Abstract:
A vehicle electronic key system includes a control device mounted on a vehicle and an electronic key for transmitting a response signal in response to reception of a request signal from the control device. The control device includes means for outputting a request signal to the electronic key in response to the ON operation of a main switch of the vehicle (request signal generation means, a transmission circuit, and a transmission antenna), means for detecting a response signal from the electronic key (a reception circuit and response signal matching means), and means for performing an alarm output (monitoring means, a third drive circuit, and an alarm lamp) when no response signal is detected in spite of the output of the request signal.

Description:
TECHNICAL FIELD 
     The present invention relates to an electronic key system for carrying out wireless communication between a transceiver (electronic key) carried by a user and a controller mounted on the vehicle, and, when an ID is compared and there is a request from the normal user, starting the engine etc., the invention being suitable for use as, for example, an electronic key system for a vehicle such as a motorcycle. 
     BACKGROUND ART 
     Japanese Laid-Open Patent Publication No. 2001-349110 and Japanese Laid-Open Patent Publication No. 2001-349117 exist as known examples of electronic key systems for a vehicle. The electronic key systems for a vehicle disclosed in these patent documents have activation means (switches) arranged in a door handle and trunk lid of the vehicle. If a user operates (activates) these activation means, communication with the electronic key commences, comparison is carried out between an ID transmitted from the electronic key and an ID registered in the controller, and a door lock is released at a stage when a result is obtained that there is ID matching. 
     Also, in the patent documents mentioned above, an activation means (a switch) is also provided in an ignition knob, and after a user has boarded a four-wheeled vehicle, communication with the electronic key is carried out again, by operation of the ignition knob, and an ID comparison is carried out in order to allow the engine to be started. Then, when results of the ID matching comparison have been obtained, the engine is started. 
     That is, with the electronic key system for a four-wheeled vehicle of the related art, the electronic key has functions for unlocking a door and allowing starting of the engine. 
     An electronic key system that has a transmission antenna provided for each door of a four-wheeled vehicle, which can release only the door that a portable device is closest to independently of the other doors of the vehicle, has also been proposed as related art (see, for example, Japanese Laid-Open Patent Publication No. 10-317754). 
     Also, as an electronic key system applied to a motorcycle, a system has been proposed that is intended to achieve reliable theft prevention, by generating a random number based on a time taken from a power supply of an immobilizer turning ON to starting of the engine (start time data), generating an authentication key and an authentication code by encoding information inherent to the key based on the random number, and carrying out authentication using the authentication key and the authentication code (see, for example, Japanese Laid-Open Patent Publication No. 2001-12123). 
     Since the electronic key is carried by the user, there is a danger of the key being dropped inside the vehicle when the key is not in use. In the case of a four-wheeled vehicle, however, there is only a very low possibility of the electronic key being lost after use, even if it is dropped inside the vehicle. 
     However, when the electronic key system for a four-wheeled vehicle is applied, as is, to a motorcycle, for example, if the electronic key is dropped by the user after starting the engine, there is not a problem if it is noticed that the key has been dropped. However, if the motorcycle moves without noticing that the key has been dropped, the key may become lost and it will not be possible to restart the engine. 
     In the related art, in a system that has been applied to a motorcycle, theft prevention is a main consideration, and for example, after boarding the vehicle once authentication of the normal user is complete, the security system is stopped. As a result, if the electronic key is dropped during travel, for instance, there is a danger that it will not be detected. 
     Naturally, even with a four-wheeled vehicle, if the electronic key is dropped close to the door, the probability of the electronic key becoming lost is high, and in this case also, it will not be possible to restart the engine. 
     The present invention has been conceived in view of these types of problems, and an object of the present invention is to provide an electronic key system for a vehicle that can output a warning if a user drops an electronic key while the vehicle is stopped or during travel, and that can reduce, to as low as possible, the probability of the electronic key being lost when dropped by the user. 
     DISCLOSURE OF THE INVENTION 
     An electronic key system of the present invention has a transmission antenna, a controller connected to the transmission antenna and a portable transceiver for transmitting ID data based on a request signal transmitted from the controller through the transmission antenna, wherein the controller receives the ID data through a receiving antenna, the electronic key system for a vehicle further comprising first means which outputs a first request signal to the portable transceiver based on activation of the controller, second means which outputs a second request signal to the portable transceiver following the first request signal, and in response to an ON operation of a start switch for the vehicle, third means for detecting the ID data from the portable transceiver based on the request signal, and fourth means for warning an operator when the ID data is not received from the portable transceiver even though the first request signal has been outputted from the first means and warning the operator when the ID data is not received from the portable transceiver, even though the second request signal has been outputted from the second means, wherein when the ID data is detected in response to the first request signal, the controller allows power to be supplied to electrical component drive circuits, and wherein when the ID data is detected in response to the second request signal, the controller allows power to be supplied to the vehicle such that vehicle travel is possible. 
     In this way, first of all a request signal is output from the first means as a result of ON operation of a start switch for the vehicle by the user. If the user is holding the portable transceiver, a request signal from the controller is received by the portable transceiver. The portable transceiver transmits an acknowledgement signal based on receipt of the request signal from the controller. The acknowledgement signal output from the portable transceiver is detected by the second means of the controller in the vehicle, and in that case, the engine is started without outputting a warning. 
     On the other hand, if the user is not holding the portable transceiver, or has dropped it and has not noticed, if the start switch is turned ON then since the second means does not detect the acknowledgement signal, regardless of whether or not a request signal is output from the first means, a warning is output through the third means and the user will notice that they are not holding the portable transceiver. 
     In this way, with the present invention, during the step of starting the vehicle, even if the user has dropped the portable transceiver, it is possible to make the user aware of this fact, and thus it is possible to reduce the probability of the portable transceiver becoming lost. 
     Also, another aspect of the electronic key system for a vehicle of the present invention, is that it includes a controller mounted on the vehicle, and a portable transceiver for transmitting an acknowledgement signal based on receipt of a request signal from the controller, the controller comprising first means for outputting a request signal to the portable transceiver every fixed period of time, second means for detecting an acknowledgement signal from the portable transceiver based on the request signal, and third means for outputting a warning when the acknowledgement signal is not detected in the second means. 
     In this way, first of all a request signal is output every fixed period of time through the first means. In the event that the user is holding the portable transceiver; an acknowledgement signal is output every fixed period of time in response to the request signal from the controller. The acknowledgement signal output from the portable transceiver is detected by the second means of the controller in the vehicle, and in this case, a warning is not output. 
     On the other hand, during travel of the vehicle, for example, in the event that the user has dropped the portable transceiver, since an acknowledgement signal is not detected by the second means, this time a warning signal is output by the third means, and thus the user can be made aware of the fact that the portable transceiver has been dropped. 
     In this way, with the present invention, in the event that the user drops the portable transceiver during travel of the vehicle, this fact can be notified to the user and it is possible to reduce the probability of the portable transceiver becoming lost. 
     Another aspect of the electronic key system for a vehicle of the present invention is that it has a controller mounted on the vehicle and a portable transceiver for transmitting an acknowledgement signal based on receipt of a request signal from the controller, the controller comprising first means for outputting a first request signal to the portable transceiver based on an ON operation of a start switch for the vehicle, second means for detecting an acknowledgement signal from the portable transceiver based on the first request signal or a second request signal, third means for outputting a warning when the acknowledgement signal is not detected in the second means in response to said first request signal, regardless of output of the second request signal from a fourth means, the fourth means for outputting the second request signal to the portable transceiver every fixed period of time during travel of the vehicle, and fifth means for outputting a warning when the acknowledgement signal is not detected in the second means in response to said second request signal from the fourth means. 
     In this way, even if the user drops the portable transceiver when starting the vehicle or during travel of the vehicle, it is possible to make the user aware of that fact, and it is possible to make the probability of losing the portable transceiver extremely small. 
     In the invention described above, the output period for the request signal from the first means can be from 10 to 100 seconds. If the output period of the request signal is short, it is possible to improve precision of checking whether or not the user is holding the portable transceiver. However, a battery is provided in the portable transceiver and the acknowledgement signal is output using electrical power from the battery. Therefore, as the period of the output signal becomes shorter, battery power is consumed and it will be necessary to replace the battery sooner. 
     By making the output period of the request signal from 10 to 100 seconds, it is possible to reduce battery consumption in the portable transceiver, and to reduce the frequency with which the battery is replaced. 
     Also, the third means can count the periods for which the acknowledgement signal is not detected, and output a warning at a point in time when the count value becomes a specified value or greater. Since there will be cases when arrival of the acknowledgement signal at the second means is delayed, or the acknowledgement signal itself may be missing due to effects of noise etc. during travel, it is possible to prevent frequent warning outputs by retaining a dead zone to a certain extent. 
     Also, in the invention described above, it is possible for the controller to further comprise sixth means for outputting a request signal to the portable transceiver based on activation of the controller, and seventh means for releasing a locked state of the vehicle when an acknowledgement signal from the portable transceiver is detected in the second means based on the request signal from the sixth means. 
     If activation of the controller is carried out before turning the start switch ON, it is possible to check whether or not the user is holding the portable transceiver in three situations, namely, when activating the controller, when turning the start switch ON, and during travel of the vehicle. 
     As a result, even if the electronic key system for a vehicle of the present invention is applied to a motorcycle, for example, in the event that the user drops the portable transceiver, that fact can be notified to the user and it is possible to make the probability of the portable transceiver becoming lost extremely low. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a structural drawing showing an electronic key system of a first embodiment; 
         FIG. 2  is a block drawing showing the structure of an electronic key; 
         FIG. 3  is a block diagram showing the structure of a controller; 
         FIG. 4A  is a side elevation showing one example of a setting position for a transmission antenna on the vehicle, and  FIG. 4B  is a plan view of the same; 
         FIGS. 5A through 5F  are timing charts showing normal processing operations of the electronic key system of the embodiment; 
         FIGS. 6A through 6F  are timing charts showing processing operations for the case where it is detected that there is no electronic key at the time of starting a vehicle, in the electronic key system of this embodiment; 
         FIGS. 7A through 7F  are timing charts showing processing operations for the case where it is detected that there is no electronic key at the time of starting a vehicle engine, in the electronic key system of this embodiment; and 
         FIGS. 8A through 8F  are timing charts showing processing operations for the case where it is detected that there is no electronic key during vehicle travel, in the electronic key system of this embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the electronic key system for a vehicle of the present invention applied to a system for a motorcycle (hereinafter, simply referred to as electronic key system embodiments) will now be described with reference to  FIGS. 1 through 8F . 
     As shown in  FIG. 1 , an electronic key system  10  of this embodiment comprises a portable transceiver  12  carried by a user, and a controller  14  mounted on the vehicle. The portable transceiver  12  can be either a key type having an IC chip built-in or a card type having an IC chip built in, however, in the case where a keyless system is adopted, the card type is mainly used. In this embodiment, a description will be given assuming that the portable transceiver  12  comprises a card type. Also, since the portable transceiver  12  is generally called an electronic key, in the following description, the portable transceiver will also be referred to as an electronic key. 
     The electronic key  12  is a card type, as described above, and as shown in  FIG. 2 , internally comprises a battery  20 , a power supply circuit  22 , a CPU  24 , a receiving circuit  26  and a transmission circuit  28 . 
     The power supply circuit  22  provides electrical power from the battery  20  to the receiving circuit  26 , transmission circuit  28  and CPU  24 . The receiving circuit  26  has a receiving antenna, not shown, and receives a request signal Sr or the like transmitted through the receiving antenna from the controller  14 , and extracts and demodulates it from a carrier wave. The demodulated signal is supplied to the CPU  24 . The carrier wave frequency of the request signal Sr is 100 kHz to 300 kHz. 
     The CPU  24  executes at least two computer programs (request signal comparison means  30  and acknowledgement signal generating means  32 ). The request signal comparison means  30  compares whether or not a signal supplied from the receiving circuit  26  is the request signal Sr, and if it is the request signal Sr, transfers control to the acknowledgement signal generating means  32 . The acknowledgement signal generating means  32  reads out ID data stored in a ROM, not shown, in response to a request from the request signal comparison means  30 , and adds an attribute representing acknowledgment to the ID data for output as transmission data Dt to the transmission circuit  28 . The transmission circuit  28  has a transmission antenna, not shown, and performs modulation of a carrier wave based on transmission data Dt supplied from the CPU  24 , for transmission as an acknowledgement signal Sa through the transmission antenna. The carrier frequency for the acknowledgement signal Sa is 200 MHz to 500 MHz. 
     On the other hand, as shown in  FIG. 3 , the controller  14  mounted in the vehicle is constituted by a system LSI, for example, and comprises a power supply circuit  40 , a CPU  42 , a receiving circuit  44 , a transmission circuit  46 , an input circuit  48 , an output circuit  50 , a first drive circuit  52  (actuator drive), a second drive circuit  54  (main relay drive), and a third drive circuit  56  (LED drive). Peripheral to this controller  14 , there are provided at least a battery  60 , a main-switch  62 , a handlebar actuator  64 , a main relay  66 , a warning lamp  68  (LED), an activation switch  70  and a transmission antenna  72 . 
     The main switch  62  has two fixed connection points  62   a  and  62   b , and one movable connection point  62   c , with one fixed connection point  62   a  being connected to the battery  60 , and the other fixed connection point  62   b  being connected to the input circuit  48  and the main relay  66 . 
     As well as the other fixed connection point  62   b  of the main switch  62 , the activation switch  70  is also connected to the input circuit  48 . A starter switch  74  is connected to this input circuit  48 , and operation is caused by an ON operation of the starter switch  74 . In the following description, description will mainly focus on the case where processing operations are carried out based on operation of the main switch  62 . 
     ON/OFF states of the activation switch  70  and ON/OFF states of the main switch  62  are supplied to the CPU  42  through the input circuit  48 . 
     The power supply circuit  40  of the controller  14  supplies electrical power from the battery  60  to the CPU  42 , the receiving circuit  44 , and the transmission circuit  46 , etc. 
     The receiving circuit  44  has a receiving antenna, not shown, and an acknowledgement signal Sa is received from the electronic key  12  through the receiving antenna, and extracted and demodulated from a carrier wave. The demodulated signal is supplied to the CPU  42 . 
     The CPU  42  executes at least four programs (request signal generating means  80 , acknowledgement signal comparison means  82 , monitoring means  84  and peripheral instruction means  86 ). 
     The request signal generating means  80  reads out request data Dr (data constituting the source of the request signal Sr) from a ROM, not shown, in response to the ON operation of the activation switch  70  and ON operation of the main switch  62 , for output to the transmission circuit  46 . Also, after starting the engine, the request signal generating means  80  reads out request data Dr from the ROM every fixed time and outputs the data. The fixed time is set to between 10 and 100 seconds, taking into consideration consumption of the battery by the electronic key  12 . 
     The transmission circuit  46  modulates a carrier wave based on the request data Dr supplied from the CPU  42 , and transmits, via the transmission antenna  72 , a request signal Sr. 
     As shown in  FIG. 4A  and  FIG. 4B , the transmittable range of the request signal Sr is a spherical range (the range shown by circle A in  FIG. 4A  and  FIG. 4B ) of a diameter of 1-1.5 m with the transmission antenna  72  fitted to the vehicle  100  at the center, and is a narrow range compared to the transmittable range of the acknowledgement signal Sa (a range of a few m radius with the electronic key  12  as the center). 
     Therefore, as shown in  FIG. 4A  and  FIG. 4B , if it is assumed that the vehicle  100  is, for example, a scooter provided with a space that can hold a helmet, not shown, below a seat  102 , it is preferable to provide the transmission antenna  72  close to the center of the vehicle  100  so that when the user boards the vehicle, opens the seat  102 , or is traveling, etc., communication is reliably established with the electronic key  12  carried by the user. 
     Here, when considering a linking line  108  of the center  104   a  of the front wheel  104  and the center  106   a  of the rear wheel  106 , the vicinity of the center of the vehicle  100  is a range from a point P 1  that is ¼ of the line to a point P 2  that is ¾ of the line, with the center  104   a  of the front wheel  104  as a reference, for example. With this embodiment, the transmission antenna  72  is arranged close to the front of the seat  102 . 
     The acknowledgement signal comparison means  82  compares whether or not a signal supplied from the receiving circuit  44  is an acknowledgement signal Sa, and if it is the acknowledgement signal Sa, it compares whether or not ID data contained in the acknowledgement signal Sa matches ID data stored in a memory, not shown. 
     The monitoring means  84  monitors presence or absence of arrival of an acknowledgement signal Sa (whether or not an ID match is detected by the acknowledgement signal comparison means  82 ) based on output of the request signal Sr. The request signal generating means  80  awaits input of the acknowledgement signal Sa from the point in time where request data Dr is output, and if an acknowledgement signal Sa does not arrive within a specified time (if an ID match is not detected by the acknowledgement signal comparison means  82 ), the count value is incremented by 1. At the point in time that the count value becomes a specified value or greater, a warning signal Se is output to the third drive circuit  56 . 
     In particular, if an acknowledgement signal Sa does not arrive within a specified time from the point in time where request data Dr is output based on the ON operation of the activation switch  70  and the main switch  62 , at that stage the warning signal Se is output to the third drive circuit  56 . 
     Also, if an acknowledgement signal Sa does arrive within a specified time from the point in time where request data Dr is output based on the ON operation of the activation switch  70 , the monitoring means  84  activates the peripheral instruction means  86 . The peripheral instruction means  86  outputs a lock release signal to the first drive circuit  52  in response to a request (lock release) from the monitoring means  84 , outputs an enabling signal to the output circuit  50 , and also outputs an ON signal to the second drive circuit  54 . 
     The first drive circuit  52  drives an actuator  64  for the handlebar in response to input of the lock release signal from the CPU  42 , and releases a locked state of the handlebar  110  (refer to  FIG. 4A  and  FIG. 4B ). 
     The output circuit  50  outputs an ignition/injection enabling signal to an ECU  111  (electronic control unit) in response to input of the enabling signal from the CPU  42 . The ECU  111  determines fuel injection amount and injection timing for the engine based on information from various sensors, in response to input of the ignition/injection enabling signal. 
     The second drive circuit  54  is put into an ON state based on input of an ON signal from the CPU  42 , and after that starts the engine and enters a travel possible state in a step where the main relay  66  is turned on by an ON operation of the main switch  62 . 
     The third drive circuit  56  drives the warning lamp  68  in response to input of the warning signal Se from the CPU  42 , and the warning lamp  68  emits light. It is possible to use an LED, for example, as the warning lamp  68 . 
     If the main switch  62  is turned OFF, the main relay  66  becomes OFF, and the engine is also stopped at the same time. If a locking operation is then carried out, for example, putting the handlebar  110  in a locked state, the comparison operation for the acknowledgement signal Sa in the controller  14  is stopped, the ignition/injection enabling signal from the output circuit  50  is stopped, and the second drive circuit  54  is turned OFF. 
     Next, four representative processing operations of the electronic key system  10  of the first embodiment will be described with reference to the timing charts of  FIG. 5A  to  FIG. 8F . A request signal Sr is a signal having a pulse string based on request data Dr, and the acknowledgement signal Sa is a signal having a pulse string based on data contained in ID data, but in  FIG. 5A  to  FIG. 8F , they have each been shown as single pulse signals to simplify description. 
     First of all, normally, if the activation switch  70  is turned ON at time t 1  in  FIG. 5A  with the user holding the electronic key  12 , then as shown in  FIG. 5C  the request signal Sr is transmitted from the controller  14  (refer to time t 2 ) and communication with the electronic key  12  commences. 
     When the user is in possession of the electronic key  12 , the request signal Sr is received by means of the receiving circuit  26  of the electronic key  12  (refer to  FIG. 2 ). As shown in  FIG. 5D , the electronic key  12  transmits an acknowledgement signal Sa in response to receipt of the request signal Sr (refer to time t 3 ). The acknowledgement signal Sa is supplied through the receiving circuit  44  of the controller  14  to the CPU  42  (refer to  FIG. 3 ), and ID data included in the acknowledgement signal Sa is compared. When it is judged that the ID data matches, the locked state of the handlebar  110  is released (unlocked) by means of the controller  14  and the first drive circuit  52 , as shown in  FIG. 5E  (refer to time t 4 ). At this time, the second drive circuit  54  is turned ON, and the ignition/injection enabling signal is output from the output circuit  50  of the controller  14  to the ECU  111 . 
     Continuing on, at time t 5  in  FIG. 5B , if the main switch  62  is turned ON with the user holding the electronic key  12 , then as shown in  FIG. 5C  the request signal Sr is transmitted from the controller  14  (refer to time t 6 ) and communication with the electronic key  12  is carried out. 
     When the user is carrying the electronic key  12 , then in the same way as described above, as shown in  FIG. 5D , the electronic key  12  transmits the acknowledgement signal Sa in response to receipt of the request signal Sr (refer to time t 7 ). The acknowledgement signal Sa is supplied to the CPU  42  via the receiving circuit  44  of the controller  14 , ID data contained in the acknowledgement signal Sa is compared, and if it is judged that ID data matches, then control transfers to the next step, namely a step where the request signal Sr is output every fixed time +τ. 
     From this stage, the vehicle  100  is traveling, and during this travel the request signal Sr is output from the controller  14  every fixed time τ. That is, communication with the electronic key  12  is carried out every fixed time τ, and an acknowledgement signal Sa is output from the electronic key  12  at substantially every fixed time τ. 
     Next, the processing operation when it has been detected that there is no electronic key  12  at the time the vehicle  100  is started will be described with reference to  FIGS. 6A through 6F . 
     First of all, at time t 11  in  FIG. 6A , if the activation switch  70  is turned ON while the user is not holding the electronic key  12 , as shown in  FIG. 6C , the request signal Sr is transmitted from the controller  14  (refer to time t 12 ), but in the controller  14  there is no receipt of an acknowledgement signal Sa corresponding to the output request signal Sr (refer to time t 13  in  FIG. 6D ). As a result, the warning signal Se is output from the monitoring means  84  to the third drive circuit  56 , and in this way, as shown in  FIG. 6F , the warning lamp  68  is lit. Naturally, in this case, processing such as lock release for the handlebar  110  etc. is not carried out (refer to  FIG. 6E ). 
     The user notices that the electronic key  12  is not being carried because of the lighting of the warning lamp  68 , and starting the engine while not holding the electronic key  12  can be avoided. 
     Next, a description will be given, with reference to  FIGS. 7A through 7F , of processing operations when the electronic key  12  has not been detected at the time of starting the vehicle  100 . 
     First of all, at time t 21  in  FIG. 7A , if the activation switch  70  is turned ON while the user is holding the electronic key  12 , then as shown in  FIG. 7C  the request signal Sr is transmitted from the controller  14  (refer to time t 22 ), and communication with the electronic key  12  commences. 
     When the user is in possession of the electronic key  12 , the request signal Sr is received by means of the receiving circuit  26  of the electronic key  12 , and as shown in  FIG. 7D , the electronic key  12  transmits an acknowledgement signal Sa (refer to time t 23 ). ID data included in the acknowledgement signal Sa is compared in the controller  14 , and when it is judged that the ID data matches, the locked state of the handlebar  110  is released, as shown in  FIG. 7E  (refer to time t 24 ). At this time, the second drive circuit  54  is turned ON, and the ignition/injection enabling signal is output from the output circuit  50  of the controller  14  to the ECU  111 . 
     Continuing on, at time t 25  in  FIG. 7B , if the main switch  62  is turned on without the user noticing that the electronic key  12  has been dropped, then as shown in  FIG. 7C  the request signal Sr is transmitted from the controller  14  (refer to time t 26 ), and there is no receipt of an acknowledgement signal Sa corresponding to the output request signal Sr in the controller  14  (refer to time t 27  in  FIG. 7D ). As a result, the warning signal Se is output from the monitoring means  84  to the third drive circuit  56 , and in this way the warning lamp  68  is lit, as shown in  FIG. 7F . 
     The user is made aware of the fact that the electronic key  12  has been dropped by the lighting of the warning lamp  68 , and it is possible to avoid the electronic key  12  becoming lost. 
     Next, a description will be given, with reference to  FIGS. 8A through 8F , of processing operations for the case where it is detected that there is no electronic key  12  during travel of the vehicle  100 . 
     First of all, processing from switching on the activation switch  70  up to switching on the main switch  62  (processing from time t 31  to time t 37 ) is the same as the processing from time t 1  to t 7  in  FIGS. 5A through 5F , and so description of this processing will be omitted. 
     If the engine is started in response to an ON operation of the main switch  62 , control passes to a step for outputting the request signal Sr every fixed time τ, as described above. From this stage, the user is traveling on the vehicle  100 , and during such travel, the request signal Sr is output from the controller  14  every fixed time τ. 
     While traveling on the vehicle  100 , if the electronic key  12  is dropped, for example, receipt of the acknowledgement signal Sa is not carried out by the controller  14  (refer to time t 38  in  FIG. 8D ). When the acknowledgement signal Sa is not received by the monitoring means  84  within a specified time from output of the request data Dr, the count value is incremented by 1. Then, in a process of sequentially outputting the request signal Sr, at time t 39  when the count value becomes a specified value or higher, a warning signal Se is output from the monitoring means  84  to the third drive circuit  56 , and in this way the warning lamp  68  is lit, as shown in  FIG. 8F . 
     The user notices that the electronic key  12  has been dropped as a result of the lighting of the warning lamp  68 , and it is possible to avoid the electronic key  12  becoming lost. 
     In this way, in the electronic key system  10  of this embodiment, in the event that the user turns the activation switch  70  on while not holding the electronic key  12 , since it will be detected that the acknowledgement signal Sa is not received in the receiving circuit  44 , regardless of output of the request signal Sr from the controller  14 , a warning is output via the monitoring means  84 , and the user will notice that he/she is not in possession of the electronic key  12 . 
     If the user drops the electronic key  12  at the time of starting the engine, for example, since the acknowledgement signal Sa is not detected in the receiving circuit  44 , regardless of output of the request signal Sr from the controller  14  in response to the main switch  62  being turned on, a warning is output via the monitoring means  84  and the user will notice that they have dropped the electronic key  12 . 
     If the user drops the electronic key  12  during travel of the vehicle  100 , since the acknowledgement signal Sa is not detected in the receiving circuit  44 , regardless of output of the request signal Sr from the controller  14  at every fixed time τ, a warning is output via the monitoring means  84  and the user will notice that they have dropped the electronic key  12 . 
     In this way, it is possible to check whether or not the user is holding the electronic key  12  during at least three stages, namely when starting the controller  14 , when turning ON the main switch  62 , and during travel of the vehicle  100 . 
     As a result, even if the electronic key system  10  of this embodiment is applied to a motorcycle, for example, in the event that the user drops the electronic key  12 , that fact can be made known to the user and it is possible to make the probability of losing the electronic key  12  extremely low. 
     In particular, with the embodiment described above, the output period τ for the request signal Sr from the controller  14  is set at 10 to 100 seconds. If the output period τ of the request signal Sr is made short, it is possible to improve precision of checking whether or not the user is in possession of the electronic key  12 . However, a battery  20  is provided in the electronic key  12  and the acknowledgement signal Sa is output using electrical power from the battery  20 . Therefore, as the output period τ of the request signal Sr becomes shorter, battery consumption increases and the battery  20  must be replaced more often. 
     By setting the output period τ of the request signal Sr at 10 to 100 seconds, it is possible to reduce consumption of the battery in the electronic key  12 , and it is possible to reduce the frequency with which the battery  20  needs to be replaced. 
     Also, with this embodiment, the periods in which the acknowledgement signal Sa is not detected during travel of the vehicle  100 , for example, are counted, and a warning is output when that count value is a specified value or higher. Since there will be times when arrival of the acknowledgement signal Sa is delayed, or the acknowledgement signal Sa itself is missing due to the effects of noise etc. during travel, it is possible to prevent frequent warning output by causing a particular dead-zone to be held. 
     With the above-described example, lighting of a dedicated warning lamp  68  has been given as an example of warning output, but it is also possible to output a warning sound using a dedicated buzzer. 
     Alternatively, it is possible to randomly light an indicator lamp inside an already existing meter, output a specified sound pattern by means of the horn, or cause a direction indicator to light up with a different lighting pattern from normal. 
     However, since the horn and the direction indicator are safety components, it is important not to operate them for the purpose of warning when starting the engine, and to use them as conventional safety components. 
     The electronic key system for a vehicle of the present invention is not limited to the above-described embodiments, and obviously various structures can be introduced without departing from the spirit and scope of the invention.