NOMENCLATURE -THE BACKBONE OF ANY DISCIPLINE ALL MESSED UP INTHE FIELD OF MEASUREMENT SYSTEMS

Submitted by Peter Stein, President, Stein Engineering Services, Inc., Phoenix, AZ

Practitioners in any discipline can only communicate effec- tively if their dictionaries are the same - if, by saying the same words they mean to express the same ideas. This de- sirable state of affairs requires a standard and common no- menclature. It is the author’s contention that this condition is totally absent in the field of engineering and dynamics of measurement systems for test and evaluation. He has es- tablished, over the last almost-50 years, such a nomencla- ture for that field and propagated it wherever he could.

The problem with the field of measurement is that people drift into it from many different disciplines. All engineering disciplines require measurement and everyone brings with them the nomenclature from the field they have migrated from into the measurement field.

My Monograph (Ref. 1) contains numerous sections with the title “Nomenclature Problem,” in which some of the more dis- turbing examples are cited. In this article I will discuss pri- marily the naming of transducer operating principles.

It may not always have been so, but today transducer oper- ating principles are identified by the energy form at the transducer input, usually hyphenated (but not always) to the energy form at the transducer output. I will use the hy- phens in this article for clarity. The response may be in the form of energy (such as electrical) or of a material property (such as resistance) associated with that energy form. Thus it seems a pretty straightforward nomenclature system which everyone can understand and leave little doubt about what is meant. Thus:

THERMO-ELECTRIC: a thermal input creates an electrical output as in a temperature-sensing thermocouple.

THERMO-MECHANICAL: a thermal input creates a me- chanical response as in a thermal expansion device.

THERMO-ELASTIC: a thermal input creates a change in elastic properties such as elastic modulus. This mechanism can be the basis of analog or frequency-modulated tempera- ture transducers.

PIEZO-ELECTRIC: a force or strain input creates an elec- trical output as in a quartz crystal or a polarized ferroelec- tric ceramic.

PIEZO-RESISTIVE: a force or strain input creates a change in electrical resistance as in a metallic or semiconductor bonded or unbonded resistance strain gage or any of the many transducers based on it.

PIEZO-CAPACITIVE: Aforce or strain input creates a change in electrical capacitance.

PIEZO-THERMAL: a force of strain input creates a tempera- ture change in the specimen to which the force is applied. This is the first of several seriously mis-named effects. It is the basis of the strain measurement method uthzed by the Ometron and Stress Photonics companies, for example and is often called the Kelvin Effect. But they have called it THERMO-ELASTIC since the development of the method in the early 198Os, simply because William Thomson Lord Kelvin so named it in the 1800s when he Grst hscussed the effect. I have been unable, since the early 198Os, to convince the Ometron group that this nomenclature is misleading and incorrect. Lord Kelvin is sacrosanct in England where the method was developed and so nothing has been done to cor- rect the misnomer. But the word THERMO-ELASTIC today implies quite a different transducer mechanism as identi- fied above.

ACTIVE AND PASSIVE

Perhaps the biggest nomenclature problems in the field to- day are the words active andpassive when referring to trans- ducer mechanisms. The IEEE defines a thermocouple as

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being passive and the ISA defines a thermocouple as being active. Those words should be banned from the vocabulary of the measurement engineer!

Some 40 years ago I coined the words self-generating refer- ring to transducers for which the energy input hec t ly cre- ates an energy output as in a Thermocouple or a thermo- electric device, and non-self-generating, referring to trans- ducers in which the input energy first produces an impedence change (electrical, mechanical, magnetic, etc.) which is then interrogated to convert it into the output energy, as in a re- sistance thermometer or thermo-resistive device.

Please note that all these definitions imply that the trans- ducer is only an energy-processing device, which misconcep- tion has permeated the field for centuries.

In fact, transducers are information and energy processing devices, of which the information portion is the most impor- tant since the usable and desired output of all measurement systems is, in fact, information. I believe I was the h s t to classlfy information flow in terminology useful to the mea- surement engineer',*; but that is not a topic for hscussion today.

The above definition of a transducer is the only viable defini- tion for purposes of the design, analysis, operation and in- terpretation of measurement systems. This statement is made in spite of the fact that arguments about how to define transducers keep raging in the literature. They make no

sense when a unified approach to the engineering of mea- surement systems is needed and is now available.

In the information flow portion of measurement systems, my pet peeve has to do with pulse modulated systems. Apulse train contains numerous frequencies as determined by its duty cycle giving a particular Fourier spectrum. But a steady state pulse train has only one repetition rate. How can we, then, talk intelligently about pulse frequency modulated (PFM) information flow? Should it not be pulse rep rate modulation to be accurate, precise and intelligible?

For a discipline which prides itself on the utmost in accu- racy and precision, we use terminology which may be 100% off the mark! Isn't it time we adopted a unified approach?

References

1. Stein, Peter K., The Unified Approach to the Engineering of Measurement Systems, Vol. I - Basic Principles, Mono- graph, Stein Engineering Services, Inc., ISBN No. 1-881472- 00-0, 134 p. soft cover, 8th printing, 1997.

2. Stein, Peter K., Sensors as Information Processors, Re- search & Development, R&D, June 1970, pp. 33-40. Publica- tion #3 from the author.

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