| Microassembly is an important technical approach to manufacture complex heterogeneous micro-electro-mechanical system(MEMS).Microgripper is the end-effector of the microassembly system and directly in contact with objects to be gripped,thus the microgripper is critical to microassembly.Since the microparts in the microassembly and microoperation are easily deformed and broken,on the one hand,the assembly force and the gripping force should be extremely small to avoid damage to the microparts,and on the other hand,different assembly forces and gripping forces should be provided for different objects to ensure reliability and consistency.So it is therefore required to measure the assembly force and the gripping force.Moreover,due to inherent problems of microassembly system such as a small depth-of-field and a narrow field-of-view,it is difficult to detect the jaw displacement information of the microgripper in the micro visual space in real-time.To achieve automated assembly tasks accurately and efficiently,the jaw displacement information must be obtained in real time.For accurate and efficient automatic microassembly tasks,the jaw displacement information must be obtained in real-time.Therefore,a displacement sensor must be integrated on the microgripper.In addition,when a circular or spherical object is gripped by a conventional microgripper relying on the rotation of the gripping jaws,a gripping force component parallel to the center line between the two gripping jaws will be generated and lead to the ejection of the manipulated object.If the gripping jaws move in parallel,the gripping task can be completed more easily and reliably.In this case,the parallel movement of gripping jaws should be ensured when designing a microgripper.In order to solve the above problems,a monolithic compliant piezoelectric-driven microgripper was reported by Wang et al(Wang et al,IEEE/ASME Transactions on Mechatronics,2013,18(1):138-147).in previous works of our team.On this basis,a piezoelectric-driven microgripper based on parallelogram flexible mechanism with integrated assembly force,gripping force and displacement sensors is developed in this dissertation.The microgripper can measure the assembly force,the gripping force and the jaw displacement with high precision in real time.Furthermore,the gripping jaws can not only move in parallel,but also maintain a high degree of parallelism when gripping a micropart and subjected to the gripping force.The major research works and achievements completed in this dissertation include:1.The importance of assembly force sensing is analyzed based on the requirements of microassembly tasks.The assembly force sensing mechanism for microgripper is designed,which integrated with assembly force sensors to realize the assembly force sensing.The gripping force sensing mechanism based on parallelogram flexible mechanism is designed,so that the gripping jaws can not only move in parallel,but also maintain a high degree of parallelism when gripping a micropart and sensing the gripping force.The gripping force sensors is integrated on the gripping force sensing mechanism to realize the sensing of the gripping force.The displacement sensor is integrated on the displacement amplification mechanism to achieve the sensing of the jaw displacement.2.According to the pseudo-rigid-body model of the microgripper,the sensing principles of assembly force,gripping force and jaw displacement are analyzed.And the models of the relationship between the surface strain of the single-notch circular flexure hinges and the assembly force,the gripping force and the displacement of the jaws are established respectively.According to the models,the influence of structural parameters,such as the length of parallelogram mechanism,width and thickness of flexible hinges,on the sensing performance of the sensing mechanism is analyzed and discussed.The finite element models of the assembly force sensing mechanism,the gripping force sensing mechanism and the displacement amplification mechanism is established,and finite element analysis results show that the theoretical models can correctly describe the sensing principle and sensing characteristics of the mechanism.3.Calibration methods for assembly force,gripping force and jaw displacement are researched.Since the assembly force and the gravity are in the same direction,a method that using a precision balance to calibrate the assembly force sensor is proposed,and the assembly force is calibrated.According to the mechanical properties of the parallelogram mechanism,a method to calibrate the gripping force sensor by measuring the deformation of the parallelogram mechanism using a non-contact displacement sensor is proposed,and the gripping force sensor is calibrated.The calibration of the displacement sensor is realized by a laser Doppler vibrometer.The calibration results show that the calibration method used in this thesis can accurately calibrate the sensor.The microassembly force sensor,gripping force sensor and jaw displacement sensor based on the parallelogram flexible mechanism have the advantages of high sensitivity,high linearity and high resolution.4.Taking the assembly of two micro-thin-wall metal cylinders as an example,the assembly process is simulated in the finite element simulation software.The assembly experiment is carried out in the microassembly system using the microgripper designed in this thesis.And the assembly force,the gripping force and the jaw displacement are obtained through the sensors.Finally,the assembly of two micro-thin-wall metal cylinders is completed.The experimental results show that the piezoelectric-driven microgripper based on parallelogram flexible mechanism with integrated assembly force,gripping force and displacement sensors designed in this thesis can accurately and reliably complete the assembly of microparts.The research works in this paper provide a theoretical basis for the design and development of microgrippers that need to integrate microassembly force,gripping force and jaw displacement sensor,and have guiding significance. |
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