1 - Motors

Believe it or not...motors...make robots move!

Crazy, I know. This is some riveting information.

Joking aside, your robot will probably use a lot of these, so it's important to know all the components, capabilities, and limitations of VEX's SmartMotor system.

Example

// Create a SmartMotor object called "sampleMotor"
SmartMotor sampleMotor = SmartMotor(
    "MotorName", // <-- This is a name for the motor for logging purposes
    1            // <-- This is the port written on the VEX brain that the motor is connected to
);

void opcontrol() override
{
    while (true)
    {
        // Get the up/down input from the left joystick
        // This is a value from -1 to 1 representing 100% down and 100% up
        float joystickInput = mainController.leftY;

        // Moves the motor when you move the left joystick up and down
        // This is a value from -1 to 1 representing 100% reverse and 100% forwards
        sampleMotor.move(joystickInput);

        // For example,
        //  -1   (-100%) will go reverse at full speed
        //  -0.5 (-50%)  will go reverse at 50% speed
        //  0    (0%)    will stop the motor
        //  0.5  (50%)   will go forwards at 50% speed
        //  1    (100%)  will go forwards at full speed

        // ...
    }
}

Motor Encoders

VEX's SmartMotors have a neat little feature that most DC motors don't have; and thats a built-in motor encoder.

A motor encoder is a sensor that detects how much the motor has actually moved.

// 1800 ticks/rev for 36:1 gears (11W)
// 900 ticks/rev for 18:1 gears (11W OR 5.5W)
// 300 ticks/rev for 6:1 gears (11W)

// Print the current encoder position in ticks to the screen
auto position = sampleMotor.getPosition();
std::cout << position << std::endl;

// Reset the position to 0
sampleMotor.setPosition(0);

note

The SmartMotor encoder is a relative encoder. That is, it will always reset to zero when the robot restarts. It cannot track the motor movement while the robot is off.

An absolute encoder could track the orientation/position of a shaft across restarts, even if it moves while the robot is off.

Motor Group

Often times, a robot will have a group of multiple motors that are geared or mechanically linked together. In cases like these, a SmartMotorGroup can be used to control all of the motors at once instead of controlling each motor individually.

Example

// Create a SmartMotorGroup object called "sampleMotorGroup"
SmartMotorGroup sampleMotorGroup = SmartMotorGroup(
    "MotorGroupName", // <-- Name of the motor group for logging
    {1, -2, 3, -4}    // <-- List the port of each motor (negative = reversed)
);

void opcontrol() override
{
    while (true)
    {
        // Get the up/down input from the left joystick
        float joystickInput = mainController.leftY;

        // Moves all of the motors depending on the joystick input
        sampleMotorGroup.move(joystickInput);

        // ...
    }
}

Brake Mode

By default, VEX SmartMotors are in coast mode. That is, when there is zero power applied to the motor, the shaft will coast until friction inevitable slows it to a stop. By enabling brake mode, you can use apply a slight braking force to the motor shaft whenever there is zero power applied.

This works because a DC motor is also an electric generator. The motor can convert the kinect energy of its shaft into electric energy which is used to apply power opposite it's the rotational direction. You can do this yourself to any DC motor by bridging the positive and negative leads.

Example

MyRobot() {
    sampleMotor.setBrakeMode(true);     // <-- Enable brake mode
    //sampleMotor.setBrakeMode(false);  // <-- Or disable brake mode
}

void opcontrol() override {
    // This will prevent the motor from rotating
    sampleMotor.move(0);

    // Alternatively, you can call the "stop()" method
    sampleMotor.stop();
}

warning

Brake mode inherently generates a lot of heat when stopping the motor. You may notice the motors thermal throttling faster than usual. Don't use this for motors that start and stop frequently during a match such as the drivetrain.

Thermal Throttling

As motors accelerate and decelerate, their coils generate a lot of heat. This heat is naturally dissipated over time, but if too much build up in a VEX SmartMotor then the power is throttled until they're cooled back down.

Motor Temp	Torque Limit
50°C (120°F)	50%
60°C (140°F)	25%
65°C (150°F)	12.5%
70°C (160°F)	0% (Motor Disabled)

You can also monitor the motor temperature in DevilLib.

// Gets the motor temperature in celsius (with 5deg of resolution)
float motorTemperature = sampleMotor.getTemperature();

Current Limiting

In addition to thermal throttling, VEX SmartMotors also throttle depending on the amount of connected motors the VEX V5 brain currently has connected. Due to V5RC's 8 motor limit, this is only applicable to VURC teams.

The current limit of each motor in amps ($C_{limit}$) based on the total number of SmartMotors connected to the brain ($M_{count}$) can be calculated with the following:

$$C_{limit} = 0.925 * \log_e\frac{12800}{M_{count}} - 4.324$$

Each motor is capped at 2.5A each.

If a custom current limit is set by the user, that is prioritized (if it smaller than that the existing limit). Doing so shares unused battery current with the other motors.

// Set a 1.0A current limit
sampleMotor.setCurrentLimit(1.0f);

Source: VEX Forum