Patent Publication Number: US-2021188219-A1

Title: Ranging device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This international application claims the benefit of priority from Japanese Patent Application No. 2018-165989 filed with the Japan Patent Office on Sep. 5, 2018, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     This disclosure relates to a ranging device. 
     Related Art 
     There is a ranging device mounted to a vehicle and configured to measure a distance to an object ahead of the vehicle. This ranging device emits transmitted waves forward, detects reflected waves of the emitted transmitted waves from the object, and thereby measures a distance to the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a block diagram of a LIDAR device according to a first embodiment; 
         FIG. 2  is a perspective view of the LIDAR device; 
         FIG. 3  is an illustration of an inside of a cover of the LIDAR device;  FIG. 4  is a flowchart of a determination process performed by a controller according to the first embodiment; 
         FIG. 5  is a cross-sectional view of the LIDAR device mounted to a vehicle; 
         FIG. 6  is a table illustrating a relationship between an energization level of a heater, an outside temperature, and a vehicle speed; 
         FIG. 7  is a block diagram of a LIDAR device according to a third embodiment; 
         FIG. 8  is a flowchart of a determination process performed by a controller according to the third embodiment; 
         FIG. 9  is a block diagram of a LIDAR device according to a fourth embodiment; and 
         FIG. 10  is a block diagram of a LIDAR device according to a fifth embodiment. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     In the above ranging device, a cover is provided in front of an emitter that emits transmitted waves or a detector that detects reflected waves to protect the emitter or the detector. However, snow adhering to the cover may decrease the measurement accuracy of the ranging device. 
     To address this issue, JP-A-1996-29535 describes that the cover of the ringing device is provided with a heater to melt the snow. 
     As a result of detailed research performed by the present inventors, regarding the ranging device in which the transmission window, though which transmitted waves or reflected waves are transmitted, is provided with a heater, it has been found that an energization level of the heater needed to keep the temperature of the transmission window at a desired temperature significantly differs depending on an outside temperature and a speed of the vehicle. More specifically, when the outside temperature is low, a high energization level is needed because more heat of the transmission window heated by the heater is lost to the outside air as compared with when the outside temperature is high. Even if the outside temperature is fixed, the heat of the transmission window heated by the heater is more rapidly lost when the speed of the vehicle is high as compared with when the speed of the vehicle is low, which needs a high energization level of the heater. Accordingly, if, in order to mitigate the reduction in measurement accuracy of the ranging device, the energization level is set high without exception such that snow adhering to the transmission window can be sufficiently removed under any condition, the transmission window may be unnecessarily heated by the heater, which may lead to increased power consumption by the heater. 
     In view of the foregoing, it is desired to have a ranging device in which a transmission window is provided with a heater, which enables appropriate control of energization of the heater. 
     One aspect of this disclosure provides a ranging device to be mounted to a vehicle, which is configured to emit a transmitted wave and detect a reflected wave from an object illuminated by the transmitted wave, thereby measuring a distance to the object. The ranging device includes a transmission window, a heater, and controller. at least one of the transmitted wave and the reflected wave is transmitted through the transmission window. the heater is configured to heat the transmission window. The controller is configured to control energization of the heater in response to an outside temperature that is a temperature outside the ranging device and a speed of the vehicle. 
     This configuration enables appropriate control of energization of the heater. 
     Hereinafter, some embodiments of the disclosure will be described with reference to the drawings. 
     1. First Embodiment 
     1-1. Configuration 
     A LIDAR device  100  illustrated in  FIG. 1  is a ranging device configured to emit light as transmitted waves and detect reflected waves from an object irradiated with light, and thereby measure a distance to the object. The term “LIDAR” is an abbreviation for Light Detection and Ranging. The LIDAR device  100  is mounted to a vehicle and used to detect various objects ahead of the vehicle. 
     The LIDAR device  100  includes a measurer  10 , a heater  20 , and a controller  30 . 
     The measurer  10  includes an emitter  11  that emits light and a detector  12  that detects reflected waves of the light. The emitter  11  emits laser light as the light. The detector  12  receives the reflected waves from the object and converts the received, reflected waves into electric signals. 
     The measurer  10  is housed within the case  110  formed of a cover  120  and a case body  130  of the LIDAR device  100  illustrated in  FIG. 2 . The emitter  11  of the measurer  10  is housed in the upper region of a space inside the case  110 . On the other hand, the detector  12  is housed in the lower region of the space inside the case  110 . 
     A transparent transmission window  121  through which light is transmitted is provided as a front portion of the cover  120 . As used herein the term “front ” means a direction in which light is emitted from the LIDAR device  100 . The transmission window  121  provides separation between the interior and the exterior of the LIDAR device  100 . 
     The heater  20  is configured to heat the transmission window  121  from the inside of the LIDAR device  100 , that is, from the inner side of the transmission window  121 . The heater  20  is provided on the inner surface of the transmission window  121  as illustrated in  FIG. 3 . The heater  20  includes an emitter-side heater  21  provided on the emitter  11  side of the transmission window  121  and a detector-side heater  22  provided on the detector  12  side of the transmission window  121 . Each of the emitter-side heater  21  and the detector-side heater  22  has a transparent conductive film Fi and a pair of electrodes LDi, LGi, where i is 1 when belonging to the emitter-side heater  21  and  2  when belonging to the detector-side heater  22 . The transparent conductive film Fi is a heater film formed of a material having transparency and electrical conductivity. For example, an indium tin oxide (ITO) film is used as the transparent conductive film Fi. 
     The controller  30  may be configured as at least one microcomputer including a central processing unit (CPU), a read-only memory (ROM), a random-access memory (RAM), an input-output interface (I/O), and a bus line connecting these components. The controller  30  includes, as functional blocks implemented by executing programs stored in the ROM, that is, virtual elements, a distance calculator  31 , a target energization level determiner  32 , an available energization level estimator  33 , a control value determiner  34 , and a heater energizer  35 . 
     The distance calculator  31  is configured to use the measurer  10  to detect a distance to the object illuminated by the light. More specifically, the distance calculator  31  determines a timing at which a reflected wave is detected based on a waveform of an electrical signal received from the detector  12 , and calculates a distance to the object based on a time difference from emission of light. The distance calculator  31  may acquire information about the object, such as an azimuth of the object, in addition to the distance to the object. 
     The target energization level determiner  32  is configured to determine an energization level of the heater  20  (hereinafter referred to as a target energization level) in response to an outside temperature, which is a temperature outside the LIDAR device  100 , and a speed of the vehicle to which the LIDAR device  100  is mounted (hereinafter referred to as a vehicle speed). In the described later process performed by the target energization level determiner  32 , energization power per unit of time is acquired as the target energization level of the heater  20 . The target energization level determiner  32  acquires the outside temperature from the outside temperature sensor  41  mounted to the vehicle. The outside temperature sensor  41  is provided on the bottom of the vehicle to detect the outside temperature of the vehicle. The target energization level determiner  32  further acquires a vehicle speed from the vehicle speed sensor  42  mounted to the vehicle. 
     The available energization level estimator  33  is configured to estimate the energization level that the battery  43  can supply (hereinafter, also referred to as an available energization level), based on a detected battery voltage of the battery  43  mounted to the vehicle. 
     The control value determiner  34  is configured to determine a control value for the heater energizer  35  described later to control energization of the heater  20 . In the present embodiment, the control value is a duty cycle, which is a ratio of an energization time to a de-energization time of the heater  20 . The control value determiner  34  determines the duty cycle in response to the target energization level determined by the target energization level determiner  32  and the available energization level estimated by the available energization level estimator  33 . Since, in the present embodiment, the battery  43  of the vehicle is directly connected to the heater  20  without being connected to the heater  20  through a constant-voltage circuit or the like, the voltage applied to the heater  20  varies with variation of the battery voltage. Therefore, in response to the energization level that the battery  43  can currently supply, the control value determiner  34  determines the duty cycle such that the actual energization level of the heater  20  becomes the target energization level determined by the target energization level determiner  32 . 
     The heater energizer  35  is configured to control energization of the heater  20  based on the control value determined by the control value determiner  34 . 
     1-2. Process 
     A determination process performed by the controller  30  will now be described with reference to the flowchart of  FIG. 4 . The determination process of  FIG. 4  is repeatedly performed every predetermined time interval after an ignition switch of the vehicle is turned on. 
     At S 11 , the controller  30  acquires an outside temperature from the outside temperature sensor  41 . 
     Subsequently, at S 12 , the controller  30  acquires a vehicle speed from the vehicle speed sensor  42 . 
     Subsequently, at S 13 , the controller  30  determines power W[W] to be supplied to the heater  20  based on the acquired outside temperature and the acquired vehicle speed. The power W is target supply power to the heater  20 . The target energization level determiner  32  is responsible for execution of S 11  to S 13 . 
     The power W is calculated according to the following equation (1), which is set up from the heat-transfer coefficient h [W/(m 2 ·K)], and a predetermined target surface temperature T 1  [K] of the heater  20  minus the outside temperature T 0  [K]. 
         W=q×A=h ×( T   1 - T   0 )× A    (1)
 
     In the equation ( 1 ), q is a heat flux [W/m 2 ] and A is a surface area [m 2 ] of the heater  20 . 
     The heat-transfer coefficient h is acquired using a Nusselt number Nu and a characteristic length L. 
     The Nusselt number Nu is a Nusselt number, assuming on-plate forced-convection which affects the top surface or the bottom surface of the case  110  with the LIDAR device  100  mounted on the vehicle. 
     The characteristic length L is a length along the travel direction of the vehicle, of at least a portion of the case  110  on the top or bottom surface of the case  110 . The characteristic length L may be appropriately set within a length along the travel direction on the top or bottom surface of case  110 . In the present embodiment, as illustrated in  FIG. 5 , the characteristic length L is, in a vertical cross section along the travel direction of the vehicle of the LIDAR device  100 , a length along the travel direction of the vehicle, of a rounded portion  123  connecting the top surface  122  of the cover  120  which is a portion of the case  110  and the front surface  121   a  of the transmission window  121 . More specifically, the characteristic length L is a length along the travel direction of the vehicle, of a portion of the case  110  from the upper edge  121   b  of the front surface  121   a  of the transmission window  121  to the front edge  122   a  of the top surface  122  of the cover  120 , where the length along the travel direction gradually deceases in the travel direction. This rounded portion  123  is a portion of the cover  120  of the LIDAR device  100  that is most susceptible to influence of a flow F of air in contact with the front surface  121   a  of the transmission window  121  toward the bumper of the vehicle during travel of the vehicle. 
     More specifically, the heat-transfer coefficient h can be calculated according to the following equations (2) to (4). 
         h=Nu×λ÷L    (2)
 
         Nu= 0.037 ×Re   4/5   ×P   1/3  ( Re&gt; 3.2×10 5 )   (3)
 
         Nu= 0.664 ×Re   1/2   ×P   1/3  ( Re  3.2×10 5 )   (4)
 
     In the equations (2) to (4), λ is the thermal conductivity of air [W/m·K], Re is the Reynolds number, and P is the Prandt 1  number. The Prandt 1  number is a ratio of the kinematic viscosity v [m 2 /s] to the thermal diffusion coefficient of air α [m 2 /s]. The Reynolds number is calculated according to the following (5). 
         Re=U×L÷v    (5)
 
     In the above equation (5), U is a vehicle speed [m/s]. 
     At S 14 , the controller  30  acquires a detected value of the battery voltage. 
     Subsequently, at S 15 , the controller  30  estimates power WO that the battery  43  can supply, based on the acquired, detected value of the battery voltage. The available energization level estimator  33  is responsible for execution of S 14  to S 15 . 
     Subsequently, at S 16 , the controller  30  determines a duty cycle based on the power W determined at S 13  and the power WO estimated at S 15 . Thereafter, the controller  30  terminates the determination process of  FIG. 4 . The control value determiner  34  is responsible for execution of S 16 . 
     Besides the determination process of  FIG. 4 , the controller  30  controls energization of the heater  20  based on the duty cycle determined in the determination process of  FIG. 4 . The heater energizer  35  is responsible for execution of this process. 
     1-3. Advantages 
     The first embodiment set forth above can provide the following advantages. 
     (1a) The controller  30  is configured to control energization of the heater  20  in response to the outside temperature and the vehicle speed, which enables appropriate control of energization of the heater  20 . 
     (1b) The controller  30  is configured to control energization of the heater  20  based on a function of the outside temperature and the vehicle speed as parameters, which enables optimization of the energization level of the heater  20  and thus enables reduction of power consumption of the heater  20 . 
     (1c) the controller  30  is configured to control energization of the heater  20  based on the detected battery voltage of the battery  43  as well, which enables reduction of variation of the actual energization level of the heater  20  with variation of the battery voltage. 
     2. Second Embodiment 
     2-1. Differences from First Embodiment 
     A second embodiment is similar in basic configuration to the first embodiment. Thus, duplicate description regarding the common configuration will be omitted and differences from the first embodiment will be mainly described below. 
     In the first embodiment, the controller  30  controls energization of the heater  20  based on a function of the outside temperature and the vehicle speed as parameters. More specifically, at S 13  of the determination process illustrated in  FIG. 4 , the controller  30  determines the power W to be supplied to the heater  20  based on the function of the outside temperature and the vehicle speed as parameters. 
     On the other hand, in the second embodiment, the controller  30  controls energization of the heater  20  based on a table in which an energization condition for the heater  20  is predefined depending on the outside temperature and the vehicle speed. More specifically, at S 13  of the determination process illustrated in  FIG. 4 , the controller  30  determines the power W to be supplied to the heater  20  with reference to an example table illustrated in  FIG. 6  that preliminarily associates the power W to be supplied to the heater  20  with the outside temperature and the vehicle speed. In the table illustrated in  FIG. 6 , the power W is set such that the lower the outside temperature, the higher the power W, and the higher the vehicle speed, the higher the power W. 
     2-2. Advantages 
     The second embodiment enables appropriate energization of the heater  20  in a relatively simple process as compared with the first embodiment. 
     3. Third Embodiment 
     3-1. Differences from First Embodiment 
     A third embodiment is similar in basic configuration to the first embodiment. Thus, duplicate description regarding the common configuration will be omitted and differences from the first embodiment will be mainly described below. 
     The third embodiment is different from the first embodiment in that, as illustrated in  FIG. 7 , the controller  30  is configured to determine at least a snowfall condition around the vehicle based on weather information acquired from a weather information receiver  44 , and control energization in response to not only the outside temperature and the vehicle speed, but also the snowfall condition. The weather information receiver  44  receives, from an external information communication system, such as Vehicle Information and Communication System (VICS), weather information in an area including at least a location where the vehicle is traveling. VICS is a registered trademark. The weather information received by the weather information receiver  44  includes information regarding the snowfall in the area. The controller  30  determines the snowfall condition around the vehicle based on the snowfall condition in the area, such as an amount of snowfall, in the weather information acquired from the weather information receiver  44 . 
     3-2. Process 
     A determination process of the third embodiment performed by the controller  30  instead of the determination process of the first embodiment will now be described with reference to the flowchart of  FIG. 8 . The determination process of  FIG. 8  is repeatedly performed every predetermined time interval after the ignition switch of the vehicle is turned on. 
     First, at S 21 , the controller  30  determines whether the weather information receiver  44  has acquired weather information in an area including a location where the vehicle is traveling. 
     If at S 21  the controller  30  determines that the weather information receiver  44  has acquired the weather information at S 21 , it proceeds to S 22  and determines whether there is snowfall in the area based on the weather information acquired by the weather information receiver  44 . 
     If at S 22  the controller  30  determines that there is no snowfall in the area, it proceeds to S 23  and corrects the target surface temperature T 1  in the above equation (1) to a target surface temperature T 1a  under normal conditions where there is no snowfall, and then proceeds to S 25 . 
     On the other hand, if at S 22  the controller  30  determines that there is snowfall, the controller  30  proceeds to S 24 . At S 24 , the controller  30  corrects the target surface temperature Ti to a target surface temperature T 1b  under snowfall conditions, and then proceeds to S 25 . The target surface temperature T 1b  under snowfall conditions is higher than the target surface temperature T 1a  under normal conditions such that the target surface temperature T 1b  under snowfall conditions increases as the amount of snowfall increases. The controller  30  corrects the target surface temperature T 1  in response to the amount of snowfall included in the information acquired by the weather information receiver  44 . This is because, during snowfall, heat of the transmission window  121  is easily taken away by snow, so it is necessary to increase the energization level of the heater  20  as compared with under normal conditions where there is no snowfall. This is also because, even during snowfall, it is necessary to increase the energization level as the amount of snowfall increases. 
     On the other hand, if at S 21  the controller  30  determines that the weather information receiver  44  has not acquired the weather information, it proceeds to S 25 . In this case, the current value of the target surface temperature T 1  is kept unchanged. 
     Subsequently, at S 25 , the controller  30  acquires an outside temperature from the outside temperature sensor  41 . 
     Subsequent S 26 , S 27 , and S 30  are the same as S 12 , S 13  and S 16  of the first embodiment, and S 28  and S 29  performed by the controller  30  on another route than the route of S 21  to S 27  are the same as S 14  and S 15  of the first embodiment. Thereafter, the controller  30  terminates the determination process of  FIG. 8 . The target energization level determiner  32  is responsible for execution of S 21  to S 27 . The available energization level estimator  33  is responsible for execution of S 28  to S 29 . The control value determiner  34  is responsible for execution of S 30 . 
     3-3. Advantages 
     The third embodiment set forth above in detail can provide the following advantages in addition to the advantages of the first embodiment. 
     (3a) In the third embodiment, the controller  30  determines at least a snowfall condition around the vehicle based on weather information acquired from the weather information receiver  44 , and controls energization in response to not only the outside temperature and the vehicle speed, but also the snowfall condition. This enables appropriate energization of the heater  20  in response to the snowfall condition. 
     (3b) More specifically, the controller  30  controls energization of the heater  20  such that the energization level under snowfall conditions is higher than the energization level under normal conditions where there is no snowfall. Therefore, snow adhering to the transmission window  121  can be rapidly melted even during snowfall, which can mitigate the reduction in measurement accuracy of the LIDAR device  100 . 
     (3c) The controller  30  controls the energization level of the heater  20  in response to the snowfall condition, such as an amount of snowfall or the like. This enables energization of the heater  20  at an appropriate energization level to the snowfall condition. 
     4. Fourth Embodiment 
     A fourth embodiment is similar in basic configuration to the third embodiment. Thus, duplicate description regarding the common configuration will be omitted and differences from the third embodiment will be mainly described below. 
     In the third embodiment, the controller  30  determines the snowfall condition based on the weather information acquired from the weather information receiver  44 . On the other hand, in the fourth embodiment, the controller  30  determines the snowfall condition based on the outside temperature and an operating state of a windshield wiper  45  as illustrated in  FIG. 9 . More specifically, when the outside temperature is below a predetermined temperature and the windshield wiper  45  is in operation, the controller  30  determines that there is snowfall. 
     In addition, the controller  30  controls the energization level of the heater  20  in response to an operating state of the windshield wiper  45 . A wiping speed at which the windshield wiper  45  wipes the transmission window  121  can be variably set in multiple levels. When the windshield wiper  45  is operating at a high wiping speed level, it is considered that the amount of snowfall is high. Thus, the controller  30  controls energization of the heater  20  such that the higher the wiping speed of the windshield wiper  45 , the higher the energization level of the heater  20 . 
     The determination process of the fourth embodiment performed by the controller  30  instead of the determination process of  FIG. 8  of the third embodiment is similar to the determination process of the third embodiment except in the following. More specifically, the controller  30  skips S 21  and begins the process with S 22 . In S 22 , as described above, based on the outside temperature and the operating state of the windshield wiper  45 , the controller  30  determines whether there is snowfall around the vehicle. In S 24 , the controller  30  corrects the target surface temperature T 1  to the target surface temperature T 1b  under snowfall conditions in response to the wiping speed level of the windshield wiper  45 . 
     The fourth embodiment can provide similar advantages as in the third embodiment. 
     5. Fifth Embodiment 
     A fifth embodiment is similar in basic configuration to the third embodiment. Thus, duplicate description regarding the common configuration will be omitted and differences from the third embodiment will be mainly described below. 
     In the third embodiment, the controller  30  determines the snowfall condition based on the weather information acquired from the weather information receiver  44 . On the other hand, in the fifth embodiment, as illustrated in  FIG. 10 , the controller  30  determines the snowfall condition based on a result of analysis of images of the surroundings of the vehicle captured by the camera  46  mounted to the vehicle. 
     The camera  46  is attached to the front inside of the vehicle. The camera  46  repeatedly captures images of an area ahead of the vehicle every predetermined time interval and outputs data of the captured images to a vehicle-mounted ECU (not shown). The vehicle-mounted ECU detects snow from the images captured by the camera  46  and analyzes a snowfall condition, such as an amount of snowfall, in the surroundings ahead of the vehicle. The controller  30  acquires a result of analysis by the vehicle-mounted ECU and performs the process based on the acquired result of analysis. 
     The determination process of the fifth embodiment performed by the controller  30  instead of the determination process of  FIG. 8  of the third embodiment is similar to the determination process of the third embodiment except in the following. More specifically, the controller  30  skips S 21  and begins the process with S 22 . At S 22 , as described above, based on a result of analysis of images captured by the camera  46  mounted to the vehicle, the controller  30  determines whether there is snowfall in the surroundings ahead of the vehicle. At S 24 , the controller  30  corrects the target surface temperature T 1  to the target surface temperature T 1b  under snowfall conditions in response to the amount of snowfall acquired from the images captured by the camera  46 . 
     The fifth embodiment can provide similar advantages as in the third embodiment. 
     6. Other Embodiments 
     Specific embodiments of the present disclosure have been described above, but the present disclosure may be implemented in various embodiments without being limited to the above embodiments. 
     (6a) In the above first embodiment, a length along the travel direction of the vehicle, of a portion of the case  110  connecting the front surface  121   a  of the transmission window  121  and the top surface  122  of the cover  120 , is used as the characteristic length L when determining the heat-transfer coefficient h, but is not limited thereto. That is, the characteristic length L, as long as it is a length along the travel direction of the vehicle, of at least a portion of the case  110  on the top or bottom surface of the case  110 , may be selected to be a length of an appropriate portion of the case  110  to the shape or the like of the LIDAR device  100 . More specifically, for example, as it is not desirable that the light transmitted by the LIDAR device  100  is blocked, the LIDAR device  100  may be mounted to the vehicle so as to protrude from the bumper of the vehicle. In such a vehicle, the characteristic length L may be a length along the travel direction of the vehicle, of a portion of the top surface or the bottom surface of the LIDAR device  100 , particularly, the top surface of the LIDAR device  100 , which protrudes from the bumper of the vehicle. This is because the portion of the LIDAR device  100  that protrudes from the vehicle is also considered to be susceptible to air flow that the traveling vehicle receives. Then, in cases where the length along the travel direction of the vehicle, of the portion of the LIDAR device  100  protruding from the vehicle, changes in the lateral direction of the vehicle, a maximum of the length may be used as the characteristic length L. 
     (6b) In each of the above embodiments, whether or not there is snowfall, the controller  30  controls energization of the heater  20  beforehand in response to the vehicle speed and the outside temperature. That is, the controller  30  may activate the heater  20  even if there is no snowfall. This is because it may be difficult to instantly heat the heater  20  if the outside temperature is too low when snow begins to fall. Thus, it is preferable to, whether or not there is snowfall, operate the heater  20  beforehand at an appropriate energization level. 
     On the other hand, from the viewpoint of suppressing power consumption, for example, the controller  30  may activate the heater  20  when determining that there is snowfall at least around the vehicle. Further, the controller  30  may activate the heater  20  only when determining that there is snowfall at least around the vehicle. 
     (6c) In the above embodiments, the controller  30  controls energization of the heater  20  in response to not only the outside temperature and the vehicle speed, but also the battery voltage. Alternatively, the controller  30  may control energization to the heater  20  without taking into account the battery voltage. 
     (6d) In the above third embodiment, as in the first embodiment, the controller  30  controls energization of the heater  20  based on the function of the outside temperature and the vehicle speed as parameters. Alternatively, as in the second embodiment, the controller  30  may control energization of the heater  20  based on a table in which an energization condition is pre-defined depending on the outside temperature and the vehicle speed. 
     More specifically, for example, two types of tables, in each of which an energization condition is pre-defined, may be prepared: one under snowfall conditions and the other under normal conditions where there is no snowfall. Numerical values in the table under snowfall conditions are set such that the energization level of the heater  20  is higher than under normal conditions. In response to determining that there is snowfall, the controller  30  acquires the energization condition with reference to the table under snowfall conditions. On the other hand, in response to determining that there is no snowfall, the controller  30  acquires the energization condition with reference to the table under normal conditions. 
     This also applies to the fourth and fifth embodiments. 
     (6e) The outside temperature used to determine the target energization level may be corrected based on a result of detection by a solar radiation sensor provided on a bottom of the vehicle carrying the LIDAR device  100 , or an on/off state of vehicle lights. Since the outside temperature sensor  41  is normally provided at a position away from the LIDAR device  100 , the outside temperature around the LIDAR device  100  and the outside temperature detected by the outside temperature sensor  41  may be different. More specifically, for example, in cases where the outside temperature sensor  41  is provided on the bottom of the vehicle where it is less susceptible to the sun, the outside temperature around the LIDAR device  100  may be higher than the temperature detected by the outside temperature sensor  41 . Therefore, in response to a result of detection by the solar radiation sensor, the outside temperature used to determine the target energization level may be corrected to be higher than the outside temperature detected by the outside temperature sensor  41 . In addition, when the vehicle lights are off, that is, during daylight hours, the outside temperature used to determine the target energization level may be corrected to be higher than the outside temperature detected by the outside temperature sensor  41  in response to the on/off state of the lights. 
     (6f) The outside temperature used to determine the target energization level may be corrected based on information regarding a road on which the vehicle is traveling. More specifically, for example, in cold climates, a temperature inside a tunnel is high as compared with a temperature outside the tunnel and the outside temperature drops at once upon exit from the tunnel. Thus, the transmission window  121  may not be heated sufficiently by the heater  20  and snow adhering to the transmission window  121  may not be melted quickly. Therefore, the controller  30  may be configured to, even during travel in a tunnel, determine the target energization level based on the outside temperature before the vehicle enters the tunnel. 
     (6g) The controller  30  may control the target energization level to be higher than the target energization level determined from the current vehicle speed if the vehicle speed is expected to increase rapidly. This is because it is intended to increase the energization level of the heater  20  beforehand taking into account that it takes time for the temperature of the heater  20  to rise. Specifically, the vehicle speed used to determine the target energization level may be an arrival vehicle speed estimated from the acceleration of the vehicle. In cases where the acceleration of the vehicle is equal to or higher than a predetermined value, the controller  30  may correct the target energization level to be higher than the target energization level determined based on the current speed. The acceleration of the vehicle can be acquired from an acceleration sensor of the vehicle. Furthermore, the target energization level may be corrected based on information regarding the road on which the vehicle will travel. More specifically, for example, since the vehicle traveling on an expressway is expected to reach a high speed, the controller  30  may correct the target energization level of the heater  20  to a high level when entering the expressway. 
     (6h) In the above third embodiment, the controller  30  may further be configured to determine whether there is contamination on the transmission window  121 , based on a result of detection by a contamination sensor provided on the transmission window  121 . If it is determined that there is no contamination on the transmission window  121 , then the heater  20  may not be activated. Even if the acquired weather information indicates that there is snowfall, there may be no snowfall in an area where the vehicle is actually traveling. If nothing is detected by the contamination sensor, there is no snow adhering to the transmission window  121 , and it may be considered that there is no snowfall around the vehicle. Therefore, if the controller  30  determines that there is no contamination, it may not activate the heater  20  regardless of contents of the weather information. The contamination sensor is usually used to drive a cleaning device, such as a washer, when contamination is detected. 
     (6i) In the above embodiments, the LIDAR device is an example of the ranging device, but the type of the ranging device is not limited to this type. More specifically, the ranging device includes, for example, a millimeter-wave radar device or an ultrasonic sensor device. 
     (6j) In the above embodiments, the LIDAR device  100  is mounted on the front side of the vehicle, but the mounting position of the LIDAR device  100  on the vehicle is not limited to this position. More specifically, for example, the LIDAR device  100  may be mounted around the vehicle, such as on the left side, the right side, or the rear side of the vehicle. 
     (6k) In the above embodiments, the transmission window  121  is a window that transmits both the transmitted waves and the reflected waves. Alternatively, the transmission window  121  may be configured such that at least either of the transmitted waves and the reflected waves are transmitted. In addition, in the above embodiments, the transmission window  121  is transparent such that light, as transmitted waves, can be transmitted. Alternatively, the transmission window  121  does not need to be transparent if it transmits the transmitted waves. The transmission window  121  can be made of various materials depending on the type of transmitted waves. 
     (6l) The functions of a single component may be distributed to a plurality of components, or the functions of a plurality of components may be consolidated into a single component. At least part of the configuration of the above embodiments may be replaced with a known configuration having a similar function. At least part of the configuration of the above embodiments may be removed. At least part of the configuration of one of the above embodiments may be replaced with or added to the configuration of another one of the above embodiments. 
     (6m) Besides the LIDAR device  100  described above, the present disclosure may be implemented in various modes, such as the controller  30  as a constituent element of the LIDAR device  100 , a program for causing a computer to serve as the controller  30 , a storage medium storing this program, and a method for controlling energization of the heater  20  in the LIDAR device  100 .