Abstract:
A method of manufacturing an exhaust temperature sensor is disclosed. It includes forming a green ceramic substrate; and printing an electrical circuit on the green ceramic substrate. The method then contemplates trimming the electrical circuit to a predetermined resistance prior to firing the green ceramic. Finally, the method contemplates firing the green ceramic substrate with the electrical circuit thereon.

Description:
The present disclosure relates to vehicle exhaust gas temperature sensors. 
     BACKGROUND OF THE INVENTION 
     A typical modern automobile includes an engine control system that provides closed loop fueling control. The control loop can include feedback paths that provide information from a number of exhaust gas sensors. These sensors generate respective signals that represent a predetermined combination of exhaust gas temperature and oxygen level, fuel/air ratio, or the like. Each sensor may be mounted in a respective housing, which is in turn mounted in a respective hole or mounting boss that allows the sensor to access the exhaust gas. Some implementations mount more than one sensor within a housing. This reduces the costs associated with making and assembling multiple housings and mounting bosses. 
     Referring now to  FIG. 1 , a cross section is shown of an exhaust gas temperature sensor  10  that is constructed in accordance with the prior art. Temperature sensor  10  employs a resistive thermal device (RTD)  12  that generates the exhaust temperature signal. RTD  12  is positioned on an alumina base  14 . RTD  12  changes resistance based on the exhaust gas temperature. An engine control circuit senses the resistance and converts it back to an exhaust gas temperature. It is therefore important that the relationship between the resistance of RTD  12  and the exhaust gas temperature is known. 
     RTD  12  can be formed of platinum, palladium, and the like. Since the exhaust gas can reach temperatures greater than 1000 degrees Celsius, protection is needed for RTD  12 . Compounds in the exhaust gas can alter the resistance of RTD  12 , which causes the relationship between resistance and exhaust gas temperature to drift. A solution to this problem is to place an alumina cover  16  over RTD  12 . Alumina cover  16  blocks the exhaust gas compounds from reaching RTD  12 . Glass  18  bonds alumina cover  16  to RTD  12 . At high enough temperatures, glass  18  becomes permeable. The exhaust gas compounds may then diffuse through glass  18  to RTD  12 . In an environment that combines high temperature with lean exhaust gas, glass  18  in immediate contact with RTD  12  can cause the relationship between resistance and exhaust gas temperature to drift. 
     Referring now to  FIG. 2 , a cross section is shown of another embodiment of an exhaust gas temperature sensor  20  that is constructed in accordance with the prior art. RTD  12  is positioned on substrate  14 . Glass  22  is inked on and fired. Glass  22  seals only the sides of cover plate  16  to the sides of substrate  14 . This arrangement can sever the direct transport mechanism that exists between glass  18  and RTD  12  in the embodiment of  FIG. 1 . However, inking glass  22  to the outside edges of cover plate  16  can allow glass ink to seep underneath cover plate  16  and contact RTD  12 . The glass will then cause the relationship between resistance and exhaust gas temperature to drift just as in the embodiment of  FIG. 1 . 
     The embodiments of  FIGS. 1 and 2  both provide methods of using glass to bind a pre-fired alumina cover plate  16  to substrate  14 . In both of the embodiments described thus far, the glass meant to form a barrier from the exhaust gas can become soft or permeable at high temperatures. As a result, even the glass allows the contaminants from the exhaust gas to reach RTD  12 . Substitutes for the glass such as alumina ink, cannot generally be used to replace the glass ink because of a shrinkage mismatch; in order for alumina to be impermeable, it must first be sintered. 
     SUMMARY OF THE INVENTION 
     A method of manufacturing an exhaust temperature sensor is presented. The method includes forming a green ceramic substrate, printing an electrical circuit on the green ceramic substrate, and trimming the electrical circuit to a predetermined resistance prior to firing the green ceramic to form a trimmed pattern and placing at least one layer of green ceramic substrate over the trimmed pattern. Finally, the method contemplates firing the green ceramic substrate with the electrical circuit thereon. In a non-limiting embodiment, trimming the electrical circuit includes measuring a resistance of the electrical circuit and comparing the resistance to the predetermined resistance. The method then determines a relationship between the predetermined resistance and a resistance of the circuit after firing. 
     An exhaust temperature sensor is described. The sensor includes a green ceramic substrate and an electrical circuit printed on the green ceramic substrate. The electrical circuit includes an obstructed area formed by a portion that is trimmed away to give the circuit a predetermined resistance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the drawings. 
         FIG. 1  is a cross section of an exhaust temperature sensor in accordance with the prior art; 
         FIG. 2  is a cross section of another exhaust temperature sensor in accordance with the prior art; 
         FIGS. 3A-3E  are perspective views of exhaust temperature sensors at corresponding stages of manufacture, in accordance with the present invention; 
         FIG. 4  is a functional block diagram of an engine control system; and 
         FIG. 5  is a flowchart of a method that calibrates the exhaust temperature sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 3A-3E , where the invention will be described with reference to specific embodiments without limiting same, a process is shown that produces resistive thermal device (RTD) exhaust gas temperature sensors. The process eliminates the glass ink and associated problems that are found in the prior art and discussed above. The resultant sensors maintain accuracy at high temperatures. The process employs high temperature co-fired ceramic (HTCC) technology. 
     Referring now to  FIG. 3A , a first step of the process is shown. A plurality of RTD elements  50  is formed of green alumina tape with a printed electrical circuit  51  of platinum ink. Platinum ink printed electrical circuit  51  may be thin film or thick film. 
     The process then proceeds to  FIG. 3B  and laminates the ink side of RTD elements  50  with a polyester sheet  52 . In practice, the polyester sheet  52  currently used is a MYLAR brand polyester sheet. This lamination step presses the platinum ink circuit  51  into the green alumina tape prior to a laser trimming. Pressing the platinum ink circuit  51  reduces its resistance and minimizes its resistance change through the remainder of the process. The reduction in resistance can be greater than a factor of 15. 
     The process then proceeds to  FIG. 3C . In a conventional manner, an ohmmeter  54  is employed during a trimming process. Trimming cuts  53  are performed on green RTD elements  50  that form a trimmed pattern to increase the resistance of printed circuit  51  by creating an obstructed portion relative to the remainder of circuit  51 . The predetermined resistance is checked using ohmmeter  54 . While it will be appreciated that trimming cuts  53  may be performed by any method, the depicted trimming cuts  53  are made with a laser which removes a portion of platinum ink circuit  51 , and are normal to the surface of each of RTD elements  50 . Once the predetermined resistance is achieved, as determined by the ohmmeter  54 , trimming is complete. 
     It is important to note that the resistance changes when RTD elements  50  are fired. The relationship between the pre- and post-firing resistances is first experimentally determined. Once the desired post-fire resistance is known or specified, then the relationship to the prefired resistance is used to determine the predetermined resistance that is the objective when trimming the green RTD elements  50 . Using this closed loop trimming method effectively provides a close tolerance for the final fired product. 
     After trimming, RTD elements  50  are placed and laminated, as seen in  FIG. 3D . Specifically, RTD elements are laminated with other green sensor elements  60 . Obviously other green sensor elements  60  may be used depending on the specific end use application, including but not limited to, oxygen sensors, particulate matter sensors or lambda sensors. Green sensor elements  60  are laminated with an associated polyester sheet  62 . A first protective layer  64  and a second protective layer  66  are employed at the top and bottom, respectively, of the lamination stack. Depending on the application, one or both of protective layers  64 ,  66  may be eliminated. The lamination stack is pressed together and, as shown in  FIG. 3E , the individual sensors are singulated and fired. In one embodiment, a six-hour hold at 1450° C. has been found to provide a resistance that does not deviate from a predetermined resistance while the sensor is in its intended use. 
     Referring now to  FIG. 4 , a functional block diagram is shown of an engine control system  70 . Engine control system  70  includes a fired RTD  50  that was produced in accordance with the process that is shown in  FIGS. 3A-3E . A resistor R rtd    73  represents one of the plurality of RTD elements  50  formed by the process described hereinabove. 
     In the system  70  shown, an engine control module (ECM)  80  reads the resistance of R rtd    73  and the resistance of a resistor R tag    74  that is described below in more detail. ECM  80  also reads a third temperature sensor R temp    75 . Temperature sensor R temp    75  can sense any one of a number of engine component or fluid temperatures, including coolant temperature, intake air temperature, battery temperature, ambient air temperature, cylinder head temperature, exhaust gas temperature and others. ECM  80  also includes a timer  90 . Timer  90  and temperature resistor R temp    75  are employed by a method  100  that is described below in more detail. 
     Tag resistor R tag    74  indicates a correction factor for the relationship between exhaust gas temperature sensed and the resistance of R rtd    73 . Tag resistor R tag    74  is located within a housing  71  that also mounts R rtd  to the vehicle exhaust system. However Tag resistor R tag    74  may be located in any other suitable location. In either case, tag resistor R tag    74  should be positioned at a location that is out of the exhaust stream, since such positioning reduces resistance changes that will otherwise occur due to temperature changes. In addition, the location of tag resistor R tag    74  should be chosen so that it remains at as constant a temperature as possible, thus it may be desirable to remotely locate tag resistor R tag    74  from housing  71 , such as for example, in a wiring harness that attaches to housing  71 . Tag resistor R tag    74  can alternatively be formed within the same laminate stack as R rtd    73  by the process shown in  FIGS. 3A-3E . Tag resistor R tag    74  can also be trimmed and formed of a material that has a low thermal coefficient of resistance (TCR). 
     Both R rtd    73  and tag resistor R tag    74  provide a resistance signal to ECM  80 . ECM  80  compares the two signals and determines how much the R rtd    73  deviates from its predetermined resistance. When the resistance of tag resistor R tag    74  indicates a percentage difference between the predetermined and actual resistance of R rtd . ECM  80  compensates for the difference in resistances based on the relationship
 
 R   t   =R   200   /R   tag (1+ aT−bT   2 ), wherein
 
     R t  is the resistance for a PT200 RTD at the sensed temperature, R 200  is the predetermined resistance of R rtd    73  (for example 200 ohms at 0 deg. C.). R tag    74  is the multiplier representing the resistance R tag    74  deviates from the desired resistance, a and b are alpha and beta values, respectively, of ink  51  that was used to form R rtd    73 , and T is the measured temperature of exhaust gas temperature, as measured by R temp    75 . 
     Referring now to  FIG. 5 , a method  100  is shown that determines the relationship between the exhaust gas temperature and the resistance of resistor R rtd    73 . Method  100  waits for R rtd    73  and R temp    75  to reach a thermal equilibrium and then calibrates R rtd    73  based on the temperature that is indicated by R temp    75 . ECM  80  may execute method  100  at anytime, including a time prior to the vehicle engine being started. 
     The control sequence of method  100  begins at block  102  and immediately proceeds to a decision block  104 . At decision block  104 , ECM  80  is fed signals by a timer  90  to determine how long the engine has been shut off, i.e. not running. If the engine has been shut off less than a predetermined amount of time then ECM  80  uses the previously determined relationship from the most recent prior calibration, as indicated at control block  106 . If the ECM  80  is new and no relationship has been stored, then ECM will use a predetermined default relationship initially stored in control block  106 , instead. Thereafter, the control sequence ends at block  108 . 
     Alternatively, if the engine has been shut off for at least a predetermined time in decision block  104 , then the control sequence reads R temp , as indicated in control block  110  to determine the present ambient temperature. The control sequence then proceeds to block  112 , where the relationship between the resistance of R rtd  and the temperature of R rtd  is determined based on the assumption that both R rtd    73  and R temp    75  are thermally soaked and at the same temperature. The determined relationship is stored for future use in control block  106  via a feed-back signal. Control then thereafter the control sequence ends at block  108 . 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.