How to get rotor speed?

[PabloRauhCorro]

Hello,

I would like to know the rotor angular rates of my quadcopter. Is it somehow possible to publish a topic with this info? How would I go about this?

There is a ros topic /motor_speed, however this is not the speed of the rotor itself. Is it possible to infer the rotor speed from the motor speed? For low motor speeds, it seems to be a simple linear relation, but I don't know if this holds true for higher motor speeds.

Any help or input would be appreciated.