I am looking to develop a RPM sensor to measure the actual RPM of quad rotor motors continuously during flight. My search so far has resulted in mostly sensors that gets the RPM from the ESC, not the actual RPM from the motor.
I have looked for a sensor that can do this without any luck. I am open to making a system that can do this, but it must be able to fit within the quadrotor frame. Any help in terms of a commercial solution (not too expensive, under $100 definitely) or a DIY solution would be greatly appreciated. Thanks!