A PROPOSED METHODOLOGY FOR THE DEVELOPMENT OF MICROGRIPPERS: AN APPLICATION TO A SILICON DEVICE ACTUATED BY SHAPE MEMORY ALLOY WIRES

In this paper a methodology is presented for the development of a silicon microgripper with actuators made by shape memory alloys (SMA). The kinematic architecture for the movement of the fingers is inspired by a four bar mechanism whose design has been evaluated through different solutions. Given the small dimensions it is not convenient to provide physical hinges for the mechanisms and therefore movements based on structural deformations have been considered. Various solutions with virtual hinges were tried, but the solution was implemented using a shear-like frame scheme. In this way it was possible to distribute the deformation necessary for the movement instead of concentrating it in small areas. This allowed to obtain wider movements at the same mechanical tension in the structural material. The fingers of the microgripper are supported by slender beams and have a translatory movement with parallel approach which is implemented by means of actuators made by wires in SMA with a diameter of 50 μm. The wires are arranged in such a way as to obtain an amplification of the movement and give rise only to the gripping force, transverse to the finger supports, without generating other load components such as compression, avoiding structural instability. The closing stroke ranges from 200 μm to 500 μm. The overall dimensions are 7,8 mm × 21 mm × 1 mm. The structural design has been realized using the finite element modeling. Particular care required the design of the actuators which, after the contraction to command the closure of the gripper, to return to the maximum length associated with the martensitic phase are subjected to traction by the elastic forces of deformation of the structural beams that support the fingers. The silicon used is a <100> monocrystalline type. The device was produced using microcutting technology with copper vapor lasers. The performances were obtained by considering different design parameters of which the effects on the different performances were studied. Finally, it was possible to deduce a flow chart that organizes all the activities that must be carried out to develop new optimized microgrippers.