For several years, Northeastern University's MicroFabrication Laboratory has been working on the development of microswitches and relays. This technology is based on the Northeastern University MEtal Micromachining (NUMEM) process which is shown schematically in Figure 1 below. With this process, we are able to make a variety of surface micromachined structures and devices. Examples of these include microrelays, microinterferometers, micromirrors and microaccelerometers. During the development of micromirrors it became apparent to us that a significant advantage could be derived from surface micromachined metallic structures. This advantage arises from the fact that metals, when stressed beyond their yield strength plastically deform. When the stress is released, the metal assumes a new deformed shape. This attribute of metal surface micromachined devices was exploited to raise a mirror off the substrate by tens of microns. Figure 2 below shows a raised mirror.
The same technology can be used for a variety of devices. In relationship to biomimetic underwater robots, it is known that many animals employ sensors based on hairs. Our interest focuses on the sensors used by lobsters to control their movement. Two sensors have been identified for study; a tilt sensor and a flow sensor. In the case of the flow sensor, the lobster has hairs in surface indentations on its claws. As the animal moves, the hairs bend against the flow of water around the claw. Nerves at the hair follicles transmit information about the position of the hairs to the lobster's brain. This sensor is very simple. It can be easily implemented with deformable MEMS.
Figure 3 shows a diagram of a sensor as fabricated using the NUMEM process. The device consists of a complex cantilever shape. A central beam is ultimately used as the hair. The surrounding two beams are switches of the type already fabricated at Northeastern. On the central beam, the beam is defined with two mechanically weak points as indicated by the indentations. Our approach is to grab the end of the beam and raise it to a new position allowing it to bend and plastically deform at the weak points. In this way, the hair is raised and becomes normal to the surface (Figure 4). The great advantage of this concept is its simplicity.
With the hair normal to the surface, forces applied to the hair will cause it to deflect as shown in Figure 5. Because the outer cantilever beams are mechanically connected to the central beam or hair, the outer beams are bent toward the substrate. The contacts at the ends of these beams eventually (with high enough force) contact the counter electrode on the substrate creating a short circuit between them. Therefore, the sensor provides an on-off signal in this configuration. The flow threshold that is detected is proportional to the stiffness of the beams and the length of the switch cantilevers.
The sensor described about is a single bit device. In most applications, several bits may be needed. With our technology, this may be accomplished in at least two ways. The first is that hairs of various lengths are fabricated. The longer ones will clearly be more sensitive. An array of 16 hairs provides four bits of information. Alternatively, a single device with multiple contacts could be created to accomplish the same function. Several configurations are shown in Figure 6.
Finally, a tilt sensor
is made by attaching a weight to the end of the hair. The tilt sensor
can be made to sense various angles by changing the original angle to which
the beam is bent (Figure 7). Further, by orienting several beams
at different angles on the substrate, tilt measurement through any solid
angle is possible.
Figure 1. A process flow diagram for the NUMEM process.
Figure 2.
SEM micrograph of a raised mirror which was first surface micromachined
using the NUMEM process and then raised off the substrate by approximately
10 microns.
Figure 3.
Top and cross-sectional views of a basic structure as fabricated using
the NUMEM process. The center cantilever has indentations to aid
in locating the bend
positions on the beam.
Figure 4. The
end of the beam is held and lifted bending the beam into its final position.
For more information on Biomimetic Robots see: Biomimetic Underwater Robot Program
Controlled Biological Systems Program< |
Office of Naval Research |
maintained by Paul M. Zavracky, zavracky@neu.edu
copyright (c) 1995 by Northeastern University. All rights reserved.