Object detection system

Abstract

The invention relates to an object detection system (1) comprising an antenna arrangement (3), comprising at least one antenna; a control device (2) adapted to control the antenna arrangement (3) to emit a radio-frequency detection signal (DI, D2) and to detect an object based on a radio-frequency response signal received by the antenna arrangement (3); and at least one passive reflector (10) adapted to at least partially reflect the detection signal (DI, D2), the reflector (10) comprising a metamaterial with a plurality of unit cells, the metamaterial comprising at least one dielectric substrate layer and a conductive structured layer disposed on an upper side thereof, whereby the reflector (10) has a frequency-dependent reflectivity, wherein the control device (2) is adapted to detect the object at least partially based on a frequency spectrum of the response signal. In order to provide efficient means for reliable radar-based object detection, the invention provides that at least one of said at least one reflector (10) has an absorption region (Al, A2) which is smaller than a frequency bandwidth of the detection signal (DI, D2) and in which the reflectivity of the reflector (10) is reduced with respect to neighbouring regions, wherein the absorption region is within the frequency bandwidth, and the control device (2) is adapted to detect the object at least partially based on identifying an influence of the absorption region on the frequency spectrum of the response signal.

Description

Object Detection SystemTechnical field

[0001] The invention relates to an object detection system using a radio-frequency detection signal.Background of the Invention

[0002] It is known to employ radar for monitoring certain areas like vehicle interiors or rooms in buildings. This includes detecting the presence and possibly also the position of a person. In some cases, a single transmitter and a single receiver can be used. When the scene to be observed is complex, higher resolution sensors are used, e.g., MIMO radar systems with several transmitting and receiving antennas. One problem for radar sensing applications is that it is often difficult to reach sufficient coverage of an area or space that is to be monitored (e.g., a room or a car interior). Depending on the setup, a person can at least temporarily be out of the field of view of the sensor. One reason may be that specific areas are weaker illuminated by the detection radiation than others. Additionally, it is possible that a person is at least partially blocked by a static or moving object and the corresponding responses (reflections and / or doppler signatures) are too weak. The sensor will thus fail to detect a person in some regions of the scene. One way to avoid this problem is to work with multiple sensors which are distributed in the room and operate simultaneously, thereby hopefully eliminating any “blind spots”. This is however costly and requires sensor fusion in a central processing unit.

[0003] AUBRY ET AL. , Reconfigurable Intelligent Surfaces for N-LOS Radar Surveillance, IEEE Transactions on Vehicular Technology, vol 70, no 10, oct 2021 , discloses a concept of using reconfigurable (i.e. switchable or programmable) reflective surfaces for sensing. The reflective surface is electronically controlled and requires various active components.

[0004] RANA SADAF ANWAR ET AL. , Frequency Selective Surfaces: A Review, MDPI applied sciences, vol. 8, issue 9, 2018, discloses reflective surfaces, referred to as frequency selective surfaces (FSS).

[0005] FATHNAN ET AL. , Bandwidth limit and synthesis approach for single resonance ultrathin metasurfaces, J. Phys. D, Appl. Phys. 53, 495304, 2020,discloses the design of a reflective surface (called reflective metasurface) with a plurality of unit cells, and specific synthesis techniques for engineering the dispersion of each unit cell.

[0006] US 2016 / 0359235 A1 discloses a holographic radar reflector, comprising a surface including a plurality of substantially first wavelength scale patterns along one or more portions of the surface, wherein the holographic radar reflector is non- specular, wherein the first wavelength scale patterns have varying dependent on amplitude or phase, and wherein the holographic radar reflector and the first wavelength scale patterns are configured to reflect electromagnetic radiation emitted from a fixed feed point in varying directions depending on the portion of the surface reflecting the electromagnetic radiation.

[0007] US 2022 / 0393362 A1 discloses a detection device for a system comprising a closed space with a metallic environment with at least one object within the closed space, the object being provided with a wave-transmitting / receiving identification element, wherein the detection device comprises an identification reader adapted to exchange messages with the identification element via at least one wave- transmitting / receiving antenna, which is adapted to be installed in the closed space in order to detect the presence of the identification element. At least one metasurface is adapted to be installed in the closed space and configured to reflect, according to at least one first-chosen law, waves that originate from said antenna and which are intended to be received by the identification element and to reflect, according to at least one second-chosen law, waves that originate from the identification element and which are intended to be received by said antenna.Object of the invention

[0008] It is thus an object of the present invention to provide efficient means for reliable radar-based object detection.

[0009] This problem is solved by an object detection system according to claim 1 .General Description of the Invention

[0010] The invention provides an object detection system. More specifically, this is a radar-based object detection system. “Detection” can refer to determining whether an object is present. It can also refer to determining a position of the object and / ora speed of the object. Here and in the following, the term “object” also includes living beings such as animals or humans. In case of a living being, the detection may also refer to vital sign monitoring, i.e. , determining whether vital signs can be detected and / or determining the current value of a certain vital sign, e.g. heart rate or respiratory rate.

[0011] The system comprises an antenna arrangement, comprising at least one antenna. Depending on the embodiment, the antenna arrangement may comprise a plurality of antennas, but using a single antenna is within the scope of the invention. Each antenna may also be referred to as a radar antenna and is configured to emit and / or receive an electromagnetic signal having a radar frequency or radio frequency. In the context of this invention a “radio frequency” may in particular be between 9 GHz and 170 GHz. In particular, each antenna may be a simple, non-directional antenna that is configured to emit the signal in all directions and / or to receive the signal from all directions. For instance, each antenna may be a patch antenna on a printed circuit board.

[0012] Furthermore, the system comprises a control device adapted to control the antenna arrangement to emit a radio-frequency detection signal and to detect an object based on a radio-frequency response signal received by the antenna arrangement. The detection signal has a radio frequency, which includes the possibility that it comprises a plurality of radio frequencies. In particular, this may be at least one frequency of the 57 - 64 GHz band, the 79 - 85 GHz band or an UWB band. It is understood that the control device is suitably connected to the antenna arrangement in order generate the detection signal and that it comprises suitable components, like an HF generator, necessary to generate the detection signal. Also, the control device is adapted to detect an object based on a response signal received by the antenna arrangement, which implies that it is electrically connected in a suitable manner. Preferably, the response signal results from at least one reflection of the detection signal. Optionally, the control device may be adapted to process, analyse and / or evaluate the response signal. While reference is made here to “a” detection signal and “a” response signal, this is to be understood as referring to at least one signal, i.e. if the antenna arrangement comprises a plurality of antennas, different detection signals may be emitted from different antennas and generally different response signals can be received by different antennas. Thecontrol device comprises hardware components but may partially be software- implemented. As mentioned above, “detecting the object” may not only refer to detecting the presence of the object, but also the position, the speed and / or other properties of the object. Specifically, the control device is adapted to detect an object within a detection space. The detection space can be an interior space of a building or a vehicle but could also be an external space.

[0013] The system also comprises at least one passive reflector adapted to at least partially reflect the detection signal, the reflector comprising a metamaterial with a plurality of unit cells, the metamaterial comprising at least one dielectric substrate layer and a conductive structured layer disposed on an upper side thereof, whereby the reflector has a frequency-dependent reflectivity. The reflector is adapted to at least partially reflect the detection signal, which implies that it has a significant reflectivity in the frequency range of the detection signal. On the one hand, “partially” refers to that a portion of the detection signal may not be directed towards the reflector and therefore cannot be reflected. Furthermore, it is possible that a part of the radio-frequency radiation is not reflected but absorbed and / or transmitted. Depending on the embodiment, the reflector may also reflect radiation that does not originate directly from the antenna arrangement, but that has already been reflected, e.g., by the reflector itself, by another reflector, or by any other object, like an object to be detected. The reflector is passive, which means that it needs no energy supply and is not switchable or controllable. Therefore, the optical properties of the reflector, and specifically its reflectivity, are fixed. The reflector comprises a metamaterial with a plurality of unit cells. The term “metamaterial” implies that the unit cells, which represent the structure of the material, are smaller than a certain wavelength. In this case, the relevant wavelength is given by the detection signal. So strictly speaking, the metamaterial is a metamaterial for at least a part of the detection signal, preferably for the entire detection signal. It should be understood that the unit cells may possibly not be clearly separated from each other by any physical features. Rather, the metamaterial can conceptually be divided into unit cells. Although other configurations are conceivable, it is preferred that the reflector has a surface along which the unit cells are disposed in a Cartesian or rectangular pattern. As will be explained in more detail below, although all unit cells preferably have the same size, their internal structure may be the same or different. Also,although the rectangular pattern makes it possible to define two orthogonal directions along which the unit cells are arranged, the surface of the reflector does not have to be planar but could be curved and / or angled. In this case, two directions can be defined which are orthogonal locally, but each direction can be oriented differently in different locations of the reflector.

[0014] The metamaterial comprises at least one dielectric substrate layer and a conductive structured layer disposed on an upper side thereof. One could say that the substrate layer and the structured layer are disposed above each other and extend laterally, wherein the terms “above” and “laterally” are not to be construed in any limiting way as to the orientation of the metamaterial with respect to the direction of gravity. Rather, these terms simply referred to the arrangement of the layers within the metamaterial. In general, “lateral” refers to the direction in which the layers extend. As indicated above, the metamaterial does not have to be planar but can be curved and / or angled, wherefore “lateral” may refer to different absolute directions depending on the location within the metamaterial. The conductive layer is structured, which means that it is not a single, uninterrupted object, but is perforated and / or comprises a plurality of separate elements, which are arranged as one layer. More specifically, one could say that the conductive layer is laterally structured. The structures are preferably smaller than the smallest wavelength of the detection signal.

[0015] It is highly preferred that the structured layer is at least partially or predominantly metallic. I.e., preferably the structured layer comprises at least one metal, e.g. copper or silver. In this case, it is also referred to as a metallic structured layer. Optionally, the conductive layer can also comprise half-metal or non-metal components, e.g. as part of an alloy or as part of a metallic ink comprising metal particles and a binder. It can be fabricated using methods like screen printing, inkjet printing, reverse offset printing, etching of aluminum laminates or the like. In general, the structured layer is made of at least one material with a conductivity of at least 102S / m, and preferably comprises a metal having a conductivity of at least 106S / m. Preferably, a thickness of the structured layer is between 5 pm and 50 pm, more preferably between 10 pm and 25 pm. The dielectric substrate layer, on the other hand, is made of at least one material that can be considered as a poor conductor or an insulator. The conductivity of such a material is below 10 S / m, below 0, 1 S / mor below 10’3S / m. The substrate layer could e.g., be made of a dielectric material commonly used in flexible printed electronics, like polyethylene terephthalate (PET), polyimide (PI), polyetherimide (PEI), polyamide (PA) and similar. These materials are suited because they have very low losses for electromagnetic radiation. Optionally, the substrate layer may be composed of different materials forming sublayers. In contrast to the structured layer, the substrate layer is normally a solid, nonperforated body without lateral structure. The thickness of any substrate layer is normally considerably greater than the thickness of the structured layer. For instance, the thickness of the substrate layer may be between 0,05 mm and 2 mm or between 0,1 mm and 1 mm. A (total) thickness of the metamaterial can preferably be between 50 pm and 2 mm. It will be understood that this thickness is negligible in comparison with the dimensions of any location or object in which the reflector may be installed.

[0016] Due to the properties of the metamaterial, the reflector has a frequencydependent reflectivity. In general, “reflectivity” designates the quantity of reflected radiation for a given incident radiation. More specifically, this can be the square of the Fresnel reflection coefficient, which is the ratio of the amplitudes of the reflected field and the incident field. For a normal reflector, e.g., an unstructured metallic surface, this reflectivity has no or negligible frequency dependence. I.e., the reflectivity may depend on the incidence angle, but not on the frequency or wavelength of the incident wave. This is different for the reflector(s) of the inventive system. Here, the reflectivity significantly depends on the frequency. Preferably, the metamaterial is designed so that a minimum reflectivity within the frequency range of the detection signal is less than 60%, less than 40% or less than 20% of a maximum reflectivity in that frequency range. It will be understood that the reflection coefficient of the reflector is also frequency dependent. Due to the frequencydependent reflectivity, the frequency spectrum of a reflected signal is different from the frequency spectrum of the incident signal, e.g., the detection signal. The reflector-specific changes in the frequency spectrum can also be described as a “spectral response” of this reflector. Due to the frequency-dependent reflectivity, the reflector or the metamaterial can also be referred to as a dispersion-controlled metasurface. The metamaterial can be designed (or “tailored”) to have a desired specific frequency-dependent reflectivity.

[0017] The design of such or similar metamaterials has been described before, and extensive research has been conducted on the influence of various design parameters of the unit cell on the reflection behavior of the reflector. For instance, information on this topic can be found in the abovementioned publications by FATHNAN ET AL. and ANWAR ET AL.

[0018] The control device is adapted to detect the object at least partially based on a frequency spectrum of the response signal. Specifically, the frequency spectrum can be the frequency-dependent intensity or the frequency-dependent amplitude of the response signal. Any reflected signal that is not reflected by a reflector (but e.g., by an object to be detected) should have a frequency spectrum that is more or less identical to the frequency spectrum of the detection signal. If the reflected signal originates from a reflector, however, the frequency spectrum is changed significantly. Accordingly, if such a reflected signal reaches the antenna arrangement as part of the response signal, its contribution to the response signal is distinguishable from other signals. This is due to the characteristic spectral response of the reflector. Since the reflectivity is frequency dependent, some parts of the reflected signal are reduced in comparison to other parts. The reflected signal may directly or indirectly contribute to the response signal. In other words, the reflected signal may be directly received by the antenna arrangement, or it may be received indirectly, e.g., after it has undergone at least one additional reflection. If a reflected signal originates from a reflector, the contribution of the reflected signal is distinguishable from other signals. In other words, it is distinguishable from any portion of the response signal that does not originate from this reflector. This distinction is based on the differences in the frequency spectrum. One could also say that if a reflected signal originates from a reflector, the contribution of the reflected signal to the response signal is identifiable within the response signal. This includes the possibility that the contribution is zero, namely if the response signal does not contain any portion that results from a reflection by the respective reflector.

[0019] It can also be possible to distinguish specific propagation paths inside of a room or vehicle from others. With corresponding signal processing techniques, it can be retrieved whether the reflector with its spectral response was part of the propagation path or not. In general, the presence or absence of the characteristic signal can be used to deduce information about the presence of an object like aperson, and preferably about the position of the object. Any information about the position can be more or less precise. In some cases, this may only include the information that the object is in a certain part or region of a room or a vehicle. In other cases, this may refer to two-dimensional or even three-dimensional coordinates of the object.

[0020] According to the invention, at least one reflector has an absorption region which is smaller than a frequency bandwidth of the detection signal and in which the reflectivity of the reflector is reduced with respect to neighboring regions, wherein the absorption region is within the frequency bandwidth and the control device is adapted to detect the object at least partially based on identifying an influence of the absorption region on the frequency spectrum of the response signal. The absorption region is a frequency region or frequency interval in which the reflectivity is low compared to lower frequencies as well as higher frequencies. Although the term “absorption region” is used to indicate that the reduced reflectivity can be due to absorption, it is also possible that it is due to transmission, i.e. , radiation passing through the metamaterial. Without being limited to this explanation, the center of the absorption region may represent a resonance frequency of the structured layer. It is possible to design the metamaterial specifically for an absorption region in a certain frequency range. This absorption region then represents a “signature” or “fingerprint” of the reflector which is easily identifiable within the response signal. In case of several reflectors, each reflector can have a different absorption region. The absorption region is within the operating bandwidth. In other words, the detection signal has a frequency band or frequency range with an operating bandwidth, and the absorption region is within the frequency band or frequency range. Preferably, a minimum reflectivity within the absorption region is less than 60%, less than 40% or less than 20% of a maximum reflectivity within the neighboring regions.

[0021] The control device is adapted to detect the object at least partially based on identifying an influence of the absorption region on the frequency spectrum of the response signal. As explained above, a reflected signal originating from the reflector has a characteristic contribution to the response signal. The absorption region influences the reflected signal and therefore it influences the response signal. It is therefore possible to identify the influence of the absorption region within the frequency spectrum of the response signal. By identifying the influence of theabsorption region, the control device can determine whether at least a portion of the response signal originates from the reflector. As mentioned above, the influence may be zero if the response signal contains no contribution from the respective reflected signal. Depending on the embodiment, the absence, presence and / or strength of such influence may indicate the presence and / or the position of the object.

[0022] It will be understood that determining the position of the object requires information about the position of the reflector in relation to the antenna arrangement and preferably also information about the geometric configuration of a detection space in which an object is to be detected, e.g. a room in a building or a vehicle interior. Such information may be explicitly or implicitly provided to the control device. The reflector can also be regarded as reference marker in the scene. Preferably, the antenna arrangement and the at least one reflector are mounted stationarily in relation to each other. In some embodiments, the reflector could be movable to a limited extent that does not impact the result of the object detection. In other embodiments, the reflector could be movable with respect to the antenna arrangement and the control device could receive information about the current position of the reflector. Information about the position of each reflector and / or the geometric configuration of the detection space, may be stored in the control device, or in a memory accessible by the control device. The control device can be adapted to detect the object at least partially based on a position of the at least one reflector with respect to the antenna arrangement and / or at least partially based on a geometric configuration of the detection space.

[0023] The inventive object detection system is suitable for a variety of applications. While the system may be used in a closed environment like a room or a vehicle interior, it can also be used in an open environment. Possible applications include, but are not limited to, automotive in-cabin sensing, vital signs monitoring, d-call, smart home, indoor monitoring in elderly care facilities, monitoring of activity in hospital rooms, people counting, occupancy detection and unattended child detection in cars and school buses, monitoring of office environment, classification of moving objects in indoor or vehicle environment. The invention may also be used for surveillance in urban areas, e.g., near bus stops, train stations or other public transport infrastructure, burglar alarm, e.g. burglar alarm for exterior (outdoor)applications, e.g., in private gardens, public buildings, industrial zones and similar. Due to its small thickness, the reflector can be easily integrated into a wall structure, e.g. into a wallpaper or lining of a vehicle interior. Integration is also facilitated by the passive nature of the reflector, which eliminates the need for energy or data connections. Therefore, even if the reflector covers a large area, it can be realized at low cost and is significantly easier to integrate than additional radar sensors.

[0024] Optionally, the metamaterial may comprise at least one additional layer. For example, a metallic base layer may be disposed on a lower side of the substrate layer, i.e. opposite the structured layer. Like the substrate layer, the base layer is preferably solid and uninterrupted. Without being limited to this effect, the function of the base layer may be to eliminate transmission through the metamaterial. The base layer may be made of the same material as the structured layer or a different material. Its thickness may be between 5pm and 100pm. Another option is that a dielectric protection layer can be disposed on the upper side of the structured layer. The protection layer is normally solid an uninterrupted and covers the entire structured layer. In those locations where the structured layer is not present, the protection layer can be in direct contact with the abovementioned substrate layer. It may be made of the same material as the substrate layer or a different material. Its thickness may be e.g. between 20pm and 100 pm.

[0025] The detection signal can be a continuous-wave signal. In this case, the detection signal can be characterized by a single frequency and corresponds to a sinusoidal oscillation. In order to identify whether the response signal is the result of a metamaterial reflection, a sequence of continuous-wave signals with different frequencies have to be used. In this embodiment, it may be impossible to deduce any range information from an analysis of the response signal, although this may be possible depending on the number and arrangement of antennas. Even without range information, an object can be detected by detecting motion via analysis of the Doppler shift of the response signal. According to another embodiment, the detection signal is a frequency-modulated signal, in particular a frequency- modulated continuous wave (FMCW) signal. Normally, the frequency of the signal is changed periodically, e.g. by applying a linear chirp (linear frequency change). However, a non-linear frequency change may be applied as well as a non-periodical frequency change. In this embodiment, range information can be deduced from acomparison of the detection signal and the response signal. At the same time, motion can be detected by analyzing the range as a function of time. According to another embodiment, the detection signal can be a pulse signal. Like with a frequency-modulated signal, range information can be deduced from analysis of the response signal, basically by analyzing the delay between the emission of a pulse and the detection of he reflected pulse. Various types of pulses may be used, e.g. rectangular, sinusoidal, Gaussian or the like. As mentioned above, if the antenna arrangement comprises a plurality of antennas, a detection signal may be applied to several antennas, in which case a different detection signal may be applied to each antenna. For example, the detection signals are applied to different antennas could differ by a time offset. It is understood that motion detection also allows for life sign monitoring. In some embodiments, the control device can be adapted to determine a position of the object by combining range information retrieved from the response signal with identifying the influence of the absorption region on the frequency spectrum of the response signal.

[0026] E.g. if a pulse signal is used, the antenna arrangement could comprise a single antenna that emits the detection signal and receives the response signal. It is mostly preferred, though, that the antenna arrangement comprises at least one transmit antenna and at least one receive antenna and the control device is adapted to control each transmit antenna to emit the detection signal and to detect the object based on a response signal received by each receive antenna. Normally, all antennas are offset from each other or spaced from each other in order to reduce the risk of electromagnetic interference. As mentioned above, the control device may generate different detection signals using different transmit antennas (if present). It is understood that generally different response signals are received by different receive antennas (if present).

[0027] According to one embodiment, the detection system comprises a building in which the antenna arrangement and the at least one reflector are installed. Specifically, they can be installed in the same room. The antenna arrangement could be mounted on a wall, a pillar, or a ceiling, possibly in a recess and hidden from view by a radiation-permeable cover. The reflector(s) could also be mounted on a wall, a pillar, a ceiling, a floor, or a window. Due to its flat shape, a reflector can be integrated into a surface, e.g. integrated or hidden behind a wallpaper. Thecontrol device can also be installed in the same building, possibly in the same room, so that the entire object detection system is installed in the building. In this case, one could also say that the building comprises the object detection system. Alternatively, the control device could also be disposed in a different location and be connected to the antenna arrangement by a wireless or wired connection. The system could be used specifically for detecting persons in the building or room, which could include localizing people and / or monitoring vital signs. According to another embodiment, the detection system comprises a vehicle in which the antenna arrangement and the at least one reflector are installed. The antenna arrangement could be mounted on or in a dashboard, a sidewall, a vehicle seat, or a ceiling. Again, the antenna arrangement could be hidden from view by a radiation- permeable cover. The reflector(s) could be mounted, e.g., on a sidewall, a vehicle seat, a ceiling, a floor, or a window. Each reflector can be easily integrated into a surface, e.g. integrated into or hidden behind a trim panel. It can also be integrated into a seat cushion. The control device will also be installed in the same vehicle, i.e. the entire object detection system is installed in the vehicle. In this case, one could also say that the vehicle comprises the object detection system. The system could be used specifically for occupancy detection, but also for vital sign monitoring, e.g. in order to detect possible health problems of the driver.

[0028] According to one embodiment, at least one of said at least one reflector is disposed to provide a reflection path between the antenna arrangement and an obscured region that is hidden from the antenna arrangement by an interposed object, so that an object in the obscured region is detectable by the control device. Here and in the following the expression “at least one of said at least one reflector” refers to “the reflector” in an embodiment with only one reflector and to “at least one of the reflectors” in case of a plurality of reflectors. The obscured region cannot be accessed directly by the antenna arrangement. One could say that it is not within the line of sight of the antenna arrangement. Strictly speaking, the region is obscured if it is not in the line of sight of any transmitting antenna and / or any receiving antenna. In such a situation, no signal can be transmitted directly into the obscured region and / or no reflected signal from the obscured region can be received directly. Therefore, no object in the obscured region can be detected without the reflector. The obscured region may be caused by a stationary object like a wall, apillar, a piece of furniture, a vehicle seat, or the like. The position, orientation and configuration of the reflector are adapted so that a reflection path is provided between the antenna arrangement and the obscured region. In other words, radiation from the antenna arrangement can be reflected into the obscured region and / or radiation from the obscured region can be reflected to the antenna arrangement. The control device can therefore detect an object in the obscured region. On the one hand, the position of the object can be partially deduced by “conventional” techniques like evaluating the time-of-flight of the signal or a phase difference. Moreover, due to the frequency-dependent reflectivity of the reflector, an object in the obscured region can be identified by the frequency spectrum of the response signal. This can e.g. be used to resolve ambiguities between the obscured region and a non-obscured region that corresponds to a similar time of flight for the signal. The control device can preferably be adapted to determine whether the object is in the obscured region or in a non-obscured region based on the influence of the absorption region on the frequency spectrum of the response signal.

[0029] In another embodiment that the control device is adapted to detect an object interposed between the antenna arrangement and at least one of said at least one reflector based on the influence of the interposed object on the frequency spectrum of the response signal which results from an effect of the interposed object on the influence of said absorption region on the frequency spectrum. An object being interposed between the antenna arrangement and the reflector may also be referred to as a (partial or total) concealment of the reflector. In the absence of an object, the reflector is in the line of sight of the antenna arrangement. Thus, the respective metamaterial reflection leads to a comparatively strong contribution to the response signal, wherefore the influence of the absorption region is also strong. If, on the other hand, an object is interposed between the reflector and the antenna arrangement, less radiation reaches the reflector and less radiation from the reflector reaches the antenna arrangement, wherefore the influence of the absorption region is weak or non-existent. Therefore, there will be a difference in the frequency spectrum of the response signal as a whole. This embodiment can be developed further in that a plurality of reflectors with different reflection characteristics, and different absorption regions, are employed. Not only can concealments of different reflectors be identified, but it may also be possible toidentify a simultaneous concealment of two (or more) reflectors. From this information, the position of the object can be deduced with higher accuracy.

[0030] A reflector may comprise a metamaterial that is “homogeneous” in that all unit cells of the metamaterial are identical. However, it is also possible to include regions with different properties in a single reflector. According to such an embodiment, at least one of said at least one reflector comprises a plurality of sections, at least two sections being structured differently so that they have different frequency-dependent reflectivities. Each section comprises a plurality of unit cells and may extend laterally in two directions. Although each section may comprise identical unit cells, at least two sections are structured differently, i.e. , they comprise different unit cells. The different structure leads to a different frequency-dependency of the reflectivity. For example, two sections may have different absorption regions. Specifically, the reflector may comprise several (e.g., two or three) types of sections, wherein a plurality of sections belongs to each type. Sections of different types can be disposed alternatingly, e.g., in case of two types a checkerboard pattern can be used. If radiation like the detection signal irradiates several types of sections at the same time, the reflected radiation will have a frequency spectrum that is influenced by the characteristics of all involved types. For example, the spectrum may show a plurality of absorption regions.

[0031] Preferably, at least one of said at least one structured layer extends laterally along a first direction and a second direction perpendicular to the first direction and comprises a plurality of conductive resonators, each unit cell comprising one resonator, and each two neighboring resonators of the structured layer being spaced-apart along at least one of the first direction and the second direction. The term “laterally” has already been explained above. In case of a planar reflector, the first and second direction represent straight axes. In case of a non-planar shape, e.g. a curved shape, the two directions can be defined locally, where they are orthogonal. However, at least one of these directions may have different orientations with respect to an absolute Cartesian coordinate system, depending on the location within the reflector. The conductive resonators are structures within the structured layer. Therefore, they are also preferably metallic, i.e., they comprise at least one metal and can predominantly consist of metal. Since the resonators are spaced from each other, they represent spatially and electrically isolated structures. Each unitcell comprises one resonator, preferably exactly one resonator. It is possible, though, that a resonator is not coherent but comprises a plurality of separate portions, which could alternatively also be regarded as individual resonators. The term “resonator” is not to be construed in any limiting way. However, each resonator can have at least one resonance frequency. If all resonators have an identical shape and size, this resonance frequency can be the center of an absorption region. By adapting the dimensions of the resonators, the reflection, absorption and / or transmission properties of the metamaterial can be adapted for various requirements. Also, the layout of the individual resonator influences the phase of the radiation in the reflection process. I.e., the geometry and dimensions of the reflector influence a phase difference between an incident wave and a reflected wave. This can also be employed to influence the reflection behavior of the reflector. In some embodiments, a lateral dimension of each unit cell along each of the first direction and the second direction is between 250 pm and 10 mm or between 500 pm and 5 mm. The dimensions along the two directions can be the same or different. A maximum lateral dimension of each resonator is preferably less than 2 mm and a minimum lateral dimension of each resonator is preferably at least 50 pm. However, it will be understood that dimensional restrictions also depend on the frequency / frequencies of the detection signal, given that for a metamaterial, each dimension of the unit cell has to be smaller than the wavelength. The shape of the resonators is not limited within the scope of the invention. Possible examples include linear, hexagonal, rectangular, triangular, T-shaped, H-shaped, cross-shaped, C- shaped, crescent-shaped, circular, star-shaped, or other. While the lateral dimensions of a unit cell are small, the lateral dimensions of the reflector can be considerably larger. For instance, a lateral dimension of a reflector may be at least 1 cm, at least 10 cm or at least 50 cm, while an area or cross-section of the reflector may be at least 1 cm2, at least 100 cm2, or at least 2.500 cm2. In some embodiments, a lateral dimension may be even more than 1 m and / or an area of the reflector may be more than 1 m2

[0032] In some embodiments, each resonator is made of a single material or material mixture, e.g. conductive ink. In this case, the conductivity (i.e. the specific conductance) of all parts of the resonator is the same. According to another embodiment, at least one of said at least one resonator comprises a plurality ofportions with different conductivity. For instance, one portion could be metallic, thus having a high conductivity, while another portion could comprise graphite, thus having a considerably lower conductivity. Including such a low-conductivity region can significantly increase the absorption around the resonance frequency, but it may also influence the resonance frequency itself. By increasing the absorption, the abovementioned absorption region can be better distinguished from the neighboring frequency regions.

[0033] As already mentioned, the reflector can be easily integrated into a variety of objects. In some embodiments, it can be virtually invisible to the naked eye. According to such an embodiment, the substrate layer is transparent for visible light and the structured layer comprises a transparent structure that comprises a plurality of line elements having a maximum width of 1 ,5 pm. The substrate layer is transparent for visible light, which may specifically refer to light having a wavelength between 380 nm and 780 nm. It will be understood that suitable transparent dielectric materials exist, e.g. polymeric materials. However, since the structured layer has to be conductive and is preferably metallic, it is made of materials that are not inherently transparent. In this embodiment, though, the structured layer is split into fine line elements which are so narrow that they are invisible or almost invisible to the naked eye. Preferably, each resonator is a transparent structure. While the overall dimensions of such a transparent resonator may be identical to a nontransparent resonator, it is composed of the abovementioned line elements. Neighboring line elements can be separated by a spacing that has a similar width, e.g. at maximum 1 ,5 pm. For instance, if the overall shape of the resonator is a square with an edge length of 100 pm, this can be achieved by 33 line elements disposed next to each other, or by a single meandering line element. The individual transparent structure, which may preferably be a transparent resonator, can have a minimum dimension of at least 50 pm or at least 100 pm. A transparent reflector as described here can be integrated into a window, e.g., a window of a room or a vehicle.

[0034] According to a preferred embodiment, at least one of said at least one reflector is a flexible sheet. In other words, the respective reflector can be bent without breaking or tearing. On the one hand, flexibility reduces the risk of damaging the reflector during installation. On the other hand, this makes it possible to integratethe reflector into various surfaces having various shapes. E.g. it can be integrated into a planar surface like a wall in a building, as well as into a curved surface like the surface of a pillar. It will be understood that the thin structure of the metamaterial facilitates flexible deformation. Also, the substrate layer can be made e.g. of a polymeric material that allows for deformation without being damaged. Moreover, the reflector may comprise at least one cutout that facilitates bending and / or folding it into a desired shape.

[0035] Metamaterials allow for reflection behavior that cannot be realized with other materials. As already mentioned, the layout of the individual resonator influences the phase of the radiation in the reflection process. By adapting the phase responses of a plurality of resonators, the direction of the reflected radiation can be influenced and - at least to some degree - tailor-made. One embodiment provides that for at least one of said at least one reflector, at least one dimension of one resonator varies for different unit cells along one of the first and second direction, whereby the reflector is adapted for anomalous reflection in which an incidence angle and a reflection angle differ in absolute value. By varying the dimension(s) of the resonator from unit cell to unit cell, the phase of the reflected radiation is influenced. Such structures are sometimes referred to as gradient metasurfaces. This is, for example, described in the abovementioned article by FATHNAN ET AL. Therefore, so-called anomalous reflection can be achieved, which is a reflection in which the absolute value of incidence angle and reflection angle are not the same as in normal reflection processes. In contrast, the absolute values are different, although the reflection angle still depends on the incidence angle. In some embodiments, the reflection angle may be frequency-dependent, while in other embodiments, it is independent of the frequency. According to one embodiment, the at least one dimension of the resonator varies linearly along at least one of the first direction and the second direction. E.g., the respective dimension is reduced by approximately 5 pm from one unit cell to the next. Optionally, this linear variation can be done periodically. Such linear variation results in a single reflection angle corresponding to a single incidence angle. One possible application for an anomalous reflection could be a situation where a reflection is desired that would require an ordinary reflector to be oriented in a way so that it cannot be integrated into a surface like a wall, a vehicle ceiling etc. With an anomalous reflection, acertain reflection angle can be generated that is - at least to some extent - independent of the orientation of the reflector.

[0036] There are a variety of possible variations of the resonator from one unit cell to the next, all of which may lead to beneficial reflection behavior. According to one embodiment, at least one dimension varies for different cells in a non-linear way along one of the first and second direction, whereby the reflector is adapted for a reflection in which a single incidence angle corresponds to a plurality of reflection angles. In other words, parallel beams of the incident radiation will not lead to parallel reflected beams. This may be used to focus or spread out the reflected radiation, i.e., to generate convergent or divergent radiation. Specifically, a parallel set of beams may be reflected and thus spread out over a certain angular range. This could be used to effectively irradiate a large region even if the detection signal hits the reflector as a narrow beam and / or if the reflector has a small surface area. The respective reflector could be referred to as a planar multidirectional reflector or even a planar omnidirectional reflector.

[0037] In order to realize the invention, it is sufficient if the reflector comprises exactly one substrate layer and one structured layer. However, it is possible to expand the structure to include more layers. According to such an embodiment, at least one of said at least one reflector comprises a plurality of substrate layers with one structured layer disposed on the upper side of each substrate layer. Thus, every structured layer, except for the uppermost one, is interposed between two substrate layers, one below and one above the structured layer. In those areas where the structured layer is interrupted, e.g., between resonators, the substrate layers can be in direct contact with each other. The structured layers could have the same structure or different structures. For example, resonators of several structured layers could be disposed above each other and be regarded as parts of a single unit cell. In this case, a unit cell extends over several structured layers and several substrate layers. With each additional structured layer, or with each additional resonator, the reflection behavior can be influenced.

[0038] It should be appreciated that although the reflector can be hidden behind a radiation-permeable cover like a headliner, a trim panel, a seat cover, a wallpaper, or the like, the presence of the cover can impact the frequency response of the reflector. For instance, as compared to the “naked” reflector (i.e., without the cover),the cover may e.g. shift a resonance frequency and / or an abovementioned absorption region, reduce the variation of the amplitude or similar. Therefore, when designing the metamaterial, i.e. , the individual unit cell and the array as a whole, the presence of the cover material must be accounted for in the model. However, if the properties of the cover material are known, its possible influences can be compensated for by the design of the metamaterial. Similarly, the presence of a protection layer may be taken into account when designing the metamaterial.

[0039] The invention further relates to a passive reflector which can be used in an inventive object detection system. The reflector is adapted to at least partially reflect a radio-frequency signal and comprises a metamaterial with a plurality of unit cells, the metamaterial comprising at least one dielectric substrate layer and a conductive structured layer disposed on an upper side thereof, whereby the reflector has a frequency-dependent reflectivity. The reflector has an absorption region in which the reflectivity of the reflector is reduced with respect to neighbouring regions. Preferred embodiments of the reflector correspond to those of the inventive object detection system.

[0040] The invention also relates to an object detection method using an object detection system comprising: an antenna arrangement, comprising at least one antenna; a control device; and at least one passive reflector comprising a metamaterial with a plurality of unit cells, the metamaterial comprising at least one substrate layer and a conductive structured layer disposed on an upper side thereof, whereby the reflector has a frequency-dependent reflectivity.

[0041] According to the method, the control device controls the antenna arrangement to emit a radio-frequency detection signal, the reflector at least partially reflects the detection signal, and the control device detecting an object based on a radio-frequency response signal received by the antenna arrangement, wherein the control device detects the object based on a frequency spectrum of the response signal. According to the invention, at least one of said at least one reflector has an absorption region which is smaller than a frequency bandwidth of the detection signal and in which the reflectivity of the reflector is reduced with respect to neighbouring regions, wherein the absorption region is within the frequencybandwidth, and the control device detects the object at least partially based on identifying an influence of the absorption region on the frequency spectrum of the response signal. All these terms have already been explained with reference to the inventive object detection system and will not be explained again. Preferred embodiments of the inventive method correspond to those of the inventive system.

[0042] According to one preferred embodiment of the method, a learning process is performed for the control device to establish a correlation between the frequency spectrum and the detection of the object. The learning process can be performed by the control device, but it may also be at least partially based on an external device that is only used for the learning process. The learning process may in some cases also be a calibration process. In any case, it serves to establish a correlation between the frequency spectrum of the response signal and the detection of the object. The term “correlation” does not mean that a correlation function in a mathematical sense has to be determined, but more generally describes the influence of the presence or absence of the object on the frequency spectrum. Depending on the embodiment, it may also relate to the influence of the position of the object or other parameters. During the learning process, a plurality of response signals are received by the antenna arrangement, each for a different situation, e.g. with and without an object, with one or several objects, with various object positions etc. For such a learning process, a calibration object like a corner reflector can be placed at different well-defined positions in various positions while recording the response signal. To discriminate such a calibration target from static reflections, the calibration object can e.g., be moved in an oscillating manner at a well-defined speed during the procedure and to detect it using range-doppler data.Brief Description of the Drawings

[0043] Further details and advantages of the present invention will be apparent from the following detailed description of not limiting embodiments with reference to the attached drawing, wherein:Fig. 1A is a top view of a first unit cell of a reflector for an inventive object detection system;Fig. 1 B is a perspective view of the unit cell from fig.1 A;Fig. 2 is a top view of a second unit cell of a reflector for an inventive object detection system;Fig. 3 is a top view of a third unit cell of a reflector for an inventive object detection system;Fig. 4A is a perspective view of a fourth unit cell of a reflector for an inventive object detection system;Fig. 4B is an explosion view of the unit cell from fig.4A;Fig. 5 is a top view of a fifth unit cell of a reflector for an inventive object detection system;Fig. 6 is a first diagram showing frequency-dependent reflectivity;Fig. 7 is a side view of a vehicle with a first embodiment of an inventive object detection system;Fig. 8 is a second diagram showing frequency-dependent reflectivity;Fig. 9 is a side view of a vehicle with a second embodiment of an inventive object detection system;Fig. 10A is a top view of a room with a third embodiment of an inventive object detection system;Fig. 10B is another top view of the room from fig. 10A;Fig. 11 is decision tree for object detection by the object invention system from fig.WA;Fig. 12A is a side view of a vehicle with a fourth embodiment of an inventive object detection system;Fig. 12B is another side view of the vehicle from fig.12A;Fig. 13 is a side view of a room with a fifth embodiment of an inventive object detection system;Fig. 14 is a top view of a first embodiment of a reflector for an inventive object detection system;Fig. 15 is a third diagram showing frequency-dependent reflectivity;Fig. 16 is a top view of a second embodiment of a reflector for an inventive object detection system;Fig. 17 is a fourth diagram showing frequency-dependent reflectivity;Fig. 18 is a top view of a third embodiment of a reflector for an inventive object detection system;Fig. 19 is a first diagram showing a location-dependent phase response;Fig. 20 is a top view of a fourth embodiment of a reflector for an inventive object detection system;Fig. 21 is a second diagram showing a location-dependent phase response;Fig. 22 is a side view of the reflector from fig.19;Fig. 23 is a top view of a fifth embodiment of a reflector for an inventive object detection system;Fig. 24 is a perspective view of a pillar with the reflector from fig.14; andFig. 25 is a top view of a sixth unit cell of a reflector for an inventive object detection system.Description of Preferred Embodiments

[0044] In Figs. 1A and 1 B show a first unit cell 11 of a reflector 10 for an inventive object detection system 1. The entire reflector is shown in fig.14 and comprises a metamaterial 20 with a periodic array of unit cells 11 arranged in a Cartesian pattern along a first direction x and a second direction y. The reflector 10, and the unit cell 11 , comprise a substrate layer 12 made of dielectric material. With respect to a vertical third direction z, a structured layer 15 is disposed on top of the substrate layer 12. The structured layer 15 comprises a plurality of metallic resonators 16, one in each unit cell 11 . This first unit cell 11 comprises an H- shaped metallic resonator 16 and is square-shaped. As can be seen e.g., in fig.7, the object detection system 1 comprises an antenna arrangement 3 that is adapted to emit a radio-frequency detection signal D1-D4. The size of the resonator 16 is of a similar size or smaller than the smallest wavelength of the detection signal D1-D4. In the reflector 10 of fig.14, all unit cells 11 comprise identical resonators. At a lower side of the substrate layer 12, a continuous metallic base layer is disposed. By way of example, thethicknesses of the substrate layer 12 can be between 0,05 mm and 1 mm, e.g., 0,15 mm. The thickness of structured layer 15 can be between 5 pm and 50 pm, e.g. 20 pm. The thickness of the base layer 19 may be between 5pm and 100pm, e.g. 50 pm. Due to its small thickness, the reflector 10 is a flexible sheet that can be bent in order to adapt to a variety of surface shapes. The shape and dimensions of the resonator 16 influence a reflectivity (or reflection coefficient) and a phase shift or phase response coinciding with a reflection process. Specifically, a frequencydependent reflectivity or a spectral response of the reflector 10 can be influenced.

[0045] In this case, by modifying the geometric dimensions of the ‘H’ shape, the reflection coefficient of the individual unit cell 11 and metamaterial 20 can be controlled. With simulation tools for electromagnetic wave propagation (full wave solvers) the dependency of geometry to reflection coefficient can be accurately analyzed. This allows controlling an amplitude and phase of a reflected signal F1- F4 by design. To some extent, also the spectral shape of the reflection coefficient can be controlled.

[0046] One important parameter to understand the nonlinear spectral response is the surface impedance which can be represented by the inductance and capacitance of the structure. Typically, the inductance can be increased by making the metallic lines that constitute the resonator 16 thinner. The capacitance can be increased by adding more parallel lines and decreasing the gap between parallel lines.

[0047] With the resonator shape shown in figs. 1A and 1 B, the resonator 16 mostly responds to microwave radiation polarized with electric field parallel to the y direction. Fig.2 shows a second unit cell 11 with a different resonator 16, which allows for reflection that is nearly independent of the polarization. This shape is sometimes referred to as Jerusalem cross in the literature.

[0048] Fig.3 shows an embodiment with a so-called “double split ring” resonator 16, i.e. , two concentric metallic rings which have openings on opposite sides. This could also be regarded as two coupled resonators 16 in a single unit cell 11. This embodiment represents a compact way of including two coupled resonators and provides more geometric parameters to tune the spectral response.

[0049] Figs.4A and 4B illustrate another way to include more than one resonator 16, 18. In this embodiment, the metamaterial 20 and the unit cell 11 include two substrate layers 12, 13 and two metallic structured layers 15, 17. Accordingly, two H-shaped resonators 16, 18 are disposed above each other with respect to the third direction z, with one substrate layer 12 disposed in between. This architecture is one possible way of keeping the geometric detail of each metallic resonator 16, 18 reasonably coarse and easy enough for cost effective fabrication while having enough degrees of freedom for shaping the spectral response.

[0050] In another embodiment not shown here, an additional very thin dielectric protection layer is disposed on the uppermost substrate layer 12 and the uppermost structured layer 15. The thickness of this protection layer can for instance on the order of 50 pm.

[0051] In another embodiment, which is shown in fig. 5, the resonator 16 comprises a plurality of portions 16.1 , 16.2 made of different conducting materials. This can also be used to further influence the spectral response. Here, the resonator comprises two metallic primary portions 16.1 which constitute a major part of the resonator 16. These may be made of a highly conducting material such as copper or a silver particle ink. A small secondary portion 16.2 of the resonator 16 has a lower conductivity and is made of non-metallic material, e.g., carbon black. By the size of the secondary portion 16.2 and the gap between the primary portions 16.1 respectively, the resistance of the overall vertical part of the “H”-structure is controlled, which essentially controls the quality factor. Fig. 6 shows the frequencydependent reflectivity using the unit cell 11 from figs. 1 A and 1 B as the dotted curve and using the unit cell 11 from fig.5 as the solid curve. Both spectra show an absorption region A1 , which can be smaller than a frequency bandwidth of the detection signal D1-D4. In this absorption region A1 , the reflectivity of the reflector 10 is reduced with respect to neighbouring regions. This effect is more pronounced for the solid curve.

[0052] Fig. 7 shows a first embodiment of an inventive object detection system 1 . In this case, the system 1 is installed in a vehicle 30 and is adapted for occupancy detection and detection of vital signs. An antenna arrangement 3 is installed on top of the dashboard while a control device 2 is installed next to it. The antenna arrangement 3 is radiating mainly in the direction of the driver and passenger onfront seats 31 of the vehicle 30. Therefore, one portion D2 of a detection signal can directly irradiate the front seats 31 . This configuration gives very reliable signal from people on the front seats 31 . However, the people on rear seats 32 are mostly in an obscured region 30.1 which is at least partially hidden behind the front seats 31. Therefore, a reflector 10 is installed in a ceiling of the vehicle 30. Thus, another portion D1 of the detection signal is reflected by the reflector and a reflected signal F1 irradiates the obscured region 30.1 .

[0053] In this example, a system 1 with a significant bandwidth is used, for instance operating in between a first frequency f1 =60 GHz and a third frequency f3=64 GHz. The frequency-dependent reflectivity of the reflector 10 is shown in fig. 8. The frequency range in which the antenna arrangement 3 operates is marked in grey. In this case, two sub-bands are used, one ranging from f1 to a second frequency f2=62 GHz, and the other ranging from f2 to f3. Due to a pronounced absorption region A1 , the reflectivity of the reflector 10 is very low in the first sub-band, while it is close to unity in the second sub-band. Objects that occur when evaluating the signal in both sub-bands consequently more likely correspond to locations near the front seats 31. These propagation paths are indicated in fig 7 with solid arrows. When the propagation path includes the reflector 10 as indicated by the dashed arrows, a response signal R1-R4 (not shown in fig.7) that is received by the antenna arrangement 3 will be significantly stronger in the second sub-band ranging from f1 to f3. This way, using the reflector 10 gives additional information that can be used in the signal processing routine for locating and classifying objects (i.e., persons). Generally speaking, the control device 2 can detect and / or locate an object based on the frequency spectrum of the response signal R1 -R4.

[0054] The reflector 10 is installed just behind the headliner material that covers the ceiling of the vehicle 30. This way, the reflector 10 remains invisible to the passengers. Headliners are typically made of foam and textile material with comparably low density and are practically transparent for microwave radiation. Here, the headliner material must be taken into account during the design of the reflector 10 since it can usually not be considered as completely transparent to the radar signal. If the properties of the headliner material are known, the metamaterial of the reflector 10 can be designed (e.g. by computer simulation) to compensate for any influences by the headliner.

[0055] Fig.9 shows an alternative embodiment, in which the antenna arrangement 3 is attached to the ceiling of the vehicle 30 above the rear seats 32. In this case, an obscured region 30.1 exists near the front seats 31 . The reflector 10 is attached to a curved surface of the dashboard. Thus, a portion D1 of the detection signal is reflected into the obscured region 30.1 , so that the response of passengers on the front seats 31 is enhanced.

[0056] Figs. 10A and 10B show another embodiment in which the system 1 is installed in an L-shaped room 40. This can for instance be a room in a hospital, retirement home or similar. The antenna arrangement 3 is mounted to a wall while the control device 2 is shown hidden inside the wall, but this is just exemplary. A reflector 10 is mounted to another wall. The room 40 comprises four different regions 40.1 , 40.2, 40.3, 40.4, one of which is an obscured region 40.4 that cannot be directly irradiated by the antenna arrangement 3. However, the reflector 10 is disposed to establish a propagation path between the antenna arrangement 3 and the obscured region 40.4. Again, due to the distinct spectral response of the reflector 10, a signal that was reflected by the reflector 10 (dashed arrows) can be distinguished from a signal that did not interact with it (solid arrows). A detected object can be located inside one of the regions 40.1 -40.4 using range information in combination with information from the frequency spectrum of the response signal R1 -R4. Fig.11 shows a decision tree that could be implemented in the control device 2. If the distance r of the object is smaller than a first radius r1 , the object is in a first region 40.1 . If the distance r is between the first radius r1 and a second radius r2, the object is in a second region 40.2. If the distance r is greater than the second radius r2, it is decided using the frequency spectrum whether the reflector 10 was included in the propagation path. If not, the object is in a third region 10.3. If so, the object is in the obscured region. This concept can be extended to more complex configurations with multiple reflectors 10 and / or multiple antenna arrangements 3. Other features can be added to the decision-making process, for instance an angular bin when using MIMO radars or radar system with channels that differ in polarization that is radiated (left-handed or right-handed circular polarization, and / or vertical and horizontal linear polarization).

[0057] When the configuration gets more complex, the decision tree can be replaced by other more advanced artificial intelligence (Al) methods. In such a case,the parameters possibly cannot be found by logical arguments. Rather, a learning process or calibration process can necessary. For such a process, a specific strongly reflecting object like a corner reflector can be placed at different well- defined positions in every region of the room while recording the response signal R1 -R4. To discriminate such a calibration target from static reflections, it can help to make the calibration target oscillating at a well-defined speed during the procedure and to detect it in the range-doppler data.

[0058] When one or several persons move from one region 40.1 -40.4 to the next, over a longer time period (such as one day), positions can be tracked and yield information for activity profiles. In conjunction to recording doppler signatures for the detection of breathing rates and the heartbeat, this can give meaningful input for vital signs monitoring. When mounted at the wall, the antenna arrangement 3 could be mounted at a height of around 100 - 170 cm in order to radiate evenly into the room 40 and radiate above small furniture like tables, chairs, etc.

[0059] Other situations in which enhancing the signal strength by using reflectors 10 can be useful include detecting objects in a trunk, on a floor of a vehicle (particularly in the case of an SUV with comparably heigh seats), covering the cabin of longer vehicles like 7-seat vans, busses, recreational vehicles, vehicles of public transport, airplanes, ferries or similar.

[0060] Figs. 12A and 12B show another embodiment of an inventive system 1 that is also installed in a vehicle 30. Like in the embodiment of fig. 9, the antenna arrangement 3 is installed in the ceiling above the rear seats 32. However, in this case, a reflector 10 is placed in the rear seats 32, just underneath the outer textile material. This way, the reflection coefficient and reflectivity of the rear seat are controlled by the design of the reflector 10. The spectral response of the reflector is designed so that it can be distinguished from other reflections in the vehicle 30. Thus, a reflected signal F1 that is reflected by the reflector 10 and constitutes a portion R1 of a response signal can be distinguished from other reflections inside the vehicle 30. If an object, in this case a person 80, occupies this seat 32, as shown in Figure 13B, the control device 2 will only record reflections caused by the person 80, not by the reflector 10. This way, occupancy of this particular seat 32 can be detected.

[0061] The concept can be extended to a configuration with more than one reflector 10 and with more than one seat 31 , 32, e.g., one in every rear seat 32. In this case, it can also be beneficial to operate the radar with more than two sub-bands. The different reflectors 10 can be different in their spectral response for better discrimination.

[0062] Fig. 13 shows an embodiment that can work well for indoor applications such as in an office environment. The detection system 1 is installed in a room 40 with rectangular outline. The antenna arrangement 3 is mounted to one of the long walls. On the opposite side of the room 40, a total of four reflectors 10, 50, 60, 70 mounted. These are adapted to reflect a significant amount of energy back to the antenna arrangement 3. Additionally, each reflector 10, 50, 60, 70 has an individual spectral response that is distinct from the others.

[0063] In the middle of the room, there is a table with six chairs 41-46. Since the propagation paths of the detection signals D1-D4 from the antenna arrangement 3 to the reflectors 10, 50, 60, 70, as well as those of the reflection signals F1 -F4 (or response signals R1 -R4, respectively) cross the table region, the signal strength is varied when a person 80 is in this area. For example, when a person 80 sits on the second chair 42, the response signal R2 from the second reflector 50 is influenced as well as the response signal R3 from the third reflector 60. Similar effects occur for the other chairs 41 , 43-46. Adding range information, even with very coarse resolution, this can be enough to decide which chairs 41 -46 are occupied and which are not.

[0064] Again, the concept can be extended to more than one antenna arrangement 3 and even more reflectors 10, 50, 60, 70. Similar decision trees as shown in fig.11 , or a trained Al-based algorithm can be used to locate and / or classify people. In consequence, location and classification can be given even with a minimalistic and cost-effective antenna arrangement 3.

[0065] Fig.14 is a top view of a first embodiment of a reflector 10. As already mentioned, it comprises a plurality of unit cells 11 arranged in a Cartesian pattern. The substrate layer 12 is thin and flexible and can be made of a dielectric material commonly used in flexible printed electronics, for instance polyethylene terephthalate (PET), polyimide (PI), polyetherimide (PEI), polyamide (PA) or a similar material. The resonators 16 of the individual unit cells 11 can be fabricatedusing methods common in flexible printed circuit technology, like screen printing, inkjet printing, reverse offset printing, etching of aluminum laminates or the like. Due to the simplistic architecture, roll to roll processes are possible (often preferred due to high throughput and low cost). In some cases, the required aperture size can be quite large, e.g., more than 1 m2. Therefore, the previously mentioned fabrication methods are beneficial compared to PCB or thin film techniques. For this embodiment, the absorption is very high at one distinct frequency, corresponding to a single absorption region A1 shown in fig.15, where the frequency band of the antenna arrangement 3 is shown in grey between a first frequency f1 and a second frequency f2. Outside of the absorption region A1 , the reflectivity is nearly 1 . The size of the unit cells 11 depends on the frequency band in use. It may e.g. be between of 0,5 mm to 5 mm.

[0066] In another embodiment, shown in fig. 16, the reflector 10 comprises two types of rectangular sections 10.1 , 10.2 with different resonators 15. The sections 10.1 , 10.2 are marked in white and grey, respectively. This is an alternative to using multi-resonance elements in every unit cell and it can simplify the design. Fig.17 shows the frequency-dependent reflectivity of this reflector 10. Due to the different sections 10.1 , 10.2 with different resonators 16, the spectrum has two absorption regions A1 , A2.

[0067] Fig.18 shows another embodiment in which the size of the resonators 16 is varied linearly along the first direction x in a periodic manner. This leads to locally varying phase response as shown in the diagram of fig.19. Accordingly, the reflector 10 causes anomalous reflection in which an incidence angle and a reflection angle differ in absolute value. For instance, radiation that is incident perpendicular to the surface of the reflector 10 will not be reflected perpendicular but at a different angle. However, with the embodiment of fig.18, plane waves are also reflected as plane waves, i.e. parallel rays stay parallel. This way, knowing e.g., the room configuration, the reflection direction can be optimized for maximum signal amplitude in an obscured region 30.1 , 40.4 and / or at the antenna arrangement 3.

[0068] Another embodiment is shown in fig.20. Here, the dimensions of the resonators 15 are varied in a nonlinear way along the first direction x. This leads to a nonlinear variation of the phase response as shown in fig.21. This means that when being illuminated with a detection signal D1 having normal incidence angle,as shown in fig.22, the reflected signal F1 is distributed into a quite large angular range. A reflecting surface with such a phase profile can be referred to as planar multidirectional reflector or even a planar omnidirectional reflector.

[0069] Another embodiment is shown in fig.23. Here, the shape of the reflector 10 is not simply rectangular but comprises a plurality of cutouts 10.3. in order to allow the integration onto curved elements such as on panels of a vehicle interior, the integration into seats or the like. This also facilitates bending and folding of the reflector 10 during operation without any risk of damage.

[0070] Fig. 24 shows the reflector 10 of fig.14 wrapped around an evenly curved surface, in this case the surface of a pillar 47, e.g., in a large office room. In this case, even with identical unit cells 11 , the reflected signal can be distributed over a wide angular range.

[0071] It should be noted that in figs. 14, 16, 18, 20, 23 and 24, the size of the unit cell 11 in relation to the reflector 10 is exaggerated. With more realistic dimensions it would be much smaller, e.g., by a factor of 50. Similarly, the number of unit cells is typically significantly larger, e.g., 1000 cells along one direction.

[0072] Fig.25 shows another embodiment of a unit cell 11 that can be used for a virtually transparent reflector 10. Thus, the reflector 10 can be disposed on or integrated inside a window of a vehicle 30 or a room 40. Apart from comprising a substrate layer 12 that is transparent for visible light, the resonators 16 of the structured layer 15 are adapted to be transparent portions. Although the overall Flshape of fig.1 A is maintained, the metallic resonator 16 is separated into multiple fine line elements 16.3, which are separated by fine gaps 16.4. The width of the line elements 16.3 is below 1 ,5 pm. The gaps 16.4 can have a similar width. Since the width of the line elements 16.3 is not much larger or even smaller than the wavelength of visible light, the resonators 16 cannot be seen with the naked eye.List of Reference Symbols1 object detection system2 control device3 antenna arrangement10, 50, 60, 70 reflector10.1 , 10.2 section10.3 cutout11 unit cell12, 13 substrate layer15, 17 structured layer16, 18 resonator19 base layer20 metamaterial30 vehicle30.1 , 40.4 obscured region31 , 32, 41-46 seat40 room40.1-40.3 region80 personA1 , A2 absorption regionD1-D4 detection signalF1-F4 reflected signalR1-R4 response signal x first direction y second direction z third direction

Claims

Claims1 . An object detection system (1 ) comprising:- an antenna arrangement (3), comprising at least one antenna;- a control device (2) adapted to control the antenna arrangement (3) to emit a radio-frequency detection signal (D1 -D4) and to detect an object (80) based on a radio-frequency response signal (R1 -R4) received by the antenna arrangement (3), and- at least one passive reflector (10, 50, 60, 70) adapted to at least partially reflect the detection signal (D1 -D4), the reflector (10, 50, 60, 70) comprising a metamaterial (20) with a plurality of unit cells (11 ), the metamaterial (20) comprising at least one dielectric substrate layer (12, 13) and a conductive structured layer (15, 17) disposed on an upper side thereof, whereby the reflector (10, 50, 60, 70) has a frequency-dependent reflectivity , wherein the control device (2) is adapted to detect the object (80) at least partially based on a frequency spectrum of the response signal (R1 -R4), characterised in that at least one of said at least one reflector (10, 50, 60, 70) has an absorption region (A1 , A2) which is smaller than a frequency bandwidth of the detection signal (D1 -D4) and in which the reflectivity of the reflector (10, 50, 60, 70) is reduced with respect to neighbouring regions, wherein the absorption region (A1 , A2) is within the frequency bandwidth, and the control device (2) is adapted to detect the object (80) at least partially based on identifying an influence of the absorption region (A1 , A2) on the frequency spectrum of the response signal (R1 -R4).

2. An object detection system according to claim 1 , characterised in that at least one of said at least one reflector (10, 50, 60, 70) is disposed to provide a reflection path between the antenna arrangement (3) and an obscured region (30.1 , 40.4) that is hidden from the antenna arrangement (3) by an interposed object, so that an object (80) in the obscured region (30.1 , 40.4) is detectable by the control device (2).

3. An object detection system according to claim 2, characterised in that the control device (2) is adapted to determine whether the object (80) is in the obscuredregion (30.1 , 40.4) or in a non-obscured region (40.1 -40.3) based on the influence of the absorption region on the frequency spectrum of the response signal (R1 -R4).

4. An object detection system according to any of the preceding claims, characterised in that the control device (2) is adapted to detect an object (80) interposed between the antenna arrangement (3) and at least one of said at least one reflector (10, 50, 60, 70) based on the influence of the interposed object (80) on the frequency spectrum of the response signal (R1 -R4), which results from an effect of the interposed object (80) on the influence of said absorption region (A1 , A2) on the frequency spectrum.

5. An object detection system according to any of the preceding claims, characterised in that at least one of said at least one reflector (10, 50, 60, 70) comprises a plurality of sections (10.1 , 10.2), at least two sections (10.1 , 10.2) being structured differently so that they have different frequency-dependent reflectivities.

6. An object detection system according to any of the preceding claims, characterised in that at least one of said at least one structured layer (15, 17) extends laterally along a first direction (x) and a second direction (y) perpendicular to the first direction (x) and comprises a plurality of conductive resonators (16, 18), each unit cell (11 ) comprising one resonator (16, 18), and each two neighbouring resonators (16, 18) of the structured layer (15, 17) being spaced-apart along at least one of the first direction (x) and the second direction (y).

7. An object detection system according to any of the preceding claims, characterised in that at least one resonator (16, 18) comprises a plurality of portions (16.1 , 16.2) with different conductivity.

8. An object detection system according to any of the preceding claims, characterised in that the substrate layer (12, 13) is transparent for visible light and the structured layer (16, 18) comprises a transparent structure (16) that comprises a plurality of line elements (16.3) having a maximum width of 1 ,5 pm.

9. An object detection system according to any of the preceding claims, characterised in that at least one of said at least one reflector (10, 50, 60, 70) is a flexible sheet.

10. An object detection system according to any of the preceding claims, characterised in that for at least one of said at least one reflector (10, 50, 60, 70), at least one dimension of one resonator (16, 18) varies for different unit cells (11 ) along one of the first (x) and second direction (y), whereby the reflector (10, 50, 60, 70) is adapted for anomalous reflection in which an incidence angle and a reflection angle differ in absolute value.

11. An object detection system according to any of the preceding claims, characterised in that at least one dimension varies for different cells (11 ) in a non-linear way along one of the first (x) and second direction (y), whereby the reflector (10, 50, 60, 70) is adapted for a reflection in which a single incidence angle corresponds to a plurality of reflection angles.

12. An object detection system according to any of the preceding claims, characterised in that at least one of said at least one reflector (10, 50, 60, 70) comprises a plurality of substrate layers (12, 13) with one structured layer (16, 18) disposed on the upper side of each substrate layer (12, 13).

13. An object detection system according to any of the preceding claims, characterised in that it comprises a building or a vehicle (30) in which the antenna arrangement (3) and the at least one of said at least one reflector (10, 50, 60, 70) are installed.

14. An object detection method using an object detection system (1 ) comprising:- an antenna arrangement (3), comprising at least one antenna;- a control device (2) and- at least one passive reflector (10, 50, 60, 70) comprising a metamaterial (20) with a plurality of unit cells (11 ), the metamaterial (20) comprising at least one dielectric substrate layer (12, 13) and a conductive structured layer (16, 18) disposed on an upper side thereof, whereby the reflector (10, 50, 60, 70) has a frequency-dependent reflectivity, wherein the control device (2) controls the antenna arrangement (3) to emit a radio-frequency detection signal (D1 -D4), the reflector (10, 50, 60, 70) at leastpartially reflects the detection signal (D1-D4), and the control device (2) detecting an object (80) based on a radio-frequency response signal (R1 -R4) received by the antenna arrangement (3), wherein the control device (2) detects the object (80) based on a frequency spectrum of the response signal (R1 -R4), characterised in that at least one of said at least one reflector (10, 50, 60, 70) has an absorption region (A1 , A2) which is smaller than a frequency bandwidth of the detection signal (D1 -D4) and in which the reflectivity of the reflector (10, 50, 60, 70) is reduced with respect to neighbouring regions, wherein the absorption region (A1 , A2) is within the frequency bandwidth, and the control device (2) detects the object (80) at least partially based on identifying an influence of the absorption region (A1 , A2) on the frequency spectrum of the response signal (R1 -R4).

15. A detection method according to claim 14, characterised in that a learning process is performed for the control device (2) to establish a correlation between the frequency spectrum and the detection of the object (80).

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