Editor’s Note: The following is the continuation of the JulyIAugust article in the Strain Gages: Back to Basics se- ries. The series deals with resistance strain gages, the transducers based on them and the signal conhtioning required and desired for them. The approach is uncon- ventional, as the first three installments have already shown, and features The Unified Approach to the Engineer- ing of Measurement Systems for Test and Evaluation.

* Path

. I

1

- 3

. 4 The following sections identify systematic procedures for identifying various sources of noise levels or other system malfunctions for diagnostic and suppression purposes.

A SYSTEMATIC SCHEME FOR NOISE DOCUMENTATION

Desired Environment Interrogating Stimulaong Response Indicated Input Environment Obtained Acoon (*)

OFF OFF Undesired Underired

OFF ON Undesired Desired

Interrogating Undesired

ON OFF Desired Undesired

ON ON VALID NOISE-FREE DATA

Input

The scheme can be carried out in one of two ways: 1. Sequentially, usually by switching - this takes time. 2. In Parallel - this takes channel capacity.

Time and/or channel capacity are the investment required for noise documentation, a form of data insurance. After all, you insure your life, your home, your car, your health, why not your data? For resistive strain gages the resis- tive, NSG-response is the desired response and all other responses are undesired. Paths 1, 2 and 3 are noise lev- els. Extensive discussions of hagnostic procedures based on the Unfied Approach are given in Ref 1.

Figure 1 illustrates the conceptual transducer model to which the procedures in Table 1 will be applied. Figures 2, 3 and 4 illustrate practical executions of some of the principles given below.

Path 1 (Fig. 1): is determined by removing the design- controlled interrogating input, also known as the bridge supply, auxiliary power source, etc. It is specified by the signal conhtioning design and may be a voltage or a cur- rent, DC or AC, sinusoidal or pulse train. A manually or computer-controllable OdOff selection is absolutely man- datory and required for data validation. Insist on i t when buying or specifying signal conhtioning. Without it you are powerless to assess data integrity.

This action kills Paths 2 and 4. A resistor with no current through it cannot produce a voltage; a capacitor with no voltage across it cannot accumulate a charge; a thermo- couple with no temperature gradient across i t cannot gen- erate a response, etc.

Fringe Benefit: In fact, removing the design-controlled in- terrogating input reveals the total self-generating response: Paths 1 and 3. If all else fails, these two phenomena can be separated from the non-self-generating responses,

ET is pleased to feature the coritiiiuatioii of the third “Back to Ba- sics” article in a series on strain gages, thanks to veteran SEM member, 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. If you have any comments about the series, or questions for Pete to address iri this series, please contact me at journals@senil.com. PD

Peter K. Stein (SEM Fellow arid 47-year member) is President of Stein Engineering Services. Znc. in Phoeriix, AZ.

ENVIRONMENTS TRANSDUCER RESPONSES

.

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

SWITCH

. Fig. 2: Note that the resistors in the bridge are also shown as

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the interrogating input voltage source.

. Paths 2 and 4, through information conversion (modula-

. tion or a carrier system) with a time-dependent design-

. controlled interrogating input (Ref. 1). But no currier sys- * tem can ever separate Path 2 from Path 4! Hence, the ne- ’ cessity for noise hagnostics to identify the problems so * that appropriate noise suppression methods can be se- . lected. Ref. 3 cites a spectacular case study from Pratt & . Whitney which illustrates the power of Information Con- + version or carrier systems. The switch 2 in Fig. 2 present * hardware executions of the above principle. If the system * output during that check returns to its initial “zero” refer- ’ ence, then the absence of self-generating responses has : been documented beyond reasonable doubt.

Septernber/October 1999 EXPERIMENTALTECHNIQUES I 3

*

* conductor-based resistive sensors. result in zero-shifts which are slow to recover, as in semi- 1

SOLENOID

VALVE

TRANSDUCER PRESSURE 0-k CONTROLLED

. If the channel being checked is part of a closed-loop con-

. trol system, then removal of the Interrogating Input will

. result in highly undesirable gyrations of that system.

. In those cases, simultaneous reversal of interrogating in- . put and output may be substituted. This procedure was

. originally adopted when the time for rebalancing a mea-

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between the two polarity connections is caused by self- generating responses (here noise levels).

. It is now necessary to kill Path 3, so that Path 1 can be . viewed and documented by itself. This separation of self- - generating responses (i.e. Path 1 from Path 3) is neces- * sary so that the source and, hence, the cure for the in&- ’ vidual components of the noise level can be identified. If ’ the choice of information conversion is made for some rea- . son, then all self-generating responses are suppressed, . regardless of source.

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I 4 EXPERIMENTALTECHNIQUES Septetnber/October 1999

sure source by means of a tube, a valve (perhaps remotely- controlled) can be inserted into that connecting tube and the pressure to the pressure transducer can be cut off at any time during the test without affecting the test. (Fig. 3). The transducer still vibrates, acting like an accelerom- eter, it is still in the same temperature gradient acting like a thermometer, and still in the same magnetic field, etc., as it was before. The author has witnessed tests where this action resulted in an increase in output!

Where it is not possible to remove the desired environ- mental stimulus from the transducer, it is necessary to use a check channel which is not exposed to the measurand but is exposed to all the undesired environments. A pres- sure transducer embedded only part way into a pipe wall, with the bottom of the blind hole vented to the environ- ment, is a close approximation to the desired effect (Fig. 4). The two pressure transducers are in substantially the same environment in terms of temperature, acceleration, magnetic fields, etc. (but not light intensity, for example), although slight differences could possibly exist. In fact, the check channel can be subjected to a known (“calibra- tion”) stimulus through the vent hole during the test, veri- fying the transfer ratio for the measuring channel.

The measurement engineer’s imagination may be chal- lenged to produce the conditions required for these checks, but it usually proves worth while, since phenomena are discovered which were not known and which obscure the real data.

Path 1 has now been identified, it is recorded and suit- able action is taken until it has been suppressed.

When is suppression sufficient? The author’s target is half the smallest scale division on the readout (half the least significant bit). Anything less than that should not be in- terpreted anyway, but other targets may be selected.

Path 2 (Fig. 1): It is now necessary to turn on the interro- gating input but to leave the stimulus from the desired environment off by whatever means that condition was achieved for Path 1. Paths 3 and 4 are now dead. Path 1 has already been suppressed, and Path 2 survives to be recorded (documented) and suppressed in its turn. The suppression methods may well be different for each of the paths and a multiplicity of suppression methods may be invoked for any one. The documentation can, of course, all occur first and the suppression, one by one, afterwards.

Path 2 documentation is an important part of the noise hagnostic I documentation procedure, because it can not be suppressed through the use of Information Conver- sion or a carrier system. If i t is of any significance it must be suppressed by one of the other five available noise sup- pression methods which are available. If it is non-repeat- able and non-reproducible, such as contact resistances in sliprings, switches and connectors, then a second noise suppression method (mutual compensation) has become useless and only four remain. A cure cannot be selected until the hsease has been correctly diagnosed!

Path 3 (Fig. 1): Removal of the interrogating design-con- trolled input will now show Path 3, since Path 1 has been

. , . . . . ,.. . . . .. . . . . ~ _ .

suppressed and Paths 2 and 4 are uninterrogated and, therefore, can produce no output.

Path 3 can be a very deceptive enemy. With strain-gage- based transducers, it represents a self-generating noise level. Those transducers, however, will produce self-gen- erating voltages directly as a response to dynamic strain. Shock or impact conditions or frequencies above about 5 KHz produce up to half a millivolt of self-generating volt- ages which are directly correlated with the measurand which also produces the resistance change in the strain gage load cell, accelerometer, pressure transducer, torque meter., etc. The noise level occurs in the same frequency range as the signal and to the same time scale, since the noise is, in fact, caused by the signal. It is an undesired self-generating manifestation of the desired non-self-gen- erating response. This situation occurs quite frequently, especially in shock and vibration tests, pyroshock and explosion tests and other transient or high-frequency dy- namic situations Refs. 1, 3, 4 and 5. In electromagneti- cally excited vibration tests, the noise levels and signals will both be at excitation frequency, and similar problems may exist.

Thermocouples, especially Chromel-P/Alumel, also pro- duce voltage outputs under dynamic strains. The effect is general for all materials although it seems to predomi- nate in ferromagnetic materials. Ref 5 . presents an ex- tensive discussion and numerous references including a case study from Pratt & Whitney in which strain-induced outputs from thermocouples severely disturbed a data acquisition system Important Note: Once Paths 1 and 3 have been allowed to merge within the transducer then no separation of them may be possible any more! Thus, the engineering of the measurement system for provably valid data must include the noise diagnostic step of de- sign-controlled input removal for documentation that this marriage did not take place. Path 4 (Fig. 1): With every- thing turned on, provably valid, noise-free data can now be harvested.

REFERENCES 1. Stein, P.K., The Unified Approach to the Engineering of Mea- surernerit Sys tem for Test & Evaluation, I - Basic Concepts. Stein Engineering Services, Inc., p. 134, 9th revision, (1998).

2. MacKay, A.L., The Harvest of a Quiet Eye, The Institute of Physics, Bristol, U. K., p. 3, (1977).

3. Stein, P.K., Our Engineering Education: The Not-So-Scien- tific Method, Proc. 1982 Measurement Science Cod., Jan. 1982, pp. 220-235. Presented at ASME Winter Annual Meeting, De- cember 3, 1979. LflMSE Alumni Newsletter No. 38, July 1992, (updated). An additional version co-authored with C.P. Wright, Proc. 13th Aerospace Testing Seminar, Oct. 1991, Inst. for Envi- ronmental Sciences. Publ. 74 from Stein Eng. Svcs.

4. Stein, P.K., Case Studies of the Proper Application of the Uni- fied Approach to the Engineering of Measurement System. Ex- cerpts from LfMSE Newsletters 36-39, Jul. 1991-Jul. 1997. Publ. 87 from Stein Eng. Svcs.

5. Spurious Signals Geiierated in Strain Gages, Thermocouple and Leads, Proc. 9th Transducer Workshop, April 1977, pp. 162- 185, Range Commanders Council Sec. Publ. 69 from Stein Eng. Svcs..