PID Logic

How robots maintain precise position and speed automatically.

What Is PID?

PID is a control loop that automatically adjusts power to reach and maintain a target. Think of it like cruise control for your robot.

Term	What It Does	Real-World Analogy
P (Proportional)	Reacts to how far off you are	"The shower is cold, turn the hot water way up"
I (Integral)	Reacts to how long you have been off	"Still cold after 30 seconds? Turn it up more"
D (Derivative)	Reacts to how fast you are approaching	"Getting warmer! Slow down so you do not overshoot"

How It Works

graph LR
    A[Target: 5000 RPM] --> B{Error = Target - Current}
    B --> C[P: error * Kp]
    B --> D[I: accumulated error * Ki]
    B --> E[D: rate of change * Kd]
    C --> F[Output = P + I + D]
    D --> F
    E --> F
    F --> G[Motor Power]
    G --> H[Encoder reads speed]
    H --> B

PID in XBot

You do not need to write PID math yourself. XBot provides PIDManager:

// Create PID with a factory (Dagger provides the factory)
PIDManager pid = pidFactory.create(
    "ShooterPID",    // Name for dashboard tuning
    4.0, 0.0, 0.0,  // P, I, D values
    1.0, -1.0        // max output, min output
);

// Use it every loop
double output = pid.calculate(targetSpeed, currentSpeed);
motor.setPower(output);

Tuning PID Values

Problem	What To Adjust
Too slow reaching target	Increase P
Oscillates (wobbles around target)	Decrease P, increase D
Never reaches target (steady error)	Add some I
Overshoots then corrects	Increase D

Properties for Tuning

XBot stores PID values as properties so you can tune them at runtime without rebuilding:

// In ShooterSubsystem constructor
this.pid = pidFactory.create("ShooterPID", propertyFactory, 4.0, 0.0, 0.0, 1.0, -1.0);

// Change "ShooterPID/P" from 4.0 to 3.0 on the dashboard
// The robot adjusts immediately without redeploying code!

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Quiz

Q1: What does the P (Proportional) term do?

• [ ] A) Predicts future error
• [ ] B) Reacts to current error (how far off you are)
• [ ] C) Accumulates error over time
• [ ] D) Sets the motor to full power

Answer

B) Reacts to current error

P adjusts output based on how far you are from the target. Big error = big adjustment. Small error = small adjustment. It is the main term that does the work.

Q2: If the motor oscillates (wobbles around the target), what should you try?

• [ ] A) Increase P
• [ ] B) Increase I
• [ ] C) Decrease P and/or increase D
• [ ] D) Do nothing, oscillations are normal

Answer

C) Decrease P and/or increase D

Oscillation means P is too aggressive. Lower P so the corrections are smaller, or increase D to dampen the overshoot.

Q3: Why does XBot store PID values as properties?

• [ ] A) To make the code slower
• [ ] B) So you can tune them without rebuilding the code
• [ ] C) Because Dagger requires it
• [ ] D) To save disk space

Answer

B) So you can tune them without rebuilding the code

Properties let you change PID values on the dashboard while the robot is running. This is essential at competitions where every second counts -- you can tune in real time instead of rebuilding and redeploying.