Survey of sticking effects for micro parts handling

When parts to be handled are less than one millimeter in size, adhesive forces between gripper and object can be signiicant compared to gravitational forces. These adhesive forces arise primarily from surface tension , Van der Waals, and electrostatic attractions and can be a fundamental limitation to part handling in a gas environment. While it is possible to fabricate miniature versions of conventional robot grippers, for example from polysilicon, it appears that it will be diicult to overcome adhesion eeects for the smallest parts. Thus, manipulation of parts on the order of 10 micron or smaller may best be done in a uid medium using techniques such as laser trapping, or di-electrophoresis. A typical robotic manipulation scenario is the sequence of operations pick, transport, and place. For parts with masses of several grams, the gravitational force will usually dominate adhesive forces, and parts will drop when the gripper opens. For parts with size less than a millimeter (masses less than 10 ?6 Kg), the gravitational and inertial forces may become insignii-cant compared to adhesive forces, which are generally proportional to surface area. When parts become very small, adhesive forces can prevent release of the part from the gripper. For example, a laser diode for an optical disk may be only 300m in size Hara et al 1993]. Figure 1 illustrates some of the eeects which can be seen when attempting to manipulate micro parts. As the gripper approaches the part, electrostatic attraction may cause the part to jump oo the surface into the gripper, with an orientation dependent on initial charge distributions. When the part is placed to a desired location, it may adhere better to the gripper than the substrate, preventing accurate placement. Adhesion forces could be due to electrostatic, van der Waals or surface tension. Electrostatic forces arise from charge generation (triboelectriication) or charge transfer during contact. Van der Waals forces are due to instantaneous polarization of atoms and molecules due to quantum mechanical eeects Is-raelachvili, 1974]. Surface tension eeects arise from interactions of layers of adsorbed moisture on the two surfaces. The goal of this paper is to survey causes of adhesion, provide estimates on the magnitude of their eeect, and to survey methods for reducing the eeect of adhesive forces. For a simple numerical example to get an idea of the scale of the adhesion forces, consider the force between a spherical object and a plane …