Big wheel low RPM speed calculation problem

[FrGe2016]

Hi

I have a robot rotating at a max of 25 RPM with 250 mm wheels. The encoders are are directly on the wheel shaft and have a low resolution of 20 tick per turns.

I am at the very beginning of my pre-operational testing. The speed loop behavior is strange.

The calculated linear velocity at start around is 1.75 m/s ( when the max speed for a 0.25 m wheel at 25 rpm is 0.33 m/s ) ???

The control loop make the wheel jog forward and backward around the starting position ???

Previously i used the same mostly the same setup with a smaller robot but at that time i had a encoder with 900 tick per turn with a motor rotating at 220 rpm with 0.06m wheels and every thing was running perfectly)

With the motor that i have it is difficult (if not imposible to have the encoder on the motor shaft )