Closed loop speed control of DC motor using back emf sening

Posted on Feb 6, 2014

A speed closed loop control of DC motor using back emf sensing. Running a DC motor in one direction, just replace the LED with DC motor and remove the current limit resistor. Please note that motordirection reversal is not possible with this configuration. The motor speed can be varied by varying the duty cycle of PWM. But a common problem

Closed loop speed control of DC motor using back emf sening
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with this schematic for DC motor control is that if the motor is loaded (mechanical load), the motor speed reduces. In other words, the system is running in open loop. If we have to make a system as closed loop, we should have a mechanism to feedback the motor speed back into the controller (PIC18F4520). Only then, the controller will know the present speed of motor and vary the duty cycle of PWM to compensate for any change in mechanical load. A DC motor when driven by external means, will generate a voltage. This voltage is also generated by the motor, when it is supplied by a DC source. This voltage is proportional to the speed of motor and is called the back emf. Best part is that the back emf is linearly proportional to the speed. To measure the back emf of motor, we stop the PWM driver for a brief period. During this brief period, the motor coasts for sometime when the current flows through the freewheeling diode. Once the energy stored in motor inductance is exhausted, the back emf build up. This back emf is scaled to suitable voltage using the potential divider. The back emf signal is then fed into the ADC input of controller. 4. Small motors with gear used for hobby robotics usually run at high speed (gear runs at low speed, gear reduction). Such motor can easily use this scheme. 3. If the motor is configured to run on both direction (using H bridge), then the back emf measurement circuit becomes complicated...

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