DcMotorEx: Velocity Control

When you call motor.setPower(0.5), you are commanding the motor to use 50% of available voltage. This sounds consistent, but there is a hidden problem: available voltage changes as the battery drains. A 50% power command at 14V drives the wheel faster than at 12V. Over a match, this causes your driving to become inconsistent. DcMotorEx solves this by giving you closed-loop velocity control.

The Problem with setPower()

setPower() is an open-loop command -- it sets a voltage fraction and hopes for the best. The REV Hub measures nothing about the actual resulting speed. If the battery drops or the robot drives into a slight incline, the motor slows down and your code has no idea.

This matters most in:

• Autonomous: a slight speed difference between left and right motors causes the robot to drift.
• Shooter flywheels: ball velocity changes if the motor RPM is inconsistent.
• Multi-motor mechanisms: motors at different positions in the mechanism may need to stay synchronized.

What Is DcMotorEx?

DcMotorEx is an extended interface that wraps DcMotor and exposes additional functionality built into the REV Expansion Hub and Control Hub firmware:

• Velocity control with a built-in PIDF controller
• Reading velocity in ticks per second
• Reading current draw in amps
• External encoder support (on certain hardware)

Getting a DcMotorEx Reference

Change the type and the hardwareMap.get() call:

// Standard motor -- no velocity control available
DcMotor motor = hardwareMap.get(DcMotor.class, "motor");
// Extended motor -- velocity control enabled
DcMotorEx motor = hardwareMap.get(DcMotorEx.class, "motor");

The motor name in the hardware map ("motor") stays the same. Only the Java type changes.

Required Run Mode: RUN_USING_ENCODER

Velocity control requires the motor to be in RUN_USING_ENCODER mode. This tells the REV Hub firmware to activate its internal PIDF loop, which reads encoder ticks and continuously adjusts voltage to maintain the target speed.

motor.setMode(DcMotor.RunMode.RUN_USING_ENCODER);

Do not use RUN_WITHOUT_ENCODER for velocity control -- the PIDF loop won't activate and setVelocity() will have no effect.

Also do not confuse this with RUN_TO_POSITION. Here is the distinction:

Setting Velocity

motor.setVelocity(500);   // 500 ticks per second

The unit is ticks per second. Positive values drive the motor in the forward direction; negative values drive it in reverse.

Common Velocity Reference Points

For a goBILDA 5202 312 RPM motor (537.7 ticks per revolution):

Max RPM = 312
Max ticks/sec = 312 / 60 * 537.7 ≈ 2797 ticks/sec
500  ticks/sec  ≈  17.8% of max speed
1000 ticks/sec  ≈  35.7% of max speed
1500 ticks/sec  ≈  53.6% of max speed

Start with a moderate value like 500 and work up as needed.

Reading Current Velocity

You can read back the motor's actual measured velocity for telemetry or feedback:

double actualVelocity = motor.getVelocity();   // Returns ticks per second
telemetry.addData("Target",  500);
telemetry.addData("Actual",  actualVelocity);

This is useful for debugging: if the actual velocity is consistently much lower than the target, the battery may be too low or the mechanism is mechanically loaded.

Full Example: Two-Motor Drive with Velocity Control

@Override
public void runOpMode() {
    DcMotorEx leftMotor  = hardwareMap.get(DcMotorEx.class, "leftMotor");
    DcMotorEx rightMotor = hardwareMap.get(DcMotorEx.class, "rightMotor");
leftMotor.setDirection(DcMotorSimple.Direction.REVERSE);
leftMotor.setMode(DcMotor.RunMode.RUN_USING_ENCODER);
    rightMotor.setMode(DcMotor.RunMode.RUN_USING_ENCODER);
waitForStart();
leftMotor.setVelocity(500);
    rightMotor.setVelocity(500);
while (opModeIsActive()) {
        telemetry.addData("Left velocity",  leftMotor.getVelocity());
        telemetry.addData("Right velocity", rightMotor.getVelocity());
        telemetry.update();
    }
leftMotor.setVelocity(0);
    rightMotor.setVelocity(0);
}

Scaling Velocity with Gamepad Input

In teleop, you often want the driver's stick to control speed. You can scale the stick value to a tick-per-second range:

while (opModeIsActive()) {
    double stick = -gamepad1.left_stick_y;   // -1.0 to 1.0
    double maxVelocity = 1500.0;             // Ticks per second at full stick
leftMotor.setVelocity(stick * maxVelocity);
    rightMotor.setVelocity(stick * maxVelocity);
telemetry.addData("Command", stick * maxVelocity);
    telemetry.addData("Left",   leftMotor.getVelocity());
    telemetry.addData("Right",  rightMotor.getVelocity());
    telemetry.update();
}

The PIDF controller in the hub firmware adjusts voltage automatically to maintain the commanded ticks per second as the battery drains.

Your Exercise

Retrieve leftMotor and rightMotor as DcMotorEx instances. Set both to RUN_USING_ENCODER mode. After waitForStart(), command both motors to 500 ticks per second using setVelocity(). Log both getVelocity() values to telemetry inside the while (opModeIsActive()) loop.