DC motor RPM accuracy/precision
For applications with only moderate requirements you can implement closed loop control without a sensor by measuring the back-EMF of the motor.
You can even have a purely analog solution where the motor is driven by a power supply with negative output resistance to compensate for the motor resistance - this was used in Philips cassettes recorders 40 years ago.
This is a similar circuit to that used. The resistor Rs should be equal to the motor resistance measured when the motor is not rotating. The voltage across Rs increases with motor current and is fed back to increase the applied voltage to compensate for the voltage dropped by the internal motor resistance. The equivalent circuit in section B of the diagram helps explain the operation.
(Image from Precisionmicrodrives)