WO1997026170A1

WO1997026170A1 - Control system for a steerable axle and a steerable axle

Info

Publication number: WO1997026170A1
Application number: PCT/GB1997/000170
Authority: WO
Inventors: Carlton John Davis; John Philip Potter
Original assignee: Carlton John Davis; John Philip Potter
Priority date: 1996-01-17
Filing date: 1997-01-17
Publication date: 1997-07-24
Also published as: AU1315897A

Abstract

A control system (1) for a steerable axle (30) of a vehicle has a first sensor (41) for outputting a first control signal representative of the amount of steer of a first steerable axle (37) and a second sensor (42) for outputting a second control signal representative of the amount of steer of a second steerable axle (30). Drive means (21) are provided for controllably steering the second steerable axle (30) in proportion to the amount of steer of the first steerable axle (37) in accordance with the first and second control signals. A device (50) is provided for clamping the steerable axle (30) in the straight ahead condition.

Description

CONTROL SYSTEM FOR A STEERABLE AXLE AND A STEERABLE AXLE

The present invention relates to a control system for a steerable axle. In road vehicles generally, and in trucks in particular, there is usually a steering axle which is connected to a steering wheel via a steering column and steering box. The steering axle is generally at the front of the vehicle. Conventionally, a fixed axle, which is often driven but may be non-driven, is usually situated at the rear of the vehicle. In trucks in particular, one or more further axles may be provided so as to spread the weight of the vehicle over a greater number of contact points and to provide a larger area of contact with the road. There is a growing demand for one or more of the previously conventionally fixed axles to be steerable so as to minimise tyre wear and stresses caused during turning of the vehicle and to improve the turning circle of the vehicle; it will be understood that a reference to a "steerable axle" means that the wheels mounted on that axle are steerable.

Some systems for controlling the amount of steer of the (further) steerable axles are known. In one system, a hydraulic ram is connected to the steering axle (at the front of the vehicle) so that the ram is displaced as the steering axle is displaced during steering. Hydraulic fluid is pumped from the hydraulic ram on the steering axle to a second hydraulic ram which is connected to the other steerable axle. Thus, as the steering axle is steered by the vehicle driver, the steerable axle is also steered.

The sizes of the various hydraulic rams and cylinders are set so that an appropriate amount of steer of the steerable axle is obtained according to the amount of steer of the steering axle. However, this system has a drawback in that if the wheelbase is varied between trucks, it is necessary to change the size of the hydraulic rams and cylinders in order to obtain the correct amount of steer of the steerable axle relative to the amount of steer of the steering axle. Furthermore, the mounting of a hydraulic ram on the steering axle makes the steering extremely heavy for the vehicle driver and can prevent the steering axle from naturally returning to the straight ahead condition if the steering wheel is released by the driver.

In another known system, cables are connected to the steering axle, the cables running down to a drive ram on the steerable axle. The use of cables is inevitably troublesome in terms of manufacture and maintenance of the vehicle as cable controls are often unreliable.

Furthermore, in both of the systems mentioned above, the further steerable axles must be positioned at the rear of the vehicle, behind the fixed axle, so that the steerable axle or axles are trailing in order for the wheels on that axle to self-straighten. This precludes the use of a steerable axle which is mid-mounted near to the fixed axle at the rear of the vehicle or in a so-called "Chinese six" in which the mid axle is positioned very close to the steering axle at the front of the vehicle. Furthermore, with the systems mentioned above, there is no way of disabling the steering feature of the steerable axles, for example so as to lock the wheels in the straight ahead position if the vehicle is travelling above some predetermined speed.

In EP-A-0350809, there is disclosed a steering axle regulation system in which a steerable axle is steered by a hydraulic cylinder. A steering error signal, being the difference between a desired steering angle and an actual steering angle of the steerable axle, is sent to a microprocessor control unit to control the overall pressure of the hydraulic system.

According to a first aspect of the present invention, there is provided a control system for a steerable axle of a vehicle, the control system comprising: a first sensor for outputting a first control signal representative of the amount of steer of a first steerable axle; a second sensor for outputting a second control signal representative of the amount of steer of a second steerable axle; and, drive means for controllably steering the second steerable axle in proportion to the amount of steer of the first steerable axle in accordance with the first and second control signals.

The drive means may include a hydraulic ram. In a hydraulic system, fail-safe valves may be provided. Such fail-safe valves may be operated to prevent supply of hydraulic fluid pressure to the hydraulic ram above a predetermined vehicle road speed. The fail-safe valves may be solenoid operated. The system may include a valve for selectively supplying hydraulic fluid to the hydraulic ram in accordance with the first and the second control signals. The valve may be a direct drive valve.

The system may have a control unit, the first and second sensors being arranged to output the first and second control signals to the control unit, the control unit being arranged to provide an output control signal to the valve in accordance with the first and second control signals. The control unit may be a microprocessor.

Alternatively, the control unit may be an analogue electronic circuit. The control unit may be able to receive a smart card containing data relevant to the vehicle on which the system is or is to be mounted. The control unit may be arranged not to send a control signal to the valve above a predetermined vehicle speed. Thus, steering of the second steerable axle is only enabled below a certain threshold speed.

The present invention also includes a vehicle having a first steerable axle and a second steerable axle, and a control system as described above. Speed detecting means may be provided for detecting the speed of the vehicle, the control system being arranged to provide full steering of the second steerable axle below a first lower speed threshold and no steering of the second steerable axle above a second upper speed threshold.

The control system is preferably arranged to vary the proportion of steer of the second steerable axle relative to the amount of steer of the first steerable axle between a maximum at the first lower speed threshold and zero at the second upper speed threshold.

The vehicle may have further steerable axles each having a respective sensor for outputting a respective control signal, the drive means being arranged to controllably steer each respective steerable axle in accordance with the first control signal and the control signal output by the sensor provided for that steerable axle.

The first steerable axle may be a steering axle. The steering axle may be steered by a mechanical linkage, such as a steering wheel, steering column and box having a rack and pinion arrangement. Alternatively, the steering axle may be steered by a non-mechanical linkage such as a hydraulic ram controlled by signals from an input steering device. According to a further aspect of the present invention, there is provided a steerable axle, the axle comprising: a steerable wheel on the axle; and, a clamping device for clamping the axle in a straight ahead condition in which the wheel is fixed against steering, the clamping device comprising: a steering lever mounted for pivotal movement with respect to the axle and having a pivotable connection to the steerable wheel so that, as the wheel is pivoted, the steering lever pivots; a steering stop; and, driving means for driving the steering stop to a position where it engages the steering lever to drive the steering lever to the straight ahead condition. A connecting rod may be connected to the steerable wheel to drivingly steer the wheel, the steering lever being pivotally connected to the connecting rod.

The steering stop is conveniently pivotally mounted with respect to the axle.

The steering stop may be arranged to pivot with the steering lever as the steering lever is pivoted away from the straight ahead condition.

The driving means preferably comprises an inflatable air bag which can be inflated to drive the steering lever to the straight ahead condition.

Two steering stops are preferably provided for engaging respective opposed sides of the steering lever, each steering stop having its own respective driving means for respectively driving each steering stop to a position where it engages the steering lever to drive the steering lever to the straight ahead condition.

The axle may have two steerable wheels, the wheels being respectively fixed to the ends of the axle, the steering lever being pivotally connected to each of the wheels.

A hydraulic ram is preferably provided for steering the wheel or wheels.

The present invention also includes a vehicle as described above in which each steerable axle is an axle as described above.

According to a yet further aspect of the present invention, there is provided an axle assembly, the assembly comprising: a king pin; a stub axle mounted for pivotal movement about the king pin; and, a rotary position transducer having a first part fixed against pivoting with respect to the stub axle and a second part fixed against pivoting with respect to the king pin, whereby pivotal movement of the stub axle with respect to the king pin causes a corresponding pivotal movement of the first transducer part with respect to the second transducer part, thereby causing said transducer to output a signal indicative of the amount of pivotal movement of the stub axle.

The first transducer part may be a housing of the rotary position transducer. The second transducer part may be a rotating control spigot for the transducer.

The king pin is fixed in use to an axle beam. A cap may be fixed to the stub axle. The first transducer part may be fixed to the cap. A spacer may be fixed between the cap and the king pin.

The present invention also includes a vehicle having a stub axle assembly as described above.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a diagrammatic representation of the main hydraulic components of an example of a system according to the present invention;

Fig. 2 is a diagrammatic representation of an example of a system according to the present invention;

Fig. 3 is a schematic plan view of a vehicle chassis; Fig. 4 is a plan view of an example of a device for clamping a vehicle axle;

Figs. 5a to 5c are plan views showing the axle clamping device in operation; and,

Fig. 6 is a partial cross-sectional view of an example of a stub axle assembly.

In Figures 1 and 2, there is shown a control system 1 for a steerable axle 30 (shown in Figure 3 and discussed below) , with the main hydraulic components being shown in more detail in Figure 1. A supply pump 2, driven by an electric motor 3 powered by a battery 4, supplies hydraulic fluid such as hydraulic oil under pressure from a reservoir 5 of hydraulic fluid to an input port 10 to the main hydraulic part of the system. The pump 2 may be mounted on the engine of the vehicle to be powered by the vehicle engine or battery or the pump may be provided as a separate component with its own power supply. The hydraulic fluid is preferably passed through a filter 6 to remove unwanted particles before entry of the hydraulic fluid to the input port 10. A pressure switch 7 detects the pressure of the hydraulic fluid being pumped and stops operation of the pump 2 if an over-pressure condition is detected.

Hydraulic fluid is passed from the input port 10 to a direct drive valve 11. The direct drive valve 11 is controlled by an input control signal 12 selectively to prevent transmission of hydraulic fluid pressure further down the system (centre position as shown in Figure 1) , to supply hydraulic fluid pressure to the right hand side of the system in Figure 1 (with the valve 11 moved to the left in Figure 1) , or to supply hydraulic fluid pressure down the left hand side of the system shown in Figure l (with the valve moved to the right in Figure 1) . The direct drive valve 11 is controlled by the input control signal 12 to cause movement of the spool of the valve 11 by an amount proportional to the input control signal 12. A direct drive valve 11 can have its spool driven in both directions away from a centred position under electric closed loop spool position control. A gas-filled hydraulic fluid accumulator 8 is connected to the direct drive valve 11 and stores hydraulic fluid under pressure so that pressure is available on demand.

A hydraulic ram 20 moves in a cylinder 21 under the action of hydraulic fluid pressure supplied to the left hand side input port 22 or right hand side input port 23 of the hydraulic cylinder 21 shown in Figure 1. As shown in

Figure 3, the hydraulic ram 20 is adjacent to the steerable axle 30 and is connected by respective left hand and right hand tie bars or connecting rods 24,25 to respective wheels 31,32 mounted on the steerable axle 30 so as to be able to steer the wheels 31,32. In the example shown in Figure 3, the steerable axle 30 is mounted in front of a fixed axle 33 which carries the driven wheels 34,35. The steerable axle 30 and fixed axle 33 are mounted towards the rear of a vehicle chassis 36. The main steering axle 37 which carries the main steering wheels 38,39 which are steered by the vehicle driver is mounted towards the front of the vehicle chassis 36.

Referring again to Figure l, in which the hydraulic circuit between the direct drive valve 11 and the hydraulic ram 20 is symmetrical on the left and right hand sides, the hydraulic fluid to and from the direct drive valve 11 passes through respective left and right and side fail-safe solenoid valves 26 and hose burst fail-safe valves 27 to the respective input ports 22,23 to the hydraulic cylinder 21. The action of the fail-safe solenoid valves 26 will be described in more detail below. The hose burst fail-safe valves 27 prevent loss of hydraulic fluid from the tank if a connecting hose fails and only allow fluid to be lost from the hydraulic cylinder 21. Pressure spikes in the hydraulic system can be caused when the vehicle strikes an obstruction such as a kerb. Thus, respective over-pressure relief valves 28 are provided to prevent undue build up of hydraulic pressure in the system 1 (say beyond 210 atmospheres) and are connected to the input ports 22,23 to the hydraulic cylinder 21 and allow hydraulic fluid to be released to a tank (not shown) if the hydraulic pressure exceeds the predetermined pressure. The direct drive valve 11 is also connected at an output port 40 of the system 1 to the tank to complete the hydraulic circuit. A pump pressure relief valve 29 is connected between the supply input port 10 to the system 1 and the tank output port 40 of the system 1 to prevent excessive hydraulic pressure being supplied into the system 1.

A rotary position transducer 41 is mounted as a first sensor to detect the amount of steer of the wheels 38,39 carried by the steering axle 37. The rotary position transducer or sensor 41 for the steering axle 37 can be ounted to detect movement of the tie bars (not shown) which move the wheels 38,39 carried by the steering axle 37. Alternatively, the first sensor 41 can be mounted on the steering column (not shown) or within the steering box housing (not shown) which drives the steering axle 37. If the rotary position transducer 41 iε mounted in the steering box housing, a suitable gear reduction can be provided to drive the rotary position transducer 41 in a way which mimics the steer of the wheels 38,39. The wheels 38,39 on the steering axle 37 may alternatively be driven by a hydraulic ram, without any mechanical linkage such as a steering column and rack and pinion arrangement to the vehicle's steering wheel or other steering input device; in such a case, the sensor 41 can itself be mounted on the steering wheel or other steering input device or in the hydraulic ram provided for the steering axle 37. In any event, the sensor 41 outputs a signal which is representative of the amount of steer of the wheels 38,39 on the steering axle 37. A further rotary position transducer or second sensor 42 is mounted on the steerable axle 30 to detect the amount of steer of wheels 31,32 carried by the steerable axle 30. The second sensor 42 can of course alternatively be mounted to detect the amount of movement of the hydraulic ram 20 or the tie bars or connecting rods 24,25 which control the amount of steer of the wheels 31,32 on the steerable axle 30. The second sensor 42 outputs a control signal representative of the amount of steer of the steerable axle 30. The first and second control signals respectively from the first and second sensors 41,42 are used to control the direct drive valve 11. The control signals from the sensors 41,42 are passed to a control unit 43. The control unit 43 may be a microprocessor which has overall control of the system 1. Alternatively, the control unit may be in the form of analogue circuits. A microprocessor has the advantage of being more flexible in that it can readily be reprogrammed according to the vehicle on which the system 1 is mounted. However, a microprocessor can emit electromagnetic radiation or be susceptible to interference from electromagnetic radiation and therefore an analogue circuit may be more straightforward to implement.

In use, as the steering axle 37 is moved to steer the steering road wheels 38,39, the amount of steer is detected by the first sensor 41 which outputs its control signal to the control unit 43 mentioned above. The control unit 43 determines the required amount of steer of the steerable axle 30. Where the control unit 43 is a microprocessor, the output control signal 12 can be calculated taking into account variable factors, such as the speed of the vehicle, the loading of the vehicle, direction of travel (forward or reverse), etc., and fixed factors such as the wheelbase, size of the wheels 31,32,34,35,38,39, the length of the axles 30,33,37, etc. Where the control unit 43 is implemented by analogue circuitry, the control signal 12 is a steering error signal which is the difference between the first and second control signals. Preferably, the first and second control signals from the first and second sensors 41,42 are respectively amplified by a different predetermined gain for each control signal before their difference is taken. This ensures that the steerable axle 30 is moved by an angle which is the correct proportion of the angle of steer of the steering axle 37. For example, the steerable axle 30 may need to be moved through 30° for a 50° movement of the steering axle 37. This is readily implemented by analogue circuitry. Where the control unit 43 is a microprocessor, the required amount of steer of the steerable axle 30 can be stored as a map of angles for various amounts of steer of the steering axle 37 and vehicle speed, etc., and again is usually a proportion of the amount of steer of the steering axle 37. The control signal 12 is applied to the direct drive valve 11 which then shifts so as to provide hydraulic fluid to either the left hand or right hand input port 22,23 of the hydraulic cylinder 21 as necessary. The hydraulic fluid pressure output by the direct drive valve 11 is proportional to the level of the input control signal 12. As fluid pressure is applied to either the left hand or right hand side of the hydraulic ram 20, the hydraulic ram 20 moves to the left or right as appropriate in order to steer the steerable axle 30. The amount of steer of the steerable axle 30 is detected by the second sensor 42 which provides its control signal to the control unit 43, thus forming a closed loop system, so that if the detected amount of steer of the steerable axle 30 differs from the requested amount of steer, appropriate correction can be made by sending the error signal to the direct drive valve 11.

Where the control unit 43 is a microprocessor, it is envisaged that a so-called smart card be used in the control unit 43, the smart card carrying data which determines the required amount of steer of the steerable axle 30 according to the amount of steer of the steering axle 37 and the dimensions of the vehicle. Where the system 1 is installed in vehicles having different wheelbases or axle lengths, etc. , the vehicle manufacturer only needs to insert the appropriate smart card according to the respective dimensions of the vehicle. This should be contrasted with the prior art system mentioned above which relies on transmission of hydraulic fluid from the steering axle to the steerable axle in which it is necessary to recalculate and change the sizes of the respective hydraulic cylinders whenever different dimension vehicles are to be supplied with the system. Where the control unit 43 is an analogue circuit, it is a simple matter to make appropriate adjustment to the gain applied to the first and second control signals so that the required amount of steer of the steerable axle 30 is achieved. Thus, with the present invention, basically the same mechanical and hydraulic components can be fitted to every vehicle and it is simply a matter of adjusting or reprogramming the control unit 43 or supplying appropriate smart cards according to the dimensions of the vehicle concerned.

The system is preferably arranged so that the steerable axle 30 is only steered below a predetermined vehicle road speed. Above the predetermined road speed, the steering axle 30 is clamped and effectively fixed against steering. Where the control unit 43 is a microprocessor, this can be set up in the program running in the control unit 43 so that no control signal 12 is sent to the direct drive valve 11 above the predetermined road speed. This control dependent on vehicle speed can also readily be achieved by the analogue version of the control unit 43. In either case, the control unit 43 is supplied with a signal indicative of the vehicle road speed from a sensor 44 mounted to detect the speed from an anti-lock brake speed ring 45 associated with one of the road wheels. Indeed, speed signals may be taken from several of the vehicle wheels in order to provide an additional measure of safety in case one of the speed sensors 44 fails for some reason. Additionally or alternatively, the system may be arranged so that the hydraulic fluid supply pump is only operational below the predetermined road speed by having the pump switched on or off according to a signal from the vehicle tachometer. Where the control unit 43 is an analogue circuit, the circuit can be arranged so that a signal indicative of vehicle speed is used to prevent the control signal 12 being sent to the direct drive valve 11 about a certain predetermined vehicle speed.

As a further safety feature, in addition to or as an alternative to the methods described above, when the vehicle is above the predetermined road speed, or on failure of the electrical power supply, the fail-safe solenoid valves 26 are de-energised to allow free flow of hydraulic fluid to and from the respective ports 22,23 to the hydraulic cylinder 21. This causes the steerable axle 30 to be hydraulically clamped in the straight ahead condition as the pressures on the left and right sides of the hydraulic cylinder 21 will equalise.

It is most preferred for the steerable axle 30 to be positively mechanically clamped in the straight ahead position when steering is not required or desired, for example because of the vehicle speed or because of a failure of a part of the system 1. An example of a suitable clamping device 50 is indicated in Figure 2 and shown in more detail in Figure 4. The axle clamping device 50 has a T-shape steering lever 51. The steering lever 51 is pivotally fixed at one end 52 to the fixed part of the steerable axle 30. Each end of the horizontal bar 53 of the T-shape steering lever 51 is pivotally fixed to a respective connecting rod 24,25 by respective pivots 54. Thus, as the wheels 31,32 of the steerable axle 30 are steered, and the respective connecting rods 24,25 moved to the left and right, the steering lever 51 is caused to pivot to the left or right about the pivot point 52. The pivoted position of the steering lever 51 is shown by dashed lines in Figure 4. Steering stops 55 in the form of lever arms 55 are pivotally fixed to the fixed part of the steerable axle 30 either side of the vertical bar 56 of the steering lever 51. The pivot point 56 of each steering stop 55 is adjacent the pivot point 52 of the steering lever 51. A first end 57 of each steering stop 55 engages the vertical bar 56 of the steering lever 51 in the straight ahead position of the steerable axle 30 as shown particularly clearly in Figures 4 and 5a. As the steering lever 51 pivots to the left or right, the associated left or right side steering stop 55 follows the steering lever 51 and is thereby caused to pivot about its pivot point 56 as shown by the dashed lines in Figure 4 and in Figures 5b and 5c.

The other end of each steering stop 55 carries a respective air bag or bellow 58, the respective air bags 58 being in contact with one another at all times. In the normal, steering condition of the system 1, the air bags 58 are not pressurised. Thus, as the steering lever 51 pivots to the left, for example, as shown in Figure 5b and by dashed lines in Figure 4, the air bags 58 offer no resistance to the pivoting. Thus, steering of the steerable axle 30 is allowed.

When steering of the steerable axle 30 is to be inhibited, both of the air bags 58 are inflated by pressurised air delivered from a vehicle air tank 59 indicated in Figure 2. A solenoid operated air valve 60, under control of signals from the control unit 43, normally prevents air being transmitted to the air bags 58. When clamping of the axle 30 is required, the air bags 58 are inflated. This causes the steering lever 51 to be driven to the straight ahead condition shown in Figure 5a by virtue of the steering stops 55 being pivoted under the action of the pressurised air bags 58. The steerable axle 30 is thereby driven to and clamped in its straight ahead condition. The solenoid air valve 60 can act in a fail- safe mode so that, on loss of power, the air bags 58 are automatically inflated in order to drive the steerable axle 30 to its straight ahead condition and clamp the axle 30 in the straight ahead condition. The solenoid air valve 60 can also be operated to clamp the axle 30 in the straight ahead condition when the vehicle speed exceeds a certain threshold.

When the axle clamping device 50 is used, a convenient mounting point for the second sensor 42 is on the pivot of the steering lever 51 as shown in the drawings. A first fixed part of the second sensor 42 can be fixed with respect to the steerable axle 30 whilst the second rotating -15-

part of the second sensor 42 can be fixed relative to the steering lever 51. In this way, the angular deflection of the steering lever 51 to the left or right, and thus the angular deflection of the wheels 31,32, can be detected and the second control signal sent to the control unit 43. Where the steerable axle 30 is to be fixed and not steerable above a predetermined speed, it is preferred to use two threshold values for the speed limit, an upper speed limit and a lower speed limit. The upper speed limit may be say 12mph (say 19 or 20kph) and the lower speed limit may be lOmph (say 16kph) . Above the upper speed limit, the steerable axle 30 is locked against steering, e.g. by allowing the solenoid valves 26 to open and/or cutting the power supply to the hydraulic fluid pump and/or stopping the supply of the control signal to the direct drive valve 11, thereby to prevent transmission of hydraulic fluid pressure to the hydraulic ram 21, and/or by using a mechanical axle clamping device 50, as described above. As the vehicle speed drops below the upper speed limit, the steerable axle 30 can be unlocked so as to become fully steerable when the lower speed limit is reached, e.g. by operating the solenoid valves 26 and/or providing power to the hydraulic fluid pump and allowing the supply of the control signal to the direct drive valve ll and by allowing air to escape from the air bags 58 of the axle clamping device 50. In order to prevent a sudden mismatch of actual amount of steer of the steerable axle 30 and the requested amount according to the amount of steer of the steering axle 37 (e.g. when the driver has already steered the steering axle 37 before the lower speed limit has been reached) , the control unit 43 can be arranged so that the steerable axle 30 is gradually moved at a "ramp" speed to achieve the desired amount of steer and is therefore not suddenly and abruptly moved to the requested amount of steer. It will be appreciated that this use of two threshold speed limits prevents the steerable axle 30 from rapidly switching between being the steerable and locked conditions as might otherwise occur if a single speed threshold is used and the vehicle speed varies slightly around that single speed threshold. In this embodiment, the actual desired angle of steer of the steerable axle 30 output by the control unit 43 is gradually increased on dropping below the upper speed limit to reach the theoretical desired angle of steer gradually. A similar process is used in reverse when the vehicle speed increases above the lower speed threshold so that, when the upper speed threshold has been reached, the steering signal sent to the direct drive valve 11 has been gradually reduced to zero.

In normal use, as the vehicle speed increases to reach the upper speed threshold, the steerable axle 30 will be in the straight ahead condition. However, if a failure of a part of the system is detected so that the air bags 58 of the clamping device 50 are inflated, the solenoid air valves 26 are de-energised so that hydraulic fluid can flow freely to and from the hydraulic cylinder 21. This means that the inflating air bags 58 do not meet any resistance in the hydraulic cylinder 21, allowing rapid and safe clamping of the steerable axle 30 in the straight ahead condition The actual amount of steer of the steerable axle 30 can be indicated to the vehicle driver by use of an appropriate display 46 in the vehicle cab. The display 46 may be a liquid crystal display or an analogue display, for example, fed with a signal from the control unit 43. A warning light 47 can be provided in the vehicle cab to indicate a system failure. A system on/off switch 48 can also be provided in the vehicle cab so that the system can be deactivated if desired for any reason.

It will be appreciated that, with the present invention, the steerable axle 30 can be positioned practically anywhere along the vehicle chassis 36. The steerable axle 30 can be positioned just in front of the driven axle 33 as shown in Figure 3, behind the driven axle 33, or much further forward on the vehicle and just behind the steering axle 37 as in a so-called Chinese six. These different positions are accommodated simply by reprogramming the control unit 43, perhaps by use of a different smart card as described above.

Furthermore, two or more steerable axles, in addition to the main steering axle, can easily be provided by the use of appropriate programming and respective control systems 1 having respective direct drive valves, hydraulic drive rams, etc., so that all of the axles on a multi-axle vehicle are steerable.

In Figure 6, there is shown a yet further example of a suitable mounting for the second sensor 42.

A stub axle assembly 61 has a king pin 62 which is solid and generally cylindrical. The king pin 62 is fixed to an axle beam 63 of the axle 30,33,37 so that the king pin 62 is unable to pivot or rotate in the axle beam 63. A stub axle 64 is provided on which a wheel (not shown) is rotatably mounted. The stub axle 64 is mounted on the king pin 62 and can pivot thereabout. To facilitate pivotal movement of the stub axle 64 on the king pin 62, low friction bushes 65 are provided on the cylindrical internal surface of the stub axle 64 which abuts the king pin 62.

A circular cup-shape cap 66 is fixed over the top of the king pin 62 and upper part of the stub axle 64. The cap 66 is fixed to the stub axle 64 by pins 67 which are preferably a friction or force fit in respective recesses 68,69 provided in the stub axle 64 and cap 66. The cap 66 is therefore fixed against pivoting movement with respect to the stub axle 64.

The second sensor 42, which is a rotary position transducer, has an outer housing 71 and a pivotable control spigot 72 which projects from the housing 71. As the spigot 72 pivots or rotates with respect to the housing 71, the rotary position transducer 42 outputs an electrical signal which is representative of (and usually proportional to) the amount of rotation of the spigot 72 with respect to the housing 71.

The housing 71 is fixed to the cap 66 by pins 73 which are a friction or force fit in respective recesses 74,75 provided in the upper surface of the cap 66 and the corresponding lower surface of the housing 71 of the transducer 42. Accordingly, the housing 71 is fixed against rotation with respect to the cap 66 and therefore is also fixed against rotation with respect to the stub axle 6 .

The spigot 72 of the transducer 42 projects through a central recess 76 provided in the cap 66 and fits into a fixing pin 77. The fixing pin 77 is a friction or force fit in a recess 78 provided in the upper surface of the king pin 62. The spigot 72 is therefore fixed against rotation with respect to the king pin 62. Conveniently, a metal spacer 79 is positioned between the upper surface of the king pin 62 and the cap 66. The spacer 79 is fixed to the king pin 62 by pins 80 which are a friction or force fit in respective recesses 81,82 provided in the king pin 62 and the spacer 79. The spacer 79 has a central through hole 83 through which the control spigot 72 of the transducer and the fixing pin 77 pass. In use, as the wheel (not shown) fixed to the stub axle 64 is turned in order to cause steering, the pivotal movement of the stub axle 64 with respect to the king pin 62 carries the housing 71 of the rotary position transducer 42 with it. As the control spigot 72 of the transducer 42 is fixed to the king pin 62, the housing 71 pivots with respect to the spigot 72. This causes the transducer 42 to output a signal representative of the amount of pivotal movement of the stub axle 64 and therefore of the wheel

(not shown) mounted on the stub axle 64. The first sensor 41 mentioned above can also be constituted in the same manner.

The fixing of the first or second rotary position transducers 41,42 in the manner described above is very straightforward and requires little modification of the components such as the king pin 62 and stub axle 64 which are already present in a conventional stub axle assembly. The rotary position transducer 41,42 can readily be removed if replacement or maintenance is required. Whilst mounting of the first and/or second sensor 41,42 in the king pin assembly described above is very convenient, it will be appreciated that the mounting of the sensor 41,42 in the axle clamping device 50 described above is also very convenient and operates in a similar manner; a first part of the sensor 41,42 is fixed against pivoting with respect to the king pin (because that first part is fixed to a fixed part of the axle as mentioned above) and a second part of the sensor 41,42 is fixed against pivoting with respect to the stub axle (because that second part follows the pivotal steering movement of the wheel mounted on the stub axle) .

Embodiments of the present invention has been described with particular reference to the example illustrated. However, it will be appreciated that variations and modifications may be made to the example described within the scope of the present invention.

Claims

1. A control system for a steerable axle of a vehicle, the control system comprising: a first sensor for outputting a first control signal representative of the amount of steer of a first steerable axle; a second sensor for outputting a second control signal representative of the amount of steer of a second steerable axle; and, drive means for controllably steering the second steerable axle in proportion to the amount of steer of the first steerable axle in accordance with the first and second control signals.

2. A system according to claim 1, wherein the drive means includes a hydraulic ram.

3. A system according to claim 2, comprising a valve for selectively supplying hydraulic fluid to the hydraulic ram in accordance with the first and the second control signals.

4. A system according to claim 3, wherein the valve is a direct drive valve.

5. A system according to claim 3 or claim 4, comprising a control unit, the first and second sensors being arranged to output the first and second control signals to the control unit, the control unit being arranged to provide an output control signal to the valve in accordance with the first and second control signals.

6. A system according to claim 5, wherein the control unit is arranged not to send a control signal to the valve above a predetermined vehicle speed.

7. A vehicle having a first steerable axle and a second steerable axle, and a control system according to any of claims 1 to 6.

8. A vehicle according to claim 7, comprising speed detecting means for detecting the speed of the vehicle, the control system being arranged to provide full steering of the second steerable axle below a first lower speed threshold and no steering of the second steerable axle above a second upper speed threshold.

9. A vehicle according to claim 8, wherein the control system is arranged to vary the proportion of steer of the second steerable axle relative to the amount of steer of the first steerable axle between a maximum at the first lower speed threshold and zero at the second upper speed threshold.

10. A vehicle according to any of claims 7 to 9, wherein the first steerable axle is a steering axle.

11. A vehicle according to claim 10, wherein the steering axle is steered by a mechanical linkage.

12. A vehicle according to claim 10, wherein the steering axle is steered by a non-mechanical linkage controlled by signals from an input steering device.

13. A vehicle according to any of claims 7 to 12, comprising further steerable axles each having a respective sensor for outputting a respective control signal, the drive means being arranged to controllably steer each respective steerable axle in accordance with the first control signal and the control signal output by the sensor provided for that steerable axle.

14. A steerable axle, the axle comprising: a steerable wheel on the axle; and, a clamping device for clamping the axle in a straight ahead condition in which the wheel is fixed against steering, the clamping device comprising: a steering lever mounted for pivotal movement with respect to the axle and having a pivotable connection to the steerable wheel so that, as the wheel is pivoted, the steering lever pivots; a steering stop; and, driving means for driving the steering stop to a position where it engages the steering lever to drive the steering lever to the straight ahead condition.

15. A steerable axle according to claim 14, comprising a connecting rod connected to the steerable wheel to drivingly steer the wheel, the steering lever being pivotally connected to the connecting rod.

16. A steerable axle according to claim 14 or claim 15, wherein the steering stop is pivotally mounted with respect to the axle.

17. A steerable axle according to claim 16, wherein the steering stop is arranged to pivot with the steering lever as the steering lever is pivoted away from the straight ahead condition.

18. A steerable axle according to any of claims 14 to 17, wherein the driving means comprises an inflatable air bag which can be inflated to drive the steering lever to the straight ahead condition.

19. A steerable axle according to any of claims 14 to 18, comprising two steering stops for engaging respective opposed sides of the steering lever, each steering stop having its own respective driving means for respectively driving each steering stop to a position where it engages the steering lever to drive the steering lever to the straight ahead condition.

20. A steerable axle according to any of claims 14 to 19, comprising two steerable wheels, the wheels being respectively fixed to the ends of the axle, the steering lever being pivotally connected to each of the wheels.

21. A steerable axle according to any of claims 14 to 20, comprising a hydraulic ram for steering the wheel or wheels.

22. A vehicle according to any of claims 7 to 13, wherein each steerable axle is a steerable axle according to any of claims 14 to 21.

23. An axle assembly, the assembly comprising: a king pin; a stub axle mounted for pivotal movement about the king pin; and, a rotary position transducer having a first part fixed against pivoting with respect to the stub axle and a second part fixed against pivoting with respect to the king pin, whereby pivotal movement of the stub axle with respect to the king pin causes a corresponding pivotal movement of the first transducer part with respect to the second transducer part, thereby causing said transducer to output a signal indicative of the amount of pivotal movement of the stub axle.

24. An axle assembly according to claim 23, wherein the first transducer part is a housing of the rotary position transducer and the second transducer part is a rotating control spigot for the transducer.

25. An axle assembly according to claim 23 or claim 24, comprising a cap fixed to the stub axle.

26. An axle assembly according to claim 25, wherein the first transducer part is fixed to the cap.

27. An axle assembly according to claim 25 or claim 26, comprising a spacer fixed between the cap and the king pin.