Control system

Abstract

A control system, for a train 200 having two pantographs 202,252 raised for collecting power from an overhead line 401, addresses the problem of degrading contact stability of the trailing pantograph at high train speeds. The control system determines, relative to a direction of travel of the train 402, which of the two pantographs is a leading pantograph 202 and which is a trailing pantograph 252. The control system receives a measurement of a speed of the train and determines, relative to the measured speed: a first uplift force for the leading pantograph against the overhead line; and a second uplift force for the trailing pantograph against the overhead line. The second uplift force is greater than the first uplift force, and the difference between the first and second uplift forces is a function of the measured speed, wherein the difference increases as the vehicle speed increases. The control system then issues command signals to the leading and trailing pantographs to respectively apply the first and second uplift forces to the overhead line.

Description

Field of the Invention The present invention relates to a control system for a train having two pantographs. Background For many railway applications, power is supplied to a train from an overhead line via a pantograph, which is located on the top of the train body, and is raised to contact the overhead line with a pantograph shoe. When a pantograph is raised and contacts the overhead line, it applies a certain amount of uplift force to the line to maintain that contact. The uplift force applied during dynamic operation causes an oscillatory disturbance in the overhead line, the effects of which can still be observed behind the pantograph for several hundred metres until the oscillations are damped and the overhead line steadied. This oscillation effect is often termed a standing wave, as the oscillations in the wire can resemble ocean waves shuttling between supporting masts along the line. Limits on the amount of uplift force between the pantograph and overhead line may be governed by legislative standards, as well as by compatibility requirements of the infrastructure manager, in order to maintain train operation while reducing occurrence of operational damage to both the network infrastructure and the train. Trains are often operated with plural pantographs raised. For example, trains may comprise multiple train units, operated in a coupled state, so that passenger transportation capacity can be increased. For such coupled operation, the train requires mechanical and operational coupling between the train units, but usually the traction systems of the different train units comprising the train are kept electrically separated and consequently require independent power supplies. As a result, each train unit comprising the train must raise at least one of its own pantographs to contact the overhead line to power its own traction system. In such a situation, a trailing pantograph (e.g. on a trailing train unit) can encounter a problem of maintaining adequate contact with the overhead line in a region affected by the standing wave caused by the interaction of a leading pantograph (e.g. on a leading train unit) with the overhead line. Specifically, the standing wave can reduce the contact stability of the trailing pantograph on the overhead line. This reduction in contact stability can lead to a loss of traction power for the trailing train unit, which in turn can cause operational inefficiencies. To address this problem, GB2567476A (hereby incorporated by reference) proposes the concept of reducing the uplift force applied to the leading pantograph relative to the uplift force applied to the trailing pantograph. However, the reduction in contact stability due to the standing wave from the interaction of the leading pantograph with the overhead line tends to become greater as a train is operated at higher speeds, and reducing the uplift force applied to the leading pantograph may not adequately account for the increased disturbance. This problem is particularly acute for high-speed trains. Therefore, a solution is needed which can control the uplift force with more flexibility and adjustability to cover a wide range of operational speeds. The present invention has been devised in light of the above considerations. Summary of the Invention It would be desirable to address the problem of degrading contact stability of the trailing pantograph at high speeds. In general terms, the present disclosure achieves this by increasing the uplift force applied to the trailing pantograph relative to the uplift force applied to the leading pantograph, and increasing the difference between the two uplift forces relative to the speed of the train. Accordingly, a first aspect of the disclosure provides a control system for a train having two pantographs raised for collecting power from an overhead line, wherein the control system is configured to perform the steps of: (a) determining, relative to a direction of travel of the train, which of the two pantographs is a leading pantograph and which is a trailing pantograph, (b) receiving a measurement of a speed of the train, (c) determining, relative to the measured speed, a first uplift force for the leading pantograph against the overhead line and a second uplift force for the trailing pantograph against the overhead line, wherein the second uplift force is greater than the first uplift force, and the difference between the first and second uplift forces is a function of the measured speed in which the difference increases as the vehicle speed increases, and (d) issuing command signals to the first and second pantographs to respectively apply the first and second uplift forces to the overhead line. Advantageously, by increasing the second uplift force relative to the first uplift force, and increasing the difference between the first and second uplift forces as a function of the measured speed, the effect of the standing wave generated by the first uplift force on the contact stability of the trailing pantograph can be mitigated even at high speeds. This in turn helps to prevent operational inefficiencies by reducing wear to the trailing pantograph, the overhead line, and other railway infrastructure. In a second aspect, the present disclosure provides a train having two or more pantographs and the control system of the first aspect. Optional features of the present disclosure will now be set out. These are applicable singly or in any combination. The control system may repeatedly perform in a loop steps (b) to (d) such that the difference between the first and second uplift forces of the command signals varies as the measured speed varies. In this way, the first and second uplift forces can be continuously updated as the train speed varies along a travel route. Each pantograph may be part of a respective pantograph system comprising: the pantograph, a pneumatic actuator to control the position of the pantograph, and a pneumatic pressure regulator to regulate an air supply to the pneumatic actuator, wherein the respective command signal is received by the pneumatic pressure regulator, which regulates the air supply to the pneumatic actuator to positionally control the pantograph to produce the respective uplift force. A pneumatically operated pantograph system such as the one described above can be controlled with ease and accuracy by the pneumatic pressure regulator. In practice, the command signals issued to the first and second pantographs to respectively apply the first and second uplift forces to the overhead line typically account for respective aerodynamic components of the uplift forces produced by the speed of the train. For example, the command signal received by a respective pneumatic pressure regulator may command the regulator to regulate the air supply to the pneumatic actuator to positionally control the pantograph to apply an uplift force that is less, by a predetermined amount, than the actual determined uplift force for that pantograph, the predetermined amount being the expected additional uplift force applied to the overhead line by aerodynamic forces acting on the pantograph at the measured speed. These additional uplift forces can be determined in advance for a given pantograph over its range of operational speeds, and conveniently may be stored in a suitable database of the control system (such as the reference database discussed in more detail below). The control system may constrain the first and second uplift forces between a lower uplift force limit, and an upper uplift force limit. For example, each uplift force limit may increase with increase in the measured speed. Advantageously, such a constraint can ensure that the first and second uplift forces remain within safe operating limits, while being compatible with uplift forces that increase as the train speed increases. The control system may constrain the first uplift force such that, for a given measured speed, the ratio A / B is less than 20% and preferably less than 15%, where A is the difference between the first uplift force and the lower uplift force limit and B is the lower uplift force limit. Such a constraint advantageously allows the first uplift force applied to the leading pantograph to be maintained within a safe operating distance of the lower uplift force limit, thereby enabling the control system to increase the difference between the first and second uplift forces while keeping them both between the lower and upper uplift force limits. The function of the measured speed, in which the difference between the first and second uplift forces increases as the vehicle speed increases, may be such that the difference increases linearly with measured speed. The control system may contain a reference database storing values of a reference uplift force against speed for a selected one of the leading and trailing pantographs, and the control system may determine the uplift force for said selected pantograph by selecting the reference uplift force corresponding to the measured speed. The control system may further store a coefficient value by which the uplift force for the other of the leading and trailing pantographs can be determined by performing a mathematical operation on the selected reference uplift force using the coefficient value. Preferably the mathematical operation is F2 = Fi + C(Fi - Fo) when the selected pantograph is the trailing pantograph or Fi = F2 - C(F2 - Fo) when the selected pantograph is the leading pantograph, where F2 is the second uplift force, Fi is the first uplift force, Fo is an uplift force for both the leading and trailing pantographs at zero measured speed, and C is the coefficient value. Conveniently, the reference uplift force is the first uplift force. The use of the reference database facilitates rapid and reliable determination of the selected uplift force, and the use of a coefficient value facilitates rapid and reliable determination of the other uplift force. Additionally, the use of the reference database, and preferably also the coefficient value, enables the control system to be rapidly and reliably updated for operation on other rail networks having different operational characteristics, and in particular different uplift force requirements. The coefficient value can be a constant. Alternatively it can be a function of measured speed, typically increasing with increasing measured speed. For example, the coefficient value can be linearly proportional to measured speed. Step (a) of the control system may also include determining a distance between the two pantographs. Moreover, in step (c) the difference between the first and second uplift forces may also be a function of said distance in which the difference increases as the distance decreases. In general, the shorter the distance between the two pantographs, the more the contact stability of the trailing pantograph is negatively impacted by the standing wave disturbance caused by the leading pantograph. Therefore, an increased difference between the first and second uplift for shorter distances can help to improve the contact stability of the trailing pantograph. Each pantograph may be a half-pantograph having an upper arm and a lower arm connected by a knuckle, with each half-pantograph being operable in either a knuckle leading orientation or a knuckle trailing orientation relative to the direction of travel. In this case, step (a) may also include determining the respective orientations of the two pantographs, and in step (c) the difference between the first and second uplift forces may be reduced when the two pantographs are in the same orientation and increased when the two pantographs are in different orientations. The orientation of the half-pantograph influences the aerodynamic drag exerted on the pantograph, and consequently impacts the uplift force that the pantograph exerts on the overhead line. When the two pantographs have different orientations, the standing wave disturbance created by the uplift forces can increase relative to when the two pantographs have the same orientation, such that an increase in the uplift force of the trailing pantograph is desirable. The control system may be further configured to raise the two pantographs preliminary to issuing the command signals to the two pantographs. The two pantographs may be part of the same train unit. Alternatively, the train may comprise two or more coupled train units, and the two pantographs are parts of different train units. In this case, the two pantographs may provide power to respective traction systems of their train units. A third aspect provides a method of controlling the pantographs of a train having two pantographs raised for collecting power from an overhead line, wherein the method includes: (a) determining, relative to a direction of travel of the train, which of the two pantographs is a leading pantograph and which is a trailing pantograph, (b) measuring a speed of the train, (c) determining, relative to the measured speed, a first uplift force for the leading pantograph against the overhead line and a second uplift force for the trailing pantograph against the overhead line, wherein the second uplift force is greater than the first uplift force, and the difference between the first and second uplift forces is a function of the measured speed in which the difference increases as the vehicle speed increases, and (d) commanding the leading and trailing pantographs to respectively apply the first and second uplift forces to the overhead line. Thus the method of this aspect corresponds to the use of the control system of the first aspect. The optional features of the present disclosure discussed above are applicable singly or in any combination, suitably adapted as appropriate, to the method of the third aspect. For example, steps (b) to (d) may be repeatedly performed in a loop such that the difference between the first and second uplift forces of the command signals varies as the measured speed varies. Each pantograph may be part of a respective pantograph system comprising: the pantograph, a pneumatic actuator to control the position of the pantograph, and a pneumatic pressure regulator to regulate an air supply to the pneumatic actuator, wherein the respective command signal is received by the pneumatic pressure regulator, which regulates the air supply to the pneumatic actuator to positionally control the pantograph to produce the respective uplift force. The first and second uplift forces may be constrained between a lower uplift force limit, and an upper uplift force limit. For example, each uplift force limit may increase with an increase in the measured speed. The first uplift force may be constrained such that, for a given measured speed, the ratio A / B is less than 20% and preferably less than 15%, where A is the difference between the first uplift force and the lower uplift force limit and B is the lower uplift force limit. The function of the measured speed, in which the difference between the first and second uplift forces increases as the vehicle speed increases, may be such that the difference increases linearly with measured speed. In step (c) the uplift force for a selected one of the leading and trailing pantographs may be determined by selecting a reference uplift force corresponding to the measured speed from a reference database storing values of the reference uplift force against speed, and the uplift force for the other of the leading and trailing pantographs may be determined by performing a mathematical operation on the selected reference uplift force using a stored coefficient value. Step (a) may also include determining a distance between the two pantographs. Moreover, in step (c) the difference between the first and second uplift forces may also be a function of said distance in which the difference increases as the distance decreases. Each pantograph may be a half-pantograph having an upper arm and a lower arm connected by a knuckle, with each half-pantograph being operable in either a knuckle leading orientation or a knuckle trailing orientation relative to the direction of travel. In this case, step (a) may also include determining the respective orientations of the two pantographs, and in step (c) the difference between the first and second uplift forces may be reduced when the two pantographs are in the same orientation and increased when the two pantographs are in different orientations. The method may further include raising the two pantographs preliminary to commanding the leading and trailing pantographs to respectively apply the first and second uplift forces to the overhead line. The two pantographs may be part of the same train unit. Alternatively, the train may comprise two or more coupled train units, and the two pantographs are parts of different train units. In this case, the two pantographs may provide power to respective traction systems of their train units. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 shows schematically a train unit, having a pantograph; Figure 2 shows schematically a train which is comprised of a leading train unit and a trailing train having a leading pantograph and a trailing pantograph respectively; Figure 3 shows graphically the relationship between speed and uplift force for the uplift force limits, as well as an illustrative example of the first and second uplift forces applied to the leading and trailing pantographs; Figure 4 shows schematically a train having two pantographs; Figure 5 shows schematically features of a control system as applied to the leading train unit in Figure 4; Figure 6 shows schematically features of the control system as applied to the trailing train unit in Figure 4; Figure 7 shows schematically a train having a single pantograph; Figure 8 shows features of the control system as applied to the train in Figure 7; Figures 9A and 9B shows schematically variation of the distance between two pantographs of a train; Figure 10 shows graphically the relationship between speed and uplift force applied by the pantographs in Figures 9A and 9B; Figures 11A-11C shows schematically variation in pantograph orientation in a train having two pantographs; and Figure 12 shows graphically the relationship between speed and uplift force applied by the pantographs in Figures 11A-11C. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. To increase the contact stability of a trailing pantograph on an overhead line, and to reduce the risk of loss of contact by the trailing pantograph, which in the context of a train formed of plural train units would result in a loss of traction power to the corresponding train unit, the present disclosure provides a control logic which may be embodied in a control system and a method that applies a reduced pantograph uplift force to the leading pantograph in the direction of travel of the train, in order to put less uplift force on the overhead line and its catenary at the passage of the leading pantograph, and hence reduce the standing wave disturbance for a trailing pantograph. This logic can be implemented by a hardware output signal which commands a pneumatic circuit for the pantograph uplift force selection. In particular, the control logic may command a first uplift force for the leading pantograph that is less than a second uplift force for the trailing pantograph, where the difference between the two uplift forces is a function of the speed of the train, and wherein the difference increases as the speed of the train increases. Figure 1 shows schematically a train unit 101, having a single pantograph 102 providing power to a traction unit 103. Although the unit 101 is shown as being formed by a single car, in general a train unit can be formed of multiple coupled cars. The pantograph is raised (and lowered) by a pneumatic actuator 104, operated through a pneumatic circuit in which an input pneumatic pressure to the pneumatic actuator 104 is provided by an air supply 105 feeding a variable pressure regulator 106. A command signal to raise or lower the pantograph 102 is sent from a train-mounted Train Control Management System (TCMS) 107 and is received by the variable pressure regulator 106, which regulates the air supply to the pneumatic actuator 104, an increase in the air supply to the pneumatic actuator 104 resulting in the pantograph 102 being raised. The speed of the train may be measured by a speed sensor 108 which inputs to the TCMS 107. The speed sensor 108 can have many embodiments, including but not limited to a local sensor housed within the train, a GPS-enabled device, or an input signal from an external server. Figure 2 shows schematically a train 200 which is comprised of coupled train units 201,251. More specifically, the train is comprised of a leading train unit 201 and a trailing train unit 251, having a leading pantograph 202 and a trailing pantograph 252 respectively, the pantographs 201,251 powering respective traction systems 203, 253 of the train units 201,251. As in Figure 1, each train unit 201,251 has a respective pneumatic actuator 204, 254, with input pneumatic pressure to the pneumatic actuators 204, 254 being supplied by respective air supplies 205, 255 regulated by respective variable pressure regulators 206, 256 commanded by respective TCMSs 207, 257. The TCMS 207 for the leading train unit 201 is connected to the TCMS 257 for the trailing train unit 257, such that they are able to send and receive messages to each other. A speed sensor 208 sends a signal containing the measured speed of the train to the leading train unit TCMS 207. However, alternative embodiments can have the speed sensor 208 mounted on any of the train units comprising the train, or, as above, the speed sensor 208 can be a GPS-enabled device or an input signal from an external server. The control logic can be combined with the TCMSs 207, 257, which recognise if the train is formed of coupled units, and if so, further recognises the configuration of the train. In other words, the TCMSs 207, 257 recognise which train unit is the leading train unit 201, which train unit is the trailing train unit 251, and apply the control logic appropriately. Under this logic, the first uplift force exerted by the leading pantograph 202 is less than the second uplift force exerted by the trailing pantograph 252, and moreover the difference between these forces varies with changes in train speed. In this way, the effect at the trailing pantograph 252 of the standing wave disturbance generated by the first uplift force exerted on the overhead line by the leading pantograph 202 is reduced. This improves the contact stability of the trailing pantograph 252 on the overhead line, particularly at high speeds. In particular, the control logic allows better use to be made of the available range of allowable uplift forces at a given speed. By reducing the first uplift force relative to the second uplift forces the amplitude of the standing wave disturbance generated by the first uplift force is reduced, and / or by increasing the second uplift force relative to the first uplift force the trailing pantograph 252 is helped to maintain electrical contact for a given amplitude of disturbance. In one implementation, the control logic determines an uplift force for a primary pantograph (a selected one of the leading and trailing pantographs) based of the speed of the train. The uplift force for a secondary pantograph (the other of the leading and trailing pantographs which is not the primary pantograph) is then determined by varying the uplift force applied to the primary pantograph, such as to fulfil the logic’s requirements that the first uplift force exerted by the leading pantograph 202 is less than the second uplift force exerted by the trailing pantograph 252, and moreover that the difference between these forces varies with changes in train speed. Preferably, the primary pantograph is the leading pantograph 202, the secondary pantograph is the trailing pantograph 252, and the uplift force for the secondary pantograph (the trailing pantograph 252) is increased relative to the determined uplift force for the primary pantograph (the leading pantograph 202). The variable pressure regulator 256 on the trailing train unit 251 can then increase the air supply 255 to its raising device 254, such that the second uplift force exerted by the trailing pantograph 252 on the overhead line is increased, relative to the first uplift force exerted by the leading pantograph. In the examples discussed below in relation to Figures 4 to 12, is the primary pantograph is the leading pantograph. Nonetheless, however, it should be noted that the primary pantograph can be the trailing pantograph 252, the secondary pantograph the leading pantograph 202, and the control logic suitably adapted so that the uplift force for the secondary pantograph (the leading pantograph 202) is reduced relative to the determined uplift force for the primary pantograph (the trailing pantograph 252). In this case, the variable pressure regulator 206 on the leading train unit 201 reduces the air supply 205 to the pneumatic actuator 204, such that the first uplift force exerted by the leading pantograph 202 on an overhead line is reduced, relative to the second uplift forces exerted by the leading pantograph. Figure 3 shows graphically example relationships between speed and uplift force for an upper uplift force limit 301 a lower uplift force limit 302, and possible first 303 and second 304 uplift forces applied respectively to the leading 202 and trailing 252 pantographs within the bounds set by those limits. The uplift force limits 301,302 may be set by operational requirements of the network infrastructure in which the train is operating. The limits can be imposed for a variety of reasons, such as to restrict damage to either the pantograph, the overhead line, or any other railway or train component. An operational envelope defining acceptable operable uplift forces is the area between the upper uplift force limit 301 and the lower uplift force limit 302 over a speed range. Both the upper uplift force limit 301 and the lower uplift force limit 302 increase with the speed of the train. In the example, below a speed of 200 km / h the difference between these limits stays constant. However, above 200 km / h the operational envelope progressively widens, i.e. the difference between the limits 301,302 increases as the train operates at higher speeds. Starting with equal first 303 and second 304 uplift forces when the train is stationary, below 200 km / h the control logic increases the difference between the uplift forces 303, 304 with increasing speed to take advantage of the difference between the limits, and then above 200 km / h further increases the difference to take advantage of the widening envelope. The control system constrains the first uplift force 303 to be close to the lower uplift force limit 302. In more detail, the first uplift force can be constrained such that, for a given speed, the ratio A / B is less than 20% and preferably less than 15%, where A is the difference between the first uplift force and the lower uplift force limit and B is the lower uplift force limit. This can be seen in Figure 3, where the first uplift force is greater than but closely follows the lower uplift force limit. This enables the difference between the uplift forces to be increased for a given speed within the operational envelope as defined by the uplift force limits. The standing wave disturbance can increase with speed due to (a) aerodynamic drag increasing the uplift force exerted by the pantograph on the overhead line, and (b) the increased speed of the train causing a greater disturbance to the overhead line, independent of effect (a). By increasing the difference between the uplift forces 303, 304 exerted by the leading 202 and trailing 252 pantographs on the overhead line as the speed of the train is increased, the control logic can mitigate the increased standing wave disturbance generated by the first uplift force on the overhead line. Figure 4 shows schematically the train 200 comprising a leading train unit 201 and a trailing train unit 251. The leading train unit 201 has a leading pantograph 202, and the trailing train unit 251 has a trailing pantograph 252, which exert respective uplift forces on an overhead line 401. An arrow indicates the direction of travel 402 of the train 200. Figures 5 and 6 schematically show features of a control system, where the TCMS determines appropriate uplift forces to be applied by the pantographs 202, 252 by potentially varying a first uplift force input with an additional uplift force input 500 from a measured speed measured by the speed sensor 208 of the train 200. For each of the leading and trailing train units 201, 251, the additional uplift force input 500 is only incorporated into the respective command signal if all conditions satisfying two AND gates are met. More particularly, if all the conditions are confirmed, the additional uplift force input 500 is incorporated with the command signal that is issued from the respective TCMS 207, 257 and received by its variable pressure regulator 206, 256, thereby commanding the application of the uplift force by its pantograph 202, 252. In more detail, Figure 5 illustrates how a command signal may be sent by the TCMS 207 to the pantograph 202 of the leading train unit 201 of the train 200. A first uplift force input is determined in the TCMS 207 in a manner described later in this description for use in forming the command signal. A speed-dependent additional uplift force input 500, determined in a manner also described later, may then be incorporated into the command signal under the control of two AND gates. A first of these AND gates 501 confirms the coupled status of the respective train unit and the raised status of its pantograph, and in this case these statuses are satisfied as the leading train unit 201 is coupled to the trailing train unit 251, and the leading pantograph 202 is raised. However, a second AND gate 502 confirming the moving status of the train and the position of the train unit in the train is in this case unsatisfied as, whilst the train 200 is moving, the respective pantograph 202 is the leading pantograph, as opposed to the trailing pantograph 252. Consequently, the second AND gate 502 prevents the additional uplift force input 500 from being incorporated into the command signal issued to the variable pressure regular 206. Therefore, the command signal instructs the variable pressure regulator to regulate the air supply 205 to the pneumatic actuator 204 in order that the leading pantograph 202 applies just the first uplift force (without any additional uplift force input 500) to the overhead line. Figure 6 by contrast illustrates how a command signal may be sent by the TCMS 257 to the pantograph 252 of the trailing train unit 251 of the train 200. The TCMS uses the same arrangement of AND gates 501,502. Similarly to the leading train unit, the first AND gate 501 is satisfied as the trailing train unit 251 is coupled to the leading train unit 201, and the trailing pantograph 252 is raised. However, now the second AND gate 502 is also satisfied, as the train 200 is moving and the respective pantograph 252 is the trailing pantograph. Consequently, the two AND gates 501,502 permit the speed-dependent additional uplift force input 500 to be incorporated into the command signal issued to the variable pressure regular 256. Therefore, the command signal instructs the variable pressure regulator 256 to regulate the air supply 255 to the pneumatic actuator 204 in order that the trailing pantograph 252 applies the second uplift force (specifically, the first uplift force combined with the additional uplift force input 500) to the overhead line. In this way, while using the same TCMS architecture, the second uplift force applied by the trailing pantograph 252 is greater than the first uplift force applied by the leading pantograph 202, and the difference between the uplift forces increases as a function of speed. Although the functionalities discussed above in relation to the examples of Figures 5 and 6 are embodied in TCMSs 207, 257, in other examples it is possible that some or all of the functionalities may be embodied instead in a control module or modules separate from the TCMSs, or in dedicated pantograph control systems. Next, Figure 7 shows schematically a train 101 similar to the train of Figure 1, i.e. having a single pantograph 102 exerting an uplift force on an overhead line 701. An arrow indicates the direction of travel 702 of the train. Figure 8 schematically how the train’s TCMS 107 determines an appropriate uplift force to be applied by its pantograph 102. The TCMS has the same architecture as that shown in Figure 5 or 6., i.e. with the potential, under certain circumstances, to vary a first uplift force input with a speeddependent additional uplift force input 500. However, as the train 101 is a single unit, not coupled to any other, the AND gate 501 is not satisfied, and this in turn prevents the AND gate 502 from being satisfied. Consequently, the two AND gates 501,502 prevent the speed-dependent additional uplift force input 500 from being incorporated into the command signal issued to the variable pressure regular 106. Therefore, the variable pressure regulator regulates the air supply 105 to the pneumatic actuator 104 in order that the pantograph 102 applies just the first uplift force (without any additional uplift force input 500) to the overhead line. The control logic may determine appropriate uplift forces for the leading and trailing pantographs by using a reference database. This database may store values of a reference uplift force against speed for the primary pantograph. For example, this may be the first uplift force for the leading pantograph. The reference database may further store a coefficient value by which the uplift force for the secondary pantograph (in this example the second uplift force for the trailing pantograph) can be determined by performing a mathematical operation on the selected reference uplift force using the coefficient value. For example, at a given measured speed the control system firstly determines the first uplift force directly from the reference database. The control system then determines the second uplift force by adding to the first uplift force an additional value, which is specifically the product of the coefficient with the difference between the first uplift value and a first uplift force value when the train is stationary, i.e. F2 = F1 + C(Fi - Fo), where F2 is the second uplift force, F1 is the first uplift force, Fo is an stationary uplift force for both the leading and trailing pantographs at zero measured speed, C is the coefficient value. F1 and C(Fi - Fo) are thus the two inputs to the “sum” function of Figures 5, 6 and 8. Alternatively, the control logic can designate the trailing pantograph as the primary pantograph and reduce the first uplift force exerted by the leading pantograph (designated as the secondary pantograph) relative to the second uplift force exerted by the trailing pantograph. In this case, the reference database may store values of a reference uplift force against speed for the second uplift force for the trailing pantograph. Consequently, in this example at a given measured speed the control system firstly determines the second uplift force directly from the reference database. The control system then determines the first uplift force by subtracting from the second uplift force the product of a stored coefficient value with the difference between the second uplift value and the stationary uplift force value, i.e. F1 = F2 - C(F2 - Fo). The control system can also account for the phenomenon of aerodynamic drag on the pantograph producing an additional uplift force caused by the speed of the train. For example, to achieve a desired applied uplift force of 100 N on the overhead line at a speed where the aerodynamic drag would generate an uplift force of 20 N, the command signal to the variable pressure regulator should be set so that the pneumatic system generates just 80 N of force. In the TCMS architecture of Figures 5, 6 and 8, one way to achieve this is simply to reduce the output of the “sum” function by the amount of the aerodynamic drag uplift force, determined e.g. by obtaining the appropriate force from a database (e.g. the reference database mentioned above) which stores values of aerodynamic drag uplift force against train speed. The control system operates by receiving a measurement of the speed of the train 200, determining appropriate values for the first and second uplift forces for the pantographs 202, 252 in the manner described above, and issuing command signals to the pantographs 202, 252 to apply the first and second uplift forces to the overhead line 401. These steps are repeatedly performed in a loop so that as the speed of the train varies during operation, the first and second uplift forces are appropriately adjusted. To help ensure stable current collection and prevent damage to the overhead line, the control logic can determine the appropriate uplift forces by additionally considering the distance between the raised pantographs. This can be particularly beneficial when there is a short separation between the two raised pantographs such that disturbance to contact stability of the trailing pantograph is more likely to occur due to the proximity of the trailing pantograph to the focal point of the standing wave disturbance. Figure 9A shows schematically a first train 200 comprising a leading train unit 201 having a leading pantograph 202, and a trailing train unit 251 with a trailing pantograph 252. The pantographs 202, 252 exert respective uplift forces on the overhead line 401. The train is travelling in a direction denoted by arrow 402. A distance 901 is defined between the leading pantograph 202 and the trailing pantograph 252. Figure 9B shows schematically a second train 200’ comprising a leading train unit 201’ having a leading pantograph 202’, and a trailing train unit 251 ’ with a trailing pantograph 252’. The pantographs 202’, 252’ exert respective uplift forces on the overhead line 401. The train is travelling in a direction denoted by arrow 402. A distance 902 is defined between the leading pantograph 202’ and the trailing pantograph 252’. The distance 901 between the leading pantograph 202 and the trailing pantograph 252 on train 200 is smaller than the distance 902 between the leading pantograph 202’ and the trailing pantograph 252’ on train 200’. Figure 10 shows graphically the relationship between uplift force and speed applied by the pantographs in Figures 9A and 9B. The leading pantographs 202, 202’ have the same uplift force against speed relationship, indicated by the solid line. This is because both the leading pantographs encounter the overhead line in a relatively undisturbed state. The trailing pantographs 252, 252’, by contrast, encounter the overhead line in a disturbed state, from the passing of the leading pantographs 202, 202’. However, as the distance 901 between pantographs 202, 252 is smaller than distance 902 between pantographs 202’, 252’, trailing pantograph 252 experiences a greater disturbance from the standing wave created by the leading pantograph 202, than trailing pantograph 252’ experiences from the standing wave created by leading pantograph 202’. Consequently, the control logic increases the difference between the uplift forces for the pantographs on train 200, compared to the difference between the uplift forces for the pantographs on train 200’. In this embodiment, this is achieved by increasing the uplift force exerted by trailing pantograph 252, relative to the uplift force exerted by trailing pantograph 252’, to mitigate the reduction in contact stability of the trailing pantograph when the distance between the pantographs is reduced. To calculate the distance between the leading and trailing pantographs, the database can store for each pantograph the length between that pantograph and a reference point on the train, such as the end of the train. In this example, the distance is then conveniently just the difference between the two lengths. If the train has three or more pantographs, with two of them being lifted and in use at any given time, and for some reason the train has to lower one pantograph and raise another one, the system can calculate the new distance between the leading and trailing pantographs if the database also stores the order of the pantographs along the length of the train. The present disclosure may relate to half pantographs, which comprise of an upper arm and a lower arm, connected by a knuckle. Half pantographs can be oriented in a knuckle leading orientation, where the knuckle is angled towards the front of the train, and the arms extend from the knuckle in directions generally rearwards therefrom. Alternatively, half pantographs can be oriented in the opposite knuckle trailing orientation. The orientation of the pantograph can vary the uplift forces exerted by the pantographs on the overhead line, and therefore the control logic can take steps to account for the change in the resultant standing waves from the uplift forces, such as to increase the contact stability of the trailing pantograph on the overhead line. Figure 11A shows schematically a first train 200 comprising a leading train unit 201 having a leading pantograph 202 in a knuckle leading orientation, and a trailing train unit 251 with a trailing pantograph 252 also in a knuckle leading orientation, the train travelling in a direction denoted by arrow 402. Figure 11B shows schematically a second train 200” comprising a leading train unit 201” having a leading pantograph 202” in a knuckle leading orientation, and a trailing train unit 251 ” with a trailing pantograph 252” in the opposite knuckle trailing orientation. Figure 11C shows schematically a third train 200”’ comprising a leading train unit 201 having a leading pantograph 202”’ in a knuckle trailing orientation, and a trailing train unit 251”’ with a trailing pantograph 252”’ in the opposite knuckle leading orientation. Figure 12 shows graphically the relationship between uplift force and speed applied by the pantographs in Figure 11. The leading pantographs 202, 202”, 202”’ have the same uplift force against speed relationship because they encounter the overhead line in the same undisturbed state. The trailing pantographs 252, 252”, 252”’, by contrast, encounter the overhead line in a disturbed state. However, when the orientations of the leading and trailing pantographs are different from each other, such as for trains 200”, 200’”, the trailing pantograph experiences a greater loss of contact stability with the overhead line, due to aerodynamic forces exacerbating the overhead wire disturbance experienced by the trailing pantograph. Consequently, the control logic increases the difference between the leading and trailing pantograph uplift forces when the pantographs have different orientations. This can be achieved by increasing the uplift force exerted by the trailing pantographs 252”, 252’” on trains 200”, 200’”, relative to the uplift force exerted by the trailing pantograph 252 on train 200. In the case that a train raises three or more pantographs at the same time (e.g. four raised pantographs, with two on a first train unit and another two on second train unit coupled to the first), the control system may control the uplift forces adjacent pantographs so that the difference between the uplift forces of each pair of adjacent pantographs increases as the vehicle speed increases. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. 5 It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it 10 will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%.

Claims

1. A control system for a train (200) having two pantographs (202, 252) raised for collecting power from an overhead line (401), wherein the control system is configured to perform the steps of:(a) determining, relative to a direction of travel of the train (402), which of the two pantographs (202, 252) is a leading pantograph (202) and which is a trailing pantograph (252),(b) receiving a measurement of a speed of the train,(c) determining, relative to the measured speed, a first uplift force for the leading pantograph (202) against the overhead line (401) and a second uplift force for the trailing pantograph (252) against the overhead line (401), wherein the second uplift force is greater than the first uplift force, and the difference between the first and second uplift forces is a function of the measured speed in which the difference increases as the vehicle speed increases, and(d) issuing command signals to the leading and trailing pantographs (202, 252) to respectively apply the first and second uplift forces to the overhead line (401).

2. The control system according to claim 1, wherein the control system repeatedly performs in a loop steps (b) to (d) such that the difference between the first and second uplift forces of the command signals varies as the measured speed varies.

3. The control system according to any preceding claim, wherein each pantograph (202, 252) is part of a respective pantograph system comprising:the pantograph (202, 252),a pneumatic actuator (204, 254) to control the position of the pantograph (202, 252), anda pneumatic pressure regulator (206, 256), to regulate an air supply (205, 255) to the pneumatic actuator (204, 254),wherein the respective command signal is received by the pneumatic pressure regulator (206, 256), which regulates the air supply (205, 255) to the pneumatic actuator (204, 254) to positionally control the pantograph (202, 252) to produce the respective uplift force.

4. The control system according to any preceding claim, wherein the first and second uplift forces (303, 304) are constrained to stay between a lower uplift force limit (302) and an upper uplift force limit (301).

5. The control system according to claim 4, wherein each uplift force limit (301,302) increases with increase in the measured speed.

6. The control system according to claim 4 or 5, wherein the first uplift force (303) is constrained such that, for a given measured speed, the ratio A / B is less than 20%, where A is the difference between the first uplift force (303) and the lower uplift force limit (302) and B is the lower uplift force limit (302).

7. The control system according to any preceding claim, wherein the function of the measured speed, in which the difference between the first and second uplift forces (303, 304) increases as the vehicle speed increases, is such that the difference increases linearly with measured speed.

8. The control system according to any preceding claim, wherein the control system contains a reference database storing values of a reference uplift force against speed for a selected one of the leading and trailing pantographs (202, 252), the control system determining the uplift force for said selected pantograph by selecting the reference uplift force corresponding to the measured speed, and the control system further stores a coefficient value by which the uplift force for the other of the leading and trailing pantographs is determined by performing a mathematical operation on the selected reference uplift force using the coefficient value.

9. The control system according to claim 8, wherein the coefficient value is a function of the measured speed.

10. The control system according to any preceding claim, wherein step (a) also includes determining a distance (901; 902) between the two pantographs (202, 252; 202’, 252’), and wherein in step (c) the difference between the first and second uplift forces is also a function of said distance (901; 902) in which the difference increases as the distance (901; 902) decreases.

11. The control system according to any preceding claim, wherein:each pantograph is a half-pantograph having an upper arm and a lower arm connected by a knuckle, and each pantograph is operable in either a knuckle leading orientation or a knuckle trailing orientation relative to the direction of travel (402),step (a) also includes determining the respective orientations of the two pantographs, and in step (c) the difference between the first and second uplift forces is reduced when the two pantographs are in the same orientation and increased when the two pantographs are in different orientations.

12. The control system according to any preceding claim, wherein the control system is further configured to raise the two pantographs (202, 252) preliminary to issuing the command signals to the two pantographs.

13. A train (200) having two or more pantographs (202, 252) and the control system of any one of the previous claims.

14. The train (200) according to claim 13, wherein the train (200) comprises two or more coupled train units (201,251), and the two pantographs (202, 252) are parts of different train units (201,251).

15. A method of controlling the pantographs of a train (200) having two pantographs (202, 252) raised for collecting power from an overhead line (401), wherein the method includes:(a) determining, relative to a direction of travel of the train (402), which of the two pantographs (202, 252) is a leading pantograph (202) and which is a trailing pantograph (252),(b) measuring a speed of the train,(c) determining, relative to the measured speed, a first uplift force for the leading pantograph5 (202) against the overhead line (401) and a second uplift force for the trailing pantograph (252) againstthe overhead line (401), wherein the second uplift force is greater than the first uplift force, and the difference between the first and second uplift forces is a function of the measured speed in which the difference increases as the vehicle speed increases, and(d) commanding the leading and trailing pantographs (202, 252) to respectively apply the first and 10 second uplift forces to the overhead line (401).19

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