5. Pneumatics#

Why it matters#

Pneumatics use compressed air to push and pull pistons. They're great for mechanisms that only need two positions, moved fast and with force:

• a claw: open or closed
• an intake: deployed or stowed
• a gearbox: high gear or low gear
• a climber hook: extended or retracted

Pneumatics are simple to control in code (usually just "extend" or "retract"), but they come with their own hardware, safety rules, and failure modes. Many teams have moved away from pneumatics in recent years, but they still show up often, so you should know how they work.

Read this#

• Wiring Pneumatics - REV Pneumatic Hub
• Pneumatics APIs

Walkthrough#

The air path#

Why two pressures?#

The tanks store air at high pressure (up to about 120 psi) so they hold a lot of it. The mechanisms run at a lower working pressure (about 60 psi) from the regulator, which keeps the force steady and stays within the rules. As the tanks drain during a match, the working pressure stays the same until the tanks drop below it.

How a cylinder moves#

A double-acting cylinder has two ports. Air into one side pushes the piston out; air into the other side pulls it in. The solenoid valve decides which side gets air.

Single vs. double solenoids:

Code it#

WPILib needs to know which control module you have. Use PneumaticsModuleType.REVPH for a REV Pneumatic Hub or PneumaticsModuleType.CTREPCM for a CTRE PCM.

A double solenoid:

import edu.wpi.first.wpilibj.DoubleSolenoid;
import edu.wpi.first.wpilibj.PneumaticsModuleType;
import edu.wpi.first.wpilibj2.command.Command;
import edu.wpi.first.wpilibj2.command.SubsystemBase;

public class Claw extends SubsystemBase {
    // Forward channel 0, reverse channel 1 on the Pneumatic Hub
    private final DoubleSolenoid piston =
        new DoubleSolenoid(PneumaticsModuleType.REVPH, 0, 1);

    public Command open() {
        return runOnce(() -> piston.set(DoubleSolenoid.Value.kForward));
    }

    public Command close() {
        return runOnce(() -> piston.set(DoubleSolenoid.Value.kReverse));
    }

    public Command toggle() {
        return runOnce(piston::toggle);
    }
}

A single solenoid:

private final Solenoid intakeDeploy = new Solenoid(PneumaticsModuleType.REVPH, 2);

public Command deploy() { return runOnce(() -> intakeDeploy.set(true)); }
public Command stow()   { return runOnce(() -> intakeDeploy.set(false)); }

Bind them to buttons in RobotContainer:

driver.leftBumper().onTrue(claw.open());
driver.rightBumper().onTrue(claw.close());

Each command is runOnce because setting a solenoid is instant: it sends the signal and is done. The air does the rest.

The compressor#

The compressor runs automatically by default whenever the robot is enabled and the tanks aren't full. You can create a Compressor object to check it or change its behavior:

private final Compressor compressor = new Compressor(PneumaticsModuleType.REVPH);
private final TelemetryTable telemetry = Telemetry.getTable("Pneumatics");   // Lesson 6.1

@Override
public void periodic() {
    telemetry.log("Compressor Running", compressor.isEnabled());
    telemetry.log("Pressure (psi)", compressor.getPressure());
}

getPressure() only works if the robot has an analog pressure sensor wired to the Pneumatic Hub.

Troubleshooting#

Try it#

With a mentor, on a robot with pneumatics:

• Find each part in the table: compressor, tanks, relief valve, regulator, gauges, solenoids, cylinders, dump valve
• Trace the air from the compressor to one cylinder
• Find which solenoid channels your team's mechanism uses
• Write open() and close() commands and bind them to two buttons
• Put the compressor status on the dashboard and watch it turn off when the tanks are full
• Vent the system with the dump valve and confirm both gauges read zero

Check yourself#