Research On Compliant Mechanism-based Piezoelectric-Driven Microgripper

With the development of precision engineering,material science,biological technology and modern medicine,micro-nano-manipulation has been paid more attention to in last twenty years.Microgripper is a typical end-effector in micro-nano-manipulation system and several driven principles are developed.The advantages of piezoelectric actuators are high displacement resolution,fast response,etc,which are corresponding with the trend of micro-nanomanipulation.Due to the advantages of simple manufacturing technique,free assembly,free clearance,free lubrication as well as easy miniaturization,compliant mechanisms become the important effectors of precision engineering.Base on the background mentioned above,the compliant mechanism-based piezoelectric-driven microgrippers are researched in this thesis.According to the overview,the key points of piezoelectric-driven microgrippers include:the strategies to enlarge grasping stoke,the coupling relation between piezoelectric actuator and mechanism,and micro-grasping's accuracy.Compared with other types of microgrippers,the miniaturization of piezoelectric-driven microgripper is challenging.The trend of miniaturization puts forward new requirements for the key points of piezoelectric-driven microgrippers.In this thesis,combining the trend as well as the knowhows of compliant mechanisms,the key points of piezoelectric-driven microgrippers are researched.The main contents are as follows.Firstly,considering compactness,parallel grasping and enlarging grasping stroke,a novel compliant orthogonal displacement amplification mechanism(DAM)configuration is designed.In the novel DAM,the parasitic movement at the output port can be eliminated without needing bidirectional symmetric input forces,which benefits the miniaturization.The configuration is analyzed and the number of undetermined structural parameters are researched.Under small deflection assumption,without considering specific type of flexure hinges,the general design equations are derived.For the novel DAM with typical flexure hinges,the static models are established.Combining the design equations with the static models,the DAM can be designed specifically.The performance evaluation parameters(including displacement amplification ratio and the parameters characterizing the parasitic movements)are modeled and analyzed.Based on the evaluation parameters,the effectiveness of design examples is verified by the small deflection-based static finite element analysis.Then,for the compliant orthogonal DAMs,large deflection geometrical nonlinearity analysis and the dimensional optimization are conducted.The large deflection analysis can be divided into three parts: the large deflection analysis of output displacement for the novel DAM;the large deflection analysis of displacement amplification ratio for the novel DAM and the traditional bridge-type mechanism;the global sensitivity analysis of large deflection nonlinearity.A static constraint limiting the large deflection nonlinearity is established.The allowable upper limit can be determined by the approximate characterization of large deflection nonlinearity;approximately characterizing a nonlinear structural parameter avoids the solution of nonlinear design equations in the optimal process,which improves the stability.For the novel DAM,the dynamic model is established and verified by modal analysis,and the dynamic constraint is further established.Considering the enlargement of grasping stroke,geometrical constraints,static constraints and dynamic constraint comprehensively,the dimensional optimal model of novel mechanism is established.Combining the optimal model and the design equations derived in the last chapter,a systematic design approach is constructed,which avoiding the problems in the design framework of last chapter: the large deflection geometrical nonlinearity can not be restraint,and experience is needed for the design of predetermined parameters.Piezoelectric stack actuator is coupling with compliant mechanism.Considering that preload is needed to eliminate the clearance between them,whereas the prestress is more significant in the miniaturized design.In this thesis,the output displacement of piezoelectric stack actuator is modeled considering stiffness and the ferroelectric nonlinearity due to the prestress.A test prototype is developed to verify the nonlinear term in the model and its simplified conditions.Considering preload and inverse piezoelectric effect,the input force of compliant mechanism is modeled.For pizeoelectric stack actuators,two typical preload force adjustment strategies are analyzed and experimentally researched in terms of preload adjustment and stiffness.A novel wedge preload mechanism which can be adjusted continuously along both directions is designed.Further,the hybrid piezoelectric driven are researched.Through the analysis of driven mode and the piezoelectric-static coupling finite element analysis,a hybrid piezoelectric-driven mechanism accumulating stroke is designed against the situation that in traditional multi-stages DAM,the nonlinear item in the coupling relation between piezoelectric stack actuator and the compliant mechanism is further amplified.The hybrid mechanism benefits the miniaturization as well.For a stiff gripper arm,the influence of position and pose at the end of transmission mechanism on the jaws movement is investigated.A compensation method for the parasitic jaws movement is established based on a hybrid piezoelectric-driven mechanism.Piezoelectric-static coupling finite element analysis is used to verified the method.Based on the researches described above,three piezoelectric-driven microgripper prototypes are designed.Optical micro-metrology is used to test the prototypes.The experimental results verify the effectiveness of researches.