Research On Sensing Mechanism And Wireless Monitoring Method For Contact Stress Measurement In Cramped Slit Between Curved Surfaces

Contact stress sensing in real time plays a significant role in condition monitoring,fault diagnosis and failure analysis.With the rapid development of sensing technology,the study on contact stress sensing mechanism and methods in a specified harsh environment has become one of the most valuable and critical research subjects.In a harsh environment such as cramped slit between two curved metal surfaces,the force-sensitive element has to meet some special and strict requirements in device size,packaging and signal transmitting method.Aimed at contact stress measurement in the cramped slit,this paper reported two passive and wireless sensing methods,namely the sensing scheme based on surface acoustic wave(SAW)device and the strain gauge,respectively.For both sensing methods,starting with the stress distribution and electromagnetic coupling characteristic simulation in the slit between metal surfaces,the influence on the stress distribution caused by the size and stiffness of implanted force-sensitive element was analyzed.For the force-sensitive element placed at different positions in the cramped slit,the level of electromagnetic shielding caused by the slit was evaluated by simulating the ability to receive external excitation signal.On a basis of simulation results,the acquisition,conversion and wireless transmission methods of measured contact stress were investigated for both sensing methods.The contents could be devided into four parts as follows.(1)Based on the passive and wireless operating mode of SAW sensor and periphery fixed thin plates model,a layered diaphragm structure of force-sensitive element was put forward to package the SAW resonator.The incluence that the thickness and Young’s modulus of each layer has on the sensitivity of force-sensitivity element was discussed.The results show that the center of diaphragm is the best position to place single-channel resonator with high sensitivity coefficient since the stress distribution in the area was uniform and successive.Meanwhile,the size of diaphragm cavity is the most crucial parameter to affect the sensor sensitivity.(2)The coupling of modes(COM)model of one-port resonator was constructed and the frequency characteristics under different geometrical sizes were analyzed on a basis of admittance curves.Besides,SAW resonator was manufactured with lift-off process,and the key issues in the process was investigated.Thin films with elasticity and flexibility were adopted to transfer load effectively between rigid surfaces.As the key components in the wireless interrogation system,the micro-strip patch antenna and transmission line were designed,optimized and manufactured.The wireless and passive experimental system shows good linearity and accuracy in the load range of 0-4MPa.However,the experiments results prove that the usage of elastic film leads to the hysteresis in the output even though the elasticity of films could improve the force transfer efficiency.(3)A force-sensitive element based on strain gauge was proposed in this paper for passive and wireless contact stress sensing in slits.The sensitivity of force-sensitive element was subject to the thickness and Young’s modulus of thin films,especially the medum layer film.The Young’s modulus of support layer affects the strain distribution but has little influence on the deflection.Thus,the stress concentration could be relieved by selecting proper Young’s modulus of support layer.Based on wireless transmission technology,the wireless interrogation system was designed and constructed,and the key signal processing steps was analyzed to realize accurate acquisition and stable transmission of contact stress signal.The experimental results show that the sensing system exhibits good linearity,accuracy and hysteresis.In summary,two sensing methods for the contact stress measurements in cramped slits were proposed in this paper since passive and wireless sensing technology was suitable and advantageous in harsh environment application.The crucial sensing procedures including the acquisition,conversion and wireless transmission of measured contact stress were investigated.Thus,the design rules and package principles of force-sensitive element in slit with large stiffness were concluded.The working rules of force-sensitive element was proved by both simulation and experimental results.The influence mechanisms on the sensor sensitivity by some parameters(thickness and Young’s Modulus of layers)were also revealed.The implementation and realization of passive and wireless contact stress sensing in crampled slits could provide theoretical foundation for the further practical application,which is of instructive significance for the sensing scheme in other similar harsh environment as well.The issue in this paper comes from the project “Study on the contact stress monitoring method in crampled slit between closed spherical surfaces”,which was financially supported by National Natural Science Foundation of China under grand number 11076007.Part of the research results has provided a theoretical basis for the further practical application of SAW sensor in cramped slit,and is of instructive significance for the design,package and testing of a commercialized sensor product in the future as well.