Patent Publication Number: US-2021190924-A1

Title: Distance measuring device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application of International Application No. PCT/JP2019/034438 filed on Sep. 2, 2019, which is based on and claims the benefit of priority from Japanese Patent Application No. 2018-165988 filed with the Japan Patent Office on Sep. 5, 2018. The contents of these applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to a distance measuring device. Distance measuring devices that are mounted on a vehicle and measure a distance to an object located in front of the vehicle include a distance measuring device that emits a transmission wave toward the front and detects a reflected wave of the emitted transmitted wave from an object to detect a distance to the object. 
     SUMMARY 
     An aspect of the present disclosure is a distance measuring device configured to emit a transmitted wave and detect a reflected wave to measure a distance to a object, and the distance measuring device includes a transmission window, a heater, and a control unit. The heater is configured to heat the transmission window. The control unit is configured to control energization of the heater according to an inside air temperature and an outside air temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a lidar device of a first embodiment. 
         FIG. 2  shows an external appearance of the lidar device. 
         FIG. 3  shows a cover of the lidar device viewed from inside. 
         FIG. 4  is a flow chart of a determination process performed by a control unit in the first embodiment. 
         FIG. 5  shows a ratio of energization time of a heater. 
         FIG. 6  is a block diagram showing a configuration of the lidar device of a second embodiment. 
         FIG. 7  is a flow chart of a determination process performed by the control unit in the second embodiment. 
         FIG. 8  is a block diagram showing a configuration of the lidar device of a third embodiment. 
         FIG. 9  is a flow chart of a determination process performed by the control unit in the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the distance measuring device, in order to protect an irradiation unit that emits a transmitted wave or a detection unit that detects a reflected wave, a cover is provided on the front of these units. However, when snow is adhered to the cover, the measurement accuracy of the distance measuring device may be reduced. 
     Thus, JP H8-29535 A discloses that a cover of a distance measuring device is provided with a heater to melt snow. 
     However, as a result of detailed studies, the inventor has found a problem that fogging may occur on the cover due to a temperature difference between the inside and the outside of the distance measuring device. When fogging occurs particularly on a transmission window of the cover through which a transmitted wave or a reflected wave is transmitted, the measurement accuracy of the distance measuring device may be reduced. 
     An aspect of the present disclosure provides a distance measuring device capable of preventing fogging of a transmission window. 
     An aspect of the present disclosure is a distance measuring device configured to emit a transmitted wave and detect a reflected wave from an object irradiated with the transmitted wave to measure a distance to the object, and the distance measuring device includes a transmission window, a heater, and a control unit. 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 from an inside of the distance measuring device. The control unit is configured to control energization of the heater according to an inside air temperature which is an air temperature inside the distance measuring device and an outside air temperature which is an air temperature outside the distance measuring device. 
     With such a configuration, energization of the heater is controlled according to the inside air temperature and the outside air temperature; thus, fogging of the transmission window can be prevented. 
     Exemplary embodiments of the present disclosure will be described below with reference to the drawings. 
     1. First Embodiment 
     1-1. Configuration 
     A lidar device  100  shown in  FIG. 1  is a distance measuring device that emits light as a transmitted wave and detects a reflected wave from an object irradiated with the light to measure a distance to the object. The term lidar is also written as LIDAR. LIDAR is an abbreviation for light detection and ranging. The lidar device  100  is mounted on a vehicle and used to detect various objects that are present in front of the vehicle. 
     The lidar device  100  includes a measurement device  10 , a heater  20 , an inside air temperature sensor  30 , and a control unit  40 . 
     The measurement device  10  includes an irradiation unit  11  that emits light and a detection unit  12  that detects a reflected wave of the light. The irradiation unit  11  emits a laser beam as light. The detection unit  12  receives a reflected wave from an object and converts the reflected wave into an electrical signal. 
     The measurement device  10  is stored inside a case  110  of the lidar device  100  shown in  FIG. 2  that includes a cover  120  and a case body  130 . The irradiation unit  11  of the measurement device  10  is stored in an upper region of a space inside the case  110 . On the other hand, the detection unit  12  of the measurement device  10  is stored in a lower region of the space inside the case  110 . 
     As a part of the cover  120 , a transmission window  121  that is transparent and through which light is transmitted is provided on the front of the cover  120 . The front here indicates a direction toward which light is emitted from the lidar device  100 . The inside and the outside of the lidar device  100  are separated from each other by the transmission window  121 . 
     The heater  20  is configured to heat the transmission window  121  from an inner portion of the lidar device  100 , i.e., from the inside of the lidar device  100 . As shown in  FIG. 3 , the heater  20  is provided on an inner surface of the transmission window  121 . The heater  20  includes an irradiation-side heater  21  that is provided on the irradiation unit  11  side of the transmission window  121 , and a detection-side heater  22  that is provided on the detection unit  12  side of the transmission window  121 . Each of the irradiation-side heater  21  and the detection-side heater  22  includes a transparent conductive film Fi and a pair of electrodes LDi and LGi. Note that i is 1 for the transparent conductive film and the electrodes belonging to the irradiation-side heater  21 , and i is 2 for the transparent conductive film and the electrodes belonging to the detection-side heater  22 . The transparent conductive film Fi is a heater film made of a material having transparency and electrical conductivity. The transparent conductive film Fi may be, for example, an ITO film. ITO is indium tin oxide. 
     The inside air temperature sensor  30  is provided inside the lidar device  100 , and detects an inside air temperature which is an air temperature inside the lidar device  100 . 
     The control unit  40  is mainly composed of a microcomputer including a CPU, a RAM, a ROM, an I/O, a bus line connecting these components, and the like, and performs various processes. As functional blocks implemented by executing a program stored in the ROM, i.e., virtual components, the control unit  40  includes a distance calculation unit  41 , a target energization amount determination unit  42 , a possible energization amount estimation unit  43 , a control value determination unit  44 , and a heater energization unit  45 . 
     The distance calculation unit  41  is configured to obtain a distance to an object irradiated with light, by using the measurement device  10 . Specifically, on the basis of a waveform of an electrical signal inputted from the detection unit  12  to the distance calculation unit  41 , the distance calculation unit  41  specifies a timing at which a reflected wave is detected, and obtains a distance to an object on the basis of a difference between the specified timing and a timing at which light is emitted. Other than the distance, the distance calculation unit  41  can obtain information on an object such as an azimuth of the object. 
     The target energization amount determination unit  42  is configured to determine a target amount of energization to the heater  20  (hereinafter also referred to as a target energization amount) according to an inside air temperature acquired from the inside air temperature sensor  30  and an outside air temperature which is an air temperature outside the lidar device  100 . As the amount of energization of the heater  20 , the target energization amount determination unit  42  may determine electric power which is an energization amount per unit time. The target energization amount determination unit  42  acquires an outside air temperature from an outside air temperature sensor  50  that is mounted on the vehicle. The outside air temperature sensor  50  is provided at a lower portion of the vehicle, and detects an air temperature outside the vehicle. 
     The possible energization amount estimation unit  43  is configured to estimate an amount of energization that can be supplied from a battery  51  of the vehicle (hereinafter also referred to as a possible energization amount) on the basis of a battery voltage detected for the battery  51 . 
     The control value determination unit  44  is configured to determine a control value for control of energization of the heater  20  by the heater energization unit  45  (described later). In the present embodiment, the control value is a duty ratio which is a ratio between energization time and non-energization time of the heater  20 . The control value determination unit  44  determines the duty ratio according to the target energization amount determined by the target energization amount determination unit  42  and the possible energization amount estimated by the possible energization amount estimation unit  43 . In the present embodiment, the battery  51  of the vehicle is directly connected to the heater  20  not through a constant voltage circuit or the like, and accordingly, a voltage applied to the heater  20  is changed due to a change in the battery voltage. Thus, according to the amount of energization that can be currently supplied from the battery  51 , the control value determination unit  44  determines the duty ratio so that the actual amount of energization of the heater  20  is the target energization amount determined by the target energization amount determination unit  42 . 
     The heater energization unit  45  is configured to control energization of the heater  20  on the basis of the control value determined by the control value determination unit  44 . 
     1-2. Process 
     A determination process performed by the control unit  40  will be described with reference to a flow chart in  FIG. 4 . The determination process in  FIG. 4  is repeatedly performed in a predetermined cycle after an ignition switch of the vehicle is turned on. 
     First, at S 11 , the control unit  40  acquires an inside air temperature and an outside air temperature from the inside air temperature sensor  30  and the outside air temperature sensor  50 , respectively. 
     Subsequently, at S 12 , the control unit  40  calculates a difference between the inside air temperature and the outside air temperature. 
     Subsequently, at S 13 , the control unit  40  determines a target energization amount on the basis of the difference between the inside air temperature and the outside air temperature. Specifically, the control unit  40  obtains a target energization amount by referring to a table in which an appropriate target energization amount is set for each difference between the inside air temperature and the outside air temperature. S 11  to S 13  correspond to a process performed by the control unit  40  as the target energization amount determination unit  42 . 
     The target energization amounts in the table are set so that the control unit  40  operates the heater  20  when an absolute value of the difference between the inside air temperature and the outside air temperature is a predetermined value or more. Thus, when the absolute value of the difference between the inside air temperature and the outside air temperature is small, the target energization amount is 0, and the control unit  40  does not operate the heater  20 . On the other hand, when the absolute value of the difference between the inside air temperature and the outside air temperature is large, fogging of the transmission window  121  is more likely to occur. In this case, the control unit  40  prevents fogging by operating the heater  20  according to the target energization amount set in advance. 
     The target energization amount when the absolute value of the difference between the inside air temperature and the outside air temperature is large is specifically set as follows. 
     When the difference between the inside air temperature and the outside air temperature is large in the case where the inside air temperature is low and the outside air temperature is high, condensation occurs on the outside of the transmission window  121  and causes fogging of the transmission window  121 . In this case, when the transmission window  121  whose temperature is lower than the outside air temperature is heated by the heater  20 , presumably, a temperature difference between the outer surface of the transmission window  121  and the outside air temperature is reduced and fogging of the transmission window  121  is less likely to occur. Thus, the target energization amount when the inside air temperature is lower than the outside air temperature and the absolute value of the difference between the inside air temperature and the outside air temperature is the predetermined value or more is set so that the temperature of the transmission window  121  approaches the outside air temperature. 
     On the other hand, when the difference between the inside air temperature and the outside air temperature is large in the case where the inside air temperature is high and the outside air temperature is low, condensation occurs on the inside of the transmission window  121  and causes fogging of the transmission window  121 . Also, in this case, when the transmission window  121  whose temperature is lower than the inside air temperature is heated by the heater  20 , presumably, a temperature difference between the inner surface of the transmission window  121  and the inside air temperature is reduced and fogging of the transmission window  121  is less likely to occur. 
     When the inside air temperature is low and the outside air temperature is high, fogging occurs on the outside of the transmission window  121 . Thus, water adhered to the transmission window  121 , which is a cause of fogging, is expected to be removed by wind blowing to the outer surface of the transmission window  121  during traveling of the vehicle. On the other hand, when the inside air temperature is high and the outside air temperature is low, fogging occurs on the inside of the transmission window  121 . Thus, unlike when fogging occurs on the outside of the transmission window  121 , adhered water droplets are less likely to be removed. 
     In this case, in order to evaporate the water adhered to the transmission window  121 , it is more preferable to significantly increase the amount of energization of the heater  20  than to merely eliminate the temperature difference between the inner surface of the transmission window  121  and the inside air temperature. Thus, the target energization amount when the inside air temperature is higher than the outside air temperature and the absolute value of the difference between the inside air temperature and the outside air temperature is the predetermined value or more is set so that the water adhered to the transmission window  121  is evaporated. 
     Therefore, in the case where the absolute value of the difference between the inside air temperature and the outside air temperature is the same, the target energization amount when the heater  20  is operated at the inside air temperature higher than the outside air temperature is always larger than the target energization amount when the heater  20  is operated at the inside air temperature lower than the outside air temperature.  FIG. 5 ( a )  shows energization of the heater  20  when the inside air temperature is higher than the outside air temperature, and  FIG. 5 ( b )  shows energization of the heater  20  when the inside air temperature is lower than the outside air temperature. 
     As shown in  FIGS. 5 ( a ) and ( b ) , a ratio of energization time when the inside air temperature is higher than the outside air temperature is usually higher than a ratio of energization time when the inside air temperature is lower than the outside air temperature. At S 14 , the control unit  40  acquires a detection value of the battery voltage. 
     Subsequently, at S 15 , the control unit  40  estimates a possible energization amount on the basis of the acquired detection value of the battery voltage. S 14  to S 15  correspond to a process performed by the control unit  40  as the possible energization amount estimation unit  43 . 
     Subsequently, at S 16 , the control unit  40  determines a duty ratio on the basis of the target energization amount determined at S 13  and the possible energization amount estimated at S 15 . S 16  corresponds to a process performed by the control unit  40  as the control value determination unit  44 . 
     Then, the control unit  40  ends the determination process in  FIG. 4 . 
     Separately from the determination process in  FIG. 4 , the control unit  40  performs a process of controlling energization of the heater  20  on the basis of the duty ratio determined in the determination process in  FIG. 4 . This process corresponds to a process performed by the control unit  40  as the heater energization unit  45 . 
     1-3. Effects 
     The first embodiment described above in detail achieves the following effects. 
     (1a) The control unit  40  is configured to control energization of the heater  20  according to the inside air temperature and the outside air temperature; thus, fogging of the transmission window  121  can be prevented. 
     (1b) Specifically, when the absolute value of the difference between the inside air temperature and the outside air temperature is the predetermined value or more, the control unit  40  operates the heater  20 . With such a configuration, the heater  20  is operated corresponding to a situation where fogging of the transmission window  121  may occur; thus, as compared with the case where the heater  20  is always operated, power consumption of the heater  20  can be reduced. 
     (1c) The control unit  40  controls energization of the heater  20  also considering the battery voltage detected for the battery  51  of the vehicle. This makes it possible to prevent a change in the actual amount of energization of the heater  20  due to a change in the battery voltage. 
     2. Second Embodiment 
     2-1. Differences From First Embodiment 
     A basic configuration of a second embodiment is the same as that of the first embodiment. Thus, common configuration will not be described and mainly differences from the first embodiment will be described. 
     As shown in  FIG. 6 , in addition to the components of the lidar device  100  of the first embodiment, the lidar device  100  of the second embodiment further includes a contamination sensor  60  that detects contamination of the transmission window  121 , and a cleaner  70  that cleans the transmission window  121 . Furthermore, in addition to the components of the control unit  40  of the first embodiment, the control unit  40  of the second embodiment further includes a contamination determination unit  46  and a cleaner driving unit  47  as the functional blocks. 
     The contamination determination unit  46  is configured to determine on the basis of a result of detection by the contamination sensor  60  whether the transmission window  121  is contaminated. The contamination sensor  60  irradiates the transmission window  121  with light that is different from light emitted from the irradiation unit  11 , and detects the degree of contamination on the basis of the reflectance of light on the transmission window  121 . When the degree of contamination detected by the contamination sensor  60  is a predetermined threshold or more, the contamination determination unit  46  determines that the transmission window  121  is contaminated. 
     The cleaner driving unit  47  is configured to drive the cleaner  70  on the basis of a result of determination by the contamination determination unit  46 . In the present embodiment, the cleaner  70  is a washer that cleans the outer surface of the transmission window  121  with a cleaning liquid. 
     A main difference between the first embodiment and the second embodiment is that when the contamination determination unit  46  determines that the transmission window  121  is not contaminated, the control unit  40  does not perform determination of the target energization amount by the target energization amount determination unit  42 , estimation of the possible energization amount by the possible energization amount estimation unit  43 , or determination of the control value by the control value determination unit  44 . Specific determination process performed by the control unit  40  will be described later. 
     2-2. Process 
     The determination process of the second embodiment performed by the control unit  40  instead of the determination process of the first embodiment will be described with reference to a flow chart in  FIG. 7 . The determination process in  FIG. 7  is repeatedly performed in a predetermined cycle after the ignition switch of the vehicle is turned on. 
     First, at S 21 , the control unit  40  determines whether the transmission window  121  is contaminated. S 21  corresponds to a process performed by the control unit  40  as the contamination determination unit  46 . 
     When the control unit  40  determines at S 21  that the transmission window  121  is not contaminated, control return to S 21 . On the other hand, when the control unit  40  determines at S 21  that the transmission window  121  is contaminated, control proceeds to S 22 . 
     Subsequent processes are the same as in the first embodiment. 
     2-3. Effects 
     The second embodiment described above in detail achieves the following effect in addition to the effects of the first embodiment described above. 
     (2a) In the second embodiment, the control unit  40  determines whether the transmission window  121  is contaminated, and when the control unit  40  determines that the transmission window  121  is not contaminated, the control unit  40  does not operate the heater  20 . Fogging of the transmission window  121  is detected as contamination of the transmission window  121 . Thus, when the transmission window  121  is not contaminated, the transmission window  121  is unlikely to be fogged. Therefore, with such a configuration, the heater  20  is not operated when the transmission window  121  is unlikely to be fogged; thus, power consumption can be reduced. 
     3. Third Embodiment 
     3-1. Differences From First Embodiment 
     A basic configuration of a third embodiment is the same as that of the first embodiment. Thus, a common configuration will not be described and differences from the first embodiment will be mainly described. 
     As shown in  FIG. 8 , the third embodiment differs from the first embodiment in that the target energization amount determination unit  42  is configured to acquire an operation mode of an air conditioning device  80  of the vehicle on which the lidar device  100  is mounted and determine the target energization amount considering the operation mode of the air conditioning device  80  in addition to the inside air temperature and the outside air temperature. 
     3-2. Process 
     A determination process of the third embodiment performed by the control unit  40  instead of the determination process of the first embodiment will be described with reference to a flow chart in  FIG. 9 . 
     S 31  to S 33  are the same as S 11  to S 13  of the first embodiment. 
     Subsequently, at S 34 , the control unit  40  determines whether the target energization amount determined at S 33  is 0. 
     When the control unit  40  determines at S 34  that the target energization amount is 0, control proceeds to S 35 , and the control unit  40  determines whether the operation mode of the air conditioning device  80  is a defrost mode. The defrost mode is a mode in which in order to remove fogging from a windshield of the vehicle, the air conditioning device  80  blows out conditioned air toward the windshield. 
     On the other hand, when the control unit  40  determines at S 34  that the target energization amount is not  0 , control proceeds to S 39 . 
     When the control unit  40  determines at S 35  that the operation mode of the air conditioning device  80  is the defrost mode, control proceeds to S 36  and the control unit  40  corrects the target energization amount, and then control proceeds to S 39 . Specifically, at S 36 , the control unit  40  corrects the target energization amount to a predetermined positive value so that the heater  20  is operated. 
     On the other hand, when the control unit  40  determines at S 35  that the operation mode of the air conditioning device  80  is not the defrost mode, S 36  is skipped and control proceeds to S 39 . 
     At S 37 , the control unit  40  acquires a detection value of the battery voltage. 
     Subsequently, at S 38 , the control unit  40  estimates a possible energization amount on the basis of the detection value of the battery voltage acquired at S 37 . 
     Subsequently, at S 39 , the control unit  40  determines a duty ratio on the basis of the target energization amount determined at S 33  and the possible energization amount estimated at S 38 , and then the control unit  40  ends the determination process in  FIG. 9 . S 31  to S 36  correspond to a process performed by the control unit  40  as the target energization amount determination unit  42 , S 37  to S 38  correspond to a process performed by the control unit  40  as the possible energization amount estimation unit  43 , and S 39  corresponds to a process performed by the control unit  40  as the control value determination unit  44 . 
     3-3. Effects 
     The third embodiment described above in detail achieves the following effect in addition to the effects of the first embodiment described above. 
     (3a) In the third embodiment, the control unit  40  operates the heater  20  when the operation mode of the air conditioning device  80  is the defrost mode. That is, the control unit  40  operates the heater  20  when a measure has been taken to remove fogging from the windshield in the vehicle. When the windshield in the vehicle is fogged, the transmission window  121  of the lidar device  100  may also be fogged. Therefore, with such a configuration, even when the target energization amount determined by the target energization amount determination unit  42  is 0, it is possible to detect fogging of the transmission window  121  and operate the heater  20 ; thus, fogging of the transmission window  121  can be prevented with high accuracy. 
     4. Other Embodiments 
     The embodiments of the present disclosure have been described above. However, it is needless to say that the present disclosure is not limited to the above embodiments and may be implemented in various forms. 
     (4a) In the above embodiments, the control unit  40  is configured to operate the heater  20  when the absolute value of the difference between the inside air temperature and the outside air temperature is the predetermined value or more and not to operate the heater  20  when the absolute value of the difference between the inside air temperature and the outside air temperature is less than the predetermined value. However, the predetermined value when the inside air temperature is lower than the outside air temperature may be a value different from the predetermined value when the inside air temperature is higher than the outside air temperature. Furthermore, the control unit  40  may be configured to operate the heater  20  only when the inside air temperature is lower than the outside air temperature or only when the inside air temperature is higher than the outside air temperature. Thus, the control unit  40  may be configured to operate the heater  20  when the inside air temperature is lower than the outside air temperature and the absolute value of the difference between the inside air temperature and the outside air temperature is the predetermined value or more, but not to operate the heater  20  when the absolute value of the difference between the inside air temperature and the outside air temperature is large in the case where the inside air temperature is higher than the outside air temperature. 
     (4b) In the above embodiments, the control unit  40  determines the target energization amount to the heater  20  on the basis of the difference between the inside air temperature and the outside air temperature, but the method of determining the target energization amount is not limited to this. Specifically, for example, the control unit  40  may obtain the target energization amount by using a map of the target energization amount in which an appropriate target energization amount is set on the basis of the inside air temperature and the outside air temperature or a function by which the target energization amount is calculated using the inside air temperature and the outside air temperature as parameters. 
     (4c) In the above embodiments, energization of the heater  20  is controlled by changing the duty ratio which is a ratio between the energization time and the non-energization time of the heater  20 , but the method of controlling energization of the heater  20  is not limited to this. Specifically, for example, energization of the heater  20  may be controlled by changing the voltage applied to the heater  20 . 
     (4d) In the above embodiments, the control unit  40  controls energization of the heater  20  also considering the battery voltage in addition to the inside air temperature and the outside air temperature, but the control unit  40  may control energization of the heater  20  without considering the battery voltage. 
     (4e) In the second embodiment, the transmission window  121  is irradiated with light different from light emitted from the irradiation unit  11 , and the degree of contamination is detected on the basis of the reflectance of light on the transmission window  121 , but the method of detecting the degree of contamination is not limited to this. For example, the degree of contamination may be detected on the basis of the reflectance on the transmission window  121  of light emitted from the irradiation unit  11  of the lidar device  100 . 
     (4f) In the third embodiment, a washer is presented as an example of the cleaner  70 , but the cleaner  70  is not limited to this. Specific examples of the cleaner  70  include a wiper that wipes off dirt on the transmission window  121  and an ultrasonic vibrator that causes dirt adhered to the transmission window  121  to fall off. 
     (4g) The outside air temperature used to determine the target energization amount may be corrected on the basis of a result of detection by a solar radiation sensor that is provided in the vehicle on which the lidar device  100  is mounted or an ON/OFF state of a light of the vehicle. The outside air temperature sensor  50  is usually provided at a position distant from the lidar device  100 . Thus, in some cases, the outside air temperature around the lidar device  100  differs from the outside air temperature detected by the outside air temperature sensor  50 . Specifically, for example, when the outside air temperature sensor  50  is provided at the lower portion of the vehicle, the outside air temperature sensor  50  is less likely to be influenced by sunlight, and accordingly, in some cases, the outside air temperature around the lidar device  100  is higher than the temperature detected by the outside air temperature sensor  50 . Thus, the outside air temperature used to determine the target energization amount may be corrected according to the result of detection by the solar radiation sensor so that the outside air temperature used to determine the target energization amount is higher than the outside air temperature detected by the outside air temperature sensor  50 . Furthermore, the outside air temperature used to determine the target energization amount may be corrected according to the ON/OFF state of the light so that when the light is OFF, i.e., during daytime, the outside air temperature used to determine the target energization amount is higher than the outside air temperature detected by the outside air temperature sensor  50 . 
     (4h) Furthermore, the outside air temperature used to determine the target energization amount may be corrected on the basis of information on a road on which the vehicle is travelling. Specifically, for example, in a cold district, an air temperature inside a tunnel is higher than an air temperature outside the tunnel, and when the vehicle exits the tunnel, the outside air temperature suddenly decreases, and accordingly, in some cases, heating of the transmission window  121  by the heater  20  is insufficient and fogging of the transmission window  121  occurs. Thus, even when the vehicle is travelling in a tunnel, the control unit  40  may determine the target energization amount on the basis of the outside air temperature before the vehicle enters the tunnel. 
     (4i) The target energization amount may be corrected on the basis of weather conditions around the vehicle. Specifically, for example, during rain or snow, heat is more likely to be removed from the transmission window  121  by rain or snow; thus, a large energization amount is required as compared with the case where there is no rain or snow. The control unit  40  may correct the target energization amount so that the amount of energization of the heater  20  is large in such a case. Information on the weather around the vehicle can be acquired from an information communication system such as VICS. VICS is registered trademark. 
     (4j) Furthermore, the target energization amount may be corrected on the basis of a vehicle speed or a speed to be reached by the vehicle that is estimated from acceleration of the vehicle. This is because heat is more likely to be removed from the transmission window  121  at a high vehicle speed than at a low vehicle speed. Furthermore, the target energization amount may be corrected on the basis of information on a road on which the vehicle is travelling. Specifically, for example, the vehicle speed is assumed to be high on an expressway; thus, the control unit  40  may correct the target energization amount so that the amount of energization of the heater  20  becomes large when the vehicle enters an expressway. 
     (4k) In the above embodiments, a lidar device is presented as an example of the distance measuring device, but the type of the distance measuring device is not limited to this. Specific examples of the distance measuring device include a millimeter wave radar device and an ultrasonic sensor device. 
     (4l) In the above embodiments, the lidar device  100  is mounted at the front portion of the vehicle, but the mounting position of the lidar device  100  in the vehicle is not limited to this. Specifically, for example, the lidar device  100  may be mounted at a peripheral portion of the vehicle such as a side or rear portion of the vehicle. 
     (4m) In the above embodiments, the transmission window  121  is a window through which both a transmitted wave and a reflected wave are transmitted, but the transmission window  121  may be configured such that at least one of a transmitted wave and a reflected wave is transmitted through the transmission window  121 . Furthermore, in the above embodiments, the transmission window  121  is transparent so that light as a transmitted wave can be transmitted through the transmission window  121 , but the transmission window  121  does not need to be transparent as long as a transmitted wave is transmitted through the transmission window  121 . Thus, the transmission window  121  may be made of various materials according to the type of transmitted wave. 
     (4n) In the above embodiments, the function of a single component may be divided into a plurality of components, or the functions of a plurality of components may be integrated into a single component. Furthermore, part of the configuration of the embodiments may be omitted. Furthermore, at least part of the configuration of the embodiments may be, for example, added to or replaced with another configuration of the embodiments. 
     (4o) Other than the lidar device  100 , the present disclosure may also be implemented in various forms such as the control unit  40  constituting the lidar device  100 , a program for functioning a computer as the control unit  40 , a medium that records the program, and a method of controlling energization of the heater  20  of the lidar device  100 .