Abstract:
An exhaust gas sensor is provided and formed by attaching the sensor&#39;s upper shield and shell. The attachment is attained by bending a protruding segment or lip over a terminal end portion of the lower shield. This produces a single sealing surface and eliminates the requirement of a conventional crimp which places high compressive forces on the sensing element..

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to exhaust gas sensors. More particularly, the present invention relates to an exhaust gas sensor with a crimp design.  
         BACKGROUND OF THE INVENTION  
         [0002]    Exhaust gas sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, exhaust gas sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases, for example, to sense when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the exhaust sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.  
           [0003]    A conventional stoichiometric sensor typically consists of an ionically conductive solid electrolyte material, a porous electrode on the sensor&#39;s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor&#39;s interior surface exposed to a known gas partial pressure. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of oxygen present in an automobile engine&#39;s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:  
       E   =       (       -   RT       4      F       )          ln        (       P     O   2     ref       P     O   2         )                         where   :                            E   =                electromotive                 force                 R   =                universal                 gas                 constant                 F   =                Faraday                 constant                 T   =                absolute                 temperature                 of                 the                 gas                   P     O   2     ref     =                oxygen                 partial                 pressure                 of                 the                 reference                 gas                   P     O   2       =                oxygen                 partial                 pressure                 of                 the                 exhaust                 gas                                 
 
           [0004]    Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture.  
           [0005]    As taught by U.S. Pat. No. 4,863,584 to Kojima et al., U.S. Pat. No. 4,839,018 to Yamada et al., U.S. Pat. No. 4,570,479 to Sakurai et al., and U.S. Pat. No. 4,272,329 to Hetrick et al., a sensor which operates in a diffusion limited current mode produces a proportional output which provides a sufficient resolution to determine the air-to-fuel ratio under fuel-rich or fuel- lean conditions. Generally, diffusion limited current sensors have a pumping cell and a reference cell with a known internal or external oxygen partial pressure reference. A constant electromotive force, typically corresponding to the stoichiometric electromotive force, is maintained across the reference cell by pumping oxygen through the pumping cell. The magnitude and polarity of the resulting diffusion limited current is indicative of the exhaust oxygen partial pressure and, therefore, a measure of air-to-fuel ratio.  
           [0006]    Where a gas-diffusion-limiting means is added to an oxygen pump, the pump current can be limited, and the limiting current is linearly proportional to the absolute value of the equilibrium oxygen concentration of the exhaust gas. In lean condition, the equilibrium oxygen concentration is larger than zero, which indicates a surplus of oxygen, and oxygen needs to be pumped out of the exhaust gas to create a limiting current. In the rich condition, the equilibrium oxygen concentration is smaller than zero, which indicates depletion of oxygen, and oxygen needs to be pumped into the exhaust gas to create a limiting current. Therefore, using the absolute value and the polarity of the limiting current, one can determine the air-to-fuel ratio of the exhaust gas.  
           [0007]    However, an oxygen pump cell will not switch its current polarity automatically if both pump electrodes are exposed to the same exhaust gas. Conventional sensor technology either uses an air reference electrode as one of the pump electrodes or utilizes an air reference electrode as a third electrode to detect the lean or rich status of the exhaust gas (by emf mode) and to switch the current polarity accordingly. In this way, wide range air-to-fuel ratios of the exhaust gas can be determined.  
           [0008]    Such conventional sensors use two types of air reference electrodes. The first type has a sizable air chamber to provide oxygen from an ambient air supply to the reference electrode (breatheable air reference). However, to avoid contamination by the exhaust gas, the air chamber requires a hermetic seal sensor package, which is expensive and is problematic in field applications. The second type is a pumped-air reference electrode. It uses a pump circuit to pump oxygen from the exhaust gas to the reference electrode. As such, it does not require a sizable air chamber connected to ambient air.  
           [0009]    One known type of exhaust sensor includes a flat plate sensor formed of various layers of ceramic and electrolyte materials laminated and sintered together with electrical circuit and sensor traces placed between the layers in a known manner. The flat plate sensing element can be both difficult and expensive to package within the body of the exhaust sensor since it generally has one dimension that is very thin and is usually made of briffle materials. Consequently, great care and time consuming effort must be taken to prevent the flat plate sensing element from being damaged by exhaust, heat, impact, vibration, the environment, etc. This is particularly problematic since most materials conventionally used as sensing element supports, for example, glass and ceramics, typically have a high modulus of elasticity and cannot withstand much bending. Hence, great care and expense is expended in preventing manufacturing failures.  
           [0010]    Accordingly, there remains a need in the art for a low cost, temperature resistant sensor package having an improved assembly and design.  
         SUMMARY OF THE INVENTION  
         [0011]    The problems and disadvantages of the prior art are overcome and alleviated by the dead headed sealed air reference sensor and method preventing contamination of a sensor. The exhaust gas sensor comprises an upper shield having an upper shield first end and an upper shield second end; an inner shield positioned with a portion of the upper shield second end; a shell having a shell first end and a shell second end positioned about a portion of the inner shield, the shell first end has a projecting edge spaced apart from the inner shield, wherein a segment is formed between the projecting edge and the inner shield; a crimp formed from a bent portion of the projecting edge of the shell about a terminal end portion of the inner shield; a lower shield affixed to the shell second end; and a sensor element extending through and within the lower shield, the shell, and the upper shield.  
           [0012]    The method of forming an exhaust gas sensor, comprises providing an upper shield having an upper shield first end and an upper shield second end; positioning an inner shield within a portion of the upper shield second end; placing a shell having a shell first end and a shell second end positioned about a portion of the inner shield, the shell first end has a projecting edge spaced apart from the inner shield, wherein a segment is formed between the projecting edge and the inner shield; forming a crimp by bending a portion of the projecting edge of the shell about a terminal end portion of the inner shield; affixing a lower shield to the shell second end; and extending a sensor element through and within the lower shield, the shell, and the upper shield.  
           [0013]    The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The present invention will now be described by way of example with reference to the following Figure, which are meant to be exemplary, not limiting, and in which:  
         [0015]    [0015]FIG. 1 is a cross-sectional side view of one embodiment of an exhaust sensor of the present invention utilizing a crimp.  
         [0016]    [0016]FIG. 2 is a prior art cross-sectional side view of an exhaust sensor utilizing a standard crimp. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    Sensor shields and shells are typically crimped to adjoin the sections to form completed exhaust sensors. An exhaust sensor disclosed herein advantageously utilizes a crimp design. Typically, exhaust sensors are constructed to endure an exhaust gas environment while protecting the sensor components. Because of the sensor component fragility, the manufacturing process can be difficult and expensive. To preserve the components, a shield and shell are formed together to form a unitary sensor. To maintain a unitary structure, shields and shell are commonly adjoined together by, for example, crimping, welding, and/or adhesives.  
         [0018]    Referring now to the FIG. 1, an exemplary exhaust sensor  10  is shown employing a crimp  58  of upper shield  20  to shell  50  in accordance with the present invention. Exhaust sensor  10  includes a housing structure generally formed of an upper shield  20  disposed above a shell  50 . A lower shield  30  is disposed beneath shell  50 . Inner shield  40  is optionally disposed within a lower portion of upper shield  20  and shell  50 . A terminal connector  60  and a portion of a sensing element  80  are disposed within upper shield  20 . Sensing element  80  is a pumped-air reference exhaust sensing element of a known type with any conventional geometry, such as a generally flat elongated rectangular shape. At a first end  82  thereof, sensing element  80  includes an exhaust constituent- responsive structure fabricated into sensing element  80  in a known manner, preferably along with a heater (not shown) of a known type.  
         [0019]    Exhaust sensor  10  advantageously utilizes a crimp  58  to adhere upper shield  20  to shell  50 . Crimp  58  is comprised of a first end  53  of shell  50  being disposed proximate to lower end  22  of the upper shield  20 . First end  53  of shell  50  is formed as a projecting edge or lip of material spaced apart from inner shield  40  and shell inner edge  51  to form segment  59 . Segment  59  is shown as a flat length, disposed substantially perpendicular to the length of upper shield  20 , but can also comprise any angle. Lower end  22  of the upper shield  20  is formed so that a terminal end portion of lower end  22  is formed to extend at an angle from the main axis of sensing element  10  (i.e., at an angle from the sensing element  80 ), with a substantially perpendicular extension of the terminal end preferred. This terminal end portion of lower end  22  is then placed juxtaposition to segment  59  to rest thereupon. Segment  59  need be of sufficient length so that when crimp  58  is formed, it will engage the terminal end portion and hold it securely within the wrapped lip of first end  53 . Optionally, a gasket  25 , for example an S-gasket, can be used between the wrapped lip end of first end  53  and the terminal end of lower end  22 . Gasket  25  can be formed as a separate piece of any suitable material for an exhaust environment. Alternatively, gasket  25  can be formed from the terminal end of lower end  22  by having the terminal end wrapped in and upon itself as a coiled, rolled, or layered section, or the like.  
         [0020]    This crimping arrangement negates the need for a high pressure crimp of upper shield  20  to inner shield  40  and of shell  50  to inner shield  40  and creates only one sealing surface. For example, a prior art high pressure crimp required a pressure directed inward toward the sensor element of about 2,000 pounds per square inch (p.s.i.) whereas the crimping arrangement of the present invention uses a vertical pressure, i.e., pressure applied in a parallel direction to the sensor element, sufficient to dispose first end  53  inward, e.g., a pressure of about 10,000 p.s.i. to about 20,000 p.s.i., and more preferably between about 14,000 to about 16,000 p.s.i. As to the one sealing surface, this is formed between upper shield  20 , shell  50 , and inner shell  40 . Whereas in the prior art, two sealing surfaces were created where shell  50  met inner shield  40  and where upper shield  20  met shell  50  (See FIG. 2).  
         [0021]    Sensor  10  can be formed by known manufacturing techniques with the exception that upper shield  20  and components therein are mated to shell  50  and components therein so that the terminal end portion of lower end  22  is positioned juxtaposition to segment  59 . Thereafter, a force is applied so that the wrapped lip of first end  53  is formed. This is particularly advantageous because the force used to form the wrapped lip is not directed inward towards sensor element  80 . Instead, the force is directed in an approximately parallel or angled direction to the length of sensor element  80 .  
         [0022]    Referring to prior art FIG. 2, first end  53  rests flatly against inner shield  40  to meet or about meet the terminal end portion of lower end  22 . Hence, a portion of inner shield  40  may be directly exposed to the exterior environment. The prior art sensor first end  53  does not form a projecting edge or lip of material to engage the terminal end of lower end  22 . Therefore, the prior art assembly utilizes inward pressure crimps  85  to attach and retain upper shield  20  to inner shield  40  and to attach inner shield  40  to shell  50 . The use of high pressure inward crimps by the prior art endanger the fragile sensor element  80  and thus increases the chance for failure and scrap due to the higher stresses placed upon the sensor.  
         [0023]    Again referring to FIG. 1, as to the remaining structure of sensor  10 , shell  50  includes a body portion  52  and a threaded portion  54  at a second end  55 . Body portion  52  is preferably shaped to accommodate a wrench or other tool for tightening threaded portion  54  into a mount for an exhaust pipe or other component of an exhaust flow system enabling a sensor chamber  31  located within lower shield  30  to be located within a flow of exhaust gasses to be measured. Additionally, shell  50  is securely disposed around inner shield  40  and holds inner shield  40  via a compressive force engagement. Formed at second end  55  of shell  50  is a shoulder  56  for contacting first end  42  of inner shield  40 , whereby inner shield  40  rests against shoulder  56  when shell  50  is secured to inner shield  40  during assembly.  
         [0024]    At a second end  84  of sensing element  80 , lower ends  104  and  106  of terminals  100  and  102 , respectively, contact external pads (not shown) on end  84  to provide electrical connection between terminals  100  and  102  and sensing element  80 . Ends  104  and  106  of terminals  100  and  102 , respectively, are maintained against second end  84  of sensing element  80  by a compressive force applied by disposing second end  84  of sensing element  80  between lower ends  104  and  106 . Preferably, terminals  100  and  102  comprise spring terminals, as is known in the art, such that the compressive force generated by disposing second end  84  between spring terminals  100  and  102  securely maintains end  84  in electrical contact therewith. While spring terminals are disclosed herein, other known terminals that allow an electrical connection may be used.  
         [0025]    Adjoined and partially encased by a bottom portion of upper shield  20 , inner shield  40  has a first end  42  and a preferably partially closed second end  44  opposite first end  42 . A centrally located annular opening  46  is provided at second end  44  and is sized to allow insertion of element second end  84  of sensing element  80  therethrough. Disposed within inner shield  40  is a central portion  83  of sensing element  80 , and a high temperature material  90 . Optionally, a pair of thermal insulating members (not shown) may be disposed against the sensing element  80  for additional support as is known in the art.  
         [0026]    To allow an electrical connection of sensing element  80 , a terminal connecter  60  can be used. The use of terminal connector  60  is known in the art and a suitable terminal connector  60  is also known in the art as an edge card connector, a clam shell connector, or the like. Terminal connector  60  typically includes a plurality of electrical terminals with each having a corresponding electrical wire connected thereto.  
         [0027]    For the purpose of illustration only, sensor  10  is shown having a pair of electrical terminals  100  and  102 , which are adapted to be connected to electrical wires  120  and  130  in a known manner. Electrical wires  120  and  130  pass through cable seal  140 , which generally comprises an elastomeric material suitable for use in a high temperature environment, e.g., spark ignition engine, without failing. Cable seal  140  is maintained in place by upper shield  20 , which has an upper end  23  forming a seal around a shoulder  142  of cable seal  140 , wherein upper shield  20  can be crimped in place around cable seal  140  to further secure the same. A central portion  24  of upper shield  20  is disposed around terminal connector  60  while a lower end  22  of upper shield  20  forms an opening preferably tightly fit around inner shield second end  44  when sensor  10  is assembled. Generally, the upper shield  20  has a geometry complimentary with the inner shield geometry, such as cylindrical, elliptical, multi-sided, or the like.  
         [0028]    In a generally preferred configuration, lower shield  30  is securely coupled to shell  50  by engaging flared open end  32  of lower shield  30  with annular recess  57 . Shell  50  is itself securely coupled to upper shield  20  and thereby to optional inner shield  40  which is further secured by shoulder  56 . Consequently, sensing element  80  is disposed through inner shield  40  with a first end  82  extending within sensing chamber  31 . Lower shield  30  defines sensing chamber  31  and disposed within lower shield  30  can be an internal shield  35 , which has an open end  36  for receiving sensing element  80  and a closed end  37  adjacent and parallel to closed end  34  of lower shield  30 . Lower shield  30  and internal shield  35  incorporate a plurality of apertures on lower shield  38  and on inner shield  40  for allowing passage of exhaust gas in and out of sensing chamber  31  so that the gasses may be sensed by receptive first end  82  of sensing element  80 .  
         [0029]    Extending from first end  42  to partially closed second end  44 , a high temperature material  90  can be concentrically disposed around sensing element  80 . As used herein, the term “high temperature material” refers to materials that are designed for use in a spark ignition engine environment, where temperatures range up to about 1,000° C. Such materials include ceramic fibrous materials, and/or metal mesh, among others. When a ceramic fibrous material is used, the orientation and size of the ceramic fibers are not critical to the practice of the present invention. High temperature material  90  may be installed in either a preform or fibrous blanket type state around at least a portion of sensing element  80  as is known in the relevant arts.  
         [0030]    Exhaust erosion of high temperature materials  90  and terminal connector  60  may be prevented in a particularly advantageous embodiment, which further comprises a disk supporting device and/or a metal mesh support, distinct from the high temperature material. These supports are capable, individually or in tandem, of providing secure support of the sensing element in the weak axis direction, and of preventing excessive exhaust erosion of sensitive sensor components.  
         [0031]    The, disk element support  170  is positioned between partially closed second end  44  of inner shield  40  and mat  90 , concentrically around sensing element  80 . Disk element support  170  may also (or alternatively) be positioned between shoulder  56  of shell  50  and mat support  90 . Also, an aperture is provided therein, through which the sensing element  80  may be inserted.  
         [0032]    Disk element support  170  is made of a material compatible with the environmental conditions of the sensor. Specifically, the disk element support  170  is capable of maintaining structural integrity in a high temperature environment (up to about 1,000° C.). Exemplary materials include metal, ceramic, talc, composites, combinations combining at least one of the foregoing and others compatible with the sensor environment.  
         [0033]    The mesh is typically located between high temperature material  90  and sensing chamber  31 . The mesh can be made from fine wire, impregnated with a filler material, e.g. clay, talc, or the like, to fill the space between the mesh fibers, and compressed into desired form.  
         [0034]    Wire material may be made of any metal, however, metals with high nickel or chrome content are preferred due to their rust resistant properties. Particularly preferred metals include  310 ,  309 , and  316  stainless steels. Suitable thickness for fine wire material used as a mesh element support is about 0.2 to about 1.2 millimeters, with about 0.4 to about 0.6 millimeters being preferred. Preferred wire densities are about 20% to about 50% of the solid density, with the filler material making up the difference, giving a solid density of about 50% to about 70%.  
         [0035]    As to the sensor&#39;s other materials, exemplary materials for the shields  20 ,  30 ,  40 , and  35  and for the shell  50  include high chrome, high nickel stainless steel, or mixtures thereof, and the like, with all steels chosen for high temperature endurance, high-strength and corrosion resistance. Terminal connector  60  may be formed of a thermoplastic or thermoset material (e.g., plastic) or ceramic durable in the high temperature environments to which exhaust sensor  10  is exposed.  
         [0036]    The present invention describes a new sensor and upper shield design and method of making the same. The crimp formed between upper shield  20  and shell  50  reduces the amount of pressure directed inward towards sensor element  80  during production. With the reduced stress, the sensor formed is less likely to fail and/or be scrapped. Furthermore, a more robust and simplified product can be produced that is less likely to leak from the adjoining of upper shield  20  to shell  50 . This is because the crimp forms only a single sealing surface between upper shield  20  and shell  50 , reducing potential leak points.  
         [0037]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.