Method for generating fog and measuring the fill level in a liquid container of a fog generator, corresponding device and household appliance

Abstract

Method for fog generation and level measurement in a liquid container of a fog generator with a reversible excitation element, in which an excitation signal is emitted from the excitation element, the excitation signal is reflected at the liquid surface, the reflected excitation signal is received by the reversible excitation element and the level is calculated from the transit time of the signal, wherein the frequency of the excitation signal is adapted to the level of the liquid.

Description

[0001] The invention relates to a method for fog generation and level measurement in a liquid container of a fog generator with a reversible excitation element, as well as a device for carrying out such a method and a household appliance with such a device.

[0002] For the purposes of this description, fog generators refer to cold foggers, which generate a mist of extremely fine liquid particles from a liquid mechanically, i.e., not through evaporation and condensation. For this purpose, a liquid container is provided containing an excitation element that can be set into mechanical vibrations at high frequencies, usually in the ultrasonic range. Such fog generators are known from a wide variety of applications. For example, in film and stage production for creating effects or as room decorations, but also in household appliances, such as dishwashers to improve cleaning performance or in tumble dryers for freshening.

[0003] For proper operation of the fog generator using cold fog, it is important to check the water level in the liquid container so that the excitation element is sufficiently covered with liquid to prevent overheating of the excitation element and to maintain a favorable operating point.

[0004] EP 1 523 926 B1 shows a dishwasher with a mist generator in which the liquid level is regulated so that the surface of the mist generator is constantly covered with liquid.

[0005] WO 96 / 00132 shows an effects device for stage performances equipped with an ultrasonic fog generator. To ensure a sufficient liquid level during operation, a control device is provided, which should also include a sensor, such as a float or an optical sensor.

[0006] US 6 357 671 B1 describes an ultrasonic fog generator that uses sonar pulses to determine the fill level of a liquid in a fog chamber.

[0007] A level monitoring system for a flowable substance in a container is described in DE 38 26 101 A1.

[0008] US 4 319 155 A1 describes a fog atomizer control system for an ultrasonic fog generator.

[0009] CN 201212831 Y shows a liquid level measuring device, wherein the liquid level measuring device has an ultrasonic transmitter and a receiver.

[0010] US 2009 / 0093983 A1 describes a method for measuring the level of a liquid or bulk material.

[0011] The invention is based on the object of simplifying the level measurement in the liquid container of a fog generator.

[0012] For this purpose, a method according to claim 1 is provided. The method has the advantage that no additional devices are required in the fog generator to measure the fill level. On the contrary, it is in principle possible to cost-effectively equip an existing fog generator with a fill level measurement capability. Preferably, the excitation element is designed as a piezo element. Piezo elements are inexpensively available and have proven reliable in fog generator applications.

[0013] According to a preferred embodiment, the excitation signal has a frequency that is lower than the frequency for fog generation. Fog generators for water are usually excited with electrical signals in the megahertz range. A typical oscillation frequency of the excitation element for fog generation is, for example, 1.7 MHz. With a speed of sound of approximately 1500 m / s, the wavelength of sound waves in water at frequencies of this magnitude is in the millimeter range. Disturbances on the water surface, e.g., waves or an inclination of the water surface relative to the plane of the excitation element, can lead to interference effects in the reflected excitation signal. This interference can impair or completely prevent reception of the reflected excitation signal, thus making a measurement of the time of flight impossible.

[0014] According to an advantageous development of the invention, the excitation signal comprises at least one wave packet that is shorter than the expected propagation time of the excitation signal. This prevents the reflected excitation signal from arriving at the excitation element before the excitation signal has been transmitted. Thus, upon arrival of the reflected excitation signal, the excitation element has completed its transmission process and can already register the first incoming edge of the reflected excitation signal. This makes the evaluation of the propagation time particularly simple.

[0015] According to one embodiment of the invention, the excitation signal comprises wave packets of different frequencies. This has the advantage that interference occurring at certain frequencies can be more easily eliminated.

[0016] According to an advantageous development, the operating parameters for the excitation element can be dynamically adjusted. This means that if the quality of the received reflected excitation signal is insufficient, one or more further measurements are carried out with different operating parameters, i.e., an entire measuring cycle is carried out. Operating parameters that can be changed include, for example, the frequency of the excitation signal or the length of the wave packet. Different operating parameters can be changed independently or independently of one another. For example, starting from initial operating parameters defined before the measuring cycle, one or more operating parameters can advantageously be changed until a sufficient or optimal reflected excitation signal is received, thus allowing a sufficient, optimal measurement result to be obtained.The predefined starting operating parameters can, for example, be determined empirically and set for a maximum, minimum, or average fill level. It can also be provided that the starting operating parameters defined before the measuring cycle are determined based on the result of the last successful measuring cycle. This means that if, for example, an average liquid level was determined in the last successful measuring cycle, this result is saved, and the starting operating parameters for the next measuring cycle are set so that they correspond to those of the successful measurement of the previous measuring cycle.

[0017] According to an advantageous embodiment of the invention, the excitation signal for a single measurement is configured in the form of a sinusoidal pulse. This is particularly advantageous for low liquid levels, as it allows for the shortest possible excitation signal duration and the excitation element to be available as a receiver again as quickly as possible.

[0018] According to a further advantageous embodiment of the invention, the excitation signal for a single measurement is configured as a frequency mixture. The frequency mixture can consist of a superposition of oscillations of different frequencies or pulses of different durations. This has the advantage of preventing measurement disruption due to interference effects. Such interference effects can occur particularly when the liquid surface and the emitting and receiving surfaces of the excitation element are not parallel to each other.

[0019] The object is also achieved by a device according to claim 6 and a household appliance according to claim 9.

[0020] Further features and advantages of the invention will become apparent from the dependent claims and the following description of an embodiment with reference to the drawings, in which Fig. 1. an embodiment of a device according to the invention in a first state, Fig. 2. an embodiment of a device according to the invention in a second state.

[0021] The schematic in Fig. The fog generator 10 shown in Figure 1 comprises a liquid container 12 and an excitation element in the form of a piezoelectric element 14. The liquid container 12 has a base 16 and side walls 18. The piezoelectric element is arranged on the base 16. The liquid container 12 is filled with a liquid volume 20, which is defined by the base 16, the side walls 18, and a surface 22, with the surface 22 being formed opposite the base 16. The liquid volume is adapted to the required amount of fog and the power of the excitation element and can, for example, be several hundred milliliters for the fog generator in a tumble dryer, typically about 200 ml, with a typical fill level of 45 mm.

[0022] To generate fog, the piezo element is activated by applying a high-frequency alternating electrical voltage, the generator voltage U genexcited to mechanical vibrations. The frequency of the generator voltage can typically be 1.7 MHz for water as a liquid. The generator voltage is generated by a high-frequency generator, which is Fig. 1 is not shown. The mechanical vibrations are transmitted by the piezo element 14 to the liquid and cause the finest liquid droplets to be ejected from the liquid volume 20 at the surface 22, forming the desired mist. The generated mist can be transported, for example, by means of a fan to the location where the mist is desired, such as in a tumble dryer onto the laundry to be wetted with the mist.

[0023] The distance from the piezo element 14 to the surface 22, in particular to an imaginary plumb point P on the surface 22, should be defined as the fill level h of the liquid container 12.

[0024] According to the invention, the piezo element 14 is reversible, meaning it can not only be excited to mechanical vibrations by applying an alternating electrical voltage, but also emits an electrical voltage when exposed to mechanical vibrations. For this purpose, an evaluation unit 26 is provided, which registers the electrical voltage from the piezo element 14. The evaluation unit is advantageously implemented in the form of a microprocessor or microcontroller and can particularly advantageously be integrated into a functional unit present in the device for other purposes, for example, the program control unit.

[0025] The method for measuring the fill level h in the liquid container 12 can be carried out as follows: The piezo element 14 is activated by applying a high-frequency electrical alternating voltage, the transmission signal voltage U send (t), to mechanical vibrations. Preferably, the transmit signal voltage U send(t) has a different frequency than the generator voltage U gen . Advantageously, the transmission signal voltage U send (t) can be provided by the same high-frequency generator. Alternatively, the transmission signal voltage U send (t) a separate high-frequency generator may be provided, which may, for example, be integrated into the evaluation unit.

[0026] The mechanical vibrations of the piezo element 14 are transmitted to the liquid and propagate through the liquid as an excitation signal in the form of waves. After traveling the path h, the excitation signal is reflected at the surface 22 of the liquid, as shown in Fig. 2. The reflected excitation signal, after having traveled the path h in the opposite direction, hits the piezo element 14 again. As a result, the reflected excitation signal at the piezo element 14 can be used as a received signal voltage U rec(t) can be detected.

[0027] In the evaluation unit, the distance traveled is calculated from the travel time of the excitation signal to the surface and back again, whereby the calculated distance corresponds to twice the fill level height h.

[0028] Firstly, the frequency of the excitation signal must be sufficiently high. For a fill level h of, say, 45 mm, the excitation signal must travel a distance of 90 mm. The time required for this is 60 µs. A wave packet consisting of, for example, five oscillations, which is to be completely transmitted before the reflected excitation signal reaches the piezo element 14 again, must therefore have a frequency of at least 83 kHz for the dimensions of the example given. Secondly, interference effects can occur due to disturbances in the water surface 22. Disturbances can arise, for example, from waves or a water surface 22 inclined relative to the emitting and receiving surface of the piezo element 14.These interference effects can lead to an attenuation of the reflected excitation signal at the receiving surface of the excitation element, so that the reflected excitation signal is registered more weakly at the piezo element 14 and a reduced received signal voltage U. rec (t). A disturbance of the water surface by fractions of a wavelength, which at a frequency of 1.5 MHz is sufficient for a few tenths of a millimeter, can result in complete signal cancellation.

[0029] In the illustrated embodiment, the excitation element is designed as a disc-shaped piezoelectric element 14, which is housed at the bottom 16 of the liquid container 12. Alternatively, the described method can also be carried out with excitation elements of a different shape. Excitation elements that follow other functional principles are also suitable for the invention if they are reversible, i.e., suitable for registering the reflected excitation signal.

[0030] The invention thus offers a simple and cost-effective way to detect the fill level in the liquid container of a fog generator. In particular, fill level measurement can be implemented without additional sensors.

Claims

[1] Method for fog generation and level measurement in a liquid container of a fog generator with a reversible excitation element, in which an excitation signal is emitted by the excitation element reversible in the method for level measurement, the excitation signal is reflected at the liquid surface, the reflected excitation signal is received by the reversible excitation element and the level is calculated from the propagation time of the signal, wherein the frequency of the excitation signal is adapted to the height of the level. [2] Method according to claim 1, wherein for level measurement the excitation signal comprises at least one wave packet which is shorter than the expected propagation time of the excitation signal. [3] Method according to claim 1 or 2, wherein for level measurement the excitation signal has a frequency which is lower than the frequency for fog generation. [4] Method according to one of the preceding claims, wherein the excitation signal for level measurement comprises wave packets of different frequencies. [5] Method according to one of claims 1 to 3, wherein the excitation signal comprises sinusoidal pulses for level measurement. [6] Device for carrying out a method according to one of the preceding claims. [7] Device according to claim 6, wherein the reversible excitation element is a piezo element. [8] Device according to claim 6 or 7, wherein the excitation element is arranged at the bottom of the liquid container. [9] Household appliance with a device according to one of claims 6 to 8.

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