Angle-resolved radar sensor

Abstract

An angle-resolved radar sensor (8) comprising a transmitting and receiving device and an evaluation device for determining positional data of a target (24) that reflects radar radiation, wherein the evaluation device is configured to recognize multipath scenarios, and in a multipath scenario, a portion of the received signal representing the target is based on the reflection of the transmitted and / or received signals at a reflective surface (42), and the evaluation device is further configured to determine the azimuth angle (y) of the reflective surface (42) itself or read it from an external source, and to determine the tangential velocity (v_θ) of the target (24) based on different Doppler shifts of the signal portions obtained on different paths.

Description

Technical Field

[0001] The present invention relates to an angular resolution radar sensor comprising a transceiver device and an evaluation device for determining position data of a target reflecting radar radiation, the evaluation device being configured to recognize a multipath scenario in which a part of the received signal representing the target is based on reflection of the transmitted signal and / or the received signal at a reflecting surface.

[0002] In particular, the present invention relates to a radar sensor used in a driver assistance system for a motor vehicle to detect a traffic environment.

Background Art

[0003] In the ambient monitoring in a driver assistance system, the distances, relative speeds, and azimuth and elevation angles of the located targets are important. By measuring the Doppler shift, a radar sensor can only measure the radial component of the relative speed. However, for recognizing lane changes and when vehicles and pedestrians cross, the tangential component of the speed is also important. This can be determined by using a radar sensor to track the distance and azimuth angle of a target over a plurality of measurement cycles, for example. As an alternative method, the radial speed is measured using a plurality of sensors from different directions. Other sensors such as LiDAR and cameras provide further alternative forms for estimating the tangential speed. However, it is also desirable to measure the tangential speed with a radar sensor. Even when a radar sensor is used simultaneously with a LiDAR sensor or a camera in the same vehicle, the object fusion can be improved by using the tangential speed measured by the radar sensor.

[0004] Known methods for estimating tangential velocity within a single measurement cycle using radar sensors utilize multiple laterally offset reflecting points of the same object. This requires the existence of multiple reflecting points with different azimuth angles. However, even then, the lateral offset is limited by the object's dimensions, making the estimation highly inaccurate.

[0005] From DE102021212376A1, the type of radar sensor mentioned at the beginning is known that can also recognize multipath scenarios, in which signals traveling from the radar sensor to the target, or signals returning from the target to the radar sensor, or both, are reflected by walls, for example, the surface of a guardrail. In this case, in addition to the direct signal path and the reflected path, there are also two so-called cross paths where only the forward beam or only the returning beam is reflected. Using a MIMO radar with at least three transmitting antennas and at least three receiving antennas, it is possible to separate the signals from the four possible paths, and thus distinguish between the actual target and the apparent target resulting from reflection, enabling more accurate measurements of distance, velocity, and angle.

[0006] Patent application DE102022209813.7 proposes a method for determining the spatial location of a reflective surface using such a radar sensor. Alternatively, the reflective surface can be recognized and the reflection associated with an actual object by utilizing estimation of surrounding structures. Here, the surrounding structures can be detected, for example, by evaluating stationary targets measured by the radar sensor, or by fusing with video or LiDAR data, for example. Further methods for locating the position of a reflective surface are described at https: / / de.mathworks.com / help / driving / ug / multipath-radar-detection-and-tracking.html. [Overview of the project] [Problems that the invention aims to solve]

[0007] The objective of the present invention is to enable accurate estimation of tangential velocity using only a single (MIMO) radar sensor within a single measurement cycle. [Means for solving the problem]

[0008] According to the present invention, this problem is solved by further configuring the evaluation device to determine the azimuth angle of the reflective surface itself or to read it from an external source and to determine the tangential velocity of the target based on different Doppler shifts of signal portions obtained on different paths.

[0009] In this process, distance, velocity, and angle information from both the direct and reflected paths from the same object are combined. The radar sensors described above, based on the background technology, are already capable of recognizing and associating reflections and determining the angle of the reflective surface. Under these preconditions, the fact that the tangential component of the target's velocity affects the Doppler shift of the radar signal on different signal paths in different ways can be utilized, thereby allowing the tangential velocity to be determined based on the measured Doppler shift.

[0010] Advantageous embodiments and advanced forms of the present invention are evident from the dependent claims. An exemplary embodiment will be described in more detail below with reference to the drawings. [Brief explanation of the drawing]

[0011] [Figure 1] This is a block diagram of the radar sensor. [Figure 2] This figure shows a scenario involving multipath propagation. [Figure 3] This is a flowchart illustrating the method according to the present invention. [Figure 4] This figure shows an example of a signal spectrum. [Figure 5] This is a diagram illustrating the method according to the present invention. [Figure 6] This is a diagram illustrating the method according to the present invention. [Figure 7] This is a diagram illustrating the method according to the present invention. [Modes for carrying out the invention]

[0012] Figure 1 shows a simplified schematic representation of the structure of the MIMO (Multi-Input Multi-Output) radar sensor 8. A frequency modulation device 10 controls an RF oscillator 12, which generates a sequence of signals in the form of frequency ramps for multiple transmitting antennas 14. An amplifier 16 is placed in each of the multiple transmitting channels, and the amplifier 16 blocks or amplifies the signal and transmits it to the associated antenna. The oscillator 12 and amplifier 16 are controlled by a multiplexing device 18, for example, according to a time and frequency multiplexing scheme, so that each of the transmitting antennas 14 transmits a frequency-modulated signal in a specific frequency subband within a specific time slot.

[0013] The transmitted signal, reflected by the target 24 (e.g., the reflection point of the vehicle), is received by multiple receiving antennas 26 and mixed with a portion of the signal from the RF oscillator 12 by a mixer 28 in each receiving channel, thereby reducing it to a lower frequency range. Then, A / D conversion is performed in the usual manner by the A / D converter 30. The digitized signal is then further processed by a digital evaluation device 32.

[0014] A radar sensor can operate, for example, according to the fast chirp principle. In this case, the frequency ramp of the transmitted signal is very steep, so the frequency of the low-frequency (beat) signal obtained during mixing depends substantially only on the signal propagation time, and therefore on the distance to the target, and has only a negligibly small Doppler shift even when the target is moving radially. When multiple such ramps are transmitted in succession, the radial motion of the target causes a phase shift from ramp to ramp. By Fourier transforming the sequence of ramps, the Doppler shift of the target, and therefore the radial velocity, can be determined.

[0015] In the evaluation device 32, a two-dimensional spectrum in the dimensions of interval and relative velocity is first calculated by Fourier transform using a known method. Then, individual targets can be identified based on this spectrum, and their intervals and relative velocities can be determined. Subsequently, angle estimation is performed in the subsequent evaluation stage.

[0016] The transmitting antenna 14 forms a transmitting array having n transmitting channels TX1, ..., TXn, and the receiving antenna 26 forms a receiving array having k receiving channels RX1, ..., RXk. For example, both arrays are two-dimensional, and therefore MIMO angle measurement is possible in both azimuth and elevation angles.

[0017] Figure 2 illustrates a scenario characterized by multipath propagation. The signal transmitted by the radar sensor 8 can propagate not only on the direct path 38 to the target 24, but also on the indirect path 40, which first travels to a reflective surface 42, such as a guardrail, and is then deflected towards the target 24. Similarly, the signal reflected from the target 24 can propagate not only to the radar sensor 8 on the direct path 44, but also on the indirect path 46, where the signal is similarly reflected by the reflective surface 42, thereby being recognized by the radar sensor as a signal from the apparent target 24'. Two paths, one through the direct path 38 or 44 and the other through the indirect path 40 or 46 in one direction, are called crosspaths. Angle estimation in the evaluation device 32 is based on a crosspath model that models not only signal propagation on direct and indirect paths, but also signal propagation on crosspaths. Details of the evaluation method based on this model are described in DE102021212376A1.

[0018] The data on the intervals, radial velocities, and angles obtained in this way form the basis for a method of determining the tangential velocity of the target 24. The essential steps of this method are shown in Fig. 3. In step S1, multi-path estimation is performed based on the cross-path model. Depending on the separation ability of the radar sensor in the interval dimension and the velocity dimension, the signals acquired for a single target and the associated apparent targets may, after two-dimensional Fourier transformation, be in the same interval / velocity cell or in two different cells. Especially in the former case, overlapping of signals transmitted from the same target but propagating on different paths and thus having different propagation times can occur.

[0019] Fig. 4 shows an example regarding the spectrum obtained in this way. The curve 48 in Fig. 4 shows the received signal output P as a function of the signal propagation distance Δ (proportional to the frequency). This curve 48 results from the superposition of four signal components indicated by the curves 50, 52, and 54. The curve 50 represents a signal component that propagates on the direct path 38 in the transmission direction and on the direct path 44 in the reception direction and thus has a maximum value at a relatively small signal propagation distance Δ. The curve 54 represents a signal component that propagates on the indirect paths 40 and 46 in both directions and thus has its maximum value at a substantially larger signal propagation distance. The curve 52 represents two essentially identical signal components corresponding to two cross-paths (38 and 46, or 40 and 44). These curves have their maximum values at a signal propagation distance equal to the average value of the positions of the maxima of the two curves 50 and 54.

[0020] In step S2, the received spectrum is analyzed to examine whether it has characteristics typical of a multi-path scenario and thus whether it can be separated into four signal components as shown in Fig. 4.

[0021] After the signal separation in step S2, the angle γ is determined in step S3, and this angle γ indicates the orientation of the reflecting surface 42, i.e., the azimuth angle formed by the reflecting surface 42 with the optical axis of the radar sensor. FIG. 2 shows a special case where γ = 0. A more general case where γ is different from 0 is shown in FIG. 5. The optical axis of the radar sensor 8 is indicated here by the reference number 56. Due to the inclination of the reflecting surface 42, the positions of the target 24 and the apparent target 24' caused by reflection are not symmetric with respect to the optical axis 56, but are symmetric with respect to the straight line n indicating the orientation of the reflecting surface 42 at the reflection point R, i.e., the location where the radiation is reflected from and back to the radar sensor 8. The (polar) coordinates of the target 24, i.e., the distance d and the azimuth angle θ, can be determined by evaluating the signal components represented by the curve 50 in FIG. 4. The coordinates of the apparent target 24', i.e., the apparent distance d' and the apparent azimuth angle θ', can be determined by evaluating the signal components represented by the curve 54. The apparent distance d' is composed of the distance d1' from the radar sensor 18 to the reflection point R and the distance d'' from the reflection point R to the target 24 or the apparent target 24' (by symmetry, both distances are equal). Here, the orientation of the reflecting surface 42 is obtained by drawing a straight line g connecting the target 24 and the apparent target 24', and then erecting a perpendicular bisector (straight line n) to the line segment from 24 to 24'. In this way, the azimuth angle γ can be determined.

[0022] Next, the subsequent calculation of the tangential velocity v_θ in step S4 will be described based on the vector diagrams shown in FIGS. 6 and 7. For the sake of simplification, these vector diagrams show the special case where γ = 0 corresponding to FIG. 2. FIG. 6 shows the radar sensor 8, the target 24, the apparent target 24', the signal path, and the perpendicular bisector n, as well as the velocity vector v of the target 24 and its tangential component, i.e., the tangential velocity v_θ. For the radial component, two different vectors v_r related to the direct path and v_r' related to the indirect path can be obtained. In FIG. 7, the related vectors are shown enlarged again.

[0023] Two vectors v_r and v_r' are relative to each other by an angle. α = 2γ - θ' - θ (1) To accomplish.

[0024] In the simplified case γ=0 shown in Figures 6 and 7, γ = -θ' - θ (2) This holds true.

[0025] Figure 7 shows two lines q and q' that intersect at the target point of vector v_r. q is perpendicular to v_r, and q' is perpendicular to v_r'. Therefore, these two lines similarly form an angle α with each other. Together with vector v, these lines form a triangle with height h. The side of this triangle connecting the target points of v and v_r has length v_θ. Therefore, h=v_θsin(α) (3) This holds true, v_θ=h / sin(α)=(v_r'-v_rcos(α)) / sin(α) (4) This holds true.

[0026] The azimuth angle γ of the perpendicular bisector n can be calculated as follows: γ=arctan((dcos(θ)-d'cos(θ')) / (d'sin(θ∋)-dsin(θ))) (5) By substituting this value into equation (1), the angle α is obtained, and then the value of the radial velocity v_θ is obtained according to equation (5).

[0027] For example, in many cases such as reflections from guardrails, the special case γ=0 holds true, which simplifies the calculations.

Claims

1. An angle-resolved radar sensor (8) comprising a transmitting and receiving device and an evaluation device (32) for determining position data of a target (24) that reflects radar radiation, wherein the evaluation device (32) is configured to recognize a multipath scenario, and in the multipath scenario, a portion of the received signal representing the target is based on the reflection of the transmitted and / or received signals at a reflective surface (42), and the evaluation device (32) is further configured to determine the azimuth angle (y) of the reflective surface (42) itself or read it from an external source, and to determine the tangential velocity (v_θ) of the target (24) based on different Doppler shifts of the signal portions obtained on different paths.

2. The radar sensor according to claim 1, wherein the radar sensor (8) is a MIMO radar sensor comprising at least three transmitting antennas (14) and at least three receiving antennas (26) for measuring the angle of the azimuth angle.

3. The radar sensor according to claim 1 or 2, wherein the evaluation device (32) is configured to calculate the azimuth angle (y) of the reflective surface (42) based on interval and angle data relating to the actual target (24) and the apparent target (24') associated therewith, which is produced by reflection.

4. A software product comprising computer-readable program code that, when loaded into the computer of a radar sensor evaluation device (32), causes the evaluation device to perform the functions of the radar sensor described in any one of claims 1 to 3.

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