Angle-resolving radar sensor

Abstract

The invention relates to an angle-resolving radar sensor (8) comprising a transmitting and receiving device and an evaluation device for determining locating data of targets (24) which reflect the radar radiation, the evaluation device being configured to recognize multipath scenarios in which a portion of the received signal representing a target is based on reflection of the transmitted signal and / or of the received signal off a reflection surface (42), characterized in that the evaluation unit is furthermore configured either to autonomously determine an azimuth angle (y) of the reflection surface (42) or to read same from an external source, and to determine the tangential velocity (v_θ) of the target (24) on the basis of varying Doppler shifts of the signal portions obtained on different paths.

Description

[0001] Description

[0002] title

[0003] Angle-resolving radar sensor

[0004] Description

[0005] The invention relates to an angle-resolving radar sensor with a transmitting and receiving device and an evaluation device for determining location data of targets that reflect the radar radiation, wherein the evaluation device is configured to detect multi-path scenarios in which a portion of the received signal representing a target is based on reflection of the transmitted and / or received signal at a reflection surface.

[0006] In particular, the invention relates to a radar sensor which is used in a driver assistance system of a motor vehicle to detect the traffic environment.

[0007] State of the art When monitoring the environment in driver assistance systems, the distance, relative speed and the azimuth and elevation angles of the located line are important. By measuring the Doppler shift, radar sensors can only measure the radial component of the relative speed. However, the tangential component of the speed is also relevant for detecting lane changes as well as crossing vehicles and pedestrians. This can be determined with a radar sensor, for example, by tracking the distance and azimuth angle of a target over several measurement cycles. In an alternative method, the radial velocities are measured using several sensors from different directions. Other sensors such as lidar and cameras offer further alternatives for estimating the tangential speed. Nevertheless, it is desirable to also measure the tangential speed with a radar sensor.Even if a radar sensor is used simultaneously with lidar sensors or cameras in the same vehicle, the tangential velocity measured by the radar sensor can then be used to improve object fusion.

[0008] A known method for estimating tangential velocity with a radar sensor within a single measurement cycle uses multiple laterally offset reflection points of the same object. This requires multiple reflection points with different azimuth angles. Even then, the lateral offset is limited by the object dimensions, making the estimation very inaccurate.

[0009] DE 10 2021 212 376 A1 discloses a radar sensor of the type mentioned above that can also detect multi-path scenarios in which the signal traveling from the radar sensor to the target, or the signal traveling back from the target to the radar sensor, or both, are reflected on a wall, such as the surface of a guardrail. In addition to the direct signal path and the reflected path, there are also two so-called cross paths, in which 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, the signals from the four possible paths can be separated, thus distinguishing real targets from apparent targets created by reflections, and allowing more precise distance, speed, and angle measurements to be made. c

[0010] Patent application DE 10 2022 209 813.7 proposes a method for determining the spatial position of a mirror surface using such a radar sensor. Alternatively, an estimate of the peripheral buildings can be used to detect mirror surfaces and assign reflections to direct objects. The peripheral buildings can be detected, for example, by evaluating stationary targets measured by the radar sensor or by fusion with video or lidar data. Another method for localizing reflection surfaces is described at: https: / / de.mathworks.com / help / driving / ug / multipath-radar-detection-and-tracking.html. Disclosure of the Invention

[0011] The object of the invention is to enable a precise estimation of the tangential velocity with only a single (MIMO) radar sensor within a single measurement cycle.

[0012] This object is achieved according to the invention in that the evaluation device is further configured to either determine an azimuth angle of the reflection 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 the signal components obtained on different paths.

[0013] This combines distance, velocity, and angle information from direct paths and mirror paths of the same object. The state-of-the-art radar sensors described above already make it possible to detect and assign reflections and determine the angle of the mirror surface. Under these conditions, one can exploit the fact that the tangential component of a target's velocity influences the Doppler shift of the radar signal in different ways along the various signal paths, allowing the tangential velocity to be determined based on the measured Doppler shifts.

[0014] Advantageous embodiments and further developments of the invention emerge from the subclaims.

[0015] An exemplary embodiment is explained in more detail below with reference to the drawing. Figure 1 shows a block diagram of a radar sensor;

[0016] Fig. 2 is a diagram illustrating a multipath propagation scenario; Fig. 3 is a flowchart for a method according to the invention;

[0017] Fig. 4 shows an example of a signal spectrum; and

[0018] Fig. 4 and 5 diagrams to explain the method according to the invention.

[0019] Fig. 1 shows a schematic and simplified structure of a MIMO (Multiple Input Multiple Output) radar sensor 8.

[0020] A frequency modulation device 10 controls an RF oscillator 12, which generates sequences of signals in the form of frequency ramps for a plurality of transmitting antennas 14. An amplifier 16 is arranged in each of the multiple transmitting channels, which either blocks the signals or forwards them in amplified form to the associated antenna. The oscillator 12 and the amplifiers 16 are controlled by a multiplexing device 18, for example, according to a time and frequency division multiplexing scheme, so that each of the transmitting antennas 14 transmits a frequency-modulated signal in a specific frequency subband within specific time slots.

[0021] The transmitted signal, reflected from a target 24 (e.g., a reflection point on a vehicle), is received by several receiving antennas 26 and mixed in each receiving channel with a portion of the signal from the RF oscillator 12 by a mixer 28, thereby bringing the signal into a low-frequency range. A / D conversion is then performed in the usual manner by an A / D converter 30. The digitized signals are then further processed in a digital evaluation device 32. The radar sensor can, for example, operate according to the rapid chirp principle. The frequency ramps of the transmitted signal are then so steep that the frequency of the low-frequency (beat) signal obtained during mixing depends practically only on the signal's propagation time and thus on the distance to the target, and exhibits only a negligible Doppler shift even with a radial movement of the target.When several such ramps are transmitted in rapid succession, the radial motion of the target results in a phase shift from ramp to ramp. A Fourier transformation of the sequence of ramps can then be used to determine the Doppler shift and thus the radial velocity of the target.

[0022] In the evaluation device 32, a two-dimensional spectrum in the dimensions of distance and relative velocity is first calculated in a known manner using Fourier transformation. Based on this spectrum, individual targets can then be identified and their distances and relative velocities determined. In subsequent evaluation stages, an angle estimate is then performed.

[0023] The transmit antennas 14 form a transmit array with n transmit channels TX1, ..., TXn, and the receive antennas 26 form a receive array with k receive channels RX1, ..., RXk. For example, both arrays are two-dimensional, allowing MIMO angle measurements in both azimuth and elevation.

[0024] Fig. 2 outlines a scenario characterized by multipath propagation.

[0025] The signal transmitted by radar sensor 8 can propagate not only along a direct path 38 to target 24, but also along an indirect path 40, which first leads to a reflective surface 42, such as a guardrail, and is then deflected toward target 24. Likewise, the signal reflected by target 24 can propagate not only along a direct path 44 to radar sensor 8, but also along an indirect path 46, where the signal is also reflected by reflective surface 42, thus simulating an apparent target 24' to the radar sensor. The two paths, on which the signal takes the direct path 38 or 44 in one direction and the indirect path 40 or 46 in the opposite direction, are referred to as cross paths.The angle estimation in the evaluation device 32 is based on a cross-path model that models not only the signal propagation on the direct path and the indirect path, but also the signal propagation on the cross paths.

[0026] Details of an evaluation method based on this model are described in DE 10 2021 212 376 A1. The distance, radial velocity, and angle data obtained in this way form the basis for a method for determining the tangential velocity of the target 24. The essential steps of this method are shown in Fig. 3. In step S1, a multi-path estimation is performed based on the cross-path model. Depending on the separation capability of the radar sensor in the distance dimension and the velocity dimension, the signals obtained for a single target and the associated decoy can lie in the same distance / velocity cell or in two different cells after the two-dimensional Fourier transformation. In particular, in the former case, superposition of the signals originating from the same target but propagating along different paths and therefore having different propagation times can occur.

[0027] Fig. 4 shows an example of a spectrum obtained in this way. Curve 48 in Fig. 4 indicates the received signal power P as a function of the signal propagation length A (proportional to the frequency). This curve 48 is created by superimposing four signal components, which are indicated by curves 50, 52 and 54. Curve 50 represents a signal component that propagates in the transmission direction on the direct path 38 and in the reception direction on the direct path 44 and therefore has a maximum at a relatively small signal propagation length A. Curve 54 represents a signal component that propagates in both directions on the indirect paths 40 and 46 and therefore has its maximum at a significantly longer signal propagation length. Curve 52 represents two essentially identical signal components that correspond to the two cross paths (38 and 46 or 40 and 44).These curves have their maximum at a signal path length that is equal to the mean value of the position of the maxima of the two curves 50 and 54.

[0028] In step S2, the received spectrum is analyzed and examined to determine whether it exhibits the characteristics characteristic of a multipath scenario and can therefore be separated into four signal components as illustrated in Fig. 4.

[0029] After the signal separation in step S2, an angle y is determined in step S3 which indicates the course of the reflection surface 42, i.e. the azimuth angle that the reflection surface 42 forms with the optical axis of the radar sensor. Fig. 2 illustrates the special case y = 0. The more general case in which y is different from 0 is illustrated in Fig. 5. The optical axis of the radar sensor 8 is designated here by 56. Due to the inclination of the reflection surface 42, the positions of the target 24 and the apparent target 24' created by reflection are not symmetrical to the optical axis 56, but symmetrical to a straight line n which indicates the course of the reflection surface 42 at the reflection point R, i.e. where the radiation is reflected from and to the radar sensor 8. The (polar) coordinates of the target 24, i.e. the distance d and the azimuth angle 0, can be determined by evaluating the signal component 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 0', can be determined by evaluating the signal component represented by curve 54. The apparent distance d' is made up of a distance d1 ' from the radar sensor 18 reflection point R and a distance d" from the reflection point R to the target 24 or to the apparent target 24' (due to symmetry, both distances are the same). The course of the reflection surface 42 is now obtained by drawing a connecting straight line g between the target 24 and the apparent target 24' and then erecting the perpendicular bisector (the straight line n) on the section from 24 to 24'. In this way, the azimuth angle y can be determined. The subsequent calculation of the tangential velocity v_0 in step S4 will now be explained using the vector diagrams shown in Figures 6 and 7. For simplicity, these vector diagrams illustrate the special case y = 0 according to Figure 2.Figure 6 shows the radar sensor 8, the target 24, the decoy 24', the signal paths, 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_0. For the radial component, two different vectors are obtained: v_r for the direct path and v_r' for the indirect path. The vectors involved are shown again in an enlarged scale in Figure 7.

[0030] The two vectors vr and vr' form an angle a = 2y - 0' - 0. (D In the simplified case y = 0 shown in Figures 6 and 7, y = -0' - 0 (2)

[0031] In Fig. 7, two straight lines q and q' are drawn that intersect at the target point of the vector v_r. q is perpendicular to v_r and q' is perpendicular to v_r'. These two straight lines therefore also form an angle a with each other. Together with the vector v, these straight lines form a triangle with a height of h. The side of this triangle, 0, that connects the target points of v and v_r, has a length of v_0. Consequently, h = v_0 sin(a) (3) and v_0 = h / sin(a) = ( v_r' - v_r cos(a) ) / sin(a) (4)

[0032] The azimuth angle y of the perpendicular bisector n can be calculated as follows: 0 y = arctan ( (d cos(0) - d'cos(0') ) / (d' sin(09) - d sin(0) ) ) (5)

[0033] By inserting this value into equation (1), one obtains the angle a and then, according to equation (5), the magnitude of the radial velocity v_0. c

[0034] In many cases, for example reflections from a guardrail, the special case y = 0 applies, so that the calculation is simplified.

Claims

Claims 1. Angle-resolving radar sensor (8) with a transmitting and receiving device and an evaluation device (32) for determining location data of targets (24) that reflect the radar radiation, wherein the evaluation device (32) is configured to detect multi-path scenarios in which a portion of the received signal representing a target is based on reflection of the transmitted and / or received signal at a reflection surface (42), characterized in that the evaluation unit (32) is further configured to either determine an azimuth angle (y) of the reflection surface (42) itself or to read it from an external source and to determine the tangential velocity (v_0) of the target (24) based on different Doppler shifts of the signal components obtained on different paths.

2. Radar sensor according to claim 1, wherein the radar sensor (8) is a MIMO radar sensor with at least three transmitting antennas (14) and at least three receiving antennas (26) for angle measurements in azimuth.

3. Radar sensor according to claim 1 or 2, wherein the evaluation device (32) is configured to calculate the azimuth angle (y) of the reflecting surface (42) based on the distance and angle data for a real target (24) and an associated decoy target (24') created by reflection.

4. Software product with computer-readable program code which, when loaded into a computer of an evaluation device (32) of a radar sensor, causes the evaluation device to execute the functions of a radar sensor according to one of claims 1 to 3.

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