Working on motor control had always been my passion. For this project, I thought of working on something simpler in hardware. Hence it came out as a speed closed loop control of DC motor using back emf sensing.

Running a DC motor is as simple as glowing a LED. The schematic is mentioned below:


Running a DC motor in one direction, just replace the LED with DC motor and remove the current limit resistor. Please note that motor - direction reversal is not possible with this configuration. The motor speed can be varied by varying the duty cycle of PWM.
But a common problem 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.
For sensing the motor speed and feeding it back to controller, usually, some transducer (sensor) is used such as: -

1. Tachogenerator (analog output).
2. Quadrature encoder (pulse output).
3. Resolver (Not used unless it is flight control or defense application).
4. Photo interrupter (common one, can be found in older ball type mouse).

These sensors are great, but all of them cost money. How do we make a closed loop control of normal DC motor, which does not come with any sensor?
Well, the technique does exist and it uses the back emf of motor to sense the speed of motor. Please refer to the diagram below: -


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.

Some advantage of using this control scheme: -

1. Cost effective (This is the most important and mostly sought after reason).
2. Easy on hardware design.
3. Can be retrofitted on systems where motor does not come along with the encoder.
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.

Where this scheme cannot be used: -

1. Cannot be used to control the motor at extremely low speeds as motor does not generate sufficient back emf.
2. Cannot be used in system where torque ripple cannot be tolerated. Stopping the PWM for back emf measurement causes torque ripple.
3. If the motor is configured to run on both direction (using H bridge), then the back emf measurement circuit becomes complicated (differential measurement).