Abstract:
A single cell oxygen sensor apparatus and method are disclosed. An yttrium-based stabilized layer having electrical terminals connected to the yttrium-based stabilized layer can be provided on a substrate, wherein the yttrium-based stabilized layer is excitable by a constant current applied to the electrical terminals. A plurality of electrodes are located on a side of the yttrium-based stabilized layer and a plurality of heater elements located on said substrate opposite said yttrium-based stabilized layer. The heater elements can maintain the yttrium-based stabilized layer at a particular temperature. A cavity is formed and located between the yttrium-based stabilized layer and the heater elements. The partial pressure of oxygen can be measured by comparing the partial pressure of oxygen within the cavity with respect to the partial pressure of oxygen in the atmosphere external to the single cell oxygen sensor apparatus.

Description:
TECHNICAL FIELD 
     Embodiments are generally related to sensor devices and techniques. Embodiments are also related to pressure sensors. Embodiments are additionally related to O 2  pressure sensors. Embodiments are also related to sensors utilized in engine control applications. 
     BACKGROUND 
     Gas sensor devices are utilized in a number of sensing applications. Dual-cell gas sensors, for example, are frequently used to measure oxygen, which is particularly important in automobile and engine systems. One type of dual-cell gas sensor measures the concentration of a gas component in a first space comprising a sealed measurement space, of which at least one wall portion consists of a separation wall which exhibits ionic conduction and is in contact at least in part via the outer side with the first space. In this type of dual-cell gas sensor, a control unit can be utilized to periodically supply during a pumping time interval, a pumping current to the separation wall so that by means of an ion current in the separation wall the gas component is removed from the measurement space. 
     In this type of device, during a filling time interval a filling current can be supplied whose polarity is opposite to that of the pumping current so that the gas component is supplied to the measurement space. The dual-cell sensor includes a detection circuit which is connected to electrode layers on either side of the separation wall, the outer electrode layer of which is in contact with the first space. This detection circuit includes a first voltage detector, which supplies a filling interrupt signal for interrupting the filling current when the electrode voltage across the said electrode layers reaches a first reference value, and a second voltage detector which supplies a pumping interrupt signal for interrupting the pumping current when the electrode voltage reaches a second reference value. The electrical charge provided in the separation wall is a measure of the concentration of the gas component. Such a gas analysis apparatus is disclosed in U.S. Pat. No. 4,384,935 entitled “Gas Analysis Apparatus” which is incorporated herein by reference. 
     During the measurement of the electrical charge provided in the separation wall it is assumed that the separation wall, as to its impedance, acts as an electrical resistance so that this charge is to be measured outside the separation wall as supplied and removed charge or as a product of a current to be measured and a time interval to be measured or with constant currents as time intervals. 
     However, when the various parameters, such as the temperature, the volume of the sealed measurement space, the chosen measurement currents and the measuring range of the concentration to be measured, have such values that the measured time intervals become comparatively small, it is found that the measurement is strongly influenced by switch-on and switch-off transients. In other words, the separation wall is not a pure resistance. It can be derived from a theoretical consideration that the equivalent circuit diagram of the separation wall comprises besides resistances also capacitances, as a result of which RC time constants and stored capacitor charges are obtained. 
     Another type of dual-cell sensing device is disclosed in U.S. Pat. No. 4,545,889 entitled “Gas Analysis Apparatus” which describes a gas analysis apparatus for measuring the concentration of a gas component in a first space. The apparatus described in U.S. Pat. No. 4,545,889 includes a sealed measurement space, of which at least one wall portion consists of a separation wall which exhibits ionic conduction. The concentration of the gas component in the measurement space is changed periodically between two values by filling and pumping currents at the separation wall. The time intervals are measured and are a measure of the concentration. However, these time intervals comprise a “dead time” caused by switch-on and switch-off (both electrical and physical) transients. When given time intervals are combined by addition and subtraction, the influence of dead times can be considerably reduced using the device of U.S. Pat. No. 4,545,889. 
     Such dual-cell type sensor devices, however, are plagued with a number of problems. First, a typical dual cell pellet type configuration is designed for flue gas environment and must be redesigned for automotive applications. Thus, the use of dual-cell type sensing devices in automotive applications is very limited. The cost of such sensors is also extremely high, particularly in the context of automotive engine control applications. Dual cell sensors are also bulky and offer a slow response, which means that such devices need to be improved considerably for engine control applications. Additionally, these type of devices are fragile and utilize glass and/or ceramic seals, which are also delicate components, which means that the devices must be re-designed for automotive applications. 
     It is therefore believed that a solution to these problems lies in eliminating the dual-cell nature of such devices and completely re-designing a much simpler and efficient device, one which is based on the use of a single cell sensing element for oxygen sensing applications. Such a device and operating method are described in greater detail herein. 
     BRIEF SUMMARY 
     The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments and is not intended to be a full description. A full appreciation of the various aspects of the embodiments disclosed can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
     It is, therefore, one aspect of the present invention to provide for an improved pressure sensor. 
     It is yet another aspect of the present invention to provide for an improved O 2  pressure sensor. 
     It is a further aspect of the present invention to provide for an improved sensor for use in engine control applications. 
     It is an additional aspect of the present invention to provide for a single cell YSZ (Yttrium stabilized ZrO 2 ) sensor and method for forming and operating the same. 
     The aforementioned aspects of the invention and other objectives and advantages can now be achieved as described herein. A single cell oxygen sensor apparatus and method are disclosed. In general, an yttrium-based stabilized layer is formed on a substrate and includes electrical terminals connected. The yttrium-based stabilized layer is excitable by a constant current applied to the electrical terminals. A plurality of electrodes are located on one or more sides of the yttrium-based stabilized layer and a plurality of heater elements are located on said substrate opposite said yttrium-based stabilized layer. The heater elements can maintain the yttrium-based stabilized layer at a particular temperature. 
     A cavity can be formed and located between the yttrium-based stabilized layer and the heater elements. The cavity maintains an oxygen; however, the constant current applied to the electrical terminals results in an immediate evacuation of the oxygen from the cavity, which permits the partial pressure of the oxygen in the cavity to be measured by halting an excitation of the constant current utilizing a fixed resistance across the electrical terminals when a voltage across the yttrium-based stabilized layer attains a particular preset value. The voltage across the yttrium-based stabilized layer and ionic leakage of the oxygen through the yttrium-based stabilized layer then decreases, which permits a measurement of a voltage decay across the yttrium-based stabilized layer to be taken and the partial pressure of the oxygen in the cavity determined with respect to a partial pressure of the oxygen in an atmosphere external to the single cell oxygen sensor apparatus. The voltage decay across the yttrium-based stabilized layer is based on a time taken by a voltage across the yttrium-based stabilized layer to decay from a particular voltage value to another voltage value as a function of a different in a partial pressure of the oxygen between the cavity and the atmosphere. The yttrium-based stabilized layer can be provided as an YSZ (Yttrium stabilized ZrO 2 ) layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the principles of the disclosed embodiments. 
         FIG. 1  illustrates a side-view of a single cell oxygen sensor apparatus, which can be implemented in accordance with a preferred embodiment; 
         FIG. 2  illustrates a graph depicting a part of the electronic signal sequence for measuring O 2  partial pressure around the single cell O 2  sensor apparatus, in accordance with a preferred embodiment; 
         FIG. 3  illustrates a schematic circuit of a circuit representative of electrical activities associated with the single cell oxygen sensor apparatus depicted in  FIG. 1  and graph depicted in  FIG. 2 , in accordance with a preferred embodiment; 
         FIG. 4  illustrates a graph indicating the voltage across ZrO 2  when loaded with a fixed resistor, in accordance with a preferred embodiment; 
         FIG. 5  illustrates an equivalent circuit between a time T 2  and T 3  discharge cycle, in accordance with a preferred embodiment; 
         FIG. 6  illustrates a side-view of a single cell oxygen sensor apparatus, which can be implemented in accordance with an alternative embodiment; 
         FIG. 7  illustrates a graph associated with the operation of sensor apparatus of  FIGS. 1  and/or  6  and indicating that the time T is a function of the difference in O 2  partial pressures between the atmosphere and the cavity; and 
         FIG. 8  illustrates a high-level flow chart of operations of a method that can be followed in order to operate the sensor apparatus of  FIGS. 1  and/or  6 , in accordance with preferred or alternative embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope of the invention. 
       FIG. 1  illustrates a side-view of a single cell oxygen sensor apparatus  100 , which can be implemented in accordance with a preferred embodiment. The single cell oxygen sensor apparatus  100  depicted in  FIG. 1  includes a substrate  104  upon which a layer  102  of Yttrium stabilized ZrO 2  can be located. A cavity  106  is located and formed between the layer  102  and the substrate  104 . One or more porous platinum electrodes  130 ,  131 ,  132 ,  134 ,  136 ,  138 ,  140 ,  142 , and  144  can be formed on one side of the layer  102 , while another set of platinum electrodes  146 ,  148 ,  150 ,  152 ,  154 ,  156 ,  158 ,  160 , and  162  can be formed on the other side of the layer  102 . 
     A group of heater elements  164 ,  166 ,  168 ,  170 ,  172 ,  174 ,  176 ,  178 , and  180  can be formed on a side of the substrate  104  opposite the layer  102  as depicted in  FIG. 1 . A first terminal  110  can be located on substrate  104  in additional to a second terminal  108 . An electrical connector  109  can be utilized to electrically connect terminal  110  to one or more of the electrodes  130 ,  131 ,  132 ,  134 ,  136 ,  138 ,  140 ,  142 , and  144  associated with the layer  102 . The substrate  104  can be preferably formed from a material such as, for example, 2MgOSiO 2 , depending upon design consideration. 
       FIG. 2  illustrates a graph  200  depicting a part of the electronic signal sequence for measuring O 2  partial pressure around the single cell O 2  sensor apparatus  100 , in accordance with a preferred embodiment. Graph  200  illustrates a representative curve  206 , which represents the voltage across ZrO 2  when excited by a constant current. Graph  200  includes a vertical axis  202  indicative of voltage and a horizontal axis  204  indicative of time. A first voltage  210  or V 1  and a second voltage  208  or V 2  is shown along the vertical axis  202 . A second time  212  or T 2  is shown on the horizontal axis  204 . The voltage difference  203  or V represents the voltage between the first voltage  210  or V 1  and the second voltage  208  or V 2 . 
       FIG. 3  illustrates a schematic circuit of a circuit  300  representative of electrical activities associated with the single cell oxygen sensor apparatus  100  depicted in  FIG. 1  and graph  200  depicted in  FIG. 2 , in accordance with a preferred embodiment. Note that in  FIGS. 1-7  illustrated herein, identical or similar parts or elements are generally indicated by identical reference numerals. Thus, circuit  300  includes a constant current source (e.g., 40 μAmps) that is electrically in parallel with the YSZ layer or cell  102  and a resistor  304  or R z . Voltage  203  across the layer or cell  102  is also depicted in circuit  300 , which represents an equivalent circuit of the apparatus  100 . 
     In general, the YSZ layer or cell  102  can be excited using the constant current source (DC)  302  through the first and second terminals  110  and  108  in order to pump all O2 from the cavity  106  to the atmosphere (external to the apparatus  100 ). This “pumping” can be accomplished by monitoring the voltage across the first and second terminals  110  and  108 . The voltage across the YSZ layer  102  varies when excited by a constant current and temperature as shown in graph  200  if  FIG. 2 . At the time T 2  where the voltage is V 2 , the O 2  in the cell will be as minimal as possible as it is excited in order to be pumped out of cavity  106 . Circuit  300  represents the equivalent electrical circuit of apparatus  100 . 
       FIG. 4  illustrates a graph  400  indicating the voltage across ZrO 2  when loaded with a fixed resistor, in accordance with a preferred embodiment. Graph  400  is similar to graph  300 , the difference being the inclusion of data per the addition of a fixed resistor. Thus, in addition to the parameters indicated in graph  200  of  FIG. 2 , the graph  400  of  FIG. 4  includes third and fourth voltages  402  and  404  (respectively V 3  and V 4 ) and second and third times  212  and  410  (respectively T 2  and T 3 ). Time  408  (T) depicted in graph  400  represents the time that is proportional to the O 2  partial pressure. Additionally, the curve  406  of graph  400  indicates that decay depends on leakage current (i.e., the load resistance). 
       FIG. 5  illustrates an equivalent circuit  500  between a time T 2  and T 3  discharge cycle, in accordance with a preferred embodiment. Circuit  500  represents an equivalent circuit of apparatus  100 .  FIG. 5  includes a voltage source  504  (i.e., v=V 2 ) in parallel with resistor  304  (R z ) and a load resistor  502  (R L ). The resistor  304  (R z ) represents the YSZ internal resistance at a particular temperature and the load resistor  502  (R L ) is the external load resistor to drive current. 
     Thus, at the pre-defined value of voltage (V 2 ), the constant current is switched off and a fixed resistor is connected across the first and second sensor terminals  110  and  108 , which will act to induce ionic/electrical leakage and thus the voltage across the sensor element drops down as shown in graph  400  of  FIG. 4 . The equivalent circuit at this condition is as shown in circuit  500  of  FIG. 5 . 
     The decay time from V 3  to V 4  (i.e., Time (T 3 -T 2 )) is the time proportional to the O 2  partial pressure difference between the cavity  106  and the surrounding area of the sensor  100 . If the O 2  Partial pressure inside the cavity  106  is zero or close to zero at time T 2  (i.e. when the voltage across the sensor is V 2 ), then the signal is proportional to the O 2  partial pressure around the sensor element or YSZ cell  102 . 
       FIG. 6  illustrates a side-view of a single cell oxygen sensor apparatus  600 , which can be implemented in accordance with an alternative embodiment. The sensor apparatus  600  depicted in  FIG. 6  is similar to that of the sensor apparatus  100 , with slight differences. For example, electrodes  146 ,  148 ,  150 ,  152 ,  154 ,  156 ,  158 ,  160 , and  162  are illustrated in  FIG. 6 . As indicated in  FIG. 6 , a heater  602  is associated with cavity  106 . The heater  602  can be composed of, for example, one or more group of heater elements  164 ,  166 ,  168 ,  170 ,  172 ,  174 ,  176 ,  178 , which were depicted earlier with respect to  FIG. 1 . Arrows  604  and  602  respectively represent oxygen evacuation and heat pumped into cavity  106 . 
       FIG. 7  illustrates a graph  700  associated with the operation of sensor apparatus  600  and indicating that the time T is a function of the difference in O 2  partial pressures between the atmosphere and the cavity  106 . The following parameters and variable apply to the illustration depicted in  FIG. 6  and graph  700  of  FIG. 7 :
         YSZ—Yttrium stabilized ZrO 2      Pt—platinum electrodes   P O2  (atm)—Partial pressure of O 2  in atmosphere   P O1  (cavity)—partial pressure of O 2  in cavity   Tp 1  and Tp 2  are terminals across YSZ       

       FIG. 8  illustrates a high-level flow chart of operations of a method  800  that can be followed in order to operate the sensor apparatus  100  and sensor apparatus  600 , in accordance with preferred or alternative embodiments. The process can begin, as indicated at block  802 . As depicted thereafter at block  804 , the YSZ layer  102  can be maintained at an elevated temperature (e.g., approximately 700 Deg C., but this depends on the size, temperature, type, etc. of the sensor element). Next, as described at block  806 , the YSZ layer or cell  102  can be excited by applying a constant current of 40 mA (depends on the size, temperature, type, etc. of the sensor element) or more or less across terminal Tp 1  and Tp 2  such that the O 2  in the cavity is evacuated completely. The completeness can then be determined as indicated at block  808  by measuring the voltage across YSZ layer or cell  102  (i.e. across terminals Tp 1  and Tp 2 ). 
     Thereafter, as indicated at blocks  810  and  812 , when the voltage across the YSZ layer or cell  102  reaches a particular preset value V 1  (e.g., 500 mV—fully evacuated condition—see  FIG. 2 ), the excitation is halted through a constant current. The YSZ element can be loaded with a fixed resistor across its terminal sTp 1  and Tp 2  as indicated at block  814  so that the current flow, the O 2  leak through YSZ layer or cell  102  (ionic leakage) and the voltage across the YSZ layer or cell  102  decreases. Next, as indicated at block  816 , the voltage decay across the YSZ layer or cell  102  can be measured. The time taken by the voltage to decay from V 2  to V 3  (e.g., see  FIG. 2 ) is a function of the difference in O 2  partial pressure between the cavity  106  and the atmosphere. 
     Next, as depicted at block  810 , if the partial pressure of O 2  in the atmosphere is greater than the partial pressure of O 2  in cavity  106 , the time taken to leak is larger, and if the partial pressure of O 2  in atmosphere is smaller or equal to the partial pressure of O 2  in cavity  106 , the time taken to leak is less. The process can then terminate, as indicated at block  820 . 
     It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.