IMU Scaling

note

This is not required for consistent autonomous routines, it only makes the correlation between what the robot thinks it's doing and what it's actually doing match more closely.

Coming from 3.2.x?

In 3.2.x, drive_imu_scaler_set() took a multiplier like 1.007. In 4.0 it was renamed to drive_imu_scaler_3600_set() and takes what the imu reads after a 3600 degree turn, so a call from 3.2.x will not compile. Remove the old line and follow the steps below to calibrate again.

drive_imu_scaler_get(), drive_imus_scalers_set() and drive_imus_scalers_get() were renamed the same way, with _3600 added.

What is it?

No imu reports exactly 1 degree of rotation for every degree the robot actually turns. Rather than hand-tune a scaling factor by trial and error, you calibrate it directly: physically turn the robot 3600 degrees (10 full rotations) and tell EZ-Template what the imu reported for that turn. Internally, the imu's readings are corrected so that value maps back to a true 3600.

Tuning

Place your robot carefully, ideally aligned with tiles on the field so you know it's facing forward, and make sure nothing is calling drive_imu_scaler_3600_set() yet, since a freshly constructed drive's imu is unscaled.

Physically turn the robot exactly 3600 degrees (10 full rotations) by hand. Don't use an autonomous routine for this. pid_turn_set() stops when the imu reads its target, so the imu would report 3600 no matter how far the robot really turned. Line the robot up with a tile edge at the start and after every rotation so you know each one was a full turn.

While you turn it, show what the imu reports on a blank page. Add this to ez_screen_task() in main.cpp, under // Add your own blank pages here!, and upload it. In the example project blank page 0 shows odometry and blank page 1 shows motor temperatures, so this is page 2. If you've added pages of your own, use the next number that's free. Then go left on the autonomous selector until you're on the page. Blank Pages explains how they work.

else if (ez::as::page_blank_is_on(2)) {
  ez::screen_print("IMU: " + ez::util::to_string_with_precision(chassis.drive_imu_get()), 1);  // What the imu has measured so far, in degrees
}

The imu reads 0 once it finishes calibrating, so wait for the loading bar to finish before you start turning the robot. Once you've finished the 10th rotation, write down the number on the screen.

Whatever that number is, pass it directly into drive_imu_scaler_3600_set(). You'll do this by adding this line of code to default_constants() in src/autons.cpp.

chassis.drive_imu_scaler_3600_set(3625.42);  // Whatever your imu printed above

No trial and error needed, this is a one-shot calibration. For drives built with the redundant IMU constructor, repeat this per imu and pass all the values to drive_imus_scalers_3600_set() at once, in the same order as the IMU ports passed to the constructor.

You're Done!

That's it! Your IMU is now tuned!