Lesson 8.5: 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 upper and lower DigitalChannel touch sensors as software limiters prevents the motor from applying power in either dangerous direction, 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 lesson combines concepts from Units 7 and 8. You must:

1. Map the slide DcMotor and both DigitalChannel limits in init().
2. Configure both digital ports as inputs.
3. Correct the motor's installed direction and select BRAKE behavior.
4. Read both active-low limit states each loop cycle.
5. Apply conditional logic that overrides driver input only in the blocked direction.

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 each signal for intuitive use: boolean atUpper = !upperLimit.getState().

The limiter logic has two safe motion branches. Positive power is allowed only while the upper limit is clear. Negative power is allowed only while the lower limit is clear. Neutral input, a blocked direction, or contradictory sensor data must all produce an explicit zero-power command.

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Annotated Code

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;
import com.qualcomm.robotcore.hardware.DigitalChannel;

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

    private DcMotor slideMotor;
    private DigitalChannel upperLimit;
    private DigitalChannel lowerLimit;

    @Override
    public void init() {
        slideMotor = hardwareMap.get(DcMotor.class, "slide");
        upperLimit = hardwareMap.get(DigitalChannel.class, "limit_upper");
        lowerLimit = hardwareMap.get(DigitalChannel.class, "limit_lower");

        upperLimit.setMode(DigitalChannel.Mode.INPUT);
        lowerLimit.setMode(DigitalChannel.Mode.INPUT);
        slideMotor.setDirection(DcMotor.Direction.REVERSE);
        slideMotor.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);
        slideMotor.setPower(0.0);
    }

    @Override
    public void loop() {
        double requestedPower = -gamepad1.left_stick_y;
        boolean atUpper = !upperLimit.getState();
        boolean atLower = !lowerLimit.getState();
        double appliedPower;

        if (requestedPower > 0.0 && !atUpper) {
            appliedPower = requestedPower;
        } else if (requestedPower < 0.0 && !atLower) {
            appliedPower = requestedPower;
        } else {
            appliedPower = 0.0;
        }

        slideMotor.setPower(appliedPower);
        telemetry.addData("Slide Power", appliedPower);
        telemetry.addData("At Upper Limit", atUpper);
        telemetry.addData("At Lower Limit", atLower);
        telemetry.update();
    }
}

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Fill-in-the-Blank Practice

1. Positive slide power must be blocked by the __________ limit, while negative power must be blocked by the lower limit.
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. Every unsafe or neutral request must result in an explicit setPower(__________) command.

Show answers

1. upper
2. ! (the logical NOT operator)
3. 0.0

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Simulator Practice

Use the simulator below to practice protecting both directions with active-low upper and lower switches and lesson-specific starter code.

1.Complete the codeWork in fullscreen when you need more room for the editor and visualizer. Where file tabs are available, hover over the tab strip and choose + to create a file or reuse saved code.

2.Run the lifecyclePress Init, then Start. Use the available gamepad or scene controls.

3.Debug the behaviorCheck telemetry, motion, requirements, and hints before opening the answer.

Telemark Unit 8.5 Simulator

Loads the lesson-specific Telemark motor challenge with starter code instead of a completed solution.

Includes live hardware feedback, telemetry checks, and gamepad input for each motor-control concept.

Best for practicing power, direction, braking, stopping behavior, and limit-switch safety in context.

A Team Applies Limits Before Power

BunyipsLib's holdable actuator supports lower and upper limits around its motor control. This lesson makes the decision visible in one method:

double safePower = requestedPower;
if (requestedPower > 0 && upperLimit.isPressed()) safePower = 0.0;
if (requestedPower < 0 && lowerLimit.isPressed()) safePower = 0.0;
motor.setPower(safePower);

The source's command and supplier layers were flattened, and the limit names were changed to match the simulator. The safety order remains: evaluate limits before writing motor power. Adapted from BunyipsLib's HoldableActuator.java at a pinned commit under the MIT License.

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