Device for calibrating a distributed sensor system and distributed sensor system with such a device

Abstract

The invention relates to a device for calibrating a distributed sensor system comprising several sensors at different positions within a vehicle. An optical source (Z1) generates an optical signal which is coupled into at least one optical fiber (F), wherein the optical fiber (F) is arranged at least partially in the vicinity of one or more sensors of the distributed sensor system. An optical detection unit (Z4) detects a calibration signal based on the interaction of the optical signal with the material and / or surface of the optical fiber (F). An evaluation unit (Z7) evaluates the detected calibration signal, thereby detecting any change in the position of one or more of the sensors of the distributed sensor system.

Description

The present invention relates to a device for calibrating a distributed sensor system, which may, for example, be configured as a photonic radar system. The present invention further relates to a distributed sensor system comprising such a calibration device. For driver assistance and safety systems in fully automated driving, the safest possible perception of the surroundings is essential. This is achieved by using sensors such as radar, lidar, and camera sensors integrated into the vehicle to capture the environment. Based on the collected sensor data, an environmental model can then be created. While cameras provide detailed visual information and enable the recognition of traffic signs, lane markings, and colors, for example, they deliver poor results in unfavorable lighting conditions, fog, or glare, and provide only inadequate distance information. Lidar-based systems, while capable of precise distance measurement, are expensive and susceptible to weather conditions. Radar sensors, on the other hand, deliver reliable and fail-safe data in all weather conditions.However, the resolution of radar sensors currently in series production in the automotive sector is limited. Current developments in distributed radar systems rely on the arrangement of transmit and / or receive modules in a spatially distributed aperture, which leads to a significant increase in resolution. A special case of distributed radar systems are photonic radar systems, in which driver signals in the GHz range can be distributed to a multitude of radar sensors via one or more optical fibers using an optical carrier signal in the THz frequency range. Here, electronic and photonic components can be co-integrated on a single semiconductor chip, enabling compact form factors for the individual radar sensors and, consequently, arrays with a large number of such radar sensors integrated into the vehicle. German patent application DE 10 2017 221 257 A1 discloses a radar system in which signal transmission between a central unit and a radar transmitter or radar receiver is implemented optically. For this purpose, a radar driver signal is optically generated in the central unit and transmitted via at least one optical fiber to at least one radar receiver and / or at least one radar transmitter. In the radar transmitter, the radar driver signal is then converted into an electrical radar driver signal and used to drive a radar transmitter. A radar echo signal received by a radar receiver is mixed with the electrical radar driver signal in a mixer of the radar receiver. The mixed signal is then modulated onto the optical driver signal by means of a modulation unit, coupled into the optical fiber, and transmitted back to the central unit.The central unit receives the modulated optical signal and evaluates it using an evaluation unit. The result is then provided as radar information. The radar transmitting and receiving units are typically arranged in front-end modules, where radar antennas for transmitting high-frequency radar signals and receiving high-frequency radar echo signals interact with an electronic circuit for generating and processing the high-frequency signals. These components are referred to as radar antennas in the following. Due to the large-scale distribution of the individual antenna elements in distributed radar systems, precise knowledge of the positions of the radar antennas is necessary for evaluating the radar information. CN 109323659 A describes a method and a device for measuring the base length of an airborne synthetic aperture radar (SAR). A measuring fiber equipped with Bragg gratings is attached to a measuring substrate, which is mounted on a wing of the aircraft and connected to the SAR antennas. It is an object of the invention to provide a device for calibrating a distributed sensor system and a distributed sensor system with such a device. This problem is solved by the independent claims. Preferred embodiments of the invention are the subject of the dependent claims. A photonic radar system according to the invention, comprising a central unit and several radar sensors that exchange signals with the central unit and are arranged at different positions of a vehicle, includes: - an optical source with which an optical signal is generated; - at least one optical fiber into which the optical signal is coupled, wherein the optical fiber is arranged at least partially in the vicinity of one or more radar sensors of the photonic radar system; - an optical detection unit with which a calibration signal is detected, which is based on the interaction of the optical signal with the material and / or surface of the optical fiber; and - an evaluation unit with which the detected calibration signal is evaluated, wherein the evaluation detects a change in the position of one or more of the radar sensors of the photonic radar system. A fiber-optic calibration system designed in this way enables the detection of changes in the sensor positions of the photonic radar system during operation, for example due to temperature or mechanical stress, and can be performed with high accuracy. Compared to electrical measurement methods, this approach offers both cost savings and weight reduction, while simultaneously ensuring resilience to electromagnetic interference. According to one embodiment of the invention, several fiber Bragg gratings are integrated along the optical fiber in the at least one optical fiber. Preferably, several fiber Bragg gratings with different Bragg wavelengths are integrated into the at least one optical fiber. In particular, by evaluating the detected calibration signal, a change in the Bragg wavelength of one of the fiber Bragg gratings can be determined, and from this a change in the position of one or more of the sensors of the distributed sensor system can be determined. According to one embodiment of the invention, the light reflected from one or more fiber Bragg gratings is evaluated. According to a further embodiment of the invention, the optical signal is coupled into the at least one optical fiber at one end and the light coupled out at the other end of the at least one optical fiber is evaluated. According to a further embodiment of the invention, a backscattered signal of the coupled optical signal is evaluated. Preferably, the central unit comprises the optical source, optical detection unit and / or evaluation unit. Finally, the invention also includes a vehicle having a photonic radar system according to the invention and a corresponding method for calibrating a photonic radar system. Further features of the present invention will become apparent from the following description and the claims in conjunction with the figures. Figure 1 shows a schematic representation of a distributed radar system; Figure 2 shows the integration of the radar system on the vehicle surface of a passenger car with two exemplary arrangements of fiber Bragg gratings for calibrating the radar sensors; Figure 3 shows a schematic representation of an embodiment in which fiber Bragg gratings of an optical fiber are used in reflection for calibrating a photonic radar system; Figure 4 shows a schematic representation of the first embodiment in the presence of a deformation of the optical fiber; Figure 5 shows a schematic representation of a further embodiment in which fiber Bragg gratings of an optical fiber are used in transmission for calibrating a photonic radar system. To better understand the principles of the present invention, embodiments of the invention are explained in more detail below with reference to the figures. It is understood that the invention is not limited to these embodiments and that the described features can also be combined or modified without departing from the scope of protection of the invention as defined in the claims. Fig. 1 shows a schematic representation of a distributed sensor system using the example of a photonic radar system, in which the calibration according to the invention can be performed. For the sake of clarity, the device according to the invention for calibrating the distributed sensor system is not shown, but will be explained in detail in connection with the other figures. The radar system comprises a central unit Z, several radar transmitters S-1, S-2, S-3,..., Sn, and several radar receivers E-1, E-2, E-3,..., En. The central unit is referred to as the backend, and the radar transmitters and receivers as frontend modules. Although the radar transmitters and receivers are shown here as separate units, they can also be implemented as combined units, each integrating both a transmitter and a receiver. The central unit Z is connected via one or more transmission media G to the radar transmitting units S-1, S-2, S-3,..., Sn and the radar receiving units E-1, E-2, E-3,..., En, wherein the transmission media can in particular be one or more optical fibers. In the case of an integrated unit on which circuits for transmitting and receiving are located, the signal only needs to be sent to this common unit. The central processing unit Z generates a frequency-modulated continuous wave (FMCW) signal and processes and evaluates the signals generated by the radar receiver units E-1, E-2, E-3, ..., En. Instead of a frequency-modulated continuous wave signal, a signal with a different waveform can also be generated. This centralized processing and evaluation of the signals also contributes to making the individual radar sensors as small and cost-effective as possible. In the central station, a radar driver signal, which consists of a radar carrier signal with frequency fcarrier and a radar ramp signal with frequency frramper, is modulated onto an optical carrier signal. The frequency of the radar driver signal is preferably only a fraction of the carrier frequency required to drive the individual radar transmitters. For example, the signal to be transmitted can be modulated with 1 / 8 or 1 / 4 of the radar carrier frequency commonly used in road vehicles. The optical carrier signal modulated with the radar driver signal is then coupled into the transmission medium G, for example, an optical fiber. The optical carrier signal modulated by the radar driver signal is coupled out of the transmission medium G by each of the individual radar transmitter units S-1, S-2, S-3,..., Sn by means of a coupling unit (not shown), and the radar driver signal is separated in each case. If this radar driver signal is only a fraction of the radar frequency used, for example, 1 / 8 or 1 / 4 of the radar frequency, it is first multiplied eightfold or fourfold, respectively, in the individual radar transmitter units S-1, S-2, S-3,..., Sn. The resulting signal then drives the respective radar transmitters in the radar transmitter units S-1, S-2, S-3,..., Sn. The individual radar transmitters include, in particular, radar antennas, each of which then transmits a radar signal. An unmodulated continuous wave signal can be transmitted to the individual radar receiving units E-1, E-2, E-3,..., En and also coupled out of the transmission medium G by means of a coupling unit. Radar echo signals are received by the individual radar receiving units E-1, E-2, E-3,..., En and modulated onto the unmodulated continuous wave signal. The resulting signal is coupled into the transmission medium G by means of further coupling units and sent back to the central unit. The central unit Z then receives the signals from the radar receivers E-1, E-2, E-3,..., En, evaluates them, and provides derived radar information. This radar information can then be further processed, for example, to create or update an environmental model. The radar transmitters and receivers can each be designed as separate electronically and photonically cointegrated chips (so-called "EPIC chips") or implemented on a single electronically and photonically cointegrated chip. Silicon photonics technology can be used for the cointegration of the electronic and photonic components, enabling the monolithic integration of photonic devices, high-frequency electronics, and digital electronics on a single chip. A hybrid implementation using separate electronic (EIC) and photonic (PIC) chips is also possible. The integration of optical components into the chip can be achieved, for example, using so-called silicon-on-insulator (SOI) regions, while the integration of electronic components can be accomplished using bulk silicon regions. In SOI regions, a thin silicon layer is separated from the silicon substrate by an insulating layer, such as silicon dioxide. Since silicon is transparent at the near-infrared wavelengths common in optical communication technology, and the refractive indices of silicon and silicon dioxide differ significantly in this wavelength range, various optical components can be implemented using SOI structures. This allows for high signal quality with low parasitic interference, particularly at high data rates.The connection of the RF circuits for the radar antennas, including the frequency multiplier, to the photonic circuit can be implemented without additional wire or flip-chip bonding. Furthermore, chips can be tested optically and electrically at the wafer level. Additionally, the scalability to large volumes in the highly integrated manufacturing of electronically and photonically integrated circuits enables a significant reduction in assembly costs and a more efficient cost structure. Figure 2 schematically shows the integration of a distributed radar system in a passenger car, using a distributed photonic radar system as an example. A multitude of fiber Bragg gratings are provided for calibrating the radar sensors. This involves the large-area integration of numerous antenna chips A with radar sensors onto the vehicle surface. For example, the windshield, rear window, and bumper can be used for integration at the front and rear, while the vehicle floor, roof, and B-pillar can be used for integration along the sides. One or more optical fibers F with integrated fiber Bragg gratings BG are rigidly connected to the relevant structures of the vehicle body by means of fixing devices (not shown), so that deformations of the body can be detected by the fiber Bragg gratings and used to calibrate the radar sensors. The fiber Bragg gratings can be arranged in the relevant structures of the vehicle body so that they are equidistant from each other. Fig. 2A shows an arrangement with a substantially meandering course of the optical fibers. In the exemplary arrangement in Fig. 2B, however, the optical fibers run horizontally at different heights. The central unit of the photonic radar system is not shown in this illustration and may, for example, be implemented as a radar control unit located in the vehicle's interior or engine compartment. Similarly, the figure omits the optical fibers for transmitting the radar driver signal from the central unit to the antenna chips and the radar echo signal from the antenna chips to the central unit. Figure 3 schematically illustrates a first embodiment of the invention, in which an optical fiber F is connected to a central processing unit Z of a photonic radar system for calibration. For the sake of clarity, components required for coupling the optical signal generated by the central processing unit Z into the optical fiber are not shown, and only components of the central processing unit Z relevant to the implementation of the invention are shown. Similarly, for clarity, a connection with a single optical fiber is shown, but the invention is equally applicable to a connection with multiple optical fibers. The optical fiber can, in particular, be a single-mode glass fiber. The optical fiber F exhibits a multitude of fiber Bragg gratings, of which four fiber Bragg gratings BG-1, BG-2, BG-3, and BG-4 are shown as examples. These fiber Bragg gratings are periodic microstructures inscribed in the core of the optical fiber, for example, a periodic modulation of the refractive index, thereby generating a resonance structure. A fiber Bragg grating selectively reflects only light of a specific wavelength, known as the Bragg wavelength, which is essentially defined by the grating period of the microstructure and the refractive index of the core. The fiber Bragg gratings BG-1, BG-2, BG-3, and BG-4 shown as examples exhibit different Bragg wavelengths λB1, λB2, λB3, and λB4, at which reflection of the optical radiation coupled into the optical fiber occurs. When broadband light is coupled into the optical fiber, only a portion of the coupled light with a limited spectral width around the Bragg wavelength of a given fiber Bragg grating is reflected. The remaining portions of the light, however, can propagate through the optical fiber without attenuation until, if necessary, different light components are reflected at subsequent fiber Bragg gratings with different Bragg wavelengths. In this way, a large number of measurement points are possible within a single optical fiber. If an optical fiber is stretched or deformed within the area of ​​a fiber Bragg grating, this leads to a change in the grating period, or refractive index, and thus in the Bragg wavelength of that fiber Bragg grating. By detecting the change in the wavelength of the reflected light, the underlying deformation can therefore be deduced in real time. The central unit Z, schematically depicted in Fig. 3, comprises a light source Z1 to provide the optical signal for calibrating the photonic radar system. This light source can be, in particular, a laser diode. The laser diode can, for example, emit laser light in the near-infrared or in other wavelength ranges where optical losses and dispersion are as low as possible. Depending on the embodiment, this can be a continuous-wave laser or a pulsed laser. Likewise, depending on the application, a narrowband, broadband, or tunable laser can be used. Alternatively, a differently designed optical source, such as an LED or an ASE (Amplified Spontaneous Emission) light source, can be used. These sources emit light generated by spontaneous emission and subsequently optically amplified by stimulated emission. The optical signal generated by the light source is optionally fed to an optical beam splitter or switch Z2, which splits the optical signal. The optical signal is then fed to an optical processing unit Z3, which may contain control functions, further optical beam splitters or switches, optical modulators, etc., and then feeds the processed signal to the optical fiber F. The light components reflected by a fiber Bragg grating of the optical fiber are extracted from the optical fiber and fed to an optical detection unit Z4 in the central processing unit Z by the optical processing unit Z3 or a separate optical control element. If a partial beam has been split off from the optical signal by the optical beam splitter or switch Z2, this is also fed to the optical detection unit Z4. This allows the reflected light components to be mixed electronically or optically with a copy of the signal previously sent into the optical fiber, thus providing an intermediate frequency signal for evaluation. After the optical detection unit Z4 has processed the signals it receives, it feeds the resulting electrical signal to a digital interface Z5, where it is digitized by analog-to-digital converters.Furthermore, optional preprocessing of the digitized signals can take place in a preprocessing unit Z6, for example, by means of an FFT to divide the signal into several frequency components. The preprocessed signal is then fed to the evaluation unit Z7, which determines changes in the Bragg wavelengths based, for example, on a spectral analysis and / or an evaluation of the intermediate frequency signal. The information thus obtained can then be further processed to determine position changes of the radar sensors of the photonic radar system. Additionally, the electrical and optical components of the central processing unit Z can be controlled via one or more optional control interfaces and units Z8, Z9. For clarity, Figures 3, 4 to 5 show several such control interfaces and units; however, implementation using a single interface is also possible. Figure 4 schematically illustrates the first embodiment when the optical fiber F is deformed. This deformation can be caused, for example, by a significant temperature change or mechanical stress, schematically indicated by the arrow in the area of ​​the fiber Bragg grating BG-2, and can lead to local stretching or deformation of the optical fiber. This results in a change in the grating period, or refractive index, of the fiber Bragg grating BG-2 and thus in the Bragg wavelength (λB2→ λ''B2) of this fiber Bragg grating. If the Bragg wavelength is known beforehand, for example, calibrated during integration, its change can therefore be monitored and correlated with local deformations. Figure 5 schematically illustrates a second embodiment of the invention, in which fiber Bragg gratings are also used in an optical fiber for calibrating a photonic radar system. In this embodiment, the measurement is of the power transmitted through the optical fiber F, instead of or including the reflected power. For this purpose, the light signal generated by the central unit Z is coupled into the optical fiber F at one end, and the light coupled out at the other end of the optical fiber is supplied to the optical detection unit Z4 in the central unit Z and evaluated to detect deformations. Another embodiment is based on the use of backscattering effects along the optical fiber, with the optional integration of reflection points via fiber Bragg gratings. Calibration can be performed by measuring the intensity of scattered light, which is based, for example, on Raman, Rayleigh, or Brillouin scattering of the incident light in the optical fiber. Optical time-domain reflectometry (OTDR) or optical frequency-domain reflectometry (OFDR) can be used to detect the scattered light. In optical time-domain reflectometry, light pulses generated by a pulsed laser source are coupled into the optical fiber and guided along its length.Of the scattered light generated in the optical fiber, a portion propagates within the fiber itself, which can be measured over time using a light-sensitive photodetector. In contrast, frequency-domain reflectometry analyzes the signal in the frequency domain, not the time domain. For this, a continuous-wave light source is used, such as a tunable CW laser diode modulated over a specific wavelength range. The device according to the invention can be used in particular in any road vehicles, such as passenger cars, commercial vehicles, trucks or buses, in which radar-based environmental sensing is used, for example for driver assistance systems or for automatic or autonomous driving functions, but is not limited to this. Application with other vehicle sensors is also possible. Reference symbol list Z Central processing unit S-1, S-2, S-3, Sn Radar transmitting units E-1, E-2, E-3, En Radar receiving units G Transmission medium F Optical fiber BG, BG-1, BG-2, BG-3, BG-4 Fiber Bragg grating A Antenna chip Z1 Light source Z2 Switch or splitter Z3 Optical processing unit Z4 Optical detection unit Z5 Digital interface Z6 Preprocessing unit Z7 Evaluation unit Z8, Z9 Control interface and unit

Claims

Photonic radar system comprising a central unit (Z) and several radar sensors, wherein the radar sensors exchange signals with the central unit (Z) and are arranged at different positions of a vehicle, comprising: - an optical source (Z1) with which an optical signal is generated; - at least one optical fiber (F) into which the optical signal is coupled, wherein the optical fiber (F) is arranged at least partially in the vicinity of one or more radar sensors of the photonic radar system; - an optical detection unit (Z4) with which a calibration signal is detected, which is based on the interaction of the optical signal with the material and / or the surface of the optical fiber (F); and - an evaluation unit (Z7) with which the detected calibration signal is evaluated, wherein the evaluation detects a change in the position of one or more of the radar sensors of the photonic radar system. Photonic radar system according to claim 1, wherein several fiber Bragg gratings (BG, BG-1, BG-2, BG-3, BG-4) are integrated along the at least one optical fiber (F). Photonic radar system according to claim 1 or 2, wherein several fiber Bragg gratings (BG, BG-1, BG-2, BG-3, BG-4) with different Bragg wavelengths are integrated in the at least one optical fiber (F). Photonic radar system according to one of claims 1 to 3, wherein a change in the Bragg wavelength of one of the fiber Bragg gratings (BG, BG-1, BG-2, BG-3, BG-4) is determined by evaluating the detected calibration signal and a change in the position of one or more of the radar sensors of the photonic radar system is determined from this. Photonic radar system according to one of claims 1 to 4, wherein the light reflected from one or more fiber Bragg gratings (BG, BG-1, BG-2, BG-3, BG-4) is evaluated. Photonic radar system according to one of claims 1 to 5, wherein the optical signal is coupled into the at least one optical fiber (F) at one end and the light coupled out at the other end of the at least one optical fiber (F) is evaluated. Photonic radar system according to one of claims 1 to 6, wherein a backscattered signal of the coupled optical signal is evaluated. Photonic radar system according to one of claims 1 to 7, wherein the central unit (Z) comprises the optical source (Z1), optical detection unit (Z4) and / or evaluation unit (Z9).

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