KRYZYS on the EvalSpace blog writes about how DC motor speed can be measured without using any external feedback device like an encoder.
During the entire cycle of the PWM modulation there are short pulses which are quite interesting, but we’ll get to them later. Now let’s focus on the “off” time. During the off time there is a small periodic waveform. Let’s call it an “off waveform”.
This waveform is created by rotor windings that are moving through magnetic field of stator magnets. I don’t want to get through details of a DC motor construction, but in general, this waveform period is the 1/6th of the of the full DC motor period (full revolution), so we can use this waveform to measure the real speed of the motor.
I built a small device using PIC16F1619 microcontroller with the Curiosity board to regulate the motor speed using this method as a feedback signal.
Above is measuring the voltage at V1 point noted below. The PWM period is 16.386 ms, and duty is set to about 50%.
The most important advantage of this method is that there is no need for any other mechanical feedback components. But there are also some disadvantages. We can measure the speed of the motor only if the “off” time of the PWM signal is long enough. It has to be at least 2 times longer than the 1/6th of the period of full motor revolution. So if the PWM duty cycle is too high, or the motor speed is too low, we won’t have a good feedback signal.
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