Sensitive resistive pressure transducer

A resistive pressure transducer wherein a flexible diaphragm is mounted in a spaced apart relationship to the upper surface of a base member. A conductive path is deposited on one surface of the diaphragm. A resistive configuration is also deposited on the same surface of the diaphragm. The resistive configuration includes four separate resistive patterns. The first and third resistive patterns are deposited in close proximity to the outer edge of the diaphragm. The first and third resistive patterns are each comprised of three radially extending resistors, spaced apart along the periphery of the diaphragm, and interconnected by conductive material. The second and fourth resistive patterns are deposited in close proximity to the center of the diaphragm. The first and third resistive patterns measure radial strain, while the second and fourth resistive patterns measure tangential strain. The four resistive patterns each forms a separate leg of a Wheatstone bridge configuration.

I. Field of Invention 
The present invention relates to resistive pressure transducers, and more 
particularly to resistive pressure transducers utilizing diaphragms having 
radially and tangentially oriented resistive patterns deposited on the 
surface of the diaphragm. 
II. Background of Invention 
Pressure transducers using flat diaphragms with strain gauges to measure 
pressure induced deflections are known in the art of pressure transducers. 
For example, F. Kavli and K. Park, U.S. Pat. No. 4,735,098, issued on Apr. 
5, 1988, and assigned to the assignee of the present application, 
discloses a pressure transducer having a diaphragm mounted in spaced apart 
relationship to a base member, with resistive material bonded to an inner 
surface of the diaphragm, such that the resistance varies as the diaphragm 
deflects under pressure. 
It is also known in the art to place the resistive material in such a 
location so as to measure tensile tangential strain at the center of the 
diaphragm and compressive radial strain at the outer edge of the 
diaphragm. In order to obtain high resistance values, it is preferable 
that the resistors have large area. As a result, pressure transducers 
having long, narrow resistors with large area were designed. For example, 
E. Skuratovsky, U.S. Pat. No. 4,932,265 issued on Jun. 12, 1990, discloses 
a pressure transducer having long narrow resistors deposited on a 
diaphragm. 
In order to accommodate the long and narrow resistors, larger diaphragms 
were required. However, as the diaphragm size becomes larger, the stresses 
in the diaphragm increase as a function of the square of the radius. 
Therefore, large diaphragms have undesirable high stresses, as well as low 
strength. 
To solve this problem with the prior art, curved resistive patterns were 
utilized. Curved resistive patterns are desirable in that the curved 
pattern allows for a large area resistor that fits within a diaphragm with 
a relatively small diameter. For example, Skuratovsky discloses a pressure 
transducer having C-shaped annular resistors mounted on the diaphragm. The 
C-shaped resistor provided the desirable large area within a relatively 
small diameter. 
However, in the prior art resistive configurations, the radial location and 
configuration of the resistors caused the resistors to pick up undesirable 
stress effects. More specifically, the resistors located at the outer edge 
of the diaphragm tended to pick up the tangential components, as well as 
the radial components of the strain. Therefore, a need existed for a high 
output resistive pressure transducer with a resistive configuration having 
large area resistors located near the outer edge of the diaphragm, yet at 
a radial distance and of a configuration such that the tangential stress 
effects on the resistors are minimized and the radial stress effects are 
enhanced, and further having large area resistor located near the center 
of the diaphragm, but at a radial distance so as the radial strain effects 
on the resistors are minimized. 
SUMMARY OF INVENTION 
One object of the present invention is to provide a resistive pressure 
transducer utilizing tangentially oriented resistive patterns deposited 
near the center of the diaphragm and radially oriented resistive patterns 
deposited near the outer edge of the diaphragm. 
A further object of this invention is to provide a resistive pressure 
transducer wherein the tangential stress effects on the radially oriented 
resistive patterns are minimized. 
Another object of this invention is to provide a resistive pressure 
transducer having a resistive configuration wherein the radial location of 
the resistors is selected such that the sensitivity and gain of the 
transducer is maximized. 
Yet another object of the present invention is to provide a resistive 
pressure transducer wherein the resistors have broad area. 
A still further object of the invention is to provide a resistive pressure 
transducer that accommodates a diaphragm with a relatively small diameter. 
These and other objects of the present invention are achieved through a 
resistive pressure transducer for measuring pressure comprising a 
substrate having an upper surface and a lower surface, a flexible 
diaphragm mounted in a spaced apart relationship to the upper surface of 
the substrate, a conductive path deposited on a first surface of the 
diaphragm, and a resistive configuration deposited on the first surface of 
the diaphragm. The resistive configuration is preferably comprised of four 
resistive patterns. The first and third resistive patterns are deposited 
in close proximity to the outer diameter of the diaphragm in order to 
measure the radial effects of the stress. The first and third resistive 
patterns are each comprised of three resistors spaced apart along the 
circumference of the disk and interconnected by the conductive material. 
The second and fourth resistive patterns are deposited in close proximity 
to the center of the diaphragm. The four resistive patterns are coupled in 
a Wheatstone bridge configuration such that the adjacent legs of the 
bridge detect strains of different signs. 
In accordance with a broader aspect of the invention, a resistive pressure 
transducer is also disclosed wherein a flexible diaphragm is mounted in a 
spaced apart relationship to a substrate. A resistive pattern is deposited 
on one surface of the diaphragm. The resistive patterns comprises a first 
resistive pattern deposited in close proximity to the outer edge of the 
diaphragm, and includes a plurality of substantially radially extending 
resistive segments, with alternate ends of successive segments being 
interconnected by conductive material, and a second resistive pattern 
deposited in close proximity to the center of the diaphragm. The 
transducer further includes means for coupling the resistive patterns to 
sense the differences in resistance of the resistive patterns as the 
diaphragm flexes. In a preferred embodiment, the resistive pattern 
material has a resistivity over 100 times the resistivity of the 
conductive material. 
These and other objects of the present invention will now become apparent 
from a review of the drawings and the following description of the 
preferred embodiments.

DETAILED DESCRIPTION 
Referring now to FIGS. 1 and 2, a pressure resistive transducer 10 of the 
present invention is shown. The transducer 10 is generally comprised of a 
base plate 12, a diaphragm 14, means 16 for mounting the diaphragm to the 
base plate in a spaced apart relationship, a layer of conductive material 
18 deposited in a pre-determined pattern on the diaphragm 14, and a 
resistive configuration 20 deposited on predetermined areas of the 
diaphragm 14, and engaging the conductive material 18. More specifically, 
the base plate 12 has a substantially planar lower surface 22 and a 
substantially planar upper surface 24. The diaphragm 14 also includes a 
substantially planar lower surface 26 and a substantially planar upper 
surface 28. The diaphragm 14 is mounted to the base plate upper surface 24 
by the process of silk screening palladium silver along an outer perimeter 
30 of the diaphragm lower surface 26 and compressing the diaphragm 14 to 
the base plate upper surface 24. The palladium silver therefore forms a 
seal 32 along the diaphragm outer perimeter 30, which maintains the 
diaphragm 14 in a spaced apart relationship from the base plate 12. 
Alternatively, another type of suitable screen epoxy may be used to seal 
the diaphragm 14 to the base plate 12 in a spaced apart relationship. 
In the present invention, the electrical components disposed on the 
diaphragm 14 include both conductive material and resistive material. The 
resistive material has a resistivity preferably one hundred times greater 
than the resistivity of the conductive material. In empirical tests, 
conductive materials have a resistivity of less than 1 ohm per unit area, 
while resistivity of the resistive materials range in the 10 kilo-ohm per 
unit area, on one-mil thick specimens. The present invention thus utilizes 
both conductive material and resistive material while prior art 
transducers use what can be characterized as resistive material only. 
The present invention has advantages over prior art transducers. For 
example, using only semi-conductive material to form the pattern on the 
diaphragm as in conventional transducers limits the pattern design and 
makes for an inefficient design. The reason is that the semi-conductive 
material must be of a certain thickness and width so that the current 
carrying cross-section is adequate for conduction. By comparison, the 
present invention uses conductors that have very low resistivity and can 
be fabricated into a greater variety of cross-sectional configurations 
without concern for limiting current flow. Current carrying capacity of 
the present invention conductors is therefore not a problem during 
engineering design as is the case in many prior art applications. 
In addition, when all of the electrical components on the diaphragm are 
made of a single material as in conventional transducers, detection of 
deflection is complicated and is inefficient because of the broad areas of 
semi-conductive material needed to provide high conductivity connections. 
In accordance with the present invention, however, higher efficiency is 
achieved because the resistance zones are only located in the inner and 
outer areas where high (and different) stresses are present, and 
conductors are employed to interconnect these resistive areas. Using 
conductive leads for interconnection improves efficiency and transducer 
sensitivity by freeing up areas for the sensing resistive materials which 
are located at the positions where stress and strain should be measured. 
Isolation of the detection components that measure the stress or strain is 
much improved in the present invention over the prior art. The present 
invention is thus a precision instrument that accurately measures stress 
or strain at particular locations on the diaphragm. 
In a preferred embodiment of the present invention, the resistive material 
is a paste made from DuPont.RTM. 1641, which exhibits a resistivity of 
about 10 kilo-ohms, or thousands of ohms, per square, or per unit area. 
Incidentally, when the resistivity of a resistive or conductive coating is 
to be referenced, it is useful to refer to the resistivity across from one 
side of a square area of the coating to the other side of the square, as 
this resistivity is constant, notwithstanding changes in the dimension of 
the square. For various applications, a resistivity ranging from five to 
seven kilo-ohms per unit area can be attained by mixing two parts 
DuPont.RTM. 1641 with one part DuPont.RTM. 1631. The resistive materials 
are understood to be an alloy of manganese, tungsten and carbon. 
In the preferred embodiment, the conductive material is a paste made from 
DuPont.RTM. 7474, which exhibits a resistivity of only 0.1 ohms per unit 
area. This low resistivity material is highly suitable as a conductor, 
because it is an alloy with a high percentage of silver. 
The base plate 12 is preferably formed of 96% alumina. The base plate 12 is 
also preferably constructed of the same material as the diaphragm 14 such 
that the temperature expansion rates of the base plate 12 and the 
diaphragm 14 are similar. Therefore, changes in temperature will not 
result in undesirable stresses in the diaphragm 14 caused by the 
difference in expansion rates between the base plate 12 and the diaphragm 
14. It is further noted that, with the base plate 12 and the diaphragm 14 
being formed of ceramic, such as aluminum oxide, the material 16 may be a 
glass frit formed of two types of glass particles, screened onto the base 
plate, and fired to mount and space the diaphragm relative to the base 
plate. 
In the preferred embodiment, the diaphragm 14 is coated with a polymer 
coating. The polymer coating prevents undesirable shorting of the circuit 
when a pressure media is applied to the resistive configuration 20. 
Referring now to FIG. 6, a top view of the diaphragm upper surface 28, with 
the conductive layer 18 and resistive configuration 20 deposited on the 
upper surface 28 is shown. The diaphragm 14 is circular with an outer 
perimeter edge 34 defining an outer diameter dimension and a corresponding 
diaphragm radius. For purposes of reference, the diaphragm 14 further 
defines a horizontal axis 36 and a vertical axis 38. 
The conductive layer 18 is deposited in a pre-determined path. In the 
preferred embodiment shown, the conductive layer includes four legs 40, 
each extending from approximately the outer perimeter edge 34 to 
approximately the center of the diaphragm 14. 
In the preferred embodiment shown, the resistive configuration 20 is 
comprised of a first resistive pattern 42, a second resistive pattern 44, 
a third resistive pattern 46, and a fourth resistive pattern 48. The first 
resistive pattern 42 is deposited in close proximity to the outer 
perimeter edge 34, and is located such that it is bisected by the vertical 
axis 38. The second resistive pattern 44 is deposited in close proximity 
to the center of the diaphragm, and is located such that it is bisected by 
the diaphragm horizontal axis 36. 
The third resistive pattern 46 is deposited in close proximity to the outer 
perimeter edge 34. The third resistive pattern 46 is preferably located 
such that it is a mirror image of the first resistive pattern 42 in 
reference to the horizontal axis 46, and therefore, is also bisected by 
the vertical axis 38. The fourth resistive pattern 48 is deposited in 
close proximity to the center of the diaphragm 14. The fourth resistive 
pattern 48 is preferably located such that it is a mirror image of the 
second resistive pattern 44 in reference to the vertical axis 38, and 
therefore, is also bisected by the horizontal axis 46. 
The first resistive patterns 42 is preferably comprised of a plurality of 
interconnected resistors 50 spaced apart along the circumference of the 
diaphragm 14. In the embodiment shown, three resistors 50 are 
interconnected to form the first resistive pattern 42. The three resistors 
50 are preferably substantially radially-extending, and interconnected by 
the conductive material 18. Preferably the conductive material 18 
interconnects alternate successive ends of the resistors 50. Similarly, 
the third resistive pattern 46 is preferably formed of three substantially 
radially extending resistors 52, spaced apart along the periphery of the 
diaphragm 26 and interconnected by the conductive material 18. The use of 
the three separate resistors 50 and 52 interconnected by the conductive 
material enhances the radial effects on the first and third resistive 
patterns 42 and 46, and serves to minimize the tangential stress effects 
on the first and third resistive patterns 42 and 46. 
The interconnection of the radially extending resistors 50 and 52 at 
alternate successive ends of the resistors by the conductive material 18 
provides one feature of the present invention. The interconnecting 
conductive material 18 allows the resistors 50 and 52 of the first and 
third resistive patterns 42 and 46 to extend a maximum distance radially 
toward the center of the diaphragm 14, without allowing the positive 
tangential components present close to the center of the diaphragm 14 to 
cancel the negative tangential components present toward the outer edge of 
the diaphragm 14. Accordingly, these radially located resistive 
configurations 50 and 52 may have a net change in resistance which is 
opposite to that of the inner resistors 44 and 48. It should be noted that 
the plurality of present resistors 50 and 52 may also be interconnected by 
the conductive material 18 at the ends near the center of the diaphragm 
14, and interconnected by resistive material at the ends near the outer 
perimeter of the diaphragm 14. 
The first resistive pattern 42 is preferably curved and defines a first 
inner curved wall 54 and a first outer curved wall 56. Similarly, the 
third resistive pattern 46 also is preferably curved and defines a second 
inner curved wall 58 and a second outer curved wall 60. The radii of 
curvature of both the inner curved walls 54 and 58 and the outer curved 
walls 56 and 60 approximate the radius of curvature of the diaphragm outer 
edge 34. 
The second and fourth resistive patterns 44 and 48 are also preferably 
curved. Therefore, the second resistive pattern defines a second inner 
curved wall 62 and a second outer curved wall 64, and the fourth resistive 
pattern defines a fourth inner curved wall 66 and a fourth outer curved 
wall 68. The radiuses of curvature for the inner curved walls 62 and 66 
and the outer curved walls 64 and 68 also approximates the radius of 
curvature for the diaphragm outer edge 34. 
Two resistors 74 are further deposited in close relation to the outer 
perimeter 30 of the diaphragm 14. In the preferred embodiment, the two 
resistors 74 are short-circuited, so as to have no effect on the pressure 
transducer 10. However, if necessary, the resistors 74 may be opened and 
used to balance the Wheatstone bridge of the pressure transducer 10. 
By way of example, possible dimensions for the resistive configuration 20 
in accordance with the present invention are given. It should be noted, 
however, that these dimensions are exemplary only of one embodiment of the 
invention. In this embodiment, the radius of the diaphragm is 0.350 
inches. The radius of the first and third resistive pattern outer curved 
walls is 0.315 inches. The radius of the first and the third resistive 
pattern inner curved walls is 0.255 inches. The radius of the second and 
fourth resistive pattern outer curved walls is 0.128 inches. The radius of 
the second and fourth resistive pattern inner curved walls is 0.058 
inches. 
Referring now to FIG. 7, an electrical schematic for the resistive pressure 
transducer of the present invention is shown. Each of the four resistive 
patterns 42, 44, 46, and 48 form a leg of a Wheatstone Bridge 
configuration, such that the adjoining legs of the bridge sense strains of 
opposite signs, as explained more fully below. 
It should be noted that the resistive pressure transducer 10 of the present 
invention functions with only the first resistive pattern 42 and third 
resistive pattern 46 deposited in the diaphragm 14 and coupled to sense 
the difference in resistance of the patterns 42 and 46 as the diaphragm 14 
flexes under pressure. The advantage, however, of utilizing four resistive 
patterns, 42, 44, 46 and 48 coupled in a Wheatstone Bridge configuration 
is that the four resistive patterns provide automatic temperature 
compensation. 
Referring now to FIG. 3, a showing of pressure being applied to the 
diaphragm upper surface 28 is illustrated. The transducer 10 is oriented 
such that the pressure media is applied downward across the surface of 
diaphragm upper surface 28. FIG. 4 is a representational view of the 
deflection in the diaphragm caused by the pressure media. For the purposes 
of FIG. 4 it is assumed that the resistive patterns are on the lower 
surface of the diaphragm; however, the same result would be achieved with 
pressure applied to the lower surface of the diaphragm of FIG. 2, bowing 
the diaphragm upward. Returning to FIG. 4, when pressure is applied to the 
diaphragm upper surface 28, the resistive patterns near the center of the 
diaphragm are elongated, and therefore increase their value. The resistors 
near the diaphragm outer perimeter 30, however, are compressed, causing 
their value to decrease. For example, in testing performed on the 
preferred embodiment of the invention as shown in FIG. 6, the measured 
resistance of the first and third resistive patterns 42 and 46 decreased 
approximately five ohms (from 14,630 ohms to 14,626 ohms in the case of 
R.sub.1, and from 14,464 to 14,458 in the case of R.sub.3), as the 
pressure on the diaphragm upper surface increased from 1 PSI to 29 PSI. 
The measured resistance of the second and fourth resistive patterns 44 and 
48 increased approximately forty-two ohms (from 12,715 ohms to 12,755 ohms 
for R.sub.2 and from 13,290 ohms to 13,334 ohms for R.sub.4) as the 
pressure on the diaphragm upper surface increased from 1 PSI to 29 PSI. 
Thus, one pair increased in resistance while the other pair decreased in 
resistance. In a similar test conducted on a prior art resistive pressure 
transducer using long, narrow resistive patterns, such as in FIG. 1 of 
U.S. Pat. No. 4,932,265 to Skuratovsky, the measured resistance of the 
resistive patterns located near the diaphragm outer perimeter increased 
approximately seven ohms, (from 10,559 ohms to 10,567 ohms for R.sub.1 and 
from 10,362 ohms to 10,362 ohms to 10,368 ohms for R.sub.3) and the 
measured resistance of the resistive patterns located near the diaphragm 
center increased approximately thirty-two ohms, (from 17,476 ohms to 
17,507 ohms for R.sub.2 and from 10,466 ohms to 10,500 ohms for R.sub.4) 
with both of these changes being increases in resistance. Therefore, the 
resistive configuration of the present invention allows for increased gain 
or sensitivity in the transducer as compared to the prior art pressure 
transducers using long, narrow resistors. 
Referring now to FIG. 5, a radial stress component curve 70 and a 
tangential stress component curve 72 for the present invention are shown. 
The stress curves 70 and 72 are graphs of the radial and tangential 
components of stress against the dimensionless radius r/a, wherein r=the 
radial dimension at which the resistor is located, and a=the diaphragm 
radius. The radial and tangential stress components, measured along the 
y-axis, are calculated by the following equation: 
##EQU1## 
wherein u is equal to Poisson's ratio, q is equal to the distribution of 
load over the diaphragm surface, a is equal to the diaphragm radius, and h 
is equal to the thickness of the diaphragm. 
The plot begins at r/a=0, the center of the diaphragm, shown on FIG. 4 as 
R=0, and extends to r/a=1, the outer edge of the diaphragm, shown on FIG. 
4 at R=OUT. Still referring to FIG. 5, the radial stress component is 
maximum at r/a=0, the center of the diaphragm, and decreases to zero at 
the point where r/a=0.628. Above the point where r/a=0.628, the radial 
stress component is a negative value. Therefore the critical point for the 
radial stress curve 70 is the point where the ratio of the radial 
dimension of the resistor to the diaphragm radius is equal to 0.628. The 
tangential stress also is maximum at r/a=0 and decreases to zero at the 
point where r/a=0.827. Above r/a=0.827, the tangential stress component is 
a negative value. Therefore, the critical point for the tangential stress 
curve 72 is the point where the ratio of the radial dimension of the 
resistor to the diaphragm radius is equal to 0.827. 
As previously explained herein, each of the resistive patterns of the 
present invention defines a measurable inner curved wall radius and a 
measurable curved wall outer radius. In order to maximize the performance 
of the pressure transducer, the ratio of the inner curved wall radius and 
the outer curved wall radius of the first and the third resisters to the 
diaphragm radius preferably correspond to a point on the radial stress 
component curve 70 wherein the radial stress component is a negative 
value. More specifically, the ratios of the radiuses of each of the first 
and the third resisters inner and outer curved walls to the diaphragm 
radius are preferably greater than 0.827, the critical point of the radial 
stress component curve 70. The radial location of the first and third 
resistive patterns as described above minimizes undesirable tangential 
effects picked up by the first and third resistive patterns. 
Furthermore, the ratios of the radii of the inner wall and outer curved 
walls of the second and fourth resistive patterns to the diaphragm radius 
preferably correspond to a point on the tangential stress component curve 
72 wherein the tangential stress component is a positive value. More 
specifically, the ratios of the inner curved wall and outer curved wall 
radii of the second and fourth resistive patterns to the diaphragm radius 
are preferably less than 0.628, the critical point of the tangential 
stress component curve 72. This radial location minimizes the undesirable 
radial effects picked up by the second and fourth resistive patterns. 
For purposes of example, the exemplary dimensions of the embodiment of the 
present invention as previously described are used to demonstrate the 
preferred radial locations of the resistive patterns and the corresponding 
effect on the radial and tangential stress component curves. The ratio of 
the first and third resistor outer diameter to the radius is 0.315/0.350, 
which equals 0.9. The ratio of the first and third resistor inner diameter 
to the radius is 0.255/0.350, which equals 0.728. Referring to the radial 
stress component curve of FIG. 5, the points on the radial stress 
component curve corresponding to both 0.9 and 0.728 are higher than the 
radial stress component curve critical point of 0.628. Therefore, both 0.9 
and 0.728 correspond to a negative value on the y-axis. 
The ratio of the second and fourth resistor outer diameter to the diaphragm 
radius is therefore 0.128/0.350, which equals 0.36. The ratio of the 
second and fourth resistor inner diameter to the diaphragm radius is 
0.058/0.350, which equals 0.165. Referring to the tangential stress 
component curve of FIG. 5, the points on the curve corresponding to 0.36 
and 0.165 are both lower than the tangential stress curve critical point 
of 0.827. Therefore, both 0.36 and 0.165 correspond to a positive value on 
the y-axis. 
Having thus described exemplary embodiments of the present invention, it 
should be noted by those skilled in the art that the within disclosures 
are exemplary only and that various other alternatives, adaptations and 
modifications may be made within the scope of the invention. Thus by way 
of example, but not of limitation, the resistive patterns may be deposited 
in different orientations with the effect of the radial dimensions of the 
resistive patterns still providing increased gain for the transducer. It 
is further noted that, with respect to the embodiment shown in FIG. 2, the 
opening through the substrate 12 may be wider than shown and may have a 
diameter equal to the inner diameter of the frit or other bonding material 
16. Further, instead of being mounted to the substrate 12, the diaphragm 
14 may be mounted between two peripheral 0-ring supports to mount 
diaphragm 12 and to seal and isolate gas or fluid pressure applied to two 
sides of the diaphragm 14. Accordingly, it is to be understood that the 
present invention is not limited to the precise construction as shown in 
the drawings and described hereinabove.