Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention relates to a capacitive type
sensor having a diaphragm that de~lects under forces for sensing
pressures or loads and wherein the diaphragm has conductive
surfaces. A capacitor plate is mounted directly to the sensing
diaphragm. The capacitor plate has surface portions spaced
from the structure mounting it to the diaphragm and positioned ;
substantially parallel ~o the diaphragm with the diaphragm in
an at rest position. The conductive surface portions of the
diaphragm and the surface portions of the capacitor plate form
- 10 a capacitance sensor.
Means are provided for loading the diaphragm in
response to a pressure or load, so that the diaphragm will
deflect. Electrical connections are provided for the capacitor
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plate and diaphragm, and as the diaphragm deflects, the capaci~
~ tance between the diaphragm and the surface portions of the -
;~ second capacitor plate changes substantia~ly.
~1 The diaphragm construction can be varied as desired
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~ so that a flush diaphragm, a fixed edge diaphragm, or a free
- edge type diaphragm can be used in combination with a capacitor
plate mounted directly to the diaphragm. The sensor construc-
` tion can be adapted for differential pressure sensing by mount-
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, ing capacitor plates on each side of the sensing diaphragm.
The sensing assembly may be precisely formed by
shimming the capacitor plate for proper spacing with respect
- to the diaphragm before final assembly so that the capacitor
plate is fixed to the diaphragm in a proper position. In the
form shown solder is used between the plate and the mounting
member. This permits the adjustment of the diaphragm after the
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`~ mounting member has been welded onto the diaphragm by melting
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the soft solder and permitting the solder to cool while the
diaphragm is held with the shims.
In a differential pressure sensing device constructed `~
with capacitor plates on opposite sides of the diaphragm, the
capacitor plates can be supported on suitable shims or insul-
ating spacers, and held with a through bolt, holding both of
the capacitor plates on opposite sides of the diaphragm. A
constant force can be applied to these capacitor plates by
spring washers as shown.
The mounting of a capacitor plate directly to the
diaphragm (which serves as the other capacitor plate) reduces
or eliminates problems due to temperature instability, thermal
coefficient of expansion, mismatches between the plate and the
diaphragm, and also a greater capacitance signal is obtained ~ ~
because larger areas of the capacitor plate can be moved or ~ ~ -
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translated with respect to the diaphragm. -
The capacitor plate-diaphragm assembly lends itsel
well to high production techniques, and thus keeping the costs
of producing pressure sensors low.
In the drawings:
Figure 1 is a vertical sectional view taken through a -
typical capacitive pressure sensor made according to the
present invention;
Figure 2 is a vertical sectional view of a modified
form of the invention used for differential pressure sensing;
and
Figure 3 is a vertical sectional view through a
pressure sensor made according to a further modified form of
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the invention.
Referring specifically to Figure 1, a pressure sensor ~ ;
indicated generally at 10 as shcwn comprises a hou~ing 11
that may be circular in cross section and generated about a
central axis, The housing has a recess or opening 12 defined
therein. The recess forms a diaphragm 13 that is integral
with the peripheral walls of the housing and closes one end
of the recess.
A pressure fitting indicated at 14 is provided with
a threaded opening 15 for receiving a pressure connection from
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a source of pressure to be measured. The pressure fitting is
fastened to the housing 11 with suitable cap screws 16. ~;
Between the pressure fitting and the housing, an isolation ~;
di~hragm 17 can be provided. This isolation diaphragm is to ~;
prevent the introduction of corroslve fluids or material that
might destroy the high tensile steel preferably used for
-, constructing the housing. The isolation diaphragm as shown
is attached or mounted directly to the diaphragm 13 with a ~ -
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layer of rubber cement 18, and a suitable spacer 19 is used
around the periphery of the isolation diaphragm to prevent i ,
` the rubber cement from being squ0ezed out. A suitable ring 20
can be used for sealing the isolation diaphragm with respect
~il to the pressure fitting as shown.
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The main sensing diaphragm 13 is thus made so that
~ it will deflect from pressures introduced through the fitting
i` 14 and opening 15, which pressu re acts on the isolation dia~
phragm and through the rubber cement layer to deflect the
sensing diaphragm 13.
On the interior of the recess or opening 12 of the
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housing 11, a capacitor plate assembly illustrated at 25 is
mounted. This capacitor plate assembly comprises a plate 26
of alumina or similar insulating material fastened to a cup
shaped metal support 27 with a layer of soft solder 28, or other
suitable fastening means. The cup shaped support 27 is welded
with a suitable spot weld to the inner surface of the sensing
diaphragm 13. The support 27 and layer of solder 28 fixes the
plate assembly 25 to the diaphragm 13. The capacitor plate 26
forms a self supported annular flange member that extends
unrestrained (except for support 27) parallel to the inn~r
surface of the diaphragm 13. The plate 26 can be positioned
close to the inner surface of the sensing diaphragm 13. The
~ outer portions of the plate 26 are coated with a suitable
:` coating of metal in a known manner to form a conductive
sensing surface 30. The metal coating is carried over the
' edges of the plate 26 to the upper surface thereof indicated
at 31 for providing electrical connections to the capacitor ~;;
plate.
The metal coating on the undersurface of the plate
26 terminates short of the layer of soft solder joining the
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plate to support 27 so that the conductive surface 30 is
~ insulated from the mounting cup 27. Thus, there is no elec-
- trical connection between the metal coating on plate 26 and
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the sensing diaphragm 13. The diaphragm 13 is made of metal
or at least has conductive surface portions facing the surface ~ -
30 of plate 26. The diaphragm could have a metal coa~ing or
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", ! layer thereon if desired. The diaphragm must have some means
to form a conductive surface which becomes one capacitor plate
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or surface of ~he sensor.
The opening 12 in the housing ll is closed with a
suitable adaptor member 36 that plugs the opening as shown,
and is welded with a suitable weld 37 to the housing. The~!
adaptor 36 includes an electrically conductive rod or lead
38, that has a spring contactor member 39 at the lower end
thereof. The spring member 39 can be a flat spring steel
fixed to the rod 38, which has a bent over lip as shown that
resiliently rests against the metal coating on the upper
- 10 surface 31 of the plate 26 to form an electrical connection
to the metal coating which extends to the coating on surface
30. The rod 38 is mounted in a central opening of the adaptor
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36 and is fixed thereto with an insulating material such as
fused glass shown at 40. Thus the rod 38 does not shift with
respect to the adapter, and yet is insulated therefrom. The
rod 38 forms one of the leads for the capacitive sensor, and
comes from the capacitor plate formed on the undersurface of
the plate 26. The spring contactor keeps in contact with the ~ ~
coating throughout deflection of the diaphragm. ~-
An electrical lead can be connected to the housing
11 to form electrical connection to the diaphragm 13.
Deflection of the diaph~agm 13 causes translatory
movement of the plate 26 along with the diaphragm, and the
bowing of the diaphragm results in a substantial change in
-~ capacitance between the two conductive surfaces comprising the
inner surface of the sensing diaphragm 13 and the undersurface
30 of the plate 26. This c~ange in capacitance provides an
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indication of deflection o~ the diaphragm.
It can be seen that any shifting of the housing or
change in size of the housing because of temperature transients
or the like will not affect the positioning of the second
capacitor plate with respect to the diaphragm 13 so that more
accurate measurements can be taken across a wider range of
temperatures. In addition a difference in coefficients of
thermal expansion between the plate 26 and the sensing diaphragm
13 does not greatly affect the operation.
A flush diaphragm sensor design can be achieved with
the form of the invention shown in Figure 1 by providing the
`~ housing assembly 11 only, with some means for attaching the
housing to the desired structure such as bolt holes or other
connections. The housing 11 even could be threaded into a
connection by providing threads on the exterior surface of the
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housing for a flush diaphragm unit.
When the sensing unit i~ manufactured, the plate 26
is metalized (coated with metal in the desired areas) first, and -
is then soldered with soft solder to the metal cup 27. The ~ ~
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; 20 cup 27 is then spot welded to the inner surface of the diaphragm
13. Any misalignment then can be corrected by shimming the
plate 26 with respect to the inner surface of the sensing
diaphragm 13. This means placing flat shims of the proper -
- thickness between the plate 26 and the inner surface of the
diaphragm, and then heating the soft solder until it softens.
The plate 26 is then repositioned in the desired location with
respect to the sensing diaphragm when the solder is cooled.
In Figure 2, a modified form of the invention is
shown for use in differential pressure sensors. The pressure
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sensor assembly illustrated generally at 50 comprises a center
housing member 51, and end caps 52 and 53, which can be welded
to the housing. The caps 52 and 53 can have connections 54
leading to sources of pressure to be measured.
The housing 51 as shown has two recesses 55 and 56,
respectively, and the recesses form a diaphragm 57. The dia-
phragm 57 has upper and lower surfaces 57A and 57B respectively,
and each of these surfaces forms a capacitor plate surface used
with the pressure sensor. The sensor assembly further includes
! 10 a capacitor plate 60 and a capacitor plate 61 on opposite sides
of the diaphragm. The plates 60 and 61 may be made of an insu-
lating material such as alumina and may be metal coated to form
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capacitor plates on the surfaces that face the surfaces 57A and
57B respectively. Suitable electrical connections such as
a layer of metal extend from the surfaces sorming the capacitor
plates adjacent the surfaces of the diaphragm to lead wires
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; 62 and 63 respectively, which are carried ou~ through openings
in the caps 52 and 53 and are fused in glass.
, :, ~^~ The plates 60 and 61 are spaced from their respective ~;
surfaces 57A and 57B by shims 64 and 65, and a through bolt 66
is mounted through provided openings in the diaphragm 57, and
in each of the plates 60 and 61. The bolt 66 can have a head -
at one end, and a nut threaded at the other. The head and the
nut act against bellville type springs67 and 68, respectively
to urge the respective plates 60 and 61 against the shims 64
and 65 with a known force to hold the assembly together. The
electrical conductive portion of plates 60 and 61 must be
insulated from bolt 66 and bolt 66 must be hermetically sealed
`~ to diaphragm 57.
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Differential pressure in the chambers 55 and 56 then
causes a deflection of the diaphragm 57 and this deflection
will cause a change in capacitance between the plates 60 and 61
and the respective adjacent surfaces 57A and 57B of the sensing
diaphragm. In this construction the capacitor plates are also
mounted directly to the diaphragm and move with the diaphragm.
As the diaphragm translates or moves it bows and the capacita~ce
between the sensing surfaces of the plates 60 and 61 and the
adjacent surfaces 57A and 57B changes to provide an indication
of the dif~erential in pressure between the respective chambers
55 and 56.
In Figure 3, a further modified form of the invention
is shown which provides a free edge diaphragm construction
as opposed to the fixed edge diaphragm construction of both
Figures 1 and Figure 2. In this form of the invention a
housing 70 has a narrow annular ring wall 71 supporting the~
outer edge of diaphragm 72 on the main portion of the housing -
and this wall 71 causes diaphragm 72 to act as a free edge
; diaphragm in a known manner. The radial bending stresses at
the outer edge of the diaphragm are reduced substantially to
zero. In this form of the invention, the diaphragm 72 has a
small lug 73 integrally formed thereon, when a chamber 74 is
made, and this lug 73 is used for fixedly mounting a capacitor
plate 75 thereon in closely spaced relationship to the under-
surface 72A of the diaphragm. The capacitor plate 75 can be
attached to the lug 73 with a suita~le glass insulating mater-
ial fused into place and indicated at 76. A support plate 77 ~ - ~
can be used for closing the chamber 74, and a lead wire 78 from ~ ~ -
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the plate 75 is provided out through a suitable opening. The
~- lead wire 78 is fused in place in glass as indicated at 79 to
seal the chamber 74.
The plate 75 may be made of conductive material and
has a surface facing the surface 72A. The housing 70 provides
an electrical connection to the diaphragm 72, which forms a
capacitor plate, and as the diaphragm 72 moves from force or
load acting on the upper surface thereof, the capacitance between
the surface 72A and the adjacent surface of the capacitor
plate 75 changes to give an indication of the amount of force
on the diaphragm. As shown, the diaphragm 72 has a load button
80 that can be used for supporting a load directly to provide
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~ for a means of measuring a concen~rated load on one side of the
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diaphragm. In this form of the invention, the plate 75 can be
made entirely of metal, and is insulated from the diaphragm by
the glass 76. Shims can be used to locate the plate 75 with
respect to the surface 72A when the glass 76 is fused into
place. After glassing or fastening the plate 75 into place the ~;
shims are withdrawn and the assembly is completed.
It should be noted that the mounting for the capacitor
plates 60 and 61 as shown in Figure 2 is suitable where vibra-
; tion and shock are not problems. In other words, the mounting
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-~ shown in Figure 2 can be used in locations where the appli-
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j-` cation is not severe from a shock load standpoint, and the
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accuracy does not have to be extremely high.
~,~ In all forms of the invention a diaphragm having
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means forming a conductive surface is subjected to loads or
forces which deflect it. A second capacitor plate is mounted
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directly to the diaphragm and translates or moves with the
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diaphragm. The second plate has conductive surface portions
spaced from ~he mounting devices which face conductive portions
of the diaphragm surface. These surface portions change in
spacing in relation to the diaphragm surface as the diaphragm
deflects.
The change in capacitance between the sensing surfaces
is quite large in relation to the diaphragm deflection, as
shown and temperature stability is enhanced. The capacitor
plates can be mounted directly to the diaphragm at position
other than the central axis, which is the position shown. Por
example, the capacitor plates may be mounted to the diaphragm
at their edges or at some fixed edge diaphragm construction.
The central mounting is a preferred form for stability and
satisfactory response.
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