PID Control Documentation

Overview

PID (Proportional-Integral-Derivative) control is a feedback control mechanism used in our robot's motion control system. LemLib implements PID controllers for both lateral (forward/backward) and angular (turning) movements.

Current PID Settings

Lateral Controller (Forward/Backward Movement)

lemlib::ControllerSettings lateral_controller(
    10,  // kP - Proportional gain
    0,   // kI - Integral gain  
    3,   // kD - Derivative gain
    3,   // Anti-windup
    1,   // Small error range (inches)
    100, // Small error timeout (ms)
    3,   // Large error range (inches)
    500, // Large error timeout (ms)
    20   // Maximum acceleration (slew)
);

Angular Controller (Turning)

lemlib::ControllerSettings angular_controller(
    2,   // kP - Proportional gain
    0,   // kI - Integral gain
    10,  // kD - Derivative gain
    3,   // Anti-windup
    1,   // Small error range (degrees)
    100, // Small error timeout (ms)
    3,   // Large error range (degrees)
    500, // Large error timeout (ms)
    0    // Maximum acceleration (slew)
);

PID Components Explained

Proportional (P)

• What it does: Provides output proportional to the current error
• Effect: Higher values = more aggressive response to error
• Tuning: Start low and increase until oscillation begins

Integral (I)

• What it does: Accumulates error over time to eliminate steady-state error
• Effect: Helps reach exact target but can cause overshoot
• Tuning: Usually kept at 0 or very low values in FRC/VEX

Derivative (D)

• What it does: Predicts future error based on rate of change
• Effect: Reduces overshoot and oscillation
• Tuning: Add after P is tuned to smooth response

Tuning Guidelines

Step-by-Step Tuning Process

1. Start with P only (I=0, D=0)
2. Increase P until robot oscillates around target
3. Reduce P slightly to eliminate oscillation
4. Add D to reduce overshoot and smooth response
5. Add I only if steady-state error exists (rare in VEX)

Common Issues and Solutions

Problem	Likely Cause	Solution
Robot overshoots target	P too high	Reduce P, add D
Robot doesn't reach target	P too low	Increase P
Robot oscillates	P too high, D too low	Reduce P, increase D
Slow response	P too low	Increase P
Steady-state error	Need integral action	Add small I value

Testing and Validation

Recommended Tests

1. Step Response: Move robot 24 inches forward
2. Turn Test: Rotate robot 90 degrees
3. Repeatability: Run same movement 10 times
4. Different Distances: Test 12", 24", 48" movements

Performance Metrics

• Settling Time: Time to reach and stay within ±1" of target
• Overshoot: Maximum distance past target
• Steady-State Error: Final error after settling
• Rise Time: Time to reach 90% of target

Advanced Features

Anti-Windup

• Prevents integral term from growing too large
• Set to 3 in our configuration
• Helps prevent overshoot when changing directions

Error Ranges

• Small Error Range: Tolerance for "close enough" (1" or 1°)
• Large Error Range: Tolerance for major corrections (3" or 3°)
• Timeouts: Maximum time to spend in each error range

Slew Rate Limiting

• Lateral: 20 units/second maximum acceleration
• Angular: 0 (disabled) for turning
• Prevents wheel slipping and mechanical stress

Code Integration

The PID controllers are integrated into the chassis object:

lemlib::Chassis chassis(
    drivetrain,           // drivetrain settings
    lateral_controller,   // lateral PID settings
    angular_controller,   // angular PID settings
    sensors              // odometry sensors
);

Future Improvements

• Tune PID values through testing
• Implement feedforward control
• Add velocity PID for smoother motion
• Create automated tuning routine
• Log PID performance data

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Last updated: 2025-08-25