DE102016208506A1 - Method for operating a sensor element for detecting at least one property of a measurement gas in a sample gas space - Google Patents

Abstract

A method is proposed for operating a sensor element (10) for detecting at least one property of a measurement gas in a measurement gas space, in particular for detecting a proportion of a gas component in the measurement gas or a temperature of the measurement gas, wherein the sensor element (10) comprises at least one solid electrolyte body (12 ), a pump cell (40), and a Nernst cell (46), wherein at a predetermined property of the sample gas, a target pumping voltage (58) of the pump cell (40) is used as a reference variable of a control for regulating a pumping voltage, wherein an actual Pumping voltage (60) and an actual pumping current (66) of the pump cell are detected, wherein the actual pumping current (66) is determined as a correction value, provided that the actual pumping voltage (60) of the desired pumping voltage (58) corresponds, wherein at a Deviation from the predetermined property of the measuring gas, the correction value is used in a regulation for controlling the pumping current.

Description

State of the art

A large number of sensor elements and methods for detecting at least one property of a measurement gas in a measurement gas space are known from the prior art. In principle, these can be any physical and / or chemical properties of the measurement gas, one or more properties being able to be detected. The invention will be described below in particular with reference to a qualitative and / or quantitative detection of a proportion of a gas component of the measurement gas, in particular with reference to a detection of an oxygen content in the measurement gas part. The oxygen content can be detected, for example, in the form of a partial pressure and / or in the form of a percentage. Alternatively or additionally, however, other properties of the measuring gas are detectable, such as the temperature.

In particular, ceramic sensor elements are known from the prior art, which are based on the use of electrolytic properties of certain solids, that is to ion-conducting properties of these solids. In particular, these solids may be ceramic solid electrolytes such as zirconia (ZrO ₂ ), in particular yttrium stabilized zirconia (YSZ) and scandium doped zirconia (ScSZ), the minor additions of alumina (Al ₂ O ₃ ) and / or silica (SiO ₂ ) ₂ ).

For example, such sensor elements can be configured as so-called lambda probes, as they are made, for example Konrad Reif (ed.): Sensors in the motor vehicle, 1st edition 2010, pp. 160-165 , are known.

With broadband lambda probes, in particular with planar broadband lambda probes, it is possible, for example, to determine the oxygen concentration in the exhaust gas over a large range and thus to deduce the air-fuel ratio in the combustion chamber. The air ratio λ describes this air-fuel ratio.

In internal combustion engines, a catalytic converter is located in the exhaust gas tract in order to comply with exhaust gas regulations. To control the internal combustion engine, a broadband lambda probe is usually arranged upstream of the exhaust gas flow of the catalytic converter and a so-called jump probe downstream of the exhaust gas flow of the catalytic converter. From the component package of the engine control, the pumping current is output and used for the lambda control. The pumping current is converted in the engine control unit into a lambda signal and this is used for the control. The previous adjustment of the broadband lambda probe upstream of the catalyst via the jump probe downstream of the catalyst. At the moment when the jump probe downstream of the catalyst, which is very accurate at λ = 1, is active and a λ = 1 is set, the broadband lambda probe reading can be corrected upstream of the catalyst. The broadband lambda probe upstream of the catalyst is more inaccurate at λ = 1 than the jump probe downstream of the catalyst, but may also measure lambda values not equal to one.

Despite the advantages of the sensor elements known from the prior art and methods for operating the same, these still contain room for improvement. Thus, in the current use of the jump probe upstream of the catalyst to adjust the broadband lambda probes upstream of the catalyst an operational lambda probe must be present.

Disclosure of the invention

A method is therefore proposed for operating a sensor element for detecting at least one property of a measurement gas in a measurement gas space, which at least largely avoids the disadvantages of known methods and in which an adjustment of the broadband lambda probes upstream of the catalyst can take place independently of a jump probe downstream of the catalyst ,

The method according to the invention is suitable for operating a sensor element for detecting at least one property of a measurement gas in a measurement gas space, in particular for detecting a proportion of a gas component in the measurement gas or a temperature of the measurement gas. The sensor element has at least one solid electrolyte body, a pump cell, and a Nernst cell. In the case of a predetermined property of the measuring gas, a desired pumping voltage of the pumping cell is used as the reference variable of a control for regulating a pumping voltage. An actual pumping voltage and an actual pumping current of the pumping cell are detected. The actual pumping current is determined as a correction value, provided that the actual pumping voltage corresponds to the desired pumping voltage. In the case of a deviation from the predetermined property of the measurement gas, the correction value is used in a regulation for regulating the pumping current.

The Korekturwert can be determined by means of an offset correction filter. The correction value can be stored in the offset correction filter. Before outputting the measured value, the pumping current can be subtracted from the correction value Getting corrected. The desired pumping voltage may be, for example, 0V. When controlling for a first desired pump current, after reaching a first control target, a first actual pump current can be detected and determined as a correction value. In a second control to a second desired pumping current, the correction value can be added to the second desired pumping current. The first actual pumping current can be 0 mA. The sensor element can be arranged in an exhaust tract of an internal combustion engine. The sensor element may be connected to a control unit of the internal combustion engine. The predetermined property of the measurement gas may be adjustable by injecting fuel into at least one combustion chamber of the internal combustion engine. The predetermined composition may be set at a stoichiometric ratio of air and fuel.

Furthermore, a computer program is proposed which is set up to carry out each step of the method according to the invention.

Furthermore, an electronic storage medium is proposed, on which such a computer program is stored.

Furthermore, an electronic control unit is proposed which comprises such an electronic storage medium.

In the context of the present invention, a solid electrolyte body is to be understood as a body or article having electrolytic properties, that is to say having ion-conducting properties, for example oxygen-ion-conducting properties. In particular, it may be a ceramic solid electrolyte. For example, the solid electrolyte body may comprise stabilized zirconia and / or scandium stabilized zirconia. The solid electrolyte body may also be formed of a plurality of solid electrolyte layers. Under a layer is to understand a uniform mass in areal extent at a certain height, which is above, below or between other elements.

In the context of the present invention, an electrode is generally to be understood as meaning an element which is able to contact the solid electrolyte such that a current can be maintained or a voltage can be measured by the solid electrolyte and the electrode. Accordingly, the electrode may comprise an element to which the ions can be incorporated in the solid electrolyte and / or removed from the solid electrolyte. Typically, the electrodes comprise a noble metal electrode which may, for example, be deposited on the solid electrolyte as a metal-ceramic electrode or otherwise be in communication with the solid electrolyte. Typical electrode materials are platinum-mesh electrodes. However, other precious metals, such as gold or palladium, are in principle applicable.

In the context of the present invention, a Nernst cell is to be understood as meaning an electrochemical measuring cell which uses a solid electrolyte as membrane between two electrodes. It uses the property of the solid electrolyte, above a certain temperature ions of the measured gas to be measured, such as oxygen ions, electrolytically to be able to transport from one electrode to the other, creating a so-called Nernst voltage. This property determines the difference in the partial pressure of the sample gas on the different sides of the membrane. In the lambda probe one side of the membrane is exposed to the sample gas, while the other side is at a reference.

In the context of the present invention, a pump cell is to be understood as meaning an electrochemical cell in which a content of a component of the measurement gas, such as oxygen, in a measuring gap on the one hand via the measurement gas, which acts through a diffusion channel, and on the other hand by the current flow of the pumping cell being affected. Depending on the polarity, the pumping current pumps measurement gas from the measurement gas side of the solid electrolyte membrane into the measurement gap or out of it. In this case, the pumping current is controlled by an external regulator so that the lambda value in the measurement gas exactly balances the measurement gas flow through the diffusion channel and keeps the measurement gas in the measurement gap constant at a predetermined value such as λ = 1. For example, a lambda value of 1 is always given when the voltage at the Nernst cell is 0.45V. When the mixture is rich, the pumping current pumps measurement gas ions into the sample gas in the measuring gap, with a lean mixture.

In the context of the present invention, a heating element is to be understood as meaning an element which serves for heating the solid electrolyte and the electrodes to at least their functional temperature and preferably to their operating temperature. The functional temperature is the temperature at which the solid electrolyte becomes conductive to ions and is about 350 ° C. Of this, the operating temperature is to be distinguished, which is the temperature at which the sensor element is usually operated and which is higher than the operating temperature. The operating temperature may be, for example, 700 ° C to 950 ° C.

The heating element may comprise a heating area and at least one feed track. In the context of the present invention, a heating region is to be understood as meaning the region of the heating element which overlaps an electrode in the layer structure along a direction perpendicular to the surface of the sensor element. Usually, the heating area heats up during operation more than the supply line, since this has a higher electrical resistance, so that they are distinguishable. The different heating can thus be realized for example by the fact that the heating area has a higher electrical resistance than the supply track. The heating area and / or the supply line are formed, for example, as an electrical resistance path and heat up by applying an electrical voltage. The heating element may for example be made of a platinum mesh.

A basic idea of the method described here is that the injection of the internal combustion engine is no longer regulated to the pumping current, but is regulated in the region near λ = 1 to a pumping voltage. The aim of the control is to set the voltage between the outer pumping electrode and the reference electrode so that a constant voltage of, for example, 450 mV results. If the voltage between the outer pumping electrode and the reference electrode in the control unit is not available, but the pumping voltage is equal to the voltage between the outer pumping electrode and the inner pumping electrode, it is possible to control to a pumping voltage of 0 V, for example, because the Nernst voltage is equal to the voltage between the reference electrode and the inner pumping electrode is already regulated by the lambda probe ASIC to a value of usually 450 mV.

The method according to the invention can also be used in many ways without jumping probe. It is conceivable, for example, that at least two exhaust gas strands of a system of internal combustion engines are brought together, and only after the merger is a jump probe installed. In this case, the broadband lambda probes that sit prior to the merge can not be offset compensated using conventional techniques. However, with the method according to the invention, offset compensation of the broadband lambda probes without the use of the jump probe is possible even in such an arrangement. Again, it is not necessary to resort to an operational jump probe. As a result, offset compensation can be carried out with the method according to the invention after a shorter period of time after starting the internal combustion engine.

Brief description of the drawings

Further optional details and features of the invention will become apparent from the following description of preferred embodiments, which are shown schematically in the figures.

Show it:

1 a cross section of a sensor element,

2 a schematic representation of control circuits according to a first embodiment of the present invention, and

3 a schematic representation of control circuits according to a second embodiment of the present invention.

Embodiments of the invention

1 shows a cross-sectional view of a sensor element 10 according to a first embodiment of the invention. This in 1 illustrated sensor element 10 can be used to detect physical and / or chemical properties of a sample gas, wherein one or more properties can be detected. The invention will be described below in particular with reference to a qualitative and / or quantitative detection of a gas component of the measurement gas, in particular with reference to a detection of an oxygen content in the measurement gas. The oxygen content can be detected, for example, in the form of a partial pressure and / or in the form of a percentage. In principle, however, other types of gas components are detectable, such as nitrogen oxides, hydrocarbons and / or hydrogen. Alternatively and / or additionally, however, other properties of the measuring gas can also be detected. The invention can be used in particular in the field of motor vehicle technology, so that the measuring gas chamber can be, in particular, an exhaust gas tract of an internal combustion engine and, in the case of the measuring gas, in particular an exhaust gas.

The sensor element 10 comprises a solid electrolyte body 12 , The sensor element 10 also has a gas access path 14 on. The gas access route 14 has a gas entry hole 16 on, extending from an outside or surface 18 of the solid electrolyte body 12 inside the solid electrolyte body 12 extends. In the solid electrolyte body 12 is an electrode cavity 20 provided to the gas access hole 16 is adjacent and connected to this. The electrode cavity 20 is formed, for example cuboid. The electrode cavity 20 is part of the gas access route 14 and can over the gas access hole 16 communicate with the sample gas space. For example, the gas access hole extends 16 as a cylindrical blind hole perpendicular to the surface 18 of the solid electrolyte body into the interior of the solid electrolyte body 12 , Between the gas access hole 16 and the electrode cavity 20 is a channel 22 arranged, which is also part of the Gaszutrittswegs 14 is. The channel 22 or the electrode cavity 20 is radial or perpendicular with respect to the gas inlet hole 16 arranged. In this channel 22 is a diffusion barrier 24 arranged. The diffusion barrier 24 reduces or even prevents a subsequent flow of sample gas from the sample gas space into the electrode cavity 20 and only allows diffusion of the sample gas. In the solid electrolyte body 12 and from the electrode cavity 20 separated is a reference gas channel 26 or exhaust duct formed.

Furthermore, the sensor element 10 a heating element 28 on. The heating element 28 is in an imaginary extension of the direction in which the gas entry hole 16 extends in the solid electrolyte body 12 below the electrode cavity 20 and the reference gas channel 26 arranged. The heating element 28 has a heating area 30 , a first feeder line 32 and a second feeder line 34 on. The first feeder line 32 is doing it with a positive pole 36 of the heating area 30 connected. The second supply line 34 is with a negative pole 38 of the heating area 30 connected.

The sensor element 10 also has a pumping cell 40 on. The pump cell 40 has one in the electrode cavity 20 arranged first electrode 42 and a second electrode 44 on. The first electrode 42 is on the surface that can be exposed to the sample gas 18 of the solid electrolyte body 12 arranged. The second electrode 44 is in the electrode cavity 20 on one of the first electrode 42 arranged facing side. The pump cell 40 further comprises the part of the solid electrolyte body 12 between the first electrode 42 and the second electrode 44 , About the diffusion barrier 24 can be a limiting current in the pumping cell 40 to adjust. The limiting current represents a current flow between the first electrode 42 and the second electrode 44 over the solid electrolyte body 12 between them.

The sensor element 10 also has a Nernst cell 46 on. The Nernst cell 46 includes a third electrode 48 and a fourth electrode 50 , The third electrode 48 is located adjacent to the heating area 30 of the heating element 28 in the electrode cavity 20 , In the reference gas channel 26 is the fourth electrode 50 arranged. The fourth electrode 50 may be referred to as a so-called pumped reference in the reference gas channel 26 be arranged. That is, the reference gas channel 26 is not a macroscopic reference gas channel but a pumped reference, ie an artificial reference. The second electrode 44 and the third electrode 48 are about the solid electrolyte body 12 coupled together. The third electrode 48 , the fourth electrode 50 and the part of the solid electrolyte body 12 between the third electrode 48 and the fourth electrode 50 form, for example, the Nernst cell 46 , By means of the pumping cell 40 For example, a pumping current through the pumping cell 40 be adjusted so that in the electrode cavity 20 the condition λ = 1 or another known composition prevails. This composition in turn is from the Nernst cell 46 detected by applying a Nernst voltage between the third electrode 48 and the fourth electrode 50 is measured. As in the reference gas channel 26 or in the fourth electrode 50 , which serves as a reference electrode, an excess of oxygen prevails, can be determined by the measured voltage on the composition in the electrode cavity 20 getting closed. According to the described construction, the first electrode 42 also as outer pumping electrode, the second electrode 44 as the inner pump electrode, the third electrode 48 as Nernst electrode and the fourth electrode 50 be referred to as a reference electrode.

As is apparent from the above description of the structure of the sensor element 10 results, this is designed as a so-called two-cell, wherein the pump cell 40 and the Nernst cell 46 are formed separately. Alternatively, the sensor element 10 be designed as a so-called unicellular, in the pumping cell 40 and Nernst cell 46 combined. Such a protozoa requires only two electrodes for its function. Compared to the construction of a two-cell described above, the first electrode is eliminated 42 and the second electrode 44 , The third electrode is used for this purpose 48 as the inner pumping electrode of the pumping cell 40 and as the Nernst electrode of the Nernst cell 46 because they are on a common line. The fourth electrode 50 serves as an outer pumping electrode of the pumping cell 40 and as a reference electrode of the Nernst cell 46 ,

2 shows a schematic representation of control circuits according to a first embodiment of the present invention. A probe not shown in detail, the sensor element 10 has, is in an exhaust tract of an internal combustion engine 52 arranged by the one injection system 54 is shown schematically. The sensor element 10 is with an electronic control unit 56 the internal combustion engine 52 connected or involved in its control and regulation. For this reason, the sensor element 10 shown as part of a controlled system. The controlled system more precisely has two control circuits, as will be described in more detail below.

The first control loop becomes a desired pumping voltage 58 the pump cell 40 used as a reference variable of a control for regulating a pumping voltage. Accordingly, in the control for regulating the pump voltage as a control variable, the pump voltage is used and a control deviation between one of the sensor element 10 detected actual pumping voltage 60 and the target pumping voltage 58 a first lambda controller 62 fed. The first lambda controller 62 Again, with a λ = 1 controller 64 connected, in turn, with the injection system 54 communicates. By means of the injection system 54 is the predetermined property of the measurement gas by injecting fuel into at least one combustion chamber of the internal combustion engine 52 adjustable. From the sensor element 10 will continue to be an actual pumping current 66 the pump cell 40 detected. The sensor element 10 also stands with an offset correction filter 68 in connection to which the detected actual pumping current can be fed. At the point 70 that with the offset correction filter 68 is related by the controller 56 detects whether the actual pumping voltage 60 the desired pumping voltage 58 equivalent.

The second control circuit becomes a desired pumping current 72 the pump cell 40 used as a reference variable of a control for regulating a pumping current. Accordingly, in the closed-loop control, the pump current is used as the control variable, and a control deviation between that of the sensor element is used 10 detected actual pumping current 66 and the desired pumping current 72 a second lambda controller 74 fed. The second lambda controller 74 Again, with the λ = 1 controller 64 connected. The λ = 1 controller 64 can toggle between the first loop and the second loop.

In a predetermined property of the measurement gas, the target pump voltage 58 the pump cell 40 is used as the reference variable of the control for regulating a pumping voltage. The target pumping voltage 58 For example, it can be 0V. To do this, the λ = 1 controller switches 64 on the first loop. If the actual pumping voltage 60 the desired pumping voltage 58 corresponds, in the method according to the invention, the actual pumping current 66 determined as a correction value. The Korekturwert is using the offset correction filter 68 determined. The correction value is in the offset correction filter 68 saved. In the case of a deviation from the predetermined property of the measurement gas, the correction value is used in the regulation for regulating the pumping current. In the regulation for controlling the pumping current, the detected actual pumping current becomes 66 corrected by subtracting the correction value.

In other words, in the method according to the invention, an offset adjustment of the sensor element 10 without additional jump probe after a catalyst or downstream of the catalyst possible. If at the senasor element 10 an offset adjustment is to take place, a predetermined property of the sample gas is required. For example, a λ = 1 control is required. The predetermined property thus becomes at a stoichiometric ratio of air and fuel for the injection system 54 specified. Here, the actual pumping voltage 60 with the desired pumping voltage 58 compared. The injection system 54 is then controlled so that the actual pumping voltage 60 set according to the setpoint, so that no deviation between actual pumping voltage 60 and target pumping voltage 58 what is in place 70 is detected. In this case, the actual pumping current 66 and the actual pumping voltage 60 measured. If now the actual pumping voltage 60 the desired pumping voltage 58 corresponds or approximately corresponds and thus 0 V or about 0 V, this means that the measured actual pumping current 66 corresponds to the offset. In this case, this value is via the offset correction filter 68 learned, for example, is a low-pass filter, on the condition "actual pumping voltage 60 approximately 0V stationary. "This learned value is then derived from the detected actual pumping current 66 subtracted and thus corrected. Because the value of the offset correction filter 68 is stored, the correction is also available when the learning process or offset adjustment has been completed and λ is no longer 1, that is, a desired pumping current 72 not equal to 0 mA is requested, for example, for a forced amplitude or a lean or rich setpoint.

3 shows a schematic representation of control circuits according to a second embodiment of the present invention. Hereinafter, only the differences from the first embodiment will be described, and like components are given the same reference numerals. In the second embodiment, there is only one common lambda controller 76 that with the injection system 54 connected is. The lambda controller 76 become as input variables a target pumping current 72 and an output value of the λ = 1 controller 64 fed. The sensor element 10 detects an actual pumping voltage 60 and an actual pumping current 66 , The λ = 1 controller 64 is the reference pump voltage as a reference variable of the first control loop 58 fed. The λ = 1 controller 64 is only active if a predetermined target pumping current as a reference variable of the second control circuit this over the site 78 is supplied as will be explained in more detail below. This is at a first desired pumping current 80 the actual pumping current 66 detected and determined as a correction value and at a second target pumping current 82 which is different from the first target pumping current 80 the correction value becomes the second target pumping current 82 added. The first target pumping current 80 for example, it can be 0 mA. Accordingly, the λ = 1 controller 64 only active if in place 78 it is detected that the desired pumping current 72 equal to the first desired pumping current 80 of 0 mA. Otherwise, the corrected pumping current is stored and the lambda controller 76 fed.

In the second embodiment, the offset error becomes the target pumping current 72 added. The overall result is stationary identical to the first embodiment. The offset is learned by the first target pumping current 80 of 0 mA is requested. The λ = 1 controller 64 then gives the corrected pumping current such that the actual pumping voltage 60 = Target pumping voltage 58 results. This actual pump current value corresponds to the offset error. If a pumping current is requested not equal to 0 mA, and thus the second target pumping current 82 , the λ = 1 controller becomes 64 deactivated and added to the previously learned corrected pumping current to the setpoint.

The advantage of this embodiment is that it is not necessary to switch between the regulation of the pumping voltage and the regulation of the pumping current, but the two regulators are arranged as a cascade control. In the learning phase, or in the λ = 1 control, the lambda controller operates 76 as an internal, fast control loop, while the λ = 1 controller 64 the pump voltage can adjust with a larger time constant. In the case of regulation to lambda not equal to 0, only the external λ = 1 controller is used 64 disabled and the controller output is held.

Since the pumping voltage increases very rapidly in the λ = 1 passage and, in addition, a λ = 1 ripple is to be expected, the pump voltage should be filtered in both embodiments. The conditions for activating the respective learning functions relate to the filtered pump voltage. The filter time constant can be in the range 10 ms to 500 ms, preferably between 300 ms and 500 ms, for example 400 ms.

The method steps described above can be carried out by means of a computer program. The computer program can be stored on an electronic storage medium. The storage medium can be part of the control unit 56 be.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Konrad Reif (ed.): Sensors in the motor vehicle, 1st edition 2010, pp. 160-165 [0003]

Claims (12)

Method for operating a sensor element ( 10 ) for detecting at least one property of a measurement gas in a measurement gas space, in particular for detecting a proportion of a gas component in the measurement gas or a temperature of the measurement gas, wherein the sensor element ( 10 ) at least one solid electrolyte body ( 12 ), a pump cell ( 40 ), and a Nernst cell ( 46 ), wherein at a predetermined property of the measuring gas a desired pumping voltage ( 58 ) of the pump cell ( 40 ) is used as a reference variable of a control for regulating a pumping voltage, wherein an actual pumping voltage ( 60 ) and an actual pumping current ( 66 ) of the pump cell, the actual pump current ( 66 ) is determined as a correction value, provided that the actual pumping voltage ( 60 ) the desired pumping voltage ( 58 ), wherein, in the case of a deviation from the predetermined property of the measuring gas, the correction value is used in a regulation for regulating the pumping current. Method according to the preceding claim, wherein the correction value is determined by means of an offset correction filter ( 68 ), the correction value in the offset correction filter ( 68 ) is stored. Method according to one of claims 1 to 2, wherein in the closed-loop control for regulating the pumping current, a detected actual pumping current ( 66 ) is corrected by subtracting the correction value. Method according to one of claims 1 to 3, wherein the desired pumping voltage ( 58 ) 0V is. Method according to one of claims 1 to 4, wherein in the control to a first desired pumping current ( 80 ) after reaching a first control target first actual pumping current ( 66 ) is detected and determined as a correction value. Method according to claim 5, wherein at a second desired pumping current ( 82 ), which differs from the first desired pumping current ( 80 ), the correction value to the second desired pumping current ( 82 ) is added. Method according to claim 5 or 6, wherein the first desired pumping current ( 80 ) Is 0 mA. Method according to one of claims 1 to 7, wherein the sensor element ( 10 ) in an exhaust tract of an internal combustion engine ( 52 ), wherein the sensor element ( 10 ) with a control device ( 56 ) of the internal combustion engine ( 52 ), wherein the predetermined property of the measurement gas by injecting fuel into at least one combustion chamber of the internal combustion engine ( 52 ) is adjustable.  The method of claim 8, wherein the predetermined property is set at a stoichiometric ratio of air and fuel.  A computer program adapted to perform each step of the method of any one of claims 1 to 9.  An electronic storage medium on which a computer program according to claim 10 is stored. Electronic control unit ( 56 ) comprising an electronic storage medium according to claim 11.

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