Radar sensor for a motor vehicle

Abstract

Radar sensor for a motor vehicle, the radar sensor having an antenna device which has a plurality of antenna elements (10, 12) arranged linearly and equidistantly along a connection line (14), wherein the distance between each two adjacent antenna elements (10, 12) is equal to half of the average wavelength λ of the transmitted radar signal, and the device has at least one triple of adjacent antenna elements, in which triple, on the one hand, the two outer antenna elements (10) in the triple and, on the other hand, the antenna element (12) located between the two antenna elements extend in opposite directions from the connection line (14), characterized in that the antenna device has at least one pair of antenna elements (12, 16; 10, 20) extending in opposite directions from the connection line (14) and the distance between which is an integer multiple of the wavelength λ, such that one of these antenna elements (18, 20) has a negative amplitude distribution with respect to the antenna elements (10, 12) of the triple.

Description

Field of the Invention

[0001] The present invention relates to a radar sensor for a motor vehicle, the radar sensor having an antenna device which has a plurality of antenna elements arranged linearly and equidistantly along a connection line, wherein the distance between every two adjacent antenna elements is equal to half of the average wavelength λ of the transmitted radar signal, and the device has at least one triple group consisting of adjacent antenna elements, in which triple group, on the one hand, the two outer antenna elements in the triple group and, on the other hand, the antenna element located between the two antenna elements extend away from the connection line in opposite directions. Background Art

[0002] In such a conventional radar sensor of this type, the antenna elements are alternately arranged on opposite sides of the connection line. Since the distance from antenna element to antenna element is λ / 2, the fed-in signal is out of phase at the positions of two adjacent antenna elements. However, since the antenna elements are arranged on opposite sides of the connection line, a positive amplitude distribution is obtained for all the antenna elements.

[0003] The antenna elements and the connection line can be constructed in microstrip line technology. However, in other radar sensors, the antenna device can also be formed by an electromagnetic waveguide (Hohlleiter-) antenna or a SIW antenna (substrate integrated waveguide). If the antenna elements are constructed in microstrip line technology, the amplitude distribution of the antenna elements can be set by changing the width and length of individual antenna patches such that the side lobes in the directional characteristic of the antenna device are largely suppressed. This applies not only to the radiated power in the transmitting antenna, but also to the direction-dependent sensitivity in the receiving antenna.

[0004] However, even in the case of good side lobe suppression, the radar sensor has a relatively unclearly bounded field of view, because at the edges of the line of sight, the transmitted power and the sensitivity as a function of the azimuth angle only decrease gradually. This can lead to undesired effects in certain application cases, such as interference signals caused by reflections of irrelevant objects at the edges of the field of view.

[0005] In autonomous driving systems for motor vehicles, the requirements for sensing mechanisms are generally increasing, and a larger number of radar sensors are often installed in a single vehicle. Here, it is becoming increasingly difficult to find a suitable mounting position on the vehicle for the radar sensor, at which the signal is not disturbed by vehicle structures in the sensor's surroundings, such as parts of the bumper, logos mounted on the bumper, etc. Due to the unclearly bounded field of view of the radar sensor, the sensitivity with respect to such interference sources is increased. Summary of the Invention

[0006] Accordingly, the object of the present invention is to provide a radar sensor with a more clearly defined field of view.

[0007] This object is achieved according to the invention in that the antenna device has at least one pair of antenna elements that extend in opposite directions from a connection line and whose distance is an integer multiple of the wavelength λ, such that one of these antenna elements has a negative amplitude distribution relative to the triple antenna element.

[0008] Since the signals fed into the two antenna elements are in phase, a negative amplitude distribution is obtained due to the relative arrangement of the antenna elements. By enabling this possibility of negative amplitude distribution, additional clearance space for beam shaping is achieved.

[0009] If a clearly defined field of view is desired, the curve describing the directional characteristic, i.e., the angular correlation, of the antenna gain has a configuration approximating a rectangular shape. The antenna gain is approximately constant within a limited angular range around the 0° direction and drops sharply at the edges of the line of sight. The relationship between the directional characteristic of the antenna array and the amplitude distribution is in principle given by Fourier transform. The Fourier transform of a rectangular function has an oscillatory behavior, so that in the amplitude distribution, in addition to positive coefficients, negative coefficients also appear. Since this negative amplitude distribution can be described using the antenna device according to the invention, a directional characteristic approximating a rectangular shape can be achieved.

[0010] This can be applied not only to the transmitting antenna, but also to monostatic antenna designs in which the antenna elements not only transmit but also receive, and can also be applied to purely receiving antennas, even in the latter case where the connection line is a receiving line and in other cases the connection line is a feeding line.

[0011] Advantageous configurations of the present invention are described in the dependent claims. Description of the Drawings

[0012] The following embodiments are elaborated in detail with the aid of the drawings.

[0013] The drawings show:

[0014] Figure 1 : the antenna device in a radar sensor according to the invention;

[0015] Figure 2 : the directional characteristics of two different antenna devices;

[0016] Figure 3 : an idealized illustration of the directional characteristics of a conventional radar sensor and a radar sensor according to the invention; and

[0017] Figure 4: An example of the installation status of a radar sensor in a motor vehicle. DETAILED DESCRIPTION

[0018] In Figure 1 an example of the antenna device according to the invention in a radar sensor is shown. In this example, the antenna device is constructed in microstrip line technology on a substrate (not shown) and includes a plurality of antenna elements 10, 12, which are arranged alternately on opposite sides of a linearly extending connection line 14 at least in the central region of the antenna device. The antenna elements 10, 12 are fed microwave energy serially via the connection line 14.

[0019] In the example shown, the left end of the connection line 14 in Figure 1 is connected to a signal source (not shown). The signal starting from this signal source is reflected at the right end of the connection line in Figure 1 , thereby generating a standing wave 16 in this connection line, which is shown as a dashed line in this figure. The wavelength λ of this wave is determined by the frequency of the radar signal to be transmitted. In practice, the frequency of the radar signal is mostly modulated within a certain frequency band. The antenna device is designed according to the frequency corresponding to the center of this frequency band. The distance between an antenna element 10 on one side of the connection line 14 and the next antenna element 12 on the opposite side of this connection line respectively corresponds to half of the wavelength λ.

[0020] The antenna elements 10, 12 are respectively at the antinode positions of the standing wave 16. Due to the distance being λ / 2, the signal at the position of the antenna element 10 on this side is out of phase with the signal at the position of the antenna element 12 on the opposite side. However, since the antenna elements 10 and 12 extend from the connection line 14 in opposite directions, the oscillating dipole moments are in phase and accordingly the emitted radar waves are also in phase. Therefore, the coefficients describing the amplitude distribution of the antenna elements 10, 12 have the same sign. For example, all the amplitude distributions of the antenna elements 10 and 12 are positive. As can be seen in this figure, the patterns of the antenna elements 10, 12 roughly map the elongation of the standing wave 16.

[0021] Every two antenna elements in the antenna element 10 and the antenna element 12 located between these two antenna elements form a triple group. In this triple group, the external antenna element 10 extends from the connection line in one direction and the middle antenna element 12 extends from the connection line in the opposite direction. The same also applies to the triple group formed by two antenna elements 12 and an antenna element 10 located between these two antenna elements. In Figure 1 the shown antenna device has a plurality of such triple groups in its middle region.

[0022] However, according to the present invention, the pattern is broken in the end region of the antenna device. There are antenna elements 18 there, which are on the same side of the connection line 14 as the antenna element 10, but the distances of these antenna elements from the antenna elements 12 on the opposite side are each an integer multiple of the wavelength λ. There are also antenna elements 20 on the same side as the antenna elements 12, but the distances of these antenna elements from the antenna elements 10 on the opposite side are also integer multiples of λ. For the antenna elements 18, 20, the amplitude distribution is negative, which can be seen in this figure: the directions in which the antenna elements 18, 20 face away from the connection line 14 are opposite to the stretching direction of the standing wave 16. Due to this discontinuous positive amplitude distribution, a more rectangular-shaped directivity is achieved.

[0023] In the example shown, the antenna elements 18, 20 with negative amplitude distribution are at opposite ends of the antenna device, and their distance from each other is λ / 2.

[0024] As can be further seen in Figure 1 , the antenna elements 10, 12, 18, 20 have different lengths and widths. By this variation in length and width, the magnitude of the amplitude is modified in a known manner such that the side lobes in the directivity are largely suppressed.

[0025] The above antenna device can be used not only in a transmitting antenna but also in a monostatic antenna design, in which the antenna elements 10, 12, 18, 20 not only transmit but also receive, and can also be used in a pure receiving antenna.

[0026] In Figure 2 , examples of the directivities of different antenna devices are shown. In Figure 2 , the curve 22 drawn in solid lines illustrates the directivity of the antenna device shown in Figure 1 . Here, the directivity is given by the antenna gain (in dB) as a function of the angle θ at which radar radiation is transmitted or received, where the 0° direction is perpendicular to the connection line 14, perpendicular to Figure 1the figure plane. For comparison, the curve 24 drawn in dashed lines illustrates the directional characteristics of a conventional antenna device in which the pattern of "antenna elements 10 and 12 alternatingly located on opposite sides" continues over the entire length of the antenna device. It can be seen that the curve 24 of the conventional antenna device has a distinct maximum at 0°, and already drops significantly to both sides at relatively small angles. In contrast, in the curve 22 of the antenna device according to the invention, the maximum is flatter. The antenna gain at 0° is slightly smaller than the curve 24, yet is approximately constant within an angular range up to about + / - 10°. In contrast, at larger angles the curve 22 drops relatively steeply, while the curve 24 attenuates significantly more slowly here. At angles near + / - 30°, the side lobes are suppressed approximately equally well in both curves.

[0027] The curve 22 of the antenna device according to the invention is significantly closer to the desired rectangular shape than the curve 24 of the conventional antenna device.

[0028] In Figure 3 the angular dependence (curve 26) of the operating range R of a radar sensor having an antenna device according to the invention and the angular dependence of the operating range of a conventional radar sensor (curve 28) are ideally shown. The radar sensor according to the invention has an approximately constant operating range in the core region II, while the operating range suddenly decreases when transitioning to the edge regions I and III. In practice, the width of the core region II is selected such that the core region covers the angular range relevant for vehicle guidance. Since almost no radar echoes are received in the edge regions I and III, interference signals from these edge regions are largely masked, thereby simplifying the analysis, processing, and interpretation of radar signals. In the course of an increasing number of vehicles being equipped with radar sensors, as another advantage, it results in fewer interference signals for the radar sensors of oncoming vehicles.

[0029] In contrast, in a conventional radar sensor (curve 28), the operating range in the edge regions I and III is still relatively large, so that the number of interference signals is correspondingly larger.

[0030] In the antenna device according to the invention, the edge regions I and III are not illuminated during transmission operation, and the energy thus released is used to achieve a higher and constant sensitivity in the core region II. In the 0° direction, the operating range of the radar sensor according to the invention is slightly smaller than that of the conventional sensor, yet this slight reduction in operating range, which is limited to a very narrow angular range, can be accepted without problems.

[0031] In Figure 4Illustrates another advantage of the antenna device according to the invention and the resulting directional characteristics. Shown here is an installation situation in which the radar sensor 30 is mounted in a motor vehicle such that the radar sensor is located between two other components 32 of the motor vehicle. These other components 32 form a reflecting surface that reflects a portion of the arriving radar echoes and deflects them onto the radar sensor 30 at a relatively large angle. The radar signals 34 reflected in this way form unwanted interference signals. However, in the radar sensor according to the invention, these reflected signals 34 are located in the edge regions I and III, in which the sensitivity is zero or very small, so that the interference signals are automatically suppressed. In this way, in particular when multiple radar sensors are installed in a motor vehicle, a greater degree of freedom in the structural design for positioning the radar sensors is achieved.

Claims

1. A radar sensor for a motor vehicle, the radar sensor having an antenna device which has a plurality of antenna elements (10, 12) that are linear and arranged equidistantly along a connection line (14), wherein, The distance between every two adjacent antenna elements (10, 12) is equal to half of the average wavelength λ of the transmitted radar signal, and the device has at least one triple group composed of adjacent antenna elements. In the triple group, on the one hand, the two antenna elements (10) located on the outer sides in the triple group and, on the other hand, the antenna element (12) located between the two antenna elements extend in opposite directions from the connection line (14). It is characterized in that the antenna device has at least one pair of antenna elements (12, 16; 10, 20) that extend in opposite directions from the connection line (14) and the distance between them is an integer multiple of the wavelength λ, so that one of these antenna elements (18, 20) has a negative amplitude distribution relative to the antenna elements (10, 12) of the triple group.

2. The radar sensor according to claim 1, in which the antenna elements (10, 12) extend in opposite directions from the connection line alternately in the middle section of the connection line (14), and at least one antenna element (18, 20) having a negative amplitude distribution is outside the middle section.

3. The radar sensor according to claim 1 or 2, in which the antenna device has at least two antenna elements (18, 20) having a negative amplitude distribution, and these antenna elements extend in opposite directions from the connection line (14).

4. The radar sensor according to claim 3, in which the two antenna elements (18, 20) having a negative amplitude distribution are at a distance of λ / 2 from each other.

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