In many railroad accidents caused by derailment there occurs at first a partial derailment of an axle or a bogie, sometimes of a single wheel of a railway vehicle. In these cases, usually an axle is displaced in such manner that one of its wheels repeatedly hits the sleepers or the bolts fastening the rail to the sleepers, or the ballast. In other, less frequent cases, it is possible that both wheels of a same axle are lifted up or displaced such that they are not subjected to impacts. In both cases it has been observed that the train may travel in such partial or incipient derailment conditions for many kilometres before a more severe damage or an incident occurs. During this period, neither the motorman nor other personnel on board of the train notice or are warned of the failure and are therefore not aware of the impending danger.

It is an object of the present invention to detect in time a derailment condition of at least one of the wheels of a railway vehicle and generate, in such an event, an alarm signal in real time, so as to be able to stop the train immediately to avoid more serious damage and intervene in good time on the damage that has occurred.

The above and other object and advantages are attained, according to a first aspect of the present invention, by a method as defined in claim 1. According to another aspect of the invention, there is provided an apparatus as defined in claim 7.

The features and advantages of the invention will be apparent from the ensuing detailed description of a few embodiments thereof, given by way of not-limiting example, reference being made to the attached drawings, in which: -figure 1 is a schematic view of a railway vehicle wheel to which there is associated an apparatus according to the invention; -figure 2 is a diagram showing in the time domain a vibration signal detected in normal travelling conditions of a railway vehicle; -figure 3 is a diagram showing in the time domain a surveillance signal obtained by processing the signal of figure 2; -figure 3A is a diagram showing in the time domain a surveillance signal obtained by processing the signal of figure 2 in accordance with an alternative embodiment of the invention; -figure 4 is a diagram showing in the time domain a vibration signal detected in derailment conditions of a wheel or an axle of a railway vehicle; and -figure 5 is a diagram showing in the time domain a surveillance signal obtained by processing the signal of figure 4.

Referring initially to figure 1, numeral 1 schematically designates the wheel of a railway vehicle travelling on a railtrack 2. A vibration sensor 3 is mounted on a stationary support element 4, secured to or integral with an element supporting the wheel axle.

The elements shown in figure 1 are generally known to those skilled in the art and will not therefore be described in further detail in the ensuing description. For example, the stationary support element 4 may be the axle box housing or an outer cover of the axle box, or the outer stationary race of the axle bearing, or a sealing shield mounted on the outer race.

Preferably, the vibration sensor 3 includes a piezoelectric accelerometer of known kind associated with an amplifier incorporated in the same body which also contains the accelerometer.

For an advantageous application of the invention it is important that all the axles of the train, and preferably all the wheels of the train, are provided with a vibration sensor 3.

Each vibration sensor is connected to an electronic processing unit, schematically designated E, mounted on board of the train. The unit E receives the vibration signal V generated by the vibration sensor 3 and processed this signal as described herein after for recognising and signalling a condition of derailment.

The processing unit E is provided with or connected to circuit means schematically designated C. The circuit means C process the vibration signal and generate a surveillance signal the instant value of which is compared with a reference threshold value indicative of a derailment condition, as explained herein after.

In normal operation conditions, i. e. in the absence of derailment as shown in figure 2, where the ordinate axis reports the vertical accelerations undergone by the wheel in a certain period of time, the processing unit receives an impulsive vibration signal V in which appear a series of impulses I1 of small amplitude caused for example by the passage of the train over points, and an isolated impulse Is of great amplitude, provoked for example by the passage of the wheel over a rigid relief on the rail.

The vibration signal V received by the processing unit is processed by the circuit means C associated with the processing unit E, resulting in the surveillance signal S shown in figure 3.

By means of the circuit means C, the instant value of the surveillance signal S is increased in a manner proportional to or anyway correlated with the amplitude of each vibration impulse and decreased according to a predetermined time law after each vibration impulse. Figures 3 and 5 show the surveillance signal generated by analog circuit means C which prolong or slow down in time the trailing edge of the impulses of the vibration signal according to a predetermined law. As shown in figure 3, in the surveillance signal S the trailing edge I1'd, I2'd of the impulses I1', 1 121 corresponding to the impulses Il and Is of the vibration signal V of figure 2.

Circuit means C of the above mentioned kind are known in the art and do not need to be described in detail herein. Suffice it to say here that the circuit means C may indifferently be of the hardware or software kind or a combination thereof.

For example they may comprise a capacitive circuit capable of producing the above cited slowing-down effect. Alternatively, the circuit means C may be digital and comprise a counter adapted for counting up or down in accordance with functions equivalent to those described in the following.

In the absence of high impulses of the kind of Is repeated closely in time, the processing unit effects no additional processing. The impulses I1'and I2'of the surveillance signal S fade out. and no alarm signal-is generated, as the processing unit recognises that an isolated impulse, however high, is not indicative of a derailment condition but simply of an isolated phenomenon not due to defective operation of the wheels but to a foreign cause, for example due to a discontinuity or relief on the rail.

In a particular embodiment of the invention, the processing unit E may be provided with filtering means F1 for preventing the surveillance signal S from being affected by small vibration impulses I1 having an amplitude lower than a reference threshold value R and which are not indicative of a derailment condition. The filtering means F1 may indifferently be of hardware or software kind, i. e. in the form of a device or a program associated with or incorporated within the processing unit E for separating data which are irrelevant for the analysis from the signal coming from the vibration sensor. According to such embodiment, the surveillance signal S takes the form shown in figure 3A, i. e. free of small amplitude impulses.

In derailment conditions, at least one of the wheels is out of the rail a repeatedly hits the ballast or the sleepers or the bolts fastening the rail to the sleepers. In such an event, the vibration signal V generated by the accelerometer 3 is of the kind shown in figure 4, having a close and repeated succession of high amplitude impulses I2 for a prolonged period of time. A typical acceleration value that can be detected in derailment conditions is to the order of several tenths or a few hundreds of g (gravity acceleration).

Upon receiving a vibration signal V of the kind shown in figure 4, the circuit means C correlate the high intensity impulses and provide a surveillance signal S of the kind shown in figure 5, the instant value of which is increased in stepwise manner.

When the value of the signal S reaches or exceeds a reference threshold value T, the processing unit E automatically generates an alarm signal A which warns the personnel on board of the train that a derailment is occurring.

As an alternative or in addition to the above discussed filtering means F1 for filtering small amplitude impulses, the processing unit E may be coupled with filtering means F2 which filter the vibration signal V in such manner as to prevent the surveillance signal S from being affected by vibration frequencies out of a range which is meaningful for the analysis. The meaningful frequencies, generated by the impacts of a derailed wheel, are comprised between zero and a few hundred Hertz. In order to cut out vibration components due to other causes, for example resonance frequencies provoked by the suspension springs, the filtering means F2 cut out of the surveillance signal S those vibrations having frequencies exceeding some hundreds of hertz, for example exceeding 500 Hz.

In case the filtering means F2 are of the software type, the can be self-programmable for instantaneously and automatically varying the limits of the range of frequencies to be cut out as a function of the travelling speed of the train. To this end, the processing unit E is set to receive a signal V indicative of the instant speed of the train provided for example by one or more speed sensors mounted at the wheels. Based on the instant value of the speed signal V, the unit E calculates the limits of the frequencies to be cut out, by varying at least the upper limit in a manner proportional to or anyway increasing with the speed.

In a particular embodiment of the invention, the alarm signal can be supplied to a further control-unit (schematically designated B in figure 1) to provoke automatic activation of the brakes and stop the train immediately.

In accordance with a particularly preferred embodiment of the invention, the control unit B which provokes automatically the activation of the brakes is set to activate the brakes in different modes, i. e. more or less sharply, as a function of the number of wheels for which an alarm signal has been generated. The braking may be effected sharply if the alarm signal concerns a single wheel or axle, whereas the train may be slowed down more gradually if the alarm signal relates to all the wheels of a bogie or a carriage, to avoid the risk that a sharp braking breaks the train in two, especially where the train has a rear locomotive.