Lesson 2: What is a Subsystem?

• Lesson 2: Subsystems

  • What is a Subsystem?

    • Constructing a Subsystem

    • Adding Methods

    • Outputting Data

What is a Subsystem?

Subsystems are the basic unit of robot organization in the command-based paradigm.

Every robot that we design has "subsystems" of hardware that work together to serve a purpose. For example, on our 2017 robot, we had a drivetrain, a climber, an agitator, a launcher, and a gear manipulator. These are treated as seperate units by the mechanics, and we adopt this structure in our code.

The purpose of this is to provide modularity - it's much more difficult to program a robot if it's all in one class file. Additionally, command-based programming allows for "locks" on subsystems, so that for example the agitator can't be both agitating and not agitating at the same time. But we'll get more into that later - the important takeaway is that subsystems are collections of hardware operating as units of the robot.

Subsystems have a general structure to them. A barebones subsystem would look like this:

import edu.wpi.first.wpilibj2.command.SubsystemBase;

public class ShooterSubsystem extends SubsystemBase {

    // Creates a new ShooterSubsystem.
    public ShooterSubsystem() {

    }

    // Example actions as methods go here...
    // public void raiseArm()

    public void outputTelemetry() {

    }
}

We'll go over every part of this.

Constructing a Subsystem

That ShooterSubsystem method is the constructor (a Java term you should be familiar with). You're going to use this to initialize the hardware controlled by your subsystem.

¡ojo! It's very bad to initialize the same hardware twice. You cannot have the same sensor or motor in two subsystems - but it's still possible to use it in both.

As a matter of fact, you've already done this with the TalonSRX motor last lesson. This is essentially the same concept - define the motor member variable outside of the constructor, and initialize it inside the constructor.

• Try doing that now with a TalonSRX. Make sure you use the correct port.

Adding Methods

We talked earlier about how subsystems are units of hardware that serve a common purpose. The subsystem controlls and coordinates this hardware.

Be careful! You want these actions to be as simple as possible. Instead of having an action that, say, makes the robot climb, break it into individual actions. If this was CHAOS (our 2019 robot), you'd have four actions - extending and retracting the front set of pneumatics, and extending and retracting the back set of pneumatics.

We allow our subsystem to be controlled by public methods. You should design a public interface that best "fits" the underlying hardware in an easy-to-use way (for the caller). Note that all the "timing" of the methods is controlled by the commands.

Commands, which we'll talk about and use in the next lesson, call into these public methods. Additionally:

Subsystems also serve as the backbone of the CommandScheduler’s resource management system. Commands may declare resource requirements by specifying which subsystems they interact with; the scheduler will never concurrently schedule more than one command that requires a given subsystem. An attempt to schedule a command that requires a subsystem that is already-in-use will either interrupt the currently-running command (if the command has been scheduled as interruptible), or else be ignored.

• You should put a couple of public methods into your subsystem that run the motor at a varying velocity and "unjam" the shooter by running the motor backwards. One should be void shootBall(double dutyCycle), and the other should be void unjam().

• Additionally, create a third method stop() - try and guess what to do with this one.

• Now put code into each method to control the TalonSRX appropriately.

• Now add some code to make the motor only run forward, regardless of input, in the shootBall method.

Outputting Data

The last method is one called outputTelemetry - this method is our way of logging data about the robot. This is called repeatedly in very short time intervals to provide us with up-to-date data. This information is sent back to the dashboard and used for debugging. You should log information about the state of your subsystem in this method.

Our convention is to use the methods dashboardPutBoolean, dashboardPutNumber, and dashboardPutString to send out logs. Each one of those takes a descriptive String, followed by either a boolean, int, or String (guess which method takes which!).

• Send back some information about what the motor is currently doing and find where that information on the Network Tables page of the dashboard.