Patent Publication Number: US-2020300991-A1

Title: Vehicle control system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2019-055022, filed Mar. 22, 2019. The contents of this application are incorporated herein by reference in their entirety. 
     FIELD 
     The present disclosure relates to a control system of a vehicle on which a LIDAR (Laser Imaging Detection and Ranging) is mounted. 
     BACKGROUND 
     US2018/0149732A discloses a LIDAR system which is mounted on a vehicle. The conventional system includes a rotary-typed LIDAR. The LIDAR includes a laser, a movable mirror, a rotating table and a photodetector. The laser emits light. The movable mirror reflects light emitted from the laser (hereinafter also referred to as “laser beam”) to irradiate surrounding environment. The rotating table rotates the movable mirror. The photodetector detects the light reflected from the surrounding environment. 
     The conventional system further includes a controller. The controller recognizes a surrounding object based on data detected by the photodetector. When an object is detected, the controller adjusts a position of the rotating table such that the laser beam is irradiated in a direction where the object is detected. 
     SUMMARY 
     According to the conventional system, immediately after the detection of the object, the rotating table is operated in a different operation mode to that prior to the detection. Therefore, detailed information of the detected object can be acquired earlier than when a continuous operation mode is selected before and after the detection. As a general theory, however, when the vehicle is traveling at high speed, it is required to acquire this detailed information quickly. Therefore, it is desirable to advance a first timing of the detection or to acquire the detailed information at the first timing. From this viewpoint of this, the conventional system have room for improvement. 
     It is an object of the present disclosure to provide a novel system capable of varying detecting mode of an object, taking into account the traveling speed of the vehicle. 
     A first aspect of the present disclosure is a vehicle control system. 
     The system comprises a vehicle speed acquisition device, a rotary-typed LIDAR and a controller. 
     The vehicle speed acquisition device is configured to acquire traveling speed of a vehicle. 
     The LIDAR is configured to acquire surrounding information of the vehicle using a laser beam. 
     The controller is configured to control a rotational movement of the LIDAR. 
     The controller is also configured to execute processing to set a cycle of the rotational movement based on the traveling speed. 
     In the setting processing, the controller is configured to set the cycle during the traveling speed is relatively fast to a longer cycle than that during the traveling speed is relatively slow. 
     A second aspect of the present disclosure further has the following features in the first aspect. 
     The system further comprises a positional information acquisition device and a map database. 
     The positional information acquisition device is configured to acquire positional information of the vehicle. 
     The map database is configured to store map information. 
     The controller is also configured to:
         determine, based on the traveling speed and the positional and map information, whether or not a stop line locate on a pathway of the vehicle within a predetermined detecting region; and   when it is determined that the stop line locates on the pathway, set the cycle a shorter cycle in the setting processing as compared with a case where it is determined that the stop line does not locate on the pathway.       

     A third aspect of the present disclosure further has the following features in the first aspect. The system further comprises a positional information acquisition device and a map database. 
     The positional information acquisition device is configured to acquire positional information of the vehicle. 
     The map database is configured to store map information. 
     The controller is also configured to:
         determine, based on the traveling speed and the positional and map information, whether or not a stop line locate on a pathway of the vehicle within a predetermined detecting region; and   when it is determined that the stop line locates on the pathway, execute deceleration control in which the traveling speed is decreased than a case where it is determined that the stop line does not locate on the pathway.       

     A fourth aspect of the present disclosure further has the following features in any one of the first to third aspects. 
     The system further comprises a positional information acquisition device and a map database. 
     The positional information acquisition device is configured to acquire positional information of the vehicle. 
     The map database is configured to store map information. 
     The controller is also configured to:
         determine, based on the traveling speed and the positional and map information, whether or not an object which blocks the laser beam locates around an intersection on a pathway of the vehicle within a predetermined detecting region; and   when it is determined that the object locates around the intersection, set the cycle a shorter cycle in the setting processing as compared with a case where it is determined that the object does not locate around the intersection.       

     A fifth aspect of the present disclosure further has the following features in any one of the first to third aspects. 
     The system further comprises a positional information acquisition device and a map database. 
     The positional information acquisition device is configured to acquire positional information of the vehicle. 
     The map database is configured to store map information. 
     The controller is also configured to:
         determine, based on the traveling speed and the positional and map information, whether or not an object which blocks the laser beam locates around an intersection on a pathway of the vehicle within a predetermined detecting region; and   when it is determined that the object locates around the intersection, execute deceleration control in which the traveling speed is decreased than a case where it is determined that the object does not locate around the intersection.       

     A sixth aspect of the present disclosure further has the following features in any one of the first to fifth aspects. 
     The system further comprises a surrounding information acquisition device. 
     The surrounding information acquisition device is configured to acquire the surrounding information. 
     The controller is also configured to:
         execute, based on information on preceding vehicle data which is included in the surrounding information, following control to follow traveling of the preceding vehicle;   determine, under a condition where an inter-vehicular distance is maintained, whether or not the following control is executed; and   when it is determined that the following control is executed under the condition, set the cycle a shorter cycle in the setting processing as compared with a case where it is determined that the following control is not executed under the condition.       

     A seventh aspect of the present disclosure further has the following features in the sixth aspect. 
     The controller is also configured to:
         determine, based on the information on the preceding vehicle, whether or not the preceding vehicle has started decelerating travel during the execution of the following control; and   when it is determined that the preceding vehicle has started the decelerating travel, set the cycle a shorter cycle in the setting processing as compared with a case where it is determined that the preceding vehicle has not started the decelerating travel.       

     According to the first aspect, the setting processing to set the cycle of the rotational movement of the LIDAR is executed. In the setting processing, the cycle during the traveling speed is relatively fast is extended over the cycle during the traveling speed is relatively slow. Therefore, it is possible to make the LIDAR rotate at a high speed as the traveling speed decreases and also make the LIDAR rotate at a low speed as the traveling speed increases. Therefore, it is possible to detect an object in the vicinity of the vehicle at an earlier timing during a low-speed driving. In addition, during a high-speed driving, it is possible to acquire the detailed information of the object which locates far from the vehicle at the first timing of the detection. 
     According to the second aspect, when it is determined that the stop line locates on the pathway, the cycle is shortened as compared with a case where it is determined that the stop line does not locate on the pathway. The stop line locates within the predetermined detecting region. In front of such the stop line, the vehicle is required to be temporarily stopped. Therefore, there is no problem even if importance for detecting the object existing in front of the stop line is lowered and the cycle is shortened. Rather, by shortening the cycle, it is possible to detect the object around the stop line at an early timing by making the LIDAR rotate at the high speed. Therefore, it is possible to improve running safety. 
     According to the third aspect, when it is determined that the stop line locates on the pathway, the deceleration control is executed to lower the traveling speed than the case where it is determined that the stop line is not locate on the pathway. If this deceleration control is executed, the cycle is shortened by the execution of the setting processing described in the first aspect. Therefore, it is possible to obtain an effect equivalent to the effect by the second aspect. 
     According to the fourth aspect, when it is determined that the object locates around the intersection, the cycle is shortened than a case where it is determined that the object does not locate around the intersection. The object is located on the pathway of the vehicle within the predetermined detecting region. If the object locates at a blind spot and rushes out into front of the vehicle from the blind spot, there is a possibility that the detection timing of the same object is delayed. In this respect, if the cycle is shortened, it is possible to make the LIDAR move at the high speed and to detect such a pop-out at an early timing. Therefore, it is possible to improve the running safety. 
     According to the fifth aspect, when it is determined that the object locates around the intersection, the deceleration control is executed to lower the traveling speed than a case where it is determined that the object does not locate around the intersection. If this deceleration control is executed, the cycle is shortened by the execution of the setting processing described in the first aspect. Therefore, it is possible to obtain an effect equivalent to the effect by the fourth aspect. 
     According to the sixth aspect, when it is determined that the following control is executed under the particular situation, the cycle is shortened compared to a case where it is determined that the following control is not executed under the same situation. This particular situation indicates a situation where the inter-vehicular distance is kept constant. It is desirable to be paid attention to the preceding vehicle while the following control is executed. In this respect, if the cycle is shortened when the following control is executed under the particular circumstance, it is possible to detect a change in a traveling state of the preceding vehicle at an earlier timing. Therefore, it is possible to improve accuracy of the following control. 
     According to the seventh aspect, when it is determined that the preceding vehicle has started the decelerating travel, the cycle is shortened as compared with a case where it is determined that the preceding vehicle has not the decelerating travel. If the cycle is shortened, it is possible to detect a change in the traveling state of the preceding vehicle due to an initiation of the decelerating travel at an earlier timing. Therefore, it is possible to improve the accuracy of the following control moreover. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a configuration example of a vehicle control system according to a first embodiment; 
         FIG. 2  is a diagram for explaining an upper limit condition of rotational speed ω per second of a movable mirror; 
         FIG. 3  is a diagram for explaining a lower limit condition of the rotational speed ω; 
         FIG. 4  is a diagram for explaining an example of setting processing of the rotational speed ω; 
         FIG. 5  is a flow chart for explaining a flow of rotational speed control processing executed by a controller; 
         FIG. 6  is a diagram for explaining a configuration example of a vehicle control system according to a second embodiment; 
         FIG. 7  is a diagram for explaining an example in which a first change condition is satisfied; 
         FIG. 8  is a diagram for explaining an example in which a second change condition is satisfied; 
         FIG. 9  is a flow chart for explaining a flow of first change processing executed by the controller; 
         FIG. 10  is a diagram for explaining the rotational speed ω set in the setting processing when the first change processing is executed. 
         FIG. 11  is a diagram for explaining a configuration example of a vehicle control system according to a third embodiment; 
         FIG. 12  is a diagram for explaining an example in which a third change condition is satisfied; 
         FIG. 13  is a diagram for explaining an example of second change processing executed when a third change condition is satisfied. 
         FIG. 14  is a flow chart for explaining a flow of second change processing executed by the controller based on a fourth change condition; 
         FIG. 15  is a diagram for explaining a configuration example of a vehicle control system according to a fourth embodiment; and 
         FIG. 16  is a flow chart for explaining a flow of processing to execute deceleration control by the controller. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure are described hereunder with reference to the accompanying drawings. However, it is to be understood that even when the number, quantity, amount, range or other numerical attribute of an element is mentioned in the following description of the embodiments, the present disclosure is not limited to the mentioned numerical attribute unless it is expressly stated or theoretically defined. Further, structures or steps or the like described in conjunction with the following embodiments are not necessarily essential to embodiments of the present disclosure unless expressly stated or theoretically defined. 
     1. First Embodiment 
     First, a first embodiment will be described with reference to  FIGS. 1 to 5 . 
     1.1 Entire Configuration of Vehicle Control System 
       FIG. 1  is a diagram showing a configuration example of a vehicle control system (hereinafter referred to simply as a “system”) according to the first embodiment. The system  100  shown in  FIG. 1  is mounted on a vehicle M 1 . Examples of the vehicle M 1  include a vehicle in which an engine is used as a power source, an electronic vehicle in which a motor is used as the power source, and a hybrid vehicle including the engine and the motor. The motor is driven by a battery such as a secondary battery, a hydrogen fuel cell, a metal fuel cell, an alcohol fuel cell, or the like. 
     As shown in  FIG. 1 , the system  100  includes a vehicle speed sensor  10 , a LIDAR  20 , and a controller  30 . 
     A vehicle speed sensor  10  is a device that acquires a traveling speed of the vehicle M 1  (hereinafter also referred to as “vehicle speed”). A wheel speed sensor is exemplified as the vehicle speed sensor  10 . The wheel speed sensor may be provided on wheels of the vehicle M 1  or on a drive shaft which rotates integrally with the wheels. The vehicle speed sensor  10  transmits the acquired data of the vehicle speed to the controller  30 . 
     The LIDAR  20  is a device that acquires information on surrounding of the vehicle M 1  using a laser beam. The LIDAR  20  includes a laser, a movable mirror, and a photodetector. The laser emits light. Number of light emissions per second is unique to the LIDAR  20 . The movable mirror reflects the emitted light (i.e., the laser beam) from the laser to irradiate surrounding environment. The direction of movement of the movable mirror may be a horizontal direction or a vertical direction. A mechanical mirror is exemplified as the movable mirror. Examples of the mechanical mirror include a polygon mirror and a small integrated mirror which uses a MEMS (Micro Electra Mechanical Systems) technique. The photodetector detects light reflected from the surrounding environment. The photodetector transmits the detected data of the reflected light to the controller  30 . 
     The LIDAR  20  further includes a driving member  21  to drive the movable mirror. The driving member  21  controls cycle of rotational movement T[s −1 ] of the movable mirror based on a control signal from the controller  30 . The cycle T is defined as a period until the movable mirror, which has started the rotational movement from a reference position, returns to the reference position via a turn-around position. For convenience of explanation, the present disclosure assumes that deflection angle of the movable mirror is 360 degrees, and the rotational speed ω [Hz] per second of the movable mirror is controlled instead of controlling the cycle T. 
     The controller  30  is a microcomputer that includes a processor, a memory, and an input interface and an output interface. The controller  30  receives various kinds of data via the input and output interface. The information received by the controller  30  includes vehicle speed and reflected light information. The controller  30  executes various controls based on the received data. 
     1.2 Configuration of Controller 
     The configuration of the controller  30  will be described. As shown in  FIG. 1 , the controller  30  includes an object recognition portion  31  and a rotational speed control portion  32  as function blocks related to the rotational speed control of the LIDAR  20 . These function blocks are realized when the processor of the controller  30  executes various types of control program stored in the memory. 
     The object recognition portion  31  executes processing to recognize objects around the vehicle M 1  based on the reflected light data. The objects around the vehicle M 1  include a moving object and a stationary object. Examples of the moving object include a vehicle, a motorcycle, a bicycle, and a walker. Examples of the stationary object include a white lane and a mark. Known processing is applied to the recognition processing. Therefore, descriptions of the recognition processing are omitted. 
     The rotational speed control portion  32  executes rotational speed control in which the rotational speed ω of the LIDAR  20  is controlled based on the information on the vehicle speed. In the rotational speed control, processing to set the rotational speed ω in accordance with the vehicle speed is executed. This setting processing will be described below. 
     1.3 Setting Processing 
     In the setting processing, the rotational speed ω is set based on an upper limit value ω 1  and a lower limit value ω 2  of the rotational speed co. Prior to the explanation of the upper limit value ω 1  and the lower limit value ω 2 , a relationship between density of the rotational speed ω and the laser beam (or the density of the data (point group)) p constituting the reflected light data) will be described. As mentioned above, the number of the irradiation of the laser beam is unique to the LIDAR  20 . Therefore, when the rotational speed ω changes under a constant vehicle speed condition, the density p changes. Specifically, when the rotational speed ω increases, the density p decreases. And when the rotational speed ω decreases, the density p increases. When the density ρ is high, information amount of the objects included in the reflected light is larger than when the density ρ is low. 
     1.3.1 Upper Limit Value ω 1   
       FIG. 2  is a diagram for explaining the upper limit condition of the rotational speed ω. The vehicle M 1  shown in  FIG. 2  travels at vehicle speed v on a traffic lane. In front of the vehicle M 1 , an object B 1  (specifically a stationary object) is present. The object B 1  is located a periphery portion of a detecting region R 1 . The detecting region R 1  is defined as an area where any object is required to be detected from a viewpoint of securing the running safety. Many parts of the detecting region R 1  extend toward a travel direction of the vehicle M 1 . An area SR 1  of the detecting region R 1  increases or decreases in accordance with the vehicle speed. Specifically, the area SR 1  increases as the vehicle speed increases whereas it decreases as the vehicle speed decreases. 
     In order to avoid contacting with the object B 1 , the vehicle M 1  must be stopped in front of the object B 1  after the object recognition portion  31  recognizes the object B 1 . A travel length L(v) from a position where the object B 1  is recognized to a position where the vehicle M 1  stops is expressed by the following equation (1) using vehicle speed v, and coefficients a and b. 
         L ( v )= av+bv   2   (1)
 
     The first term on the right side of the equation (1) represents an idle traveling length, and the second term on the right side of the equation (1) represents a braking length. The idle traveling length is defined as a length that the vehicle M 1  travels from the recognition of the object B 1  until a brake device of the vehicle M 1  begins to work. The braking length is defined as a length that the vehicle M 1  travels from the brake device begins to work until the vehicle M 1  stops. 
     Here, a circle CI whose radius is the travel length L(v) is considered. The circumference length of the circle CI is represented by 2ηL(v). The density ρ at a position separated from the vehicle M 1  by the travel length L(v) is expressed by the following equation (2) using the number N of the laser beam per second, the rotational speed ω and the travel length (v). 
       ρ=( N /ω))/2π L ( v )  (2)
 
     Furthermore, a lowest value ρ min of the density ρ required to recognize the objects around the vehicle M 1  is considered. The lowest value ρ min is able to set from a software configuration to execute the recognition processing and a configuration of the LIDAR  20 . Then, if at least the density ρ is equal to or larger than the lowest value ρ min, the object B 1  is able to be recognized, and the contacting with the object B 1  is able to be avoided by a deceleration operation after the recognition. That is, if the density ρ satisfies the condition show with the following equation (3), it is possible to avoid contacting with the object B 1 . 
       ρ□ρ min   (3)
 
     The condition shown in the following equation (4) is derived from the equations (2) and (3). This condition is defined as the upper limit condition of the rotational speed co. 
       ω≤ N /(2π L ( v )ρ min )  (4)
 
     The rotational speed ω when the values of the left side and the right side of the expression (4) are equal corresponds to the upper limit value ω 1 . 
     1.3.2 Lower Limit Value ω 2   
       FIG. 3  is a diagram for explaining the lower limit condition of the rotational speed ω. The vehicle M 1  shown in  FIG. 3  travels at a constant vehicle speed v speed on the traffic lane from time t 1  to time t 2 . In front of the vehicle M 1 , an object B 2  (specifically a stationary object) is present. The object B 2  has been detected by the LIDAR  20  not only at the time t 1  but also at the time t 2 . An interval Δt from the time t 1  to the time t 2  corresponds to the cycle T. The positions at which the object B 2  have been detected are a periphery portion of the detecting region R 1  at the time t 1  and a vicinity of the vehicle M 1  at the time t 2 . 
     In order to treat the object B 2  as same object, a length S between the detected position of the object  132  at the time t 1  and that at the time t 2  requires to be less than or equal to a tolerance Smax. The tolerance Smax can be set from the software configuration to execute the recognition processing and the configuration of the LIDAR  20 . That is, if the length S satisfy the condition of the following equation (5), the object B 2  can be treated as the same object. 
     
       
         
           
             
               
                 
                   
                     
                       
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     The condition shown in the following equation (6) is derived from the equation (5). This condition is defined as the lower limit condition of the rotational speed ω. 
       ω≥ v/S  max  (6)
 
     The rotational speed ω when the values of the left side and the right side of the expression (6) are equal corresponds to the lower limit value ω 2 . 
     1.4 Example of Setting Processing 
       FIG. 4  is a diagram for explaining an example of the setting processing of the rotational speed ω. When the conditions shown with the equations (4) and (6) are drawn on a plane having the vehicle speed v as the horizontal axis and the rotational speed ω as the vertical axis, a range of the rotational speed ω which satisfies the upper limit and the lower limit conditions at the same time is specified. In the example shown in  FIG. 4 , the rotational speed ω is set as an average ωave between the upper limit value ω 1  and the lower limit value ω 2 . However, if constraints on the configuration of the movable mirror (e.g., upper limit and lower restrictions) are separately imposed, the rotational speed ω is modified to fit within a movable range specified by the constraints. 
     The average wave is set based on the upper limit value ω 1  and the lower limit value ω 2  obtained by referring to an upper limit map and a lower limit map individually. The upper limit map is a control map in which the upper limit value ω 1  is set for each vehicle speed v. The lower limit map is a control map in which the lower limit value ω 2  is set for each vehicle speed v. These control maps are preset and stored in the memory. 
     The average wave may be set directly by referring to an average map. The average map is a control map in which the average wave is set for each vehicle speed v. The vehicle speed v, the upper limit value ω 1  and the lower limit value ω 2  may be separately calculated based on the equations (4) and (6), and the average ωave may be calculated based on the calculated values. 
     As can be seen from the tendency of the average wave shown in  FIG. 4 , the average wave during the vehicle speed v is relatively high becomes slower than that during the vehicle speed v is relatively low. That is, when the rotational speed ω is set to the average wave by the execution of the setting processing, the movable mirror rotates at higher speed as the vehicle speed v decreases whereas the movable mirror rotates at lower speed as the vehicle speed v increases. 
     As described in the section 1.3, the density ρ decreases as the rotational speed ω increases, whereas the density ρ increases as the rotational speed ω decreases. Furthermore, when the density ρ is high, the information amount of the objects included in the information on the reflected light becomes larger than when the density ρ is low. Therefore, when the rotational speed ω is set to the average wave, the density ρ increases as the vehicle speed v increases, and thus the information amount increases. 
     1.5 Example of Specific Processing 
       FIG. 5  is a flow chart for explaining a flow of processing to execute the rotational speed control executed by the controller  30 . The processing routine shown in  FIG. 5  is repeatedly executed at a predetermined control cycle while the vehicle M 1  is traveling. 
     In the processing routine shown in  FIG. 5 , first, the information on the vehicle speed is acquired (step S 10 ). The information on the vehicle speed is that transmitted from the vehicle speed sensor  10 . 
     Subsequent to the step S 10 , the upper limit value ω 1  and the lower limit value ω 2  are acquired (step S 11 ). The upper limit value ω  1  is obtained by referring to the upper limit map. The lower limit value ω  2  is obtained by referring to the lower limit map. 
     Subsequent to the step S 11 , the rotational speed ω is calculated (step S 12 ). The rotational speed ω is calculated by averaging the upper limit value ω 1  and the lower limit value ω 2  (i.e. the average wave). 
     Subsequent to the step S 12 , it is determined whether or not the rotational speed ω is within the movable range (step S 13 ). When the rotational speed ω calculated in the step S 12  is between the lower and upper restrictions, it is determined that the rotational speed ω is within the movable range. Otherwise, it is determined that the rotational speed ω is outside the movable range. 
     If the determination result of the step S 13  is negative, the rotational speed ω is changed (step S 14 ). The change of the rotational speed ω is executed by considering content of the determination of the step S 13 . Specifically, if the rotational speed ω is less than lower restriction, the rotational speed ω is changed to the lower restriction. If the rotational speed ω is higher than upper restriction, the rotational speed ω is changed to the upper restriction. 
     Subsequent to the step S 13  or S 14 , a control signal is output (step S 15 ). The control signal is output in accordance with the rotational speed ω set in the step S 13  or S 14 . The control signal is input to the driving member  21 . In this way, the rotational speed of the movable mirror is controlled. 
     1.6 Advantageous Effects 
     According to the first embodiment described above, the setting processing is executed in the rotational speed control. The setting processing allows the movable mirror to rotate faster as the vehicle speed decreases. Therefore, it is possible to detect the objects in the vicinity of the vehicle M 1  at an earlier timing during a low-speed driving. Furthermore, it is possible to rotate the movable mirror at lower speed as the vehicle speed increases. Therefore, it is possible to acquire the detailed information of the object which locates far from the vehicle M 1  at the first detecting time during a high-speed driving. 
     2. Second Embodiment 
     Next, a second embodiment will be described with reference to  FIGS. 6 to 10 . Explanation of the same configuration as that of the first embodiment will be omitted as appropriate. 
     2.1 Entire Configuration of Vehicle Control System 
       FIG. 6  is a diagram for explaining a configuration example of the vehicle control system according to the second embodiment. The system  200  shown in  FIG. 6  is mounted on the vehicle M 1 . The system  200  includes the vehicle speed sensor  10 , the LIDAR  20 , the controller  30 , a map database  40 , and sensors  50 . 
     The map database  40  is a data base in which high-precision map information is stored. The map information includes information on positional information and road shape of roads (e.g., information on road type such as straight and curve, and curvature of the curve). The positional information on the roads also includes information on intersections and divergent points. The map information also includes information on marks on the roads (e.g., compartment lines, stop lines and crosswalks) and that installed at breakdown lanes (e.g., information on no-parking areas, legal speed and stop lines). The map information also includes information on appendages on the roads (e.g., location, geometry and width). The appendage includes constructions (e.g., guard fences, marks and light fixtures) installed around the roads and buildings founded next to the roads. 
     The map database  40  is formed in a storage device (e.g., a hard disk and a flash memory) mounted on the vehicle M 1 . The map database  40  may be formed in a computer of a facility (e.g., a management center) that is capable of communicating with the vehicle M 1 . 
     The sensors  50  includes a GPS (Global Positioning System) receiver, an external sensor, and an internal sensor. 
     The GPS receiver is a device that receives signals from more than two GPS satellites. The GPS receiver is also a device to require information on position of the vehicle M 1 . The GPS receiver calculates the position and orientation of the vehicle M 1  based on the received signals. The GPS receiver transmits the calculated data to the controller  30 . 
     The external sensor is a device to acquire the information on the surrounding of the vehicle M 1 . The LIDAR  20  is also a type of the external sensor. However, the LIDAR  20  is not included in the external sensor referred to herein. Examples of the external sensor include a millimeter wave radar and a camera. The millimeter wave radar detects the objects around the vehicle M 1  by using radio waves. The camera images surrounding conditions of the vehicle M 1 . The external sensor transmits the detected data to the controller  30 . 
     The internal sensor is a device that acquires a traveling state of the vehicle M 1 . The vehicle speed sensor  10  is also a type of internal sensor. However, the vehicle speed sensor  10  is not included in the internal sensor referred to herein. Examples of the internal sensor include an acceleration sensor and an acceleration yaw rate sensor. The acceleration sensor detects acceleration of the vehicle M 1 . The yaw rate sensor detects yaw rate around a vertical axis of a center of gravity of the vehicle M 1 . The internal sensor transmits the detected data to the controller  30 . 
     2.2 Configuration of Controller 
     The configuration of the controller  30  will be described. As shown in  FIG. 6 , the controller  30  includes an object recognition portion  31 , a rotational speed control portion  32 , a vehicle position estimation portion  33 , and a coefficient change portion  34 . These function blocks are function blocks associated with the rotational speed control. These function blocks are realized when the processor or the controller  30  executes various types of control program stored in the memory. 
     The vehicle position estimation portion  33  executes processing to estimate actual position of the vehicle M 1  based on the data from the sensors  50  and the map information. In the estimate processing, the information from the GPS receiver is compared with the map information, whereby the position of the vehicle M 1  on a map is specified. The specified position of the vehicle M 1  is transmitted to the coefficient change portion  34 . 
     The coefficient change portion  34  executes processing to change a coefficient α based on the information on the vehicle speed, the positional information of the vehicle M 1  and the map information. The coefficient α is a weighting coefficient used in the setting process of the rotational speed ω. This coefficient change processing will be described below. For convenience of explanation, the coefficient change processing of the present embodiment will be referred to as “first change processing”. 
     2.3 First Change Processing 
     As described above, in the setting processing, the rotational speed ω is set based on the upper limit value ω 1  and the lower limit value ω 2 . By using the upper limit value ω 1 , the lower limit value ω 2 , and the coefficient α, an arithmetic expression of the rotational speed ω is expressed by the following expression (7). 
       ω=α·ω1+(1−α)·ω2  (7)
 
     A default value α 0  of the coefficient α is set to 0.5. In the first change processing, when a first change condition or a second change condition is satisfied, the coefficient α is changed to a value larger than the default value α 0 . The first and second change conditions will be described below. 
     2.3.1 First Change Condition 
     The first change condition is satisfied when there is a stop line on a pathway of the vehicle M 1  within the detecting region R 1 .  FIG. 7  is a diagram for explaining examples in which the first change condition is satisfied. In  FIG. 7 , a stop line SL is drawn on a road in front of the vehicle M 1 . If such the stop line SL is present, the vehicle M 1  is required to be temporarily stopped in front of the stop line SL. If the vehicle M 1  stops in front of the stop line SL, an importance to detect the objects existing in front of the stop line SL is reduced. For this reason, when the stop line SL is present, a detecting region R 2  is drawn whose shape misses a portion in front of the normal detecting region R 1 . 
     2.3.2 Second Change Condition 
     The second change condition is satisfied when there is the laser beam blocking object around an intersection on the pathway of the vehicle M 1  within the detecting region R 1 .  FIG. 8  is a diagram for explaining examples in which the second change condition is satisfied. The vehicle M 1  shown in  FIG. 8  is proceeding towards an intersection IN. An object B 3  (specifically, a building) exists around the intersection IN. When such the object B 3  is present, a barrier area BA is formed where the laser beam is not irradiated. Then, an overlapping region OR between the barrier area BA and the detecting region R 1  becomes a blind spot. 
     2.3.3 Specific Example of First Change Processing 
       FIG. 9  is a flow chart for explaining the flow of first change processing executed by the controller  30 . The processing routine shown in  FIG. 9  is repeatedly executed at a predetermined control cycle while the vehicle M 1  is traveling. 
     In the processing routine shown in  FIG. 9 , first, information is acquired (step S 20 ). The information to be acquired is the information on the vehicle speed, the positional information and the map information. In the processing of the step S 20 , an area of the detecting region R 1  (i.e., the area SR 1 ) is specified based on the information on the vehicle speed. The stop line SL or the barrier area BA is also specified based on the positional information and the map information. 
     Subsequent to the step S 20 , it is determined whether or not the first or second change condition is satisfied (step S 21 ). In the processing of the step S 21 , it is determined whether or not the change condition described in  FIG. 7 or 8  is satisfied based on the area SR 1 , the stop line SL and the barrier area BA specified in the step S 20 . 
     If it is determined that the first or second change condition is satisfied, the coefficient α is changed (step S 22 ). Specifically, the coefficient α is changed to the value larger than the default value α 0 . 
     2.4 Advantageous Effects 
       FIG. 10  is a diagram for explaining the rotational speed ω set in the setting processing when the first change processing is executed.  FIG. 10  shows the tendency of the rotational speed ω when the coefficient α is set 0.5 (i.e., the default value α 0 ) and that of the rotational speed ω when the coefficient α is set 0.7. The tendency when the coefficient α is set 0.7 corresponds to that when the first change processing is executed. As can be seen from comparing the two tendencies while fixing the vehicle speed condition, when the first change processing is executed, the value of the rotational speed ω increases. Therefore, when the first change processing is executed, the movable mirror rotates at the higher speed. 
     As described in  FIG. 7 , the presence of the stop line SL reduces the importance to detect the objects existing in front of the stop line SL. Further, as described in  FIG. 8 , when the barrier area BA is formed, the overlapping region OR becomes the blind spot. In this respect, if the movable mirror is rotated at the higher speed by executing the first change processing, it is possible to detect the objects around the stop line SL at an earlier timing. In addition, it is possible to detect a pop-out of an object from the blind spot at an early timing. Therefore, the running safety can be improved. 
     3. Third Embodiment 
     Next, a third embodiment will be described with reference to  FIGS. 11 to 14 . Explanation of the same configuration as that of the first or second embodiment will be omitted as appropriate. 
     3.1 Entire Configuration of Vehicle Control System 
       FIG. 11  is a diagram for explaining a configuration example of the vehicle control system according to the third embodiment. The system  300  shown in  FIG. 11  is mounted on the vehicle M 1 . The system  300  includes the vehicle speed sensor  10 , the LIDAR  20 , the controller  30 , the map database  40 , the sensors  50  and a traveling device  60 . 
     The traveling device  60  automatically drives the vehicle M 1  in accordance with control signals from the controller  30 . The traveling device  60  includes a driving force outputting device, a steering device and a brake device. The driving force outputting device generates a driving force for traveling. The steering device turns the wheels. The brake device generates a braking force to be applied to the wheels. 
     3.2 Configuration of Controller 
     The configuration of the controller  30  will be described. As shown in  FIG. 11 , the controller  30  includes the object recognition portion  31 , the rotational speed control portion  32 , the vehicle position estimation portion  33 , the coefficient change portion  34 , and a following control portion  35 . These function blocks are function blocks associated with the rotational speed control. These function blocks are realized when the processor of the controller  30  executes various types of control program stored in the memory. 
     The following control portion  35  executes following control to follow the traveling of a preceding vehicle M 2  by the operation of the traveling device  60 . The preceding vehicle M 2  may be recognized based on the information on the reflected light or may be recognized based on the information from the external sensor. The preceding vehicle M 2  may be recognized based on an integration of these information. As processing to execute the following control, a known processing is applied. Therefore, descriptions of the processing of the following control are omitted. 
     The coefficient change portion  34  executes processing to change the coefficient α based on the information on the preceding vehicle M 2 . This coefficient change processing will be described below. For convenience of explanation, the coefficient change processing of the present embodiment will be referred to as “second change processing”. 
     3.3 Second Change Processing 
     In the second change processing, when a third condition or a fourth change condition is satisfied, the coefficient α is changed to the value larger than the default value α 0 . Hereinafter, the third and fourth change conditions and the processing examples executed when these change conditions are satisfied will be described. 
     3.3.1 Third Change Condition 
     The third change condition is satisfied when the following control is executed and an inter-vehicular distance is kept constant. The inter-vehicular distance is a distance between the preceding vehicle M 2  and the vehicle M 1 .  FIG. 12  is a diagram for explaining examples in which the third change condition is satisfied. The vehicle M 1  shown in  FIG. 12  automatically travels based on the execution of the following control. The vehicle speed v of the vehicle M 1  is equal to that of the preceding vehicle M 2 . Therefore, an inter-vehicular distance LM is kept constant. 
       FIG. 13  is a diagram for explaining an example of the second change processing executed when the third change condition is satisfied. In the example shown in  FIG. 13 , the shorter the inter-vehicular distance LM is, the larger value the coefficient α is changed. The lowest value of the coefficient α is the default value α 0 . The coefficient α is set to the smallest value when the inter-vehicular distance LM is maintained at a longest inter-vehicular distance that can be set in the following control. 
     3.3.2 Fourth Change Condition 
     The fourth change condition is satisfied when the following control is executed and the preceding vehicle M 2  decelerates its speed. The decelerating travel of the preceding vehicle M 2  may be recognized based on the information on the reflected light or may be recognized based on the information from the external sensor. If the preceding vehicle M 2  decelerates its speed, the preceding vehicle M 2  may tack or stop. 
       FIG. 14  is a flow chart for explaining the flow of the second change processing executed by the controller  30  based on the fourth change condition. The processing routine shown in  FIG. 14  is repeatedly executed at a predetermined control cycle during the following control is executed. 
     In the processing routine shown in  FIG. 14 , first, the information on the preceding vehicle M 2  is acquired (step S 30 ). The information to be acquired is the information indicating a traveling state of the preceding vehicle M 2 . 
     Subsequent to the step S 30 , it is determined whether or not the preceding vehicle M 2  decelerate its speed (step S 31 ). The processing of the step S 31  is processing to determine whether or not the fourth change condition is satisfied. 
     If it is determined that the fourth change condition is satisfied, the coefficient α is changed (step S 32 ). Specifically, the coefficient α is changed to the value larger than the present value α 1 . As described in the third change condition, during the execution of the following control, the coefficient α is set in accordance with the inter-vehicular distance LM. Therefore, when the coefficient α is changed, the changed value becomes larger than the present value α 1  (i.e., α 1 □α 0 ). 
     3.4 Advantageous Effects 
     According to the third embodiment described above, the second change processing is executed. In a situation where the third change condition is satisfied, it is desirable to be paid attention to the preceding vehicle M 2 . In this respect, according to the second change processing, the coefficient α is changed to the value larger than the default value α 0  when the third change condition is satisfied. Then, as described in  FIG. 10 , the rotational speed ω set in the setting processing increases, and the movable mirror rotates at the higher speed. Therefore, it is possible to detect a change in the traveling state of the preceding vehicle M 2  at an earlier timing. Therefore, it is possible to improve accuracy of the following control. 
     Also, in a situation where the fourth change condition is satisfied, the preceding vehicle M 2  may tack or stop. Therefore, in such the situation, it is desirable to be paid more attention to the preceding vehicle M 2  than in the situation where the third change condition is satisfied. In this respect, according to the second change processing, the coefficient α is changed to the value larger than the present value α 1  when the fourth change condition is satisfied. Therefore, it is possible to detect the change in the traveling state of the preceding vehicle M 2  due to an initiation of the decelerating travel at an earlier timing. Therefore, it is possible to improve the accuracy of the following control. 
     4. Fourth Embodiment 
     Next, a fourth embodiment will be described with reference to  FIGS. 15 and 16 . Explanation of the same configuration as that of the first to third embodiment will be omitted as appropriate. 
     4.1 Entire Configuration of Vehicle Control System 
       FIG. 15  is a diagram for explaining a configuration example of the vehicle control system according to the fourth embodiment. The system  400  shown in  FIG. 15  is mounted on the vehicle M 1 . The basic configuration of the system  400  is the same as that of the system  300  described in  FIG. 11 . The system  400  and the system  300  differ in the configuration of the controller  30 . 
     4.2 Configuration of Controller 
     The configuration of the controller  30  will be described. As shown in  FIG. 15 , the controller  30  includes the object recognition portion  31 , the rotational speed control portion  32 , the vehicle position estimation portion  33 , and a deceleration control portion  36 . These function blocks are function blocks associated with the rotational speed control. These function blocks are realized when the processor of the controller  30  executes various types of control program stored in the memory. 
     The deceleration control portion  36  executes deceleration control of the vehicle M 1  based on the information on the vehicle speed, the positional information of the vehicle M 1  and the map information. The deceleration control is to decelerate the vehicle M 1  by the operation of the brake device. The deceleration control will be described below. 
     4.3 Deceleration Control 
     The deceleration control is executed when the first or second change condition is met. These change conditions are as described in the items 2.3.1 and 2.3.2. The deceleration control may be executed until the vehicle M 1  stops, or may be executed temporarily. For example, when the first change condition is satisfied, the deceleration control is executed until the vehicle M 1  stops. When the second change condition is satisfied, the deceleration control is executed temporarily. When the deceleration control is executed, the vehicle speed is lowered. The lower the vehicle speed is, the faster the movable mirror rotates due to the execution of the setting processing. The setting processing is as described in the above first embodiment. 
       FIG. 16  is a flow chart for explaining a flow of processing to execute the deceleration control by the controller  30 . The processing routine shown in  FIG. 16  is repeatedly executed at a predetermined control cycle while the vehicle M 1  is traveling. 
     In the processing routine shown in  FIG. 16 , the processing of steps S 40  and S 41  is executed. The processing of these steps is the same as that of the steps S 20  and S 21  shown in  FIG. 9 . 
     If it is determined in the step S 41  that the first or second change condition is satisfied, control amount of the brake device is calculated (step S 42 ). In the processing of the step S 42 , the control amount is calculated in accordance with the change condition satisfied in the step S 41 . For example, if the first change condition is satisfied, the control amount is calculated such that the vehicle M 1  stops in front of the stop line SL. If the second change condition are met, the control amount is calculated such that at least the vehicle speed is reduced. 
     4.4 Advantageous Effects 
     According to the fourth embodiment described above, the deceleration control is executed when the first or second change condition is satisfied. Therefore, it is possible to rotate the movable mirror at a high speed without changing the coefficient α. Therefore, it is possible to obtain the same effects as when the first change processing described in the second embodiment is executed.