This tutorial will helps you how the PWM works. You will learn what is PWM, the usage of PWM also how to program a DC Motor with PWM. Some Robots need to control the motor speed for its to work. Just use PWM system and you will get your motor’s speed adjustable…
One of the easiest ways of generating an analog voltage from a digital value is by pulse-width modulation ( PWM ). I PWM a high frequency square wave is generated as a digital output. For example, a port bit continuously swiched on and off at a reltive high frequency. The signal is fed to a low pass filter. The voltage at the otuput of filter is equal to ther Roo Mean Squeare ( RMS ) of the squeare wave signal. The RMS of the square wave signal may then be varied by changing the duty cycle of the signal. A cycle is initiated by a low to high transition of the signal and terminates at the next such transition. During one cycle if the time the signal stays high is equal to the time the signal stays low, then the duty cycle is said to be 50 percent.
Figure 1. Duty Cycle 30 %
The following circuit shows a DAC constructed with PWM. The Program controls the speed of a DC motor by pulse width modulation ( PWM ). Bit P3.0 Drives a switching transistor as shown in the circuit diagram. Motor is swiched on for a period of time, and then off. The fraction of time the motor is on is call the duty cycle.
Figure 2. Pulse width modulation to Control Motor Speed
This program uses a byte to store the time the motor is on, that is the number of cycles out of 256 cycles while the motor is on. a value of 10 means that the motor is on 10 cycles and off 246 cycles.The duty cycle value is stored in the internal register labeled dCycle. The complement of the duty cycle is similarly stored in register labeled dCycleC. ( Download File : motor.zip )
dCycle equ 30hdCycleC equ 31h count equ 32h Analog0 equ 33h Analog1 equ 34h PWM bit P3.0 Channel bit P3.1 PortADC equ P0 DCLB equ 000h DCUB equ 0F0h MotorCondition bit 20h ; org 0h ljmp start org 0bh ljmp Timer_Interrupt0 start: call Init_Interrupt_Timer0 clr PWM ; Turn off motor ;================== ; Main Routine ;================== Forever: call ADC call Update sjmp Forever ; ;====================== ;Subrutine Timer Interruption ;====================== Timer_Interrupt0: JB MotorCondition,MotoroFF Setb PWM Mov TH0,dCycle Setb MotorCondition reti ; Motoroff:clr PWM mov TH0,dcycleC clr MotorCondition reti ; ;========================= ;Subrutine Initialize Timer Interrupt ;timer 0 as counter 8 bit mode 2 ;======================== Init_interrupt_Timer0: mov TH0,#dCycle Mov TMOD,#00000010b ; setb ET0 ; Enable Timer 0 Interrupt Setb EA ; Master Interrupt Enable setb TR0 ; start rock and roll timer 0 Setb MotorCondition mov count,#50 mov dCycle,#0 mov dCycleC,#0FFh ret ; ;======================== ;Subrutine Update - Update duty cycle ;========================= Update: djnz count,upDone mov A,analog1; reference voltage mov b,analog0; tachometer voltage clr c subb a,b jnz update1 ret Update1: jc decrease call higherDC ret Decrease: call lowerDC UpDone: ret ; ;==================== ;Subrutine lowerDC ;==================== LowerDC: mov a,dCycle cjne a,#DCLB,DecDC ret DecDC: dec dCycle inc dCycleC ret ; ;=================== ;Subrutine HigherDC ;=================== HigherDC: mov a,dCycle cjne a,#DCUB,IncDC ret IncDC: Inc dCycle dec dCycleC ret ; ;============= ;Subrutine ADC ;============= ADC: Clr channel ; Pick channel X0 from multiplexer 4051 mov Analog0,PortADC Setb channel ;Pick channel X1 from multiplexer 4051 mov Analog1,PortADC ret ; end