COMMUNICATION SYSTEM

Description

[0001] The invention relates to a communication system according to the preamble of claim 1.

[0002] Such a communication system is known, for example, from DE 10 2013 002 227 B4. It enables high-bandwidth, interference-free communication between a vehicle moving along a predetermined path and a stationary station. An antenna is mounted on the vehicle such that it protrudes through a slot into the cavity of a waveguide and can receive and / or transmit electromagnetic waves propagating along the waveguide as the vehicle moves. A corresponding antenna on a stationary station is located at one end of the waveguide.

[0003] In a communication system of this type, as in any communication system based on the propagation of electromagnetic waves, the power of a received signal depends on the distance between the transmitting and receiving antennas. The transmit power and the length of the transmission path must be matched so that, on the one hand, at the maximum possible distance between transmitter and receiver, a signal-to-noise ratio sufficient for interference-free message transmission is still guaranteed, and on the other hand, at the minimum possible distance between transmitter and receiver, the receiver is not overloaded by an excessively high received signal power, which could disrupt message transmission or even damage the receiver. The latter condition limits the maximum transmit power, and consequently, the former condition limits the maximum length of the transmission path.

[0004] In the aforementioned document, the transmitting and receiving equipment of both the fixed station and the vehicles with their respective antennas are connected via an attenuator. Attenuators are well-known components in telecommunications and are available on the market in various designs with fixed or variable attenuation, as can be seen in Bill Schweber's review article "RF Attenuators: For When You Have Too Much of a Good Thing" in the magazine "Electronic Products." In the aforementioned communication system, a signal always passes through two attenuators on its way from the transmitter to the receiver: one between the transmitter and the transmitting antenna, and one between the receiving antenna and the receiver. By designing these attenuators, a level adjustment can be made for largely interference-free operation, given the output power of a transmitter and the input sensitivity of a receiver.However, the previously explained limitation of the maximum length of the transmission path remains in effect even with this concept.

[0005] In view of this state of the art, the invention is based on the objective of increasing the maximum length of the transmission distance.

[0006] This problem is solved according to the invention by a communication system with the features of claim 1. Advantageous embodiments are specified in the respective dependent claims.

[0007] According to the invention, in a communication system for communication between a vehicle guided along a predetermined path and a stationary station, a slotted waveguide extending parallel to the path of the vehicle is used, into which at least one antenna connected to a transmitting and receiving device of the vehicle and at least one antenna connected to a transmitting and receiving device of the stationary station project, wherein the antenna of the vehicle is moved in the longitudinal direction of the slot during the movement of the vehicle, a damping unit with adjustable attenuation is connected between the antenna of the vehicle and a transmitting and receiving device of the vehicle, and a switching device is provided in the damping unit.by means of which the vehicle's antenna can be connected to the vehicle's transmitting and receiving equipment either directly or via at least one intermediate attenuator.

[0008] This allows the communication range to be increased by increasing the transmitter power and preventing the receivers from being overloaded by the attenuator when the signal strength received by the antennas would otherwise rise excessively as the vehicle approaches the stationary station due to the increased transmission power. A switching device of this type allows for the simple implementation of an attenuator. It permits a stepwise change in attenuation, with a sufficient number of steps achieving adequate accuracy in the attenuation setting.

[0009] The damping unit advantageously has an analog or digital interface, via which its damping can be adjusted by means of a control signal supplied to it at the interface. In this case, the damping adjustment can be made on the vehicle side and thus decentrally, and does not require the transmission of additional control signals via the slotted waveguide.

[0010] It is particularly advantageous that the damping unit is connected to a damping controller, to which the vehicle's transmitter and receiver can supply a signal indicating the power of the received signal, and the damping controller is configured to adjust the damping of the damping unit depending on the power of the received signal. This power is the direct criterion that necessitates a variation in damping, because below a certain distance between transmitter and receiver it would be too high for the receiver.

[0011] The damping unit can also be connected to a damping controller, which can receive a signal indicating the vehicle's position from a control unit in the vehicle. The damping controller can then be configured to adjust the damping of the damping unit depending on the vehicle's position. The vehicle's position, and thus the length of the transmission path, can either be used as an alternative to the power of the received signal as a criterion for adjusting the damping, since the signal power decreases in a defined manner with increasing transmission path length, or the two criteria can be combined, for example, as part of a plausibility check.

[0012] A practical implementation of a damping unit consists of a continuously variable damping element, the value of which can be adjusted by an analog signal. An interface of the damping unit is provided to derive the analog signal from the control signal supplied to it. The advantage of this implementation is that the continuously variable damping allows its value to be optimally adjusted at all times.

[0013] The switching device, by means of which the vehicle's antenna can be connected to the vehicle's transmitting and receiving equipment either directly or via at least one intermediate attenuator, is expediently operable via an interface of the attenuator unit by an attenuation control of the vehicle. One possibility for stepless variation of the attenuation is to provide at least two attenuators with different attenuation values, and to allow the antenna to be connected to the vehicle's transmitting and receiving equipment either directly or via one of the attenuators by means of the vehicle's switching device.Another possibility for stepwise variation of the attenuation is to provide at least two attenuators, and to allow the antenna to be connected to the vehicle's transmitting and receiving equipment either directly or via one of the attenuators or via a series connection of several attenuators by means of the switching device.

[0014] The vehicle can also be equipped with two antennas arranged one behind the other in the direction of travel at a predetermined distance from each other. In this case, the vehicle's transmitting and receiving equipment has two antenna connections, and an attenuator with adjustable attenuation is connected between each antenna and the vehicle's transmitting and receiving equipment. Such an arrangement is advantageous for a communication system intended to provide a long transmission link for a long-distance transport system. In this case, the slotted waveguide consists of several sections, each with a gap between them. Using only a single antenna would interrupt communication if the antenna moved within this gap.By using two antennas in series, each equipped with a switchable attenuator, an interruption of communication at such a gap can be avoided.

[0015] Advantageously, in this case, the damping units are connected to a multi-output damping controller, allowing their damping to be adjusted independently. For this purpose, the vehicle's transmitter and receiver can supply the damping controller with a signal indicating the power of the received signals, and the damping controller is configured to adjust the damping of the damping units depending on the power of the received signals. Alternatively, the vehicle's control unit can supply the damping controller with a signal indicating the vehicle's position, and the damping controller is configured to adjust the damping of the damping units depending on the vehicle's position. A combination of both criteria is also possible, for example, for the purpose of a plausibility check.

[0016] Exemplary embodiments of the invention are described below with reference to the drawings. These show Fig. 1 a schematic cross-sectional view of a slotted waveguide with an antenna projecting into it, Fig. 2 a schematic longitudinal section view of a slotted waveguide in the direction of arrows AA in Fig. 1 with a block diagram of the further components of the communication system according to the invention, Fig. 3 a block diagram of a first embodiment of the vehicle-side components of the communication system according to the invention Fig. 2 , Fig. 4 a block diagram of a second embodiment of the vehicle-side components of the communication system according to the invention of Fig. 2 , Fig. 5 a block diagram of a third embodiment of the vehicle-side components of the communication system according to the invention of Fig. 2 and Fig. 6 Fig. 2corresponding view of a second embodiment of the communication system according to the invention.

[0017] Fig. 1 Figure 1 shows a schematic cross-sectional view of a slotted waveguide 1, as used in a communication system for communication between a vehicle 4 guided along a predetermined path and a stationary station 5, and / or between several such vehicles, according to the prior art. An antenna 3 of the vehicle projects through the slot 2 into the slotted waveguide 1 to radiate and receive electromagnetic waves propagating along the waveguide 1. As the vehicle moves along its predetermined path, the antenna 3 moves with it in the longitudinal direction of the slotted waveguide 1. The vehicle 4 may, in particular, be guided by rails along a designated track.

[0018] How Fig. 2As shown, an antenna 6 of the stationary station 5 also protrudes into the slotted waveguide 1. It is connected to a transmitting and receiving device 7, hereinafter referred to as transceiver 7. The antenna 6 of the stationary station 5 could also protrude into the slotted waveguide 1 through slot 2, like the antenna 3 of the vehicle 4, but this is not required, as it does not need to be movable. The transceiver 7 is connected to a control unit 8 of the stationary station 5, which controls the entire transport system, which may also include several vehicles 4, and / or monitors its operation, for example, using video signals and status messages. The control unit 8 can perform a controlling function, but this is not mandatory; it can also serve solely for the purpose of monitoring operation.For this purpose, continuous bidirectional data communication takes place between the control unit 8 of the stationary station 5 and a control unit 9 of the vehicle 4, which controls the movement of the vehicle 4, via the slotted waveguide 1 as a transmission channel.

[0019] How Fig. 2As further shown, the control unit 9 of the vehicle 4 is connected to a transmitting and receiving unit 10, hereinafter referred to as transceiver 10, on board the vehicle 4. This transceiver 10 is connected to the antenna 3 of the vehicle 4 via an attenuator 11. The attenuator 11 is reciprocal, i.e., it attenuates a signal in both possible directions of transmission from the transceiver 10 to the antenna 3 of the vehicle 4 and vice versa, equally. The attenuation of the attenuator 11 is adjustable via a control line 12. An attenuator 13 is connected to this control line and is connected to both the transceiver 10 and the control unit 9.The maximum attenuation of the attenuation unit 11 is designed such that, at the minimum possible distance of the vehicle 4 from the stationary station 5, when the attenuation unit 11 is set to its maximum attenuation, neither the input of the transceiver 10 of the vehicle 4 is overloaded when the transceiver 7 of the stationary station 5 transmits, nor the input of the transceiver 7 of the stationary station 5 is overloaded when the transceiver 10 of the vehicle 4 transmits.

[0020] As vehicle 4 moves away from the stationary station 5, the level of the signals receivable by antennas 3 and 6 decreases at a constant transmit power of transceivers 7 and 10. At a certain distance, this level reaches a point where the signal-to-noise ratio at the receive input of at least one of the two transceivers 7 and 10 becomes too low for interference-free message transmission. Before this occurs, the attenuator 13 of vehicle 4 adjusts the attenuation of the attenuator 11 to a lower value in time. This increases the signal power receivable by each of the two transceivers 7 and 10 sufficiently to ensure that the resulting increase in the signal-to-noise ratio at the respective receive input of both transceivers 7 and 10 continues to enable largely interference-free message transmission.

[0021] The maximum distance over which the vehicle 4 can communicate with the stationary station 5 is significantly extended by the possibility according to the invention of varying the attenuation between the antenna 3 and the transceiver 11 of the vehicle 4 depending on the distance by a targeted adjustment of the attenuation of the attenuation unit 11, compared to a configuration without an attenuation unit 11 or a configuration with an attenuator with a fixed attenuation value connected in the signal path between the antenna 3 and the transceiver 10.

[0022] The distance-dependent adjustment of the attenuation of the attenuator 11 is preferably carried out by measuring the signal level at the receive input of the transceiver 10, since the signal level is the direct criterion for the necessity of changing the attenuation. This signal level is continuously reported to the attenuator 13 by the transceiver 10. Based on this information, the attenuator 13 adjusts the attenuation of the attenuator 11 to the optimal value via the control line 12.

[0023] However, the distance between the vehicle 4 and the stationary station 5 can also be used as a criterion for changing the damping of the damping unit 11. For example, in a transport system of this type, in which a communication system according to the invention is designed as a component, it is common to arrange optical markers along the route, by means of which the control unit 9 of a vehicle 4 can determine its position along the route and thus also the distance of the vehicle's antenna 3 from the antenna 6 of the stationary station 5 by means of optical sensors.

[0024] This information can be used either as an alternative or in addition to the current level of the signal arriving at the antenna input of the transceiver 10 by the attenuator 13 to decide whether to change the attenuation of the attenuator 11. For this purpose, the attenuator 13 is connected to the control unit 9 of the vehicle 4. In principle, either the received signal level or the vehicle position would be sufficient as a criterion for adjusting the attenuation.

[0025] The transmit power of transceivers 7 and 10 is always selected according to the maximum required range of the communication system. When vehicle 4 is at its maximum distance from the stationary station 5, the attenuation of the attenuator 11 is set to its minimum possible value. If vehicle 4 moves from this position towards the stationary station 5, resulting in a corresponding increase in the power of the signals received by transceivers 7 and 10, the attenuation of the attenuator 13 is continuously increased via the control line 12. This ensures that the signal power at the receive inputs of transceivers 7 and 10 remains within the permissible range until vehicle 4 reaches its minimum possible distance from the stationary station 5.

[0026] A first embodiment of the vehicle-side part of a communication device according to the invention with an adjustable damping unit 11 shows Fig. 3 This attenuator 11 consists of two attenuators 14 and 15 with different fixed attenuation values, a short line 16 with very low attenuation, two switches 17 and 18, and an interface 19. Each of the three terminals—where the antenna 3 is connected to switch 17 and the transceiver 10 to switch 18—can be selectively connected to one of three other terminals by means of switches 17 and 18. On each of these two switches 17 and 18, the first of these three other terminals is the first attenuator 14, the second attenuator 15 is connected to the second of these three terminals, and the short line 16 is connected to the third of these three terminals.

[0027] The control line 12 leading from the attenuator 13 to the attenuator 11 is connected to the interface 19, which derives internal control signals for controlling switches 17 and 18 from a digital control signal supplied via the control line 12. Switches 17 and 18 are connected to these switches via their respective control lines. By selecting the switch positions of switches 17 and 18 using the control signal supplied via the control line 12, the attenuator 13 of the vehicle 4 can connect the antenna 3 to the transceiver 10 either via one of the two attenuators 14 or 15, or directly via line 16. Due to the different attenuation values ​​of the attenuators 14 and 15, it is possible to vary the attenuation between two different values ​​when either attenuator 14 or 15 is connected in the signal path between the antenna 3 and the transceiver 10.It is understood that switches with more than three terminals on one side and more than two attenuators with different attenuation values ​​can also be used to achieve an even finer gradation of the attenuation depending on the distance between the vehicle 4 and the stationary station 5.

[0028] A second embodiment of the vehicle-side part of a communication device according to the invention with an adjustable damping unit 11 shows Fig. 4 In this embodiment, two attenuators 24 and 25, each with fixed attenuation, can be selectively connected individually or in series between the antenna 3 and the transceiver 10 of the vehicle 4 by means of four switches 20, 21, 22, and 23. A first attenuator 24 and a first short line 26 are connected to the antenna 3 via a first switch 20, just as in the first embodiment. Fig. 2A second attenuator 25 and a second short line 27 of the same type are connected to the transceiver 10 via a second switch 23, exactly as in the first embodiment. Fig. 2 connected. Between the attenuators 24 and 25 and the lines 26 and 27, two switches 21 and 22 are connected in series, through which the first attenuator 24 can be connected in series with either the second attenuator 25 or the second line 27, or the first line 26 can be connected in series with either the second attenuator 25 or the second line 27.

[0029] Thus, by selecting the positions of the four switches 20, 21, 22, and 23, the antenna 3 can be connected to the transceiver 10 either directly, i.e., via the two lines 26 and 27; or via the first line 26 and the second attenuator 25; or via the first attenuator 24 and the second line 27; or via the series connection of both attenuators 24 and 25. If the attenuators 24 and 25 have different attenuation values, the attenuation can be varied in three stages if one or both of the attenuators 24 or 25 are connected in the signal path between the antenna 3 and the transceiver 10. It is understood that more than two combinations of one attenuator and one line can also be connected in series via switches to achieve even finer attenuation gradations depending on the distance between the vehicle 4 and a fixed station 5.

[0030] Here too, switches 20 to 23 are controlled via control lines from an interface 28, to whose input the control line 12 is connected, via which the damping unit 11 is connected to the damping controller 13 of the vehicle 4.

[0031] A third embodiment of the vehicle-side part of a communication device according to the invention with an adjustable damping unit 11 shows Fig. 5In this embodiment, an attenuator 29 with continuously variable attenuation is provided. This attenuator 29 contains an electrically adjustable ohmic resistor in the form of a diode, the value of which can be changed by applying a DC voltage. The DC voltage can be easily separated from the high-frequency message signal to be attenuated by filtering. The interface 30 of the attenuator 11 generates the DC voltage for adjusting the attenuation of the variable attenuator 29 from the signal supplied to it by the attenuator 13 via the control line 12.

[0032] It is also possible to combine the previously described embodiments of a variable damping unit 11. Thus, series and parallel circuits of damping elements that can be varied by means of switches can be combined, and damping elements with continuously variable damping can be used within a series and / or parallel circuit that can be varied by means of switches, in order to allow both a large variation in damping and a precise setting of a desired damping value.

[0033] In all embodiments, the control line 12 can be a simple two-wire line to which the attenuator 13 applies an analog signal in the form of a variable voltage, the magnitude of which is proportional to the value of the attenuation to be set. Alternatively, it can be a multi-core control line 12 over which a digital signal is transmitted, in which the value of the attenuation to be set is encoded. In this case, interfaces 19 and 28 function as decoders, and interface 30 functions as a digital-to-analog converter. In an embodiment with switchable attenuators, it would also be possible to use an analog voltage for adjusting the attenuation, which is converted into control signals for switches by interfaces 19 and 20 based on threshold comparisons. Another alternative for implementing the control line 12 is a data line for serial data transmission.In this case, interfaces 19, 28 and 30 can, for example, contain a USB, Ethernet or RS485 interface, and the damping controller 13 is also equipped with one.

[0034] In the Figures 3 and 4 The switches shown are preferably not mechanical, but electronic switches in the form of semiconductor components, in particular transistors. The various components of the damping unit 11 can be arranged on a common substrate and connected to each other by conductors. This common substrate can also be a silicon chip on which all components are integrated using semiconductor technology.

[0035] A second embodiment of a communication system according to the invention shows Fig. 6 In this version, unlike the embodiment of Fig. 2The vehicle 104 is provided with two damping units 111A and 111B with independently adjustable damping and two antennas 103A and 103B. The two damping units 111A and 111B are of the same type as the damping unit 11 of the first embodiment, which was previously described based on the Figures 3 to 5 as described. They are generally set to the same attenuation. The transceiver 110 of the vehicle 104 has two different antenna connections, to each of which an antenna 103A or 103B is connected via an intermediate attenuation unit 111A or 111B, respectively. Accordingly, two control lines 112A and 112B extend from the attenuation controller 113 of the vehicle 104, namely one to each of the adjustable attenuation units 111A and 111B. As in the first embodiment, the transceiver 110 and the attenuation controller 113 are also connected to the control unit 109 of the vehicle 104.

[0036] The second embodiment is intended for long-distance transport systems, which requires that the slotted waveguide 101 consists of at least two different sections 101A and 101B separated by a gap 131, wherein in the Fig. 5 In the example shown, two separate stationary transceivers 107A and 107B are provided, each with its own antenna 106A and 106B, which are each assigned to one of the different sections 101A and 101B of the slot waveguide 101 and are connected to a common control unit 108 of the stationary station 105.

[0037] A gap 131 may also be necessary if, although sufficient communication range is ensured with only a single stationary transceiver 107A and a single antenna 106A, a margin for thermal expansion of sections 101A and 101B is required due to their length. In this case, antennas are arranged at the gap 130 at the opposing ends of the different sections 101A and 101B of the slotted waveguide 101, respectively, and are connected by a flexible conductor. This arrangement of passive elements enables signal transmission between the different sections 101A and 101B of the slotted waveguide 101. The additional transceiver 107B and the additional antenna 106B are omitted in this configuration.

[0038] The two antennas 103A and 103B of vehicle 104 are offset from each other in its direction of travel such that, at a gap 131 between two sections 101 and 101B of the slotted waveguide 101, one antenna 103A always projects into one section 101A and the other antenna 103B into the other section 101B of the slotted waveguide 101. This prevents an interruption of communication when vehicle 104 travels over a gap 131 between two different sections 101A and 101B of the slotted waveguide 101. The attenuators 111A and 111B provided in the two signal paths between each of the antennas 103A and 103B of vehicle 104 and the respective associated connection of the transceiver 110 are identical. The signal paths are independent of each other and can be adjusted to different attenuation values ​​if required.The latter could be of interest if a gap 131 between two different sections 101A and 101B were located at different distances from the two fixed antennas 106A and 106B.

[0039] Since the position of each gap 131 is known, the control unit 109 of the vehicle 104 can, during travel, when approaching a gap 131 (which can be detected, for example, by scanning markings arranged along the travel path using optical sensors), send a control signal to the attenuation controller 113 in a timely manner, if necessary. Upon receiving this control signal, the attenuation controller 113 adjusts the attenuation of the second signal path to a suitable value. Communication can then be switched to the second signal path at the appropriate time. It is understood that the second embodiment of the invention is not limited to use with a slotted waveguide 101 consisting of two sections 101A and 101B, but can be used with slotted waveguides composed of any number of sections.

Claims

1. Communication system for communication between a vehicle (4; 104) travelling along a predetermined path and a fixed station (5; 105), using a slotted waveguide (1; 101) extending parallel to the path of the vehicle (4; 104), in which at least one antenna (6; 106A, 106B) connected to a transmitting and receiving device (7; 107A, 107B) of the fixed station (5; 105) and at least one antenna (3; 103A, 103B) of the vehicle (4; 104), wherein the antenna (3; 103A, 103B) of the vehicle (4; 104) is moved in the longitudinal direction of the slotted waveguide (1; 101) as the vehicle (4; 104) moves, characterised in that between the antenna (3; 103A, 103B) of the vehicle (4; 104) and a transmitting and receiving device (10; 110) of the vehicle (4; 104) an attenuation unit (11; 111A, 111B) with adjustable attenuation is connected, and the attenuation unit (11; 111A, 111B) comprises switching means (17, 18; 20, 21, 22, 23) with which the vehicle's (4; 104) antenna (3; 103A, 103B) can be selectively connected either directly or via at least one intermediate attenuator (14, 15; 24, 25) to the transmitting and receiving device (10; 110) of the vehicle (4; 104).

2. Communication system according to claim 1, characterised in that the attenuation unit (11; 111A, 111B) comprises an analogue or digital interface (19; 28; 30) via which its attenuation can be adjusted by means of a control signal supplied to it at the interface (19; 28; 30).

3. Communication system according to claim 1 or 2, characterised in that the attenuation unit (11; 111A, 111B) is connected to an attenuation controller (13; 113), to which a signal indicating the power of the received signal can be supplied by the transmitting and receiving device (10; 110) of the vehicle (4; 104), and in that the attenuation controller (13; 113) is arranged to adjust the attenuation of the attenuation unit (11; 111A, 111B) as a function of the power of the received signal.

4. Communication system according to any one of claims 1 to 3, characterised in that the attenuation unit (11; 111A, 111B) is connected to an attenuation controller (13; 113), to which a signal indicating the position of the vehicle (4; 104) can be supplied by a control unit (9; 109) of the vehicle (4; 104), and in that the attenuation controller (13; 113) is arranged to adjust the attenuation of the attenuation unit (11; 111A, 111B) as a function of the position of the vehicle (4; 104).

5. Communication system according to any one of claims 1 to 4, characterised in that the switching device (17, 18; 20, 21, 22, 23) can be actuated via an interface (19; 28) of the attenuation unit (11; 111A, 111B) by an attenuation controller (13; 113) of the vehicle (4; 104).

6. Communication system according to any one of claims 1 to 5, characterised in that at least two attenuators (14, 15) with different attenuation levels are provided, and in that the antenna (3; 103A, 103B) can be connected by the switching device (17, 18) of the vehicle (4; 104) either directly or via one of the attenuators (14, 15) to the transmitting and receiving device (10; 110) of the vehicle (4; 104).

7. Communication system according to any one of claims 1 to 5, characterised in that at least two attenuators (24, 25) are provided, and in that the antenna (3; 103A, 103B) can be connected by the switching means (20, 21, 22, 23) to the transmitting and receiving device (10; 110) of the vehicle (4; 104) either directly or via one of the attenuators (24, 25) or via a series connection of several attenuators (24, 25).

8. Communication system according to any one of claims 1 to 7, characterised in that two antennas (103A, 103B) are arranged on the vehicle (104) one behind the other in the direction of travel at a predetermined distance from one another, in that the transmitting and receiving device (110) of the vehicle (104) has two connections for antennas (103A, 103B), and that an attenuation unit (111A, 111B) with adjustable attenuation is connected between each antenna (103A, 103B) of the vehicle (104) and the transmitting and receiving device (110) of the vehicle (104).

9. Communication system according to claim 8, characterised in that the attenuation units (111A, 111B) are connected to an attenuation controller (113) with multiple outputs, by means of which their attenuation levels can be adjusted independently of one another.

10. Communication system according to claim 9, characterised in that a signal indicating the power of the received signals can be supplied to the attenuation controller (13; 113) by the transmitting and receiving device (110), and that the attenuation controller (113) is arranged to adjust the attenuation of the attenuation units (111A, 111B) as a function of the power of the received signals.

11. Communication system according to claim 9 or 10, characterised in that a signal indicating the position of the vehicle (104) can be supplied to the attenuation controller (113) by a control unit (109) of the vehicle (104), and in that the attenuation controller (113) is arranged to adjust the attenuation of the attenuation units (111A, 111B) as a function of the position of the vehicle (104).