Driving Precise Distances with Encoders

An encoder is a sensor built into most FTC motors that counts how far the motor shaft has turned. It reports this as a number of ticks. If the count goes from 0 to 1000, the shaft turned a known amount, so the wheel moved a known distance. That is how a robot can measure distance instead of guessing from time.

Every motor has a fixed number of ticks per revolution of its output shaft. It depends on the motor and its gearbox, so look it up on the manufacturer's spec sheet. For example, one common goBILDA 312 RPM motor lists about 537.7 ticks per output revolution. Your motor may be different.

From ticks to inches

When a wheel makes one full revolution, the robot rolls forward by the wheel's circumference, which is pi times the diameter. Divide ticks per revolution by the circumference and you get ticks per inch. Here is a worked example with example numbers:

1. 1
   Ticks per revolution

   From the spec sheet: 537.7 ticks per output revolution (example motor).
2. 2
   Wheel diameter

   A 96 mm wheel. Convert to inches: 96 / 25.4 = 3.78 inches.
3. 3
   Circumference

   pi x 3.78 = about 11.87 inches travelled per wheel revolution.
4. 4
   Ticks per inch

   537.7 / 11.87 = about 45.3 ticks per inch.
5. 5
   Target for 24 inches

   24 x 45.3 = about 1087 ticks. One field tile is 24 inches, so this drives one tile.

Encoder run modes

Order matters with RUN_TO_POSITION. First reset the encoder, then call setTargetPosition(), then switch to RUN_TO_POSITION, and only then set the power. If you switch to RUN_TO_POSITION before setting a target, the SDK reports an error. The power you set acts as the maximum speed on the way to the target.

1static final double TICKS_PER_INCH = 45.3; // replace with your calibrated value
2
3private void driveInches(double inches, double power, double timeoutS) {
4    int target = (int) Math.round(inches * TICKS_PER_INCH);
5
6    leftDrive.setMode(DcMotor.RunMode.STOP_AND_RESET_ENCODER);
7    rightDrive.setMode(DcMotor.RunMode.STOP_AND_RESET_ENCODER);
8    leftDrive.setTargetPosition(target);
9    rightDrive.setTargetPosition(target);
10    leftDrive.setMode(DcMotor.RunMode.RUN_TO_POSITION);
11    rightDrive.setMode(DcMotor.RunMode.RUN_TO_POSITION);
12
13    ElapsedTime timer = new ElapsedTime();
14    leftDrive.setPower(Math.abs(power));
15    rightDrive.setPower(Math.abs(power));
16
17    while (opModeIsActive() && timer.seconds() < timeoutS
18            && leftDrive.isBusy() && rightDrive.isBusy()) {
19        telemetry.addData("Target", target);
20        telemetry.addData("Left / Right", "%d / %d",
21                leftDrive.getCurrentPosition(), rightDrive.getCurrentPosition());
22        telemetry.update();
23    }
24
25    leftDrive.setPower(0);
26    rightDrive.setPower(0);
27    leftDrive.setMode(DcMotor.RunMode.RUN_USING_ENCODER);
28    rightDrive.setMode(DcMotor.RunMode.RUN_USING_ENCODER);
29}

isBusy() returns true while a motor is still moving toward its target. The loop stops as soon as either motor finishes, the timeout runs out, or the OpMode is stopped. The timeout is your safety net: if an encoder cable is unplugged, the count never changes and the robot would otherwise drive forever. For a negative distance, the target becomes negative and the robot drives backward.

Calibrate with a tape measure

The calculated value is a starting point. Real wheels compress slightly and roll a little differently from the spec. To calibrate, drive 48 inches with your helper, measure the real distance and correct the value: new ticks per inch = ticks driven / inches actually travelled. Repeat until the robot lands within about half an inch, and write the final number in your engineering notebook.