A hybrid-type electrostatically driven microgripper with an integrated vacuum tool

Abstract

Reliable and accurate pick and release of microobjects is a long-standing challenge at the microscale manipulation field. This paper presents a new hybrid type of microelectromechanical systems (MEMS) microgripper integrated with an electrostatic mechanism and vacuum technology to achieve a reliable and accurate manipulation of microobjects. The microgripper is fabricated by a surface and bulk micromachining technology. Vacuum pipelines are constructed by bonding technique. The pick and release micromanipulation of microobjects is accomplished by electrostatic driving force caused by comb structure and an auxiliary air pressure force from air pump. A deflection of 25 μm at the tip of the gripper is achieved with the applied voltage of 80 V. The performance of this new hybrid type of microgripper was experimentally demonstrated through the manipulation of 100–200 μm polystyrene balls. Experimental results show that this microgripper can successfully fulfil the pick and release micromanipulation. Theoretical analyses were conducted to have a more comprehensive understanding of the release principle. Based on this paper, the new kind of hybrid microgripper will possibly provide an effective solution to the manipulation of submicrometer-sized objects.

Introduction

Manipulation of microobjects is of great interest and challenge to the scientists and engineers which enables the assembly of microsystems. Due to recent development of microelectromechanical systems (MEMS), there is an increasing demand of micromanipulation techniques [1]. Microgripper is one of the key tools to realize the micromanipulation and microassembly of microobjects [2].

Since 1991 when Kim [3] designed a polysilicon electrostatic microgripper, which features a flexible cantilever comb-drive arm with bidirectional actuation and an over-range protector, research has been focused on the development of electrostatic comb-drive microgrippers. Volland and Heerlein [4] in 2002 designed an electrostatically driven microgripper with a system of elastic spring beams which is capable of converting the linear motion of the microactuator to rotational gripping motion. A microgripper based on silicon bulk micromachining was designed by Li [5] in 2003, and an electrostatically driven microgripper was designed and fabricated for blood vessel manipulation by Wierzbicki and Houston [6] in 2006.

At the microscale, adhesive forces such as van der Waals forces, surface tension forces and electrostatic forces are dominant in manipulating microobjects over the gravitational force [7]. Because of the existence of adhesive forces, the releasing of microobjects by a microgripper encounters a lot of difficulties [8].

Hence, several approaches have been proposed in the literature to solve the releasing problem.

• Reducing the adhesion such as roughing surface to reduce the van der Waals force [9], [10].

• Using inertial effects [11], [12] or by rolling [13].

• Using vacuum tool that generates positive pressure to overcome the adhesion between the tool and the microobject [14], [15], [16].

Comparing with others, vacuum tools are considered to be a good strategy to solve releasing problem. The positive pressure generated by the vacuum tool can overcome the adhesion between tools and microobjects. This approach has been successfully implemented by Zesch in 1997 [14] and Chen in 2004 [15].

However, vacuum tools are limited in microobject manipulation. One problem is that it cannot provide sufficient constraint force, especially the friction force, required in assembly operations. Another problem is that it does not satisfy different orientation operation requirements and different shapes of the microobjects. Hence, one possible solution is to use the direct mechanical gripping approach such as two-fingered electrostatic comb-drive gripper.

In order to handle microobjects effectively and efficiently, we design and fabricate a hybrid-type electrostatic comb-drive silicon microgripper integrated vacuum tool. This hybrid-type microgripper combines the advantages of both electrostatic comb-drive microgripper and vacuum tool.

Particularly, the arm tip of the hybrid-type microgripper actuated by the electrostatic comb fingers can satisfy the different orientation operation requirements and be suitable for different shapes of the microobjects. Bulk micromachining technology is used to fabricate the hybrid-type microgripper, integrating electrostatic comb drive and vacuum together. Vacuum gas pipes embedded in microgripper is used to realize stable and reliable pick and release manipulation of the microdevices.

In this paper, the structure and the working mechanism of the hybrid-type microgripper are introduced firstly. Finite element analysis (FEA) of the gripper is performed to predict both the static and dynamic performance of the microgripper. The structure of the vacuum tool is designed according to the analysis of the force acting on the microobjects during pick and place operations. In addition, the fabrication of the microgripper is also described in detail. Two control systems are designed with one for electrostatic drive control, and the other for working gas pressure control. Finally, microobjects ranging from 100 μm to 200 μm are picked and released by this microgripper to demonstrate the performance of this microgripper.