Gas sensor with orientation insensitivity

A gas sensor for being placed into a gas stream such that the gas sensor is insensitive to any specific rotational orientation about a longitudinal axis of the sensor within the gas stream. The sensor includes 1) a base having an axis that is perpendicular to the gas stream, 2) a sensor element on the base, 3) a catalyzed sensor element on the base proximate the sensor element, for creating an exothermic reaction upon contacting the gas stream thereby forming a heated gas stream portion, and 4) the catalyzed sensor element and the sensor element are positioned on the base with a sufficient axial separation therebetween so that as the base rotates about the axis, the heated gas stream portion will not contact the sensor element. In particular, the base has a second axis being perpendicular to the axis and separating the sensor element from the catalyzed sensor element. Additionally, the invention provides a device that may have both the sensor element and the catalyzed sensor element including a longitudinal axis. Wherein, both the sensor element and the catalyzed sensor element may have many different shapes. Both the sensor element and the catalyzed sensor element may have two or three sides that are coextensive with at least one void. As a result of having a void the base may include a bridge that connects at least one side of the sensor element and the catalyzed sensor element to the base.

BACKGROUND OF THE INVENTION 
CO-PENDING PATENT APPLICATIONS 
This application is related to the following: 
1) copending U.S. application Ser. No. 08/872,817, entitled, A GAS SENSOR 
WITH MULTIPLE EXPOSED ACTIVE ELEMENTS, attorney docket no. CTS-1508, filed 
Jun. 11, 1997, 
2) copending U.S. application Ser. No. 08/872,987, entitled A GAS SENSOR 
WITH MULTI-LEVEL INSENSITIVITY CIRCUITRY, attorney docket no. CTS-1518, 
filed Jun. 11, 1997, and 
3) copending U.S. application Ser. No. 60/017,112, entitled, FUEL SYSTEM 
LOW CURRENT RHEOSTAT, attorney docket no. CTS-1491, filed May 9, 1996. 
The aforementioned are assigned to the assignee named in the present 
application and are herein incorporated by reference in their entirety. 
1. Field of the Invention 
This invention relates to a gas sensor, and specifically to a sensor that 
can rotate about an axis without having a reference sensor element exposed 
to excess heat generated by a proximate catalyzed sensor element. 
2. Description of the Related Art 
Various devices are well known for combustible gas detectors used to detect 
the presence of combustible gases such as those found in car engines. 
Typical circuits are configured to include at least one sensing element 
that may be a wire having a catalytic coating. The sensing element was 
used as one of four legs of a wheatstone bridge circuit. The other three 
legs consisted of two resistors and a compensator element. The compensator 
element was identical to the sensing element except that it did not bear a 
catalytic coating. 
A current or voltage was applied to the bridge circuit to heat the surface 
of the catalytic coating affixed to the sensing element. Since the 
resistance values of the other three legs of the bridge were known, the 
resistance in the sensing element could be determined as the current or 
voltage was passed through the bridge. 
When the sensing element was exposed to a combustible gas, such as 
hydrocarbon, the catalytic coating would begin to burn, increasing the 
temperature of the sensing element. As the temperature of the sensing 
element increased, the resistance of the element increased. Accordingly, 
the current passing through the element decreased. By comparing the 
resistance level of the sensing element to the resistance level of the 
compensator element, the presence of a combustible gas could detected. 
Since the amount of gas present caused a nearly linear increase or 
decrease in the resistance of the sensing element, the quantity of the gas 
could be accurately determined by calibrating the change in resistance. 
This is the basic principal of operation of a catalytic combustible gas 
sensor. It is noted that often the sensing element must be at a 
predetermined elevated temperature to properly cause the catalyst reaction 
with the designated gas. 
3. Related Art 
Examples of patents related to the present invention are as follows, and 
each patent is herein incorporated by reference for the supporting 
teachings: 
U.S. patent statutory registration no. H454, is a chemical agent leak 
detector and a method of using the same. 
U.S. Pat. No. 5,400,643, is a gas sensor based on semiconductor oxide, for 
gaseous hydrocarbon determination. 
U.S. Pat. No. 5,388,443, is an atmosphere sensor and method for 
manufacturing the sensor. 
U.S. Pat. No. 5,365,216, is a catalyst monitoring device using EGO sensors. 
U.S. Pat. No. 5,363,091, is a catalyst monitoring device using EGO sensors. 
U.S. Pat. No. 5,211,053, is a hot gas sensor device with improved thermal 
isolation from carrier plate. 
U.S. Pat. No. 5,012,671, is a gas detecting device. 
U.S. Pat. No. 4,991,424, is an integrated circuit heatable sensor. 
U.S. Pat. No. 4,984,446, is a gas detecting device and gas detecting system 
using the same. 
U.S. Pat. No. 4,928,513, is a sensor. 
U.S. Pat. No. 4,839,767, is an element and device for detecting internal 
faults in an insulating gas charged electrical apparatus. 
U.S. Pat. No. 4,816,800, is an exhaust gas sensor. 
U.S. Pat. No. 4,674,319, is an integrated circuit sensor. 
U.S. Pat. No. 4,377,944, is an integrated gas sensitive unit comprising a 
gas sensitive semiconductor element and a resistor for gas concentration 
measurement. 
U.S. Pat. No. 3,901,067, is a semiconductor gas detector. 
The foregoing patents reflect the state of the art of which the applicant 
is aware and are tendered with the view toward discharging applicants' 
acknowledged duty of candor in disclosing information that may be 
pertinent in the examination of this application. It is respectfully 
stipulated, however, that none of these patents teach or render obvious, 
singly or when considered in combination, applicant's claimed invention. 
4. A Related Problem 
Referring to FIG. 1, there is a previous design from the present inventors 
that illustrates one problem to be overcome by the additionally now 
disclosed preferred embodiments. Specifically, there is a sensor 10 having 
a base 12 with signal conditioning circuitry 14 that receives signals via 
traces 24 and 26 from sensing element structures 17 and 19, which are 
located on either side of longitudinal axis 11. Uniquely, the sensing 
element structures 17 and 19 are made up of sensing elements 20 and 21 
that are located on parallel bridges 22 and thermally isolated by voids 
18. In operation, one skilled in the art will realize that all of the 
electrical signals will be skewed if the heated air flow 27 first 
interacts with the catalyzed sensing element and then contacts the 
reference sensor. The air contacting the reference sensor will have been 
heated by the catalytic reaction, thus skewing any resulting signals. To 
avoid this problem, great care must be given to ensure that the air 27 
does not contact the sensor elements in this fashion. In particular, the 
sensor would have to be placed so the air either hits both sensors 
simultaneously or hits the reference sensor first. In either case it is 
very difficult to make sure that the sensor is so arranged when placing it 
in an exhaust gas air stream. However, the present invention has overcome 
the need for careful placement of the sensor 10 in air stream 27 and thus 
eliminating the potential signal skewing problem. 
This and other problems will be solved by the preferred embodiments of the 
invention. A review of the specification, drawings, and claims will more 
clearly teach a skilled artisan of other problems that are solved by the 
preferred embodiments. 
SUMMARY OF THE INVENTION 
It is a feature of the invention to provide a gas sensor for being placed 
into a gas stream. The sensor includes 1) a base having an axis, 2) a 
sensor element on the base, 3) a catalyzed sensor element on the base 
proximate the sensor element to have a similar temperature to the sensor 
element, for creating an exothermic reaction upon contacting the gas 
stream thereby forming a heated gas stream portion, and 4) the catalyzed 
sensor element and the sensor element are positioned on the base so that 
as the base rotates about the axis, the heated gas stream portion will not 
contact the sensor element. 
A further feature of the invention includes the base having a second axis 
being perpendicular to the axis and separating the sensor element from the 
catalyzed sensor element. 
Still a further feature of the invention may include having the catalyzed 
sensor being a mirror image of the sensor element. 
An additional feature of the invention may be a device that has both the 
sensor element and the catalyzed sensor element with a longitudinal axis. 
Wherein both of the sensor elements may have parallel longitudinal axis. 
Wherein both of the sensor elements may be parallel to the axis of the 
base. Wherein both of the sensor elements may have co-extensive 
longitudinal axes. 
A further feature of the invention may be that both the of sensor elements 
to be arcuate in shape. 
Yet, an additional feature is that both the sensor element and the 
catalyzed sensor element may be at an angle to the axis. 
Still a further feature of the invention is that the base has at least one 
void therein. Wherein both the sensor elements may have two or three sides 
that are coextensive with at least one void. 
A further feature of the invention is that the base may include a bridge 
that connects at least one side of the sensor element and the catalyzed 
sensor element to the base. 
A further feature of the invention is to provide a device that has a 
ceramic substrate and a glass layer for adhering a catalyst support layer 
to the substrate. The catalyst support structure is comprised of high 
surface area ceramic particles. A catalytic material is deposited on the 
catalytic support structure for reacting with the gas to be sensed. 
The invention resides not in any one of these features per se, but rather 
in the particular combination of all of them herein disclosed and claimed. 
Those skilled in the art will appreciate that the conception, upon which 
this disclosure is based, may readily be utilized as a basis for the 
designing of other structures, methods and systems for carrying out the 
several purposes of the present invention. Further, the abstract is 
neither intended to define the invention of the application, which is 
measured by the claims, neither is it intended to be limiting as to the 
scope of the invention in any way.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention provides a gas sensor for determining gas 
concentrations in an air stream. Referring to FIG. 2, there is a preferred 
embodiment of the gas sensor 10. Specifically, the gas sensor 10 has a 
base 12, which has a longitudinal axis 11 that is oriented to be at an 
angle to the gas stream 27, illustrated as being perpendicular. There is 
also a reference sensor element 20 on base 12, and a catalyzed sensor 
element 21 proximate the reference sensor element 20. As known in the art, 
the catalyzed sensor element 21 creates an exothermic reaction upon 
contacting the gas stream thereby forming a heated gas stream portion 29. 
The gas stream flowing past the sensor element 20 would not create a 
heated gas stream portion 29'. The base also has an extension portion 16, 
located between the signal conditioning circuitry 14 and the sensor 
regions 17, 19, for creating a distance between the circuitry 14 and the 
heated sensor regions 17, 19. Although extension portion 16 is illustrated 
as being relatively short, in reality it could be relatively long to 
protect the circuitry 14 from the detrimental high temperatures associated 
with the operational temperature ranges of the sensor elements 20, 21. The 
sensor regions 17,19 could be operating, for example, from 200 to 500 plus 
degrees Celsius for proper operation. However, the conditioning circuitry 
14 would need to be operated around a maximum of 150 degrees Celsius for 
optimum signal processing. Thus, by regulating the length of extension 
section 16, it is possible to keep the signal conditioning circuitry 14 in 
a proper operational temperature range. 
In the present embodiment, both sensor elements 20, 21 are located upon two 
separate bridge sections 22a, 22b. Additionally, these bridge sections are 
isolated from any heat sink effects from the base 12 by voids 18 located 
on at least either side of the bridges. In this arrangement, it is 
possible to have both sensor elements 20, 21 closer in temperature so that 
any change in electrical resistance would not be due to ambient gas stream 
heat. Thus, only exothermic heat from the catalytic reaction on the 
measuring sensor element will cause a notable difference between the two 
resistances of the two sensor elements. It is advantageous to have both 
sensor elements to be close in temperature to avoid having compensating 
circuitry and other means for adjusting for the temperature differences. 
With various designs of the bridges, voids, and sensor elements, it is 
possible to have temperature differences below 80 degrees Celsius and 
optimumly below 50 degrees Celsius when operating in 200 to 600 or more 
degrees Celsius. It is noted that the ideal situation would be to have no 
difference in temperature between the sensor elements except for the 
exothermic catalytic reaction effects. 
Of particular note, horizontal axis 13 separates sensor regions 17 and 19. 
It is this separation that provides for the advantage of orientation 
insensitivity. Specifically, the sensor 10 may rotate about axis 11 and in 
no position will the heated gas stream portion 29 affect the reference 
non-catalyzed sensor 20. This is a great advantage over the previous 
design considered by the inventors in FIG. 1, where the sensor is very 
orientation sensitive for proper operation. Of course, this situation only 
works if the gas stream is substantially perpendicular to the longitudinal 
axis 11. Also keep in mind that the gas stream most likely will already be 
heated but a skilled artisan will realize that the catalytic reaction with 
the gas will further heat the gas stream, thus creating the "extra" or 
catalyticly heated gas stream portion 29. 
Referring to FIG. 3 there is illustrated several potential designs for the 
sensor. In particular, sensors 40, 50, 60, 70, 80, 90, 100, 110, and 120 
all have a substrate 12, a sensor element 20 and a catalyzed sensor 
element 21, a longitudinal axis 11, a horizontal axis 13, and some type of 
void that thermally isolates the sensor elements from the base. 
There are several distinguishing features to separate the various types of 
sensor configurations. There are the single bridge sensors, typically 
sensor 70 and 80 incorporate a single bridge 72 or 82 respectively that 
connects the two sensor elements 20, 21 to the main body of the base 12 
via a single bridge. There are the single bridge sensors 40, 50, 110, and 
120, which have both sensor elements on a single bridge but allow for 
electrical connection of the sensor elements to the main body of the base 
along two or more paths. There are the two bridge sensors 60, 90, and 100, 
which have the two sensor elements located on two separate bridges that 
are basically separate from each other. There are the axially balanced 
sensors 40, 50, 60, 80, and 100, which have an equal amount of sensor 
element mass located on either side of the longitudinal axis 11. There is 
the offsetting sensor design 90, that has the sensor element on one side 
of the longitudinal axis 11 and opposite to the other sensor element. 
There is the asymmetrical sensors 70, 110, and 120, which have both 
sensors located on one side of the longitudinal axis 11. All of the sensor 
designs are horizontally balanced sensors, where each sensor element is 
located on opposite sides of the horizontal axis 13. 
Referring to FIG. 4, there is illustrated a housing for holding the sensor 
10. In particular there is a hollow air pervious porous cap 30 for 
encapsulating the sensing elements 20, 21. There is also an attachment 32 
for coupling the cap 30 to a spacer 34. The spacer serves the purpose of 
extending the electronics housing 36 far enough away from the cap 30, 
since the cap region is the hottest area and the housing holds the 
conditioning circuitry 14, which requires lower temperatures for proper 
operation. The electronics housing 36 protects the conditioning circuitry 
14 and provides support for coupling the sensor to output wires to 
communicate with remote analysis circuitry (not shown). The whole assembly 
is mounted onto an exhaust pipe just after a catalytic converter. Of 
course only the porous cap 30 should be located in the exhaust pipe to 
remove the electronics from the hot temperatures. 
The present invention provides a gas sensor having a multilayered 
structure. The structure is ideally suited to sensing hydrocarbons and 
nitrogen oxides in an automobile exhaust system. Regarding FIG. 5, there 
is a plan view of the gas sensor 10 showing a portion of a substrate 
(base) 12. Substrate 12 is preferably made out of a ceramic material but 
other suitable dielectric materials may be utilized. Only the portion of 
substrate 12 containing a catalytic support structure 50 and glass 
adhesion layer 46 has been included in FIG. 5. 
The remaining portion of substrate 12 can take on any desired configuration 
that will supply the necessary structural and thermal properties for the 
sensor. For instance, the structure must be strong enough to survive the 
shock and vibration attendant in an automobile exhaust system. In 
addition, the thermal properties must be such that any catalytic reactions 
occurring on catalytic support structure 50 can be detected by a thermally 
sensitive resistor element 42 located on substrate 12 (i.e. the substrate 
must not extract so much heat from the catalytic reaction that there is no 
resulting temperature increase in resistor element 42). 
Located on substrate 12 and electrically connected to resistor element 42, 
are conductors 44 and 45. Conductors 44 and 45 are connected to circuitry 
(not shown) that can detect resistance changes from accompanying voltage 
drops along the length of resistor element 42. 
In FIG. 6, a cross section taken through resistor element 42 is depicted. 
Resistor element 42 can be deposited on substrate 12 using any 
conventional thick or thin film technique as long as the deposit is robust 
enough to withstand the environment of an auto exhaust system and the 
thermal coefficient of resistivity is high enough so that the resistor 
will respond to temperature changes from catalytic reactions. The material 
used to form resistor element 42 can be selected using these same 
criteria. In the preferred embodiment, it was found that platinum is a 
suitable material for resistor element 42 and that screen printing proved 
to be a suitable deposition method. 
Conductors 44 and 45 can likewise be deposited using any conventional thick 
or thin film technique. Gold was selected as the conductor material for 
the preferred embodiment. 
A layer of glass 46 is deposited over the resistor element 42. One way of 
forming glass layer 46 is to mix powdered glass with an organic solvent 
and screen print the mixture on the substrate. The glass layer can also be 
formed using a doctor blade or brushing the mixture on. The layer of glass 
46 is then dried but not fired yet. This provides a firm surface on which 
to deposit the catalytic support structure 50, but still enables the glass 
to act as an adhesion promoter when the structure is subsequently fired. 
The catalytic support structure 50 is comprised of high surface area 
particles such as powdered alumina. The particles can be calcined before 
applying them to the sensor structure to help assure that they have a high 
surface area for receiving a catalyst coating. The alumina particles can 
be combined with aluminum hydroxide or a similar substance to form a paste 
for application. The paste can be applied with thick film techniques such 
as screen printing. 
After catalytic support structure 50 is applied the entire assembly is 
fired at the proper firing profile for the glass employed. This will 
reflow the glass and cause it to firmly adhere to both the alumina 
particles and substrate 12. It is important that the glass bond very 
firmly to both the substrate and catalytic support because if the alumina 
particles flake off, the sensor will no longer function. In principal, any 
glass film formation, including many commercially available varieties such 
as GA-4 from Nippon Electric Glass, can be used as described above, 
provided it has the property of adhering to both substrate 12 and the 
catalytic support structure 50. A temperature of 700 degrees centigrade 
for 1 hour is sufficient to reflow the GA-4 glass. 
The final step is to apply a catalyst to catalytic support structure 50. In 
the preferred embodiment for a hydrocarbon sensor, platinum is used for 
the catalyst. The platinum is applied as a chloroplatinic acid solution 
using a dropper or other suitable technique. Afterwards the entire 
structure is again fired at about 500 degrees centigrade for 1 hour to 
reduce the acid to platinum. 
The final catalytic support structure, as shown by the enlarged view in 
FIG. 7, is comprised of alumina particles 54 adhered to glass layer 46. 
The particles vary in size and shape and the surface may include pores 56. 
When the chloroplatinic acid is applied and dried as described above, the 
surfaces of particles 54, including the surfaces of pores 56, will be 
covered by a very fine layer of platinum. 
In FIG. 8, a cross section taken through resistor element 42 is depicted. 
Resistor element 42 can be deposited on substrate 12 using any 
conventional thick or thin film technique as long as the deposit is robust 
enough to withstand the environment of an auto exhaust system and the 
thermal coefficient of resistivity is high enough so that the resistor 
will respond to temperature changes from catalytic reactions on the 
overlaid support structure. The material used to form resistor element 42 
can be selected using these same criteria. In the preferred embodiment, it 
was found that platinum was a satisfactory material for resistor element 
42 and that screen printing proved to be a suitable deposition method. 
As shown in FIG. 9 and 10, the catalyst support structure 64, comprises a 
mixture of alumina particles 74 and powdered glass 72. In the preferred 
embodiment, the mixture includes 20% LaRoche V700 alumina and 80% GA-4 
glass from Nippon Electric Glass. The alumina is calcined at about 600 
degrees Centigrade for 1 hour before it is added to the mixture. This 
helps assure that the alumina will have a high surface area for a catalyst 
coating. Sufficient screening agent is added to the mixture to obtain a 
paste like consistency. The screening agent used in the preferred 
embodiment is comprised of an organic solvent, a rheology modifying solid 
and a wetting agent. 
The mixture is deposited over the resistor element 42. Screen printing is 
one suitable method of depositing the mixture; although it can also be 
deposited using a doctor blade, brushing etc. After catalytic support 
structure 64 is applied, the entire assembly is heated at a temperature 
that will reflow the glass employed. A temperature of 700 degrees 
centigrade for 1 hour is sufficient to reflow the GA-4 glass 72, and cause 
it to firmly adhere to both the alumina particles 74 and substrate 12 as 
shown in FIG. 10. It is important that the glass bond very firmly to both 
the substrate and catalytic support because if the alumina particles flake 
off, the sensor will no longer function. 
The final step is to apply a catalyst to catalytic support structure 64. In 
the preferred embodiment for a hydrocarbon sensor, platinum is used for 
the catalyst. The platinum is applied as a chloroplatinic acid solution 
using a dropper or other suitable technique. Afterwards the entire 
structure is reheated at a temperature that is high enough to reduce the 
acid to platinum. A temperature of 500 degrees centigrade was used for the 
preferred embodiment. 
Alumina particles 74 vary in size and shape and the surface may include 
pores 76. When the chloroplatinic acid is applied and dried as described 
above, the surfaces of particles 74, including the surfaces of pores 76, 
will be covered by a very fine layer of platinum. Of course, some platinum 
will also adhere to the surfaces of glass 72. 
Operation of the Sensor 
The key to the operation of the sensor is the catalytic reaction of the gas 
to be sensed and the ability of the resistor element to respond to this 
reaction by a resulting change in its resistance. For example, as a 
hydrocarbon gas contacts the platinum catalyst, a chemical reaction occurs 
in which the hydrocarbon is combusted and heat is generated. The greater 
the quantity of hydrocarbons, the more heat is produced, thus causing the 
resistance of resistor element 42 to rise accordingly. The resistance of 
resistor element 42 is then compared to the resistance of a reference 
sensor (not shown), which is in the same environment and of the same 
design, except that it is not covered with a catalyst. The difference in 
the resistance between resistor element 42 and the reference sensor (not 
shown) is due to the heat generated by the catalytic reaction. The 
resistance difference indicates the concentration of hydrocarbons in an 
exhaust stream. 
Variations of the Preferred Embodiment 
Although the illustrated embodiments discuss the arrangement of the sensor 
and signal conditioning circuitry 14 to be on a single base, one skilled 
in the art will realize that the preferred embodiment would work with most 
any arrangement. For example, the signal conditioning circuitry 14 could 
be on a separate base, where the sensor element containing base is, for 
example, solder connected to the signal conditioning circuit containing 
base. Additionally, the second base containing the conditioning circuitry 
could also be a printed circuit board and not ceramic material like the 
sensor element base. 
Although, only nine designs for the sensor were illustrated in FIG. 3, one 
skilled in the art would be able to envision many variations. 
Additionally, even though the preferred embodiment discusses a horizontal 
and longitudinal axis, a skilled artisan would not be constrained by the 
descriptive wording of horizontal and longitudinal. In fact, the sensor 
may not even have an axis that is longer than the other, it could even be 
shorter, in which case the word longitudinal would be inaccurate. Of 
course, a skilled artisan would be able to use the preferred embodiment to 
detect numerous types of gases by using various catalysts and heating 
methods. 
Although the preferred embodiment discusses the location of the catalyzed 
sensor to be closest to the far end of the sensor, i.e. sensor 21, it is 
equally workable to have the catalyzed sensor to be located furthest away 
from the top of the sensor base, i.e. sensor 20 location. Thus, a reversal 
of the positions is often needed dependent upon the orientation of the 
overall gas sensor in the gas stream. In these variations, it is still 
possible to have the heated gas stream portion 29 not contact the 
reference sensor 20 or 21 dependent upon the orientation design. 
A further variation of the preferred embodiment is to have the longitudinal 
axis at most any angle to the gas stream that would allow rotation about 
the axis that would not have the catalyzed heated gas stream portion 
contact the reference sensor. This arrangement works especially well if 
the whole sensor were inserted into the gas stream at a right angle. 
However, for example, the sensor housing could be at an acute angle 
oriented any way in the gas stream. If oriented toward the gas stream, of 
course the catalyzed sensor would be located below the non-catalyzed 
sensor (i.e. further away from the one end of the base). In this 
arrangement, the sensor base 12 could be rotated about the longitudinal 
axis without having the heated gas stream potion contacting the reference 
sensor elements. 
While the invention has been taught with specific reference to these 
embodiments, someone skilled in the art will recognize that changes can be 
made in form and detail without departing from the spirit and the scope of 
the invention. The described embodiments are to be considered in all 
respects only as illustrative and not restrictive. The scope of the 
invention is, therefore, indicated by the appended claims rather than by 
the description. All changes that come within the meaning and range of 
equivalency of the claims are to be embraced within their scope.