Patent Publication Number: US-2005127920-A1

Title: Method and system for impedance measurement of a zeolite-based ammonia sensor

Description:
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/529,368(DEL01 PP489), entitled “METHOD AND SYSTEM FOR IMPEDANCE MEASUREMENT OF A ZEOLITE-BASED AMMONIA SENSOR,” which was filed Dec. 12, 2003, and which is hereby incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD  
      The present invention is generally directed to measuring the impedance of a sensor and, more specifically, to measuring the impedance of a zeolite-based ammonia sensor.  
     BACKGROUND OF THE INVENTION  
      Various sensors have been developed to detect chemical elements and/or chemical compounds in a gas stream. For example, one zeolite-based sensor exhibits a complex impedance that is dependent on the concentration of ammonia (NH3) in a gas stream presented to the sensor. In one particular implementation, a zeolite-based sensor has been positioned within an exhaust gas stream of a diesel engine to provide feedback, as to the concentration of ammonia in the exhaust gas stream, to a control unit. Based upon the concentration of the ammonia in the exhaust gas stream, the control unit may cause reduction of the injection of urea, which acts to reduce nitrogen oxide (NOx) emission levels from the diesel engine, into the exhaust gas steam.  
      In many applications, in determining an impedance of a sensor it is undesirable to bias the sensor with a signal that has a direct current (DC) component, as the DC component may cause ion migration or other chemical reactions in the sensor. Ion migration in a sensor may alter the impedance of the sensor, thereby providing an incorrect indication of the level of a gas within a gas stream. A traditional approach for determining the impedance of a sensor has utilized a system that has sourced a sinewave voltage excitation to an input of the sensor and has observed the resulting sinusoidal current. In general, such systems have captured both the amplitude and phase relationship of the sensor voltage and the sensor current, such that both a real and imaginary part of a sensor impedance could be determined. However, it should be appreciated that a system designed according to the traditional approach can be relatively expensive and may experience difficulty in providing a desired accuracy over an extended period of time.  
      At least one zirconia-based wide range air fuel (WRAF) sensor has been designed to provide an impedance that can be correlated with a temperature of the sensor. In systems implementing a WRAF sensor that provides an impedance that can be correlated with the temperature of the WRAF sensor, the impedance has been determined through a number of techniques. One technique for determining an impedance of a WRAF sensor has been to periodically apply two successive pulses that have opposite polarity. In this technique, two current readings are taken during a first electrical pulse to obtain information that can be utilized to determine both reactive and resistive components of the impedance. Another technique for determining an impedance of a WRAF sensor has been to periodically couple a known resistive load between an input of the sensor and ground and measure a resultant voltage across the resistive load. However, neither of these techniques have generally been extended beyond zirconia-based WRAF sensors.  
      What is needed is a less expensive, less error prone technique for determining the impedance of a sensor, e.g., a zeolite-based ammonia sensor, that may experience ion migration.  
     SUMMARY OF THE INVENTION  
      One embodiment of the present invention is directed to a technique for measuring the impedance of a sensor that is subject to ion migration. Initially, a first electrical pulse is applied to an input of the sensor, whose impedance varies according to a first gas concentration in a gas stream. Next, a second electrical pulse is applied to the input of the sensor. The energy of the first and second electrical pulses is approximately the same, e.g., within +/−five percent, and the first and second electrical pulses have opposite polarity. Next, a sensor load current is determined during at least one of the first and second electrical pulses to provide a first sensor current. Then, the sensor load current during the same one of the first and second electrical pulses is determined to provide a second sensor current. Finally, at least one component of the impedance of the sensor is determined based upon the first and second sensor currents.  
      According to another embodiment of the present invention, a technique for measuring the impedance of a sensor subject to ion migration includes a number of steps. Initially, a first electrical pulse is applied to an input of the sensor, whose impedance varies according to a first gas concentration in a gas stream. Next, a sensor load current is determined during the first electrical pulse to provide a sensor current. Then, at least one component of the impedance of a sensor is determined based upon the first sensor current.  
      According to another embodiment of the present invention, the first sensor current is determined during the first electrical pulse at a first time and the second sensor current is determined during the first electrical pulse at a second time that occurs after the first time. According to still another embodiment of the present invention, the first sensor current is determined during the first electrical pulse, while the sensor load current is slewing, and the second sensor current is determined during the first electrical pulse when the sensor load current has approximately reached a steady-state value, e.g., within +/−five percent of the steady-state value.  
      According to still another embodiment of the present invention, the gas stream is an exhaust gas stream associated with a diesel engine. According to another aspect of the present invention, at least one component of the impedance is a resistive component. According to still another aspect of the present invention, the at least one component of the impedance includes a reactive component and a resistive component. According to another aspect of the present invention, the sensor is a zeolite-based sensor. According to a different embodiment of the present invention, the first gas concentration is an ammonia concentration and the gas stream is an exhaust gas stream associated with a diesel engine.  
      These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
       FIG. 1  is a diagram of an exemplary system for removing nitrogen oxide (NOx) from an exhaust stream of a diesel engine;  
       FIG. 2  is a chart depicting impedance curves for a sensor at different ammonia (NH3) concentrations;  
       FIG. 3  is an electrical schematic of a circuit for providing pulses with opposite polarity to a sensor for determining at least one sensor impedance component, according to one embodiment of the present invention;  
       FIG. 3A  depicts exemplary signals associated with the circuit of  FIG. 3 ;  
       FIG. 4  is an electrical diagram of a circuit for providing pulses with opposite polarity to a sensor for determining at least one sensor impedance component, according to another embodiment of the present invention;  
       FIG. 4A  depicts exemplary signals associated with the circuit of  FIG. 4 ;  
       FIG. 5  is an electrical schematic of another circuit for applying pulses of opposite polarity to a sensor for determining at least one sensor impedance component, according to a different aspect of the present invention; and  
       FIG. 5A  depicts exemplary signals associated with the circuit of  FIG. 5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Various embodiment of the present invention are directed to techniques for measuring an impedance of a sensor that is subject to ion migration. It should be appreciated that the specific technique implemented will depend upon the type of sensor. For example, in sensors that are very sensitive to charge imbalance it can be advantageous to apply a first electrical pulse to an input of the sensor followed by application of a second electrical pulse to balance the charge on the sensor. According to one embodiment of the present invention, a first electrical pulse is provided to an input of a sensor whose impedance varies according to a first gas concentration in a gas stream. Next, a second electrical pulse is applied to the input of the sensor.  
      For charge balance, it is desirable for the energy of the first and second electrical pulses to be approximately the same, e.g., within +/−five percent of each other, and for the first and second electrical pulses to have opposite polarities. Then, a sensor load current is determined during at least one of the first and second electrical pulses to provide a first sensor current. Next, the sensor load current is determined during the same one of the first and second electrical pulses to provide a second sensor current. Finally, at least one component of the impedance of the sensor is determined based upon the first and second sensor currents.  
      According to another embodiment of the present invention, a technique for measuring the impedance of a sensor subject to ion migration includes the application of only a single electrical pulse to the sensor. It should be appreciated that this technique may not be satisfactory for sensors that have relatively long recovery times. According to this embodiment, a first electrical pulse is applied to an input of the sensor, whose impedance varies according to a first gas concentration in a gas stream. Next, a sensor load current is determined during the first electrical pulse to provide a sensor current. Then, at least one component of the impedance of a sensor is determined based upon the sensor current.  
       FIG. 1  depicts an exemplary system  100 , which removes nitrogen oxide (NOx) from an exhaust gas associated with a diesel engine  10  of a motor vehicle. As is shown, a probe S 1 , e.g., a zeolite-based ammonia sensor, is positioned within an exhaust gas pipe  102  of the system  100  downstream of a selective catalytic reduction (SCR) catalyst. An ammonia sensor interface unit  106  is coupled to the probe S 1  and to a dosing controller  108 . The ammonia sensor interface unit  106  provides an indication, which is based on the impedance of the sensor S 1 , to the dosing controller  108  as to the level of the ammonia in an exhaust gas stream associated with diesel engine  10 .  
      When ammonia breakthrough is detected in the exhaust gas stream, the dosing controller  108  may command a treatment unit  104  to decrease the amount of a urea solution provided into the exhaust stream carried by the exhaust pipe  102 . In this manner, the system  100  performs selective catalytic reduction (SCR) to remove nitrogen oxide (NOx) from the exhaust stream of the diesel engine  10 , while monitoring for ammonia breakthrough.  
      With reference to  FIG. 2 , graphs  202  and  204  show a number of impedance curves of an exemplary zeolite-based ammonia sensor. Each of the graphs  202  and  204  include impedance curves that show that both of the real and imaginary parts of the impedance change with frequency and the concentration of NH3 in the sample gas being measured. This frequency dependence of both of these components points out the need to correctly select and control the actual times of the two waveform measurement samples to accurately calculate the real and imaginary parts of the sensor impedance and relate it to the desired indication of NH3 concentration.  
      With reference to  FIGS. 3 and 4 , an interface circuit  300  that may be utilized to provide first and second electrical pulses to facilitate determination of a sensor impedance and exemplary signals at various locations in the circuit  300 , respectively, are shown. In this embodiment, the first and second electrical pulses have opposite polarity and approximately equal energy, e.g., energies within +/−5 percent of each other. As is shown in  FIG. 3 , dual monostable multivibrators U 2 A and U 2 B are utilized to provide the first electrical pulse and the second electrical pulse, respectively. A pulse generator (not shown) is coupled to and provides a trigger on a trigger input +T of the multivibrator U 2 A. An output ({overscore (Q)}) of the multivibrator U 2 A is coupled to a trigger input +T of the multivibrator U 2 B. A resistor R 2  and a potentiometer R 4 , in conjunction with a capacitor C 2 , are utilized to set the pulse width associated with the multivibrator U 2 A. A resistor R 1  and a potentiometer R 3 , in conjunction with a capacitor C 1 , are utilized to set the pulse width associated with the multivibrator U 2 B.  
      It should be appreciated that the pulse widths are generally sensor type specific. In general, if one wants to determine a complex sensor impedance it is desirable to take two sensor current readings. In this case, a first current reading is taken while the sensor current is slewing and a second current reading is taken when the sensor current has approximately reached a steady-state, e.g., within +/−5 percent of a steady-state value. At any rate, the first electrical pulse may be terminated upon taking the second current reading. It should also be appreciated that if the resistive component of the impedance is all that is desired, only one current reading, taken when the sensor current is approximately at a steady-state value, is required. In either situation, a single pulse may be utilized to determine a sensor impedance, depending upon a sensor type. That is, a single pulse may be used to determine the impedance of a sensor that recovers relatively quickly from a charge imbalance.  
      Responsive to the pulse generator, a signal TP 3  is provided at the output ({overscore (Q)}) of the multivibrator U 2 A and a signal TP 2  is provided at the output of the multivibrator U 2 B. A signal VCELL shows an exemplary signal provided to a simulated version of sensor S 1 , which is represented by a potentiometer RS in parallel with a capacitor CS. A signal ICELL represents the current at test point TP 8 , within the feedback loop of the operational amplifier U 1 B.  
      The interface circuit  300  may be set up in a current mode or a voltage mode. In the current mode, a resistor RT 1  is provided inside the loop associated with the operational amplifier U 1 B and a resistor RT 2  is replaced with a short. In the voltage mode, the resistor RT 1  is replaced with a short and the resistor RT 2  is provided outside the loop of the amplifier U 1 B. As the amplifier U 1 B is being operated in a closed loop configuration, the voltage outside the loop is a known value, as determined by the components associated with the amplifier U 1 B. The current flowing through the resistor RT 1  can then be determined by measuring the voltage at the test point TP 8 , as the value of the resistor RT 1  is also known.  
      As mentioned above, the reactive and resistive components of the sensor can then be calculated when two current readings are taken. For example, if the components associated with amplifier U 1 B are set such that the output of the amplifier U 1 B goes to one volt, the voltage at the test point TP 8  will be one IR (where I is the sensor current and R is the resistance of the resistor RT 1 ) drop higher than the output voltage, which allows the sensor current to be readily calculated.  
      The transistors Q 1 , Q 2  and Q 3 , and their associated components, perform a level shifting function to allow the symmetrical driving of the transistors Q 4  and Q 5 . As is shown, the transistor Q 4  is a PNP transistor and the transistor Q 5  is an NPN transistor and these transistors form switches which pull-up and pull-down a voltage that is provided at test point TP 4 . It should be appreciated that the pull-up and pull-down voltages are mirror images of each other and are set by the operational amplifier U 1 A and its associated components. The amplifier U 1 A is configured as a voltage follower to generate a DC voltage at test point TP 6 . An operational amplifier U 1 D and its associated components may be provided to vary a low side of resistor RG from ground (at test point TP 1 ).  
      The voltage at test point TP 6  is divided in half during the pull-up portion of the waveform and the output of the operational amplifier U 1 C is divided in half on the pull-down portion of the waveform. As the voltage is divided in half, the next stage, which consists of operational amplifier U 1 B and its associated components, is set for a gain of two. Accordingly, the transistors U 1 B and U 1 C are inside the loop and are therefore short circuit protected. A diode package D 1  is also included to provide transient switching protection. An RC filter, made up of resistor RF and capacitor CF, provides output feedback to an input of the amplifier U 1 C and may be implemented to achieve charge balance, if desired. Transistors QSC 1  and QSC 2  and their associated components provide short circuit protection for the output of the amplifier U 1 B.  
      With reference to  FIG. 4 , an interface circuit  400  that includes a microcontroller U 2 C and pulse driver circuit  402  is depicted.  FIG. 4A  depicts exemplary waveforms associated with the circuit  400 . The circuit  400  functions in a similar manner to the circuit  300  of  FIG. 3 , with the exception that the monostable multivibrators U 2 A and U 2 B are replaced with the microcontroller U 2 C and the microcontroller U 2 C monitors the voltage at the test point TP 8  to determine a sensor current, as the circuit  400  is configured for the current mode. Another difference between the circuit  400  of  FIG. 4  and the circuit  300  of  FIG. 3  is that the operational amplifier U 1 D and its associated components have been removed and the low side of the resistor RG has been connected directly to ground.  
       FIG. 5  depicts an interface circuit  500  that is similar to the circuit  400 , with the exception that the circuit  500  operates in the voltage mode, i.e., a voltage across the sensor is measured to determine a sensor current.  FIG. 5A  shows exemplary signals associated with the circuit  500 . As is shown in  FIG. 5 , a non-inverting input of a unity gain operational amplifier U 1 E is coupled through a 10K ohm resistor to an input of the sensor S 1 . An output of the amplifier U 1 E is coupled to an input of the microcontroller U 2 C, such that the microcontroller U 2 C can determine the voltage across the sensor S 1 . It is also contemplated that a matched pair of pull-up and pull-down constant current sources may be utilized to determine the impedance of a sensor.  
      Accordingly, a method and system have been described herein that advantageously provide a less expensive, less error prone technique for determining the impedance of a sensor, e.g., a zeolite-based ammonia sensor, that may experience ion migration.  
      The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.