The invention relates to a device for the detection of mechanical forces exerted on a rail such as a railway rail, in particular for determining pressure and mass changes on rail systems, such as trains, trams, mobile cranes on rails, etc., as a result of which deformations occur in such rail systems, in particular deflection of a rail.

In practical applications, where a high safety level is required, for example in the case of rail transport with trains and trams, highly reliable sensors for detecting mechanical forces are required for performing signalling functions and other applications, whereby passing rail vehicles, for example, have to be detected in a reliable manner.

Accordingly, it is an object of the present invention to provide a highly reliable device for measuring or detecting mechanical forces, with which the device's own operation can be continuously monitored so as to be able to use it as a so-called"fail safe"safety device.

In order to accomplish that objective the invention provides a device which is characterized by a mechanical detection element coupled to the rail, sensor means comprising at least two elements which are in mechanical contact with each other, the elements each comprising terminal electrodes, wherein one element is of a first type, arranged for exerting a mechanical force under the influence of an electric signal applied to its terminal electrodes, and wherein another element is of a second type arranged for providing an electric signal at its terminal electrodes under the influence of a mechanical force applied to the element, wherein the detection element and the sensor means are in engagement with each other, in such a manner that mechanical forces exerted on the rail during use are transferred to the sensor means by the detection element, wherein an alternating and/or pulsating electric signal is continuously applied to the terminal electrodes of an element of the first type, and wherein the electric signal provided at the terminal electrodes of an element of the second type is a measure for the mechanical forces exerted on the rail.

The device according to the invention may in fact be characterized as regards the sensor means that are used by a"driving part" and a"detecting part", wherein a sensor element of the above first type

is associated with the driving part, and a sensor element of the above second type is associated with the detecting part.

In use, in accordance with the invention, the sensor means are controlled in such a manner that an alternating and/or pulsating electric signal (voltage or current) is continuously applied to the terminal electrodes of an element of the first type (that is, the driving part), and that the electric signal provided at the terminal electrodes of an element of the second type (that is, the detecting part) is used for electrically processing external mechanical forces which are being exerted on the mechanical detection element.

In use, at least the detecting part of the device is subjected to a continuous, internal, alternating mechanical force, which is caused by the driving part, as well as to an external mechanical force, which is to be detected or sensed by the mechanical detection element.

In the case of sensor elements of identical construction, for example, the electric signal provided by the detecting part will be practically identical as regards its shape to the electric signal which is applied to the driving part. External mechanical forces which are exerted on the mechanical detection element will cause a change or modulation of the electric signal delivered at the terminal electrodes of the detecting part. The change is a measure for the external mechanical force that is exerted on the sensor via the mechanical detection element.

The"self-monitoring"properties of the device according to the invention consist hereof that the detecting part provides an electric signal, even if no external force is exerted on the mechanical detection element, under the influence of the internal mechanical force which is continuously exerted thereon by the driving part. By monitoring the electric signal which is thus delivered by the detecting part, the correct and reliable operation of the combination of elements can be checked. The mechanical construction of the device is thereby such that the so-called"fail safe"properties are ensured.

The absence of an electric signal on the terminal electrodes of an element of the second type, that is, the detecting part, is a direct indication of incorrect operation of the sensor, which may for example be caused by the absence of a signal on the terminal electrodes of the driving part, a failure of the sensor itself, for example caused by fracture of the sensor, or a failure in the connections and the

processing circuits. In particular this continuous self-monitoring operation makes the device according to the invention highly suitable for applications wherein a high degree of reliability and safety is required, as is the case in the aforesaid rail systems.

In a first embodiment of the device according to the invention, the detection element is bar-shaped, and extends substantially parallel to the rail, wherein the ends of the detection element and the sensor means are fixedly coupled to the rail, in such a manner that a difference in the degree of deflection of the rail and the bar-shaped detection element caused by mechanical forces exerted on the rail results in a detectable force exerted on the sensor means.

This embodiment of the device according to the invention makes it possible to detect, in a reliable manner, the force which is exerted on the mechanical detection element as a result of deflection of the rail caused by the wheel load from a rail vehicle whereby, in another embodiment of the invention, the sensor means are provided between the bearing surface of the rail and the bar-shaped detection element.

In a second embodiment of the device according to the invention, which also uses a bar-shaped mechanical detection element which extends substantially parallel to the rail and which is fixedly coupled to the rail with its ends, adjusting means coupled to the rail and engaging the bar-shaped element are provided between the bearing surface of the rail and the bar-shaped element, and wherein the sensor means are fixedly coupled to the bar-shaped detection element.

Also in this case it applies that the different degree of deflection of the rail and the elongated, bar-shaped detecting element caused by the wheel load from a rail vehicle will result in a detectable force exerted on the sensor means. The adjusting means are preferably arranged for exerting an adjustable mechanical bias force on the detection element, by means of which the detection characteristics of the device according to the invention can be optimally adjusted and be optimally adapted to the specific properties of the construction in which it is used.

In a third embodiment of the device according to the invention, the detection element is an elastic bearing plate provided between the rail and a mounting bracket thereof, and the sensor means are mounted in or on the mounting bracket, in such a manner that mechanical forces which are exerted on the rail in use will result in a detectable

force being exerted on the sensor means via the bearing plate.

In a fourth embodiment of the device according to the invention, the detection element is bar-shaped, and is arranged between the bearing surface, or head, and the lower flange of the rail, and the sensor means are connected to the bar-shaped detection element in such a manner that deflection of the rail under the influence of mechanical forces exerted thereon during use will result in a detectable force exerted on the sensor means.

In a preferred embodiment of the device according to the invention, the sensor elements of the first and the second type are of a piezo-electric material. The advantage of using piezo-electric material is the absence of moving mechanical parts, which has a positive effect on the reliability of the device.

It is noted that a sensor of this type is known per se from US patent No. 4,546,658. However, the use thereof in a device for measuring mechanical forces exerted on a rail with a continuous self- monitoring function as regards its correct operation is not disclosed by this US patent.

The invention is not limited to the use of sensor elements of a piezo-electric material, however, in accordance with another embodiment it may also comprise sensor elements of the first type, which are capable of exerting a mechanical force by electrodynamic means, in particular elements which are based on the exertion of electromagnetic forces and comprising at least one electric conductor and at least one element of a magnetic material, which are movably disposed with respect to each other. It is also possible to use capacitive elements or other types of electrodynamic elements as sensor means in the device according to the invention.

In yet another embodiment of the device according to the invention, sensor elements of the second type are used, which are capable of generating a voltage or a current by electromagnetic means, in particular elements comprising at least one electric conductor and at least one element of a magnetic material, such that movement of the conductor and the element with respect to each other will generate an electric voltage or current in the conductor. Also in this case it applies that elements causing an electric signal by capacitive means or otherwise under the influence of a mechanical force being exerted thereon, may be

used as a detection element of the sensor means in the device according to the invention.

In accordance with the invention, various types of sensor elements of the first and the second type can be mechanically coupled, whereby both electrodynamically and electromagnetically operating elements may be coupled to an element of a piezo-electric material, or whereby pairs of for example piezo-electric elements and electrodyna- mically/electromagnetically operating elements can be combined to form a single sensor. In particular a piezo-ceramic element as the driving part with a permanent magnet being (for example fixedly) coupled thereto, wich is freely positioned in a coil, whereby the permanent magnet and the coil constitute the detecting part of the sensor means of the device. This, to provide sensors which are optimally suited for a specific application.

The i nventi on al so rel ates to a sensor whi ch i s arranged for use in a device as described above.

The invention will be explained in more detail hereafter with reference to embodiments of the sensor means and the mechanical detection elements according to the invention as shown in the figures, wherein: Figure 1 schematically shows a preferred embodiment of a sensor intended for use in the device according to the invention which is built up of piezo-electric elements.

Figure 2 schematically shows another embodiment of a sensor intended for use in the device according to the invention, which is based on electrodynamically/electromagnetically operating elements.

Figures 3,4,5 and 6 show schematic embodiments of devices according to the invention for measuring mechanical forces, which are intended for use in rail systems.

Figure 1 shows, partially in elevation and partially in sectional view, an embodiment of a sensor 1 for use in the device according to the invention, wherein the individual constituting parts are shown in a spaced-apart relationship to provide a better understanding of the invention.

Sensor 1 comprises a housing 2, in which a first element 3 of a piezo-electric material and a second element 4 of a piezo-electric material are accommodated. The two elements 3,4 are disposed adjacently to each other in their contraction and expansion direction. In the figure

it is assumed that the contraction and expansion of the two elements takes place in vertical direction, seen in the plane of the drawing. That is, in the direction of the longitudinal axis 16, transversely to the planes of the elements 3,4, which may be disc-shaped, for example.

Terminal electrodes 5-8 connect to the elements 3, 4 on either side thereof, whereby the terminal electrodes 5,6 are in electrical contact with the opposite surfaces of the first element 3, and the terminal electrodes 7,8 are in electrical contact with the respective opposite surfaces of the second element 4 in the assembled condition of the sensor. An insulator 9 is present between the electrodes 6,7, whilst an insulator 10 engages the terminal electrode 5 and an insulator 11 engages the terminal electrode 8. The housing 2 is free from the terminal electrodes 5-8 and the piezo-elements 3,4.

The insulators 9,10,11 preferably have a shape which is adapted to the elements 3,4, for example a disc shape, as shown in the figure. The insulators 9,10,11 are made of a material having a certain E-modulus, whereby the various insulators may have different E- moduli. The terminal electrodes 5-8 are preferably configured to be as thin as possible, but they may also have a specified thickness and a specified E-modulus, if desired, and they may provide a desired capacitive action. This, in order to enable an optimum transfer of the external mechanical forces exerted on the insulators 10,11 to the elements 3,4.

The terminal electrodes 5-8 are provided with contact terminals 12- 15, respectively.

In the assembled state, the first and the second elements 3,4 are mechanically in contact with each other under the influence of a specified mechanical bias force Fl, which may for example be provided by means of a housing or covering 2 which closely surrounds the stack of elements 3-11.

It is possible to omit the terminal electrode 8 and the insulator 11 in certain applications, whereby the element 4 contacts directly the housing/covering 2, which is provided with a contact terminal 15, which may be used as an earthing point, for example.

The operation of the sensor 1 is as follows.

In use, an essentially alternating and/or pulsating electric voltage of a specified frequency, which is the base signal for sensor 1, is applied to the contact terminals 12,13 of the terminal

electrodes 5,6, which are in electrical contact with the first element 3.

The base signal will cause the first element 3 of piezo-electric material, for example piezo-ceramic material, to undergo an elastic contraction or expansion, at a frequency which is essentially identical to the frequency of the base signal, which is inherent to the physical properties of the piezo-electric material. In the illustrated embodiment, the contraction and expansion of the first element 3 takes place along the longitudinal axis 16 of the sensor 1.

As a result of the expansion or contraction of the first element 3, a mechanical contraction or expansion force is exerted on the second element 4, which is in mechanical contact with the first element 3, and which results in a potential difference at the contact terminals 14,15 of the terminal electrodes 7,8, which are in electrical contact with the second element 4, and is inherent to the physical properties of the piezo-electric material used. All this as symbolically indicated by +/-signs.

In principle, the frequency of the electric response signal which is delivered at the terminals 14,15 will be identical to that of the base signal which is applied to the terminals 12,13. The amplitude of the response signal depends, among others, on the amplitude of the base signal, the piezo-electric materials which are used and the dimensions of the elements 3,4. Preferably, the same materials and dimensions are selected for the elements 3,4.

In use, the first element 3 thus exerts a mechanical driving force on the second element 4, so that the first element 3 forms the"driving part"of the sensor or the device.

If an external force F2 is exerted on the sensor 1 in the direction of the longitudinal axis 16, this will cause an additional contraction or expansion of the assembly, which is directly expressed in a change in the response signal. The change in the response signal may be processed as an amplitude variation or as a frequency variation, depending on the speed at which the force F2 varies within a specified interval. In the latter case, the frequency at which the force F2 varies will be superposed on the frequency of the base signal. In the case of a non-periodically varying force F2, the additional contraction or expansion which is caused thereby will manifest itself as a superposed amplitude on the amplitude caused by the base signal. All this inline with

the concepts of frequency modulation, phase modulation and amplitude modulation, which are known in the field of electrical engineering.

Because of the fact that an external force F2 manifests itself in the form of a change in the response signal of the second element 4, this element is also referred to as the"detecting part"of the sensor 1 or the device according to the invention.

In the situation wherein no external forces are being exerted on the sensor 1, the detecting part will continuously provide an electric response signal at its contact terminals 14,15 for as long as a base signal is being applied to the contact terminals 12,13. The absence of this response signal indicates a failure in the sensor and/or in the base signal being applied and/or in the connecting lines to the contact terminals 14,15, which provides an indication that the sensor 1 does not operate in a reliable manner. Thus, a"continuous self-monitoring"action for measuring mechanical forces is provided, as is required for applications in rail transport, wherein a high degree of reliability is required.

Those skilled in the art will appreciate that the driving part and the detecting part of the sensor 1 may be built up of different elements of piezo-electric material, whereby not only the driving part and the detecting part may be arranged adjacently to each other, but whereby also pairs of detecting parts and pairs of driving parts may be arranged adjacently to each other, for example. In other words, another first element 3 or another second element 4 may be arranged in a mechanically adjacent relationship between the first element 3 and the second element 4. This to provide a sensor which is optimally adapted for detecting mechanical forces in a specific application. The electrodes 6, 7 of the sensor 1 may also be provided in the form of a common electrode, such that the central insulator 9 may be omitted.

Those skilled in the art will appreciate that several modifications and additions are possible without departing from the basic "fail safe"construction and the operation of the device according to the invention.

Figure 2 shows, partially in elevation and partially in sectional view, an alternative embodiment of a sensor 20 for use in a device according to the invention, which comprises a housing or covering 21, in which a driving part 22 and a detecting part 23 are accommodated.

The driving part 22 consists of an electric conductor 24 in the shape of a coil comprising contact terminals 25 and 26, which coil 24 is wound round a yoke 27 of a magnetic material, having a bore 28. An element 29 of a magnetic material, for example a cylindrical element of a permanent magnetic material, is movably disposed within the bore 28.

In the illustrated embodiment, the detecting part 23 is of a similar construction as the driving part 22, that is, it consists of a conductor 30 in the shape of a coil comprising contact terminals 31, 32, which coil is wound round a yoke 33 of magnetic material, which is provided with a bore 34, in which a cylindrical core 35 of a permanent magnetic material is movably disposed.

The core 29 of the driving part 22 is fixedly coupled to the housing 21 with one end, at 36, and is connected at its other end, via a spring 37, to the core 35 of the detecting part 23, whose other end is fixedly connected to the housing 21 at 38.

The driving part 22 operates according to the electrodynamic principle, that is, core 29 is caused to move in its longitudinal direction by applying the base signal to the contact terminals 25,26. This movement is transmitted to the core 35 of the detecting part 23 via the spring 37, as a result of which an electric voltage is generated in the coil 30, which voltage is available on the contact terminals 31, 32. The detecting part 23 operates according to the known electromagnetic principle.

Comparible to sensor 1, the application of a base signal at the contact terminals 25,26 will result in a response signal at the contact terminals 31,32. If an external mechanical force is exerted on housing 21 in longitudinal direction of the cores 29,35, this will result in a changed force being exerted on the spring 37, and consequently to a deviation in the response signal at the contact terminals 31 and 32.

Those skilled in the art will appreciate that the driving part 22 which is shown in Figure 2 may be configured in various ways, for example comprising more than one coil 24, or a differently shaped magnetic yoke, or a core 29 wound with wire, etc. The same applies to the detecting part 23. Instead of using an electromagnetic detecting part 23, it is also possible, for example, to use a detecting part of a construction which is known per se in practice and whose operation is based on capacitance changes. Furthermore, it is feasible to form combinations of

elements of a piezo-electric material and electrodynamically/- electromagnetically operative elements (not shown) in order to obtain an intended sensor operation.

In particular a piezo element as the driving part, to which a permanent magnet is fixedly connected, which permanent magnet, forming the detecting part, is positioned in a coil in such a manner as to be freely movable therein. The movement of the permanent magnet with respect to the coil caused by the contractions and expansions of the driving piezo element generates signals, which can be further processed to provide an indication of a force which is being exerted. Conversely, the coil may also be connected to the piezo element, and the permanent magnet may be fixedly disposed.

Figure 3 shows a device 40 according to a first embodiment of the invention, which is in particular suitable for detecting mechanical forces that are exerted on a rail 17, which is supported on sleepers 18 and 19.

In the illustrated embodiment, the device 40 according to the invention comprises a mechanical detection element 41 having the shape of a bar, which extends in the longitudinal direction of the rail 17 between its bearing surface or head 46 and its lower flange 47. The bar 41 is mechanically coupled, in a rigid manner, to the rail 17 at its ends 42,43. All this as schematically illustrated in a usual manner by short sloping lines. The bar 41 mechanically acts on a sensor 45, which is built up in the above-described manner, and which is mechanically coupled to the rail 17 in a rigid manner between the bearing surface 46 thereof and the bar 41.

In use, the bar 41 and the rail 17 will undergo different degrees of deflection, for example when a train, a tram or a construction crane moves over the rail 17, which results in a detectable force being exerted on the sensor 45.

In a second embodiment of the device according to the invention, which is schematically shown in Figure 4, the sensor 45 is mounted in or on the bar 41. Adjusting means 48 are fixedly connected to the rail 17, and are in engagement with the bar 41. Also in this embodiment different degrees of deflection of the rail 17 and the bar 41 will result in a detectable force being exerted on the sensor 45 via the adjusting means 48.

A biassing force that may be required can be exerted on the sensor 45 through the bar 41 via the adjusting means 48, for example an adjustment bolt.

A third embodiment of the device is shown in Figure 5, wherein the sensor 5 is built into a rail support 49, and whereby an elastic bearing plate 50 is provided the between lower flange 47 of the rail 17 and the rail support 49. The bearing plate operates as the mechanical detection element of the device according to the invention.

The compression of the elastic bearing plate 50 by a passing train, a tram or a mobile crane moving over the rail 17, for example, will result in a detectable force being exerted on the sensor 45.

A fourth embodiment of the device according to the invention, which is shown in Figure 6,1 i kewi se uses a mechanical detection element taking the shape of a bar 51, which is mounted between the bearing surface or head 46 and the lower flange 47 of the rail, wherein the sensor 45 is mounted in or on the bar-shaped detection element 51. The sensor 45 is mounted in such a manner that the bending force exerted by a wheel 52 of a rail vehicle moving over the rail 17, as schematically indicated by arrow 53, will result in a detectable force exerted on the sensor 45.

The mechanical properties of the bar-shaped detection element can be selected in accordance with the bending forces that are to be expected.

Self-resonance and self-vibration occurring in the device according to the invention, which in fact do not form part of the signal to be measured, can be effectively compensated by using existing techniques. The detection element 41 may have any suitable form adapted to a specific application.

In summary, the invention provides a"fail safe"device, which is capable of monitoring itself for proper operation, which is intended for use in applications wherein a high degree of reliability and safety is required.