Motors and Servos in Code

A DC motor in the FTC SDK is a DcMotor object. The most important method is setPower, which takes a number from -1.0 to 1.0. 1.0 is full power in one direction, -1.0 is full power in the other, and 0 means no power. Power is roughly the share of battery voltage sent to the motor, not an exact speed: a robot with a weak battery or a heavy load moves slower at the same power. Values outside the range are not useful, so keep your calculations between -1 and 1.

1DcMotor intake = hardwareMap.get(DcMotor.class, "intake");
2intake.setDirection(DcMotor.Direction.FORWARD);
3intake.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);
4
5waitForStart();
6
7intake.setPower(0.5);    // half power, one direction
8sleep(1000);             // wait 1000 ms = 1 second
9intake.setPower(-0.5);   // half power, the other direction
10sleep(1000);
11intake.setPower(0);      // stop

Direction: why one side is reversed

On a drivetrain the left and right motors are usually mounted as mirror images. If you give both the same positive power, they spin the same way relative to their own shafts, which means one side drives forward and the other backward, and the robot spins in place. The fix is setDirection(DcMotor.Direction.REVERSE) on one side. Which side depends on your motors and gearboxes, so you must test it. The goal is simple: positive power on every drive motor should push the robot forward.

Zero-power behavior: brake or float

Servos: moving to a position

A standard servo moves to a position and holds it. In code you call setPosition with a number from 0.0 to 1.0. These numbers are not degrees: 0.0 and 1.0 are the two ends of the servo's range, and how many degrees that covers depends on the servo model and how it is programmed. A servo usually stays limp until it receives its first setPosition command. You can read back the last commanded position with getPosition, but this is not a sensor reading.

1// Class-level constants (inside the OpMode class, outside runOpMode)
2static final double CLAW_OPEN   = 0.30;   // example values: find yours by testing
3static final double CLAW_CLOSED = 0.65;
4
5// Inside runOpMode()
6Servo claw = hardwareMap.get(Servo.class, "claw");
7
8waitForStart();
9
10while (opModeIsActive()) {
11    if (gamepad1.a) {
12        claw.setPosition(CLAW_OPEN);
13    } else if (gamepad1.b) {
14        claw.setPosition(CLAW_CLOSED);
15    }
16}

A continuous rotation servo spins like a small motor instead of moving to a position. Configure it as Continuous Rotation Servo and control it as a CRServo with setPower, for example spinner.setPower(1.0). Using the wrong class or the wrong configuration type is a common source of 'my servo does nothing' bugs.

1. 1
   Center first

   Before attaching the servo horn or arm, command the servo to 0.5 so the mechanism is mounted in the middle of the range.
2. 2
   Move in small steps

   Use a test OpMode where the D-pad changes the position by 0.01 and telemetry shows the current value.
3. 3
   Find the limits

   Move slowly until the claw is just open and just closed. Stop before the mechanism hits a hard stop.
4. 4
   Record and name

   Write the values down and store them as named constants such as CLAW_OPEN instead of typing numbers everywhere.

• Forgetting to reverse one side, so the robot spins when it should drive straight.
• Using Servo in code when the port is configured as a continuous rotation servo, or the opposite.
• Typing magic numbers like 0.47 in five places instead of one named constant.
• Commanding a servo past the mechanism's limit, which strains and heats the servo.