Patent Publication Number: US-2021190605-A1

Title: Correction amount setting apparatus, ultrasonic object detecting apparatus, correction amount setting method, and non-transitory computer-readable recording medium having correction amount setting program stored therein

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The entire disclosures of Japanese Patent Application No. 2019-231790 filed on Dec. 23, 2019, Japanese Patent Application No. 2019-231794 filed on Dec. 23, 2019, Japanese Patent Application No. 2019-231797 filed on Dec. 23, 2019, Japanese Patent Application No. 2019-231799 filed on Dec. 23, 2019, Japanese Patent Application No. 2019-231802 filed on Dec. 23, 2019, and Japanese Patent Application No. 2020-159869 filed on Sep. 24, 2020 are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a correction amount setting apparatus, an ultrasonic object detecting apparatus, a correction amount setting method, and a non-transitory computer-readable recording medium having a correction amount setting program stored therein. 
     BACKGROUND ART 
     A known ultrasonic object detecting apparatus (also referred to as sonar) is mounted on a vehicle and detects an object that is present around the vehicle by transmitting and receiving an ultrasound wave. 
     Typically, the ultrasonic object detecting apparatus of this type transmits an ultrasound wave and receives a reflection wave that returns from the outside, and compares the intensity of the reflection wave with a threshold for determining whether an object is present (hereinafter, this threshold is referred to as “object determination threshold”), thereby determining whether an object is present. 
     In the related art, in the ultrasonic object detecting apparatus of this type, considering the temperature dependency of an attenuation amount of a sound wave that propagates in the air, a temperature sensor has calculated an estimated outside air temperature outside the vehicle, and in accordance with the estimated outside air temperature, the object determination threshold or the sensitivity to the reflection wave has been corrected (hereinafter, this processing is also referred to as “temperature compensation processing”) (for example, see PTL 1 and PTL 2). 
     CITATION LIST 
     Patent Literature 
     PTL 1 
     Japanese Patent Application Laid-Open No. 2014-089071 
     PTL 2 
     Japanese Patent Application Laid-Open No. 2016-085040 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the ultrasonic object detecting apparatus of this type, if the estimated outside air temperature at the last temperature compensation processing largely deviates from the actual outside air temperature at the current time point, the object determination threshold becomes incorrect, and, as a result, a temporary abnormality (e.g., erroneous detection or detection failure) may occur. Accordingly, while the outside air temperature is changing, there is a demand for performing temperature compensation processing by calculating a more accurate estimated outside air temperature as early as possible. 
     However, the vehicle may travel in various environments. For example, when a vehicle is heated in a parking lot under a burning sun, when the temperature sensor is iced in a snowstorm, or when a vehicle enters an underground parking lot that is cooled by air-conditioning from a street under a burning sun, the temperature detected by the temperature sensor (hereinafter, this temperature is referred to as “detected temperature”) may be extraordinarily high or extraordinarily low, which largely deviates from the outside air temperature, or detected temperatures indicated by a plurality of temperature sensors in a vehicle may largely differ from each other. 
     To address such as issue, for example, PTL 1 describes that temperature compensation is performed under the condition that a vehicle has continuously traveled at a speed higher than or equal to a reference speed for a duration that is longer than or equal to a reference duration. According to this method, it is possible to perform temperature compensation processing by using a sensor value of the temperature sensor that is cooled by traveling wind to a temperature close to the outside air temperature. However, the related art according to PTL 1 has an issue of incapability of performing temperature compensation in a case where, for example, the vehicle travels on a jammed road. 
     In addition, PTL 2 describes that temperature compensation processing is performed by using the lowest detected temperature of detected temperatures detected by a plurality of temperature sensors. However, in a situation where the temperature sensors are iced in a snowstorm, for example, the lowest detected temperature is likely to be a temperature influenced by icing. Thus, by using the related art according to PTL 2, if the vehicle travels in a snowstorm, for example, there is an issue that temperature compensation might not be performed correctly. 
     The present disclosure has been made in view of the above issues and is directed to providing a correction amount setting apparatus that may perform more appropriate temperature compensation processing, an ultrasonic object detecting apparatus, a correction amount setting method, and a non-transitory computer-readable recording medium having a correction amount setting program stored therein. 
     Solution to Problem 
     The present disclosure mainly solving the problems mentioned above provides a correction amount setting apparatus for setting a correction amount for at least one sound wave sensor that is mounted on a vehicle and that detects an obstacle by transmitting and receiving a sound wave, the correction amount being a correction amount of a sensitivity to a reflection wave or a threshold for determining whether an obstacle is present, the correction amount setting apparatus comprising: 
     a correction amount calculator that acquires information regarding a detected temperature from a temperature sensor that detects an outside air temperature around the vehicle and determines the correction amount based on the detected temperature; and 
     a correction amount setter that sets the correction amount for the at least one sound wave sensor, 
     wherein the correction amount setter sets the correction amount for the at least one sound wave sensor at a first timing, the first timing being a time at which, from start of traveling of the vehicle, the vehicle, after having been accelerated to a speed higher than a first reference speed, is decelerated to the first reference speed. 
     Further, in another aspect, the present disclosure provides an ultrasonic object detecting apparatus comprising: the correction amount setting apparatus described above. 
     Further, in another aspect, the present disclosure provides a correction amount setting method for setting a correction amount for a sound wave sensor that is mounted on a vehicle and that detects an obstacle by transmitting and receiving a sound wave, the correction amount being a correction amount of a sensitivity to a reflection wave or a threshold for determining whether an obstacle is present, the correction amount setting method comprising: 
     first processing in which information regarding a detected temperature is acquired from a temperature sensor that detects an outside air temperature around the vehicle, and the correction amount is determined based on the detected temperature detected by the temperature sensor; and 
     second processing in which the correction amount is set for the sound wave sensor, 
     wherein, in the second processing, the correction amount is determined and set for the sound wave sensor at a timing, the timing being a time at which, from start of traveling of the vehicle, the vehicle, after having been accelerated to a speed higher than a first reference speed, is decelerated to the first reference speed. 
     Further, in another aspect, the present disclosure, a correction amount setting program for setting a correction amount for a sound wave sensor that is mounted on a vehicle and that detects an obstacle by transmitting and receiving a sound wave, the correction amount being a correction amount of a sensitivity to a reflection wave or a threshold for determining whether an obstacle is present, the correction amount setting program comprising: 
     first processing in which information regarding a detected temperature is acquired from a temperature sensor that detects an outside air temperature around the vehicle and the correction amount is determined based on the detected temperature detected by the temperature sensor; and 
     second processing in which the correction amount is set for the sound wave sensor, 
     wherein, in the second processing, the correction amount is determined and set for the sound wave sensor at a timing, the timing being a time at which, from start of traveling of the vehicle, the vehicle, after been accelerated to a speed higher than a first reference speed, is decelerated to the first reference speed. 
     Advantageous Effects of Invention 
     The correction amount setting apparatus according to an embodiment of the present disclosure can perform more appropriate temperature compensation processing. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example of the configuration of an ultrasonic object detecting apparatus according to an embodiment of the present disclosure; 
         FIG. 2  illustrates an example of a state in which the ultrasonic object detecting apparatus according to an embodiment of the present invention is mounted on a vehicle; 
         FIG. 3  illustrates examples of an object determination threshold stored in a threshold memory and a reception signal stored in a waveform memory according to an embodiment of the present disclosure; 
         FIG. 4  is an explanatory diagram of an operation of a sonar ECU according to Embodiment 1; 
         FIG. 5  is a flowchart illustrating an example of an operation of the sonar ECU according to Embodiment 1; 
         FIG. 6  is an explanatory diagram of temperature compensation processing performed by the sonar ECU according to Embodiment 2; 
         FIG. 7  is an explanatory diagram of temperature compensation processing performed by the sonar ECU according to Embodiment 2; 
         FIG. 8  is a flowchart illustrating an example of an operation of the sonar ECU according to Embodiment 2; 
         FIG. 9  is an explanatory diagram of the temperature compensation processing performed by the sonar ECU according to Embodiment 4; 
         FIG. 10  is a flowchart illustrating an example of an operation of the sonar ECU according to Embodiment 4; 
         FIG. 11  is an explanatory diagram of an operation of the sonar ECU according to Embodiment 5; 
         FIG. 12  is a flowchart illustrating an operation of the sonar ECU according to Embodiment 5; 
         FIG. 13  is an explanatory diagram of an operation of the sonar ECU according to Embodiment 6; 
         FIG. 14  is a flowchart illustrating an operation of the sonar ECU according to Embodiment 6; 
         FIGS. 15A and 15B  illustrate behaviors of detected temperatures from temperature sensors in a plurality of ultrasound wave sensors, respectively, mounted on the vehicle according to Embodiment 7; 
         FIG. 16  is a flowchart illustrating an operation of the sonar ECU according to Embodiment 7; 
         FIG. 17  is a flowchart illustrating an operation started by the sonar ECU according to Embodiment 8 when the vehicle is turned on with a key (e.g., when the vehicle is activated); and 
         FIG. 18  is a flowchart illustrating an operation performed by the sonar ECU according to Embodiment 8 while the vehicle is traveling. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, preferred embodiments of the present disclosure will be described in detail with reference to the attached drawings. Note that components having substantially the same functions are denoted by the same reference numerals so as to omit repeated description in the specification and the drawings. 
     (Basic Configuration of Ultrasonic Object Detecting Apparatus) 
     Now, an example of a basic configuration of an ultrasonic object detecting apparatus according to an embodiment of the present disclosure will be described with reference to  FIGS. 1 to 3 . 
       FIG. 1  illustrates an example of the configuration of ultrasonic object detecting apparatus  1  according to an embodiment of the present disclosure.  FIG. 2  illustrates an example of a state in which ultrasonic object detecting apparatus  1  according to an embodiment of the present disclosure is mounted on vehicle C. 
     Ultrasonic object detecting apparatus  1  includes ultrasound wave sensors  10 A to  10 H (each of which corresponds to “sound wave sensor” in the present invention) and sonar electronic control unit (ECU)  20  (corresponding to “correction amount setting apparatus” in the present invention). 
     Ultrasonic object detecting apparatus  1  is mounted on vehicle C. Herein, ultrasound wave sensors  10 A to  10 D are provided in a front part of vehicle C, and ultrasound wave sensors  10 E to  10 H are provided in a rear part of vehicle C. Ultrasound wave sensors  10 A to  10 H are typically provided to be exposed to the outside of vehicle C. 
     Note that ultrasound wave sensors  10 A to  10 H have substantially the same configuration. Unless ultrasound wave sensors  10 A to  10 H are distinguished from one another, they are simply referred to as “ultrasound wave sensor  10 ” or “ultrasound wave sensors  10 ” in the following description. 
     Ultrasound wave sensor  10  and sonar ECU  20  can transmit and receive necessary data and control signals to and from each other via on-board network  100  (e.g., a communication network conforming to the CAN communication protocol). 
     Vehicle C further includes, in addition to ultrasonic object detecting apparatus  1 , second temperature sensor  30  and vehicle ECU  40 . Second temperature sensor  30  is provided in an air-conditioning apparatus and detects an outside air temperature. Vehicle ECU  40  generally controls a driving state of vehicle C. Sonar ECU  20  can receive information regarding the driving state of vehicle C, and in particular, speed information and traveling direction information via on-board network  100 . Sonar ECU  20  is configured to be capable of communicating with second temperature sensor  30  and vehicle ECU  40 . 
     Note that sonar ECU  20  is, for example, a microcomputer including a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), a communication interface, and the like. Sonar ECU  20  implements the functions described below by, for example, the CPU referring to control programs and various kinds of data stored in the ROM and the RAM. 
     [Configuration of Ultrasound Wave Sensor  10 ] 
     Ultrasound wave sensor  10  includes transmitter/receiver  11 , drive circuit  12 , reception circuit  13 , controller  14 , and temperature sensor  15 . 
     Transmitter/receiver  11  externally transmits an ultrasound wave on the basis of a driving signal from drive circuit  12 . Transmitter/receiver  11  receives a reflection wave returning from the outside and outputs a signal indicating an echo intensity of the reflection wave to reception circuit  13 . Transmitter/receiver  11  is constituted by, for example, a piezoelectric element that mutually converts an electric signal and an ultrasound wave. 
     Drive circuit  12  generates a pulsed driving signal and outputs the driving signal to transmitter/receiver  11 . Note that the operation of drive circuit  12  is controlled on the basis of a transmission command from controller  14  (transmission/reception controller  14   a ). 
     Reception circuit  13  performs amplification processing and A/D conversion processing on the signal indicating the echo intensity of the reflection wave input from transmitter/receiver  11 , and outputs the signal subjected to the amplification processing and the A/D conversion processing (hereinafter, this signal is referred to as “reception signal”) to controller  14  (waveform memory  14   c ). 
     Controller  14  includes transmission/reception controller  14   a , communicator  14   b , waveform memory  14   c , threshold memory  14   d , and determiner  14   e . Note that controller  14  is, for example, a microcomputer including a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), a communication interface, and the like. 
     Upon acquisition of an operation instruction signal from sonar ECU  20  through communicator  14   b , transmission/reception controller  14   a  causes drive circuit  12  and reception circuit  13  to operate. Note that transmission/reception controller  14   a , for example, issues a command of a timing for generating a driving signal and a pulse width of the driving signal for drive circuit  12 . In addition, transmission/reception controller  14   a  may be capable of adjusting a gain (i.e., the sensitivity to the reflection wave) used by reception circuit  13  to amplify the echo intensity of the reflection wave. 
     Communicator  14   b  communicates with sonar ECU  20  via on-board network  100 . Communicator  14   b  receives, from sonar ECU  20 , a transmission instruction signal or a correction amount of an object determination threshold, for example. In addition, communicator  14   b  transmits, to sonar ECU  20 , a sensor value of temperature sensor  15  or a result of determination performed by determiner  14   e  as to whether an object is present, for example. 
     Waveform memory  14   c  sequentially stores reception signals received from reception circuit  13 . Typically, waveform memory  14   c  stores time-series data of the signal intensity of the reception signals. 
     Threshold memory  14   d  stores an object determination threshold for determining whether an object is present. Note that the object determination threshold is, as described above, a reference value of the signal intensity of the reception signal for determining whether an object is present. 
       FIG. 3  illustrates examples of the object determination threshold stored in threshold memory  14   d  and the reception signal stored in waveform memory  14   c  according to an embodiment of the present disclosure. Note that in  FIG. 3 , the horizontal axis represents the time from when transmitter/receiver  11  transmits an ultrasound wave until when transmitter/receiver  11  receives a reflection wave thereof, and the vertical axis represents the signal intensity [dB] of the reception signal or the signal intensity [dB] of the object determination threshold. 
     In  FIG. 3 , graph L 1  represents the object determination threshold for a low outside air temperature (e.g., 10° C.), graph L 2  represents the object determination threshold for a high outside air temperature (e.g., 30° C.), graph L 1   a  represents a temporal change in the signal intensity of the reception signal for a low outside air temperature, and graph L 2   a  represents a temporal change in the signal intensity of the reception signal for a high outside air temperature. 
     The longer the time from when transmitter/receiver  11  transmits an ultrasound wave until when transmitter/receiver  11  receives a reflection wave thereof, the smaller the object determination threshold stored in threshold memory  14   d  is, for the following reason. The longer the time from when transmitter/receiver  11  transmits an ultrasound wave until when transmitter/receiver  11  receives a reflection wave thereof, the more distant an object is from ultrasound wave sensor  10 . The more distant the object is, the larger the attenuation of the ultrasound wave is. 
     The object determination threshold stored in threshold memory  14   d  herein is configured to be corrected on the basis of the correction amount specified by sonar ECU  20 , considering the temperature dependency of the attenuation amount of the ultrasound wave. Specifically, through this correction processing, the object determination threshold stored in threshold memory  14   d  is set to a relatively small value if the outside air temperature is high or is set to a relatively large value if the outside air temperature is low. 
     Note that, for example, a reference value of the object determination threshold for an average outside air temperature (e.g., 20° C.) is stored in controller  14 , and the correction amount of the object determination threshold transmitted from sonar ECU  20  is added to or subtracted from the reference value of the object determination threshold, and the resultant value is set in threshold memory  14   d . Note that the memory structure of threshold memory  14   d  for enabling temperature compensation may be any structure, and, for example, the object determination threshold for each temperature may be stored in advance in the storage (e.g., the ROM) of controller  14 . Alternatively, the correction amount of the object determination threshold may be added to or subtracted from the reference value of the object determination threshold by sonar ECU  20 , and then the object determination threshold subjected to addition/subtraction of the correction amount may be transmitted from sonar ECU  20  and stored in the storage (e.g., the RAM) of controller  14 , for the following reason. To transmit the correction amount for addition/subtraction of the correction amount is the same as to transmit the threshold subjected to addition/subtraction for storage, in setting the correction amount for the sound wave sensor. 
     Determiner  14   e  compares the signal intensity of the reception signal stored in waveform memory  14   c  with the object determination threshold stored in threshold memory  14   d . If the signal intensity of the reception signal is greater than or equal to the object determination threshold, determiner  14   e  determines that an object is present; if the signal intensity of the reception signal is less than the object determination threshold, determiner  14   e  determines that no object is present. In addition, if an object is detected, determiner  14   e  transmits information regarding the object to sonar ECU  20 . 
     At this time, if determiner  14   e  determines that an object is present, on the basis of a reception timing (a time difference from when transmitter/receiver  11  transmits an ultrasound wave until when transmitter/receiver  11  receives a reflection wave that is greater than or equal to the object determination threshold), determiner  14   e  may calculate the distance from vehicle C to the object. Note that the distance from ultrasound wave sensor  10  to the object is ½ of a value obtained by multiplying, by the speed of sound, the time difference from when transmitter/receiver  11  transmits an ultrasound wave until when transmitter/receiver  11  receives a reflection wave that is greater than or equal to the object determination threshold). 
     Temperature sensor  15  is a temperature detector built in ultrasound wave sensor  10  and detects an atmosphere temperature around vehicle C, and more specifically, an atmosphere temperature around transmitter/receiver  11 . Temperature sensor  15  is provided for the purpose of detecting the outside air temperature and detects the outside air temperature in a normal state. However, being built in ultrasound wave sensor  10 , temperature sensor  15  may be influenced by the temperature inside ultrasound wave sensor  10  or the temperature of a bumper that is in contact with ultrasound wave sensor  10 . In addition, if a water drop or snow is attached to ultrasound wave sensor  10 , temperature sensor  15  may be influenced by the temperature of the attached matter. For example, temperature sensor  15  may be a thermistor. Temperature information detected by temperature sensor  15  is transmitted to sonar ECU  20  through controller  14  (communicator  14   b ). Note that temperature sensor  15  may also be referred to as any of temperature sensors  15 A to  15 H in the following description if temperature sensors  15  in plural ultrasound wave sensors  10 A to  10 H, respectively, are distinguished from one another. 
     [Configuration of Sonar ECU  20 ] 
     Sonar ECU  20  includes sensor operation commander  20   a , correction amount calculator  20   b , and correction amount setter  20   c.    
     Sensor operation commander  20   a  transmits an operation command signal to ultrasound wave sensor  10 . Upon acquiring the operation command signal from sensor operation commander  20   a , ultrasound wave sensor  10  starts an operation of transmitting an ultrasound wave and receiving a reflection wave thereof. 
     On the basis of the speed of vehicle C, sensor operation commander  20   a  herein determines whether ultrasound wave sensor  10  is to operate. Specifically, only if the speed of vehicle C is less than or equal to a predetermined speed (corresponding to a second reference speed described later), sensor operation commander  20   a  causes ultrasound wave sensor  10  to operate, for the following reason. Since the distance for which an obstacle can be detected by using an ultrasound wave is short, if an obstacle is detected by using an ultrasound wave while vehicle C is traveling at a high speed, emergency braking may be too late. In addition, sensor operation commander  20   a  is configured not to cause ultrasound wave sensor  10  to operate from the time point at which vehicle C is turned on with a key until when temperature compensation processing is performed. Note that the “predetermined speed” is, for example, 12 to 18 km/h. 
     Correction amount calculator  20   b  acquires, from temperature sensors  15  of all ultrasound wave sensors  10 , temperature information (i.e., detected temperatures) regarding the outside air temperature around vehicle C (i.e., around ultrasound wave sensors  10 ). Correction amount calculator  20   b , for example, successively acquires the detected temperatures from all temperature sensors  15  while vehicle C is traveling. 
     Note that correction amount calculator  20   b  may also acquire a detected temperature from second temperature sensor  30  that is provided in vehicle C and that detects the outside air temperature. 
     As a value to be used as the outside air temperature when calculating the correction amount (hereinafter, this value is referred to as “reference temperature”), correction amount calculator  20   b  may select one of the plurality of detected temperatures and use the temperature as the reference temperature, or may use the average of the plurality of detected temperatures as the reference temperature to calculate the correction amount of the object determination threshold. 
     Note that the storage (e.g., the RAM) of sonar ECU  20  successively stores information regarding the detected temperature detected by temperature sensor  15  acquired by correction amount calculator  20   b , and on the basis of a transition of the detected temperature stored in the storage, correction amount calculator  20   b  may calculate an estimated outside air temperature and may use the estimated outside air temperature as the reference temperature to calculate the correction amount of the object determination threshold. Alternatively, correction amount setter  20   c  may typically use the detected temperature from temperature sensor  15  at the current time point as the reference temperature to calculate the correction amount of the object determination threshold for ultrasound wave sensor  10 . 
     Correction amount calculator  20   b  may alternatively perform the above operation for calculating the correction amount by, for example, reading the correction amount of the object determination threshold in accordance with the reference temperature (the estimated outside air temperature or the detected temperature at the current time point) from a correction data table that is stored in advance in the storage (e.g., the ROM) of sonar ECU  20 . The correction data table stores, for example, for each reference temperature, the correction amount of the object determination threshold in accordance with the time from when transmitter/receiver  11  transmits an ultrasound wave until when transmitter/receiver  11  receives a reflection wave (see  FIG. 3 ). Note that the correction data table stores, for example, a correction amount from the object determination threshold for the average outside air temperature (e.g., 20° C.). The more the reference temperature deviates from the average outside air temperature, the larger the stored correction amount is. 
     Correction amount setter  20   c  performs correction amount setting processing in which the correction amount of the object determination threshold calculated by correction amount calculator  20   b  is transmitted to ultrasound wave sensor  10  so as to perform the temperature compensation processing in the end. In addition, if sonar ECU  20  is configured to add or subtract the correction amount of the object determination threshold to or from the reference value of the object determination threshold, sonar ECU  20  transmits, to controller  14 , the object determination threshold subjected addition/subtraction of the correction amount so as to perform the correction amount setting processing in the end. Note that the target to be corrected by correction amount setter  20   c  may be, instead of the object determination threshold to be set in threshold memory  14   d , the sensitivity to the reflection wave (i.e., a gain) of reception circuit  13 . 
     Sonar ECU  20  according to an embodiment of the present disclosure has features in the timing at which correction amount setter  20   c  performs the temperature compensation processing and in the use of the reference temperature used when correction amount calculator  20   b  calculates the correction amount. Now, various embodiments of the temperature compensation processing employed by sonar ECU  20  according to an embodiment of the present disclosure will be described. 
     Embodiment 1 
     Now, the configuration of sonar ECU  20  according to Embodiment 1 will be described below with reference to  FIGS. 4 and 5 . 
     Sonar ECU  20  (correction amount setter  20   c ) according to this embodiment has a feature in transmitting the correction amount of the object determination threshold at a timing at which, from the start of traveling of vehicle C (including a case of start from a parking state), vehicle C, after having been accelerated to a speed higher than a first reference speed, is decelerated to the first reference speed. 
       FIG. 4  is an explanatory diagram of an operation of sonar ECU  20  according to this embodiment. Note that  FIG. 4  illustrates a change in the speed of vehicle C. In  FIG. 4 , the horizontal axis represents the time, and the vertical axis represents the speed [m/sec] of vehicle C. Note that T 1  represents a start timing of the temperature compensation processing, and T 2  represents an end timing of the temperature compensation processing in  FIG. 4 . 
     As described above, for example, if vehicle C is heated in a parking lot under a burning sun, the detected temperature indicated by temperature sensor  15  becomes extraordinarily high, and the object determination threshold becomes incorrect. As a result, a temporary abnormality (erroneous detection or detection failure) may occur. In addition, such a situation may also occur when vehicle C is parked for a long time in a traffic jam under a burning sun. 
     Even if vehicle C is heated under a burning sun, for example, sonar ECU  20  according to this embodiment performs the temperature compensation processing for the object determination threshold so that the object determination threshold can be corrected to an appropriate value as early as possible and at an appropriate timing. That is, sonar ECU  20  starts to monitor the speed of vehicle C when vehicle C starts to travel and performs the temperature compensation processing at a timing at which vehicle C is decelerated to the first reference speed from a state of traveling at a speed higher than the first reference speed (V 1  in  FIG. 4 ). Note that sonar ECU  20  according to this embodiment successively acquires the speed of vehicle C at the current time point from vehicle ECU  40 . 
     The “first reference speed” (V 1  in  FIG. 4 ) herein to be used as the reference of the timing for performing the temperature compensation processing for the object determination threshold is a speed higher than or equal to a predetermined speed to be used as a reference for causing ultrasound wave sensor  10  to operate (hereinafter, this predetermined speed is also referred to as “second reference speed”) (V 2  in  FIG. 4 ). As the “first reference speed”, preferably, a speed higher than the second reference speed by 3 to 10 km/h, and for example, 20 to 25 km/h is set. 
     If vehicle C travels at a speed higher than or equal to the first reference speed, temperature sensor  15  is sufficiently cooled by traveling wind, and the detected temperature indicated by temperature sensor  15  is assumed to have converged to the actual outside air temperature. The timing at which vehicle C is decelerated to the first reference speed corresponds to the timing at which or immediately before ultrasound wave sensor  10  starts to operate. 
     That is, since the temperature compensation processing is performed at a timing at which vehicle C is decelerated to the first reference speed after having been accelerated to a speed higher than the first reference speed, the object determination threshold can be corrected to an appropriate value as early as possible and immediately before ultrasound wave sensor  10  needs to start to operate. 
     In addition, this can reduce the frequency for performing the temperature compensation processing. For example, this can limit the frequency for performing the temperature compensation processing to only the first time at which vehicle C is decelerated to the first reference speed after having been accelerated to a speed higher than the first reference speed from the start of traveling of vehicle C by being turned on with a key. Thus, unlike in PTL 1, the temperature compensation processing can be prevented from being unnecessarily repeated while vehicle C is traveling at a high speed. This can prevent unnecessary power from being consumed, unnecessary electromagnetic wave from being radiated, unnecessary channel capacity from being generated, a threshold memory from being degraded in accordance with high-frequency data overwriting, or the like. 
     In addition, if the first reference speed is set to a speed higher than the second reference speed by 3 to 10 km/h, the temperature compensation processing can be completed immediately before ultrasound wave sensor  10  starts to operate. That is, this can prevent a delay in detecting an object by ultrasound wave sensor  10 , which is caused by an interruption time of object detection for the temperature compensation processing. Typically, the deceleration rate of vehicle C has a practical upper limit (emergency braking: according to the international standard for autonomous emergency braking system (AEBS), the deceleration rate is 4 m/s 2  or higher, and in a typical design, the deceleration rate is 0.5G=about 4.9 m/s 2  even at full braking by emergency braking being activated under a good road condition. It is allowed to decrease the deceleration rate under a bad road condition.). Thus, for example, if the first reference speed is 25 km/h and the second reference speed is 20 km/h, at least 300 msec can be spared while vehicle C is decelerated by 5 km/h. A duration of about 300 msec may be sufficient to determine the correction amount of the object determination threshold and to end transmission of data regarding the correction amount. 
     Note that sonar ECU  20  according to this embodiment may perform the temperature compensation processing only once, which is the first time at which vehicle C is decelerated to the first reference speed. However, since the outside air temperature around vehicle C is monitored on a regular basis, the temperature compensation processing may be performed every time vehicle C is decelerated to the first reference speed after having been accelerated to a speed higher than the first reference speed. 
     In addition, to prevent the temperature compensation processing from being performed in a state where temperature sensor  15  is not cooled sufficiently, sonar ECU  20  preferably sets a condition on the traveling duration during which vehicle C travels at a speed higher than the first reference speed. In this case, the configuration may be such that, at a timing at which vehicle C is decelerated to the first reference speed after having been accelerated to a speed higher than the first reference speed, for example, sonar ECU  20  determines whether the traveling duration during which vehicle C travels at a speed higher than the first reference speed is longer than or equal to a reference duration. If the traveling duration is longer than or equal to the reference duration, the temperature compensation processing is performed; if the traveling duration is shorter than the reference duration, the temperature compensation processing is not performed. Note that in this case, it is preferable to employ a setting method according to Embodiment 3 as a method for setting the time range of the reference duration. 
       FIG. 5  is a flowchart illustrating an example of an operation of sonar ECU  20  according to this embodiment. 
     In step S 11 , sonar ECU  20  determines whether the speed of vehicle C exceeds the first reference speed. If the speed of vehicle C exceeds the first reference speed (S 11 : YES), sonar ECU  20  advances the process to step S 12 ; if the speed of vehicle C does not exceed the first reference speed (S 11 : NO), sonar ECU  20  ends the process in the flowchart illustrated in  FIG. 5 . 
     In step S 12 , sonar ECU  20  determines whether vehicle C is decelerated to the first reference speed, and waits for vehicle C to be decelerated to the first reference speed (S 12 : NO). When vehicle C is decelerated to the first reference speed (S 12 : YES), sonar ECU  20  advances the process to step S 13 . 
     In step S 13 , sonar ECU  20  acquires information regarding the detected temperature from temperature sensor  15 . 
     In step S 14 , on the basis of the information regarding the detected temperature at the current time point acquired in step S 13 , sonar ECU  20  determines the correction amount of the object determination threshold and transmits data regarding the correction amount to ultrasound wave sensor  10 . At this time, if ultrasound wave sensor  10  stores the reference value of the object determination threshold, sonar ECU  20  may transmit the correction amount of the object determination threshold to ultrasound wave sensor  10  as the data regarding the correction amount; if ultrasound wave sensor  10  does not have the reference value of the object determination threshold, sonar ECU  20  may add or subtract the correction amount of the object determination threshold to the reference value of the object determination threshold and transmit the object determination threshold subjected to addition/subtraction of the correction amount to ultrasound wave sensor  10  as the data regarding the correction amount. Regardless of whether the data regarding the correction amount is the correction amount itself or data subjected to the addition/subtraction of the correction amount, the timing at which the data regarding the correction amount is transmitted to ultrasound wave sensor  10  is the timing at which the correction amount for ultrasound wave sensor  10  is set. 
     Sonar ECU  20  according to this embodiment, for example, repeatedly performs the operation in this flowchart at predetermined time intervals (e.g., intervals of 100 ms) while vehicle C is traveling. 
     Through the above process, the temperature compensation processing can be performed at an appropriate timing without unnecessarily repeating the temperature compensation processing. The above description mainly illustrates the timing at which sonar ECU  20  sets the correction amount of the object determination threshold. However, the timing at which sonar ECU  20  calculates the correction amount is not necessarily the same timing with the timing at which sonar ECU  20  sets the correction amount. In the flowchart illustrated in  FIG. 5 , the information regarding the detected temperature is acquired after vehicle C has been decelerated to the first reference speed. However, the correction amount calculation processing can be performed independently of the correction amount setting processing. Thus, for example, sonar ECU  20  may repeat calculation of the correction amount of the object determination threshold at predetermined time intervals (e.g., intervals of 1 s), and sonar ECU  20  may set the most recent correction amount at the timing of step S 14  for ultrasound wave sensor  10 , for the following reason. Since the transition of the detected temperature is typically gradual, even if the time point for acquiring the detected temperature and calculating the correction amount changes from the time point immediately before the correction amount setting to the time point 1 second before the correction amount setting, the result of correction is not largely different. The time from when vehicle C is decelerated to the first reference speed until when ultrasound wave sensor  10  starts to operate is short, and thus, by separately performing the correction amount calculation processing in advance, a more inexpensive processing apparatus can implement the function. 
     [Effects] 
     As described above, at a timing at which vehicle C is decelerated to the first reference speed (e.g., 20 to 25 km/h) after having been accelerated to a speed higher than the first reference speed from the start of traveling of vehicle C, sonar ECU  20  according to this embodiment determines the correction amount of the object determination threshold and sets the correction amount for ultrasound wave sensor  10 . 
     Thus, according to sonar ECU  20  according to this embodiment, the object determination threshold can be corrected to an appropriate value as early as possible and at an appropriate timing. In particular, sonar ECU  20  according to this embodiment is useful in that the timing for performing the temperature compensation processing for the object determination threshold can be limited to the timing immediately before ultrasound wave sensor  10  starts to operate and in that the frequency for performing the temperature compensation processing can be reduced. 
     Thus, unlike PTL 1, the temperature compensation processing can be prevented from being unnecessarily repeated while vehicle C is traveling at a high speed. This can prevent unnecessary power from being consumed, unnecessary electromagnetic wave from being radiated, unnecessary channel capacity from being generated, a threshold memory from being degraded in accordance with high-frequency data overwriting, or the like. 
     Embodiment 2 
     Now, the configuration of sonar ECU  20  according to Embodiment 2 will be described below with reference to  FIGS. 6 to 8 . Sonar ECU  20  (correction amount calculator  20   b ) according to this embodiment has a feature in using, as the reference temperature to be referred to when performing temperature compensation, an estimated value estimated from a transition of the detected temperature from temperature sensor  15 . 
       FIGS. 6 and 7  are explanatory diagrams of the temperature compensation processing performed by sonar ECU  20  according to Embodiment 2. 
       FIG. 6  illustrates an example of a transition of the detected temperature from temperature sensor  15  when temperature sensor  15  in a high-temperature state is gradually cooled by traveling wind against vehicle C. In  FIG. 6 , the horizontal axis represents an elapsed time after the start of cooling of temperature sensor  15 , and the vertical axis represents the detected temperature based on the actual outside air temperature (hereinafter referred to as “actual temperature”). 
     As described above, in a case where vehicle C is heated in a parking lot under a burning sun, temperature sensor  15  is incapable of detecting an accurate outside air temperature at the start of traveling of vehicle C. Thus, the timing for determining the correction amount of the object determination threshold is preferably after temperature sensor  15  has been cooled by traveling wind and become to function normally. However, if temperature compensation is performed after waiting for the time at which the detected temperature from temperature sensor  15  is expected to have converged sufficiently, the time point for starting temperature compensation is late. In addition, in a case of simply using the detected temperature from temperature sensor  15  obtained when the reference duration that is specified in advance has elapsed, a temperature difference may remain between the detected temperature and the actual temperature. 
     From the above viewpoint, sonar ECU  20  (correction amount calculator  20   b ) according to this embodiment estimates the actual outside air temperature on the basis of a temporal transition of the detected temperature from temperature sensor  15  and determines the correction amount of the object determination threshold on the basis of the estimated outside air temperature. This enables earlier and more accurate temperature compensation. 
     The amount of heat to move by air cooling is generally in proportion to the difference between the detected temperature from temperature sensor  15  and the actual temperature, and thus, the temperature difference generally decreases in accordance with an exponential function. Herein, in a case where the detected temperature from temperature sensor  15  converges toward the actual temperature in accordance with the exponential function, as illustrated in  FIG. 6 , when the detected temperature at a certain time point Tx is Vx, Δt a  is present that satisfies the detected temperature at a timing Tx−Δt a  is Vx+ΔV and the detected temperature at a timing Tx−2Δt a  is Vx+3ΔV. This Δt a  is a half-life of the temperature difference between the detected temperature from temperature sensor  15  and the actual temperature. In this case, the detected temperature from temperature sensor  15  converges toward Vx−ΔV, and the value Vx−ΔV can be estimated to be the actual temperature. The convergent value Vx−ΔV of the temperature change is referred to as convergent temperature. Starting from the timing Tx−2Δt a , when the first Δt a  passes, the temperature difference between the detected temperature from temperature sensor  15  and the actual temperature is reduced by half, which is from 4ΔV to 2ΔV, and when another Δt a  passes, the temperature difference between the detected temperature from temperature sensor  15  and the actual temperature is reduced by half, which is from 2ΔV to ΔV. Thus, in other words, the timing Tx is a time point at which twice the half-life has passed. 
     Thus, from the start of temperature detection by temperature sensor  15 , sonar ECU  20  according to this embodiment monitors a transition of the detected temperature from temperature sensor  15  and detects the timing Tx corresponding to the time point at which twice the half-life has passed. In addition, sonar ECU  20  estimates, as the actual outside air temperature (i.e., the actual temperature), Vx−ΔV obtained by subtracting ΔV from the detected temperature Vx at the timing Tx corresponding to the time point at which twice the half-life has passed, and performs temperature compensation for the object determination threshold by using the estimated outside air temperature. 
     At this time, sonar ECU  20  may monitor a change in a temperature change rate (i.e., the temperature change amount per unit time) of the detected temperature from temperature sensor  15 , and, on the basis of the change in the temperature change rate, may determine the timing Tx corresponding to the time point at which twice the half-life has passed. 
       FIG. 7  illustrates an example of the change in the temperature change rate of the detected temperature from temperature sensor  15  when temperature sensor  15  in a high-temperature state is gradually cooled by traveling wind against vehicle C. In  FIG. 7 , the horizontal axis represents an elapsed time after the start of cooling of temperature sensor  15 , and the vertical axis represents the temperature change rate of the detected temperature from temperature sensor  15 . Note that Ta in  FIG. 7  represents the timing corresponding to the time point at which twice the half-life has passed. 
     Since the change in the temperature difference is an exponential function, the temperature change rate, which is the differential thereof, is also an exponential function. When the temperature change rate is referred to as a temperature change per unit time, the half-life is also referred to as “the time until the temperature change rate changes from an initial value (v 0  in  FIG. 7 ) to a half value (v 0 ×½ in  FIG. 7 )” (=Δt a  in  FIG. 7 ). That is, from the change in the temperature change rate indicated by the detected temperature from temperature sensor  15 , correction amount calculator  20   b  according to this embodiment can find the timing Ta at which the temperature change rate becomes from the initial value to ½, thereby determining the half-life. If the detected temperature from temperature sensor  15  at the timing Ta is Vx, and the temperature at the time point Ta−Δt a , which is earlier than the timing Ta by the half-life Ata, is Vx+ΔV, it may be assumed that the detected temperature from temperature sensor  15  converges toward the convergent temperature=Vx−ΔV. 
       FIG. 8  is a flowchart illustrating an example of an operation of sonar ECU  20  according to this embodiment. Note that the process in this flowchart is started when, for example, vehicle C is turned on with a key. 
     In step S 21 , sonar ECU  20  starts to acquire a sensor value (i.e., the detected temperature) from temperature sensor  15 . In addition, from the time point at the start of acquisition of the sensor value of temperature sensor  15 , sonar ECU  20  successively calculates a change rate (i.e., the temperature change rate per unit time) of the sensor value at each time point. 
     In step S 22 , on the basis of the sensor value of temperature sensor  15 , sonar ECU  20  waits for the temperature change rate of the detected temperature at the current time point to be reduced to ½ of the temperature change rate of the detected temperature at the cooling start time (i.e., a reference of the temperature change rate for determining whether the temperature change rate is reduced by half) (S 22 : NO). When the temperature change rate of the detected temperature at the current time point is reduced to ½ of a reference of the temperature change rate of the detected temperature at the cooling start time (S 22 : YES), sonar ECU  20  advances the process to step S 23 . Note that, at this time, for example, sonar ECU  20  may assume that the time point at which vehicle C starts to travel or the time point at which vehicle C is accelerated to the first reference speed is the cooling start time, and may use the temperature change rate of the detected temperature at this time point as the reference of the temperature change rate of the detected temperature at the cooling start time, or may assume that the time point at which the temperature change rate of the detected temperature peaks is the cooling start time, and, using the temperature change rate of the detected temperature at this point as the reference, may wait for the temperature change rate of the measured detected temperature to be reduced to ½ of the reference. 
     In step S 23 , sonar ECU  20  determines that the time taken for the temperature change rate of the detected temperature at the current time point to be reduced to ½ of the temperature change rate of the detected temperature at the cooling start time is the half-life until the detected temperature from temperature sensor  15  converges to the actual temperature, and estimates the actual temperature (i.e., the convergent temperature estimated from the half-life) by substantially the same method as that described with reference to  FIG. 6 . 
     In step S 24 , on the basis of the actual temperature estimated in step S 23 , sonar ECU  20  performs the temperature compensation processing for the object determination threshold. That is, at this time, on the basis of the actual temperature estimated in step S 23 , sonar ECU  20  determines the correction amount of the object determination threshold and sets the correction amount for ultrasound wave sensor  10 . 
     Note that the flowchart in  FIG. 8  illustrates a case in which sonar ECU  20  calculates the estimated value estimated from the detected temperature from temperature sensor  15  (i.e., the convergent temperature estimated from the half-life) only once. However, sonar ECU  20  preferably updates the estimated value at predetermined time intervals (e.g., intervals of 1 s). In this case, from the change in the temperature change rate of the detected temperature from temperature sensor  15 , sonar ECU  20  may find a timing at which the temperature change rate is twice the temperature change rate at this time point to repeat determination of the half-life and estimation of the actual temperature. The most recent measured value is closer to the actual temperature than the previous measured value is, and thus, by repeating such estimation, it is possible to estimate the actual temperature more accurately from the detected temperature from temperature sensor  15 . In addition, by repeating such estimation, even if the vehicle speed or the outside air temperature changes, it is possible to estimate the actual temperature accurately from the detected temperature from temperature sensor  15 . 
     In addition, to match with the timing for performing the temperature compensation processing described in Embodiment 1, sonar ECU  20  may perform the correction amount setting processing at a timing at which vehicle C is decelerated to the first reference speed after having been accelerated to a speed higher than the first reference speed. In this case, sonar ECU  20  may determine the correction amount of the object determination threshold on the basis of the outside air temperature estimated at the time point at which the correction amount setting processing is performed or may use the most recent correction amount obtained by repeated calculation at predetermined time intervals. Furthermore, if sonar ECU  20  performs the processing for adding/subtracting the correction amount of the object determination threshold to set the resultant value as the object determination threshold, sonar ECU  20  may perform addition/subtraction at the time of calculating the correction amount or may perform addition/subtraction immediately before transmitting the threshold to ultrasound wave sensor  10 . In any case, the timing at which data regarding the correction amount (the correction amount or the threshold subjected to addition/subtraction of the correction amount) is transmitted to ultrasound wave sensor  10 , not the timing at which addition/subtraction is performed, is the timing at which the correction amount setting processing for setting the correction amount for the sound wave sensor is performed. 
     [Effects] 
     As described above, on the basis of the change in the outside air temperature detected by temperature sensor  15 , sonar ECU  20  according to this embodiment estimates the actual outside air temperature, and determines the correction amount of the object determination threshold on the basis of the estimated outside air temperature. 
     This enables earlier and more accurate temperature compensation for the object determination threshold. 
     Embodiment 3 
     Now, the configuration of sonar ECU  20  according to Embodiment 3 will be described below. Sonar ECU  20  (correction amount calculator  20   b  and correction amount setter  20   c ) according to this embodiment has a feature in determining the timing for performing the temperature compensation processing on the basis of the transition of the detected temperature detected by temperature sensor  15 . 
     For example, on the basis of the transition of the detected temperature from temperature sensor  15 , sonar ECU  20  according to this embodiment determines the timing at which the detected temperature from temperature sensor  15  converges to a temperature around the actual temperature, and performs the temperature compensation processing by using the detected temperature from temperature sensor  15  at the timing. In other words, when vehicle C starts to travel, sonar ECU  20  according to this embodiment sets a waiting time until performing the temperature compensation processing. 
     The method by which sonar ECU  20  according to this embodiment determines the timing at which the detected temperature from temperature sensor  15  converges to a temperature around the actual temperature is substantially the same as the method described in Embodiment 2. That is, on the basis of the transition of the detected temperature from temperature sensor  15  or the change in the temperature change rate of the detected temperature from temperature sensor  15 , sonar ECU  20  determines the half-life from the time point at which cooling of temperature sensor  15  is started (e.g., the time point at which vehicle C starts to travel at a speed higher than or equal to the first reference speed) until the detected temperature from temperature sensor  15  converges to the actual temperature. In addition, from the half-life, sonar ECU  20  determines the timing at which the detected temperature from temperature sensor  15  converges to a temperature around the actual temperature. 
     After the half-life has passed, the temperature compensation processing may be performed at any timing. For example, the temperature compensation processing may be performed at an end timing of 80% temperature change of the temperature change in a duration until the detected temperature from temperature sensor  15  at a time point at which vehicle C is turned on with a key converges to the actual temperature or a timing at which the difference between the detected temperature from temperature sensor  15  and the actual temperature falls within a predetermined allowable range, such as 5 degrees. 
     Subsequently, sonar ECU  20  according to this embodiment waits for the time to pass until the determined timing for performing the temperature compensation processing, and then acquires information regarding the detected temperature from temperature sensor  15 . At the timing, sonar ECU  20  performs the temperature compensation processing by using the detected temperature detected by temperature sensor  15 . 
     [Effect] 
     As described above, on the basis of the transition of the detected temperature from temperature sensor  15 , sonar ECU  20  according to this embodiment estimates the timing at which the detected temperature converges to a temperature around the actual temperature and determines the timing for performing the temperature compensation processing. 
     This enables earlier and more accurate temperature compensation for the object determination threshold. 
     Note that the method by which sonar ECU  20  according to this embodiment sets the waiting time may be applied as, for example, a method by which sonar ECU  20  according to Embodiment 1 sets a time range of the reference duration of the traveling duration (i.e., the traveling duration during which vehicle C travels at a speed higher than the first reference speed). In this case, after vehicle C has started to travel, when the speed of vehicle C exceeds the first reference speed, on the basis of the transition of the detected temperature detected by temperature sensor  15 , sonar ECU  20  may set the time range of the reference duration. This can make sure to prevent the temperature compensation processing from being performed in a state where temperature sensor  15  is not sufficiently cooled. 
     Embodiment 4 
     Now, the configuration of sonar ECU  20  according to Embodiment 4 will be described below with reference to  FIGS. 9 and 10 . Sonar ECU  20  (correction amount calculator  20   b  and correction amount setter  20   c ) according to this embodiment has a feature in integrating a duration during which, from when vehicle C starts to travel (e.g., from when vehicle C is turned on with a key), vehicle C travels at a speed higher than a third reference speed (hereinafter, this duration is also referred to as “traveling duration”), determining a correction amount by using a detected temperature detected when the integrated value of the traveling duration becomes longer than or equal to a threshold duration, and setting the correction amount for ultrasound wave sensor  10 . 
     The “third reference speed” herein is substantially the same speed as the “first reference speed” described in Embodiment 1 and is 20 to 25 km/h, for example. In addition, the “threshold duration” is 3 to 15 minutes, for example, and may be the waiting time until performing the temperature compensation processing calculated by the method described above in Embodiment 3. 
       FIG. 9  is an explanatory diagram of the temperature compensation processing performed by sonar ECU  20  according to this embodiment. In  FIG. 9 , the horizontal axis represents an elapsed time after the time point at which vehicle C is turned on with a key, and the vertical axis presents the speed of vehicle C. Note that, as an example,  FIG. 9  illustrates a change in the speed when vehicle C stops repeatedly due to a traffic jam or traffic lights. 
     As described above, in a case where vehicle C is heated in a parking lot under a burning sun, temperature sensor  15  is incapable of detecting an accurate outside air temperature at the start of traveling of vehicle C. Thus, the timing for determining the correction amount of the object determination threshold is preferably after temperature sensor  15  has been cooled by traveling wind and become to function normally. 
     However, if the timing for determining the correction amount of the object determination threshold is simply set as a timing at which the traveling duration during which vehicle C travels at a speed higher than or equal to the reference speed becomes longer than or equal to the threshold duration, vehicle C is incapable of traveling continuously for a long time due to a traffic jam or traffic lights, and, in a case where the traveling duration of a single time does not exceed the threshold duration, the object determination threshold is not corrected for a long time. Even in a case where vehicle C is incapable of traveling continuously due to a traffic jam or traffic lights, however, if vehicle C is repeatedly subjected to fresh outside air along with traveling, temperature sensor  15  radiates heat accumulatively. 
     Thus, sonar ECU  20  (correction amount setter  20   c ) according to this embodiment is configured to integrate the traveling duration during which, from when vehicle C starts to travel (e.g., from when vehicle C is turned on with a key), vehicle C travels at a speed higher than the third reference speed (ΔE 1 , ΔE 2 , and ΔE 3  in  FIG. 9 ), and to perform the temperature compensation processing when the integrated traveling duration becomes longer than or equal to the threshold duration. This enables earlier temperature compensation processing after temperature sensor  15  has become to function normally. 
     At this time, sonar ECU  20  (correction amount setter  20   c ) preferably determines the correction amount, after the integrated value of the traveling duration during which vehicle C travels at a speed higher than the third reference speed becomes longer than or equal to the threshold duration, at a timing at which vehicle C is decelerated to the third reference speed (&gt;the reference speed for causing ultrasound wave sensor  10  to start to operate). Thus, the temperature compensation processing can be performed as early as possible and immediately before ultrasound wave sensor  10  needs to start to operate. 
     Note that, at this time, sonar ECU  20  may change a time range of the threshold duration on the basis of a change in the speed of vehicle C during a time slot in which vehicle C travels at a speed higher than the third reference speed. For example, in a case where the speed when vehicle C travels at a speed higher than the third reference speed is largely higher than the third reference speed, sonar ECU  20  may change the time range of the threshold duration to a short duration in accordance with the speed. In addition, in a case of using the method described above in Embodiment 3, if, due to traveling at a high speed, a measured temperature change rate is higher than the temperature change rate at the time the threshold duration is initially calculated, sonar ECU  20  may change the time range of the threshold duration to a short duration in accordance with the most recent temperature change rate. 
     In addition, sonar ECU  20  may initialize the integrated value of the traveling duration every time the temperature compensation processing is performed, and may repeatedly perform the temperature compensation processing while vehicle C is traveling. 
       FIG. 10  is a flowchart illustrating an example of an operation of sonar ECU  20  according to this embodiment. 
     In step S 31 , sonar ECU  20  acquires the speed of vehicle C from vehicle ECU  40 . 
     In step S 32 , sonar ECU  20  determines whether the speed of vehicle C is higher than the third reference speed. If the speed of vehicle C is higher than the third reference speed (S 32 : YES), sonar ECU  20  advances the process to step S 33 ; if the speed of vehicle C is lower than or equal to the third reference speed (S 32 : NO), the sonar ECU  20  ends the process in the flowchart in  FIG. 10  without performing any processing. 
     In step S 33 , sonar ECU  20  updates the integrated value of the traveling duration that is currently stored in the storage. 
     In step S 34 , sonar ECU  20  determines whether the integrated value of the traveling duration becomes longer than or equal to the threshold duration. If the integrated value of the traveling duration becomes longer than or equal to the threshold duration (S 34 : YES), sonar ECU  20  advances the process to step S 35 ; if the integrated value of the traveling duration is not longer than or equal to the threshold duration (S 34 : NO), sonar ECU  20  ends the process in the flowchart in  FIG. 10  without performing any processing. 
     In step S 35 , sonar ECU  20  determines whether vehicle C is decelerated to the third reference speed, and waits for vehicle C to be decelerated to the third reference speed (S 35 : NO). When vehicle C is decelerated to the third reference speed (S 35 : YES), sonar ECU  20  advances the process to step S 36 . 
     In step S 36 , sonar ECU  20  acquires information regarding the detected temperature from temperature sensor  15 , and performs the temperature compensation processing on the basis of the detected temperature. 
     Sonar ECU  20  according to this embodiment, for example, repeatedly performs the operation in this flowchart at predetermined time intervals (e.g., intervals of 100 ms) while vehicle C is traveling. The above description mainly illustrates the timing at which sonar ECU  20  sets the correction amount of the object determination threshold. However, the timing at which sonar ECU  20  calculates the correction amount is not necessarily the same timing with the timing at which sonar ECU  20  sets the correction amount. In the flowchart illustrated in  FIG. 10 , the information regarding the detected temperature is acquired after vehicle C has been decelerated to the third reference speed. However, the correction amount calculation processing can be performed independently of the correction amount setting processing. Thus, for example, sonar ECU  20  may repeat calculation of the correction amount of the object determination threshold at predetermined time intervals (e.g., intervals of 1 s), and sonar ECU  20  may set the most recent correction amount at the timing of step S 36  for ultrasound wave sensor  10 , for the following reason. Since the transition of the detected temperature is typically gradual, even if the time point for acquiring the detected temperature and calculating the correction amount changes from the timing immediately before the correction amount setting to the timing 1 second before the correction amount setting, the result of correction is not largely different. The time from when vehicle C is decelerated to the third reference speed until when ultrasound wave sensor  10  starts to operate is short, and thus, by separately performing the correction amount calculation processing in advance, a more inexpensive processing apparatus can implement the function. 
     [Effects] 
     As described above, sonar ECU  20  according to this embodiment integrates the traveling duration during which, from when vehicle C starts to travel, vehicle C travels at a speed higher than the third reference speed, determines the correction amount of the object determination threshold when the integrated value of the traveling duration becomes longer than or equal to the threshold duration, and sets the correction amount for ultrasound wave sensor  10 . 
     Thus, for example, even if temperature sensor  15  is overheated when vehicle C starts to travel, the temperature compensation processing can be performed by using the detected temperature from temperature sensor  15  after temperature sensor  15  has radiated heat. In particular, correction amount setter  20   c  according to this embodiment determines the timing for performing the temperature compensation processing on the basis of the integrated value of the traveling duration during which vehicle C travels at a speed higher than the third reference speed, and thus, even in a case where vehicle C intermittently travels or stops (e.g., in a case where vehicle C travels on a jammed road), it is possible to prevent a situation where the temperature compensation processing is not performed for a long time. 
     Embodiment 5 
     Now, the configuration of sonar ECU  20  according to Embodiment 5 will be described below with reference to  FIGS. 11 and 12 . Sonar ECU  20  (correction amount calculator  20   b  and correction amount setter  20   c ) according to this embodiment has a feature in determining whether a detected temperature detected by temperature sensor  15  that is built in ultrasound wave sensor  10  is normal, and, if the detected temperature is likely to be abnormal, suppressing a temperature compensation amount (i.e., the correction amount) of the object determination threshold. 
     Vehicle C may travel in various environments, and temperature sensor  15  in ultrasound wave sensor  10  is provided to be exposed to the outside of vehicle C. While vehicle C is traveling, due to water splash on the road, water drops may be attached to temperature sensor  15 , or, due to snow falling, ice and snow may be attached thereto. In addition to ice and snow, direct sunlight, exhaust gas of other vehicles traveling around vehicle C, and the like are likely to influence temperature sensor  15 . If ice and snow, direct sunlight, exhaust gas of other vehicles traveling around vehicle C, or the like influences temperature sensor  15  as above, the detected temperature detected by temperature sensor  15  may differ from the actual outside air temperature around vehicle C. As a result, the object determination threshold may be corrected to an inappropriate value, which may cause erroneous detection or detection delay of ultrasound wave sensor  10 . Sonar ECU  20  according to this embodiment can prevent such a situation. 
       FIG. 11  is an explanatory diagram of an operation of sonar ECU  20  according to this embodiment. 
     By using the detected temperature from second temperature sensor  30  provided at such a position so as not to be exposed to the outside of vehicle C for comparison, sonar ECU  20  according to this embodiment determines whether the detected temperature detected by temperature sensor  15  (referred to as “first temperature sensor  15 ” for convenience of description in this embodiment) that is built in ultrasound wave sensor  10  is normal. 
     Note that sonar ECU  20  according to this embodiment acquires temperature information regarding the outside air temperature from each of first temperature sensor  15  and second temperature sensor  30 . 
     In this embodiment, temperature sensor  15  that is built in ultrasound wave sensor  10  is referred to as “first temperature sensor  15 ” for convenience of description. 
     Second temperature sensor  30  herein is, for example, a temperature sensor that is included in an air-conditioning apparatus or intake apparatus (either is not illustrated) of vehicle C and that detects the outside air temperature. Although being inside the exterior of vehicle C and not exposed to the outside, second temperature sensor  30  is provided at a position appropriate for measuring the outside air temperature (e.g., in an air passage of the air-conditioning apparatus for introducing air to the inside of the vehicle from the outside of the vehicle or an intake filter part of the intake apparatus). Thus, unlike first temperature sensor  15 , second temperature sensor  30  is unlikely to be influenced by ice and snow attached thereto, direct sunlight, exhaust gas of other vehicles, or the like. That is, compared with first temperature sensor  15 , second temperature sensor  30  is advantageous in measuring the outside air temperature stably, although the responsivity to the change in the outside air temperature around vehicle C is lower. Second temperature sensor  30  may be any temperature sensor that is inside the exterior of the vehicle body and is suitable for measuring the outside air temperature. Thus, second temperature sensor  30  is not limited to one attached to the air-conditioning apparatus or the intake apparatus but may be a temperature sensor attached to another apparatus or may be an independent temperature sensor not attached to another apparatus. However, using an existing temperature sensor produces an effect of suppressing the entire cost, and thus, this embodiment describes a configuration of using temperature information obtained by the temperature sensor attached to the air-conditioning apparatus or the intake apparatus. 
     On the other hand, since first temperature sensor  15  is provided at such a position as to be exposed to the outside of vehicle C, the responsivity to the change in the outside air temperature around vehicle C is high. For example, if vehicle C enters a tunnel where the outside air temperature is low, first temperature sensor  15  can detect an accurate temperature at a relatively early stage. 
     Sonar ECU  20  according to this embodiment in a normal state (which means, in this embodiment, a state where the difference between the detected temperature from first temperature sensor  15  and the detected temperature from second temperature sensor  30  is less than a first threshold temperature), as described in the basic configuration of ultrasonic object detecting apparatus  1 , determines the correction amount of the object determination threshold on the basis of the detected temperature from first temperature sensor  15 . In this case, for example, by referring to a correction data table that is stored in advance in the storage (e.g., the ROM) of sonar ECU  20  by using the detected temperature from first temperature sensor  15  as the reference temperature, sonar ECU  20  acquires a correction amount corresponding to the detected temperature from first temperature sensor  15  and sets the correction amount for ultrasound wave sensor  10  without modifying it. 
     However, sonar ECU  20  (correction amount calculator  20   b ) according to this embodiment determines whether the difference between the detected temperature from first temperature sensor  15  and the detected temperature from second temperature sensor  30  is greater than or equal to the first threshold temperature, and if the difference is greater than or equal to the first threshold temperature, sonar ECU  20  modifies the correction amount of the object determination threshold such that the temperature compensation amount for the reference value of the object determination threshold is suppressed more than in a case where the difference is less than the first threshold temperature (i.e., in a normal state). Specifically, for example, if the difference is greater than or equal to the first threshold temperature, by referring to the correction data table, sonar ECU  20  acquires the correction amount corresponding to the detected temperature from first temperature sensor  15  and then modifies the correction amount to be smaller and sets the correction amount for ultrasound wave sensor  10 . Note that, in the correction data table, for example, by using the object determination threshold for an average outside air temperature (e.g., 25° C.) as the reference value, the correction amount for each temperature from the reference value is set, and to reduce the correction amount means to suppress the temperature compensation amount. 
     This processing is performed in order to prevent the correction amount of the object determination threshold from being determined on the basis of an abnormal temperature detected if first temperature sensor  15  is influenced by, for example, rainwater attached thereto. That is, correction amount calculator  20   b  according to this embodiment can prevent abnormal correction of the object determination threshold in accordance with abnormality of the detected temperature from first temperature sensor  15 . 
     In addition, if the difference between the detected temperature from first temperature sensor  15  and the detected temperature from second temperature sensor  30  is greater than or equal to the first threshold temperature, sonar ECU  20  (correction amount calculator  20   b ) according to this embodiment is configured to further determine whether the difference is greater than or equal to a second threshold temperature (note that the second threshold temperature &gt;the first threshold temperature), and, if the difference is greater than or equal to the second threshold temperature, to not perform the temperature compensation processing for the object determination threshold (see  FIG. 11 ) for the following reason. If the difference between the detected temperature from first temperature sensor  15  and the detected temperature from second temperature sensor  30  is extraordinarily large, the detected temperature from first temperature sensor  15  is assumed to be based on attached matter such as ice and snow, not on the outside air temperature around vehicle C. That is, the above configuration can prevent obviously incorrect temperature compensation. 
     Note that the threshold temperature for the difference between the detected temperature from first temperature sensor  15  and the detected temperature from second temperature sensor  30  is set to the same value in a case where the detected temperature from second temperature sensor  30  is higher than the detected temperature from first temperature sensor  15  and in a case where the detected temperature from second temperature sensor  30  is lower than the detected temperature from first temperature sensor  15  in  FIG. 11 . However, the threshold temperature may alternatively be set to different values in a case where the detected temperature from second temperature sensor  30  is higher than the detected temperature from first temperature sensor  15  and in a case where the detected temperature from second temperature sensor  30  is lower than the detected temperature from first temperature sensor  15 . 
       FIG. 12  is a flowchart illustrating an operation of sonar ECU  20  according to this embodiment. 
     In step S 41 , sonar ECU  20  acquires information regarding the detected temperature from first temperature sensor  15 . 
     In step S 42 , sonar ECU  20  acquires information regarding the detected temperature from second temperature sensor  30 . 
     In step S 43 , sonar ECU  20  determines whether the difference between the detected temperature from first temperature sensor  15  and the detected temperature from second temperature sensor  30  is less than the first threshold temperature. If the difference is less than the first threshold temperature (S 43 : YES), sonar ECU  20  advances the process to step S 45 ; if the difference is greater than or equal to the first threshold temperature (S 43 : NO), sonar ECU  20  advances the process to step S 44 . 
     In step S 44 , sonar ECU  20  determines whether the difference between the detected temperature from first temperature sensor  15  and the detected temperature from second temperature sensor  30  is less than the second threshold temperature. If the difference is less than the second threshold temperature (S 44 : YES), sonar ECU  20  advances the process to step S 46 ; if the difference is greater than or equal to the second threshold temperature (S 44 : NO), sonar ECU  20  advances the process to step S 47 . 
     In step S 45 , sonar ECU  20  performs the temperature compensation processing without suppressing the temperature compensation amount. At this time, for example, sonar ECU  20  sets a correction amount for ultrasound wave sensor  10  without modifying it, the correction amount being determined by referring to the correction data table on the basis of the detected temperature from first temperature sensor  15 . 
     In step S 46 , sonar ECU  20  performs the temperature compensation processing with the temperature compensation amount suppressed. At this time, for example, sonar ECU  20  modifies a correction amount to a half value, the correction amount being determined by referring to the correction data table on the basis of the detected temperature from first temperature sensor  15 , and sets the correction amount for ultrasound wave sensor  10 . 
     In step S 47 , sonar ECU  20  determines that the temperature compensation processing is not to be performed. At this time, for example, the sonar ECU  20  sets ultrasound wave sensor  10  in a non-operating state in order to prevent erroneous detection of ultrasound wave sensor  10 . 
     Sonar ECU  20  according to this embodiment, for example, repeatedly performs the operation in this flowchart at predetermined time intervals (e.g., intervals of 1 s) while vehicle C is traveling. Note that the correction amount calculation processing and the correction amount setting processing are performed concurrently in this flowchart. However, the correction amount calculation processing and the correction amount setting processing can be performed separately and independently. Thus, for example, to match with the timing for setting the correction amount described in Embodiment 1, the correction amount may be set at a timing at which the condition on the vehicle speed is satisfied. 
     [Effects] 
     As described above, sonar ECU  20  according to this embodiment determines whether the difference between the detected temperature from first temperature sensor  15  and the detected temperature from second temperature sensor  30  is greater than or equal to the first threshold temperature, and, if the difference is greater than or equal to the first threshold temperature, determines the correction amount of the object determination threshold to be set for ultrasound wave sensor  10  such that the temperature compensation amount for the reference value of the object determination threshold (e.g., a value at a normal temperature) becomes smaller than that in a case where the difference is less than the first threshold temperature. 
     Thus, while the responsivity of temperature compensation to the change in the outside air temperature around vehicle C is maintained, it is possible to prevent the object determination threshold from being corrected to an abnormal value due to a disturbance factor (e.g., ice and snow attached to ultrasound wave sensor  10 ) other than the outside air temperature. 
     Embodiment 6 
     Now, the configuration of sonar ECU  20  according to Embodiment 6 will be described below with reference to  FIGS. 13 and 14 . Sonar ECU  20  (correction amount calculator  20   b ) according to this embodiment has a feature in switching, depending on situation, a detected temperature to be referred to at the time of the temperature compensation processing from among detected temperatures detected by temperature sensors  15 A to  15 H that are built in plural ultrasound wave sensors  10 A to  10 H, respectively, provided in vehicle C. 
     Vehicle C may travel in various environments, and a traveling route of vehicle C may include an underground parking lot or a tunnel. In such an underground parking lot or a tunnel, a temperature difference may be present in the outside air temperature around vehicle C from the outdoor traveling route in many cases (during summer in particular, the temperature in such an environment is lower than that in the outdoor environment). Thus, the outside air temperature around vehicle C may suddenly change while vehicle C is traveling. In such a case, correction of the object determination threshold to be set for ultrasound wave sensor  10  may delay, which may cause obstacle erroneous detection or obstacle detection failure. Correction amount calculator  20   b  according to this embodiment can prevent such a situation. 
       FIG. 13  is an explanatory diagram of an operation of sonar ECU  20  according to this embodiment. 
     In a normal state (which means a case where the outside air temperature around vehicle C does not suddenly change in this embodiment), sonar ECU  20  (correction amount calculator  20   b ) according to this embodiment determines the correction amount of the object determination threshold by referring to detected temperatures from all temperature sensors  15 A to  15 H. At this time, correction amount calculator  20   b  determines the correction amount of the object determination threshold by using, for example, the average of the detected temperatures detected by temperature sensors  15 A to  15 H, or the second lowest detected temperature (refer to Embodiment 7 described later) of the detected temperatures detected by temperature sensors  15 A to  15 H. Note that the same correction amount of the object determination threshold is typically set for plural ultrasound wave sensors  10 A to  10 H. 
     Note that under a predetermined condition, sonar ECU  20  (correction amount calculator  20   b ) according to this embodiment determines the correction amount of the object determination threshold on the basis of only detected temperatures detected by temperature sensors  15  that are built in ultrasound wave sensors  10  provided on the front side with respect to the traveling direction of vehicle C among plural ultrasound wave sensors  10 A to  10 H. 
     The “predetermined condition” herein is typically a case where, while vehicle C is traveling, the outside air temperature around vehicle C is assumed to have suddenly changed or may suddenly change. Specifically, sonar ECU  20  determines that the following case corresponds to the “predetermined condition”. For example, a temperature difference between the detected temperatures indicated by temperature sensors  15  (e.g., one of temperature sensors  15 A to  15 D) that are built in ultrasound wave sensors  10  (e.g., one of ultrasound wave sensors  10 A to  10 D) provided on the front side with respect to the traveling direction of vehicle C among plural ultrasound wave sensors  10 A to  10 H and the detected temperatures indicated by temperature sensors  15  (e.g., one of temperature sensors  15 E to  15 H) that are built in ultrasound wave sensors  10  (e.g., one of ultrasound wave sensors  10 E to  10 H) provided on the rear side with respect to the traveling direction of vehicle C among plural ultrasound wave sensors  10 A to  10 H is greater than or equal to a third threshold temperature (e.g., 5° C.). Note that the traveling direction is not limited to ahead of the vehicle but may be behind the vehicle when the vehicle is moving backward. That is, when the vehicle is moving backward, ultrasound wave sensors  10  provided on the front side with respect to the traveling direction of vehicle C are ultrasound wave sensors  10 E to  10 H, and ultrasound wave sensors  10  provided on the rear side with respect to the traveling direction of vehicle C are ultrasound wave sensors  10 A to  10 D. Information on the traveling direction of vehicle C can be acquired via the on-board network  100 . 
     If sonar ECU  20  according to this embodiment determines that the current situation corresponds to the “predetermined condition”, sonar ECU  20  determines the correction amount of the object determination threshold by using only the detected temperatures from temperature sensors  15  (e.g., temperature sensors  15 A to  15 D) that are built in ultrasound wave sensors  10  (e.g., ultrasound wave sensors  10 A to  10 D) provided on the front side with respect to the traveling direction of vehicle C. Note that, in this case, sonar ECU  20  determines the correction amount of the object determination threshold by using, for example, the average or the second lowest detected temperature of the detected temperatures detected by temperature sensors  15  (e.g., temperature sensors  15 A to  15 D) in ultrasound wave sensors  10  (e.g., ultrasound wave sensors  10 A to  10 D) provided on the front side with respect to the traveling direction of vehicle C. 
     Sonar ECU  20  according to this embodiment performs the above processing because, in a case where vehicle C enters an underground parking lot or a tunnel having a temperature difference from the outdoor environment while traveling, the detected temperatures from temperature sensors  15  that are built in ultrasound wave sensors  10  provided on the front side with respect to the traveling direction of vehicle C converge to the actual outside air temperature around vehicle C earlier than the detected temperatures from temperature sensors  15  that are built in ultrasound wave sensors  10  provided on the rear side with respect to the traveling direction of vehicle C. The reason for this is that air in a region where vehicle C enters directly hits the side surfaces of the front side with respect to the traveling direction of vehicle C along with traveling of vehicle C, whereas vortices are formed around the side surfaces of the rear side with respect to the traveling direction of vehicle C, and part of outdoor air before vehicle C enters the tunnel or the like is retained thereon. In addition, in a case where a freight car moves backward to enter or approach a refrigeration facility for loading, such as a liquefied natural gas storage place or a cold storage, from the rear part of the vehicle body, cold air directly hits the rear surface of vehicle C, whereas part of not-cooled air is retained on the front surface of vehicle C. 
     Note that sonar ECU  20  according to this embodiment detects a state in which vehicle C enters an underground parking lot. Thus, for example, sonar ECU  20  may acquire, from vehicle ECU  40 , traveling-route slope information indicated by a slope sensor (not illustrated) mounted on vehicle C, information on the location and elevation of vehicle C indicated by a global positioning system (GPS) receiver (not illustrated) mounted on vehicle C, or information regarding the location of a tunnel or a refrigeration facility, and, on the basis of such information, may determine whether the current situation corresponds to the above “predetermined condition”. In this case, for example, on the basis of such information, if the slope of the traveling route on which vehicle C is traveling is greater than to a threshold angle, or if the elevation of the traveling route on which vehicle C is traveling changes by a threshold elevation or more within a predetermined distance or a predetermined duration, correction amount calculator  20   b  may determine that the current situation corresponds to the above “predetermined condition”, or, on the basis of the fact that vehicle C is near the refrigeration facility, may determine that the current situation corresponds to the above “predetermined condition”. 
       FIG. 14  is a flowchart illustrating an operation of sonar ECU  20  according to this embodiment. Note that this operation is performed while vehicle C is moving forward. 
     In step S 51 , sonar ECU  20  acquires information regarding the detected temperatures from temperature sensors  15 A to  15 H from all ultrasound wave sensors  10 A to  10 H, respectively, mounted on vehicle C. 
     In step S 52 , sonar ECU  20  determines whether the difference between the detected temperatures from temperature sensors  15 A to  15 D in ultrasound wave sensors  10 A to  10 D on the front side with respect to the traveling direction and the detected temperatures from temperature sensors  15 E to  15 H in ultrasound wave sensors  10 E to  10 H on the rear side with respect to the traveling direction is greater than or equal to the third threshold temperature. If the difference is less than the third threshold temperature (S 52 : NO), sonar ECU  20  advances the process to step S 53 ; if the difference is greater than or equal to the third threshold temperature (S 52 : YES), sonar ECU  20  advances the process to step S 54 . 
     In step S 53 , sonar ECU  20  determines the correction amount of the object determination threshold by using the average of the detected temperatures from temperature sensors  15 A to  15 H in all ultrasound wave sensors  10 A to  10 H and sets the correction amount for ultrasound wave sensors  10 A to  10 H. 
     In step S 54 , sonar ECU  20  determines the correction amount of the object determination threshold by using the average of the detected temperatures from temperature sensors  15 A to  15 D in ultrasound wave sensors  10 A to  10 D on the front side with respect to the traveling direction and sets the correction amount for ultrasound wave sensors  10 A to  10 H. Note that, while vehicle C is moving backward, sonar ECU  20  determines the correction amount of the object determination threshold by using the average of the detected temperatures from temperature sensors  15 E to  15 H in ultrasound wave sensors  10 E to  10 H on the front side with respect to the traveling direction. The traveling direction of vehicle C is acquired via the on-board network  100 . 
     Sonar ECU  20  according to this embodiment, for example, repeatedly performs the operation in this flowchart at predetermined time intervals (e.g., intervals of 1 s) while vehicle C is traveling. Note that the correction amount calculation processing and the correction amount setting processing are performed concurrently in this flowchart. However, the correction amount calculation processing and the correction amount setting processing can be performed separately and independently. Thus, for example, to match with the timing for setting the correction amount described in Embodiment 1, the correction amount may be set at a timing at which the condition on the vehicle speed is satisfied. 
     [Effects] 
     As described above, under the predetermined condition, sonar ECU  20  according to this embodiment determines the correction amount of the object determination threshold by selectively using the detected temperatures from temperature sensors  15  that are built in ultrasound wave sensors  10  provided on the front side with respect to the traveling direction of vehicle C from among temperature sensors  15 A to  15 H that are built in plural ultrasound wave sensors  10 A to  10 H, respectively, and on the basis of the detected temperatures detected by temperature sensors  15 . 
     Thus, even if vehicle C enters or exits from an underground parking lot or a tunnel having a temperature difference from the outdoor environment, the temperature compensation processing for the object determination threshold can be performed so as to correspond to the outside air temperature early. 
     Embodiment 7 
     Now, the configuration of sonar ECU  20  according to Embodiment 7 will be described below with reference to  FIGS. 15A, 15B, and 16 . Sonar ECU  20  (correction amount calculator  20   b ) according to this embodiment has a feature in selecting the second lowest detected temperature from among detected temperatures detected by temperature sensors  15 A to  15 H in plural ultrasound wave sensors  10 A to  10 H, respectively, and determining the correction amount of the object determination threshold on the basis of the second lowest detected temperature. 
     Vehicle C may travel in various environments, and ultrasound wave sensor  10  is provided to be exposed to the outside of vehicle C. While vehicle C is traveling, due to water splash on the road, water drops may be attached to temperature sensor  15  built in ultrasound wave sensor  10 , or, due to snow falling, ice and snow may be attached thereto. In addition, depending on the driving state of vehicle C, temperature sensor  15  may be thermally influenced by a heat source (e.g., engine or muffler) mounted on vehicle C. In such a case, the object determination threshold may be corrected to an incorrect value, and ultrasound wave sensor  10  may cause obstacle erroneous detection or obstacle detection failure. Correction amount calculator  20   b  according to this embodiment can prevent such a situation. 
       FIGS. 15A and 15B  illustrate examples of behaviors of detected temperatures from temperature sensors  15  in plural ultrasound wave sensors  10 , respectively, mounted on vehicle C. For example, in a case where vehicle C is hit by snow or sleet, individual ultrasound wave sensors  10  may or may not be hit by snow or sleet at random. In addition, if ultrasound wave sensors  10  are hit by snow or sleet, it may or may not be attached to ultrasound wave sensors  10 .  FIG. 15A  illustrates behaviors of detected temperatures from temperature sensors  15  in four ultrasound wave sensors  10  among plural ultrasound wave sensors  10  mounted on vehicle C.  FIG. 15B  illustrates a graph obtained by selectively connecting, with lines, the lowest detected temperature (thick line) and the second lowest detected temperature (broken line) at the respective timings among the detected temperatures from four temperature sensors  15  illustrated in  FIG. 15A . 
     As described above, the detected temperature from temperature sensor  15  that is built in ultrasound wave sensor  10  may indicate a value different from the actual outside air temperature due to a disturbance factor (e.g., ice and snow attached thereto or the heat source mounted on vehicle C). For example, in a case where ultrasound wave sensor  10  is influenced by ice and snow attached thereto, when snow or sleet hits and is attached to ultrasound wave sensor  10 , as illustrated in  FIG. 15A , the detected temperature from temperature sensor  15  that is built in ultrasound wave sensor  10  suddenly decreases to the chilling temperature. However, if snow or sleet melts to fall down or does not hit, the detected temperature gradually increases and converges toward the outside air temperature. 
     In a case where ultrasound wave sensors  10  are influenced by the heat source mounted on vehicle C, the lowest temperature of temperatures detected by temperature sensors  15 A to  15 H that are included in plural ultrasound wave sensors  10 A to  10 H, respectively, mounted on vehicle C may be the least thermally influenced by the heat source (e.g., engine or muffler) mounted on vehicle C and may be the closest to the outside air temperature, however, in a case where ultrasound wave sensors  10  influenced by ice and snow attached thereto, the lowest temperature may be the most distant from the outside air temperature and the most inappropriate temperature. Typically, if an event that changes at random is assumed, the temporal change in the lowest temperature of temperatures detected by plural temperature sensors  15  may be a mixture of transitional sudden changes in many cases, whereas the second lowest temperature is a mixture of convergence to a stationary state and has more stable behavior than the lowest temperature (see  FIG. 15B ). In addition, in a case where a thermal influence of the heat source (e.g., engine or muffler) mounted on vehicle C is assumed, ultrasound wave sensors  10  are provided at positions away from one another, and the number of thermally influenced temperature sensors  15  is limited. Thus, even if the second lowest temperature is selected, the case where the second lowest temperature is selected may be expected to be not largely different from the case where the lowest temperature is selected. 
     From the above viewpoint, sonar ECU  20  (correction amount calculator  20   b ) according to this embodiment selects the second lowest detected temperature of the detected temperatures from temperature sensors  15 A to  15 H in plural ultrasound wave sensors  10 A to  10 H, respectively, mounted on vehicle C and determines the correction amount of the object determination threshold from the detected temperature. 
     Note that, among temperature sensors  15 A to  15 H in plural ultrasound wave sensors  10 A to  10 H, respectively, temperature sensor  15  indicating the second lowest detected temperature temporally changes as illustrated in  FIG. 15B . Thus, when determining the correction amount of the object determination threshold, correction amount calculator  20   b  acquires the detected temperatures from temperature sensors  15 A to  15 H in plural ultrasound wave sensors  10 A to  10 H, respectively, and then determines the correction amount of the object determination threshold by using the second lowest detected temperature as the reference temperature. 
       FIG. 16  is a flowchart illustrating an operation of sonar ECU  20  according to this embodiment. 
     In step S 61 , sonar ECU  20  acquires information regarding the detected temperatures from temperature sensors  15 A to  15 H in plural ultrasound wave sensors  10 A to  10 H, respectively. 
     In step S 62 , sonar ECU  20  selects the second lowest temperature of the detected temperatures from temperature sensors  15 A to  15 H in plural ultrasound wave sensors  10 A to  10 H, respectively. 
     In step S 63 , sonar ECU  20  performs the temperature compensation processing by using the selected temperature. 
     Sonar ECU  20  according to this embodiment, for example, repeatedly performs the operation in this flowchart at predetermined time intervals (e.g., intervals of 1 s) while vehicle C is traveling. Note that the correction amount calculation processing and the correction amount setting processing are performed concurrently in this flowchart. However, the correction amount calculation processing and the correction amount setting processing can be performed separately and independently. Thus, for example, to match with the timing for setting the correction amount described in Embodiment 1, the correction amount may be set at a timing at which the condition on the vehicle speed is satisfied. 
     [Effects] 
     As described above, sonar ECU  20  according to this embodiment selects the second lowest detected temperature of the detected temperatures detected by temperature sensors  15 A to  15 H in plural ultrasound wave sensors  10 A to  10 H, respectively, and determines the correction amount of the object determination threshold on the basis of the second lowest detected temperature. 
     This enables the temperature compensation processing for the object determination threshold by using, as the reference, a temperature closer to the actual outside air temperature, and thus can increase the detection accuracy of an obstacle. 
     Note that this embodiment illustrates a case where sonar ECU  20  selects the second lowest detected temperature of the detected temperatures from temperature sensors  15 A to  15 H in all ultrasound wave sensors  10 A to  10 H mounted on vehicle C. However, in a case where a temperature sensor or temperature sensors with a high temperature-detection accuracy can be identified in advance from among temperature sensors  15 A to  15 H in all ultrasound wave sensors  10 A to  10 H mounted on vehicle C, sonar ECU  20  may narrow down temperature sensor  15  or temperature sensors  15  to be referred to from among temperature sensors  15 A to  15 H in ultrasound wave sensors  10 A to  10 H. 
     For example, while vehicle C is driving, if the outside air temperature around vehicle C is assumed to have suddenly changed, correction amount calculator  20   b  may refer to only the detected temperatures from temperature sensors  15  in ultrasound wave sensors  10  provided on the front side with respect to the traveling direction of vehicle C among ultrasound wave sensors  10 A to  10 H mounted on vehicle C, and, by using the second lowest detected temperature of these detected temperatures as the reference temperature, may determine the correction amount of the object determination threshold. 
     On the other hand, if ultrasound wave sensors  10  with a large heat generation amount in operation are used, sonar ECU  20  may refer to only the detected temperatures from temperature sensors  15  in ultrasound wave sensors  10  provided on the rear side with respect to the traveling direction of vehicle C, and, by using the second lowest detected temperature of these detected temperatures as the reference temperature, may determine the correction amount of the object determination threshold, for the following reason. Typically, while vehicle C is driving, ultrasound wave sensors  10  provided on the rear side with respect to the traveling direction of vehicle C are in a non-operating state. 
     Embodiment 8 
     Now, the configuration of sonar ECU  20  according to Embodiment 8 will be described below with reference to  FIGS. 17 and 18 . This embodiment illustrates an example of a temperature compensation method obtained by combining the temperature compensation methods in the above embodiments. 
       FIG. 17  is a flowchart illustrating an operation started by sonar ECU  20  according to this embodiment when vehicle C is turned on with a key (e.g., when vehicle C is activated). 
     In step S 101 , first, sonar ECU  20  initializes a system. 
     In step S 102 , sonar ECU  20  acquires temperature information regarding the outside air temperature from temperature sensors  15 A to  15 H that are built in ultrasound wave sensors  10 A to  10 H mounted on vehicle C. 
     In step S 103 , sonar ECU  20  determines whether a difference between the detected temperatures (e.g., the average) detected in step S 102  and the detected temperature detected the last time and stored in the RAM or the like is greater than a predetermined value (e.g., 5° C.). If the difference is greater than the predetermined value (S 103 : YES), sonar ECU  20  advances the process to step S 104 ; if the difference is less than or equal to the predetermined value (S 103 : NO), sonar ECU  20  ends the process in the flowchart. Note that this processing is provided in order to skip the temperature compensation processing if the current outside air temperature is substantially equal to the detected temperature detected the last time. 
     In step S 104 , sonar ECU  20  calculates reference duration Th 1  defining the condition regarding the traveling duration. Threshold duration Th 1  is the reference duration described in Embodiment 1 for determining the duration during which vehicle C travels at a speed higher than the first reference speed. At this time, as described in Embodiment 3, for example, on the basis of the transition of the detected temperature from temperature sensor  15 , sonar ECU  20  determines the time until the detected temperature from temperature sensor  15  converges to a temperature around the actual temperature, and sets the time as reference duration Th 1 . 
     In step S 105 , sonar ECU  20  determines whether vehicle C is decelerated to the first reference speed after having traveled at a speed higher than the first reference speed. If this traveling condition is satisfied (S 105 : YES), sonar ECU  20  advances the process to step S 106 ; if this traveling condition is not satisfied (S 105 : NO), sonar ECU  20  returns the process to step S 104  and performs substantially the same processing again. Note that this processing corresponds to the processing for determining the timing for performing temperature compensation described in Embodiment 1. 
     In step S 106 , sonar ECU  20  determines whether the traveling duration during which vehicle C travels until vehicle C is decelerated to the first reference speed after having been accelerated to a speed higher than the first reference speed is longer than reference duration Th 1  calculated in step S 104 . If the traveling duration is longer than reference duration Th 1  calculated in step S 104  (S 106 : YES), sonar ECU  20  advances the process to step S 107 ; if the traveling duration is shorter than or equal to reference duration Th 1  calculated in step S 104  (S 106 : NO), sonar ECU  20  returns the process to step S 104  and performs substantially the same processing again. 
     In step S 107 , sonar ECU  20  acquires temperature information regarding the detected temperatures from temperature sensors  15 A to  15 H that are built in ultrasound wave sensors  10 A to  10 H, respectively, and calculates the reference temperature on the basis of this information. Note that, at this time, for example, sonar ECU  20  determines, as the reference temperature, the average of the detected temperatures from temperature sensors  15 A to  15 H that are built in plural ultrasound wave sensors  10 A to  10 H, respectively. 
     In step S 108 , by using the reference temperature calculated in step S 107 , sonar ECU  20  performs the temperature compensation processing for each of plural ultrasound wave sensors  10 A to  10 H. Note that, after having performed the temperature compensation processing, sonar ECU  20  transitions to an operation in the flowchart in FIG.  18 . 
     Through the above process, from when vehicle C starts, the temperature compensation processing for ultrasound wave sensor  10  can be performed at an appropriate timing without unnecessarily repeating the temperature compensation processing. 
       FIG. 18  is a flowchart illustrating an operation performed by sonar ECU  20  according to this embodiment while vehicle C is traveling. 
     In step S 111 , sonar ECU  20  acquires temperature information regarding the outside air temperature from temperature sensors  15 A to  15 H that are built in ultrasound wave sensors  10 A to  10 H mounted on vehicle C. 
     In step S 112 , sonar ECU  20  determines whether vehicle C is decelerated to the first reference speed after having traveled at a speed higher than the first reference speed. If this traveling condition is satisfied (S 112 : YES), sonar ECU  20  advances the process to step S 113 ; if this traveling condition is not satisfied (S 112 : NO), sonar ECU  20  returns the process to step S 111  and performs substantially the same processing again. 
     In step S 113 , sonar ECU  20  determines whether a difference between the detected temperatures detected in step S 111  and the detected temperature detected the last time and stored in the RAM or the like is greater than a predetermined value (e.g., 5° C.). If the difference is greater than the predetermined value (S 113 : YES), sonar ECU  20  advances the process to step S 114 ; if the difference is less than or equal to the predetermined value (S 113 : NO), sonar ECU  20  returns the process to step S 111  and performs substantially the same processing again. 
     In step S 114 , among temperature sensors  15 A to  15 H in plural ultrasound wave sensors  10 A to  10 H mounted on vehicle C, sonar ECU  20  calculates a difference between the detected temperatures from temperature sensors  15  in ultrasound wave sensors  10  provided on the front side with respect to the traveling direction and the detected temperatures from temperature sensors  15  in ultrasound wave sensors  10  provided on the rear side with respect to the traveling direction, and determines whether the difference is greater than the third threshold temperature (e.g., 5° C.). If the difference is greater than the third threshold temperature (S 114 : YES), sonar ECU  20  advances the process to step S 115 ; if the difference is less than or equal to the third threshold temperature (S 114 : NO), sonar ECU  20  advances the process to step S 117 . Note that this processing is determination processing so as to correspond to a situation where the outside air temperature around vehicle C suddenly changes while vehicle C is traveling, as described in Embodiment 6. 
     In step S 115 , sonar ECU  20  determines whether the slope of the traveling route on which vehicle C is traveling is greater than a threshold angle. If the slope is greater than the threshold angle (S 115 : YES), sonar ECU  20  advances the process to step S 116 ; if the slope is less than or equal to the threshold angle (S 115 : NO), sonar ECU  20  advances the process to step S 117 . Note that this processing is determination processing so as to correspond to a situation where the outside air temperature around vehicle C suddenly changes while vehicle C is traveling, as described in Embodiment 6. 
     In step S 116 , from among plural ultrasound wave sensors  10 A to  10 H mounted on vehicle C, sonar ECU  20  selects only temperature sensors  15 A to  15 D in ultrasound wave sensors  10 A to  10 D provided on the front side with respect to the traveling direction and calculates a reference temperature from the detected temperatures from temperature sensors  15 A to  15 D. Note that, at this time, for example, sonar ECU  20  calculates, as the reference temperature, the average of the detected temperatures from temperature sensors  15 A to  15 D in ultrasound wave sensors  10 A to  10 D provided on the front side with respect to the traveling direction. 
     In step S 117 , sonar ECU  20  determines a reference temperature from the detected temperatures from temperature sensors  15 A to  15 H in all plural ultrasound wave sensors  10 A to  10 H mounted on vehicle C. Note that, at this time, for example, sonar ECU  20  calculates, as the reference temperature, the average of the detected temperatures from temperature sensors  15 A to  15 H in all ultrasound wave sensors  10 A to  10 H. 
     In step S 118 , by using the reference temperature determined in step S 116  or step S 117 , sonar ECU  20  performs the temperature compensation processing for each of plural ultrasound wave sensors  10 A to  10 H. 
     For example, the process in the flowchart in  FIG. 18  is repeatedly performed at predetermined time intervals (e.g., intervals of 1 s) while vehicle C is traveling. Note that the correction amount calculation processing and the correction amount setting processing are performed concurrently in this flowchart. However, the correction amount calculation processing and the correction amount setting processing can be performed separately and independently. Thus, for example, to match with the timing for setting the correction amount described in Embodiment 1, the correction amount may be set at a timing at which the condition on the vehicle speed is satisfied. 
     Through the above process, even in a situation where the atmosphere temperature suddenly changes, temperature compensation for the object determination threshold can be performed appropriately. 
     Other Embodiments 
     The present invention may be modified in various manners without being limited to the above embodiments. 
     The above embodiments illustrate various examples of temperature compensation performed by sonar ECU  20 . However, the examples of temperature compensation performed by sonar ECU  20  illustrated in the embodiments may be implemented independently or in combination in various manners. 
     In addition, although the above embodiments illustrate, as an example, correction amount setter  20   c  correcting the object determination threshold, the target of temperature compensation may alternatively be the sensitivity to the reflection wave (i.e., a gain) of reception circuit  13 . In this case, correction amount setter  20   c  may set the correction amount such that the sensitivity is increased as the outside air temperature is higher and the sensitivity is decreased as the outside air temperature is lower, in contrast to the case of correcting the object determination threshold. 
     Furthermore, although the functions of sonar ECU  20  are implemented by processing of the CPU in the above embodiments, some or all of the functions of sonar ECU  20  may alternatively be implemented by, in place of or in addition to processing of the CPU, processing of a digital signal processor (DSP) or a dedicated hardware circuit (e.g., an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA)). 
     While various embodiments have been described herein above, it is to be appreciated that various changes in form and detail may be made without departing from the spirit and scope of the invention(s) presently or hereafter claimed. 
     INDUSTRIAL APPLICABILITY 
     The correction amount setting apparatus according to an embodiment of the present disclosure can perform more appropriate temperature compensation processing. 
     REFERENCE SIGNS LIST 
     
         
         C Vehicle 
           1  Ultrasonic object detecting apparatus 
           10 A to  10 H Ultrasound wave sensor 
           11  Transmitter/receiver 
           12  Drive circuit 
           13  Reception circuit 
           14  Controller 
           14   a  Transmission/reception controller 
           14   b  Communicator 
           14   c  Waveform memory 
           14   d  Threshold memory 
           14   e  Determiner 
           15 A to  15 H Temperature sensor (first temperature sensor) 
           20  Sonar ECU 
           20   a  Sensor operation commander 
           20   b  Correction amount calculator 
           20   c  Correction amount setter 
           30  Second temperature sensor 
           100  On-board network