Alarm System For Monitoring A Plurality Of Serially Connected Objects Utilizing Signal Translation Techniques

Abstract

An arrangement for causing an alarm in a guarding device, for example a burglary alarm device, includes a central guarding unit which sends out control signals through a loop to a number of objects to be guarded. The objects are connected in series with the guarding unit and upon unwarranted manipulation with the loop, for example upon attempts to by-pass an object, an alarm is obtained. Each object to be guarded comprises a signal translating means which translates the signal obtained through the loop in a manner characteristic of the respective object. The central guarding unit compares the control signal sent out from the guarding unit, with the signal sent back from the objects to be guarded. At least one of said two signals is translated before the comparison in order to obtain the same translation which has been obtained in the object to be guarded. During a comparison when an inequality between the two signals is sensed the alarm device is operated.

Description

This invention refers to an arrangement for causing an alarm in the guarding unit of a guarding device, for example a burglary alarm device, in which a central guarding unit sends out a control signal through a loop to a number of objects to be guarded which are connected in series with the guarding unit, upon unwarranted manipulation with the loop, for example upon attempts to by-pass one or more objects to be guarded.

In such devices attempts to defeat the alarm system are generally made by by-passing or shunting one or more objects to be guarded.

In order to make it possible to discover such attempts it has been proposed among other things that each object to be guarded should be provided with an alternator, the output signal of which is superimposed on a direct current supplied from the central guarding unit through the loop. Supervision of both the direct and the alternating current is in this case carried out in the guarding unit. ( Compare, for example, U.S. Pat. No. 3,010,100.)

The disadvantage of such an arrangement is that an unauthorized person can find out very easily the frequency and amplitude of the alternating current, can by-pass the object to be guarded and also feed an identical alternating current to the loop from a separate generator without this being discovered in the guarding unit.

An object of the invention is to provide an arrangement in which a number of objects to be guarded can be supervised by means of one single loop and in which the alarm system cannot be defected by by-passing one or more objects to be guarded and where an object to be guarded which is exposed to unwarranted manipulation, for example by-passing, is immediately identified in the guarding unit.

The invention will be described more fully with reference to the accompanying drawing in which the sole FIGURE shows an example of a guarding device to which the principle of the invention has been applied.

The guarding device consists of a central guarding unit C, to which a number of objects to be guarded B1, B2 . . . Bn are connected in series through a current loop a .

Each object to be guarded includes in the known manner an alarm detector and an alarm contact which are diagrammatically shown as a relay function R1, R2 . . . Rn. The alarm detectors as well as the alarm contacts are assumed to be inaccessible to undue actuation.

In order to make it possible to discover unwarranted manipulation with the loop a , for example attempts to by-pass an object to be guarded, a control signal is sent out from the guarding unit C through the loop. According to the principle of the invention this control signal is translated in each of the objects to be guarded, in a predetermined manner characteristic of the respective object to be guarded. The signal obtained is then compared, taking in consideration the translations carried out by the objects to be guarded, with the control signal sent out from the guarding unit C. Thus upon a correct functioning of the device the signals compared will be equal. If, however, one of the objects to be guarded is by-passed, the expected translation of the control signal in this object will be lacking upon the comparison and alarm is initiated in the guarding unit.

The control signal sent out through the loop a to the objects to be guarded B1, B2 . . . Bn is supposed to be a pulse train which is generated by for example a random sequence generator G. Each object to be guarded includes, besides said alarm detector and alarm contact, a signal translating means for translating the incoming signal before it is forwarded. In this example the signal translating means consists of an exclusive-or-circuit E11, E12 . . . E1n to which are supplied on one hand the pulse train incoming to the respective object to be guarded through the loop a , and on the other hand a pulse train individual for each object to be guarded. This pulse train is generated by a generator G11, G12 . . . G1n according to a previously known pattern. Thus by means of such an arrangement the control signal is translated in each object to be guarded B1,B2 . . . Bn in a manner such that in practice copying can be made impossible for an unauthorized person.

The translated signal is supplied through the loop a from the object Bn to be guarded to the central guarding unit C. According to the principle of the invention this signal is to be compared with the control signal sent out from the guarding unit considering the known translations carried out in the objects to be guarded. This can be solved in many different ways. In the present example a comparison circuit K is provided, to one of the inputs of which the pulse train incoming through the loop is supplied. To the second input of the comparison circuit K a pulse train is supplied from the generator G which train is subjected to the same known translations as the pulse trains transmitted through the loop a . The translations occur in a signal code translating means SK which according to the example comprises a number of exclusive-or-circuits E21, E22 . . . E2n and pulse generators G21, G22 . . . G2n corresponding to the number of objects to be guarded. These pulse generators are identical with the pulse generators G11, G12 . . . G1n in the objects to be guarded. Thus the pulse train obtained from the signal code translating means SK is identical with the pulse train incoming through the loop provided that no object to be guarded has been by-passed. Upon unequality between these comparison circuits, i.e., when an object to be guarded has been by-passed, an alarm signal is generated which may actuate an alarm device L in the guarding unit.

In order to make it possible to carry out in the guarding unit the identifying of for example an object to be guarded which has been by-passed, the guarding unit C is provided with an identifying means IM. This is assumed, according to the example, to consist of parallel comparison branches the number of which corresponds to the number of objects to be guarded. Each of these comparison branches contains an exclusive-or-circuit E31, E32 . . . E3n to one of the inputs of which the pulse train incoming through the loop a is supplied. To the second input of the respective exclusive-or-circuit a pulse generator G31, G32 . . . G3n is connected. These pulse generators are identical with the pulse generator G11, G12.. G1n of the respective object to be guarded. The pulse train obtained on the respective exclusive-or-circuit E31, E32 . . . E3n is conducted to one of the inputs of an individual comparison circuit K1, K2 . . . Kn. To the other input of the respective comparison circuit the pulse train incoming from the signal code translating means SK is supplied.

Normally the pulse trains compared in the respective comparison circuit K1,K2 . . . Kn are different while the pulse trains incoming to the comparison circuit K are equal. If, however, one of the objects to be guarded is by-passed , the pulse train incoming through the loop a will not have the translation corresponding to the object to be guarded which has been by-passed. This is discovered upon the comparison in the comparison circuit K upon which the alarm device L is operated. In the identifying means IM a translation of the pulse train incoming through the loop a will take place in each of the comparison branches in accordance with the translations in the respective object to be guarded. If thus the translation in one of the objects to be guarded does not take place due to the fact that the object to be guarded has been by-passed, a corresponding translation will occur in that one of the comparison branches which corresponds to the object to be guarded which has been by-passed. Identical pulse trains will be fed to the corresponding comparison circuit K1,K2 . . . Kn, an equality signal being generated actuating an alarm device L1,L2 . . . Ln.

Alarm in the guarding unit C is obtained not only when an object to be guarded has been by-passed but also when an alarm is detected in one of the objects to be guarded. In this case the corresponding alarm detector R1,R2 . . . Rn is activated, its alarm contact interrupting the connection from the respective pulse generator G11,G12 . . . G1n. No translation of the signal through the loop a occurs in this object to be guarded. In the same manner as described above an alarm will be obtained in this case in the guarding unit C from the alarm device L as well as from that one of the alarm devices L1,L2 . . . Ln which corresponds to the object to be guarded giving the alarm. To be able to identify both an object to be guarded which is giving alarm and an object to be guarded which has been by-passed with the same equipment, it is accordingly necessary that the alarm detector R1,R2 . . . Rn and its alarm contact should be connected in such a way that the same translations of the signal through the loop a fail to occur in both cases.

It should be emphasized that the signal code translating means SK can very well be connected in series with the loop a between the object to be guarded Bn and the comparison circuit K instead of between the pulse generator G and the comparison circuits K, K1, K2 . . . Kn as is shown in the FIGURE. In this case the pulse generator G is connected directly to the upper inputs of the comparison circuits K, K1,K2 . . . Kn.

Claims

We claim:

1. An alarm system comprising a central guarding unit, a plurality of objects to be guarded and a loop connecting said objects to be guarded in series with said central guarding unit, said central guarding unit including means for transmitting a control signal through said loop to said different objects to be guarded, each of said objects to be guarded comprising a signal translating means including a signal generator, a function generator provided with a first input for receiving said control signal transmitted through said loop and a second input for receiving the signal generated by said signal generator, said function generator generating on an output a signal which is a definite function of the input signals, each of said objects furthermore including an alarm detector having means for disconnecting said signal generator from the respective input of said function generator upon manipulation of the object to be protected to cause an interruption of the signal translation in the respective signal translating means, and said central guarding unit further comprising a comparison circuit for comparing the transmitted control signal with the control signal returning through the loop from the objects to be guarded after having subjected at least one of said signals to a further translation, and at least one signal code translating means for carrying out said further translation in order to obtain equality between the two compared signals if none of the objects is influenced and an alarm device which is activated upon inequality between said two compared signals.

2. The system according to claim 1 in which the signal code translating means translates the same control signal which is supplied to said loop and has an output connected to one input of said comparison circuit in order to supply translated signals to said input. selected

3. The system according to claim 1 in which the signal code translating means translates the control signal received through the loop from the objects to be guarded and has an output connected to one input of said comparison circuit in order to supply translated signals to said input.

4. The system according to claim 1 in which the central guarding unit further comprises identifying means for identifying which of said objects has caused alarm, said identifying means comprising parallel branches whose number equals the number of objects to be guarded, each of said branches comprising a signal translating means each identical with a corresponding signal translating means of one of said objects and a comparison circuit in series, said signal translating means each having an input for receiving the signal incoming from said loop and an output connected to a first input of the respective comparison circuit in order to supply translated signals to said input, each of said comparison circuits having a second input for receiving said control signal so upon equality between the two compared signals in any one of the comparison circuits an indication is given as to which of said objects has caused alarm.

5. An alarm system comprising: signal generating means having an output for generating coded combinations of signals; a plurality of objects to be guarded, each of said objects comprising a signal generator for generating coded combinations of signals; a function generator having first and second inputs and an output for transmitting from said output coded combinations of signals related to coded combinations of signals received at said inputs, said first input being the signal input of the object and said output being the signal output of said object; an alarm detector means for connecting said signal generator to the second input of said function generator as long as the object is undisturbed; means for serially connecting the signal input of one object to the signal output of another object to form an open loop of serially connected objects with the signal input of the object at one end of said loop being the loop signal input and the signal output of the object at the other end of said loop being the loop signal output; a comparator means having first and second inputs for comparing coded combinations of signals received at said inputs; first connecting means for connecting the output of said signal generating means to said loop signal input; second connecting means for connecting the output of said signal generating means to the first input of said comparator means; third connecting means for connecting said loop signal output to said second input of said comparator means, one of said connecting means being a translator means having an input and an output for transmitting from said output coded combination signal translations of coded combinations of signals received at said input and alarm generating means connected to said comparator means for giving an alarm when the comparator means indicates a predetermined relationship between the coded combinations of signals being compared.

6. The alarm system of claim 5 wherein said function generator is an exclusive- or circuit.