Internal-combustion engines – Charge forming device – Including exhaust gas condition responsive means
Reexamination Certificate
1999-09-07
2001-02-27
Ro, Bentsu (Department: 2837)
Internal-combustion engines
Charge forming device
Including exhaust gas condition responsive means
C123S672000, C123S693000, C701S109000
Reexamination Certificate
active
06192874
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to apparatuses for measuring the oxygen content of a gaseous medium, the apparatuses being of the type comprising:
a sensor capable of delivering a voltage representative of the ratio between a reference oxygen pressure and the oxygen pressure in a volume of the sensor which communicates with the gaseous medium via a porous wall; and
monitoring and control means enabling a pumping current to be delivered to cause oxygen to migrate away from or into said volume.
Numerous apparatuses of that type are already known, such as those described in EP-A-0 507 149 and U.S. Pat. No. 4,932,238. The sensor has at least one sensitive element constituted by a solid electrolyte plate of a type that allows oxygen ions to migrate, the plate being placed between two porous electrodes.
Such a sensor can be the subject of numerous embodiments.
FIG. 1
is a diagram of a sensor that can be considered as having two cells. A first cell referred to as a “pumping” cell
10
p
is sandwiched between two electrodes
12
and
14
. The pumping cell
10
p
is fixed to a second cell
10
s
, referred to as the “sensitive” cell, via a porous intermediate sheet so as to define a volume
18
. Oxygen in the gaseous medium tends to penetrate into the volume
18
so as to bring the oxygen partial pressures into equilibrium. The passage of a current I
p
through the pumping cell tends to cause the oxygen contained in the volume to migrate, and thus to maintain the partial pressure therein at a determined value. A final plate
20
can be placed in contact with the sensitive element
10
s
and can be made of the same material so as to deliver a constant reference pressure; its usefulness appears below.
Between the electrodes on either side of the sensitive element
10
s
there thus appears a measurement voltage V
s
representative of the ratio between the partial pressures of oxygen in the volume
18
and in the plate
20
in contact with the cell
10
s
. Appropriate solid electrodes, and in particular of doped zirconium oxide or zirconia, have characteristics such that the voltage V
s
varies in substantially logarithmic manner with oxygen partial pressure in the volume
18
. Conventionally, the current I
p
is controlled so as to maintain V
s
at a constant value, in which case I
p
is representative of the oxygen partial pressure in the gaseous medium. A heating resistor
21
serves to raise the cells to a suitable temperature.
In another embodiment, that can be described as a single cell embodiment and as shown in
FIG. 2
, the volume
18
is defined solely by the porous intermediate sheet
16
and by the cell
10
p
. The reference oxygen partial pressure is then that of atmospheric air, which is in contact with the cell
10
p
. Under such circumstances, variation of I
p
as a function of V
p
for different oxygen partial pressures has the general appearance shown in FIG.
3
. Insofar as it is desired to remain at all times within a rectilinear portion of the characteristic, the value to which V
p
is servo-controlled must depend to some extent on the partial pressure of oxygen in the gas and on the impedance of the cell.
A major application of the invention lies in determining the air/fuel ratio admitted into an internal combustion engine on the basis of the composition of the exhaust gas, and more particularly on the basis of the partial pressure of the residual oxygen in the exhaust gas.
Until now, use has been made above all of sensors of the kind shown in FIG.
1
. Often the monitoring and control means are constituted by an analog loop for servo-controlling V
s
to a constant value, associated with a microcontroller which deduces the oxygen partial pressure and the instantaneous richness of the mixture from the value of the current I
p
. That solution suffers from drawbacks. The accuracy with which richness is measured is limited by the accuracy with which I
p
is measured. Embodiment in hard-wired form reduces possibilities of adjustment and matching.
SUMMARY OF THE INVENTION
It is an object of the invention to provide improved apparatus another object is to satisfy practical requirements better than previously known apparatuses, at low cost, and enabling richness to be measured accurately.
To this end, the invention provides in particular apparatus of the above-defined type in which the monitoring and control means have a digital controller receiving said voltage on its input and delivering the pumping current to the sensor in the form of a current that varies continuously and progressively without interruptions, and governed by the digital controller in such a manner as to servo-control said voltage on a determined value. The architecture can be such that the microcontroller has means for acting on the current I
p
, thereby providing greater flexibility in adjusting the servo-control, good protection for the sensor, and finer management of putting into action and of degraded mode.
An advantageous solution consists in providing a digital controller that delivers a control voltage which is time modulated (e.g. by pulse width modulation) and in providing a lowpass filter and a current generator at the input of the sensor. Such modulation is easily performed by means of a digital controller. The presence of the filter avoids the need to apply interrupted pulses to the sensor which would reduce its lifetime.
The sensors have integrating behavior. It is advantageous to compensate it by subjecting said voltage representative of the ratio to processing that introduces a derivative component, prior to application to the digital controller. This increases the signal-to-noise ratio and enables the controller to reduce response time. In general, an amplifier circuit is provided to amplify the proportional component, and also the derivative component if present, prior to application to the digital controller.
The invention is applicable to apparatuses in which the sensor has a single cell separating the volume which communicates with the gaseous medium from a zone in which there exists a reference pressure; the representative voltage is then taken from the two electrodes on either side of the cell, and the pumping current is applied to the same cell. Nevertheless, the invention is more usually applied to apparatuses in which the sensor has two cells. Such a sensor has a volume defined by a porous intermediate sheet, by a first cell or “pumping” cell separating the volume from a zone occupied by the gas whose oxygen partial pressure is to be measured, and by a second cell or “sensitive” cell in contact with the reference pressure, with the representative voltage being taken from the electrodes on either side of the second cell. The pumping current then passes through the two electrodes on either side of the pumping cell.
In general, it can be considered that the main advantages of the apparatus stem from the fact that the digital controller has complete control over the pumping current I
p
, both in normal servo-control mode and during transient stages (starting up, protecting the probe, diagnostics, . . . ), and that it serves to produce the current rather than to read back a current that is produced by external means as in present architectures.
Also, the apparatus:
firstly takes advantage of an analog portion that delivers an amplified signal on the basis of the sensor voltage V
s
, which signal fortunately has a derivative component of accuracy that is not degraded by analog-to-digital conversion situated downstream from the preparation of said derivative component; and
also makes it possible to produce the current I
p
by time modulation in the digital controller, which modulation, in association with a lowpass filter and a current controller governed by the filtered command delivers a pumping current I
p
that is variable continuously, progressively, and without interruptions, under the control of the digital controller.
Said current I
p
is then produced in a form that does not require an integrated digital-to-analog converter, even though such a converter can cons
Aimard Frédéric
Dekoninck Christophe
Delgrange Louis
Herbin Luc
Neyrat Pierre
Larson & Taylor PLC
Ro Bentsu
Sagem SA
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