Airside vehicles and methods

Abstract

The present invention relates to a non-driven airside vehicle 1. The non-driven airside vehicle comprises a steering system 11, 12, 13a, 13b, 14 and / or a braking system 16a, 16b. The steering system is configured to enable the non-driven airside vehicle to travel in a direction having a component substantially perpendicular to a towing force on the non-driven airside vehicle. The non-driven airside vehicle further comprises a control system configured to output one or more commands to the steering system and / or the braking system to provide control of steering and / or braking of the non-driven airside vehicle. Also disclosed is a vehicle train comprising one or more non-driven airside vehicle, methods of operating a vehicle train, and a method of retrofitting an existing non-driven airside vehicle. The steering system may comprise a steering axle, a wheel mounted to each end of the steering axle, and a steering actuator configured to pivot the steering axle relative to the towing force on the non-driven airside vehicle. There is further claimed a method of docking airside vehicles of a vehicle train and a method of retrofitting an existing non-driven airside vehicle.

Description

TEHCNICAL FIELD The present invention relates to a non-driven airside vehicle, a vehicle train comprising one or more non-driven airside vehicle, methods of operating a vehicle train, and a method of retrofitting an existing non-driven airside vehicle. BACKGROUND Airside vehicles are used to transport baggage, cargo, passengers, and ground equipment around an airport. An example of an airside vehicle is an airside baggage or cargo dolly. Airport infrastructure typically comprises fixed structures arranged close together. For example, a baggage hall typically comprises rows of lateral conveyers with limited space between the lateral conveyers. Airside dollies are required to manoeuvre within these limited spaces in order to collect baggage or cargo from the lateral conveyers. This means that the size of individual airside dollies is limited. In order to transport more baggage or cargo than can be transported by a single airside dolly, multiple airside dollies are typically mechanically coupled together in a line and towed using a towing vehicle, such as a tug or baggage, thereby forming a vehicle train comprising the towing vehicle and the airside dollies. A vehicle train comprising multiple airside vehicles is typically difficult to manoeuvre. For example, a change in direction of the towing vehicle may result in undesirable swaying of the airside vehicles which can be difficult to bring under control. In addition, there will be an inevitable delay between a change in direction of the towing vehicle and a corresponding desired change in direction of the airside dollies. A particular problem arises when it is necessary to align the airside dollies with a lateral, such as a lateral conveyer or a lateral platform at a loading bay. Due to the delay in a change in direction of the airside dollies, the vehicle train must be manoeuvred to align with a lateral in advance of the lateral, which requires additional space within the airport environment. Such a manoeuvre is also unlikely to be performed perfectly, resulting in impact between one or more of the airside dollies in the train and the structure of the lateral. The above manoeuvrability problems means that the number of airside vehicles that can be towed in a vehicle train is limited, which means that the throughput of baggage, cargo, passengers, and equipment within an airport environment is limited. This in turn has implications on ground handling efficiency and ultimately cost. One solution to the above problems is to provide one or more self-propelled autonomous airside dollies, such as those disclosed in WO2020128442A2, which can be manoeuvred independently outside of a vehicle train. However, this solution may not always be appropriate, for example where an airport environment lacks the necessary network and communication infrastructure to support one or more autonomous airside dollies. Another solution is to increase the length of the mechanical couplings between vehicles in a vehicle train. This may help with stability but increases the overall length of the vehicle train. This means that fewer vehicles in the vehicle train can align with a fixed length lateral, meaning that fewer vehicles can be loaded or unloaded with baggage or cargo. There is therefore a need for further solutions to the manoeuvrability issues of nondriven airside vehicles. SUMMARY OF INVENTION According to a first aspect of the invention, there is provided a non-driven vehicle comprising a steering system and / or a braking system, and a control system. The control system is configured to output one or more commands to the steering system and / or the braking system. As referred to herein, a ‘non-driven’ vehicle is a vehicle which does not comprise means to generate its own motive power. In other words, a non-driven vehicle is not self-propelled, i.e., it does not have means to move itself. As such, a non-driven vehicle requires a motive force from an external source for the non-driven vehicle to move. Such motive force may be provided by another vehicle towing or pushing the non-driven vehicle. The non-driven vehicle of the first aspect of the invention may comprise a trailer, a box trailer, a flatbed trailer, a dolly, a caravan, a horse box, or the like. The steering system may comprise a mechanical, hydraulic, pneumatic, electrical, or electromechanical steering system. The braking system may comprise a mechanical, hydraulic, pneumatic, electrical, or electromechanical braking system. The control system may comprise one or more controllers, one or more processors, one or more memories, and / or one or more other suitable electronic control means. Typically, non-driven vehicles such as those described above, do not have any controllable steering or braking means. As such, there is limited control over the movement of a typical non-driven vehicle in use. This results, for example, in phenomenon such as swaying and jack-knifing of the non-driven vehicle as it is towed by a towing vehicle. The turning circle of the combined towing vehicle and non-driven vehicle is also limited. Providing a controllable steering and / or braking means, as in the first aspect of the invention, advantageously provides improved control of the movement of the non-driven vehicle. The non-driven vehicle may comprise a non-driven airside vehicle. The non-driven airside vehicle may comprise an airside dolly. The airside dolly may comprise an airside baggage dolly or an airside cargo dolly. The maximum length of the airside dolly may be in the range of l-7m, 3-5m, or 3.5-4.5m. The maximum width of the airside dolly may be in the range of 0.5-3.5m, l-3m, or 1.5-2.5m. The unladen mass of the airside dolly may be in the range of 300-700kg, 400-600kg, or 450-550kg. The load capacity of the airside dolly may be up to 10,000kg, 7500kg, 5000kg, 2000kg, 1500kg, 1000kg, or 500kg. The problem of controlling the movement of a non-driven vehicle is particularly prevalent in non-driven airside vehicles. Non-driven airside vehicles operate in environments in which space is limited and there is large number of fixed immovable structures to navigate around. Typical non-driven airside vehicles do not comprise any controllable steering and / or braking means, resulting in, for example, loss of control of a vehicle train comprising multiple non-driven airside vehicles and collisions between non-driven airside vehicles and fixed structures. A particular problem is that it is not feasible to perform a reversing manoeuvre of a vehicle train comprising multiple conventional non-driven airside vehicles as the vehicle train inherently experiences jack-knifing when reversing. The controllable steering and / or braking means of the first aspect of the invention provides means for addressing these problems. The steering system may be configured to enable the non-driven vehicle to travel in a direction having a component substantially perpendicular to a towing force on the nondriven vehicle. The steering system may be configured to enable the non-driven vehicle to travel in a direction which is oblique to a forward or reverse direction of travel of the non-driven vehicle. The steering system may be configured to enable the non-driven vehicle to travel in a direction which is substantially perpendicular to a forward or reverse direction of travel of the non-driven vehicle. The steering system may comprise a crab-steering system. The steering system may be operable in a normal mode, in which the steering system is configured to enable the non-driven vehicle to travel substantially in the direction of a towing force on the non-driven airside vehicle, and a docking mode or enhanced steering mode, in which the steering system is configured to enable the non-driven airside vehicle to travel in a direction having a component substantially perpendicular to a towing force on the non-driven airside vehicle. The steering system being configured in the above manner advantageously allows the non-driven vehicle to be more accurately and more closely aligned with a fixed structure adjacent the non-driven vehicle, such as a docking lateral or the like. In use, a towing vehicle may be used to tow the non-driven vehicle in the normal mode to a position approximately aligned with a docking lateral. The non-driven vehicle may then be configured into the docking mode such that the non-driven vehicle is able to travel in a direction having a component towards the docking lateral. As the towing vehicle moves in a forward direction, exerting a forward towing force on the non-driven vehicle, the non-driven vehicle will move diagonally towards the docking lateral, thereby becoming more closely aligned with the docking lateral. The enhanced steering mode advantageously provides the non-driven vehicle with a tighter turning circle in use and enables a number of control modes, such as jack-knife mitigation and sway correction, as described further below. The steering system may comprise a steering axle, at least one wheel mounted to the axle, and a steering actuator. The steering actuator may be configured to pivot the steering axle relative to the towing force on the non-driven vehicle. The steering system may comprise a wheel mounted to each end of the axle. The pivot point of the axle may be located between the ends of the axle. The steering system of any of these embodiments advantageously provides a simple means of enabling the docking mode or enhanced steering mode described above without the need, for example, for complex steering linkages or the like. The non-driven vehicle may comprise a drawbar. The axle may be pivotably mounted relative to the drawbar. The axle may be pivotably mounted directly to the drawbar. The steering actuator may be arranged to pivot the axle relative to the drawbar. The steering system of any of these embodiments advantageously enables an existing non-driven vehicle, which already comprises a drawbar and an axle, to be retrofitted with the steering system. The steering axle and the drawbar may be arranged to pivot together relative to the chassis of the non-driven vehicle. The steering actuator may comprise rod of variable length. The steering actuator may be configured to pivot the steering axle by means of altering the length of the rod. A first end of the rod may be mounted to the steering axle and a second end of the rod, opposite the first end, may be mounted to the drawbar. The rod may comprise a hydraulic cylinder. In other embodiments, the steering actuator may comprise an alternative hydraulic actuator, a pneumatic actuator, a mechanical actuator, an electrical actuator, an electromechanical actuator, or any other suitable actuator. The axle may be a front axle of the non-driven vehicle. This may provide advantages in terms of steering dynamics. The non-driven vehicle may comprise a fixed rear axle, which may also provide advantages in terms of steering dynamics. The combination of a steerable front axle and a fixed rear axle may be particularly advantageous. In other embodiments, the steering system may comprise a rear steering axle in addition to or as an alternative to the front steering axle. The non-driven vehicle may comprise one or more rear wheels. The braking system may be configured to exert a braking force on the one or more rear wheels. This may provide advantages in terms of vehicle dynamics. The combination of a steerable front axle and a fixed rear axle where the braking system is configured to exert a braking force on the one or more rear wheels may be particularly advantageous as it may provide improved vehicle stability when steering and braking at the same time. For example, if a braking force were exerted on a wheel mounted to the steerable front axle during steering of the non-drive vehicle, the steering of the vehicle may be affected by the braking force. The braking system may alternatively or additionally be configured to exert a braking force on one or both of the wheels mounted to the steering axle. The braking system may be configured to provide differential braking. Differential braking may comprise exerting a different braking force on different wheels. The control system may be configured to receive one or more inputs from one or more external sources and / or provide one or more outputs to one or more external sources. The control system may be configured to output one or more commands to the steering system and / or the braking system in dependence on the one or more inputs from one or more external sources. The control system may therefore be configured to enable monitoring of the non-driven vehicle by an external system and / or remote control of the steering system and / or braking system of the non-driven vehicle. The one or more external sources may comprise one or more other vehicle and / or a central controller of a baggage handling system or other appropriate system. The non-driven vehicle may comprise one or more proximity sensors. The control system may be configured to output one or more commands to the steering system and / or the braking system in dependence on one or more signals received from the one or more proximity sensors. The one or more proximity sensors may be configured to detect the presence of an object within the surroundings of the respective non-driven vehicle. The one or more proximity sensors may be configured to measure a distance between the respective non-driven vehicle and an object within the surroundings of the respective non-driven vehicle. An object within the surroundings of the respective non-driven vehicle may be a fixed object or a moving object; the one or more proximity sensors may be capable of tracking a moving object within the surroundings of the respective non-driven vehicle, i.e., the one or more proximity sensors may be capable of detecting the presence of a moving object and measuring a temporal distance between the respective non-driven vehicle and the moving object. This may enable operation of the non-driven vehicle in any mode of operation in which it is desirable to maintain a minimum distance between the non-driven vehicle and objects within the surroundings of the non-driven vehicle, for example when trying to avoid collisions between the non-driven vehicle and objects within the surroundings of the non-driven vehicle, or when trying to maintain a minimum distance between the non-driven vehicle and another vehicle when the non-driven vehicle is operating in proximity to one or more other vehicles, such as when in a vehicle train. This may also enable operation of the non-driven vehicle in any mode of operation in which it is desirable to minimise a distance between the non-driven vehicle and an object within the surroundings of the non-driven vehicle, for example in a docking mode where the object is a docking lateral. According to a second aspect of the invention, there is provided a vehicle train comprising a towing vehicle and one or more non-driven vehicles according to the first aspect of the invention. The towing vehicle may be a human-operated non-autonomous towing vehicle. For example, where the or each non-driven vehicle is a non-driven airside vehicle, the towing vehicle may comprise a conventional baggage tractor or tug. In use, a human operator may operate the towing vehicle to tow the or each non-driven vehicle in the vehicle train. The vehicle train can operate in a conventional manner, in which a change in speed or direction of the towing vehicle results in a corresponding change in speed or direction of the or each non-driven vehicle in the vehicle train. However, the steering system and / or braking system and the control system of the or each non-driven vehicle provide improved manoeuvrability, as described further below. The steering system of at least one of the non-driven vehicles in the vehicle train may be configured to enable the non-driven vehicle to travel towards the centre of a turning circle of the vehicle train in a direction having a component substantially perpendicular to a towing force on the non-driven airside vehicle. This may advantageously reduce the size of the turning circle of the vehicle train compared to if the or each non-driven vehicle was only enabled to travel in the direction of the towing force on the respective non-driven vehicle. In addition, it means that shorter mechanical couplings between vehicles in the vehicle train can be used to achieve the same turning circle compared to a conventional vehicle train, thereby reducing the overall length of the vehicle train. The towing vehicle may comprise a control system configured to receive one or more inputs from the control system of at least one of the non-driven vehicles. This may enable the control system of the towing vehicle to monitor one or more of the nondriven vehicles in the vehicle train. The control system of the towing vehicle may be configured to provide one or more outputs to the control system of at least one of the non-driven vehicles. The control system of the towing vehicle may be configured to provide a braking request to the control system of at least one of the non-driven vehicles. The control system of the or each non-driven vehicle may be configured to output a braking command to the braking system of the respective non-driven vehicle in response to the braking request. The braking system may exert a braking force on one or more wheels of the respective nondriven vehicle in response to the braking command. The braking system may exert a different braking force on two or more different wheels in response to the braking command. The control system of the towing vehicle may be configured to provide a steering request to the control system of at least one of the non-driven vehicles. The control system of the or each non-driven vehicle may be configured to output a steering command to the steering system of the respective non-driven airside vehicle in response to the steering request. The steering system may be configured to actuate the steering actuator in response to the steering command. The control system of the towing vehicle may therefore be configured to control the steering and / or braking of one or more non-driven vehicles within the vehicle train. This may be dependent on any suitable input provided to the control system of the towing vehicle, including one or more inputs provided by a human operator or one or more inputs from one or more sensors or other control systems. Providing control of the one or more non-driven vehicles via the towing vehicle means that movements of the one or more non-driven vehicles can be controlled in response to movements of the towing vehicle, thereby improving the manoeuvrability of the vehicle train as a whole. It will be appreciated that, in use, the control system of the towing vehicle may only provide a steering request and / or braking request to one or more, but not all, of the non driven vehicles in the vehicle train. The control system of the towing vehicle may also provide a different steering request and / or braking request to two or more of the nondriven vehicles in the vehicle train. The steering and / or braking of each non-driven vehicle in the vehicle train may be controlled differently in order to affect an intended behaviour of the vehicle train as a whole. Communication between the control system of the towing vehicle and the control system of the or each non-driven vehicle may be provided by any suitable means, including wireless or wired means. The towing vehicle may comprise one or more proximity sensors. The control system of the towing vehicle may be configured to provide one or more outputs to the control system of at least one of the non-driven vehicles in dependence on one or more signals received from the one or more proximity sensors of the towing vehicle. This may enable control of the steering system and / or braking system of the or each non-driven vehicle in dependence on the proximity of the towing vehicle to an object within the surroundings of the towing vehicle, for example to avoid the object. The vehicle train may comprise two or more non-driven vehicles according to the first aspect of the invention. The vehicle train may comprise three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, or ten or more non-driven vehicles according to the first aspect of the invention. In a conventional vehicle train, such as an airside vehicle train, the manoeuvrability problems discussed in the background above become worse as the number of non-driven vehicles in the train increases. As such, the number of non-driven vehicles is limited. In the example of an airside vehicle train, the number of non-driven vehicles is typically limited to four. The manoeuvrability improvements provided by embodiments of the present invention enable a vehicle train comprising many more non-driven vehicles, for example up to ten or more non-driven vehicles in the example of an airside vehicle train. Where the vehicle train comprises two or more non-driven vehicles, the control system of the towing vehicle may be configured to provide a braking request to the control system of at least two of the non-driven vehicles. The control system of each of the at least two non-driven vehicles may be configured to output a braking command to the braking system of the respective non-driven vehicle in response to the braking request to cause synchronised braking of the at least two non-driven vehicles. Synchronised braking may comprise the braking system of two or more of the nondriven vehicles exerting a braking force on one or more wheels of the respective non-driven vehicle at the same time. This may be in addition to causing independent braking of the two or more non-driven vehicles at a different time, such as immediately before or immediately after synchronised braking. Independent braking may comprise the braking system of one or more, but fewer than all, of the two or more non-driven vehicles exerting a braking force on one or more wheels of the respective non-driven vehicle at a given time. Providing synchronised braking of at least two of the non-driven vehicles advantageously mitigates jack-knifing during stopping or deceleration of the vehicle train. Synchronised braking is particularly advantageous during an emergency stop of the vehicle train, i.e., during heavy braking, when the risk of jack-knifing is increased. The control system of at least one of the non-driven vehicles may be configured to receive one or more inputs from the control system of at least one other of the nondriven vehicles and / or provide one or more outputs to the control system of at least one other of the non-driven airside vehicle. At least two of the non-driven vehicles in the vehicle train may therefore be in communication with each other. Communication between non-driven vehicles in the vehicle train be provided by any suitable means, including wireless or wired means. The control system of at least one of the non-driven vehicles may be configured to provide a braking request and / or a steering request to the control system of at least one other of the non-driven vehicles. The control system of at least one of the non-driven vehicles may be configured to receive one or more signals from the one or more proximity sensors of at least one other of the non-driven vehicles. The or each nondriven vehicle may be configured to respond to the braking request and / or the steering request and the one or more signals from the one or more proximity sensors as described above. This may enable at least one non-driven vehicle within the vehicle train to adjust its steering and / or braking independence on the behaviour of at least one other nondriven vehicle within the vehicle tram. It will be appreciated that, in use, the control system of a non-driven vehicle may only provide a steering request and / or braking request to one or more, but not all, of the other non-driven vehicles in the vehicle train. The control system of anon-driven vehicle may also provide a different steering request and / or braking request to one or more other of the non-driven vehicles in the vehicle train. The steering and / or braking of each nondriven vehicle in the vehicle train may be controlled differently in order to affect an intended behaviour of the vehicle train as a whole. At least one of the non-driven vehicles may comprise a target sensor configured to detect a target on an object within the surroundings of the non-driven vehicle. The target may comprise a stop target. The object within the surroundings of the non-driven vehicle may comprise another vehicle, such as another vehicle in a vehicle train if the nondriven vehicle is operating as part of a vehicle train. The target may comprise a paint marking, a computer readable code, a hole, a protrusion, a reflector, or any other suitable target. The target sensor may be integrated with or the same as the one or more proximity sensors. The control system of the or each non-driven vehicle may be configured to provide a train braking request and / or a train steering request to the control system of the towing vehicle in dependence on the target sensor detecting the target. The control system of the towing vehicle may be configured to provide a vehicle braking request and / or a vehicle steering request to the control system of at least one of the non-driven vehicles in dependence on the train braking request and / or the train steering request. The control system of the or each non-driven vehicle may be configured to output one or more commands to the steering system and / or the braking system of the respective non-driven vehicle in dependence on the vehicle braking request and / or the vehicle steering request. In one example, in use, a stop target may be provided on a docking lateral. Detection of the stop target by the stop target sensor of one of the non-driven vehicles may indicate that the respective non-driven vehicle is approaching a predetermined docked position. In response, the control system of the respective non-driven vehicle may provide a train braking request to the control system of the towing vehicle in order stop the vehicle train when the respective non-driven vehicle reaches the docked position. The towing vehicle and / or at least one of the non-driven vehicles may comprise one or more sway sensors configured to generate one or more sway signals indicative of sway of the vehicle train. The control system of at least one of the non-driven vehicles may be configured to output one or more commands to the steering system and / or the braking system of the non-driven vehicle in dependence on the one or more sway signals to correct the sway. The one or more sway sensors may comprise one or more steering angle sensors configured to measure changes in the angle of the steering axle of at least one of the non-driven vehicles over time. The one or more steering angle sensors may generate one or more sway signals in response to a sinusoidal change in the angle of the steering axle of the at least one non-driven vehicle as measured by the one or more steering angle sensors. The one or more sway sensors may comprise one or more wheel speed sensors configured to measure a difference in speed between two or more wheels of at least one of the non-driven vehicles. The one or more wheel speed sensors may generate one or more sway signals in response to a difference in speed between two or more wheels of at least one of the non-driven vehicles as measured by the one or more wheel speed sensors. The one or more sway sensors may comprise one or more accelerometers configured to detect sway of the vehicle train. In other embodiments, the one or more sway sensors may comprise any suitable sensor(s) configured to detect sway of the vehicle train. Where at least one of the non-driven vehicles comprises one or more sway sensors, the control system of the or each non-driven vehicle may be configured to provide a sway correction request to the control system of the towing vehicle in response to the one or more sway signals. The control system of the towing vehicle may be configured to provide a vehicle steering request and / or a vehicle braking request to the control system of at least one of the non-driven vehicles in dependence on the sway correction request to correct the sway of the vehicle train. The control system of the or each non-driven vehicle may be configured to output one or more commands to the steering system and / or the braking system of the respective non-driven vehicle in dependence on the vehicle steering request and / or the vehicle braking request. Where the towing vehicle comprises one or more sway sensors, the control system of the towing vehicle may be configured to provide a vehicle steering request and / or a vehicle braking request to the control system of at least one of the non-driven vehicles in response to the one or more sway signals to correct the sway of the vehicle train. The control system of the or each non-driven vehicle may be configured to output one or more commands to the steering system and / or the braking system of the respective nondriven vehicle in dependence on the vehicle steering request and / or the vehicle braking request. In a conventional vehicle train, sway mitigation might be achieved by increasing the length of the mechanical couplings between vehicles in the vehicle train. This increases the overall length of the vehicle train. Embodiments of the present invention provide means of sway mitigation which do not require increasing the length of the mechanical couplings between vehicles in a vehicle train. Preferably, at least one of the non-driven vehicles in the vehicle train comprises one or more proximity sensors as described above. These proximity sensors are utilised in the following embodiments of the invention. At least one of the non-driven vehicles in the vehicle train may be arranged such that the vehicle in front of the non-driven vehicle in the vehicle train falls within the field of view of the proximity sensor of the non-driven vehicle. The or each non-driven vehicle may be arranged such that the vehicle in front always falls within the field of view of the proximity sensor of the non-driven vehicle, or only falls within the field of view of the proximity sensor of the non-driven vehicle during turning of the vehicle train, such as turning of the vehicle train beyond a predetermined turning angle. The control system of at least one of the non-driven vehicles may be configured to output one or more commands to the steering system and / or the braking system of the non-driven vehicle in dependence on the one or more proximity signals to achieve a predetermined target distance between the non-driven vehicle and an object within the surroundings of the non-driven vehicle. This may enable at least one of the non-driven vehicles in the vehicle train to avoid an obstacle within the surroundings of the nondriven vehicle. Where the towing vehicle also comprises one or more proximity sensors, the control system of the towing vehicle may be configured to provide one or more outputs indicative of the predetermined target distance to the control system of at least one of the non-driven vehicles in dependence on one or more signals received from the one or more proximity sensors of the towing vehicle. In use, the towing vehicle may be operated to manoeuvre around an object such that there is a clearance distance between the towing vehicle and the object. The control system of the towing vehicle may determine the value of the clearance distance based on one or more signals received from the one or more proximity sensors of the towing vehicle. The control system of the towing vehicle may then communication the clearance distance to the control system of at least one of the non-driven vehicles. The control system of the or each non-driven vehicle may then output one or more commands to the steering system and / or the braking system of the respective non-driven vehicle so as to achieve the same clearance distance between the non-driven vehicle and the object as the non-driven vehicle is towed past the object. The control system of at least one of the non-driven vehicles may be configured to output one or more commands to the steering system of the non-driven vehicle to enable the non-driven vehicle to travel towards a docking lateral in a direction having a component substantially perpendicular to a towing force on the non-driven vehicle. This enables the towing vehicle to tow the non-driven vehicle in a forward direction while the non-driven vehicle moves laterally towards the docking structure at the same time as moving forward. This advantageously enables closer alignment of the non-driven vehicle with the docking structure. The one or more commands may be in dependence on one or more signals received from the one or more proximity sensors of the nondriven vehicle, or on one or more signals received from the one or more proximity sensors of another vehicle of the vehicle train, such as the towing vehicle or another non-driven vehicle. The control system of at least one of the non-driven vehicles may be configured to output one or more commands to the steering system and / or the braking system of the non-driven vehicle in dependence on the one or more proximity signals received from the one or more proximity sensors of the non-driven vehicle to maintain a predetermined minimum distance between the non-driven vehicle and an adjacent vehicle in the vehicle train. The control system is preferably configured to operate in this manner during both forward and reverse travel of the vehicle train. The predetermined minimum distance is preferably selected so as to mitigate jack-knifing or collision between vehicles in the vehicle train during operation. The adjacent vehicle in the vehicle train may be another non-driven vehicle or it may be the towing vehicle, depending on the position of a nondriven vehicle in the vehicle train. A predetermined minimum angle may be used in place of or in addition to the predetermined minimum distance. In a conventional vehicle train, jack-knife mitigation might be achieved by increasing the length of the mechanical couplings between vehicles in the vehicle train. This increases the overall length of the vehicle train. Embodiments of the present invention provide means of jack-knife mitigation which do not require increasing the length of the mechanical couplings between vehicles in a vehicle train. The control system of at least one of the non-driven vehicles may be configured to provide a train braking request to the control system of the towing vehicle. The train braking request may be dependent on one or more signals received from the one or more proximity sensors of the non-driven vehicle. The control system of the towing vehicle may be configured to provide a vehicle braking request to the control system of at least one of the non-driven vehicles in dependence on the train braking request. The control system of the or each non-driven vehicle may be configured to output one or more commands to the braking system of the respective non-driven vehicle in dependence on the vehicle braking request. At least one individual non-driven vehicle in the vehicle train may therefore request braking of the vehicle train as a whole. For example, the one or more proximity sensors of one of the non-driven vehicles may indicate that there is an obstacle within the surroundings of the vehicle train and respond by providing a train braking request to the towing vehicle in order to stop the vehicle train and avoid any collision with the obstacle. In other embodiments, a train braking request may not be dependent on one or more signals received from one or more proximity sensors; for example, one of the nondriven vehicles may provide a train braking request in response to a malfunction detected on the respective non-driven vehicle. The control system of at least one of the non-driven vehicles may be configured to provide a train steering request to the control system of the towing vehicle. The train steering request may be dependent on one or more signals received from the one or more proximity sensors of the non-driven vehicle providing the steering request. The control system of the towing vehicle may be configured to provide a vehicle steering request to the control system of at least one of the non-driven vehicles in dependence on the train steering request. The control system of the or each non-driven vehicle may be configured to output one or more commands to the steering system of the respective non-driven vehicle in dependence on the vehicle steering request. At least one individual non-driven vehicle in the vehicle train may therefore request steering of the vehicle train as a whole. For example, the one or more proximity sensors of one of the non-driven vehicles may indicate that there is an obstacle within the surroundings of the vehicle train and respond by providing a train steering request to the towing vehicle in order to steer the vehicle train around the obstacle. The control system of at least one of the non-driven vehicles may be configured to provide a vehicle braking request to the control system of at least one other of the nondriven vehicles. The vehicle braking request may be dependent on one or more signals received from the one or more proximity sensors of the non-driven vehicle providing the vehicle braking request. The control system of the or each other non-driven vehicle may be configured to provide a braking request to the control system of the respective non-driven vehicle in dependence on the vehicle braking request. The control system of the or each non-driven vehicle may be configured to output one or more commands to the braking system of the respective non-driven vehicle in dependence on the braking request. The control system of at least one of the non-driven vehicles may be configured to provide a vehicle steering request to the control system of at least one other of the nondriven vehicles. The vehicle steering request may be dependent on one or more signals received from the one or more proximity sensors of the non-driven vehicle providing the steering request. The control system of the or each other non-driven vehicle may be configured to provide a steering request to the control system of the respective nondriven vehicle in dependence on the vehicle steering request. The control system of the or each non-driven vehicle may be configured to output one or more commands to the steering system of the respective non-driven vehicle in dependence on the steering request. At least one non-driven vehicle of the vehicle train may therefore be configured to request braking and / or steering of at least one other non-driven vehicle of the vehicle train. For example, the one or more proximity sensors of one of the non-driven vehicles may indicate that there is in an obstacle in the surroundings of the respective non-driven vehicle. The non-driven vehicle may then provide a vehicle steering request to an adjacent non-driven vehicle in the vehicle train to enable the adjacent non-driven vehicle to steer around the obstacle. It will be appreciated that the vehicle train of the second aspect of the invention may comprise one or more other non-driven vehicle in addition to one or more non-driven vehicles of the first aspect of the invention. The one or more other non-driven vehicles may not comprise a steering system, a braking system, or a control system. The one or more other non-driven vehicles may comprise one or more conventional non-driven vehicle, such as a conventional airside dolly. Any reference to at least one of the non-driven vehicles of the vehicle train may refer to all of the non-driven vehicles of the vehicle train. The towing vehicle may be capable of operating in a semi-autonomous or fully autonomous mode. The towing vehicle may be capable of operating in both a human-operated mode, in which a human operator controls the towing vehicle either remotely or while travelling with the towing vehicle, and a semi-autonomous or fully autonomous mode. The towing vehicle may comprise a cab configured to allow a human operator to travel with the towing vehicle. The towing vehicle may be propelled by means of an internal combustion engine and / or one or more electric motors. According to a third aspect of the invention, there is provided a method of docking one or more non-driven airside vehicles of a vehicle train, comprising: towing the or each non-driven airside vehicle in a forward direction to align the or each non-driven airside vehicle with a docking structure; controlling a steering system of the or each non-driven airside vehicle to enable the non-driven airside vehicle to travel in a docking direction, wherein the docking direction is towards the docking structure and has a component substantially perpendicular to the forward direction; and further towing the or each non-driven airside vehicle in the forward direction to cause the non-driven airside vehicle to travel in the docking direction. The method may comprise stopping the vehicle train when the or each non-driven airside vehicle is a predetermined distance from the docking structure. Where there is more than one non-driven airside vehicle in the vehicle train, the predetermined distance may different or may be the same for each non-driven airside vehicle. According to a fourth aspect of the invention, there is provided a method of turning a vehicle train, the vehicle train comprising a towing vehicle and one or more non-driven vehicles, the method comprising controlling a steering system of the or each non-driven vehicle to cause the respective non-driven vehicle to travel towards the centre of a turning circle of the vehicle train in a direction having a component substantially perpendicular to a towing force on the respective non-driven vehicle. According to a fifth aspect of the invention, there is provided a method of braking a vehicle train, the vehicle train comprising a towing vehicle and two or more non-driven vehicles, the method comprising controlling a braking system of each of the two or more non-driven vehicles to cause synchronised braking of the two or more non-driven airside dollies. According to a sixth aspect of the invention, there is provided a method of mitigating jack-knifing of a vehicle train, the vehicle train comprising a towing vehicle and one or more non-driven vehicles, the method comprising controlling a steering system and / or braking system of the or each non-driven vehicle to maintain a predetermined minimum distance between the or each non-driven airside vehicle and an adjacent vehicle in the vehicle train. The method may comprise controlling a steering system and / or braking system of the or each non-driven vehicle to maintain a predetermined minimum distance between the or each non-driven airside vehicle and an adjacent vehicle in the vehicle train during a reversing manoeuvre. According to a seventh aspect of the invention, there is provided a method of mitigating sway in a vehicle train, the vehicle train comprising a towing vehicle and one or more non-driven vehicles, the method comprising detecting sway in the vehicle train and controlling a steering system and / or braking system of the or each non-driven vehicle to correct the sway. The one or more non-driven vehicles of the third, fourth, fifth, sixth, or seventh aspect of the invention may comprise one or more non-driven vehicles of the first aspect of the invention. The vehicle train of the third, fourth, fifth, sixth, or seventh aspect of the invention may comprise vehicle train of the second aspect of the invention. According to an eight aspect of the invention, there is provided a method of retro-fitting an existing non-driven vehicle, the non-driven vehicle comprising a drawbar, the method comprising: pivotably mounting an axle to the drawbar, wherein a wheel is mounted on each end of the axle; and fitting a steering actuator to the non-driven vehicle such that steering actuator is arranged to pivot the axle relative to the drawbar. The non-driven vehicle may comprise a conventional airside dolly. The non-driven vehicle may comprise the non-driven vehicle of the first aspect of the invention. In any of the above aspects or embodiments of the invention, the non-driven vehicle may instead by a driven vehicle. The driven vehicle may comprise a self-propelled airside dolly or a towing vehicle for a vehicle train. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 show a schematic plan view of a non-driven airside dolly according to an embodiment of the invention; Figures 2a to 2e each show a schematic plan view of the airside dolly of Figure 1; Figures 3a to 3c each show a schematic plan view of a prior art non-driven vehicle; Figures 4a and 4b each show a schematic plan view of a prior art vehicle train; Figure 5a shows a schematic plan view of a vehicle train according to an embodiment of the invention; Figures 5b to 5k illustrate a process of docking the airside dolly of the vehicle train of Figure 5a; Figures 6a to 6c each show a schematic plan view of a vehicle train according to another embodiment of the invention; Figures 7a and 7b each show another schematic plan view of the vehicle train of Figure 6a; Figures 8a to 8f each show another schematic plan view of the vehicle train of Figure 6a; Figure 9 shows a method of docking one or more non-driven airside vehicles of a vehicle train according to an embodiment of the invention; Figure 10 shows a method of turning a vehicle train according to an embodiment of the invention; Figure 11 shows a method of braking a vehicle train according to an embodiment of the invention; Figure 12 shows a method of mitigating jack-knifing of a vehicle train according to an embodiment of the invention; and Figure 13 shows a method of mitigating sway in a vehicle train according to an embodiment of the invention. DETAILED DESCRIPTION Figure 1 shows a schematic plan view of a non-driven vehicle 1 according to an embodiment of the invention. In this embodiment, the non-driven vehicle 1 is an airside dolly. The outline of the chassis of the airside dolly 1 is illustrated by the dashed line labelled 110. The airside dolly 1 comprises a steering system and a braking system. The steering system comprises a steering axle 11, a pivot joint 12, a first wheel 13a mounted to a first end of the steering axle 11, a second wheel 13b mounted to a second end of the steering axle 11, opposite the first end, and a steering actuator 14. In this embodiment, the steering axle 11 is a front axle of the airside dolly 1 and the first and second wheels 13a, 13b are first and second front wheels. The airside dolly 1 further comprises a drawbar 15. The steering axle 11 is pivotably mounted to a first end of the drawbar 15 via the pivot joint 12. As illustrated by the rotational arrows in Figure 1, the pivot joint 12 enables the steering axle 11 to pivot both clockwise and anti-clockwise, as viewed from above, about the first end of the drawbar 15. The steering actuator 14 is arranged to pivot the steering axle 11 relative to the drawbar 15 to adjust the steering angle of the first and second wheels 13a, 13b. In use, a second end of the drawbar 15, opposite the first end, is pivotably mounted to a towing vehicle or another airside dolly in a vehicle train. In this embodiment, the steering actuator 14 comprises a rod of variable length in the form of a hydraulic cylinder, a first end of which is pivotably mounted to the steering axle 11 and a second end of which, opposite the first end, is pivotably mounted to the drawbar 15. Extending the hydraulic cylinder causes the steering axle 11 to pivot clockwise about the pivot joint 12 as shown in Figure 1 as the hydraulic cylinder pivots about its ends. Retracting the hydraulic cylinder causes the steering axle 11 to pivot anti-clockwise about the pivot joint 12 as shown in Figure 1 as the hydraulic cylinder pivots about its ends. In other embodiments, the hydraulic cylinder may be replaced by any other suitable actuator, such as any suitable hydraulic, pneumatic, or electromechanical actuator. The airside dolly 1 further comprises a braking system comprising first and second electric brakes 16a, 16b. The airside dolly 1 comprises first and second rear wheels 17a, 17b. The first electric brake 16a is configured to exert a braking force on the first rear wheel 17a and the second electric brake 16a is configured to exert a braking force on the second rear wheel 17a. It will be appreciated that the braking system of the present embodiment is merely exemplary, and that the airside dolly 1 may comprise any suitable braking system configured to decelerate the airside dolly 1 in use. For example, other types of brakes, such as hydraulic or pneumatic brakes, may be employed. In this embodiment, the rear wheels 17a, 17b are mounted on a fixed axle such that the rear wheels 17a, 17b are not steerable, i.e., the orientation of the rear wheels 17a, 17b is fixed relative to the chassis of the airside dolly 1. However, in other embodiments, the rear wheels 17a, 17b may be steerable, either by the same pivotable axle mechanism as the steerable axle 11 and the front wheels 13a, 13b or by an alternative mechanism. The airside dolly 1 further comprises a control system 18. The control system 18 may comprise any suitable arrangement of one or more controllers and / or one or more processors, and one or more memories. Suitable control systems will be apparent to those skilled in the art. The control system 18 is configured to output commands to the steering actuator 14 of the steering system and the electric brakes 16a, 16b of the braking system, as illustrated by the dotted lines extending between the control system 18 and the steering actuator 14 and the control system 18 and the electric brakes 16a, 16b in Figure 1. The control system 18 is further configured to receive inputs from one or more external sources and output commands to the steering actuator 14 and the electric brakes 16a, 16b in dependence on the one or more inputs. The control system 18 is also configured to provide outputs to one or more external sources. The control system 18 may comprise any suitable arrangement of wireless or wired interfaces to enable the control system 18 to receive inputs and provide outputs. The airside dolly 1 further comprises a proximity sensor 19. The proximity sensor 19 is configured to detect the presence of an object within the surroundings of the airside dolly 1 and measure a distance between the airside dolly 1 and an object within the surroundings of the airside dolly 1. The control system 18 is configured to receive signals from the proximity sensor 19 indicative of the presence of an object within the surroundings of the airside dolly 1 and the distance between the airside dolly 1 and an object within the surroundings of the airside dolly 1, as illustrated by the dotted line extending between the control system 18 and the proximity sensor 19 in Figure 1. The control system 18 is configured to output one or more commands to the steering actuator 14 and the electric brakes 16a, 16b in dependence on the signals received from the proximity sensor 19. The field of view of the proximity sensor 19 is illustrated by the dotted line labelled 191. As shown, the proximity sensor 19 is able to detect objects in front of the airside dolly 1 within a predetermined range. The proximity sensor 19 is configured to track moving objects within the surroundings of the airside dolly 19, as well as detect stationary objects within the surroundings of the airside dolly 19. In other embodiments, the proximity sensor 19 may be replaced by an array of multiple proximity sensors configured to detect objects in front of, behind, and / or to the sides of the airside dolly 1. Figures 2a to 2e each show a schematic plan view of the airside dolly 1 of Figure 1. For clarity, not all features of the airside dolly 1 are shown in Figures 2a to 2e. Figures 2a to 2c show the airside dolly 1 in a normal mode in which the steering system is configured to enable the airside dolly I to travel substantially in the direction of a towing force on the airside dolly 1. The direction of a towing force F on the airside dolly 1 and the direction D in which the airside dolly 1 is configured to travel is shown by the labelled arrow in Figures 2a to 2c. In the normal mode, the front wheels 13a, 13b are aligned with the towing force F. In the configuration shown in Figure 2a, the front wheels 13a, 13b are aligned with the rear wheels 17a, 17b. As such, the airside dolly 1 will move in the direction of the towing force F on application of the towing force F. As well as the steering axle 11 being pivotable relative to the drawbar 15 about the pivot joint 12, the steering axle 11 and the drawbar 15 are together pivotable about the pivot joint 12 relative to the chassis 110 of the airside dolly 1. In the configuration shown in Figure 2b, the steering axle 11 and the drawbar 15 have been pivoted clockwise about the pivot joint 12 relative to the chassis 110 of the airside dolly 1 from the position shown in Figure 2a. In the configuration shown in Figure 2c, the steering axle 11 and the drawbar 15 have been pivoted anti-clockwise about the pivot joint 12 relative to the chassis 110 of the airside dolly 1 from the position shown in Figure 2a. On application of the towing force F on the airside dolly 1 in the configuration show in Figure 2b or Figure 2c, the chassis 110 of the airside dolly 1 will initially pivot about the pivot joint 12 until the rear wheels 17a, 17b become aligned with the front wheels 13a, 13b. Once the rear wheels 17a, 17b and the front wheels 13a, 13b are aligned, the airside dolly 1 will move in the direction of the towing force F. Figures 2d and 2e show the airside dolly 1 in a second mode, termed a docking mode or an enhanced steering mode, in which the steering system is configured to enable the airside dolly 1 to travel in a direction having a component substantially perpendicular to a towing force on the airside dolly 1. The direction of a towing force F on the airside dolly 1, the direction component P substantially perpendicular to the towing force F, and the direction D in which the airside dolly 1 is configured to travel is shown by the labelled arrows in Figures 2d and 2e. As shown, the direction of travel D has a component in the direction of the towing force F as well as the perpendicular component P. As such, the airside dolly 1 is configured to move diagonally in the direction D as shown in Figures 2d and 2e. In order to reconfigure the airside dolly 1 from the normal mode to the docking mode / enhance steering mode, the hydraulic cylinder of the steering actuator 14 is extended or retracted from the position shown in Figures 2a to 2c. To achieve the configuration shown in Figure 2d, the hydraulic cylinder 14 is extended from the position shown in Figures 2a to 2c and to achieve the configuration shown in Figure 2e, the hydraulic cylinder 14 is retracted from the position shown in Figures 2a to 2c. Figures 3a to 3c each show a schematic plan view of a prior art non-driven vehicle 13. In this example, the prior art non-driven vehicle 13 takes the form of a conventional airside dolly. The prior art airside dolly 13 comprises a front axle 3 1, a first front wheel 33a mounted to a first end of the front axle 31, a second front wheel 33b mounted to a second end, opposite the first end, of the front axle 31, a drawbar 35, and first and second rear wheels 37a, 37b. The outline of the chassis of the prior art airside dolly 3 is illustrated by the dashed line labelled 310. The drawbar 35 is fixedly mounted to the front axle 31, i.e., the front axle is not able to move relative to the drawbar 35. The drawbar 35 and the front axle 31 are together pivotably mounted relative to the chassis 310 of the prior art airside dolly 3. The prior art airside dolly 3 is only able to travel in the direction of a towing force on the airside dolly 3. The direction of a towing force F on the prior art airside dolly 3 and the direction D in which the airside dolly 3 is configured to travel is shown by the labelled arrow in Figures 3a to 3c. In the configuration shown in Figure 3b, the steering axle 31 and the drawbar 35 have been pivoted clockwise relative to the chassis 310 from the position shown in Figure 3a. In the configuration shown in Figure 3c, the steering axle 31 and the drawbar 35 have been pivoted anti-clockwise relative to the chassis 310 from the position shown in Figure 3a. Movement of the prior art airside dolly 3 on application of the towing force F is as described above with reference to Figures 2a to 2c. Figures 4a and 4b each show a schematic plan view of a prior art vehicle train. The prior art vehicle train comprises a towing vehicle and four non-driven vehicles 3a to 3d according to Figures 3a to 3c. Mechanical couplings (not shown) extend between the vehicles in the vehicle train in a known manner. The non-driven vehicles 3a to 3d do not comprise any independent steering or braking means. Steering and braking of the prior art vehicle train is affected only by steering and braking of the towing vehicle 2. Figure 4a illustrates problems that may occur under braking of the prior art vehicle train. Non-driven vehicles 3a to 3c are undergoing sway, i.e., misalignment with the direction of travel of the towing vehicle 2. This causes instability of the vehicle train 2 as a whole. Non-driven vehicles 3c and 3d are undergoing jack-knifing, i.e., non-driven vehicles 3c and 3d have become misaligned with the direction of travel of the towing vehicle 2 to the extent that non-driven vehicle 3d has collided with non-driven vehicle 3c. As well as under braking, sway and jack-knifing may also occur during turning or a change of direction of the vehicle train, particularly under high speeds. Figure 4b illustrates a problem that may occur during turning of the prior art vehicle train. Figure 4b shows the path 21 taken by the towing vehicle 2 during a turn around an obstacle O. As shown, the towing vehicle 2 has taken a relatively tight turn, passing closely past the obstacle O. Figure 3b shows the projected path 31 of the final nondriven vehicle 3d in the vehicle train. Due to the inherent dynamics of the vehicle train, the final non-driven vehicle 3d will not follow exactly the path of the towing vehicle 2 and will instead follow a tighter turn, resulting in collision of the final non-driven vehicle 3d with the obstacle 0. Figure 5a shows a schematic plan view of a vehicle train according to an embodiment of the invention. The vehicle train comprises a towing vehicle 4 and the airside dolly 1 of Figure 1. For clarity, not all the features of the airside dolly 1 are labelled in Figure 5a. The towing vehicle 4 comprises a control system 41, a proximity sensor 42, first and second front wheels 43a, 43b, first and second rear wheels 44a, 44b, and a drive system 45. The outline of the chassis of the towing vehicle 4 is illustrated by the dashed line labelled 46. The field of view of the proximity sensor 42 is illustrated by the dotted line labelled 421. The second end of the drawbar 15 of the airside dolly 1 is pivotably mounted to the rear of the towing vehicle 4 by a suitable tow hitch or the like. The control system 41 is configured to receive signals from the proximity sensor 42 indicative of the presence of an object within the surroundings of the towing vehicle 4 and the distance between the towing vehicle 4 and an object within the surroundings of the towing vehicle 4, as illustrated by the dotted line extending between the control system 41 and the proximity sensor 42. The control system 41 is also configured to receive one or more inputs from and provide one or more outputs to the control system 18 of the airside dolly 1, as illustrated by the dotted line extending between the control system 41 and the control system 18 of the airside dolly 1. The control system 41 may comprise any suitable arrangement of wireless or wired interfaces to enable the control system 41 to receive inputs and provide outputs. In other embodiments, the towing vehicle 4 may not comprise the control system 41 and / or the proximity sensor 42. In some embodiments, the towing vehicle 4 may comprise one or more additional sensors, such as LIDAR, GPS sensors, and cameras, in communication with the control system 41 to enable operation of the towing vehicle 4 in an autonomous or semi-autonomous mode. The towing vehicle 4 may comprise a control interface configured to enable a human operator to provide inputs to the control system 41. In such embodiments, the towing vehicle 41 may comprise a cab to enable a human operator to travel with the towing vehicle 41 as they provide inputs to the control interface to drive the towing vehicle 4. The drive system 45 is configured to provide motive power to the rear wheels 44a, 44b of the towing vehicle 4. The drive system 45 may comprise any suitable prime mover, such as an internal combustion engine and / or an electric motor, and any suitable transmission means for delivering torque from the prime mover to the rear wheels 44a, 44b. The towing vehicle 4 further comprises a steering system (not shown) configured to steer the front wheels 43a, 43b to change the direction of travel of the towing vehicle 4. It will be appreciated that the front wheels 43a, 43b, rear wheels 44a, 44b, and drive system 45 are merely exemplary, and that the towing vehicle 4 may comprise any other suitable means of propulsion. It will be appreciated that the towing vehicle 4 as described above is merely illustrative and that other embodiments may comprise any suitable towing vehicle, such as a conventional diesel-powered airport tug. Figures 5b to 5k illustrate a process of docking the airside dolly 1 of the vehicle train of Figure 5a at a docking lateral L. Each of Figures 5b to 5k show a schematic plan view of the vehicle train. For clarity, not all features of the airside dolly 1 or the towing vehicle 4 are shown. In Figures 5b to 5f, the airside dolly 1 is in the normal mode as described above with reference to Figures 2a to 2c. Figures 5b to 5f show the towing vehicle 4 manoeuvring towards the docking lateral L, towing the airside dolly 1 with it. For example, where the towing vehicle 4 is in a human-operated mode, a driver of the towing vehicle 4 may drive the towing vehicle 4 towards the docking lateral L. Figure 5g shows the vehicle train in a position in which the airside dolly 1 is aligned with the docking lateral L and spaced from the docking lateral L by a distance close to a desired clearance distance. As the vehicle train moves into this position, the docking lateral L enters the field of view 191 of the proximity sensor 19 of the airside dolly 1. The proximity sensor 19 generates a signal indicative of the proximity of the airside dolly 1 to the docking lateral L. This signal is received by the control system 18 of the airside dolly 1. In response, the control system 18 outputs a command to the steering system of the airside dolly 1 to configure the steering system into the docking mode described above with reference to Figures 2d and 2e. Figures 5h to 5j show the vehicle train with the steering system of the airside dolly 1 in the docking mode. As the towing vehicle 4 moves forward, in the direction F as shown in Figures 5h to 5j, the airside dolly 1 will travel in the direction D towards the docking lateral L. As shown in Figures 5i and 5j, the hydraulic cylinder of the steering actuator 14 is configured to continue to extend as the towing vehicle 4 travels in the direction F and the airside dolly 1 travels in the direction D. This enables the towing vehicle 4 to travel in the direction F without the airside dolly I pulling the towing vehicle 4 in the direction P. This helps to ensure that the towing vehicle 4 maintains its intended path of travel. As shown in Figure 5j, the airside dolly 1 has reached the desired clearance distance between the airside dolly 1 and the docking lateral L. As a result, the proximity sensor 19 of the airside dolly 1 generates a signal indicative of the desired clearance distance having been reached. This signal is received by the control system 18 of the airside dolly 1. In response, the control system 18 of the airside dolly 1 outputs a signal to the control system 41 of the towing vehicle to indicate that the desired clearance distance has been reached. This allows the towing vehicle 4 to stop, thereby stopping the vehicle train with the airside dolly 1 at the desired clearance distance from the docking lateral L. The control system 18 of the airside dolly 1 may alternatively, or additionally, output a command to the braking system of the airside dolly 1 to apply the electric brakes 16a, 16b. If at any point during the docking process an obstacle enters the field of view 191 of the proximity sensor 19 of the airside dolly 1, the proximity sensor 19 of the airside dolly 1 generates a signal indicative of the presence of the obstacle. This signal is received by the control system 18 of the airside dolly 1. The control system 18 of the airside dolly 1 outputs a signal to the control system 41 of the towing vehicle to indicate that an emergency stop is required to avoid a collision with the obstacle. This allows the towing vehicle 4 to perform an emergency stop, thereby stopping the vehicle train. The control system 18 of the airside dolly 1 may alternatively, or additionally, output a command to the braking system of the airside dolly 1 to apply the electric brakes 16a, 16b. Where the towing vehicle 4 is operating in a human-operated mode, the signal from the control system 18 of the airside dolly 1 may result in a notification appearing on a control interface of the towing vehicle 4, indicating to a driver of the towing vehicle 4 to stop the vehicle train. Where the towing vehicle 4 is operating in an autonomous mode, the signal from the control system 18 of the airside dolly 1 may result in automatic stopping of the towing vehicle 4 and hence the vehicle train. Figure 6a shows a schematic plan view of a vehicle train according to another embodiment of the invention. The vehicle train comprises the towing vehicle 4 of Figure 5a and multiple airside dollies Ia-1N of Figure 1. For clarity, not all the features of the vehicle train 4 or the airside dollies la-lx are shown. The second end of the drawbar of the first airside dolly la in the vehicle train is pivotably mounted to the rear of the towing vehicle 4, as shown in Figure 5a. The first airside dolly la is mechanically coupled to the second airside dolly lb in the vehicle train in a similar manner, the second airside dolly lb is mechanically coupled to the third airside dolly 1c and so on. The airside dolly ln, represented by the dashed outline, indicates that there may be any suitable number of airside dollies 1 in the vehicle train, with airside dolly In being the final airside dolly in the vehicle train. For example, there may be up to ten or more airside dollies 1 in the vehicle train. As indicated by the dotted line extending between the towing vehicle 4 and each of the airside dollies Ia-1N, the control system of each of the airside dollies Ia-1N is in communication with the control system of the towing vehicle 4. This enables control of the steering system and / or braking system of each of the individual airside dollies la-1n so as to affect the behaviour of the vehicle train as a whole. For example, the control system of the towing vehicle 4 may provide a braking request to the control system of each of the airside dollies la-In. In response, the control system of each of the airside dollies Ia-1N may output a braking command to the braking system of the respective airside dolly 1 to cause synchronised braking of the airside dollies la-In. In some embodiments, the control system of each of the airside dollies la-In may, additionally or alternatively, be in communication with the control system of each of the other airside dollies la-ln. Figures 6b and 6c each show another schematic plan view of the vehicle train of Figure 6a. For clarity, not all the features of the towing vehicle 4 or the airside dollies la-In are shown. Figures 6b and 6c illustrate a process of steering the vehicle train to avoid an object within the surroundings of the vehicle train in the form of an obstacle O. As shown in Figure 6b, the towing vehicle 4 is driven past the obstacle O at a suitable clearance distance. The obstacle O enters the field of view 421 of the proximity sensor of the towing vehicle 4 and the proximity sensor generates a signal indicative of the clearance distance between the obstacle 0 and the towing vehicle 4. The signal is received by the control system of the towing vehicle 4 and the control system provides an output to the control system of each of the airside dollies Ia-1N indicative of the clearance distance. The clearance distance is adopted by the airside dollies Ia-1N as a target distance between the airside dollies la-In and the obstacle O. Figure 6c shows the first airside dolly la in the vehicle train passing the obstacle O after the towing vehicle 4 has been driven passed the obstacle O. As the first airside dolly la passes the obstacle O, the obstacle O enters the field of view 191a of the proximity sensor of the first airside dolly la and the proximity sensor generates a signal indicative of the distance between the obstacle O and the first airside dolly la. This signal is received by the control system of the first airside dolly la and the distance between the obstacle 0 and the first airside dolly la is compared with the target distance. The control system of the first airside dolly la then outputs commands to the steering system and / or braking system of the first airside dolly la to control the movement of the first airside dolly la to achieve the target distance between the first airside dolly la and the obstacle O. This process is then repeated for each of the other airside dollies ib-lx in the vehicle train as the dollies pass the obstacle O. The result of the above-described process is that each of the airside dollies Ia-1N in the vehicle train substantially follows the same path, indicated by arrow 47, past the obstacle O. In contrast to the prior art vehicle train as shown in Figure 4b, this means that any collision between any of the airside dollies la-1 n and the obstacle O can be avoided. It will be appreciated that, due to the mechanical coupling between the vehicles in the vehicle train, steering and / or braking of one of the vehicles in the vehicle train will affect the movement of the other vehicles in the vehicle train. In other words, the forces on each vehicle within the vehicle train are affected by the movement of the other vehicles in the vehicle train. However, because each of the airside dollies la-ln is configured to control its own steering and / or braking to maintain the target distance from the obstacle 0, these forces are inherently taken into account. The vehicle train is also operable to mitigate jack-knifing of the vehicles within the vehicle train. As shown in Figures 6b and 6c, each vehicle within the vehicle train falls within the field of view of the proximity sensor of the airside dolly immediately behind it. The towing vehicle 4 falls within the field of view 191a of the proximity sensor of airside dolly la, airside dolly la falls within the field of view 191b of the proximity sensor of airside dolly lb, etc. In the embodiment of Figures 6b and 6c, the vehicles within the vehicle train are arranged such that each vehicle always falls within the field of view of the proximity sensor of the airside dolly immediately behind it. In other embodiments, the vehicles may be arranged such that each vehicle only falls within the field of view of the proximity sensor of the airside dolly immediately behind it during turning of the vehicle train, as shown in Figures 6b and 6c, beyond a predetermined turning angle. In operation, the proximity sensor of each airside dolly la-1 generates a signal indicative of the distance between the respective airside dolly la-In and the airside dolly immediately in front. This signal is received by the control system of the respective airside dolly and is compared with a predetermined distance. The predetermined distance represents a minimum distance between vehicles within the vehicle train to be maintained so as to avoid jack-knifing. The control system of the respective airside dolly then outputs commands to the steering system and / or braking system of the airside dolly to control the movement of the airside dolly to maintain the predetermined distance from the vehicle in front. This jack-knife mitigation may be employed at the same time as the process of steering the vehicle train to avoid the obstacle O as described with reference to Figures 6b and 6c, and may be employed at the same time as any other process, such as a docking process or braking process. In particular, the jack-knife mitigation may be employed during a reversing manoeuvre of the vehicle train. The control system of each airside dolly la-1 is also operable to request steering and braking of the vehicle train as a whole. The control system of each airside dolly la-In is configured to provide train braking requests and train steering requests to the control system of the towing vehicle 4 in dependence on signals from the proximity sensor of the respective airside dolly. The control system of the towing vehicle is configured to provide vehicle braking requests and vehicle steering requests to the control system of each airside dolly la-1 in dependence on the train braking requests and the train steering requests respectively. The control system of each airside dolly Ia-1N is configured to output commands to the steering system and braking system of the respective airside dolly in dependence on the vehicle braking requests and the vehicle steering requests respectively. For example, an object may enter the field of view of the proximity sensor of one of the airside dollies la-lx in use. The proximity sensor will generate a signal indicative of the distance between the object and the airside dolly. This signal is received by the control system of the airside dolly and the control system uses this signal to determine if steering and / or braking of the vehicle train as a whole is required in order to avoid a collision with the object. For example, with reference to Figure 6c, the obstacle O enters the field of view 191a of the proximity sensor of airside dolly la. The control system of the airside dolly la may determine that if an adjustment to the steering and / or braking of the vehicle train as a whole is not made, then a collision may occur between airside dolly lb and the obstacle O as the vehicle train travels past the obstacle O. As such, the control system of the airside dolly la will output a train braking requests and / or a train steering request to the control system of the towing vehicle 4. The control system of the towing vehicle 4 will then output a vehicle braking request and / or a vehicle steering request to one or more of the airside dollies Ia-1N, as appropriate, to ensure that a collision between airside dolly lb and the obstacle O is avoided. In another example, an object may enter the field of view of the proximity sensor of one of the airside dollies la-lw, and the control system of the airside dolly may determine that the best way to avoid a collision with the object is to affect an emergency stop of the vehicle train. As such, the control system of the airside dolly will output an emergency braking request to the control system of the towing vehicle 4. The control system of the towing vehicle 4 will then output a vehicle braking request to the control system of each of the airside dollies la-lw to affect immediate and synchronised braking of the airside dollies la-lw. Braking of the towing vehicle 4 will also be affected by appropriate means in order to bring the vehicle train to a stop. It will be appreciated that during control of steering and braking of the vehicle train as a whole, as described above, different steering and / or braking requests may be sent to the control system of each of the airside dollies la-lw, and a steering and / or braking request may not be sent to one or more of the airside dollies la-lw. For example, two or more of the airside dollies la-lw may be steered differently, i.e., the steering system of each of two or more of the airside dollies la-lw may be controlled to provide a different steering angle of the steerable wheels of each of the two or more of the airside dollies la-lw. Each of the airside dollies la-lw is also configured to control its own steering and braking within the vehicle train. The control system of each airside dolly is configured to output commands to the steering system and the braking system of the respective airside dolly in dependence on signals received from the proximity sensor of the respective airside dolly to achieve a predetermined target distance between the respective airside dolly and an object within the surroundings of the respective airside dolly. For example, referring to Figure 6c, the obstacle O enters the field of view 191a of the proximity sensor of airside dolly la. The control system of the airside dolly la may determine that steering and / or braking of the airside dolly la is required in order to avoid a collision between the airside dolly la and the obstacle O. The control system of the airside dolly la will then output commands to the steering system and / or braking system of the airside dolly la as appropriate in order to avoid a collision. This process may be carried out at the same time as or separately from steering and / or braking of the vehicle train as a whole as described above. In other embodiments, the towing vehicle 4 and / or at least one of the airside dollies lain may comprise one or more sway sensors configured to generate one or more sway signals indicative of sway of the vehicle train. In such embodiments, the control system of each airside dolly I a-1 is configured to output one or more commands to the steering system and / or the braking system of the respective airside dolly in dependence on the one or more sway signals to correct the sway. Figures 7a and 7b each show another schematic plan view of the vehicle train of Figure 6a. For clarity, not all the features of the vehicle train are shown. Figures 7a and 7b illustrate enhanced steering of the vehicle train in which the vehicle train can be turned in a tighter turning circle. In Figure 7a, each of the airside dollies la-In is shown in the normal mode as described above with reference to Figures 2a to 2c. In Figure 7b, each of the airside dollies la-lx is shown in the enhanced steering mode as described above with reference to Figures 2d and 2e. Both Figures 7a and 7b show the vehicle train in a right turn. In Figure 7b, the steering system of each of the airside dollies Ia-1N enables each airside dolly Ia-1N to travel towards the centre of the turn, in other words, ‘turn into’ the turn. As such, the turning circle of the vehicle train as shown in Figure 7b is tighter than the turning circle of the vehicle train as shown in Figure 7a. It will be appreciated that two or more of the airside dollies la-In may be steered by a different amount during this process. In other words, the steering angle of the steerable wheels of each of airside dollies la-In may not be the same. It will be appreciated that the docking process described above with reference to Figures 5a to 5j can be applied in the same manner with any of the airside dollies la-1 n of the vehicle train of Figures 6a to 6c and Figures 7a and 7b. For example, the docking process may be used to dock the final airside dolly In of the vehicle train. It will also be appreciated that any features described above with reference to Figures 6a to 6c and Figures 7a and 7b are equally applicable to the vehicle train of Figures 5a to 5j. Figures 8a to 8f each show another schematic plan view of the vehicle train of Figure 6a. For clarity, not all the features of the towing vehicle 4 or the airside dollies la-In are shown. Figures 8a to 8f illustrate a process of docking the final airside dolly In of the vehicle train at a lateral L. In this embodiment, the lateral L is what may be known as a Jumbo Container Pallet Loader (JCPL), but in other embodiments any suitable lateral may be used. The lateral L comprises a stop target S. Figure 8a shows the towing vehicle 4 approaching the lateral L at a suitable clearance distance di. The lateral L enters the field of view 421 of the proximity sensor of the towing vehicle 4 and the proximity sensor generates a signal indicative of the clearance distance d between the lateral L and the towing vehicle 4. The is signal is received by the control system of the towing vehicle 4 and the control system provides an output to the control system of each of the airside dollies Ia-1N indicative of the clearance distance di. The clearance distance di is adopted by the airside dollies la-In as a target distance between the airside dollies la-In and the lateral L. Figures 8b to 8e show each airside dolly la-ln passing the lateral L as the vehicle train turns to the right. In doing so, the clearance distance di is maintained between the lateral L and each airside dolly la-ln in the same manner as described above with reference to Figures 6b and 6c and the obstacle O. Figure 8e shows the lateral L entering the field of view 19 IN of the proximity sensor of the final airside dolly In of the vehicle train. The proximity sensor will generate a signal indicative of the presence of the lateral L and this signal is received by the control system of the final airside dolly In. In response, the control system outputs a command to the steering system of the final airside dolly In to configure the steering system into the docking mode described above with reference to Figures 2d and 2e. The final airside dolly In will then travel towards the lateral L in the manner described above with reference to Figures 5g to 5j. Figure 8f shows the stop target S entering the field of view 19 IN of the proximity sensor of the final airside dolly In of the vehicle train. In response, the proximity sensor, or an alternative stop target sensor if present, generates a signal indicative of detection of the stop target sensor. The control system of the final airside dolly In receives this signal and then outputs a train braking request to the control system of the towing vehicle 4. This affects braking and stopping of the vehicle train in the manner described above with reference to Figures 6b and 6c. The position of the stop target S is selected so as to cause the vehicle train to stop when the final airside dolly In has reached an intended docking position. Figure 9 shows a method 9 of docking one or more non-driven airside vehicles of a vehicle train according to an embodiment of the invention. The vehicle train may be the vehicle train of any of the previously described embodiments. The method 9 comprises: towing 91 the or each non-driven airside vehicle in a forward direction to align the or each non-driven airside vehicle with a docking structure; controlling 92 a steering system of the or each non-driven airside vehicle to enable the non-driven airside vehicle to travel in a docking direction, wherein the docking direction is towards the docking structure and has a component substantially perpendicular to the forward direction; and further 93 towing the or each non-driven airside vehicle in the forward direction to cause the non-driven airside vehicle to travel in the docking direction. The method further comprises stopping 94 the vehicle train when the or each non-driven airside vehicle is a predetermined distance from the docking structure. In other embodiments, step 94 may be omitted. Figure 10 shows a method 10 of turning a vehicle train according to an embodiment of the invention. The vehicle train comprises a towing vehicle and one or more non-driven vehicles. The vehicle train may be the vehicle train of any of the previously described embodiments. The method 10 comprises: controlling 101a steering system of the or each non-driven vehicle to cause the respective non-driven vehicle to travel towards the centre of a turning circle of the vehicle train in a direction having a component substantially perpendicular to a towing force on the respective non-driven vehicle. Figure 11 shows a method 20 of braking a vehicle train according to an embodiment of the invention. The vehicle train comprises a towing vehicle and two or more non-driven vehicles. The vehicle train may be the vehicle train of any of the previously described embodiments. The method comprising controlling 201 a braking system of each of the two or more non-driven vehicles to cause synchronised braking of the two or more nondriven airside dollies. Figure 12 shows a method 30 of mitigating jack-knifing of a vehicle train according to 5 an embodiment of the invention. The vehicle train comprises a towing vehicle and one or more non-driven vehicles. The vehicle train may be the vehicle train of any of the previously described embodiments. The method 30 comprises controlling 301a steering system and / or braking system of the or each non-driven vehicle to maintain a predetermined minimum distance between the or each non-driven airside vehicle and an 10 adjacent vehicle in the vehicle train. Figure 13 shows a method 40 of mitigating sway in a vehicle train according to an embodiment of the invention. The vehicle train comprise a towing vehicle and one or more non-driven vehicles. The vehicle train may be the vehicle train of any of the 15 previously described embodiments. The method 40 comprises detecting 401 sway in the vehicle train and controlling 402 a steering system and / or braking system of the or each non-driven vehicle to correct the sway.

Claims

1. A non-driven airside vehicle, comprising:a steering system, wherein the steering system is configured to enable the nondriven airside vehicle to travel in a direction having a component substantially perpendicular to a towing force on the non-driven airside vehicle, and / or a braking system; anda control system, wherein the control system is configured to output one or more commands to the steering system and / or the braking system.

2. The non-driven airside vehicle of claim 2, wherein the steering system comprises a steering axle, a wheel mounted to each end of the steering axle, and a steering actuator configured to pivot the steering axle relative to the towing force on the non-driven airside vehicle.

3. The non-driven airside vehicle of claim 2, comprising a drawbar, wherein the steering axle is pivotably mounted relative to the drawbar, and wherein the steering actuator is arranged to pivot the steering axle relative to the drawbar.

4. The non-driven airside vehicle of claim 2 or claim 3, comprising one or more rear wheels, wherein the steering axle is a front axle of the non-driven airside vehicle, and wherein the braking system is configured to exert a braking force on the one or more rear wheels.

5. The non-driven airside vehicle of any preceding claim, wherein the control system is configured to receive one or more inputs from one or more external sources and / or provide one or more outputs to one or more external sources.

6. The non-driven airside vehicle of claim 5, wherein the one or more external sources comprises one or more other vehicle.

7. The non-driven airside vehicle of claim 5 or claim 6, wherein the one or more external sources comprises a central controller of a baggage handling system.

8. The non-driven airside vehicle of any preceding claim, comprising one or more proximity sensors, wherein the control system is configured to output one or more commands to the steering system and / or the braking system in dependence on one or more signals received from the one or more proximity sensors.

9. A vehicle train comprising a towing vehicle and one or more non-driven airside vehicles according to any preceding claim.

10. The vehicle train of claim 9, wherein the steering system of at least one of the non-driven airside vehicles is configured to enable the non-driven airside vehicle to travel towards the centre of a turning circle of the vehicle train in a direction having a component substantially perpendicular to a towing force on the non-driven airside vehicle.

11. The vehicle train of claim 9 or claim 10, wherein the towing vehicle comprises a control system configured to receive one or more inputs from the control system of at least one of the non-driven airside vehicles and / or provide one or more outputs to the control system of at least one of the non-driven airside vehicles.

12. The vehicle train of claim 11, wherein the towing vehicle comprises one or more proximity sensors, wherein the control system of the towing vehicle is configured to provide one or more outputs to the control system of at least one of the non-driven airside vehicles in dependence on one or more signals received from the one or more proximity sensors of the towing vehicle.

13. The vehicle train of claim 11 or claim 12, wherein the control system of the towing vehicle is configured to provide a braking request to the control system of at least two of the non-driven airside vehicles, and wherein the control system of each of the at least two non-driven airside vehicles is configured to output a braking command to the braking system of the respective non-driven airside vehicle in response to the braking request to cause synchronised braking of the at least two non-driven airside vehicles.

14. The vehicle train of any of claims 11 to 13, comprising one or more non-driven airside vehicles according to claim 8.

15. The vehicle train of claim 14, wherein the control system of at least one of the non-driven airside vehicles is configured to output one or more commands to the steering system and / or the braking system of the non-driven airside vehicle in dependence on the one or more proximity signals received from the one or more proximity sensors of the non-driven airside vehicle to achieve a predetermined target distance between the non-driven airside vehicle and an object within the surroundings of the non-driven airside vehicle.

16. The vehicle train of claim 15, when dependent on claim 12, wherein the control system of the towing vehicle is configured to provide one or more outputs indicative of the predetermined target distance to the control system of at least one of the non-driven airside vehicles in dependence on one or more signals received from the one or more proximity sensors of the towing vehicle.

17. The vehicle train of any of claims 14 to 16, wherein the control system of at least one of the non-driven airside vehicles is configured to output one or more commands to the steering system of the non-driven airside vehicle to enable the nondriven airside vehicle to travel towards a docking lateral in a direction having a component substantially perpendicular to a towing force on the non-driven airside vehicle.

18. The vehicle train of any of claims 14 to 17, wherein the control system of at least one of the non-driven airside vehicles is configured to output one or more commands to the steering system and / or the braking system of the non-driven airside vehicle in dependence on the one or more proximity signals received from the one or more proximity sensors of the non-driven airside vehicle to maintain a predetermined minimum distance between the non-driven airside vehicle and an adjacent vehicle in the vehicle train.

19. The vehicle train of any of claims 14 to 17, wherein the control system of at least one of the non-driven airside vehicles is configured to provide a train braking request and / or a train steering request to the control system of the towing vehicle in dependence on the one or more signals from the one or more proximity sensors of the non-driven airside vehicle, wherein the control system of the towing vehicle isconfigured to provide a vehicle braking request and / or a vehicle steering request to the control system of at least one of the non-driven airside vehicles in dependence on the train braking request and / or the train steering request, and wherein the control system of the or each non-driven airside vehicle is configured to output one or more commands to the steering system and / or the braking system of the respective non-driven airside vehicle in dependence on the vehicle braking request and / or the vehicle steering request.

20. The vehicle train of any of claims 9 to 19, wherein at least one of the non-driven airside vehicles comprises a stop target sensor configured to detect a stop target on an object within the surroundings of the non-driven airside vehicle, wherein the control system of the or each non-driven airside vehicle is configured to provide a train braking request and / or a train steering request to the control system of the towing vehicle in dependence on the stop target sensor detecting the stop target, wherein the control system of the towing vehicle is configured to provide a vehicle braking request and / or a vehicle steering request to the control system of at least one of the non-driven airside vehicles in dependence on the train braking request and / or the train steering request, and wherein the control system of the or each non-driven airside vehicle is configured to output one or more commands to the steering system and / or the braking system of the respective non-driven airside vehicle in dependence on the vehicle braking request and / or the vehicle steering request.

21. The vehicle train of any of claims 9 to 20, wherein the towing vehicle and / or at least one of the non-driven airside vehicles comprises one or more sway sensors configured to generate one or more sway signals indicative of sway of the vehicle train, wherein the control system of at least one of the non-driven airside vehicles is configured to output one or more commands to the steering system and / or the braking system of the non-driven airside vehicle in dependence on the one or more sway signals to correct the sway.

22. The vehicle train of any of claims 9 to 21, wherein the towing vehicle is an autonomous vehicle or a semi-autonomous vehicle.

23. The vehicle train of any of claims 9 to 21, wherein the towing vehicle is a non-autonomous vehicle.

24. A method of docking one or more non-driven airside vehicles of a vehicle train, comprising:towing the or each non-driven airside vehicle in a forward direction to align the or each non-driven airside vehicle with a docking structure;controlling a steering system of the or each non-driven airside vehicle to enable the non-driven airside vehicle to travel in a docking direction, wherein the docking direction is towards the docking structure and has a component substantially perpendicular to the forward direction;further towing the or each non-driven airside vehicle in the forward direction to cause the non-driven airside vehicle to travel in the docking direction; andstopping the vehicle train when the or each non-driven airside vehicle is a predetermined distance from the docking structure.

25. A method of retro-fitting an existing non-driven airside vehicle, the non-driven airside vehicle comprising a drawbar, an axle, and a wheel mounted on each end of the axle, the method comprising:pivotably mounting the axle relative to the drawbar; andfitting a steering actuator to the non-driven airside vehicle such that steering actuator is arranged to pivot the axle relative to the drawbar to adjust the steering angle of the one or more steerable wheels.

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