Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
.
The presen~ invention relates to diaphragms for
pressure sensors and the method OI forming same.
~his application is a diYisional of Canadian Patent
Application SoN~ 333~006 filed August 1, 19790
Various configurations of diaphragms have been
used in a wide variety of pressure sensor devices with
th~ primary ob~ect of these diaphragms being to seal the
inside of the devlce from the surrounding environment
while all4win~ the force collector $o move in response to
the measurecl pressure. To thi~ end, metal sheet-like
elements generally of circular configuration have been used.
The contour of such diaphragms is also generally symmetri-
cal about a central axis with concentric, convex and con-
cave deformations in the sheet. The deformations act to
reduce the force re9uired to bend the diaphragm in response
to movement of the central force collecting piston of the
pressure sensor~ The deformations also are intended to
relieve radial stress in the diaphragm as the ~orce collect-
ing piston moves axially under pressure load.
The r~sistance which is encountered with such
~iaphragms is generally non-linear. This is in part because
the radial strain experienced by such diaphragms is related
to ~he axial movement o~ the force collecting piston by the
- - approximated relationship that the elongated radial width of
the active portion of the diaphragm is equal to the square
root of the sum of the s~uares of t~e relaxed radial wiath
of the active portion of the diaphgram and the axial dis-
placement of the force collecting piston~ Such a non-linear
relationship affecting the performance o~ the diaphragm
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T~2643-239-1 ~ 1 -
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results in a n~n-linear response of the sensor to pressure.
Calibration of the instrument over its operating range.
Another difficulty encountered by thin metal
diaphragms is stress concentration at both the annular
mounting rim o~ th~ senso~ and the force collecting piston.
5 Diaphragm failure is frequently experienced at these points
rather than at some intermediatP point therebetween. The
abrupt change from the flexible unsupported portion of the
diaphragm to the rigid components of the sensor is primarily
responsible for these problems.
In attempting to overcome the foregoin~ diffi-
culties, i.e., a non-linear spring rate of the diaphragm
and high stress loadings at the attachment poînts, various
configurations have previously been employed. ~owever,
efforts at solving these problems have generally resulted
15 in an improvement in diaphragm performance with respect to
one consideration at the expense of the other consideration.
Consequently, no real solution to the improvement of over-
all diaphragm performance has heretofore been found~
! '
~0 SUMMARY OF THE I~VENTTON
The present invention is directed to improved
diaphragm configurations for pressure sensor~ and the
methods of forming same. Through the present invention, the
non-linear radial resistance of a diaphragm can be greatly
- 25 reduced as well as the high stress concentrations at the
attachment points7 The special configuration of the present
invention can be advantageously formed through a two-step
process without undue stretching of the diaphragm material.
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~l~5~3~j
To accompli~h the substantial reduction in non-
linear radial resistance of the diaphragm, a specific radial
profile has been developed which includes, as viewed from the
pressure receiving side of the diaphragm, a broad~ convex
section associatea at ~he inner and outer edges thereof with
concave sections having smaller radii of curvature than the
broad, convex,portion. The concave sections extend away from
the convex section to portions ~hich approach a perpendicular
orientation relative to the plane of the diaphragm. This
1~ orientation ma~ be characterized as creating substantially
cylindrical portion~.
From the foregoing configuration, two effects are
obtained. E`irst, the broad, convex portion will tend to
flatten out with increasing pressure. This results in com-
pression loading on the concave sections. As pressure in-
creases and as the center convex sec~ion tends to flatten
out, the force collecting piston of the transducer moves axiallyO
This axial movement results in a radial tension loading of
the ~iaphrasm. Through empirical analysis, diaphragms can be
developed using the present configuration which tend to
offset the compression loading of the convex portion with the
tension loading caused by the axial movement of the force
collecting piston. The concave sections add significantly to
the reduction in flexure rigidity of the diaphragm and provide
little resistance ~o the flattening of the center convex
section. Also, the broad, convex section provides increased
raaial compliance irrespectiYe of its pressure responsive
characteristic as compared to the sinesoidal convolutions o~
conventional diaphragms. Thus, the non-linear resistance to
the required radial extension of the diaphragm can be sub-
stantially overcome.
~s~
Second,stresses at the attachment points of
the diaphra9m are also reduced by the configuration of
the present inven~ion- As stated above, the concave
sectionS of the diaphragm extend upwardly to substantially
5 cylindrical portions. Once having approached such a
cylindrical configuration, a bend may be formed in the
diaphragm which is preferably about a~minimum radius of
curvature for the material of the diaphragm. From this
bend on each edge of the diaphragm, a radially extending
10 flange is provided. The flange provides an area for
attachment of the diaphragm to the pressure sensor.
Finally, the diaphragm is welded or otherwise fixed to
the pressure sensor at the flanges of the diaphragm as
closely as possible to the sharp bends of the diaphragm.
15 Alternately, the diaphragm may be directly welded to the
pressure sensor where the concave sections approach a
substantially cylindrical configuration The substan-
tially cylindrical configuration provides great rigidity
; to the diaphragm at that location. The rigidity is such
20 that it overcomes any stress concentration which might
otherwise be transmitted to the attachment area. Instead,
the other portions of the diaphragm between the generally
cylindrical portions will take the strain. Naturally, the
sharp bend also provides added rigidity to the flange and
25 attachment points.
In order to form the sharp bend and maintain a
relatively uniform wall thickness, across the width of the
diaphragm a method has been developèd by the present in-
vention which avoids formation of the sharp be~d during
30 formation of the convex and concave annular sections.
During the formation of these sections, the annular sheet
of metal being formed into the diaphragm tends to migrate
radially inwardly from the oute~ edge and radially out-
wardly from the inner edge in defining the annular rings.
If the sharp bend is to be forTned in the same process,
5 the dies tend to hold onto both the inner and outer edges
of the annular sheet. Under such circumstances, the
center portions of the sh~et may tend o stretch exces-
sively, leading to a non-unifor~ wall thickness of the
resulting diaphragm. By the present invention, the sharp
10 bends formed at the inner and outer edges of the diaphragm
are completed by a second forming step.
Accordingly, it is an object of the present in-
vention to provide an improved diaphragm for pressure
sensing devices.
It is another object of the present invention
to prov~de a diaphragm for pressure sensors having mini-
mum resistance to the operation of the sensor.
It is yet another object of the present inven-
tion to provide a diaphragm for pressure sensors having
20 low stress concentrations at the attachment points of the
diaphragm.
It is yet another object of the present inven-
tion to provide an improved method for forming diaphragms
of the present invention.
Other and further objects and advantages will
appear ~ereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional elevation of a
30 diaphragm of the present invention shown in place in a
~ SC~076.
pressure sensor.
Figure 2 is a cross-sectional elevation of a
detail of the pressure sensor of Figure 1.
Figure 3 is a cross-sectional elevation of a
sezond embodiment of the diaphragm showing attachment
to the pressure sensor~ - -
Figure 4 is a graph showing the percent devia-
tion in resistance force of the diaphragm with increasing
pressure.
10Figure 5 is another embodiment of a diaphragm
of the present invention shown in cross-sectional eleva-
tion.
Figure 6 is a second application shown in cross-
sectional elevation of the diaphragm of Figure 1.
1~Figure 7 is a third application shown in cross-
sectional elevation of the diaphragm of Figure 1.
Figure 8 is a cross-sectional elevation of a die
used in the first step of forming a diaphragm of the pres-
ent invention.
20Figure 9 is a die used in the second step of
forminy a diaphragm of the present invention.
DETAILED_DES~RIPTION OF THE PREFERRED EMBODIMENT
Turning in detail to the drawings and in parti-
cular the embodiment illustrated in Figures 1 and 2, a
25 portion of a pressure sensor is disclosed. The term
"pressure sensor" as used herein is any device which is
responsive to pressure by having the sensed pressure
operate to deflect a piston resistive to such deflection
from its rest position. As illustrated, the pressure sen-
30 sor may have an annular rim 10 ~or attachment to the outer
edge of a diaphragm. The rim may or may not have a step in
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the upper surface thereof to insure proper pl~cement of
the diaphra9m on the rim- A force collecting piston 12
is shown to be concentrically mounted relative to the
annular rim lO at approximately the same level. A shaft
5 14 extends to the recording portion of the sensor.
A diaphragm, generally designated 16, is shown
here to be annular in overall plan. As may be preferred
in some applications, the diaphragm may not have a central
hole therethrough. The presence or absence of a central
10 hole in the diaphragm is of little consequence to the
overall operation of the annular portion or element which
constitutes the active part of the diaphragm.
The operation of the pressure sensor re~uires
that the piston 12 move downwaldly in response to in-
15 creased pressure on the outside of the sensor. Resis-
tance to movemen~ of the piston 12 is usually provided by
a spring or other mechanism which exhibits a substantially
constant ratio between displacement and resisting force.
In other words, the resistance provided by the sensor it-
20 self generally is designed to exhi~it a fixed spring con-
stant. At the same time, the diaphragm is designed to
provide as little resist~nce to movement of the piston 12
as possible. Diaphragms are also generally designed to ex-
perience a minimum amount of sag between their inner and
25 outer support. For convenience the convexity and con-
_ cavity of the diaphragm`will be defined here in terms of
the surface which is on the pressure side of the diaphragm.
Naturally, as the diaphragm is of generally uniform thick-
ness, these features will be reversed on the sensor side
30 of the diaphragm.
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The diaphragm of the preferred emb~diment as
seen in Figure 1 incorporating the present invention em-
ploys a hroad, relatively shall~w, convex span 18 forming
a central annular section. Radially in either direction
5 from convex span 18 are shorter, deeper, con~ave spans 20
and 22. The annular concave spans ~0 and 22 continue to
substantially cylindrical portions 24 and 26~ In the pre-
ferred embodiment of Figure 1, a sharp, convex bend ~8 and
30 extends from each substantially cylindrical portion 24
10 and 26 respectively. The sharp, convex bends 28 and 30 are
preferably about a minimum radius for the material empl~yed
as the diaphragm. Lastly, attachment flan~es 32 and 34
extend from the sharp, convex bends 2~ and 30 in radial
directions for association with the piston 12 and the
15 annular rim 10 of the pressure sensor. In the alternate
embodiment af Figure 3, attachment is made to the inner
surface of the annular rim 10; and therefore, the sharp,
convex bends 28 and 30 and the att:achment flanges 32 and 34
become unnecessary.
,. ~0TQ achie~e a minimum effect of the diaphragm
on the overall resistance to movement of the force collect-
ing piston 12, counter-balancing mechanisms have been de-
signed into the diaphragm of the present invention. To
this end, an effort has been made to minimize the resist-
25 ance to radial extension of the diaphragm required as the
- force collecting piston 12 moves axially and hence away
from the annular rim 10. Secondly, the diaphragm of the
present invention has been designed to itself respond to
the accumulated pressure on the press~re side ~f the sen-
30 sor. This pressure response of the diaphragm causes
radial expansion of the diaphragm to keep up with t~e
- 8 -
~5~7~6
reguired elongation as the central force collecting piston
12 moveS under the accumulated pressure. The concave sec-
tions 20 and 22 minimize bending stresses by providing an
extended length over which bendiny stra~s may occur; and
the convex section exhibits substantial radial compliance.
Thus, these sections provide maximum flexibility of the
diaphragm in the required directions. The central, convex
span 18 is relatively broad and shallow in order that it
will respond to pressure on the pressure side of the sen-
sor. As pressure is increased on the diaphragm, theconvex span lB will tend to flatten. This flattening of
the span 18 will result in radial compression loading or
outward movement of the concave spans 20 and 22. At the
same time, as pressure increases on the sensor, the piston
12 will tend to move axially. This movement will require a
radial elongation of the diaphragm 16. Thus, convex span
18 will act to meet that requirement by itself being flat-
tened by the pressure. The concave sections minimize the
bending required in accommodating bo$h the extension of the
2~ diaphragm and the flattening of the convex portion thereof.
~ either the flattening of the convex portion
of the diaphragm nor the overall radial extension of the
diaphragm are linear functions. Consequently, the con-
siderations necessary in calculating the appropriate
dimensions for such a diaphragm become prohibitively com-
plicated. Through empirical testing, appropriate relation-
ships can be found for any given diaphragm size normally
associated with such pressure sensors. Tests have demon-
strated that certain approxi~ate relationships can be
employed as a basis from which to start empirically fine
7~
tuning the design of a diaphra~m to fully realize the
advantages of the present invention. Tne diaphra~ms
which have been tested are of the type suitable for use
in sensorS where eighty to ninety percent of the systems
5 stiffness is in a force gaging device such as a strain
gage equiped cantilever beam~ Only ten to twenty per-
cent of the stif~ness is in the diaphragm.
In ~escribing SUCh relationships, certain
de~initions are necessary~ The active span of the dia-
10 phragm oi the preferred embodiment is the distance betweenthe two cylindrical portions 24 and 26. It is within this
area that s~bstantially all of the diaphragm flexure occurs.
l~hrough empirical methods, a first a~proximate radius of
curvature ~or the two concave sections 20 ana 22 has been
15 found to be around one eighth of the active span length. A
~irst approximate radius of curvature of the convex span 18
nas been found to be acceptable when approximately equal to
the active span length. When properly matched and empiri-
cally tuned, a response curve of the overall sensor may be
20 arrived at such as shown in Figure 4. In Figure 4, the
deviation of the spring constant from its constant value
is shown across the full range of pressure for the pressure
sensor. Naturally, hysteresis inevitably affects the per-
formance of such a device but the device may be adjusted to
25 give zero deviation at zero pressure and maximum pressure
as shown.
To eliminate concentration ~f bending stresses at
the attachment points of the diaphragm, the concave portions
20 and 22 extend up to substantially cylindrical portions 24
30 and 26. Ihese portions 24 and 26 act to resist bending and
radial stresses and thus do not transmit such stresses
-- 10 --
:
~5~7~6
through to the attachment points of the diaphragm. In the
case of the embo~iment as shown in Figure 3/ this effect oE
the cylindrical portion is used exclusively to reduce bend-
ing load on both the diaphragm adjacent the attachment point
5 and the bond itself. In Figure 3, the diaphragm is shown to
be welded to the inside of the annular rim 10 at 36.
In Figure 2, the cylindrical portions 24 and 26
are employed with the sharp, convex bends 28.and 30 and the
radial attachment flanges 32 and.34 to provide riqid resist-
ance to transmission of bendin~ stresses directly to theattachment locationO It is also advantageous that the
attachment.point shown as weld 38 is as close as pract-
ical to the inner edge of the rim 10. This prevents any
cantilevering effect of the diaphragm at the inner corner
of the rim 10. Resistance to bending is maximized in
the sharp, convex bend 28 and 30 of the diaphragm by having
the radius of curvature approach the minimum possible for
the diaphragm material and thickness used. As the thickness
of such diaphragms generally ranges from .0015 to .0110
inches, the radius of this sharp bend may preferably range
from .003 inches to .010 inches. Through the use of the
concave sections 20 and 22 with the cylindrical portions 24
and 26 and the sharp bends 28 and 30/ maximum compliance is
obtained with minimum stress concentration. As a result,
the pressure responsive characteristic of the convex span 18
may be allowed to operate as freely as possible to accommo-
date the necessary radial extension of the diaphragm under
loading.
As a variation on the present embodiment, a plur-
ality of diaphragm elements are disclosed in Figure 5 It
can be seen from Figure 5 that a plurality of complete
elementS having the concave sections 20a and 22a and the
convex sectio~ 18a may be used to arrive at the same result
as the single element shown in Fi~ure 1. The diaphragm 16a
of Figure 5 iS shown to be positioned between annular rim
5 lOa and force collecting piston 12a.
Two uses for the diaphragm as illustrated in
Figure 1 are shown in Figures 6 and 7. In each of these
- Figures, diaphragms are placed in series to accomplish
greater axial flexibility of the instrument. As both the
10 outer rims 40 and 42 and the inner elements 44 through 52
do not expand or contract radially, the same compensa~ing
effect of the flattening of the central convex span 18 is
required. It should be noted tha~ not all of the dia-
phragms in Figure 7 are oriented to have the convex sec-
1~ tion face the pressure input. Instead, the system isbalanced to provlde two in eacll direction, making the
device reversible with maximum compliance provided by
the present diaphragm co~figuration.
Figures 8 and 9 show th~e manufacture of a dia-
20 phragm of the present inven~ion. In Figure 8, a first dieis employed to define the complete proile of the diaphragm
inside of the two sharp, convex bends which exist at the
inner and outer edges of the diaphragm when completed. A
more moderate bend is made. Shown in phantom in Figure 8
25 is the original sheet prior to forming. As can be seen,
substantial migration of the metal occurs in both radial
directions with the formation of the central portion of
the diaphragm.
In Figure 9, a second die 56 is shown which is used
30 to simply form the outer, sharp convex bends 28 and 30 once
--12 -
7~
mi9ration of the sheet material into`the central portion of
the die is no longer a consideration, This two step pro-
cedure has been found to make more uniform the overall
thickneSS and sonstruction of the diaphragm when complete.
Thus, a diaphragm and method of making same are
5 disclosed herein which haYe the ability to provide a minimum
e~fect on the spring constant of the pressure sensor, re-
duce bending stresses at the attachment points of the dia-
phragm and make more uniform the overall diaphragm construc-
struction. While embodiments and applications of this in-
10 vention have been shown and described, it would be apparent
to those skilled in the art that many more modifications
are possible without departing from the inventive concepts
herein described. The invention, therefore, is not to be
restricted except by the spirit of the appended claims.
