| The snap-action thermostat is a switching element to control temperature which is wildlyused in household appliances, such as water fountains, rice cookers, microwave ovens, electricovens and so on. The reliability of snap-action thermostat affects the performance and theproduct’s grade directly. The life of domestic thermostat remains one hundred thousand times atpresent which is far behind the foreign products’ level at about two hundred thousand times. Toimprove the market competitiveness of the domestic household appliances, it badly needs toimprove the reliability of the snap-action thermostat.For such a switching element like the snap-action thermostat, contact failure is one of themost common failure modes, which takes80%of the total failure. Contact bounce is widely existin electrical switches, it’s a main reason that leads to contact failure. Contact bounce causesmechanical wear, starts arc, exacerbates the contacts electrical wear and material erosion, evenadheres the contacts together, which seriously reduces the lifetime and the reliability of theelectrical switches. Thus, it needs to investigate the contact bounce of the snap-action thermostat,analysis the main factors that may cause contact bounce to provide a basis for evaluating theresults of the optimal design and preventing the contact bounce.Flat spring is one of the key elements to ensure the proper functioning of the snap-actionthermostat, the mechanical properties of the flat spring have a great influence on the operationcharacteristics and contact bounce of the snap-action thermostat. Currently, the calculationmethod based on small deformation theory is wildly used to calculate the mechanical propertiesof the flat spring, ignoring the geometric nonlinear of this thin-walled element as the flat spring,so the results have inconformity with the real results. In this paper, the geometric nonlinear finiteelement method is taken to calculate the mechanical properties of the flat spring, based on this,the optimal design of the flat spring is performed to improve the dynamic bounce characteristics,which has a practical implication to increase the reliability of the snap-action thermostat.In this dissertation, the KSD301snap-action thermostat is taken as an object. At first thecontact bounce is studied, the influence factors of the contact bounce is analyzed. Then themechanical property is calculated. At last the structure dimension of the flat spring is optimaldesigned with the goal that the pre-pressure of the flat spring is the biggest under its constraints.The following is the main contents of this research:Chapter1presents the purpose and significance of this research, give an overview of thedomestic and overseas research status about the contact bounce phenomenon, summaries the calculation methods and the problems exist at present of the mechanical characteristics of therelay system, at last the main research contents of this paper is given.Chapter2gives a brief introduction to the structure, main components and operatingprinciple of the snap-action thermostat at first. Then build the ADAMS simulation model toanalysis the movable contact’s velocity, displacement and contact force between the movable andimmovable contact as the bounce happens. At last the laser displacement measurement method istaken to test the movable contact’s displacement and make a comparison with the result of thesimulation method to detect the accuracy of the simulation model. The results show that thesimulation model is suitable with an enough accuracy.Chapter3based on the simulation model of the contact bounce and the orthogonalexperimental design method, designs the orthogonal experimental program, which takes thebounce parameters and the flat spring’s mechanical characteristics as target and the structuredimension of the flat spring as variables. The influence level and laws between the variables andtarget is analyzed using the variance Analysis method.Chapter4takes into consideration that the flat spring used in thermostat is a thin-walledelements， which has a large deformation during the action process, so takes the geometricnonlinear finite element method to calculate the static contact characteristics of the flat spring.Then perform an experiment to a flat spring sample. At last makes a comparison among theresults of the experiment, the calculation method based on the small deformation theories and thegeometric nonlinear finite element analysis to explain the feasibility and accuracy of thegeometric nonlinear finite element analysis method.Chapter5builds an optimal model which taking the pre-pressure of the flat spring as goaland the stiffness, max stress and the dimension restrictions as restrict principles, and this optimalmodel is calculated by using the fmincon optimal function in MATLAB software. At lastcompare the contact bounce phenomenon before and after the optimal design to explain theability of the optimized flat spring to prevent the contact bounce.Chapter6summaries the contents of the entire dissertation and presents the researchcontents of the next research. |
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