Experiment No. 5: DC Motor Speed Control using Pulse Width Modulation (PWM)

Introduction

PIC12F683 has a built-in PWM module. The PWM output (CCP1 pin) is multiplexed with GPIO.2 (pin 5). So the TRISIO<2> bit must be cleared to make the CCP1 pin an output.
The objective of this experiment is to control the speed of a DC motor with an input from a potentiometer. This will be achieved in two steps:

• Read analog value (potentiometer output) through AN0 channel and generate a PWM wave at CCP1 pin (5) with duty cycle proportional to the analog value.
• Feed the PWM to motor driver transistor and observe the speed.

mikroC library routines for PWM

PWM1_Init

PWM1_Set_Duty

PWM1_Start

PWM1_Stop

Since PWM1_Set_Duty library function takes duty cycle input as an 8-bit integer, the digital value of analog signal, which is 10-bit, must be converted to 8-bit first. This will give 256 speed levels (0-255).

Experimental Setup
Connect POT2 output to pin 7 (AN0) of PIC12F683 and pin 5 (CCP1) to the input of motor driver transistor.

Software

/*
  PIC12F683 Experiment Board
  Experimen No. 5 : DC Motor Speed Control using PWM
  Date: 07/28
  /2010
*/
unsigned short DutyCycle=0;
unsigned int adc_value;


void main() {
CMCON0 = 7;
TRISIO = 9;  // GPIO 0 and 3 Inputs; Rest are O/Ps
ANSEL = 0x01;
GPIO = 0;


PWM1_Init(5000); // PWM module initialization (5KHz)
PWM1_Start(); // Start PWM1 module with Zero DC
PWM1_Set_Duty(DutyCycle);
do {
 adc_value = ADC_Read(0);
 DutyCycle = adc_value >> 2;  // Convert 10-bit ADC value to 8-bit
 PWM1_Set_Duty(DutyCycle);
  } while(1);
}


Output

The speed of the DC motor is controlled with the potentiometer.