Indi an Journal of Pure & Applied Physics VoL 37. June 1999, pp. 482-489

A PC -XT/AT based low temperature display and control system of an optical cryostat

N B Manik, T K Ballabh & A N Basu

Condensed Matter Physics Research Centre. Department of Physics. Jadavp ur Un iversity. Calcutta 700 032

Received 12 October 1998: revi sed 8 March 1999: accepted 17 May 1999

The design and fabrication of the hardware and soft ware of a simple and low cost AID interface schcme through which a TC (thermocouple) and diode sensor connected to a PC - XT/AT or 8088 ~p kit has been described in thi s paper. Th is unit is now bei ng used in low temperature control system of an optica l cryostat using a TC as temperature sensor. For cold junction compensation, silicon diode, IN 4148 operat ing at constant voltage instead of the constant current has been used and the add iti on for the compensation process is done under the program con trol. The variation of thermo emfofTC with temperatu re is nonlinear. The process for calibrat ion of nonlinear signal by analog means is rather difficult and it i, known that all the exis ting sensors have either static or dynamic nonlinear response wi t!l respect to inp ut variables. Here ins!<.:ad of linearizing the signal which lowers the accuracy of the measurements. a pol ynomial fit tu th e experimental points is used for the purpose of ca libra tion . The temperature val ue corresponding to a thermo emf is obtained from this re lati on . Accuracy of measurement of thi s system with a 8 bit ADC over a temperature range from - 196°C (boi ling temperature of liquid nitrogen) to + 100°C is about 1.16 0c. l3ut splitt ing the temperatu re region from -196°C to + 100°C in seven equal steps enables one to ac hieve a rcsoluti on almost equal to 0.35 0c. Se lection of differcnttemperaturc ranges by solhvan.: is au tomat ic. For control a proport ional control algorithm has been adopted and the set temperature remains steady at any value for about half an hour with a flu ctuation of about ± 0.35 °C within the above range of temperature.

1 Introduction Several prec ision temperature contro llers t

-IJ includ­

ing the authors design of a PWM temperature control­lers l4 us ing different control algorithms have been reported which operate within a small range of tempera­ture. For controlling of temperature of an opti ca l cryostat over a wide range from -196°C to + 100°C by a PC using the TC as temperature sensor including a di ode as compensation device for co ld junction has not yet been reported . The followi ng secti on desc ribes a PC­XTI AT based low temperature d isp lay-cum-controll ing system.

The system is used to control and di splay the tem­perature of an optica l cryostat with a Chromel-Alumel TC as temperature sensor with comm onl y used Si rect i­fying diode IN-4148 as the compensation device for the TC.

2 Principle The basic function of the present unit is to accept

analog signal from the sensors, di giti ze the input data, process the data according to the needs and store these data in memory. The calibration curve of temperature versus thermo emf which is nonlinear is prev iously

defined . The temperature correspond ing to a thermo em f from the TC sensor is ca lcu lated from this relation and displayed.

Us ually a pi ece-wise I inear approximation is used for the calibration of a non linear s igna l15

- ' ? But this tech­nique makes the circuit design complex and also lowers the accuracy of the measurement. Moreover this tech­nique is not app li cab le in a wide var iety of situations. It is rest ri cted only for few types of s igna l where the nonlinearity is not hi gh. Here to find out the relati on between the thermo emf and temperature the authors have first ca librated th e output ofa Ch romel-Alumel TC for some known temperature. The experimental points are fitted by the curve fitting method and the relation of therm o emf with temperature is obta ined . By a polyno­mi al fit up to s ixth order terms with these experimental points the relation of the TC output versus temperature comes out in the forlll,

T = 273.65 + 26.82 1 £ - 0.833 £ 2 -.0. 11 2 £ ' + 0.033 E + 0.003 £ 5 - 0.00 I E' ... ( I)

where £ and T are the therm o emf and temperature in Absolute sca le respectively. The fitted curve is shown by a dotted line in Fig. I . The deviat ion from the fitted

MANIK et al.: PC-XT/AT BASED LOW TEMP. DISPLAY 483

curve of E-T and as obtained from experimental data is negligible for all practical purposes.

Control of temperature by using the ON-OFF con­troller within a small experimental space results in tem­perature fluctuation. For smooth control of temperature the different complex algorithms are adopted9

-13 some

of which are very accurate and sensitive though the realisation of these controllers a re comp lex . The authors have developed a very s imple but effective technique to control the heater based on proporti onal control algo­rithm. For control of temperature, the instant value is

600

500

..;...400

'" ~ :J

~ 300 .. a. E ~ 200

100

OLI ____ L� __ ~_.I ____ ~IL---_7'----~I~--~1 -15 -10 -5 0 5 10 15

Ther mo emf (mV)

Fig. I - Pll lynomial fi t of th t: thamo t:mf variat ion \\ ith templ: r<lturc

Compensation ~_--, Voltage

I DC

l Power Supply

CH1

CH2

compared with the set value of temperature as fixed by the user. The heater will be switched ON or OFF if the spec imen temperature is below or above the set tempera­ture respectively and the ON or OFF time will be pro­portional to the difference of the current va lue and the set point of temperature. In other words the duty cyc le of the heater current is adjusted and the amount of energy fed by the heater to the system depends proport ionally on the difference between the current value and the set value of temperatures.

3 Hardware of the system Fig. 2 shows a bl oc k diag ram of the PC based tem­

perature di sp lay and control system. Input signal from the TC and diode sensors are read by the ~lP to disp lay the temperature . The keyboard a llows to enter the set te mperature. O ne heater is placed inside the cryostat . Depending on the statu s of the set temperature and current temperature the heater current is manipul ated . For operat ion of the AID olle reference voltage and clock freq uency is generated .

To build the hardware for thi s work. the authors have used a PC- XTIAT or 8088 rP kit. The analog to dig ital (AID) converter (AD -0809) is interfaced to the expan­s ion slot shown in Fig .3 of the PC-XT/AT via the I/O lines l8

. The design was done in a doubl e si ded PCB board .

Fig.4 shows the detail s of the circuit d iagram for this scheme. AID conve:ier converts the analog signal from

COMPUTER

I/O

I----'I=---i P o R

Fig. 2 - nl"ek diagrall111fth ~ illt~rfac~ "fthl: I C and diodc S<':Jl S(lr

484 INDIAN] PURE APPL PHYS. VOL 37. JUNE 1999

SIGNAL NAME

GNO

+5V

-5VOC

-12V

.12 V

lOW lOR

+ALE

-

-

r-I- 81 ~

I-

~

I-

~

t-I-

l-

t- Bl0 l-

t-l-

t-- I-

l-

t-l-

I-

t- B20 l-

t-l-

t-l-

I-- t-l-

t-

'--I- 8 31

"-

Al-r--

-----

-Al0

-

A20

A31 '--

I"

f-f---

SIGNAL NAME

+07 +06 +05 -04 -03 +02 - 01 -DO

+A9 -AS +A7 +A6 +AS +A4 -A3 -A2 -AI +AO

PONENT COM '\ SIDE

Fig . 3 - Pin nUI1lb.: rs with l1 a I1l': ~ I"or p.:rirhcral slot s on IB M I'C l1lotherboard

the TC and diode senso r to dig ital va lues whi ch ca n be read and processed by th e ~lP .

3. 1 PPI chip connection

The ADC was connected to the computer via the programmable periph eral interface (PPI ) chip 8255 . The add ress lines A2 - A 9 of th e expansion slot of the PC bus (shown in Fig. 3) are decoded by a nand gate 7430 to se lec t th e chip 8255. The output pin 8 of the 743 0 goes tn the chip select ion (CS) pin ( 15) of the 825 5. Thi s chip is se lected when the signal leve l at pin no 15 become Im \ . So as sho\\ n in FigA th e signal leve l of th e pin A2 -A9 are to be se t hi gh to select 8255. The add ress line .1\0 and A I (Fig. 3) \\ hich are connected to the pin 8 and 911f8255 arc used to address th e different ports 01"8255. To ~ il o \\ th e diffe rent addrl:ss bu s used (F ig. 3) to select tile difkrl:nt port s of our design a tab le is constru cted. T:lhk I silo\\ s th c address o f" dillerent ports.

\\ he n.' A() -1\15 arc the add ress line of tile p i> and X stand s for don·t ca re (i .e. it may be e ithl:r () or I)

3.2 Function of the port

For a control word of value 98h the different fun cti on of the ports and corresponding pins used in our design ... are g iven in Table 2.

The RD and WR lines from the expansion bus are connected to the pin 36 and 5 of the 8255 .

3.3 AID converter connection

AID 0809 is used in our des ign. It is a 8 bit AID converter with maxi mum conversion speed of J 0 f-l S.

The A DC-0809 AID converter used here can rece ive

eight channe l input analog signal in a Illultiplexing way. -;­

For th e operation of th e AID it requ ires th e exte rnal c loc k and the reference vo ltage.

3.4 neference vo lt age so urce and clock for the AID

A preci se reference vo ltage source of 300 111 V is constructed by ll sing an IC LM-723 (Fig. 4). For the

Table I - ;\dJn;ss 01" dirti:r':ll t Ports

Not CS lin.: Port S.:i<:clioll Port L1 s..:d addr.:"

;\ 15 - .'\9 AX A 7 ;\6 AS A4 ;\3 A2 A I 1\0 /\ 10

I I I 0 ( ) I'urt .'\ XI J"c

0 Port Il X l l"d

() I'ort e- X If.:

Cont rol Xli"f Port

Tahle 2 - Function of til..: differ.: nt ports Co ntrol \\-ord = 9XI1

Port s Typ<: Pin No Fum:lioll (l fX 255

Port /\ Input 1.2.3.·U7. IX. I lata nus 39. ·W (nit n = I. SG.

nit 7 ~ MSB )

Port B Ou trut I <J .:W.2 1.22.2 TranSistor 23 .2-U5 'ill il cio

IX ( ·ontrlll I 1e, ll l'l"

1-1 0 1

I'm t C ( II i ()utput 15 'it

16 :\ 1.1.

17 :\ I J.\

l'ortC(L) In put I ' .' I (l(

......

MANIK et al.: PC-XT/AT BASED LOW TEMP. DISPLAY 485

clock frequency of the AID, a relaxation oscillator using an IC op-amp LF-35\ is used to produce a 630 kHz clock frequency.

3.5 Selection of the input channel and data read by AID

The ADC-0809 AID converter used here can receive

returns the low EOC signal. The /lP monitor the EOC and when EOC signal from the AID converter become low the /lP sends output enable (OE) and reads a integer value of the input signal from the AID through the Port A of 8255 which is connected to the data bus of the /lP.

eight input analog signal in a mUltiplexing way. The 8 3.6 Temperature sensor with amplifier

input signal channel can be addressed by a 3 bit address Chromel-alumel TC is used as a temperature sensor. to activate the ADC, ADS, ADA pins. Sy sending the The output of the TC at liquid nitrogen (LN2 ) tempera-proper address to ADC, ADS, ADA the desired input ture is -5 .73 mY and at 100 °C is about 4.23 mY . It is channel is selected. Here two input channels viz. one for amplified by a factor 100 by the op-amp LF 351. So the TC voltage and another for diode voltage for com pen- output varies from -573 mY to 423 mY with a variation sati on have been used. So only one address line is used . of temperature from -196 °C to + I 00 0c. To get a ADS and ADC are grounded. The selection of the unipolar positive voltage variation the authors add a channel is given below voltage equal to 573 mY. So after th is add it ion the output

ADA = 0 for diode (Channe I I) voltage varies from 0 to 996 mY. = I for TC (Channel 2) A chromel-alumel single junction TC is connected

To address the TC or diode channel the desired value to the non- inverting input (pin 3) of the FET input of ADA is first sent. Then the ALE input is set to high op-amp IC LF-351. The op-amp is designed for a gain to se lect the channel. To read data connected to the input of 5. The output pin 6 of thi s op- amp goes to the channel of the AID, the start conversion signal of the non-inverting pin 3 of a next FET input op-amp LF 351. AID input is sent high and then low. After receiving the The feedback resistance of this amplifier is a 200 K start conversion (SC) signal , the AID starts the conver- trimmer. It is so adjusted that the gain factor of this slon process and at the end of conversion (EOC) it amplifier be equal to 20. So. a total gain ofthe TC output

COMPENSATION VOLTAGE REFERENCE VOLTAGE CLOCE r --------+i2v------------i r-------+liV--l r-------l00K--j

: :: lOil: I :

: 6 :: I

I I I I I : I I 22K I L____________ L ___________ ~

I L..f---i-' I

I I I

L __________ J HEATING ARRANGEMENT

ADDER 1-- ---

Fig.I.

Fi ~ . .t -l klaiicirclIil dia~ralll

Vl :::> III

I .... ~

486 INDIAN J PURE APPL PHYS. VOL 37, JUNE 1999

TC S~msor E,.:a(t,-t2 } +

l CH 1 AMP AOOITION

E, +K,E2 E=at,

Whent K, = gain

Compensation E2=bt2 J CH2 factor=a/b

Unit

Fig. 5 - Block diagram of the compensation principle

as obtained by the two amplifier is 100. The output of

the second op-amp is connected to Channe l 2 of the AD

converter. The offset of the op- amp need not be adjusted

immediately. The offset will also be taken into account

by adjusting this calibration factor through the software.

3.7 Cold junction compensation

The principle of temperature measurement by TC is

to measure the difference of temperature of the two

j unctions in terms of thermo emf E. The expression for

E is 2 E==a(/l - t2)+b(tI-/2) ... (3 .1)

where a and b are constants and II and {2 are the temperature of the two junctions of the TC.

In standard laboratory measurement, one junction is

kept at the specimen temperature, Is ( or II ) wh ile the

other is kept in the ice bath at constant temperature, 0°

C. The necessity of maintaining an ice bath outside or within the laboratory may be e li m inated by the compen­sation unit which enables measurement w ith a s ingle

junction TC. Elimination of the ice co ldjullction neces­

sitates the compensation process of addition or subtrac­

tion of voltage corresponding to thermo em f of that TC

at ambient temperature la (or t2 ) to the voltage of a single

junction TC. The ambient temperature, la (or 12) varies

from + 10°C +40° C. Here the compensating voltage is added through

software and the ~cheme for this compensation is shown

in a block diagram Fig. 5. The compensating diode generates a voltage EI == bt2,

where (2 is the ambient temperatu re, which is read

through Channel I. The output of the TC sensor gener­

ates a vo ltage E2 == a( II - 12 ), where I I is the specimen

tem perature which is read through Channel 2. The out­

put of Channel I is sui tably multiplied by a ga in factor

K == alb and added to the reading of Channel 2 to get a

output voltage E, wh ich is direct ly proportional to the

specimen temperature.

1400·00

1200·00

1000·00

E 800·00

> 600·00

400·00

200·00 '---'-------'----'---_----'-_ _ L-_--L...J

-200·00 -150·00 -100·00 -50·00 ().OO 50·00 Temperature (oe l

Fig. 6 - Diode voltage variation with temperature when the diode is biased with constant vo ltage

To generate this compensation voltage a S i rectifying diode IN 4148 w ith a series resistance of I K is forward biased by a constant voltage source I ? The sensing vo lt­age is measured across this series res istance . The vari­ation of this vo ltage by varying the temperature of the diode is shown in Fig.6 for bias voltage of 1.00 V. The variation is linear with a sens itivity of the order of 1.43 mV/o C.

The d iode is operated from a precise reference volt­age source constructed by IC LM-723 (Fig.4) . The IC voltage regulator LM 723 has a linear fall o f voltage with tcmperature of about 0.002%. The value of the resis­tanceR2 is adjusted to get an output voltageofl.OV . Thi s output voltage which is linear over the temperature range l

? 10-40 °C is taken from pin 10 of IC -723. The diode IN-4 148 is connected in series with sensing resis­tance of 1 K. This vo ltage is connected to the C hannel I of the AID converter. The attenuation or am plifi cation is again done by a calibration factor K under the program control.

MANIK et at.: PC-XT/AT BASED LOW TEMP. DISPLAY 487

3.8 Selection of the different temperature range

To enhance the resolution of the measurement the total temperature range is split into different regions.

The aim of this design is to measure the temperature from -196°C to + I 00 0c. The thermo emf after the adder

amplifier (LF 351) varies from 0 to 996 mY, i.e. 0 mY represents - 196°C and 996 mY represents + 100°C. The range of this output voltage is divided in seven equal steps of 99617 == 142.2 mY each. After each reading

Stor. the equation for calibration

. Pickup the data corresponding to AID from Cal.

No

Yes Return to DOS

Continue

Fig. 7 - Flowchart of the temperature measurement and control acti on

corresponding to a multiple of this voltage, the ~P keeps a signature and sends a signal to operate a transistor which results in a subtraction of voltage corresponding to this temperature from the adder amplifier (Fig. 4) and the previous value of the temperature is held . Due to this subtract ion of this voltage the input signal value is reduced to 142.2 mY. Now the system reads a voltage 0 for actual value of 142.2 mY . At every successive 142.2 mY it switches one transistor leaving a signature and lowers the input signal level.

One SL 100 transistor is connected through each bit from 2 to 7 of Port B. After having a high pulse at its base the SL 100 starts conduction and a voltage is sensed across a I K resistance connected to em itter and ground . Part of this voltage is inverted by a unity gain amplifier designed by the IC op-amp LF-351 and trimmed to the value corresponding to the AID value and goes to the adder op-amp IC LF 351 for lowering the voltage level equa l to 142.2 mY. By this technique, i.e . dividing the total measured region of temperature in different steps one can realise a higher accuracy with the same AID.

3.9 Control heater

A control heater of lOW is used to del iver power to the control chamber i.e. the optical cryostat. From the output port C bit 4 is connected to the base of an op to iso lator MCT 2E. The emitter output drives a PNP power transistor MJ-2955 for providing necessary cur­rent and maintaining constant temperature to the iso­lated optical cryostat. To control the power delivered to the heater, the authors have used a 25Y dc supply with low ripple content which can deliver a current of 2A. The duty cycle of the pul ses determines the average heat generated by the heater.

4 Software Iprogram structure Fig. 7 shows the flow chart from one way in which

the program for a temperature display cum control sys­tem can operate . After reading the TC input voltage from Channel 2 the diode voltage for compensation is read immediately from Channel I. The two values are then added and the result is used to calculate the temperature va lue from Eq (I). A keyboard in the system allows the user to enter set point values under the program control.

The software includes several basic subroutines for different functions. The software written in assembly language to realise a fast action can be downloaded to a ~P kit. Delay (for controlling the interval between two data readings), AID read (for reading the data from the AID port), A to I (to convert the ASCII value entering from the keyboard to an integer), I to A (to convert the

488 INDIAN J PURE APPL PHYS. VOL 37, JUNE 1999

integer value to the ASCII value for display on the monitor), Readkey (to read data from the keyboard), Prints (to print an integer value in the monitor) and Search ( to search the corresponding temperature value from the calibration table for a particular thermo emf value) are the important procedures used here.

The keyboard procedure allows the user to control the data to change the calibration and set the different control temperatures. The keyboard procedure allows the user to change the set points, to stop a process or to examine the value of process variables at any time.

5 Performance/Accuracy In this desig', the temperature varies from -196°C to

+ 100 °C i.e. the total temperature span is 296 0c. The analog output of the TC after suitable addition varies from 0 m V to 996 m V due to th is temperature variation i.e. 0 m V represents -196 °C and 996 m V correspond to + I 00 0c. This temperature range is split in seven equal steps of296/7= 42.2 °C and the thermo emf also divided in equal seven steps of 996/7 = 142.2 m V. So the required full scale reference voltage is 142.2 m V. But in practice it is found that for safe linear conversion with maximum input voltage of 142.2 m Va reference voltage 300 m V is appropriate. Further reduction of reference voltage for maximizing the resolution get defeated by nonlinearity in AD conversion. So for a 300 mV refer­ence and 142.2 mV maximum input voltage which cor­responds to the change of each 42.3 °C with the 8 bit ADC, the resolution is equal to (42.2 /256)*(300/ 142.2) = 0.35 0c. Th is resolution is not un iform throughout the entire temperature region as the sensitivity of the TC is not uniform throughout the above temperature span.

It is also to be mentioned that without splitting the temperature region a maximum resolution of the order of 296/256 = 1.15 °C is achieved with suitably appl ied reference voltage to the AD and the gain of the TC output.

200 G

eQ; 100 L. ::>

~ 0

5.1 Calibration

For the measurement of the specimen temperature the authors have used a single junction TC. The calibra­tion is done under the program control. For calibration the authors have used two constants K 1 and X2. K 1 is used to add a suitable diode voltage for the cold junction compensation. For easy calibration turn off the Channel 2 (disconnect the TC) and read only the diode data through Channel I. By selecting the proper value of K I adjust the output temperature equal to the room tempera­ture. Th is is the voltage requ ired for compensation . Now turn on the Channel 2 (i .e. TC sensor) and adjust the calibration factor K2 to get the room temperature. The thermometer is now cal ibrated . To ensure the cal ibration put the TC at any other temperature say ice or steam temperature and adjust the value of K2 to get the corre­sponding temperature.

The performance of the designed controller has been tested by assembling the module for measurement and control of an optical cryostat developed by the authors in this department. The controller has been tested at different tem peratures usi ng a cal ibrated electron ic ther­mometer. It is capable of maintaining the temperature stable within ± 0.35 °C over this temperature range for about half an hour. The time-temperature control char­acteristics are shown in Fig.8. After pouring LN2 in the cryostat it takes about 20 minutes to reach the lowest temperature which is about 78 K (LN2). To attain differ­ent temperatures the controller is switched on . The horizontal line along the temperature axis of Fig. 8 indicates the constancy of the temperature against time . From these curves it is clear that the temperature may be controlled anywhere in between the LN2 temperature to + 100°C for a reasonable period of time . In Fig. 8 time scale is represented in units of minutes to show the long time performance. Practically there is little fluctuation if the time is expressed in units of seconds. This curve is drawn taking the average value of the experimental points. It requires further investigation to study the other

~-100 ~ -200~~ __ -L __ -L __ ~ __ ~~L-~ __ -L __ ~ __ ~ __ L-~

20 40 60 80 100 120 140 160 180 200 220 240 Time (min)

Fig. 8 - Time-temperature characteristics of the controller

t

MANIK el al.: PC-XT/AT BASED LOW TEMP. DISPLAY 489

performance of the controller like speed of response, the lag time etc.

The specification of our developed AID system is : (i) 8 bit AID converter with conversion speed up to

251lS (ii) Acquisition frequency = 25 kHz (iii) 8 channel for different input signal Besides the present purpose this unit can be used for

other purposes with slight modification: (i) storage of data from different sensors with different delay loops, (ii) by using different calibration curves whether linear or nonlinear this unit can be used as different measuring instruments like vacuum meter, level meter, photo meter etc. In the present measurement better resolution of temperature measurement is achieved with a 8 bit ADC by splitting the total temperature region in different steps which is done automatically. For higher resolution an ADC with higher bit can be used . But this technique can be applied everywhere to split the total temperature region to enhance the resolution . The controller is inex­pensive and accurate to - 0.35 °C over the temperature range from - 196°C to + IOO°C.

6 Conclusion This paper describes electronic modules designed

and constructed for stabilising and maintaining the ther­mal balance inside any isolated system with optimum performance and control. The reliability and perform­ance depend on several factors such as thermal cycling, lag in heat transfer between the heating coil and sample, detector, delay factor between the sensors and sample, the effective thermal mass of the optical cryostat , differ­ent inherent noise such as non-uniform heat transfer, thermal vibrations, damping forces or other dissipative disturbances within or outside the cryostat. Little change of temperature may occur due to thermal noise of the sensors and input common mode rejection ratios of the op-amps . A drift also comes from the switching tran sis­tor. The temperature error caused by all these facto rs is of the order of 0.35 0c.

Acknowledgmen t The authors wish to thank Dr S C Mukherjee, Advi­

sor and Scientist of Advanced Centre of Cryogenic Research Centre, Calcutta and Dr A K Rakshit, Electri­cal Engineering Department of Jadavpur University, Calcutta for their valuable suggestion and cooperation in developing this work.

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