Lift Subsystem

A subsystem that is found on almost all FTC robots in most seasons is a linear slide, also known as a lift. Here, you will learn how to program your own lift Subsystem.

Step 1: Create your class

The first step to creating your Subsystem is setting up the structure for it. There should only be one instance of each subsystem class. To do this, we will make our subsystem a singleton class. Here is the most basic structure that can be copy+pasted to create all of your subsystems.

Creating our subsystem as an object makes it a singleton:

object MySubsystem: Subsystem() {

}

In this case, let's call our subsystem Lift.

Step 2: Create your motor

Next, we need to set up a motor to power our lift. This is easy to do using the MotorEx class. Let's start by creating a variable to store our motor. It should be of type MotorEx.

We're using lateinit var here because we can't initialize our motor until the OpMode has been initialized, since the hardware map isn't created until an OpMode has been initialized.

lateinit var motor: MotorEx

We also need a Controller, since we want to move our motor. Let's use a PID controller. I recommend using a P value of 0.005 to start, and leaving I and D at zero. You should come back and tune it later, though.

val controller = PIDFController(0.005, 0.0, 0.0, StaticFeedforward(0.0))

Next, we need a name. This is the name specified in the hardwareMap, so that NextFTC can find the motor. I called mine lift_motor, so that's the name I'm using. Use whatever name you've set in your configuration:

val name = "lift_motor"

Finally, the last step to setting up a motor is instantiating the motor in the initialize() function. Let's do that now:

override fun initialize() {
    motor = MotorEx(name)
}

That's all you need to do to create a motor in NextFTC! To recap:

• Every motor needs to be stored in a variable
• Every motor needs a Controller
• Every motor needs a name
• Every motor must be instantiated in the initialize() function.

We're not quite done, though. We still need to create our first commands!

Step 3: Create commands

The last step when you create a Subsystem is to create the commands you'll be using. This process varies with each subsystem. Here, I'll walk you through creating three commands that each move the lift to a different height: toLow, toMiddle, and toHigh.

TIP

It's recommended to create a variable to store each encoder position. However, that takes up more space, so I won't be doing that here.

To control our motor, we will be using the RunToPosition command.

Let's create our first RunToPosition command.

There are a few different ways to implement commands, but the cleanest and recommended way is using getter methods. We will create properties (of type Command) and return instances of classes whenever we reference those properties:

val toLow: Command
        get() = RunToPosition(motor, // MOTOR TO MOVE
            0.0, // TARGET POSITION, IN TICKS
            controller, // CONTROLLER TO IMPLEMENT
            this) // IMPLEMENTED SUBSYSTEM

Note the last parameter: subsystem. This is what tells NextFTC which commands should be allowed to run at the same time. If it weren't set, toLow would be able to run at the same time as other commands that use the Lift subsystem -- so there would be multiple things fighting to set the motor's power. Generally, you just need to pass this as the subsystem -- there are exceptions with more complicated custom commands.

Pretty easy, right? Let's duplicate it and update our variable name and target position to create our other two commands:

val toMiddle: Command
        get() = RunToPosition(motor, // MOTOR TO MOVE
            500.0, // TARGET POSITION, IN TICKS
            controller, // CONTROLLER TO IMPLEMENT
            this) // IMPLEMENTED SUBSYSTEM

val toHigh: Command
    get() = RunToPosition(motor, // MOTOR TO MOVE
        1200.0, // TARGET POSITION, IN TICKS
        controller, // CONTROLLER TO IMPLEMENT
        this) // IMPLEMENTED SUBSYSTEM

Final result

That's it! You've created your first Subsystem! Here is the final result:

object Lift: Subsystem() {
    lateinit var motor: MotorEx

    val controller = PIDFController(0.005, 0.0, 0.0, StaticFeedforward(0.0))

    val name = "lift_motor"

    val toLow: Command
        get() = RunToPosition(motor, // MOTOR TO MOVE
            0.0, // TARGET POSITION, IN TICKS
            controller, // CONTROLLER TO IMPLEMENT
            this) // IMPLEMENTED SUBSYSTEM

    val toMiddle: Command
        get() = RunToPosition(motor, // MOTOR TO MOVE
            500.0, // TARGET POSITION, IN TICKS
            controller, // CONTROLLER TO IMPLEMENT
            this) // IMPLEMENTED SUBSYSTEM

    val toHigh: Command
        get() = RunToPosition(motor, // MOTOR TO MOVE
            1200.0, // TARGET POSITION, IN TICKS
            controller, // CONTROLLER TO IMPLEMENT
            this) // IMPLEMENTED SUBSYSTEM

    override fun initialize() {
        motor = MotorEx(name)
    }
}