Patent Publication Number: US-2020298838-A1

Title: Parking guidance apparatus and parking guidance system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 U.S.C. § 119 to Japanese Patent Application 2019-050145, filed on Mar. 18, 2019, the entire content of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     This disclosure generally relates to a parking guidance apparatus and a parking guidance system. 
     BACKGROUND DISCUSSION 
     Information obtained on a basis of an image of surroundings of a vehicle that is acquired by an onboard camera is provided to guide the vehicle to a predetermine area. Such technique is disclosed, for example, in JP2010-188744A. 
     The information included in the image usually varies depending on brightness in environment where the image is captured. In a case where a vehicle is guided using the aforementioned technique in autonomous (semi-autonomous) driving, for example, accuracy in guiding the vehicle may vary depending on environment. 
     A need thus exists for a parking guidance apparatus and a parking guidance system which are not susceptible to the drawback mentioned above. 
     SUMMARY 
     According to an aspect of this disclosure, a parking guidance apparatus provided at a vehicle, the parking guidance apparatus includes a reception processor receiving a transmission wave transmitted from a ground apparatus that is fixedly provided on a ground, the reception processor receiving the transmission wave through at least two onboard receivers fixedly provided at different positions from each other at the vehicle, the at least two onboard receivers including a first onboard receiver and a second onboard receiver that are different from each other, an estimation processor estimating a position of the ground apparatus relative to a position of the vehicle in accordance with a time difference between a first timing and a second timing, the first timing at which the transmission wave is received by the reception processor through the first onboard receiver, the second timing at which the transmission wave is received by the reception processor through the second onboard receiver, and a guide processor specifying a parking area in accordance with the position of the ground apparatus estimated by the estimation processor and guiding the vehicle towards the parking area, the parking area conforming to the position of the ground apparatus. 
     According to another aspect of this disclosure, a parking guidance apparatus includes a ground apparatus fixedly provided on a ground and transmitting a transmission wave and a parking guidance apparatus provided at a vehicle. The parking guidance apparatus includes a reception processor receiving a transmission wave transmitted from the ground apparatus, the reception processor receiving the transmission wave through at least two onboard receivers fixedly provided at different positions from each other at the vehicle, the at least two onboard receivers including a first onboard receiver and a second onboard receiver that are different from each other, an estimation processor estimating a position of the ground apparatus relative to a position of the vehicle in accordance with a time difference between a first timing and a second timing, the first timing at which the transmission wave is received by the reception processor through the first onboard receiver, the second timing at which the transmission wave is received by the reception processor through the second onboard receiver, and a guide processor specifying a parking area in accordance with the position of the ground apparatus estimated by the estimation processor and guiding the vehicle towards the parking area, the parking area conforming to the position of the ground apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed description considered with the reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic view illustrating a configuration of a parking guidance system according to an embodiment disclosed here; 
         FIG. 2  is a block diagram illustrating configurations of a parking guidance apparatus and a ground apparatus according to the embodiment; 
         FIG. 3  is a schematic view illustrating an example where the parking guidance apparatus receives a transmission wave from a guide device through object detectors according to the embodiment; 
         FIG. 4  is a schematic view illustrating a first method employed by the parking guidance apparatus for estimating a position of the guide device according to the embodiment; 
         FIG. 5  is a schematic view illustrating a second method employed by the parking guidance apparatus for estimating a position of the guide device according to the embodiment; 
         FIG. 6  is a schematic view illustrating a third method employed by the parking guidance apparatus for estimating a position of the guide device according to the embodiment; 
         FIG. 7  is a schematic view illustrating an example where the parking guidance apparatus receives a transmission wave from a stopper device according to the embodiment; and 
         FIG. 8  is a flowchart illustrating a processing executed by the parking guidance apparatus and the ground apparatus according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment is explained with reference to the attached drawings. Configurations of the embodiment, and effects and results derived from such configurations are examples and are not limited to descriptions below. 
     As illustrated in  FIG. 1 , a parking guidance system according to the embodiment guides a vehicle  1  to be parked or halted in autonomous (semi-autonomous) driving within a parking area A provided on the ground and serving as a predetermined region where the vehicle  1  is parked or halted. 
     As illustrated in  FIG. 1 , the parking guidance system according to the embodiment includes a parking guidance apparatus  100  serving as an onboard apparatus that is mounted at the vehicle  1  including a pair of front wheels  3 F and a pair of rear wheels  3 R, and a ground apparatus  200  provided on the ground so as to conform to the parking area A. 
     The parking guidance apparatus  100  is mounted inside the vehicle  1  to serve as an electronic control unit (ECU) corresponding to a microcomputer that includes the same or similar hardware for a typical computer, such as a central processing unit and memory, for example. The parking guidance apparatus  100  brings the vehicle  1  to be parked within the parking area A after the vehicle  1  enters through an entrance X in autonomous (semi-autonomous) driving along a path C illustrated in  FIG. 1 , for example, using plural object detectors  110  provided on an exterior of the vehicle  1 . 
     As illustrated in  FIG. 1 , for example, the plural object detectors  110  include four object detectors  111 F,  112 F,  113 F, and  114 F provided at a front end part of the vehicle  1 , four object detectors  111 R,  112 R,  113 R, and  114 R provided at a rear end part of the vehicle  1 , two object detectors  111 S and  112 S provided at a left end part of the vehicle  1 , and two object detectors  113 S and  114 S provided at a right end part of the vehicle  1 . Each of the object detectors  110  is constituted by a sonar that detects information related to an object that may be positioned in surroundings of the vehicle  1 , specifically, detects a distance thereto by transmitting and receiving ultrasonic waves. 
     According to the present embodiment, arrangements and the number of object detectors  110  are not limited to those illustrated in  FIG. 1  and may be appropriately specified and changed. 
     The ground apparatus  200  is placed in the vicinity of a boundary B (i.e., a rectangle indicated with a broken line in  FIG. 1 ) of the parking area A. Specifically, the ground apparatus  200  includes guide devices  210  and  220  provided at positions P 1  and P 2  abutting on the boundary B of the parking area A, and a stopper device  230  provided at a position P 3  abutting on the boundary B of the parking area A. Each of the guide devices  210  and  220  serves as a first ground device and the stopper device  230  serves as a second ground device. 
     The positions P 1  and P 2  correspond to opposed ends of the entrance X in a width direction thereof, the entrance X through which the vehicle  1  enters the parking area A. In  FIG. 1 , the entrance X is positioned at an upper end portion (upper side) of the rectangle indicated with the broken line corresponding to the boundary B of the parking area A. The parking area A is thus defined to be a rectangular area with a predetermined space including the end portion formed by the entrance X. 
     The position P 3  is away from the position P 1  by a predetermined distance in a direction where the vehicle  1  enters the parking area A (i.e., an entry direction of the vehicle  1 , refer to an arrow provided at an end of the path C illustrated in  FIG. 1 ). The position P 3  is specified to be opposed to the object detector  112 S of the vehicle  1  in a state where the vehicle  1  is positioned within the parking area A (see  FIG. 6 ) after entering the parking area A. 
     In  FIG. 1 , the positions P 1 , P 2 , and P 3  are all disposed abutting on the boundary B of the parking area A. Instead, the positions P 1 , P 2 , and P 3  may be away from the boundary B of the parking area to a certain amount as long as the positions P 1 , P 2 , and P 3  are placed in the vicinity of the boundary B of the parking area A. 
     According to a known technique, information obtained on a basis of an image of surroundings of a vehicle that is acquired by an onboard camera is provided to guide the vehicle to a predetermine area. 
     The information included in the image usually varies depending on brightness in environment where the image is captured. In a case where a vehicle is guided using the aforementioned technique in autonomous (semi-autonomous) driving, for example, accuracy in guiding the vehicle may vary depending on environment. 
     The parking guidance system according to the present embodiment enables the vehicle  1  to be accurately guided with no influence of environment in a state where the parking guidance apparatus  100  mounted at the vehicle  1  and the ground apparatus  200  provided on the ground operate together with functions as illustrated in  FIG. 2  (or hardware). 
     As illustrated in  FIG. 2 , in the present embodiment, the ground apparatus  200  includes the guide devices  210 ,  220 , and the stopper device  230 . 
     The guide devices  210 ,  220 , and the stopper device  230  of the ground apparatus  200  are explained below. 
     The guide device  210  includes a transmitter-receiver  211  and a controller  212 . The guide device  220  includes a transmitter-receiver  221 , a controller  222 , and a communicator. The stopper device  230  includes a transmitter-receiver  231 , a controller  232 , and a communicator. 
     Components having identical names include the same functions and operations. In the following, the construction of the guide device  210  of the ground apparatus  200  provided at the position P 1  illustrated in  FIG. 1  is explained as an example. 
     The transmitter-receiver  211  includes an oscillator  211   a  including a piezoelectric element, for example, that oscillates to receive and transmit ultrasonic waves. The transmitter-receiver  211  at least functions as a ground transmitter that transmits the ultrasonic wave from the ground. 
     The controller  212  controls the guide device  210 . The controller  212  controls transmission and reception of ultrasonic waves performed by the transmitter-receiver  211  and communication performed by the communicator, for example. 
     The oscillator  211   a  of the transmitter-receiver  211  of the guide device  210  and an oscillator  221   a  of the transmitter-receiver  221  of the guide device  220  transmit transmission waves serving as ultrasonic waves at least in a direction opposite to the entry direction of the vehicle  1  to the entrance X of the parking area A. At this time, the transmission wave from the transmitter-receiver  211  and the transmission wave from the transmitter-receiver  221  are transmitted so as not to interfere with each other. An oscillator  231   a  of the transmitter-receiver  231  of the stopper device  230  transmits a transmission wave (ultrasonic wave) in a different direction from the direction where the transmission wave is transmitted from each of the transmitter-receivers  211  and  221 . 
     A construction of the parking guidance apparatus  100  is explained below. 
     As illustrated in  FIG. 2 , the parking guidance apparatus  100  includes a reception processor  101 , an estimation processor  102 , and a guide processor  103 . The reception processor  101 , the estimation processor  102 , and the guide processor  103  may be partially or fully achieved in function by co-operation of hardware and software, specifically, by a processor of the ECU constituting the parking guidance apparatus  100  to read and execute program stored at a memory, or achieved by hardware such as an exclusive circuit, for example. 
     The reception processor  101  controls reception of ultrasonic waves through transmitter-receivers  110   a  provided at the respective object detectors  110 . Each of the transmitter-receivers  110   a  is provided as a device such as a piezoelectric element, for example, for transmitting and receiving the ultrasonic wave. The transmitter-receiver  110   a  includes an oscillator  110   b  including a piezoelectric element, for example, that oscillates to receive and transmit the ultrasonic wave. The transmitter-receiver  110   a  thus at least functions as an onboard receiver receiving the ultrasonic wave at the vehicle  1  from the ground apparatus  200 . 
     The estimation processor  102  performs an estimation processing for estimating positions of the guide devices  210  and  220  relative to the position of the vehicle  1  in accordance with an output value of a driving state sensor  130  that detects a driving state of the vehicle  1  and a reception result of transmission waves transmitted from the ground apparatus  200 , the reception result being obtained by the reception processor  101 . The driving state sensor  130  includes a wheel speed sensor that detects a rotation speed (the number of rotations) of each front wheel  3 F and rear wheel  3 R and a steering angle sensor that detects a steering angle of the vehicle  1 , for example. The output value of the driving state sensor  130  is usable for calculations of the present position of the vehicle  1  and a moving distance of the vehicle  1  by odometry, for example. 
     The guide processor  103  specifies the entrance X through which the vehicle  1  is expected to enter and the parking area A including the entrance X as illustrated in  FIG. 1 , for example, in accordance with the positions of the guide devices  210  and  220  estimated by the estimation processor  102  to perform a guidance control for guiding the vehicle  1  in autonomous (semi-autonomous) driving towards the entrance X and the parking area A. The autonomous (semi-autonomous) driving in the guidance control is achieved by controlling a driving control system mounted at the vehicle  1 . The driving control system includes a braking system  121  controlling braking of the vehicle  1 , an acceleration system  122  controlling acceleration of the vehicle  1 , a steering system  123  controlling steering of the vehicle  1 , and a gear change system  124  controlling gear change of the vehicle  1 , for example. 
     According to the present embodiment, the guidance control is achieved as explained below by cooperation between the parking guidance apparatus  100  and the ground apparatus  200 . 
     The reception processor  101  of the parking guidance apparatus  100  receives transmission waves transmitted from the guide devices  210  and  220  of the ground apparatus  200  through the transmitter-receivers  110   a  of the plural (specifically, two) object detectors  110  provided at a lateral end portion of the vehicle  1  in the following manner for achieving the guidance control performed by the guide processor  103 . 
     As illustrated in  FIG. 3 , the guide device  210  transmits the transmission wave to a range R 310  that expands from the position P 1  with the center constituted by an arrow A 310  that extends in an opposite direction to an arrow A 352  indicating an entry direction of the vehicle  1  to the entrance X, for example. The guide device  220  transmits the transmission wave to a range R 320  that expands from the position P 2  with the center constituted by an arrow A 320  in an opposite direction to the arrow A 352 , for example. 
     In a case where the vehicle  1  moves along a direction indicated by an arrow A 532  to cross the front of the guide device  210  as illustrated in  FIG. 3 , both the object detectors  111 S and  112 S provided at the left end portion of the vehicle  1  opposed to the guide device  210  are brought to be positioned within the range R 310  serving as a transmission range of the transmission wave from the guide device  210 . At this time, the estimation processor  102  of the parking guidance apparatus  100  estimates the position of the guide device  210  using any of three methods explained below in accordance with a time difference between a first timing and a second timing. Specifically, the transmission wave from the guide device  210  is received at the first timing through the object detector  111 S serving as the first onboard receiver. Additionally, the transmission wave from the guide device  220  is received at the second timing through the object detector  112 S serving as the second onboard receiver. 
     A first method employed by the parking guidance apparatus  100  for estimating the position of the guide device  210  is explained with reference to  FIG. 4 . In an example illustrated in  FIG. 4 , a point P 400  corresponds to the position of the guide device  210 , a point P 401  corresponding to the position of the object detector  111 S, a point P 402  corresponding to the position of the object detector  112 S. A distance D 401  between the points P 400  and P 402  corresponds to a transmission distance of the transmission wave between the guide device  210  and the object detector  112 S. A distance D 402  between the points P 400  and P 401  corresponds to a transmission distance of the transmission wave between the guide device  210  and the object detector  111 S. 
     As illustrated in  FIG. 4 , a distance D 403  is defined as a distance difference between the distance D 401  and the distance D 402 . Such distance difference corresponds to a value obtained by multiplying a time difference between the first timing and the second timing by speed of sound. The distance difference is thus acquirable by measuring the time difference. 
     A distance D 400  between the points P 401  and P 402  corresponds to an interval between the object detectors  111 S and  112 S. Such interval is acquirable as a predetermined value based on specifications of the vehicle  1 . 
     The position of the guide device  210  relative to the position of the vehicle  1  is calculated (estimated) with the distance D 400  and the distance D 403 . An angle α indicating the direction of the guide device  210  relative to the vehicle  1  is approximately calculated by the following calculation using the distances D 400  and D 403 . 
     Specifically, a distance between the vehicle  1  and the guide device  210  is sufficiently small relative to a distance by which the transmission wave moves (transfers) per time unit under circumstances which are expected or assumed in the embodiment. A triangle formed by the point P 401 , the point P 402 , and the point P 403  that is away from the point P 402  by the distance D 403  towards the point P 400  may be approximated to a right triangle with an angle β at the point P 403 . The aforementioned approximation leads to a relational expression of cos α=D 403 /D 400 . The angle α indicating the direction of the guide device  210  relative to the vehicle  1  is thus calculated using the distance D 400  and the distance D 403 . 
     The distance D 401  and the distance D 402  are calculated by the following calculation using the distance D 400 , the distance D 403 , and the angle β. The position of the guide device  210  relative to the vehicle  1  is calculated accordingly. 
     Specifically, the distance D 403  between the point P 402  and the point P 403  corresponds to the time difference between the first timing and the second timing. Thus, the distance D 402  between the point P 400  and the point P 401  and a distance between the point P 400  and the point P 403  (i.e., a distance D 401 -D 403 ) satisfies a formula: D 402 =D 401 −D 403 . Additionally, a formula (D 402 ) 2 =(D 400 ) 2 +(D 401 ) 2 −2×D 400 ×D 401 ×cos α is established by cosine formula for a relationship between the distance D 400  and the distance D 401 . The distance D 401  and the distance D 402  indicating the position of the guide device  210  relative to the vehicle  1  are thus calculated in the aforementioned formulas using the angle α, the distance D 400 , and the distance D 403 . 
     The estimation processor  102  when employing the first method acquires the time difference between the first timing at which the transmission wave is received through the object detector  111 S and the second timing at which the transmission wave is received through the object detector  112 S in a state as illustrated in  FIG. 3  where the transmission wave from the guide device  210  reaches both the object detectors  111 S and  112 S. The estimation processor  102  then calculates the distance difference between the transmission distance of the transmission wave between the guide device  210  and the object detector  111 S (i.e., a first transmission distance) and the transmission distance of the transmission wave between the guide device  210  and the object detector  112 S (i.e., a second transmission distance) in accordance with the time difference and estimates the position of the guide device  210  relative to the vehicle  1  from the distance difference. 
     The first method is usable in a state where the two object detectors  110  receive the transmission wave from the same guide device of the ground apparatus  200 . For example, when the vehicle  1  further moves along the direction indicated by the arrow A 532  in  FIG. 3 , both the object detectors  111 S and  112 S are positioned within the range R 320  serving as a transmission range of the transmission wave from the guide device  220 . In such state, the estimation processor  102  estimates the position of the guide device  220  relative to the vehicle  1  by the first method based on the time difference between the first timing at which the transmission wave is received through the object detector  111 S and the second timing at which the transmission wave is received through the object detector  112 S. 
     In  FIG. 3 , a state where the object detector  111 S is positioned within the range R 320  and the object detector  112 S is positioned within the range R 310  is presumable. The first method is not applicable to such state because the transmission wave from the guide device  220  and the transmission wave from the guide device  210  are received respectively by the two object detectors  111 S and  112 S. 
     In  FIG. 3 , the range R 310  and the range R 320  partially overlap each other. Thus, the transmission wave from the guide device  210  and the transmission wave from the guide device  220  are expected to be both received by the same object detector  110 . In this case, the transmission wave from the guide device  210  and the transmission wave from the guide device  220  interfere with each other, which may inhibit the parking guidance apparatus  100  from identifying the guide devices  210  and  220 . 
     In light of the foregoing, the guide devices  210  and  220  according to the embodiment transmit the respective transmission waves that are coded to include different identification information from each other so as to restrain the aforementioned interference. The parking guidance apparatus  100  identifies the transmission wave from the guide device  210  and the transmission wave from the guide device  220  accordingly. The other method than using the identification information for restraining the interference may include appropriately adjusting directionality of transmission waves from the guide devices  210  and  220  so as to minimize the overlapping area of transmission ranges thereof. 
     According to the first method, in a case where the state where the transmission wave from the guide device  210  is received by the two object detectors  110  ( 111 S and  112 S) and the state where the transmission wave from the guide device  220  is received by the two object detectors  110  ( 111 S and  112 S) occur one time each, the positons of the guide devices  210  and  220  are able to be estimated. The first method thus achieves simple and prompt processing. 
     Nevertheless, the state where the transmission wave from the guide device  210  is received by the two object detectors  110  may occur twice or more and the state where the transmission wave from the guide device  220  is received by the two object detectors  110  may occur twice or more. 
     Thus, according to the embodiment, the estimation of the position of each of the guide devices  210  and  220  by the first method is performed at least twice as illustrated in  FIG. 5 , so that the position of each of the guide devices  210  and  220  is more accurately estimated on a basis of more information. 
     A second method employed by the parking guidance apparatus  100  for estimating the positions of the guide devices  210  and  220  is explained with reference to  FIG. 5 . In  FIG. 5 , the object detectors  111 S and  112 S are only illustrated as the configuration of the vehicle  1  for purposes of simple explanation. 
     First, the estimation of the position of the guide device  210  in the second method is explained. As illustrated in  FIG. 5 , the estimation processor  102  estimates the direction of the guide device  210  relative to the vehicle  1  using the similar approximation to the first method in a first case where the object detectors  111 S and  112 S are placed at a position P 501  and a position P 502  respectively and in a second case where the object detectors  111 S and  112 S are placed at a position P 511  and a position P 512  with the movement of the vehicle  1 . The aforementioned direction of the guide device  210  serves as information corresponding to the angle α illustrated in  FIG. 4 , for example. 
     The estimation processor  102  estimates a candidate for the position of the guide device  210  in accordance with the direction that is estimated in each of the first case and the second case. The candidate for the position of the guide device  210  serves as information corresponding to a straight line passing through the point P 400  and the point P 402  illustrated in  FIG. 4 , for example. 
     The estimation processor  102  then estimates a point where the candidate for the position of the guide device  210  estimated in the first case and the candidate for the position of the guide device  210  estimated in the second case conform to each other as the actual position of the guide device  210 . At this time a positional relation between a first position serving as the position of the vehicle  1  in the first case and a second position serving as the position of the vehicle  1  in the second case needs to be obtained. Such positional relation is obtainable from a moving distance of the vehicle  1  between the first position and the second position that is calculated on a basis of an output value of the driving state sensor  130 . 
     The position of the guide device  210  is accurately estimated using the two estimation results according to the second method. 
     The estimation of the position of the guide device  220  according to the second method is performed in the same manner as above. Specifically, as illustrated in  FIG. 5 , the estimation processor  102  estimates the direction of the guide device  220  in the first case where the object detectors  111 S and  112 S are placed at positions P 521  and P 522  and the direction of the guide device  220  in the second case where the object detectors  111 S and  112 S have moved to positions P 531  and P 532  with the movement of the vehicle  1 . The estimation processor  102  estimates a candidate for the position of the guide device  220  in accordance with each of the aforementioned directions. The estimation processor  102  then estimates a point where the candidate for the position of the guide device  220  estimated in the first case and the candidate for the position of the guide device  220  estimated in the second case conform to each other as the actual position of the guide device  220 . 
     The second method is similar to the first method in terms of usage of approximation to estimate the direction of each of the guide devices  210  and  220  for estimating each position of the guide devices  210  and  220 . Thus, there is yet room for improving accuracy of estimation of positions of the guide devices  210  and  220  in the second method. 
     The estimation processor  102  of the parking guidance apparatus  100  according to the embodiment accurately estimates positions of the guide devices  210  and  220  as illustrated in  FIG. 6  without using the approximation utilized in the first method and the second method. 
     A third method employed by the parking guidance apparatus  100  for estimating the positions of the guide devices  210  and  220  is explained with reference to  FIG. 6 . A point P 600  corresponds to the position of the guide device  210  or  220 , a point P 601  corresponding to the position of the object detector  111 S, a point P 602  corresponding to the position of the object detector  112 S. A distance D 601  between the points P 600  and P 602  corresponds to a transmission distance of the transmission wave between the guide device  210  or  220  and the object detector  112 S. A distance D 602  between the points P 600  and P 601  corresponds to a transmission distance of the transmission wave between the guide device  210  or  220  and the object detector  111 S. 
     A distance D 603  serving as a distance difference between the distance D 601  and the distance D 602  is calculated on a basis of a time difference between the first timing at which the transmission wave from the guide device  210  or  220  is received through the object detector  111 S and the second timing at which the transmission wave from the guide device  210  or  220  is received through the object detector  112 S in the same manner as the first and second methods. 
     Mathematically, a point that satisfies a condition where a difference between distances from such point to two given points is constant is positioned on a hyperbola with foci constituted by the aforementioned two points. In  FIG. 6 , the point P 600  that satisfies a condition where a difference between distances from the point P 600  to the points P 601  and P 602  is constantly equal to the distance D 603  is located on a hyperbola L 600  with foci constituted by the points P 601  and P 602  in x-y coordinate including an origin ◯ that is a middle point between the point P 601  and the point P 602 . 
     In  FIG. 6 , the object detectors  111 S and  112 S are arranged at a left end portion of the vehicle  1  and the distance D 601  is longer than the distance D 602 . The point P 600  is thus supposed to be on one curve L 601  of the hyperbola L 600  (i.e., a part of the hyperbola L 600 ). The one curve L 601  of the hyperbola L 600  is estimated as a candidate for the position of the guide device  210  or  220 . 
     According to the third method, the hyperbola (or a part thereof) that satisfies the aforementioned condition is specified for each of the guide devices  210  and  220  at least two times. An intersection of the two hyperbolas specified for the guide device  210  is estimated to be an actual position of the guide device  210 . Additionally, an intersection of the two hyperbolas specified for the guide device  220  is also estimated to be an actual position of the guide device  220 . 
     In the third method, a positional relation between the position of the vehicle  1  obtained when the hyperbola is specified first time (i.e., the first position) and the position of the vehicle  1  obtained when the hyperbola is specified second time (i.e., the second position) is necessarily acquired in the same way as the second method. Such positional relation is acquirable from the moving distance of the vehicle  1  between the first position and the second position that is calculated on a basis of an output value of the driving state sensor  130 . 
     According to the embodiment, the estimation processor  102  of the parking guidance apparatus  100  estimates, in any of the first to third methods, the position of each of the guide devices  210  and  220  based on a time difference between the first timing at which the transmission wave from the guide device  210 ,  220  is received through the object detector  111 S serving as the first onboard receiver and the second timing at which the transmission wave from the guide device  210 ,  220  is received through the object detector  112 S serving as the second onboard receiver. 
     Specifically, the estimation processor  102  calculates, in any of the first to third methods, a distance difference between the transmission distance of the transmission wave between the guide device  210 ,  220  and the object detector  111 S and the transmission distance of the transmission wave between the guide device  210 ,  220  and the object detector  112 S in accordance with a time difference between the first timing and the second timing. The estimation processor  102  then estimates the position of the guide device  210 ,  220  in accordance with the aforementioned distance difference. 
     Specifically, in the first method, the estimation processor  102  estimates the position of each of the guide devices  210  and  220  using the aforementioned approximation by calculating one time the distance difference based on the time difference in the aforementioned manner. 
     Additionally, in the second and third methods, the estimation processor  102  calculates at least twice the distance difference based on the time difference in the aforementioned manner for each of the guide devices  210  and  220 . The estimation processor  102  estimates a candidate for the position of each of the guide devices  210  and  220  in accordance with the aforementioned distance difference. The estimation processor  102  estimates the position of each of the guide devices  210  and  220  based on a first candidate estimated first, a second candidate estimated second, and a positional relation between the position of the vehicle  1  obtained at the first estimation and the position of the vehicle  1  obtained at the second estimation. In the second method, the approximation similar to the first method is utilized to estimate the positions of the guide devices  210  and  220 . In the third method, the positions of the guide devices  210  and  220  are estimated using the hyperbola, without using the approximation. 
     In the aforementioned explanation for the first to third methods, the positions of the guide devices  210  and  220  are estimated using the object detectors  111 S and  112 S provided at the left end portion of the vehicle  1 . Alternatively, the positions of the guide devices  210  and  220  may be estimated using the other object detectors than the object detectors  111 S and  112 S in a state where such object detectors are able to receive transmission waves from the guide devices  210  and  220 . The number of object detectors  110  used for estimating the position of each of the guide devices  210  and  220  is not limited to two. The position of the guide device  210 ,  220  may be estimated with three or more than three object detectors  110 , for example. 
     The reception processor  101  may control the object detectors  111 S and  112 S to tentatively stop transmission of transmission waves for detecting the distance as sonars and to exclusively serve as receivers, so that reception of reception waves from the guide devices  210  and  220  is not disturbed. Such operation may be triggered by a predetermined operation by a driver to start the guide control, for example. 
     In a case where the positons of the guide devices  210  and  220  are estimated, the guide processor  103  specifies the entrance X where the vehicle  1  is expected to enter and the parking area A including the entrance X. Specifically, the guide processor  103  specifies the entrance X on the assumption that the positions of the guide devices  210  and  220  correspond to the opposed ends of the entrance X. The guide processor  103  specifies the parking area A based on the specified entrance X in accordance with the definition that the parking area A is a rectangular area with a predetermined space including the entrance X as an end portion. 
     The guide processor  103  determines an entry line of the vehicle  1  to the parking area A via the entrance X and starts the guide control so that the vehicle  1  enters the parking area A from the entrance X along the aforementioned entry line. The entry line includes a position or a direction of the vehicle  1  so that the vehicle  1  is inhibited from deviating from the opposed ends of the entrance X when entering the entrance X in autonomous (semi-autonomous) driving. 
     In the embodiment, the ground apparatus  200  includes the stopper device  230  in addition to the guide devices  210  and  220 . The stopper device  230  is also able to transmit the transmission wave. The transmission wave transmitted from the stopper device  230  is usable to halt or park the vehicle  1  at a predetermined position within the parking area A in a manner illustrated in  FIG. 7  after the vehicle  1  enters through the entrance X. 
     In  FIG. 7 , the parking guidance apparatus  100  according to the embodiment receives the transmission wave from the stopper device  230 . The vehicle  1  is positioned within the parking area A by moving along the entry direction indicated by the arrow A 352  to enter the parking area A through the entrance X in autonomous (semi-autonomous) driving. 
     The stopper device  230  is arranged to be opposed to the object detector  112 S of the vehicle  1  when the vehicle  1  is positioned within the parking area A. At timing where the vehicle  1  reaches a position within the parking area A corresponding to the position P 3  of the stopper device  230  as illustrated in  FIG. 7 , the signal level of the transmission wave from the stopper device  230  (see an arrow A 330 ) shows a peak level, the transmission wave being received through the object detector  112 S provided at the left end portion of the vehicle  1 . The guide control that is terminated at the aforementioned timing enables the vehicle  1  to be appropriately stopped within the parking area A so as not to deviate therefrom. 
     In the embodiment, the reception processor  101  continues to receive the transmission wave from the ground apparatus  200  (specifically, the stopper device  230 ) after the vehicle  1  enters the parking area A. The guide processor  103  terminates the guide control to stop (park) the vehicle  1  around timing where the signal level of the transmission wave from the stopper device  230  shows a peak level. 
     The parking guidance apparatus  100  and the ground apparatus  200  according to the embodiment including the aforementioned configurations perform a processing as illustrated in  FIG. 8 . In the following, the second or third method is employed for estimating the positions of the guide devices  210  and  220 . 
     As illustrated in a flowchart in  FIG. 8 , the guide devices  210 ,  220 , and the stopper device  230  of the ground apparatus  200  transmit transmission waves at S 801 . In  FIG. 8 , the transmission waves from the ground apparatus  200  are repeatedly transmitted. Instead, the transmission of the transmission waves may be started or finished in response to some trigger such as a reception of instruction from the parking guidance apparatus  100 , for example. 
     The reception processor  101  of the parking guidance apparatus  100  receives the transmission wave from one of the guide devices  210  and  220  arranged at the opposed ends of the entrance X, specifically, receives the transmission wave from one of the guide devices  210  and  220  positioned closer to the vehicle  1  at S 811 . In the following explanation, the transmission wave from the guide device  210  is received at the reception processor  101  as an example. 
     The estimation processor  102  of the parking guidance apparatus  100  estimates the first candidate for the position of the guide device  210  at S 812  using the second or third method in accordance with the reception result of the transmission wave obtained at S 811 . 
     The reception processor  101  of the parking guidance apparatus  100  again receives the transmission wave from the guide device  210  at S 813 . At this time, it is a precondition that the position of the vehicle  1  at the time of reception of the transmission wave at S 813  is different from the position of the vehicle  1  at the time of reception of the transmission wave at S 811 . 
     The estimation processor  102  of the parking guidance apparatus  100  estimates the second candidate for the position of the guide device  210  at S 814  using the second or third method in accordance with the reception result of the transmission wave at S 813 . 
     The estimation processor  102  of the parking guidance apparatus  100  then estimates at S 815 , as the position of the guide device  210 , a position where the estimation result obtained at S 812  and the estimation result obtained at S 814  conform to each other while reflecting the moving distance of the vehicle  1  from S 812  to S 814 . 
     The parking guidance apparatus  100  performs a processing in the same manner as S 811  to S 815  for the other one of the guide devices  210  and  220  of which position is not yet estimated (i.e., the guide device  220  in  FIGS. 8 ) at S 816  to S 820 . 
     Specifically, the reception processor  101  of the parking guidance apparatus  100  receives the transmission wave from the guide device  220  at S 816  and estimates the first candidate for the position of the guide device  220  at S 817  in accordance with the reception result obtained at S 816 . 
     The reception processor  101  of the parking guidance apparatus  100  again receives the transmission wave from the guide device  220  at S 818  and estimates the second candidate for the position of the guide device  220  at S 819  in accordance with the reception result of the transmission wave obtained at S 818 . 
     The estimation processor  102  of the parking guidance apparatus  100  then estimates at S 820 , as the position of the guide device  220 , a position where the estimation result obtained at S 817  and the estimation result obtained at S 819  conform to each other while reflecting the moving distance of the vehicle from S 817  to S 819 . 
     The guide processor  103  of the parking guidance apparatus  100  specifies at S 821 , the entrance X and the parking area A in accordance with the estimation result obtained at S 815  and the estimation result obtained at S 820 . 
     The guide processor  103  of the parking guidance apparatus  100  then starts the guide control for guiding the vehicle  1  to enter the parking area A through the entrance X along the entry line at S 822 . Specifically, the guide processor  103  determines the entry line indicating the position and/or the direction of the vehicle  1  so that the vehicle  1  is inhibited from deviating from the opposed ends of the entrance X in autonomous (semi-autonomous) driving when the vehicle  1  enters the entrance X, in accordance with the entrance X and the parking area A which are specified at S 821 . The guide processor  103  then starts the guide control. 
     The reception processor  101  of the parking guidance apparatus  100  receives the transmission wave from the stopper device  230  at S 823 . 
     The guide processor  103  of the parking guidance apparatus  100  terminates the guide control around or in the vicinity of timing at which the signal level of the transmission wave received at S 823  shows the peak level at S 824 . The vehicle  1  is stopped or parked at an appropriate position within the parking area A. The present processing is terminated accordingly. 
     The parking guidance apparatus  100  according to the embodiment mounted at the vehicle  1  includes the reception processor  101 , the estimation processor  102 , and the guide processor  103 . The reception processor  101  receives transmission waves transmitted from the guide devices  210  and  220  serving as the ground apparatus  200  and being provided fixedly on the ground, through at least two object detectors  110  serving as the two onboard receivers and being provided at different positions from each other at the vehicle  1 . The estimation processor  102  estimates positions of the guide devices  210  and  220  relative to the vehicle  1  in accordance with the time difference between the first timing at which the transmission wave is received by the reception processor  101  through one object detector  110  serving as the first onboard receiver and the second timing at which the transmission wave is received by the reception processor  101  through another object detector  110  serving as the second onboard receiver. The guide processor  103  specifies the parking area A provided conforming to the positions of the guide devices  210  and  220  in accordance with the positions of the guide devices  210  and  220  estimated by the estimation processor  102  to guide the vehicle  1  towards the parking area A. 
     Specifically, the reception processor  101  receives, through at least the aforementioned two object detectors  110 , the transmission wave transmitted from each of the guide devices  210  and  220  serving as the ground apparatus  200  and arranged at positions corresponding to the opposed ends of the entrance X opposed in the width direction thereof. The guide processor  103  estimates the position of the guide device  210  based on the transmission wave transmitted from the guide device  210  and estimates the position of the guide device  220  based on the transmission wave transmitted from the guide device  220 . The guide processor  103  specifies the entrance X and the parking area A based on the positions of the guide devices  210  and  220  to guide the vehicle  1  towards the parking area A so that the vehicle  1  enters the parking area A through the entrance X. 
     According to the embodiment including the aforementioned construction, the parking guidance apparatus  100  specifies the positions of the guide devices  210  and  220  using the transmission waves from the guide devices  210  and  220  to appropriately specify the parking area A provided conforming to the positions of the guide devices  210  and  220 , without using an image, for example, which may be easily influenced by environment. The vehicle  1  is thus accurately guided to the parking area A without being influenced by environment. 
     The estimation processor  102  calculates a distance difference between a transmission distance of the transmission wave between the guide device  210  ( 220 ) and the first onboard receiver and a transmission distance of the transmission wave between the guide device  210  ( 220 ) and the second onboard receiver in accordance with the time difference between the first timing and the second timing. The estimation processor  102  then estimates the position of the guide device  210  ( 220 ) based on the aforementioned distance difference. The position of the guide device  210  ( 220 ) is simply estimated by calculating the distance difference between the two transmission distances accordingly. 
     Specifically, the estimation processor  102  calculates the distance difference based on the aforementioned time difference at least in two cases including the first case where the vehicle  1  is located at the first position and the second case where the vehicle  1  is located at the second position different from the first position, and estimates the candidate for the position of each of the guide devices  210  and  220  based on the calculated distance difference. The estimation processor  120  then estimates the position of each of the guide devices  210  and  220  based on the first candidate estimated in the first case, the second candidate estimated in the second case, and the positional relation between the first position and the second position. The position of each of the guide devices  210  and  220  may be accurately estimated by specifying the position at which the two candidates obtained by at least two estimations conform to each other. 
     The estimation processor  102  obtains the positional relation between the first position and the second position based on the moving distance of the vehicle  1  between the first position and the second position that is calculated from the output value of the driving state sensor  130  provided at the vehicle  1  for detecting the driving state of the vehicle  1 . The positional relation between the first position and the second position that is required for specifying the position of each of the guide devices  210  and  220  by at least two estimations is easily obtainable from the output value of the driving state sensor  130 . 
     In the embodiment, the reception processor  101  receives the transmission waves through at least two object detectors  110  (for example, the object detectors  111 S and  112 S) fixedly provided at different positions from each other at the lateral end portion of the vehicle  1 . The position of each of the guide devices  210  and  220  may be easily specified using at least two object detectors  110  provided at the lateral end portion of the vehicle  1  where the transmission wave from each of the guide devices  210  and  220  may be easily received. 
     The reception processor  101  employs, as at least two onboard receivers receiving the transmission waves from the guide devices  210  and  220 , the object detectors  110  constituted by sonars that detect information related to an object in surroundings of the vehicle  1  by transmitting and receiving sound waves. The aforementioned construction inhibits an exclusive configuration for receiving the transmission waves from the guide devices  210  and  220  and enables easy reception of the transmission waves from the guide devices  210  and  220  using the sonars. 
     The aforementioned embodiment is achieved with transmission and reception of ultrasonic waves. Alternatively, the embodiment is achievable with transmission and reception of sound waves, millimeter waves, and electromagnetic waves, for example. 
     In the embodiment, the single guide device is provided at the position corresponding to each of the opposed ends of the entrance of the parking area, i.e., two guide devices are provided as the ground apparatus. The number of guide devices is not limited to two. The single guide device is acceptable in a state where the guide device is associated with the parking area (and the entrance) on a one-to-one basis so that the parking area (and the entrance) is appropriately specified in accordance with the estimation result of the position of the guide device. In the same manner, three or more than three guide devices are acceptable as long as the positions of such guide devices are appropriately specified in accordance with the estimation result of the positions of the guide devices. 
     In the embodiment, the stopper device is provided in addition to and separately from the two guide devices which are employed for specifying the parking area and the entrance X. The stopper device is provided to stop or park the vehicle at an appropriate position within the parking area. In a modified example, the stopper device may not be provided. Specifically, the vehicle may be stopped at an appropriate position without the stopper device being provided in a state where a process for specifying a target stop position of the vehicle at a predetermined position within the parking area A is performed together with the process for specifying the parking area based on the position of the guide device. 
     In the embodiment, the guide control is terminated around timing at which the signal level of the transmission wave from the stopper device that is received through the object detector shows the peak level so as to stop the vehicle at an appropriate position within the parking area. The embodiment also includes a construction where the position of the stopper device is estimated in the same manner as the estimation of positions of the guide devices and an appropriate position within the parking area is determined in accordance with the estimation result for stopping the vehicle, so that the guide control is terminated around timing at which the vehicle reaches the aforementioned appropriate position. 
     In the embodiment, the transmission wave from the ground apparatus is received through the object detectors functioning as sonars. Alternatively, the transmission wave from the ground apparatus may be received by exclusive onboard receivers that are separately provided from the sonars. 
     The embodiment and the modified examples thereof are not limited to include the aforementioned configurations. Omissions, replacements, and changes may be appropriately conducted for the embodiment and the modified examples thereof, for example. 
     According to the embodiment, a parking guidance apparatus  100  provided at a vehicle  1  includes a reception processor  101  receiving a transmission wave transmitted from a ground apparatus  200  that is fixedly provided on a ground, the reception processor  101  receiving the transmission wave through at least two object detectors  110  fixedly provided at different positions from each other at the vehicle  1 , the at least two object detectors  110  including a first object detector  111 S and a second object detector  112 S that are different from each other, an estimation processor  102  estimating a position of the ground apparatus  200  relative to a position of the vehicle  1  in accordance with a time difference between a first timing and a second timing, the first timing at which the transmission wave is received by the reception processor  101  through the first object detector  111 S, the second timing at which the transmission wave is received by the reception processor  101  through the second object detector  112 S, and a guide processor  103  specifying a parking area A in accordance with the position of the ground apparatus  200  estimated by the estimation processor  102  and guiding the vehicle  1  towards the parking area A, the parking area A conforming to the position of the ground apparatus  200 . 
     In addition, the estimation processor  102  estimates the position of the ground apparatus  200  based on a distance difference between a first transmission distance of the transmission wave between the ground apparatus  200  and the first object detector  111 S and a second transmission distance of the transmission wave between the ground apparatus  200  and the second object detector  112 S, the distance difference being calculated on a basis of the time difference between the first timing and the second timing. 
     Further, the estimation processor  102  calculates the distance difference based on the time difference in at least two cases including a first case where the vehicle  1  is in a first position and a second case where the vehicle  1  is in a second position different from the first position, estimates a candidate for the position of the ground apparatus  200  based on the distance difference, and estimates the position of the ground apparatus  200  based on a first candidate serving as the candidate estimated in the first case, a second candidate serving as the candidate estimated in the second case, and a positional relation between the first position and the second position. 
     Furthermore, the estimation processor  102  acquires the positional relation between the first position and the second position in accordance with a moving distance of the vehicle  1  between the first position and the second position, the moving distance being calculated on a basis of an output value of a driving state sensor provided at the vehicle  1  for detecting a driving state of the vehicle  1 . 
     Furthermore, the reception processor  101  receives the transmission waves transmitted from a first ground device  210  and a second ground device  220  each of which serves as the ground apparatus  200  through the at least two object detectors  110 , the first ground device  210  and the second ground device  220  being provided at positions corresponding to opposed ends of an entrance X of the parking area A through which the vehicle  1  enters, the opposed ends of the entrance X being opposed in a width direction of the entrance X. The guide processor  103  estimates a position of the first ground device  210  based on the transmission wave transmitted from the first ground device  210  and estimates a position of the second ground device  220  based on the transmission wave transmitted from the second ground device  220 . The guide processor  103  specifies the entrance X and the parking area A based on the position of the first ground device  210  and the position of the second ground device  220  estimated by the estimation processor  102  and guides the vehicle  1  towards the parking area A to cause the vehicle  1  to enter the parking area A through the entrance X. 
     The entrance X and the parking area A may be thus easily specified by specifying the positions of the two ground devices  210  and  220 . 
     Furthermore, the reception processor  101  receives the transmission waves through the at least two object detectors  111 S,  112 S fixedly provided at different positions from each other at a lateral end portion of the vehicle  1 . 
     Furthermore, the reception processor  101  receives the transmission wave transmitted from the ground apparatus  20 ) through sonars serving as the at least two object detectors  111 S,  112 S and detecting information related to an object in surroundings of the vehicle  1  by transmitting and receiving a sound wave. 
     According to the embodiment, a parking guidance apparatus  100  includes a ground apparatus  200  fixedly provided on a ground and transmitting a transmission wave and a parking guidance apparatus  100  provided at a vehicle  1 . The parking guidance apparatus  100  includes a reception processor  101  receiving a transmission wave transmitted from the ground apparatus  200 , the reception processor  101  receiving the transmission wave through at least two object detectors  110  fixedly provided at different positions from each other at the vehicle  1 , the at least two object detectors  110  including a first object detector  111 S and a second object detector  112 S that are different from each other, an estimation processor  102  estimating a position of the ground apparatus  200  relative to a position of the vehicle  1  in accordance with a time difference between a first timing and a second timing, the first timing at which the transmission wave is received by the reception processor  101  through the first object detector  111 S, the second timing at which the transmission wave is received by the reception processor  101  through the second object detector  112 S, and a guide processor  103  specifying a parking area A in accordance with the position of the ground apparatus  200  estimated by the estimation processor  102  and guiding the vehicle  1  towards the parking area A, the parking area A conforming to the position of the ground apparatus  200 . 
     The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.