The invention relates to vehicle security systems which

provide for the remote operation of a vehicle security

device.

The remote operation of vehicle door locks and engine

immobilization systems has gained popularity over the last

few years and is becoming available on an increasing number

of vehicles . A typical remote control arrangement for

locking and unlocking a vehicle door takes the form of a hand

held transmitter which operates a receiving device in the

vehicle. More recently, car thieves have been using

electronic devices (known as code grabbing devices) capable

of receiving a signal transmitted from a hand held

transmitter and recording the signal. The recorded signal

can subsequently be used to unlock the vehicle after the user

leaves an area where the vehicle is parked.

A basic lock/unlock system for a vehicle utilizes a single

fixed code 10 as shown in Fig.l which is transmitted both to

lock and unlock the vehicle. It has been found relatively

easy to receive and record such a code for subsequent

unlocking of the vehicle.

in a slightly more secure system shown in Fig.2, the transmitted signal includes a fixed code 11 and portion in the form of a lock/unlock bit or bits 12. However, once the

code 10 has been recorded by a code grabbing device, a thief

can change the lock command bit or bits 12 to an unlock

command and unlock the vehicle.

Accordingly systems have been designed in which the codes

stored in the transmitter and receiver change continually,

the transmitter and receiver being arranged to change at the

same time and in the same way to remain in synchronism. However it is a problem with such systems that the

transmitter and receiver can gradually drift out of

synchronism to such an extent that the transmitted signal is

not accepted by the receiver.

An object of the present invention is to provide a system in

which the gradual loss of synchronism between the stored

codes in the transmitter and receiver does not affect the

operation of the system.

According to the present invention there is provided a

vehicle security system including 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, the receiver including code generating

means operable to provide a code associated with or related

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 one of the

transmitter and the receiver provides a tolerance which

increases with time lapse to accommodate a gradual de-

synchronization between the timing of code changes over a

period of time.

Preferably, operation of the transmitter within range of the

receiver will re-synchronize the timing of the code changes

in the transmitter and receiver. It is proposed to use a

clock such as a crystal clock in the transmitter and

preferably also in r.he receiver which would normally remain

in synchronism for several days following the last operation

of the transmitter. The clocks will provide timing accuracy

between the transmitter and receiver. However, the receiver

may be arranged to provide a tolerance which is effectively

zero at synchronism and which increases with time lapse

between transmitter operations. In that way, the

intelligence in the receiver enables it to open up a window

of allowable timing error in the synchronization of code

changes if the receiver has not received a signal from the

transmitter for a number of days or weeks.

If synchronism has been entirely lost, for example, due to

battery failure in the transmitter, means may be provided for

producing a re-synchronizing signal for bringing nhe coding

in the transmitter and receiver back into synchronism. It is

highly desirable that such a re-synchronizing signal is not

transmitted accidentally when, for example, leaving the

vehicle as the re-synchronizing signal could be of use to a

thief with a code grabbing device. To reduce the risk of

accidentally producing a re-synchronizing signal, the re-

synchronizing means may be operated by a battery compartment:

cover on the transmitter.

The code generator for the transmitter may be a roiling code

generator or encryption device and the same preferably

applies to the receiver.

If desired the transmitted signal may include a portion which

provides code changes within the interval at determined

times. Such a portion of the transmitted signal may provide

changes in code at regular intervals.

The present invention also provides a vehicle securrity

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, the receiver including code generating means operable

to provide a code associated with or related 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

transmitter and receiver are arranσed such that one of them

can communicate to the other the length of said intervals to

be used.

The communication of the length of the intervals may, for

example, take place on the first operation of the transmitter

in an initial pairing procedure.

Preferably the length of said intervals varies in time.

The present invention further provides a vehicle security

system including a system for remote operation according to

the invention.

A system for the remote operation of a lock in accordance

with the invention will now be described by way of example ^'

with reference to the remaining accompanying drawings in

which: -

Fig.3 shows a block diagram arrangement of a transmitter and

receiver in accordance with the invention

Fig.4 shows a transmitter/receiver arrangement for operating

the door of a vehicle in accordance with the present

invention,

Fig.5 shows a modified form of transmitted signal,

Fig.6 shows a time scale illustrating part of the modified

transmitted signal and

Fig.7. shows a signal used to communicate from the transmitter

to the receiver information subsequently used for generating

security codes.

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 for

operating vehicle door locks 37 if those signals match each

other.

At intervals e.g. every 10 seconds, the code generated by the

rolling code generator 26 in the transmitter 20 is changed.

Similarly, at the same intervals the code generated by the

rolling code generator 34 of the receiver is changed. 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

generated at that time and will either match or be otherwise

related to the code generated in the receiver at that time to

permit the control logic 35 to produce the output signals 36.

In the simplest form of the invention the intervals are

regular, and are of a length, e.g. 10 seconds, which is

permanently programmed into the transmitter and receiver.

However, in order to increase the security of the system it

can be made possible for the length of the intervals to be

used to be initially programmed into the transmitter, and to

be communicated to the receiver when they are first used

together as a pair. This allows the intervals to be of any

fixed length. Alternatively the length of each interval can

vary from the previous one in accordance with an algorithm,

the algorithm and a 'seed' first value being held in the

transmitter, and communicated to the receiver when they are

first used together as a pair. In a further alternative the

length of the intervals can cycle through a sequence of

different lengths, the sequence being programmed into the

transmitter and communicated to the receiver in the same way.

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. A signal for achieving this is

shown in Figure 7. The first portion 60 of the signal is a

code which would be recognized by the receiver as an

instruction to pick up the following information and store it

for use in generating codes. The next portion 62 of the

signal includes a random sequence of interval lengths which

are to be cycled through by the transmitter and receiver.

The advantage of pairing up the transmitter and receiver in

this way is that each pair can have different values for the

seed intervals or different algorithms or sequences of

ι n

intervals without the need to coordinate the manufacture and

supply of the transmitters and receivers.

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, the clocks 25, 33 in the transmitter

and receiver will be re-synchronized.

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, circuitry is included in

the control logic 35 of the receiver to enable the receiver

to open up a window of allowable timing error. This would

allow the door locks to be operated even if the codes in the

transmitter and receiver were up to a number of steps out of

synchronism, that number increasing with time from the last

transmitter operation. Therefore, the allowable error in

synchronization will increase with time from the last

lock/unlock signal of the transmitter. With such an

increasing window of allowable timing error, complete loss of

synchronization is unlikely to occur very often. However, 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. By using the battery

compartment cover 42 in that way, there is little risk that a

user of the transmitter will accidentally transmit the re-

synchronization signal.

As mentioned above, the rolling codes will change in response

to the operation of the clocks 25, 33. An alteration in

rolling code requires the processing of an algorithm but a

change in code at, say, every 10 seconds can be achieved

within the system. However it is envisaged that an astute

thief could possibly grab the transmitted code and make use

of it in the 10 second period. To reduce the risk of that,

it is proposed that each signal 50 (Fig.5) includes not only

a rolling code 52 (and, if desired, fixed code bits such as

transmitter identification 14, a lock/unlock bit or bits 15

and even a vehicle identification 16) but also a code portion

53 which changes at intervals within the 10 second period P

(Fig.6) . As shown in Fig.6 the portion 50 changes eight

times in the ten second period P on a regular basis (or, if

desired a pseudo random basis e.g., 1, 3, 7, 2, 5 etc. within

interval P) giving a thief with a grabbing device virtually

no time in which to make use of the grabbed signal. This

will apply both to the transmitter 20 and the receiver 30.

Such an arrangement will help when re-synchronizing the

transmitter 20 and the receiver 30 as the point of re-

synchronization will be more accurately identified within the

main code change cycle or period P. Also with such an

arrangement re-synchronization could be achieved by, for

example, pressing one of the transmitter buttons 23, 24 a

number of times in quick succession which would be recognized

by the receiver 30 as a re-synchronizing signal structure.

A further advantage of the present invention is that pressing

of the transmitter buttons will have no effect on the content

of the rolling code transmitted as the code is changed by

operation of the clock 25 and not by the buttons 23, 24.

Therefore accidental operation of the lock/unlock buttons

will not take the transmitter out of synchronism with the

receiver.

Instead of using a rolling code in the transmitter and

receiver, the code to be transmitted can be in the form of an

encryption algorithm which effectively scrambles the content

of the transmitted signal which is then unscrambled in the

receiver. Information on public encryption codes is

available in text books for example Cryptography; A New

Dimension in Computer Data Security by Meyer, C and S. Matys

1982 New York, John Wiley &. Sons, Inc.

The term "security device" used herein embraces a lock,

alarm, vehicle immobilizes, latch release or similar security

device.