None.
1. Field of the Invention
The present invention generally relates to a device capable of large conduction changes when locally compressed and decompressed. Specifically, the present invention is a sensor of sufficiently high sensitivity so as to enable pressure and stress measurements.
2. Description of the Related Art
Sensors are critical to the performance of keyboards, intrusion detection systems, and fluid control systems.
Musical keyboards employ carbon contacts to detect the depression of a key and a capacitive strip to measure how hard and fast the key is depressed so as to simulate the dynamic response of an instrument. Keys are susceptible to dirt, moisture, and other contaminants. Mechanical solutions adversely effect pressure detection and penalize the dynamic range of the instrument.
Intrusion systems include optical and laser devices for detection purposes. Devices are difficult to conceal and readily defeatable. Furthermore, such systems are limited in their mapping capability and therefore do not provide a complete record of an intrusion, including, but not limited to, location and time, to resolve direction of travel and speed.
Fluid control systems for cooling, fire suppression, and fuel transport require inline sensors to accurately measure pressure for the active control of valves and pumps. However, such systems must function within a harsh environment including rapidly changing pressures. Diaphragm-based gages offer reliable pressure measurement, even when surrounded by corrosive and high temperature fluids, but are generally too bulky for inline use. Furthermore, low sensitivity and high cost limit the application of diaphragm-based devices to all but the most critical systems.
High-sensitivity pressure sensors are sorely needed for the above referenced applications, as well as for other applications, including submarines, torpedoes, sonobuoys, industrial and commercial keyboards, doors, and switches. Composites having one or more pressure variable properties are critical to a new class of sensors for use within the applications above.
Polymer-metal composites having variable resistance are described in U.S. Pat. No. 4,028,276. Compositions experience an actual change in electric properties, namely, resistance, when compressed by a mechanical load. Practical applications of these materials require a complete and uniform compression of the composite cross its presented area. As such, a large force is required for proper function. Resultantly, polymer-metal composites lack the fidelity necessary to accurately measure pressure and stress and thereby limited to sensing gross magnitudes and differentials.
Pressure conduction composites exhibit a change in conductance in response to a mechanical load. Unlike polymer-metal composites, pressure conduction composites do not experience an actual resistance change. Rather, compression alters the spatial arrangement of conductive particles within a non-conductive matrix as so to enable a change in conduction between particles about the critical percolation threshold of the material system. Small localized mechanical loads or pressures cause a very large change in the “effective” resistance at the output of the composite. This highly localized sensitivity greatly improves the signal-to-noise ratio and usable signal strength from the composite. Furthermore, pressure conduction composites are inherently resistant to the deleterious effects of dirt, moisture, and other contaminants.
Therefore, what is currently required is a high-sensitivity sensor including a pressure conduction composite capable of detecting and measuring pressure and/or stress.
Furthermore, what is currently required is a high-sensitivity sensor including a pressure conduction composite that is sufficiently robust to avoid the deleterious effects of harsh environments.
Furthermore, what is currently required is a high-sensitivity sensor including a pressure conduction composite having a compact, conformal form of minimal volume.
Furthermore, what is currently required is a high-sensitivity sensor including a pressure conduction composite that is easily configurable to enable a matrix arrangement for large spatial applications.
An object of the present invention is to provide an electrically and mechanically simple sensor that minimizes signal-processing requirements.
A further object of the present invention is to provide a sensor that is sufficiently robust to avoid the deleterious effects of harsh environments.
A further object of the present invention is to provide a sensor having a compact, conformal form of minimal volume.
A further object of the present invention is to provide a sensor that is easily configurable into a matrix arrangement for large spatial applications.
In its simplest form, the present invention is comprised of a pair of locally resilient conductive layers and a locally resilient pressure conduction composite disposed between and contacting both conductive layers.
In alternate embodiments, the present invention may include at least three locally resilient conductive layers and at least two locally resilient pressure conduction composites, each having a critical percolation threshold. Each composite is disposed between and contacting two locally resilient conductive layers in a multi-layered fashion.
In yet other embodiments, the present invention may include a locally resilient pressure conduction composite, a flexible substrate completely surrounding the composite so as to seal it therein, and a pair of electrical leads contacting the composite and terminating outside of the flexible substrate.
Composites are composed of a plurality of conductive particles electrically isolated within a non-conductive matrix. Conductive particles are loaded at a volume fraction approaching the critical percolation threshold of the material system so as to exhibit a conductance that greatly increases with pressure. Passive and active biasing may be applied via a variety of mechanical, electromechanical, and magnetic devices. Sensors may be arranged to form an array architecture including planar and conformal configurations.
The term “locally resilient” refers to the movement of composite and layers immediately adjacent to a mechanical load. For example, a compression event includes a volumetric reduction of the matrix and localized elastic deformation of the outer layers. A decompression event includes a volumetric expansion of the matrix and localized elastic recovery of the outer layers.
The flexible and pliable composites within the present invention are the functional equivalent of a highly pressure responsive resistor. Since composites may be tailored to a variety of pressure ranges, composites may be color coded to visually identify performance characteristics.
The described invention provides several advantages over the related arts. The invention is inherently more sensitive resulting in more accurate and detailed measurements. The noise component within the usable signal from the present invention is far lower than conventional sensors. The invention is extremely rugged, inexpensive, scalable in size and form, flexible and conformal, and amenable to conductive and non-conductive coatings to improve durability in harsh environments.
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:
a is a schematic diagram showing a pressure conduction composite, composed of a non-conductive matrix having conductive particles electrically isolated therein, sandwiched between a paired arrangement of conductive and optional non-conductive layers.
b is a schematic diagram showing sensor from
Referring now to
Conductive layers 5, 6 include a variety of materials, such as metals and composites, and a variety of generally planar structures, including plates, foils, films, foams, weaves, and braids. The pressure conduction composite 2 is composed of conductive particles 3 within a non-conductive yet pliable and resilient matrix 4. The matrix 4 is a generally planar solid that surrounds and isolates the conductive particles 3 so as to maximize resistance and minimize conductance thereby preventing current flow between conductive layers 5, 6 at an ambient or biased pressure. Non-conductive layers 51, 52 isolate the sensor 1 so as to prevent electrical current loss from the sensor 1 and to shield the sensor 1 from electrical current external to the device. Non-conductive layers 51, 52 include a variety of non-conducting materials, such as polymers and composites, and a variety of generally planar structures, including plates, foils, films, foams, weaves, and braids. It is preferred for conductive layers 5, 6 and non-conductive layers 51, 52 to be pliable.
Referring now to
The term “locally resilient” refers to the movement of the pressure conduction composite 2, one or more surrounding conductive layers 5, 6, and one or more optional non-conductive layers 51, 52 under and adjacent to a mechanical load. For example, a compression event includes a volumetric reduction of the matrix 4, spatial displacement of conductive particles 3, and localized elastic deformation of conductive layers 5, 6 and nonconductive layers 51, 52. A decompression event includes a volumetric expansion of the matrix 4 to its original or nearly original volume, spatial displacement of conductive particles 3 to their original or nearly original locations, and localized elastic recovery of conductive layers 5, 6 and nonconductive layers 51, 52. Compression and decompression may be assisted by a variety of mechanical, electromechanical, and magnetic devices.
Referring now to
Referring again to
The critical percolation threshold is the pressure at and above which the pressure conduction composite 2 exhibits a very large decrease in resistance, which may be as large as six orders of magnitude. Pressures near yet less than the critical percolation threshold ensure the conducting particles 3 to have a spatial separation sufficiently small so as to allow current flow between the conducting layers 5, 6. Pressure conduction composites 2 operating near the critical percolation threshold ensure a sufficiently distinctive change in conduction over a range of pressures so as to allow for the precise measurement of pressure and/or stress.
The matrix 4 may be composed of one or more electrically resistive, compressible and resilient materials including, but not limited to, polymers and elastomers. It is preferred for the matrix 4 to be temperature resistant. Exemplary materials include formulations of polyethylene, polystyrene, polyvinyldifluoride, polyimide, epoxy, polytetrafluorethylene, silicon rubber, polyvinylchloride, and combinations thereof. Preferred embodiments of the present invention were composed of the elastomer RTV R3145 manufactured by the Dow Corning Company.
Conductive particles 3 may include one or more electrically conductive materials including, but not limited to, metals, metal-based oxides, nitrides, carbides, and borides, and carbon black. It is preferred that conductive particles 3 resist deformation when compressed and have a melt temperature sufficiently above the thermal conditions generated during current flow and interrupt. Exemplary materials include aluminum, gold, silver, nickel, copper, platinum, tungsten, tantalum, iron, molybdenum, hafnium, combinations and alloys thereof, Sr(Fe,Mo)O3, (La,Ca)MnO3, Ba(Pb,Bi)O3, vanadium oxide, antimony doped tin oxide, iron oxide, titanium diboride, titanium carbide, titanium nitride, tungsten carbide, and zirconium diboride.
The pressure conduction composite 2 is fabricated via known methods. For example, the pressure conduction composite 2 may be prepared from high-purity feedstocks, mixed, pressed into a solid, and suffused with oil. Conductive layers 5, 6 are thereafter bonded to the pressure conduction composite 2 via an adhesive or vulcanization process. It was preferred to adhesively bond conductive layers 5, 6 to the pressure conduction composite 2 via an electrically conductive epoxy. Non-conductive layers 51, 52 are likewise bonded to the conductive layers via a thermally resistant adhesive, preferably a pliable epoxy.
Feedstocks include both powders and liquids. Conductive particles 3 were exclusively solid particulates. For example, it was preferred for the feedstock comprising the conductive particles 3 to be a fine, uniform powder, examples including 325-mesh titanium diboride and titanium carbide. The non-conductive matrix 4 was fabricated with either a fine, uniform powder or a liquid with sufficient viscosity to achieve adequate dispersion of conductive particles 3 after mixing. Powder-based formulations were mechanically mixed and compression molded using conventional methods. Polytetrafluorethylene and other polymers may require sintering within an oven to achieve a structurally durable solid. Powder-liquid formulations, examples including titanium diboride or titanium carbide and a silicone-based elastomer, were vulcanized and hardened within a die under low uniaxial loading at room temperature.
In some embodiments, it may be desired to impregnate the pressure conduction composite 2 with a liquid via a method referred to as suffusion. The pressure conduction composite 2 is submerged within a bath of one or more inorganic oils, preferable silicone based, thereby allowing complete infiltration of the liquid into the otherwise solid pressure conduction composite 2. The exposure time of the pressure conduction composite 2 is influenced by the dimensional properties and composition of the solid. For example, a pressure conduction composite 2 having a thickness of 0.125-inch, a width of 0.200-inch, and a length of 0.940-inch and composed of titanium diboride or titanium carbide at a volume fraction of 66 percent and RTV R3145 at a volume fraction of 34 percent was adequately suffused after a 48 hour period.
Conductive layers 5, 6 and non-conductive layers 51, 52 are adhered to the pressure conduction composite 2 either before or after suffusion. If before suffusion, conductive layers 5, 6 and non-conductive layers 51, 52 are placed within a die along with sufficiently mixed composition comprising the pressure conduction composite 2 in the desired sequential order. For example, a matrix 4 composed of a silicone elastomer was adequately bonded to two 0.020-inch thick brass plates and polymer non-conductive layer 51, 52 by curing the otherwise liquid elastomer at room temperature between 3 to 24 hours or at an elevated temperature between 60 to 120 degrees Celsius for 2 to 10 hours. If after suffusion, a silicone adhesive is applied between pressure conduction composite 2 and conductive layers 5, 6 and non-conductive layers 51, 52 and thereafter mechanically pressed until the adhesive is cured.
In some embodiments, it may be advantageous for the pressure conduction composite 2 to be porous. Porosity may be required to tailor the mechanical stiffness, elastic properties and cooling characteristics of the pressure conduction composite 2 without adversely degrading electrical conductance and resistance of the element. Furthermore, porosity may improve the compliance and sensitivity of the sensor 1.
Pores may include a variety of shapes including, but not limited to, spheres, ellipsoids, cylinders, and irregular shapes. Referring now to
Pores may be formed by a variety of manufacturing methods. For example, cavities may be mechanically drilled into the pressure conduction composition 13. Pores may be introduced during mixing of matrix 4 and conductive particles 3 feedstocks via the introduction of gas bubbles. It is likewise possible to include microspheres composed of either a low-density foam or a gas or fluid filled spheres during the mixing process. Also, cavities may be formed during curing of the matrix 4 in an oven whereby localized heating and phase transitions yield void formation and growth.
In yet other embodiments, it may be advantageous to apply a waterproof or heat resistant coating known within the art over sensors 1 described herein to prevent direct contact with the surrounding medium.
A more complex embodiment of the sensor 1 in
Referring now to
Inner conductive layers 18a–18c physically and electrically separate adjacent pressure conduction composites 19a–19d within the multi-layer sensor 16. A voltage is selectively applied to one or more outer conductive layers 17a, 17b and inner conductive layers 18a–18c so as to allow for current flow to one or more outer conductive layers 17a, 17b and/or one or more inner conductive layers 18a–18c. Current flow across one or more pressure conduction composites 19a–19d may be arranged to form a conduction logic circuit facilitating two or more sensitivity ranges for pressure and stress. For example, it may be desired to have two or more pressure conduction composites 19a–19d tuned to one or more separate pressure-conduction or stress-conduction ranges. The multi-layer sensor 16 in
Referring now to
It is preferred for the outer layers 21 and 22 to be composed of a flexible, thermally resistant, and non-conducting material, one example being a polyimide. Outer layers 21 and 22 completely cover and surround the pressure conduction composite 23 and electrical leads 24 and 25 so as to prevent their contact with the surrounding environment. Outer layers 21 and 22 are joined via an adhesive or thermally bonded so as to provide a continuous seam 28 about their mutual perimeters. Electrical contacts 26 and 27 are electrically connected to the electrical leads 24 and 25, respectively, and traverse the seam 28 between outer layers 21 and 22 without compromising the seal there between. Electrical contacts 26, 27 facilitate communication of sensor 20 data to acquisition equipment.
In preferred embodiments, the pressure conduction composite 23 was hermetically sealed between a pair of Kapton® thin films, sold by the DuPont Corporation, having copper traces and contact pads along one side thereon so as to form the electrical contacts 26 and 27. Contact pads mechanically and electrically contacted the pressure conduction composite 23 within the sensor 20. Traces and pads were pre-etched onto the Kapton thin film via known flex circuit techniques. Kapton outer layers 21 and 22 were adhered to the pressure conduction composite 23 via a conductive silver epoxy. A pair of pads was provided along the thin film opposite of the pads contacting the pressure conduction composite 23 so as to form the electrical contacts 26 and 27.
The sensor 1 in
The type, density, size, and mass fraction of the conductive particles 3 and matrix 4 within the pressure conduction composite 2 determine the functional relationship between pressure and voltage at node “A”. For example, it is possible for the pressure conduction composite 2 to communicate a voltage that is linearly proportional to the mechanically applied pressure within the sensor 29. Likewise, it is possible for the pressure conduction composite 2 to communicate a voltage that is non-linear in relation to the applied pressure. However, it was preferred for the change in conductance to be sufficiently large so as to be distinguishable from electrical noise within the circuit to minimize signal filtering and amplification.
Referring now to
While a variety of commercially available microcontrollers 35 are applicable to the present invention, the MPS430 sold by Texas Instruments, Inc. contains onboard ADCs and communications interface required for remote data uploading to a master controller. The MPS430 supports a variety of asynchronous serial communication protocols compatible with the present invention.
The wire interface 36 may be attached to the MPS430 controller as a flexible Serial Communications Interface (SCI). The SCI allows for both synchronous and asynchronous communication protocols, including RS-232, RS422, RS485, SPI and I2C.
The wireless interface 37 may include, by way of example, a Wi-Port module sold by Lantronix, Inc. capable of communicating data in an asynchronous format over an 802.11b Ethernet network. The Wi-Port also contains internal firmware allowing connection to a variety of TCP/IP protocol stacks including ARP, UDP, TCP, ICMP, Telnet, TFTP, AutoIP, DHCP, HTTP, and SNMP with or without 128 bit WEP encryption for security purposes.
In some applications, it may be advantageous to limit external mechanical loads to one surface along the sensor 38. Referring now to
Referring now to
The low profile and compactness of the present invention lend itself to arrayed configurations. Referring now to
The rigid element 50 may include a keyboard housing or a floor. In the former application, sensors 49 are applied to the housing so as to provide a plurality of touch sensitive keys or interact with conventional keys. In the latter application, sensors 49 are applied to a floor to form an intrusion detection system. Sensors 49 may be covered by carpet or have an exterior finish representing a specific floor type and style. An intruder activates individual sensors 49 within the floor via the progression of footsteps. Individual signals from the sensors 49 are thereafter communicated via wire or wireless means to a central computer so as to provide location, path and speed data to security personnel.
The description above indicates that a great degree of flexibility is offered in terms of the present invention. Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.
This application is based upon and claims priority under 35 U.S.C. § 119(e) from U.S. Provisional Patent Application No. 60/512,335 filed Oct. 17, 2003, entitled Pressure Conduction Devices, the contents of which are hereby incorporated by reference.
Number | Name | Date | Kind |
---|---|---|---|
3806471 | Mitchell | Apr 1974 | A |
6323751 | Duggal et al. | Nov 2001 | B1 |
6411191 | Shea et al. | Jun 2002 | B1 |
6646540 | Lussey | Nov 2003 | B1 |
Number | Date | Country |
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WO 200079546 | Dec 2000 | WO |
Number | Date | Country | |
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20050081640 A1 | Apr 2005 | US |
Number | Date | Country | |
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60512335 | Oct 2003 | US |