Robot Tuning Guide
Measure the robot. Set up the sensors. Tune the movement. Then build your auton.
1. Measure the robot
Your code must match the real robot. Wrong numbers can't be fixed with PID.
Drive wheel diameterMeasure the wheel on the field. Worn wheels can be smaller.
Tracking wheel diameterMeasure the wheel your odometry uses.
Tracking wheel offsetDistance from the wheel to the robot's center of turning.
Track widthDistance between the left and right wheels.
Robot widthNeeded to plan paths that fit.
Robot lengthFront to back.
Don't guess. A wrong wheel size makes the robot drive too far or too short every time.
Measure each one a few times. Write the numbers down before you type them in.
2. Odometry
Odometry tracks where the robot is. Check these four things:
1
Wheel diameter. It sets how far one spin goes.
2
Wheel position. Where the tracking wheel sits on the robot.
3
Sensor direction. Forward should count up.
4
Heading. Turn the robot and check the reading.
Tip: Fix odometry before you touch PID.
3. IMU
1
Mount it tight. It must not wiggle.
2
Calibrate with the robot still.
3
Test turns. Turn 90°, 180° and 360°. Compare to the reading.
If turns are always off, check the IMU first. Don't change turn PID yet.
4. EZ-Template
It has separate PID for driving, heading and turning. It also has a built-in PID tuner.
Drive
chassis.pid_drive_set(24_in, 110);
chassis.pid_wait();That's a distance and a max speed (0–127).
EZ PID
void initialize() {
// Drive
chassis.pid_drive_constants_set(10, 0, 100);
// Heading
chassis.pid_heading_constants_set(3, 0, 20);
// Turn
chassis.pid_turn_constants_set(3, 0, 20);
}| Term | What it does |
|---|---|
| P | Pushes harder when far from the target. |
| I | Adds push if you stay off target. |
| D | Slows you down as you get close. |
Set P first. Add D to stop overshoot. Only add I if you really need it.
EZ Odom
void autonomous() {
chassis.pid_targets_reset();
chassis.drive_imu_reset();
chassis.drive_sensor_reset();
chassis.odom_xyt_set(0_in, 0_in, 0_deg);
chassis.drive_brake_set(MOTOR_BRAKE_HOLD);
chassis.pid_odom_set({{24_in, 24_in}, ez::fwd, 110});
chassis.pid_wait();
}Start is (0, 0) at 0°. Right is +X. Forward is +Y. Clockwise is +theta.
Check that the planner's axes match your code.
EZ Slew
Slew limits how fast speed ramps up. It gives smoother starts.
chassis.slew_drive_set(true);
chassis.slew_drive_constants_set(5_in, 50);
chassis.pid_drive_set(24_in, 110);
chassis.pid_wait();Tune PID first. Don't use slew to hide bad PID.
5. LemLib
Same plan: set up the robot, set up sensors, then tune.
1
Motors. Check ports, directions and gearing.
2
Tracking wheels. Enter diameter and offset.
3
IMU. Mount it and match the code.
4
Chassis. Put it all together.
Setup
Change the ports, sizes and offsets to match your robot.
// Drivetrain
lemlib::Drivetrain drivetrain(
&leftMotors,
&rightMotors,
11.5, // track width
lemlib::Omniwheel::NEW_275, // wheel
450, // wheel RPM
2 // horizontal drift
);
// IMU
pros::Imu imu(6);
// Tracking wheel
pros::Rotation verticalEncoder(5);
lemlib::TrackingWheel vertical(
&verticalEncoder,
lemlib::Omniwheel::NEW_275,
-2.5 // offset
);
// Odometry sensors
lemlib::OdomSensors sensors(
&vertical, // vertical 1
nullptr, // vertical 2
nullptr, // horizontal 1
nullptr, // horizontal 2
&imu
);PID
One controller for driving. One for turning.
// Forward / backward
lemlib::ControllerSettings lateral_controller(
10, // kP
0, // kI
3, // kD
3, // anti-windup
1, // small error range
100, // small error timeout
3, // large error range
500, // large error timeout
20 // slew
);
// Turning
lemlib::ControllerSettings angular_controller(
2, // kP
0, // kI
10, // kD
3, // anti-windup
1, // small error range
100, // small error timeout
3, // large error range
500, // large error timeout
0 // slew
);
lemlib::Chassis chassis(
drivetrain,
lateral_controller,
angular_controller,
sensors
);These numbers are only a start. Tune them for your robot.
Calibrate
void initialize() {
chassis.calibrate();
}Move
void autonomous() {
chassis.setPose(0, 0, 0); // start X, Y, heading
chassis.moveToPoint(0, 24, 3000); // X, Y, timeout (ms)
chassis.turnToHeading(90, 2000); // heading, timeout (ms)
chassis.moveToPoint(24, 24, 3000);
chassis.waitUntilDone();
}Read position
lemlib::Pose pose = chassis.getPose();
double x = pose.x;
double y = pose.y;
double heading = pose.theta;LemLib PID
Tune one thing at a time:
1
Straight driving.
2
Turning. Try 90° and 180°.
3
Staying straight.
4
Point to point.
Drives fine but turns badly? Leave drive PID alone. Check turn PID and the IMU.
void autonomous() {
chassis.setPose(0, 0, 0);
chassis.moveToPoint(0, 24, 3000);
chassis.waitUntilDone();
chassis.turnToHeading(90, 2000);
chassis.waitUntilDone();
}LemLib Odom
Good odometry needs correct wheel sizes and offsets.
1
Put the robot on a known spot.
2
Reset the position in code.
3
Drive a distance you know.
4
Compare the code's position to the real one.
5
Repeat in another direction.
Error grows with distance? Check wheel diameter. Error appears in turns? Check offsets and heading.
LemLib vs EZ-Template
Don't copy PID numbers between them.
| Idea | EZ-Template | LemLib |
|---|---|---|
| Drive | Drive PID | Lateral controller |
| Turn | Turn PID | Angular controller |
| Heading | Heading PID | Built in |
| Paths | Odom / Boomerang / PP | moveToPoint, moveToPose |
| Ramp up | Slew | Controller slew |
6. Test order
1
Check the robot. Wheels, axles, sensor mounts.
2
Measure it.
3
Check sensors. Right direction?
4
Check odometry.
5
Tune drive PID.
6
Tune turn PID.
7
Tune point to point.
8
Build the auton.
7. Fixes
| Problem | Check first |
|---|---|
| Goes too far or too short | Wheel diameter |
| Drives in an arc | Drivetrain alignment, motor setup, heading |
| Overshoots | More D, less P |
| Shakes back and forth | P is too high |
| Stops short | Gains, exit settings, friction |
| 90° turn is off | IMU, track width, turn PID |
| Position drifts after turns | Tracking offsets, heading |
| Different every run | Battery, wheel slip, loose sensors |
8. Checklist
Remember: Measure, set up, calibrate, tune, test. Don't change numbers at random.