Patent Application
20100180681
High precision accelerometers that use some form of magnetic rebalancing are typically bulky, expensive, and include a moving area that may not be completely sealed such that particulate contamination occurs when magnets are added to the accelerometer system. D'Arsonval MEMS accelerometers have been developed to address these deficiencies. Further, D'Arsonval MEMS devices performing other functions have been developed. However, there is an increasing need to improve the operating efficiency of the D'Arsonval MEMS devices.
The present invention increases flux density in a D'Arsonval Micro-Electro-Mechanical Systems (MEMS) device. An exemplary embodiment of the increased flux density D'Arsonval Micro-Electro-Mechanical Systems (MEMS) device includes a housing, a proof mass suspended within the housing by at least one torsional flexure, a second torsional rebalancing magnet, and a current coil disposed on the proof mass. A portion of the current coil is disposed between the first torsional rebalancing magnet and the second torsional rebalancing magnet. A field generated in response to a current in the current coil interacts with a magnetic field generated by the first torsional rebalancing magnet and the second torsional rebalancing magnet. The magnetic field generates a rebalancing force that stabilizes a position of the proof mass.
In accordance with still further aspects of the invention, a method used by an exemplary embodiment senses movement of a proof mass. The proof mass has an axis of rotation about at least one flexure and at least one pair of rebalancing magnets. A portion of a current coil is positioned between the pair of rebalancing magnets. A magnetic field is generated by the pair of rebalancing magnets such that a magnetic flux field flows approximately orthogonally to the axis of rotation of the proof mass. The method includes injecting current through the current coil to to generate flux that interacts with the magnetic flux field and generating a rebalancing force that is applied to the proof mass to stabilize a position of the proof mass.
Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
The increased flux density D'Arsonval MEMS device 100 includes a proof mass 202 suspended by at least one torsional flexure 204 within a housing 206 and two sets of torsional rebalancing magnets 208a, 208b, 208c, 208d. The first torsional rebalancing magnet 208a and the second torsional rebalancing magnet 208b are located proximate to a first end of the proof mass 202. The third torsional rebalancing magnet 208c and the fourth torsional rebalancing magnet 208d are located proximate to an opposing second end of the proof mass 202.
The two sets of torsional rebalancing magnets 208a, 208b, 208c, 208d make use of a Lorentz force by passing a current 212 through a current coil 214 that lies on both sides of a rotational axis 210 of the proof mass 202. The current 212 injected through the current coil 214 generates a magnetic flux field 216 so that D'Arsonval type movement about the rotational axis 210 rebalances the proof mass 202. An optional magnetic shield (not shown) may be present around the increased flux density D'Arsonval MEMS device 100 and/or the MEMS system 102 in some embodiments for use in a multi-sensor environment. In some embodiments, the magnetic shield may be integrated into, or may be part of, the housing 206.
Any suitable MEMS device may be implemented as an increased flux density D'Arsonval MEMS device 100. In the exemplary embodiment of
A plurality of electrodes 220a, 220b, 220c, 220d sense movement (and/or forces) of the proof mass 202 in response to an acceleration and/or a rotation of the increased flux density D'Arsonval MEMS device 100. A change in position of the proof mass 202 with respect to the electrodes 220a, 220b, 220c, 220d causes a capacitance change between the proof mass 202 and the electrodes 220a, 220b, 220c, 220d. Changes in capacitance are interpreted by the controller 104 to determine the acceleration of and/or the rotation of the increased flux density D'Arsonval MEMS device 100.
The housing 206, in the exemplary embodiment of
The cover portion 224, in this exemplary embodiment, has a plurality of non-magnetic standoffs 230a, 230b, 230c, 230d. The two sets of torsional rebalancing magnets 208a, 208b, 208c, 208d are secured to the non-magnetic standoffs 230a, 230b, 230c, 230d, respectively, using any suitable securing means. The dimensions of the non-magnetic standoffs 230a, 230b, 230c, 230d are such that, after fabrication of the proof mass and other components, the cover portion 224 is secured into position in a manner that positions the two sets of torsional rebalancing magnets 208a, 208b, 208c, 208d in their designed locations.
The side portions 226 may facilitate precise positioning of the torsional rebalancing magnets 208a, 208b, 208c, 208d between the current coil 214. The side portions 226 may be part of or integrated into the cover portion 224, the bottom portion 222, or may be separate portions. The side portions 226, the bottom portion 222, and the cover portion 224 are secured together using any suitable securing means, thereby forming the housing 206. In an exemplary embodiment, anodic bonding is used to secure the side portions 226, the bottom portion 222, and/or the cover portion 224 with each other.
The housing 206 may be optionally sealed by design or the side portions 226, the bottom portion 222, and the cover portion 224. Further, other portions (not shown) may be used in the housing 206. For example, but not limited to, electrical leads, contacts or the like may be fabricated into the side portions 226, the bottom portion 222, and/or the cover portion 224 as needed to provide the communication of the various elements of the increased flux density D'Arsonval MEMS device 100 with the drive electronics 108, the sense electronics 106, the controller 104, and/or other devices (not shown).
The current coil 214 lies on top of or within the proof mass 202. Portions 214a, 214b of the planar current coil 214 are located between each set of torsional rebalancing magnets 208a, 208b and 208c, 208d. The current coil 214 may be a single layer spiral coil in an exemplary embodiment that is oriented along a plane of the proof mass 202. In one embodiment, the current coil 214 includes approximately 10 turns that are each approximately 45 micrometers (microns) wide, with a spacing of approximately 15 microns between turns and a thickness of approximately 0.5 millimeters. However, different numbers of turns, widths, spacing, and thicknesses for the coil 214 may also be used. Although only a single current coil 214 is shown for clarity, additional current coils (not shown) may also be used.
In the exemplary embodiment illustrated in
The current coil 214 is connected to the drive electronics component 108 (
Any suitable material may be used for the torsional rebalancing magnets 208a, 208b, 208c, 208d. Samarium cobalt provides a relatively high coercive force and a relatively high flux per magnet volume ratio. In an exemplary embodiment, a Samarium Cobalt (SmCo) magnet is used.
Each of the set of the torsional rebalancing magnets 208a, 208b, 208c, 208d has a body portion 404 and a face portion 406. The external surface of the face portion 406 is configured to improve the uniformity of the generated flux. Here, the shape of the external surface of the face portion 406 is generally parabolic. Any suitable shape of the external surface of the face portion 406 may be selected to optimize the generated flux in any desired pattern.
In some embodiments, the body portion 404 and the face portion 406 are separate portions that have been joined. In other embodiments, the body portion 404 and the face portion 406 are formed as a single piece.
Generally, embodiments of the increased flux density D'Arsonval MEMS device 100 are formed by starting with a silicon wafer that is patterned and etched to produce several pendulous proof masses 202 with the torsional flexures 204 attached to support the proof masses 202. The silicon wafer is then oxidized to produce a dielectric layer to support metallization. Further patterning and metallization steps create the electrodes 220a, 220b, 220c, 220d and the basic element for the spiral current coil 214.
In an exemplary embodiment, the inner trace of the current coil 214 is brought out via the placement of a dielectric layer across the current coil 214. The current coil 214 is formed using a suitable metallization step. Glass wafers with the same diameter as the silicon wafer are patterned, etched, and metalized to produce recesses in the surface that act as second plates of the differential capacitive pickoffs and further serve to control device damping. A first glass wafer is then aligned with the silicon wafer and anodically bonded to form the lower capacitor plates. This assembly is then anodically bonded to a second glass wafer to form the upper capacitors for the differential capacitive pickoff. The two sets of torsional rebalancing magnets 208a, 208b, 208c, 208d are attached to the respective non-magnetic standoffs 230a, 230b, 230c, 230d. The wafer assembly is then diced so that individual accelerometers can be accessed for packaging, testing, and incorporation into an accelerometer system. Magnetic fields established by the sets of torsional rebalancing magnets 208a, 208b, 208c, 208d interact with the field created when the current is injected into the current coil 214.
While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.
