EP3724965A1

EP3724965A1 - Object detection system for a wireless power transfer system

Info

Publication number: EP3724965A1
Application number: EP18819095.3A
Authority: EP
Inventors: Tom Lambertus Johannes GOOSSENS; Geraldi WAHYULAKSANA
Original assignee: Prodrive Technologies BV
Current assignee: Prodrive Technologies Innovation Services BV
Priority date: 2017-12-12
Filing date: 2018-12-12
Publication date: 2020-10-21
Also published as: WO2019115631A1

Abstract

Wireless power transfer system (300) comprising a first power transfer unit (10) configured for inductive power transfer. The first power transfer unit comprises a first ultrasound transducer (12), a second ultrasound transducer (11), and a controller (17). The first ultrasound transducer is positioned for receiving a first ultrasonic signal, emitted by the second ultrasound transducer, and the controller is configured to determine a presence of foreign objects based on the first ultrasonic signal.

Description

OBJECT DETECTION SYSTEM FOR A WIRELESS POWER TRANSFER SYSTEM

Wireless power transfer systems generate an electromagnetic field that may pose a safety risk for objects (e.g. living, conducting) that are exposed to it. Therefore, these charging systems may comprise object detection systems as a safety feature. This is of particular interest in situations where the generated electromagnetic field is of considerable strength. This is typically the case in situations where the level of transferred power is high, such as the wireless charging of electric vehicles.

US 2016/0028265A1 describes an object detection system wherein ultrasound transducers positioned in an array are used in a monostatic arrangement, i.e. the transducer emitting a signal is also used for receiving the signal. The general principle is that the ultrasonic signal is reflected by the object and that this reflected signal comprises information regarding the object. Such a system using a monostatic configuration has several disadvantages. Firstly, if an object is non-reflective it will not be detected by the system. Secondly, there is a dead zone around the transducer, because a transducer that transmits a burst signal cannot measure an incoming signal until sometime after the emission of the signal has ended. Therefore, the transducers need to be placed at a larger distance to the protected area and/or a redundancy of ultrasound transducer is provided, so that an object may be detected by multiple transducers. Thirdly, the incident angle of the emitted acoustic wave with the underbody of the car must be shallow, otherwise the underbody will be detected as an object. Therefore, the sensor either needs to protrude from the ground assembly or be placed at a larger distance. Fourthly, an object located at an elevated level with respect to the surface of the charging station may not be detected.

An object detection system and method according to the appended claims aims at overcoming the disadvantages of the prior art because it uses ultrasound transducers positioned in a bistatic configuration. A first ultrasound transducer is positioned for receiving a first ultrasonic signal emitted by a second ultrasound transducer. The first and second ultrasound transducers are advantageously spaced apart, such as positioned along a perimeter of a first power transfer unit. Advantageously, a plurality, e.g. three or more ultrasound transducers are provided, distributed along the perimeter. The ultrasound transducers are advantageously configured for receiving, emitting, or for receiving and emitting ultrasonic signals. A controller is configured to determine a presence of foreign objects based on the first ultrasonic signal. Advantageously, a first portion of the first ultrasonic signal is received by the first ultrasound transducer indirectly, i.e. following reflection by a surface which is advantageously arranged facing the first and second ultrasound transducers and spaced apart therefrom. The surface can be a vehicle underbody and/or the external surface of a second power transfer unit which is arranged to be inductively coupled to the first power transfer unit, e.g. for power transfer. In addition, a second portion of the first ultrasonic signal is advantageously directly received by the first ultrasound transducer, i.e. without being reflected, e.g. by the surface of reflection. When an object is situated between the first ultrasound transducer and the second ultrasound transducer, any one or both of the first portion and the second portion will be disturbed and the controller can be configured to detect the object on the basis of the disturbed signal of the first portion and advantageously also the second portion of the first ultrasonic signal.

This type of configuration overcomes the disadvantages of the prior art mainly because (1 ) absorption of the transmitted signal by a foreign object can also lead to the detection of the object, (2) the receiving transducer does not have a dead zone which enables placement of the transducers closer to the protected area, (3) reflections against for instance the underbody of a car may be used to direct the acoustic wave to the receiver instead of being detected as an object and (4) the entire volume, that is exposed to a field of relevant strength, can be monitored.

In an object detection system according to the present invention, at least part of the signal is advantageously received by the first ultrasonic transducer without being reflected by a surface. This direct signal may be beneficial for embodiments of the invention, wherein the ultrasound transducers may not protrude the housing of the power transfer unit and are therefore arranged to emit and receive signals substantially vertically, because a system according to the present invention also enables the sole use of reflected signal to detect objects.

Advantageously, the first and second ultrasound transducers are arranged as transceivers. Such an arrangement may improve detection reliability, because it allows for inversing the operation. Hence, the second ultrasound transducer is advantageously positioned for receiving a second ultrasonic signal, emitted by the first ultrasound transducer, and wherein the controller is configured to determine the presence of foreign objects based on a comparison of the first ultrasonic signal and the second ultrasonic signal. The comparison can comprise evaluating a similarity of the first ultrasonic signal and the second ultrasonic signal. Techniques for evaluating such a similarity can comprise comparing a parametric description of the first ultrasonic signal and a parametric description of the second ultrasonic signal, e.g. performing a cross-correlation and/or comparing impulse responses of the two signals or portions thereof. The cross-correlation can be evaluated between an envelope and/or an impulse response of (at least a portion of) the first ultrasonic signal and (at least a portion of) the second ultrasonic signal.

FIGs. A1 A-B show a schematic representation of a top view of an object detection system 100 according to the invention. FIG. 1A shows two ultrasound transducers 1 1 1 emitting an ultrasonic signal 1 16 towards two other ultrasound transducers 112, wherein the ultrasonic signal transmission from emitter to receiver is not disturbed. FIG. 1 B shows the same, but here the direct and/or reflected transmission of the ultrasonic signal 116 from emitter to receiver is disturbed by an object 13.

FIG. 2A-B show a schematic representation of a side view of an object detection system 200 according to the invention. FIG. 2A shows an ultrasound transducer 11 emitting an ultrasonic signal towards another ultrasound transducer 12, wherein the ultrasonic signal transmission from emitter to receiver is not disturbed. FIG. 2B shows the same, but here an object 13 disturbs the transmission of some reflections of the ultrasonic signal from emitter to receiver.

FIG. 3A-B show the envelope of the readout of six ultrasonic transducers, the first signal (rxO) being the signal of the transducer acting as emitter and the subsequent five signals (rx1-rx5) being the signals of the transducers acting as receiver. FIG. 3A shows the envelope of the read-out of the ultrasound transducers without an object disturbing signal transmission from the emitter to the seven receivers. FIG. 3B shows the envelope of the read-out of the ultrasound transducers with an object disturbing signal transmission from the emitter to the seven receivers. The envelopes in FIG. 3A and FIG. 3B can be processed to obtain a parametric description of the received signal, which can be used to determine the presence or absence of an object.

FIG. 4 shows a schematic representation of a side view of an object detection system 300 according to aspects of the present invention.

FIGs. 5A-B show the object detection system of FIG. 4 when an object disturbs signal reception by the ultrasound transducers.

FIG. 6 illustrates a plot of a response sensed by a receiving transducer for an ultrasonic signal emitted by another ultrasound transducer. The response includes a portion of the ultrasonic signal that is received directly without being reflected as well as a portion of the ultrasonic signal that is received following reflection by a surface. FIG. 7 illustrate the response of FIG. 6 (shown in solid line) and the response of the same signal when a foreign object is present (dotted line). Referring to FIG. 4, an object detection system 300 comprises a power transmission unit 10. Power transmission unit 10 is mounted on a floor 9. Power transmission unit 10 comprises an induction coil 14 for inductive power transfer to a body 20, e.g. a vehicle, which is shown in an operating position on top of the power transmission unit 10, operable to receive power from induction coil 14. Body 20 may comprise a second power transfer unit 22 with a coil 221 for receiving power from coil 14. Ultrasound transducers 1 1 and 12 are shown mounted in the power transmission unit 10 at opposite sides of the induction coil 14, although this is no requirement. In practical cases, a plurality, e.g. three, four or more ultrasound transducers are arranged along a perimeter of the power transfer unit 10 and any of these ultrasound transducers may act as receiver 12 and any other one may act as emitter 1 1. Advantageously the ultrasound transducers 1 1 , 12 are configured as transceivers, i.e. they can act both as emitter and as receiver.

Ultrasound transducers 1 1 , 12 are advantageously flush mounted with an external surface 15 of power transmission unit 10. External surface 15 is advantageously substantially parallel with the floor 9 and advantageously substantially perpendicular with a winding axis 141 of the induction coil 14. Winding axis 141 is advantageously arranged substantially perpendicular to the level of floor 9. The flush mounting avoids accumulation of dirt and water or ice on top of the ultrasound transducersl 1 , 12, which may affect operation.

A first ultrasound transducer 12 is configured for capturing ultrasonic signals and will be referred to as receiver. A second ultrasound transducer 11 is advantageously configured for emitting ultrasonic signals and will be referred to as emitter. Both ultrasound transducers 1 1 and 12 advantageously have a field of emission respectively sensing having a wide angle. The ultrasound transducers 1 1 , 12 are advantageously positioned facing the external surface 21. Their position is advantageously such, that the field of emission, respectively sensing of the transducers has an axis, such as an axis of symmetry, which is arranged substantially vertically, i.e. substantially perpendicular to the surface 15 across which inductive power transfer occurs. As a result, an acoustic signal 16 emitted by emitter 11 can have a first portion 161 which arrives at the receiver 12 following reflection at a surface 21 of body 20 which faces the external surface 15 and is spaced apart therefrom. The acoustic signal 16 advantageously comprises a second portion 162 which arrives directly at the receiver 12 without being reflected.

Receiver 12 therefore advantageously senses two portions 161 , 162 of a same ultrasonic signal 16 emitted by the same emitter 1 1 , and which arrive at different instants of time at receiver 12 since the two portions propagate along different paths having different lengths. The two portions can be separated during signal processing in a controller 17 of the power transmission unit 10 by applying different time windows to the signal sensed by receiver 12. Generally speaking, the second portion 162 (direct signal) will be received by the receiver 12 first, followed by the first portion 161 (reflected signal). It is possible that the two time windows partially overlap, such that an overlap portion may be processed both in the first portion and in the second portion. Referring to FIG. 6, the response 60 sensed by receiver 12 for the ultrasonic signal 16 can be processed to extract or separate the response 62 relating to the second portion 162 (direct signal) and the response 61 relating to the first portion 161 (reflected signal), e.g. by extracting different or time-shifted time windows from the response 60. Controller 17 is configured for controlling operation of the emitter 11 and of the receiver 12. Controller 17 is further operable to receive the ultrasonic signal 16 captured by the receiver 12 and to process it in order to evaluate the presence of a foreign object in the space between surfaces 15 and 21. The first portion 161 and second portion 162 of signal 16 are advantageously processed separately by the controller 17. Alternatively, both portions 161 and 162 can be processed integrally as a single signal.

The second portion 162 can furthermore be used for determining a speed of sound and/or a signal attenuation, since the distance between the emitter 1 1 and the receiver 12 is known. The speed of sound and signal attenuaion is typically affected by environmental conditions, such as ambient temperature and humidity. Using the measured attenuation and/or speed of sound may improve object detection strategies, and/or object localization. The measured speed of sound may also be used to adapt the time windows for separating the first portion and the second portion signals. Additionally, the measured speed of sound may be used to estimate signal attenuation due to air absorption.

Referring to FIGs. 5A-B, when a foreign object 13 is interposed between surface 15 and surface 21 , either the first portion 161 , the second portion 162, or both can be disturbed or blocked. The foreign object can therefore be sensed by processing the first portion 161 and the second portion 162 of the ultrasonic signal 16. Referring to FIG. 7, the presence of foreign object 13 will alter the response 70 sensed by receiver 12 for the ultrasonic signal 16, as compared to the response 60 when no foreign object is present between the surfaces 15 and 21. As a result, it is obtained that foreign objects located on the unit 10 or on the floor (as shown in FIG. 5A) and objects located at an elevation (as shown in FIG. 5B) may be detected reliably with a reduced number of ultrasound transducers. A more compact and more cost effective wireless power transfer system is obtained.

In order to improve detection reliability and/or reduce the number of ultrasound transducers, the controller can alternatively or additionally be configured for inversing operation of the ultrasound transducers 11 and 12. That is, ultrasound transducer 12 will emit a second ultrasonic signal, which may or may not be identical to the first ultrasonic signal emitted by transducer 11 as previously described. The second ultrasonic signal is received by ultrasound transducer 1 1. In this case, both transducers act as transceivers. The second ultrasonic signal as sensed by transducer 1 1 can comprise a directly received second portion, and a reflected first portion, analogously as with the first ultrasonic signal. Both portions may or may not be processed separately. In addition, or alternatively, the controller may be configured to jointly process the first portions of the first and second ultrasonic signals on the one hand, and the second portions of the first and second ultrasonic signals on the other.

It will be convenient to note that, although the transducers and controller have been described as comprised in a wireless (contactless) power transfer unit arranged on the floor (referred to as a ground unit), this is no requirement. It is alternatively possible to provide the transducers and/or controller in the second power transfer unit 22 arranged on the moving body 20.

Controller 17 is advantageously configured to apply signal processing methods in order to process the first and/or the second ultrasonic signal. One suitable method of processing involves determining an energy loss in the received signal as compared to the emitted signal. However, additional information may be gathered when processing the first portion and the second portion of signal 16 separately, e.g. an improved localization of the foreign object may be obtained. By way of example, the controller can be configured to normalize the first ultrasonic signal and/or the second ultrasonic signal. Normalization can comprise modifying either one or both (direct and reflected) portions of the ultrasonic signal to cope with signal attenuation due to environmental conditions, e.g. humidity, temperature, etc.. Signal attenuation due to environmental conditions may be estimated or determined by measuring a speed of sound.

Controller 17 is advantageously configured to implement a method for object detection as described herein. Methods for object detection according to aspects of the present invention advantageously comprise operations of sending/emitting a first signal 16, preferably a burst signal, by the second ultrasound transducer 11 and receiving the first signal by the first ultrasound transducer 12. The first signal advantageously propagates through an air space in which foreign objects must be detected, e.g. the space between external surfaces 15 and 21.

In a following operation, the signal as received (e.g., response 60 or 70) is processed. Controller 17 can be configured to evaluate the first ultrasonic signal. The evaluation can include evaluating a parametric description of the ultrasonic signal. The evaluation can include evaluating descriptive statistics (or properties) of the first ultrasonic signal and/or an autocorrelation of the first ultrasonic signal, e.g. based on an envelope and /or an impulse response of the first ultrasonic signal. In one example, the operation advantageously comprises normalizing the signal as received. Next, an envelope of the received signal may be calculated. Additionally, or in the alternative, an impulse response of the signal is calculated. A similarity between different parts of the calculated signal(s) separated by a time interval can be evaluated, e.g. by autocorrelation.

It is additionally, or alternatively possible to obtain a second signal by inversing the roles of the first and second ultrasound transducers, i.e. obtaining a signal in the reverse direction. Same operations as normalization, envelope and/or impulse response calculation can be effected on the second signal.

A similarity between (a parametric description of) the first and the second signal can be evaluated, e.g. by cross-correlation. The cross-correlation can be evaluated between an envelope and/or an impulse response of the first ultrasonic signal and the second ultrasonic signal. Additionally, or in the alternative, a similarity between successive signals emitted by one ultrasound transducer and received by another ultrasound transducer is advantageously evaluated, e.g. by cross-correlation. Evaluating a similarity between successive signal transmissions may be useful for detecting moving objects and/or for removing false positives/negatives.

Any combination of the above operations on the first and/or second signals can be combined into a parametric and/or statistical description and a decision can be inferred from the obtained description, e.g. using a machine learning algorithm, such as gradient tree boosting.

In one example, the method for object detection comprises the following steps:

1. Send burst;

2. Receive burst;

3. Signal processing;

4. Summarize the signals using parametric or statistical descriptions; and

5. Decide if there is an object. The step of signal processing can comprise a signal normalization and/or determining a signal envelope. Optionally, the signal processing can comprise determining an impulse response and/or performing cross-correlation and/or auto-correlation.

It will be convenient to note that the above method steps may be performed separately on the first and second portions (direct and reflected signal portions) of the first and/or second ultrasonic signal.

The above example will now be described in more detail in relation to object detection in a wireless charging system for a vehicle. A first transducer transmits a burst of sound at an ultrasonic frequency, e.g. 40 kHz, by means of a block-wave signal applied to its clamps. For common piezo transducers, the electro-mechanical properties of the transducer and its driving electronics have the effect of bandpass filtering the initial block-wave signal around its resonance frequency as it is transferred into a soundwave through the air. The sound is received by a second transducer either directly or indirectly via reflection against the underbody of a vehicle and/or the floor or surface of the first power transfer unit. This received signal is affected by the acoustic space in the neighborhood of the transducers, for instance, by the height of the car, the shape and material of the underbody, and by the presence of a foreign object in between the first and second transducer. It is the task of the post processing to judge if a foreign object is present or not by cues in the received acoustical signal. The received signal can be normalized by the mean, standard deviation or zero mean in order to obtain invariance to environmental influences, such as air temperature, underbody height, underbody material.

The carrier frequency contains little information to this end, so it is advantageously removed using an envelope detector. A convenient way to achieve this is by applying Hilbert transform to the received signal y(t), or Y(f) in the frequency domain, to get the analytical signal y_a(t):

and hence

ym( = ly_a( I

Where T ^-1 is the inverse Fourier transform. The absolute of the analytical signal yields y_m(t), which is the instantaneous amplitude or envelope signal.

The acoustic space between the transducers can be considered as a linear time- invariant system in the case of a stationary source and receiver. Using either a measurement of the transmitted signal or a model of the transmitted signal (x(t)) it is possible to obtain the impulse response of the acoustic space via the process of deconvolution on either the received signal or its envelope. For example, using the method of Wiener deconvolution:

no E(f)

H(0 =

X(f)

Where E(f) is an error or noise signal. The wiener filter g(t ) is applied to estimate the error of a noisy process in the frequency domain, so the G(/) is a representative of impulse response H(f) can be calculated with: xJf)^y(f)

G(/) =

\xiO\²\no\ ² And hit) or the impulse response of the system is the inverse Fourier transform of Gif^') = H (/).

When the roles of the first and second transducers are reversed, the signal in the reverse direction can be obtained. The similarity between the two signals can be evaluated for use in the foreign object detection. One way to quantitatively measure the similarity is to calculate the cross-correlation of the two signals. if * 90 = 1 f^*it)g{t + Odt

J

Where * denotes the cross-correlation operation, / and g are the two signals and f^* denotes the complex conjugate of /.

The presence of a foreign object can be inferred from the above representations of the received signal, such as the envelope signal, the impulse response, and cross- correlation, using parametric descriptions or descriptive statistics of these signal representations. For instance the received energy, area under curve, the standard deviation, variance, peak-to-average, average, magnitude, sum of squares, time of arrival, curve fitting parameters.

These parameters can be influenced by a number of environmental factors (e.g. temperature, wind, humidity) that affect the wave propagation (e.g. speed of sound, signal attenuation ). These changes might be significant enough to cause false identification of foreign objects. This influence can be compensated by having some measured information (e.g. speed of sound, attenuation) related to the environment condition, (e.g. temperature). This information may advantageously be measured on the available ultrasound signals. By taking these measurements into account, influence of the environmental conditions can be estimated and corrected for, thus making the detection more robust. Preferably, the system is calibrated for these influences (and other influences, such as system variables, e.g. sensitivities) by means of a regular calibration based on the non-reflected ultrasonic signal (i.e. direct path between transducers). Preferably, the wireless power transfer system comprises at least three ultrasonic transducers to achieve a redundant set of transducer pairs for performing such calibration. For instance, a wireless power transfer system comprising 6 ultrasonic transducers has 15 sets of transducer pairs. The benefit of such a redundancy is that calibration can be performed even if a limited number of non-reflected ultrasonic signals is blocked by an object.

Claims

1. Wireless power transfer system comprising:

a first power transfer unit configured for inductive power transfer, comprising:

a first ultrasound transducer,

a second ultrasound transducer, and

a controller

characterized in that

the first ultrasound transducer is positioned for receiving a first ultrasonic signal, emitted by the second ultrasound transducer, and

the controller is configured to determine a presence of foreign objects based on the first ultrasonic signal.

2. Wireless power transfer system according to claim 1 , wherein the first ultrasound transducer and the second ultrasound transducer are positioned such that a first portion of the first ultrasonic signal is reflected by a surface of reflection arranged opposite the power transfer unit and spaced apart from the power transfer unit prior to being received by the first ultrasound transducer.

3. Wireless power transfer system according to claim 2, wherein the first ultrasound transducer and the second ultrasound transducer are positioned such that a second portion of the first ultrasonic signal is configured to be directly received by the first ultrasound transducer without being reflected by the surface.

4. Wireless power transfer system according to claim 2 or 3, wherein the controller is configured to determine the presence of the foreign objects based on at least the first portion of the first ultrasonic signal.

5. Wireless power transfer system according to claim 3 or 4, wherein the controller is configured to determine a speed of sound and/or a signal attenuation based on the second portion of the first ultrasonic signal.

6. Wireless power transfer system according to any one of the claims 3 to 5, wherein the controller is configured to separate and to separately process the first portion and the second portion of the first ultrasonic signal.

7. Wireless power transfer system according to any one of the preceding claims, wherein the second ultrasound transducer is configured to emit ultrasonic signals with an axis of emission oriented substantially vertically.

8. Wireless power transfer system according to any one of the preceding claims, comprising a second power transfer unit configured for inductive coupling with the first power transfer unit across an air gap, wherein the first power transfer unit comprises a first external surface and the second power transfer unit comprises a second external surface, the first and second external surfaces configured to be positioned in a working position at opposite sides of the air gap and substantially parallel to each other.

9. Wireless power transfer system according to claim 8, wherein the first ultrasound transducer and the second ultrasound transducer are flush mounted with the first external surface.

10. Wireless power transfer system according to claim 8 or 9, comprising a surface of reflection of the first ultrasonic signal adjoining the second power transfer unit, wherein in the working position, the surface of reflection, the first ultrasound transducer and the second ultrasound transducer are positioned such that a first portion of the first ultrasonic signal is reflected by the surface of reflection prior to being received by the first ultrasound transducer.

11. Wireless power transfer system according to claim 10, wherein the surface of reflection is a surface of a vehicle underbody and/or the second external surface.

12. Wireless power transfer system according to any one of the preceding claims, wherein the second ultrasound transducer is positioned for receiving a second ultrasonic signal, emitted by the first ultrasound transducer, and wherein the controller is configured to determine the presence of foreign objects based on a comparison of the first ultrasonic signal and the second ultrasonic signal.

13. Wireless power transfer system according to any one of the preceding claims, wherein the controller is configured to evaluate a parametric description of the first ultrasonic signal.

14. Wireless power transfer system according to claim 13, wherein the parametric description comprises one or more of a group consisting of peak-to-average, variance, magnitude, average, sum of squares, area under curve, and time of arrival.

15. Wireless power transfer system according to any one of the claims 3 to 6, wherein the controller is configured to evaluate a parametric description of the first ultrasonic signal and to apply the parametric description to the first portion of the first ultrasonic signal, the second portion of the first ultrasonic signal and/or the ratio between the second portion and the first portion.

16. Wireless power transfer system according to any one of the preceding claims, wherein the first ultrasound transducer and the second ultrasound transducer are located at spaced apart positions along a perimeter of the first power transfer unit.

17. Combination of a vehicle and the wireless power transfer system of any one of the preceding claims, wherein, in a working position, an underbody of the vehicle is configured to reflect at least a first portion of the first ultrasonic signal which is received by the first ultrasound transducer, and wherein the controller is configured to evaluate the presence of the foreign objects based at least in part on the first portion.

18. Use of a vehicle underbody for object detection in a wireless power transfer system, wherein the vehicle underbody reflects signals emitted from a first ultrasonic transducer towards a second ultrasonic transducer and wherein the reflected ultrasonic signal is used for detecting an object under the vehicle.

19. Method for object detection in a wireless power transfer system comprising:

receiving a first ultrasonic signal by a first ultrasound transducer following emission by a second ultrasound transducer and,

evaluating a presence of a foreign object on the basis of the first ultrasonic signal.

20. Method according to claim 19, wherein a first portion of the first ultrasonic signal is reflected by a surface opposite and spaced apart from a first power transfer unit of the wireless power transfer system prior to being received by the first ultrasound transducer.

21. Method according to claim 20, wherein a second portion of the first ultrasonic signal is emitted from the second ultrasound transducer and received directly by the first ultrasound transducer without being reflected.

22. Method according to claim 21 , wherein in processing the first ultrasonic signal, the first portion and the second portion are separated, preferably by application of different time windows on the first ultrasonic signal.

23. Method according to any one of claim 19 to 22, wherein evaluating the presence of the foreign object comprises evaluating a parametric description of the first ultrasonic signal.

24. Method according to claim 23 in conjunction with claim 21 or 22, wherein the parametric description of the first ultrasonic signal is evaluated based on a calibration measurement of the second portion of the first ultrasonic signal, wherein the calibration measurement comprises one or more of: measuring a speed of sound based on the second portion and measuring an attenuation of the second portion.

25. Method according to any one of claims 19 to 24, comprising receiving a second ultrasonic signal by the second ultrasound transducer following emission by the first ultrasound transducer and reflection by the surface, and evaluating the presence of the foreign object on the basis of the second ultrasonic signal.