Add coupling ratio

[thenetworkgrinch]

TBD on how to calculate it but apply using CTRE template base.

[thenetworkgrinch]

https://www.chiefdelphi.com/t/ctre-start-of-2024-open-alpha-for-phoenix-6/441193/19

Theory is turns_encoder:turns_drive found empirically by rotating the module 360deg and marking the bottom center before and after. Theorhetically by encoder * 2nd * 3rd = wheel rot and encoder / 1st = drive rot

[thenetworkgrinch]

source here https://api.ctr-electronics.com/phoenix6/release/java/src-html/com/ctre/phoenix6/mechanisms/swerve/SwerveModule.html#line.228

[thenetworkgrinch]

• Add glossary of coupling ratio's to docs

[thenetworkgrinch]

rotate a few times for best measirement

[thenetworkgrinch]

(if you use drive rotations as the basis instead of wheel rotations) is to lock the drive wheel in place, rotate the azimuth a few times, look at the number of rotations reported by the drive motor, drive rotations / azimuth rotations = coupling ratio

[thenetworkgrinch]

Taken from discord

(or other ctre folks)- I think there might be a mistake in the coupling correction code in phoenix 6's SwerveModule. Here's a derivation to help explain:

P_raw = drive motor position in rotations
P_comp = drive motor position, compensated for coupling error (ie, the true distance travelled by the wheel, in units of motor rotations)
V_raw = drive motor velocity in rotations per second
V_comp = drive motor velocity, compensated for coupling error (ie, the true velocity of the wheel, in units of motor rotations per second)
azimuth_rot = rotations of azimuth
azimuth_vel = velocity of azimuth in rotations per second
coupling_ratio = drive motor rotations per rotation of azimuth

P_comp = P_raw - (azimuth_rot * coupling_ratio)

We can extend this to velocity like so:

V_comp = V_raw - (azimuth_vel * coupling_ratio)

We then invert the function to solve for V_raw given V_comp (in the case of converting from a desired wheel speed to the necessary motor speed)

V_raw = V_comp + (azimuth_vel * coupling_ratio)

The first function matches the encoder correction implementation on line 250 of SwerveModule:

drive_rot -= angle_rot * m_couplingRatioDriveRotorToCANcoder;

However the inverse equation doesn't match the drive velocity correction implementation on line 324-327. Instead of adding the correction factor (azimuth_vel * coupling_ratio) to the desired speed V_comp, it is subtracted:

double azimuthTurnRps = m_steerVelocity.getValue();
/* Azimuth turn rate multiplied by coupling ratio provides back-out rps */
double driveRateBackOut = azimuthTurnRps * m_couplingRatioDriveRotorToCANcoder;
velocityToSet -= driveRateBackOut;

This would have the effect of doubling the coupling error when setting velocity.