The present invention generally relates to a train suspension system for a train vehicle and particularly to a train suspension system designed to improve ride comfort. The present invention also relates a train suspension system for a train vehicle designed to ease the railway gauging problem.

It is well known that the forward speed of trains is restricted by the 'hunting' motion, which corresponds to the lateral vibration of trains running at high speed. Therefore, trains have an upper speed limit, called the 'critical speed'. Several attempts have been made in the past to increase the critical speed of trains. For example, Wang, Fu-Cheng and Liao, Min-Kai (2010) 'The lateral stability of train suspension systems employing inerters', Vehicle System Dynamics, 38:5, 619 have attempted to improve the critical speed by using 'inerters' in the railway suspension systems.

An 'inerter', as disclosed for example in US7316303B, represents a mechanical two-terminal element configured to control the mechanical forces at the terminals such that they are proportional to the relative acceleration between the terminals. The inerter, together with a spring and a damper, provides a complete analogy between mechanical and electrical elements, which allows arbitrary passive mechanical impedances to be synthesised. Inerters have been increasingly used in mechanical systems such as car suspension systems to improve system performance.

In the design of train suspension systems is there is a trade-off between ride comfort and other important performance measures. 'Ride comfort' generally refers to the vibration and motion inside a vehicle which affects the comfort, safety and health of the passengers. In particular, the level of ride comfort in trains is indicated by the maximum lateral body acceleration (Mace) of the vehicle. The smaller the Mace, the greater the ride comfort. The railway gauging problem is concerned with the clearance between the railway vehicle and track-side structures such as bridges, tunnels, signals etc. The railway gauging problem includes the need to assess the required clearance in a rigorous manner with the possibility to allow larger freight and passenger trains. The cost of rebuilding the railway infrastructure (for example to provide a larger clearance throughout the United Kingdom railway system to allow larger continental railway vehicles) is prohibitive.

The assessment of the required clearance in the railway gauging problem needs to take account of several factors such as the absolute position of the track, the steady state deflections of the vehicle during curving, and maximum transient deflections in transitional track sections. The faster a train runs, the larger is the required clearance between vehicle and infrastructure when curving. The present invention seeks to overcome the drawbacks of the prior art, improving ride comfort and limiting the lateral movement of the vehicle body, thereby reducing the railway gauging problem.

According to the present invention there is provided a suspension system for a train vehicle, wherein the train vehicle comprises a body and at least one wheelset having an axle which defines a lateral direction, wherein the suspension system is arranged, in use, to transmit forces in the lateral direction, the suspension system comprising at least one inerter, such that, in use, the movement of the body in the lateral direction is minimised.

According to the present invention there is also provided a method of reducing movement of a body of a train vehicle in a lateral direction, wherein the train vehicle comprises a body and at least one wheelset having an axle which defines a lateral direction, the method comprising the step of:

providing a suspension system arranged, in use, to transmit forces in the lateral direction, the suspension system comprising at least one inerter, such that, in use, the movement of the body in the lateral direction is minimised. According to the present invention inerter devices are used to minimise the movement of the body in the lateral direction defined by the wheelset axle (i.e. a direction perpendicular to the track which defines the direction of travel of the train). The term 'movement' of the body is used to describe either the lateral displacement of the vehicle body when curving or the maximum lateral body acceleration (Mace) to thereby improve ride comfort. Many modern techniques of simulation allow the vehicle/infrastructure interface to be analysed accurately, and in doing so place less requirement for large margins of safety. By limiting the lateral travel of the vehicle body, the present invention may also contribute to solving the railway gauging problem.

'Minimising' the lateral displacement or the Mace means that these values are reduced below values which are achievable with conventional damping technology while maintaining acceptable values of other performance metrics, such as, for example, least damping ratio (Ldmp). Preferably, the performance metrics have predetermined ranges. Some examples of 'acceptable values' of the maximum body acceleration (Mace), and least damping ratio (Ldmp) will be given below. However, it will be appreciated that 'acceptable values' as well as relevant performance metrics may vary according to the use and type of railway vehicle.

The suspension system may be a 'single stage suspension system', which includes a single suspension system between the body and the wheelsets. In this case, the at least one inerter may be connected within the suspension system, i.e. between a wheelset and a body of the train vehicle. The at least one inerter forms part of the suspension system of the train vehicle, which transmits forces between the wheelset and the body in the lateral direction. Alternatively, the suspension system may be a 'two stage suspension system', which includes a primary lateral suspension system and a secondary lateral suspension system. In this case, the at least one inerter may be connected within the primary and/or secondary suspension system, i.e. between the wheelset and a bogie and/or between the bogie and the body. The at least one inerter forms part of the primary or the secondary lateral suspension system of the train vehicle, which transmits forces between the bogie and the wheelset in the lateral direction; or the bogie and the body, respectively.

The at least one inerter may be connected to at least one damper. 'Damper' means any mechanical device or structure which has the effect of 'damping'. 'Damping' represents any effect that tends to reduce the amplitude of oscillations in an oscillatory system. For example, in mechanical systems, friction is one such damping effect. It will be appreciated that a connection of the inerter with such damper may be direct, where the damper is a mechanical device, or indirect, where the 'damper' represents a structural damping effect. In preferred embodiments, the secondary suspension system comprises an inerter in series or in parallel with a damper.

Specific examples of the invention will now be described in greater detail with reference to the following figures in which:

Figure 1 represents a plan view of a conventional train system;

Figure 2 is a table listing parameters and default settings of a 7-degrees of freedom model of the train system shown in Figure 1 ;

Figure 3 represents a plan view of a system in accordance with the present invention, in which the primary and secondary lateral suspensions Y1 , Y2 and Y3 are mechanical networks comprising inerters as shown in Figures 4(b), 4(c) and Figures 5(b), 5(c);

Figure 4 shows the conventional suspension layout (a) and the proposed layouts (b) and (c) incorporating an inerter b _sy for the secondary suspension Y1 ;

Figure 5 shows the conventional suspension layout (a) and the proposed layouts (b) and (c) incorporating an inerter b _py for the primary suspensions Y2 and Y3;

Figure 6 is a table listing results for minimizing the Mace by using an inerter in the secondary lateral suspension;

Figure 7 is a graph showing (a) the lateral body acceleration, and (b) the least damping ratio against velocity for the results tabulated in Figure 6; Figure 8 is a table listing results for maximizing the secondary lateral spring stiffness by using an inerter is the secondary lateral suspension;

Figure 9 is a graph showing (a) the least damping ratio and (b) the lateral body acceleration, against velocity for the results tabulated in Figure 8;

Figure 10 is a table showing results for minimizing the lateral body movement by using an inerter in the secondary lateral suspension;

Figure 1 1 is a graph comparing the time domain response of vehicle body lateral movement for the results tabulated in Figure 10;

Figure 12 represents a plan view of a two-axle railway vehicle with the suspension in the lateral direction being replaced by a lateral suspension Y(s), which represents a mechanical network comprising inerters, as shown for example in Figures 13(b) and 13(c);

Figure 13 shows a conventional suspension layout S1 (a) and proposed layouts S2 (b) and S4 (c) incorporating the inerter device for the lateral suspension;

Figure 14 is a table listing parameters and default settings of the 6- degrees of freedom model of the train system shown in Figure 1 ;

Figure 15 is a table listing results for minimizing the lateral acceleration by using the inerter device in the primary lateral suspension;

Figure 16 is a graph showing the lateral body acceleration against velocity for the results tabulated in Figure 15; and

Figure 17 is the power spectral density of lateral body acceleration under the excitation from random track irregularity for structures S1 , S2 and S4 with parameter values tabulated in Figure 15.

A first example of the invention is described below with reference to Figures 1 to 1 1.

Figure 1 represents a conventional train system 1 comprising a vehicle body v, one bogie frame g, and two solid axle wheelsets w, wherein each wheelset comprises two wheels either side of the axle. The body v is equivalent to the body of half a vehicle or carriage in a high speed train vehicle. The bogie g is used to carry and guide the body along a track or line. Bogies have traditionally been used in train designs as a 'cushion' between vehicle body and wheels to reduce the vibration experienced by passengers or cargo as the train moves along the track.

The wheelsets w and bogie g are connected by a primary suspension system K _p /C _p . Only longitudinal (x direction) and lateral (y direction) connections are represented in Figure 1. Any suitable suspension system may be used, such as a steel coil or steel plate framed bogie g with laminated spring axlebox suspension. The (lateral and longitudinal) connections of the primary suspension system K _p /C _p are represented by equivalent 'spring-damper' circuits, each circuit comprising a spring of stiffness K _p in parallel with a damper of damping constant C _p .

A secondary suspension system K _s /C _s is included between the body v and the bogie g, e.g. making use of an air suspension. The secondary suspension system K _s /C _s may also be represented by equivalent 'spring-damper' circuits, wherein each circuit comprises a spring K _s in parallel with a damper C _s .

Accordingly, the train system 1 shown in Figure 1 represents an example of a 'two stage suspension system', which includes a primary suspension system and a secondary suspension system. The train system, however, may be a 'single stage suspension system', which includes a single suspension system between the body and the wheelsets, as shown in Figure 12 and described in the second example below. The longitudinal connections in the system of Figure 1 contribute to the yaw modes and only these contributions are accounted for in the model described below. Vertical, longitudinal and roll modes are not included in this model. The conventional train system 1 of Figure 1 may be described by a seven degrees-of freedom (7-DOF) model including lateral and yaw modes for each wheelset (y _w i ;9wi ;yw2;9w2) and for the bogie frame (y _g ;9 _g ), and a lateral mode for the vehicle body (yv). System 1 may be modeled by Eqs. (1 ) - (7) listed below, with parameters defined in Table 1 shown in Figure 2:

A state-space form can be derived from equations (1 ) - (7) as given by:

, where

The vector w is used to define the inputs from the railway track (curvature, cant and track lateral stochastic displacement). When entering a curve, the track cannot change from straight to the nominal value of the radius (Ri;R2) and cant angle (9d;9c2) immediately. A conservative assumption is made in that Ri;R2 and 9ci;9c2 are ramped with 3 seconds transition time. In fact, for high speed trains a longer transition time is appropriate depending on the vehicle and track type. The straight track lateral stochastic inputs (yti;yt2) are of a broad frequency spectrum with a relatively high level of irregularities

In the example provided below, y _t i (t) is defined to be the output of a second order filter H (s) = (21.69 s ² + 105.6s + 14.42)/( s ³ + 30.64s ² +24.07s) whose input is a process with a single sided power spectrum given by: in which A _v is the track roughness factor, f _s is a spatial frequency in cycles/metre. The body lateral acceleration is quantified in terms of the root mean square (r.m.s.) acceleration J1 , and evaluated using the covariance method, time domain simulation method and frequency calculation method. The results by the three methods are all consistent. For the frequency calculation, Ji is expressed by:

where

T _d is the time delay of the track input between the front and rear wheelsets, which equals 2WV seconds, where is the semi-longitudinal spacing of the wheels and V is the system's speed in the longitudinal direction x.

A nominal speed V is assumed to be equal to 55 m/s. Using the default suspension layout and parameter settings, with velocity V varying between 1 m/s and 55m/s, it can be calculated that the least damping ratio (Ldmp) equals 6.45% (which is achieved at the nominal speed). Using the covariance method, it can also be calculated that, with yn and yt2 as input, the maximum lateral body acceleration (Mace) equals 0.2204 m/s ² when the velocity equals 55 m/s.

To improve ride comfort, a system according to the present invention minimizes the maximum lateral body acceleration (Mace) by including inerters in the lateral suspension, as will be described below.

In accordance with the present invention, the system 2 of Figure 3 comprises the same elements of the conventional system 1 of Figure 1 described above, and additionally comprises inerter devices in the lateral connections of the secondary suspension systems (y direction). Figure 3 together with Figure 4(c) show a system in accordance with a first embodiment of the present invention, which includes an inerter in series with a damper C _sy . Figure 3 together with Figure 4(b) show a system in accordance with a second embodiment of the present invention, which includes an inerter in parallel with a damper C _sy .

In its most general form, an 'inerter' represents a mechanical two-terminal element comprising means connected between the terminals to control the mechanical forces at the terminals such that they are proportional to the relative acceleration between the terminals. Inerters are defined by the following equation:

, where F is the applied force and b is either a fixed term or a variable function representing the 'inertance' of the system; v and V2 are the corresponding velocities of the two terminals.

In the 7-DOF model defined above according to equations (1 ) - (7), the maximum lateral body acceleration (Mace) is minimized, while the Ldmp is kept above 6.45%. All parameter values have been constrained to be within physically reasonable ranges, for example, the spring stiffness cannot be arbitrarily large.

Table 2 of Figure 6 summarises the results obtained when minimising the Mace for a conventional system, a system in accordance with the second embodiment of the present invention using an inerter in parallel with a damper in the lateral secondary suspension system, and a system in accordance with the first embodiment of the present invention using an inerter in series with a damper in the lateral secondary suspension system. Optimisation has been done with the secondary suspension only, and then with both primary and secondary suspensions. b _sy in Table 2 of Figure 4 represents the inertance of the inerters in the lateral secondary suspension. It can be seen that if optimization is only carried out for the secondary suspension, 1.45% advantage can be obtained by the first embodiment of the invention, using a secondary series inerter-damper scheme (Figure 3 with Figure 4(c)). If optimization is carried out for both primary and secondary suspensions, up to 12.84% performance benefit can be obtained by the second embodiment of the invention, using a secondary parallel inerter-damper scheme (Figure 3 with Figure 4(b)).

Figure 7(a) shows the Mace plotted as a function of velocity, for a conventional system optimized only over the secondary suspension (plotted in a continuous curve) and optimized over both the primary and secondary suspension (plotted in a dot dashed curve). Figure 7(a) also shows the Mace results for a system in accordance with the first embodiment of the present invention using an inerter in series with a damper in the lateral secondary suspension system optimized only over the secondary suspension (plotted in dashed curve), and optimized over both the primary and secondary suspension (plotted in starred curve). Figure 7(a) also shows the Mace results for a system in accordance with the second embodiment of present invention (plotted in a dotted curve). Figure 7(b) is a graph showing the least damping ratio Ldmp as a function of velocity corresponding to the results shown in Figure 7(a). It may be seen from Figure 7(b) that the constraint on the Ldmp (being above 6.45%) is satisfied.

The present inventors have carried out similar experiments on systems with different nominal parameter values, using inerters in the primary lateral suspension system K _p /C _p (Figure 3 with Figure 5(b) or (c)) which can provide further performance benefit on the ride comfort.

Systems in accordance with embodiments of the invention may also be used to limit the lateral movement of the vehicle body v. Firstly, it is possible to maximise the secondary lateral stiffness K _sy per axle box (attached to one side of a wheelset) which results in less quasi-static movement of the vehicle body v when the train is running on a curved track (also referred to as 'curving'). Secondly, it is possible to minimise the maximum lateral movement of the vehicle body, Max (y _v ) during curving. The transition responses for transitional track sections are also taken into account. Optimisations have been carried out using a vehicle speed V = 55 m/s.

The restrictions are for Ldmp and Mace to be at least as good as the nominal values and for all parameter values to be within physically reasonable ranges, as will be described in more detail below. The present inventors have also verified that the root mean square (r.m.s.) of the secondary suspension deflection with the straight track lateral stochastic displacement at the front and rear wheelsets (y _t i, yt ₂ ) as input is similar to that of the conventional suspension layout.

The first embodiment 2 of the present invention as shown in Figure 3 with Figure 4(c) has the inerter b _sy connected in series with the damper C _sy in the secondary lateral suspension system K _s /C _s . Table 3 of Figure 8 shows the results for maximising K _sy while keeping the Ldmp and Mace values at least as good as the nominal values. It can be seen from Table 3 that with conventional systems using a traditional suspension layout (without inerters, as shown in Figure 1 ), when the optimization is carried out over only the secondary suspension, the default parameter setting is very close to optimum values; when optimization is carried out over both the primary and secondary suspension, up to 19.8% improvement can be obtained. With the first embodiment in accordance with the invention (using a series inerter-damper structure in the secondary lateral suspension as shown in Figure 3 with Figure 4(c)), when optimization is carried out only over the secondary suspension, 6.5% improvement can be obtained; when the system is optimised over both the primary and secondary suspension, the stiffness K _sy can be increased by about 28%. Figures 9(a) and (b) describe the least damping ratio and body lateral acceleration against velocity respectively. It can be seen that both restrictions on least damping ratio (Ldmp) and maximum body acceleration (Mace) are satisfied. Table 4 of Figure 10 shows the results for minimising the maximum lateral movement of the vehicle body, Max (y _v ). It may be seen from Table 4 that when optimisation is carried out over only the secondary suspension, a 3.63% improvement can be obtained by the first embodiment of the present invention as shown in Figure 3 with Figure 4(c); when optimisation is carried out over both the primary and secondary suspension, the results can be improved by a 20% using the first embodiment of the present invention as shown in Figure 3 with Figure 4(c), compared with the optimised conventional system. Figure 1 1 shows the time domain response of vehicle body lateral movement when running into a curve at a velocity V= 55 m/s, corresponding to the results tabulated in Table 4 of Figure 10. The inventors have checked that the Ldmp and Mace restrictions are all satisfied. Plots similar to Figure 9 can be obtained.

With the same schemes and parameter settings as in Table 4 of Figure 10, the inventors of the present invention has checked that the percentage improvement of lateral body movement when curving stays at the same level with vehicle velocities different from 55 m/s.

It will be appreciated that the embodiments described above are merely exemplary and that it is possible to have many combinations of inerters with dampers or other mechanical parts of the lateral suspension systems. In particular, it is possible to include series and/or parallel damper-inerter combinations in the primary and/or secondary suspension systems. Embodiments in accordance with the invention may comprise inerter-damper combinations at one or more connection points between the wheelsets w and bogie g, as well as between the bogie and body v shown in Figure 3. For example, based on the inventors' calculation, the model with different nominal parameter values (e.g. higher value of primary lateral spring stiffness), using the inerter in the primary lateral suspension (Figure 3 with Figure 5(b) or 5(c)) can provide larger performance benefits. Another example of the invention is described below with reference to

Figures 12 to 17.

Figure 12 represents a two-axle train system comprising a vehicle body v and two solid axle wheelsets w. Unlike the 'two stage suspension system' shown in Figure 1 (and described above), the system of Figure 12 is an example of a 'single stage suspension system'. Conventionally, the vehicle body v and the wheelsets w and are connected by a lateral suspension system k/(k1 -c) represented by S1 in Figure 13. Only longitudinal (x direction) and lateral (y direction) connections are represented in Figure 12.

The train system of Figure 12 may be described by a six degrees-of freedom (6-DOF) model including lateral and yaw modes for each wheelset and vehicle body. This system may be modelled by equations (8) to (13) listed below, with parameters defined in the Table 5 of Figure 14.

The straight track lateral stochastic inputs are the same as defined for the the 'two stage suspension system' 1 shown in Figure 1 (and described above on page 8). A nominal speed V is assumed to be equal to 31 m/s. Similar to the lateral suspension system described in the first example above, this suspension system can also be replaced by a suspension incorporating an inerter, for example included in mechanical networks S2 or S4 as shown Figure 13. Figure 12 together with Figure 13(c) show a system in accordance the invention which includes an inerter in series with a damper c. Figure 12 together with Figure 13(b) show a system in accordance with yet another embodiment of the present invention, which includes an inerter in parallel with a damper c.

Table 6 shown in Figure 15 presents the optimisation results over the lateral body acceleration (representing ride comfort). As in the first example (with results shown in Table 3 of Figure 8), K _sy is maximised while the Ldmp and Mace values are kept at least as good as the nominal values. It can be seen from Table 6 that by including a suspension with an inerter as arranged in the S2 or S4 structures, the ride comfort can be improved by 30.7% and 42.9%, respectively.

Figure 16 shows the lateral body acceleration as a function of velocity with layouts S1 (thick, black solid line), S2 (thin, grey solid line) and S4 (starred line) and parameter values tabulated in Figure 15. Figure 17 presents the power spectral density of the lateral body acceleration with layouts S1 (thick, black solid line), S2 (thin, grey solid line) and S4 (starred line) and parameter values tabulated in Figure 15 at nominal speed (31 m/s). It can be seen from Figures 16 and 17 that by using inerters in the lateral suspension system (S2 or S4), a significant improvement in ride comfort can be achieved.