Encoder Basics

So far, you have controlled motors by setting a power level and hoping for the best. But how does your robot know how far it has actually moved? The answer is encoders -- sensors built into FTC motors that measure exactly how much the motor shaft has rotated.

What is an Encoder?

An encoder is a sensor attached to a motor that produces electrical pulses as the shaft rotates. By counting these pulses (called ticks or counts), you can determine exactly how far the motor has turned.

Every FTC-legal motor has a built-in encoder. You do not need to attach any extra hardware -- the encoder signals travel through the same cable as the motor power.

Counts Per Revolution (CPR)

Different motors produce different numbers of ticks per revolution. This value is called Counts Per Revolution (CPR) and varies by motor model:

Reading the Encoder

To read the current encoder position, use getCurrentPosition():

DcMotor motor = hardwareMap.get(DcMotor.class, "driveMotor");
int position = motor.getCurrentPosition();

This returns the total number of ticks the encoder has counted since it was last reset. The value can be positive or negative depending on the direction of rotation.

Resetting the Encoder

Before you start counting, you typically want to reset the encoder to zero. This is done by setting the run mode:

motor.setMode(DcMotor.RunMode.STOP_AND_RESET_ENCODER);
motor.setMode(DcMotor.RunMode.RUN_USING_ENCODER);

The first line resets the encoder count to zero and stops the motor. The second line puts the motor back into a mode where you can drive it normally while the encoder continues to count.

Converting Ticks to Distance

Knowing the tick count is useful, but what you really want to know is: how far has the robot moved? To calculate this, you need two pieces of information:

1. CPR -- ticks per revolution of the motor
2. Wheel circumference -- how far the robot moves in one wheel revolution

revolutions = ticks / CPR
distance = revolutions * wheelCircumference
wheelCircumference = pi * wheelDiameter

For example, with a goBILDA 312 RPM motor (537.7 CPR) and a 96mm diameter wheel:

wheelCircumference = pi * 0.096m = 0.3016m

If the encoder reads 537 ticks:

revolutions = 537 / 537.7 = 0.9987 (almost exactly one revolution)
distance = 0.9987 * 0.3016m = 0.3012m (about 30 cm)

The robot has moved roughly 30 centimeters -- one full wheel rotation.

Displaying Encoder Data

During development, always display encoder values on telemetry:

int position = motor.getCurrentPosition();
double revolutions = position / 537.7;
double distance = revolutions  (Math.PI  0.096);
telemetry.addData("Position", position);
telemetry.addData("Revolutions", "%.2f", revolutions);
telemetry.addData("Distance (m)", "%.3f", distance);
telemetry.update();

Watching the encoder value change in real time helps you understand how the numbers relate to physical movement. Try pushing the robot by hand and watching the ticks increase!

Your Exercise

In this exercise, you will read an encoder value and display it on telemetry. The motor "driveMotor" has a simulated encoder position of 537 ticks.

Your task:

1. Get the motor from the hardware map.
2. After waitForStart(), read the current encoder position using getCurrentPosition().
3. Display the position on telemetry with the caption "Position".
4. Call telemetry.update().

Give it a try!