Abstract:
A radar apparatus includes a first processing unit, a second processing unit, and a speed determining unit. Of these units, the first processing unit determines at least speed including ambiguity caused by phase folding back within a predetermined speed measurement range, from phase rotation of frequency components detected in time-series for a same target, using a beat signal obtained by transmitting and receiving, a plurality of times, a predetermined first modulation wave. The second processing unit determines at least speed that is uniquely determined within the speed measurement range, from Doppler frequency included in frequency components indicating a target, using a beat signal obtained by transmitting and receiving a predetermined second modulation wave. The speed determining unit determines a measurement result for speed expressed by a resolution obtained by the first processing unit by comparing a calculation result of the first processing unit and a calculation result of the second processing unit.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a radar apparatus that uses a chirp wave and a signal processing method of the radar apparatus. 
       BACKGROUND ART 
       [0002]    Radar apparatuses are currently used in various fields. A fast chirp modulation (FCM) -type radar apparatus is known as one such radar apparatus. This radar apparatus uses a chirp wave of which the frequency continuously increases or decreases, as a radar wave. Furthermore, this radar apparatus measures distance and speed by performing a two-dimensional fast Fourier transformation (FFT) with respect to a beat signal generated from transmission and reception signals thereof. In the FCM scheme, a distance to a target is determined from the frequency of the beat signal generated from the transmission and reception signals. In addition, in the FCM scheme, a relative speed to the target (also referred to, hereafter, as simply speed) is determined from phase rotation of a frequency component continuously detected regarding the same target. However, the speed determined from phase rotation includes ambiguity caused by phase folding back in which when a detected phase is θ, the actual phase may be θ+2π·n (n being an integer) and therefore cannot be identified. 
         [0003]    Regarding the foregoing, a technology is proposed in which the ambiguity in the detected speed (that is, speed ambiguity: synonymous with the above-described ambiguity caused by phase folding back) is reduced by sampling of the beat signal being performed at a high speed (refer to PTL 1). That is, to reduce the ambiguity in the speed, a cycle period of the chirp wave is required to be shortened. To do so, the duration of each individual chirp is shortened, while increasing the slope (rate of change) of the frequency in the chirp wave and ensuring a variation width of the frequency. Furthermore, to ensure the number of samplings required for a frequency analyzing process during the shortened period, a higher speed for sampling is required. 
       CITATION LIST 
     Patent Literature 
       [0004]    [PTL  1 ] U.S. Pat. No.  8 , 436 , 763   
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    However, to perform sampling at a high speed, an analog-to-digital (AD) converter that is higher in component cost is required to be used. In addition, it is difficult to provide such high-speed AD converters in existing on-board radar apparatuses and the like in which cost reduction is demanded. 
         [0006]    The present invention has been achieved in light of the above-described issues. An object of the present invention is to provide a technology for resolving ambiguity in detected speed without increasing the sampling speed in a radar apparatus that uses a chirp wave. 
       Solution to Problem 
       [0007]    A radar apparatus of the present invention includes a first processing unit, a second processing unit, and a speed determining unit. The first processing unit uses a beat signal obtained by transmitting and receiving, a plurality of times, a predetermined first modulation wave. Furthermore, the first processing unit determines at least speed including ambiguity caused by phase folding back within a predetermined speed measurement range, from phase rotation of frequency components detected in time-series for a same target. The second processing unit uses a beat signal obtained by transmitting and receiving a predetermined second modulation wave. Furthermore, the second processing unit determines at least speed that is uniquely determined within the speed measurement range, from Doppler frequency included in frequency components indicating a target. The speed determining unit determines a measurement result for speed expressed by a resolution obtained by the first processing unit by comparing a calculation result of the first processing unit and a calculation result of the second processing unit. 
         [0008]    As a result of a configuration such as that described above, the present invention can acquire a high-resolution measurement result for speed, without increasing the sampling speed of the beat signal. 
         [0009]    Reference numbers within the parentheses recited the scope of claims indicate corresponding relationships with specific means according to an embodiment described hereafter as an aspect, and do not limit the technical scope of the present invention. 
         [0010]    In addition, the present invention can be obtained through various aspects in addition to the above-described radar apparatus, such as a system of which the radar apparatus is a constituent element, a program for enabling a computer to function as the radar apparatus, and a speed measurement method. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    In the accompanying drawings: 
           [0012]      FIG. 1  is a block diagram of an overall configuration of an on-board system; 
           [0013]      FIG. 2  is an explanatory diagram related to a modulation scheme for radar waves to be used; 
           [0014]      FIG. 3  is a flowchart of a target detection process performed by a processing unit; 
           [0015]      FIG. 4  is an explanatory diagram of an overview of two-dimensional fast Fourier transformation (FFT); 
           [0016]      FIG. 5  is a flowchart of a distance and speed determining process; and 
           [0017]      FIG. 6  is an explanatory diagram of a determining method for speed. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    An embodiment to which the present invention is applied will hereinafter be described with reference to the drawings. 
       Overall Configuration 
       [0019]      FIG. 1  shows an overall configuration of the entirety of an on-board system  1  to which a radar apparatus is applied, according to the embodiment. As shown in  FIG. 1 , the on-board system  1  includes a radar apparatus  10  and a driving assistance electronic control unit (ECU)  100 . The on-board system  1  is mounted in a vehicle, such as a four-wheel automobile. A signal processing method of the present invention is performed by the radar apparatus. Therefore, the signal processing method will be described together with a description of the functions of the radar apparatus. 
         [0020]    The radar apparatus  10  emits radar waves and receives reflected waves. In addition, the radar apparatus  10  observes a distance R to a target that is an object ahead that has reflected the radar waves, a speed V of the target, and an orientation θ of the target, based on a reception signal thereof. Furthermore, the radar apparatus  10  inputs observation values (Rz, Vz, and θz) to the driving assistance ECU  100 . 
         [0021]    The driving assistance ECU  100  performs various processes for assisting in driving of the vehicle by a driver, based on the observation values (Rz, Vz, and θz) of each target inputted from the radar apparatus  10 . As processes related to driving assistance, for example, there is a process for displaying a warning to the driver that an approaching object is present, and a process for performing vehicle control to avoid collision with an approaching object through control of a brake system, a steering system, or the like. 
         [0022]    The radar apparatus  10  includes a transmission circuit  20 , a distributor  30 , a transmission antenna  40 , a reception antenna  50 , a reception circuit  60 , a processing unit  70 , and an output unit  80 . 
         [0023]    The transmission circuit  20  is a circuit for supplying a transmission signal Ss to the transmission antenna  40 . The transmission circuit  20  inputs a millimeter-wave-band high-frequency signal to the distributor  30  that is positioned upstream of the transmission antenna  40 . Specifically, as shown in  FIG. 2 , the transmission circuit  20  alternately repeats a first modulation period in which a high-frequency signal (chirp) that is frequency-modulated such that the frequency changes in the shape of a sawtooth-waveform is generated a plurality of times, and a second modulation period in which a high-frequency signal (frequency-modulated continuous wave [FMCW]) that is frequency-modulated such that the frequency increases and decreases in the shape of a triangular waveform is generated. Furthermore, the transmission circuit  20  inputs the generated high-frequency signals to the distributor  30 . 
         [0024]    That is, the radar apparatus  10  operates as an FMC-type radar that transmits and receives a chirp wave (first modulated wave) during the first modulation period. In addition, the radar apparatus  10  operates as an FMCW radar that transmits and receives the FMCW (second modulated wave) during the second modulation period. A rate of change (slope of the graph in  FIG. 2 ) of the frequency of the high frequency signal generated during the first modulation period is set to be greater than that of the frequency of the high frequency signal generated during the second modulation period. In particular, during the first modulation period, the rate of change of the frequency is set such that the frequency of a beat signal generated based on reflected waves from a target that is separated by a predetermined distance or more is a value that is sufficiently large enough that a Doppler frequency corresponding to a detection upper limit value for relative speed can be ignored. In addition, during the second modulation period, the rate of change of the frequency is set such that the speed can be uniquely identified over the overall detection range of relative speed. 
         [0025]    As shown in  FIG. 1 , the distributor  30  performs power distribution of the high-frequency signal inputted from the transmission circuit  20  to a transmission signal Ss and a local signal L. 
         [0026]    The transmission antenna  40  emits a radar wave ahead of the vehicle based on the transmission signal Ss supplied from the distributor  30 . The radar wave has a frequency that corresponds to the transmission signal Ss. 
         [0027]    The reception antenna  50  is an antenna for receiving a radar wave (reflected wave) reflected by a target. The reception antenna  50  is configured as a linear array antenna in which a plurality of antenna elements  51  are arrayed in a single row. A reception signal Sr of the reflected wave from each antenna element  51  is inputted to the reception circuit  60 . 
         [0028]    The reception circuit  60  processes the reception signal Sr inputted from each antenna element  51  configuring the reception antenna  50 . Furthermore, the reception circuit  60  generates and outputs a beat signal BT for each antenna element  51 . Specifically, for each reception element  51 , the reception circuit  60  mixes the reception signal Sr inputted from the antenna element  51  and the local signal L inputted from the distributor  30  using a mixer  61 , and thereby generates and outputs the beat signal BT for each antenna element  51 . 
         [0029]    However, the process until the beat signal BT is outputted includes a process for amplifying the reception signal Sr, a process for removing unnecessary signal components from the beat signal BT, and a process for converting the beat signal BT to digital data. In this way, the reception circuit  60  converts the generated beat signal BT for each antenna element  51  to digital data and outputs the digital data. The outputted beat signal BT for each antenna element  51  is inputted to the processing unit  70 . Hereafter, A/D conversion data of the beat signal BT acquired during the first modulation period is referred to as first modulation data, and A/D conversion data of the beat signal BT acquired during the second modulation period is referred to as second modulation data. 
         [0030]    The processing unit  70  is composed of a known microcomputer that includes a central processing unit (CPU)  70 A that functionally performs various means related to the present invention, a read-only memory (ROM)  70 B that serves as a recording medium, a random access memory (RAM)  70 C, a coprocessor  70 D that performs a fast Fourier transformation (FFT) process and the like, and a clock  70 E. The ROM  70 B stores therein, in advance, codes expressing various programs (such as a target detection process) described hereafter. Therefore, the CPU  70 A is configured to be capable of calling up a program stored in the ROM  70 B to a work area thereof and running the program. In addition, the RAM  70 C is used to temporarily store data when the CPU  70 A is performing a process. The coprocessor  70 D handles fast Fourier transformation as a dedicated process, as described hereafter. The clock  70 E generates a clock signal for operation. 
         [0031]    In this way, the processing unit  70  analyzes the beat signal BT for each antenna element  51 , and thereby performs the various processes that are provided, including the target detection process for calculating the observation values (Rz, Vz, and θz) for each target that reflects the radar waves. 
       Target Detection Process 
       [0032]    The target detection process performed by the processing unit  70  (that is, the CPU  70 A) will be described with reference to a flowchart in  FIG. 3 . 
         [0033]    The present process is repeatedly performed when the on-board system  1  is started. When the present process is started, at S 110 , the processing unit  70  determines whether or not acquisition of the second modulation data (that is, the beat signals of the FMCW) is completed. When acquisition of the second modulation data is not completed, the processing unit  70  waits by repeating the same step. In addition, when acquisition of the second modulation data is completed, the processing unit  70  proceeds to S 120 . 
         [0034]    At S 120 , the processing unit  70  performs FFT processing (frequency analysis processing) of the second modulation data and generates a power spectrum, for each antenna element  51  and for each up-chirp and down-chirp. The power spectrum indicates the power of the reflected wave in each frequency bin. 
         [0035]    At S 130 , the processing unit  70  calculates an average power spectrum that is the average of the power spectrums for each up-/down-chirp. Furthermore, the processing unit  70  extracts a peak (identified by the frequency bin) at which the power is equal to or greater than a predetermined peak detection threshold, from the average power spectrum. 
         [0036]    At S 140 , the processing unit  70  performs a pair-matching process to combine peaks, of the peaks in the up-chirp and the peaks in the down-chirp, based on reflected waves from the same target. A specific method for performing the pair-matching process varies and is a known technique. Therefore, a description thereof is omitted. 
         [0037]    At S 150 , the processing unit  70  calculates the distance and the relative speed using a method known in FMCW radars, based on the frequencies of the two peaks combined in the pair-matching process. 
         [0038]    At S 160 , the processing unit  70  determines whether or not acquisition of the first modulation data is completed. When acquisition of the first modulation data is not completed, the processing unit  70  waits by repeating the same step. In addition, when acquisition of the second modulation data is completed, the processing unit  70  proceeds to S 170 . 
         [0039]    At S 170 , the processing unit  70  performs a two-dimensional FFT on the first modulation data. Specifically, as shown in  FIG. 4 , first, the processing unit performs a first FFT process (frequency analysis process) and generates a power spectrum for each chirp. Next, the processing unit  70  collects the processing results of all chirps for each frequency bin, and performs a second FFT process. The frequencies of the beat signals (the components that form peaks in a power spectrum) detected in each chirp by the reflected waves from the same target are all the same. However, when the target and the own vehicle have relative speed, the phase of the beat signal slightly differs with each chirp. That is, from the result of the second FFT process, a power spectrum in which the frequency component based on the above-described rotation speed of phase is the frequency BIN (speed bin) is determined for each frequency BIN (distance BIN) acquired as the result of the first FFT process. Hereafter, the power spectrum is referred to as a two-dimensional power spectrum. 
         [0040]    At S 180 , the processing unit  70  extracts a peak from the two-dimensional power spectrum. At subsequent S 190 , the processing unit calculates the distance and speed from the distance BIN and speed BIN of the extracted peak. 
         [0041]    At S 200 , the processing unit  70  compares the distances and speeds based on the same target determined at S 150  and S 190 , and thereby performs a process to determine the distance and the speed. At subsequent S 210 , the processing unit  70  inputs, to the driving assistance ECU  100  via the output unit  80 , a combination of the process result (Rz, Vz) and an orientation θz of the target separately determined using either of the first modulation data and the second modulation data as the measurement result (Rz, Vz, θz). As a result, the processing unit  70  ends the present process. 
       Distance and Speed Determining Process 
       [0042]    The distance and speed determining process performed by the processing unit  70  at S 200  will be described with reference to  FIG. 5 . 
         [0043]    In the present process, at S 310 , the processing unit  70  compares the calculation results for distance and speed based on the second modulation data, and the calculation results for distance and speed based on the first modulation data. Furthermore, in the present process, the processing unit  70  combines the calculation results that are in close distance and extracts the combinations. 
         [0044]    At S 320 , for each extraction result at S 310 , from the calculation result for speed based on the first modulation data, that is, speed including ambiguity caused by phase folding back, the processing unit  70  determines a plurality of speeds within a speed detection range corresponding thereto. Furthermore, from the plurality of speeds, the processing unit  70  selects the speed closest to the calculation result for speed based on the second modulation data as a true value. As a result, the processing unit  70  uses the value as the speed of an object present at the distance of interest and ends the present process. 
       Operation 
       [0045]    Here,  FIG. 6( a )  is an example of the calculation result (second modulation output) for distance and speed based on the second modulation data. In addition,  FIG. 6( b )  is an example of the calculation result (first modulation output) for distance and speed based on the first modulation data. As shown in  FIG. 6 , the distance resolutions are both similar. The speed resolution is higher in the first modulation output than the second modulation output. However, the second modulation output enables the speed to be uniquely determined over the overall speed measurement range. The first modulation output has ambiguity resulting caused by phase folding back (four fold back portions P 1  to P 4  occur within the speed measurement range in  FIG. 6 ). Therefore, the speed cannot be uniquely determined. Here, through comparison of speed between those of the same distance, as shown in  FIG. 6( c ) , the speed closest to the speed in the second modulation output (here, the speed within the fold back portion P 2 ), of the plurality of speeds determined from the first modulation output, is used as the speed of the target. 
       Effect 
       [0046]    As described above, according to the present embodiment, the speed is determined (speed ambiguity is resolved) through comparison of the first modulation output in which resolution is high but the speed cannot be uniquely determined, and the second modulation output in which resolution is low but the speed can be uniformly determined. Therefore, according to the present embodiment, a high-resolution measurement result for speed can be acquired without increasing the speed for sampling of the beat signal. As a result, the reliability of speed detection in the radar apparatus  10  can be improved. 
         [0047]    According to the present embodiment, a first processing means is functionally configured by steps S 160  to S 190  shown in  FIG. 3 . In addition, steps S 160  to S 190  correspond to a first process. Furthermore, a second processing means is functionally configured by steps S 110  to S 150  shown in  FIG. 3 . In addition, steps S 110  to S 150  correspond to a second process. Furthermore, a speed determining means is functionally configured by step S 200  shown in  FIG. 3 . In addition, step  5200  corresponds to a speed determining process. 
       Other Embodiments 
       [0048]    An embodiment of the present invention is described above. However, the present invention is not limited to the above-described embodiment. Various embodiments may be used. 
         [0049]    (1) According to the above-described embodiment, the FMCW is used as the second modulation data. However, according to the above-described embodiment, all that is required is that the speed can be measured using Doppler frequency and, for example, a multi-frequency CW may be used. 
         [0050]    (2) Each constituent element of the present invention is conceptual and is not limited to the above-described embodiments. For example, a function provided by a single constituent element may be dispersed among a plurality of constituent elements. Functions provided by a plurality of constituent elements may be integrated in a single constituent element. In addition, at least a part of a configuration according to the above-described embodiments may be replaced by a publicly known configuration having a similar function. Furthermore, at least a part of a configuration according to an above-described embodiment may be added to or replace another configuration according to another above-described embodiment. 
       Reference Signs List 
       [0000]    
       
           1 : on-board system 
           10 : radar apparatus 
           20 : transmission circuit 
           30 : distributor 
           40 : transmission antenna 
           50 : reception antenna 
           51 : antenna element 
           60 : reception circuit 
           61 : mixer 
           70 : processing unit 
           70 A: CPU 
           70 B: ROM 
           70 C: RAM 
           70 D: coprocessor 
           70 E: clock 
           80 : output unit 
           100 : driving assistance ECU