06-04-2012, 03:34 PM
Speed Control of DC Motor using DSP
Abstract :
Speed control of a DC Motor using Pulse Width Modulated signals from ePWM unit of TMS320F28335 DSP is proposed here. With pulse width modulation we can represent any analog signal as a digital signal of fixed amplitude, fixed carrier frequency. The pulse width of this modulated signal depends on the amplitude of the original analog signal.
The speed of a DC motor is directly proportional to the supply voltage, so if we reduce the supply voltage to half the rated value, the motor will run at half the speed. The speed controller works by varying the average voltage sent to the motor. Adjusting the voltage sent to the motor is one option to achieve this. But this is quite inefficient. A better way is to switch the motor's supply on and off very quickly. If the switching is fast enough, the motor doesn't notice it, it only notices the average effect. When the switch is closed, the motor sees full rated value, and when it is open it sees 0 Volts. If the switch is open for the same amount of time as it is closed, the motor will see an average of half of the input voltage, and will run more slowly accordingly. Instead of a switch circuit we can use ePWM unit of the DSP to generate pulse width modulated signals of the required duty ratio to achieve the same effect. We can achieve speed regulation corresponding to 0 – 100% duty cycle of the ePWM signal.
In our project we will be achieving speed regulation of a DC shunt motor using the ePWM unit of the DSP TMS320F28335. The duty cycle of the ePWM unit will be varied in order to vary the average voltage as seen by the motor. Correspondingly the speed of the motor varies. A look up table will be included to portray the speed variation with duty ratio.