flip the signal for intuitive use.
3. Preventing a motor from stalling against a physical limit improves robot __________ by avoiding the sustained high-current draw that burns out motor windings.

Show answers

1. conditional (comparison / logical)
2. ! (the logical NOT operator)
3. reliability (longevity / lifespan)

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Template Challenge

Robot Scenario: Your robot has a lift motor "lift" and a touch sensor "limit" at the bottom. Allow the driver to move the lift with gamepad1.left_stick_y, but stop the motor if the stick is commanded downward (negative power after negation) and the limit switch is triggered.

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.LinearOpMode;
import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
import com.qualcomm.robotcore.hardware.DcMotor;
import com.qualcomm.robotcore.hardware.DigitalChannel;

@TeleOp(name="Safe_Lift_Challenge")
public class SafeLift extends LinearOpMode {

    @Override
    public void runOpMode() {
        DcMotor       lift  = hardwareMap.get(DcMotor.class, "lift");
        DigitalChannel limit = hardwareMap.get(DigitalChannel.class, "limit");

        limit.setMode(DigitalChannel.Mode.INPUT);
        lift.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);

        waitForStart();

        while (opModeIsActive()) {
            double power = -gamepad1.left_stick_y;

            // INSERT CODE HERE: Read the limit switch and invert for active-low
            boolean pressed = // INSERT CODE HERE

            // INSERT CODE HERE: If pressed AND power is negative, set power to 0

            lift.setPower(power);
            telemetry.addData("Power",   power);
            telemetry.addData("Pressed", pressed);
            telemetry.update();
        }
    }
}

Show answer

while (opModeIsActive()) {
    double power = -gamepad1.left_stick_y;

    boolean pressed = !limit.getState(); // Invert active-low signal

    if (pressed && power < 0) {
        power = 0.0; // Block downward movement at the bottom limit
    }

    lift.setPower(power);
    telemetry.addData("Power",   power);
    telemetry.addData("Pressed", pressed);
    telemetry.update();
}

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