Method for monitoring a hydraulic pressure system and application of the method

Abstract

Method for monitoring a section (20) of a hydraulic pressure system (10), wherein a pressure of a fluid is built up in the section (20) by an upstream pump and wherein the fluid exits the section (20) via a valve (25) which opens with a valve control frequency and a duty cycle, characterized in that a pressure profile is detected by a pressure sensor (17) arranged downstream of the pump and upstream of the section (20) at least during one period of the valve control frequency, and that frequency components in the frequency spectrum of the pressure profile are determined by means of a Fourier transform.that if the frequency spectrum determined in this way matches the frequency spectrum of an intact subsection (20), it is concluded that the subsection (20) is intact, and that if the frequency spectrum determined in this way deviates from the frequency spectrum of an intact subsection (20), a defect in the subsection (20) is concluded, wherein the duty cycle of the opening valve (25) is changed from a first value to a second value, or that a period is considered in which the duty cycle changes, that local maxima of the frequency components in the frequency spectrum are determined, and that a defect in the subsection (20) is concluded if the amplitude of the local maxima decreases quasi-monotonic with increasing frequency.

Description

[0001] The invention relates to a method for monitoring a section of a hydraulic pressure system, wherein a pressure of a liquid is built up in the section by an upstream pump and wherein the liquid exits the section via a valve which opens with a valve control frequency and a duty cycle. State of the art

[0002] Selective catalytic reduction (SCR) is sometimes used to clean the exhaust gases of internal combustion engines of nitrogen oxides. In this process, a urea solution is added to the exhaust gas as a reducing agent, which is converted into ammonia and carbon dioxide. The ammonia then reacts with the nitrogen oxides on a catalyst to produce nitrogen and water vapor. The urea solution is drawn from a storage tank, pressurized to the operating pressure by a pump in a delivery module, and fed via a pressure line to a metering module with a valve. The valve is controlled with a product-specific frequency and a suitable duty cycle to meter the required amount of urea solution to reduce the amount of nitrogen oxides present in the exhaust gas. The frequency can be, for example, in the range of 1 Hertz, and the duty cycle between 0.5% and 99.5%.

[0003] However, the dosing process only functions correctly if there are no blockages or leaks in the system; for example, a blocked valve prevents the urea solution from being dosed, and a leak in the supply lines or the valve prevents the pressure required for proper function from building up. Therefore, methods are known that examine at least sections of the pressure system for leaks and blockages, such as in the valve.

[0004] A disadvantage of the currently used methods is that they only function from a product-specific minimum duty cycle up to a maximum duty cycle of the valve. The minimum duty cycle can be in the range of 10% and the maximum duty cycle in the range of 90%. However, in the operation of an SCR system, duty cycles of less than 10% and greater than 90% may occur.

[0005] US Patent 5,499,538 A discloses a diagnostic system for an internal combustion engine to detect a faulty or defective fuel pump.

[0006] EP 2 147 203 B1 relates to a method and a system for monitoring the functionality of an injector for injecting a fluid into an engine exhaust system, a so-called After Treatment Injector (ATI).

[0007] It is therefore an object of the invention to provide a method by which leaks and blockages of sections of a pressure system can be detected even at small duty cycles of, for example, less than 10% and at large duty cycles of, for example, greater than 90%. Disclosure of the invention

[0008] The object of the invention is achieved by using a pressure sensor arranged downstream of the pump and upstream of the section to detect a pressure profile at least during one valve actuation period, by determining frequency components in the frequency spectrum of the pressure profile via a Fourier transform such that, if the frequency spectrum determined in this way matches that of an intact section, it can be concluded that the section is intact, and if the frequency spectrum determined in this way deviates from that of an intact section, it can be concluded that the section is defective. Because the pressure sensor is located upstream of the section to be monitored, and thus outside the system being monitored, a leak or blockage in the pressure system can be detected by analyzing the frequency spectrum of the pressure signal.This applies when the valve is activated and is neither fully open nor fully closed during the test period. By increasing the duty cycle from 0% (closed valve), this method can differentiate between a defective and a functioning pressure system.

[0009] For example, in an advantageous embodiment, increasing the duty cycle from 0% to 0.5% may be sufficient to successfully apply the method. The same applies if the duty cycle is reduced from 100% (valve open). The Fourier transform of the pressure signal can also be performed using the Fast Fourier Transform (FFT) method if the number of measurement points for the pressure signal in the considered period is a power of two. If only individual periods of the valve actuation are considered, the method can be used for both a valve actuated periodically at a fixed frequency and a valve actuated at a variable frequency.

[0010] In a non-inventive embodiment of the invention, it is provided that an intact section is concluded if, within the specified frequency spectrum in the region of the valve actuation frequency, a frequency component with an amplitude higher by a predetermined value or factor compared to the adjacent frequency components is present, and that a defect in the section is concluded if, in the region of the valve actuation frequency, no frequency component with an amplitude higher by a predetermined value or factor compared to the adjacent frequency components is present. The pressure surges of the opening valve at one end of the section under consideration lead to pressure fluctuations reaching the pressure sensor at the opposite end of the section of the pressure system. If the pressure system is blocked, the pressure fluctuations at the valve actuation frequency cannot reach the pressure sensor.If there is a leak in the pressure system, the pressure fluctuations at the valve actuation frequency are reduced, and the frequency component determined in the Fourier analysis is lower compared to the contributions at other frequencies than in an intact pressure system. Besides a blockage in a pressure line or a pressure line connector, a blockage in the valve itself can also be a cause of the lack of pressure fluctuations.

[0011] Another embodiment, not according to the invention, provides that a predetermined number of frequencies with the highest frequency components are determined within a predetermined first frequency range around the valve control frequency. An intact section is concluded if one of the frequencies lies within a predetermined second frequency range around the valve control frequency, and a defect in the section is concluded if none of the frequencies lies within the predetermined second frequency range around the valve control frequency. For example, if the frequencies for the five highest frequency components are determined and none of the frequencies are close to the valve control frequency, a defect can be assumed. This evaluation can be performed for a single period of valve control or, with a constant control frequency and constant duty cycle, over several periods.

[0012] The monitoring method according to the invention provides that the duty cycle of the opening valve is changed from a first value to a second value, or that a period during which the duty cycle changes is observed, that local maxima of the frequency components in the frequency spectrum are determined, and that a defect in the sub-section is inferred if the amplitude of the local maxima decreases quasi-monotonic with increasing frequency. In an intact pressure system, after the Fourier transform of the pressure signal, maxima appear in the frequency components at higher frequencies in the frequency spectrum, which are higher than individual maxima at lower frequencies.

[0013] Distinguishing between an intact and a defective section of a pressure system is achieved more reliably by determining the number of valve actuation frequency periods during which a defect in the section is detected, and by concluding that a defect exists in the section when this number of periods reaches a predetermined value. If a defect is detected in, for example, five consecutive valve actuation periods, a defective system can be assumed with a high degree of certainty. If the pressure system is subsequently detected as intact, the detection count is restarted.

[0014] The described method is particularly suitable for monitoring a section between a conveying module and a valve contained in a metering module within a hydraulic pressure system for supplying a reducing agent to the exhaust stream of an internal combustion engine for the selective catalytic reduction of nitrogen oxides. No additional components are used; only the pressure signal from a pressure sensor integrated into the conveying module is evaluated.

[0015] The method is generally advantageously suited for monitoring a section of a hydraulic pressure system with at least one pressure line, at least one valve controllable with a predefinable duty cycle, and at least one pressure sensor. In one embodiment, the system may be provided with a pressure sensor. Brief description of the drawings

[0016] The invention will be explained in more detail below with reference to an embodiment illustrated in the figures. The figures show: Fig. 1. A hydraulic pressure system for dosing reducing agent in schematic representation, Fig. 2 a first frequency spectrum of a pressure curve in an intact hydraulic pressure system, Fig. 3 a second frequency spectrum of the pressure curve in a defective hydraulic pressure system, Fig. 4 a third frequency spectrum of the pressure profile in an intact hydraulic pressure system when the duty cycle changes.

[0017] Fig. Figure 1 shows a schematic representation of a hydraulic pressure system 10 designed for metering a reducing agent into the exhaust manifold of an internal combustion engine. The system comprises a tank 11 and a delivery module 16 connected via a delivery line 13 and a return line 12 by means of a first low-pressure connector 14 and a second low-pressure connector 15. The delivery module 16 is connected via a first pressure line connector 21, a pressure line 22, and a second pressure line connector 23 to a metering module 24, which includes a valve 25 and an outlet 26. These components, connected to the delivery module 16 on the high-pressure side, are grouped in a section 20 of the hydraulic pressure system 10, which, according to the invention, is to be monitored for leaks and blockages. A pressure sensor 17 is provided in the delivery module 16 to determine the system pressure. This sensor is arranged upstream of the section 20 to be monitored.

[0018] During operation, a pump in the pumping module 16 draws the reducing agent via the suction line 13 and delivers it via the pressure line 22 to the valve 25, from where it is metered into the exhaust gas duct via the outlet 26. Depending on the required amount of reducing agent in the exhaust gas, the valve is opened with a suitable duty cycle. For example, in a system, the duty cycle can be selected between 0.5% and 99.5%. The opening frequency of the valve 25 is adapted to the system components and can, for example, be in the range of 1 Hertz. Any unused reducing agent can be returned to the tank 11 via the return line 12. To monitor the correct function of section 20, a Fourier transform of the output signal from the pressure sensor 17 is performed to determine its frequency spectrum.

[0019] Fig. Figure 2 shows a first frequency spectrum 30 of the output signal of the pressure sensor 17 for an intact section 20 of the pressure system 10. In the Fourier transform, certain frequency components, such as the magnitudes or the values ​​of the Fourier transforms, are plotted along a frequency component axis 31 above a frequency axis 33. Since an intact section 20 is present, the value of the frequency component at the valve actuation frequency 32 is significantly higher than the value of surrounding frequency components. This is a consequence of the pressure fluctuations at the pressure sensor 17 caused by the pressure fluctuations at the pressure sensor 17, which occur synchronously with the valve opening. A prerequisite for carrying out the procedure is that the valve 17 is neither permanently closed (duty cycle 0%) nor permanently open (duty cycle 100%).

[0020] Fig. Figure 3 shows a second frequency spectrum 40 of the output signal of the pressure sensor 17 in a defective section 20 of the pressure system 10. The same designations are used as in Fig. 2. Additionally, an example frequency component 41 is entered next to the valve control frequency 32, which has a higher value than the frequency component at the valve control frequency 32. From this, it can be concluded that subsection 20 has a defect.

[0021] Fig.Figure 4 shows a third frequency spectrum 42 of the output signal of the pressure sensor 17 for an intact section 20 of the pressure system 10, as it appears in the Fourier transform when the duty cycle of the valve 25 is increased from 0% to 0.5%. A first, second, and third maximum 43, 44, 45 are marked in the representation of the frequency components over the frequency axis 34. A characteristic of an intact section 20 of the pressure system 10 is that at a higher frequency, a higher maximum (e.g., the third maximum 45) occurs than the second maximum 44. In contrast, a characteristic of a defective section 20 is that the amplitude of the local maxima 43, 44, 45, and any other maxima that may occur, decreases quasi-monotonically with increasing frequency.

Claims

[1] Method for monitoring a section (20) of a hydraulic pressure system (10), wherein a pressure of a liquid is built up in the section (20) by an upstream pump and wherein the liquid exits the section (20) via a valve (25) which opens with a valve control frequency and a duty cycle, characterized by, that a pressure sensor (17) arranged downstream of the pump and upstream of the subsection (20) is used to detect a pressure profile at least during one period of the valve control frequency, that frequency components in the frequency spectrum of the pressure profile are determined via a Fourier transform, that if the frequency spectrum determined in this way matches the frequency spectrum of an intact subsection (20), it is concluded that the subsection (20) is intact, and that if the frequency spectrum determined in this way deviates from the frequency spectrum of an intact subsection (20), a defect in the subsection (20) is concluded, wherein the duty cycle of the opening valve (25) is changed from a first value to a second value, or that a period is considered in which the duty cycle changes, that local maxima of the frequency components in the frequency spectrum are determined, and that a defect in the subsection (20) is concluded.when the height of the local maxima decreases quasi-monotonic with increasing frequency. [2] Method according to claim 1, characterized by , that a number of periods of the valve control frequency in which a defect of the subsection (20) is detected is determined and that a defect in the subsection (20) is inferred when the number of periods thus determined reaches a predetermined value. [3] Application of the method according to claim 1 or 2 for monitoring a section (20) between a conveying module (16) and a valve (25) contained in a metering module (24) in a hydraulic pressure system (10) for supplying a reducing agent to an exhaust stream of an internal combustion engine for the selective catalytic reduction of nitrogen oxides. [4] Application of the method according to claim 1 or 2 for monitoring a section of a hydraulic pressure system comprising at least one pressure line and at least one valve controllable with a predefinable duty cycle and at least one pressure sensor.

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