IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-28, NO. 4, DECEMBER 1979

The software for the microprocessor is implemented into2 kB EPROM, and the hardware needed for this terminal isimplemented onto a single board. Thus it can be installedinside the CRT terminal with added connectors for measur-ing instruments and an acoustic coupler. If we have theIEEE Standard 488 Bus [5] for the interface betweeninstruments and the microprocessor, only one connectorwill be sufficient.

ACKNOWLEDGMENTThe authors would like to express their gratitude to K.

Akiyama, T. Ikeda and Y. Nakashizuka for their support

during the software development. We are also grateful to N.Onda and S. Yamazu for their assistance in experiments.

REFERENCES[1] H. Cole, "System/7 in a hierarchical laboratory automation system,"

IBM Syst. J., vol. 13, no. 4, p. 307, 1974.[2] A. A. Guido, H. Cole and L. B. Kreighbaum, "Interactive laboratory

automation aid: The research device coupler," Proc. IEEE, vol. 63, p.1509, Oct. 1975.

[3] SDK-85 System Design Kit User's Manual, Intel Corporations, 1977.[4] S. Ohkawa and H. Yamamoto, "Precision measurements of high

standing-wave ratio and extremely small phase shift," in Proc. 1976Conf. Precision Electromagnetic Measurement, p. 159.

[5] IEEE Standard 488-1975, Digital Interface for ProgrammableInstrumentation, IEEE Apr. 1975.

Development of a Standard Cell ReferenceGroup with Microprocessor-Controlled

IntercomparisonsERNESTO ARRI, MEMBER, IEEE, AND UMBERTO POGLIANO

Abstract-An automatic system is described to intercompare thestandard cells of a reference group. A microcomputer controls theselection of cells or external sources, the electromotive force (EMF)difference measurement, and the protection devices. The pro-grammed cell selection is performed by actuating suitable low-thermal rotary switches driven at limited speed. TheEMF differenceis measured through an automatic potentiometric device at nanovoltlevel. Protection is provided by both software and hardware.The system designed is intimately connected to a new thermostatic

air bath and aimed at improving the uncertainty of the comparisonsto the limit imposed by thermal noise to a few nanovolts. It is intendedboth for the maintenance of the voltage unit alongside a Josephsonstandard and for first-level dissemination of the same unit. Prelimin-ary experimental results on the performance ofthe automatic systemare reported.

I. INTRODUCTION

IN NATIONAL laboratories locking the voltage unit to-the fundamental constant 2e/h by means of a Josephson-

junction primary standard has supplanted the traditionalmaintenance through a reference group of numerous stan-dard cells.

However, a reference group, even with much fewer cells, isstill necessary as a physical analog memory for day-to-day

Manuscript received May 16, 1979; revised July 23, 1979.This work was supported in part by the Consiglio Nazionale delle

Ricerche (CNR), Italy.The authors are with the Istituto Elettrotecnico Nazionale "Galileo

Ferraris" (IEN), Turin, Italy.

maintenance of the volt during the interval between twosuccessive comparisons with the Josephson standard.Groups of several cells will also continue to be used for thedissemination of the unit.The analog memory of a reference group is the average

value of its cell electromotive forces (EMF's). In assigningthis value, a great amount of repetitive, tedious and time-consuming comparisons with high uniformity are required;therefore, methods for automatic intercomparisons havebeen recently proposed [1]-[4].At the Istituto Elettrotecnico Nazionale (IEN) work is in

progress for setting up a new reference group devoted bothto direct comparisons with the Josephson standard and tofirst-level dissemination of the volt. For this new group achange from oil to air thermostatic bath technique is underway and microcomputer control of cell selection, intercom-parison, and protection is introduced. The main aimspursued are to improve the uncertainty of the comparisonsto the limit set by thermal noise (a few nanovolts), toeliminate the manual operator, and to reduce the measure-ment time.

II. THE AUTOMATED INTERCOMPARISON SYSTEMThe entire system is illustrated in block diagram form in

Fig. 1. For purposes of description it can be divided in thefollowing functional parts.

1) Thermostatic Air Enclosure: Since, as shown in Fig. 1,

0018-9456/79/1200-0311$00.75 C) 1979 IEEE

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-28, NO. 4, DECEMBER 1979

| 1 | ~~~~~~~~~STAGEl

. I - 1CONTROL

I v.DRIVE SELECIION

§ MOOlT LOSIC

EXTEINALIWI DIVIDER SAOURC

rrAMPIIER DIGITALL - - - - - - - - J CONVERTER -

Fig. 1. Block diagram ofthe microprocessor-controlled intercomparisonsystem and thermostatic air bath for a standard cell reference group.

TOMI N ICOMPUTER

the automatic system is intimately connected with thethermostatic system, a brief preliminary description for thislatter is necessary, even though it is still under construction.1The air bath has a circular cylindrical geometry and is

designed as a two-stage thermostatic enclosure with innerand outer cylindrical walls of multilayer type with alternatethermal-conductor-thermal-insulator structure. The firstexternal stage can provide both heating and cooling bymeans of four Peltier-element actuators and also includesthe thermally more critical components of the automaticsystem, as indicated in Fig. 1. With the external ambienttemperature of + 1°C around 20°C, the temperature insidethe first stage is kept by feedback at 19.99°C with a toleranceof a few millikelvin.The second internal stage contains the saturated standard

cells maintained at 20.0000°C. In fact, at this temperaturesaturated cells have an absolute temperature coefficient ofabout -40 uV/K and a differential one about ten timeshigher. The design of the enclosure aims at obtaining atemperature uniformity better than 20 uK in the inner stageand maintaining a stable temperature within 100pK at mostwith a long thermal time constant. For this, the internalstage has in its central part a core of thermal conductingmaterial (aluminum alloy) with a high thermal capacity, inwhich 21 cells are placed in seven special 3-cell containers.The temperature control is actuated only by heating bymeans of two resistive cylindrical layers placed externally onthe inner wall near the top and bottom. Moreover, each3-cell container is removable with minimum thermal troublefor the second stage. Simulations with mathematical modelshave proved that the central-core temperature disuniform-ity, due to an opening of the first stage for the substitutionof single cells, is limited to a few tens of microkelvin.

2) Cell Selection: For comparison purposes the selection

'A paper on the air bath is under preparation.

between the 21 cells and external sources, connected in theusual back-to-back configuration, is controlled by amicrocomputer2 and carried out by two 32-position switchboxes placed inside the external stage of the thermostaticenclosure. The low-thermal rotary switches3 ofthe boxes areprovided with an additional deck for sensing their positionand are driven, through long thermally insulated shafts, bygeared synchronous motors outside the air bath. The limitedspeed (4 r/min) of the shafts reduces the contact heating andminimizes the thermal EMF's: experimental tests haveshown values lower than 30 nV with an extinction time(reduction down to 10 percent of initial value) ofabout 1.5 s.The microcomputer selects the two internal or external

sources to be compared by program, actuating the driveinotors through a suitable selection logic.

3) EMF Intercomparison: The EMF difference ismeasured by an automatic potentiometric system at nano-volt level controlled by the same LSI 11 microcomputer.2The system has been already described for the first version[5]; the present version only has minor modifications for thespecific task. The system reproduces the techniques ofmanual potentiometers starting with the unbalance signal ofa galvanometric photocell amplifier, in which the micro-computer provides a suitable strategy of null balance actingon a programmable compensation source and the galvan-ometer sensitivity. In such a way, the current in the compari-son circuit is continuously kept at an almost zero value.The difference from the earlier version [5] is that in this

application the sensitivity selection is realized by means ofanother low-thermal rotary switch3 located inside the ther-mostatic enclosure and driven by an external motor likethose used for cell selection. As the switch is ofmake-before-break shorting type, it allows the insertion in the compari-

2 Digital Model LSI 11.3Leeds & Northrup Type 31, 16-position.

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ARRI AND POGLIANO: STANDARD CELL REFERENCE GROUP

son circuit of resistors of value suitable for the differentsensitivities without opening the circuit, which can causetroubles. Moreover, the same switch can both open thecircuit and reverse the galvanometer terminals for makingcomparisons in both directions.

4) Protection: In order to avoid damages to the cells, themicrocomputer also controls suitable protection devices.The management program checks the comparison circuit inthe critical instants of the balance procedure, providing forcircuit opening when either the chosen maximum compari-son current is exceeded or the galvanometer sensitivity iskept at its highest value for too long. Further protectionsystems are actuated not only by software but also byhardware, as for example the protection against lack oflamplight for galvanometer photocells.

All the protection systems act through the rotary switch ofthe galvanometer sensitivity. The switch is connected to itsdrive motor through a friction clutch actuated by an electro-magnet and is pulled back by means ofa return spring to theparticular position that opens the comparison circuit. Whenone of the security conditions fails, the electromagnet isdenergized by the microcomputer, the clutch is disconnectedand the switch opens the comparison circuit.

5) Input-Output: The management program and thesuccession of both the cell pairs to be compared and therelated probable EMF differences, as first balance values(based on the last EMF's determined), are inputted, bymeans of a paper-tape reader (Fig. 1).

For each comparison the identification numbers of thecompared sources, the comparison type (direct or reverse)and the balance result are outputted, on Teletype and papertape (Fig. 1).

Alternatively the output interface of the microcomputercan be connected to a minicomputer for data elaborationand storage and for certification of external sources (Fig. 1).

III. OPERATION

For each cell pair four comparisons are made: direct andreverse in both directions by reversing the galvanometerterminals and hence eliminating systematic errors otherthan those arising in the switches. The measurement cycle ofeach comparison consists of three basic steps.

1) On the basis of the input data (source pair and theirprobable EMF difference) the microcomputer providesboth for putting the selection switches in the requiredposition and for presetting the programmable source to theforeseen initial value. In case the knowledge of the previousEMF value of a cell is lacking, the programmable source isset to zero.

2) Meanwhile the microcomputer determines the relativezero of the galvanometer [5].

3) Successively the microcomputer controls the balanceat increasing sensitivities and then, also through suitablecorrections [5], determines the EMF difference, which isprinted and punched.

IV. PRELIMINARY RESULTSThe automatic system exhibited a comparison resolution

of 1 nV for cell EMF differences lower than 100 puV. Theimplementation of the thermostatic bath has just begun, thesystem has not yet been tested with a cell group. However, ithas been systematically checked for all its functions. Inparticular sources where a voltage of approximately 50 uVand an internal resistance of about 1 kQ have beenmeasured, actuating the switches in various settings: therepeatibility of the results was of about 3 nV.

V. CONCLUSIONSThe system described for automatic intercomparisons

between the cells of a reference group by means of amicrocomputer, which controls cell selection, comparison,and protection, seems very promising on the basis of theresults of preliminary experimental tests. Besides the elimin-ation of the manual operator and the reduction of themeasurement time, the third main advantage, i.e., loweruncertainties at nanovolt level in the comparisons, will becompletely proved as soon as the implementation and thepreliminary tests on the intimately connected thermostaticair enclosure are accomplished. The whole system will thenbe tested for almost a year using a cell working group beforebeing dedicated to its planned use with a new referencegroup alongside a Josephson primary voltage standard.

ACKNOWLEDGMENTThe authors are indebted to M. Angelino for suggestions

due to his experience in cell comparisons and M. Negro forparticipation in the construction. Dr. F. Cabiati made a bigcontribution in designing the thermostatic air bath.

REFERENCES[1] H. Hirayama and Y. Murayama, "Automatic measuring system for a

control of standard cells," IEEE Trans. Instrum. Meas., vol. IM-21, pp.379-384, Nov. 1972.

[2] A. F. Dunn, "Automatic intercomparison of standard cells," IEEETrans. Instrum. Meas., vol. IM-23, pp. 278-282, Dec. 1974.

[3] D. W. Braudaway and R. E. Kleimann, "A high-resolution prototypesystem for automatic measurement of standard cell voltage," IEEETrans. Instrum. Meas., vol. IM-23, pp. 282-286, Dec. 1974.

[4] S. Harkness and C. H. Dix, "A new NPL voltage standard with com-puter controlled measurement," in Proc. EUROMEAS 77 (Brighton,England), pp. 158-160, 1977.

[5] E. Arri, U. Pogliano, and G. Righini, "Microprocessor-controlledpotentiometric system for nanovolt measurements," IEEE Trans.Instrum. Meas., vol. IM-27, pp. 372-376, Dec. 1978.

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