DC motor control using PIC

 When using PIC to control DC motor, the motion of motor such as forward, reverse,and speed of rotation can be programmed. If the motion of motor only required forward and reverse without considering the speed of rotation, motor can be control through relay module. the are two example of relay module which can be use by PIC to control DC motor. First figure is to control only On and OFF of motor which also compatible to other devices that are using switch. Second figure can be used to control forward and reverse motion of DC motor.

 

By using relay module, it is very easy to control the motion of motor. The programming part only require ON and OFF of output PORT.

Example of simple forward reverse motor control using 2 switches connected to RD0 and RD1:

main()

{

while (1)

{

TRISD = 0b00000011;

PORTB = 0b00000000;

TRISB = 0b00000000;

PORTB = 0b00000000;

if (PORTDbits.RD0 == 1 && PORTDbits.RD1 == 0)

 PORTB = 0b00000010;

if (PORTDbits.RD0 == 0 && PORTDbits.RD1 == 1)

 PORTB = 0b00000010;

else

PORTB = 0b00000000;

 }

}

 When DC motor require speed control, the speed of DC motor can be control by various techniques including Pulse Width Modulation (PWM), power transistor, driver IC etc. The diagram below is example of DC motor controlled by transistor and PWM.

 

Whenever there is input voltage supplied to Base pin of the transistor, the transistor will turned ON and let the current flow through. with different speed of PWM provided, the speed of motor can be adjusted. The faster the PWM signal, the faster the motor will rotate.

 

Analog - Digital convertion with PIC

PIC 18F4550 come with ADC in PORT A which reads analog input such as:

1. voltage

2. temperature

3. light intensity

4. resistance

5. moisture 

6. humidity etc

Analog signal is converted into digital form by referring to reference voltage which can be set to Vdd (same as source voltage) or any other external reference voltage source. ADC Module has high-voltage reference (Vref+) and low-voltage reference (Vref-). The reference voltage value can be chose to be Vdd, VSS, RA2 or RA3. ADC Module from Channel 0-7 (AN0-AN7) will read the voltage value detected from the chosen channel.

 Table below shows how to configure PORT A to either digital or analog I/O.

For example: configure only AN0 of PORT A as analog I/O while others are digital. An analog signal is send to AN0. The code should look like below:

ADCON0 = 0B00000001 //choose channel AN0

ADCON1 = 0B00001110  // only AN0 is analog

**ADON = turn ON ADC

**GO/DONE = conversion status

For PIC18, it is difference compare to PIC16 where PIC18 need to configure ADCON2.

ADCON2 = 0B10001010 is use for 22pF ceramic capacitor pair and 20MHz-40Mhz crystal.

The following steps should be followed to perform an A/D conversion:
1. Configure the A/D module:
• Configure analog pins, voltage reference and digital I/O (ADCON1)
• Select A/D input channel (ADCON0)
• Select A/D acquisition time (ADCON2)
• Select A/D conversion clock (ADCON2)
• Turn on A/D module (ADCON0)

2. Configure A/D interrupt (if desired):
• Clear ADIF bit
• Set ADIE bit
• Set GIE bit

3. Wait the required acquisition time (if required).

4. Start conversion:
• Set GO/DONE bit (ADCON0 register)

5. Wait for A/D conversion to complete, by either:
• Polling for the GO/DONE bit to be cleared
OR
• Waiting for the A/D interrupt

6. Read A/D Result registers (ADRESH:ADRESL); clear bit ADIF, if required.

7. For next conversion, go to step 1 or step 2, as required. The A/D conversion time per bit is defined as TAD. A minimum wait of 3 TAD is required before the next acquisition starts.