WO2005115810A2

WO2005115810A2 - Immobilizer device for vehicles and antiheft system equipped with such a device

Info

Publication number: WO2005115810A2
Application number: PCT/IB2005/001418
Authority: WO
Inventors: Mario Nava; Giovanni Pedruzzi; Gianfranco Corna
Original assignee: Mitzi S.R.L.
Priority date: 2004-05-27
Filing date: 2005-05-25
Publication date: 2005-12-08
Also published as: WO2005115811A1; WO2005115810A3

Abstract

An immobilizer device, as well as a method and a system for preventing the theft and/or unauthorized handling of vehicles and/or trailers, are described. The control of the vehicle's pneumatic actuators is provided in such a manner as to carry out constant checking to counter attempted tampering. Pressure is monitored in the brake feed line (42) and is maintained at the valve higher that the atmoespheric pressure downstream of a feed line shut-off valve (2) by means of a controlled discharge valve (1) ans a pressure sensor (5).

Description

"IMMOBILIZER DEVICE FOR VEHICLES AND ANTITHEFT SYSTEM EQUIPPED WITH SUCH A DEVICE" **************************** Field of the Invention The present invention generally concerns an antitheft system for vehicles in general and, in particular, an immobilizer device, as well as a plant and method for preventing the theft and/or tampering of vehicles such as tractors, trailers and semi-trailers. Background of the Invention With regards to vehicles such as trailers or similar, a large part of the antitheft systems of known art provide for the immobilization of the vehicle/trailer by acting on the parking brake's pneumatic system that, in order to be excluded, needs air under pressure supplied by a compressor or by one or more pressurized air chambers installed on the vehicle. To avoid the trailer being hooked up to any other vehicle not authorized to take it, until now only antitheft systems that contemplate the releasing of the parking brake by authorized operators, or in any case only when the trailer is connected to an identifiable vehicle from those authorized, have been proposed. For example, United States patent no. US-B1 -6338534 contemplates the use of solenoid valves that only allow the parking brake of a trailer to be released in the case where the signal output by a transmitter unit on the tractor is recognized as valid by the receiver unit on the trailer. United States patent no. US-A-5378929 discloses an antitheft system wherein the solenoid valves controlling the pneumatic supply to the parking brake circuit are actuated using radio frequency via a remote control. The solenoid valves completely discharge the pneumatic circuit so as to not only prevent operation of the parking brake of the vehicle or trailer, but also the operation of any pneumatic drive member present on the vehicle and the trailer. The antitheft systems according to known art, although permitting the immobilization of the vehicle and/or relative trailer, can be hazardous for traffic as they act indistinctly on all of the pneumatic equipment of the vehicle/trailer on which they are installed. In addition, the traditional systems require that the entire pneumatic system of the vehicle be designed specifically to operate according to the described technique, with obviously increased design and production costs. The traditional systems have been shown to be ineffective in preventing their fraudulent disconnection by vehicle thieves. In fact, it is known that by tampering with the vehicle's pneumatic circuit, via plugs and bypasses, it is possible to exclude traditional antitheft devices, rendering them ineffective. Summary of the Invention In general, the object of the present invention is that of providing an antitheft system that is capable of ensuring not only safe immobilization of the vehicle and/or trailer on which it is installed, but also self-protection against attempts of tampering. A further object of the present invention is that of providing an antitheft system that can be also efficaciously installed on mass production vehicles, without it being necessary to modify parts of the brake control system, whether pneumatic or hydraulic. These and other objects are achieved by the present invention, which concerns an immobilizer device for a vehicle, or trailer, equipped with a feed line of pressurized fluid running to an actuator of the parking brake, the device including at least one shut-off valve of said fluid in said feed line, at least one discharge valve, situated downstream of said shut-off valve, for discharging said feed line and at least one pressure sensor, characterized in that said discharge valve is set to reduce the pressure of said fluid in said feed line, downstream of said shut-off valve, to a reference value higher than atmospheric pressure and that said pressure sensor measures the pressure of said fluid in said feed line downstream of said shut-off valve. An immobilizer device according to the present invention can be usefully installed on vehicles and/or trailers, the pneumatic parking brake of which can be activated when the pressure of the compressed air on the relative feed line is minimized. The device allows the air pressure in the feed line of the vehicle's brake to be reduced, downstream of the shut-off valve, to a reference pressure higher than atmospheric pressure, but still compatible with actioning the parking brake, and allows residual pressure variations of the air in the line, downstream of the shut-off valve, to be measured with respect to the above-stated reference pressure. If the measured residual pressure deviates from the reference value, the device commands the antitheft plant to generate an alarm signal. For example, in trailers equipped with a "spring brake chamber" pneumatic parking brake, a pressure value of 1.8 bar in the compressed air feed circuit is sufficient to allow operation of the parking brake. Therefore, a valid reference pressure for trailers with a pneumatic brake is 1.8 bar (180 KPa), a pressure that is higher than atmospheric pressure. In vehicles equipped with a hydraulically operated parking brake, a device according to the present invention allows the pressure of the liquid, oil for example, in the relative feed line to be measured. If this value deviates from the reference value, the device commands the antitheft plant to generate an alarm signal. When an immobilizer device according to the present invention is applied to hydraulic circuits, the discharge valve does not discharge into the atmosphere, but directly into the hydraulic fluid reservoir. Attempted tampering or unauthorized operations on the braking system aimed at releasing the vehicle's brake easily provoke pressure changes in the fluid downstream of the shut-off valve. Advantageously, the immobilizer device detects these variations and, therefore, detects the attempted theft. With respect to what is provided for by traditional systems, an immobilizer device according to the present invention is difficult to bypass. In fact, the reference pressure is higher than that of the atmosphere and, in consequence, any attempt to apply a bypass along the fluid feed line to isolate the device and release the parking brake, would cause a change in the pressure of the fluid measured by the device in the line. Preferably, the pressure sensor measures the pressure of the fluid, compressed air for example, in a continuous manner or at predefined time intervals. In the latter case, the pressure measurement frequency of the relative sensor can be chosen so that tampering cannot be attempted between two successive pressure measurements. The device includes a control unit for controlling the various components. In particular, the control unit compares the values of the reference pressure and the pressure measured by the sensor. If the difference between these values exceeds a predefined limit, the device acts on the antitheft plant to generate an alarm signal. For example, the immobilizer device itself, or in any case the antitheft plant as a whole, can be equipped with a siren or a device for remote transmission of the signal. The discharge valve discharges the fluid from the relative line into the atmosphere if the fluid is compressed air. If the fluid is a liquid, the device may not provide for discharge (the pressure in the circuit is not minimized) or discharge can be made to a specially provided reservoir. At least one portion of the feed line, the shut-off valve, the discharge valve and the pressure sensor are housed in a single container. Preferably, the container is made of die-cast aluminium and is provided with a hermetic cover. The connections of the portions of feed line inside the container with the portions of the feed line outside the container are made through the cover. In this way, the opening of the container, or rather the removal of the cover, can only take place by also interrupting the feed line, bringing the pressure of the fluid inside it to atmospheric pressure and thus generating the alarm signal. For example, at least one part of the portion of the feed line inside the container can be defined by the coupling of the container cover with other portions of the container itself and, in this case, the removal of the cover causes the opening of the line. In accordance with a preferred aspect of the present invention, the shut-off valve and the discharge valve are solenoid valves with a bistable control. The operation of the valves can be performed via the control unit, which in turn can respond to commands sent from the outside, in a coded form for example. The pressure sensor is preferably of the electronic, semiconductor type and can be interfaced with the control unit. A device in accordance with the invention provides a socket for an external power supply. The device can also include a source of power inside the container for powering the device's electrical components, the valves, the pressure sensor and the control unit for example, even when the external power supply is not present. The internal power source could be a backup battery. The immobilizer device, or in any case the antitheft plant in which it is installed, preferably includes a signalling device, for example a siren positioned inside the container. The siren can be powered by the backup battery and the external power supply. The siren signals any residual pressure variations within the feed line with respect to the reference pressure. These variations occur when the immobilizer device or the antitheft plant are tampered with. An immobilizer device according to the present invention can also be installed on trailers, tractors, and in general on vehicles equipped with a hydraulic or pneumatic parking brake for guaranteeing immobilization. Furthermore, the immobilizer device can be usefully installed also on fluid feed lines to the suspension of a vehicle or a trailer, or on a fluid feed line to the actuation device for engaging the vehicle's clutch. The device can detect pressure variations in the fluid used in the suspension units or for operating the vehicle's clutch due to use of the vehicle itself. An immobilizer device according to the present invention can also be installed on motorized vehicles in series with the clutch control circuit. The present invention also concerns a method for immobilizing a vehicle or a trailer equipped with a parking brake fed by the relative pneumatic circuit under pressure, characterized by including the steps of: - reducing the pressure of the fluid in said circuit, downstream of a first section, until a reference pressure higher than atmospheric pressure is reached,

- isolating at least one portion of said circuit downstream of said first section, - detecting the pressure variations of said fluid in said circuit, downstream of said first section, with respect to said reference pressure. In the case in which the vehicle is equipped with a hydraulically operated parking brake, the method according to the present invention may not include the pressure reduction step in the related circuit. A reduction in circuit pressure, in fact, could cause brake release in many types of vehicles.

The method includes the step of detecting pressure variations in the liquid in the circuit with respect to a reference pressure. The method contemplates that the step of detecting fluid pressure variations is carried out continuously or, alternatively, at intervals of time. When the detected variations are greater than a certain value an alarm signal is generated. The alarm signal can be acoustic and/or visual and can be transmitted over a long distance. The alarm can follow the immobilization of the vehicle. The immobilizer device is also capable of recognizing tampering on the electrical control line and of generating an alarm signal.

The use of bistable valves ensures that the immobilization effect continues even if the electrical power supply is completely removed. The electronic control unit, of the microprocessor type for example, can be electrically connected to an electronic driver circuit associated with the immobilizer device or the antitheft plant. Advantageously, the electronic control unit can include at least one section for the exchange of encoded data and commands in radio frequency with at least one remote transceiver device. The possibility is thus achieved of complete interaction between the electronic control unit present on the vehicle/trailer and a remote device, so as to make information regarding the state of the vehicle/trailer and its load available at any time, such as making it possible to send commands to the antitheft plant installed on the trailer in the case where, for example, abnormal events are detected. In accordance with an aspect of the present invention, a method is proposed for preventing the theft of and/or tampering with a vehicle/trailer, in which at least the immobilization of the vehicle/trailer by acting on the respective parking brake's pneumatic system is provided, and in which control of the immobilization function is provided via at least one microprocessor-based electronic control unit that is electrically connected to an electronic driver circuit associated with at least one immobilizer device of the above-described type, characterized in that it includes the exchange of encoded data and commands in radio frequency between the electronic control unit and at least one remote transceiver device. Brief Description of the Drawings Further characteristics and advantages of the present invention will be become clearer from the description that follows, given for purely illustrative and non limitative purposes, with reference to the attached drawings, where: - Figure 1 is a logical diagram that illustrates a possible form of embodiment of an immobilizer device for an antitheft plant according to the present invention and the relative connections to the control circuit of the parking brake of a vehicle, - Figure 2 is a diagram that illustrates a possible form of embodiment of an immobilizer device for an antitheft plant according to the present invention, - Figure 3 is a diagram that illustrates a possible form of embodiment of an antitheft plant according to the present invention and the relative transmission and reception connections, and - Figure 4 is a simplified diagram of the pneumatic circuit of the parking brake of a trailer equipped with an antitheft plant according to the present invention. Modes for carrying out the Invention Figure 1 represents the layout of an immobilizer device A according to the present invention applied to a vehicle or a trailer equipped with a pneumatic parking brake. The parking brake is controlled by one or more pneumatic actuators

M, of the type known by the name "spring brake chamber" for example, fed with compressed air through a feed line 4. The line 4 is in turn fed from a chamber of compressed air W, having a volume in the range of 50 - 80 litres, for example. The pressure of the air during normal operation, for example, could be 8 bar, with a maximum circuit pressure of approximately 12 bar. When the air pressure inside the line 4 is close to the atmospheric pressure, the actuators M operate the parking brake. When the air pressure in the relative line 4 is restored, that is to say when it is returned to the values corresponding to normal vehicle running, the actuators M release the parking brake. In other words, the parking brake is active when the pressure in the line 4 is insufficient for operation and, vice versa, the parking brake is not active when the line 4 is under pressure. The immobilizer device A is positioned along the feed line 4 upstream of the actuator M. In the diagram illustrated in Figure 1 , the immobilizer device A is connected to the line 4 in correspondence to connection 41 upstream and in correspondence to connection 42 downstream. Downstream of the section 42, the line 4 can feed more than one actuator M. The immobilizer device A includes at least one shut-off valve 2, at least one discharge valve 1 and an air-pressure detection sensor 5. The valves 1 and 2 are preferably solenoid valves with a bistable control. The operation of the valves 1 and 2 is controlled by a control unit 10 (Figure 2). The immobilization of the vehicle/trailer, or rather the engaging of the parking brake and the activation of the immobilizer device A, can take place according to two modes. The operation of the immobilizer device A according to a "rapid" first mode contemplates the following steps. The shut-off valve 2 is activated by the control unit 10 to block the flow of compressed air along the feed line 4 arriving from the chamber W. In consequence, the portion of the circuit downstream of the shut-off valve is isolated from the upstream portion of the same valve 2. The control unit 10 controls the opening of the discharge valve 1 that, through the limiter 12, bleeds off compressed air into the atmosphere via the outlet 6, reducing the pressure in the isolated portion of circuit until a reference value corresponding to the activation of the actuator M (brake chamber) is reached, but still higher than atmospheric pressure. In practice, the discharge valve 1 is set to maintain a residual pressure in the line 4 downstream of the shut-off valve 2. The pressure sensor 5 measures the air pressure in the isolated section of the circuit, or rather in the line 4 downstream of the valve 2. The sensor 5 can measure the residual pressure in the immobilizer device A continuously or at predefined time intervals. The pressure sensor 5 sends a signal indicating the measured pressure to the control unit 10. The control unit 10 compares the measured pressure with the reference pressure, or rather determines any variations in residual pressure in the isolated circuit with respect to the reference pressure preset on the discharge valve 1. The variations are indicative of possible attempts of tampering on any portion of the immobilizer device A. Any kind of tampering with the immobilizer device, for the purpose of applying a bypass for example, would cause a drop and/or increase in pressure in the line 4 downstream of the valve 2, independently of the manner and point at which tampering was attempted. The control unit 10, upon determining a variation in residual pressure greater than a certain value, generates an alarm signal. The alarm signal can be transmitted over a long distance via opportune interfaces, radio or GSM for example, or can be an acoustic signal generated by a siren connected to the control unit, etc. The operation of the immobilizer device A in a "complete" second mode contemplates the following steps. The discharge valve 1 is activated first by the control unit. The air pressure drops throughout the feed line 4 to the reference value. Subsequently, the shut-off valve 2 is activated to isolate the downstream portion of the line 4 and to keep the entire pneumatic circuit downstream of the immobilizer device A under control. The monitoring modes are identical to those contemplated in the case of the rapid operation mode. A reference pressure value suitable for applications of this type, namely for application to vehicles equipped with a "brake chamber" for the pneumatic engagement of the parking brake, is 1 .8 bar (180 kPa). Figure 2 shows a possible layout for the immobilizer device A. In this embodiment, the immobilizer device A is equipped with a siren 9 suitable for generating an acoustic alarm signal in cases of attempted unauthorized releasing of the vehicle's parking brake. The internal siren 9, of the piezoelectric type, is powered by the internal battery and is activated upon command from the control unit 10 or when the immobilizer device A, controlled remotely or from the outside, is isolated by an external monitoring centre. The immobilizer device A shown in Figure 2 is provided with a backup battery 1 1 that guarantees power to the various electrical/electronic components of the immobilizer device A. The type of battery 1 1 can be of gel, lead, etc. The immobilizer device A is also provided with a socket 8 for connecting an external power supply and a circuit section 7 that allows the immobilizer A to be interfaced with monitoring or control devices (diagnostics, unlocking, etc.). The battery 11 is preferably 12 VDC. The external power supply can also be 12 VDC or 24 VDC. The quiescent electrical absorption of the immobilizer device A must be extremely small in order to guarantee long- term operation, typically less than 2.5 mA. The reference pressure takes into account possible variations in atmospheric pressure, due to changes in altitude for example. The immobilizer device A can also be provided with a temperature compensation system to correct the signal generated by the pressure sensor 5. The immobilizer A can also be applied to vehicles equipped with a hydraulically controlled parking brake. The principle of operation of the immobilizer device does not change: the control unit 10 compares the pressure measured by the sensor 5 in the hydraulic circuit (containing oil for example) with a reference value. Any variations are interpreted by the control unit 10 as attempts to release the parking brake. In this case, the valve 1 does not discharge into the atmosphere, but into the reservoir of the hydraulic circuit. The principle of operation of the immobilizer device according to the present invention, with opportune variations in the operation sequence, can also be applied to positive braking systems, or rather systems that contemplate the presence of pressure to maintain the braking action. In Figure 3, a trailer 100 is represented on which an immobilizer A' according to the present invention is installed and configured for immobilizing the trailer 100 by acting on the pneumatic circuit of the relative parking brake. The immobilizer A' includes at least one microprocessor-based electronic control unit 30. The electronic control unit 30 preferably incorporates its own independent power supply, a backup battery for example, capable of guaranteeing operation for a predetermined duration. The immobilizer device A' is substantially similar to that described in Figures 1 and 2. In accordance with the invention, the electronic control unit 30 includes at least one transceiver section for allowing the exchange of encoded data and commands in radio frequency with at least one remote transceiver device, such as a mobile phone 50 for example, or a remote device 60 located in a monitoring centre 65. The connection between the electronic control unit 30 present on the trailer and the remote devices 50 or 60 is preferably implemented via the various links 70 (only one is shown in Figure 3 for simplicity) of the GSM network. The part of the system installed on the trailer 100 also includes an external portion 31 enclosed in a safety casing for containing at least the antenna for GSM network connection, an acoustic warning device, such as a siren for example, and at least one shock sensor capable of detecting any attempts of forcing. The shock sensor present in the external portion 31 is connected to the electronic control unit 30 to send a signal to it representing the current tampering status on the external portion 31. Other types of sensors suitable for generating signals representing one or more fault and/or tampering conditions on the trailer 100 are also connected to the electronic control unit 30. For example, perimeter protection sensors 32 (mechanical limit switches, magnetic actuators, impact sensors, etc.) can be provided for signalling, for example, the open status of doors on the trailer 100, as can volumetric protection sensors 33 for detecting, for example, the presence of people inside the body of the trailer 100, or at least in the access area to the load compartment. At least one camera 34 capable of acquiring images in correspondence to certain events, for example, the opening of doors, the detection of movement in the load compartment or similar, can also be connected to the electronic control unit 30. The acquisition of images by the camera 34 can also be remotely controlled, for example, by the monitoring centre 65. Other types of sensors can also be connected to the electronic control unit 30, for example, inclination sensors for detecting any movements of the trailer that cause tilting, or even environmental protection sensors, such as sensors capable of detecting changes in temperature and/or humidity inside the load compartment for example, also for preventing possible damage to the transported goods. In Figure 4, the layout of a pneumatic circuit for a parking brake is shown that includes an immobilizer device A' installed, for example, on a trailer 100 such as that shown in Figure 3.

In the form of embodiment shown here, the immobilizer device A' is installed on the pneumatic circuit and acts through the pipe 4 on two actuators 22 of the brakes of two respective wheels 70 on the same side of the vehicle. The pressurized air provided by the driving vehicle is fed to a chamber 23, equivalent to the chamber W. The immobilizer device A', shown in greater detail in Figure 3, preferably includes two direct-drive solenoid valves for activating and deactivating operation of the parking brake, thereby permitting the immobilization of the trailer 100. At least one electronic driver circuit connected to the electronic control unit 30 can be usefully integrated inside the casing of the immobilizer device A'. Alternatively, both circuits could be incorporated in the immobilizer device A'. The operation of an antitheft plant according to the present invention will now be described. The activation and deactivation of the system can be performed remotely via a mobile phone 50 that has been previously authorized by including its identification information on a list maintained at the monitoring centre 65. The same commands can also be transmitted from a fixed telephone, as long as it has been authorized. In general, activation performed by an operator entails the activation of both the immobilizer device A' and the other functions implemented via the sensors. It could also be made possible to activate just the functions implemented by the sensors from the monitoring centre 65, while leaving the immobilizer device A' deactivated. Furthermore, it is preferable that all of the protection functions can be activated also when the trailer 100 is in movement. In the case in which the trailer 100 is isolated from the GSM network, any deactivation of the antitheft plant is prevented. In this case, procedures are contemplated that allow the deactivation of the system only on the basis of procedures and codes determined by the supervisor. The operator assigned to transferring the trailer must therefore contact the supervisor, via the fixed telephone network for example, to receive the necessary codes and/or authorizations for deactivating the system. The immobilizer device A' can therefore also include means of data entry (for example, keyboards or similar) that allow the codes and/or authorization keywords to be entered. Every event detected by the connected sensors is stored in the electronic control unit 30 and transmitted to the monitoring centre 65 immediately, or in any case at established times or upon request. In the same manner, every operation performed is stored in the electronic control unit 30, together with an identification code for the operator who performed the operation. In any case, all of the events and operations are stored and transmitted to the monitoring centre 65. In this centre, a recording is made of these notifications received from the trailer 100, together with information regarding the date and time of transmission and/or reception. The images acquired from the camera 34 are also stored in the electronic control unit 30 and sent immediately to the monitoring centre 65, or in any case as soon as possible.

With regards to the operation of the immobilizer device A', the solenoid valves allow the interception, and thus the blocking, of the pneumatic supply to the actuators 22 associated with each brake or pair of brakes on one of the sides of the vehicle, producing in effect the engagement of the parking brake of the trailer 100. The connection between the driver circuit and the electronic control unit 30 can be implemented by means of a pair of non-polarized wires through which both power and commands are received. The exchange of data between the driver and control circuits is implemented via a suitable protocol. To guarantee the inviolability of the command. A reciprocal verification procedure is implemented between the electronic control unit 30 and the driver circuit. This procedure, based for example on the generation and processing of random numbers, allows the necessary information to be extracted for determining the opening or the closing of the parking brake's pneumatic circuit, which takes place by actioning one of the two solenoid valves 25 of the immobilizer device A'. The solenoid valves guarantee switching in a temperature range from -40 to +85°C without requiring lubrication. A system according to the present invention is also suitable for being used to locate the trailer 100. In fact, each message returned by the electronic control unit 30 installed on the trailer 100, for example an SMS message, can contain the current geographic position, based on the GSM cell used for the connection.

Claims

1. An immobilizer device for a vehicle, or a trailer, equipped with a feed line (4) for a fluid under pressure to an actuator (M) of the parking brake, the device including at least one shut-off valve (2) of said fluid in said feed line (4), at least one discharge valve (1 ), situated downstream of said shut-off valve (2) for discharging said feed line (4), and at least one pressure sensor (5), characterized in that said discharge valve (1 ) is set to reduce the pressure of said fluid in said feed line (4), downstream of said shut-off valve (2), to a reference value higher than atmospheric pressure and said pressure sensor (5) measures the pressure of said fluid in said feed line (4) downstream of said shut-off valve (2).

2. The immobilizer device according to claim 1 , characterized in that said pressure sensor (5) measures the pressure continuously, and that it includes a control unit (10) for comparing the values of said reference pressure and of said pressure measured by said pressure sensor (5).

3. The immobilizer device according to claim 1 or claim 2, characterized in that at least one portion of said feed line (4), said shut-off valve (2), said discharge valve (1 ) and said pressure sensor (5) are housed in a single container.

4. The immobilizer device according to claim 3, characterized in that at least one part of the portion of feed line (4) inside said container is defined by elements of said container.

5. The immobilizer device according to any of the preceding claims, characterized in that said shut-off valve (2) and said discharge valve (1 ) are solenoid valves with a bistable control and said pressure sensor (5) is electronic.

6. The immobilizer device according to any of the preceding claims, characterized in that it also includes a signalling device (9) suitable for generating an alarm signal.

7. The immobilizer device according to any of the preceding claims, characterized in that it also includes a power source (11 ) for powering said shut-off valve (2), said discharge valve (1), and/or said pressure sensor (5), said control unit (10) and/or said signalling device (9).

8. The immobilizer device according to any of the preceding claims, characterized in that said fluid is a gas or a liquid.

9. Use of the immobilizer device according to any of the preceding claims to immobilize a vehicle or a trailer and prevent theft.

10. Use of the immobilizer device according to any of claims 1 to 8 to immobilize a vehicle or a trailer equipped with a hydraulically operated parking brake, in which said discharge valve (1 ) discharges into a reservoir of the hydraulic circuit.

11. A method to immobilize a vehicle or a trailer equipped with a parking brake fed by the relative pneumatic circuit under pressure, characterized b including the steps of:

- reducing the pressure of the fluid in said circuit, downstream of a first section, until a reference pressure higher than atmospheric pressure is reached,

- isolating at least one portion of said circuit downstream of said first section,

- detecting the pressure variations of said fluid in said circuit, downstream of said first section, with respect to said reference pressure.

12. The method according to claim 11 , characterized in that said step of detecting pressure variations in said fluid is carried out continuously.

13. The method according to claim 11 o 12, characterized by including the step of generating an alarm signal when the measured pressure of said fluid is different from said reference pressure.

14. The method according to any of claims 11 to 13, characterized by including a further step of:

- intercepting said fluid in said first section of said circuit and isolating at least one portion of the circuit downstream of said first section.

15. An antitheft plant for a vehicle or a trailer, including at least one immobilizer device according to any of claims 1 to 8, characterized by including at least one microprocessor-based electronic control unit (30) which is electrically connected to an electronic driver circuit associated with immobilizer device (A, A'), wherein said electronic control unit (30) includes at least one section for the exchange of encoded data and commands in radio frequency with at least one remote transceiver device (50, 60 and 70).

16. The antitheft plant according to claim 15, wherein said remote transceiver device consists of a telephone handset connected to the fixed network.

17. The antitheft plant according to claim 15, wherein said remote transceiver device consists of a mobile phone (50) connected to the GSM network.

18. The antitheft plant according to claim 15, wherein said remote transceiver device is incorporated in a central monitoring unit (65).

19. The antitheft plant according to claim 15, wherein said electronic control unit (30) includes at least one power section to selectively supply said electronic driver circuit with sufficient electric power to operate the immobilizer device (A, A').

20. The antitheft plant according to claim 15, wherein one or more sensors (5, 32, 33) are connected to said electronic control unit for generating signals representing one or more fault and/or tampering conditions on the vehicle or trailer.

21. The antitheft plant according to claim 15, wherein at least one camera (34) is connected to said electronic control unit for taking pictures of said vehicle or trailer.

22. The antitheft plant according to any of the preceding claims 15 to 21 , wherein said electronic control unit (30) includes at least one memory card for storing information regarding the operations carried out on said vehicle/trailer and/or the events detected by sensors and/or the pictures taken by said at least one camera.

23. The antitheft plant according to claim 15, wherein said solenoid valves (1 , 2) are of the direct-drive type.

24. The antitheft plant according to claim 15, wherein at least one part of the pneumatic distribution unit of the parking brake plant is integrated in said immobilizer device (A, A').

25. The antitheft plant according to claim 15, wherein at least said electronic driver circuit is integrated in said immobilizer device (A, A').

26. The antitheft plant according to any of the preceding claims, wherein at least one an acoustic warning device (9) is provided for signalling the occurrence of one or more predetermined or remotely controlled events.

27. A method for preventing the theft of and/or tampering with a vehicle or a trailer, in which at least the immobilization of the vehicle/trailer by acting on the respective parking brake's pneumatic system is provided, and in which the control of the immobilization function is provided by at least one microprocessor-based electronic control unit (30) which is electrically connected to an electronic driver circuit associated with at least one immobilizer device (A, A'), characterized by including the exchange of encoded data and commands in radio frequency between said electronic control unit (30) and at least one remote transceiver device (50, 60, 70).

28. The method according to claim 27, wherein said remote transceiver device consists of a telephone handset connected to the fixed network.

29. The method according to claim 27, wherein said remote transceiver device consists of a mobile phone (50) connected to the GSM network.

30. The method according to claim 27, wherein said remote transceiver device is incorporated in a central monitoring unit (65).

31. The method according to claim 27, wherein said electronic control unit (30) selectively supplies an electrical power sufficient to permit the activation of said immobilizer device (A, A').

32. The method according to claim 27, wherein said electronic control unit (30) receives and processes signals representing one or more fault and/or tampering conditions on the vehicle/trailer originating from one or more sensors (5, 32, 33).

33. The method according to claim 27, wherein said electronic control unit (30) receives and processes the images of said trailer taken by at least one camera.

34. The method according to any of claims 27 to 33, wherein the storing of the information related to the operations carried out on said vehicle/trailer and/or the events detected by said sensors and/or the pictures taken by said at least one camera is provided in a memory of said electronic control unit (30).

35. The method according to any of claims 27 to 34, wherein the transmission of the information related to the events detected by said sensors and/or the pictures taken by said at least one camera is provided to said central monitoring unit (65).

36. The method according to claim 27, wherein the immobilization of said vehicle/trailer is carried out by acting on at least one pair of direct-drive solenoid valves (1 , 2).