Study On The Electrothermal SU-8Microgripper

To promote the MEMS technologies and the studies of microscale and nanoscale effects, it’s necessary to develop the micro-gripping system which is suitable for the micro-assemblies of complicated MEMS devices and the micro-manipulations of microscale and nanoscale objects. As the end execution part of the micro-gripping system, the microgripper is the interface between the micro-gripping system and the gripped object. Therefore, the performances of the micro-gripper system are directly determined by the clamping capacity of the microgripper which has become a reseach focus currently. Based on the analysis of the current research achievements of micro-gripping devices, it can be known that electrothermal microgrippers have many advantages such as compact structure, easily control and relatively large gripping force. And with the characteristics including a relatively high coefficient of thermal expansion, low coefficient of thermal conductivity, good biocompatibility and simple processing, SU-8is suitable to be the material of electrothermal microgrippers. For these reasons, the design method, fabrication processes and performace tests of the electrothermal SU-8microgripper is studied in this thesis.A new systematic design procedure for the compliant mechanisms of electrothermal microgrippers is presented. The explicit selection terms for the compliant mechanisms topology design are summarized and proofed firstly. Based on the selection terms, the structural synthesis of the symmetrical one-DOF6-bar,8-bar and10-bar rigid-body mechanisms for the microgripper compliant mechanism topology design is carried out, and66reasonable mechanisms are obtained. In order to avoid the reliance on designer’s experience and improve the design efficiency, the stiffness parametric modeling of microgripper is carried out based on the stiffness matrix model method. And then the geometric size optimizes are implemented based on the proposed models. A normally closed type (A type) and a normally open type (B type) electrothermal SU-8microgripper are designed using the systematic design procedure. According to ANSYS software simulation results, the obtained microgrippers meet the design requirements, and their performances are better than the empirical designs. The amplification ratios of A type and B type microgripper are respectively30.6and14.8. Then, the SU-8electrothermal microgripper with nanoscale jaws is designed based on the topology optimization method and the pseudo-rigid model method.Considering the polymer electricity and thermal coupling and the thermal expansion uneven distribution of multi-material structure, a novel SU-8chevron electrothermal micro-actuator with three-layer symmetrical sandwich structure is proposed. Without the out-of-plane actions, the novel micro-actuator has the advantages of low driving voltage, low working temperature and compact structure. Then, the designed microgrippers are successfully fabricated using the entire piece releasing process and two-sided sputtering process process. And the critical process parameters affecting the processing quality are optimized. A fabrication process combining micro fabrication process and nanoimprint process is proposed for the fabrication of the designed microgripper with nanoscale jaws. A preliminary study of the proposed process is carried out.A micro-manipulation and performance test system of fabricated A type and B type microgripper is developed for the measurements of jaw displacement output characteristics and the actuator temperature characteristics. The test experiments are carried out at25℃in clean laboratory. The experimental results demonstrate that for A type microgripper, a jaw gap change of107.5μm requires only73.6mV,25.61mW and only44.92℃average temperature increase at the actuator, and the jaws response time is about0.3s. For B type microgripper, with195mV,111.1mW and53.7℃average temperature increase at the actuator, a71.5μm jaws gap change is obtained, making jaws to be cloesd. The jaws response time of B type microgripper is about0.23s. During both performance tests, the out-of-plane actuations of jaws are less than500nm, which verifies the rationality of the micro-actuator structure. It can be known that A type microgripper has the maximum driving efficiency (4.18μm/mW) in the reported SU-8electrothermal microgrippers. And A type and B type microgripper require lower voltages than others.This thesis presents a direct measuring method of microgripper gripping force based on SU-8micro-cantilever sensors with integrated copper piezoresistive strain gauge. The corresponding measuring device is developed and the direct measurements of microgripper gripping forces are implemented. Then, the micro gripping forces of two developed microgrippers are measured by the developed device and the jaw stiffness calibrations are also carried out. According to the experiment results, A type microgripper has a jaw stiffness of about2.83N/m and the jaw stiffness of B type microgripper is about7.22N/m. The calibration results meet the design requirements. With a simple structure, appropriate size and measurement accuracy, the low cost micro-cantilever sensor is suitable for the microgripper gripping force measurements.In order to test the gripping performances, micro-assembly experiments of specimen of fine hair and asio otus covert feather barbule for micro-tensile testing, and micro-manipulation of PS balls, micro blood vessel specimen and cyanobacteria cell are successfully implemented using A or B type microgripper. The test experiments are carried out at25℃in clean laboratory. The experiment results demonstrate that the developed microgrippers can accomplish many micro-manipulation and micro-assembly experiments of microscale objects and biological samples with a variety of shapes and sizes.