2011 International Conference on Electronic &Mechanical Engineering and Information Technology

PWM Speed Control System of DC motor Based on AT89S51

Zhijun LiuSchool of Electrical and Information Engineering

Liaoning Institute of Science and TechnologyBenxi, China

Izj73@126.com

Lianzhi JiangSchool ofElectrical and Information Engineering

Liaoning Institute of Science and TechnologyBenxi, China

bxjlz1963@163.com

Keywords-AT89S51; DC motor; PWM

Abstract-This paper presents a new approach of DC motorPWM speed control, using microcomputer as controller, afterdiscussing conventional cascade speed control. It introducesthe principle of DC motor PWM speed control and the detailsof the realization of the approach base on AT89S51 single-chipmicrocomputer. Different duty cycle quadrate waves areproduced by the PI of AT89S51. Different speed grades andthe direction are depended on different buttons. Experimentshave proved that this system is of higher performance.

I. INTRODUCTION

In recent years, with scientific and technological progressand social development, the electronic technology isdeveloping rapidly, to achieve the portability and low costand energy efficient, and the noise limit, a DC motor is usedwidely, so, the study of DC motor speed adjustablel'<' ismore practical significance. The traditional method ofcontrol speed was that the resistance is strung in the rotorcircuit or adjust the voltage of electrical machinery circuit ,the two methods is easy, but they exist some shortcomings:The smooth character is bad and the characteristic is soft inlow speed ,The motor speed will be changed larger when theload is changed; The motor speed is very hard to get a lowwhen the load is light ;The larger the I resistance are, thegreater its losses are, the Efficiency reduce noticeably.Therefore, a new kind of speed control method is calledPWM(pulse width modulation) speed regulating system[3]has been widely used in the motor control speed. It is beingapplied to every field and has been product-FrequencyConverter. With the wide use of PWM technological, thepower energy can make full use of and the circuit efficient isvery high. This paper utilizes the timing of themicrocontroller timer function, outputs analog PWM signalat the P1.0 pin, to adjust the duty cycle according to thenumber of different pulse high, thus to achieve thegovernor's role.

III. PWM SR PRINCIPLE AND CIRCUIT

Bridge choppert' shown in Figure 1, the equivalentcircuit, allows both positive and negative direction of the

motor in the SR, transistors V~,VT4 turn on, VT2 , VT3 cut

off and the motor turns forward. Similarly, VT2 , VT3 turn

off the motor rolls back. Suppose turning off V~ ,VT4 after

it has turned on ~ seconds and then a period of dead zone,

when VT2 , VT3 turn on for T2 seconds, cut off it

immediately. So repeatedly, at both ends of the armaturevoltage.

Where: Ua is the armature terminal voltage; la is

armature current; Ra is equivalent resistance of armature

circuit; ¢ is each level of magnetic flux.

There are so many ways to speed. For instance, armaturevoltage control method can be used and excitation controlmethod, but excitation control method by the magneticsaturation in the low speed limit, at high speed by changingthe restrictions to the spark and the commentator, the largerfield coil inductance will affect the dynamic response,therefore armature voltage control method is often currentlyused.

Armature voltage can be adjusted series resistance of thearmature loop method, but this is inefficient, and has beenreplaced by the chopper, which consists of the AC powersupply, AC adapter with DC rectifier circuit and then thePWM chopper the chopper. Chopper motor loss not only issmall, but also gives fast dynamic response, so it is widelyused.

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II. DC MOTOR SPEED CONTROL PRINCIPLE

Expression for DC motor speed (r / min)

Ua-laRan==-----

Ce¢

Figure 1. Bridge chopper circuit

978-1-61284-088-8/11/$26.00 ©2011 IEEE 1301 12-14 August, 2011

Waveform is shown in Figure 2.

Va

forward, key 4 the fastest .There are 4 levels, while touchingthe No. 5,6,7,8 button, the motor turns back. Therefore theduty cycle a ranges from 1/ 8 to 8 / 8 .

V"!lOOk

Figure 4. the number of different high pulse

3a ==-

8

J

B. Pl. 0 pin realization ofthe output waveform

There are two timer/counters TO and Tl inside AT89S51,in whichTO is set to work 1, counted every 16 bit. At thismoment, the mode value of TO is 65536, the system of thetime period T is 1 f.1S , then the initial value[8-1O] TOis:

TO = 65536 _!...- = 65536 _ 30msT If.1S (3)

== 35536 == 8ADOHTherefore THO = 8AH; TLO = ODOHTO timer is initialized as following:

MOV TMOD, # 01HMOV THO, # 8AHMOV TLO, # ODOHSetb TROSetb ETOSetb EA

Figure 3. Single-chip microcomputer control speed principle

A PI.l

T P12

Assuming a pulse is 30ms, a square wave pulse of 8cycles is period of 240ms. Setting the frequency of AT89S51is 11.0952MHZ, when the pl.0 pin outputs a high level in aperiod, the duty cycle is a == 1/8 , the motor reverses in

high speed. When a == 0.5 , the motor stops running.

When a == 5 / 8 , the motor runs at low speed. When a == 1,the motor runs forward at full speed. As is shown in Figure 4.

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tT

Figure 2. the armature voltage waveform

U = T[Ud _ TZUd T[ - (T - T[)Ud

a T T T

2T -T== 1 Ud == (2a -l)Ud

TFor a is the duty cycle 0 < a < 1 so

- U d < U a < +Ua : change the size of U a by changing

the size of a . When 0 < a < 0.5 ,the motor turns back.

When a == 0, the motor has the maximum speed of reversal;

when 0.5 < a < 1, the motor turns forward, as a == 1, itmoves in the maximum speed.

IV. THE SINGLE CHIP MICROCOMPUTER CONTROL DC

MOTOR CONTROL METHOD

SR ~art of the circuit controlled by AT89S51 MCVshown!" ] in Figure 3, S is the motor start switch, outputwaveforms of different duty cycle with the P1.0 pin, make 3x 3 ranks of the keyboard with P1.2 r--.J P1.7, which key 0 isthe motor stop button, regulate speed in 8 grades, which 1 r--.J

4 are the motor forward speed, 5 to 8 are for the motor speedreversal. P1.1 , the smooth speed, rated speed can be set inthe host computer; when the S key is broken, Pl.0 is high,

the opt coupler turns on, VI; base is extremely low,

VTs close, V~, VT4 , VT6 turn, VT2 , VT3 , VT7 stop, +5 V

power supply forms a pathway through the motor at bothends having the positive voltage, the motor is transferred.Conversely, when P1.0 is low, the opt coupler turns off,

VI; base IS extremely high, VTs conducts, thus

VT2 , VT3 , VT7 lead pass, V~, VT4 ,VT6 cut off, reverse

voltage across the motor and the motor moves back.

A. Pl.O pin output waveform

AT89S51 outputs different duty cycle square wave withPl.0 pin, and makes the ranks of3 x 3 keyboard with Pl.2 r--.J

P1.7, shown in Figure 3. When pressing the keys 1,2,3,4, themotor is transferred, transfer speed of key 1 is the slowest

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C. Flowchart

DC motor speed control flow chart shown in Figure 5:Interrupt service

procedure

main

Cycle R7=8-Key number

y

Outpt 8-R7Low

level

NStop?

Figure 5. Flow chart shown

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V Conclusions

The paper takes advantage of AT89S51as the processorDC motor PWM speed control system to improve the controlprecision. It is proved to be operated so simple that thesystem can be applied in small motor control system.

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