Abstract:
A radar sensor for motor vehicles, including at least one transmitter and receiver device for transmitting and receiving a frequency-modulated radar signal, an analyzer unit for computing the distances and relative velocities of the located objects, and an integrated Doppler radar system for independent measurement of the relative velocities.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a radar sensor for motor vehicles, including at least one transmitter and receiver device for transmitting and receiving a frequency-modulated radar signal and an analyzer unit for computing the distances and relative velocities of the located objects. 
       BACKGROUND INFORMATION 
       [0002]    Such radar sensors are frequently used in motor vehicles in a driver assistance system, such as an ACC system (adaptive cruise control), for automatic radar-assisted distance control. 
         [0003]    A typical example of a radar sensor of the type mentioned above is an FMCW radar (frequency modulated continuous wave), where the frequency of the transmitted radar signal is periodically modulated with a specific ramp slope. The frequency of a signal that has been reflected by a radar target and is then received by the radar antenna at a certain point in time therefore differs from the frequency of the signal that is transmitted at this point in time by an amount which is dependent, on the one hand, on the signal propagation time and, thus, on the distance of the radar target and, on the other hand, on the Doppler shift and, thus, on the relative velocity of the radar target. In the radar sensor, the received signal is mixed with the signal transmitted at this point in time. The mixed product so obtained is a low-frequency signal, whose frequency corresponds to the difference in frequency between the transmitted and the received signal. This low-frequency signal is then digitized in the analog-to-digital converter with a suitable time resolution. The digitized data is recorded during a certain recording period, which corresponds, for example, to the length of the ramp with which the transmitted signal is modulated. The data set so obtained is then transformed into a spectrum using an algorithm known as the “fast Fourier transform” (FFT). In this spectrum, each detected radar target is represented by a peak, which stands out, more or less distinctly, from the background noise level. By repeating this procedure using different ramp slopes, it is possible to eliminate the ambiguity between the propagation time-dependent frequency shift and the Doppler shift, thus allowing computation of the distance and relative velocity of the radar target. 
         [0004]    In motor vehicles, it is usual to use an angular-resolution radar sensor, which generates a plurality of radar lobes that are slightly angularly offset from each other, and the above-described signal processing and analysis is then performed separately for each individual radar lobe, preferably in parallel channels. 
         [0005]    For traffic safety reasons, the radar sensor should allow other vehicles and obstacles to be located as reliably as possible. Furthermore, efforts are being made to enhance the functionality of driver assistance systems with the long-term objective being to provide fully autonomous vehicle control. As new and increasingly more complex functions are added to the driver assistance system, the level of reliability required of the radar sensor increases correspondingly. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention has the advantage of increasing the reliability of the radar sensor. To this end, in accordance with the present invention, the radar sensor has an integrated Doppler radar, which allows the relative velocities of the located objects to be measured independently. In this manner, the redundancy of the system is increased, and, by matching the relative velocities measured by the Doppler radar to the relative velocities computed by the analyzer unit based on the frequency-modulated signal, any errors in the transmitter and receiver device and/or in the analyzer unit can be quickly detected, so that suitable countermeasures can be initiated. In addition, the present invention makes it easier to eliminate ambiguities, especially when locating several objects simultaneously. During analysis of the spectra obtained using the frequency-modulated signal, misinterpretations, which can easily occur, especially in the case of very noisy signals, can therefore be quickly and reliably detected and corrected. 
         [0007]    A particularly simple and inexpensive design of the redundant radar sensor can be achieved by using essentially the same components for the Doppler radar system as those already present in the frequency-modulated radar system, for example, an FMCW radar. 
         [0008]    In order to generate the radar signal for the Doppler radar, preferably, a reference oscillator is used, which, at the same time, is used to control the frequency during the generation of the frequency-modulated signal. 
         [0009]    In a particularly preferred embodiment, the reference oscillator is formed by a dielectric resonator (DRO) operating at a frequency whose integral multiple is near the operating frequency band of the oscillator used to generate the frequency-modulated signal. For example, if the operating frequency band is from about 76 to 77 GHz, then the reference oscillator has a frequency of, for example, 12.65 GHz, 19 GHz or 24.5 GHz, which is equivalent to one-sixth, one-fourth or one-third of the mid-frequency of the operating frequency band, respectively. For frequency control purposes, the harmonic of the reference oscillator near the operating frequency band is fed to a harmonic mixer and mixed with the frequency-modulated signal. The mixed product is then equivalent to the difference between the modulated frequency and the fixed reference frequency (of the harmonic), and is used as a feedback signal for frequency control, for example, in a phase locked loop (PLL). 
         [0010]    The fundamental frequency of the reference oscillator is used directly as the transmitting frequency for the operation of the Doppler radar. In this embodiment, therefore, there is no need to provide a special oscillator for the Doppler radar. Another advantage is that the frequency of the Doppler radar is only a fraction of the frequency of the FMCW radar, so that interference, such as noise signals, rain or snow, and the like, have different effects on the two radar systems and, therefore, interferences in one system can be detected and, if necessary, compensated for by the other system. 
         [0011]    In a typical design of an angular-resolution radar sensor, the antenna has a plurality of antenna elements (patches) disposed in the focal plane of a lens in laterally offset relation to each other, so that the radar lobes generated by the individual patches and converged by the lens are angularly offset from each other. Preferably, the same lens is used for the Doppler radar, an additional patch being disposed in the focal plane, or slightly offset therefrom, said additional patch being connected to the reference oscillator and matched to the frequency thereof. Due to stronger diffraction effects at the lower frequency of the reference oscillator, the radar lobe generated by the additional patch is less strongly converged, so that an additional angular range can be covered by the one additional patch. Additional beam expansion can be achieved, if desired, by disposing this patch slightly out of focus. 
         [0012]    The radar sensor repeats the radar measurements periodically, typically with a period on the order of 100 ms. However, the plurality of frequency ramps with which the signal of the FMCW radar is modulated altogether make up only a fraction of this period, for example about 15 ms. During the remaining time, which is needed, for example, for signal analysis, no frequency control is required, so that the reference oscillator can be used as a signal source for the Doppler radar during this time period. 
         [0013]    For signal analysis purposes, it is also possible to use essentially components that are already present. Usually, each antenna patch used for generating the angularly offset, frequency-modulated radar beams has a separate preamplifier associated therewith, which amplifies the low-frequency signal (intermediate frequency signal) of the corresponding mixer. The additional patch provided for the Doppler radar has a separate mixer associated therewith. However, to amplify the intermediate frequency signal produced by this mixer, one of the other preamplifiers can be used during the operation of the Doppler radar. Similarly, the already present hardware can be used to transform the intermediate frequency signal of the Doppler radar into a spectrum by fast Fourier transform. In this process, it is only necessary to adapt the parameters of the transformation algorithm with respect to the smaller frequency of the Doppler radar. However, in order to add redundancy to the system, it is also possible to use a separate processor to compute the spectrum for the Doppler radar. 
         [0014]    The downstream analysis software simply needs to be enhanced with a module which computes the relative velocities of the located objects from the spectrum of the Doppler radar and compares them to the relative velocities determined by the FMCW radar. When the radar sensor operates without error, the independently determined relative velocities must be consistently correlatable with each other. If this is not possible, then an error exists in the system. In the simplest case, the system is then shut down or restarted, and a warning is issued to the driver. If the error cannot be corrected by a restart, the system is completely shut down, and the driver is suitably prompted to go to a garage. 
         [0015]    However, in a further embodiment of the present invention, the relative velocities independently determined by the Doppler radar can also used to automatically correct errors of the FMCW radar and/or to eliminate ambiguities in the results of the FMCW radar, which would otherwise not be able to be removed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]      FIG. 1  is a block diagram of a radar sensor according to the present invention. 
           [0017]      FIG. 2  is a frequency/time diagram for illustrating the operation of the radar sensor of  FIG. 1 . 
           [0018]      FIG. 3  is a distance/velocity diagram for illustrating a method for analyzing the measurement results. 
       
    
    
     DETAILED DESCRIPTION  
       [0019]    The radar sensor shown in  FIG. 1  includes an oscillator driver  10  which, using a voltage signal, controls the oscillation frequency of a controllable oscillator  12 . The frequency of oscillator  12  so controlled is in an operating frequency band from about 76 to 77 GHz. The output signal of oscillator  12  is supplied to a plurality, in the example shown four, of mixers  14 , which are each connected to an antenna patch  16 . Antenna patches  16 , to which the signal of oscillator  12  is supplied via mixers  14 , are disposed in the focal plane of a lens  18  in laterally offset relation to each other, so that the radar radiation emitted from the patches is converged into four beams that are slightly angularly offset from each other. When one of these beams hits a radar target, then the reflected signal is focused through lens  18  back onto the antenna patch  16  from which the beam was emitted. The received signal then returns to mixer  14 , where it is mixed with the signal that is supplied to the mixer from oscillator  12  at this point in time. The mixed product so obtained is an intermediate frequency signal whose frequency (on the order of about 100 kHz) corresponds to the difference in frequency between the received signal and the signal of oscillator  12 . The intermediate frequency signals of the four mixers  14  are amplified in a four-channel preamplifier  20 , digitized in an analog-to-digital converter  22 , and then transformed into spectra in a first processor  24  by fast Fourier transform (FFT). 
         [0020]    The frequency of oscillator  12  is modulated in a ramped form by means of an oscillator driver  10 , and controlled in a closed loop during this process. For frequency control purposes, a reference oscillator  26  is used, for example a dielectric resonator (DRO), whose frequency is, for example, one-third of the mid-frequency of the operating frequency band of oscillator  12 , which, in the example under discussion, is therefore about 24.5 GHz. The third harmonic of the frequency of the reference oscillator  26  is fed to a harmonic mixer  28 , where it is mixed with the signal of oscillator  12 . The mixed product, which thus indicates the difference between the instantaneous frequency of oscillator  12  and the fixed frequency of reference oscillator  26 , is fed back via a phase locked loop (PLL)  30  to oscillator driver  10 , and thus serves as a feedback signal for frequency control. 
         [0021]    In  FIG. 2 , the graph  32  drawn with bold solid lines indicates the frequency f of oscillator  12  as a function of time t. A complete measuring cycle of the radar sensor has the period T. At the start of this measuring cycle, oscillator  12  is active and its frequency is modulated, for example, with a rising ramp  34 , which is followed by a falling ramp  36 , whose slope can be of the same magnitude as ramp  34 . Then, a further rising ramp  38  follows, whose slope is, for example, only half the slope of ramp  34 . After that, oscillator  12  is inactive for the rest of the measuring cycle, so that reference oscillator  26  is no longer needed for frequency control. Using a switch  40  (such as a PIN diode switch or a MEM switch), reference oscillator  26  is then connected to a further mixer  42 , via which the fundamental frequency of the reference oscillator is transmitted to an additional antenna patch  44  disposed on the optical axis of lens  18 . Antenna patch  44  is larger than antenna patches  16  because it transmits a radar signal of greater wavelength, according to the fundamental frequency of reference oscillator  26 . As symbolically indicated in  FIG. 1 , antenna patch  44  may be disposed at a position slightly before the focal plane of lens  18 , so that the radar beam generated by this patch diverges more strongly. This radar beam, whose frequency is not modulated, allows the relative velocities of the objects located by it to be measured according to the principle of a Doppler radar. 
         [0022]    Here too, the radar echo is focused through lens  18  back onto antenna patch  44 , and the received signal is mixed in mixer  42  with the signal of reference oscillator  26 . The mixed product is supplied to one of the four channels of preamplifier  20 , preferably to a channel belonging to an antenna patch  16  whose radar lobe deviates only slightly from the optical axis of lens  18 . The preamplified intermediate frequency signal of mixer  42  is then digitized and transformed into a spectrum in the same manner as was done before with the signals of mixers  14 . 
         [0023]    In  FIG. 2 , the graph  46  drawn with dashed lines shows the frequency of the signal transmitted by antenna patch  44  as a function of time. It can be seen that the signals of antenna patches  16 , one the one hand (graph  32 ), and of antenna patch  44 , on the other hand, are offset in time. Therefore, when the intermediate frequency signal of mixer  42  is to be amplified and analyzed, preamplifier  20 , analog-to-digital converter  22 , and first processor  24  are not busy with analyzing the signals from antenna patches  16 . 
         [0024]    Therefore, the radar sensor described integrates the functions of an angular-resolution FMCW radar (antenna patches  16 ) and of a Doppler radar, which does not provide angular resolution (antenna patch  44 ). In the example shown, the spectra computed by processor  24  for both sub-systems are further analyzed in a second processor  48 . In each measuring cycle, three spectra, which are recorded during the three ramps  34 ,  36  and  38 , are obtained in each of the four channels of the FMCW radar. Each radar target detected in the particular channel appears in this spectrum in the form of a peak at a frequency which is dependent on both the distance and the relative velocity of the radar target. A module  50  of processor  48  computes therefrom the distances d i  and relative velocities v i  of the located radar targets, as will be explained in greater detail hereinafter. 
         [0025]    Moreover, since generally each radar target is detected by several of the four radar beams, it is also possible to compute the azimuth angle φ i  of the objects by comparing the amplitude and/or phase relation between the different channels in module  50 . 
         [0026]    When, after closing switch  40 , the Doppler radar is active and the corresponding spectrum has been computed in processor  24 , this spectrum is analyzed in another module  52  of second processor  48 . This is symbolized in  FIG. 1  by a switch  54  coupled to switch  40 , although in practice, module  52  will be a software module which is only invoked when the computation of the spectrum is complete. In the spectrum recorded by the Doppler radar too, each of the located objects appears as a peak at a characteristic frequency, and an independent value v i ′ for the relative velocity of the object can be computed from this frequency. 
         [0027]    Assuming that the Doppler radar detects all objects detected by the four radar beams of the FMCW radar together, there must be a substantially identical value v i ′ for each value v i  computed by module  50 . This is checked in second processor  48 , as symbolized by a comparator module  56  in  FIG. 1 . 
         [0028]    A failure of the independently determined relative velocities to match suggests a malfunction of the radar sensor. Such a malfunction can be a transient failure, which may be that one of the objects detected by the angular-resolution FMCW radar is located outside the detection range of the Doppler radar, or vice versa. Such errors can be ignored if they occur only sporadically. However, an increase in cases where the Doppler radar locates more objects than the FMCW radar suggests partial blindness of the FMCW radar, and a warning should be issued to the driver. Similarly, a breakdown or malfunction of one of mixers  14  may also be detected. 
         [0029]    Since the data of the FMCW radar and of the Doppler radar are digitized in analog-to-digital converter  22 , sporadically occurring digitization errors due to interference signals or the like will also manifest themselves in comparator module  56 . Since the algorithm for the fast Fourier transform in the Doppler radar system works with other parameters than in the FMCW radar system, any errors in the computation of the spectra generally will generally also become apparent. 
         [0030]    Finally, errors may also occur in the computation of the distances and relative velocities in module  50 , especially when the signal quality is poor. Such errors can occur especially when the peaks present in the different spectra are not correctly associated with the real objects. This causes errors in the computed distances and azimuth angles as well as in the computed relative velocities. Such errors can therefore also be detected in comparator module  56  and immediately corrected if necessary. 
         [0031]    This is explained in greater detail below with reference to  FIG. 3 . For the sake of simplicity, only one of the four channels of the FMCW radar is discussed and, furthermore, it is assumed that exactly two radar targets are being located in this channel. Therefore, the three spectra recorded for the three ramps  34 ,  36 ,  38  each contain two peaks at different frequencies. However, it is not clear from the outset, which peak belongs to which object. 
         [0032]    The mid-frequency of each peak, however, defines a relationship between distance d and relative velocity v of the object in question. In the diagram of  FIG. 3 , this relationship can be represented by a straight line. For the spectra recorded during rising ramp  34 , falling straight lines  34 A and  34 B are obtained, respectively, since the distance- and frequency-dependent components of the frequency shift add together. Therefore, the higher the relative velocity, the smaller must be the distance. For falling ramp  36 , rising straight lines  36 A,  36 B are obtained accordingly. These four straight lines intersect in four points, and the pair of values (v, d) belonging to each of these four points is a candidate for a real object. However, since only two real objects are present, the ambiguity is only eliminated when adding two additional straight lines  38 A,  38 B, which result from ramp  38 . These are falling straight lines again, but they are steeper because the slope of ramp  38  is smaller. Ideally, three straight lines  34 A,  36 A,  38 A and  34 B,  36 B,  38 B, respectively, intersect in one point, which then indicates the distance and relative velocity of a real object. In this manner, relative velocities v 1  and v 2  are obtained for the two objects with the aid of module  50 . 
         [0033]    If the system operates properly, the same relative velocities v 1  and v 2  must be obtained by module  52 , as is symbolized in  FIG. 3  by dashed vertical lines. 
         [0034]    In reality, because of measuring errors, the three straight lines, for example,  34 A,  36 A and  38 A, belonging to the same object often do not meet exactly in one point. Therefore, in some circumstances, it may be difficult to decide which point should be taken as the intersection point of the straight lines. Using the additional information obtained with the aid of the Doppler radar and module  52  makes this decision much easier.