Railway Car Roll Control System

Abstract

This invention relates to a control system for controlling rotation or roll about a longitudinal axis of a variable trim railway vehicle body, so as to compensate for lateral acceleration as the vehicle moves over a curved track. A gyroscope records tilt of an axle about a longitudinal axis of the vehicle and the gyroscope signal, after integration, is passed through a threshold device to a gate or switch to open the latter at a predetermined threshold level and thereby cause a rate signal, derived from a tachometer on the vehicle, to be transmitted to a servomechanism for rotating the vehicle body.

Description

BACKGROUND OF THE INVENTION

This invention relates to railway vehicle trim control systems, applicable to a railway vehicle having a body which is adapted for controlled rotation about a longitudinal axis.

Railway vehicles with variable lateral trim are known, in which the vehicle body, when travelling around a curve, is made to rotate about a longitudinal axis so as to counterbalance, with a component of its weight, the centrifugal force acting upon the vehicle, so that the passengers experience, even at high speed, a relatively limited lateral acceleration ideally between 0.6 and 1.0 m/sec.sup.2 (2-3 ft/sec.sup.2).

The simplest form of such compensation is where the vehicle body is suspended pendulum-fashion and is freely rotatable about the longitudinal axis, this axis being higher than the centre of gravity of the body. When the vehicle travels around a curve the body rotates under the action of the centrifugal force to an equilibrium position in which the centre of gravity of the body lies on the resultant of the centrifugal force and the weight, so that no lateral reaction is experienced by any vehicle passenger.

Such a simple pendulum system has been little used in practice since it has the disadvantage that the body rotates about its axis under the influence of the centrifugal force at too low an acceleration, on account of the high inertia of the suspended body, and consequently the body cannot reach its position of equilibrium within the usual relatively short time during which the vehicle is travelling over the transition sections of a track between straight and fully cambered curve sections. Compensation for the centrifugal force is complete in the full curve, but inadequate along the transition portions of the track, and consequently the passengers may experience intense lateral acceleration as the vehicle travles over the transition portions, which is very unpleasant even though of short duration.

Consequently servo-assisted rotation control systems are favoured, in which, by the use of sufficiently powerful and fast-acting servo-controls, the vehicle body can be rotated by the required amount at each stage of its movement around a curve.

In early assisted rotation control systems, intervention of the servo-controls is controlled in response to lateral acceleration of the vehicle body, sensed, for example, by a pendulum with a longitudinal axis, or by an accelerometer mounted on the vehicle body.

With such systems, however, no means exist for ascertaining whether the sensed lateral acceleration results from centrifugal force, or from disturbances caused by irregular motion of the vehicle. In particular, the lateral oscillation or "snaking" phenomenon which is nearly always exhibited by a moving railway vehicle could give rise to undesirable intervention of the servo-control system. In order to avoid this, it is necessary to damp the movement of the pendulum, or to filter the accelerometer output signal, to exclude lateral accelerations of high frequency. Such measures, however, result in a slowing down of the response to the rotation control system and a reduction in the time available for carrying out the actual trim variation. In practice, the available time is very short, particularly for a high speed railway vehicle.

Thus a vehicle travelling at 200 Km/h (124 mph) covers a parabolic transition track 110 metres (361 ft.) long between a straight and a full curve track section in about 2 seconds. In this time it is necessary to detect the presence of the curve and bring the servo-control into action to rotate the whole body about a longitudinal axis. This rotation clearly must include a phase of angular acceleration and a phase of angular deceleration. It will therefore be apparent that it is necessary to reduce to a minimum the delay between the start of the track curvature and occurrence of the variation in trim under the action of the servo control system.

A similar problem arises at the end of a curve, in the transition track section between a fully curve and a straight track section.

This problem with which the present invention is concerned is illustrated graphically in FIGS. 1 to 4 of the attached drawings.

FIG. 1 shows the lateral acceleration acting upon a vehicle body (curve 1), travelling over a curve in a railway track at constant speed, plotted against time, and also the compensated lateral acceleration (curve II) of the said body when the body is rotated about a longitudinal axis, in the case where, as usually occurs in practice, the angle of rotation of the body is, for reasons of construction and bulk, limited to angles less than a certain maximum angle, so that lateral acceleration is not wholly compensated;

FIG. 2 shows the variation in amplitude a of a typical non-filtered signal derived from an accelerometer and representing the lateral acceleration shown in curve 1 of FIG. 1;

FIG. 3 shows the accelerometer signal of FIG. 2 after filtering with upper cut-off frequency equal to 0.5 Hz(cps) and after retardation by an overall delay of one second, and

FIG. 4 shows the lateral acceleration a felt by the passenger when rotation of the vehicle body about the longitudinal axis is controlled on the basis of the signal of FIG. 3, also plotted against time.

The actual rotation of a vehicle body under control of a signal of the type shown in FIG. 3 will be further retarded by 0.1 - 0.2 sec., due to the response time to the rotation servomechanism, so that, taking the difference between the centrifugal acceleration and the rotation of the body, one will have, for the lateral acceleration a experienced by the passenger, a very uneven pattern, certainly not conducive to comfort, as shown in FIG. 4, where the highest permissible lateral acceleration level is indicated by a broken line at about 0.08 g.

A main object of this invention is to provide a rotation control system for the body of a railway vehicle with variable lateral trim, which is capable of detecting with a minimal delay (around 0.1 - 0.2 seconds) the start and finish of a curve in a railway track along which the vehicle is travelling, including the transition track sections at the entry to and exit from the curve, and which is adapted to cause rotation of the vehicle body about a longitudinal axis to compensate for the lateral acceleration of the vehicle as it passes over the track, so that passengers are not subjected to accelerations greater than a predetermined threshold.

SUMMARY OF THE INVENTION

According to the invention there is provided a control system for controlling the rotation of a railway vehicle body about a longitudinal axis as the vehicle travels along a curved track, comprising a gyroscope adapted to be mounted in relation to an axle of the vehicle so as to be responsive to and to provide an output signal representative of tilt of the axle about a longitudinal axis of the vehicle, an integrator arranged to integrate said gyroscope output signal, signal forming means including a tachometer responsive to the vehicle speed for providing a rate signal determining a desired rate of rotation of the vehicle body, said rate signal being supplied to a servomechanism for effecting rotation of the vehicle body through a gate or switch which is controlled by an enabling or control signal provided by a threshold device when the output of the integrator reaches a predetermined threshold level.

The integrator is preferably of the type whose output signal returns to zero when the input signal is removed.

According to a further preferred embodiment of the invention the threshold device is connected to the gate through a persistence device adapted to prolong the output signal of the threshold device by a predetermined time interval following the end of the said output signal. Preferably the system is normally controlled by the rate signal from the signal forming means, and in which the system is controlled, after a predetermined time from the detected start of a curve in the track, by the output signal of a chain including, in series, a first lateral accelerometer arranged, in use of the system, on a bogie of the vehicle, a first low-pass filter, and a differentiator.

The control system may also include a second lateral accelerometer mounted on the vehicle body, a second low-pass filter connected to the output of the second accelerometer, and a second threshold device connected to the output of the low-pass filter, the second threshold device being connected to the servomechanism through a gate which is controlled by the output of the first threshold device applied to the gate through an inverter.

THE DRAWINGS ILLUSTRATING THE INVENTION

FIG. 1 is of a graph showing the lateral acceleration acting upon a vehicle body (curve I), travelling over a curve in a railway track at constant speed, plotted against time, and also the compensated lateral acceleration (curve II) of the said body when the body is rotated about a longitudinal axis in the case where the angle of rotation of the body is limited to angles less than a certain maximum angle so that the lateral acceleration is not wholly compensated;

FIG. 2 is a graph showing the variation in amplitude of a typical non-filtered signal derived from an accelerometer representing the lateral acceleration shown in curve I of FIG. 1;

FIG. 3 is a graph showing the accelerometer signal of FIG. 2 after filtering with upper cut-off frequency signal equal to 0.5Hz and after retardation by an overall delay of one second;

FIG. 4 is a graph showing the lateral acceleration felt by a passenger when rotation of the vehicle body about the longitudinal axis is controlled on the basis of the signal of FIG. 3:

FIG. 5 shows diagrammatically in perspective part of a bogie or track of a railway vehicle equipped with a lateral trim control system according to the invention;

FIG. 6 is a block schematic diagram of one embodiment of a control system according to the invention;

FIG. 7 is a circuit diagram of one of the blocks in the diagram of FIG. 6;

FIG. 8 is a block schematic diagram of a second embodiment of a control system according to the invention;

FIG. 9 is a block schematic diagram of a third embodiment of a control system according to the invention;

FIG. 10 illustrates diagrammatically in elevation the camber of a curved railway track, and

FIGS. 11 - 17 represent diagrammatically the waveforms of different signals in the control system according to the invention, plotted against time, illustrating the operation of the system.

FIG. 18 is a cross-sectional view of a railway vehicle showing the relationships of the axle, wheels, gyroscope and tachometer relative to a vehicle body and servo-mechanism, the body and servo-mechanism being shown in phantom lines.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 5 and 18 show two wheels 10, 12 of a railway vehicle (not shown) connected by an axle 14. A bridge 16 which supports the body 41 of the railway vehicle is carried by the axle 14.

The bridge 16 carries a housing 18 in which a gyroscope (not shown in FIG. 5) is mounted, the axis of rotation of the gyroscope being contained in a vertical transverse plane of the vehicle and being oriented in said plane in a direction preferably parallel to or orthogonal to the axis of rotation b -- b of the axle 14. That is to say, the axis of rotation of the gyroscope is either along the axis a -- a or along the axis d -- d of FIG. 5. In any event, the gyroscope is arranged in such a way as to register the tilt of the axle 14 or of the bogie on which the axle is carried in the said transverse vertical plane, that is, about a longitudinal axis c -- c.

The suspension of the gyroscope housing 18 consists of a shaft 22 rotatably supported by two brackets on the bridge 16 and arranged parallel to the axis b -- b of the axle 14, and two arms 24, 26 fixed to the shaft 22 and pivotally connected at their free ends to the housing 18 by means of coaxial pins 25, 27 integral with the housing 18, the common axis of the pins 25, 27 being aligned with the axis a -- a.

The housing 18 rests on a flat portion of the bridge 16 by way of an interposed resilient pad 28. The housing 18 could alternatively be mounted on the frame of one of the bogies of the railway vehicle or any other suitable place on the truck or running gear.

In this way the axis a -- a of the housing 18 always remains parallel to the axis b -- b of the axle 14. The resilient pad 2 protects the gyroscope against vibrations and high frequency dynamic forces, whilst the suspension of the gyroscope housing 18 supports the housing 18 against lateral and longitudinal movement, leaving it free to move vertically, that is, by rotation about the axis of the shaft 22. The axis a -- a of the gyroscope therefore remains constantly parallel to the axis b -- b of the axle 14, whilst still being able to effect displacement parallel to the axle 14. Consequently the signal generated by the gyroscope will be proportional only by the torque acting on the gyroscope in the vertical plane, due to tilt of the axle 14 about the longitudinal axis c -- c, as a result of camber of the track, that is, superelevation of one rail of the track relative to the other.

FIG. 10 illustrates diagrammatically the variation in camber of a railway track having a curved path, represented as the variation with time t of the height h or superelevation of one rail relative to the other for a geometrically perfect pair of rails laid in a curve, as experienced by a vehicle passing over the track at constant speed. In other words, FIG. 10 represents diagrammatically the elevation E of the outside rail as viewed from the inside rail of the curve. The central portion of the curve, shown parallel to the axis t in FIG. 10 is the full curve portion of constant camber, and in this portion, and in the straight portions at each end of the curve, the track has a constant camber, so that the speed of tilt of a vehicle axle about the longitudinal axis of the vehicle is zero in these portions. In the transition sections of track, however, at the entry to and exit from the full curve portion, the speed of tilt T of the axle about the longitudinal axis of the vehicle would ideally be constant and equal in each case to iV/S, where i is the gradient, in mm/metre or inches foot, of the outside track relative to the inside track of the curve, S is the distance in mm. or inches between the bearing points of the vehicle wheels on the two respective rails -- that is, the gauge of the track -- and V is the speed of the vehicle in m/sec. (or ft./sec).

The gyroscope output signal representing the speed of rotation of the axle about the longitudinal axis of the vehicle would therefore ideally have the form indicated in FIG. 11.

In practice, however, the rails of the track have an unevenness which both on straight and on full curve (constant camber) portions give rise to lateral tilt of the vehicle axle about the longitudinal axis of the vehicle, these variations being superimposed upon those due to the change in camber of the track in the transition sections, so that the resultant gyroscope output signal has the form shown in FIG. 12.

In order to avoid untimely intervention of the trim control system, the signal determining the start of the compensating rotation of the vehicle body is not the angular velocity of the axle about the longitudinal axis but the integral in time of the said speed when this is higher than a certain value.

The integration of the angular velocity signal gives a signal related to the angle of the axle to the horizontal, or, multiplying by "S", the superelevation h of one rail (the outside rail of the curved track) relative to the other.

In practice, even with the most perfect integrating instrument the resulting integral does not maintain a fixed zero line, but undergoes a progressive "drift", so that after a certain time there is no longer any correspondence between the integrated signal and the effective superelevation h.

In order to avoid this, and to provide a signal which will indicate reliably and promptly the varying camber transition sections adjoining a curved track the control system shown in FIG. 6 is used.

In FIG. 6, 30 indicates the gyroscope mounted in the housing 18 in FIG. 5. The gyroscope 30 provides a velocity signal representative of the angular velocity of the axle 14 about a longitudinal axis C -- C (FIG. 5) of the vehicle. This angular velocity signal is passed to a limiter 32 which cuts off this signal at a maximum amplitude corresponding to the highest rotational speed of the axle 14 about the axis C--C due to track camber or superelevation (about 0.08 rad/sec.). In this way one excludes high transitory values of h caused by, for example, localised subsidence below individual sleepers of the track, or by points in bad condition.

The output signal from the limiter 32 is passed to an integrator 34, of the type in which the integral is returned to zero every time the input signal, that is to say the limited angular velocity signal, goes through zero. An integrator of this type is described in the Applicants' German Patent Application filed Offenlegungsschrift, published Aug. 10, 1972 2204072. The output of the integrator 34 therefore has the form shown in FIG. 13.

A threshold h.sub.o is fixed below which the integrated signal does not give rise to any effective control signal. The threshold h.sub.o must be a little greater than the value of the integrator output corresponding to the greatest track inclination and constitutes the threshold value of a threshold device 36. The threshold device 36 receives the output of the integrator 34 and provides a constant amplitude positive or negative signal g' whenever the integrator output exceeds a positive or negative threshold level respectively indicated by the dashed lines h.sub.o and - h.sub.o in FIG. 13. The output of the threshold device 36 is represented in FIG. 14.

A persistence device 38 (FIG. 6) provides an output signal P consisting of positive or negative constant amplitude pulses or duration .DELTA.t.sub.o (0.10 - 0.15 sec), each pulse originating immediately after the output of the threshold device 36 drops to zero, thus eliminating the disadvantage of frequent disappearance of the signal if the track curve is in a bad condition, and has points at which the track camber is constant and then suddenly reverses. The contribution made by the persistence device 38 is represented in FIG. 15; this contribution is added to the output of the threshold device 36 (FIG. 14) to give a combined output signal C as shown in FIG. 16.

A tacho-generator 52 installed on the vehicle to record its speed of travel supplies a tachometer signal to a forming circuit 52' which determines the speed of rotation of the vehicle body which, as described below, gives an output rate signal representative of the desired rotational speed of the body, the magnitude of which is dependent upon the speed of the vehicle.

The rate signal ouput of the forming circuit 52' provides one input 39a of an AND gate 39, the enabling or control input 39b of which consists of the output signal of the persistence device 38. When the gate 39 is opened by the control input 39b it passes the rate signal from the forming circuit 52' to a servomechanism 40 controlling the rotation of the vehicle body about the longitudinal axis, so that the body is then rotated at a predetermined rate, related to the speed of the vehicle.

The servomechanism 40 as shown in FIG. 18 may be of known type, either electric or hydraulic, and should preferably include counter-reactive (negative feedback) means for more exact positioning of the body 41. The combined output signal of FIG. 16 has a value of other than zero only during travel of the vehicle along the transition track sections and each pulse of this signal is delayed relative to the start and the finish of the said transition sections by intervals of very short duration (0.1 - 0.2 sec.). As soon as this signal is detected, the vehicle body is rotated by the servomechanism 40 in the same direction as that in which tilt of the axle 14 is sensed. Upon cessation of the signal rotation of the body by the servomechanism 40 is also stopped.

In FIG. 7 there is shown a simplified circuit diagram of a preferred embodiment of the forming circuit 52' which includes a number of differential amplifiers 200-1, 200-2, - - - , 200-n. Each of the differential amplifiers has a positive input consisting of a tachometric signal V provided by the tachometer 52 and a negative input consisting of respective fixed fiducial voltages V.sub.1, V.sub.2, - - - V.sub.n, of successively increasing value. The respective outputs of the amplifiers 200-1, 200-2, - - - , 200-n are connected to earth via Zener diodes 202-1, 202-2, - - - , 202-n and are also connected to a common point 204 through the respective resistors 206-1, 206-2, - - - , 206-n. An output operational amplifier including an amplifier 208 and resistors 210, 212, amplifies with calibrated gain the combined output signal present at the point 204, determining the rate of rotation applied to the body.

The contribution to the combined output signal at point 204 provided by each amplifier 200 and Zener diode 202 is zero when the tachometric signal V is less than the respective fiducial voltage V.sub.1, V.sub.2, - - - , V.sub.n while it is positive and constant when the tachometric signal V is greater than the fiducial voltage V.sub.1 . . V.sub.n. Therefore the output signal V.sub.o of the amplifier 208 increases in a stepwise manner in relation to the tachometric signal V, as illustrated graphically in FIG. 17. The parameters which define the graph of FIG. 17 are, for example, those recorded in the following table, where V is the speed of the vehicle, and .theta.' the corresponding angular speed of rotation required of the body.

______________________________________ V .theta.' (km/h) (m/h) (rad/sec) ______________________________________ <100 62 0.00 111 69 0.01 121 75 0.02 130 81 0.03 138 86 0.04 145 90 0.05 151 94 0.06 156 97 0.07 160 99 0.08 163 101 0.09 >163 101 0.10 ______________________________________

The integrated gyroscopic signal determines the duration of the rotation applied to the body; for a vehicle moving along parabolic transition track sections it is necessary, by means of other controls, to regulate in a more exact fashion the speed of rotation of the body.

Since the angle of rotation of the body has to be proportional to the non-compensated lateral acceleration a.sub.nc due to the camber of the track it is necessary that the speed of rotation of the body shall also be proportional to the rate of variation of the said acceleration a.sub.nc.

FIG. 8 shows the block diagram of a system of rotational control of the body for achieving such a proportional rate of rotation. A lateral accelerometer 44 is mounted on the bogie of the vehicle to record its lateral acceleration. The output signal of the accelerometer 44 passes through a low-pass filter 46, which filters the signal with an upper cut-off frequency of 0.5 Hz. The output signal of the filter 46 is passed through a differentiator 48 to give a signal proportional to the rate of change of the lateral acceleration, this rate signal being stored in a memory 150 which has a set/reset input 152.

The tachometric output signal of the forming circuit 52' is passed to a first input of a two-position selector switch 154, a second input of which receives the output of the memory 150. The selector switch 154 normally transmits the tachometric signal from the forming circuit 52' to an output point 156. A timer 158, controlled by the output signal of the persistence device 38, supplies, after a predetermined time of about one second from the start of the track curve, a control signal to the selector switch 154 in order to commute the switch 154 to the second input, passing to the output point 156 the stored signal from the memory 150, and at the same time activating the memory 150 to store the value of the signal at its input at that instant.

The AND gate 39 in FIG. 6 is in FIG. 8 replaced by a three-position selector switch 160. The selector switch 160 is arranged to transmit to the servomechanism 40 the output signal from the selector switch 154 either directly or after inversion in an inverter 162, according to the position of the switch 160, which is determined by the output signal of the persistence device 38, supplied via line 164. When the output of the persistence device 38 is zero, the selector switch 160 is in its neutral position, as shown in FIG. 8; when the output of the device 38 is positive the selector switch 160 transmits the output signal from switch 154 direct to the servomechanism 40, and when the output of the device 38 is negative the selector switch 160 transmits the inverted output signal to the servomechanism 40.

Control of the rotation of the body about the axis C--C therefore takes place in two phases.

In the first phase, no signal reaches the memory 150 from the lateral accelerometer 44, since the low-pass filter 46 introduces a delay of about one second in the output signal of the lateral accelerometer 44. Rotation of the body therefore occurs in the first phase under the control of the forming circuit 52' only, as in the schematic arrangement shown in FIG. 6.

When a period of about one second has elapsed from entry into the curve, the signal proportional to the rate of change of non-compensated lateral acceleration a.sub.nc reaches the memory 150 from the differentiator 48 and simultaneously the memory 150 is activated, together with commutation of the selector switch 154. Rotation of the body now continues under the control of the output signal from the lateral accelerometer 44.

FIG. 9 shows a more complete embodiment of a system of rotational control of the body of a railway vehicle about a longitudinal axis. In addition to the components of the system of FIG. 8, the system of FIG. 9 includes a lateral accelerometer 56 which registers the residual transverse acceleration a.sub.res acting upon the body. In this case, too, the output signal of the accelerometer 56 is passed through a low-pass filter 58 with a cut-off frequency of 0.5 Hz(cps). The output signal of the filter 58, which is proportional to a.sub.res, is passed to a threshold device 59 adapted to provide a predetermined calibrated signal which is positive or negative according to whether the output signal of the accelerometer 56 is positive or negative respectively. This calibrated signal is passed via an AND gate 60 to provide an alternative input to the servomechanism 40. In effect the output of the AND gate 60 and the output of the selector switch 160 are connected to the servomechanism 40 via an OR gate. The AND gate 60 has a control input constituted by the output signal of the persistence device 38, inverted by means of an inverter 171.

In contrast to the embodiment of FIG. 8, the persistence device 38 in the embodiment of FIG. 9 is not directly connected to the output of the threshold device 36, but is connected to the latter through one or two routes: one, direct, route comprises an AND gate 166 having a control input constituted by the output signal of the threshold device 59, inverted by an inverter 168; the other, indirect, route comprises a time 170 and an AND gate 172 having a control input constituted by the directly applied output signal of the threshold device 59.

The output signal of the AND gate 60 when applied to the servomechanism 40 causes reverse rotation of the body back into the normal position, with a very low speed, of the order of 0.005 rad/sec. This serves the purpose of returning the body to its normal position when the speed of the vehicle decreases as the vehicle moves along a curve of constant radius, or when the gyroscopic signal is nil, for example if the vehicle stops on a curve.

The timer 170 interposed between the threshold device 36 and the persistence device 38 only works if the residual transverse acceleration a.sub.res has a sign conflicting with the gyroscopically sensed acceleration, and has the object of introducing a delay very roughly proportional to the acceleration a.sub.res acting upon the body, between perception of the integrated gyroscopic signal and the start of the rotation of the body. In this fashion the rotation imparted to the vehicle body approximates as closely as possible to the variation with time of the non-compensated acceleration a.sub.nc, and one can reduce to a minimum the end of track phase displacement in the case in which the compensation of the lateral acceleration is not complete.

Claims

I claim:

1. A control system for controlling the lateral trim of a railway body which is rotatable about a longitudinal axis on a supporting truck in which at least one vehicle axle is supported for rotation as the vehicle travels along a curved track, said system comprising;

a gyroscope mounted on said truck in close relation to said axle of the vehicle for providing an electrical output signal representative of the tilt of said axle about a longitudinal axis of the vehicle, integrator means connected to said gyroscope for integrating said output signal, said integrator means being of the type whose output signal returns to zero when the input signal is removed,

a threshold device connected to the integrator means to provide an enabling control signal when the integrated signal reaches a predetermined threshold level,

signal forming means including a tachometer adapted to be mounted on the vehicle to provide a signal representative of the vehicle speed, said signal forming means providing from said speed signal a rate signal adapted to be applied to a servo mechanism for effecting the desired rate of rotation of the vehicle body about a longitudinal axis of the body in response to said rate signal,

gating means connected to said first forming means and having a control input connected to said threshold device whereby said gating means is adapted to pass said rate signal to said servo mechanism when said threshold device produces the enabling control signal,

said system further comprising in series, a first lateral accelerometer adapted to be mounted upon said truck of the vehicle, a first low-pass filter, and a differentiator, said system being controlled after a predetermined time from the detected start of a curve in the track, by the output signal of said differentiator.

2. The control system claimed in claim 1 and further including a limiter connected at the input of the integrator means and limiting the amplitude of the gyroscope output signal which is integrated to amplitudes less than the greatest possible angular speed of rotation of the said vehicle axle about said longitudinal axis when the vehicle is moving over a curved track.

3. The control system claimed in claim 1, including a persistence device through which the threshold device is connected to said gating means, said persistence device prolonging the output signal of the threshold device by a predetermined time interval following the end of the said output signal.

4. The control system claimed in claim 1, and further including a second lateral accelerometer adapted to be mounted on the vehicle body, a second low-pass filter connected to the output of the second accelerometer, and a second threshold device connected to the output of said low-pass filter, a gate controlled by the output of said first threshold device and an inverter interposed between said gate and said first treshold device, said second threshold device being connected to said servomechanism through said gate.

5. The control system claimed in claim 4, and further including a persistence device through which the threshold device is connected to said gating means, said persistence device prolonging the output signal of the threshold device by a predetermined time interval following the end of the said output signal, a delay circuit interposed between said first threshold device and said persistence device and controlled by the output signal of said second threshold device to cause a delay in the application of the enabling control signal to the gating means when the output signal of said second threshold device is greater than the threshold level.

6. The control system claimed in claim 1, in which said gating means is a selector switch the operation of which is controlled by said control input from said threshold device.