Research On Contact Resistance Modeling And Measurement Evaluation Methods For Contacts Considering Asperity Features

Contacts as the executive components are used to realize the current conduction and disconnection functions in the high and low voltage apparatus.The real mechanical contact only occurs in the very small part of the apparent area due to the roughness nature of the metal surface.And the produced contact resistance would result in the joule heat and signal insertion loss.In recent years,the low and stable contact resistance has become the requirement for the high-end electrical apparatus with the continuous development of control relays in the direction of“high reliability and long life”.The contact resistance problem has the typical complexity and uncertainty features,which is the intersection of the metal materials science,elastoplastic mechanics and interface physics.The analysis methods for the static contact resistance are not completely suitable for the research and application under the micro-scale conditions.Thus,it is difficult to calculate the contact resistance accurately and evaluate the electrical contact performance scientifically.This dissertation not only realizes the accurate modeling of contact resistance based on the rough surface reconstruction by asperities,but also realizes the automatic measurement of contact resistance and evaluation of the electrical contact performance by developing the in-situ observation and measurement system for the contact interface and the high-throughput precision measurement system for contact resistance of contacts.Firstly,the theoretical model of contact resistance for contacts with smooth surface is established.The calculation model of contact resistance is built based on the contact mechanics theory and electrical contact theory for the point contact and line contact.And the corrected contact resistance model considering the contact size features is proposed according to the finite-element simulation results.The influences of the contact contour and contact load on the contact resistance are analyzed.It is determined that the line contact is more conductive than the point contact under the low contact load.Furthermore,the calculation models of contact resistance and total resistance are built based on the cantilever beam theory.It is found that the contact force would increase and the contact resistance would decrease when the action position of push rod approaches the movable contact（or the fixed end）.Secondly,the numerical modeling method for contact resistance of contacts with rough surface is proposed.The multi-point ellipsoid-peak（MPEP）method is proposed according to the surface topography feature of the real contact,and the center coordinates and curvature radius are calculated by the least square method to realize the numerical reconstruction of the rough surface.It is determined that the surface resolution scale should be less than 1/10 of the equivalent contact diameter.Furthermore,the numerical calculation model of contact resistance for the rough surface is built based on the asperity contact unit.It is found that the number of the contact spot is the root reason for the contact resistance decrease by taking the electrical contact between the rough curved contact and smooth plane contact as an example.And the contact resistance could decrease effectively by increasing the number of the contact spot when the contact area is a constant.Then,in order to clarify the influence of asperities on contact spots and the real contact area,an in-situ measurement system for the contact interface is designed and built to realize the real-time observation of the contact interface between the curved contact and plane sapphire.And the image processing algorithm for extracting features of contact spots is also developed to realize the calculation of the real contact area.The results indicate that both the number increasement and fusion extention of contact spots would make the real contact area increase linearly.Subsequently,an innovative high-throughput precision measurement system for contact resistance of contacts is designed.The motion control module of the movable contact and the flxible loading module are developed.The adaptive movable contact fixture and multi-station static contact clamping device are also designed.The weak signal conditioning technology and the high-throughput automatic measurement method are proposed.Furthermore,the evaluation method for the measurement uncertainty of the contact resistance is investigated and the measurement accuracy is also verified by the standard resistance.The measurement results indicate that increasing the load current and contact force could decrease the measurement uncertainty of the contact resistance for contacts.Finally,the evaluation method for the electrical contact performance of contacts based on the relationship between contact resistance and contact load is proposed.The contact resistance variation law for micro-relays under the weak contact force is investigated experimentally.It is determined that the height difference of asperities is the root reason for the sudden decrease of the contact resistance.And the characteristic parameters in stages III and IV are extracted from the contact resistance curves to realize the quantitative evaluation of the electrical contact performance by considering the surface roughness.In addition,the contact resistance curve of the silver-based contact materials for power relays is also acquired.And the characteristic parameters are extracted from the contact resistance curves for Ag,Ag Sn O₂ and Ag Ni to realize the quantitative evaluation of the electrical contact performance by considering physical properties of contact materials and surface treatment process.This research not only has academic significance on the development of macro-and micro-scale static electrical contact theory,but also has reference value to improve the surface state and electrical contact performance for contacts.