THE NEW SERIES

This series of articles will deal with res i s tance s t r a i n gages, t h e transducers based on them and the signal conditioning required and desired for them. The approach will be unconventional, a s th i s f i rs t ins ta l lment a l ready shows, a n d feature The Unified Approach to the

Peter K. Stein Engiiieering of Measurement Systems for Test and Evaluation, which the author has developed over the last 40 years.

INTRODUCTION

First and foremost, bonded resistance strain gages are resistors. The material of which they are made may be metallic or semi-conductor, wire or foil, bulk material or deposited film; wires may be round or oval, and strain gage geometry may be one of many hundreds of shapes depending on application and imagination. A convenient summary of the myriad of choices cited above is: strain gages are made of resistive filanients.

Resistors will change the i r resistance with every conceivable variable in the world: not only mechanical strain, but also temperature, magnetic field, hydrostatic pressure, light intensity (if semiconductor), humidity (for certain materials), etc. This versatility of resistive responses gives rise to problems in correctly interpreting the real meaning of the resistance change in a strain gage, which may well be par t of a strain-gage-based t ransducer , such a s a load cell, torquemeter , accelerometer, pressure transducer, etc. Test procedures and the measurement system must be selected and planned so as to permit unequivocal assignment of an observed resistance change to a specific phenomenon acting on the strain gage, such as mechanical strain, or on the transducer. However, that is a topic for a future article in this series.

This first installment of the Strain Gage series will emphasize a different aspect of strain gages. If you think of resistors or resistive transducers as squiggle symbols Al\l\l\l\l\ on your circuit diagram, don’t!

All resistive transducers are also voltage generators and through a multiplicity of causes. These voltages change polarity when you reverse the leads which connect your resistor to the rest of the circuit - ergo, your resistor has a polarity in terms of the output it injects into your signal conditioning. This polarity reversal is sometimes helpful in minimizing noise levels because these voltage do add and subtract even within a bridge circuit.

Editor’s Note: ET is pleased to introduce a new educational “Back to Basics ”department 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. I f you have any comments about the series, or questions for Pete to address i n this series, please contact me, PB journals&eml.coni.

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

All resistive transducers and Wheatstone Bridge circuits should always incorporate a voltage-source symbol superimposed on every resistor. That way you will be forced to think about the problem and remember some of the solutions. It can make a big difference which way you connect a strain gage into its circuit!

Non-ohmic resistors, of course, have a different resistance depending on the polarity of the interrogating current, but that is not the phenomenon being discussed here. But a l l piezo-resistive semiconductor transducers (accelerometers, pressure transducers, load cells, etc.) should have their resistance checked periodically in both polarities, since, with time, alloy migration and other contaminating mechanisms may affect the previously ohmic connections inside the transducer.

MECHANISMS OFVOLTAGE GENERATION

All resistive transducers a re capable of generating vol tages due to env i ronmen ta l s t imul i , such as thermoelectric signals from the copper-Constantan connections a t the terminals of the strain gage (Case Study 1, also Ref. 1, p. 50-53). Note that copper-Karma has a thermoelectric effect a n order of magnitude lower than copper-Constantan!’ Copper-platinum thermo- electric effects have been noted for res i s tance thermometers3. It is a mistake to believe that just because the connections are close together and of “identical” material pairs differentially coupled, that there will be no thermo-electric voltage generated. For four reasons and a demonstration of why this should not be so, see Ref. 1, p . 54. There a re no identical twins in the measurement This is a topic for a future article.

Voltages may be generated magnetically due to time- varying transducer position (vibration) andlor time- varying magnetic fields. In vibration studies with magnetic exciters, the noise level thus generated is a t the same frequency, to the same time scale and correlated with the signal, impossible to even diagnose, and certainly not to suppress, once the noise and the signal have been allowed to merge’. Only the UnifiedApproach presents methods for the detection and elimination of such noise levels, as will be seen in future articles.

Strain-induced voltages, a dynamic (not DC) effect, are present in all materials, more so in ferromagnetic than non-ferromagnetic ones above about 5 kHz. Thus, all impact, explosion, pyro-shock and other transient tests with resistive transducers will be contaminated with voltages generated by mechanisms other than those which produce the resistance change in the transducer. This effect is usually between 10% to 80% of the signal. I t cannot be swept under the rug, (i.e. neglected and hidden) a s it is often the size of the rug! (Refer to Case Studies 2, 3, 4, 5 and 62*5,‘;). They cannot be detected or eliminated with frequency-selective filtering, time- domain, or statistical techniques - the traditional “tools of the trade.” They are not yet understood or explained. But they can be minimized to be negligible by proper design of the signal conditioning and often by the use of high-frequency carrier systems through the Uiiified Approach. See Case Study 6’.

March/Apd 1999 EXPERIMENTALTECHNIQUES 13

Other mechanisms, such as triboelectric effects in cables, electrically generated voltages, and many others, may find their way into the data and be misinterpreted as belonging to the data.

Thus, whether for the measurement of high speed transients, pyro-shock, explosions, or for steady state testing, such as vibration exciters for modal analysis and observations of operating machinery, or for simple static testing, resistor polarity and some of the diagnostic checks to be discussed in future installments a re absolutely crucial to data integrity.

Case Study No. 1: Lockheed Missiles & Space Co., Sunnyvale, CA, 1962’O. A tine-shaped part in the actuator chain in the exhaust of a Polaris missile firing was instrumented to measure the transmitted forces. Strain gages were placed only in mutual perpendicular pairs located as stacked-T-rosettes on both sides of both legs of the tine. With mutually perpendicular gages in adjacent bridge arms, the s t ra ins will add and any temperature-induced resistance changes will subtract. All internal bridge lead wires were exactly the same length so no output from unbalanced lead wires was possible. When tested in an oven, the “load cell” showed unacceptably large outputs. But the outputs were about the same whether bridge supply was on or o f f , showing that the major portion of the output was a voltage, not a strain. They had completely forgotten the 16 copper- Constantan thermocouples in the 8-gage bridge! The use of a CEC System “D’ 20 kHz carrier suppressed these voltage giving less than 5 micro-strain zero shift through the full temperature range of the test.

A note about the new International Definition of Thermal Zero Output. The definition is, unfortunately, nonsense. The output voltage due to temperature from a strain gage transducer will include the thermoelectric voltages when the bridge is fed with DC and exclude them if the bridge is fed from a sine-wave carrier. The recognition that the entire transducer and its boundary conditions must be specified completely before a definition can be written, did not occur to the definition writers! They had no conceptual model of how a transducer works. The Unified Approach to the Engineeringof Measurement Systeriis for Test & Evaluation provides such a conceptual model.

Reference 10 shows two drastically different thermal-zero characteristics: one for the DC-fed transducer and one for the carrier-fed transducer!

CONCLUSION

The most convenient methods for determining whether or not contaminating voltages exist in your transducer during a test, are to either disconnect the interrogating input (bridge supply), or to reverse the polarity of bridge input and output at the same time.

If the interrogating input is disconnected, no voltages generated by resistance change iii the circuit can possibly appear. Where there is no current, there cannot be a resistively-induced voltage. Any signal which emerges at the transducer output is a n undesired response t o whatever environmental stimuli act on the transducer, i.e. it is a noise level.

I4 EXPERIMENTALTECHNIQUES March/Aprilmg

If bridge input and output are simultaneously reversed, then one half the difference in output is attributable to that same noise level mechanism. If there are no self- generated voltages present, there will be no change in output.

So far as the author is aware, there are fewer than 20 manufacturers of s ignal conditioning for resistive transducers in the United States which permit removal of bridge power during a test either manually or by computer program. There seems to be only one which permits simultaneous bridge power and bridge output reversal. The requirement for one of these two features leaves out all the big signal conditioning and data acquisition manufacturers! Jus t check!

Do iiot use any signal coiiditioiiiiig for which one of these operations is not possible!!! Demand their inclusion when specifying equipment. After all, you ask a measurement system for the facts not for its opinion!

REFERENCES Some references are not cited in the paper, but are provided as additional resources. 1.

2.

3.

4.

5.

6.

7.

8.

9.

Stein, Peter K., The Unified Approach to the Engineering ofMeasurement Systems for Test & Evaluation, 134pp., 8- 1/2“ x 11“ softcover, 7th revised printing 1995. Stein Engineering Services, Inc., 5602 E. Monte Rosa, Phoenix,

Stein, Peter K.. Spurious Signals Generated in Strain Gages, Thermocouples and Leads. Proc. 9th Transducer Workshop, April 1977. Range Commanders Council, White Sands Missile Range, NM 88002. Dawson J., Hyman, L.G. , and Sheppard, J . Notes on Possibly Large Thermocouple E f f e c t s f r o m Copper Instrumentation Wires, Cryogenics, Dec. 1980, p . 728. Stein, Peter K., Sensors/Transducers /Detectors: The Basic Me as u re me n t S y s t e in Go nip0 ne n ts . Pro c . 1 9 72 Jo i n t Measurement Conference, Boulder, GO, June . I.S.A., Research Triangle Park, NC. Stein, Peter K., Pyro-Shock, Impact, Explosions and other High-speed Transient: Some Thoughts on “TQM” - Total Quality Measurements, Proc. 14th Aerospace Testing Seminar, March 9, 10, 11, 1993, Manhattan Beach, CA, f rom Inst i tute of Environmental Sciences, 940 East Northwest Highway, Mount Prospect, I L 60056. Tavares, M.R., Tsuchida, T.M., Barthel, C.A., and M.E. Voss, The Acquisition of Valid Strain Gage Data for Live Fire Testing, Proc. 16th Aerospace Testing Seminar. March 1996, Paper IX-3, p . 301, Institute for Environmental Sciences, 940 East Northwest Highway, Mount Prospect, IL 60056. Wyant, Robert, quoted as Case Study No. 5, in Stein, Peter K. Sixty Years of Bonded Resistance Strain Gages, 1936- 1996, Measurements & Control, April 1996, pp. 131-140. Stein, Peter K., A New Conceptual Model for Cornponerits in Measurement /Control Systems: Practical Application to Thermocouples, Proc. 5th Int’l Syrnp. on Temperature 1971, I. S. A, , Research Triangle Park, NC,1973. Reed , R.P., Val idat ion Diagnost ics for Defect ive Thermocouple Circuits, Proc. 6th Syrnp. on Temperature 1981. I.S.A.. Research Triangle Park, NC, 1983.

AZ 85018, U.S.A.. ISBN NO. 1-881472-00-0.

- 10. Petersen, P. L., Perrin, F.L., and Trask, W.H., Strain Gage

Thrust Link Monitors Vector Control During Static Firing of Polaris Motor, Lockheed Missiles & Space Go., Sunnyvale, CA, Strain Gage Readings, Vol. V, No. 4, 0ct.-Nov. 1962, also as L f / M S E Reprint 38-A, from the author.

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