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A Conversion of Wheatstone Bridge to Current-Loop Signal Conditioning for Strain Gages

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A Conversion of Wheatstone Bridge to Current-Loop Signal Conditioning for Strain Gages https://ntrs.nasa.gov/search.jsp?R=19970024858 2020-06-16T02:20:48+00:00Z View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by NASA Technical Reports Server NASA Technical Memorandum 104309 A Conversion of Wheatstone Bridge to Current-Loop Signal Conditioning For Strain Gages Karl F. Anderson NASA Dryden Flight Research Center Edwards, California 1995 National Aeronautics and Space Administration Dryden Flight Research Center Edwards, California 93523-0273 A CONVERSION OF WHEATSTONE BRIDGE TO CURRENT-LOOP SIGNAL CONDITIONING FOR STRAIN GAGES Karl F. Anderson* NASA Dryden Flight Research Center P. O. Box 273 Edwards, California 93523-0273 ABSTRACT Alcal current change through the gage during calibration, A Current loop circuitry replaced Wheatstone bridge cir- gage current, A cuitry to signal-condition strain gage transducers in more than 350 data channels for two different test programs at Iref reference resistance current, A NASA Dryden Flight Research Center. The uncorrected J circuit board jack test data from current loop circuitry had a lower noise level than data from comparable Wheatstone bridge circuitry, JP circuit board jumper were linear with respect to gage-resistance change, and OUT were uninfluenced by varying lead-wire resistance. The output current loop channels were easier for the technicians to set OVP over-voltage protection up, verify, and operate than equivalent Wheatstone bridge R resistors on the circuit card channels. Design choices and circuit details are presented in this paper in addition to operational experience. R initial strain gage resistance, _2 NOMENCLATURE AR resistance change, _2 AR/R resistance change-to-gage resistance ratio C capacitors on the circuit card l_cal apparent gage resistance change caused by DACS data acquisition and control system Mcal, _2 EUcal engineering units represented by calibration gg gage resistance, _2 EX excitation Rcal calibration resistance, _2 GF gage factor Rref reference resistance, f_ F fuse Rref a apparent reference resistance, f_ FLL Flight Loads Laboratory U integrated circuit components I current, A v_ gage voltage, V IN input Vo output voltage, V M current change, A AVcal output voltage change caused by A[cal, V Vrel reference voltage, V set-point voltage, V *Senior Measurement Systems Engineer Vsp INTRODUCTION The NASA Dryden FLL has a 1,280-channel data acqui- sition and control system (DACS) in current use known The Flight Loads Laboratory (FLL) at the NASA Dry- locally as the "DACS II." The system is equipped with 640 den Flight Research Center has used large-scale (more channels of Wheatstone bridge signal conditioning, each than 1,000 data channels), computer-controlled data with a plug-in card containing bridge completion and acquisition systems since it opened in 1967. The FLL test shunt calibration components and a presampling filter. A programs frequently involve strain measurements during prototype current-loop signal-conditioning circuit was high-temperature (higher than 2,000 °F) test operations. designed to replace the DACS II Wheatstone bridge cir- Until recently, the ubiquitous Wheatstone bridge circuit cuit. The prototype was built on an external, solderless was the only reasonable choice for static strain-gage signal breadboarding system that connected with shielded cables conditioning. Current-loop circuit topology was invented in place of the Wheatstone bridge card. This circuit was at NASA Dryden in 1992, demonstrated in the laboratory, demonstrated to the FLL technicians and engineers. and reported in comparison with the Wheatstone bridge. 1 The prototype current-loop signal-conditioning circuit Significant improvements have been made at NASA provided measurement data from the DACS II with a stan- dard deviation of less than two data counts. This standard Dryden in the circuitry that implement the current loop paradigm. 2 These improvements are as follows: deviation is less than 5 gV of deviation caused by electri- cal noise, representing typically less than 2 gin/in of strain. Deviation from perfect linearity was unobservable • An effective method has been found to design a stable within the resolution and accuracy of the DACS II. Perfor- voltage-difference measuring system without the mance was stable and fully predicted by current loop the- switching circuitry initially used in practical current- ory, and random variations in lead-wire resistance had no loop implementations appreciable influence on the output. • Regulating loop current by controlling the reference After witnessing the prototype demonstrations and voltage to be constant has been found to be especially reviewing its performance data, the technicians and engi- advantageous neers were convinced the current-loop signal conditioning offered superior performance and more versatility than the • Various means have been designed to accomplish an Wheatstone bridge. Several of these improvements pro- analog offset that is a function of the excitation cur- vided such key benefits that the test engineers for two test rent level programs involving approximately 350 strain measure- ments chose to require current-loop signal conditioning for • End-to-end electrical calibration for strain is accom- their test programs. A printed circuit board (fig. 1) was plished without the need to know the initial resistance developed that directly replaces the old DACS II Wheat- of the strain gage stone bridge circuitry with new current loop circuitry. This paper reports the design of this modification, the test data, Ratiometric measurement of the output with respect and user experience with several hundred current-loop to the reference voltage removes the need to regulate measurement channels installed in the DACS II. loop current DESIGN REQUIREMENTS Signal conditioning has been demonstrated that sepa- rately indicates strain and temperature using the same The system requirements were to include current-loop four connecting wires. 3 These innovations are the signal conditioning and to avoid system changes beyond subject of various patent applications, and a patent 4 the new current-loop printed circuit board while retaining has been issued on the fundamental current-loop cir- the operational features, measurement accuracy, and preci- cuit topology sion of the existing DACS II. The DACS II allowable sys- tem measurement errors, based on the connection of a A CONVERSION OF WHEATSTONE BRIDGE TO CURRENT-LOOP SIGNAL CONDITIONING FOR STRAIN GAGES Karl E Anderson* NASA Dryden Flight Research Center E O. Box 273 Edwards, California 93523-0273 ABSTRACT Alcal current change through the gage during calibration, A Current loop circuitry replaced Wheatstone bridge cir- gage current, A cuitry to signal-condition strain gage transducers in more than 350 data channels for two different test programs at reference resistance current, A NASA Dryden Flight Research Center. The uncorrected J circuit board jack test data from current loop circuitry had a lower noise level than data from comparable Wheatstone bridge circuitry, JP circuit board jumper were linear with respect to gage-resistance change, and OUT were uninfluenced by varying lead-wire resistance. The output current loop channels were easier for the technicians to set OVP over-voltage protection up, verify, and operate than equivalent Wheatstone bridge R resistors on the circuit card channels. Design choices and circuit details are presented in this paper in addition to operational experience. R initial strain gage resistance, f2 NOMENCLATURE AR resistance change, AR/R resistance change-to-gage resistance ratio C capacitors on the circuit card ARcal apparent gage resistance change caused by DACS data acquisition and control system A/cat, f_ EUcal engineering units represented by calibration Rg gage resistance, f_ EX excitation Rcal calibration resistance, _2 GF gage factor Rref reference resistance, F fuse Rref a apparent reference resistance, f2 FLL Flight Loads Laboratory U integrated circuit components I current, A v_ gage voltage, V IN input Vo output voltage, V M current change, A AVcat output voltage change caused by Alcal, V Vre_ reference voltage, V set-point voltage, V *Senior Measurement Systems Engineer % INTRODUCTION The NASA Dryden FLL has a 1,280-channel data acqui- sition and control system (DACS) in current use known The Flight Loads Laboratory (FLL) at the NASA Dry- locally as the "DACS II." The system is equipped with 640 den Flight Research Center has used large-scale (more channels of Wheatstone bridge signal conditioning, each than 1,000 data channels), computer-controlled data with a plug-in card containing bridge completion and acquisition systems since it opened in 1967. The FLL test shunt calibration components and a presampling filter. A programs frequently involve strain measurements during prototype current-loop signal-conditioning circuit was high-temperature (higher than 2,000 °F) test operations. designed to replace the DACS II Wheatstone bridge cir- Until recently, the ubiquitous Wheatstone bridge circuit cuit. The prototype was built on an external, solderless was the only reasonable choice for static strain-gage signal breadboarding system that connected with shielded cables conditioning. Current-loop circuit topology was invented in place of the Wheatstone bridge card. This circuit was at NASA Dryden in 1992, demonstrated in the laboratory, demonstrated to the FLL technicians and engineers. and reported in comparison with the Wheatstone bridge. 1 The prototype current-loop
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