Abstract:
A circuit for improving the resolution of an oxygen sensor in a vehicle exhaust system. The circuit expands a limited output voltage range of an oxygen sensor to full voltage range of an analog-to-digital (A/D) converter, prior to input of the expanded signal into the A/D converter. Utilization of the full range of converter provides improved resolution for analyzing the analog signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to sensor measurements in automobile control systems and, more particularly, to a system for enhancing the precision of an analog sensor reading in an automobile control system. 
     2. Discussion of Related Art 
     Current automobile engines are internal combustion engines that use a mixture of fuel and air to generate their driving power. Complete fuel combustion produces only carbon dioxide and water as its products; however, the conditions within an engine do not correspond to the idealized requirements necessary to produce complete combustion. Incomplete combustion produces other products that may include: carbon monoxide, hydrogen gas, hydrocarbons, nitrogen gas, oxygen gas, and various nitrous oxides. Some of these gases are commonly found in the atmosphere and pose few or no health risks. Others can be toxic, creating a desire to reduce such toxic emissions. 
     The United States and many other countries have strict standards regulating the emissions from automobiles. Catalytic converters transform toxic chemicals into safer compounds. They convert CO, H 2 , and HC into CO 2  and H 2 O and also convert nitrous oxides into nitrogen gas and oxygen gas before these gases are emitted from the automobile. Catalytic converters, however, do not completely convert toxic byproducts of incomplete combustion into less harmful substances before emission into the atmosphere. The higher the efficiency of the catalytic converter, the more toxic gases are converted into safer forms before they are emitted into the atmosphere. The efficiency of a catalytic converter relates directly to the composition of its intake gases, and the composition of the intake gases is determined by the combustion conditions, including the air/fuel mixture ratio input to the engine. 
     The mixture of fuel and air used in the combustion chamber of an engine is regulated through a feedback mechanism. A sensor is placed in the exhaust manifold to measure the oxygen content in the expunged gases. The oxygen content of the combusted mixture varies with respect to where the engine is operating in relation to the stoichiometric point. Typically, the operating point of the engine is called the stoichiometric air/fuel ratio, and this corresponds to the point where the exact quantity of fuel needed for completed combustion is added to the air flow. The stoichiometric point yields the most efficient catalyst operation and produces the least amount of toxic byproducts. The varying operating characteristics of the vehicle change the efficiency of the combustion process and require altering the current fuel flow to maintain engine operating at or near the stoichiometric point. The oxygen sensor output enables optimization of the fuel-air ratio fed into the engine. Optimizing the fuel-air mixture entering the engine changes the combustion conditions and achieves more complete combustion, thereby operating the engine closer to the stoichiometric point. 
     Oxygen sensors used in most vehicles provide a voltage output that varies in accordance with the amount of oxygen in the combustion product. An analog-to-digital (A/D) converter receives the oxygen sensor output and generates a digital value input to a digital microprocessor. The microprocessor controls the air/fuel ratio and constantly adjusts the mixture entering the combustion chamber in order to maintain the engine operating near the stoichiometric point. Constant adjustment is required because changing engine and environmental conditions alter the efficiency of the combustion process, even for a constant fuel-air mixture ratio. The voltage output of the oxygen sensor varies with the amount of oxygen found in the combustion products. 
     A typical oxygen sensor functions as a switching device. The switching device outputs less than 0.25 volts when the input air/fuel ratio to the engine is leaner than stoichiometric and outputs greater than 0.75 volts when input air/fuel ratio to the engine is richer than stoichiometric. Due to the physics of the chemical reaction within the oxygen sensor, output voltages are typically limited to less than 1.0 volts. 
     In the area of ±1 percent of stoichiometric, the output waveform is very steep. In the area outside ±1 percent of stoichiometric, the output waveform is nearly flat. Within the area of ±1 percent of stoichiometric, minor changes in the oxygen content found in combustion products result in significant changes in the output voltage of the oxygen sensor. Conversely, outside of ±1 percent of stoichiometric, even significant changes in the oxygen content in the combustion products result in predictably small changes in the output voltages of the oxygen sensor. The steep characteristic of the oxygen sensor in the stoichiometric region makes measuring the prevailing operating point difficult. 
     As discussed above, most controllers utilize a A/D converter to covert the analog output voltage of the oxygen sensor into a digital value for use by an electronic engine controller. A typical A/D converter converts a voltage range that varies between 0 and 5 volts into an 8-bit digital value for use by the engine controller. An 8-bit digit value can vary between 0 and 255, yielding 256 gradations or counts. The 256 counts in the typical A/D converter translate into approximately 0.0196 volts per count. Because the normal output of the oxygen sensor varies between a voltage range of 0 to 1 volts, only counts 0 to 51 of the 256 possible counts are utilized to determine the value of the analog signal received from the oxygen sensor. Thus, only approximately 20 percent of the total range of the A/D converter is utilized. This limited resolution reduces the level of oxygen sensor output detail input to the engine control system. This reduced resolution is particularly important in the critical zone around stoichiometric where minor variations in the oxygen content of the combusted products result in large variations in the voltage output by the oxygen sensor. 
     Thus, there is a need to improve the resolution of the oxygen sensor signal applied to the A/D converter in the engine control system. 
     SUMMARY OF THE INVENTION 
     A control system for regulating the fuel and air mixture used in an engine. The control system includes an engine producing drive power through combustion of fuel and air. An analog sensor connected to the engine monitors a concentration of gases produced through the combustion of fuel and air in the engine. The analog sensor generates an analog signal that varies in accordance with the concentration. The output signal is within a first predetermined voltage range. An amplifier receives the analog signal and amplifies the analog voltage to generate an amplified signal. The amplified signal is within a second predetermined voltage range, wherein the second voltage range is greater than the first voltage range. An analog-to-digital (A/D) converter receives the analog signal and generates a digital signal that varies in accordance with the amplified signal. The A/D converter converts input voltages varying within the second voltage range. A microprocessor receives the digital signal from the A/D converter and produces a mixture signal that varies in accordance with a desired fuel and air mixture, wherein the desired mixture varies in accordance with the analog signal. 
     These and other advantages and features of the present invention will become readily apparent from the following detailed description, claims and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the exhaust and air/fuel control system of an automobile; 
     FIG. 2 is a graph of the relative equivalence ratio in the combustion exhaust versus voltage output by the oxygen sensor; 
     FIG. 3 is a block diagram of the analog-to-digital converter and microprocessor of the controller of FIG. 1; and 
     FIG. 4 is a circuit diagram of the amplifier of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to the precision enhancement of reading an analog sensor in an automobile control system is merely exemplary in nature and is in no way intended to limit the invention or its applications or uses. 
     FIG. 1 is a flow diagram of the exhaust and fuel control system  10  of a vehicle. Fuel  12  and air  14  are fed separately into the intake manifold  16  where they are mixed together. The air/fuel mixture is fed into engine  18  where it is combusted to produce drive power for the vehicle. The combustion of fuel  12  and air  14  produces various byproducts that are expelled from the engine  18  after combustion. The combustion byproducts are generically termed the combustion exhaust  20 ,  26 . The combustion exhaust  20  is fed into a catalytic converter  22 . Catalytic converter  22  reacts the various toxic byproducts from the exhaust gases into safer compounds before emission as exhaust  26 . Optionally, a second catalytic converter  28  performs a similar function as first catalytic converter  22  to further remove toxic byproducts from exhaust  26  before emission as exhaust  30 . The efficiency of catalytic converters  22 ,  28  varies with the composition of the combustion exhaust  20 ,  26 . The composition of the respective combustion exhaust  20 ,  26  varies with the fuel-air mixture and the engine&#39;s operating conditions. 
     A first sensor  24  monitors combustion exhaust  20  emitted from the engine  18 . A second sensor  32  monitors exhaust  26  output by first catalytic converter  22 . Sensors  24 ,  32  examine the byproducts produced by the combustion process and feed this information back to the air/fuel mixture control module  34 . The air/fuel mixture control module  34  adjusts the ratio of the fuel  12  and air  14  in the mixture sent to the engine  18  and thereby alters the composition of the combustion exhaust  20 ,  26 . The adjustment of the air/fuel mixture allows the engine  18  to operate closer to the stoichiometric point. At this point, catalytic converters  22 ,  28  operate at or near peak efficiency so that the vehicle emits the least amount of toxic byproducts. 
     Sensors  24 ,  32  are embodied as oxygen sensors. Sensors  24 ,  32  measure the amount of oxygen present in the exhaust gas emitted from engine  18  after combustion. Sensors  24 ,  32  operate as a voltage source and produce an output between approximately 0 and 1 volts based on the amount of oxygen present in the respective combustion exhaust  20 ,  26 . The less the amount of oxygen present (lower air/fuel ratio) in the combustion exhaust  20 ,  26 , the greater the voltage outputted by respective sensors  24 ,  32 . The amount of oxygen present in the combustion exhaust  20 ,  26  enables determination where in relation to the stoichiometric point the engine  18  is operating and how the air/fuel mixture should be adjusted to move engine  18  closer to the stoichiometric operating point. 
     FIG. 2 shows a typical operating curve of an exemplary oxygen sensor, such as oxygen sensors  24 ,  32 . Operation will be described with respect to oxygen sensor  24 , but is equally applicable to oxygen sensor  32 . The x-axis represents the equivalence ratio of combustion exhaust, and the y-axis represents and the output voltage of oxygen sensor  24 . The stoichiometric operating point M represents the point at which the combustion in the engine  18  is closest to complete. At this point the catalytic converters  22 ,  28  operate most efficiently. The equivalence ratio range P to F represents a rich mixture of fuel to oxygen. In this range, relatively little oxygen is present after the combustion process. The equivalence ratio range P to A represents a lean mixture of fuel to oxygen. In this range, the amount of oxygen emitted after the combustion process is relatively great. In both of these ranges the combustion of the engine  18  is not fully complete, and while this does not greatly affect the performance of the engine  18 , the efficiency of the catalytic converters  22 ,  28  drops and fewer compounds are removed from the exhaust gas. 
     The stoichiometric operating point M corresponds to a set voltage output Q from the oxygen sensor. It should be noted that this point does not necessarily correspond to exactly half the value of the maximum output of the oxygen sensor and this point may vary along the curve, between N and O, during normal vehicle operation. 
     The range of the curve from N to O around the stoichiometric point M is very steep. Moving from point N to O on the curve represents a small change in the equivalence ratio of the combustion exhaust. The steep portion of the curve spans an equivalence ratio of C to D. This small air/fuel ratio change represents a significant voltage change from points I to J on the y-axis. Because a small equivalence ratio change corresponds to a significant oxygen sensor output voltage change, utilizing only a small range of an A/D converter to cover the entire output voltage range of the oxygen sensor reduces the precision in determining at which point along the air/fuel curve the engine  18  is operating. This is particularly relevant when attempting to take measurements along the steep portion of the curve. 
     FIG. 3 depicts a block diagram for converting the analog signal output by the oxygen sensor to a digital signal to enable adjustment of the air/fuel mixture input to engine  18 . The block diagram of FIG. 3 will be described with respect to oxygen sensor  24 , but is equally applicable to oxygen sensor  32 . The analog output of the sensor  24  is input to amplifier circuit  50 . The output of amplifier circuit  50  is input to analog-to-digital (A/D) converter  40 . A/D converter  40  converts the analog signal output by amplifier circuit  50  into a digitally encoded n-bit word. As described herein A/D converter  40  is an 8-bit A/D converter. 
     A/D converter  40  operates using a supply voltage  44  and a ground reference  46 . The 8-bit word defines 2 8 =256 counts, where a zero count corresponds to zero volts and a 256 count corresponds to 5 volts. Preferably, the signal output by amplifier circuit  50  ranges from ground reference  46  to the supply voltage  44  to yield maximum resolution. 
     As discussed above, oxygen sensors  24 ,  32  generally output a voltage within the range of 0 to 1 volts. As also discussed above, this implies that the full range of A/D converter  40  spans 0 to 51 counts, or approximately twenty percent of the overall possible resolution. Accordingly, amplifier circuit  50  receives the signal output by oxygen sensor  24  and scales the signal output by oxygen sensor  24  to a full input range for A/D converter  40 . In this particular example, the full range of A/D converter  40  is 0 to 5 volts. Because the full range of the signal output by oxygen sensor  24  is 0 to 1 volts, a scale factor or gain of  5  is applied to the signal in order to expand the signal to the full 0 to 5 volts range of A/D converter  40 . The signal output by oxygen sensor  24  is thus scaled to the full input range of A/D converter  40  so that the full 256 available counts can be used to determine the oxygen content of the exhaust gas. The 8-bit word is then input to microprocessor  48  which determines an air/fuel error mixture signal which is output by control module  34  of FIG.  1 . 
     FIG. 4 depicts a circuit diagram for amplifier circuit  50  of FIG.  3 . The signal output by oxygen sensor  24  is applied to the non-inverting terminal of an operational amplifier  52  through a resistor R 1 . The inverting terminal of operational amplifier  52  is connected to ground through a resistor R 2 . Operational amplifier  52  is powered by a voltage signal V+ and also includes a reference voltage connected to ground. The output of operational amplifier  52  defines an amplified signal which is then input to A/D converter  40 . The output from operational amplifier  52  is fed back to the inverting terminal through a feedback resistor R 3 . A capacitor C 1  is inserted in the feedback loop in order to minimize noise. Through proper selection of resistors R 3  and R 2 , the gain of amplifier circuit  50  can be varied in accordance with the function (1+R 3 /R 2 ). In the present embodiment, the components of amplifier circuit  50  have the following values: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component 
                 Value 
               
               
                   
                   
               
             
             
               
                   
                 R1 
                  1 kΩ 
               
               
                   
                 R2 
                 15 kΩ 
               
               
                   
                 R3 
                 60 kΩ 
               
               
                   
                 C1 
                 0.0015 μf 
               
               
                   
                   
               
             
          
         
       
     
     The exemplary values discussed in the above table define an amplifier circuit  50  having a gain of 5. Accordingly, the signal output by oxygen sensor  24  having a voltage range of 0 to 1 volt has been expanded by amplifier circuit  50  to the full range of A/D converter, 0 to 5 volts. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.