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Development and Application of Resistance Strain Force Sensors

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sensors Review Development and Application of Resistance Strain Force Sensors Yinming Zhao 1,2, Yang Liu 3, Yongqian Li 3,* and Qun Hao 1 1 School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, China; [email protected] (Y.Z.); [email protected] (Q.H.) 2 Beijing Changcheng Institute of Metrology & Measurement, Beijing 100095, China 3 Key Laboratory of Micro/Nano Systems for Aerospace of Ministry of Education, Northwestern Polytechnical University, Xi’an 710072, China; [email protected] * Correspondence: [email protected] Received: 17 August 2020; Accepted: 25 September 2020; Published: 15 October 2020 Abstract: Resistance strain force sensors have been applied to monitor the strains in various parts and structures for industrial use. Here, we review the working principles, structural forms, and fabrication processes for resistance strain gauges. In particular, we focus on recent developments in resistance stress transfer for resistance strain force sensors and the creep effect due to sustained loads and/or temperature variations. Various error compensation methods to reduce the creep effect are analyzed to develop a metrology standard for resistance strain force sensors. Additionally, the current status of carbon nanotubes (CNTs), silicon carbide (SiC), gallium nitride (GaN), and other wide band gap semiconductors for a wide range of strain sensors are reviewed. The technical requirements and key issues of resistance strain force sensors for future applications are presented. Keywords: resistance strain force sensor; resistance strain gauge; stress transfer; creep effect; carbon nanotubes (CNTs); piezoresistive effect 1. Introduction The resistance strain gauge was invented by Simmons and Ruge in 1938 [1], and it has been nearly 80 years since resistance strain force sensors began to be produced in 1942. Force sensors based on resistance strain gauges have been widely used in the stress tests of various components and structures in the aerospace, transportation and automobile industries, civil engineering, and even the medical field. For example, in civil bridge structures, the great amount of strain acting on the bridge structure over a long time will fatigue the structure and yield a fracture. By monitoring the structural strain distribution within bridges, early failure predictions can be made, and the reliability and service life of the bridge structure can be improved [2,3]. Based on a combination of physical quantity-sensitive elements and resistance strain gauges, the physical quantities such as the load and the moment acting on the structure can be obtained indirectly, and other physical quantities such as strain, displacement, bending moment, torque, and acceleration can be measured directly. A strain sensor based on a resistance strain gauge can measure not only the strain on the surface of the structure but also the strain inside the structure. When the strain gauge is embedded in poured concrete structures, the strain state inside these structures can be monitored in real time through external measuring lines [4]. The structural parameters of a resistance strain force sensor directly affect its measurement characteristics. The structural parameters of the sensitive gates and the manufacturing process determine the static and dynamic measurement errors of a force sensor. Although resistance strain force sensors have been developed for a few decades, to promote their use in engineering applications, research remains ongoing to satisfy the increasing demands of their characteristics. One of these Sensors 2020, 20, 5826; doi:10.3390/s20205826 www.mdpi.com/journal/sensors Sensors 2020, 20, 5826 2 of 18 Sensors 2020, 20, x 2 of 18 ofresearch these research topics is topics the dependence is the dependence of the outputof the ou characteristicstput characteristics on the on transverse the transverse effects, effects, the endthe endeffect, effect, and theand grid the grid wire wire length length of strain of strain gauges. gauges. Grid Grid wire wire spacing spacing influences influences the measurement the measurement error errorfluctuation. fluctuation. Further, Further, the deflection the deflection of the gridof the wire grid length wire andlength diameter and diameter results inresults matrix instrain matrix transfer strain transfererrors. The errors. creep The eff creepects due effects to sustained due to sustained loading loading and/or temperature and/or temperature variations variations have led have to various led to variouserror compensation error compensation methods methods to help developto help develop a metrology a metrology standard standard for resistance for resistance strain force strain sensors, force sensors,in particular in particular for dynamic for measurementdynamic measurement of strain force. of strain To improve force. theTo measurementimprove the measurement accuracy and accuracylong-term and stability long-term of the stability strain gauge of the force strain sensor gauge system, force sensor the finite system, element the methodfinite element has been method used hasto optimize been used the to structural optimize parametersthe structural of theparameters sensitive of gate the for sensitive reducing gate matrix for reducing strain transfer matrix errors.strain transferIn this paper, errors. we In review this paper, the recent we review progress the recent of the progress micro–nano of the structure micro–nano of resistance-strain-sensitive structure of resistance- strain-sensitivegrids and the strain grids transferand the characteristicsstrain transfer ofcharacteristics the strain-type of the force strain-type sensor. Further,force sensor. the technicalFurther, thedevelopments technical developments and key problems and of key resistance problems strain of forceresistance sensors strain for their force future sensors application for their demands future applicationare explored. demands are explored. 1.1. Resistance-Strain-Sensitive Resistance-Strain-Sensitive Grids Strain-type force force sensors sensors are are mainly mainly composed composed of an of elastomer, an elastomer, a strain a strain gauge, gauge, an adhesive, an adhesive, wire, wire,and a andsubsequent a subsequent signal signalprocessing processing circuit. circuit.Under ap Underplication, application, the resistance the resistance strain gauge strain is gaugeadhered is toadhered the surface to the of surface the metal of the elastomer metal elastomer [1]. When [1]. th Whene elastomer the elastomer is deformed is deformed by an by external an external force, force, the strainthe strain is transferred is transferred to the to the resistance resistance strain strain gaug gaugee through through the the adhesive. adhesive. The The resistance resistance strain strain effect effect or piezo-resistance eeffectffect ofof the the resistance resistance strain strain gauge gauge leads leads to to a changea change in thein the resistance resistance values values of the of thesensitive sensitive grids. grids. Finally, Finally, the transformation the transformation circuit transforms circuit transforms the varying the resistance varying into resistance corresponding into voltagecorresponding or current voltage signals or current [5]. Figure signals1 shows [5]. Figure the structure 1 shows of the a metalstructure foil of resistance a metal foil strain resistance gauge straincommonly gauge used commonly in strain-type used in force strain-type sensors. force The metalsensors. foil The is etched metal intofoil gridsis etched and into bonded grids to and the polymerbonded to substrate the polymer film, substrate which is anfilm, electrical which insulator.is an electrical insulator. Figure 1. StructureStructure of of a metal-foil-type resistance strain gauge: ( aa)) top top view view and and ( (b)) side side view. The resistance-sensitive grids of metal-foil-type resistance strain gauges are generally made of constantan, nichrome, and other metal materials, whichwhich are the sensitive elemen elementsts of the stress–strain sensor. TheThe substratesubstrate films films are are made made of of a polymer a polymer material material (polyimide, (polyimide, epoxy epoxy resin, resin, and phenolicand phenolic resin) resin)or a glass or a fiber glass film, fiber which film, have which good have electrical good electrical insulation, insulation, moisture resistance,moisture resistance, and heat resistance.and heat Theresistance. substrate The film substrate supports film the supports sensitive the grids sensitive and maintains grids and their maintains relative their position. relative During position. the Duringmeasurement the measurement process, the substrateprocess, filmthe transferssubstrate thefilm generated transfers strain the withingenerated the elastomerstrain within into the elastomersensitive grid. into Thethe sensitive coating film grid. protects The coating the sensitive film protects grid fromthe sensitive mechanical grid damage from mechanical and corrosion damage due andto humidity, corrosion which due to keeps humidity, the performance which keeps of the strainperformance gauge stable. of the Thestrain coating gauge film, stable. sensitive The coating grids, film,andsubstrate sensitive filmgrids, are and bonded substrate with film a thin are layer bonded of adhesive with a thin [6,7 layer]. of adhesive [6,7]. For a rectangular sensitive grid with length L, cross-sectional area A, and resistivity ρ, the resistance is defined as R = ρL/A. When the action of axial force leads to the wire deformation ΔL, the relative
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