Patent Publication Number: US-2022229176-A1

Title: Object detection device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation application of U.S. application Ser. No. 16/905,189 filed on Jun. 18, 2020, which is based on and claims priority under 35 U.S.C. § 119 to Japanese Patent Application 2019-115727, filed on Jun. 21, 2019, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to an object detection device. 
     BACKGROUND DISCUSSION 
     Conventionally, in a technique of detecting information about an object based on transmission and reception of radar, constant false alarm rate (CFAR) processing is known as processing for reducing noise called clutter that occurs due to reflection by an object that is not a detection target. In outline, CFAR processing is processing for acquiring a difference signal based on a difference between a value (signal level) of a processing target signal according to a reception wave and an average value of values of the processing target signal. In the existing technique using such CFAR processing, the reception wave as a transmission wave reflected by and returned from an object that is a detection target is detected based on a comparison between the value of the difference signal and a fixed threshold. 
     Here, in the existing technique as described above, it is desirable that the fixed threshold which is a comparison target of the value of the difference signal obtained as a result of CFAR processing is set to such a magnitude that the reception wave as the transmission wave reflected by and returned from the object that is a detection target is detected but clutter is not detected. 
     However, in general, a degree of clutter is not constant, and may vary, for example, depending on an environment such as a state of a ground (road surface) on which the object that is a detection target is provided. Accordingly, in the existing technique as described above in which the fixed threshold is used as the comparison target of the value of the difference signal, depending on the environment, not only the reception wave as the transmission wave reflected by and returned from the object that is a detection target but also clutter may be detected. 
     A need thus exists for an object detection device capable which is not susceptible to the drawback mentioned above. 
     SUMMARY 
     An object detection device as an example of this disclosure includes a transmission unit that transmits a first transmission wave, a reception unit that receives a first reception wave as the first transmission wave that is reflected by and returned from an object, a signal processing unit that samples a first processing target signal according to the first reception wave and acquires a difference signal based on a difference between a value of the first processing target signal for at least one sample according to the first reception wave received at a certain detection timing, and an average value of values of the first processing target signal for a plurality of samples according to the first reception wave received in at least one of a first period and a second period of a predetermined time length that exist before and after the detection timing, a threshold setting unit that sets a threshold which is a comparison target with the value of the difference signal, based on variation in the values of the first processing target signal for the plurality of samples, and a detection unit that detects information about the object at the detection timing based on a comparison result between the value of the difference signal and the threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed description considered with the reference to the accompanying drawings, wherein: 
         FIG. 1  is an exemplary and schematic diagram illustrating an appearance of a vehicle including an object detection system according to an embodiment when viewed from above; 
         FIG. 2  is an exemplary and schematic block diagram illustrating schematic hardware configurations of an electronic control unit (ECU) and an object detection device according to the embodiment; 
         FIG. 3  is an exemplary and schematic diagram for describing an outline of a technique used by the object detection device according to the embodiment to detect a distance to an object; 
         FIG. 4  is an exemplary and schematic block diagram illustrating a detailed configuration of the object detection device according to the embodiment; 
         FIG. 5  is an exemplary and schematic diagram for describing an outline of cell averaging constant false alarm rate (CA-CFAR) processing and threshold setting processing that can be executed in the embodiment; 
         FIG. 6  is an exemplary and schematic diagram for describing an outline of greatest of constant false alarm rate (GO-CFAR) processing and the threshold setting processing that can be executed in the embodiment; 
         FIG. 7  is an exemplary and schematic diagram illustrating an example of a signal which is a source of a difference signal obtained as a result of CFAR processing according to the embodiment; 
         FIG. 8  is an exemplary and schematic diagram illustrating an example of a difference signal obtained as a result of CFAR processing according to the embodiment; 
         FIG. 9  is an exemplary and schematic diagram for describing a setting value that can be set in the embodiment; 
         FIG. 10  is an exemplary and schematic diagram illustrating an example of a data structure of a road surface estimation table used for estimating a shape of a road surface in the embodiment; 
         FIG. 11  is an exemplary and schematic diagram illustrating an example of a data structure of a setting value table used for acquiring the setting value in the embodiment; and 
         FIG. 12  is an exemplary and schematic flowchart illustrating a series of processing executed by the object detection device according to the embodiment to detect the distance to the object. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment and a modification example of this disclosure will be described with reference to the drawings. The configurations of the embodiment and modification example described below, and actions and effects provided by the configurations are merely examples, and the embodiment disclosed here is not limited to the contents described below. 
     EMBODIMENT 
       FIG. 1  is an exemplary and schematic view illustrating an appearance of a vehicle  1  including an object detection system according to an embodiment when viewed from above. As described below, the object detection system according to the embodiment is an on-board sensor system that performs transmission and reception of sound waves (ultrasonic waves) and acquire a time difference between the transmission and reception to detect information about an object (for example, obstacle O illustrated in  FIG. 2  described later) including human beings present in the surroundings. 
     As illustrated in  FIG. 1 , the object detection system includes an electronic control unit (ECU)  100  mounted inside the four-wheel vehicle  1  including a pair of front wheels  3 F and a pair of rear wheels  3 R, and object detection devices  201  to  204  mounted on the exterior of the vehicle  1 . 
     In the example illustrated in  FIG. 1 , as an example, the object detection devices  201  to  204  are installed at different positions, for example, on the rear bumper at the rear end of a vehicle body  2  as the exterior of the vehicle  1 . 
     In the embodiment, hardware configurations and functions of the object detection devices  201  to  204  are the same. Accordingly, in the following, for simplicity, the object detection devices  201  to  204  may be collectively referred to as an object detection device  200 . 
     In the embodiment, an installation position of the object detection device  200  is not limited to the example illustrated in  FIG. 1 . The object detection device  200  may be installed, for example, on the front bumper at the front end of the vehicle body  2 , may be installed on the side surface of the vehicle body  2 , or may be installed on two or more of the rear bumper, the front bumper, and the side surface. In the embodiment, the number of object detection devices  200  is not limited to the example illustrated in  FIG. 1 . 
       FIG. 2  is an exemplary and schematic block diagram illustrating the hardware configurations of the ECU  100  and the object detection device  200  according to the embodiment. 
     As illustrated in  FIG. 2 , the ECU  100  has the same hardware configuration as a normal computer. More specifically, the ECU  100  includes an input and output device  110 , a storage device  120 , and a processor  130 . 
     The input and output device  110  is an interface for realizing transmission and reception of information between the ECU  100  and the outside (object detection device  200  in the example illustrated in  FIG. 1 ). 
     The storage device  120  is a main storage device such as a read only memory (ROM) or a random access memory (RAM), and/or an auxiliary storage device such as a hard disk drive (HDD) or a solid state drive (SSD). 
     The processor  130  controls various processing executed in the ECU  100 . The processor  130  includes an operation device such as a central processing unit (CPU). The processor  130  realizes various functions such as automatic parking by reading and executing a computer program stored in the storage device  120 . 
     On the other hand, as illustrated in  FIG. 2 , the object detection device  200  includes a transmission and reception unit  210  and a control unit  220 . 
     The transmission and reception unit  210  includes a vibrator  211  such as a piezoelectric element, and realizes transmission and reception of ultrasonic waves by the vibrator  211 . 
     More specifically, the transmission and reception unit  210  transmits an ultrasonic wave generated according to vibration of the vibrator  211  as a transmission wave, and receives, as a reception wave, the vibration of the vibrator  211  that is caused by the ultrasonic wave transmitted as the transmission wave being reflected by and returned from an object existing outside. In the example illustrated in  FIG. 2 , the obstacle O installed on a road surface RS is illustrated as an object that reflects the ultrasonic waves from the transmission and reception unit  210 . 
     In the example illustrated in  FIG. 2 , a configuration in which both transmission of a transmission wave and reception of a reception wave are realized by a single transmission and reception unit  210  including a single vibrator  211 . However, the technique of the embodiment is naturally applicable to a configuration in which a configuration on a transmission side and a configuration on a reception side are separated, such as a configuration in which a first vibrator for transmitting a transmission wave and a second vibrator for receiving a reception wave are separately provided. 
     The control unit  220  has the same hardware configuration as a normal computer. More specifically, the control unit  220  includes an input and output device  221 , a storage device  222 , and a processor  223 . 
     The input and output device  221  is an interface for realizing transmission and reception of information between the control unit  220  and the outside (ECU  100  and transmission and reception unit  210  in the example illustrated in  FIG. 1 ). 
     The storage device  222  includes a main storage device such as the ROM and the RAM and/or an auxiliary storage device such as the HDD and the SSD. 
     The processor  223  controls various processing executed in the control unit  220 . The processor  223  includes, for example, an operation device such as a CPU. The processor  223  realizes various functions by reading and executing a computer program stored in the storage device  222 . 
     Here, the object detection device  200  according to the embodiment detects the distance to the object by a technique called a so-called a time of flight (TOF) method. As described in detail below, the TOF method is a technique for calculating the distance to an object as information about the object in consideration of a difference between the timing (more specifically, timing at which the transmission wave begins to be transmitted) at which the transmission wave is transmitted and the timing (more specifically, the reception wave begins to be received) at which the reception wave is received. 
       FIG. 3  is an exemplary and schematic diagram for describing an outline of a technique used by the object detection device  200  according to the embodiment to detect a distance to an object. More specifically,  FIG. 3  is a diagram exemplarily and schematically illustrating, in a graph format, a temporal change in a signal level (for example, amplitude) of an ultrasonic wave transmitted and received by the object detection device  200  according to the embodiment. In the graph illustrated in  FIG. 3 , the horizontal axis corresponds to time, and the vertical axis corresponds to the signal level of the signal transmitted and received by the object detection device  200  via the transmission and reception unit  210  (vibrator  211 ). 
     In the graph illustrated in  FIG. 3 , a solid line L 11  represents an example of a signal level of a signal transmitted and received by the object detection device  200 , that is, an example of an envelope curve illustrating a temporal change in a degree of vibration of the vibrator  211 . From the solid line L 11 , matters that the vibrator  211  is driven by time Ta from timing t 0  and vibrates, so that transmission of the transmission wave is completed at timing t 1 , and thereafter, during time Tb until timing t 2 , vibration of the vibrator  211  due to inertia continues while being attenuated can be read. Accordingly, in the graph illustrated in  FIG. 3 , the time Tb corresponds to so-called reverberation time. 
     The solid line L 11  reaches a peak, where the degree of vibration of the vibrator  211  exceeds (becomes equal to or larger than) a predetermined threshold Th 1  represented by the one-dot chain line L 21 , at timing t 4  at which time Tp has elapsed from the timing t 0  at which the transmission of the transmission wave starts. The threshold Th 1  is a value set in advance to identify whether the vibration of the vibrator  211  is caused by the reception of the reception wave as the transmission wave reflected by and returned from the object that is a detection target (for example, obstacle O illustrated in  FIG. 2 ) or a value set in advance to identify whether the vibration of the vibrator  211  is caused by the reception of the reception wave as the transmission wave reflected by and returned from the object outside the object (for example, road surface RS illustrated in  FIG. 2 ). 
     Although an example in which the threshold Th 1  is set as a constant value that does not change over time is illustrated in  FIG. 3 , in the embodiment, the threshold Th 1  may be set as a value that changes over time. 
     Here, the vibration having a peak exceeding (or equal to or larger than) the threshold Th 1  can be regarded as being caused by the reception of the reception wave as the transmission wave reflected by and returned from the object that is a detection target. On the other hand, the vibration having the peak equal to or less than (or less than) the threshold Th 1  can be regarded as being caused by the reception of the reception wave as the transmission wave reflected by and returned from an object other than an object that is a detection target. 
     Accordingly, from the solid line L 11 , matters that the vibration of the vibrator  211  at the timing t 4  is caused by the reception of the reception wave as the transmission wave reflected by and returned from the object that is a detection target can be read. 
     On the solid line L 11 , the vibration of the vibrator  211  is attenuated after the timing t 4 . Accordingly, the timing t 4  corresponds to the timing at which the reception of the reception wave as the transmission wave reflected by and returned from the object that is a detection target is completed, in other words, the timing at which the last transmission wave at timing t 1  returns as a reception wave. 
     In the solid line L 11 , the timing t 3  as the start point of the peak at the timing t 4  corresponds to the timing at which the reception of the reception wave as the transmission wave reflected by and returned from the object that is a detection target starts, in other words, the timing at which the transmission wave first transmitted at the timing t 0  returns as a reception wave. Accordingly, in the solid line L 11 , time ΔT between the timing t 3  and the timing t 4  becomes equal to time Ta as the transmission time of the transmission wave. 
     Based on the description as above, in order to obtain the distance to the object that is a detection target by the TOF method, it is necessary to obtain time Tf between the timing t 0  at which the transmission wave starts to be transmitted and the timing t 3  at which the reception wave starts to be received. The time Tf can be obtained by subtracting the time ΔT equal to the time Ta as the transmission time of the transmission wave from the time Tp as a difference between the timing t 0  and the timing t 4  at which the signal level of the reception wave reaches the peak exceeding the threshold Th 1 . 
     The timing t 0  at which the transmission wave starts to be transmitted can be easily specified as the timing at which the object detection device  200  starts its operation, and the time Ta as the transmission time of the transmission wave is determined in advance by setting or the like. Accordingly, in order to obtain the distance to the object that is a detection target by the TOF method, it is ultimately important to specify the timing t 4  at which the signal level of the reception wave reaches the peak exceeding the threshold Th 1 . Then, in order to specify the timing t 4 , it is important to accurately detect the reception wave as the transmission wave reflected by and returned from the object that is a detection target. 
     By the way, conventionally, in a technique of detecting information about an object based on transmission and reception of waves such as radar and sound waves, constant false alarm rate (CFAR) processing is known as processing for reducing noise called clutter that occurs due to reflection by an object that is not a detection target. CFAR processing is processing for acquiring a difference signal based on a difference between the value (signal level) of the processing target signal according to the reception wave and an average value of the values of the processing target signal. In the existing technique using such CFAR processing, the reception wave as the transmission wave reflected by and returned from the object that is a detection target is detected based on a comparison between a value of the difference signal and a fixed threshold. 
     Here, in the existing technique as described above, it is desirable that the fixed threshold which is a comparison target of the value of the difference signal obtained as a result of CFAR processing is set to such a magnitude that the reception wave as the transmission wave reflected by and returned from the object that is a detection target is detected but clutter is not detected. 
     However, in general, the degree of clutter is not constant, and may change variously, for example, depending on the environment such as a state of the road surface on which the object that is a detection target is provided. Accordingly, in the existing technique as described above in which a fixed threshold is used as the comparison target of the value of the difference signal, depending on the environment, not only the reception wave as the transmission wave reflected by and returned from the object that is a detection target but also clutter may be detected. 
     Therefore, in the embodiment, by configuring the object detection device  200  as follows, it is possible to accurately detect the reception wave as the transmission wave reflected by and returned from the object that is a detection target while appropriately reducing clutter, regardless of the environment. 
       FIG. 4  is an exemplary and schematic block diagram illustrating a detailed configuration of the object detection device  200  according to the embodiment. In the example illustrated in  FIG. 4 , the configuration on the transmission side and the configuration on the reception side are separated, but such an illustrated mode is merely for convenience of description. Accordingly, in the embodiment, as described above, both the transmission of the transmission wave and the reception of the reception wave are realized by the single transmission and reception unit  210  including the single vibrator  211 . However, although the description as above is repeated, the technique of the embodiment is naturally applicable to a configuration in which the configuration on the transmission side and the configuration on the reception side are separated. 
     As illustrated in  FIG. 4 , the object detection device  200  includes a wave transmitter  411 , a code generation unit  412 , a carrier wave output unit  413 , a multiplier  414 , and an amplification circuit  415 , as the transmission side configuration. The wave transmitter  411  is an example of a “transmission unit”. 
     The object detection device  200  includes a wave receiver  421 , an amplification circuit  422 , a filter processing unit  423 , a correlation processing unit  424 , an envelope processing unit  425 , a CFAR processing unit  426 , a threshold processing unit  427 , a threshold setting unit  428 , a detection unit  429 , and a setting value acquisition unit  430 , as the reception side configuration. The wave receiver  421  is an example of a “reception unit”, and the CFAR processing unit  426  is an example of a “signal processing unit”. 
     In the embodiment, at least a part of the configuration illustrated in  FIG. 4  can be realized by dedicated hardware (analog circuit), and the remaining part can be realized as a result of the cooperation of hardware and software, more specifically, as a result of the processor  223  of the object detection device  200  reading and executing the computer program from the storage device  222 . In the embodiment, each configuration illustrated in  FIG. 4  may operate under control of the control unit  220  of the object detection device  200  itself, or may operate under control of an external ECU  100 . 
     First, the configuration on the transmission side will be briefly described. 
     The wave transmitter  411  is configured by the vibrator  211  described above, and transmits a transmission wave according to a transmission signal (after amplification) output from the amplification circuit  415  by the vibrator  211 . 
     Here, in the embodiment, based on the configuration described below, the wave transmitter  411  transmits the encoded transmission wave after encoding the transmission wave so as to include identification information of a predetermined code length. 
     The code generation unit  412  generates a signal corresponding to identification information to be added to the transmission wave, for example, a pulse signal corresponding to a code of a bit string consisting of a sequence of 0 or 1 bits. 
     The carrier wave output unit  413  outputs a carrier wave as a signal to which identification information is to be added. For example, the carrier wave output unit  413  outputs a sine wave having a predetermined frequency as a carrier wave. 
     The multiplier  414  executes modulation of the carrier wave so as to add identification information by multiplying the output from the code generation section  412  and the output from the carrier wave output unit  413 . As a modulation method, a plurality of methods such as phase modulation, amplitude modulation, and frequency modulation can be used alone or in combination of one or more of the plurality of methods. Then, the multiplier  414  outputs the modulated carrier wave, to which the identification information is added, to the amplification circuit  415  as a transmission signal that is a source of the transmission wave. 
     The amplification circuit  415  amplifies the transmission signal output from the multiplier  414  and outputs the amplified transmission signal to the wave transmitter  411 . 
     Next, the configuration on the reception side will be briefly described. 
     The wave receiver  421  is configured by the vibrator  211  described above, and receives the transmission wave reflected by the object as a reception wave by the vibrator  211 . 
     The amplification circuit  422  amplifies the received signal as a signal according to the reception wave received by the wave receiver  421 . 
     The filter processing unit  423  performs filtering processing on the reception signal amplified by the amplification circuit  422  to reduce noise. In the embodiment, the filter processing unit  423  may acquire information about a frequency of the transmission signal and further perform frequency correction on the reception signal so as to match the frequency of the transmission signal. 
     The correlation processing unit  424  acquires a correlation value corresponding to the degree of similarity of the identification information between the transmission wave and the reception wave, for example, based on the transmission signal acquired from the configuration on the transmission side and the reception signal after processing by the filter processing unit  423 . The correlation value can be obtained based on a generally well-known correlation function or the like. 
     The envelope processing unit  425  obtains an envelope of a waveform of the correlation value signal as a signal based on the correlation value acquired by the correlation processing unit  424 , and outputs the signal to the CFAR processing unit  426  as the processing target signal. 
     The CFAR processing unit  426  acquires a difference signal by performing CFAR processing on the processing target signal output from the envelope processing unit  425 . As described above, in outline, CFAR processing is processing for acquiring a difference signal based on the difference between the value (signal level) of the processing target signal and an average value of values of the processing target signal, in order to reduce clutter contained in the processing target signal. 
     The CFAR processing unit  426  according to the embodiment samples the processing target signal according to the reception wave and acquires the difference signal based on the difference between a value of the processing target signal for (at least) one sample according to the reception wave received at a certain detection timing, and an average value of the values of the processing target signal for a plurality of samples according to the reception wave received in at least one of a first period and a second period of a predetermined time length that exist before and after the detection timing, by executing CFAR processing. 
     As CFAR processing, a plurality of processing having different properties such as cell averaging constant false alarm rate (CA-CFAR) processing, greatest of constant false alarm rate (GO-CFAR) processing, smallest of constant false alarm rate (SO-CFAR) processing can be considered. In this regard, the CFAR processing unit  426  according to the embodiment selectively executes the plurality of types of CFAR processing according to settings so that at least CA-CFAR processing and GO-CFAR processing are used properly. Details of CA-CFAR processing and GO-CFAR processing will be described later in detail, and thus the description thereof is omitted here. 
     The threshold processing unit  427  compares the value of the difference signal acquired by the CFAR processing unit  426  with the threshold, and determines whether or not the identification information of the transmission wave and the reception wave are similar to each other at a predetermined level or higher based on the comparison result. 
     Here, as described above, if a fixed threshold is used as a comparison target of the value of the difference signal, depending on the environment, not only the reception wave as the transmission wave reflected by and returned from the object that is a detection target but also clutter may be detected. 
     Therefore, in order to eliminate inconvenience described above, the threshold processing unit  427  according to the embodiment uses a threshold dynamically set by the threshold setting unit  428  according to the environment as a comparison target with the value of the difference signal. 
     That is, the threshold setting unit  428  according to the embodiment dynamically sets an appropriate threshold according to the environment in consideration of the variation in the value of the processing target signal which is a target of CFAR processing. More specifically, the threshold setting unit  428  sets a threshold as a comparison target with the value of the difference signal, based on the variation in the value of the processing target signal for a plurality of samples according to the reception wave received in at least one of a first period and a second period of a predetermined time length that exist before and after the detection timing described above. Although details will be described later, an indicator indicating variation is calculated based on different criteria depending on the type of CFAR processing executed by the CFAR processing unit  426 . 
     Then, the detection unit  429  specifies the timing at which the degree of similarity of the identification information between the transmission wave and the reception wave reaches a predetermined level or more, that is, the timing (for example, timing t 4  illustrated in  FIG. 2 ) at which the signal level of the reception wave as the transmission wave returned by reflection reaches the peak exceeding the threshold based on the processing result by the threshold processing unit  427 , and detects the distance to the object by the TOF method. 
     Here, an outline of CFAR processing and the threshold setting processing executed in the embodiment will be described. As described above, the CFAR processing unit  426  according to the embodiment selectively executes the plurality types of CFAR processing according to settings so that at least CA-CFAR processing and GO-CFAR processing are used properly. Then, the threshold setting unit  428  according to the embodiment calculates the indicator indicating the variation in the value of the processing target signal by different criteria according to the type of CFAR processing executed by the CFAR processing unit  426 , and based on the arithmetic result, dynamically sets the threshold according to the environment such as a condition of the road surface that causes clutter. 
     First, CA-CFAR processing and the threshold setting processing based on CA-CFAR processing will be described. 
     As illustrated in  FIG. 5  below, CA-CFAR processing is processing for acquiring a difference signal based on a difference between the value of the processing target signal for one sample according to the reception wave received at a certain detection timing and an average value of values of the processing target signal for a plurality of samples according to the reception wave received in both the first period and the second period that exist before and after the detection timing. 
       FIG. 5  is an exemplary and schematic diagram for describing an outline of CA-CFAR processing that can be executed in the embodiment and the threshold setting processing based on CA-CFAR processing. 
     As illustrated in  FIG. 5 , in CA-CFAR processing, first, a processing target signal  510  is sampled at predetermined time intervals. Then, a arithmetic unit  511  of the CFAR processing unit  426  calculates a total sum of values of processing target signals for N samples according to a reception wave received in a first period T 51  existing before a certain detection timing t 50 . An arithmetic unit  512  of the CFAR processing unit  426  calculates a total sum of values of processing target signals for N samples according to a reception wave received in a second period T 52  existing after the detection timing t 50 . 
     Then, an arithmetic unit  520  of the CFAR processing unit  426  adds up arithmetic results of the arithmetic units  511  and  512 . Then, an arithmetic unit  530  of the CFAR processing unit  426  divides the arithmetic result of the arithmetic unit  520  by 2N, which is the sum of the sample number N of the processing target signal in the first period T 51  and the sample number N of the processing target signal in the second period T 52  and calculates an average value of the values of the processing target signal in both the first period T 51  and the second period T 52 . 
     Then, an arithmetic unit  540  of the CFAR processing unit  426  subtracts the average value as the arithmetic result of the arithmetic unit  530  from the value of the processing target signal at the detection timing t 50  and acquires a difference signal  550 . 
     On the other hand, an arithmetic unit  560  of the threshold setting unit  428  subtracts the average value as the arithmetic result of the arithmetic unit  530  from the respective values of the processing target signals for 2N samples in the first period T 51  and the second period T 52 . Then, an arithmetic unit  570  of the threshold setting unit  428  computes a square root of a square of the total sum of the arithmetic results for 2N samples of the arithmetic unit  560  divided by 2N to calculate a standard deviation as an indicator indicating the variation in the value of the processing target signal in both the first period T 51  and the second period T 52 . Then, the threshold setting unit  428  sets a threshold  580  which is a comparison target of the difference signal  550  by executing processing such as multiplying the arithmetic result of the arithmetic unit  570  by a predetermined gain. 
     In the example illustrated in  FIG. 5 , the first period T 51  and the second period T 52  may be set before and after the detection timing t 50  without a time gap, or may be set before and after the detection timing t 50  with a time interval of several samples. According to the latter setting, it is possible to obtain a processing result that emphasizes information at the detection timing t 50  by ignoring information immediately before and after the detection timing t 50 . 
     Next, the GO-CFAR processing and the threshold setting processing based on the GO-CFAR processing will be described. 
     As illustrated in  FIG. 6  below, the GO-CFAR processing is processing for acquiring the difference signal based on a difference between the value of the processing target signal for one sample according to the reception wave received at a certain detection timing and a larger average value of an average value of the values of the first processing target signal for a plurality of samples according to the reception wave received in the first period that exists before the detection timing and an average value of the values of the processing target signal for a plurality of samples according to the reception wave received in the second period that exists after the detection timing. 
       FIG. 6  is an exemplary and schematic diagram for describing an outline of GO-CFAR processing that can be executed in the embodiment and the threshold setting processing based on GO-CFAR processing. 
     As illustrated in  FIG. 6 , also in GO-CFAR processing, similar to CA-CFAR processing, first, a processing target signal  610  is sampled at predetermined time intervals. Then, an arithmetic unit  611  of the CFAR processing unit  426  calculates a total sum of the values of the processing target signals for N samples according to a reception wave received in a first period T 61  existing before a certain detection timing t 60 . An arithmetic unit  612  of the CFAR processing unit  426  calculates the total sum of the values of the processing target signals for a plurality of N samples according to a reception wave received in a second period T 62  existing after the detection timing t 60 . 
     Then, the arithmetic unit  621  of the CFAR processing unit  426  divides the arithmetic result of the arithmetic unit  611  by the number of samples N of the processing target signal in the first period T 61 , and calculates an average value of the values of the processing target signal in the first period T 61 . An arithmetic unit  622  of the CFAR processing unit  426  divides the arithmetic result of the arithmetic unit  612  by the number of samples N of the processing target signal in the second period T 62 , and calculates an average value of the values of the processing target signal in the second period T 62 . 
     Then, the CFAR processing unit  426  extracts a larger average value  630  of the average value as the arithmetic result of the arithmetic unit  621  and the average value as the arithmetic result of the arithmetic unit  622 . Then, an arithmetic unit  640  of the CFAR processing unit  426  subtracts the extracted larger average value  630  from the value of the processing target signal at the detection timing t 50  to acquire a difference signal  650 . 
     On the other hand, an arithmetic unit  661  of the threshold setting unit  428  subtracts the average value as the arithmetic result of the arithmetic unit  621  from each of the processing target signals for N samples in the first period T 61 . Then, an arithmetic unit  671  of the threshold setting unit  428  computes a square root of a square of the arithmetic result of N samples of the arithmetic unit  661  divided by N to calculate a standard deviation as an indicator indicating the variation in the value of the processing target signal in the first period T 61 . 
     An arithmetic unit  662  of the threshold setting unit  428  subtracts the average value as the arithmetic result of the arithmetic unit  622  from each of the processing target signals for N samples in the second period T 62 . Then, an arithmetic unit  672  of the threshold setting unit  428  computes a square root of a square of the arithmetic result of N samples of the arithmetic unit  662  divided by N to calculate a standard deviation as an indicator indicating the variation in the value of the processing target signal in the second period T 62 . 
     Then, the threshold setting unit  428  extracts the standard deviation corresponding to the larger average value  630  extracted by the CFAR processing unit  426 , of the standard deviation as the arithmetic result of the arithmetic unit  661  and the standard deviation as the arithmetic result of the arithmetic unit  662 . Then, the threshold setting unit  428  executes processing such as multiplying the extraction result by a predetermined gain, and sets a threshold  680  based on the standard deviation corresponding to the larger average value  630  as the comparison target of the difference signal  550 . 
     Also in the example illustrated in  FIG. 6 , similarly as in the example illustrated in  FIG. 5 , the first period T 61  and the second period T 62  may be set before and after the detection timing t 60  without a time gap, or may be set before and after the detection timing t 60  with a time interval of several samples. 
     In this way, the CFAR processing unit  426  according to the embodiment selectively executes a plurality of types of CFAR processing according to the settings so that at least CA-CFAR processing and GO-CFAR processing are used properly. Then, the threshold setting unit  428  according to the embodiment calculates the indicator indicating the variation in the value of the processing target signal by different criteria according to the type of CFAR processing executed by the CFAR processing unit  426  and dynamically sets the threshold according to the environment such as the condition of the road surface that causes clutter, based on the arithmetic result. 
     In the description above, although SO-CFAR processing is also referred to, the SO-CFAR processing is basically the same as GO-CFAR processing except that the smaller one of the value for the first period and the value for the second period is extracted, and thus, further description is omitted here. 
     Hereinafter, the difference signal obtained as a result of CFAR processing will be described in more detail. 
     First, two signals that are the source of a difference signal will be described by illustrating examples of specific waveforms. 
       FIG. 7  is an exemplary and schematic diagram illustrating an example of a signal that is a source of a difference signal obtained as a result of CFAR processing according to the embodiment. 
     In the example illustrated in  FIG. 7 , a solid line L 700  indicates an example of the temporal change in the value of the processing target signal as one of the signals that is the source of the difference signal, more specifically, the time change in the value (signal level) of the signal input to the +side of the arithmetic unit  540  at each detection timing t 50  in the example illustrated in  FIG. 5  and the signal input to the +side of the arithmetic unit  640  at each detection timing t 60  in the example illustrated in  FIG. 6 . 
     In the example illustrated in  FIG. 7 , a one-dot chain line L 701  indicates an example of a temporal change in the value of the processing target signal which is the target of subtraction from the processing target signal in CA-CFAR processing, more specifically, the temporal change in the value of the signal input to the −side of the arithmetic unit  540  in the example illustrated in  FIG. 5 . A two-dot chain line L 702  indicates an example of a temporal change in the value of the processing target signal which is the target of subtraction from the processing target signal in GO-CFAR processing, more specifically, the temporal change in the value of the signal input to the −side of the arithmetic unit  640  in the example illustrated in  FIG. 6 . 
     In the embodiment, when CA-CFAR processing is executed, the difference signal at each time is acquired by subtracting the value of the signal indicated by the one-dot chain line L 701  from the value (signal level) of the signal indicated by the solid line L 700  at each time, and when GO-CFAR processing is executed, the difference signal is acquired by subtracting the value of the signal indicated by the two-dot chain line L 702  from the value of the signal indicated by the solid line L 700  at each time. 
     In the embodiment, the value of the difference signal is not a negative value and is always processed to be a value of zero or more. Accordingly, in the embodiment, for example, in a section in which the value indicated by the one-dot chain line L 701  (same for the two-dot chain line L 702 ) is larger than the value of the signal indicated by the solid line L 700 , the value of the difference signal is calculated as zero without being calculated as a negative value. Needless to say, in a section in which the value of the signal indicated by the solid line L 700  is equal to or less than the value indicated by the one-dot chain line L 701  (same for the two-dot chain line L 702 ), the value of the difference signal is calculated as a value of zero or more. 
     Based on the computation described above, in the embodiment, the temporal change in the value (signal level) of the difference signal is, for example, in a form as illustrated in  FIG. 8  below. 
       FIG. 8  is an exemplary and schematic diagram illustrating an example of a difference signal obtained as a result of CFAR processing according to the embodiment. 
     In the example illustrated in  FIG. 8 , a solid line L 801  indicates an example of the temporal change in the value of the difference signal. As illustrated by the solid line L 801 , the value of the difference signal reaches a peak P 801  at a certain time t 80 , and fluctuates in a value range smaller than the peak P 801  in periods T 81  and T 82  before and after the time t 80 . 
     The time t 80  corresponds to the timing at which the signal level of the reception wave as the transmission wave reflected by and returned from the object that is a detection target, which is extracted as a result of clutter reduction by CFAR processing, reaches a peak (the timing t 4  illustrated in  FIG. 3 ) and the periods T 81  and T 82  correspond to clutter that is reduced by CFAR processing and that occurs due to the reflection of the road surface or the like. 
     Here, in the embodiment, as described above, at least two CFAR processing, that is, CA-CFAR processing and GO-CFAR processing are used separately, but in  FIG. 8 , only one example of the change in the value of the difference signal is illustrated by the solid line L 801 . However, as will be described below, in the embodiment, since the difference signal indicating a temporal change as illustrated by the solid line L 801  can be obtained by both CA-CFAR processing and GO-CFAR processing, in  FIG. 8 , the difference signal obtained as a result of GO-CFAR processing and the difference signal obtained as a result of CA-CFAR processing are illustrated without distinction. 
     More specifically, as illustrated in  FIG. 7 , the value of the signal (see one-dot chain line L 701 ) which is the target of subtraction from the processing target signal (see solid line L 700 ) in CA-CFAR processing and the value of the signal (two-dot chain line L 702 ) which is the target of subtraction from the processing target signal (see solid line L 700 ) in GO-CFAR processing indicate substantially the same temporal change except that the former tends to be smaller than the latter. Accordingly, a difference signal indicating substantially the same temporal change can be obtained by both CA-CFAR processing and GO-CFAR processing. 
     However, as described above, in GO-CFAR processing, a value larger than that in CA-CFAR processing tends to be subtracted from the value of the processing target signal, and thus the difference signal obtained as a result of GO-CFAR processing tends to have a zero value for a longer time than the difference signal obtained as a result of CA-CFAR processing. 
     In the example illustrated in  FIG. 8 , if the threshold illustrated by a solid line L 811  is set, it is possible to detect the time t 80 , at which the value of the difference signal reaches the peak P 801  and which corresponds to the timing at which the signal level of the reception wave as the transmission wave reflected by and returned from the object that is a detection target reaches the peak, without detecting periods T 81  and T 82  corresponding to clutter. 
     Here, although the description as above is repeated, when the degree of clutter does not change, even if a fixed threshold is used as the threshold described above, no particular inconvenience occurs. However, the degree of clutter is not constant and may change variously depending on the environment, and thus, depending on the environment, for example, as in the example illustrated in  FIG. 8 , a value (signal level) of clutter in the period  181  may reach a peak P 802  larger than a threshold illustrated by the solid line L 811  such as a one-dot chain line L 812  illustrated in  FIG. 8 . In such a case, if the threshold indicated by the solid line L 811  is not changed, not only the peak P 801  which is the original detection target but also the peak P 802  corresponding to clutter will be detected. 
     Therefore, in the embodiment, the threshold setting unit  428  dynamically sets an appropriate threshold according to the environment in consideration of the variation in the value of the processing target signal that is the target of CFAR processing, as described above. Since the variation in the value of the processing target signal affects the degree of clutter, if the threshold is dynamically set (changed) in consideration of the variation, it is possible to appropriately set the threshold that detects only the original detection target and does not detect clutter. 
     For example, in the example illustrated in  FIG. 8 , in an environment in which a clutter value reaches the peak P 802  in the period T 81 , the threshold setting unit  428  sets a threshold indicated by the one-dot chain line L 812  so as to correspond to the environment. Since the threshold indicated by the one-dot chain line L 812  is larger than the peak P 802 , according to the threshold indicated by the one-dot chain line L 812 , it is possible to accurately detect only the original detection target without detecting clutter. 
     By the way, as described above, the difference signal at a certain detection timing is acquired based on a value obtained by subtracting the average value of the values of the processing target signal in at least one period before and after the detection timing from the value of the processing target signal at the detection timing. Then, when the value of the processing target signal is smaller than the average value, the value of the difference signal is calculated as zero. Based on this, if a computation method for acquiring the difference signal is adjusted so that a value equal to or larger than a setting value having a magnitude of zero or greater as illustrated in the following  FIG. 9  is always subtracted from the value of the processing target signal, the time during which the value of the difference signal becomes zero can be lengthened, and clutter reduction can be effectively realized. 
       FIG. 9  is an exemplary and schematic diagram for describing setting values that can be set in the embodiment. 
     In the example illustrated in  FIG. 9 , a solid line L 900  indicates an example of a temporal change in the value (signal level) of the processing target signal. A line L 901  composed of a one-dot chain line L 901   a  and a broken line L 901   b  illustrates an example of a temporal change in the value of the signal which is the target of subtraction from the processing target signal in order to acquire the difference signal in CA-CFAR processing. A line L 902  composed of a two-dot chain line L 902   a  and a broken line L 902   b  illustrates an example of a temporal change in the value of the signal which is the target of subtraction from the processing target signal in order to acquire the difference signal in GO-CFAR processing. A solid line L 903  illustrates an example of setting values for lengthening the time during which the value of the difference signal becomes zero to effectively realize clutter reduction. 
     Similar to the example described above, in the example illustrated in  FIG. 9 , a basic computation method for acquiring the difference signal is to subtract the value illustrated by the line L 901  or L 902  from the value illustrated by the solid line L 900 . However, in the embodiment, in order to effectively realize clutter reduction by lengthening the time during which the value of the difference signal becomes zero, the computation method for acquiring the difference signal is adjusted based on the setting value such that a value equal to or larger than the setting value indicated by the solid line L 903  is always subtracted from the value indicated by the solid line L 900 . The setting value is smaller than the peak of the value indicated by the solid line L 900 , but is set to be larger than the value of the foot portion that fluctuates in a small value range closer to zero than the peak. 
     That is, in the example illustrated in  FIG. 9 , the difference signal based on CA-CFAR processing is acquired based on the result of subtracting the value indicated by the one-dot chain line L 901   a  from the value indicated by the solid line L 900  in a section in which the value indicated by the line L 901  is the value indicated by the one-dot chain line L 901   a  that exceeds the setting value indicated by the solid line L 903 , and is acquired based on the result of subtracting the value indicated by the solid line L 903  from the value indicated by the solid line L 900  in a section in which the value indicated by the line L 901  is the value indicated by the broken line L 901   b  that is equal to or less than the setting value indicated by the solid line L 903 . 
     Similarly, in the example illustrated in  FIG. 9 , the difference signal based on GO-CFAR processing is acquired based on the result of subtracting the value indicated by the two-dot chain line L 902   a  from the value indicated by the solid line L 900  in a section in which the value indicated by the line L 902  is the value indicated by the two-dot chain line L 902   a  that exceeds the setting value indicated by the solid line L 903 , and is acquired based on the result of subtracting the value indicated by the solid line L 903  from the value indicated by the solid line L 900  in a section in which the value indicated by the line L 902  is the value indicated by the two-dot chain line L 902   b  that is equal to or less than the setting value indicated by the solid line L 903 . 
     Here, in the example illustrated in  FIG. 9 , the setting value illustrated by the solid line L 903  is illustrated as a fixed value. However, a degree of fluctuation of the value indicated by the solid line L 900  differs depending on a shape of the road surface. That is, depending on the shape of the road surface, a situation in which the value indicated by L 900  constantly changes to exceed the setting value indicated by the solid line L 903  may occur, and thus the setting value needs to be dynamically set according to the shape of the road surface. 
     Then, returning to  FIG. 2 , in the embodiment, the wave transmitter  411  transmits the transmission wave for estimating the shape of the road surface before transmitting the transmission wave for distance detection by the TOF method as described above. Then, the setting value acquisition unit  430  acquires the setting value based on the processing target signal output from the envelope processing unit  425  according to the reception wave received by the wave receiver  421 , that is, the transmission wave for estimating the road surface shape, which is returned by reflection. 
     The transmission wave for distance detection and the transmission wave for estimating the shape of the road surface are basically the same wave from the viewpoint of whether or not encoding is performed, but in the following, in order to clearly distinguish the two transmission waves, the transmission wave for distance detection may be referred to as a first transmission wave, and the transmission wave for estimating the shape of the road surface may be referred to as a second transmission wave, if necessary. The reception wave as a result of the first transmission wave returning by reflection may be referred to as a first reception wave, and the reception wave as a result of the second transmission wave returning by reflection may be referred to as a second reception wave. Furthermore, the processing target signal corresponding to the first reception wave may be referred to as a first processing target signal, and the processing target signal corresponding to the second reception wave may be referred to as a second processing target signal. 
     In the embodiment, the setting value acquisition unit  430  estimates the shape of the road surface by referring to a road surface estimation table  430   a  based on information about the second processing target signal according to the second reception wave and acquires the setting value according to the shape of the road surface by referring to a setting value table  430   b  based on the estimation result. The road surface estimation table  430   a  is an example of “first setting information”, and the setting value table  430   b  is an example of “second setting information”. 
     More specifically, if the setting value acquisition unit  430  acquires the second processing target signal, the setting value acquisition unit  430  specifies an empirical formula that most closely approximates the second processing target signal. Such an empirical formula is selected by computing a coincidence degree with each of a plurality of empirical formulas set in advance. The target of approximation by the empirical formula is not the entire section of the second processing target signal, but a part of the specific section in which the second reception wave as the second transmission wave reflected by and returned from the object, which is other than a detection target, such as road surfaces, which is estimated according to an installation position and installation attitude of the object detection device  200 , is received. 
     Then, when the empirical formula that best approximates the second processing target signal (a part of the specific section among the sections of the temporal change) is specified, the setting value acquisition unit  430  calculates an indicator (for example, standard deviation) indicating the variation in the difference between the value of the signal and the value of the second processing target signal represented by the empirical formula. The setting value acquisition unit  430  also calculates the average characteristic based on the moving average of the values of the second processing target signal. 
     The setting value acquisition unit  430  estimates the shape of the road surface by referring to the road surface estimation table  430   a  having a data structure illustrated in  FIG. 10  by considering encoded information indicating whether or not the second transmission wave and the second reception wave are encoded as information about second processing target signal, in addition to the average characteristic and the variations calculated by the computation described above. The encoded information is known in advance when the second transmission wave is transmitted. 
       FIG. 10  is an exemplary and schematic diagram illustrating an example of a data structure of the road surface estimation table  430   a  used for estimating the shape of the road surface in the embodiment. 
     In the embodiment, as illustrated in  FIG. 10 , the road surface estimation table  430   a  is data indicating a correspondence relationship between the encoded information indicating whether or not the second transmission wave and the second reception wave are encoded, the average characteristic based on the moving average of the values of the second processing target signal, variation in the difference between the value of the signal expressed by the empirical formula, of the plurality of empirical formulas set in advance, that best approximates the second processing target signal (part of a specific section among the sections of the temporal change) and the value of the second processing target signal, and the shape of the road surface. The road surface estimation table  430   a  is stored in advance in the setting value acquisition unit  430  as fixed data determined by an experiment or the like. 
     Accordingly, when the encoded information that is known in advance at the time of transmitting the second transmission wave is specified and the average characteristic and the variation calculated according to the reception of the second reception wave are specified, the setting value acquisition unit  430  can estimate an appropriate shape of the road surface by referring to the road surface estimation table  430   a  based on the specified result. 
     Then, when the estimation of the shape of the road surface based on the road surface estimation table  430   a  is completed, the setting value acquisition unit  430  refers to the setting value table  430   b  having a data structure illustrated in  FIG. 11  and acquires a setting value according to the shape of the road surface. 
       FIG. 11  is an exemplary and schematic diagram illustrating an example of the data structure of the setting value table  430   b  used for acquiring the setting values in the embodiment. 
     As illustrated in  FIG. 11 , in the embodiment, the setting value table  430   b  is data indicating the correspondence relationship between the shape of the road surface and the setting value. The setting value table  430   b  is stored in advance in the setting value acquisition unit  430  as fixed data determined by an experiment or the like. 
     Accordingly, if the setting value acquisition unit  430  estimates the shape of the road surface by referring to the road surface estimation table  430   a , the setting value acquisition unit  430  can acquire the appropriate setting value by further referring to the setting value table  430   b  based on the estimation result. 
     Returning to  FIG. 2 , the setting value acquisition unit  430  outputs the setting values acquired based on the road surface estimation table  430   a  and the setting value table  430   b  to the CFAR processing unit  426 . With this configuration, the setting value is taken into consideration when the difference signal is acquired by the CFAR processing unit  426 , and the difference signal with effectively reduced clutter can be acquired. 
     A flow of processing executed in the embodiment will be described below. 
       FIG. 12  is an exemplary and schematic flowchart illustrating a series of processing executed by the object detection device  200  according to the embodiment to detect information about an object. 
     As illustrated in  FIG. 12 , in the embodiment, first, in S 1201 , the wave transmitter  411  transmits a transmission wave according to the transmission signal generated by the code generation unit  412 , the carrier wave output unit  413 , the multiplier  414 , and the amplification circuit  415  toward the outside of the vehicle  1 . The transmission wave transmitted in S 1201  is the second transmission wave for estimating the shape of the road surface. 
     Then, in S 1202 , the wave receiver  421  receives a reception wave. The reception wave received in S 1202  is the second reception wave as a result of the second transmission wave transmitted in S 1201  being reflected by an object existing outside the vehicle  1 , and the received signal corresponding to the second reception wave is output to the envelope processing unit  425  after being subjected to amplification by the amplification circuit  422 , suppression of noise by the filter processing unit  423 , and acquisition of a correlation value by the correlation processing unit  424 . 
     Then, in S 1203 , the envelope processing unit  425  obtains the envelope of the waveform of the correlation value signal based on the correlation value acquired by the correlation processing unit  424  and generates a processing target signal corresponding to the temporal change in the value of the envelope. The processing target signal generated in S 1203  is the second processing target signal according to the second reception wave received in S 1202 . 
     Then, in S 1204 , the setting value acquisition unit  430  acquires information necessary for estimating the shape of the road surface based on the second processing target signal generated in S 1203 . As described above, the information necessary for estimating the shape of the road surface is three types of information of the encoded information indicating whether or not the second transmission wave and the second reception wave are encoded, the average characteristic based on the moving average of the values of the second processing target signal, and the variation in the difference between the value of the signal expressed by the empirical formula, of the plurality of empirical formulas set in advance, that best approximates the second processing target signal (part of a specific section among the sections of the temporal change) and the value of the second processing target signal. 
     Then, in S 1205 , the setting value acquisition unit  430  estimates the shape of the road surface using the road surface estimation table  430   a . More specifically, the setting value acquisition unit  430  estimates the shape of the road surface according to the information acquired in S 1203  by referring to the road surface estimation table  430   a  based on the information acquired in S 1203 . 
     Then, in S 1206 , the setting value acquisition unit  430  acquires the setting value using the setting value table  430   b . More specifically, the setting value acquisition unit  430  acquires the setting value according to the shape of the road surface by referring to the setting value table  430   b  based on the estimation result in S 1205 . The setting value acquired in S 1206  is output to the CFAR processing unit  426  and used for CFAR processing in S 1210  described later. 
     Then, in S 1207 , the wave transmitter  411  transmits a transmission wave according to the transmission signal generated by the code generation unit  412 , the carrier wave output unit  413 , the multiplier  414 , and the amplification circuit  415  to the outside of the vehicle  1 . The transmission wave transmitted in S 1207  is the first transmission wave for detecting the distance to the object existing outside the vehicle  1 . 
     Then, in S 1208 , the wave receiver  421  receives the reception wave. The reception wave received in S 1208  is the first reception wave received as a result of the first transmission wave transmitted in S 1207  being reflected by an object existing outside the vehicle  1 , and the received signal corresponding to the first reception wave is output to the envelope processing unit  425  after being subjected to amplification by the amplification circuit  422 , suppression of noise by the filter processing unit  423 , and acquisition of a correlation value by the correlation processing unit  424 . 
     Then, in S 1209 , the envelope processing unit  425  obtains the envelope of the waveform of the correlation value signal based on the correlation value acquired by the correlation processing unit  424 , and generates a processing target signal corresponding to the temporal change in the value of the envelope. The processing target signal generated in S 1209  is the first processing target signal according to the first reception wave received in S 1208 . 
     Then, in S 1210 , the CFAR processing unit  426  executes CFAR processing. More specifically, the CFAR processing unit  426  acquires a difference signal based on the difference between the value (signal level) of the first processing target signal generated in S 1209  and the average value of the values of the first processing target signal. However, when the average value of the values of the first processing target signal is less than the setting value acquired in S 1206 , the target that takes the difference from the value of the first processing target signal is not the average value but the setting value. Although the method of calculating the average value of the values of the first processing target signal differs depending on whether CA-CFAR processing is executed, GO-CFAR processing is executed, or another CFAR processing is executed, since a specific example of the method of calculating the average value has already been described, further description is omitted here. 
     On the other hand, in S 1211 , the threshold setting unit  428  acquires the variation in the value of the processing target signal. More specifically, the threshold setting unit  428  calculates the standard deviation as an indicator indicating the variation in the value of the first processing target signal generated in S 1209 , based on different criteria depending on the type of CFAR processing executed in S 1210 . Since a specific example of how to calculate the standard deviation according to the type of CFAR processing has already been described, further description is omitted here. 
     Then, in S 1212 , the threshold setting unit  428  sets a threshold which is a comparison target of the value of the difference signal acquired in S 1210 , based on the variation in the value of the processing target signal acquired in S 1211 . More specifically, the threshold setting unit  428  sets the threshold by executing processing such as multiplying the standard deviation of the value of the processing target signal acquired in S 1211  by a predetermined gain. 
     Then, in S 1213 , the threshold processing unit  427  executes threshold processing for comparing the value of the difference signal acquired in S 1210  with the threshold set in S 1212 . 
     Then, in S 1214 , the detection unit  429  detects the distance to the object that causes reflection of the transmission wave, based on the result of the threshold processing in S 1213 . More specifically, the detection unit  429  regards the timing at which the difference signal acquired in S 1210 , which is specified as a result of the threshold processing in S 1213 , exceeds the threshold set in S 1212  as the timing at which the reception wave is received as a result of the first transmission wave being reflected by and returned from the object that is a detection target, and detects the distance to the object by the TOF method. Then, the process ends. 
     As described above, the object detection device  200  according to the embodiment includes the wave transmitter  411 , the wave receiver  421 , the CFAR processing unit  426 , the threshold setting unit  428 , and the detection unit  429 . The wave transmitter  411  transmits the first transmission wave. The wave receiver  421  receives the first reception wave as the first transmission wave reflected by and returned from the object. The CFAR processing unit  426  samples a first processing target signal according to the first reception wave and acquires a difference signal based on a difference between a value of the first processing target signal for at least one sample according to the first reception wave received at a certain detection timing, and an average value of the values of the first processing target signal for a plurality of samples according to the first reception wave received in at least one of a first period and a second period of a predetermined time length that exist before and after the detection timing. The threshold setting unit  428  sets a threshold which is a comparison target with the value of the difference signal, based on variation in the value of the first processing target signal for a plurality of samples. The detection unit  429  detects information about the object at the detection timing based on a comparison result between the value of the difference signal and the threshold. 
     According to the configuration described above, even when there is a change in the environment, based on the variation in the value of the first processing target signal, for example, it is possible to dynamically set an appropriate threshold such that a reception wave as a transmission wave reflected by and returned from an object that is a detection target is detected but clutter is not detected. Accordingly, according to the configuration described above, it is possible to accurately detect the reception wave as the transmission wave reflected by and returned from the object that is a detection target, while appropriately reducing clutter regardless of the environment. 
     In the embodiment, the CFAR processing unit  426  selectively executes a plurality of types of CFAR processing including CA-CFAR processing and GO-CFAR processing according to settings. CA-CFAR processing is processing for acquiring a difference signal based on a difference between the value of the first processing target signal for at least one sample according to the first reception wave received at the detection timing described above and an average value of values of the first processing target signal for a plurality of samples according to the first reception wave received in both the first period and the second period described above. GO-CFAR processing is processing for acquiring the difference signal based on a difference between the value of the first processing target signal for at least one sample according to the first reception wave received at the detection timing described above and a larger average value of an average value of the values of the first processing target signal for a plurality of samples according to the first reception wave received in the first period described above and an average value of the values of the first processing target signal for a plurality of samples according to the first reception wave received in the second period described above. According to such a configuration, flexibility can be increased by properly using a plurality of types of CFAR processing. 
     In the embodiment, when the CFAR processing unit  426  executes CA-CFAR processing, the threshold setting unit  428  sets a threshold based on a standard deviation as an indicator indicating variation in the value of the first processing target signal for a plurality of samples according to the first reception wave received in both the first period and the second period. When the CFAR processing unit  426  executes GO-CFAR processing, the threshold setting unit  428  sets the threshold based on the standard deviation corresponding to the larger of the average values among a standard deviation as an indicator indicating the variation in the value of the first processing target signal for a plurality of samples according to the first reception wave received in the first period, and a standard deviation as an indicator indicating the variation in the value of the first processing target signal for a plurality of samples according to the first reception wave received in the second period. According to such a configuration, an appropriate threshold can be set in consideration of an appropriate standard deviation according to a type of CFAR processing. 
     In the embodiment, the wave transmitter  411  transmits the first transmission wave after encoding the first transmission wave so as to include predetermined identification information. Then, the CFAR processing unit  426  uses the correlation value signal based on the correlation value indicating the degree of similarity of the identification information between the first transmission wave and the first reception wave as the first processing target signal to acquire the difference signal. According to such a configuration, by using the correlation value signal as the first processing target signal, the difference signal can be acquired in a form that makes it easy to determine whether or not the first transmission wave and the first reception wave are similar to each other. 
     Furthermore, in the embodiment, the wave transmitter  411  transmits the second transmission wave before transmitting the first transmission wave. The wave receiver  421  receives the second reception wave as the second transmission wave reflected by the road surface as the object and returned before receiving the first reception wave. Here, the object detection device  200  further includes the setting value acquisition unit  430  that estimates a shape of the road surface based on information about a second processing target signal according to the second reception wave and acquires a setting value according to the shape of the road surface based on the estimation result. Then, the CFAR processing unit  426  acquires the difference signal based on a difference between the value of the first processing target signal for at least one sample according to the first reception wave received at the detection timing and a larger one of the average value of the values of the first processing target signal for a plurality of samples according to the first reception wave received in at least one of the first period and the second period and the setting value. According to such a configuration, the difference signal in which clutter according to the shape of the road surface is appropriately reduced can be acquired by always subtracting a value of at least equal to or larger than the setting value from the value of the first processing target signal. 
     In the embodiment, the setting value acquisition unit  430  estimates the shape of the road surface, based on encoded information indicating whether or not the second transmission wave and the second reception wave are encoded, an average characteristic based on a moving average of the values of the second processing target signal, and variation in a difference between a value of a signal expressed by an empirical formula that most closely approximates the second processing target signal among the plurality of empirical formulas set in advance and the value of the second processing target signal, as information about the second processing target signal, and acquires the setting value based on the estimation result. According to such a configuration, the shape of the road surface can be appropriately estimated based on three types of information of the encoded information, the average characteristic, and the variation, and the setting value can be appropriately acquired based on the estimation result. 
     In the embodiment, the setting value acquisition unit  430  estimates the shape of the road surface by using the road surface estimation table  430   a  stored in advance as information indicating a correspondence relationship between the encoded information, the average characteristic, the variation in the difference, and the shape of the road surface and acquires the setting value using the setting value table  430   b  stored in advance as information indicating a correspondence relationship between the road surface shape and the setting value. According to such a configuration, the shape of the road surface can be easily estimated only by referring to the road surface estimation table  430   a  based on three types of information of the encoded information, the average characteristic, and the variation, and the setting value can be easily acquired only by referring to the setting value table  430   b  based on the estimation result. 
     Modification Example 
     In the embodiment described above, although the technique of the disclosure is applied to a configuration for detecting information about an object by transmitting and receiving ultrasonic waves, the technique of the disclosure can also be applied to a configuration in which information about an object is detected by transmitting and receiving sound waves, millimeter waves, radar, electromagnetic waves, and the like as waves other than ultrasonic waves. 
     In the described above embodiment, although, as a target to which the technique of the disclosure is applied, an object detection device that detects a distance to an object is illustrated, the technique of the disclosure can also be applied to an object detection device that detects only the presence or absence of an object as information about the object. 
     Furthermore, in the embodiment described above, although the road surface estimation table  430   a  as an example of the first setting information and the setting value table  430   b  as an example of the second setting information are configured as separate data, both may be configured as a single piece of data. That is, in the technique of the disclosure, the estimation of the shape of the road surface and acquisition of the setting value may be executed based on single data indicating the correspondence relationship of the five types of information of the encoded information, the average characteristic, the variation, the shape of the road surface, and the setting value. 
     Although the embodiment and the modification example of the disclosure have been described above, the embodiment and the modification example are merely examples, and are not intended to limit the scope of the invention. The novel embodiment and modification example can be implemented in various forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. The embodiment and modification example described above are included in the scope and the gist of the invention, and are also included in the invention described in the claims and the scope equivalent thereto. 
     An object detection device as an example of this disclosure includes a transmission unit that transmits a first transmission wave, a reception unit that receives a first reception wave as the first transmission wave that is reflected by and returned from an object, a signal processing unit that samples a first processing target signal according to the first reception wave and acquires a difference signal based on a difference between a value of the first processing target signal for at least one sample according to the first reception wave received at a certain detection timing, and an average value of values of the first processing target signal for a plurality of samples according to the first reception wave received in at least one of a first period and a second period of a predetermined time length that exist before and after the detection timing, a threshold setting unit that sets a threshold which is a comparison target with the value of the difference signal, based on variation in the values of the first processing target signal for the plurality of samples, and a detection unit that detects information about the object at the detection timing based on a comparison result between the value of the difference signal and the threshold. 
     According to the configuration described above, even when there is a change in the environment, based on the variation in the value of the first processing target signal, for example, it is possible to dynamically set an appropriate threshold such that a reception wave as a transmission wave reflected by and returned from an object that is a detection target is detected but clutter is not detected. Accordingly, according to the configuration described above, it is possible to accurately detect the reception wave as the transmission wave reflected by and returned from the object that is a detection target, while appropriately reducing clutter regardless of the environment. 
     In the object detection device described above, the signal processing unit may selectively execute, according to settings, a plurality of types of constant false alarm rate (CFAR) processing, which include cell averaging constant false alarm rate (CA-CFAR) processing that acquires the difference signal based on a difference between the value of the first processing target signal for the at least one sample according to the first reception wave received at the detection timing and the average value of the values of the first processing target signal for the plurality of samples according to the first reception wave received in both the first period and the second period and greatest of constant false alarm rate (GO-CFAR) processing that acquires the difference signal based on a difference between the value of the first processing target signal for the at least one sample according to the first reception wave received at the detection timing and a larger one of the average value of the values of the first processing target signal for the plurality of samples according to the first reception wave received in the first period and the average value of the values of the first processing target signal for the plurality of samples according to the first reception wave received in the second period. According to such a configuration, flexibility can be increased by properly using a plurality of types of CFAR processing. 
     In this case, when the signal processing unit executes the CA-CFAR processing, the threshold setting unit may set the threshold based on a standard deviation as an indicator indicating variation in the values of the first processing target signal for the plurality of samples according to the first reception wave received in both the first period and the second period and when the signal processing unit executes the GO-CFAR processing, the threshold setting unit may set the threshold based on a standard deviation corresponding to the larger one of the average values among a standard deviation as an indicator indicating the variation in the values of the first processing target signal for the plurality of samples according to the first reception wave received in the first period and a standard deviation as an indicator indicating the variation in the values of the first processing target signal for the plurality of samples according to the first reception wave received in the second period. According to such a configuration, an appropriate threshold can be set in consideration of an appropriate standard deviation according to a type of CFAR processing. 
     In the object detection device described above, the transmission unit may transmit the first transmission wave after encoding the first transmission wave so as to include predetermined identification information, and the signal processing unit may use a correlation value signal based on a correlation value indicating a degree of similarity of identification information between the first transmission wave and the first reception wave as the first processing target signal to acquire the difference signal. According to such a configuration, by using the correlation value signal as the first processing target signal, the difference signal can be acquired in a form that makes it easy to determine whether or not the first transmission wave and the first reception wave are similar to each other. 
     In the object detection device described above, the transmission unit may transmit a second transmission wave before transmitting the first transmission wave, the reception unit may receive a second reception wave as the second transmission wave reflected by and returned from a road surface as the object before receiving the first reception wave, and the object detection device may further include a setting value acquisition unit that estimates a shape of the road surface based on information about a second processing target signal according to the second reception wave and acquires a setting value according to the shape of the road surface based on the estimation result, and the signal processing unit may acquire the difference signal based on a difference between the value of the first processing target signal for the at least one sample according to the first reception wave received at the detection timing and a larger one of the average value of the values of the first processing target signal for the plurality of samples according to the first reception wave received in at least one of the first period and the second period and the setting value. According to such a configuration, the difference signal in which clutter according to the shape of the road surface is appropriately reduced can be acquired by always subtracting a value of at least equal to or larger than the setting value from the value of the first processing target signal. 
     In this case, the setting value acquisition unit may estimate the shape of the road surface, based on encoded information indicating whether or not the second transmission wave and the second reception wave are encoded, an average characteristic based on a moving average of the values of the second processing target signal, and variation in a difference between a value of a signal expressed by an empirical formula that most closely approximates the second processing target signal among the plurality of empirical formulas set in advance and the value of the second processing target signal, as the information about the second processing target signal, and acquire the setting value based on the estimation result. According to such a configuration, the shape of the road surface can be appropriately estimated based on three types of information of the encoded information, the average characteristic, and the variation, and the setting value can be appropriately acquired based on the estimation result. 
     In this case, the setting value acquisition unit may estimate the shape of the road surface using first setting information stored in advance as information indicating a correspondence relationship between the encoded information, the average characteristic, the variation in the difference, and the shape of the road surface and acquire the setting value using second setting information stored in advance as information indicating a correspondence relationship between the road surface shape and the setting value. According to such a configuration, the shape of the road surface can be easily estimated only by referring to the first setting information based on three types of information of the encoded information, the average characteristic, and the variation, and the setting value can be easily acquired only by referring to the second setting information based on the estimation result. 
     In the object detection device described above, the transmission unit and the reception unit may be integrally configured as a transmission and reception unit including a single vibrator capable of transmitting and receiving sound waves. According to such a configuration, the configuration for transmitting and receiving the transmission wave and the reception wave can be simplified. 
     In the object detection device described above, the detection unit may detect a distance to the object as information about the object. According to such a configuration, the distance to the object, which is one of pieces of the information related to the object, can be detected. 
     The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.