This series of articles deals with re- sistance strain gages, the transduc- ers based on them and the signal conditioning required and desired for them. The approach is unconven- tional, as the first three installments have already shown, and features The Unified Approach to the engi- neering of measurement systems for test & evaluation.

Peter K . Srein The first three parts dealt with how

strain gages and strain-gage-based transducers respond to their environment and the desired and undesired environment-response conditions which may exist.

This installment deals with how information flows through transducers of any kind, including strain-gages and the transducers based on them.

INFORMATION FLOW IN ALLTRANSDUCERS

AND STRAIN GAGES INCLUDING STRAIN-GAGE-BASED

IN FORMAT10 N FORMATS: UN IF I E D AP PROACH

For the engineering of measurement systems according to the Unified Approach, it is more useful to consider information as existing onpatterirs ofproperties of wave shapes of measurands, than as bits and bytes. Note, this installment will not consider patterns since they are less crucial to the performance of the measurement system than the properties of the wave shape and be- cause space is limited. See Ref. 1 for more detail.

WAVE SHAPES

Commercially, there are three dominant wave shapes: 1. DC or constant levels 2. Sine waves 3. Pulse trains

The concept is open-ended and, if triangular waves should ever gain popularity, they could be included, for example. Pulse trains could be zero-based or zero-cen- tered, a difference of only a DC level which is sometimes important.

PROPERTIES

The most common properties of these wave shapes are summarized in Table I. When information is contained in the total number of elapsed periods or cycles (such as counting fringes in optical systems), the author prefers

Editor’s Note: ET ispleased to feature the fourth “Back to Basics” article in a series on strain gages, thanks to veteran SEMmeniber, Peter K. Stein. This series is intended for the novice, and as a refresher for all others. Each article iri the series will address a specific topic. I f y o u have any cortitnents about the series, or ques- tions for Pete to address in th i s series, please contact rne at pat@senil.coni. PD

Peter K. Stein (SEM Fellow and 47-year rneniber) is President of Stein Engineering Services, Inc. i n Phoenix, AZ.

TABLE I: INFORMATION CARRYING METHODS

WAVE SHAPE INDIVIDUAL PROPERTY

CONSTANT LEVEL AMPLITUDE

SINE WAVE AMPLITUDE FREQUENCY PHASE PLANE OF POLARIZATION DIRECTION OF PROPAGATION WAVE LENGTH OR VELOCITY (PERIOD)

PULSE TRAINS AMPLITUDE REPETITION RATE POSITION DUTY CYCLE (PERIOD)

(*) PWM = Pulse Width Modulation, PDM = Pulse Duration Modulation, Duty Cycle is defined as the ratio of pulse on-time divided by the period.

to consider these as sine-wave-modulated with a 360- degree phase shift increment or pulse-position-modu- lated with position increments of integer periods, rather than as a separate property - but tha t is only a prefer- ence.

ATERMINOLOGY PROBLEM

In the vernacular, the term pulse frequency modulation is used, implying that pulses have a frequency. Only sine waves have frequencies; pulse trains contain many fre- quencies, but they have a repetition rate. For a disci- pline as concerned with precision as measurement engineering, the terminology should also be precise, and i t is not. But in this survey paper, the author will try to meet his own criteria.

IMPORTANT OBSERVATIONS

Even with the properties of wave shapes as shown (with- out even considering patterns), there are eleven combi- nations. The following concepts are critically important to the Unified Approach:

1. It is quite possible to have eleven measurement sys- tems, each working on one of these combinations, re- cently calibrated, and all attached to the same process, making eleven different measurements; none of which is necessarily correct. Each of these combinations has unique properties as different from each other as night from day. The combinations must be selected to fit the conditions of the measurement problem as will be eluci- dated below.

2. The word “digital” is conspicuous only by its ab- sence. Digital data are already pattern processes - in space or time, parallel or serially.

3. Note that the word “analog” is used in a very specific sense here, quite different from conventional usage. If

Nouember/Lkember 1999 EXPERIMENTALTECHNIQUES 2 I

the world is considered only as either digital or analog, then the system-performance comparisons below can- not be understood or explained.

SAMPLE PROBLEM

Here is a sample problem statement: 11i a remote loca- tiori (several hundred meters away) measure the tempera- ture on a specirnen which has high arnbieiit static arid dyiiarnic fields, such as a motor, geiierator, transformer, welder,etc. The measurement is to be made without the effects of:

1. Electromagnetically induced voltages appearing in

2. Cable capacitance effects appearing in the data; 3. Self-heating of the temperature transducer being

4. Magneto-resistive effects on the transducer or its

5. Thermoelectric effects in resistance thermometers; 6 . Lead-resistance effects; 7 . Electrically induced voltages (electric field effects).

The example uses ternperature rather than strain mea- surement, but all comments relating to the resistance therrriorneter would apply to the resistance strain gage and it gives me an opportunity to compare therrnocouples to resistance therrnorrieters from the viewpoint of the Unified Approach.

The discussion here assumes some background that some readers may not yet have, but which will be pre- sented in future sections. So hang in there and simply believe what I write.

the data;

used;

leads;

CASE I: RESISTANCETHERMOMETER

If a resistance thermometer were selected, it should be platinum or copper, since balco and nickel (and also the commonly used “ D alloy in strain gages - isoelastic, RTM Chatillon Co.) would show sizable magneto-resistive ef- fects. It should not be interrogated from signal condi- tioning with a DC design-controlled input, since DC (analog) systems cannot separate resistance-change in- formation from voltage-change information (generated here by thermo-electric, electrically induced and mag- netically induced voltages, see the first two installments in the series).

Time-varying, design-controlled inputs can perform that separation through the use of Information Conversion principles (modulation or carrier techniques). Sine waves, however, are very sensitive to cable capacitance effects in long runs of cable, and also do not have a good Figure of Merit for self-heating (the ratio of peak-peak to rms value which is only 2.8; for DC it is 1, even worse).

For a pulse train, the self-heating Figure of Merit is 11 (duty-cycle) which can be made as small as desired with low-duty-cycle pulse trains. The frequency response of the desired temperature measurement would dictate the repetition rate of the pulse train. Pulse-interrogated re-

’ sistive transducer systems are also much less sensitive : to cable capacitance effects than sine-wave interrogated . systems.

. The choice of a low-duty-cycle pulse train as design-con- * trolled interrogating input (bridge supply) would fulfill ’ performance criteria 1, 2, 3, 5 and 7 above. Material : selection would solve criterion 4, although even plati- , num has some magnetoresistive properties, but copper . transducers or lead wires would not, near room tem- . perature. There are even copper-resistance-thermom- . eters available on the commercial market which could ’ be considered.

. The choice of a three or four lead-wire signal condition-

. ing system would satisfy the lead resistance criterion,

. 6. Twisted leads should be used. It goes without saying

. that a Zero Interrogating Input switch position should ’ be available on the signal conditioning for noise diag- : nostics and documentation purposes (See Installment 1).

CASE 1I:THERMOCOUPLE

Were a thermocouple used, the materials should be such that magnetic fields do not affect the thermoelectric char- acteristics or the resistance of the thermocouple (or lead) wires. Chromel-P vs. alumel and iron vs. constantan would not be good choices. Copper vs. constantan suf- fers from the notoriously high thermal conductivity of copper. I t is for that reason tha t Bob Moffat of Stanford University quite seriously states: “Yes, I use copper-con- stantan thermocouples - for wrapping packages, but not for temperature measurement.” Again, i t appears that a platinum-based transducer is in order.

Lead wire resistances and their changes can be sup- pressed by the sideways promotion noise suppression method (i.e. zero current flow in the leads when read- ings are taken as in a null-balance system). Magneti- cally induced voltages would have to be minimized by twisting the leads with a twist wave-length very much smaller than the spatial gradient of the magnetic field source.

. Great care must be taken with the linearization scheme ‘ of the thermocouple data, because a pure sine wave pass- * ing through a nonlinear (linearizing) component will . create a DC level which then masquerades as a thermo- . electric signal.

. Electrostatic shielding would be unimportant for the re- ’ sistance thermometer fed from a time-varying interro- ’ gating input as mandated above, but critical for the . thermocouple, which may also require magnetic shield- . ing in addition. In a carrier system, closely filtered to . carrier frequency, any noise voltages at lower or higher . frequency are suppressed by frequency-selective filter- . ing.

: CONCLUSIONS ‘

’ The resistance-thermometer example illustrates the dra- matic performance capability differences of signal con-

22 EXPERIMENTALTECHNIQUES Nouerriber/Decertiber 1999

ditioning depending on the property of the wave shape selected to carry the information through the first stage of the system. Here only amplitude-modulated examples were considered.

Were a modulated thermocouple possible (Ref. l), then most noise voltages could be elegantly suppressed with the traditional Information Conversion (carrier) system as discussed for the resistance thermometer.

Many other considerations in solving the posed prob- lems are neglected here. The information carrying con- siderations are emphasized.

COMMERCIAL HARDWARE

Commercial hardware which supplies pulse trains of any kind as interrogating input for resistance-transducer sig- nal conditioning is not very common, but they do exist.

Manufacturers of such systems, either manually or com- puter-programmable, number less than the fingers on your hands, as far as the author is aware.

THE MORAL OFTHE STORY

Every link in the measurement chain must be carefully selected, evaluated and designed to provide the specific performance demanded in any particular application. I hope that the above to examples convince you that you do not simply pick up signal conditioning hardware in- discriminately, plug i t in, push the CAL button take data and believe it. That process is called rneterosis in analog systems with meters and digitosis in digital ones! W

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