Device for the electrical power supply of an oxygen pump forming part of a linear oxygen probe

Abstract: The device comprises a bridge of transistors (T1, T2, T3, T4) controlled by a microprocessor (µP). The latter processes a measurement signal (Vs) delivered by a measurement cell (1) of the probe in order to control the direction and duration of the passage through an oxygen pump (2) of a current I of predetermined strength, brought under the control of (Vs). A periodic ajternating control, with a variable cyclic ratio, of the current sent through the probe is provided by appropriate switching of the transistors in the bridge. Application to the measurement of the air/fuel ratio in the mixture supplied to an internal combustion engine.

Description: The present invention relates to a device for the electrical power supply of an oxygen pump forming part of a linear oxygen probe and, more particularly, to such a probe used for the measurement of the oxygen concentration in the exhaust gases from an internal combustion engine. It is known that an important parameter in the operation of an internal combustion engine is the air coefficient lambda (λ) representing the air:fuel ratio in the mixture supplied to the engine, divided by the ratio corresponding to a stoichiometric mixture. The importance of this parameter comes from its effect on the engine power, on its specific consumption and, above all, on the composition of the exhaust gases from the engine, particularly noxious gases. Antipollution laws of ever greater severity oblige engineers to provide a regulation of this coefficient λ, making it possible to obtain as complete a combustion of the mixture as possible. To this effect, the composition of the latter is measured indirectly by detection of the amount of oxygen present in the exhaust gases. Given that these gases contain oxygen even when the mixture is "rich" (λ < 1), the measurement of this amount of oxygen enables the composition of the mixture supplied to the engine to be measured. For this purpose, a sensor known under the name of "oxygen probe" or "lambda probe" is used, the said probe being placed in the current of exhaust gases from the engine. Conventionally, such a probe consists of a mass of zirconium or titanium oxide placed between two electrodes, one of the electrodes being exposed to the exhaust gases through a protective layer of porous ceramic, while the other electrode is in contact with the open air. A voltage is observed between the electrodes which is a function of the difference in the oxygen ion concentration at the level of the two electrodes. In particular, a jump in voltage is observed at around λ = 1, due to the particular properties of the material used, zirconium oxide or titanium oxide. As a result of this jump, such a probe supplies a signal which in practice takes on only two states, representative of a mixture in which either λ < 1 or λ > 1. It is therefore impossible to measure the real departure of the mixture from its stoichiometric composition. To overcome this drawback, an oxygen probe described as "linear" has been designed which is capable of supplying a signal substantially linearly proportional to the coefficient λ both above and below the stoichiometric composition. Such a linear oxygen probe normally comprises, as represented in Figure 1 of the appended drawing, a measurement cell 1 and an oxygen pump 2 separated by a measurement cavity 3, the measurement cell 1 being itself bounded on its other face by a reference cavity 4. The measurement cell 1 consists of a standard oxygen probe comprising a body 5 made of zirconium or titanium oxide for example, and two electrodes 6 and 7 placed against this body, in the measurement cavity 3 and in the reference cavity 4 respectively. The measurement cell delivers across these electrodes a voltage Vs which depends on the amount of oxygen present in the measurement cavity. The latter is in communication with the exhaust gases from the engine, as is a volume 8 which surrounds an electrode 9 of the oxygen pump whose other electrode 10 lies in the measurement cavity 3. The oxygen pump 2 comprises, like the measurement cell 1, a mass of zirconium or titanium oxide placed between the electrodes 9 and 10. Passing a current I p through the oxygen pump 2 causes oxygen to arrive in the measurement cavity or causes it to be extracted from this cavity, depending on the direction of the current. By bringing this current and its direction under the control of the voltage Vs delivered by the measurement cell 1, a closed loop regulation of the oxygen concentration in the measurement cavity is ensured by a supply of oxygen to, or a withdrawal of oxygen from, this cavity depending on the oxygen concentration in the exhaust gases entering this cavity. The current I p is then a substantially linear function of this oxygen concentration and thus of the coefficient λ representative of the composition of the air-fuel mixture. As an example of a probe which operates according to this principle, the probe sold under the name NTK UEGO by the Japanese company NGK Spark Plug Co. Ltd may be quoted. The problem is then to bring the current I p supplied to the pump under the control of the voltage Vs delivered by the measurement cell. For this purpose, use is conventionally made of a complex PID-type analog circuit consisting of many analog amplifiers and precision components. Such an analog circuit is subject to drifting. Its complexity and the presence of precision components make the cost prohibitive. Moreover, since the functioning of the feedback control is fixed by the structure of the circuit, this functioning cannot be modified by using various strategies programmed in advance, as is the case with digital systems controlled by microprocessor for example. Document EP 0 079 085 does not use feedback control but sets a given oxygen concentration in the reference cavity by exhausting completely then filling it to the predetermined concentration by way of a controlled quantity of current. The voltage Vs gives thereafter an indication whether the actual concentration of the gas to be measured is above or under the predetermined concentration. Such a system is slow and cannot give a real time measure of oxygen concentration. Document EP 0 025 625 discloses a device providing a constant current to the oxygen pump. This device comprises a changeover device adapted to reverse the direction of the current through the pump each time the voltage Vs crosses a given value. According to this strategy, illustrated in Figure 2B, the measure of the successive times t₁, t₂, etc. necessary to make the voltage Vs switch over are a function of the oxygen concentration. However, this strategy has several drawbacks. Firstly, the inertia of the probe is such that the measured time t₂ suffers from an overshoot in the previous time t₁. Secondly, the measurement depends on the precision of the adjustment in the value of the current. Finally, around λ = 1, the times to be measured become very short, which entails a reduction in the precision of the measurement just where it is most needed. The present invention therefore has as its object the production of a device for the electrical power supply of an oxygen pump forming part of a linear oxygen probe which does not have these drawbacks. More particularly, the invention has as its object the production of such a device which has a simple structure with a small number of components, which is reliable and which is cheap to produce. The present invention also has as its object the production of such a device making it possible to ensure the precise regulation of the current supplied to the oxygen pump, with correction for drifts in temperature. The present invention has yet again as its object the production of such a device incorporating means for enabling its functioning to be controlled according to various strategies. These objects of the invention are achieved, together with others which will emerge in the remainder of the present description, with a device for the electrical power supply of an oxygen pump forming part of a linear oxygen probe used for the measurement of the oxygen concentration in the exhaust gases from an internal combustion engine, this probe comprising in addition a measurement cell sensitive to the amount of oxygen present in a measurement cavity of the probe in communication with the exhaust gases and the oxygen pump in order to deliver a corresponding measurement signal, this signal controlling the electrical power supply to this pump. The device is of the type having a source of electrical energy and means for connecting this source to the oxygen pump so as to provide it with an alternating current, and, according to the invention, these means are adapted to provide a periodic alternating current of fixed frequency and of variable cyclic ratio controlled by the signal delivered by the measurement cell, the value of said cyclic ratio being a measure of the oxygen concentration in the exhaust gases. According to one characteristic of the present invention, the said means comprise an "H" bridge of transistors consisting of two pairs of transistors, the emitter-collector circuits of the transistors in each pair being connected in series, the oxygen pump being connected between the terminals common to the emitter-collector circuits of the transistors of each pair, the pairs of emitter-collector circuits in series each being connected between the source of electrical energy and earth through a common measurement resistor. In addition, the base of the transistor of each pair, whose emitter is connected to said common resistor, is connected to a common reference voltage generator in order to control the strength of the current flowing through the oxygen pump. These means also comprise means for selectively and individually controlling the conduction the transistors in the bridge in such a way the sign of the average current through the pump is reversed after each switchover of the measurement signal. In each pair of transistors of the bridge, the transistors are of opposite types, the means for controlling the conduction of the transistors simultaneously blocking one transistor of each pair and unblocking the other transistor by a control common to their bases, the controls of the two pairs of transistors being determined by the measurement signal in order to control the direction and duration of the passage of the current produced by the source of electrical energy through the oxygen pump. The means for selective connection of the source of electrical energy to the oxygen pump are designed so as to be able to put into operation several different strategies for the electrical supply to this oxygen pump. Other characteristics and advantages of the present invention will emerge on reading the description which follows and on examining the appended drawing in which: Figure 1 is a diagram of the device for the electrical power supply of an oxygen pump forming part of a linear oxygen probe of the type described in the preamble to the present description, Figures 2A to 2D represent various wave forms useful in the description of various strategies for controlling the current through the oxygen pump, and Figure 3 is a graph illustrating another strategy for regulating the current through the oxygen pump. Reference is made to Figure 1 in the appended drawing, which is a diagrammatic representation of the linear oxygen probe described in the preamble to the present description. Figure 1 also represents the device for the electrical power supply of the oxygen pump of the probe according to the present invention. This device consists essentially of a source of electrical energy delivering a voltage +V and of means for selectively connecting this source in series with the electrodes 9, 10 of the oxygen pump 2, in such a way as to bring under the control of the measurement signal Vs delivered by the measurement cell 1, the direction and duration of the passage through the pump of a current Ip of predetermined strength. These means comprise essentially an "H" bridge of transistors (T1, T2, T3, T4) and a microprocessor µP selectively controlling the conduction of the transistors in the bridge by signals emitted at the output pins C1 and C2. The microprocessor is itself supplied by the output signal Vs from the measurement cell 1 through an analog-digital convertor CAN. The microprocessor is duly programmed in order to emit at the pins C1 and C2 signals making it possible to adjust the average current of the pump 2 in such a way as to regulate the oxygen concentration in the measurement cavity 3 and hence the value of the output voltage Vs from the measurement cell 1. This regulation is implemented by the extraction of oxygen from the measurement cavity 3 under the action of the oxygen pump 2. Thus, when the oxygen concentration in the exhaust gas from an internal combustion engine (which fills the cavities 8 and 3 of the probe) is representative of an air/fuel mixture supplied to the engine characterized by an air coefficient λ > 1, there is an extraction of oxygen from the cavity 3 under the action of the pump, I p being suitably adjusted by the device according to the invention, as will be seen in what follows, so as to bring the oxygen concentration in the cavity 3 back to a value corresponding to λ = 1. Conversely, if the oxygen concentration corresponds to an air coefficient λ < 1, oxygen is pumped into the cavity through the oxygen pump excited by a current I p with a direction which is the reverse of that corresponding to the extraction of oxygen from this cavity. In this way, the current through the oxygen pump is representative of the difference between the composition of the air/fuel mixture and the composition corresponding to λ = 1, the variation of I p as a function of this difference being substantially linear. According to the previous art, I p is adjusted by complicated PID-type analog circuits. The present invention proposes a digital control for this current, which is simpler and more flexible, as will be seen later. To this effect, the "H" bridge of transistors (T1, T2, T3, T4) is designed to provide a switching of the direction of the current I p in the oxygen pump, under the control of signals emitted at the pins C1 and C2 of the microprocessor µP. The bridge consists of two pairs of bipolar transistors, (T1, T3) on the one hand and (T2, T4) on the other. The transistors of the same pair are of opposite types. Thus, the transistors T1 and T2 are of the PNP type, for example, while the transistors T3 and T4 are of the NPN type. In each pair, the emitter-collector circuits of the two transistors are connected in series between a line at the voltage +V and earth. The common terminals of the emitter-collector circuits of the transistors T1, T3 on the one hand and of T2, T4 on the other, are connected respectively to the electrodes 9 and 10 of the oxygen pump 2. Load resistors R1 and R2 are connected to the emitters of the transistors T1 and T2 respectively, between the latter and the line at +V. The emitters of the transistors T3 and T4 are connected to each other and their common point is connected to earth through an earthing resistor R7. The bases of the transistors T1, T3, T2, T4 are loaded by resistors R3, R5, R4, R6 respectively. The points common to the resistors R4 and R6 on the one hand and to R3 and R5 on the other are connected to the pins C1 and C2 respectively of the microprocessor µP. The device also comprises a generator of a reference voltage consisting of two resistors R8 and R9 placed in series between the line at +V and earth. The central point of the voltage divider formed in this way is connected to the bases of the transistors T3 and T4. Diodes D1 and D2, connected in a forward-biased configuration between the bases of T3 and T4, respectively, and the central point of the voltage divider (R8, R9), provide a compensation of the variations in the base-emitter voltages V be of the transistors T3 and T4 due to temperature variations, for a purpose that will be explained later. With the circuit which has just been described, it is clear that if, for example, the microprocessor µP establishes a zero voltage on its pin C1, the transistor T4 is blocked, whereas the transistor T2 is saturated. In this way, the voltage +V is applied, through the resistor R2 and the emitter-collector circuit of the transistor T2, to the electrode 10 of the oxygen pump. If, in addition, the microprocessor simultaneously establishes on its pin C2 a voltage V₁ of suitable magnitude, this voltage causes the blockage of the transistor T1 of the other pair of transistors (T1, T3) and the unblocking of the transistor T3. The electrode 9 of the oxygen pump 2 is then connected to earth through the collector-emitter circuit of the transistor T3 and the earthing resistor R7. In this way, a potential difference is set up between the electrodes 9 and 10 of the oxygen pump and a current I p flows in the pump in the direction opposite to that of the arrow shown between the electrodes 9 and 10, which denotes a direction for the current chosen arbitrarily as the positive direction. The current which then passes into the probe is that which, having passed through the resistor R2 and the transistor T2, flows to earth through the transistor T3 and the fixed resistor R7. It is clear that the strength I of the current in this circuit is then determined by the bias of the base of the transistor T3, which then functions as a current generator, i.e. by the voltage at the central point of the divider bridge R8, R9. In this connection, it will be noted that the diode D1 compensates for a variation with temperature of the voltage V be of the transistor T3 in order to prevent in this way a drift of the bias voltage of this transistor, and thus to maintain the current I p at a predetermined value. It is clear that, by reversing the voltages set up above on the pins C1 and C2 of the microprocessor, the direction of the current in the oxygen pump 2 is also reversed. Similarly, by applying equal or zero voltage signals to the pins C1 and C2, any passage of current through the oxygen pump is cut off. The table below compiles the various situations mentioned above. It can thus be seen that, according to the invention, it is possible to fix both the direction and duration of the passage through the pump 2 of a current of predetermined strength I. Also according to the invention, the microprocessor µP is duly programmed to emit at the pins C1 and C2 control signals enabling the average current passing through the oxygen pump 2 to be brought under the control of the measurement signal Vs digitized by the convertor CAN. As a result of such control, the average value of the current is, at each instant, representative of the air coefficient λ of the air/fuel mixture supplied to the internal combustion engine. The maximum value Imax of the current passing through the pump is fixed by the voltage divider R8, R9, the measurement resistor R7 and the supply voltage +V of the bridge of transistors (T1, T2, T3, T4). Because the diodes D1 and D2 exactly compensate the variations with temperature of the voltages V be of the transistors T3 and T4, the current I through the oxygen pump 2 has almost the same absolute value Imax in both of the directions in which this current flows. Thus, by applying appropriate signals to the pins C1 and C2, the microprocessor can adjust with precision the average current passing through the pump between two values +Imax and -Imax. Various strategies are then possible for bringing this current under the control of the voltage signal Vs. According to a strategy illustrated in Figure 2C, the means adopted for selectively connecting the source of electrical energy to the oxygen pump 2 provide a periodic supply with a variable cyclic ratio to the pump of a predetermined current Imax whose direction is reversed at each switchover of the measurement signal Vs. Since the control has a period T, the average current over a period during which the current Imax is applied for a time t₃ has the form : Iaverage = t₃/T x Imax, if Vs is positive Iaverage = t₃/T x (-Imax), if Vs is negative. This strategy has the drawback that it makes the measurement of Iaverage depend on the precision of +Imax and -Imax. Moreover, near λ = 1, the cyclic ratios are small because the time t₃ is very much less than T. Certain periods might even pass by in their entirety with a zero current, which impairs the speed and precision of the measurements. The graph of Figure 2D illustrates a preferred control strategy of the device according to the present invention. According to this, the means for selectively connecting the source of electrical energy to the oxygen pump provide an alternating periodic supply with a variable cyclic ratio, the sign of the average current being reversed after each switchover of the measurement signal Vs. Thus, referring to Figure 2D, over a period during which a current +Imax has been applied for a time t₄, we have: Iaverage = Imax x t₄/T -+Imax x (T-t₄)/T or, with a cyclic ratio r = t₄/T, Iaverage = Imax x (2r - 1). Near λ = 1, Iaverage must be small, with the cyclic ratio r then becoming established at about 50%. Because Imax and -Imax are fixed by the same voltage divider (R8, R9), an error in Imax is compensated by the same error in (-Imax), and the final measurement of λ that is obtained does not depend on the precision in the maximum strength of the current delivered by the device according to the invention, which is an undoubted advantage of the strategy described above. All the strategies mentioned so far have worked with values of Imax fixed in absolute value. It is also possible to make this parameter vary, for example as a function of the range that occurs in the richness of the air/fuel mixture, in order to increase the dynamic range of the measurement. It is easy to obtain, with control by microprocessor, a variation in Imax by switching between several resistors R7 or several divider bridges R8, R9, for example. The graph of Figure 3 illustrates the variation of the average current through the oxygen pump as a function of λ when Imax is switched over a greater value when λ changes to the value a. Over the region a near λ = 1, it is possible to obtain a high resolution since the ratio of variations ΔIaverage/Δr is small. On the other hand, in the region b which extends beyond the region a , it is possible to have a large dynamic range since the ratio ΔIaverage/Δr is high. It is now evident that the device for the power supply of the oxygen pump of a linear oxygen probe according to the invention has many advantages. Firstly, the device has great simplicity and contains a small number of components, which has a beneficial effect on its manufacturing cost. The precision of the measurements that are obtained is high because only the three resistors R7, R8, R9 and the voltage +V affect the initial precision of the device. Moreover, since the latter is controlled by a microprocessor, this imprecision can be compensated by a numerical calibration consisting in storing in the microprocessor correction parameters as a function of the observed initial drift. The temperature drift of the circuit is very small because of the compensation of the voltages V be of the transistors T3 and T4 by the diodes D1 and D2, such a temperature drift depending on no more than the differential drift of the resistors, on the voltage V and on the differential variation in the gains of the transistors. The device is reliable since the circuit is self-protected against short circuits to earth thanks to the limitations of current established by the resistors R1 and R2, and against short circuits of the power supply, constituted generally by the battery of the vehicle in automobile electronics. The device according to the invention also possesses a great flexibility in operation because a microprocessor is used to ensure the control of the probe and because this microprocessor can be programmed in such a way as to establish one or another of several control strategies of varying degrees of sophistication. It is known that an oxygen probe must be fitted with means of heating since the signal provided is only reliable above a certain temperature, for example of the order of 200°C. Since the microprocessor can also carry out the control of the heating of the probe and the measurement of the signal Vs delivered by this probe, this microprocessor can thus ensure the control of the whole system. Of course, the invention is not limited to the embodiment described and illustrated, which has only been given as an example. In particular, the invention is not limited to a device produced with bipolar technology and could just as easily be produced with MOS technology, for example.

Claims: Device for the electrical power supply of an oxygen pump forming part of a linear oxygen probe used for the measurement of the oxygen concentration in the exhaust gases from an internal combustion engine, this probe comprising in addition a measurement cell (1) sensitive to the amount of oxygen present in a measurement cavity (3) of the probe in communication with the exhaust gases and the oxygen pump (2) in order to deliver a corresponding measurement signal (Vs), this signal controlling the electrical power supply to this pump, said device being of the type having a source of electrical energy and means for connecting this source to the oxygen pump so as to provide it with an alternating current, and being characterized in that said means are adapted to provide a periodic alternating current of fixed frequency and of variable cyclic ratio controlled by the signal (Vs) delivered by the measurement cell (1), the value of said cyclic ratio being a measure of the oxygen concentration in the exhaust gases. Device according to Claim 1, where the said means comprise an "H" bridge of transistors consisting of two pairs (T1, T3) and (T2, T4) of transistors, the emitter-collector circuits of the transistors in each pair being connected in series, the oxygen pump (2) being connected between the terminals common to the emitter-collector circuits of the transistors of each pair, characterized in that the pairs of emitter-collector circuits in series are connected between the source of electrical energy and earth through a common measurement resistor (R7) and in that the base of the transistor (T3, T4) of each pair, whose emitter is connected to said common resistor, is connected to a common reference voltage generator (+V, R8, R9) in order to control the strength of the current flowing through the oxygen pump. Device according to Claim 2, characterized in that the said means also comprise means (µP) for selectively and individually controlling the conduction of transistors in the bridge at a given frequency with a cyclic ratio determined in such a way that the sign of the average current through the pump is reversed after each switchover of the measurement signal (Vs). Device according to either Claim 2 and 3, characterized in that it comprises means for varying selectively the strength of the current flowing through the oxygen pump. Linear oxygen probe having an oxygen pump, characterized in that it comprises a device for the electrical power supply of the pump according to any one of the preceding claims.

Inventors: METRICH PIERRE MARC ROLAND, METRICH, PIERRE MARC ROLAND
Applicants: SIEMENS AG, SIEMENS AKTIENGESELLSCHAFT
Application year: 1990
Main classification: G01N27
IPC codes: G01N27