HOW TO SELECT A CARRIER FREQUENCY FOR VOLTAGE- NOISE SUPPRESSION IN RESISTIVE MEASUREMENT SYSTEMS THROUGH INFORMATION CONVERSION IN TEN EASY STEPS

Impedance-based transducers re- quire a design-controlled interrogat- ing input in order to produce an out- put. Thus strain-gage-based trans- ducers and strain gages require a bridge supply or excitation signal the nature of which is determined by the measurement system designer.

The electrical supply could be DC, Si- nusoidal or a Pulse Train and might be constant voltage or constant cur- rent. In other words, it is a design-

controlled input - an important concept in The Unified Ap- proach to the Eiigineering of Measurement Systems.

Peter K. Stein

If the problem to be solved is to separate voltage (self-gener- ating) transducer responses such as described in the first article in this series, from resistive (non-self-generating) re- sponses as described in the second article in this series, then the design-controlled interrogating input must be a time- varying, non-DC quantity. What kind of responses exist in any particular test can be determined from the two articles on noise diagnostics in this series.

If undesired voltage/current responses exist, then the sys- tem can be designed to reject them, such as thermal emfs, magnetically-induced voltages, electrically induced voltages, EMI, RFI, tribo-electrically generated voltages, strain-in- duced voltages, etc., from all resistive responses. This sepa- ration can only be achieved by a carrier or modulation tech- nique which involves a time-varying excitation. Note, how- ever, that all resistive responses are transmitted, even those caused by undesired environmental stimuli such as contact resistances or thermo-resistive effects in strain gages. To separate undesired impedance responses from desired ones requires a different noise suppression technique.

THE METHOD AND ITS APPLICATION

The mathematical proof of the above statements can be found in Ref. 1. This article discusses how to select a carrier fre- quency so that the voltage responses can be suppressed and the impedance responses transmitted. This is the remark-

Editor’s Note: ET is pleased to feature the fifth “Back to Basics” article in a series on strain gages, thanks to veteran SEMmernber, Peter K . Stein. This series is intended for the riouice, and as a refresher for all others. Each article in the series will address a specific topic. I fyou have any comments about the series, or ques- tions for Pete to address in this series, please contact me at pat@seml.coni. PD

Peter K. Stein (SEM Fellow arid 47-year member) is President of Stein Engineering Seruices, Oic. in Phoenix, AZ.

able power of carrier or modulated systems. What is done, in effect, is to convert analog impedance information into am- plitude modulated impedance information (see the last ar- ticle in this series) while leaving the voltage responses in their original analog form. The Unified Approach calls this process information conversion, which is mathematically identical to carrier or modulatioir techniques but in prin- ciple and application, considerably hfferent.

In effect the impedance information is moved from its origi- nal frequency range into a band centered around the carrier frequency while the voltage information remains in its origi- nal analog form. I t is then possible to use a band-pass fre- quency-selective filter to suppress the analog (voltage) re- sponses and to retain only the frequency-converted imped- ance information. That information must then be demodu- lated back into Analog form, as will be described.

This article will not go into the mathematical derivation, but only describe the step-by-step procedure for achieving the rejection of the voltage responses in a resistance-measuring system.

Step 1: Feed the transducer with no interrogating input at all - i.e. a short circuit for voltage-supplied transducers or an open circuit for current-supplied transducers.

Step 2: Expose the transducer to the operating environment - i.e., run your test.

Step 3: Record the transducer response in such a way that it can be frequency-analyzed. A wave shape record is helpful but not essential.

Step 4: Determine the maximum frequency present. Call this frequency f-a-max.

Step 5: Connect a DC supply to the transducer and repeat your test. Again, frequency analyze the output. Again, a wave shape record is helpful but not essential.

Step 6: Determine the maximum frequency of that response and call it €s mBx.

Step 7: The conservatively selected carrier frequency required to separate the voltage responses from the resistance re- sponses is:

where m is a filter-design factor usually chosen between 3 and 10 dependmg on the filter-design talent available. Com- monly a figure m = 4 is used. I t has been used in the case studies cited below.

JanuarylFebruary 2000 EXPERIMENTALTECHNIQUES I 7

Step 8: The band-pass filter required to separate the ampli- . tude-modulated-impedance-information from the analog-volt- age-information must have a bandwidth: fc +I- f, m(Lv

Step 9: If the information is to be demodulated into Analog : form, a second criterion applies and the higher of the two . carrier frequencies must be used: f p = @ + I ) (Ref. 3). lz is . another filter design factor, this time for a low-pass filter and is usually selected to be the same as m.

Step 10. Any carrier system, sine wave or pulse, with a car- : rier frequency higher than the larger one prescribed by the . formulas, with the appropriate filter, will solve the problem . and suppress the voltage noise.

It's that simple!

Sometimes frequencies in the megahertz are required and may not be available - but that just means that the problem *

can not be solved with carrier systems with the present tech- '

nology. In Case Study 2, Pratt & Whitney had to build their own half-Megahertz carrier system for strain gages run . through sliprings, to solve their problem.

Note that it is necessary to run the prescribed hagnostic . tests before a design can be finalized - THAT is always nec- essary. You can not solve a problem without having diag- : nosed it. That was also the requirement in the noise-diag- . nostics articles.

A FEW CASE STUDIES

Case Study 14: A thrust link to monitor vector control during '

the firing of a Polaris Missile at Lockheed Missiles & Space : Co., was carefully designed to minimize to acceptable levels . all RESISTIVE responses of the 8 Constantan strain gages . and connecting copper lead wires during the thermal tran- sient. Initial tests showed an output many times the permit- a

ted response when no bridge voltage was applied, but sub- '

stantially the same output was observed with a DC bridge : voltage. This conhtion identfied a very small resistive re- . sponse and a very large voltage response from the link dur- . ing the thermal transient test - 16 copper-Constantan ther- mocouples! A 20 kHz carrier system was used (because it *

was available, not because it was necessary) and all voltage '

responses were suppressed successfully. Note, without their noise diagnostics they would never have known that the origi- . nal data were totally contaminated and useless.

Case Study 2': In testing a strain-gage-instrumented idler gear in the re-designed JT3D engine gearbox, Prat t & '

Whitney found stresses of +I- 65,000 psi (448 MPa) a t two : frequencies closely centered about 50,000 Hz. None of these . observations were expected nor could they be explained . theoretically. Since production lines were down this became . a crash program. The noise diagnostics, finally run in des- a

peration, revealed a tremendously high voltage response '

without any bridge power to the gage, higher in amplitude : than the response with gage excitation ON, and both with a . frequency spectrum up to 100 kHz. Note that their noise di- . agnostics revealed a noise level higher than the signal plus noise, which means that the signal and noise were out of .

phase and subtracting! And both were in the same frequency range.

When a carrier system was designed and built (it could not be purchased commercially) with a carrier frequency of 500 kHz (see the above equations), the problem was solved and explained by a giant one-per-rev tooth impact of 50,000 psi amplitude but without ANY apparent resonances - those had all been artifacts of the self-generating voltage response. (Ref. 2).

Case Study 3': The strains on a vibrating cantilever beam, electromagnetically excited a t 60 Hz, showed a voltage re- sponse with zero bridge power, very much higher than the signal obtained when DC-bridge power was applied. (The noise was larger than the signal plus the noise! Which again meant signal and noise were out of phase and subtracting.) The noise diagnostics showed a voltage response with a fre- quency range from 60 Hz to 600 Hz. The total response with bridge power on showed a response Gom 60 Hz to 2060 Hz (the 4th mode of vibration of the beam). From the equations above calculate the required carrier frequency to separate the voltage responses from the impedance responses and the one to demodulate i.e. re-convert the information from SAM back into analog form: 4460 to separate, 10,300 to demodu- late. A 25 kHz carrier system which was available, success- fully accomplished both aims.

WARNING: Once in a while a paper gets published which maintains that DC-fed systems must be used when thermal emfs, magnetically induced voltages, etc., are a problem. Usually the papers are based on a non-understanding of the basic phenomena and can lead readers to hsaster. One such publication is cited as Case Study 10 in Ref. 2.

CONCLUSION

The remarkable power of non-DC carrier systems of suppress- ing self-generating voltage responses from non-self-generat- ing impedance responses while suppressing the voltage-re- sponses can be exploited to great advantage when designing measurement systems for practical field applications in which such noise levels usually exist.

As in all measurement engineering design, a pre-test diag- nostic study is necessary to identify the parameters for which the measurement system must be designed.

It is interesting that within the last half century the com- mercially-available-instrument-pendulum has swung from almost 100% carrier systems in the 1940s and 1950s through almost 100% DC-fed systems in the 1970s and 1980s, and now slowly to a balanced position where carrier systems are again beginning to be commercially available in constant voltage, constant current, DC, Sine or Pulse fed form, in any combination and even computer programmable in any which way desired.

It is only possible to achieve Success Through Engineered Instrumentatiou!

I8 EXPERIMENTALTECHNIQUES Jawary l February 2000

REFERENCES

1. Stein, Peter K., The Uiiified Approach to the Eiigiiteeriiig of Measurerneiit Systems for Test & Evaluation: I - Basic Concepts, 9th Ed., 1998. Stein Engineering Services, Phoe- nix, AZ 85018. ISBN No. 1-881472-00-0.

. '

2. Our Engineering Education: The Not-So-Scientific Method, *

Proc. 1982 Measurement Science Conf., Jan 21-22, 1982, pp. '

220-235, Alta Lorna, CA. Presented at ASME Winter Annual '

Meeting, December 1979. LDMSEAlumni Newsletter No. 38, : July 1992, (up-dated). A de-fanged version co-authored with . Charles P. Wright, in Proc. 13thAerospace Testing Seminar, . Oct. 1991, Inst. for Environmental Sciences. Full version: . Lf/MSE Publ. 74, from the author.

3. Stein, Peter K., Iiiforrnatioii Coiiversioii as a Noise Sup- 1 pressioii Method, Lf/MSE Reprint 66, July 1975, from the . author's organization.

4. Petersen, P.L., Perrin, F.L., Trask, W.H., Strain Gage *

Thrust Link Monitors Vector Control During Static Firing of '

Polaris Motor, Strain Gage Readings, Vol. V, No. 4, Oct-Nov. 1962. Also in Lf/MSE Reprint 38A from the author 's . organization.1

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