Abstract:
An electrochemical gas sensor is provided for a motor vehicle. The gas sensor contains a digital controller and a detection circuit. The digital controller captures, by a feedback input, a value of the voltage applied to the inside of the gas sensor cell. The output of the digital controller provides a control value for the current flowing in the gas sensor cell. The detection circuit is used to detect the properties of the gas sensor cell and to adjust the dynamic control properties of the digital controller corresponding to the properties of the gas sensor cell.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an electrochemical gas sensor and a method for adjusting an electrochemical gas sensor. Electrochemical gas sensors are used to measure concentrations of a predetermined gas component in a gas. In the case of internal combustion engines in motor vehicles for instance, the concentration of oxygen in the exhaust gas is measured. This measurement is used to determine whether the air/fuel ratio of the internal combustion engine has the desired value. 
     DE 10 2005 033 263 A1 proposes a gas concentration measuring device, with which the impedance of the gas sensor element is measured with the aid of a computation circuit for controlling activation and diagnosing the gas sensor element. To measure gas concentration the current flowing through the gas sensor or the voltage present across the gas sensor must be regulated. This serves to put the gas sensor in a state in which the concentration can be measured. 
     It has proven that gas sensors are subject to major production variations, so that their electrical properties, such as impedance, differ by up to 20% for instance from gas sensor to gas sensor. To ensure that the control circuit does not become unstable even when there are major deviations, the controller must be designed so that it responds relatively slowly. This means that the air/fuel ratio is not stable all the time, resulting in the unnecessary consumption of a large quantity of fuel. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is therefore to provide an electrochemical gas sensor that can be controlled more quickly. 
     It is also the object of the invention to provide a method for adjusting such an electrochemical gas sensor. 
     These objects are achieved by the subject matter of the independent claim. Further advantageous embodiments will emerge from the subclaims. 
     The invention relates to a control circuit for an electrochemical gas sensor for a motor vehicle, with which a digital controller is provided, featuring a feedback input and an output. The digital controller receives a value for the voltage present across a gas sensor cell by way of the feedback input. At the output the digital controller supplies a control value for a current flowing into the gas sensor cell. 
     A detection circuit is provided for detecting the properties of the gas sensor cell. It is also provided for adjusting the dynamic control properties of the digital controller according to the properties of the gas sensor cell. 
     The dynamic control properties of the digital controller are characterized for example by a z-transformed 
     
       
         
           
             
               
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     During adjustment the parameters a 1 , a 2 , b 1  and b 2  are changed. The dynamic properties of the digital controller characterize how quickly and in some instances the overshoots with which the digital controller responds to deviations in the setpoint value. 
     The electrochemical gas sensor has the advantage that the digital controller can be adjusted according to the properties of the gas sensor cells. It is thus possible to tailor the controller property in each instance to the installed gas sensor cell, with the result that the digital control circuit is adjusted so that it responds quickly when there are deviations, without endangering the stability of the control loop. 
     The control parameters can also be tailored to the operating state of the gas sensor and temperature. It is also possible to use sensors of different types, for instance from different manufacturers, as the digital controller with the detection circuit can be tailored automatically. 
     In one embodiment the detection circuit is provided at least partially in a microcontroller. The microcontroller can manage a plurality of data, which is compared with the detected properties of the gas sensor cell. 
     The detection circuit is preferably configured so that it detects the properties of the gas sensor cell based on the voltage occurring across the gas sensor cell, when a current step is present at the output of the digital controller. Step responses are characteristic of the dynamic behavior of control elements and thus allow ready conclusions to be drawn about the control properties of the measured gas sensor cell. 
     In a further embodiment the detection circuit is also configured to detect the temperature of the gas sensor cell. Temperature is also a major influence on the behavior of the gas sensor cell. It is therefore helpful to consider the properties of the gas sensor cell at the measured temperature, to draw a conclusion about the sensor type for instance. 
     In a further embodiment the property of the gas sensor cell is calculated based on the impedance of the gas sensor cell, in order to characterize the gas sensor cell as accurately as possible. 
     In a preferred embodiment the detection circuit features an analog/digital converter for converting the voltage present across the gas sensor cell. Conversion to a digital value allows the digital calculation of gas sensor properties, which simplifies the comparison with already known gas sensors. 
     It is particularly appropriate to employ the described electrochemical gas sensor, if the gas sensor cell contains a Nernst cell, since such a cell is produced by a plurality of manufacturers, in each instance with major production variations. 
     The invention also relates to the use of an inventive electrochemical gas sensor in a motor vehicle to control the air/fuel mixture of an engine of the motor vehicle. The inventive provision of the detection circuit allows more precise adjustment of the control circuit for the gas sensor. This allows the electrochemical gas sensor to be controlled more dynamically, making it faster, which ultimately reduces fuel consumption. 
     The invention also relates to a method for adjusting an electrochemical gas sensor, with an inventive control circuit first being provided for an electrochemical gas sensor. The output of the digital controller is controlled so that the current at the output generates a step of maximum gradient. The properties of the gas sensor cell are then detected based on the voltage at the feedback input of the digital controller. This method can be used to characterize the gas sensor cell, in order to adjust the properties of the digital controller as a function of the properties of the gas sensor cell. 
     In one preferred embodiment the method is used to check the exhaust gas flow of an engine and the steps of activating the output and detecting the properties of the gas sensor cell are performed at least when the engine is started up. It is thus checked every time the engine is started up, whether the properties of the gas sensor cell have changed, for example due to aging, or whether a new gas sensor cell has been fitted during a repair. The digital controller is then tailored to the changed properties of the gas sensor cell. 
     In a further embodiment the control properties are adjusted a number of times during the trip, to tailor them to any temperature changes. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows an inventive control circuit for a gas concentration sensor. 
         FIG. 2  shows the frequency behavior of an inventive gas compensation sensor. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an inventive control circuit for electrochemical gas concentration sensors. The assembly  1  consisting of a control circuit  100  and a Nernst cell  9  contains a Nernst cell  9 , as well as a digital controller  2  and a setpoint value generator  30 . The digital controller  2  controls a current source  20 , which determines the current Ip flowing into the Nernst cell  9 . The digital controller receives a setpoint value for controlling the current regulator  20  at its first input  3 . All the elements in  FIG. 1 , which are not part of the Nernst cell  9 , are part of the control circuit  100 . 
     The Nernst cell  9  contains a first resistor  11 , which represents the resistance across the supply lines to the Nernst cell, and a second resistor  12 , which represents the internal resistance of the Nernst cell  12 . A voltage  10  drops across the Nernst cell  9 , being identified by the circle. The first resistor  11 , the second resistor  12  and the voltage  10  are connected to one another in series. The first connection of the voltage  10  is connected to ground  36  here, while its second connection is connected to a first connection of the second resistor  12 . 
     The second connection of the second resistor  12  is connected to the first connection of the first resistor  11 , the second output of which is connected to the output of the current source  20 . The first input of the sampling element  16  is connected to ground  36  and its second input is connected to the second connection of the second resistor  12 . The voltage Vs is generated between these two inputs of the sampling element  16  and then converted to a digital value in said sampling element. This digital value is fed to the input  5  of the digital controller  2  by way of the feedback path  160 . 
     The proposed gas sensor is a lambda control oxygen sensor. It ensures that the engine is operated with a constant air/fuel mixture. The voltage Vs is adjusted to a constant value. The digital controller  2  controls the current source  20  at its output  7  so that the current Ip brings about a voltage Vs, which is equal to the voltage in the setpoint value generator  30 . The size of the current Ip is a measure of the lambda value. 
     The voltage Vs is also measured by the limiter  17 . This limiter  17  measures whether the voltage Vs exceeds a specified threshold value. If so, the limiter  17  outputs a signal value to the digital controller  2 , which receives this value at its input  6 . If the voltage Vs exceeds the previously specified threshold value, the current Ip is reduced. 
     The current Ip is measured using a measurement circuit identified with the reference character  38  in the figure. The measured value Imess is a measure of the lambda value to be set in the engine controller and is output to the microcontroller of the engine controller. 
     The microprocessor  14  contains a detection circuit for identifying the properties of the Nernst cell  9 . When the engine starts up, the digital controller  2  controls the current source  20  so that it performs a current step from 0 mA for example to 5 mA. The driver of the current source  20  is dimensioned so that the rise is as steep as possible. The profile of the voltage Vs responds correspondingly to this current step. As described above, the voltage Vs is sampled by the sampling element  16  and converted to digital values using the AD converter  18  present in the sampling element. The sequence of digital values is output to the detection circuit  14 . The detection circuit  14  identifies properties of the Nernst cell  9  from the voltage profile Vs. 
     Nernst cells  9  from different manufacturers and different production batches differ in their transmission behavior. The measured step response characterizes the transmission behavior of the Nernst cell  9  and is used as the basis for control evaluation. The frequency profiles of different types of Nernst cells are stored in the storage unit  21 . The detection circuit  14  compares the voltage profiles received from the sampler  16  with the voltage profiles stored in the storage unit  21 . 
     Once the detection circuit  14  has identified the correct sensor type, it adjusts the properties of the digital controller  2 . To this end it changes the transmission characteristic of the digital controller  2 . The digital transmission characteristic of the digital controller  2  is characterized by amount and phase by a Bode diagram for example. The characteristics are adjusted by means of an amount/phase calibration according to the properties of the Nernst cell. 
     Sensors are generally manufactured with significant production variations. Deviations of up to 20% in transmission behavior are not uncommon. The detection circuit  14  can be used to detect whether such deviations from a normal value are present. If so, the digital controller  2  is also adjusted so that the control behavior of the system as a whole is optimized. 
     The use of the digital control structure in particular allows the necessary adaptations of the control parameters to be extracted directly from the step response of the sensor by means of software. The changed control parameters can be programmed directly into the digital controller  2 . The filter structure has the advantage that the control characteristic can be tailored easily to different states of the connected linear lambda probe. If a different type of probe is connected, it is only necessary to change the data in the detection circuit  14 , which is much easier and quicker than a hardware change. 
     If the detection circuit  14  is programmed so that it can also evaluate parameters such as trim resistance, heating resistances and the step response of the probe, it is also possible to tailor the control characteristic in a fully automatic manner to different predefined sensor types or even to tailor the controller characteristic individually in a fully automatic manner to the attached sensor. 
     The fully automatic calibration of the controller characteristic allows intelligent differentiation between different types of linear lambda sensors and automatic adaptation of the control circuit. 
     The controller characteristic is tailored individually to the respective connected linear lambda sensor in order to tailor control behavior taking into account the frequency characteristic of the sensor due to production variance, aging and temperature fluctuations for instance. 
       FIG. 2  shows a Bode diagram of the control characteristic of an open control circuit of a gas sensor according to  FIG. 1 . A plurality of simulations have been carried out, in which the properties of the digital controller  2  and the gas sensor  9  were varied according to their production deviations. It has proven that the amplitude and frequency profiles of the open control circuit are very dependent on the parameters of the digital controller  2  and the gas sensor  9 . 
     The phase reserve at amplification 0 dB is read off to assess the stability of the control circuit. The phase reserve varies between 60 and 100° in the simulations shown in  FIG. 2 . In order to be able to increase the phase reserve, the properties of the digital controller  2  are made dependent on the properties of the gas sensor  9 . This reduces the variation in the amplitude and frequency profiles of the Bode diagram. This allows a higher value to be achieved for the phase reserve.