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Lesson 10.2: Calculating Distances based on Ticks Per Revolution Data

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.

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 -- 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 actually stores the motor's resolution for you inside motor.getMotorType().getTicksPerRev(). This is worth using because if your team ever swaps the 20:1 gearbox for a 40:1, the constant updates automatically without you having to hunt down a hardcoded number somewhere in your code.

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 -- the remainder gets thrown away, and you lose precision. Storing one of the values as a double promotes the entire calculation to floating point automatically.

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 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);

        // Pull the resolution directly from the motor's config object
        double ticksPerRev = driveMotor.getMotorType().getTicksPerRev();

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

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

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. The SDK method getMotorType().__________() provides the ticks-per-revolution value for the configured motor.
  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. getTicksPerRev()
  3. double

Template Challenge​

Robot Scenario: You have a winch motor with a drum diameter of 2.0 inches. Calculate and report the number of motor rotations and the total cable length reeled in (in inches) to the Driver Station.

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="Winch_Rotations_Challenge")
public class WinchRotations extends OpMode {

    private DcMotor winch;
    private static final double DRUM_DIAMETER_IN = 2.0;

    @Override
    public void init() {
        winch = hardwareMap.get(DcMotor.class, "winch");
        // INSERT CODE HERE: Store the ticks per revolution from the motor type
    }

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

        // INSERT CODE HERE: Calculate rotations and distance in inches
        // INSERT CODE HERE: Display both values via telemetry
    }
}
Show answer
private DcMotor winch;
private static final double DRUM_DIAMETER_IN = 2.0;
private double ticksPerRev;

@Override
public void init() {
    winch = hardwareMap.get(DcMotor.class, "winch");
    ticksPerRev = winch.getMotorType().getTicksPerRev();
}

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

    double rotations  = winch.getCurrentPosition() / ticksPerRev;
    double distanceIn = rotations * (Math.PI * DRUM_DIAMETER_IN);

    telemetry.addData("Rotations",      "%.2f", rotations);
    telemetry.addData("Cable In (in)",  "%.2f", distanceIn);
}

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