GB2304950A

GB2304950A - Vehicle security system

Info

Publication number: GB2304950A
Application number: GB9517336A
Authority: GB
Inventors: Jeremy John Greenwood
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1995-08-24
Filing date: 1995-08-24
Publication date: 1997-03-26
Anticipated expiration: 2015-08-24
Also published as: GB9517336D0; DE19631331A1; GB2304950B

Description

2304950 A VEHICLE SECURITY SYSTRM The invention relates to vehicle

security systems that provide for the remote operation of a vehicle security device.

It is known from GB 2 276 023 to provide a vehicle security system in which a transmitter and a receiver each have clocked rolling codes stored therein which change according to a common sequence. Upon receipt of a code by the receiver, that code is compared with that stored in the receiver. If the received code is within a predetermined number of codes of that stored in the receiver, the code is deemed to be valid and the receiver causes some function of the security system to be operated. The number of codes which can separate a received code from the stored code for it to be deemed valid increases with time to accommodate a gradual de-synchronization between the timing of code changes over a period of time.

It can, in some circumstances, be a disadvantage of this system that the received code needs to be checked against a large number of codes in the receiver.

Accordingly the present invention provides a vehicle security system comprising a security device, a transmitter for transmitting command signals, the transmitter including code generating means operable to provide a code forming at least part of said transmitted signal, the code being automatically changeable at intervals, and a receiver which receives the transmitted signal and causes operation of the security device, the receiver including code generating means operable to provide a code corresponding to the transmitted 2 code to facilitate operation of the security device, the code in the receiver being automatically changeable at intervals substantially in synchronism with the timing of code changes in the transmitter, wherein the length of said intervals increases with time lapse to accommodate a gradual de- synchronization between the timing of code changes over a period of time.

Preferably the code stored in the receiver changes to move through a sequence of codes, and the security device is only operated if the code sent by the transmitter corresponds to a code within a predetermined number of codes in the sequence of the code stored in the receiver. For example the security device may be only operated if the code sent by the transmitter corresponds to the code stored in the receiver or the next or previous code in the sequence.

The increase in length of the intervals may be stepped.

Preferably the interval between code changes can be reset to an initial value upon communication of a reset signal to at least one of the transmitter and the receiver.

Preferably the increasing of the intervals does not start for a predetermined period from any resetting.

Preferably the reset signal is produced by movement of a key in an ignition lock of the vehicle.

A preferred embodiment of the present invention will now be described by way of example with reference to the remaining accompanying drawings in which:- 3 Fig. 1 shows a block diagram arrangement of a transmitter and receiver in accordance with the invention; Fig. 2 shows a transmitter/receiver arrangement for operating the door of a vehicle in accordance withthe present invention, A hand held transmitter 20 comprises a control logic 22 operated either by a lock button 23 or an unlock button 24. The transmitter includes a crystal clock 25, a rolling code generator 26 and the control logic and rolling code generator provide inputs for a transmitter circuit 27.

A receiver 30 comprises a receiver circuit 32, a crystal clock 33 a rolling code generator 34 and a control logic 35. The receiver circuit 32, the clock 33, and the rolling code generator 34 provide inputs for the control logic 35, which compares the signals received from the receiver circuit 32 and the code generator 34 and provides outputs 36 to a security control unit 37 which controls operation of vehicle door locks, engine immobilization, alarm siren and volumetric and other intrusion detectors.

The code generated by the rolling code generators 26, 34 in the transmitter 20 and receiver 30 are stored in memory in the respective control logic circuits 22, 35. The code generators 26, 34 generate new codes, at set intervals, each of which replaces the code currently stored. The clocks 25, 33, starting in synchronism, provide. timing accuracy common to the transmitter 20 and receiver 30 for the transmitter and receiver code change operation. The signal transmitted by the transmitter 20 will incorporate the code stored in the 4 transmitter control logic 22 at that time and will correspond to the code stored in the receiver at that time, either by being the same as it or by being otherwise related to it.

The codes produced by the code generators 26, 34 progress through a sequence which is common to the transmitter and the receiver. When the control logic 35 in the receiver compares the received signal with that stored in its memory to determine whether it is valid, it accepts the received signal as valid it its code corresponds to the current code stored in its memory, or if it corresponds to the previous or next code in the sequence. This allows for a slight de-synchronization of the clocks 25, 33.

Because of the accuracy of the crystal clocks 25,33 it is envisaged that they will remain in synchronism for several days. Nevertheless, each time the lock button 23 or unlock button 24 is pressed and a valid signal is received by the receiver from the transmitter the clocks 25, 33 in the transmitter and receiver can be re-synchronized.

Alternatively, if the system is of the type where the transmitter can be caused to send a command signal upon use of the ignition key in the ignition lock to passively re mobilize the vehicle, the resynchronization can take place when that happens.

If a battery 40 in the transmitter 20 fails and is removed from the transmitter, the re-closing of a battery cover 42 after replacing the battery will cause a re synchronization signal to be generated by the control logic 22 and transmitted to the receiver 30 which will cause complete re-synchronization of the transmitter and receiver.

If no signal is received from the transmitter 20 for a long period of time there is a risk that the clocks 25, 33 will move out of synchronism. Therefore the code generators 26, 34 are arranged to increase the length of the intervals between code changes over time. This increases the amount of discrepancy which can exist between the clocks without the code stored in the transmitter being outside the range of valid codes accepted by the receiver.

In this example, the way in which the intervals increase is as follows: after a resetting of the codes the intervals at which the codes are changed is constant, set at five seconds for an initial period of three hours. Thereafter the intervals increase by five seconds every hour until they reach a maximum length of an hour. They then remain constant until the next resetting. Alternatively the intervals could increase continuously, or inmore or less steps.

The resetting of the intervals can take place at any or all of the same times when re- synchronization of the clocks occurs as described above.

In order to increase the security of the system it can be made possible for the length of the intervals to be used, or the way in which they will vary with time, to be initially programmed into the transmitter, and to be communicated to the receiver when they are first used together as a pair. The communication from transmitter to receiver can be brought about by including in the control logic in the transmitter instructions to send the required information on the first operation of the transmitter.

6 The term "security device,, used herein embraces, for example, a lock, alarm, vehicle immobilizer, latch release or intrusion detectors of any sort. It will be appreciated that there are also several other types of security device to 5 which this term applies.

Claims

17 A vehicle security system comprising a security device, a transmitter for transmitting command signals, the transmitter including code generator means operable to provide a code forming at least part of said transmitted signal, the code being automatically changeable at intervals, and a receiver which receives the transmitted signal and causes operation of the security device in response thereto, the receiver including code generating means operable to provide a code corresponding to the transmitted code to facilitate operation of the security device, the code in the receiver being automatically changeable at intervals substantially in synchronism with the timing of code changes in the transmitter, wherein the length of said intervals increases with time lapse to accommodate a gradual de-synchronization between the timing of code changes over a period of time.

2. A vehicle security system according to claim 1 wherein the code stored in the receiver changes to move through a sequence of codes, and the security device is only operated if the code sent by the transmitter corresponds to a code within a predetermined number of codes in the sequence of the code stored in the receiver.

3. A vehicle security system according to claim 2 wherein the security device is only operated if the code sent by the transmitter corresponds to the code stored in the receiver or the next or previous code in the sequence.

8 A vehicle security system according to. any foregoing claim wherein the increase in length of the intervals is stepped.

5. A vehicle security system according to any foregoing claim wherein the interval between code changes can be reset to an initial value upon communication of a reset signal to at least one of the transmitter and the receiver.

A vehicle security system according to claim 5 wherein the increase in length of the intervals only begins after a predetermined initial period from a resetting of the interval.

7. A vehicle security system according to claim 5 or claim 6 wherein the reset signal is produced by movement of a key in an ignition lock of the vehicle.

8. A vehicle security system substantially as described herein with reference to the accompanying drawings.