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Odometry Documentation

Overview​

Odometry is the process of tracking the robot's position and orientation on the field using sensor data. Our robot uses a combination of tracking wheels and an IMU (Inertial Measurement Unit) to achieve precise position tracking through LemLib.

Hardware Configuration​

Tracking Wheels​

Our robot uses two tracking wheels for odometry:

// Horizontal tracking wheel (perpendicular to robot's forward direction)
lemlib::TrackingWheel horizontal_tracking_wheel(
    &horizontal_sensor,           // Rotation sensor on port 8
    lemlib::Omniwheel::NEW_275,  // 2.75" omni wheel
    -5.75                        // Distance from tracking center (inches)
);

// Vertical tracking wheel (parallel to robot's forward direction)
lemlib::TrackingWheel vertical_tracking_wheel(
    &vertical_sensor,            // Rotation sensor on port 9
    lemlib::Omniwheel::NEW_275, // 2.75" omni wheel
    -5.75                       // Distance from tracking center (inches)
);

IMU (Inertial Measurement Unit)​

pros::Imu imu(7);  // IMU on port 7 for heading measurement

Sensor Configuration​

lemlib::OdomSensors sensors(
    &vertical_tracking_wheel,   // Vertical tracking wheel 1
    nullptr,                    // Vertical tracking wheel 2 (not used)
    &horizontal_tracking_wheel, // Horizontal tracking wheel 1
    nullptr,                    // Horizontal tracking wheel 2 (not used)
    &imu                       // Inertial sensor
);

How Odometry Works​

Basic Principle​

  1. Tracking wheels measure linear movement in X and Y directions
  2. IMU provides accurate heading (rotation) measurement
  3. LemLib combines this data to calculate robot position
  4. Position is updated continuously in real-time

Coordinate System​

  • X-axis: Left/Right movement (positive = right)
  • Y-axis: Forward/Backward movement (positive = forward)
  • Theta: Robot heading in degrees (0° = facing forward)

Tracking Wheel Placement​

Vertical Tracking Wheel​

  • Purpose: Measures forward/backward movement (Y-axis)
  • Placement: Perpendicular to robot's main axis
  • Offset: -5.75" from robot center
  • Why offset matters: Accounts for wheel placement relative to robot's center of rotation

Horizontal Tracking Wheel​

  • Purpose: Measures left/right movement (X-axis)
  • Placement: Parallel to robot's main axis
  • Offset: -5.75" from robot center
  • Critical for: Detecting sideways drift during turns

Calibration Process​

Initial Calibration​

void initialize() {
    chassis.calibrate(); // Calibrates IMU and resets position to (0,0,0)
}

What Calibration Does​

  1. IMU Calibration: Determines IMU bias and drift
  2. Position Reset: Sets current position as origin (0, 0, 0°)
  3. Sensor Zeroing: Resets all tracking wheel encoders

Calibration Best Practices​

  • Keep robot completely still during calibration
  • Place robot on level surface
  • Allow 2-3 seconds for calibration to complete
  • Don't move robot until calibration finishes

Position Tracking Display​

Our code displays real-time position on the brain screen:

pros::Task screen_task([&]() {
    while (true) {
        pros::lcd::print(0, "X: %f", chassis.getPose().x);     // X position
        pros::lcd::print(1, "Y: %f", chassis.getPose().y);     // Y position  
        pros::lcd::print(2, "Theta: %f", chassis.getPose().theta); // Heading
        pros::delay(20);
    }
});

Accuracy Factors​

Sources of Error​

  1. Wheel Slippage: Tracking wheels slipping on field tiles
  2. Mechanical Play: Backlash in gears and mounting
  3. IMU Drift: Small errors accumulating over time
  4. Wheel Wear: Changes in effective wheel diameter
  5. Field Conditions: Debris, uneven tiles, or damage

Improving Accuracy​

  • Proper wheel tension: Ensure tracking wheels contact ground firmly
  • Quality bearings: Reduce friction and play
  • Rigid mounting: Minimize flex in tracking wheel assemblies
  • Regular calibration: Recalibrate between matches
  • Wheel maintenance: Keep tracking wheels clean and round

Common Issues and Troubleshooting​

Position Drift​

Symptoms: Robot position slowly drifts even when stationary Causes:

  • IMU not properly calibrated
  • Tracking wheels slipping
  • Mechanical issues

Solutions:

  • Recalibrate IMU
  • Check tracking wheel contact with ground
  • Verify sensor wiring

Incorrect Heading​

Symptoms: Robot thinks it's facing different direction Causes:

  • IMU mounting orientation
  • IMU calibration issues
  • Electromagnetic interference

Solutions:

  • Ensure IMU is mounted flat and secure
  • Calibrate away from motors and metal
  • Check for loose connections

Inaccurate Distance Measurement​

Symptoms: Robot reports wrong X/Y position Causes:

  • Wrong wheel diameter in code
  • Tracking wheel slippage
  • Incorrect offset measurements

Solutions:

  • Verify wheel diameter (should be 2.75")
  • Measure and verify offset distances
  • Check wheel mounting and tension

Testing and Validation​

  1. Stationary Test: Leave robot still for 30 seconds, position should not drift
  2. Straight Line Test: Drive 48" forward, measure actual vs. reported distance
  3. Square Test: Drive in 24"×24" square, should return to start position
  4. Rotation Test: Spin 360°, heading should return to 0°
  5. Complex Path: Drive figure-8 pattern, check for cumulative error

Acceptable Tolerances​

  • Linear accuracy: ±1" after 48" movement
  • Angular accuracy: ±2° after 360° rotation
  • Drift rate: less than 0.1" per second when stationary

Integration with Motion Control​

Odometry enables advanced autonomous features:

  • Pure Pursuit: Follow curved paths smoothly
  • Point-to-Point: Drive directly to field coordinates
  • Path Following: Execute pre-planned routes
  • Field-Relative Control: Move relative to field, not robot

Future Improvements​

  • Add sensor fusion with vision system
  • Implement Kalman filtering for better accuracy
  • Add wheel slip detection
  • Create field position validation system
  • Log odometry data for analysis

Last updated: 2025-08-25