Microcontr�ller Performance for DC Motor Speed Control System PDF

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National Power a nd Energy Conference (PECon) 2003 Proceedings, Bangi, Malaysia 104 Microcontr� ller Performance for DC Motor Speed Control System y. S. E. Ali, S. B. M. Noor, S. M. Uashi and M. K Hassan A bstrat-The electric drive systems used In Industrial for the co ntrol of d c drives. uta, el.a...

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Microcontr�ller Performance for DC Motor Speed Control System jadhav sushmita

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National Power a nd Energy Conference (PECon) 2003 Proceedings, Bangi, Malaysia

104

Microcontr� ller Performance for DC Motor Speed Control System y. S. E. Ali, S. B. M. Noor, S. M. Uashi and M. K Hassan

A bstrat-The electric drive systems used In Industrial

for the

co ntrol

performance and reliability requirements. The de 1I10tor Is 111 attractive piece of equipment In many Industrial 8111111c8110n5

control

algor i thms

appllutlons

are

Increasingly

requIred

to

meet

hIgher

requiring varIable speed and load characteristics due to Its ease

of controllability. Mlcrocontrollers provide a suitable means of meeting these needs. Cetainly , pat of the recent activity on mlcrocontrolles tan be ascribed to their newness and challenge. (n this paper, Implementation of the MC68HClI E9 mh:rotontroUer for speed control of de motor fed by a de chopper has been Investigated. The chopper Is driven by \ high

'rrequenty

PWM

sIgnal. Controlling the

PWM

duty eyele Is

equivalent to controlling the motor termin a l voltaRe, which In

turn adJush directly the motor speed. ExperImental resnlts have

been

obtained

that

show

the

employment

of

mlcrocontroller for speed control Bnd over current protection

of a de motor.

Kywords-de

motor

drives,

Mltrocontroller,

Speed

utilized the

are stored and

implemented by the

microprocc�sot of the microcomputer. The system employs the use ofa Thyristor, which is controlled using the sotware

imp le mented on the microcomputer.

The lise of stand-alone microcontroller for the speed contr ol

of dc motor is past gaining ground. Nicolai and Castagnet [5] have shown in t heir paper how a mi cro controller can be used for sp e ed control. The operatio n of the system can be summarized as: the drive fom a

rectiied voltage, it consists

of chopper driven by a PWM signal generated from a microconroller unit (MCU). The motor voltage control is

achieved by measurin g the rectiied mains voltage with the analog to digi t al converter pre sent on the mi crocontroller and adjusting the PWM signal d uty cy cle accordingly �

is reported brief d es c rip tion of

Another system tha t uses a microprocessor

in the work of Khoel and H adid i (6} a

the system is as follows: The microprocessor computes the

1. INTRODUCTION the use of power e lecronics for the control of ele ctric

machines ofers not only better performance c aused by

precise conrol and fast response, but also maintenance, a�d ease of implementation. In parallel with the advance In power elecronic there have been greM advances in microcontroller-based control systems due to the microcontroller flexibility and versatility. This is because al l the control algorituns are implemented in the sotware. [I] For the continuous control system, the commonly used optimum design approach is usually based on frequency response or zeroes-poles pl acement [2,3]' H owever the relationship between the specifications in the frequency doain and that in the time . domain is not straightforward, and som etimes is hard to be pred icted. Adjustable speed ives may be operated over a wide range by conrolling amature or ield excitation. Speeds below rated by armaure voltage control and above rated using field excitation variation, development of various so li d stale ,

sitching devices in the fom of diodes, transistor and

thristo r along with various an alog/d igital chips used in iring/controlling circuits, have made dc drives �ore

l

c o nro

. in i nnumerable areas of apphcahons

A re asonable number of w orks have been found in the literaure, regarding the employment of solid- state devices Y. S. E. Ali. S. B.

Depatment of

[4]

.

control.

accessible for [3].

of d c drives. uta, el.at

microcomputer 10 conlrol the speed of a dc motor. The

M. Nor, S. M. \.,hi and M. K. It""n arc with and Electronic Engineering, Faculty of

Electrical

Engineering Univcsili Pua Malaysia. 43400 UPM Serdang. Selangor. Malaysia (E�all: [email protected], [email protected])

O-?803-8208-0f03/$17.00 ©Z003 IEEE.

actu al speed

of the motor by sensing the

teminal voltage the

and the current, it then compares the acual speed of

motor with the reference speed and generates a suitable

control signal which is fed into the triggering unit. This unit

drives a H-bridge P ower MOSFET amplifier, which in un sup plies a PWM voltage to the dc molor. The objective of this paper is to explore the approach of designing a microcontroller�based closed loop controller. The inte rfa ce circuit and the soware are all designed to achieve a better performance. The system is designed with a

curent flow monitor

that can protect the dc motor from high cutrents due to ov erload ing. he microco ntrol l er system is equipped wi th an LCD display and a kepad and sotware was written to monitor

the registers on the LCD and read commands from the kepad. Thus, by using the u ser interface module (UIM) the operator can view andlor change all the control and monitori ng variables of the controller program.

105

II.

HARDWARE DESIGN

Fig. I shows the schematic diagram of the

automatic speed control system of the de

"

M�sur:d Current lOnVfA

G8HCIIE9

0-220

0-20V Oplinl CUplr

PA5

complet e

motor.

nov

20V

DC SOURCE

CURRENT

PE4

C M',

" OIlp:r

l

OPTO-COUPLER ISOLATION

PAG

SENSOR

'urreni Sensor

Mesured S-�d

Fig. I. Block diagram of automatic speed control system. The 68HClIE9 microeontroller implements the coiltrol

algoritlm1 by conditioning the spe ed and current signals and

perfO1l1S the spee d regulation according to speed reference

Fig. 2. DC motor control hardware block .

fed through the kepad. The software includes a routine to

read the motor current and sends emergency shutdown signal to protect the dc motor from over current, also this

signal can be activated manually by inserting a designated character by the keypad, which causes a software interrupt

and executes the emergency shutdown routine.

and

curre n t

interface. Changing the teninal voltage by means of dc,to­

de

The hardware control system includes the dc shunt moto r, power circuit, MC68HCII E9 microcontroller, speed

sensor (shaft,encoder),

The hardware of the microcontroller includes mainly

the MC68HCIIE9 system with LCD and keypad for user

sensor. The system

c hopper

hardware block diagram is shown in Fig. 2.

The conventional dig i tal pro po rtio n MCU technique

and the pulse width modulation (PWM) technique are adopted in dc motor control system. An optical

e ncoder

was

used to measure the speed of the motor. The output of the encoder is a stream of pulses with variable' frequency

according to the speed of the motor. The resolution of the encoder in this work was 500PPR.

current

s ensor .

s ignal

to

It se n ses the current and feeds the

microcontroller.

circuit)

Ill.

Port

PE4

of

that is controlled by the

The

SOFTWAE DESIGN

M68HCl lmicrocontrolier

(MCU)

can

control

speed of a DC motor accurately with minimum hardware at

low cost. The flow chart shown in Fig. 3 describes the main

program. The program is written using assembly. It can be divided into three main parts. The initialization is the

beginning of the software to

The current sensing was accomplished by using Hall

effect current

(the powe r

microcontroller generated PWM signal controls the speed of the motor.

The second part

(lIO

speed from the iD ports

initialize timers, I/O Ports.

scan) is reading the current and a nd

input output capture (IOC)

the

respectively. The microcontroller checks the current and

microcontroller is dedicated for the current signal and a

jump to shutdown routine that stops the motor if over load

ID port. The opto,couplers were used to isolate the high

from the keypad. From the speed sensor and the speed

continuous conversion mode where used to read from the

voltage circuits from the l ow voltage controlling signal s .

The de motor is the plant that will be controlled. The rating

occurs.

Otherwise the microcontroller reads the commands

reference (from keypad) the microcontroller calculates the

error and perfors the calculatio� of the control algorithm

of the motor should be chosen according to the rat ing of the

output and then it calculates the PWM signa, width, this is

ratings 2400RPM, 220V, 2.2A, O.37kW is used.

This part ends by updating the timers and· counters to

power circuit switch. For this study a dc shunt motor with

the last part of the program, which called program scan.

generate the PWM. T hen the microcontroller repeats the 1/0

scan and then the program scan. The motor can be stopped manually by keying a designated character at any time_

106 The MC V has a built-in timer and co unter register

"TeNT" which is a 16-bit register that can be incremented

at 0.5,2,4, or 8.ls by coniguring the prescaler unit. 0.5-1s is selected for .better granularity.

Our objective is how to satisfy the 0% to 100% duty

cycle using the MCV. The TeNT is incremented every 0.5.ls and will rollover every 32.77ms

Initialization Timers,

10

When the contents of TCNT equals the conten ts of OCx register, the control word speciied in TCTLI r egister will

Pot, etc ...

be performed automatically to update the associated pin in

port A, the compare flag OCxF set, and an interrupt will

be

ge nera ted if the OCxl intemtpt enable is set. In the TCTLI register one oflhe following actions con be selected:

•

Dues lIul arecl OCx pin

•

Clear OCx pin on successful compate

Toggle OCx pin on successful compare

•

•

Set OCx pin on successful compare

In implementing the program the following ports and

control words were used: I. Two-output compare register OC2 and OC3 are used and both use the toggle OCx pin action. Two square

waves

at the

same

frequency

generated on OC2 (PA6) and OC3 (PA5).

.

be

PA6 and PA5 will be XORed by an extenal XOR

2.

3

will

.

gate to generate the PWM output (PWMOVT). The duty

cycle

is detenined by the phase shit

between the outputs at PA6 and p A5. PA6 ou put is

ixed while PA5 is shited according to the duty cycle.

4.

Two inteupt service routines TOC21SR and TOC3ISR are programmed to hand le the interrupts ofOC2 and OC3 respectively.

The frequency is conigurable within the range 1KHz to

16Hz.The duty cycle can be changed over a ull span

(from 0% to 100%). Two output compare registers are used

in cooperation with an extenal XOR to generate the output

Yes

PWMOVT.

Cakulate the errot.

Calculate tke Control Algorithm. Update the counter� and timers and generate

PWM.

TeNT

Fig. 3. Main pogram nowcharts E

Clock A.

Full Range Frequency Independent Duy Cycle When designing a PWM unit using the

MeU

two

factors should be considered PWM duy cycle, and PWM

requency. Both are heavily dependent on the operating speed ofthe MCU. The PWM requency, in this work, is kept constant factor.

it

directly afects the

DC motor stability and

sensibility to changes in is input voltage. However th e requency can be

ed

ch ng

anually within ripper and lower

limits to make tne system lexible and able 10 operate

motors with diferent ratings and speeds.

Fig. 4. PWM

interupl service routines.

Fig. 5 shows the isolated PWM block, it accepts two inputs. Duty Cycle that is the output of the speed control

algorithm and inputted to this

block and Frequency, which is and inputted to this block

entered through keypad by user via user interface.

107

\

1 �__J",;.4 .

. . .. .... ... ?A6 XR P&l, " DIy1%

PA6

Speed Measuremellt

C�' .,�:v

T

t t·

Fig. 5 PWM lock B.

r

. r ' ;...!1. i

PWMM

S.OO 1

chI f

M 40] s

i.SO

21 'un 20JJ 0,:$1:33

Pulse width c�n be measured by using the input capture

register that are available in the 68HCllE9 and can be

programmed to capture the positive or negative

e dge

of the

(b)

input signal. Once the edge is detected the value of TNT

will be cop i ed automatically to the TIC

'

fig. 6. (a) Output at PAS and PA6 at duty 10%, {h) The coresponding PWM ompul.

T imer Input

Capture " register lmd will not be overwritten until the next

edge. Two registers

used TIC2 and TIC3. TIC3 is used to

capture the positive edge and save the time Tl, and TtC2 is used to capture the negative edge and save the time T2. The

mea sured speed (MS) can then be calculated using tbe

equation:

MS"( 60* 2)/«T2-T 1 )*PPR) where PPR

=

Chopper Output The dc chopper was driven by

signal

obtained

from the

a

high frequency PWM The voltage

microcontroller.

supplied to the motor is proportional to the PWM duty cycle

generated and fed into the chopper. Figure 7 shows the main

(1)

o utput

obtained

experimentally,

which

calculated value is VouP (79.881100)*254

pulse pear revoluti on

IV.

B.

=

is

The

RESULTS & DISCUSSION Cht Me.n \9.& I

The ha rdw a re system has been developed and tested

under laboratory conditions. The

microcontroller

based

closed loop control was implemented and applied on a dc

shunt motor. The results be discussed as follows.

A.

201 V.

202.89V.

ob

t ained are

:ht �M".n 2Q.t I

as expected which can

ChI +Duty 79,88 "

PWM Oil/pilI

Cht

freq l.oookHZ

Fig. 6(a) shows the output of microcontrolJer generated

square wave signals at port PAS and PA6. The signal at PA6

is shifted by a percentage corresponding to the duty cycle.

Figure 6(b) shows the corresponding PWM output generated

ll IU.GI

M� -C111 j-�l0l j."lo.ooooo 5 I

by XORing the signals at PAS and PA6.

2 J ul 2003 01:31 :22

Fig. 7. DC Chopper Output. i t

. . ·t ..

., I"

'j

:,�,

PAS "

�

"

I"

" . ,' ,., 'iH"

PA6 Shatol by Dy 1%

,.,.,.,.,

",

::

.. .,.

•••

, ' ..I ,

t

'"

"1':'.1

j

t ch'ii '�Y" ''''),u''Y' 'tM�· '!�chl'-'' rso!

The output voltage obtained from chopper is fed into

Ch1 Heq 500.0 Hz Chl +Uu1y 5).00. Ch2 Fr�, SO),Q H' C.,Ul{1 50.00 % .

(a)

s

I

.

Speed Output and Motor Response

This shows the

o

utp ut

of the motor ald its speed

response at diferent conditions.

The

operator inputs the

desired speed through the keypad in the auto-inode. The system will adjust th� duty cycle automatically according to the value inputted tl�rough the keypad.

,

u....I.oOOOQ

the motor.

2Q lun 2JQl 0*: 11:29

Fig. 8 shows the output of motor with speed rising from

zero to a predeterined speed comand of 100Qrpm. It be seen that a

ste ady

can

rise is achieved with almost small

108 overshoot and then the speed settles to required speed in less than 1.0 sec and almost zero ste ady state erro r .

Th e response of motor to sudden change from

a

speed

of 200rpm to 1000rpm is shown in Fig. 10. Percentage overshoot is much less at high speed.

.-- ----,

----�

Response to St ep 1000RPM

� �

.,

11200

"

Speed Respon�c

� �ooo ' �2500

]

-

800

----

�.--

400

Fig. 8.

1500·

Speed Response

500

-- ._-- -"� ----

time(O.3s/div)

o

time( 1 sldiv)

from starling to I OOORPM. Fig. 11. Decreases in speed.