BACKGROUND Field of the invention The present invention relates to a semi-active dynamic damping device for means of transport, in particular to an electro-hydraulic device for high-speed trains.

As known, a damping device, also called"damper", is generally a mechanism which allows the damping or braking of the motion of a mass to which the same is bound.

Dampers can be of a passive, active or semi-active type. A passive damper is a mechanic/hydraulic device designed to follow a fixed behaviour law, i. e. which reacts to external stresses in a predefined manner according to features and tolerances univocally established a priori ; the damping force provided by a passive damper depends exclusively by the modulus and direction of the relative speed at which the two attachment points of the above damper are displaced.

A semi-active damper instead is capable of reacting in a differentiated manner (hence no longer exclusively according to the modulus and direction of the relative speed of the attachment points), i. e. according to different laws depending on the desired external parameters; in particular, the damper follows different reaction laws according to the specific working conditions, passing from a characteristic curve to the other in a continuous or modular manner within the required range, to optimise performances. Commutation between one characteristic and the other occurs on the basis of environmental conditions, which are detected through a series of sensors or mechanical members specifically

designed for the purpose and which are used by electronic central control units instantly determining the optimal damping curve to be set.

A device of this type becomes active when a source of (hydraulic, electric or pneumatic) power is also provided, which hence enables the damper to react"actively"as said, generating reacting forces to the applied external forces.

The invention originates in the field of semi-active dampers and hence, in the following, reference will be made exclusively to this type of device.

The main feature of a semi-active damper is the presence of servo-assisted valves which allow to control the damping characteristics through an activation electric input signal, supplied and set on the basis of a certain control strategy algorithm which is capable of identifying the work conditions and of correspondingly choosing the optimal damping to maximise comfort.

A field in which these devices have a specific application, which will be dealt with in the following description, without it being considered limiting, is that of high-speed trains and, in particular, the damping of lateral motions of railway vehicles.

It is understood that this device can be adopted also on other systems which currently mount a passive damper, where it is desired to improve comfort and safety performances.

It is known that-in order to guarantee adequate levels of ride comfort for passengers-special attention is devoted to the controlled damping of car body movements of high-speed trains of the"Pendolino"type. These trains, in fact, are designed so that a rotational tilt of the car body, around a rotation centre, is possible and may be

controlled through specific electro-hydraulic systems (which will not be dealt with in the present description), to allow the traveling along curves at higher speeds and with higher levels of comfort compared to what is normally allowed by conventional railway trains.

The task of the lateral damping system is to provide a lateral damping reaction to the car body of the railway vehicle, according to the relative lateral speed between the car body and the bogie.

In order to provide an adequate level of comfort and efficiency of the system, as mentioned above, it is intended to use a semi-active damper, wherein the ratio between the force and the relative lateral speed may be varied according to the train speed and to other defined external conditions, for example the condition of the railway line, the type of railway layout, etc. The lateral damping system is therefore to be constructed so as to be able to vary its behaviour according to the detected external conditions, maintaining good journey quality across the whole range of travelling speeds.

The requirements that this device is to meet are numerous, even more so since it is applicable in a critical industry, where it is necessary to guarantee a high level of operational safety together with a perfect behaviour optimisation for greatest comfort.

First of all, the operation of the servo-assisted valve is to guarantee operation controlled according to the working conditions. In particular, the control of the valve is to occur effectively to adapt the operation characteristic (force/speed curve) of the damper to the train running dinamic conditions. This adjustment can be continuous across the whole range of use or discrete for increases which are spaced apart from one another as

required by working conditions. Transition from one characteristic to the other can occur discretely, normally providing a maximum damping characteristic and a minimum damping characteristic, and successively defining a series of intermediate characteristics. The choice of the various features is defined by the electronic unit of the train and then implemented by the control unit of the semi-active damper to guarantee the desired adjustment of the damping.

Fig. 1 shows four exemplary curves which may be obtained with a semi-active device, wherein it is possible to switch between one another with the intervention of the damper control unit upon request from the train electronic unit. This diagram represents the simplest case of discrete control which may be carried out by the semi-active device.

A passive damper is capable of obtaining only one predefined damping characteristic, as said previously; the semi-active damper represented here instead distinguishes itself for its ability to produce a number of damping characteristics which may be defined according to external conditions and which may be verified during use and hence to guarantee the reliability of the required damping value for any operation condition.

On the other hand, currently available semi-active dampers have damping characteristics which may not be verified during use and hence they cannot guarantee the reliability of the required damping. In other words, although the various characteristics required to the device are defined a priori, so far it has not been possible to guarantee that the device, in any operation condition (for example for any control history sequence), follows a repeatable behaviour and most of all a behaviour strictly adhering to the required actual characteristic.

It is hence an object of the present invention to

provide a semi-active damping device which is particularly effective in any condition, to ensure the maximum level of ride comfort and safety when applied to a means of transport, in particular a railway vehicle.

A further object is to provide a semi-active damping device which also allows safe operation according to any predetermined characteristic in any control or electrical failure condition (fail-safe).

SUMMARY OF THE INVENTION Such objects are achieved through the device and the method described in their essential features in the attached main claims.

Other inventive aspects of the device are described in the dependent claims.

According to the invention, in particular, the feedback circuit controlling the servovalve uses as adjustment parameter the force applied to the damper piston, detected through a load cell. The Applicant in fact was able to establish that this system is the only one which allows to keep under control, in the dynamic sense of the word, the desired damping.

The novel idea is indeed to employ, in the semi-active device, force sensors in cooperation with speed sensors continuously reading and monitoring force and speed valves, so that the control unit may use these two parameters to continuously and accurately adjust to the desired force/speed characteristic, obtaining a plurality of damping actions which are really the ones demanded by design requirements (and not vaguely similar to the ones required, as is the case in currently available semi-active systems).

According to a further aspect of the invention, the servovalve of the device is designed so that, in case of

electric/electronic fault of the control system, it automatically configures itself so as to guarantee connection of the chambers of the damper with a mechanic damping device (fail-safe valve) which hence restores the fixed operation conditions of a conventional passive damper.

BRIEF DESCRIPTION OF THE DRAWINGS Further features and advantages of the device according to the invention will in any case be more evident from the following detailed description of a preferred embodiment thereof, given by way of example and illustrated in the accompanying drawings, wherein: fig. 1 is a diagram showing an example of four damping characteristics of a semi-active damper; fig. 2 is a diagrammatical elevation front view of an exemplary railway bogie showing a possible application of the device according to the invention; fig. 3 is a diagrammatical view of a device according to the invention; and fig. 4 is a diagrammatical view showing the structure and the functioning of the device according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION The semi-active damper subject of the invention, in the specific application which is going to be dealt with as an example, may be arranged horizontally in the upper part of a railway bogie (fig. 2). The damper D is mounted connected at the ends thereof, through suitable bushes, on one side to the bogie and on the other side to a structure of the swinging car body of the train. In particular, the damper belongs to the lateral suspension which is necessarily used in these high-speed trains to improve lateral ride comfort at high speed.

As can be seen in fig. 3, the semi-active damper of the invention consists of the following elements.

- A double-acting cylinder/piston assembly to allow functionality in both directions, filled with hydraulic oil and within which flows a piston P which separates two chambers 1 and 2; - Two heads 3 and 4 for connection to the train bogie and car body, respectively; - A servovalve assembly for control of the damping in normal conditions of use, with a special arrangement for operation of the semi-active damper in fail safe' conditions. In other words, the flow of hydraulic oil between the two chambers 1 and 2 is controlled by the servovalve assembly 8 which determines, through the displacement of a spool 10, damping control in normal conditions of use and at the same time is capable of diverting the hydraulic flow towards a secondary circuit wherein a mechanic assembly 7 is provided for damper operation in fail-safe conditions.

- A speed sensor 5 and a force sensor 6 (such as a load cell) for detection of the parameters essential for device control and operation. The signal detected by these two sensors is transmitted to the electronic control unit which uses it for servovalve control: in this way, the control unit is able to control servovalve operation according to the desired force/speed characteristic and to use, through a feedback loop, the detected force and speed parameters on the damper to optimise its intervention.

- A mechanic assembly, in order to guarantee adequate operation of the semi-active damper even in case of a system fault 7. Such assembly operates in case of lack of power, or fault of an electronic component of the system, sensors, control units, etc.

- An electronic control unit which determines the operation of the servoassisted valve 8 according to the signal coming from an electronic central unit of the train: in particular, according to some external parameters (train speed, condition of the railway line, and so on), the electronic central unit requires from the control unit of the semi-active damper the imposition of a certain characteristic curve.

Fig. 4 schematically shows the connections between the double-acting cylinder of the damper, the servovalve and the fail-safe unit 10., The operation of the device of the invention can be illustrated with reference to such drawing. a) Conditions of normal use: during normal operation, the chambers 1 and 2 of the damper are connected with the ports P and A of the servovalve: the passageway of the hydraulic fluid between these two ports is adjusted by the spool 10 of the servovalve, whose position is in turn determined by the electronic control unit which establishes an activation electric command value to be sent to a motor M shifting the spool 10 according to the required damping.

As already described, the values of actual damping are detected instant by instant by sensors applied to the inside of the damper (speed sensor and force sensor) and fed back into the control loop: the control unit is hence capable of instantly and automatically adjusting the electric command value value (output) to be sent to the servovalve in order to obtain the required damping by comparing the detected values with the desired values.

In these conditions the port B of the servovalve is always closed, hence the circuit which connects it to the fail- safe valve is not active. b) Conditions of use in fail-safe mode: in case of

electric or electronic fault, the servovalve is no longer powered and the inner spool 10, through the action of a spring 11 acting at one end of the same, automatically moves into a condition wherein the port A is closed and the port B is fully opened, providing communication between ports P and B.

The system therefore continues to operate through the mechanic fail-safe valve since the chambers 1 and 2 of the damper are connected with each other through the secondary circuit for the mechanic adjustment of the damping, or fail-safe valve, 12.

In the mechanic component 12, the hydraulic circuit is constructed so as to ensure fixed operation according to a pre-established design-characteristic curve: in this mode the device of the invention behaves as a passive-type damper.

The servoassisted valve according to the invention is designed to be controlled so as to accomplish the wanted operation in normal conditions, and it is capable of bringing the system into the fail-safe condition-i. e. equivalent to that of a passive system having a single working characteristic-in case of lack of power.

Correspondingly, the hydraulic/mechanical part affected by the secondary circuit, as already mentioned, is designed to obtain the desired type of operation required when the damping electronic control system is out of order (purely passive damper).

The servoassisted valve and fail-safe valve assembly is completely external to the damper and may be removed, which allows fast development and easy servicing, guaranteeing at the same time the same functions of a passive mechanical damper currently manufactured and employed.

The complex system within which the device of the invention is integrated provides a general operation reported in the following.

According to the conditions (detectable through external sensors already provided aboard the train, such as <BR> <BR> accelerometers, etc. ) instantly faced by the means of transport and to the type of operation that one wants to correspondingly obtain, the general electronic central unit of the high-speed train sends signals to the control unit of the semi-active damper in order to obtain specific damping conditions. In other words, the general electronic central unit defines the input'value of the signal to be provided to the control unit of the damper which controls the servoassisted valve of the damper and, according to the above mentioned parameters, guarantees the required damping (for example curve int. 2 in fig. 2).

The control central unit of the damper detects instant by instant the functional parameters which are essential for correct device operation, such as: - instant speed of the damper piston, through the speed sensor 5 integrated in the damper; - force applied to the piston, through the load cell 6 arranged in the proximity of the attachment 3.

These parameters are fed back to the control unit of the damper in order to accurately and consistently obtain the desired value on the characteristic curve for successive correction iterations.

As already described in detail above, in case of electric or electronic fault, upstream of the device, the mechanical valve assembly 12 automatically intervenes to guarantee system operation even in these conditions: the mechanical assembly is sized and constructed so as to reach the characteristic curve predefined for the fail-safe

condition.

It is understood, however, that the invention is not limited to the specific arrangement illustrated above, which represents only a non-limiting example of the scope of the invention, but that a number of variants are possible, all within the reach of a skilled person in the field, without departing from the scope of the invention.