GB2448471A - Vehicle with backup mode for four wheel steering - Google Patents

Abstract

An automatically controlled vehicle, for example for use in a personal rapid transit system, comprises front and rear wheels 2, 4 which are steerable by means of steering actuators 16, 18. In normal operation, a coupling 20 is disengaged, so that the front and rear wheels 2, 4 are steered independently of each other. In the event of failure of one of the steering actuators 16, 18, the coupling 20 is engaged, and both front and rear wheels 2, 4 are steered, for example in opposite directions from each other, by means of the remaining functional steering actuator. The steering actuators 16, 18 are typically electric motors. In one embodiment, a rear steering actuator 18 is shifted in order to disconnect a clutch (46 see fig 3) and to connect the rear actuator 18 to the front actuator 16 via the coupling 20. In another embodiment, the coupling is provided by closing a vent valve 44 to allow linking between front and rear hydraulic cylinders (34, 36 see fig 2).

Description

VEHICLE STEERING ARRANGEMENT

This invention relates to an automatically controlled vehicle having front and rear steerable wheels.

In this specification references to a vehicle "axle" mean wheels which are rotatable about a common axis in the straight ahead position, regardless of whether or not they are interconnected by a physical axle structure.

Conventional road vehicles are steered using their front axles only. Significantly improved manoeuvrability can be achieved if both front and rear axles are steered independently. This configuration is not generally utilized because a human driver cannot manage the control task of independently steering both axles. Some road vehicles have been produced where both front and rear axles are steered -in these the relationship between front and rear axle steering is typically fixed so that a single input device, the steering wheel, can be used to steer the vehicle.

Automatically controlled vehicles are known, which navigate by means of steering inputs generated by a control system without a human driver. In such vehicles, there is no such limitation on the control of the steering and independent steering of the front and rear axles can be employed to optimize vehicle dynamics and manoeuvrability.

Such is the case for vehicles used in personal rapid transit (PRI) systems. In a PRT system, automatically guided vehicles run on a dedicated trackway between stations, each vehicle accommodating a single person or group of people and travelling directly to a specified destination station without shopping at intermediate stations.

In automatically controlled vehicles steering is typically a critical function: failure of the steering system is likely to lead to the vehicle leaving its intended path with the possibility that a collision will result.

A strategy that may be employed to reduce the likelihood of steering system failure in an automated vehicle is to provide redundancy in the steering actuation system. This is typically achieved by duplicating at least the actuation elements of the steering system.

Where a vehicle is configured to have independent front and rear axle steering at least two steering actuators are required, one for each axle. Thus to provide full steering actuation redundancy four steering actuators might be required, resulting in an expensive and complex system.

PRT vehicles running in their guideway typically have different steering requirements during different phases of their journeys. In station and depot areas where speeds are low, tight radius turns are required so that compact station and depot layouts minimizing use of real estate can be employed. Independent four wheel steering is highly desirable in these areas. Between stations, on the guideway, where vehicles travel at higher speeds, cornering radii are very much larger and consequently vehicles do not need independent four wheel steering. Instead, simple front axle only steering or coupled four wheel steering will suffice.

A known PRT system operates at Morgantown, West Virginia, USA ("Still in a Class of its Own" -Progressive Engineer, March 2002). PRT vehicles such as those used at Morgantown benefit from four wheeled steering. The implemented system uses a mechanical linkage coupling front and rear axle steering. This gives a fixed relationship between front and rear steer angles in which the rear axle steers in the opposite direction to the front. This provides benefits in minimizing the vehicle's turning circle allowing compact stations and other manoeuvring areas, and minimizing the inswing and outswing in corners, allowing the width of the guideway to be kept constant in all places. This is a crucial benefit for this application in which the vehicle's mechanical steering arrangement requires that the vehicle keeps a near constant distance from the guideway kerbs at all times. An additional benefit is that the rear wheels follow the same path along the ground as the front wheels, minimizing the width of running surface required.

This steering arrangement has some disadvantages, however, principally that it results in poor stability of the vehicle at speed.

By contrast, fully independent four wheeled steering systems can provide all of the benefits listed above together with improved stability at high speed and the ability to perform complex low speed manoeuvres, such a crabbing (in which both front and rear axles steer in the same direction) allowing optimal manoeuvring into tight spaces. This latter benefit is important in enabling optimized PRT station layout.

Because the steering system requirements of PRT systems vary during different phases of their journey the level of redundant functionality required to maintain critical steering functionality also varies.

The vehicle typically only requires independent front and rear steering functionality for complex station and depot manoeuvres. Because these are carried out at low speed, failure under these conditions is unlikely to produce a serious hazard. By contrast independent steering functionality is not required on the guideway but, since speeds are typically higher, steering failure under these conditions is likely to be significantly more hazardous.

Thus for a PRT vehicle having dual steering actuation systems allowing independent steering of the front and rear axles for station manoeuvres at low speed, a system may be implemented such that, when operating at higher speed either actuator may be employed alone to provide sufficient control of the vehicle steering for safe operation on the guideway.

US 5092419 discloses an all-wheel steering control system for a driver-operated vehicle. Steering inputs to front wheels are transmitted to a steering actuator for the rear wheels, which are thus also steered. In a failure mode, the steering actuator for the front wheels is decoupled from that for the rear wheels, and the rear wheels are returned to a straight-ahead position. However, failure of the steering actuator for the front wheels results in complete steering failure and the vehicle is inoperable. Such a system is consequently not suitable for use in an automatically controlled vehicle requiring redundancy in the steering system.

According to the present invention there is provided an automatically controlled vehicle having front and rear wheels which are steerable by respective front and rear steering actuators, whereby the front and rear wheels are steerable independently of one another, coupling means being provided for selectively coupling the rear steering actuator to the front wheels thereby to assist or replace the front steering actuator in steering the front wheels.

In one embodiment in accordance with the present invention, engagement of the coupling means not only couples the rear steering actuator to the front wheels, but also couples the front steering actuator to the rear wheels. Consequently, in the absence of any failure, both steering actuators cooperate to steer the front and rear wheels in a fixed relationship with one another. In the event of failure of the front steering actuator, engagement of the coupling enables the rear steering actuator to steer the front wheels, so that the vehicle remains operable.

The coupling may be a mechanical coupling, such as a clutch mechanism which may be a friction clutch or a dog clutch.

The steering actuators may comprise motors with driveshafts connected to respective steering mechanisms. In such an arrangement the coupling means, when engaged to couple the rear steering actuator to the front wheels, may provide a mechanical connection between the driveshafts.

In alternative embodiments, the coupling means may provide a hydraulic or pneumatic coupling between the rear steering actuator and the front wheels. For example, steering mechanisms of the front and rear wheels may be provided with cylinder and piston units which are hydraulically or pneumatically interconnected by fluid lines, the control means comprising a valve arrangement which is able to vent the fluid lines to prevent hydraulic or pneumatic coupling between the cylinder and piston units.

The coupling means may operate so that, when the rear steering actuator is coupled to the front wheels, it remains also coupled to the rear wheels. With such an arrangement, the coupling between the front and rear wheels may be such that they steer in opposite directions from each other.

Alternatively, coupling of the rear steering actuator to the front wheels may be accompanied by disengagement or decoupling of the rear steering actuator from the rear wheels. The rear wheels may be provided with a self-centring mechanism so that, when disengaged or decoupled from the rear steering actuator, they assume a substantially straight-ahead position.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-Figure 1 shows, in diagrammatic form, a vehicle steering system; Figure 2 corresponds to Figure 1 but shows an alternative embodiment; and Figure 3 corresponds to Figure 1 but shows a third embodiment.

As shown in Figure 1, front wheels 2 and rear wheels 4 of a vehicle are steerable about kingpins 6, 8 by steering mechanism 10, 12.

The vehicle is a driverless vehicle, and its operational functions are controlled by a control unit 14. The control unit 14 has an internal memory containing data for use in navigating the vehicle, but also receives input signals from sensors provided on the vehicle, and from a supervisory control outside the vehicle. The vehicle is primarily intended for use as a passenger-carrying vehicle in a PRT system.

Although the control unit 14 controls many functions of the vehicle, for the purposes of the present description it is shown connected only to front and rear steering actuators 16, 18 and a coupling 20.

The steering actuators 16, 18 may take various forms, but in the embodiment of Figure 1 they comprise electric motors providing a rotational output by means of output shafts 22, 24 which extend to both sides of the motors 16, 18, and are connected at one end to the respective steering mechanism 10, 12, and at the other end to one part of the coupling 20.

By appropriate control of the coupling 20, the vehicle may be operated in various modes. In normal operation, the coupling 20 may be disengaged so that the steering actuators 16, 18 can operate independently of each other to control the front and rear wheels 4 respectively. This provides considerable flexibility in small areas such as stations. If the wheels 2, 4 are steered in opposite directions from each other, tight cornering radii can be achieved, while steering both front and rear wheels 2, 4 in the same direction can enable the vehicle to "crab", enabling it to align itself properly with a platform. When travelling at speed, the rear wheels 4 can be controlled to provide little or no steering so that direction changes are accomplished largely or completely by the front wheels 2.

In the event of failure of one of the steering actuators 16, 18, the coupling 20 may be engaged allowing the coupling 16, 18 which remains operational to control the steering of both the front wheels 2 and the rear wheels 4. The steering layout may be such that, in such circumstances, the front and rear wheels 2, 4 are steered in opposite directions from each other. This arrangement thus allows safe steering of the vehicle along its guideway, possibly at reduced speed. The vehicle would also be manoeuvrable in stations and depots, although with less flexibility than is available when the front and rear wheels 2, 4 are steered independently. Thus, steering control of both the front and rear wheels 2, 4 by a single one of the steering actuators 16, 18 provides sufficient functionality for the vehicle operating in a PRT system, even if it suffers a single steering actuation failure. The vehicle can successively complete its journey to a destination station and reach an easily accessible berth to enable its passengers to disembark. Consequently, undue disruption to the network (which would occur if the vehicle has to stop on the guideway) is avoided. More importantly, the possibility of collision with other vehicles of the network or with stationary parts of the infrastructure is avoided.

Figure 2 shows an alternative embodiment in which parts corresponding to those of Figure 1 are designated by the same reference numbers.

In the system of Figure 2, the steering mechanisms 10, 12 are again operated by steering actuators 16, 18 in the form of electric motors controlled from the control unit 14.

One each of the front and rear wheels 2, 4 is connected by an arm 26, 28 to a piston 30, 32 which is movable in a cylinder 34, 36. The ends of the cylinders 34, 36 from which the piston rods emerge are interconnected by a line 38, and the opposite ends of the cylinders 34, 36 are interconnected by a line 40. Each of the lines 38, 40 can be vented to a reservoir 42 through a valve 44.

In normal operation, the valve 44 is displaced to the right from the position shown in Figure 2, so that the lines 38, 40 are vented to the reservoir 42. The steering actuators 16, 18 are controlled independently by the control unit 14, so that the steering arrangement adapts to the operating circumstances, as with the embodiment of Figure 1.

If one or other of the steering actuators 16, 18 fails, this failure is detected by the control unit 14 which displaces the valve 44 to the position shown in Figure 2. The lines 38, 40, and the respective chambers of the cylinders 34, 36 thus form closed circuits, so that displacement of either one of the pistons 30, 32 as a result of steering action driven by the functioning steering actuator 16, 18, is transferred through the other piston 30, 32, to the other wheels 2 or 4. Consequently, the functioning steering actuator 16, 18 serves to steer both the front and rear wheels 2, 4 in a fixed relationship, for example so that the front and rear wheels 2, 4 steer in opposite directions.

Figure 3 shows a modification of the arrangement shown in Figure 1. Again, the same reference numbers are used to designate the same components.

The driveshaft 24 of the rear steering actuator 18 is provided at one end with one part of the coupling 20, and at the other end with one part of a second coupling 46. The other part of the coupling 46 is Connected to the input of the rear steering mechanism 12.

The rear steering actuator 18 is mounted in a displaceable manner, and can be moved in the lengthwise direction of its driveshaft 24, by means of a lever 48.

In the condition shown in Figure 3, the rear steering actuator 18 is displaced to the right, so that the coupling 46 is engaged. Consequently, the driveshaft 24 is connected to the steering mechanism 12 and operation of the steering actuator 18 will steer the rear wheels 4. This is the normal operating condition of the steering arrangement, in which the front and rear wheels 2, 4 are steered independently of each other.

If the front steering actuator 16 fails, the control unit 14 provides a signal to an actuator (not shown) for the lever 48 to displace the rear steering actuator 18 to the left as shown in Figure 3 to engage the coupling 20. At the same time, this movement disengages the coupling 46, so the connection between the steering actuator 18 and the steering mechanism 12 is discontinued. The rear steering actuator 18 then controls the front steering mechanism 10 to maintain safe operation of the vehicle. A self-centring mechanism 50, for example in the form of a spring, acts on the rear wheels 4 to return them to the straight-ahead position when no steering input is provided through the shaft 24. Consequently, the vehicle will operate with front-wheel steering in the failure mode.

Should the rear steering actuator 18 fail, the self-centring mechanism 50 will again return the rear wheels 4 to the straight-ahead position. Optionally, the rear steering actuator 18 can be displaced to an intermediate position, in which both of the couplings 20, 46 are disengaged.

It will be appreciated that Figures 1 to 3 represent the embodiments of the invention in diagrammatic form to represent the functions and interconnections of the various components, and that the physical form of the individual components will differ from that shown in the Figures.

Claims (13)

1. An automatically controlled vehicle having front and rear wheels which are steerable by respective front and rear steering actuators, whereby the front and rear wheels are steerable independently of one another, coupling means being provided for selectively coupling the rear steering actuator to the front wheels thereby to assist or replace the front steering actuator in steering the front wheels.

2. A vehicle as claimed in claim 1, in which engagement of the coupling operatively connects the front steering actuator to the rear Wheels.

3. A vehicle as claimed in claim 1 or 2, in which the coupling is a mechanical coupling.

4. A vehicle as claimed in claim 3, in which the coupling is a clutch mechanism.

5. A vehicle as claimed in claim 3 or 4, in which the steering actuators comprise motors with driveshafts connected to respective front and rear steering mechanisms, the coupling being operable to interconnect the driveshafts of the steering actuators.

6. A vehicle as claimed in claim 1 or 2, in which steering mechanisms of the front and rear wheels are provided with respective cylinder and piston units, the coupling means being operable to connect regions on opposite sides of the piston of each cylinder and piston unit with the corresponding regions of the other cylinder and piston unit.

7. A vehicle as claimed in claim 6, in which respective regions of the cylinder and piston units are connected to one another by fluid lines, the coupling means comprising a valve arrangement for selectively venting the lines to a fluid reservoir.

8. A vehicle as claimed in any one of the preceding claims, in which the rear steering actuator remains connected to the rear wheels when coupled to the front wheels.

9. A vehicle as claimed in claim 8, in which, when the rear steering actuator is coupled to the front wheels, the front and rear wheels are steered in opposite senses from each other.

10. A vehicle as claimed in any one of claims ito 7, in which, when the rear steering actuator is coupled to the front wheels, the rear steering actuator is disengaged from the rear wheels.

11. A vehicle as claimed in claim 10, in which means is provided for positioning the rear wheels in a straight-ahead position when the rear steering actuator is disengaged from the rear wheels.

12. An automatically controlled vehicle substantially as described herein with reference to, or as shown in, Figure 1, Figure 2 or Figure 3 of the accompanying drawings.

13. A personal rapid transit system including vehicles in accordance with any one of the preceding claims.