CN109863392B

CN109863392B - Control unit for operating a lambda probe

Info

Publication number: CN109863392B
Application number: CN201780064866.0A
Authority: CN
Inventors: A-K·米特塔施; M·克劳斯
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2016-10-20
Filing date: 2017-09-19
Publication date: 2022-01-04
Anticipated expiration: 2037-09-19
Also published as: CN109863392A; EP3529602A1; DE102016220649A1; KR20190066609A; WO2018072949A1

Abstract

A control unit for operating a lambda probe having an electrochemical cell (401), wherein the electrochemical cell (401) has a first electrode (342), a second electrode (343) and a solid electrolyte arranged between the electrodes (342, 343), wherein the lambda probe is held in a limiting current operation by means of a pump voltage (303) applied between the electrodes (342, 343) of the electrochemical cell (401), whereby a pump current (304) is generated through the electrochemical cell (401) which is proportional to the residual oxygen in the exhaust gas, wherein an input potential (UA) is applied to the first electrode (342) of the electrochemical cell (401), wherein the input potential (UA) is tracked as a function of the generated pump current (304), wherein the pump current (304) is determined by means of a measuring resistor (345) connected in series with the electrochemical cell (401), wherein the second electrode (343) of the electrochemical cell (401) is connected to a virtual ground (355) directly or via the measuring resistance, wherein the potential of the virtual ground (355) is varied as a function of the input potential (UA).

Description

Control unit for operating a lambda probe

Technical Field

The invention relates to a control unit for operating a lambda probe having an electrochemical cell.

Background

DE 102013224811 a1 already discloses a control unit for operating a cell broadband λ probe of an exhaust gas aftertreatment system of an internal combustion engine, which control unit is held in a limiting current mode by means of a pump voltage, as a result of which a pump current proportional to the residual oxygen in the exhaust gas is generated, wherein the pump voltage is tracked as a function of the generated pump current.

The pump voltage is determined from the pump current by means of the stored characteristic curve, which is determined by means of a measuring resistor connected to a virtual ground, which acts as a current source and/or sink (Stromsenke) and supplies a constant voltage.

Disclosure of Invention

The invention is based on the following tasks: the accuracy, the operational safety and the compatibility with a predefined lambda probe of a previously known control unit are improved.

This object is achieved by a control unit having the features of claim 1. Here, the present invention is based on the following findings: in order to operate a lambda probe with electrochemical cells in limiting current operation, there are conflicting boundary conditions with regard to the pump voltages applied to it. On the one hand, the potential of the virtual ground (also referred to below as virtual ground) must be chosen as small as possible in order not to impose a very large burden on the voltage budget given by the available supply voltage, so that sufficient tracking of the input potential is no longer completely possible in the case of large pump currents. On the other hand, even in rich operation (fetttobject), the potential of the virtual ground must always be greater than the voltage drop of the maximum desired pump current over the measuring resistance.

According to the invention, such conflicts are resolved by: the potential of the virtual ground is changed according to the input potential.

In particular, when the input potential decreases, the potential of the virtual ground increases, and when the input potential increases, the potential of the virtual ground decreases.

In particular in lean operation (magerbrieb), for example when a relatively large pump current occurs and a voltage drop (associated therewith) occurs across the measuring resistor, the potential of the virtual ground is relatively low, so that limiting current operation can be maintained.

On the other hand, in particular in rich operation, the potential of the virtual ground is relatively high and may exceed the voltage drop of the pump current over the measuring resistor.

In any case, a measuring resistor with a relatively high resistance value (for example at least 20 to 600 ohms) can be used with the method according to the invention.

For example, it is possible and advantageous to vary the potential of the virtual ground as a function of the input potential in such a way that the input potential assumes a value

When the potential of the virtual ground decreases, the potential of the virtual ground increases by the value, for example. It is also possible that the potential of the virtual ground changes in dependence on the input potential such that when the input potential is at

And

when the value in between decreases, the potential of the virtual ground is, for example, such as

And is increased.

The potential of the virtual ground can be changed quickly and reliably in accordance with the input potential by an analog circuit or a digital circuit, for example, by: the virtual ground is implemented by a circuit having at least one operational amplifier, the inverting input of which and its output are connected to one another via a first resistor, and the inverting input of which is also connected to the input potential via a second resistor.

In a further embodiment, provision can be made for a constant potential to be applied to the non-inverting input of the operational amplifier, so that the potential of the virtual ground has the value VM ═ UB- (UA-UB) × R1/R2, where VM is the potential of the virtual ground, UB is the constant potential applied to the non-inverting input of the operational amplifier, UA is the input potential, R1 is the ohmic resistance of the first resistance, and R2 is the ohmic resistance of the second resistance.

It is preferable that: 1/3R1< R2<3R1, in particular R1 ═ R2.

Drawings

Figure 1 shows a block diagram of a control unit according to the invention,

fig. 2 shows the following analog circuit: the analog circuit is used for changing the potential of the virtual ground VM according to the input potential UA.

Detailed Description

Fig. 1 shows a control unit according to the invention with a circuit arrangement according to the invention or with such a circuit arrangement for operating a preferably previously described cell limiting current detector 330. A known limiting current detector 330 is connected to the circuit arrangement via an IPE connection 343 (inner pump electrode, second electrode) and the mentioned ALE connection 342 (outer pump electrode or exhaust electrode, first electrode).

The limiting current probe 330 is used to generate a pump current (Ip) 304. As is known, the pump current 304 determined by the limiting current probe 330 is a measure of the residual oxygen content in the exhaust gas. When Ip is 0, λ is 1. If the air-fuel mixture is relatively lean (λ >1), the pump current 304 flows inside the probe from the exhaust electrode (ALE)342 to the Internal Pump Electrode (IPE) of the limiting current probe 330. Conversely, if the mixture is relatively rich (λ <1), the pump current 304 flows in the opposite direction.

The voltage generator 315 is used to generate a pump voltage (Up)303 by means of a filter, in particular a low-pass filter 305, wherein the value of the pump voltage 303, which is dependent on the respective pump 304, can be adjusted ("Up tracking") in a known manner by means of a Pulse Width Modulation (PWM) signal 307. In the present embodiment, the pump current 304 is provided by a pump current generator 405, which may be disconnected from the subsequent part of the circuit by a fourth switch (T4) 400.

Such a pump current generator 405 and fourth switch 400 are not necessarily necessary, as illustrated by the dashed line 380, since the pump current 304 may in principle also be provided by the microcontroller 310.

The PWM signal 307 is provided by a voltage generator 315 and is in the frequency range of about 20 to 30kHz in the present embodiment. It should be noted, however, that the invention or its application is not limited to this frequency range. The generator 315 is disposed in an internal or external microcontroller (μ C) 310. Furthermore, the pump voltage 303 thus adjusted can be smoothed by the first low-pass filter 305. The pump voltage 303 is re-read back into the microcontroller through a second low pass filter 306 and an analog to digital converter (ADC) 320.

The pump current 304 is determined by means of the measuring resistor 345. If necessary, for example for reasons of accuracy, the voltage value dropped across the measuring resistor can be increased, for example, by means of a differential amplifier (not shown) or by means of the ADC 320 already present in the microcontroller 310. The pump voltage 303 to be generated is generated by the pump current 304 with an adjustable time delay by means of a characteristic 325 stored in the microcontroller 310.

In order that the pump current (Ip)304 can flow in the two current directions 395 mentioned, a "virtual ground" (VM)355 is arranged behind the measuring resistor 345. In the present exemplary embodiment, virtual ground 355 serves not only as a current source but also as a current sink and provides a potential behind measuring resistor 345, which therefore provides the following potential: this potential is not directly dependent on the pump current 355 and is fixed or constant for that matter. If necessary, for example for accuracy reasons, the voltage value can be read back again by an additionally arranged third low-pass filter 370. As illustrated by the dashed line 375, the read back of the VM value is merely preferred, but not necessary for the way the circuit arrangement operates.

In order for current to flow within the probe from the second electrode 343 to the first electrode 342 as well, the voltage level on the ALE 342 must be less than the voltage level on the IPE 343. Since only in this case can this reverse current flow be achieved in the rich exhaust gas. According to the invention, the value of the constant voltage (i.e. the potential of the virtual ground 355) depends on the input potential UA: when the input potential UA decreases, the potential of the virtual ground increases, and vice versa.

According to the present invention, because the input potential UA is tracked in accordance with the generated pump current 304, the potential of the virtual ground 355 is also caused to be indirectly affected by the pump current 355. However, this effect is only indirect and does not change the nature of the virtual ground: virtual ground is a fixed potential as follows: the fixed potential is not directly dependent on the pump current Ip and is fixed or constant for this reason.

The dependence of the potential of the virtual ground 355 on the input voltage UA can be realized by an analog circuit shown in fig. 2. Alternatively, a digital circuit may be implemented.

In this example, both the pump current generator 405 and the virtual ground 355 are implemented as operational amplifier circuits.

In this case, the virtual ground 355 is assigned an operational amplifier OP1, whose inverting input OP1 and output OP1A are connected to one another via a first resistor R1, and whose inverting input OP1 is also connected to the input potential UA via a second resistor R2. In this example, the input potential UA is given by a PWM input signal 307 smoothed by a low-pass filter 305. UB is a constant potential applied to the non-inverting input OP + of the operational amplifier OP 1. This potential can be provided by a separate voltage source or be taken off from another already mentioned voltage by means of a voltage divider. The potential of virtual ground 355 has a value VM, where:

VM＝UB-(UA-UB)*R1/R2

in a special example, R1 ═ R2, i.e. VM ═ 2 UB-UA.

The measuring resistor may have an ohmic resistance of 20 to 600 ohms.

By selecting the resistances R1 and R2, not only a symmetric handling of the electrochemical cell 401, but also an asymmetric handling of the electrochemical cell can be achieved. In particular, it is possible to realize: the virtual ground 355 is only changed in the partial region controlled by the input voltage UA and has a constant value (i.e., independent of the input voltage) in the other cases.

It is obvious that the circuit arrangement according to the invention can be constructed not only with discrete components but also integrated in an application-specific integrated circuit (ASIC).

Claims

1. A control unit for operating a lambda probe having an electrochemical cell (401), wherein the electrochemical cell (401) has a first electrode (342), a second electrode (343) and a solid electrolyte arranged between these electrodes (342, 343), wherein the lambda probe is held in a limiting current operation by means of a pump voltage (303) applied between the electrodes (342, 343) of the electrochemical cell (401), whereby a pump current (304) is generated through the electrochemical cell (401) which is proportional to the residual oxygen in the exhaust gas, wherein an input potential (UA) is applied to the first electrode (342) of the electrochemical cell (401), wherein the input potential (UA) is tracked as a function of the generated pump current (304), wherein the pump current (304) is determined by means of a measuring resistor (345) connected in series with the electrochemical cell (401), wherein the second electrode (343) of the electrochemical cell (401) is directly connected to a virtual ground (355), wherein the potential of the virtual ground (355) is varied depending on the input potential (UA).

2. The control unit according to claim 1, characterized in that the potential of the virtual ground (355) increases when the input potential (UA) decreases, and/or in that the potential of the virtual ground (355) decreases when the input potential (UA) increases.

3. The control unit according to claim 1 or 2, characterized in that the potential of the virtual ground (355) is changed in dependence on the input potential (UA) by means of an analog circuit or a digital circuit.

4. Control unit according to any of the preceding claims, characterized in that the virtual ground (355) is implemented by a circuit with at least one operational amplifier (OP1), the inverting input (OP1-) of which and the output (OP1A) of which are connected to each other by a first resistor (R1), and the inverting input (OP1-) of which is also connected to the input potential (UA) by a second resistor (R2).

5. The control unit of claim 4, characterized in that a constant potential (UB) is applied to the non-inverting input (OP1+) of the operational amplifier (OP1), so that the potential of the virtual ground (355) has the following value:

VM＝UB-(UA-UB)*R1/R2，

wherein VM is a potential of the virtual ground (355), UB is a constant potential (UB) applied to a non-inverting input terminal (OP1+) of the operational amplifier (OP1), UA is the input potential (UA), R1 is an ohmic resistance of the first resistor (R1), and R2 is an ohmic resistance of the second resistor (R2).

6. Control unit according to any of the preceding claims, characterized in that the input potential (UA) is provided by an operational amplifier (OP2) operating with negative feedback.