Patent Publication Number: US-2007124028-A1

Title: Vehicle appliance control system

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a vehicle appliance control system provided on a vehicle.  
      2. Description of the Related Art  
      Conventional vehicle appliance control systems-comprise a secondary communication device driving electrically-driven appliances provided in a vehicle and a communication cable serving as a signal transmission path provided between the secondary communication device and a main communication device delivering an operation command to the secondary communication device such as coaxial cable, optical fiber cable or the like. JP-A-2000-69009 discloses one of such conventional vehicle appliance control systems.  
      The number of electrically-driven appliances provided on an automobile has recently been increasing with progress in the automobile technology. As a result, the number of communication cables connecting the secondary and main communication devices to each other has also been increasing. This causes an increase in the wiring space or in the weight of the automobile.  
     SUMMARY OF THE INVENTION  
      Therefore, an object of the present invention is to provide a vehicle appliance control system which can reduce the wiring space in the vehicle and the weight of the vehicle.  
      The present invention provides a vehicle appliance control system provided in a vehicle including an electrically-driven appliance, a passive appliance and a plurality of components, the system comprising a main communication device having a main modulator superimposing, on electromagnetic waves, information including an operation command for operating the electrically-driven appliance and outputting the electromagnetic waves as surface waves of the components, and a plurality of secondary communication devices having respective secondary demodulators obtaining the information from the electromagnetic waves output by the main modulator to have propagated along surfaces of the components, the secondary communication devices being connected to the electrically-driven appliances or the passive appliances, being capable of communicating with the main communication device, and operating the electrically-driven appliances based on the operation command from the main communication device.  
      According to the above-described arrangement, the main communication device superimposes information including an operation command on electromagnetic waves and outputs the electromagnetic waves. Each secondary communication device receives the electromagnetic waves thereby to obtain the information. Thus, since information is transmitted and received by a wireless manner, the wiring space in the vehicle and the weight of the vehicle can be reduced as compared with the conventional arrangement which uses a dedicated communication cable for transmission and reception. Furthermore, since the electromagnetic waves output from the main communication device are surface waves propagating on the surfaces of a plurality of components, power consumption upon output of information can be reduced as compared with the conventional wireless communication by radio waves. Additionally, since the electromagnetic waves output as the surface waves from the main communication device are difficult to be received by a secondary communication device provided on another vehicle, the reliability of communication can be improved.  
      In one embodiment, the system further comprises an secondary modulator provided in each secondary communication device for superimposing, on the electromagnetic waves, the information to be supplied to the main communication device and outputting the electromagnetic waves as the surface waves of the components and a main demodulator provided in the main communication device for obtaining the information from the electromagnetic waves output by the secondary modulator to have propagated along the surfaces of the components.  
      According to the above-described system, each secondary communication device superimposes information on electromagnetic waves and outputs the electromagnetic waves. The main communication device receives the electromagnetic waves thereby to obtain the information. Thus, since the information directed to the main communication device is transmitted and received by a wireless manner, the wiring space in the vehicle and the weight of the vehicle can be reduced as compared with the conventional arrangement which uses a dedicated communication cable for transmission and reception. Furthermore, since the electromagnetic waves produced from each secondary communication device are surface waves propagating on the surfaces of a plurality of components, power consumption upon output of information can be reduced as compared with the conventional wireless communication by radio waves. Additionally, since the electromagnetic waves output as the surface waves from each secondary communication device are difficult to be received by a main communication device provided on another vehicle, the reliability of communication can be improved.  
      In another embodiment, a vehicle appliance control system provided in a vehicle including a battery having a positive pole, an electrically-driven appliance and a passive appliance, the system comprising a single main communication device having a main high-frequency current generating device capable of generating high-frequency current supplied to a power line connected to the positive pole of the battery and superimposing on the high-frequency current information containing an operation command for operating the electrically-driven appliance, and a plurality of secondary communication devices having respective secondary information extracting devices obtaining information from the high-frequency current flowing through the power line, the secondary communication devices connected to the electrically-driven appliance or passive appliance, capable of communicating with the main communication device, and operating the electrically-driven appliance by power supply from the battery based on the operation command from the main communication device.  
      According to the above-described system, the main communication device superimposes information containing an operation command on high-frequency current, outputting the high-frequency current onto a power line. Each secondary communication device obtains the information from the high-frequency current flowing through the power line. Since the power line provided in the vehicle is used as a transmission path for the information containing the operation command, the wiring space in the vehicle and the weight of the vehicle can be reduced as compared with the conventional arrangement which uses a dedicated communication cable for transmission and reception of information. Additionally, since the information output from the main communication device is not received by a secondary communication device provided on another vehicle, the reliability of communication can be improved.  
      In further another embodiment, the system further comprises an secondary high-frequency current generating device provided on each secondary communication device capable of generating high-frequency current supplied to the power line connected to the positive pole of the battery and superimposing on the high-frequency current information delivered to the main communication device, and a main information extracting device provided on the main communication device extracting the information from the high-frequency current flowing through the power line, thereby obtaining information containing the operation command.  
      According to the above-described system, each secondary communication device superimposes information on high-frequency current, outputting the high-frequency current onto a power line. The main communication device obtains the information from the high-frequency current flowing through the power line. Since the power line provided in the vehicle is used as a transmission path for the information directed to the main communication device, the wiring space in the vehicle and the weight of the vehicle can be reduced as compared with the conventional arrangement which uses a dedicated communication cable for transmission and reception of information. Additionally, since the information output from each secondary communication device is not received by a secondary communication device provided on another vehicle, the reliability of communication can be improved.  
      In further another embodiment, the system further comprises an abnormal information transmitter provided in the secondary communication device for carrying out a process according to the operation command from the main communication device and thereafter superimposing, on the electromagnetic waves, information about presence of an abnormality of the electrically-driven appliances, transmitting the electromagnetic waves to the main communication device. Consequently, the main communication device can obtain information about presence or absence of an abnormality of the electrically-driven appliances and the information that the passive appliance has been operated.  
      In further another embodiment, the system further comprises electromagnetic wave intensity detectors provided in the secondary and main communication devices for detecting intensity of the electromagnetic waves output by the secondary and main modulators respectively, and an output changing unit changing frequencies of electromagnetic waves output by the secondary and main modulators according to results of detection by the electromagnetic wave intensity detectors respectively. Consequently, the frequencies of electromagnetic waves output from each secondary modulator and the main modulator can be changed according to the intensity of electromagnetic waves output from the secondary and main modulators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other objects, features and advantages of the present invention will become clear upon reviewing the following description of the embodiment, with reference to the accompanying drawings, in which:  
       FIG. 1  is a conceptual illustration of a vehicle appliance control system in accordance with a first embodiment of the present invention;  
       FIG. 2  is a block diagram showing an electrical arrangement of a main communication device and first and second secondary communication devices;  
       FIG. 3  is a block diagram showing an electrical arrangement of the main communication device and third and fourth secondary communication devices;  
       FIG. 4  is a block diagram showing an electrical arrangement of the vehicle appliance control system in accordance with a second embodiment of the present invention;  
       FIG. 5  is a block diagram showing an electrical arrangement of the main communication device and the secondary communication devices; and  
       FIG. 6  is a conceptual illustration of the vehicle appliance control system in a modified form. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A first embodiment of the present invention will be described with reference to FIGS.  1  to  3 . Referring to  FIG. 1 , reference numeral  10  designates a vehicle appliance control system of the embodiment. The system  10  constitutes an automotive local area network (LAN) provided in an automobile  100  serving as a vehicle in the invention. The system  10  comprises a single main communication device  11  and a plurality of, for example, four, secondary communication devices  121  through  124 . These communication devices  11  and  121 - 124  are dispersed at predetermined positions in the automobile  100 .  
      The secondary communication devices  121 - 124  include respective electrical connectors  20  to which various appliances are connected. More specifically, for example, a headlight  30  serving as a lamp in the invention is connected to the electrical connector  20  of the first secondary communication device  121 . A wiper motor  31  is connected to the electrical connector  20  of the second secondary communication device  122 . A door sensor  32  for detecting an open or closed state of a door of the automobile  100  is connected to the electrical connector  20  of the third secondary communication device  123 . A seatbelt sensor  33  for detecting a fastened or unfastened state of a seatbelt is connected to the electrical connector  20  of the fourth secondary communication device  124 . Each of the headlight  30  and wiper motor  31  serves as an electrically-driven appliance in the invention. Each of the door sensor  32  and the seatbelt sensor  33  serves as a passive appliance in the invention. The first to fourth secondary communication devices  121 - 124  are assigned with respective identifiers (IDs) to discriminate one secondary communication device from the others.  
      The electrically-driven appliance should not be limited to the foregoing. For example, the electrically-driven appliance may include lights such as tail lights, an interior light, signal lights and the like, electric motors such as power window motors, door mirror motors and the like, air conditioner, car audio system or the like. Furthermore, the passive appliance should not be limited to the foregoing but may include a passenger sensor, a corner sensor, a tire air pressure sensor or the like.  
      Referring to  FIG. 2 , the main communication device  11  includes a signal processor  13  as a major component. The signal processor  13  is provided with a central processing unit (CPU) executing various programs, a read only memory (ROM) storing the programs and a random access memory (RAM) temporarily storing data, none of which are shown. The signal processor  13  produces predetermined main communication data. Furthermore, the main communication device  11  is provided with a modulator  14  which takes in electromagnetic waves serving as carrier waves generated by an oscillator  15  and main communication data generated by the signal processor  13 . The electromagnetic waves are modulated by the main communication data or the main communication data is superimposed on the electromagnetic waves. The modulated electromagnetic waves are delivered from an input/output circuit  16  as surface waves propagating along components of the automobile  100  such as body  50  and chassis  51 . The modulator  14  of the main communication device  11  serves as a main modulator in the invention.  
      The input/output circuit  16  takes in the electromagnetic waves (surface waves) produced by the secondary communication devices  121 - 124  and propagating along the surface of the automobile  100  (the surfaces of the body  50  and chassis  51 ). The demodulator  18  extracts secondary communication data from the electromagnetic waves and outputs the secondary communication data to the signal processor  13 . The input/output circuit  16  may carry out, for example, an amplifying process and noise rejection process for the electromagnetic waves supplied into and out of the main communication device  11 . The demodulator  18  of the main communication device  11  serves as a main demodulator in the invention.  
      As shown in  FIGS. 2 and 3 , each of the secondary communication devices  121 - 124  includes a signal processor  13 , a modulator  14 , an oscillator  15 , an input/output circuit  16  and a demodulator  18  as the main communication device  11  does. Each signal processor  13  of the secondary communication devices  121 - 124  includes a ROM (not shown) storing data of a specific identifier (ID). The modulator  14  of each of the secondary communication devices  121 - 124  serves as a secondary modulator in the invention, whereas the demodulator  18  serves as a secondary demodulator in the invention.  
      The signal processor  13  of the first secondary communication device  121  connected to the headlight  30  is provided with a rewritable memory  19 , which stores data of a normal current value of the headlight  30 , as shown in  FIG. 2 . In the same manner, the second secondary communication device  122  connected to the wiper motor  31  includes a rewritable memory  19  storing data of a normal current value of the wiper motor  31 . The aforesaid normal current values are individually written onto the respective memories  19  by data transmission as carrier waves of electromagnetic waves from the main communication device  11 . This eliminates manufacture of dedicated secondary communication devices storing different contents of the memories  19  according to the types of the electrically-driven appliances. In other words, one type of secondary communication device can be used commonly for a plurality of types of electrically-driven appliances, whereupon the versatility of secondary communication device can be improved.  
      The main and secondary communication devices  11  and  121 - 124  are connected via a power cable  21  to an external power supply such as an automotive battery  22 . When a power switch (not shown) is turned on, electric power is supplied from the automotive battery  22  to the main and secondary communication devices  11  and  121 - 124 , so that the devices are activated. Furthermore, electric power is supplied from the secondary communication devices  121 - 124  via the electrical connectors  20  to the vehicle appliances  30 - 33  connected to the secondary communication devices  121 - 124 , respectively. The power cable  21  and each electrical connector  20  serve as a power supply line in the invention.  
      A cutoff switch  26  normally on-state is provided in the middle of the electrical connector  20  for supplying electric power from the first and second secondary communication devices  121  and  122  to the headlight  30  and wiper motor  31  respectively. Furthermore, each of the first and second secondary communication devices  121  and  122  is provided with a cutoff circuit  25  for changing the cutoff switch  26  to an off-state when each of the power supply lines to the headlight  30  and the wiper motor  31  is short-circuited. Thus, even when the power supply line is short-circuited, the headlight  30  or the wiper motor  31  can be prevented from failure due to overcurrent.  
      The operation of the vehicle appliance control system  10  will now be described. For example, when the driver turns on an operation switch of the wiper of the automobile  100  provided with the vehicle appliance control system  10 , the signal processor  13  of the main communication device  11  obtains information about the operation of the switch. The signal processor  13  then generates main communication data including the ID of the second secondary communication device  122  to which the wiper motor  31  is connected and starting command for the wiper motor  31 , delivering the main communication data to the modulator  14 . The main communication data serves as information including an operation command in the invention. The modulator  14  modulates the electromagnetic waves taken in from the oscillator  15  on the basis of the main communication data and outputs the waves to the input/output circuit  16 . The input/output circuit  16  carries out an amplifying process and a noise rejection process for the electromagnetic waves and thereafter delivers the processed electromagnetic waves to the body  50  of the automobile  100 , for example.  
      The electromagnetic waves delivered to the body  50  propagate as surface waves along the surfaces of the body  50  and chassis  51 , reaching various parts of the automobile  100 . The electromagnetic waves are then taken in by the input/output circuits  16  of the secondary communication devices  121 - 124 . Each input/output circuit  16  carries out an amplifying process and a noise rejection process for the taken-in electromagnetic waves and thereafter delivers the processed electromagnetic waves to the demodulator  18 . The demodulator  18  demodulates the electromagnetic waves, thereby extracting the main communication data. The extracted main communication data is taken in by the signal processor  13  of each of the secondary communication devices  121 - 124 . The signal processor  13  determines whether the ID contained in the main communication data corresponds with ID stored in the ROM of its own. When the main communication device  11  has transmitted the main communication data for operating the wiper, the signal processor  13  of the second secondary communication device  122  determines that the ID contained in the main communication data corresponds with the ID stored in the ROM of its own, and drives the wiper motor  31  in response to the starting command contained in the main communication data, whereby the wiper is operated.  
      On the other hand, the secondary communication devices  121 ,  123  and  124  other than the second secondary communication device  122  each determine that the ID contained in the main communication data does not correspond with the ID stored in the ROM of its own, ignoring the starting command contained in the main communication data. As a result, an erroneous activation of the headlight  30  and wiper motor  31  can be completely prevented.  
      When the wiper is operated in response to the starting command, the signal processor  13  of the second secondary communication device  122  measures an electrical current flowing into the wiper motor  31 , determining whether the measured current value is within a range of normal current value which is stored in the memory  19  of the signal processor  13 . When the measured current value is within the normal current value range, the signal processor  13  determines that the wiper motor  31  is normal. When out of the normal current value range, the signal processor  13  determines that the wiper motor  31  is abnormal. More specifically, when the measured current value is smaller than a lower limit of the normal current value, the signal processor  13  determines that a break has occurred. When the measured current value is larger than an upper limit of the normal current value, the signal processor  13  determines that a short circuit has occurred. When the signal processor  13  determines that a short circuit has occurred, the second secondary communication device  122  operates the cutoff circuit  25  (see  FIG. 2 ) so that the cutoff switch  26  is turned off to stop power supply to the wiper motor  31 . The signal processor  13  serves as an abnormality determining unit in the invention.  
      The signal processor  13  of the second secondary communication device  122  generates secondary communication data containing the aforesaid result of abnormality determination and the ID, delivering the secondary communication data to the modulator  14 . The modulator  14  modulates the electromagnetic waves as carrier waves taken from the oscillator  15  on the basis of the secondary communication data or superimposes the secondary communication data on the electromagnetic waves. The modulator  14  then delivers the electromagnetic waves to the input/output circuit  16 . The input/output circuit  16  carries out an amplifying process and noise rejection process for the electromagnetic waves, delivering the processed electromagnetic waves to the body  50 . The aforesaid secondary communication data serves as information to the main communication device in the invention. The signal processor  13  and the modulator  14  of the secondary communication device  122  constitute an abnormality information transmitting unit in the embodiment.  
      The electromagnetic waves delivered from the second secondary communication device  122  to the body  50  propagate as surface waves along the surfaces of the components of the automobile  100  such as the body  50 , chassis  51  and the like, reaching various parts of the automobile  100 . The electromagnetic waves are then taken in by the main communication device  11 . The electromagnetic waves taken in by the main communication device  11  are demodulated by the demodulator  18 , whereby the secondary communication data is extracted. The extracted secondary communication data is taken in by the signal processor  13 . The signal processor  13  obtains information about presence or absence of abnormality in the wiper motor  31  and information as to whether the wiper motor  31  has normally been operated, from the result of abnormality determination contained in the secondary communication data. A process for the operation of the headlight  30  is the same as described above.  
      The main communication device  11  generates main communication data toward the third secondary communication device  123  at predetermined intervals for the purpose of confirming an on-off state of the door sensor  32 . More specifically, in the same manner as in the wiper starting command, the main communication device  11  produces main communication data containing the ID of the third secondary communication device  123  and on-off state confirmation command, superimposing the main communication data on electromagnetic waves. The main communication device  11  then delivers the main communication data to the body  50 . The electromagnetic waves propagate as surface waves along the surfaces of the body  50  and chassis  51 , reaching various parts of the automobile  100 . The electromagnetic waves are then taken in by the secondary communication devices  121 - 124 . However, only the signal processor  13  of the third secondary communication device  123  obtains the information about on-off state of the door sensor  32  in response to the on-off state confirmation command of the main communication data extracted from the electromagnetic waves. The signal processor  13  of the third secondary communication device  123  produces secondary communication data containing the information about on-off state of the door sensor  32 . The signal processor  13  then superimposes the secondary communication data on electromagnetic waves and delivers the electromagnetic waves to the body  50 . The main communication device  11  takes in the electromagnetic waves delivered from the third secondary communication device  123  and propagated along the surfaces of the body  50  and chassis  51 , extracting the secondary communication data. The signal processor  13  of the main communication device  11  obtains information about an open or closed state of the automotive door, based on the information about on-off state of the door sensor  32  contained in the extracted secondary communication data. For example, when the door is open, a predetermined warning lamp is activated. When the on-off state of the seatbelt sensor  33  is confirmed, the same process as described above is carried out.  
      According to the foregoing embodiment, communication data is superimposed on the electromagnetic waves. The electromagnetic waves are then transmitted and received between the main communication device  11  and each of the secondary communication devices  121 - 124 . In other words, wireless communication is carried out between the main communication device  11  and each of the secondary communication devices  121 - 124 . Accordingly, the wiring space in the automobile  100  and the weight of the automobile  100  can be reduced as compared with the conventional arrangement which uses a dedicated communication cable for transmission and reception. Furthermore, since the electromagnetic waves output by the main or secondary communication devices  11  and  121 - 124  are surface waves propagating on the surfaces of the components (the body  50 , chassis  51  or the like) of the automobile  100 , power consumption upon output of information can be reduced as compared with the conventional wireless communication by radio waves. Additionally, since the electromagnetic waves output as the surface waves from the main communication device  11  or each of the secondary communication devices  121 - 124  are difficult to be received by a main or secondary communication devices provided on another automobile, the reliability of communication can be improved.  
       FIGS. 4 and 5  illustrate a second embodiment of the invention. The second embodiment differs from the first embodiment mainly in the use of a power cable  21  as an information transmission path. Since the other arrangement in the second embodiment is the same as that in the first embodiment, identical or similar parts are labeled by the same reference symbols as those in the first embodiment and duplication of description will be eliminated.  
      Referring to  FIGS. 4 and 5 , the vehicle appliance control system  90  of the second embodiment comprises the main communication device  11 , a plurality of secondary communication devices  121 - 124  and couplers  170 - 174  provided with the communication devices  11  and  121 - 124 . The couplers  170 - 174  electrically couples the power cable  21  (serving as a power line in the invention) connected to the positive pole  24  of the automotive battery  22  and supplying electric power to the electrically-driven appliance (the headlight  30 , wiper motor  31  or the like) and each of the input/output circuits  16  of main and secondary communication devices  11  and  121 - 124  therebetween by making use of electromagnetic induction. More specifically, the couplers  170 - 174  comprise induction coils (not shown) as a major component and supplies, by electromagnetic induction, electric power supplied from the battery  22  through the power cable  21  into the main and secondary communication devices  11  and  121 - 124 , the headlight  30  and the wiper motor  31 , latter two of which serve as the electrically-driven appliances. Furthermore, the couplers  170 - 174  receive with the induction coils the electromagnetic waves from the input/output circuits  16  of the communication devices  11  and  121 - 124  and generate high-frequency current in the power cable  21  by the electromagnetic induction.  
      The modulator  14  of the main communication device  11  and the coupler  170  constitute a main high-frequency current generating device in the invention. The modulators  14  of the secondary communication devices  121 - 124  and the couplers  171 - 174  constitute secondary high-frequency current generating devices in the invention respectively. Furthermore, the demodulator  18  of the main communication device  11  and the coupler  170  constitute a main information extracting device in the invention. The demodulators  18  of the secondary communication devices  121 - 124  and the couplers  171 - 174  constitute secondary information extracting devices in the invention respectively.  
      The system  90  operates as follows. When the driver turns on the wiper switch in the automobile  100  provided with the system  90 , the signal processor  13  of the main communication device  11  obtains information about the operation of the wiper switch. The signal processor  1 - 3  then produces main communication data containing the ID of the second secondary communication device  122  to which the wiper motor  31  is connected and a starting command for the wiper motor  31  in the same manner as in the first embodiment. The signal processor  13  superimposes the main communication data on the electromagnetic waves and then delivers the electromagnetic waves to the input/output circuit  16 . The input/output circuit  16  carries out an amplifying process and a noise rejection process for the electromagnetic waves and then delivers the processed electromagnetic waves to the coupler  170 .  
      In the coupler  170 , the induction coil thereof having received the electromagnetic waves generates high-frequency current. The high-frequency current flows into the power cable  21  to be supplied to the couplers  171 - 174  of the secondary communication devices  121 - 124 . The couplers  171 - 174  deliver electromagnetic waves according to the high-frequency currents. The electromagnetic waves are supplied to the input/output circuits  16  of the secondary communication devices  121 - 124 . The main communication data are extracted from the electromagnetic waves in the same manner as in the first embodiment. The extracted main communication data are supplied into the signal processors  13  of the secondary communication devices  121 - 124 . Each signal processor  13  determines whether the ID contained in the main communication data corresponds with the ID stored on the ROM. When the main communication data for operating the wiper has been transmitted from the main communication device  11 , the signal processor  13  of the second secondary communication device  122  determines that the ID contained in the main communication data corresponds with the ID stored on the ROM. The wiper motor  31  is driven in response to the starting command contained in the main communication data, so that the wiper is operated.  
      On the other hand, the secondary communication devices  121 ,  123  and  124  other than the second secondary communication device  122  each determine that the ID contained in the main communication data does not correspond with the ID stored in the ROM of its own, ignoring the starting command contained in the main communication data. As a result, an erroneous activation of the headlight  30  and wiper motor  31  can completely be prevented.  
      When the wiper is operated in response to the starting command, the signal processor  13  of the second secondary communication device  122  measures a current flowing into the wiper motor  31 , determining whether the measured current value is within a range of normal current value, in the same manner as in the first embodiment. When determining that a short circuit has occurred, the second secondary communication device  122  operates the cutoff circuit  25  (see  FIG. 2 ) so that the cutoff switch  26  is turned off to stop power supply to the wiper motor  31 .  
      The signal processor  13  of the second secondary communication device  122  generates secondary communication data containing the aforesaid result of abnormality determination and the ID, superimposing the secondary communication data on the electromagnetic waves and delivering the electromagnetic waves to the input/output circuit  16  in the same manner as in the first embodiment. The input/output circuit  16  carries out an amplifying process and noise rejection process for the electromagnetic waves, delivering the processed electromagnetic waves to the coupler  172 . The induction coil of the coupler  172  having received the electromagnetic waves generates high-frequency current, which flows into the power cable  21 . The signal processor  13 , the modulator  14  and the coupler  172  of the secondary communication device  122  constitute an abnormality information transmitting unit in the invention.  
      The high-frequency current generated by the coupler  172  of the second secondary communication device  122  is supplied via the power cable  21  into the coupler  170  of the main communication device  11 . The coupler  170  outputs electromagnetic waves according to the supplied high-frequency current. The output electromagnetic waves are supplied to the input/output circuit  16  of the main communication device  11 . The demodulator  18  of the main communication device  11  then extracts secondary communication data from the supplied electromagnetic waves. The signal processor  13  obtains, from the result of abnormality detection contained in the secondary communication data, information about presence or absence of abnormality in the wiper motor  31  and information as to whether the wiper motor  31  has normally been operated. A process for the operation of the headlight  30  is the same as described above.  
      The main communication device  11  generates main communication data toward the third secondary communication device  123  at predetermined intervals for the purpose of confirming an on-off state of the door sensor  32 , for example. More specifically, in the same manner as in the wiper starting command, the main communication device  11  produces main communication data containing the ID of the third secondary communication device  123  and on-off state confirmation command of the door sensor  32 , superimposing the main communication data on electromagnetic waves. The main communication device  11  then delivers the main communication data to the coupler  173 . In the coupler  173 , the induction coil thereof having received the electromagnetic waves generates high-frequency current. The high-frequency current flows into the power cable  21  to be supplied to the couplers  171 - 174  of the secondary communication devices  121 - 124 . The couplers  171 - 174  generate the electromagnetic waves, thereafter being supplied to the input/output circuits  16  of the secondary communication devices  121 - 124 . However, only the signal processor  13  of the third secondary communication device  123  obtains the information about on-off state of the door sensor  32  in response to the on-off state confirmation command of the main communication data. The signal processor  13  of the third secondary communication device  123  produces secondary communication data containing the information about on-off state of the door sensor  32 . The signal processor  13  then superimposes the secondary communication data on electromagnetic waves and delivers the electromagnetic waves to the coupler  173 . The coupler  173  generates high-frequency current from the electromagnetic waves by electromagnetic induction, delivering the high-frequency current to the power cable  21 . The high-frequency current is supplied via the power cable  21  into the coupler  170  of the main communication device  11 . The coupler  170  converts the supplied high-frequency current to electromagnetic waves and extracts the secondary communication data from the electromagnetic waves. The signal processor  13  of the main communication device  11  obtains information about an open or closed state of the door based on the information about the on-off state of the door sensor  32  contained in the extracted secondary communication data. For example, when the door is open, a predetermined warning lamp is activated. When the on-off state of the seatbelt sensor  33  is confirmed, the same process as described above is carried out.  
      According to the second embodiment, the main communication device  11  superimposes communication data containing an operation command on the high-frequency current, delivering the high-frequency current to the power cable  21 . The secondary communication devices  121 - 124  obtains the communication data from the high-frequency current flowing through the power cable  21 . Since the power cable  21  provided in the automobile  100  is used as a transmission path for the communication data, the vehicle appliance control system can reduce the wiring space in the vehicle and the weight of the vehicle as compared with the conventional arrangement which uses a dedicated communication cable for transmission and reception of information. Additionally, since the communication data delivered from the main communication device  11  is not received by the other secondary communication devices  121 - 124  of the automobile  100 , the reliability of communication can be improved.  
      The invention should not be limited to the above-described embodiments, but the technical scope of the invention encompasses the following modified forms of the embodiments. The invention may further be practiced without departing from the gist thereof.  
      (1) Electric characteristics of the body  50  and chassis  51  of the automobile  100  are not constant but can change according to the circumstances. For example, an easiness of electricity to flow differs between the conditions where it is dry and wet. The easiness can also vary depending upon the temperature. Accordingly, for example, the intensity of electromagnetic waves delivered by the main and secondary communication devices  11  and  121 - 124  is detected by field intensity detectors  60  (serving as electric field intensity detectors) provided in the respective main and secondary communication devices  11  and  121 - 124 . Results of detection are fed back to the output changing units  61  of the main and secondary communication devices  11  and  121 - 124 . Frequencies of electromagnetic waves delivered by the modulators  14  may be changed based on the results of detection by the output changing units  61  respectively. Consequently, communication can be stabilized irrespective of conditions of the body  50  and the chassis  51 .  
      Each output changing unit  61  may be arranged to select an optimum frequency according to the electric field intensity from among a predetermined frequency range or a plurality of frequencies. The main and secondary communication devices  11  and  121 - 124  are arranged to be capable of receiving electromagnetic waves covering the entire frequency.  
      (2) In the first embodiment, the first and second secondary communication devices  121  and  122  measure current values of the electrically-driven appliances such as the headlight  30 , the wiper motor  31  and the like. The abnormality of the appliances is determined on the basis of the measured current values. The results of determination are delivered as secondary communication data. However, the measured current values may directly be delivered as the secondary communication data, instead. In this case, the electrically-driven appliances can be maintained and prevented from breakage when the main communication device  11  is arranged to analyze each measured current value and estimate occurrence of abnormality in the future such as break, short circuit or the like in the power line.  
      (3) In the first embodiment, the electromagnetic waves delivered from each of the communication devices  11  and  121 - 124  propagate along the surfaces of the body  50  and chassis  51 . However, the electromagnetic waves may propagate along the surface of the power cable  21  so that each of the communication devices  11  and  121 - 124  obtains communication data from the surface waves, instead.  
      (4) In the second embodiment, the communication devices  11  and  121 - 124  convert the high-frequency current flowing through the power cable  21  to the electromagnetic waves by the respective couplers  170 - 174 , thereby taking the electromagnetic waves in. However, when the high-frequency current flows, the communication devices  11  and  121 - 124  may receive electromagnetic waves generated near the surface of the power cable  21  and obtain communication data from the electromagnetic waves.  
      (5) A vehicle on which the vehicle appliance control system  10  or  90  is provided needs not be limited to the four-wheel automobile  100 . For example, the vehicle may include motorcycles, industrial vehicles such as forklifts or aircrafts.  
      The foregoing description and drawings are merely illustrative of the principles of the present invention and are not to be construed in a limiting sense. Various changes and modifications will become apparent to those of ordinary skill in the art. All such changes and modifications are seen to fall within the scope of the invention as defined by the appended claims.