This series of articles deals with re- sistance strain gages, the transduc- ers based on them and the signal con- ditioning required and desired for them. The approach is unconven- tional, as the first two installments have already shown, and features The Unified Approach to the Engi- neering of Measurement Systems for Test and Evaluation which the au- thor has developed over the last 40+ years.

Peter K . Slein

In the first article in this series, the case was put that you should consider all resistors as voltage generators and to select a test procedure and/or signal conditioning which allows you to determine whether or not such con- taminating voltages are present during your test. Since

Editor’s Note: ET ispleased to feature this third “Back to Basics” article in a series on strain gages, thanks to veteran SEMniernber, Peter K. Stein. This series is intended for the novice, and as a refresher for all others. Each article in the series will address a specific topic. I f you have any coninierits about the series, or ques- tions for Peter to address in this series, please contact me at journals@serri 1. coin. PB

Peter K. Stein (SEM Fellow arid 47-year member) is President o f Stein Engineering Services, Iiic. in Phoenix, AZ.

such voltages are often of the same order of magnitude, with the same frequency content and to the same time scale as the signal, such checks are absolutely vital to data integrity.

The second article considered causes and effects of resis- tance changes due to undesired environment stimuli and some of the methods which can be used to control those effects .

It is now necessary to discuss procedures for document- ing to which environments the strain gage (or strain- gage-based transducer) is responding during a particu- lar test, and whether the responses are resistive (desired) or voltaic (undesired).

N 0 I SE D IAG N OSTl C S AND DOC U M E NTATlO N

Noise: The definition of noise in this series is radically different from commonly found definitions and has noth- ing to do with statistics or random motion of electrons. Since all transducers respond in every way in which they can to all stimuli present in the test environment, it is possible to define a desired environment, i.e. t he rneasurand, which is the physical or chemical quantity to be observed. Let this quantity be mechanical strain as an example.

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It is also possible to define undesired environrnents, which ’

include the rest of the physical or chemical universe act- : ing on the transducer.

It is likewise possible to define desired responses which, . for strain gages and strain-gage-based transducers, are *

resistive (see Part I1 of the series), and undesired re- sponses which are voltages generated directly in the : strain gage by the environments (see Part I of the se- . ries). In keeping with the nomenclature of the unified . approach, voltage responses shall be called self-generat- . ing (SG) and resistive responses shall be called non-self- . generating (NSG). That is the preferred generic termi- . nology although for strain gages the vernacular alterna- tives of voltage and resistive are acceptable.

Noise, in this series, is defined as undesired environment . response combinations of which there are three - Paths 1, 2, and 3 as shown in Fig. 1. The presentation here is ’

simplified by not considering whether the responses are : additive (causing zero shifts) or multiplicative (causing . transfer-ratio or calibration changes); or whether they . are permanent or temporary. There are, in truth, 16 en- - vironment-response combinations possible in every link . in the measurement chain, but we are restricting our- . selves to those four combinations which, usually, are the : most significant contributors to transducer outs. For a . more general discussion see Ref. 1.

ENVIRONMENTS TRANSDUCER RESPONSES

DESIRE0 (MEASURAND)

VOLTAGE RESPONSES

NON-SELF-GENERATING &= RESISTIVE RESPONSES *

UNDESIRED I

DESIGN-CONTROLLED INTERROGATING INPUT AUXILIARY (BRIDGE SUPPLY SUPPLY) 8 -

WITH ONOFF SWITCH

JTPUT

Fig. I : Environment-response combinations in a strain-gage- based transducer: conceptual model

In this entire series the world is assumed to be deter- -

ministic and not random - where there was an effect, : there must have been a cause. Whether or not the world . is either, is not a t issue here. The fact is that measure- . ment systems can be designed, executed and controlled very nicely by taking the deterministic approach. I t is . perhaps well to recall the statement by Ernest Ruther- *

ford, 1st Baron (1871-1937), Nobel Prize winner for : Chemistry, 1908, for his work on radioactivity (Ref. 2): .

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16 EXPERIMENTALTECHNIQUES July/August 1999

' A A BACKTO BASICS

Path

I

2

3

TABLE I: NOISE LEVEL DOCUMENTATION -When the additive, temporary non-self-generating response is desired

Desired Environment Interrogating Stimulating Response Indicated Input Environment Obtained Action (*)

OFF OFF Undesired Undesired

OFF ON Undesired Desired

Interrogating Undesired

ON OFF Desired Undesired Input

4 ON ON VALID NOISE-FREE DATA

(*) As determined f rom the Noise Suppression methods applicable to the problem

If your experiment needs statistics you ought to have done a better experiment.

The philosophy of noise diagnostics.

Please note that the process is called noise diagnostics or noise documentation and not noise hunting. Hunting is a statistical process - sometimes you get your deer, sometimes you don't and you never know when. The process described here is a methodical, systematic pro- cedure, not a hit or miss proposition. You always get your answer if you can follow the rules and invest the required time andlor channel capacity.

A measureinent system is guilty by assurnption! Proofs of innocence are demanded. This is not the American Democratic Procedure where an accused criminal is pre- sumed innocent until proven guilty beyond reasonable doubt by a jury of hislher peers. Measurement System design presuming innocence is a luxury we cannot af- ford. Democracy has no place in the Engineering of Mea- surement Systems! (But please note the restriction).

The purpose of a measurement is to obtain information about an on-going process in such a way that the pro- cess does not realize it is being observed. That defini- tion is identical to the mission of a spy organization! We are in the business of spying and might as well uti- lize the tried and proven techniques of that ancient pro- fession:

' 2. The most important channels of information are : checked all of the time. The checking is accomplished by . second-order spies that spy on the spies - check channels . in the Measurement profession. There are a variety of . check channel types, to be discussed in the next article.

* The basic propositions are quite simple:

. 1. Produce a set of conditions from which no output is

. expected. If that set has been appropriately selected, * whatever output is obtained can be directly associated . with a specific source of noise or system malfunction, ' which can then be suppressed according to the system- : atic methodology to be developed.

. 2. Produce a set of conditions from which a known out-

. pu t is expected. If that set has been appropriately cho-

. sen, whatever deviation from the known output is ob- ' tained, can be directly associated with a specific source

of noise or system malfunction, which can then be cured.

. This article, including discussion of systematic proce- - dures for identifying various sources of noise levels or * other system malfunctions, will continue in the Septem- * berloctober 1999 issue of Experimental Techniques' : Strain Gage series..

1. All channels of information are checked some of the time.

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