| Magnetorheological torque servo (MRT) is a new type of smart transmission device,which can realize the transmission of force or torque and the stepless change of rotationalspeed. It has many advantages such as controllable output, simple structure, quick response,low consumption energy and so on. Therefore, MRT has a great potential to be used inmechanical engineering field especially in force control, overload protection, infinitelyvariable speed transmission.In this dissertation, the theory and model of force-position-posture decoupling has beenstudied systematically based on MRT to resolve the problem of force-position-posturecoupling in automatic polishing system for aspheric parts. The study has been supported bythe National Natural Science Foundation of China under grant No.50705044, the NaturalScience Foundation of Zhejiang Province under grant No. Y1080217 and the NingboMunicipal KeyLab Open Fund under grant No. 2007A22005. The prototype of MRT has beendesigned and it's various performace has been studied. The principle and model offorce-position-posture decoupling based on MRT has been proposed. The polishing system inwhich the force-position-posture is decoupled has been developed and some experiments havebeen done. The main research contents, methods and conclusions in this dissertation are asfollows:1. The study on the design and performance test of MRT. The concept of MRT has beenexplained, the prototype of MRT has been designed, the calculation models of the transfertorque and the gap magnetic flux density of MRT are established based on Bingham liquidmodel and electromagnetic theory, respectively. By means of finite element emulation, themagnetic field distribution of MRT is obtained and the factors which influence the magneticflux density are analyzed. The various performance of MRT such as no load characteristics,static characteristics, constant torque and dynamic characteristic have been studied byexperiments. Emulational and experimental results show that the effect of the input speed onthe output torque is very small and can been negligible. In certain zone, there is a good linearrelation between output torque and input current. Under the constant current, the outputtorque is constant. The dynamic model of MRT can be expressed as the first-order systemwith time-delay. 2. The study on the problem of force-position-posture coupling. After analyzing theinteraction relation among the polishing force, the position and posture of polishing tool andthe deformation between them based on the contact mechanics and probability statistics, themechanism of force-position-posture coupling is revealed. On the basis of studying on theelastic and plastic deformation among the polishing pad, aspheric part and grains, thecoupling model of force-position-posture is established. The research results show that thereis a coupling relation among the polishing force, the position and posture of polishing tool.The position and posture of polishing tool is affected by the normal displacement between thepolishing tool and aspheric part. The normal displacement is inversely proportional to thecurvature radius of aspheric part, the radius of polishing tool and rigidity of the material ofpolishing tool and aspheric part, but is proportional to the polishing force and abrasive grain.The change of the normal displacement must be compensated real-time during the feed ofpolishing tool in order to ensure there is no change at the position and posture of polishingtool.3. The principle and method study on the control of force-position-posture decoupling.The principle of the force-position-posture decoupling is proposed based on MRT afteranalyzing the control requirements of force-position-posture decoupling. The polishing forceof the force-position-posture decoupling polishing system is analyzed. Material removalfunction is established based on Preston equation. The research results show that the polishingforce can be controlled independently by means of MRT. The curve of material removal iscontinuous and smooth, and the maximum amount of material removed is obtained at thecenter of contact area, while the minimum amount of material removed is obtained at the edgeof contact area, which meets the condition of surface figure convergence.4. The model study on the control of force-position-posture decoupling based on MRT.The model of material removal rates is established for force-position-posture decouplingpolishing system. The control models of the polishing force based on MRT and the rotationspeed based on NC lath is established with the target of achieving the constant materialremoval rates. The control model of the position and posture of polishing tool is establishedtoo. The research results show that the polishing force can be controlled by means of MRTunder the change of the curvature radius of aspheric part, and the position and posture ofpolishing tool can be accurately controlled by NC lathe. The problem offorce-position-posture coupling can be resolved theoretically.5. The study on the development of the hardware and software and the dynamic modelof force-position-posture decoupling polishing system. The prototype offorce-position-posture decoupling polishing system is developed, after analyzing the transferprocess of polishing force, the dynamic model of polishing system is established. The controlsoftware of polishing force is designed based on LabVIEW. The path generation process and method of polishing tool are given.6. The emulational and experimental studies on the polishing torque control. Therelationship between the actual polishing force and the actual output torque of MRT is studiedby some experimental tests. The simulation and experiments are carried out to control theactual output torque of MRT applying the PID control strategy and fuzzy PID control strategy.The research results show that the dynamic model of the polishing force between thepolishing tool and aspheric part is the first-order system with a time delay. There is a 0.08sdelay of the response time of the polishing force than that of the output torque of MRT.Moreover, there is an 11% loss of output torque after transferring form the MRT to polishingtool. Although the traditional PID controller can meet the control requirements, the fuzzy PIDcontroller can get more satisfying control effec. The regulation time of the output torque isshortened 0.6s, and the overshoot is decreased 5% when use the fuzzy PID controllercompared with common PID controller. It can be concluded that the the fuzzy PID controllerfully satisfies the control requirements of the polishing force tracking real time.7. The experimental studies on NC polishing for various parts. The parts with thematerial of metal and nonmetallic and with the shape of plane surface, spherical surface andaspheric surface are polished, mirror-like plane surface and aspheric surface with a roughnessof 0.015μm and 0.028μm are obtained. The experimental results indicate that this decouplingpolishing system developed shows a good stability and widespread adaptability for the shapeand material of part. The unevenness of roughness can be decreased from 102% beforepolishing to 21% after polishing using the polishing force control method and rotational speedcontrol method. The material can be removed uniformly.In short, the theoretical and experimental results of this dissertation show that theMRT-based decoupling theory and method can independently, actively and real time controlthe polishing force, and can effectively resolve the problem of force-position-posture coupling.This study will greatly improve the polishing efficiency and quality, and provide a new wayof the application of the MR smart device in the filed of ultra precision machining...
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