Reseach And Application Of Magnetorheological Fluids Devices For Aspheric Surface Polishing

Magnetorheological fluid (MRF) is a new type of smart material, whose fluidness could change rapidly in response to an external magnetic field, therefore it has a great potential to be used in mechanical engineering field and attracted the interest of a large number of scholars. Because of the tremendous potential application of MRF in providing simple, quiet, rapid-response interfaces between electronic controls and mechanical systems, those devices using MRF have attracted considerable interests in mechanical transmission field and mechatronic systems. This material can be used in brake, clutch, damper, soft-start, stepless speed regulation and tension Control.Now the difficulty of the aspheric surface polishing is the hybrid control of force and position. The problem is the control of force and position are mixed and can?ˉt be easily decoupled. If the polishing is manipulated by robot, the control gain will be insufficient to keep the accuracy. The paper puts forward that a MRF device should be introduced in the control system, which can be used as a torque-servo device and control the polishing force. The advantages of MRF will benefit on stability and accuracy of the polishing system. So we focus on the Rotary MRF Transmission device.Based on the performance of MRF, the design of MRF devices and the research of MRF applications on mechanical field, the paper introduced the MRF to the compliant control system. The technology of providing a compliant polishing force by MRF servo device was studied in this paper. The main contents of research include the following aspects: MR effect, MRF transmission technology, rheological model, structure design of MRF device, the optimal design of MRF device, the magneto circuit calculating, and the theory of aspheric surface compliant polishing. After theoretical research we made some experiments about the output performance of MRF devices and the effect of compliant polishing on aspheric surface servo by MRF device. The paper is composed of following six chapters:1. The First chapter introduced the history and development trend of MRF, MRF applications, and the strategies of compliant control. Some technologies of polishing aspheric surface also are mentioned. Their drawbacks and advantages were been discussed in detail. The performance of MRF and the benefit of using MRF servo device in compliant control were described. The main subject and contents of the paper were prompted.2. Two theories about MR effect were proposed at the beginning of Chapter 2. After that, some influencing factors were listed. This included magnetic properties, yield stress, viscosity, response time, and so on? In the middle part of this chapter, the Bingham?ˉs model, which is widely-used in describing the rheological properties, was been discussed. The last part introduced the MRF used in this project. we fit the a polynomial curve about the parameters of MRF given by producer and get the function between yield stress(|óB) and intensity of magnetic field(B).3. According to the literature, we discussed three different working modes of MRF device and compared the Force-Calculation Formula of disc-type and valve-type MRF device. To decide which type of MRF device is more suitable for the compliant force control, we introduced two parameters as evaluation indicators. One is K, ratio of controllable force, and another is Tmax, maximum output force. We found that the disc-type?ˉs K better than the valve-type?ˉs K but the maximum output force of the valve-type surpassed the maximum output force of the disc-type. According to the requirement of the compliant control system, the disc-type MRF device is more appropriate. To verify the decision, we made a disc-type MRF damper and measured its output torque. The experimental result accords with the decision.When the working mode has decided, the torque calculation function can be derived from the Bingham model. The key structure parameters of MRF device can be calculated too. The clearance between working surfaces of the disc and the housings is a very important factor and has large influence to output torque. We designed three different clearance solutions and analyzed their performance under same conditions by Ansys, the finite-element analysis software. The simulation result shows that the wedge-shaped disc can output significant larger torque than other shape disc. The increase rate could be 11.9 percent. At the end of the chapter, the principle of magnet circuit design is discussed and the number of coil is obtained by calculating the intensity of magnetic field.4. Focus on the demands of MRF device used in compliant torque serve, we discussed all output parameters of it, which include the torque, exciting current, the rotate speed of axis, and the relationship of those parameters. In this chapter we actually designed a rotate disc-type MRF damper and testing system. According to the result of testing system, we obtained the function about the output torque and exciting current and analyzed the nonlinear error. Then we found the error is less than 2.19 percent. The testing system and the MRF device run about half an hour without a feedback control, the result data shows that the range of output torque error is less than 2.8 percent under same exciting current. That experiment is the basis of following torque control system. 5. At first some advanced polishing technologies of aspheric surface are listed and analyzed. According to the analysis to the working model of aspheric surface we draw a conclusion that under same conditions, such as same material and polishing slurry, three main factors in polishing are normal pressure, the relative velocity between parts and cutting tool, resident time. By installing MRF device in the torque-servo system, the whole working process can be decoupled to two independent motions, the force-servo motion and the position-control motion. Because the two motions are orthogonal the compliant control system is easier to realize than general servo device.Based on the decoupling of the compliant control system we can set a special feed path of the tool carriage. If the tool carriage traces this path, the cutting tool will ensure the pressure is normal to the aspheric surface. Then we studied other two key factors: relative velocity between parts and cutting tool, resident time. A suitable model was being presented to keep a constant removal amount. The paper gives an example to verify the model and prompts a principle to design the working conditions. At last the paper analyzed the possible errors between real pressure and ideal pressure and the influence of those errors to the roughness of aspheric surface.6. In this chapter the compliant polishing system is divided into four main subsystems: MRF torque servo subsystem, polishing tool subsystem, transmission subsystem and torque feedback subsystem. All the subsystem were designed and matched with the whole system and fixed on the tool carriage of the NC lathe. The structure graph and flow chart of the control system was drawn. According to the feedback torque signal, the value of appropriate exciting current is calculated online by two functions, one is the function between the servo torque and the Y coordinate of the polishing point, and another is the function between the output torque and the exciting current. The direction of pressure is ensured by the prearranged feed path of tool carriage, the polishing pressure and resident time are also controlled by the MRF torque servo system and polishing tool, So the removal amount will be a constant and the roughness of the aspheric surface will much less than the design value of the nc lathe. The experiment log shows that the torque fluctuating range of MRF device is less than 2.6 percent of the whole torque. It meets the requirement of the servo torque of compliant polishing system.By using a roughness tester to measure the surface of aspheric parts which is polishing by our compliant system, we found the profile accuracy of the NC lathe will be improved from 15|ìm to 1.2|ìm, and the effect is great and repeatable. So we may draw a conclusion that the MRF torque-servo compliant polishing system working on the NC lathe will lead an obviously improvement of the profile accuracy. According to the function 5.10, the diameter of part which can be polishing on the compliant system only depends on the working range of the tool carriage. So the system can be used in the batch production of polishing work of aspheric surface whose highest demands of accuracy is micron. Finally, the thesis research and the achievements of this paper are summarized, and a few questions are pointed out which is worthy of further studies.In this project MRF devices are used for torque-servo system, and applied to compliant polishing system for aspheric surface. Research about this kind of application of MRF has not been formally reported. The original application of MRF will further broaden the application field of this smart material.