Motors, Servos and Sensors

A robot needs two kinds of electrical parts. Actuators create motion: DC motors and servos. Sensors collect information: how far a wheel has turned, which way the robot is facing, whether a game element is in the intake. Good robots combine both, so the code can move something and then check that it moved the right amount.

DC motors and gearboxes

A DC motor spins continuously when you give it power. In code you set power from -1.0 (full reverse) to 1.0 (full forward), with 0 meaning stop. A bare FTC motor spins very fast, around 6000 RPM, but with very little twisting force, or torque. That is too fast and too weak to drive a robot directly, so almost every FTC motor has a gearbox attached. The gearbox trades speed for torque. You choose the gearbox based on the job.

Encoders: counting rotation

Most FTC motors include an encoder, a sensor on the motor that counts small steps of rotation called ticks. Your code reads the count with getCurrentPosition(). For example, if the encoder gives 28 ticks per turn of the motor shaft and the gearbox is 20:1, the output shaft makes one full turn every 28 x 20 = 560 ticks. If you know the wheel size, you can turn ticks into distance travelled. Encoders let you drive a set distance in Autonomous or move an arm to an exact angle with RUN_TO_POSITION.

Servos: standard and continuous rotation

A standard servo does not tell your code where it really is. getPosition() only returns the last position you asked for. If something blocks the servo, the code will not know. A continuous rotation servo cannot be told to go to an angle at all. Some servos, such as the REV Smart Robot Servo, can be switched between the two modes with a programmer tool.

Common sensors

• Encoder: counts motor rotation. Used for distance, arm angle and speed.
• IMU (inertial measurement unit): measures rotation and tilt. The Control Hub has one built in, and it is mostly used to read the robot's heading (which way it is facing).
• Distance sensor: measures how far away an object or wall is, for example in centimetres.
• Color sensor: measures the color and brightness of a nearby surface, useful for detecting game elements or lines on the field.
• Touch sensor or limit switch: reports pressed or not pressed. Often used to stop a lift at the bottom or detect when a mechanism reaches its end.

1package org.firstinspires.ftc.teamcode;
2
3import com.qualcomm.robotcore.eventloop.opmode.LinearOpMode;
4import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
5import com.qualcomm.robotcore.hardware.DcMotor;
6import com.qualcomm.robotcore.hardware.Servo;
7import com.qualcomm.robotcore.hardware.TouchSensor;
8
9@TeleOp(name = "Hardware Test")
10public class HardwareTest extends LinearOpMode {
11    @Override
12    public void runOpMode() {
13        DcMotor motor = hardwareMap.get(DcMotor.class, "test_motor");
14        Servo servo = hardwareMap.get(Servo.class, "test_servo");
15        TouchSensor touch = hardwareMap.get(TouchSensor.class, "touch");
16
17        motor.setMode(DcMotor.RunMode.STOP_AND_RESET_ENCODER);
18        motor.setMode(DcMotor.RunMode.RUN_WITHOUT_ENCODER);
19
20        waitForStart();
21
22        while (opModeIsActive()) {
23            // Pushing the stick up gives a negative value, so flip it
24            motor.setPower(-gamepad1.left_stick_y * 0.5);
25
26            if (gamepad1.a) servo.setPosition(0.2);
27            if (gamepad1.b) servo.setPosition(0.8);
28
29            telemetry.addData("Encoder ticks", motor.getCurrentPosition());
30            telemetry.addData("Touch pressed", touch.isPressed());
31            telemetry.update();
32        }
33    }
34}