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Claims  |
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The embodiments of the invention in which an exclusive property or
priviledge is claimed are defined as follows:
1. Apparatus for measuring fluid pressure in a fluid line, comprising:
(a) a pressure transducer comprising a semiconductor circuit on which are
disposed a plurality of electrical contacts, and a pressure sensitive
surface, said pressure sensitive surface experiencing a strain as a
pressure is applied thereto, the strain causing the pressure transducer to
produce an output signal, which varies as a function of the pressure,
through the plurality of electrical contacts disposed on the semiconductor
circuit;
(b) a housing defining a cavity in which the pressure transducer is
disposed and including a connector adapted to attach to the fluid line,
the housing further including a fluid passage extending through the
housing, proximate to the pressure sensitive surface of the pressure
transducer and providing communication of fluid pressure between the
connector and said pressure sensitive surface;
(c) a plurality of insulated electrical conductors through which electrical
signals are conveyed to and from the pressure transducer; and
(d) an integrally formed chip carrier mounted in the cavity, said pressure
transducer being attached to the chip carrier, said chip carrier
including:
(i) a fluid interface between the fluid passage and the pressure
transducer, said interface including means for transmitting fluid pressure
between the fluid passage and the pressure sensitive surface;
(ii) a quick-connect electrical interface between the plurality of
electrical conductors and the plurality of electrical contacts on the
semiconductor circuit of the pressure transducer; and,
(iii) means for sealing the pressure transducer within the cavity so that
one side of the pressure sensitive surface is exposed to the means for
transmitting fluid pressure and so that the opposite side thereof is
isolated from the fluid passage and the fluid pressure therein.
2. The apparatus of claim 1, wherein the quick-connect electrical interface
of the chip carrier includes a plurality of leads connected to the
electrical contacts on the semiconductor circuit of the pressure
transducer, and wherein the leads are formed in a "U" shape and engage the
electrical conductors by spring force.
3. The apparatus of claim 2, wherein a bond area at an end of each lead is
disposed on the carrier proximate the pressure transducer and is connected
to the electrical contacts on the semiconductor circuit with fine wires.
4. The apparatus of claim 1, wherein the chip carrier comprises a generally
planar surface, having a recess formed therein in which the pressure
transducer is mounted, and an aperture formed in the recess over which the
pressure sensitive surface of the pressure transducer is disposed.
5. The apparatus of claim 4, wherein the recess includes a shelf on which
is disposed a plurality of lead bond areas, said bond areas being
connected to a plurality of leads that extend from the chip carrier and
outwardly of an end thereof.
6. The apparatus of claim 4, wherein a nib projects from a surface of the
chip carrier opposite that in which the recess is formed, said nib being
disposed around the aperture and extending into engagement with a
receptacle formed in the housing, in fluid communication with the fluid
passage.
7. The apparatus of claim 6, wherein said means for transmitting fluid
pressure comprises a substantially incompressible gel, and wherein the nib
is filled with the gel.
8. The apparatus of claim 1, wherein the housing comprises a first section
on which the electrical conductors are terminated and a second section on
which the connector is disposed, said first and second sections being
sealingly joined after the chip carrier and pressure transducer are
mounted in the cavity.
9. The apparatus of claim 1, wherein the pressure transducer comprises a
piezoresistive type transducer, including a plurality of resistors
connected in a Wheatstone bridge configuration.
10. The apparatus of claim 9, wherein the semiconductor circuit further
comprises means for temperature compensating the resistance of the
resistors.
11. The apparatus of claim 1, wherein the insulated electrical conductors
include a resistor, which is connected in parallel with the output signal
from the pressure transducer to reduce the apparent impedance of the
pressure transducer relative to a device connected to monitor its output
signal.
12. In a catheter system, a pressure sensing apparatus adapted to attach to
a catheter and operative to produce an electrical signal indicative of the
magnitude of fluid pressure in the catheter, said apparatus comprising:
(a) pressure sensing means, including a first and a second surface, for
sensing a differential fluid pressure applied between the first surface
and the second surface thereof and producing an electrical signal
indicative of the magnitude of the fluid pressure, said second surface
thereof including contact means for connecting the electrical signal to a
plurality of discrete conductors;
(b) housing means for enclosing the pressure sensing means, including:
(i) connector means for providing a sealed connection to a line in fluid
communication with the catheter system,
(ii) passage means for conveying a fluid from the connector means to a
point within the housing means proximate the first surface of the pressure
sensing means, and
(iii) an electrical cable for conveying the electrical signal from the
pressure sensing apparatus; and
(c) carrier means, disposed within the housing means, for:
(i) integrally defining a sealed aperture disposed between the first
surface of the pressure sensing means and the passage means, including
pressure transmitting means disposed in the aperture for transmitting
fluid pressure from the fluid in the passage means to the first surface of
the pressure sensing means,
(ii) mounting the pressure sensing means within the housing means in sealed
relationship with the fluid passage means, and
(iii) conducting the electrical signal from the plurality of discrete
conductors to the electrical cable through a plurality of spring biased
leads, which are disposed on the carrier means.
13. The apparatus of claim 12, wherein the electrical cable comprises a
plurality of insulated wires, each wire being stripped of insulation at an
end portion that is terminated in the housing means, said leads in the
carrier means being formed in a "U" shaped loop that is spring biased to
engage the end portions of the wires, forming a conductive contact
therewith.
14. The apparatus of claim 12, wherein the housing means comprise a first
and a second section, opposed mating surfaces of the first and the second
section defining therebetween a cavity in which the carrier means and
pressure sensing means are disposed.
15. The apparatus of claim 14, wherein the first section of the housing
means is attached to the electrical cable and the second section is
attached to the connector means.
16. The apparatus of claim 14, wherein the pressure sensing means are
mounted on the carrier means and the plurality of discrete conductors are
attached between the spring biased leads and the contact means forming an
assembly, and wherein said assembly is sealed into the cavity by mating
the first section of the housing means to the second section.
17. The apparatus of claim 14, wherein the carrier means include a nib
disposed around said aperture proximate the first surface of the pressure
sensing means and projecting into the passage means, said pressure
transmitting means being disposed within said nib.
18. The apparatus of claim 17, wherein the pressure transmitting means
comprise a substantially incompressible gel, which seals the aperture,
preventing fluid communication between the passage means and the first
surface of the pressure sensing means.
19. The apparatus of claim 17, wherein the housing means include an
adhesive channel, said first and second sections of the housing means
being joined with an adhesive, said adhesive flowing through the adhesive
channel to join and the seal the nib to the housing means.
20. The apparatus of claim 12, wherein the pressure sensing means comprise
a thin film semiconductor circuit disposed on the periphery of the second
surface and a pressure sensitive diaphragm disposed on the center of the
first and the second surfaces.
21. A method for assembling a pressure sensing device useful for measuring
pressure in a catheter system, said pressure sensing device including a
pressure transducer having a pressure sensing surface and a plurality of
electrical contacts, a chip carrier, an electrical cable including a
plurality of conductors, and a housing, comprising the steps of:
(a) attaching the pressure transducer to the chip carrier;
(b) bonding wires to the electrical contacts and to a plurality of lead
areas disposed on the chip carrier, providing an electrically conductive
path therebetween;
(c) forming a plurality of leads that are attached to the chip carrier,
generally into a "U" shape;
(d) filling an aperture disposed on the clip carrier adjacent the pressure
sensing surface of the pressure transducer with a substantially
incompressible gel;
(e) mounting the chip carrier with the pressure transducer attached in a
recess formed in a first portion of the housing, with the aperture in
fluid communication with a catheter connector formed on the housing, said
catheter connector adapting the housing for connection to the catheter
system;
(f) positioning the cable in an opening formed within the housing so that
the formed leads engage the plurality of conductors in the electrical
cable; and
(g) securing a second portion of the housing to the first portion, so that
the pressure transducer and chip carrier are sealed within the housing.
22. The method of claim 21, wherein the aperture is defined by a projecting
nib disposed on the chip carrier, the step of mounting the chip carrier in
a first portion of the housing including the step of seating the
projecting nib into a mating receptacle formed on the interior of the
first portion of the housing.
23. The method of claim 22, wherein the projecting nib is filled with the
incompressible gel.
24. The method of claim 21, wherein the second portion of the housing is
secured to the first portion thereof with an adhesive.
25. Apparatus for mounting a pressure transducer comprising:
(a) a baseplate having a recessed portion on which a generally planar
surface is formed, said recessed portion including an aperture that
extends through the baseplate;
(b) a plurality of electrically conductive leads that extend outwardly from
an end of the baseplate, into the recessed portion; and
(c) a lead bond area disposed within the recessed portion, on which is
terminated an end of each of the leads that are adapted to be electrically
connected to the pressure transducer.
26. The apparatus of claim 25, wherein the baseplate is adapted to mount
the pressure transducer in an automated process by its attachment to a
thin flexible strip in which a plurality of similar baseplates are
attached and carried.
27. The apparatus of claim 26, wherein the baseplate is attached to the
thin flexible strip during its manufacture.
28. The apparatus of claim 26, wherein the plurality of electrically
conductive leads are formed from the thin, flexible strip during
manufacture of the apparatus.
29. The apparatus of claim 25, wherein the aperture extends through a nib
that projects outwardly of the baseplate, and which is adapted to be
filled with a substantially incompressible gel.
30. The apparatus of claim 25, wherein the lead bond area is disposed on a
shelf formed in the recessed portion of the baseplate, and the leads
extend through the baseplate, the ends of the leads being plated with a
precious metal of relatively higher electrical conductivity than a metal
from which the leads are formed.
31. The apparatus of claim 25, wherein the baseplate is generally
rectangular in shape and comprises a top portion and a bottom portion
disposed on opposite surfaces of the thin flexible strip. |
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Claims |
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Description |
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TECHNICAL FIELD
The present invention generally pertains to a fluid pressure sensor and,
more specifically, to a blood pressure transducer.
BACKGROUND OF THE INVENTION
Knowledge of a patient's blood pressure is often essential to properly
assess the patient's medical condition. Continuous monitoring of the blood
pressure enables medical personnel to immediately detect changes in the
cardiovascular system indicating stress, and to respond rapidly with the
appropriate action. Depending on whether a doctor wishes to monitor
arterial or venous pressure, a needle may be inserted into an artery or
vein, and a very small diameter tube or catheter run through the needle so
that its open end is exposed to the fluid pressure of blood at a desired
location in the body. If it is necessary to measure blood pressure in an
organ, e.g., inside one of the chambers of the heart, the catheter is
simply moved through the blood vessel until it is positioned at the
desired location. A sterile solution fills the catheter. The pressure of
the cardiovascular system at that point in the body is transmitted through
the sterile solution to a fluid pressure sensing device connected to the
catheter outside the patient's body. The pressure sensing device typically
includes a pressure transducer that produces an electrical signal
proportional to the fluid pressure of blood at the open end of the
catheter. An analog or digital display, or a cathode ray tube connected to
the transducer provides an indication of the patient's blood pressure in
response to the electrical signal.
In the past, the pressure sensors used to measure and/or monitor blood
pressure have been too expensive to routinely discard after a single use.
Instead, the pressure sensors have normally been sterilized for reuse on
several patients. The time and labor expense involved in preparing
conventional blood pressure sensors for reuse is significant. An increased
emphasis on stemming the rising cost of medical treatment has created a
demand for low cost disposable pressure sensors.
One such disposable blood pressure sensor is described in U.S. Pat. No.
4,576,181. This pressure sensor includes a shell-like housing in which a
pressure transducer is directly mounted, covered by an opaque plate that
blocks light from reaching a semiconductor surface of the transducer. The
opposite surface of the pressure transducer is mounted over a passage in
the housing, providing communication with a fluid-filled catheter. A
portion of the passage is filled with a gel which serves to hydraulically
couple the fluid pressure to the pressure transducer. Mounted within the
housing next to the pressure transducer is a dielectric substrate,
including a thick film hybrid temperature compensation circuit with pads
on which leads entering the housing through an insulated cable are
soldered.
Although the disposable blood pressure sensor described above is
substantially less expensive than those reusable sensors previously in
use, its material and manufacturing labor costs are significantly more
than might be desired. Components such as the thick film hybrid circuit
provided on the dielectric substrate mounted adjacent the transducer
substantially increase its cost. In addition, the labor required to hand
assemble the pressure transducer chip within the housing, and to attach
the chip to the hybrid circuit and the hybrid circuit to the cable is
significant. The hydraulic pressure coupling gel must be injected through
one end of the catheter connector into the passage communicating fluid
pressure to the transducer. It is difficult to constrain the gel to the
desired passage, because it is injected in the form of a low viscosity
liquid having a tendency to run.
In consideration of these problems, the present invention seeks to provide
an even lower cost disposable blood pressure transducer than that of the
above-described prior art. A further object of this invention is to
provide a pressure transducer that may be easily assembled with minimal
operations requiring hand labor. Yet a further object is to provide an
assembly including a premounted pressure transducer that may be readily
interfaced to a fluid pathway and electrical leads. These and other
objects and advantages of the present invention will be apparent from the
attached drawings and the description of the preferred embodiments that
follow.
SUMMARY OF THE INVENTION
In accordance with the present invention, an apparatus for measuring
pressure in a fluid line includes a solid-state pressure transducer
comprising a semiconductor circuit and a pressure sensitive surface.
Strain resulting from pressure applied to the pressure sensitive surface
while an electrical signal is applied to the pressure transducer through a
plurality of contacts disposed on the semiconductor circuit causes the
pressure transducer to produce an output signal which varies as a function
of the applied pressure.
The apparatus further includes a housing defining a cavity in which the
pressure transducer is disposed, and comprising a connector adapted to
attach to the fluid line. A fluid passage extends through the housing to a
point proximate the pressure sensitive surface of the pressure transducer.
Disposed within the cavity is a chip carrier on which is mounted the
pressure transducer. The chip carrier includes a fluid interface between
the fluid passage and the pressure transducer, and provides means for
transmitting fluid pressure between the fluid passage and the pressure
sensitive surface.
A plurality of insulated electrical conductors are provided to convey
electrical signals to and from the pressure transducer. Associated with
the chip carrier is a quick-connect electrical interface between the
plurality of electrical conductors and the plurality of connections on the
semiconductor circuit of the pressure transducer. The chip carrier further
includes means for exposing the pressure sensitive surface to the means
for transmitting fluid pressure and isolating an opposite side of that
surface from the fluid passage and fluid pressure therein.
Other aspects of this invention are related to a method for assembling a
pressure sensing device as defined above, and apparatus for mounting a
pressure transducer (i.e., the chip carrier).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view showing a first embodiment of the blood
pressure transducer;
FIG. 2 is an exploded view of the first embodiment, wherein only a portion
of the electrical lead terminated in the blood pressure transducer is
shown;
FIG. 3 is a cut-away view showing the first embodiment of the blood
pressure transducer in a side elevational view, (wherein the lock nuts on
the connectors have been removed for clarity);
FIG. 4 is a plan view of the lower portion of the housing used in the first
embodiment of the blood pressure transducer;
FIG. 5 is a cross-sectional view of the lower portion of the housing taken
along section lines 5--5 of FIG. 4;
FIG. 6 is a cross-sectional view of the lower half of the housing, taken
along section lines 6--6 of FIG. 4;
FIG. 7 is a plan view showing the interior of the upper portion of the
blood pressure transducer housing;
FIG. 8 is a cross-sectional view of the upper portion of the housing, taken
along section lines 8--8 of FIG. 7;
FIG. 9 is a plan view of a pressure transducer chip carrier;
FIG. 10 is a cross-sectional view of the chip carrier taken along section
lines 10--10 of FIG. 9;
FIG. 11 is an isometric view showing sequential assembly of the chip
carrier on a metal strip;
FIG. 12 is a schematic representation of a solid-state pressure transducer
chip used in the present invention;
FIG. 13 shows the solid-state pressure transducer in a cross-sectional view
taken along section lines 13--13 of FIG. 12;
FIG. 14 is an isometric view showing a second embodiment of the blood
pressure transducer; and,
FIG. 15 is an electrical schematic diagram showing a resistor switching
circuit provided in connectors used with the lead of the blood pressure
transducer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A first embodiment of the blood pressure transducer is shown in FIG. 1,
generally denoted by reference numeral 20. Blood pressure transducer 20 is
intended for use with a catheter, one end of which is inserted into a vein
or artery of patient during a medical procedure. The other end of the
catheter may be connected to a supply of saline solution or other medical
fluid, requiring that the pressure sensor be inserted in the catheter
line, i.e., in series connection with the medical fluid supply. To
facilitate such use, blood pressure transducer 20 includes a flow-through
passage 22, which extends between two catheter connectors 24, disposed at
opposite ends of a lower housing portion 30. Internally threaded nuts 28
are provided (seated in grooves 60--see FIG. 3) on each connector to
engage the catheter line. The center of flow-through passage 22 is defined
by an integral longitudinally extending tubular section 26 on the bottom
of lower housing 30. An upper housing section 32 is attached to lower
housing section 30 and is connected to a flat multiconductor lead 40. Both
upper and lower housing sections are molded of plastic, the upper housing
section 32 being opaque to block light transmission.
Multiconductor lead 40 includes four conductors, which carry electrical
signals to and from a solid-state pressure transducer chip (not shown in
FIG. 1) enclosed within housing sections 30 and 32. The other end of
multiconductor lead 40 is terminated in a modular connector 48 of the type
commonly used with telephones and includes a press-to-release lever 50,
that is adapted to engage a suitable female jack into which connector 48
is inserted. In addition to conductors for conveying electrical signals,
lead 40 includes a lumen (not shown), extending between the interior of
upper housing 32 and an orifice 52 disposed adjacent modular connector 48.
The lumen provides a passage for equalizing the pressure inside upper
housing section 32 with ambient air pressure. Lead 40 further includes a
rubber shield 54. The shield is normally slid over modular connector 48,
protecting it from fluids or other contamination.
Details of the first embodiment of the blood pressure transducer 20 are
shown in FIGS. 2 through 8. The internal configuration of the first
embodiment, and in particular, the interior of housing sections 30 and 32
is most clearly shown in FIG. 2. With reference thereto, a chip carrier 34
is mounted inside lower housing section 30, positioning a solid-state
pressure transducer chip 36 attached thereto in fluid communication with
flow-through passage 22. Rather than mounting solid-state pressure
transducer chip 36 directly to the interior surface of lower housing 30,
the chip is adhesively attached to chip carrier 34 so that a pressure
sensitive diaphragm on the lower surface of the chip overlies an orfice 66
formed within the chip carrier. Solid-state pressure transducer chip 36 is
electrically connected to a plurality of formed flat leads 38, which
extend from a shelf 76. The shelf is molded into chip carrier 34 adjacent
the pressure transducer chip. The advantages of the chip carrier and
details concerning the solid-state pressure transducer chip and its
attachment to the chip carrier are explained below.
The flat leads on the chip carrier are generally formed into a "U-shape,"
and extend upwardly in parallel alignment with each other above shelf 76.
Stripping insulation from the end of lead 40 exposes the ends of the four
electrical conductors 42. Since conductors 42 comprise stranded wire, they
are twisted and tinned with solder. The exposed conductors are inserted
within a slot 44 formed in the upper housing section 32 so that they each
lie within separate channels 46, which are defined by parallel aligned
ribs 47 on the inner surface of upper housing section 32. Lead 40 is
adhesively secured within slot 44. Channels 46 are slightly wider than
formed flat leads 38 and are disposed to align with these leads so that
the end of each lead is seated within one of the channels 46 upon mating
of upper housing section 32 with the lower housing section 30. Details of
upper housing section 32 are shown in FIGS. 7 and 8.
With reference to the cut-away view of FIG. 3, the relationship of the chip
carrier 34, solid-state pressure transducer chip 36, one of the flat leads
38 and one of the electrical conductors 42 is clearly shown. From this
illustration, it is apparent that each of the formed flat leads 38 makes
electrical contact with one of the electrical conductors 42 upon assembly
of the two housing sections 30 and 32. In fact, the inherent elasticity of
formed flat leads 38 causes a continuous spring force to be exerted
against the bare ends of conductors 42 by the leads, resulting from the
slight displacement of the ends of the flat leads that occurs during the
assembly process. This spring force virtually eliminates any intermittent
contact problems between the formed flat leads 38 and electrical
conductors 42. In addition, the "self-aligning" relationship between
formed flat leads 38 and electrical conductors 42 as they fit within their
respective channels 46 greatly facilitates efficient electrical
interconnection between chip carrier 34 and multiconductor lead 40, and
eliminates any hand soldering of the electrical conductors.
On the lower surface of chip carrier 34 is formed a round projecting nib
62. This nib is sized to fit snugly within an orifice 68, that is formed
within lower housing section 30. Orifice 68 provides access into
flow-through passage 22. A passage 66 is defined through the center of nib
62, providing fluid communication between fluid in a catheter line (not
shown) that is attached to connectors 24 and a pressure sensing diaphragm
106 of solid-state pressure transducer chip 36. However, the fluid within
the catheter line is not allowed to directly contact the pressure
transducer chip. Instead, passage 66 is filled with a silicon gel 64 (or
other suitable incompressible gel), which serves to couple the pressure of
fluid within flow-through passage 22 to the solid-state pressure
transducer chip 36. Details of the lower housing section 30 are shown in
FIGS. 4 through 6, and of the chip carrier, in FIGS. 9 and 10.
Chip carrier 34 is provided as a prefabricated subassembly, on which
solid-state pressure transducer chip 36 is premounted, passage 66 through
nib 62 is filled with gel 64, and flat leads 38 formed with the required
"U-shape." The chip carrier subassembly is mounted in lower housing 30 by
insertion of nib 62 into orifice 68. Adhesive is applied to a plurality of
grooves 84 that are disposed in the upper interior surface of lower
housing 30. The adhesive runs down grooves 84 and seals the interface
between the exterior surface of nib 62 and the inner surface of orifice
68. Capillary attraction draws the adhesive from grooves 84 into voids
between these surfaces. For this reason, a relatively thin adhesive is
preferably used, such as a low viscosity polyurethane adhesive. The final
step in the assembly process involves adhesively securing the upper
housing section 32 to the lower housing section 30 using a cyanoacrylate
adhesive applied to the outer surface of a lip 80 that is formed around
the periphery of upper housing section 32. Lip 80 is then inserted within
the peripheral extent of the lower housing and held in place until the
adhesive sets.
It should be apparent that relatively little labor is required to assemble
pressure transducer 20, due primarily to the use of the subassembly
comprising chip carrier 34 in which the solid-state pressure transducer
chip 36 is premounted, and also due to the ease with which the electrical
connection between chip carrier 34 and electrical conductors 42 is
effected, as described above. Consequently, the manufacturing (labor) cost
of assembling blood pressure transducer 20 is relatively low compared to
the prior art. In addition, unlike the pressure transducers used in the
prior art, a solid-state pressure transducer is used in the present
invention, which does not require a separate thick film hybrid circuit
board on which discrete components are mounted. Instead, solid-state
pressure transducer chip 36 includes a thin film compensation circuit that
is fitted into the existing semiconductor environment on the chip.
Details of the pressure sensing circuit used on solid-state pressure
transducer chip 36 are shown schematically in FIGS. 12 and 13. (The thin
film compensation circuit is not shown in these Figures.) Solid-state
pressure transducer chip 36 is generally conventional in its operation as
a piezoresistive silicon pressure sensor, and includes P-type regions
comprising sensing resistors 100 and 102, disposed in an N-type silicon
wafer 104. The center of the N-type silicon wafer 104 comprises a
relatively thin pressure sensing silicon diaphragm 106. Layers of silicon
oxide 108 insulate the surface of silicon wafer 104.
The P-type regions comprising sensing resistors 100 and 102 are disposed in
silicon wafer 104 adjacent the top surface of silicon diaphragm 106,
around the edge of an underlying cavity 70, and are connected by
conductors 110 in a standard Wheatstone bridge circuit. The thin film
hybrid circuit used to compensate solid-state pressure transducer chip 36
for changes in temperature comprises a plurality of trimming resistors
(not shown) incorporated on silicon wafer 104 in regions 72, disposed
around the periphery of the chip, outside conductors 110.
Application of unequal fluid pressures on opposite surfaces of silicon
diaphragm 106 causes it to deflect, the resulting strain changing the
relative resistances of the sensing resistors 100 and 102. If the pressure
transducer chip is exposed to the same pressure on opposite surfaces of
the silicon diaphragm 106, the resistance of resistors 100 and 102 are
substantially equal. Accordingly, under this balanced condition, if a
voltage is applied to nodes 114 at diagonally opposite corners of the
bridge, the potential difference between nodes 112 at the other two
corners is approximately zero. However, if the center of silicon diaphragm
106 is deflected by unequal fluid pressures, applied to opposite surfaces,
the potential difference between nodes 112 changes in direct proportion to
the differential pressure, the resistance of sensing resistors 100
increasing, and the resistance of sensing resistors 102 decreasing by
equal amounts, due to the unbalanced condition of the Wheatstone bridge.
Pressure transducer 36 thus provides an output voltage between nodes 112
that is proportional to gage pressure when a pressure to be measured is
applied to one surface of the center of silicon diaphragm 106, and ambient
pressure is applied to the opposite surface.
A further aspect of the present invention relates to the automated
manufacture of the subassembly comprising chip carrier 34, shown
sequentially in steps A through D in FIG. 11. Initially, a thin, flexible,
metal strip 116 is run through a progressive die (not shown), forming a
plurality of cut-out rectangular areas in which are defined the plurality
of leads 38 (step "A"). Metal strip 116 is nickel-plated to provide a base
coat for additional metal plating that is applied to leads 38.
Specifically, the inwardly extending ends 78 of leads 38 are gold plated,
providing a surface for attachment of gold "fly" wires 74, which are used
to connect leads 38 to gold plated conductive pads (not shown) on the
solid-state pressure transducer chip 36. The opposite ends of leads 38,
still attached to metal strip 116, are tinned with a lead/tin solder or
gold plated to provide a conductive path when in contact with electrical
conductors 42 of lead 40.
In step "B," the lower and upper portions 90 and 92 of chip carrier 34 are
integrally molded around metal strip 116 so that a shelf 76 formed thereon
underlies the gold plated ends 78, and so that the edges of the lower
portion are generally aligned with the edges of one of the die-cut
openings within metal strip 116. Upper portion 92 is generally
rectangular, having a relatively wide slot formed in one end. (For
clarity, the upper portion 92 is shown cut away in the view of step 8.)
The ends of leads 38 are then die-cut away from metal strip 116 in step
"C." An opening is defined by the slot in the upper portion, exposing a
surface within the lower portion on which the solid-state pressure
transducer chip 36 is mounted, with cavity 70 overlying passage 66, (in
step "D"). In addition, gold plated pads on the upper surface of the
pressure transducer chip 36 are electrically connected to the gold plated
ends 76 of flat leads 38 by means of gold fly wires 74. Once chip carrier
34 is fully assembled on metal strip 116 in a subassembly generally
denoted by reference numeral 86, it is inverted and passage 66 on nib 62
is filled with silicon gel. It will be apparent that sub-assembly 86 may
be fabricated in a substantially automated process. The metal strip with
attached sub-assemblies 86 may be rolled-up on a reel (not shown) for
later use in fabricating blood pressure transducer 20.
Prior to use in fabricating the blood pressure transducer 20, the metal
strip 116 (on which are disposed the completed chip carrier subassemblies
86) is run through a punch which removes each chip carrier 34 from the
strip and forms flat leads 38 into the required "U-shape" described above.
The pre-assembly of chip carrier 34 and solid-state pressure transducer
chip 36 into subassembly 86 thus greatly facilitates the ease of
fabrication of the blood pressure transducer 20.
Turning now to FIG. 14, a second embodiment of the blood pressure
transducer generally denoted by reference numeral 120 is shown. Blood
pressure transducer 120 differs from the first embodiment 20 primarily
with respect to the orientation of flow-through passage 22 and capillary
line connectors 24. In the second embodiment, a lower housing section 130
includes an integral tubular portion 126 through which flow-through
passage 22 extends. The tubular section 126 extends across the width of
lower housing section 130, instead of longitudinally along its length, as
was the case with respect to tubular section 26 on lower housing section
30 of the first embodiment. In all other respects, blood pressure
transducers 20 and 120 are substantially the same. For this reason, the
interior configuration of lower housing section 130 and upper housing
section 32, and the mounting of chip carrier 34 therein are not shown. The
same reference numerals are used on elements common to both embodiments.
A further aspect of the present invention is shown in the schematic diagram
of FIG. 15. Certain blood pressure monitoring devices have a relatively
low input impedance requirement, and will not respond to a DC voltage from
a high impedance source, such as solid-state pressure transducer chip 36.
A cable 140 terminated in a modified female connector 138 is provided for
such monitoring devices, as shown in FIG. 15. Female connector 138
includes a resistor 150, which is connected at one end to one of the
conductors 142 within cable 140. The other end of resistor 150 is
connected through lead 144 to a normally open switch contact 146. When
modular connector 48 is plugged-in to female connector 138, the terminal
end of the uppermost electrical conductor makes contact with its
respective conductor 142, and forces the end 148 of that conductor to
close against switch contact 146. Engagement of modular connector 48 and
female connector 138 thus places resistor 150 in parallel with the
outermost conductors in both lead 40 and cable 140. Since the outermost
conductors of lead 40 carry the output signal from the solid-state
pressure transducer chip 36, the monitoring device "sees" the required
impedance on its input provided by the parallel resistance of resistor
150. Typically resistor 150 has a resistance of about 375 ohms, providing
an output impedance to the monitoring device of about 300-350 ohms.
In prior art blood pressure transducers, an impedance matching resistor
similar in function to resistor 150 is typically mounted in the transducer
housing. There are at least three significant advantages which result from
mounting resistor 150 in female connector 138 rather than in housing
sections 30/130 and 32: (a) although blood pressure transducers 20 and 120
are disposable, cable 140 is not, being instead left connected to the
monitoring device; thus, there is only a one-time cost for resistor 150
during the useable life of cable 140;(b) the housing sections 30/130 and
32 may be made smaller in size by mounting resistor 150 in female
connector 138; and (c) most importantly, the heat dissipated by resistor
150 does not affect the thermal stability of solid-state pressure
transducer chip 36 as it would if co-mounted inside the same housing. With
respect to the third advantage (c), tests have shown that compared to
prior art devices in which the impedance matching resistor is mounted
inside the blood pressure transducer housing, the present invention
exhibits less than 30% of the thermal drift during the initial warmup
period, and stability is achieved in about one-third the time (80 seconds
versus 240 seconds).
While the present invention has been disclosed with respect to preferred
embodiments thereof, those of ordinary skill in the art will understand
that further modifications to the invention may be made within the scope
of the claims that follow hereinbelow. Accordingly, it is not intended
that the scope of the invention be limited to what has been disclosed
above but, instead, should be determined entirely by reference to the
claims that follow.
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