Note: Descriptions are shown in the official language in which they were submitted.
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PRESSURE TRANSIVfITTER WITH PRESSURE SENSOR MOUNT
BACKGROUND OF THE INVENTION
The present invention relates to pressure transmitters for use in
industrial process control and monitoring applications. More specifically, the
present invention relates to a pressure sensor mount which provides electrical
isolation in a pressure transmitter.
Pressure transmitters, and other pressure sensing instruments,
include a pressure sensor that senses the pressure of a process fluid. The
pressure sensor provides an electrical output to an electrical circuit that
generates a pressure transmitter (or pressure instrument) output.
Some types of pressure sensors require electrical isolation from a
body of the transmitter. Depending upon the technology employed to implement
the pressure sensor, in some instances it can be difficult to mount the
pressure
sensor within the transmitter body while also providing electrical isolation
from
the transmitter body.
SUMMARY
1 5 A pressure
transmitter with pressure sensor mount, includes
pressure measurement circuitry. A metal body of the pressure transmitter has a
pressure coupling configured to couple to a process pressure. A pressure
sensor
is configured to provide an output related to the process pressure to the
pressure
measurement circuitry. A conduit is coupled to the pressure sensor and
configured
to apply an applied pressure corresponding to the process pressure to pressure
2 0 sensor. A non-
conductive spacer is disposed between the metal bids and conduit
and is configured to electrically isolate the conduit from the metal body. The
non-
conductive spacer has an opening formed therein and is arranged to convey the
applied pressure from the metal body to the conduit.
In one aspect, there is provided a pressure transmitter with pressure
2 5 sensor mount, comprising:
pressure measurement circuitry;
a metal body of the pressure transmitter having a pressure coupling
configured to couple to a process pressure;
a pressure sensor configured to receive the process pressure and
30 responsively provide
a sensor output related to the process pressure to the
pressure measurement circuitry the sensor output electrically coupled to the
measurement circuitry;
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an electrically conductive conduit having a first end coupled to the
pressure sensor and configured to apply an applied pressure, received at a
second
end thereof, corresponding to the process pressure to pressure sensor, the
conduit
being coupled within the metal body of the pressure transmitter by a non-
conductive compression joint; and
a non-conductive spacer disposed between the metal body and the
second end of the conduit and configured to electrically isolate the conduit
from
the metal body of the pressure transmitter, the non-conductive spacer having
an
opening formed therein and arranged to convey the applied pressure from the
metal body to the conduit.
In one aspect, there is provided a method of coupling a pressure sensor
in a pressure transmitter to a process fluid pressure, the method comprising:
receiving the process fluid pressure;
conveying the process fluid pressure to an electrically isolated pressure
1 5 sensor through a metal body of the pressure transmitter, a conductive
conduit and
a non-conductive spacer, the metal body having a pressure coupling configured
to couple to the process fluid, the conductive conduit having a first end
disposed
proximate the pressure sensor and a second end, the non-conductive spacer
being
disposed between the second end conduit and the metal body; and
coupling the conduit to an opening in the non-conductive spacer.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a partial cutaway view of a pressure transmitter in a
process control or monitoring system.
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Figure 2 is an enlarged cross sectional view of the pressure
transmitter of Figure 1 showing a pressure sensor mount in accordance with the
present invention.
Figure 3 is a cross sectional view showing the pressure sensor
mount of Figure 2 in greater detail.
Figure 4 is a cross sectional view of another example embodiment
of a pressure sensor mount.
Figure 5 is a cross sectional view of another example embodiment
of a pressure sensor mount.
DETAILED DESCRIPTION
The present invention provides a mount for a pressure sensor in a
pressure transmitter which can be used to electrically isolate the pressure
sensor
from a body of the pressure transmitter.
Figure 1 is a diagram showing a process control or measurement
system 10 which includes a pressure transmitter 12 coupled to process piping
14
which carries a process fluid 16. (Transmitter 12 is a measurement component
of system 10.) The process fluid 16 applies a pressure P to the pressure
transmitter 12. Pressure transmitter 12 provides an output, for example on a
two-
wire process control loop 20 to a remote location such as a control room 22.
The
process control loop 20 can operate in accordance with any appropriate
protocol.
In one configuration, process control loop 20 comprises a two-wire process
control loop in which an analog current level is used to represent a "process
variable" related to the process pressure P. In another example
implementation,
the process control loop 20 carries a digital value which is related to the
process
pressure P. Examples of such protocols include the HART or Foundation
FieldBus communication protocol. Another example process control loop
comprises a wireless communication link. In such a configuration, element 20
represents a wireless communication link between transmitter 12 and process
control room 22.
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Transmitter 12 includes a pressure sensor (pressure sensor die) 40
which can operate in accordance with any appropriate technique. Example
techniques include micro machine configurations, for example, which have an
element with an electrical property which changes in response to applied
pressure. A process coupling 42 couples a body or housing 18 of transmitter 12
to process piping 14. This allows the process pressure P to be applied to an
isolation diaphragm 50 of transmitter 12. The pressure P causes a deflection
in
the diaphragm 50 which is transmitted through a capillary tube 52 which
carries
an isolation fluid to the pressure sensor 40. The capillary tube 52 extends
1 0 through a pressure sensor module 54 which also supports pressure sensor
40.
Sensor module 54 includes a sensor mount 38 which is configured to mount
pressure sensor 40. Pressure sensor 40 provides an electrical output 60 to
measurement circuitry 62. Measurement circuitry 62 connects to a terminal
block 70 which couples to the process control loop 20. In one example
configuration, process control loop 20 is also used to provide power to
circuitry,
such as measurement circuitry 62, of transmitter 12.
Figure 2 is an enlarged cross sectional view of a portion of
transmitter 12 showing sensor mount 38 in greater detail. In Figure 2, the
isolation diaphragm 50 is visible in greater detail and defines a cavity 80
2 0 between the diaphragm 50 and sensor module 54 which couples to
capillary tube
or conduit 52 in header 100. As pressure P is applied from the process fluid,
the
isolation diaphragm 50 exerts a pressure against an fill fluid contained in
cavity
80 and capillary tube 52 such that the pressure P is transferred to pressure
sensor
40. Note that in Figures 1 and 2, the sensor mount 38 and pressure sensor 40
are
2 5 not shown to scale and are enlarged for illustration purposes.
Typical techniques used to mount a pressure sensor die that
require electrical isolation from the transmitter body rely on expensive
components and processes. These processes are often difficult to implement and
may lead to reliability problems. The present invention offers a technique for
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mounting a pressure sensor die in a pressure transmitter while providing
electrical isolation between the die and the transmitter body.
Examples of processes and components that have been used to
mount pressure sensor dies include metal plating, metal to ceramic joints,
adhesives, and others. However, these techniques may experience significant
reliability issues. For example, metal plating is often used to allow
components
to be soldered together. However, poor quality plating may result in poor
wetting or poor adhesion of the plating to the component. This results in
failures
during the manufacturing process as well as reduced reliability in the
finished
product. Similarly, metal to ceramic joints may have quality problems. For
example, metal brazing to ceramic may suffer from adhesion failure. Thin films
on ceramic may also have quality issues associated with surface finish,
surface
damage from machine and grinding processes as well as contaminants which are
not easily removed from the ceramic due to the porosity of the material.
In contrast, the present invention uses more established processes
and materials that offer improved reliability and quality. For example, in
various
embodiments, the present invention uses a compression glass-to-metal seal and
soldering or welding of components.
According to one embodiment, Figure 3 is an enlarged cross
sectional view of sensor mount 38 for mounting pressure sensor die 40. Mount
38 includes a header body 100 which is typically stainless steel or the like.
A
cavity 102 is formed in the header body 100 which carries the sensor die 40. A
via or capillary tube 82 extends through the header 100 as illustrated in
Figure 2.
A dielectric spacer 110 is positioned at the bottom of the cavity 102 and has
an
opening 112 formed therein which is aligned with via 82. A metal tube 120
having an opening formed therethrough is aligned with the opening in
dielectric
spacer 112 and couples capillary tube 82 to a die (pressure sensor) mount tube
122. Tube 120 is secured in cavity 102 by hermetic seal glassing 130. In this
configuration, the dielectric spacer 110 functions only as a spacer. Thus,
nothing
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needs to be attached to spacer 110 which eliminates some of the difficulties
of
prior art designs. This allows the material to be a ceramic material without
compromising the seal. The glass seal formed by glassing 130 above the
dielectric spacer 110 provides a seal between the body 100 and the tube 120.
This configuration replicates the multiple glass-to-metal seals used for
electrical
pin feed-throughs in prior art configurations. This tube seal is preferably
formed
simultaneously with an electrical pin glass-to-metal seal with metal tube 120
and
electrical connection pins (not shown). These components may be fabricated
from alloy 52 to have an appropriate thermal expansion coefficient which is
slightly less than the thermal coefficient of glassing 130. The glass-to-metal
seals are all under heavy compression stress from these stainless steel header
body yielding very high reliability seals. The tube 120 extends above the
glass
130.
In one embodiment, a die mount tube 122 is sealed to tube 120
1 5 and is preferably selected to have a thermal expansion which is
substantially
matched to that of the sensor die 40, Tube 122 includes an opening formed
therethrough which is aligned with the opening of metal tube 120 and can be
soldered, braised, or welded or otherwise attached as appropriate, to the tube
120. Note that soldering or braised techniques may have complications in their
implementation as they may block the sensor bole through the tubes. Welding
can reduce this problem and, in one specific configuration, resistance
projection
welds may be preferred.
The sensor die 40 is preferably mounted to the tube 122 having a
matched thermal expansion coefficient. The mounting may be through any
appropriate technique such as a soldered joint or the like. Examples include
techniques used to solder silicon die to Koval-Tm tubes or brushings.
Electroless
and/or electrolytic metal plated layers are not required. Metal films formed
through sputtering can be used to provide a solder wedding surface as
required.
This configuration results in a robust mount for the pressure sensor die 40
using
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well established materials and processing techniques as well as providing
electrical isolation between the pressure sensor die 40 and the body 100.
Figure 4 is a cross sectional view of another embodiment in
which the pressure sensor mount tube 122 of Figure 3 is not used. In such a
configuration, tube 120 is attached directly pressure sensor die 40. Such a
configuration may be implemented if tube 120 is compatible with the material
used in pressure sensor die 120. For example, if a solder joint may be
employed,
the pressure sensor die 40 can be soldered mounted directly to the end of the
tube 120. However, this typically may not be possible because the glassing
material 130 may require the material of tube 120 to have a thermal expansion
coefficient which is greater than that which may be directly mounted to a
pressure sensor die made of silicon.
Figure 5 shows another example embodiment of a pressure
sensor mount 150. In the embodiment of Figure 5, the pressure from the process
fluid is applied to a cavity 102 which surrounds the pressure sensor die 40.
In
this configuration, cavity 102 receives the process pressure through capillary
tube 82, which is transmitted across isolation diaphragm 50 using a fill
fluid,
such as oil or the like. Elements in Figure 5 which are similar to those
discussed
earlier have retained their numbering. Pressure sensor mount 150 includes a
process coupling 152 coupled to a header body 154. In this configuration,
opening 112 couples to a tube 156 which is coupled to a reference pressure
which may, for example, comprise an ambient pressure. In the embodiment of
Figure 5, a metal connection pin 170 is also shown. Connection pin 170 extends
through a via 172 in the header body 154 and into the cavity 102. A wire 174
is
used to provide an electrical connection between pin 170 and the pressure
sensor
die 40. This coupling can be completed through, for example, wire bonding
techniques. A hermetic glass seal 176 is used to seal the metal pin 170 in via
172
and thereby seal cavity 102.
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Although the present invention has been described with reference
to preferred embodiments, workers slcilled in the art will recognize that
changes
may be made in form and detail without departing from the scope of
the invention. Although a pressure sensor is illustrated as a pressure sensor
die in
the above discussion, other configurations may also be implemented. Further,
although the capillary tube and isolation diaphragms are shown, other
configurations can be employed including directly coupling to the process
fluid.
The various conduits and tubes shown above are illustrated as being circular
and
concentric. However, the invention is not limited to these configurations. The
metal tube and die mount tube provide one example configuration conduit used
to
couple the pressure sensor pmvided by the pressure sensor die to the process
pressure. In some configurations, the dielectric spacer may be of any non-
conductive material having a relatively high electrie.al resistance and the
material is
not limited to a dielectric material. Note that the material used to adhere
the
metal tube to the header body should be of a non-conductive material to
prevent an
electrical connection to be formed therebetween. The glass mounts described
herein can be used to form a compression joint with the metal body. The
pressure
sensor can comprise any appropriate sensor and, in one configuration,
comprises a
MEMS (microelectro-mechanical system) pressure sensor.
