Methods for determining the concentration of a liquid

Abstract

Method for determining the concentration of a liquid (3), wherein an ultrasonic transducer (72) and at least two ultrasonic reflector surfaces (731, 732) are arranged on a common, integrally formed support element (73) at different distances from the ultrasonic transducer (72), characterized in that in a measuring operation the concentration is determined from a first transit time of an ultrasonic signal between the ultrasonic transducer (72) and a first ultrasonic reflector surface (731), without reflection of the ultrasonic signal at a further ultrasonic reflector surface (732), and in a calibration operation a second transit time of an ultrasonic signal between the first ultrasonic reflector surface (731) and at least one further ultrasonic reflector surface (732) is determined (93, 94).

Description

[0001] The present invention relates to a method for determining the concentration of a liquid. Furthermore, the present invention relates to a computer program with program code that executes each step of the method when executed on a computer, as well as to a machine-readable storage medium that stores the computer program. Finally, the invention relates to an electronic control unit configured to execute the method. State of the art

[0002] To reduce the proportion of nitrogen oxides in the exhaust gas of an internal combustion engine, especially a diesel engine, it is known to install an SCR (Selective Catalytic Reduction) catalyst in its exhaust system. This reduces the nitrogen oxides contained in the exhaust gas to nitrogen in the presence of ammonia as a reducing agent. To provide the ammonia, a reducing agent solution is metered into the exhaust system upstream of the SCR catalyst. Typically, an aqueous urea solution (urea-water solution; HWL) is used for this purpose, which contains urea as an ammonia-releasing reagent. A 32.5% HWL is known as AdBlue. ® commercially available.

[0003] Emissions protection legislation requires monitoring of the quality, i.e. the concentration of the urea solvent used, using a quality sensor, in particular to detect possible misfuelling by the driver. This misfuelling can occur, for example, with water or with a diluted urea solvent. If such misfuelling is detected, the driver is warned and ultimately operation of the vehicle is restricted by a so-called "inducement". The quality sensor can have an ultrasonic transducer and an ultrasonic reflector surface so that the concentration of the urea solvent can be calculated from the propagation time of an ultrasonic signal between the ultrasonic transducer and the ultrasonic reflector surface. For this purpose, the distance between the ultrasonic transducer and the ultrasonic reflector surface must be known. For this purpose, such quality sensors are calibrated by the manufacturer in order to eliminate unavoidable manufacturing influences.However, downstream assembly processes can lead to a change in the measuring distance and thus impair the measurement accuracy.

[0004] DE 10 2018 202 587 A1 proposes arranging several ultrasonic reflector surfaces on a common, one-piece support element. This allows an ultrasonic signal emitted by an ultrasonic transducer to be reflected back with different propagation times during measurement, enabling measurements using the differential measurement principle.

[0005] DE 10 2014 213 233 A1 discloses a device and a method for determining the speed of sound in a fluid in a fluid container. A sound transducer and two reflector elements are arranged in a fluid chamber. A control unit controls the sound transducer to generate a predetermined sound signal, to determine a first signal propagation time between a transmission time and a reception time of a first reflection of the sound signal, and to determine a respective further reflection of the sound signal at the respective further reflector element between the transmission time and a reception time. A sound speed within the fluid is determined based on the first signal propagation time and the respective further signal propagation time of the sound signal.

[0006] DE 10 2014 109 843 A1 discloses a method for determining the fill level of a liquid in a tank. For this purpose, an ultrasonic level sensor emits an ultrasonic wave, which is reflected by at least two reference surfaces arranged one below the other. Various propagation velocities and transit times in the tank are measured. A fill level is determined using a measured transit time and a selected propagation velocity.

[0007] From DE 20 2013 006 909 U1 a sensor device for detecting properties of a fluid medium by means of ultrasonic signals is known. Disclosure of the invention

[0008] In the method for determining the concentration of a liquid, in particular a UWL, which is stored in a tank, for example a reducing agent tank, it is provided that an ultrasonic transducer and at least two ultrasonic reflector surfaces, in particular exactly two ultrasonic reflector surfaces, are provided. The ultrasonic reflector surfaces are arranged on a common, one-piece carrier element at different distances from the ultrasonic transducer, wherein the carrier element is arranged in the tank. The ultrasonic transducer and the ultrasonic reflector surfaces together form a quality sensor for determining the concentration of the liquid. This quality sensor is operated in the method in a measuring mode and a calibration mode: During measurement, the concentration is determined from the first transit time of an ultrasonic signal between the ultrasonic sensor and a first ultrasonic reflector surface. The ultrasonic signal is not reflected by another ultrasonic reflector surface. This measurement mode, in which only one transit time of the ultrasonic signal is evaluated, is computationally inexpensive and can be performed in a similar way to the measurement mode of a quality sensor that has only a single ultrasonic reflector surface in addition to its ultrasonic transducer.

[0009] During calibration, a second transit time of the ultrasonic signal between the first ultrasonic reflector surface and at least one further ultrasonic reflector surface is determined. To determine the concentration during measurement, it is necessary to know the measuring distance between the ultrasonic transducer and the first ultrasonic reflector surface. If the ultrasonic transducer and the support element are mounted together on a plastic surface, for example, on the surface of a liquid conveying module, the measuring distance may deviate from the original calibration due to subsequent assembly processes in the tank. The distance between the ultrasonic reflector surfaces, however, is defined by the support element. It can therefore be used for calibration during measurement.

[0010] To calculate the second propagation time, it is preferred that a third propagation time of an ultrasonic signal between the ultrasonic transducer and a different ultrasonic reflector surface be determined than the one used to determine the first propagation time. At least one reflection of the ultrasonic signal occurs at at least one additional ultrasonic reflector surface. Furthermore, the first propagation time is also determined. The second propagation time is then calculated as the difference between the third propagation time and the first propagation time. In this difference calculation, the influence of the distance between the ultrasonic transducer and the ultrasonic reflector surfaces is eliminated.

[0011] It is particularly preferred that, when measuring the third transit time, the additional ultrasonic reflector surface at which the ultrasonic signal is reflected on the path between the ultrasonic transducer and the other ultrasonic reflector surface is the first ultrasonic reflector surface. In this way, it is sufficient for the method to be carried out if only two ultrasonic reflector surfaces are formed on the carrier element.

[0012] The distance between the ultrasonic transducer and the first ultrasonic reflector surface is preferably determined during calibration from a ratio between the second propagation time and the first propagation time. The ratio between the distance to be determined and the known propagation path of the ultrasonic signal between the ultrasonic reflector surfaces, which is determined by the geometry of the carrier element, corresponds to the ratio between the second propagation time and the first propagation time.

[0013] It is preferable that the calibration procedure be performed during the initial commissioning of the ultrasonic transducer. This eliminates the need for a manufacturer-side calibration. Even if a manufacturer-side calibration is present, a calibration procedure during the initial commissioning of the ultrasonic transducer makes it possible to compensate for deviations in the distance between the ultrasonic transducer and the first ultrasonic reflector surface that may have occurred after the manufacturer-side calibration due to downstream assembly processes.

[0014] If the tank is a reducing agent tank of a motor vehicle's SCR catalyst system, it is further preferred that the calibration operation be carried out under specified driving conditions of the motor vehicle. The driving conditions can be specified so that the most favorable measurement conditions prevail. This can, in particular, involve a homogeneous temperature distribution (low temperature gradient) or stable driving conditions (constant acceleration).

[0015] During measurement and calibration, the ultrasonic transducer preferably emits burst signals in a frequency range between 0.5 MHz and 10.0 MHz. Burst signals between 1.0 MHz and 2.0 MHz are particularly preferred. This frequency range allows for good spatial focusing of the ultrasonic field and precise determination of the propagation times without causing unacceptably high absorption of the sound energy in the liquid.

[0016] The computer program is configured to perform each step of the method, particularly when running on a computer or an electronic control unit. It enables the implementation of different embodiments of the method on an electronic control unit without requiring structural changes. For this purpose, it is stored on the machine-readable storage medium.

[0017] By loading the computer program onto a conventional electronic control unit, an electronic control unit is obtained which is configured to determine a concentration of a liquid using the method. Short description of the drawings

[0018] An embodiment of the invention is illustrated in the drawings and is explained in more detail in the following description. Fig. 1 shows a schematic representation of a reducing agent tank in which a liquid is stored, the concentration of which can be determined by means of an embodiment of the method according to the invention. Fig. Figure 2 shows a schematic plan view of a quality sensor installed in the reducing agent tank according to Fig. 1 is arranged. Fig. 3 shows a path of an ultrasonic signal in the quality sensor according to Fig. 2 in an embodiment of the method according to the invention. Fig. Figure 4 shows another path of an ultrasonic signal in the quality sensor according to Fig. 2 in an embodiment of the method according to the invention. Fig. 5 shows a flowchart of an embodiment of the method according to the invention. Embodiment of the invention

[0019] Fig. 1 shows elements of an SCR catalyst system 1 of a motor vehicle (not shown). This system has a reducing agent tank 2. A liquid 3, which is a UWL, is stored therein. A withdrawal module 4 is arranged at the bottom of the reducing agent tank 2. It carries a conveying device 5, by means of which the liquid 3 can be sucked from the reducing agent tank 2 and transported via a line 6 to a metering valve of the SCR catalyst system 1. To determine the concentration of the liquid 3, a quality sensor 7 is arranged on the withdrawal module 4 and is controlled by an electronic control unit 8.

[0020] As in Fig. 2, the quality sensor 7 has a base 71, which in the present embodiment is made of HDPE (high-density polyethylene). An ultrasonic transducer 72 is arranged on this base. Furthermore, a support element 73, which in the present embodiment is made of stainless steel, is arranged on the base 71 and carries two ultrasonic reflector surfaces 731, 732. The first ultrasonic reflector surface 731 is arranged at a distance d from the ultrasonic transducer 72. It is located opposite the ultrasonic transducer 72 such that it can reflect an ultrasonic signal emitted by the latter both back to the ultrasonic transducer 72 and onto the second ultrasonic reflector surface 732. The second ultrasonic reflector surface 732 is arranged such that ultrasonic signals emitted by the ultrasonic transducer 72 cannot hit the second ultrasonic reflector surface 732 without prior reflection.

[0021] The ultrasonic transducer 72 generates a burst signal with a frequency of 1.0 MHz in the present embodiment by means of a piezoelectric crystal. If the signal is emitted from an output point 721 of the ultrasonic transducer 72, it strikes the first ultrasonic reflector surface 731. As shown in Fig. 3, part of the signal is reflected back to the output point 721 and received there by the piezo crystal.

[0022] Fig. Figure 4 shows that another portion of the signal is reflected from the first ultrasonic reflector surface 731 to the second ultrasonic reflector surface 732. From there, the ultrasonic signal is reflected back to the first ultrasonic reflector surface 731, which reflects it further to the ultrasonic transducer 72, where it is received by the piezoelectric crystal at an impact point 722. The different travel times allow the two echoes to be distinguished from each other.

[0023] Fig. 5 shows that after a start 90 of an embodiment of the method according to the invention, a first check 91 is performed to determine whether the ultrasonic transducer 72 has been put into operation for the first time. If this condition is met, the calibration operation is carried out. If there is no initial start-up, a second check 92 is performed to determine whether a driving condition of the motor vehicle exists that requires a calibration operation. If this condition is met, the calibration operation is also initiated. In the calibration operation, an ultrasonic signal is emitted by the ultrasonic transducer 72. First, a first propagation time of the ultrasonic signal between the ultrasonic transducer 72 and the first ultrasonic reflector surface 731 is determined 93 according to Fig. 3. After the first echo of the ultrasonic signal has been evaluated in this way, a second travel time of the ultrasonic signal is determined 94 via the path according to Fig. 4. Next, a calculation 95 of the transit time of the ultrasonic signal between the two ultrasonic reflector surfaces 731, 732 is performed. For this purpose, the difference between the second transit time and the first transit time is calculated. To determine 96 a distance between the ultrasonic transducer 72 and the first ultrasonic reflector surface 731, the third transit time and the first transit time are set in a ratio to one another. This ratio corresponds to the ratio between the distance d between the ultrasonic transducer 72 and the first ultrasonic reflector surface 731 and the distance between the two ultrasonic reflector surfaces 731, 732. With the distance d now known, a calibration 97 of the quality sensor 7 is performed. The process then switches to a measuring mode 98. If the need for calibration was not determined in either of the two tests 91, 92, the measuring mode begins immediately.

[0024] In the measuring mode, the ultrasonic transducer sends out 72 ultrasonic signals and only the first echo of these ultrasonic signals is recorded according to the measuring distance in Fig. 3 is evaluated. Thus, only the first transit time of the ultrasonic signals is available, from which the concentration of urea in the liquid 3 is calculated using the now known distance d. If this concentration falls below a threshold value, an induction of the vehicle is performed.

Claims

[1] Method for determining a concentration of a liquid (3), wherein an ultrasonic transducer (72) and at least two ultrasonic reflector surfaces (731, 732) are arranged on a common, one-piece carrier element (73) at different distances from the ultrasonic transducer (72), characterized by in that in a measuring operation the concentration is determined from a first propagation time of an ultrasonic signal between the ultrasonic transducer (72) and a first ultrasonic reflector surface (731) without a reflection of the ultrasonic signal at a further ultrasonic reflector surface (732), and in a calibration operation a second propagation time of an ultrasonic signal between the first ultrasonic reflector surface (731) and at least one further ultrasonic reflector surface (732) is determined (93, 94). [2] Method according to claim 1, characterized bythat the second transit time is calculated by determining a third transit time of an ultrasonic signal between the ultrasonic transducer (72) and another ultrasonic reflector surface, wherein at least one reflection of the ultrasonic signal occurs at at least one further ultrasonic reflector surface, and a difference between the third transit time and the first transit time is calculated (95). [3] Method according to claim 2, characterized by that when measuring the third transit time, the further ultrasonic reflector surface is the first ultrasonic reflector surface (731). [4] Method according to one of claims 1 to 3, characterized by that in calibration mode a distance between the ultrasonic transducer (72) and the first ultrasonic reflector surface is determined (96) from a ratio between the second transit time and the first transit time. [5] Method according to one of claims 1 to 4, characterized bythat the calibration operation is carried out when the ultrasonic transducer (72) is put into operation for the first time. [6] Method according to one of claims 1 to 5, characterized by that the carrier element (73) is arranged in a reducing agent tank (2) of an SCR catalyst system (1) of a motor vehicle and the calibration operation is carried out under predetermined driving conditions of the motor vehicle. [7] Method according to one of claims 1 to 6, characterized by that the ultrasonic transducer (72) emits burst signals in a frequency range between 0.5 MHz and 10.0 MHz during measuring and calibration operation. [8] A computer program comprising program code adapted to perform each step of the method according to any one of claims 1 to 7 when executed on a computer. [9] A machine-readable storage medium on which a computer program according to claim 8 is stored. [10] Electronic control device (8) which is designed to determine a concentration of a liquid (3) by means of a method according to one of claims 1 to 7.

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