CROSSWIND STABILISATION METHOD AND ASSOCIATED RAIL VEHICLE ., .. . . .. . . . ... . . " , . . . . , . . .. :. .. . . . TECHNICAL FIELD OF THE INVENTION [0001] This invention relates to the crosswind stability of a rail vehicle, in 5 particular a high-speed rail vehicle, e.g. an intercity rail vehicle, and/or a vehicle subjected to high crosswind loads, e.g. a double-deck rail vehicle. BACKGROUND ART [0002] The crosswind stability of rail vehicles is influen;ed by the shape of the ., .vehicle body, the inertial properties of the vehicle body and the running gear frames, 10 and by the vehicle suspension systems. A rail vehicle provided with a passive suspension system and exposed to a crosswind reacts with a sway motion of the vehicle body. A yaw motion of the vehicle body can also be observed, in particular under the impact of a gust. Measurements of vehicle response to crosswind show that in particular the lateral stiffness of the secondary suspension, and to somewhat less 15 extent the roll stiffness, influence the stability of the vehicle under the induced loads. However, the magnitude of the aerodynamic loads is often so high that it proves impossible to sufficiently adapt the passive suspension system to crosswind without compromising or even deteriorating the ride comfort. [0003] Rail vehicles are today increasingly equipped with active suspensiurl . . 20 systems for ride comfort purposes. When such a vehicle is exposed to high crosswind . -. loads, the active secondary suspension may somewhat reduce the impact of crosswind on the vehicle. The magnitude and suddenness' of the aerodynamic loads - on the vehicle, however, are often such that the response of the active secondary suspension is insufficient or inappropriate. 25 [0004] There is therefore a need foe a. inbre specific response to crosswind on rail . . . vehicles. SUMMARY OF THE INVENTION [0005] According to one aspect of the invention, there is provided a method for stabilising a rail vehicle comprising a vehicle body resting on two longitudinally- . - spaced running gears, each of the run.ning gears comprising a running gear frame, a primary suspension between the ..running: gear frame and..a..set-af..wheelsa, nd a . . secondary suspension comprising one or more lateral actuators between the running gear frame and the vehicle body, the method comprising: - processing signals from sensors directly or indirectly measuring a wheel unloading condition to detect crosswind and a windward side; and - controlling at least one of the one or more lateral actuators of at least one, or both of the running gears to move the vehicle body according to a crosswind sta.bility-oriented co.ntro1 strategy towards the windward side in response to the detected crosswind. [0006] The sideward movement imparted to the vehicle body reduces the wheel unloading on the windward side, which minimises overturning risks and increases the stability of the vehicle. The proposed method may take advantage of an existing active suspension system or use dedicated .actuators, in particular one or more 15 dedicated lateral actuators, which are not used in the absence of crosswind. [0007] The wheel unloading can be assessed as a normalised deviation (Q-Qo)/Qo of the actual vertical'wheel-rail force Q from a corresponding static force QO on a horizontal track. However, directly measuring the wheel-rail force requires instrumented wheelsets, which is costly and impractical in daily operation. Hence, the 20 sensors preferably measure a vertical deflection and/or force of the primary suspension. Directly measuring the vertical deflection of the primary suspension, in particular, proves particularly easy and appropriate. - , [0008] While the wheel unloading can be measured or assessed on a single wheel, it is preferred to compute a left-side loading value and a right-side loading value for 25 each running gear or'for the two running gears of the rail vehicle. [0009] The signal processing may further include comparing at least one of the lek-side loading value and right-side loading value to an unloading threshold to decide an occurrence of crosswind. The unloading threshold is preferably determined r . -rp-fl7 - ~ f ~ --~ ~ ---~ - ~ - ~ T ~ ~ T ~ - ~-7q ~ & - ~ - @ - : 7 ' ~ - -B-Y-_-L_ as a result of a previous processing of signals from the sensors in a static situation, i.e. at standsfill o"n a horizontal track'or'at constant speed on a straight horizontal track. [0010] Preferably, the signal processing further includes comparing the left-side loading value to a right-side loading value to determine a windward direction. 5 [0011] The signals are preferably filtered with a low-pass filter, preferably a 1st order low-pass filter, preferably with a cut-off frequency between 0,l and 4 Hz, to avoid unnecessary time delays in the response. . . . [0012.] , The crosswind stability-oriented control strategy preferably includes . -. controlling.,,~lte ast one of the 'lateral actuators with a predetermined crosswind 10 stability-oriented constant set value upon detection of the crosswind. This strategy is particularly simple to implement. The predetermined constant crosswind stabilityoriented set value can be the maximum force or deflection value achievable with the lateral actuator. [0013] Alternatively, if the lateral actuator is sufficiently powerful, it can be 15 controlled with a set force value or deflection value which is the sum of a predetermined crosswind stability-oriented constant mean value and of a superimposed dynamic value. In particular, the superimposed dynamic value can be determined according to a known comfort-oriented control strategy. The dynamic value should not interfere with the constant mean value and should therefore have no 20 frequency component under a given split frequency. If the known comfort-oriented . control.strategy generates low frequency signals, it may be necessary to process the set value computed according to the comfort-oriented contiol strategy through a . ' high-pass filter with a cut-off frequency at the split frequency, which is preferably more than 0,l Hz and less than 3Hz. 25 [0014] If one of the running gears is provided with more than one lateral actuator, one of the lateral actuators can be dedicated, to the crosswind stability-oriented control strategy while another can be controlled according to the comfort-oriented control strategy,.after processing through a high-pass filter if necessary in order not to interfer'e with the first actuator. [0015] The secondary suspension .may be provided with left and right vertical - - . . . . .--. - ;.actuaS~rs.I n.. such a- case, the crosswind stability-oriented control strategy may ., . . ' - --' include controlling at least one vertical actuator of the secondary suspension to tilt the vehicle body towards the windward side in response to the detected crosswind. 5 In particular, it may include lifting the vehicle body on a leeward side of the vehicle and lowering the vehicle body on the windward side of the .v e. hicle. [0016] At least one of the vertical actuators can be controlled with a predetermined constant set value upon detection o'f the crosswind. This predetermined constant set value can be a maximum force or deflection value of the . . 10 vertical actuator. [0017] At least one, and preferably all, of the vertical actuators can be controlled with a set force or deflection value, which is the sum of a predetermined constant mean value and of a superimposed dynamic value. In particular, the superimposed dyn. a.m ic value can be determined according to a known comfort-oriented control 15 strategy. As discussed above in connection with the lateral actuator, the dynamic ' * value for controlling the vertical actuators should not interfere with the constant mean value and should therefore have no frequency co'mponent under a given split frequency. If the- known comfort-oriented control strategy generates low frequency signals, it may be necessary to process the set value computed according to the 20 comfort-oriented control strategy through a high-pass filter with a cut-off frequency at the split frequency, which is preferably more than 0,l Hz and less than 3Hz. .... . .. . .-<..- .- . [0018] Preferably, the method includes phasing in the crosswind stability- - , oriented control strategy and simultaneously phasing out a comfort-oriented control strategy in a transition phase at detection of the crosswind. The transition phase 25 should be short enough to react quickly to the crosswind, but also to ensure a . . relatively smooth transition between the two control approaches. . . . r 5 . 1 . [0019] The method preferably also includes processing the signals from sensors measuring a vertical deflection and/or force of the primary suspension to detect an end of the crosswind. In particular, it may include comparing at least one of the left- ' . 30 side loading value and right-side loading value to an end-of-unloading threshold to p,~~-~--- ~-FF@-77-&TX272-~-1-&p1-R-~-2-9 - detect the end of the crosswind. The end-of-unloading threshold may be equal to the unloading threshold. The method