PRESSURE TRANSDUCER WITH CONDUCTIVE POLYMER BRIDGE

Abstract: A pressure transducer has a thin central portion formed of a molded thermoplastic resin. A conductive polymer forms the resistive elements of a Wheatstone bridge printed and baked onto the molded substrate.

Description: This invention relates to a pressure sensor of the type that is used in a blood pressure transducer. The blood pressure transducer of the type disclosed in copending application Serial No. 009,643, filed January 27, 1987, employs a silicon sensor. The silicon sensor is very small, being a fraction of a square inch in area. It is circular in cross section and has a Wheatstone bridge strain gauge formed of piezo-resistive elements diffused into the silicon substrate. The silicon substrate is chemically etched away on the surface opposite the Wheatstone bridge so as to form a thin silicon diaphragm that flexes when subjected to pressure causing the resistors in the Wheatstone bridge strain gauge to be stressed to produce an indication of the applied pressure. The manufacturing process for the silicon sensor is necessarily expensive. Further, the sensor is temperature-sensitive and therefore requires a temperature compensation circuit. Strain gauges have been formed in other ways. It is known to form a strain gauge by depositing a thick film resistor on alumina. The resistor must be baked at a high temperature of the order of 800 °F (425 °C). Hence, a ceramic such as alumina is required as the substrate. It is also known to form strain gauges from conductive polymers, again using alumina as a substrate. These thick film resistor and conductive polymer strain gauges have all been massive compared to the tiny silicon sensor. An objective of the present invention has been to provide a less expensive sensor to replace the silicon sensor. European Patent Application 0196696 describes a pressure sensor comprising a deformable diaphragm Supported by a non-deformable guard ring integrally formed therewith. A strain gauge is adhered to the diaphragm on the side opposite that which is exposed to the pressure to be sensed. The paper in Microelectronics Journal, Vol. 17, No. 1, Jan./Feb., 1986 by A. Di Flore et al describes the printing of carbon polymer resistors onto a flexible plastic membrane supported by an alumina substrate. Diaphragm movement is sensed by detecting resistance changes due to the piezo resistance effect. A pressure transducer in accordance with the invention comprises a moulded thermoplastic substrate having a thin flexible section and at least one resistor on the flexible section, the resistor changing its resistance when the substrate is flexed characterised in that the substrate is formed from a high temperature thermoplastic and in that the resistor is a conductive polymer resistor which is printed on the flexible section. In a preferred embodiment the transducer is formed by molding a diaphragm from a high temperature thermoplastic. The diaphragm is molded in a cup shape having thick edges and a thin, flexible central portion onto which the strain gauge is to be formed. With the molding of the plastic, the expensive diffusion process and etching steps required with a silicon sensor are eliminated. A Wheatstone bridge of conductive polymers is screen printed onto the surface of the thin central diaphragm of the molded element. The conductive polymers can be trimmed to a desired resistance value using a Nd-YAG laser or abrasive-type trimmers. The conductive polymers are baked onto the substrate at over 400 °F (200 °C). Metal conductors are printed on the surface of the substrate at the ends of the resistor elements. These metal conductors are connected by a conductive epoxy to lead wires by which the sensor is connected into its monitoring circuit. Among the advantages of this sensor are that it is much less expensive than the silicon sensor, being a small fraction of the cost of the silicon sensor. The sensor is substantially temperature-insensitive and hence requires no temperature compensation network as is required with the silicon sensor. The several features and advantages of the present invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which: Fig. 1 is a plan view of a sensor of the present invention; Fig. 2 is a cross-sectional view taken along lines 2-2 of Fig. 1; and Fig. 3 is a cross-sectional view similar to that of Fig. 2 of an alternative embodiment. Referring to the drawings, the sensor is indicated at 10 and has a substrate 11 formed of a thermoplastic resin. The preferred material for the substrate is Ryton ® R4 (40% glass, polyphenylene sulfide) manufactured by Phillips Chemical Co. Division of Phillips Petroleum. Other materials are polyethersulfone (VITREX PES by Imperial Chemical Industries), polyetheretherketone (VITREX PEEK) by Imperial Chemical Industries, polyetherimide (ULTEM ® 1000 by General Electric) and liquid crystal polymer (VECTRA ® A150 by Celanese Advanced Technology Company). The substrate has a thick perimeter 12 having a series of stress-relieving cavities 13 spaced around its circumference. The perimeter is preferably about 0.150" (3.8 mm) thick. The central portion consists of a thin diaphragm 15 that has a thickness in the range of 5 to 15 mils (0.13 to 0.38 mm). The thickness of the diaphragm is governed by two major considerations. The thicker the diaphragm, the greater is the linearity of the output. The thinner the diaphragm, the greater is its sensitivity. Therefore, a thickness of about 10 mils (0.25 mm) producing a sensitivity of 15 µv/v/mmHg (2 mv/v/Nm⁻²) is preferred. The diaphragm is preferably about 0.500" (13 mm) in diameter. The diameter of the entire substrate is about 0.800" (20 mm). Referring particularly to Fig. 1, conductive polymer resistors 20, 21, 22 and 23 are screen-printed on the diaphragm. Printed conductor material such as paladium silver is applied as at 30 to the ends of each of the resistive elements 21-23. Leads 31 are connected to the printed conductor material by a conductive epoxy 32. In the alternative embodiment of Fig. 3, the element differs only from the element of the previous embodiment in the thick center portion 40. The center portion is preferably a thickness in the range of 0.080" to 0.090" (2.0 to 2.3 mm) and is ¼" (6.4 mm) in diameter. Sensors having rigid centres are known. The rigid centres are used for increasing the effective area and for generating high stresses in the diaphragm at lower deflection levels, thus improving the overall performance of the diaphragm.

Claims: A pressure transducer comprising a moulded thermoplastic substrate having a thin flexible section and at least one resistor on the flexible section, the resistor changing its resistance when the substrate is flexed characterised in that the substrate (11) is formed from a high temperature thermoplastic and in that the resistor (20, 21, 22, 23) is a conductive polymer resistor which is printed on the flexible section (15). A pressure transducer as claimed in Claim 1 wherein the substrate (11) includes a thick border (12) which surrounds the thin, flexible section (15). A pressure transducer as claimed in either Claim 1 or 2 wherein a plurality of conductive polymer resistors (20, 21, 22, 23) is printed and baked on the flexible section (15) forming a resistance bridge and wherein conductors (30) are connected to the ends of the resistors (20, 21, 22, 23). A transducer as claimed in Claim 3 in which the conductors (30) are printed on the substrate (11) and leads (31) are connected to the conductors (30) with a conductive epoxy (32). A transducer as claimed in any one of Claims 2 to 4 in which the thick border (12) is circular. A transducer as claimed in any one of Claims 2 to 5 in which the centre of the thin section (15) has a thick integrally-moulded boss (40) forming a rigid centre. A transducer as claimed in any of Claims 2 to 6 in which the thin section (15) has a thickness in the range of about 5 to 15 mils (0.13 to 0.38 mm).

Inventors: FRANK, THOMAS P.
Applicants: MEDEX, INC.
Application year: 1988
Main classification: G01L9
IPC codes: G01L9, G01L1