U.S. patent application number 11/078974 was filed with the patent office on 2006-09-14 for media isolated absolute pressure sensor.
Invention is credited to Walter Czarnocki.
Application Number | 20060201255 11/078974 |
Document ID | / |
Family ID | 36969387 |
Filed Date | 2006-09-14 |
United States Patent
Application |
20060201255 |
Kind Code |
A1 |
Czarnocki; Walter |
September 14, 2006 |
MEDIA ISOLATED ABSOLUTE PRESSURE SENSOR
Abstract
A method and media-isolated absolute pressure sensor apparatus
includes a first sensor (101) to measure a pressure difference
between an isolated media (P1) and a second media (Pa). A second
sensor (103) measures an absolute (relative to vacuum) pressure of
the second media (Pa). Each sensor (101, 103) has its own offset
and slope response. An equalizer (217, 219) matches the slopes of
the sensors (101, 103), wherein a summing circuit (225) can add the
substantially same slope outputs to provide an output signal (227)
indicative of an absolute pressure measurement of the isolated
media (P1). Offset and temperature compensation of each sensor can
also be provided.
Inventors: |
Czarnocki; Walter; (Hoffman
Estates, IL) |
Correspondence
Address: |
MOTOROLA, INC.
1303 EAST ALGONQUIN ROAD
IL01/3RD
SCHAUMBURG
IL
60196
US
|
Family ID: |
36969387 |
Appl. No.: |
11/078974 |
Filed: |
March 10, 2005 |
Current U.S.
Class: |
73/720 |
Current CPC
Class: |
G01L 19/02 20130101;
G01L 27/005 20130101; G01L 9/065 20130101; G01L 15/00 20130101 |
Class at
Publication: |
073/720 |
International
Class: |
G01L 9/04 20060101
G01L009/04 |
Claims
1. A media-isolated absolute pressure sensor apparatus comprising:
a first pressure sensor operable to measure a first pressure
difference between a first pressure and a second pressure applied
across the first pressure sensor, the first pressure sensor
responsive to the first pressure difference by providing a first
output having a first slope response relative to the first pressure
difference; a second pressure sensor operable to measure a second
pressure difference between the second pressure and substantially a
vacuum applied across the second pressure sensor, the second
pressure sensor responsive to the second pressure difference by
providing a second output having a second slope response relative
to the second pressure difference; at least one equalizer circuit
for adjusting the slope response of at least one of the first and
second sensors such that the first slope is substantially the same
as the second slope, and providing an at least one adjusted output
corresponding of the first and second sensors; and a summing
circuit for adding the substantially same slope outputs associated
with the first and sensor sensors so as to provide an output signal
indicative of an absolute pressure measurement of the first
pressure.
2. An apparatus in accordance with claim 1 further comprising an
offset circuit for providing at least one of a first and second
offset signal for the corresponding first and second sensor
outputs.
3. An apparatus in accordance with claim 1 further comprising an
offset-temperature and span-temperature compensation circuit
coupled to each of the first and second pressure sensors for
providing an offset-span temperature compensation signal derived
from the first and second pressure sensors.
4. An apparatus in accordance with claim 1 wherein the equalizer
includes two equalizers, one for each of the first and second
sensors.
5. An apparatus in accordance with claim 1 wherein the first
pressure sensor is a piezo-resistive device.
6. A media-isolated absolute pressure sensor apparatus comprising:
a first pressure sensor operable to measure a first pressure
difference between a first pressure of an isolated media and a
second pressure applied across the first pressure sensor, the first
pressure sensor responsive to the first pressure difference by
providing a first output having a first slope response relative to
the first pressure difference; a second pressure sensor operable to
measure a second pressure difference between the second pressure
and substantially a vacuum applied across the second pressure
sensor, the second pressure sensor responsive to the second
pressure difference by providing a second output having a second
slope response relative to the second pressure difference; a first
and second equalizer circuit for adjusting the slope response and
offset of the respective first and second sensors such that the
first slope is substantially the same as the second slope, and
providing respective adjusted first and second outputs; and a
summing circuit for adding the adjusted first and second outputs so
as to provide an output signal indicative of an absolute pressure
measurement of the isolated media.
7. An apparatus in accordance with claim 6 further comprising an
offset-temperature and span-temperature compensation circuit
coupled to each of the first and second pressure sensors for
providing an offset-span-temperature-compensation signal derived
from the first and second pressure sensors.
8. An apparatus in accordance with claim 6 wherein the first and
second pressure sensors are piezo-resistive devices.
9. A method for providing an absolute pressure measurement of an
isolated media, the method comprising the steps of: providing a
first pressure sensor operable to measure a first pressure
difference between a first pressure and a second pressure applied
across the first pressure sensor, the first pressure sensor
responsive to the first pressure difference by providing a first
output having a first slope response relative to the first pressure
difference, and a second pressure sensor operable to measure a
second pressure difference between the second pressure and
substantially a vacuum applied across the second pressure sensor,
the second pressure sensor responsive to the second pressure
difference by providing a second output having a second slope
response relative to the second pressure difference; adjusting the
slope response of at least one of the first and second sensors such
that the first slope is substantially the same as the second slope;
providing an at least one adjusted output corresponding of the
first and second sensors; and summing the substantially same slope
outputs associated with the first and sensor sensors so as to
provide an output signal indicative of an absolute pressure
measurement of the first pressure.
10. A method in accordance with claim 9 further comprising the step
of: subtracting a first and a second offset from the corresponding
first and second sensor outputs.
11. A method in accordance with claim 10 further comprising the
step of: applying offset-temperature and span-temperature
compensation, derived from the first and second pressure sensors,
to provide offset-span temperature-compensation of the first and
second sensors.
12. A method in accordance with claim 9 wherein the step of
adjusting adjusts the slope response of both the first signal and
the second signal.
13. A method in accordance with claim 9 wherein the providing step
includes providing the sensors as first and second piezo-resistive
devices.
Description
FIELD OF THE INVENTION
[0001] The present invention is generally directed to the field of
pressure sensors, and specifically for media-isolated absolute
pressure sensors.
BACKGROUND OF THE DISCLOSURE
[0002] In contemporary automotive systems it is often desirable to
measure an absolute pressure at a location. For instance, it is
desirable to measure a pressure across a sharp edge orifice in an
EGR (exhaust gas reflow) system in order to determine flow. Often,
as in this case, the media can be very harsh. Because of this
adverse environment, isolation from the medium, here exhaust gas,
is desirable to ensure that the sensor, typically semiconductor
based, survives and functions properly over a long period of
time.
[0003] Absolute pressure sensors are normally built with the vacuum
inside a sensor's cavity, and have a sensed pressure applied to
their top side. This side can be identified as the one that
contains things for interconnection (metal wire bonding pads, wire
bonds etc). As can be imagined the top side is sensitive to the
chemical/physical contamination which can adversely affect
operation of the sensor, causing severe degradation in sensors
accuracy or even catastrophic failures.
[0004] One prior art solution to protect the sensor's top side is
to use special semiconductor films (for example, nitride
passivation) and outside films or gels. This kind of protection has
its limitations. For example, extreme media, such as vehicle's
exhaust gas, can still harm the sensing element despite being
protected.
[0005] Another prior art approach to isolate the media from the
sensor is using stainless steel diaphragms for sensing a pressure
coupled by oil to a conventional semiconductor based pressure
sensor. The stainless steel diaphragm provides the necessary
isolation between the harsh media and the pressure sensor, and the
oil provides the transfer of pressure to the pressure sensor. The
oil medium used in this approach adds error to a pressure
measurement because in the manufacturing process is difficult if
not impractical to eliminate all air pockets. These air pockets add
error to the pressure transfer between the stainless steel
diaphragm sensing the media harsh pressure and the actual pressure
sensor. Also, the oil pressure transfer performance is degraded
with increasing temperature and time because of changes in oil
viscosity and leakage of oil. Furthermore, using the oil filled
approach is difficult to manufacture because the oil needs to be
hermetically sealed between the stainless steel diaphragm and the
pressure sensor.
[0006] Another prior art approach isolated the media isolated by
enclosing a differential pressure sensor in the hermetically sealed
vacuum can. This way a reference pressure is kept at zero whereas
sensed pressure is applied from sensors back side. The back side of
the sensing element, composed of the sealed silicon is not affected
by the sensed media. However, this kind of solution is expensive,
leak prone and not well suited for mass production.
[0007] What is needed is an improved media-isolated absolute
pressure sensor, that is more accurate, easier to manufacture, low
cost, and has an improved field performance over time and
temperature variations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The features of the present invention, which are believed to
be novel, are set forth with particularity in the appended claims.
The invention, together with further objects and advantages
thereof, may best be understood by making reference to the
following description, taken in conjunction with the accompanying
drawings, in the several figures of which like reference numerals
identify identical elements, wherein:
[0009] FIG. 1 is a cross-sectional view of a first and second
pressure sensor both mounted on a common assembly, in accordance
with the invention;
[0010] FIG. 2 is a system block diagram illustrating a structural
relationship used to convert signals provided by the pressure
sensors shown in FIG. 1 to form an absolute pressure sensor;
[0011] FIG. 3 is a system block diagram illustrating an alternate
embodiment of a structural relationship used to convert signals
provided by the pressure sensors shown in FIG. 1 to form an
absolute pressure sensor;
[0012] FIG. 4 is a graphical representation of the response slopes
of the sensors; and
[0013] FIG. 5 is a flow chart demonstrating a method in accordance
with a preferred embodiment of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0014] The present invention describes a method and apparatus using
two sensing elements, differential and absolute. Absolute sensing
is achieved by proper signal conditioning. A sensed absolute
pressure is applied to the differential sensor back side, thus
media isolation is achieved and harsh media can be applied thereto.
As a result, the present invention provides an absolute pressure
sensor that is more accurate, easier to manufacture, low cost, and
has an improved field performance over time and temperature
variations.
[0015] A media-isolated absolute pressure sensor apparatus and
corresponding method combines a first signal provided by a first
pressure sensor, indicative of a difference between a first
pressure and second pressure applied across the first pressure
sensor, and a second signal provided by a second pressure sensor,
indicative of a difference between the second pressure and a
substantial vacuum applied across the second pressure sensor. The
first and second signals are combined to form an absolute pressure
sensor. Responsive to a pressure span, or a range of pressures, the
first signal has a slope response different than a slope response
of the second signal. This is particularly evident where one sensor
is a differential sensor and the other an absolute sensor, unlike
the prior art where similar type differential sensors are used. An
equalizer (slope adjustment) circuit enables an adjustment of the
slope response of the first signal to correspond to the slope
response of the second signal, and provides a slope adjusted first
signal dependent on the adjusted slope response. A summing circuit
provides an output signal dependent on the slope adjusted first
signal and the second signal, where the output signal is indicative
of an absolute pressure sensed between the first pressure sensor
and the second pressure sensor, which is relative to a vacuum,
making the system an absolute pressure sensor.
[0016] Features of the present invention include providing a
structure that enables the combining of two pressure sensors to
form a absolute pressure sensor that has its critical elements
isolated from harsh media. Furthermore, the structure enables
compensation of span and offset errors associated with each sensor.
Given this teaching, this structure can be easily expanded to
include more than two sensors. These and other benefits of the
present invention will be better appreciated with a review of the
accompanying figures.
[0017] FIG. 1 is a cross-sectional view of a first and second
pressure sensor both mounted on a common assembly, in accordance
with the present invention. A first pressure sensor 101 and second
pressure sensor 103 are both mounted in a common sensor assembly
100. Preferably, these pressure sensors 101 and 103 are constructed
of silicon and are of the piezo-resistive type. The first pressure
sensor 101 is affixed to the housing 105 over a port 109. This can
be accomplished using a thin layer of adhesive 107 (e.g. RTV
adhesive), or by other means, that is disposed on the housing, such
as a plastic, ceramic or metal housing, and wherein the sensor 101
is then applied to the adhesive. Alternatively, the sensor can be
electrostatically bonded to a glass pedestal disposed on the
housing, and the glass pedestal bonded to the 105 using soldering
or alternatively using an adhesive 107. A bond wire 111
electrically connects the sensor 101 to a signal processing circuit
113 that is also bonded to the housing. It should be noted that
there are actually several bond wires but one is shown for
clarity.
[0018] The second pressure sensor 103 is anodically
(electrostatically) bonded under a vacuum to a glass or silicon
pedestal 104 to encapsulate a substantial vacuum 121 therewithin.
The pedestal 104 is then affixed to the housing 105, using any one
of various techniques described herein or known in the art. For
example, the pedestal 104 can affixed to the housing 105 using a
thin layer of adhesive. The type of adhesive or bonding technique
used should be chosen to provide a permanent bond to whatever the
material of the housing 105 is used. A bond wire 111 electrically
connects the second sensor 103 to a signal processing circuit 113
that is also bonded to the housing. It should be noted that there
are actually several bond wires but one is shown for clarity.
[0019] The above-described structure is encapsulated in a housing
105. The housing 105 could be a hermetically sealed housing.
However, the present invention is most economically applied where
the housing is open to atmosphere (P.sub.A), and therefore the
housing can be made of a low cost thermoset or thermoplastic.
Indeed the entire housing need only consist of the base portion.
Reference number 110 indicates an unprotected side of the sensor
assembly 100, and reference number 120 indicates a protected, or
media-isolated side of the sensor assembly 100. The unprotected
side 110 is considered unprotected because if the harsh media
present on the protected side 120 was exposed to the bond wire 111
it would chemically attack it and the bond wire would rapidly
deteriorate and fail. The protected side 120 of the sensor assembly
is considered protected because the port will be sealed and
isolated to the harsh media to be measured. The unprotected side
110 of the both pressure sensors is not exposed to the harsh
media.
[0020] A first pressure P1 117, is applied to pressure port 109 on
the protected side 120 of the first pressure sensor 101. A second,
typically ambient, pressure P.sub.A 119, is provided on the
unprotected side 110 of the sensor assembly and is common to both
the first and second pressure sensors 101 and 103. A substantial
vacuum 121 is contained within the second sensor. A signal
processing circuit 113 used to combine and process outputs from the
two pressure sensors 101 and 103 as detailed in FIGS. 2 and 3. The
circuit 113 will also have connections external to the package (not
shown) to output the absolute pressure signal to other circuits, as
needed.
[0021] FIGS. 2 and 3 are system block diagrams illustrating
alternate relationships used to convert signals provided by the
pressure sensors 101 and 103 shown in FIG. 1 to form a absolute
pressure sensor that provides an output signal 227 whose response
will be dependent on a sum of the first sensor pressure
(P.sub.1-P.sub.A) and the second sensor pressure (P.sub.A-0). The
sum provides an absolute pressure P.sub.1 since the sum is now
referenced to a vacuum. It should be noted that the circuitry
representing the structure shown in FIGS. 2 and 3 can be physically
combined on the signal processing circuit 113 of FIG. 1.
[0022] As pressure P.sub.1 117 and P.sub.A 119 are applied to the
sensor assembly 100, the piezo-resistive device of the first
pressure sensor 101 outputs a first signal 207, 209 indicative of a
difference between the first pressure P.sub.1 117 and the second
pressure P.sub.A 119 applied across the first pressure sensor 101.
The piezo-resistive device of the second pressure sensor 103
provides a second signal 213, 215 indicative of a difference
measurement between the second pressure P.sub.A 119 and vacuum
(P.sub.A-0) applied across the second pressure sensor 103,
resulting in an absolute measurement of P.sub.A. Ordinarily these
two signals 207, 209 and 213, 215 have different slopes over a
pressure span because the present state of the art manufacturing
processes do not allow the manufacture of sensors that have
perfectly identical slopes. In addition, the differential sensor
101 will be subject to a different pressure span than the absolute
pressure sensor 103. Thus over a range of pressure the first signal
207, 209 responds with a slope and over the a different range of
pressure the second signal 213, 215 provided by the second pressure
sensor 103 responds with a slope different than the slope provided
by the first pressure sensor 101 (see FIG. 4). An important step in
synthesizing the output signal 227 is the matching of the slope
response of the two signals 207, 209 and 213, 215 provided by the
two pressure sensors 101, 103.
[0023] An equalizer circuit (slope adjustment circuit) 217
conditions the first pressure sensor signal 207, 209 and provides a
slope adjusted first signal 221. Another equalizer (slope
adjustment) circuit 219 conditions the second pressure sensor
signal 213, 215 and provides a slope adjusted second signal 223. In
a minimal implementation, only the first slope adjustment circuit
217 is necessary because the slope of the first pressure sensor
signal 207, 209 need only be adjusted to match the slope of the
second pressure sensor signal 213, 215, so the second pressure
sensor signal 213, 215 slope can be fixed. In this case, the slope
response of the slope adjusted first signal 221 is adjusted to
correspond to the slope response of the second pressure sensor
signal 213, 215. In a preferred embodiment, the slope of the second
pressure sensor signal 213, 215 can also be adjusted and a slope
adjusted second signal 223 is provided from the slope adjustment
circuit 219, to provide another degree of freedom in manufacturing
the sensor 100.
[0024] Next the slope adjusted first signal 221 is added with the
slope adjusted second signal 223 by a summing circuit 225. To
understand the relevant aspects of combining the slope adjusted
first signal 221 and the slope adjusted second signal 223, a brief
review of equations determining the combination will be reviewed as
follows with respect to FIGS. 1 and 4.
[0025] The response of the slope adjusted first signal 221 can be
expressed as: V.sub.d1=offset.sub.1+m.sub.1(P.sub.1-P.sub.A) where
V.sub.d1 is the output signal of the first sensor (in this case a
voltage), offset.sub.1 is a pressure independent (constant) term of
the slope adjusted first signal 221 derived from the first pressure
sensor 101, and m.sub.1 is a pressure slope of the slope adjusted
first signal 221 derived from the first pressure sensor 101.
P.sub.1-P.sub.A is a differential pressure applied across the first
pressure sensor 101 with P.sub.1 117 applied from the protected
side 120 and P.sub.A 119 applied from the unprotected side 110.
[0026] The response of the slope adjusted second signal 223 is
described as: V.sub.d2=offset.sub.2+m.sub.2(P.sub.A-0) where
V.sub.d2 is the output signal of the second sensor (again a voltage
in this example), offset.sub.2 is a pressure independent (constant)
term of the slope adjusted second signal 223 derived from the
second pressure sensor 103, and m.sub.2 is a pressure slope of the
slope adjusted second signal 223 derived from the second pressure
sensor 103. P.sub.A-0 is a differential pressure applied across the
first pressure sensor 103 with P.sub.A 119 applied from the
unprotected side 110 against the vacuum 121, resulting in an
absolute pressure measurement.
[0027] If an adjustment is made in such a way that
m.sub.1=m.sub.2=m, then the adding done by the summing circuit 225
of the slope adjusted first signal 221 and the slope adjusted
second signal 223 will produce resultant signal V=V.sub.d1+V.sub.d2
that is dependent on the first pressure P1 117 relative to vacuum.
In addition, signal V is completely independent to the second
pressure Pa 119 which is common to the first pressure sensor 101
and second pressure sensor 103. Thus a absolute pressure sensor
that responds to the protected side 120 pressures only is created.
This deterministic result can be expressed in the following
equation.
V=V.sub.d1+V.sub.d2=offset.sub.1+offset.sub.2+m(P.sub.1-0)
[0028] As is indicated above, the summing circuit 225 provides the
necessary addition of the slope adjusted first signal 221 and the
slope adjusted second signal 223. In addition, the summing circuit
225 can be coupled to an offset temperature compensation circuit
237 which allows temperature compensation of the offset term of
both pressure sensors 101 and 103 at the same time. Furthermore,
the summing circuit 225 possesses means for adjustment of the total
circuit gain and offset at a reference temperature.
[0029] An additional component of the block diagrams shown in FIGS.
2 and 3 is a voltage span temperature compensation circuit
comprising a network 229 and a network 233 coupled to each of the
first and second pressure sensors 101, 103. These can include
resistors coupled between power supply signals 203, 205, as shown
in FIG. 2 or other temperature compensation networks as shown in
FIG. 3. The voltage span is the span of voltage that is exhibited
between the lowest and highest pressures that can be seen by the
sensor. The span temperature compensation circuit 203, 229, 233,
and 205 provides span temperature compensation for the first and
second pressure sensors 101, 103 at the same time. Signals present
at reference numbers 231 and 235 derived from the span temperature
compensation circuit 203, 229, 233, and 205 are provided to the
offset temperature compensation circuit 237 which in turn provides
a combined offset span temperature compensation signal 239.
[0030] All of the offsets, slopes, spans and temperature
performances of the sensors can be determined during manufacturing
and programmed into the signal processing circuit 113 before
shipping to provide the necessary compensation to provide an
accurate absolute pressure sensor. The signal processing circuit
can use analog, or preferably digital, techniques to provide the
equalization, offset, span and temperature compensation. In
practice, the entire sensor assembly 100 is exposed to two
different temperatures to determine the offsets and slopes of the
sensors, temperature performance, and drift.
[0031] Referring to FIG. 5, the present invention also provides a
method for providing an absolute pressure measurement of an
isolated media. The method includes a first step 400 of providing a
first pressure sensor consisting of a piezo-resistive device
operable to measure a first pressure difference between a first
pressure and a second pressure applied across the first pressure
sensor, as detailed above. The first pressure sensor is responsive
to the first pressure difference by providing a first output having
a first slope response relative to the first pressure difference.
Similarly, a second pressure sensor consisting of a piezo-resistive
device operable to measure a second pressure difference between the
second pressure and substantially a vacuum applied across the
second pressure sensor. The second pressure sensor is responsive to
the second pressure difference by providing a second output having
a second slope response relative to the second pressure
difference.
[0032] A next step 402 includes applying span temperature
compensation and adjusting the slope response of at least one of
the first and second sensors such that the first slope is
substantially the same as the second slope. Preferably, the slope
response of both the first signal and the second signal are
adjusted.
[0033] A next step 404 includes providing an at least one adjusted
output corresponding of the first and second sensors.
[0034] A next step 406 includes subtracting a first and a second
offset from the corresponding first and second sensor outputs.
[0035] A next step 408 includes applying offset-temperature,
derived from the first and second pressure sensors, to provide
offset-span temperature-compensation of the first and second
sensors.
[0036] A next step 410 includes summing the substantially same
slope outputs associated with the first and sensor sensors so as to
provide an output signal indicative of an absolute pressure
measurement of the first pressure.
[0037] In conclusion, an improved media-isolated absolute pressure
sensor, that is more accurate, easier to manufacture, and has
better field performance over time and temperature variations has
been detailed. It overcomes the deficiencies of prior art
approaches by replacing the hermetic media isolation techniques
with a simpler differential/absolute sensor configuration.
Furthermore, a simplified approach for calibrating multiple sensors
and combining their outputs to electronically form an absolute
signal provides a substantial manufacturability and field
performance advantage.
[0038] While the present invention has been particularly shown and
described with reference to particular embodiments thereof, it will
be understood by those skilled in the art that various changes may
be made and equivalents substituted for elements thereof without
departing from the broad scope of the invention. In addition, many
modifications may be made to adapt a particular situation or
material to the teachings of the invention without departing from
the essential scope thereof. Therefore, it is intended that the
invention not be limited to the particular embodiments disclosed
herein, but that the invention will include all embodiments falling
within the scope of the appended claims.
* * * * *