User manual

54 l Robotics experiment with PIC microcontroller

Robo-PICA can control the speed movement by send the signal to the enable

pin (EN) of motor driver IC, L293D. Refer the figure 4-1 (in this chapter), EN pin of L293D is

connected to RC2/CCP1 and RC1/CCP2 port pins of PIC16F887. Both port pins are PWM

output port. Builders can write the program to control the PWM output signal for adjust-

ment motor speed.

PWM operation

Normal driving motor technique is apply the voltage to motor directly. The motor

works in full speed. Sometime this speed faster. Then the simple method to control motor

speed is control the voltage applied to motor. The populate technique is PWM (pulse-

width modulation). This technique will control the width of the positive pulse. The volt-

age is applied to motor as average value. Ratio of positive pulse width and totally

pulse width is called Duty cycle. Its unit is percentage (%)

4.8V 4.8V

2.4V

Volt

Time

Volt

Time

50%

duty cycle

Average voltage = 2.4V

Figure A4-1 : Shows average voltage output of PWM

(A) Full volatge apply. (B) 50% duty cycle PWM

Activity 4

Speed control of Robo-PICA

(A) (B)

4.8V

3.6V

Volt

Time

4.8V

1.2V

Volt

Time

75%

duty cycle

Avera

e volta

e = 3.6V

25%

duty cycle

Avera

e volta

e = 1.2V