WO1999023463A1

WO1999023463A1 - Noninvasive pressure sensing assembly

Info

Publication number: WO1999023463A1
Application number: PCT/US1998/019730
Authority: WO
Inventors: Michael D. Lockwood; James Y. Chon; Michael D. Morgan
Original assignee: Alcon Laboratories, Inc.
Priority date: 1997-11-03
Filing date: 1998-09-22
Publication date: 1999-05-14
Also published as: AU9232198A

Abstract

The present invention improves upon prior art pressure sensors by providing a noninvasive pressure sensing assembly capable of accurately indicating relatively minute pressure changes. The present invention generally includes separating a pressure chamber from a pressure transducer by a thin, compliant membrane. The unique shape of the membrane assures that minute movements in the membrane are transferred to the measuring pressure transducer. Any change in pressure within the chamber will vary the amount of force exerted by the membrane against the transducer which, when any preload is deducted, will indicate the pressure within the chamber.

Description

NONINVASIVE PRESSURE SENSING ASSEMBLY

Background of the Invention

The present invention relates generally to pressure sensors and more specifically to noninvasive pressure sensing.

5 Various devices have been developed over the years for measuring or sensing the pressure in a volume of fluid. Many of these devices have a load cell containing probe or other sensing apparatus that must physically contact the fluid being measured. While in many mechanical applications (for example, an oil pressure sensor used on an internal combustion engine), physical contact between the probe and the fluid raises no particular 0 concerns, such contact is undesirable in medical applications where the fluid may be a virally or microbially contaminated biological fluid. Under these circumstance, if the probe is allowed to contact the biological fluid, the probe must either be discarded or sterilized prior to reuse. Therefore, in medical applications, it is important that the pressure sensor not contact the fluid being measured. s Several noninvasive pressure sensors have previously been disclosed in U.S. Patents

Nos. 1,718,494, 2,260,837, 2,510,073, 2,583,941 and 3,805,617, the entire contents of which are incorporated herein by reference. These devices use a metal disk moving within the electromagnetic field of an energized coil to sense pressure changes. As the iron disk moves closer or farther from the coil, the current flow through the coil varies, and these o current fluctuations can be used to calculate pressure changes. While these devices are satisfactory for measuring relatively large pressure changes, more minute pressure changes do not cause the current to fluctuate to a sufficient degree to provide an accurate and reliable indicator of pressure variation.

Other pressure sensors avoid contacting the fluid being tested by using a test 5 chamber separated into two parts by a flexible diaphragm. The fluid volume being measured is contained on one side of the chamber and the pressure sensor is in communication with the second side of the chamber. Any increase or decrease in the fluid pressure causes the diaphragm either to expand into the second side of the chamber or to be pulled into the fluid part of the chamber, thereby increasing or decreasing the pressure o in the second side of the chamber an amount corresponding to the change in fluid pressure in the first side of the chamber. While these diaphragm type pressure sensors do not invade the test fluid and can be used to detect relatively small pressure changes, the accuracy of such sensors relies to a great extent on the compliance or elastic properties of the diaphragm, properties that can be hard to control during manufacture and that may change over time as the diaphragm is repeatedly stretched and relaxed.

5 Another noninvasive pressure sensor described in PCT Patent Application No. WO

93/24817 (corresponding to U.S. Patent No. 5,392,653) uses a flexible diaphragm with an attached magnet. By attaching an iron disk to the diaphragm, the diaphragm is mechanically coupled to the transducer. In order for the transducer to measure the pressure accurately, the diaphragm is extremely flexible. Nevertheless, variation in the o flexibility of the diaphragm affect the accuracy of the pressure measurements. In addition, this assembly relies on firm contact between the magnet and the transducer, variations of which will also affect the accuracy of the pressure measurement.

Accordingly, a need continues to exist for an inexpensive, reliable and accurate pressure sensor capable of detecting relatively small pressure changes in a fluid without 5 contacting the fluid.

Brief Summary of the Invention

The present invention improves upon prior art pressure sensors by providing a noninvasive pressure sensing assembly capable of accurately indicating relatively minute pressure changes. The present invention generally includes separating a pressure chamber o from a pressure transducer by a thin, compliant membrane. The unique shape of the membrane assures that minute movements in the membrane are transferred to the measuring pressure transducer. Any change in pressure within the chamber will vary the amount of force exerted by the membrane against the transducer which, when any preload is deducted, will indicate the pressure within the chamber. 5 Accordingly, one objective of the present invention is to provide a noninvasive pressure sensing assembly.

Another objective of the present invention is to provide a relatively inexpensive pressure sensing assembly.

Yet another objective of the present invention is to provide a pressure sensing o assembly wherein the pressure sensor is preloaded by a flexible membrane. Still another objective of the present invention is to provide a pressure sensing assembly that can measure pressures less than ambient pressure.

Still another objective of the present invention is to provide a pressure sensing assembly that will measure pressures different than ambient pressure. These and other advantages and objectives of the present invention will become apparent from the detailed description, drawings and claims that follow.

Brief Description of the Drawings

FIG. 1 is an exploded, perspective view of the pressure sensing assembly of the present invention. FIG. 2 is an exploded cross-sectional view of the pressure sensing assembly of the present invention taken along line 2-2 in FIG. 1.

FIG. 3 is a perspective view of the assembled pressure sensing assembly of the present invention.

FIG. 4 is a cross-sectional view of the pressure sensing assembly of the present invention taken along line 4-4 in FIG. 3.

FIG. 5 is a perspective view of the assembled pressure sensing assembly of the present invention similar to FIG. 3, but showing a pressure transducer in contact with the assembly.

Detailed Description of the Invention

As can be seen in FIGS. 1 - 4, pressure sensing assembly 10 generally includes housing 12, diaphragm 14, retaining ring 16. Housing 12 defines pressure chamber 20 containing a fluid having a pressure to be measured. Housing 12 may be made of any suitable material, such as metal, glass or plastic, may be of any suitable size or shape, has an opening 24 and may contain pressure port 22, through which the pressure in chamber

20 may be varied. Opening 24 of housing 12 is sealed fluid tight by diaphragm 14. Diaphragm 14 may be made of any suitably compliant material having good dimensional stability, but 17-7 PH condition "C" stainless steel having a thickness of between 0.001 and 0.005 inches is preferred, with 0.003 inches being most preferred. Flat portion 26 of diaphragm 14 preferably has a diameter of between 0.50 inches and 1.50 inches with 0.97 inches being most preferred. Preferably, the ratio of the thickness to the diameter of diaphragm 14 is between 0.01 to 0.001, with 0.003 being most preferred.

As best seen in FIG. 2, diaphragm 14 is formed in the shape of a "U", having sidewall 28 bent transversely from flat portion 26. Flat portion 26 may be smooth or may contain corrugations (not shown). Sidewall 28 preferably are between 0.020 inches and 0.250 inches deep, with 0.180 being most preferred. The ratio of diaphragm 14 thickness to sidewall 28 depth is preferably between 0.250 to 0.004, with 0.017 being most preferred. The use of sidewall 28 isolates the stresses induced by deflections in flat 0 portion 26 as is discussed below.

Diaphragm 14 rests within recess 30 in housing 12 and is retained by ring 16, as best seen in FIGS. 2 and 4. Ring 16 may be attached to housing 12 by any suitable method, for example, ultrasonic welding, and ridge 34 may be used for this purpose. Alternatively, ridge 34 may be sized and shaped so that ridge 34 retains diaphragm 14 on s housing 12 by suitable heat staking or ultrasonic welding methods. Diaphragm 14 may also be attached to housing 12 by other suitable methods, such as an adhesive, that does not require the use of ring 16.

In use, assembly 10 is assembled as shown in FIGS. 3-5 and made to contact transducer 18 so that a preload is applied against transducer 18 and flat portion 26 of 0 diaphragm 14 is deflected slightly inward. Transducer 18 may be any suitable load cell having small full scale deflection (e.g. 0.0001 inches to 0.01 inches), stable output under constant compression load, small linearity and repeatability errors and temperature stable, for example, a Sensotec Model Number 31 load cell having a five pound working range. The pressure within chamber 20 may be varied through port 22. Variations in pressure 5 within chamber 20 will caused flat portion 26 of diaphragm 14 to deflect inwardly or outwardly, and these minute movements will be detected by transducer 18 as a variation on the load applied to transducer 18. The pressure within chamber 20 can be calculated by deducting the static preload on transducer 18 from the transient loads applied to transducer 18. o As diaphragm 14 is preloaded by transducer 18, bending of diaphragm 14 occurs which causes flat portion 26 to bend inward. This bending tends to decrease the diameter of diaphragm 14. If diaphragm 14 were flat and did not contain sidewall 28, as in the prior art, this decrease in diameter will generate very large shear forces along the seal between the diaphragm and the pressure chamber (see for example, seal 260 in FIG. 6 of U.S. Patent No. 5,392,653). This shear force will not only weaken the integrity of the seal, but can cause movement of the diaphragm, thereby affecting the accuracy of the assembly. In contrast, the preloading as well as in and out movement of flat portion 26 in response to pressure changes within chamber 20 results in movement of sidewall 28 that is transverse to the movement of flat portion 26, so that no force is transmitted to the weld or seal at edge 32. As a result, assembly 10 is more accurate and less likely to fail.

This description is given for purposes of illustration and explanation. It will be apparent to those skilled in the relevant art that modifications may be made to the invention as herein described without departing from its scope or spirit.

Claims

We claim:

i 1. A noninvasive pressure sensing assembly, comprising:

2 a. a housing having an opening; and

3 b. a generally "U"-shaped diaphragm sealing the opening, the diaphragm

4 having a flat portion and a sidewall.

1 2. The assembly of claim 1 wherein the flat portion has a thickness and a

2 diameter and the ratio of the thickness to the diameter of the flat portion is between 0.01

₃ to 0.001.

i 3. The assembly of claim 1 wherein the flat portion has a diameter of between

2 0.50 inches and 1.50 inches.

i 4. The assembly of claim 1 wherein the sidewall has a depth of between 0.020

2 inches and 0.250 inches.

1 5. A noninvasive pressure sensing assembly, comprising:

2 a. a housing having an opening;

3 b. a generally "U"-shaped diaphragm sealing the opening, the diaphragm having a flat portion and a sidewall; and

5 c. a retaining ring retaining the diaphragm within the housing.

1 6. The assembly of claim 5 wherein the flat portion has a thickness and a

3 to 0.001.

i 7. The assembly of claim 5 wherein the flat portion has a diameter of between

2 0.50 inches and 1.50 inches.

i 8. The assembly of claim 5 wherein the sidewall has a depth of between 0.020

2 inches and 0.250 inches.