Optimizable ultrasonic drying

Abstract

An ultrasonic drying apparatus for drying a sheet of material is provided. The apparatus comprises an ultrasonic generator configured to generate ultrasonic waves; an actuator configured to modify a height position of the ultrasonic generator with respect to the sheet of material to be dried; a sensor configured to measure moisture data indicative of a water content of the sheet of material; and a controller configured to control the actuator for adjusting the height position of the ultrasonic generator based on the moisture data measured by the sensor.

Description

OPTIMIZABLE ULTRASONIC DRYINGTECHNICAL FIELD

[0001] Embodiments of the present disclosure relate to an ultrasonic drying apparatus for drying a sheet of material. Further embodiments relate to a method of controlling a height position of an ultrasonic generator with respect to a sheet of material to be dried.BACKGROUND

[0002] The production of paper requires a considerable amount of energy. Within the paper making process, removing water from pulp is particularly energy-intensive. Typically, at least part of the moisture is being removed by employing thermal drying (heating). Similar problems can be found with respect to other industries where a wet film has to be dried.

[0003] An alternative method employs ultrasonic devices to induce vibrations in the paper using piezo-electric devices. When in contact with a paper to be dried, the piezo-electric devices allow to atomize moisture from the paper, which may further be removed by an air stream or convection. However, the efficacy of moisture removal strongly depends on operating parameters of the piezo-electric device.

[0004] Thus, improved ultrasonic drying apparatuses might be desired.DISCLOSURE OF THE INVENTION

[0005] In the view of the foregoing, the present disclosure is directed to an ultrasonic drying apparatus for drying a sheet of material, and a method of controlling a height position of an ultrasonic generator with respect to a sheet of material to be dried.

[0006] According to an aspect of the present disclosure, an ultrasonic drying apparatus for drying a sheet of material is provided. The apparatus comprises an ultrasonic generator configured to generate ultrasonic waves; an actuator configured to modify a height position of the ultrasonic generator with respect to the sheet of material to be dried; a sensor configured to measure moisture data indicative of a water content of the sheet of material;and a controller configured to control the actuator for adjusting the height position of the ultrasonic generator based on the moisture data measured by the sensor.

[0007] According to another aspect of the present disclosure, a method of controlling a height position of an ultrasonic generator with respect to a sheet of material to be dried is provided. The method comprises measuring moisture data of a sheet of material; and automatically adjusting the height position of the ultrasonic generator with respect to the sheet of material based on the measured moisture data.

[0008] According to some embodiments, the sheet of material comprises a paper sheet. The paper sheet may comprise a sheet of paper with high degrees of moisture. In particular, the sheet of paper may have at least 40 wt% of moisture, at least 60 wt% of moisture, at least 80 wt% of moisture or at least 90 wt% of moisture. According to some embodiments, the sheet of material comprises a fabric. The fabric may comprise synthetic and / or natural yarns.

[0009] According to some embodiments, the sheet of material may comprise a metal foil. In particular, the sheet of material may comprise a coated metal foil. The metal foil may be coated with a functional coating that changes physical and / or chemical properties of the metal foil. Exemplarily, the metal foil may be a battery electrode foil.

[0010] According to some embodiments, the ultrasonic generator may comprise a piezoelectric transducer. In typical embodiments, the ultrasonic generator may comprise a plurality of piezoelectric transducers. In some embodiments, the ultrasonic generator may comprise a power supply, in particular configured to supply the ultrasonic generator with an alternating current (AC) voltage at a desired frequency. Typically, the ultrasonic generator comprises a resonance frequency. Typically, the ultrasonic generator is operated at or close to the resonance frequency. In some embodiments, the frequency and / or amplitude of the ultrasonic waves generated by the ultrasonic generator may be fixed. Typically, the frequency and / or amplitude of the ultrasonic waves generated by the ultrasonic generator may be adjusted. Typically, the ultrasonic generator is operated at a frequency of at least 20 kHz, at least 50 kHz or at least 80 kHz. Typically, the ultrasonic generator is operated at a frequency of at most 100 kHz, at most 150 kHz, at most 200 kHz, at most 250 kHz or at most 300 kHz. In particular, the ultrasonic generator is operated at a frequency between 50 kHz and 200 kHz.

[0011] According to some embodiments, a height position of the ultrasonic generator may be modified. The height position may be defined as a position perpendicular to a plane of a portion of the sheet of material to be dried. In other words, the height position may be defined as a distance of the ultrasonic generator, or of a component of the ultrasonic generator, to the sheet of material on an axis perpendicular to the sheet of material at the position of the ultrasonic generator. In some embodiments, the height position may be defined as a relative value of a distance to the sheet of material. In some embodiments, the height position may be defined as an absolute value, exemplarily with respect to a factory floor or a fixed machine, such as a paper-making machine.

[0012] According to some embodiments, the height position influences a contact tightness of the ultrasonic generator with respect to the sheet of material. In particular, a height position that is relatively closer to the sheet of material increases the contact tightness. For an optimum drying performance, an optimized contact tightness is required. Typically, for a contact tightness too small, a reduced drying can be observed. For a contact tightness too large, mechanical damage to the sheet of material may be caused. The contact tightness may comprise a contact pressure, in particular a pressure the ultrasonic generator applies onto the sheet of material, or a stress present in the sheet of material due to an interaction with the ultrasonic generator. In some embodiments, the contact tightness may comprise a pressure exerted on the ultrasonic generator by the sheet of material.

[0013] The actuator configured to modify a height position may be based on an electric signal or a pneumatic or a hydraulic signal. In particular, the actuator may comprise a hydraulic actuator, a pneumatic actuator or an electric motor. In some embodiments, the actuator may comprise a piezo stage. In some embodiments, the actuator may provide predefined height positions and may comprise a finite number of height positions. In some embodiments, the actuator may operate continuously.

[0014] In some embodiments, the ultrasonic drying apparatus, in particular the actuator and / or the ultrasonic generator, comprises a distance sensor, configured to determine a distance between the ultrasonic generator and the sheet of material. The actuator may receive a distance signal from the distance sensor and may control the distance between the ultrasonic generator and the sheet of material, in particular to maintain a desired distancereceived by the actuator. Exemplarily, a control loop may control settings of the actuator based on the distance signal and the desired distance.

[0015] In some embodiments, the ultrasonic generator and / or the actuator is arranged on a top side of the sheet of material. The top side of the sheet of material may be the side facing away from the factory floor. In some embodiments, the ultrasonic generator and / or the actuator is arranged on a bottom side of the sheet of material. The bottom side of the sheet of material may be the side facing the factory floor.

[0016] In some embodiments, the sheet of material may comprise a coating, in particular a coating to be dried. The ultrasonic generator and / or the actuator may be arranged facing the coating; in other words, the ultrasonic generator may be in contact with the coating. Directly contacting the coating may advantageously allow to improve an efficiency of the drying process, but may induce a risk of mechanically influencing or disturbing the coating. In some embodiments, the ultrasonic generator may be in contact with a carrier film of the sheet of material. Typically, the carrier film carries the coating and provides a more homogeneous surface than the coating. A homogenous surface may provide advantages when aiming for a reliable contact with a small contact tightness variation.

[0017] In some embodiments, the ultrasonic drying apparatus comprises a sensor, in particular a moisture sensor. The moisture sensor may comprise an infrared sensor, a nucleonic sensor and / or a microwave sensor. The sensor may comprise a camera sensor and / or a temperature sensor.

[0018] In some embodiments, the ultrasonic drying apparatus comprises a transport system for transporting the sheet of material in a length direction. In particular, the transport system is configured to transport the sheet of material along the ultrasonic generator and the sensor. The length direction typically is the direction in which the sheet of material is being transported by the transport system; in other words, the length direction may be a transport direction. Typically, the length direction is the direction in which the sheet of material has the largest extension. The transport system may comprise at least one transport roll, and in particular a plurality of transport rolls. The sheet of material can be guided by the transport rolls, in particular to follow a transport path. Typically, the transport system defines thegeometric position of the sheet of material. In particular, the transport system may define the height of a sheet of material with respect to the factory floor.

[0019] In some embodiments, the transport system may be configured to vary the height of the sheet of material. The transport system may vary the tension of the sheet of material, which may influence a sagging of the sheet of material between components of the transport system, in particular between transport rolls. In some embodiments, the transport system may vary the transport velocity of the sheet of material.

[0020] In some embodiments, the transport system comprises a transport system controller, configured to control parameters of the transport system. In particular the transport system controller may control the tension of the sheet of material, the transport velocity and / or the height of the sheet of material. The transport system controller may comprise an interface to communicate with a distributed control system (DCS) of an industrial process, in particular a DCS of a paper making process, or of a battery electrode coating process. The transport system controller may provide information on parameters of the transport system, in particular the height and / or the velocity of the sheet of material to the DCS or to the controller of the ultrasonic drying apparatus.

[0021] In some embodiments, the ultrasonic generator, the actuator, the moisture sensor and / or the controller are attached to the transport system. In particular, the ultrasonic generator and / or the actuator may particularly be attached to the transport system such that a variation in the height of the sheet of material does not influence the relative height position of the ultrasonic generator with respect to the sheet of material. In particular, the moisture sensor may be attached to the transport system such that measurement conditions, exemplarily a field of view of the moisture sensor, does not change with variation of the transport system.

[0022] In some embodiments, the moisture sensor is arranged downstream of the ultrasonic generator. In other words, the moisture sensor is arranged such that the moisture in a section of the sheet of material is measured after the section of the sheet of material passing the ultrasonic generator. In particular the moisture sensor is arranged downstream of the ultrasonic generator along the length direction of the sheet of material. Typically, the field of view of the moisture sensor covers an area of the sheet of material impinged by theultrasonic waves of the ultrasonic generator. Arranging the moisture sensor downstream of the ultrasonic generator advantageously allows to assess the performance of the ultrasonic generator. In particular, the moisture content can directly be correlated with parameters of the ultrasonic generator, exemplarily with the height position of the ultrasonic generator with respect to the sheet of material, with the frequency of operation of the ultrasonic generator and / or with the amplitude of the ultrasonic waves generated by the ultrasonic generator.

[0023] In some embodiments, the moisture sensor is arranged upstream of the ultrasonic generator. In other words, the moisture sensor is arranged such that the moisture in a section of the sheet of material is measured prior to the section of the sheet of material passing the ultrasonic generator. In particular the moisture sensor is arranged downstream of the ultrasonic generator along the length direction of the sheet of material. Typically, the field of view of the moisture sensor covers an area of the sheet of material to be impinged by the ultrasonic waves of the ultrasonic generator. In other words, the moisture sensor is arranged at substantially the same position in a width direction, the width direction being perpendicular to the transport length, as the width direction of the ultrasonic generator. Arranging the moisture sensor upstream of the ultrasonic generator advantageously allows to modify settings or parameters of the ultrasonic generator based on the measured moisture of the sheet of material. In particular, fluctuations in the moisture content may be addressed and / or a more homogenous moisture content along the length direction may be achieved, in particular to provide a consistent moisture profile in the sheet of material for possible processes occurring after the drying process.

[0024] In some embodiments, the ultrasonic drying apparatus comprises a plurality of moisture sensors. In particular, the ultrasonic drying apparatus may comprise at least one moisture sensor upstream of the ultrasonic generator and at least one moisture sensor downstream of the ultrasonic generator. The at least one moisture sensor upstream of the ultrasonic generator may measure the moisture data indicative of a water content of the sheet of material prior to the ultrasonic generator. The controller of the ultrasonic drying apparatus may use the moisture data of the sheet of material to optimize parameters of the ultrasonic generator, in particular the frequency and / or amplitude of the ultrasonic waves generated by the ultrasonic generator. The controller of the ultrasonic drying apparatus may use the moisture data of the sheet of material to control the actuator for adjusting the height positionof the ultrasonic generator, in particular to achieve an optimized or predefined moisture level after ultrasonic generator. Typically, a minimum moisture level is desired. In some embodiments, a specific moisture level may be defined to be reached, exemplarily a moisture level for which a subsequent process is optimized.

[0025] In some embodiments, the ultrasonic drying apparatus comprises a reference moisture sensor, in particular upstream of the ultrasonic generator. The reference moisture sensor may be employed to calibrate another moisture sensor, in particular a moisture sensor downstream of the ultrasonic generator. Reference moisture data may be provided by the reference moisture sensor. In some embodiments, reference moisture data may be provided by the DCS.

[0026] In some embodiments, the ultrasonic drying apparatus comprises a plurality of moisture sensors, a plurality of ultrasonic generators and a plurality of actuators. Typically, each of the plurality of ultrasonic generators is associated with one of the plurality of actuators. The plurality of moisture sensors may be arranged upstream and / or downstream of the plurality of ultrasonic generators.

[0027] In some embodiments, the plurality of moisture sensors is arranged in a moisture sensor pattern. In particular, the moisture sensor pattern may comprise at least one line of moisture sensors arranged substantially perpendicular to the length direction. The plurality of moisture sensors may be arranged such that the field of view of each of the plurality of moisture sensors overlaps with the field of view of another one of the plurality of moisture sensors, in particular such that the sheet of material is being measured in the entire width of the sheet of material.

[0028] In some embodiments, the plurality of ultrasonic generators, particularly with each ultrasonic generator being associated with one of the plurality of actuators, may be arranged in an ultrasonic generator pattern. The ultrasonic generator pattern may comprise at least one line of ultrasonic generators. The plurality of ultrasonic generators may be arranged such that an operating area of each of the plurality of ultrasonic generators overlaps with an operating area of another one of the plurality of ultrasonic generators, in particular such that the sheet of material is being dried along the entire width of the sheet of material.

[0029] In some embodiments, the ultrasonic generator patterns may comprise a multiple lines of ultrasonic generators with each line being parallel to each other and perpendicular to the length direction. The ultrasonic generator pattern may follow a two-dimensional Bravais lattice pattern. In particular, the ultrasonic generator pattern comprise an oblique, a rectangular, in particular a square, a centered rectangular, or a hexagonal lattice. In some embodiments, distances between neighboring ultrasonic generators may vary, in particular throughout the width of the sheet of material. In particular, the density of ultrasonic generators may be higher at the center of the sheet of material or at the edges of the sheet of material.

[0030] In some embodiments, the moisture sensor pattern at least substantially matches the ultrasonic generator pattern. In particular, for each of the plurality of ultrasonic generators, one of the plurality of moisture sensors is present. Typically, for each section of the sheet of material dried by one of the plurality of ultrasonic generators, at least one of the plurality of moisture sensors is present upstream and / or downstream.

[0031] In some embodiments, the number of ultrasonic generators exceeds the number of moisture sensors. In particular, the density of ultrasonic generators may exceed the density of moisture sensors. The controller of the ultrasonic drying apparatus may employ the moisture data measured by one moisture sensor to control multiple actuators and / or ultrasonic generators.

[0032] In some embodiments, the controller receives moisture data from the plurality of moisture sensors and controls the plurality of ultrasonic generators and / or the plurality of actuators based upon the moisture data received from the plurality of moisture sensors. In particular, the controller may control each of the plurality of actuators based on data of at least two, at least three or at least five of the plurality of moisture sensors. Exemplarily, the controller may create a model of the moisture distribution and / or of the ultrasonic generator performance and may control the plurality of actuators based upon the model.

[0033] In some embodiments, the controller receives measurement data from the moisture sensor, and in particular from the plurality of moisture sensors. The moisture data may be received directly from the moisture sensor or may be received via a DCS. In some embodiments, the plurality of moisture sensors may provide a consolidated measurement tothe controller. Exemplarily, a moisture distribution, in particular with interpolated and / or averaged data, may be provided to the controller.

[0034] In some embodiments, the controller may receive data from the distance sensor of the actuator. The controller may correlate the distance signal provided by the distance sensor with the moisture signal provided by the moisture sensor. In some embodiments, the controller may receive information on the transport system, in particular from the transport system controller, exemplarily on the velocity of the sheet of paper. In some embodiments, the controller may receive information from the DCS, in particular on the type of material of the sheet of material, in particular information on the composition of the sheet of material, exemplarily on the composition of a sheet of paper. In some embodiments, the DCS may provide information on a desired moisture degree and / or contribution after the drying of the sheet of paper, exemplarily for a subsequent process.

[0035] In some embodiments, the controller controls the actuator, and in particular the plurality of actuators, based on the information received from distance sensors, the transport system controller and / or the DCS.

[0036] Controlling the actuator may comprise controlling the actuator in a closed-loop control scheme, in particular with a closed-loop controller. The closed-loop scheme may be based on the moisture data. In other words, a closed-loop feedback-based control is being employed. Exemplarily, the control scheme may use the moisture data as an output variable, a predefined moisture content as a setpoint and the height position of the ultrasonic generator as an input adjustment. Typically, the control scheme is being adapted for optimizing the height position of the ultrasonic generator for the predefined moisture content. In some embodiments, a minimum moisture level may be set as an optimization target.

[0037] In some embodiments, the closed-loop scheme may employ a plurality of parameters as the input adjustment. In particular, the closed-loop scheme may employ the height position of the ultrasonic generator of a plurality of ultrasonic generators as an input adjustment, in particular for the plurality of ultrasonic generators being arranged subsequently in length direction. In some embodiments, the closed-loop scheme may employ an amplitude and / or frequency of the ultrasonic waves generated by the ultrasonic generator as an input adjustment. In other words, the controller may adjust the amplitude and / orfrequency of the ultrasonic waves generated by the ultrasonic generator to optimize the moisture content of the sheet of paper.

[0038] In some embodiments, the closed-loop scheme may employ information received from the DCS to adjust the setpoint. In particular, the DCS may provide information on a desired degree of moisture for a specific material and / or with respect to a subsequent manufacturing process. In some embodiments, the setpoint may be different for different material compositions.

[0039] In some embodiments, for a plurality of moisture sensors, a plurality of ultrasonic generators and a plurality of actuators, the setpoint may be different for different ultrasonic generators arranged at a different position.

[0040] In some embodiments, the controller may comprise a proportional-integral- derivative (PID) controller. The controller may receive controlling parameters for the proportional gain constant, the integral gain and / or the derivative gain from the DCS. In some embodiments, a performance parameter of the controllers may be determined and controlling parameters may be exchanged between the plurality of controllers based upon the performance parameter.

[0041] In some embodiments, a trained machine learning (ML) model may be applied on the measured moisture data of the sheet of material. In particular, the trained machine learning model may be implemented in the controller.

[0042] The ML model may be trained using training data, in particular, labeled training data. In the training data, different settings of the actuator, which are particularly associated with the height position of the ultrasonic generator and / or of the ultrasonic generator may be labelled. In particular, the settings may be labeled with the associated moisture data indicative of the water content of the sheet of material. The training data may comprise test runs, where a plurality of settings are applied. The training data may comprise a plurality of materials and / or a plurality of initial moisture levels.

[0043] In some embodiments, the training data may employ field drying processes. The field drying processes may comprise drying processes at a plurality of sites and / or employing a plurality materials to be dried, in particular at user sites.

[0044] In some embodiments, the trained ML model may be specific for a material of the sheet of material and / or a setting of the actuator and / or the ultrasonic generator. Exemplarily, the trained ML model may be specific for a predetermined frequency and / or amplitude of the ultrasonic generator. In some embodiments, the trained ML model may be specific for the drying of paper in a paper making process or for the drying of a battery cathode material sheet. In some embodiments, the trained ML model may be applicable to a plurality of materials of the sheet of material and / or to a plurality of settings of the ultrasonic generator and / or of the actuator. In some embodiments, the trained ML model may be selected based on information received from the DCS.

[0045] In some embodiments, training the ML model with training data comprises mapping input data, in particular settings or parameters of the actuator, the ultrasonic generator, the transport apparatus and / or information received from the DCS, to corresponding known moisture data indicative of the water content of the sheet of material to provide the trained ML model.

[0046] In some embodiments, a method of controlling a height position of an ultrasonic generator with respect to a sheet of material to be dried comprises measuring moisture data of a sheet of material and automatically adjusting the height position of the ultrasonic generator with respect to the sheet of material based on the measured moisture data.

[0047] Typically, the ultrasonic generator is arranged upstream to a position at which the moisture data is being measured. In some embodiments, the position of the ultrasonic generator and the position at which the moisture data is being measured differs at least with respect to the transport direction or length direction of the sheet of material.

[0048] In some embodiments, the moisture data may be processed, in particular by applying a filter to and / or averaging at least part of the moisture data. Applying a filter may comprise applying an FFT-filter, a percentile filter or median filter and / or a Savitzky-Golay filter, in particular a moving average filter. In some embodiments, the filter may comprise a high-pass, a low-pass or a band-pass filter. Applying a filter may advantageously allow to reduce measurement noise in the measured moisture data and may improve the control of the height position of the ultrasonic generator.

[0049] Averaging at least part of the moisture data may comprise averaging the moisture data over a predefined time, exemplarily over at least 0.1 s, 0.5 s, 1 s, 2 s, 5 s, 10 s, 30 s or 1 minute. Averaging the moisture data over a predefined time may comprise calculating a moving average or calculating an average at predefined regular points of time, exemplarily every 0.1 s, 0.5 s, 1 s, 2 s, 5 s, 10 s, 30 s or 1 minute. In some embodiments, averaging at least part of the moisture data may comprise averaging moisture data obtained from a plurality of moisture sensors. In particular, an average water content may be calculated from the moisture data obtained from the plurality of moisture sensors. In some embodiments, averaging at least part of the moisture data may comprise calculating a weighted average. Averaging the moisture data over a predefined time may allow for a steadier control of the height position of the ultrasonic generator.

[0050] In some embodiments, the method further comprises automatically adjusting an amplitude of ultrasonic waves emitted by the ultrasonic generator based on the measured moisture data. In particular, the amplitude of the ultrasonic waves emitted by the ultrasonic generator may be increased for a high water content determined, in particular for a water content higher than a predefined level. The amplitude of the ultrasonic waves emitted by the ultrasonic generator may be increased for a low water content determined, in particular for a water content lower than a predefined level.

[0051] In some embodiments, the method further comprises automatically adjusting a frequency of the ultrasonic waves of the ultrasonic generator. In particular, the frequency may be changed with a varying composition of the material of the sheet of material.

[0052] In some embodiments, the method comprises automatically adjusting the frequency and the amplitude of the ultrasonic waves. The automatic adjustment comprises adjusting in reaction to a variation in the measured moisture data.

[0053] Embodiments of the present disclosure provide an improved ultrasonic drying apparatus. The drying performance is improved by applying a live feedback from a moisture sensor. The height position, which is connected to the contact tightness, of the ultrasonic generator is adjusted in response to the moisture data to ensure an improved and consistent drying performance. The present disclosure allows for an improved drying of sheets ofmaterial, and in particular paper sheets, and reduces the energy consumption of the drying process. Predefined drying levels can be reached more reliably.BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The accompanying drawings relate to embodiments of the disclosure and are described in the following:FIG 1 schematically illustrates an ultrasonic drying apparatus according to embodiments described herein;FIG 2a schematically illustrates an ultrasonic drying apparatus according to embodiments described herein;FIG 2b schematically illustrates the ultrasonic drying apparatus of Fig. 2a from a different perspective;FIG 3 schematically illustrates an ultrasonic drying apparatus according to embodiments described herein;FIG 4 schematically illustrates a method of controlling a height position of an ultrasonic generator according to embodiments described herein.DETAILED DESCRIPTION OF EMBODIMENTS

[0055] Reference will now be made in detail to the various embodiments of the disclosure, one or more examples of which are illustrated in the figures. Generally, only the differences with respect to individual embodiments are described. Each example is provided by way of explanation of the disclosure and is not meant as a limitation of the disclosure. Further, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield a further embodiment. It is intended that the description includes such modifications and variations. In the figures, elements may be depicted with exaggerated dimensions to improve the comprehensibility of the detailed description of embodiments. Further, some elements may be depicted with enlarged dimensions while other elements in the same figure are depicted, relatively, with reduced dimensions.

[0056] Fig 1 schematically illustrates an ultrasonic drying apparatus 100 for drying a sheet of material 2. The apparatus 100 comprises an ultrasonic generator 10, an actuator 20, amoisture sensor 30 and a controller 40. The ultrasonic generator 10 is connected with the actuator 20 such that the actuator 20 can modify a height position of the ultrasonic generator 10 with respect to the sheet of material 2. In particular, the actuator 20 can move the ultrasonic generator 10 perpendicular to a length direction L in which the sheet of material 2 travels.

[0057] The moisture sensor 30 is arranged above the sheet of material 2 and on a different side of the sheet of material 2 than the ultrasonic generator 10. In embodiments not explicitly shown in the Figures, the moisture sensor 30 may be on the same side as the ultrasonic generator 10. The moisture sensor 30 is configured to measure data indicative of a water content of the sheet of material 2 within the field of view 31 of the moisture sensor 30. The sheet of material 2 is transported along a length direction L by a transport system 50. In Fig. 1, the transport system 50 comprises two transport rolls 50.

[0058] The controller 40 receives data indicative of a water content of the sheet of material 2 from the moisture sensor 30. The controller 40 analyses the data and controls the actuator 20 for adjusting the height position of the ultrasonic generator 10 based on the moisture data measured by the moisture sensor 30. The controller 40 may receive a feedback from the actuator 20 or the ultrasonic generator 10, in particular on an actual height position of the ultrasonic generator 10 with respect to the sheet of material 2.

[0059] In Fig. 2a, illustrates an ultrasonic drying apparatus 100 for drying a sheet of material 2. In Fig. 2a, the ultrasonic generator 10 is attached in contact with the sheet of material 2 and induces ultrasonic waves in the sheet of material 2. At the position of the ultrasonic generator 10, water 4 is atomized and ejected out of the sheet of material 2. Subsequent to an ultrasonic drying process, the sheet of material 2 is transported along the length direction L to pass through the field of view 31 of the moisture sensor 30. In the embodiment shown in Fig. 2a, subsequent to the ultrasonic drying process, the sheet of material 2 passes through a press section 51, where a pressure is being applied on the sheet of material 2 and a further drying section 52, where typically heat is being applied to the sheet of material 2. In embodiments not shown in the Figures, the order of the ultrasonic drying process, the press section 51 and the further drying section 52 may be different. Inparticular, the sheet of material 2 may pass through the press section 51 prior to the ultrasonic drying process.

[0060] In Fig. 2b, the ultrasonic drying apparatus 100 of Fig. 2a is shown from a different perspective. The ultrasonic generator 10 has a circular shape and typically has a circular effective working area. Subsequent to the ultrasonic generator 10, the sheet of material 2 passes along the moisture sensor 30. The moisture sensor 30 is arranged such that a section of the sheet of material 2 passing the field of view of the moisture sensor 30 has been within the working area of the ultrasonic generator 10 beforehand.

[0061] Fig. 3 shows an ultrasonic drying apparatus 100 with a plurality of ultrasonic generators 10a- 10g, a plurality of actuators 20a-20g and a plurality of moisture sensors 30a- 30e. The plurality of actuators 20a-20g and the plurality of moisture sensors 30a-30e is connected to the controller 40; in particular, each of the plurality of actuators 20a-20g and each of the plurality of moisture sensors 30a-30e is connected to the controller 40. Each of the plurality of ultrasonic generators 10a- 10g is associated with one of the plurality of actuators 20a-20g. The plurality of ultrasonic generators lOa-lOg is arranged in a pattern, in particular in a hexagonal pattern. The plurality of moisture sensors 30a-30e is arranged in a pattern, in particular in a hexagonal pattern. In some embodiments, the pattern of the plurality of moisture sensors 30a-30e may be different from the pattern of the plurality of actuators 20a-20g. Typically, the fields of view of the plurality of moisture sensors 30a-30e do not overlap substantially.

[0062] Fig. 4 shows a method 400 of controlling a height position of an ultrasonic generator with respect to a sheet of material to be dried. The method 400 comprises measuring 410 moisture data of a sheet of material. Subsequently, the method optionally comprises applying a filter 412 to the moisture data and / or averaging 414 at least part of the moisture data. Based on the measured 410, and optionally filtered 412 and / or averaged 414, moisture data, the height position of the ultrasonic generator with respect to the sheet of material is automatically adjusted 420. The method 400 may additionally comprise automatically adjusting an amplitude and / or a frequency of ultrasonic waves emitted by the ultrasonic generator based on the measured 410, and optionally filtered 412 and averaged 414, moisture data.

Claims

CLAIMS1. An ultrasonic drying apparatus (100) for drying a sheet of material (2), the apparatus (100) comprising: an ultrasonic generator (10) configured to generate ultrasonic waves; an actuator (20) configured to modify a height position of the ultrasonic generator (10) with respect to the sheet of material (2) to be dried; a moisture sensor (30) configured to measure moisture data indicative of a water content of the sheet of material (2); and a controller (40) configured to control the actuator (20) for adjusting the height position of the ultrasonic generator (10) based on the moisture data measured by the moisture sensor (30).

2. The apparatus (100) of claim 1, wherein the sheet of material (2) is a paper sheet.

3. The apparatus (100) of claim 1, wherein the sheet of material (2) comprises a fabric or a metal foil.

4. The apparatus (100) of claim 3, wherein the sheet of material (2) comprises a coated metal foil.

5. The apparatus (100) of any of the preceding claims, further comprising a transport system (50) for transporting the sheet of material (2) in a length direction (L).

6. The apparatus (100) of any of the preceding claims, wherein the moisture sensor (30) is arranged downstream of the ultrasonic generator (10).

7. The apparatus (100) of any of the preceding claims, wherein the apparatus (100) comprises a plurality of moisture sensors (30a-30e), a plurality of ultrasonic generators (10a- 10g) and a plurality of actuators (20a-20g); andthe controller (40) is configured to control each of the plurality of actuators (20a- 20g) based on data of at least two of the plurality of moisture sensors (30a-30e).

8. The apparatus (100) of any of the preceding claims, wherein the controller (40) configured to adjust an amplitude and / or a frequency of the ultrasonic waves generated by the ultrasonic generator (10) based on the moisture data of the sheet of material (2) measured by the moisture sensor (30).

9. The apparatus (100) of any of the preceding claims, wherein the controller (40) is configured to control the actuator (20) in a closed-loop control scheme.

10. The apparatus (100) of any of the preceding claims, wherein the controller (40) is configured to receive information from a distributed control system; and the controller (40) is configured to control the actuator based on the information received from the distributed control system.

11. The apparatus (100) of any of the preceding claims, wherein controlling the actuator (20) comprises applying a trained machine learning model on the moisture data of the sheet of material (2) measured by the moisture sensor (30).

12. A method (400) of controlling a height position of an ultrasonic generator (10) with respect to a sheet of material (2) to be dried, the method (400) comprising: measuring (410) moisture data of a sheet of material, and automatically adjusting (420) the height position of the ultrasonic generator (10) with respect to the sheet of material (2) based on the measured moisture data.

13. The method (400) of claim 12, further comprising applying (412) a filter and / or averaging (414) to at least part of the moisture data.

14. The method (400) of any of claims 12 to 13, further comprising automatically adjusting (430) an amplitude and / or a frequency of ultrasonic waves emittedby the ultrasonic generator (10) based on the measured moisture data.

15. The method (400) of any of claims 12 to 14 using the apparatus (100) of any one of claims 1 to 11.

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