Anti-tamper device and relative method for detecting tampering with a component

Abstract: An anti-tamper device (10) for components (9) mounted in casings (1) comprising: generator means (11) to generate a detection signal that is propagated inside the casing (1) in given conditions, detector means (12) sensitive to said detection signal propagated in the casing (1), warning/alarm means (20) connected to said detector means, said warning/alarm means being for signalling the variations of said detecting signal detected by the detecting means, the generator means (11) and the detector means (12) being situated inside the body (8) of the component in order to be protected from disturbances.

Description: The present invention refers to an anti-tamper device and related method for the detection of tampering with a component. The invention is intended in particular for devices for the protection of sensors in surveillance systems, such as anti-burglar or anti-intrusion systems. In this field of application the tendency to use as sensors components suitable to be mounted like normal electrical modules in the flush-mounted casings of the electrical systems of buildings has become widespread. Precisely because of their typical position, for example at the same height as the switches of an electrical system, these sensors are very exposed to tampering, such as attempts to neutralize them. For this reason, some standards require that special anti-tamper functions be installed. A known solution provides for the use of magnetically operated devices consisting of a fixed part, integral with the recessed casing, housing a permanent magnet. The module, which is the component of the security system, is provided with a reed relay kept in the closed position by the magnetic field generated by the magnet mounted on the fixed part integral with the casing. Movement of the component from the position in which it is mounted causes the relay to move away from the magnet, with a consequent opening of the contact of the relay and production of an alarm signal. This solution, though having the advantage of low power consumption, has various drawbacks. Placement of the fixed part of the anti-tamper device in the flush mounted casing is rather awkward, amongst other things, on account of the further reduction in the free space (already not normally very large) available inside the casing for passage of the wires. Moreover, it is difficult to achieve a precise positioning of the anti-tamper device, as required to be sure of obtaining closure of the relay contact. In many cases, in fact, incorrect positioning of the anti-tamper device or even failure to fit it in place cannot be detected immediately. Furthermore each component must be supplied together with the relative fixed part of the anti-tamper device, which leads to additional costs. Additionally, it can happen that the fixed part of the device is not compatible with the type of flush-mounted casing in which it has to operate. Also known to the art are anti-tamper devices of the optoelectronic type, comprising a transmitter to generate a signal that is propagated inside the casing and a detector to detect said signal and any changes in the signal due to tampering. The transmitter and the detector are mounted on the outer rear wall of the module, consequently the detector detects the signal emitted by the transmitter and reflected by the rear wall of the casing. Being disposed inside the casing containing the electrical modules, the transmitter and the detector can be damaged, for example by the wires passing inside the casing. Such devices in any case have the drawback of detecting the signal inside the casing; consequently their operation is closely influenced by the configuration and the structure of the casing. In fact the casings and in particular their rear walls can be made of various materials, for example plastics having different surface or chromatic characteristics. Or else the casing may have been accidentally smeared, before or after flush mounting on the wall, with various materials such as plaster, paint and the like. The depth of embedding of the casing in the wall too can vary from one instance to another. Since the component is generally supported by the front plate of the casing, applied flush with the wall, it can be situated at different distances with respect to the rear wall of the casing. Furthermore, inside the casing there are usually the wires for connection of the component in question, any wires of other components present in the casing, and possibly wires passing through the casing but not belonging to any of the components. A tamper-proof device applicable to components mounted in a flush-mounted box is disclosed, for example, by document EP-A-0 748 003. An object of the present invention is to overcome the above-mentioned drawbacks by providing an anti-tamper device that is efficient and safe irrespective of the type, configuration and structure of the casing inside which the component that must not be tampered with is mounted. Another object of the present invention is to provide such an anti-tamper device that is economical and simple to make. These objects are achieved in accordance with the invention with the characteristics listed in appended independent claim 1. Another object of the present invention is to provide a method for detection of tampering with components that is efficient and immune from disturbances due to the configuration of the inside the casing in which the components are housed. This object is achieved in accordance with the invention with the characteristics listed in appended independent claim 15. Advantageous embodiments of the invention are apparent from the dependent claims. The anti-tamper device according to the invention is suitable to be applied to components inserted like modules in a casing. Said anti-tamper device provides a transmitter or generator to transmit a signal inside the casing, a receiver or detector to detect the signal that is propagated in the casing and alarm means which, on the basis of the signal detected, signal any tampering with the component. The peculiarity of the anti-tamper device according to the invention is due to the fact that the signal which is propagated inside the casing is detected inside the body of the component. Consequently said signal is better protected from disturbances due to the shape of the casing and the presence of wires inside the casing. The transmitter and the receiver are advantageously positioned inside the body of the component. In this manner the transmitter and the detector are protected against any breakage or damage caused by wires disposed inside the casing that can interfere therewith or from knocking against the walls of the casing during assembly. Furthermore, in this manner the transmitter and the detector have greater immunity against electrostatic discharge generated by the movement of the wires inside the casing. Another advantage of the anti-tamper device according to the invention is represented by the fact that the alarm signal that tampering has occurred is obtained by means of a control unit in which software is provided that implements a comparison of the signal detected with a threshold signal that is continuously updated. Said software-implemented comparison system is more flexible and reliable than the comparison systems of known anti-tamper devices that are developed by means of hardware comparators. Further characteristics of the invention will be made clearer by the detailed description that follows, referring to a purely exemplary and therefore non limiting embodiment thereof, illustrated in the appended drawings, in which: Figure 1 is an axonometric exploded view, diagrammatically illustrating a possible application of the anti-tamper device according to the invention; Figure 2 is a cross sectional view illustrating a possible arrangement for mounting of the anti-tamper device according to the invention; Figure 3 is a block diagram showing the circuit structure of the anti-tamper device according to the invention; Figure 4 is a time diagram illustrating the wave shapes of some signals generated by the anti-tamper device according to the invention. The anti-tamper device according to the invention is described with the aid of the figures. Figure 1 shows as a whole a casing 1 of the type currently used in domestic and industrial electrical systems. The casing 1 is intended to be flush-mounted in a wall with its open front side 2 facing toward the outside of the wall. The casing 1 is closed by means of a cover plate 3 comprising a plurality of apertures 4 forming respective seats for mounting of the modules 5. The modules 5 consist of a shell 8 that encloses a component 9, such as, for example, a switch, an electrical socket, a sensor etc. By way of example, in the present embodiment a sensor is considered as a component. The outer surface of the plate 3 is generally closed by a cover element 6 (the so-called "switch plate") provided with respective apertures 7 in alignment with the seats 4 of the plate 3 provided for mounting the modules 5 that are to be housed in the casing 1. As shown in Figure 2, the anti-tamper device designated as a whole with reference numeral 10 is mounted inside the shell 8 of the module 5 that contains the component 9. The anti-tamper device 10 consists of an optoelectronic pair comprising an emitter 11 and a receiver 12. The emitter 11 emits an optical radiation, typically in the infrared range, that is propagated inside the casing 1. The emitter 11 can be a LED, for example, that emits in the infrared or ultraviolet range. The receiver 12, which detects at least a fraction of the radiation generated by the emitter 11, can be a photodetector, such as a photodiode, with spectral characteristics complementary to those of the emitter 11. The radiation generated by the emitter 11 diffuses inside the casing 1 and is reflected and diffused according to a somewhat complex propagation line distribution determined by a combination of factors such as the type and shape of the casing, the possible presence of numerous sources of diffusion/reflection such as wires, etc. The intensity of the radiation detected by the receiver 12 thus depends on the sum of these factors. According to the invention, one or more windows or apertures 13 are situated at least in the rear wall 14 of the shell 8 of the module. The windows 13 can be left open or covered with a light-transparent material. The windows 13 influence the propagation of the radiation which, from the inside of the casing 1, passes into the shell 8 of the module 5. Thanks to the action of these windows 13, a stationary wave which is detected by the receiver 12 is generated inside the shell 8. Said wave detected by the receiver 12 is used to detect possible tampering with the module 5 or the casing 1. When it is installed, the anti-tamper device 10 is set according to the conditions of propagation of the radiation that goes from the emitter 11 to the receiver 12, in the specific situation of installation. Any change in said conditions of propagation of the radiation, which can be induced by even a minimal movement of the component 9, such as tampering or total removal, for example, causes a change in the signal detected by the detector 12. Consequently the detector sends a control signal to a control unit 20 (Figure 3) which emits an alarm signal indicating the attempt to tamper with the component 9. The control unit 20 can be a microcontroller such as a CPU already provided in the component 9 if it is a sensor, or a CPU used for the anti-tamper device 10. With reference to Figure 3, at an interval T with a given periodicity (for example 130 milliseconds) the control unit 20 sends an activation signal to a pilot circuit 21. The pilot circuit 21 can be obtained for example by means of a digital-to-analog converter of the R-2R type which performs a digital-to-analog conversion using four resistors R1, R2, R3 and R4 with different values. The pilot circuit 21 sends the emitter (LED) 11 a constant current piloting signal V1 so that the emitter 11 turns on for a time of about 0.5 milliseconds. The anti-tamper device 10 provides a data bus to receive and send messages. The data bus is a logical entity that allows management and control of each individual device, including the anti-tamper device 10. The data bus may be radiofrequency or cabled. The transmission or reception time of a message by the data bus, depending on the length of the message, is about 1.5-20 milliseconds. The data bus is organized to manage the reception/transmission event as a top priority event. That is to say if the transmission event is simultaneous with the reception event, control of the bus is taken by the reception event. For this reason the on time must not be considered constant (T=130 milliseconds). This on time is in fact variable, since the reception/transmission process, as stated above is a top priority process. Since the processes of interarrival of messages are independent and can be schematised according to a Poisson statistical distribution, these intervals in which the emitter remains on must be considered as the mean values of a Gaussian curve with a variance of 1 millisecond, which expresses the time necessary for transmission/reception on the bus of the device. With reference to Figure 4, the top diagram represents, in an arbitrary scale of ordinates, the activation pulse V1 applied at intervals T (equal to about 130 milliseconds) to the pilot circuit 21 of the emitter 11. The receiver 12 thus reads the status of the wave reflected by the walls of the casing 1 every 130 milliseconds. The signal received by the receiver 12 is sent to an amplifier block 22 that brings it to voltage levels between 1-4 volts. The voltage output signal from the amplifier block 22, designated as V2, is sent to the control unit 20, in which it is sampled and digitally filtered, as will be further explained below. The amplifier block 22 can be made with an operational amplifier having the inverting input (-) connected to the receiver 12 and the non-inverting input (+) connected to a voltage source Vref of about 1 Volt which serves to eliminate the non-linearity of the operational amplifier. The amplifier block 22 comprises a feedback loop 23 consisting of an analog filter which serves to remove disturbance and amplify the useful signal. The feedback block 23 amplifies only the reflected wave, whilst it leaves any disturbance outside the useful frequency band unchanged. The feedback block 23 can be made by means of a resistor R and a condenser C placed in parallel. The output signal V2 from the integrator block is sent to the control unit 20. The control unit 20 comprises an analog-to-digital converter 33 that converts the analog signal V2 to a digital signal. Inside the control unit 20 the digital signal indicating the voltage V2 detected is filtered by means of digital filters and compared with a threshold signal that will set off a possible alarm. FIR filtering techniques are used to obtain said threshold signal. Said threshold will be updated dynamically so as to compensate the variations of the components 9 and the slow variations due to the dust or anything else present on the casing 1. Inside the control unit 20 a comparator is implemented by means of a special software and carries out a comparison logic of the mathematical type comparing the digital signal indicating the voltage V2 detected with the dynamically updated threshold value. Operation and initialisation of the anti-tamper device 10 is described below. When a wave strikes the receiver 13, after having been amplified, it gives rise to a voltage signal V2 proportional to the received light intensity. The voltage signal V2 is measured by the control unit 20 by means of the analog-to-digital converter 33. The initialisation phase of the anti-tamper device is activated by a control signal sent by the control unit 20 by means of the data bus. In this initialisation phase the value of the voltage measurement indicative of the light intensity is taken as the reference (mean flow). Furthermore the voltage value when the transmitter 11 is not emitting is also measured. This value serves to make a differential measurement that increases the immunity to disturbances. By measuring V2, when the transmitter 11 is not transmitting, a signal indicative of the background noise (V_background noise) is obtained. By measuring V2, when the transmitter 11 is transmitting, a signal that includes the wave reflected by the walls of the casing and the background noise (V_background noise + V_reflected wave) is obtained. A value indicative of the signal of the anti-tamper device (V_anti-tamper = V_reflected + V_background noise_ V_background noise) is then determined. If V_anti-tamper is measured in the initialisation phase a mean signal value (V_mean anti-tamper) is obtained. The V_ mean anti-tamper value is updated every minute to compensate for very slow variations of the components of the system. In the initialisation phase a threshold value (V_threshold) indicative of tampering with the anti-tamper device is set by means of the bus. Very fast variations of the system, such as an attempt at tampering for example, cause the condition (V_anti-tamper - V_mean anti-tamper) > V_threshold. If said threshold value (V_threshold) is exceeded, the control unit 20 generates an alarm signal. The initialisation phase activated by the bus serves to bring the voltage on the control unit 20 to a value between 2.5V and 3.5V, which coincides with the range of maximum sensitivity of the receiving part 12. Said voltage value is automatically calculated by selecting the current on the emitter 11, by means of one of the resistors R1, R2, R3 and R4 of the pilot circuit 21, so as to obtain the desired voltage value on the analog-to-digital converter 33. If said initialisation conditions prove to be insufficient, the installer can disconnect the anti-tamper device and repeat the initialisation operations. Alternatively, a reflecting mirror can be inserted in the casing 1, preferably in the inner part of the rear wall of the casing, in order to restore the system to optimal working conditions. Infrared emitters matched with the reception frequency of the receivers are preferably used. That is to say, if an emitter emits with a wavelength of 880 nanometers, the receiver also will have the maximum sensitivity at that wavelength. Thanks to the fact that V_anti-tamper = V_reflected wave - V_background noise is calculated, the system proves to be insensitive to ambient light. In fact there is a great need to make the system independent of ambient light, since initialisation of the anti-tamper system is usually done without the switch plate 6 and without the supporting plate 3, therefore ambient light enters the casing 1. Numerous modifications and changes of detail within the reach of a person skilled in the art can be made to the present embodiment of the invention without departing from the scope of the invention, expressed by the appended claims.

Claims: A tamper detecting device (10) for components (9), enclosed in a shell (8) mounted in a flush-mounted casing (1), comprising: generator means (11) to generate a detection signal that is propagated inside said casing (1) in given conditions; detection means (12) sensitive to said detection signal propagated in said casing (1), so that any tampering with the component (9) produces a variation in said given conditions and a consequent variation of said detection signal detected by said detection means; alarm means (20) connected to said detection means (12) to signal the changes in said detection signal detected by said detection means, characterised in that at least said detection means (12) are disposed inside said shell (8) of the component (9). A device according to claim 1, characterised in that said generator means (11) are further disposed inside said shell (8) of the component (9). A device according to claim 1 or 2, characterized in that said shell (8) of the component (9) has at least one window (13) for the propagation of said detection signal from the inside of the shell (8) of the component toward said casing (1) and vice versa. A device according to any one of the preceding claims characterised in that said generator means (11) generate, as detection signal, an optical signal in the infrared or ultraviolet range. A device according to any one of the preceding claims characterised in that said generator means (11) are activated at variable intervals of time. A device according to any one of claims 1 to 4 characterized in that said generator means (11) are activated at time intervals having a given periodicity ( T ). A device according to claim 6, characterized in that said given periodicity corresponds to a period (T) of the order of about 130 milliseconds. A device according to any one of claims 5 to 7, characterized in that at each activation, said generator means (11) are activated for a time interval of the order of 0.5 milliseconds. A device according to any one of the preceding claims, characterised in that said generator means (11) have associated pilot means (20, 21) acting with a constant current/voltage for the entire duration of activation of said generator means (11). A device according to claim 9, characterised in that said pilot means are a control unit (20) which sends a constant current/voltage signal at variable intervals of time to a digital-to-analog converter (21) comprising four resistors (R1, R2, R3, R4) for adjustment of said constant current/voltage signal. A device according to any one of the preceding claims, characterised in that said detection device (12) is combined with an amplifier block (22) for amplification of the detected signal. A device according to claim 11, characterised in that said amplifier block (22) has an operational amplifier to the inverting input (-) of which said detector (12) is connected and to the non-inverting (+) input of which a voltage generator (Vref) is connected to eliminate the non-linearity of the operational amplifier. A device according to claim 11 or 12, characterised in that said amplifier block (22) has a feedback loop (23) formed by means of an analog filter to remove the disturbances and amplify the useful signal. A device according to claim 13, characterized in that said analog filter of the feedback loop (23) comprises a resistor (R) in parallel with a condenser (C). A method for detection of tampering with components (9), enclosed in shells (8) mounted in a flush-mounted casing (1), comprising the following steps: transmission of a known signal inside the casing (1), detection of a V_anti-tamper signal comprising said known transmitted signal, comparison of said detected V_anti-tamper signal with a preset V_mean anti-tamper signal, generation of an alarm signal when the result of the comparison exceeds a pre-set V_threshold signal, characterised in that said detection step is performed inside the shell (8) of said component (9). A method according to claim 15, characterised in that said detection step comprises detection of a V_reflected + V_background noise signal inside the shell (8) of the component (9) during transmission and detection of a V_background noise signal inside the shell (8) of the component ( 9 ) in the absence of transmission. A method according to claim 16, characterised in that said detected V_anti-tamper signal is given by the difference between the V_ reflected + V_background noise signal reflected inside the casing (1) during transmission and the V_background noise signal detected in the absence of transmission. A method according to any one of claims 15 to 17, characterised in that said pre-set V_mean anti-tamper signal is a signal obtained in the initialisation phase of the anti-tamper device and updated dynamically by a control unit (20).

Inventors: GUSI PIERO CAMILLO, GUSI, PIERO CAMILLO
Applicants: VIMAR SPA
Application year: 2001
Main classification: G08B29
IPC codes: G08B29