Method and arrangement for the analysis of gas properties

Abstract

Method for measuring the speed of sound in a gas for detecting gas properties, in particular the gas composition, the temperature and / or the humidity, designed as a method for determining the humidity of the atmosphere in a cooking chamber of a cooking appliance, with a sound transmitter (1) and a sound receiver (2), both of which are mounted on a common body (3), comprising: - providing a body (3) whose speed of sound is higher than the speed of sound in the gas, - arranging the transmitter (1) and the receiver (2) on this body (3), - operating the transmitter (1) for at least one period of time in an "on" state such that the transmitter (1) transmits an acoustic signal, and operating the transmitter (1) for at least one period of time in an "off" state such that the transmitter (1) does not transmit an acoustic signal, - operating the receiver (2) in an "off" state for at least a period of time during the "on" state of the transmitter (1) and operating the receiver (2) in an "on" state for at least a period of time during the "off" state of the transmitter (1), - Integrating the signal from the receiver (2) by an amplifier (11), calculating the speed of sound and determining the properties of the gas based on the speed of sound.

Description

[0001] The present invention relates to the detection of gas properties in a cooking appliance, in particular the detection of the gas composition, the temperature and / or the humidity of a gas, by measuring the speed of sound using a sound transmitter and a sound receiver, both of which are mounted on a common body. The present invention further relates to a method for determining the humidity of the atmosphere inside a cooking chamber of a cooking appliance. The invention further relates to a sound-speed-based gas sensor arrangement which is configured to measure gas properties, in particular the gas composition, the temperature and / or the humidity of a gas, and which comprises a transmitter, a receiver, and a signal processing unit.

[0002] The use of the speed of sound in a gas to measure its temperature / humidity or its composition is well known, since only its temperature and composition affect the speed of sound. By measuring the speed of sound, one can therefore obtain the temperature for a gas with a known composition or the composition of a gas at a known temperature. The measurement can be performed using ultrasound or non-ultrasound. The term "sound" therefore includes both types of sound. If the temperature of the gas is known, it is also possible to determine the humidity. The speed of sound is usually measured by determining the time it takes for an acoustic signal to travel the distance between a transmitter and a receiver.This can be done by transmitting pulsed signals and measuring the time it takes for the signal to reach the receiver, or by transmitting a continuous signal and measuring the phase angle between the excitation of the transmitter and the signal of the receiver.

[0003] In certain measurement sensing applications, it is advantageous if the transmitter and receiver are mounted on a common body to create a self-contained sensor. The transmitter and receiver can be mounted opposite each other or parallel to each other, with the sound traveling from the transmitter via a reflector to the receiver. In such a case, this common body transfers part of the sound energy directly from the transmitter to the receiver without passing through the measuring gas. Due to the high speed of sound in solids, especially metals, the wavelength of sound in solids is about an order of magnitude longer than in gases, so that the sound traveling over the body has a different phase angle at the receiver than the sound traveling through the gas.Typically, the change in the speed of sound with changing temperature in a solid is relatively small compared to the speed of sound in a gas and has an opposite sign.

[0004] Since both the structure-borne sound and the gas sound have the same frequency, the receiver generates a signal whose amplitude and phase are composed of both components. Without additional information, it is therefore impossible to separate the signal transmitted through the gas.

[0005] As long as the amplitude transmitted through the body is less than a few percent of the total sound amplitude at the receiver, it has little impact on the quality of the measurement. However, especially when both transducers are mounted close to each other in parallel, the amplitude of the structure-borne sound is considerable. It can be reduced by mechanical means, i.e., by constructing the body to limit and / or dampen sound transmission. However, such a handcrafted solution is complex and / or problematic at elevated temperatures.

[0006] Further systems and methods known from the state of the art relating to the determination of properties based on sound measurements are briefly listed below.

[0007] DE 20 2006 021 163 U1 discloses a device for biogas plants for determining the concentrations and / or flow of individual components of a gas mixture through a pipeline by means of a sound velocity measurement.

[0008] US 2013 / 0 013 227 A1 describes a method for measuring air flow in a smoke detector. For this purpose, the speed of sound is determined using a sound propagation time measurement.

[0009] From US 4 095 457 A, a method for determining properties in a liquid is also known, in which an ultrasonic transmitter and an ultrasonic receiver are attached to a liquid-conducting pipe.

[0010] Due to disadvantages of the methods and systems known from the prior art, it is an object of the present invention to propose another possibility for measuring the speed of sound in a gas for detecting gas properties, in particular the temperature and / or humidity, in which a transmitter and a receiver are used, both mounted on a common body, thereby avoiding the above-mentioned disadvantages.

[0011] This object is achieved according to the claimed invention. Main claim 1 claims a method for measuring the speed of sound, designed as a method for determining the humidity of the atmosphere in a cooking chamber of a cooking appliance, and claim 10 claims a sound-speed-based gas sensor arrangement of a cooking appliance. Advantageous embodiments are claimed in the respective subclaims.

[0012] According to the invention, the influence of structure-borne sound on sound velocity measurements is suppressed by temporal signal separation. The method comprises the following features: - Providing a body whose speed of sound is higher than the speed of sound in the gas, - Arranging the transmitter and the receiver on this body, - operating the transmitter for at least one period of time in an "on" state such that the transmitter transmits an acoustic signal, and operating the transmitter for at least one period of time in an "off" state such that the transmitter does not transmit an acoustic signal, - operating the receiver in an "off" state for at least a period of time during the "on" state of the transmitter and operating the receiver in an "on" state for at least a period of time during the "off" state of the transmitter, - Integrating the receiver signal through an amplifier, calculating the speed of sound and determining the temperature and / or humidity of the gas based on the speed of sound.

[0013] The common body can generally consist of any solid that offers a speed of sound higher than the speed of sound in a gas. This requirement is particularly well met by metal. A preferred material is steel, which has a speed of sound of about 4000 m / s. The transmitter and receiver can be arranged on the body in parallel or opposite each other using an additional reflector. By operating the transmitter in the above-mentioned "on" state while the receiver is simultaneously operated in an "off" state, and vice versa, the contributions transmitted by the gas and those transmitted by the body can be separated in time. Based on the speeds of sound in the different materials, for example, the transmitter is operated for a first period of time while the input from the receiver to the amplifier is switched off.During the next period, which may differ in duration from the first period, the transmitter is switched off while the receiver signal is measured by the amplifier. Since the last structure-borne sound, depending on the speed of sound, reaches the receiver almost immediately after the transmitter is switched off, the body-borne contribution to the receiver signal is reduced to a certain amount that depends on the type of material and the arrangement of the transmitter and receiver. The body-borne contribution can be further reduced by introducing a delay of a few microseconds between switching off the transmitter and switching on the receiver. Such a delay might be necessary if internal reflections of the sound within the body delay the travel time of the structure-borne sound.

[0014] Since the overall sound level of the measurement depends heavily on the number of signal oscillations the amplifier is capable of integrating, the receiver's "on" time should correspond to the propagation time of the gas sound. For the same reason, the transmitter's "on" time should extend over the same period, so that the duty cycle of both the transmitter and receiver is 50% with a phase shift of π. A lock-in amplifier allows continuous integration of the receiver signal over longer periods. The amplitude measured by the lock-in amplifier is half of a continuous signal, while the phase angle information is fully preserved. In this case, the reference channel between the function generator driving the transmitter and the lock-in amplifier must be open at all times.

[0015] According to a preferred embodiment of the invention, in case of internal reflections, the "on" state of the receiver starts with a delay after the end of the "on" state of the transmitter, as mentioned above.

[0016] It is also possible that the transmitter and receiver are never operated in their respective "on" states at the same time, or in other words that the transmitter and receiver are operated alternately.

[0017] According to a further preferred embodiment of the invention, the duration of the receiver's "on" state corresponds to the travel time of the sound through the gas. The travel time of the sound depends on the distance the sound must travel between the transmitter and the receiver and on the speed of sound in the gas. With an "on" state that corresponds to the travel time, an optimal amount of the signal is available for further determination. Accordingly, it is advantageous if the duration of the transmitter's "on" state corresponds to the travel time of the sound through the gas.

[0018] According to a further advantageous embodiment, the duty cycle in the "on" state of the receiver and in the "on" state of the transmitter is maintained for the same period of time in order to obtain optimal determination conditions, the duty cycle of the transmitter and the receiver each being 50% with a phase shift of π.

[0019] As mentioned above, the amplifier allows the receiver signal to be integrated over multiple switching periods. In a preferred embodiment, the amplifier integrates the receiver signal over longer periods.

[0020] Overall, it is possible to determine the difference in the propagation times or the phase angle difference between the excitation of the transmitter and the signal of the receiver.

[0021] According to a preferred embodiment, the properties of the gas, in particular the gas composition, the temperature and / or the humidity, are calculated at least from the phase angle difference between the excitation of the transmitter and the signal of the receiver.

[0022] In order to achieve the required temporal separation which enables the determination of the changes in the speeds of sound with high resolution according to the first advantageous embodiment, the invention comprises a mechanical body whose speed of sound is at least five times, preferably ten times, higher than the speed of sound in the gas.

[0023] In a further preferred embodiment of the invention, the transmitter and receiver are arranged on this body in such a way that the sound emitted by the transmitter reaches the receiver via an acoustic reflector. This results in a significantly shorter path for the structure-borne sound compared to an arrangement in which the transmitter and receiver are opposite each other, which facilitates temporal separation.

[0024] This effect is further improved by arranging the transmitter and the receiver at a distance of less than 10 mm, preferably in the range of 4 mm, according to a further preferred embodiment.

[0025] A particular application of the method described above is the measurement of the humidity of the atmosphere in a cooking chamber of a cooking appliance, in particular a combi steamer, i.e. a cooking appliance in which food is cooked using hot air and / or steam. The heating device and the steamer are controlled by a control unit. For precise operation of the cooking appliance, i.e. to achieve satisfactory cooking results, the humidity of the atmosphere inside the cooking appliance must be determined and fed to the control unit. In known methods, a pressure difference along an element of the cooking appliance, for example a fan, is measured. However, this measurement is influenced by external parameters such as barometric pressure and is thus disturbed. According to the preferred embodiment, the method described above is independent of such disturbances.To determine the humidity, only the temperature of the atmosphere in the cooking chamber needs to be known, for example, by measuring it. The determined humidity value is then fed to the control unit.

[0026] The sensor arrangement, which is designed to measure gas properties based on the speed of sound, in particular the gas composition, the temperature and / or the humidity of a gas, comprises devices designed to carry out the method claimed and described above.

[0027] In particular, the sensor arrangement, which is configured to measure gas properties, in particular the temperature and / or humidity of a gas, based on the speed of sound, comprises a transmitter, a receiver, and a signal processing device, as is known. According to the invention, the arrangement comprises a sound transmitter and an acoustic receiver, both mounted on a common body, wherein the signal processing device operates the transmitter in an "on" state for at least a period of time such that the transmitter transmits an acoustic signal, and the signal processing device operates the receiver in an "off" state for at least a period of time such that the transmitter does not transmit an acoustic signal.wherein the signal processing device operates the receiver in an "off" state for at least a period of time during the "on" state of the transmitter and in an "on" state for at least a period of time during the "off" state of the transmitter, and wherein the signal processing device, in particular a microprocessor of the signal processing device, integrates the signal of the receiver, calculates the speed of sound and, based on the speed of sound, determines the gas properties, such as the gas composition, the temperature and / or the humidity of the gas, and provides a corresponding output signal for further processing.

[0028] According to a preferred embodiment of the gas sensor arrangement, the acoustic signal from the transmitter reaches the receiver via an acoustic reflector, wherein the acoustic reflector is preferably a wall of a duct, a wall of an air baffle or a wall of a housing of a chamber / cooking chamber and the gas to be measured is located in this duct, this chamber or this cooking chamber.

[0029] The present invention therefore makes it possible to suppress the influence of structure-borne sound on a sound velocity measurement at low cost and with high reliability. This enables the method and arrangement to be used in applications where the determination of gas properties, such as gas composition, temperature, and / or humidity, was either not possible or did not function with the required sensitivity or accuracy.

[0030] Embodiments of the invention are described in detail below in conjunction with the drawings. However, the invention is not limited to the examples described in conjunction with the drawings and includes all embodiments covered by the claims and the description alone or in conjunction with one another. The figures show: - Fig. 1 is a schematic diagram of a gas sensor arrangement comprising a transmitter and a receiver on a common body and reflection devices arranged at a distance from the transmitter and the receiver such that the sound excited by the transmitter travels through the gas to the receiver via the reflection devices to provide a gas-transmitted signal, - Fig. 2 shows a further schematic diagram of a gas sensor arrangement with a transmitter and a receiver on a common body, the transmitter and the receiver being arranged so that the sound travels directly from the transmitter to the receiver through the gas between the transmitter and the receiver to provide a gas-transmitted signal, - Fig. 3 the principle diagram from Fig. 2 together with a block diagram of a signal processing unit comprising signal processing devices, and - Fig. 4 a diagram of the clock times of the transmitter and the receiver for different receiver clock times.

[0031] The common body principle with the transmitter and the receiver with or without reflection devices can be used in all applications where a reliable system is required for systems operating in extreme environmental conditions, such as applications for internal combustion engine exhaust systems or applications where measurements over wide temperature ranges are required.

[0032] In Fig. 1 shows a mounting body 3 with a transmitter 1 and a receiver 2 mounted on this body 3. The transmitter 1 and the receiver 2 are mounted such that the propagating sound 4 from the transmitter 1 to the receiver 2 passes over an acoustic reflector 6 before reaching the receiver 2. This sound provides a signal transmitted through the gas. The propagating sound 5 provided by the transmitter 1 travels via the body 3 to the receiver 2, thereby providing a signal transmitted through the body.

[0033] In Fig. 2 shows a mounting body 3 in which the transmitter 1 and the receiver 2, which are mounted on this body 3, are arranged in a position opposite each other, wherein the sound propagation 4 takes place directly from the transmitter 1 to the receiver 2 without a reflector in the sound path. In contrast to the arrangement of Fig. 1, the transmitter 1 and the receiver 2 are arranged at a much greater distance from each other. The distance between the transmitter 1 and the receiver 2 in the arrangement according to Fig. 1 is less than 10 mm, preferably about 4 mm, whereas the distance in the arrangement of Fig. 2 is in the order of 50-100 mm. It is essential that the propagation times, ie the time required for the sound to travel from the transmitter 1 through the different media (gas or solid) to the receiver 2, differ considerably in order to determine the speed of sound of the gas with sufficient accuracy after processing the received signals. Fig. 1, the sound propagating directly through the gas from transmitter 1 to receiver 2 provides the signal transmitted via the gas, and the sound propagating through the body from transmitter 1 to receiver 2 provides the signal transmitted through the body. The acoustic reflector 6 can be a wall of a duct, a wall of a housing of a chamber / cooking chamber, or an air baffle located within the cooking chamber. The gas to be measured is located within this duct, chamber, or cooking chamber (not shown).

[0034] In Fig. Figure 3 shows a block diagram illustrating a signal processing unit 7 comprising a microprocessor 13 and a sound function generator 8, which in this embodiment provides ultrasound. The sound function generator 8 is connected to the transmitter 1. The receiver 2 is connected to a receiver preamplifier / AD converter 10 of the signal processing unit 7. The signal processing unit 7 also includes a switching function generator 9, which controls the duty cycle of the sound function generator 8 and the receiver preamplifier / AD converter 10. The sound function generator 8 is connected to a lock-in amplifier 11 and to the receiver preamplifier / AD converter 10. The sound function generator 8 supplies the lock-in amplifier 11 with a corresponding reference signal.The lock-in amplifier 11 determines the phase angle between the reference signal supplied by the sound function generator 8 and the receiver signal from the receiver preamplifier / AD converter 10. The microprocessor 13 of the signal processing unit 7 reads the output signal from the lock-in amplifier 11 and from an external temperature measuring device 12 and provides a humidity value output 14 in the form of a corresponding signal for further processing. In a further advantageous embodiment, the lock-in amplifier 11 can be digitally integrated into the microprocessor 13.

[0035] In an exemplary embodiment, the transmitter 1 and the receiver 2, which are shown in Fig. 1 are mounted very close to each other (4 mm apart) and parallel to each other on the steel body 3. The speed of sound in steel is approximately 4000 m / s, which means that each structure-borne sound takes 1 µs to travel from the transmitter to the receiver.

[0036] The sound propagates through the gas via the acoustic reflector 6 over a total distance of 40 mm. At a speed of sound in the gas of the order of 400 m / s, the sound transmitted through the gas requires 100 µs to travel from transmitter 1 to receiver 2.

[0037] The two contributions can therefore be separated in time, as in Fig. 4. In this figure, the first diagram shows the duty cycles of transmitter 1 over time. The second, middle diagram shows the duty cycle of receiver 2 with no delay relative to an "on" state of transmitter 1, while the third, bottom diagram shows the duty cycle of receiver 2 with a delay compared to the switch-off of transmitter 1. In the above example with the mentioned dimensions and material, transmitter 1 is operated for 100 µs while the input from receiver 2 to amplifier 10 is switched off. During the next 100 µs, transmitter 1 is switched off while the signal from receiver 2 is measured by amplifier 10. Since the last structure-borne sound reaches receiver 2 1 µs after transmitter 1 is switched off, the body-borne contribution to the receiver signal has been reduced to 1% if both contributions have the same amplitude.The contribution transmitted by the body can be further reduced by introducing a delay of a few microseconds between turning off transmitter 1 and turning on receiver 2, as shown in the bottom diagram of . Fig. 4. Such a delay might be necessary if internal reflections of the sound within the body 3 delay the transition time of the structure-borne sound.

[0038] Since the overall sound level of the measurement depends largely on the number of signal cycles over which the amplifier can integrate, the "on" time of receiver 2 should match the propagation time of the sound transmitted through the gas. For the same reason, the "on" time of transmitter 1 should cover the same time period, so that the duty cycle of transmitter 1 and receiver 2 is each 50% with a phase shift of π.

[0039] At an operating frequency of 50 kHz, the receiver signal is thus integrated by amplifier 11 over 5 periods. However, using a lock-in amplifier 11, continuous integration of the receiver signal over longer periods is possible. The amplitude measured by lock-in amplifier 11 is half of a continuous signal, while the phase angle information is fully maintained. In this case, the function generator 8, which drives transmitter 1, must continuously supply a reference signal to lock-in amplifier 11.

Claims

[1] Method for measuring the speed of sound in a gas for detecting gas properties, in particular the gas composition, the temperature and / or the humidity, designed as a method for determining the humidity of the atmosphere in a cooking chamber of a cooking appliance, with a sound transmitter (1) and a sound receiver (2), both of which are mounted on a common body (3), comprising: - providing a body (3) whose speed of sound is higher than the speed of sound in the gas, - arranging the transmitter (1) and the receiver (2) on this body (3), - operating the transmitter (1) for at least one period of time in an "on" state such that the transmitter (1) transmits an acoustic signal, and operating the transmitter (1) for at least one period of time in an "off" state such that the transmitter (1) does not transmit an acoustic signal, - operating the receiver (2) in an "off" state for at least a period of time during the "on" state of the transmitter (1) and operating the receiver (2) in an "on" state for at least a period of time during the "off" state of the transmitter (1), - Integrating the signal from the receiver (2) by an amplifier (11), calculating the speed of sound and determining the properties of the gas based on the speed of sound. [2] Method according to claim 1, characterized by starting the “on” state of the receiver (2) with a delay after the end of the “on” state of the transmitter (1). [3] Method according to claim 1 or 2, characterized by that the duration of the "on" state of the receiver (2) corresponds to the travel time of the sound from the transmitter (1) to the receiver (2) through the gas and / or the duration of the "on" state of the transmitter (1) corresponds to the travel time of the sound from the transmitter (1) to the receiver (2) through the gas. [4] Method according to one of the preceding claims, characterized by the integration of the signal from the receiver (2) by the amplifier (11) over longer periods of time. [5] Method according to one of the preceding claims, characterized by calculating the properties of the gas, in particular the temperature and / or humidity, from the phase angle difference between the excitation of the transmitter (1) and the signal of the receiver (2). [6] Method according to one of the preceding claims, characterized by providing a mechanical body (3) whose speed of sound is at least five times, preferably ten times higher than the speed of sound in the gas. [7] Method according to one of the preceding claims, characterized by arranging the transmitter (1) and the receiver (2) on the body (3) in such a way that the sound emitted by the transmitter (1) reaches the receiver (2) via an acoustic reflector (6). [8] Method according to one of the preceding claims, characterized by arranging the transmitter (1) and the receiver (2) at a distance of less than 10 mm. [9] Gas sensor arrangement of a cooking appliance, which is arranged to measure gas properties based on the speed of sound, in particular the gas composition, the temperature and / or the humidity of a gas, according to the method according to one of claims 1 to 8. [10] Gas sensor arrangement of a cooking appliance, which is designed to measure the humidity of a gas in a cooking chamber of the cooking appliance, with a sound transmitter (1) and a sound receiver (2), both of which are mounted on a common body (3), and a signal processing device (7), wherein the signal processing device (7) operates the transmitter (1) for at least a period of time in an "on" state such that the transmitter (1) transmits an acoustic signal, and the signal processing device (7) operates the receiver (2) for at least a period of time in an "off" state such that the transmitter (1) does not transmit an acoustic signal, wherein the signal processing device (7) operates the receiver (2) in an "off" state for at least a period of time during the "on" state of the transmitter (1) and in an "on" state for at least a period of time during the "off" state of the transmitter (1), and wherein the signal processing device (7),in particular a microprocessor (13) of the signal processing device (7), which integrates the signal of the receiver (2), calculates the speed of sound and, based on the speed of sound, determines gas properties, in particular the gas composition, the temperature and / or the humidity of the gas, and delivers a corresponding output signal. [11] Gas sensor arrangement according to claim 10, characterized by that the acoustic signal from the transmitter (1) reaches the receiver (2) via an acoustic reflector (6), wherein the acoustic reflector is preferably a wall of a duct or a wall of a housing of a chamber, and that the gas to be measured is located in this duct or chamber. [12] Gas sensor arrangement according to claim 10 or 11, characterized bythat the transmitter (1) and the receiver (2) are both mounted side by side on the common body (3), preferably as close to each other as mechanically possible and / or at a distance of less than 10 mm.

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