Lesson 10.2: Calculating Distances based on Ticks Per Revolution Data

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From Raw Numbers to Real Distances

Raw ticks are useful for monitoring, but you cannot tell your robot to "drive 24 inches" using ticks alone without a conversion. This lesson covers exactly how to build that conversion so your autonomous code talks in real-world units rather than abstract encoder counts.

The math only requires two pieces of information: how many ticks your motor produces per full output shaft revolution, and how far the robot travels per revolution of that shaft.

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The Conversion Formula Explained

Here is how the chain works. Once the shaft completes one full revolution, a wheel of diameter d travels a distance equal to its circumference, or Math.PI * d. So if you know how many ticks equal one revolution, you can calculate distance like this:

rotations  = ticks / ticksPerRevolution
distance   = rotations * (Math.PI * wheelDiameter)

The SDK stores the configured motor resolution in motor.getMotorType().getTicksPerRev(). That is useful when code intentionally follows the Robot Controller motor configuration. For an autonomous distance routine, it is also common to declare a named TICKS_PER_REV constant beside the wheel diameter so every conversion input is visible and reviewable. If the motor or gearbox changes, update the Robot Controller configuration and this constant together.

One subtle but important point: getCurrentPosition() returns an int, but ticksPerRev should be stored as a double. If you divide an int by an int in Java, you get integer division. Java discards the remainder, and you lose precision. Storing one of the values as a double promotes the entire calculation to floating point automatically.

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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;

@TeleOp(name="Ticks_To_Distance_Demo")
public class TicksToDistanceDemo extends OpMode {

    private DcMotor driveMotor;

    // Wheel diameter in inches. Measure your actual wheel.
    private static final double TICKS_PER_REV = 537.7;
    private static final double WHEEL_DIAMETER_IN = 3.78;

    @Override
    public void init() {
        driveMotor = hardwareMap.get(DcMotor.class, "left_drive");
        driveMotor.setMode(DcMotor.RunMode.RUN_WITHOUT_ENCODER);
    }

    @Override
    public void loop() {
        driveMotor.setPower(-gamepad1.left_stick_y);

        // Convert raw ticks into rotations, then into inches
        double rotations = driveMotor.getCurrentPosition() / TICKS_PER_REV;
        double distanceIn = rotations * (Math.PI * WHEEL_DIAMETER_IN);

        telemetry.addData("Ticks Per Rev",  TICKS_PER_REV);
        telemetry.addData("Rotations",      "%.2f", rotations);
        telemetry.addData("Distance (in)",  "%.2f", distanceIn);
    }
}

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Fill-in-the-Blank Practice

1. To find how far a wheel has traveled, you must multiply the number of rotations by the wheel's __________.
2. A readable autonomous conversion stores motor resolution in a named __________ constant.
3. Dividing an int by a double in Java results in a __________ data type, preserving the decimal portion of the result.

Show answers

1. circumference (Math.PI * diameter)
2. TICKS_PER_REV
3. double

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Simulator Challenge

Use the simulator below to complete the winch conversion code. Fill in the missing lines so raw encoder ticks become motor rotations and cable length in inches.

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 10.2 Simulator

Loads the lesson-specific Telemark encoder challenge with incomplete starter code for students to finish.

Includes live mechanism feedback, telemetry checks, and encoder-state validation for each lesson concept.

Connects to the mechanical track

The gear ratio and wheel diameter in this conversion are decisions made on the mechanical side.

See Lesson 6.2: Calculating Gear Ratios Across Multiple Stages.

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