“For controlling DC motor speed, often the PWM method is used. Using this method you can accelerate or decelerate the motor speed, but without any feedback, the speed of the motor strongly depends on the conditions, like the motor load, or the voltage source.
If you want to know the exact speed of the motor, you can use a feedback device (e.g. an encoder). With information from the feedback device, you can adjust the duty cycle of the PWM signal to keep the motor at the desired speed.
In this article I would like to show how the DC motor speed can be measured without using any external feedback device. Take a look at the schematic below. It’s a standard circuit for a PWM controlled DC motor.”