Measuring and testing – Gas analysis
Reexamination Certificate
2002-12-16
2004-07-20
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Gas analysis
C073S023310, C073S031050
Reexamination Certificate
active
06763697
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a device and a method for operating a linear lambda probe of an internal combustion engine.
A linear lambda probe is used to determine the oxygen concentration in the exhaust gas of an internal combustion engine. It has two pairs of electrodes and a measuring chamber which is connected to the exhaust gas stream via a diffusion barrier. The first pair of electrodes (measuring electrodes) are arranged between the measuring chamber and air and are used to measure the oxygen concentration in the measuring chamber. The second pair of electrodes (pump electrodes) are arranged between the measuring chamber and the exhaust gas stream. This permits oxygen ions to be pumped out of the measuring chamber, or into it, when a current Ip with a corresponding polarity is applied.
In this way it is possible to generate a dynamic equilibrium between the flow of oxygen through the diffusion barrier and the flow of oxygen ions through the pair of pump electrodes. The oxygen concentration in the measuring cell which can be determined using the measuring electrodes is suitable here as a regulating criterion. A preferred value is, for example, 450 mV for &lgr;=1. The pump current Ip which flows in this case is a measure of the oxygen concentration in the exhaust gas (and also for &lgr; after numerical conversion).
Hitherto, the application of the linear lambda probe has been restricted to the nonsupercharged, stoichiometric mode of operation (Pa=1 bar, &lgr;=1) of the engine. Correspondingly, only small pump currents are necessary to maintain the equilibrium (&lgr;=1) in the measuring cell (|IP|<~2.5 mA).
For lean engines, an operating mode up to &lgr;=4 is provided, which requires a drastically increased pump current Ip. When operating in a supercharged (turbo) engine, an exhaust gas pressure of up to 2 bar is produced. The pressure sensitivity of the probe leads to a further increase in the maximum necessary pump current of up to ±12 mA.
The dynamic resistance of the diffusion barrier has a temperature dependence which leads to errors in the transmission ratio. This is countered by measuring the probe temperature and regulating it by means of a heating element which is installed in the probe. For reasons of cost, a separate thermal element is dispensed with here. Instead, the highly temperature-dependent internal resistance Ris of the probe is measured.
A customary measuring method for determining the internal resistance Ris of the probe is to apply to the probe terminal Vs+ a measuring signal M formed from a square wave alternating current—for example 500 &mgr;Ass (peak-to-peak) with a frequency fm of, for example, 3 kHz. This alternating current brings about an alternating voltage of 500 &mgr;Ass*100 &OHgr;=50 mVss with an internal resistance Ris of the probe of 100 &OHgr;, for example. This alternating voltage is amplified in an amplifier, for example by the factor ten, and then rectified. The direct voltage Vri which has been produced in this way can be used and further processed as a measure of the probe temperature.
A known evaluation circuit is illustrated in FIG.
1
and will be described in more detail below.
This circuit has certain disadvantages:
When the evaluation circuit is supplied with a supply voltage Vcc=5 V which is generally already available, a center voltage Vm of approximately 2.5 V is produced. The voltage chain which is present at the probe is obtained as:
Vm<|Rc*Ip+Vp|+Vsat
; where
Rc=30 to 100 &OHgr;=overall calibration resistance (manufacturer-dependent),
Vp=−350 to +450 mV; polarization voltage of the pump cell,
Vsat=100 to 200 mV; saturation voltage of the pump current source P; this limits the maximum possible pump current Ip to <10 mA, and therefore does not correspond to the requirements (Ip=±12 mA);
a common mode signal (Vm±2 V) is superimposed on the pump current Ip. The measurement is falsified by up to ±0.3%% by the finite common mode expression of real integrated amplifiers (for example 65 dB);
in addition, the polarization voltage of the pump cell (−350 mV when &lgr;<1) results in a zero crossover point error &Dgr;Ip of approximately 5 &mgr;A. As the pump current Ip is the primary measurement signal of the oxygen probe, these errors are included directly in the overall precision of the pump current Ip. This limits the precision of lambda control and thus constitutes a significant problem;
a further fundamental problem of this circuit arrangement is the reciprocal influence between the Nernst voltage Vs and the square wave voltage Vr which is produced from the measurement of the internal resistance. This square wave voltage Vr also appears at the input of the controller R and thus constitutes a control error. The controller will attempt—within the scope of its bandwidth—to compensate this control error. To do this, it changes the pump current Ip, which in turn has effects on Vs. As the pump current Ip is the measurement variable for &lgr;, the primary probe signal Vs is falsified. In turn the change in Vs is superimposed on the square wave voltage Vr. The effect of this is to cause the signal roof of the square voltage Vr to slope, thus bringing about a considerable amplitude error during the rectification;
when EMC interference signals occur there is also a considerable deviation of the actual measured value of the internal resistance Ris.
SUMMARY OF THE INVENTION
The object of the invention is to specify a method for operating a linear lambda probe which makes available values of the pump current Ip which correspond to the requirements set, said method avoiding the described common mode error and significantly improving the precision of the measurement of the pump current so that there is no reciprocal influence between the Nernst voltage Vs and the square wave voltage Vr, and the precision of the measurement of the internal resistance is improved, and which lambda probe remains operational even at a low battery voltage (Vb less than or equal to +6 V). The object of the invention is also to specify a device for carrying out this method.
This object can be achieved according to the invention by a method for operating a linear lambda probe having a first terminal, a second terminal, a third terminal and a fourth terminal, comprising the steps of:
generating a current with a square-wave profile and relatively low frequency from an oscillator signal with a frequency,
supplying the current at the first terminal as a measurement signal,
tapping a sum voltage between the first and second terminals, whose upper and lower envelopes determine an upper value and a lower value,
referring the sum voltage to the difference of a predefined center voltage and a predefined reference voltage, and
forming the mean value corresponding to the difference between a Nernst voltage and reference voltage from the upper value and lower value of the sum voltage and
converting the mean value into a proportional pump current which brings about, at the calibration resistor of the lambda probe, a voltage drop which is used as a measure of the oxygen concentration.
The step of generating the current can be performed by means of frequency division.
Another method for operating a linear lambda probe of an internal combustion engine having a first terminal, a second terminal, a third terminal and a fourth terminal, comprises the steps of:
a current with a square-wave profile and relatively low frequency which is derived from an oscillator signal with a frequency by means of frequency division is supplied at the first terminal as a measurement signal which brings about a square-wave voltage which drops across the internal resistor of the probe and forms, with the Nernst voltage which can be tapped between the first and second terminals, a sum voltage whose upper and lower envelopes determine an upper value and a lower value,
the sum voltage is referred to
Baker & Botts L.L.P.
Larkin Daniel S.
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