Abstract:
Access to a motor vehicle is controlled by periodically transmitting an interrogation signal from a control circuit on the vehicle. Upon receiving the interrogation signal, a hand-held remote control transmits a reply signal to the control circuit. The control circuit measures the strength of the reply signal and the activates a first function, such as unlocking a door of the vehicle, when the strength exceeds a first threshold level. Thereafter, when the signal strength exceeds a second threshold level, the control circuit activates a second function, for example enabling the engine to be started. When the signal strength drops below a third threshold activates a third function, such as locking the door.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to system for remotely controlling access to motor vehicles, and more particularly to wireless remote control systems that can be used to unlock vehicle doors, start the engine and operate other functions on the vehicle. 
     Automobiles have used keys which mechanically operate locks to limit access to the vehicle and starting the engine to only authorized persons. More recently remote keyless entry (RKE) systems have been provided that use a small radio frequency (RF) transmitter, often having the shape of a key ring fob, to access the vehicle. This RF transmitter has a number of push button switches allowing the driver to control different functions of the vehicle, such as lock and unlock the doors, arm a security system or open the trunk. These transmitters also have been proposed to control starting the vehicle engine. When a given push button switch is operated, the transmitter sends an RF signal which carries a digital numerical code and a designation of the function to be performed. A receiver in the vehicle receives the transmitter signal, verifies that the numerical code designates an authorized transmitter for that particular vehicle and if so, signals the vehicle control circuits to perform the prescribed function. 
     Although the digital numerical code makes it extremely difficult for unauthorized persons to gain access to the motor vehicle, concern has been expressed that someone with a radio receiver and a digital signal analyzer could eavesdrop on the radio transmissions and obtain the security numbers. Particular brands of vehicles use a specific single radio frequency. Thus a thief could “stake out” a valuable vehicle to await the return of the driver and learn the transmission necessary to operate the vehicle. Those security numbers then could be utilized to steal that vehicle at a later point in time. Thus, as the technology available to thieves advances, so too must the signal processing employed by the RKE system. Therefore, there exists a need for a more secure radio frequency system that allows keyless entry. 
     Bidirectional radio frequency communication has been used for some time in cordless telephones. The term “cordless telephone” as used in the telecommunication industry, means a telephone comprising a base station and a hand-held transceiver unit. The base station is connected by wires to a terrestrial telephone line serving the owner&#39;s premises. A hand-held transceiver carried by the user communicates by radio frequency signals with the single base station that is up to approximately 300 meters away. 
     The Digital Enhanced Cordless Telecommunications (DECT) protocol was developed in the mid-1980&#39;s as a pan-European standard for cordless telephones and has been adapted for use outside the European Union. The DECT standard protocol has been used for simultaneous bidirectional communication between a base station and a hand-held transceiver of cordless telephones. This standard utilizes ten frequencies for communication. The exchange of signals over each frequency is divided into frames  10  each having twenty-four slots as shown in FIG.  1 . The twelve slots in the first half  14  of each frame are used for communication from a hand-held transceiver to the associated base station, while the twelve slots in the second frame half  16  are used for communication from the base station and the hand-held transceiver. 
     When a user desires to use activates the cordless telephone to make an outgoing call, the hand-held transceiver searches for a frequency that has a matching slots in each frame half which are not being used by another cordless telephone system. This is accomplished by the hand-held transceiver listening for digital signals being sent in each slot of the frame at each of the assigned frequencies. When a vacant pair of slots, such as  18  and  19 , is found, the hand-held transceiver sends a message initiation signal on the selected frequency during slot  18  in the first half of a message frame. 
     While the hand-held transceiver is performing these functions, the base station is scanning the ten frequencies and listening during each of the twelve slots in the first half  14  of the message frames at each frequency. When the base station hears a message initiation signal that is addressed to it, i.e. containing the proper identification data, the base station sends a response to the transceiver in the associated slot  19  in the second half of a frame at the same frequency and bidirectional communication is established. A reverse procedure occurs when the base station receives an incoming call via the terrestrial telephone line. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide a remote keyless vehicle entry system which does not require manual activation by the user. 
     Another object is to provide secure mechanism for enabling a driver to start a motor vehicle without a conventional key. 
     A further aspect of the present invention is to provide a mechanism for automatically locking the doors upon the driver exiting the motor vehicle. 
     Still another aspect of the present system allows the vehicle doors and trunk to be unlocked and locked. 
     These and other objectives are satisfied by a passive keyless access system having a remote control adapted to be carried by the driver. Access to the vehicle is controlled by the remote control transmitting a signal from outside the vehicle to a control circuit on the vehicle. The control circuit measures the strength of the signal and activates a first function of the vehicle when the strength exceeds a first predefined threshold. Thereafter when the strength exceeds a second predefined threshold, the control circuit activates a second vehicle function. For example, the first function can be unlocking a vehicle door and the second function can be enabling the engine to be started. In that example, the first predefined threshold is greater than the second predefined threshold. 
     In the preferred embodiment of the present invention, transmission of the signals utilizes the Digital Enhanced Cordless Telecommunications (DECT) protocol. As an optional feature the control circuit can activate a third function of the vehicle when the signal strength drops below a third predefined threshold, the control circuit activates a second function of the vehicle, such as locking the doors as the driver has left the vehicle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a message frame of the Digital Enhanced Cordless Telecommunications (DECT) wireless telephone protocol; 
     FIG. 2 is a pictorial diagram of a remote keyless entry (RKE) system for a motor vehicle; 
     FIG. 3 is a block schematic diagram of the remote keyless entry system; 
     FIG. 4 is a flowchart of the passive entry and starting operations of a control circuit in the motor vehicle; and 
     FIG. 5 is a flowchart of operation of a hand-held remote control. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to FIG. 2, a keyless motor vehicle control system  20  comprises a driver&#39;s remote control  21 , which preferably has the form of a key ring fob carried by a driver, and a control circuit  22  located in the motor vehicle  23 . As will be described, the remote control  21  exchanges a radio frequency signals with the control circuit  22 , which responds by activating designated functions of the motor vehicle  23 . 
     As shown in detail in FIG. 3, the control circuit  22  in the motor vehicle includes a microcomputer  24  with an internal microprocessor, memory in which the control program and data are stored, and input/output circuits. A standard clock circuit  26  supplies timing pulses to the microcomputer  24 . The service technician is able to place the microcomputer into different functional modes by operating a plurality of manual input switches  27  and  28 . Alternatively the configuration of the control circuit  22  can be defined by downloading data via the radio frequency link. 
     The control circuit  22  operates several functions on the motor vehicle, such as locking and unlocking the doors, unlatching the trunk lid, and starting the engine, for example. For that functionality, the microcomputer  24  is interfaced to the corresponding actuating devices on the motor vehicle  23 . In some cases, the various functions are controlled by an another computer in the motor vehicle to which microcomputer  24  sends operating commands via a parallel communication bus  36 . In other motor vehicles, microcomputer  24  has individual output lines  30  connected directly to the control devices for the respective functions being operated. Specifically, separate wires may be coupled to actuators which lock and unlock the doors, unlatch the trunk lid and start the engine. 
     A serial output port  32  and a serial input port  34  of the microcomputer  24  are connected to a first radio frequency transceiver  35  which utilizes the Digital Enhanced Cordless Telecommunications (DECT) standard. In a general sense, the first radio frequency transceiver  35  modulates a standard RF frequency carrier with the serial digital data received from output port  32  and transmits that modulated radio frequency signal via an antenna  37 . The first transceiver  35  also receives and demodulates radio frequency signals received by the antenna  37  to recover serial digital data carried by that signal. The recovered data is sent to the microcomputer input port  34 . A meter  39  is connected to the first transceiver  35  to measure the strength of the received radio frequency signal and provide an indication of that signal strength to microcomputer  24  via another input port  33 . Alternatively the circuitry in the remote control  21  also may have a signal strength meter for the radio frequency signal that it receives. 
     The first transceiver  35  of the control circuit  22  is designed to communicate with a second radio frequency transceiver  40  and antenna  42  both located within the remote control  21 . As will be described, both transceivers  40  and  35  are designed to utilize the DECT protocol and are similar to devices found in cordless telephones. The second transceiver  40  has a receiver section which demodulates the received radio frequency signal to recover digital data carried by that signal and the recovered data is sent in a serial format to an input register  44 . The input register  44  converts the serial data stream from the second transceiver  40  into a parallel format which is read by a controller  46 . The controller  46  may be a hardwired device that sequentially performs the remote control procedure to be described or a programmable device which executes a software program to implement that procedure. The controller  46  of the remote control  21  is connected to an electrically erasable programmable read only memory (EEPROM)  48  which stores data to be transmitted to the motor vehicle control circuit  22  when the remote control  21  is interrogated. 
     A clock circuit  52  also provides timing signals for the remote control  21 . A plurality of user operable switches  54  are connected to different input lines to the controller  46  in order for the driver to select the specific functions to be performed on the motor vehicle. For example, a separate switch can be provided for the functions of unlocking and locking the doors, unlatching the trunk lid, and starting the engine. 
     The remote control  21  also includes an encryptor  50  connected to the controller  46  to encrypt a remote control security number for transmission to the control circuit  22 . The encryptor  50  utilizes a secret-key cryptography algorithm to encrypt data for sending to the control circuit. For example, the algorithm specifies a sequence of a plurality of logical operations which are performed on a known seed number and a challenge number received from the control circuit to produce a resultant number for transmission by the remote control. Several suitable cryptography algorithms are described by Mehrdad Foroozesh in an article entitled “Protecting Your Data With Cryptography,”  UNIX Review , November 1996, volume 14, number 12, page 55(6), which description is incorporated herein by reference. Such encryption techniques and algorithms are commonly used to encrypt computer data being transmitted over common carriers. It should be understood that other encryption techniques may be used. 
     Digital output data is sent by the controller  46  in parallel form to a parallel-in/serial-out output register  56 . The serial data from the output register  56  is applied to the input of a transmitter section in the second transceiver  40  which modulates a radio frequency signal which that data. The resultant RF signal is sent via the antenna  42  to the control circuit  22  in motor vehicle. The signal transmitted by the second transceiver  40  has a range of about 300 meters. The components of the remote control preferably are powered by a battery (not shown). 
     When the driver desires the vehicle perform a given function the corresponding switch  54  can be operated on the remote control  21 . This results in the controller  46  sending a signal to the control circuit  22  which designates the selected function. This communication process is similar to that used in previous remote controls for motor vehicles, except that it employs the DECT protocol. The designation of the selected function is stored in a table within the EEPROM  48  and the functions associated with each switch  54  are programmable via the RF link provided by second transceiver  40 . 
     In addition, the present keyless motor vehicle control system  20  has a passive mode of operation in which the driver does not have to operate a switch  54  on the remote control  21 . Instead, a predefined sequence of functions is performed by the driver merely approaching the motor vehicle  23 . 
     With reference to FIGS. 3 and 4, the motor vehicle control circuit  22  in the passive mode periodically, every six seconds for example, emits an interrogation signal at steps  60  and  62 . The interrogation signal contains a unique identification code for that particular motor vehicle and is transmitted sequentially on all ten of the DECT frequencies. This process begins by scanning each of the ten DECT frequencies. If the control circuit  22  does not hear a message frame on a given frequency, then it forms a new message frame and selects an arbitrary pair of time slots to use. If a particular frequency already is carrying DECT messages, the control circuit  22  listens during the message frames for an available pair of frame slots, one that does not already contain message data. If none is found, the control circuit  22  selects the next DECT frequency. When an available pair of time slots, such as the third time slots  18  and  19  in each half of the message frame shown in FIG. 1, is found, the control circuit  22  transmits the interrogation signal in the time slot  19  during the second half  16  of the message frame. 
     The microcomputer  24  determines at step  64  whether a reply signal has been received and if so, whether the reply signal contains an authorized remote control identification number. If either is not the case, the program execution returns to step  60  to delay before sending the interrogation signal again at step  62 . 
     While this is occurring, the remote control  21 , at step  90  of FIG. 5, listens on a single DECT frequency for a interrogation signal which contains the proper motor vehicle identification code. Alternatively, the interrogation signal could be sent on a single one of the DECT frequencies by the control circuit  22  and the remote control  21  could scan all of the DECT frequencies listening for that signal. However, with this latter approach the remote control  21  consumes more power which drains its battery faster. 
     When the driver is within transmission range, for example approximately 300 meters of the motor vehicle  23 , the remote control  21  receives interrogation signal and at step  92  determines whether that signal contains the identification code for the motor vehicle that the remote control is to operate. If that is the case, the remote control  21  responds with a predefined reply signal at step  94 . The reply signal, is sent at the same frequency as the interrogation signal and during a slot in the second half of the message frame that corresponds to the slot of the first frame half that contained the interrogation signal. That reply contains a unique identification number assigned to the remote control and stored in its EEPROM  48 . The reply data is sent via output register  56  to the second transceiver  40  from which it is transmitted to the control circuit  22  in the adjacent motor vehicle. As noted previously, any of several well known data encryption algorithms may be employed to exchange data between the control circuit  22  and the remote control  21  for greater security. 
     Upon receiving a valid reply signal at step  64  in FIG. 4, the microcomputer  24  in the control circuit  22  advances to steps  66  and  68  at which another interrogation signal begins to be transmitted at shorter intervals, every 60 milliseconds for example. The remote control  21  continues to send a reply signal upon each receipt of this interrogation signal. 
     At this juncture, the microcomputer  24  determines at step  70  whether a reply signal has been received and if so, whether it contains an authorized remote control identification number. If either is not the case, the program execution returns to step  60  at the beginning of the passive mode. However, when a valid reply signal is received by the control circuit  22  at step  70 , the procedure advances to step  72 . The microcomputer  24  now examines the input from meter  39  to determine whether the strength of the received reply signal exceeds a predefined first threshold level X, as occurs when the remote control  21  is within a meter of the motor vehicle, for example. If the signal strength is below this first threshold, i.e. the driver is too far from the motor vehicle, the program returns via step  84  to step  66 . Alternatively the measurement of the signal strength could be performed at the remote control  21  with an indication of that signal strength being transmitted to the control circuit  22 . In yet another variation, both the remote control  21  and the control circuit  22  could measure the strength of their respectively received signals with the control circuit  22  comparing the measurements to determine the proximity of the remote control. 
     When the signal strength exceeds this first threshold X, microcomputer  24  at step  74  checks whether a flag, designated START FLAG, has been set previously. When the START FLAG has not been set, the doors of the motor vehicle are unlocked at step  76  and the START FLAG then is set at step  78 , before the process returns to step  66 . If the START FLAG is found set at step  74 , the procedure advances to step  80  where the microcomputer  24  examines the input from meter  39  to determine whether the strength of the received reply signal exceeds a predefined second threshold level Y. The strength of the reply signal will exceed the second threshold level Y when the remote control  21  is inside the motor vehicle. In that case, the microcomputer  24  enables the starting circuit of the motor vehicle  23  so that the driver may start the engine by activating a manual switch on the dashboard. Thus the driver with the proper remote control  21  for the motor vehicle may start the engine without using a key. The procedure then returns to step  60 . If the signal strength is found to be below threshold Y at step  80 , i.e. the driver is not inside the motor vehicle, the program returns to step  66  without enabling the starting circuit. 
     After the starting circuit has been enabled, the keyless control system  20  continues to exchange interrogation and reply signals between the control circuit  22  and the remote control  21 . While the driver is within the motor vehicle, the strength of the reply signal will exceed the first threshold level X at step  72  causing the program execution pass through steps  74 ,  80  and  82  and return to step  66 . Eventually the driver will manually turn-off the engine and exit the motor vehicle  23 . As the driver walks away from the vehicle, the signal strength drops below the first threshold level X. When this occurs, the procedure branches to step  84  where the START FLAG is found to be set. As a result, the START FLAG will be reset at step  86  before the doors are locked at step  88 . Then the program returns to step  60  to await the driver again approaching the motor vehicle for reentry.