WO2014090409A1

WO2014090409A1 - Circuit for heating an oxygen sensor

Info

Publication number: WO2014090409A1
Application number: PCT/EP2013/003761
Authority: WO
Inventors: Jacques Rocher; Philippe Avian
Original assignee: Continental Automotive France; Continental Automotive Gmbh
Priority date: 2012-12-13
Filing date: 2013-12-12
Publication date: 2014-06-19
Also published as: FR2999859A1; FR2999859B1

Abstract

The invention relates to a circuit (300) for heating an oxygen sensor comprising: a heating element (302), a controllable voltage source (301) comprising a voltage converter (401) to be connected to a voltage source (400) that is suitable for generating a variable voltage at the terminals of the heating element; a control unit (402) configured to control said voltage source, such as to generate a variable voltage that ensures that the current passing through said heating element has an intensity that is lower than a predefined threshold value (It); said circuit is characterized in that the voltage converter is a chopping down-converter comprising an inductive element (404), a freewheeling diode (403) and an electronic switch (303) controlled by the control unit, said electronic switch being controlled by means of a modulation signal, the pulse width of which has a cyclic ratio that is determined on the basis of said predetermined variable voltage.

Description

Heating circuit of a lambda probe

The invention belongs to the field of control of injection in motor vehicles, and more particularly relates to a heating circuit of a lambda probe.

According to the prior art, a lambda probe (also called "probe 0 ₂ ") is used in the exhaust line of an internal combustion engine of a motor vehicle to measure the oxygen content of burnt gases. The measurement performed by the lambda probe allows the injection control and control system to control in closed loop the respective proportions of an oxidant / fuel mixture inside the internal combustion engine of the motor vehicle so that the efficiency of said internal combustion engine is optimal.

The lambda probe operates optimally from a temperature threshold T _t of the order of 300 ° C. From this temperature threshold T _t , the lambda probe quickly provides sufficiently accurate measurements. When the temperature of the probe is below the temperature threshold T _t , a heating circuit heats the lambda probe in order to more quickly obtain an operating temperature equal to or greater than said temperature threshold T _t . Typically, this kind of situation occurs on a cold internal combustion engine, starting for example.

FIG. 1 schematically represents a heating circuit 100 according to the prior art. Said heating circuit 100 comprises a heating element 101 having a first terminal connected to a battery 102 of the motor vehicle and a second terminal connected to a control module 103. The control module 103 comprises an electronic switch 104 having a first terminal connected to the heating element 101 and a second terminal connected to ground. The control module 103 further comprises a control unit 105 controlling said electronic switch 104.

When the temperature of the lambda probe is below the temperature threshold T _t , the control unit 105 closes the electronic switch 104 and a current flows in the heating element 101 which then produces heat for heating the probe lambda. The heating element 101 is of low impedance, of the order of a few ohms to a few tens of ohms. This impedance varies as a function of the temperature of said heating element 101. Typically, the value of the impedance of the heating element 101 varies between about 1.5 Ω when the heating element 101 is cold, that is to say when the temperature of said heating element 101 is substantially between -30 ° C and -20 ° C, and about 15 Ω ohms when the heating element 101 is hot, i.e. when the temperature of said heating element 101 is substantially between 350 ° C and 600 ° C. FIG. 2 shows a curve 200 representing the variation of the intensity of the current I passing through the heating element 101 as a function of time t, during a heating phase at the beginning of which said heating element 101 is cold. As shown by said curve 200, the impedance variation of the heating element 101 implies a variation of the intensity of the current flowing through the heating element 101. During the first seconds, the intensity passing through the heating element 101 can reach a ten amperes, the voltage supplied by the battery 102 being between 12 V and 14 V. Then, the intensity of the current flowing through the heating element 101 decreases gradually to a value equal to about 1 A or 2 A after 10s or 15s.

The electronic switch 104 must be dimensioned for the worst case, that is to say it must be able to withstand a current of 10 A. It is therefore large.

In addition, the control module 103 includes a current overcurrent detection system, to protect the electronic switch 104 from a possible short circuit. However, when the temperature is very low, the current flowing through the heating element 101 can reach a very high intensity, causing the overcurrent detection system to open the electronic switch 104 to protect the electronic switch 104 of the module. When the current returns to a zero value, the control unit 105 of the control module 103 again commands a closing of the electronic switch 104. This sequence of opening / closing of the electronic switch 104 can be repeated. This succession of overcurrent detection has the effect of weakening the electronic switch 104. In addition, the closures and successive openings increase the heating time of the lambda probe by the heating circuit 100.

The invention aims to solve all or part of the aforementioned problems.

To this end, the invention relates to a heating circuit of a lambda probe, said heating circuit comprising:

A heating element adapted to produce heat when a current flows through said heating element,

A controllable voltage source comprising a direct voltage DC voltage converter intended to be connected to a DC voltage source, said controllable voltage source being adapted to generate a variable voltage across the heating element,

A control unit configured to control said source of controllable voltage so as to generate a variable voltage ensuring that the current passing through said heating element is of intensity less than or equal to a predefined threshold value (It),

The heating circuit according to the invention is remarkable in that the DC voltage converter is a voltage-reducing switching converter comprising an inductive element, a free-wheeling diode and an electronic switch controlled by the control unit. control, said electronic switch being controlled by means of a pulse width modulated signal whose duty cycle is determined according to said determined variable voltage.

In particular embodiments, the heating circuit may include one or more of the following features, which may be considered individually or in any technically operative combination.

The heating circuit according to the invention comprises means adapted to measure the current flowing through the heating element, the control unit being configured to control said source of controllable voltage so as to generate, at the terminals of said heating element, a voltage variable determined according to current measurements made by said means adapted to measure the current flowing through the heating element.

The heating circuit comprises means adapted to measure the temperature of the lambda probe, the control unit being configured to control said controllable voltage source so as to generate, at the terminals of said heating element, a variable voltage determined according to measurements. temperature effected by said means adapted to measure the temperature of the lambda probe.

The heating circuit comprises a plurality of heating elements connected in parallel, each of said heating elements being associated with a dedicated lambda probe and each of said heating elements being associated with a dedicated electronic switch.

The invention will be better understood on reading the following description, given by way of non-limiting example, and with reference to the figures which represent:

- Figure 1: a schematic representation of a lambda probe heating circuit according to the state of the art;

FIG. 2: a graphical representation of the intensity of a current flowing through a heating element of a lambda probe heating circuit according to the state of the art as a function of time;

- Figure 3: a schematic representation of a lambda probe heating circuit according to an exemplary embodiment of the invention;

- Figure 4: a schematic representation of a lambda probe heating circuit according to an embodiment of the invention; - Figure 5: a graphical representation of the voltage across a heating element of a lambda probe heating circuit according to an embodiment of the invention;

- Figure 6: a schematic representation of a heating circuit of several lambda probes according to an exemplary embodiment of the invention.

In these figures, identical references from one figure to another designate identical or similar elements. For the sake of clarity, the elements shown are not to scale unless otherwise stated.

FIG. 3 shows a heating circuit 300, according to the invention, of a lambda probe comprising a heating element 302, a control unit 402 and a controllable voltage source 301. The heating element 302 is adapted to produce the heat when a current flows through said heating element 302. In one example, the heating element 302 is a resistor.

The heating of the lambda probe is carried out during recurrent phases, called "heating phases" of the heating circuit 300. During said heating phases, the current flow in the heating element 302 is regulated in order to increase the temperature. of the lambda probe. Outside of said heating phases, the flow of current in the heating element 302 is interrupted or the flow of current in the heating element 302 is regulated in order to maintain the temperature of the lambda probe.

The controllable voltage source 301 is connected in series with the heating element 302. The control unit 402 is configured to control said controllable voltage source 301 so as to generate, at the terminals of the heating element 302, a variable voltage ensuring that the current flowing through said heating element 302 during each heating phase is of intensity less than or equal to a predefined threshold value T _t .

The heating circuit 300 further comprises, in the example illustrated in Figure 3, a means adapted to measure the current flowing through the heating element 302 and / or a means adapted to measure the temperature of the lambda probe. Thus, the variable voltage to be generated by the controllable voltage source 301 is determined by the control unit 402 which controls said controllable voltage source 301 as a function of current measurements made by said means adapted to measure the current flowing through the heating element 302 and / or as a function of temperature measurements made by said means adapted to measure the temperature of the lambda probe.

Alternatively, the variable voltage to be generated by the controllable voltage source 301 is determined by the control unit 402 which controls said controllable voltage source 301 as a function of information provided by the lambda probe. The temperature measurements make it possible in particular to control the beginning and the end of the heating phase and, in some embodiments, to select a predetermined voltage profile as described in more detail below.

The current measurements make it possible in particular to estimate the impedance of the heating element 302, and thus to determine the variable voltage to be generated, ensuring that the current flowing through the heating element 302 does not exceed the predefined threshold value _t .

According to one implementation, in a first step, the means adapted to measure the current flowing through the heating element 302 measures the current flowing through the heating element 302 and / or the means adapted to measure the temperature of the lambda probe measures the temperature of the lambda probe. In a second step, the heating circuit 300 determines the variable voltage to be generated across the heating element 302 as a function of the temperature measurement and / or as a function of the current measurement. In a third step, the controllable voltage source 301 generates the variable voltage determined in the second step at the terminals of the heating element 302.

In a preferred embodiment of the heating circuit 300, during each heating phase, the variable voltage generated at the terminals of the heating element 302 by the controllable voltage source 301 increases gradually until a nominal voltage is reached. V _N. Said nominal voltage V _N is that supplied by the battery / alternator of the motor vehicle's onboard network.

The term "gradually increases" a growth in the broad sense of the variable voltage generated during a heating phase. In other words, the generated variable voltage can be constant during one or more time intervals of the heating phase.

The progressive increase in the variable voltage generated by the controllable voltage source 301 for example follows a predefined voltage profile. Such a predefined voltage profile is for example stored in a non-volatile memory of the control unit 402. This predefined voltage profile corresponds to a voltage profile previously determined by simulation or experimentation to ensure that the current flowing through the heating element 302 does not exceed the predefined threshold value l _t whatever the operating conditions of the heating circuit 300, in particular regardless of the temperature conditions of the lambda probe. In one example, several voltage profiles are predefined, associated with respective different temperatures of the lambda probe at the beginning of a heating phase. In fact, the lower the temperature of the lambda probe, the lower the impedance of the heating element 302. Consequently, the lower the temperature of the lambda probe at the beginning of a heating phase, the lower the variable voltage generated. Thus, by having several predefined voltage profiles associated with temperatures respective different from the lambda probe at the beginning of a heating phase, it will be possible to accelerate the heating of the lambda probe. Each voltage profile is advantageously predefined as corresponding, for the initial temperature considered, to the maximum voltage that can be generated without the current flowing through the heating element 302 exceeds the predetermined threshold value _t .

It is thus understood that, when one or more predefined voltage profiles are used, it is not necessary to measure the current flowing through the heating element 302.

In one embodiment, the controllable voltage source 301 includes a direct voltage DC voltage converter 401 for connection to a DC voltage source 400.

In one embodiment (see FIG. 4), the direct voltage DC voltage converter 401 is a voltage-reducing switching converter comprising an electronic switch 303 controlled by the control unit 402, an inductive element 404 connected in series. with the heating element 302 and said electronic switch 303, and a free wheeling diode 403. In one example, the electronic switch 303 is a MOS transistor.

In one example, as shown in FIG. 4, a first terminal of the electronic switch 303 is connected to the DC voltage source 400. A second terminal of the electronic switch 303 is connected to the cathode of the freewheeling diode 403 and to a first terminal of the inductive element 404. The anode of the freewheeling diode 403 is connected to ground. A second terminal of the inductive element 404 is connected to a first terminal of the heating element 302 and a second terminal of the heating element 302 is connected to ground.

The variable voltage V _R to be generated across the heating element 302, determined in the second step, is generated by controlling the electronic switch 303 of said down-converter by means of a pulse width modulated signal. generated by the control unit 402. The duty cycle of said pulse width modulated signal is determined as a function of said variable voltage, this duty cycle thus being variable.

More precisely, the pulse width modulated signal emitted by the control unit 402 controls the open or closed state of the electronic switch 303. In one example, a high state of the logic signal transmitted by the control unit 402 corresponds to in the closed state of the electronic switch 303 and a low state of the logic signal emitted by the control unit 402 corresponds to the open state of the electronic switch 303. When the electronic switch 303 is closed, the current flowing through the inductive element 404 increases linearly and no current flows through the freewheeling diode 403. When the electronic switch 303 is open, the freewheeling diode 403 becomes conductive and the current flowing through the inductive element 404 decreases. The succession of a high state and a low state of the logic signal emitted by the control unit 402 is called the switching cycle.

When the converter 401 is in steady state, the energy stored in the inductive element 404 is the same at the beginning and at the end of each switching cycle. As a result, the current flowing through the inductive element 404 is the same at the beginning and at the end of each switching cycle, which implies that the output voltage of the converter 401 is substantially equal to the voltage delivered by the DC voltage source 400. multiplied by the duty cycle.

FIG. 5 shows a curve 600 of evolution of the voltage across the heating element 302 during a heating phase, when the voltage delivered by the DC voltage source 400 is equal to 14 V. This voltage at the terminals of the heating element 302 is the output voltage of the converter 401. During the first seconds after switching on the heating circuit 300, the heating element 302 is cold. The value of the impedance of said heating element 302 is, in one example, substantially equal to 2 Ω. The voltage obtained at the terminals of the heating element 302 may be between 0 V and 5 V. When the heating element 302 is hot, the value of the impedance of said heating element 302 is, again in this example, substantially equal to 15 Ω. The voltage obtained at the terminals of the heating element 302 may be 12 V to 14 V. This change in voltage across the heating element 302 makes it possible to obtain in this example a current value passing through the heating element 302. the order of 2 At whatever the temperature, and therefore regardless of the value of the impedance of said heating element 302.

Thus control of the current by the heating circuit makes it possible to reduce the dimensions of the electronic switch 303, to reduce the heating time of the lambda probe and to eliminate the risk of deterioration of the lambda probe at startup.

As shown in FIG. 7, the heating circuit is associated in one embodiment with several lambda probes. The heating circuit then comprises a plurality of heating elements 302 connected in parallel, each of said heating element 302 being associated with a dedicated lambda probe. Each heating element 302 is further associated with a dedicated electronic switch 700.

A first terminal of each heating element 302 is connected to the converter 401. A second terminal of each heating element 302 is connected to a first terminal of the associated electronic switch 700. A second terminal of each electronic switch 700 is connected to ground. Thus, a single heating circuit 300 makes it possible to heat several lambda probes at the same time or to successively provided that the electronic switches 700 associated with the lambda probes to be heated are closed.

Claims

1. Heating circuit (300) of a lambda probe, said heating circuit (300) comprising:

A heating element (302) adapted to produce heat when a current flows through said heating element (302),

A controllable voltage source (301) having a direct voltage DC voltage converter (401) for connection to a DC voltage source (400), said controllable voltage source (301) being adapted to generate a variable voltage at the terminals of the heating element (302),

A control unit (402) configured to control said controllable voltage source (301) so as to generate a variable voltage ensuring that the current flowing through said heating element (302) is of intensity less than or equal to a predefined threshold value ( l _t ),

characterized in that the DC voltage direct voltage converter (401) is a voltage step down converter having an inductive element (404), a freewheeling diode (403) and an electronic switch (303) controlled by the control unit (402), said electronic switch (303) being controlled by means of a pulse width modulated signal whose duty cycle is determined according to said determined variable voltage.

2. Circuit according to claim 1, characterized in that it comprises means adapted to measure the current flowing through the heating element (302), the control unit (402) being configured to control said source of controllable voltage (301). ) so as to generate, at the terminals of said heating element (302), a variable voltage determined according to current measurements made by said means adapted to measure the current flowing through the heating element (302).

3. Circuit according to claim 1 or 2, characterized in that it comprises a means adapted to measure the temperature of the lambda probe, the control unit (402) being configured to control said controllable voltage source (301). so as to generate, at the terminals of said heating element (302), a variable voltage determined according to temperature measurements made by said means adapted to measure the temperature of the lambda probe.

4. Circuit according to one of claims 1 to 5, characterized in that it comprises a plurality of heating elements (302) connected in parallel, each of said elements heater (302) being associated with a dedicated lambda probe and each of said heating elements (302) being associated with a dedicated electronic switch (700).