20-06-2013, 03:23 PM
Hybrid Design Electrothermal Polymeric Microgripper with Integrated Force Sensor
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Abstract:
Microgrippers are typical MEMS devices used to pick, hold and transport micro-objects.Microgrippers are widely used in the field of micro-assembly, micro-surgery and manipulation of micro-particles. Thermal microgrippers are widely used for large displacement, high accuracy and repeatability. In this paper, a hybrid design electrothermal microgripper (Figure 1), based on Poly Methyl Methacrylate (PMMA) with integrated force sensor is designed and analysed using COMSOL Multiphysics software. The new design is designed by combining asymmetric arm structure and bi-layer structure to minimize the undesired out of plane displacement of the microgripper. The in-plane, out of plane and curl displacements, stress, strain and temperature has been analysed and the results are discussed (Figure 2). A piezoelectric force sensor (Figure 3), based on Poly Vinylidene Fluride (PVDF), which is to be integrated with microgripper to resolve the gripping force exerted by the microgripper on the micro-objects, is also designed and analysed. The force sensor is analysed for charge density for various gripping forces, stress, strain and displacements along X,Y, and Z axis (Figure 4). The hybrid design gives 1.6 μm in-plane displacement and 0.3 μm out of plane displacement at 0.1 V applied voltage.
INTRODUCTION
The field of mechanical micro machining deals with producing miniature 3D components using a variety of engineering materials in the sub-millimeter (1-999μm) range and bridges the gap between nano-scale and macro-scale manufacturing. The demand for micro machined components such as micro shafts, micro nuts, micro spiral inductors, micro motors etc. has been rapidly increasing in nuclear, aerospace, automotive, optical, military and microelectronics packaging. Miniaturization of components leads to material saving, energy saving, increased functionality, quick response etc. [1]
DESIGN AND SIMULATION
Design of microgripper:
The hybrid design microgripper is designed using Comsol Multiphysics software. The hybrid design is a combination of asymmetric arm and bi-layer structure. The major parts of the microgripper are the fixed part (anchor), hot and cold under arms, flexure, gripper arms and heaters. The hot and cold arms vary in cross sectional area in order to obtain different temperature for thermal expansion. The flexure is designed in such a way that it should be as thin as possible but not thinner than the thin arm because the temperature of the flexure could be higher than that of the hot arm which might result in overheating. Also, in order to keep it elastically deflecting, the flexure is designed long enough. However it is taken care that the flexure is not too long that the deflection of the thermal actuator tip is not reduced due to resistivity of additional material. The overall dimension of the microgripper is 400x300x20 μm.
SIMULATION AND ANALYSIS RESULTS
The designed microgripper is analyzed using Comsol Multiphysis for stress, strain, displacements, and temperature and the results are obtained. At 0.1 V the hybrid design microgripper gives good in-plane displacement of 1.6 μm and very less out of plane displacement of 0.3 μm as shown in figures 3 and 4. The displacement for different voltages is plotted in figure 5. Thus more positional accuracy can be obtained. Desired displacement can be obtained by changing the applied voltage. The maximum stress obtained is 5.4x107 Nm-2 and the strain obtained is4.7x10-3 as shown in figures 6 and 7. The maximum temperature obtained is 323 K and the temperature at the gripper arm tips is 320 K which is very less and suitable for most of the applications. Temperature analysis is shown in figure 8.
CONCLUSION
The microgripper is an important component of the system to hold, pick, manipulate and assemble mechanical micro components. Thermal microgrippers are widely used for large displacement, high accuracy and repeatability. They are designed using asymmetric arm and bi-layer structure. These structures have disadvantages such as divergent displacement and more out of plane movement. These disadvantages can be overcome by using a hybrid design structure using a combination of asymmetric arm and bi-layer structure. The hybrid microgripper has been designed and analyzed by Comsol Multiphysics software and the results are discussed.