A Multiplexed Temperature Data Acquisition System with an overall precision of +-25 ppM has been designed using state-of-the-art electronics to accurately read temperature between 2.4 K and 600 K from pre-calibrated transducers such as germanium, silicon diode, thermistor or platinum temperature sensors

Farah, Y.

Sondericker, J.H.

UNT Digital Library

PMKSKNIVD AT CKC/1CMC MKRT1NS, MASSACHUSETTS INSTITUTE OF TECHNOLOGY,AUGUST 13-16, I»»8 .,,BNL 36969BNL—36969DE85 017847A PRECISION CRYOGENIC TEMPERATURE DATA ACOUISITION SYSTEM*Y. Farah and J.H. SonderickerBrookhaven National Laboratory,Upton, New YorkABSTRACTA Multiplexed Temperature Data Acquisition System with an overallprecision of ±25 PPM has been designed using state-of-the-art elec-tronics to accurately read temperature between 2.4 K and 600 K frompre-calibrated transducers such as germanium, silicon diode, thermistoror platinum temperature sensors.INTRODUCTIONIncluded in the design of the BNL 24.8 KW helium refrigerator areabout 400 temperature transducers of different types (diodes,germaniums, platinums and thermistors) used to read temperature between2.4 K and 600 K. Due to the high accuracy demanded of these devices,1an ultra high precision temperature data acquisition system is necessaryto resolve small temperature differences.Because of the high number of transducers, a multiplexed system isnecessary to keep the system compact, thus reducing hardware costs whilepreserving the precision of the system.A block diagram is shown in Fig. 1. IE consists of four front ends(3 interfaced to the refrigerator and one at the compressor facility)which read temperature in analog form, digitize the signals and shipthem to an 8086/8087 based microcomputer. There the raw data islinearized by use of corresponding polynomial coefficients and finallyshipped to an ANATEC 11/34 process control computer for furtherprocessing. A maximum of 168 temperature transducers can be processedby one front end.SYSTEM DESCRIPTIONFront End Section MASTEREach front end (Fig. 2) consists of a EURO-CARD crate which holds 12analog input boards and a precision reference voltage module. A MULTIBUS*Work performed under the auspices of the U.S. Department of Energy.DiS;j!!BUIIQH Of THIS MCiKftNT !S (JKLIMITEflREFRIGERATOR BUILDINGTEMP XOUCERSOIOOE5:266PLATINUMS: 4 2THERMISTORS: 4 2GERMAN! <S; 14FRONT END#2IGERM.MCOULE3THER. MODULE3PLAT.MOOULEFRONT END#310 OIODEMODULESFRONT ENO• 49 OIOOEMODULESCOMPRESSOR BUILDINGANATEC(DEC 11/341BASEOMICROCOMPUTERPLATINUMS:42I4TEMR.XOUCERS'ANALOG MUXV/F CONVERTERMODULE* IMOOULEENA DECODERI4TEMRXDUCERS'MODULE# 1 2ENA OECODEf)PREOSIONRER VOLTFig. 1 - System Block Diagram2.500 V4 LINES12 V/FINPUTS8085 BASEDMICROCOMPUTERSERIALOUT12 ENABLELINES4 OECOOERLINESTO 8 0 8 6 / 8 0 8 7BASED MICROCOMPUTERFig. 2 - Typical Front End Block Diagramcard cage contains an 8085/8232 microcomputer card and two digital cardsinterfacing signals between the microcomputer card and the analogmodules.Analog SectionAn analog input board consists of low level voltage analog multi-plexers, amplifiers, an ultra linear voltage to frequency converter andoptical isolators used to separate grounds between the analog and digi-tal sections. The analog board can connect to a maximum of 14 tempera-ture transducers of the same type since gain and offset arc fixed on aper board basis.The precision reference voltage module delivers an ultra stable 2.5volts to the 12 analog modules Which in turn each produce a 10 yA con-stant current (1 mA in the case of platinum transducers). The constantcurrent signal is multiplexed by 16 analog switches, holding the valueto within a precision of about ±10 PPM. Fourteen of these outputs areconnected to the temperature sensors via a 4 wire connection in the caseof platinum and germanium transducers or 2 wire in the case of diodesand thermistors. The first of the other two multiplexers on the boardis used for switching the constant current into a precision resistor tccompensate for drift, and the second to compensate for amplifier driftand offset. An intentional Va^d *•• added to the output voltage beforethe V/F converter to insure that offset and drift voltage is the same po-larity as the signal since the V/F converter input is unipolar.Fig. 3 shows a simplified representation of the analog circuit, inwhich I is the constant current and multiplexed R is either one of the14 unknown transducers, the reference resistance, or the offset resis-tance. From Fig. 3:- K(RxI) • VIn case of R " 0:In case of R • Rref !( Q f fm U + Vzero (off + drift) add "Vref ' K(Rref * * V(off + drift) + Vadd *(1)(2)(3)n PULSESTO DIGITALSECTIONFig. 3 - Simplified Representation of the Analog CircuitIn case of R - Rx:V ^ - K(Rx x I) • V ( Q f f + d r . f t ) •Subtracting (2) from f3) and (2> from (4) yields:Vref " V.eroVRx ~ VzeroDividing (6) by (5) yields:zerox r** V - - V x ref (7)ref seroEquation (7) shows that the precision of reading Rx (the unknownsensor) is related to the reference resistance Rreft and the precisionof the V/F converter which translates Vj^, V r e f and V r e r o into pulsessent to the digital section. The voltage to frequency converter used inthe analog module exhibits a non-linearity of ±2 PPM in the frequencyrange of 100 HZ to 6000 HZ. To make up for lost resolution, a dualcounting technique is used as explained in Appendix A.Digital SectionThe digital section contains an 8085 processor, an 8232 double preci-sion floating point processor, 4 K bytes of program memory, 2 K bytes ofdata memory, counters, drivers and optical isolators. A memory residentprogram drives the analog modules, enabling the first of the 16 channelson all 12 analog boards. Amplifier offset is digitized (n^ pulses andshipped to the digital section. The second channel is activated andencodes the reference resistor voltage which measures the value of theconstant current (njj) pulses. The multiplexer steps through the 14 tem-perature transducers one at a time which provides (n^i pulses. Afterthe 16 channels have been scanned, the cyclfc repeats itself.The dual counter technique,2>3 as described in Appendix A, willenable the V/F (n) Pulses and the high frequency (N) pulses to be readat the same time by the processor. A 6 MHZ high frequency clock is usedto achieve a resolution of 1 PPM when the counter is gated for a 175msec period.By substituting equation (Al) in equation (7):\ef / Nref - no / No « f " « >All mathematical manipulations are done by the 8232 floating pointprocessor using 64 bit double precision arithmetic. The value, Rref» ^ssoftware trimmed in test, and later fixed in program memory.The result of the calculation (Rx) for each temperature transduceris serially transferred to the 8086/8087 microcomputer in the form of a64 bit floating point double precision number along with address andstart, stop, and check bits. The check bits are encoded using the cy-clic error detecting code defined by the 12th degree polynomial:^G(x) - X 1 2 + X11 + X3 + X2 + X + 1 . (9)A total of 89 bits per temperature transducer are transmitted to the8086/8087 section at 0.5 megabaud rate. Isolated current loop connec-tion permits communication up to a distance of 1000 feet.8086/8087 Microcomputer SectionThe microcomputer section consists of an 8086 Microprocessor, an8087 Numc.ic Data Processor (NDP), 32 K bytes of static RAM and 120 Kbytes of EEPR0M of which 8 K bytes are reserved for program memory. Theremainder of memory (112 K bytes) is used for temperature transducerlinearizing polynomial coefficients which are generated whilecalibrating each transducer. Each transducer calibration curve occupies320 bytes of memory divided into A segments, each consisting of onebreakpoint value and nine coefficients A0 through A9.The 8086 processor is programmed to scan the A analog front ends.When a message is detected, the 8086 will check the validity of the mes-sage. The address is checked and the polynomials associated with thisaddress are evoked by the 8087 NDP which determines the proper segmentand evaluates the temperature from the polynomial:Tx - A 8(Rx)8 + A ?(Rx)7 + A 6(Rx)6 + Ag(Rx)5 + A ^ R x ) 4 + A,j(Rx)3+ A 2(Rx)2 + Aj(Rx)1 + Ag (10)The temperature is placed i.i 16 bit form and transmitted with a 9bit address to an ANATEC PDP-11/34 processor along with 12 redundantbits for error checking, and one start and 3 stop bits for synchroniza-tion.ANATEC - 11/34 Interface SectionThe process computer interface hardware contains A K bytes of staticRAM in which Tx engineering units data is placed from the 8080/8087microcomputer section. Each temperature value occupies 16 bits of mem-ory of which 1A bits are mantissa and 2 are decoded for floating decimalpoint position. The process computer reads and unpacks the word, de-codes and places the temperature data in internal float tables which areaccessed by the control system.CONCLPSIONA very high precision multiplexed system for reading temperatures be-tween 2.A K and 600 K is described. The overall precision of the systemis greater than ±25 PPM including variations with time and temperature.The data acquisition system is self calibrating, eliminating the needfor adjustment and fine tuning.Future improvements will increase system precision to ±16 PPM andthe polynomial coefficients will be placed on hard disk memory for flexi-bility. Changing polynomial coefficients will become a simple task whenreplacement of a temperature transducer becomes necessary. Also frontend capacity will be extended to accomodate a maximum of 22A transducers.APPENDIX AHigh Resolution A/D Converter^The most straightforward method of achieving 1 PPM resolution is touse a 0 to 1 MHZ V/F converter and simply count the number of pulses ina one second period. Unfortunately, 1 MHZ V/F converters withCONVERT COMMANDFREQUENCY FROM VFCCOUNT GATEGATED FGATED SYSTEM CLOCK11 1 1 1 1 1 1J1 1 1 1i i n i i i i n u ni I i1 I11111111i iI I In nun1I11I i i"COUNTS"COUNT*CONVERT COMMANDCLCOUNTGATEREFERENCECLOCKCOUNTER A n f • •§- F CLOCKCOUNTER 8Fig. Al - V/F Converter Timing Diagramnonlinearities in the PPM range are simply nonexistent. An alternativeis to use a low frequency V/F which produces (n) pulses and to feedclock pulses (N), at a rate of 1 MHZ or higher into the counting pro-cess. A D-type flip-flop can be harnessed to the count gate of a dualcounter scheme, to synchronize the clock with the V/F converter pulsesas in Fig. Al. The input voltage will then be proportional to the ratio(n/N) of the numbers generated by both counters:viNa<r (ADV/F converter LM 331 exhibits a nonlinearity of ±2 PPM or better be-tween 100 HZ and 6 KHZ, thus by restricting its output to that range,and by using the dual counter technique, high resolution and accuracycan be gained with low frequency V/F converters.ACKNOWLEDGMENTSThe authors wish to thank the Cyrogenic Electronic Support Group es-pecially Mr. W. Hirsel for the construction of the prototypes and Mr.J. C. Osterlund for providing the transducer's coefficients and PROM pro-gramming .REFERENCES1. J. H. Sondericker, Production and Use of High Grade Silicon 3iodeTemperature Sensors, "Advances in Cryogenic Engineering, Vol.27," Plenum Press, New York (1982) p. 1163.2. R. A. Pease and L. Distaso, V/F Converter and Microcontroller Pro-duce High-resolution ADC, Electronic Design, September 13, 1980.3. B. Trump, VCF's take their place in A/D Conversion, ElectronicProducts, January 1, 1985.4. W. W. Peterson and D. T. Brown, Cyclic Codes for Error Detection,Proceedings of the IRE, January 1961.

Precision cryogenic temperature data acquisition system

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Precision cryogenic temperature data acquisition system

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