A known vehicle, as described in WO 96/15656 comprises an elongate middle section having a chassis section at each end. Each chassis section carries two steerable drive wheels. Two engines are provided, one carried on each chassis section to drive the wheels of that chassis section. The vehicle can be driven in a first mode (road mode) longitudinally of said elongate section or a second mode (field mode) transversely of the elongate section. Each wheel is mounted by an arm, each arm being connected to each respective wheel within the height of the wheel.

On each chassis section one arm is connected to one side of its respective wheel and the other arm is connected to the other side of its respective wheel such that when changing from road mode to field mode the wheels are rotated in the same direction.

Each arm is connected to two pivot points within the wheel on the upright diametrical axis of the wheel.

This vehicle suffers from several problems. First, when the vehicle is being driven in road mode, i. e. longitudinally of the elongate section, the vehicle operator can lose control of the vehicle if hydraulic power fails. This is because the steering system requires"fly by wire"control. That is, the steering wheel in the cab is not connected directly to the hydraulic actuators steering the wheels. It seems likely, that future regulations will require farm vehicles to be steerable and stoppable even after power failure.

A machine of this type, in order to aid manoeuverability, should be able to (a) rotate around one wheel, and (b) rotate about an axis centrally between the wheels.

The machine disclosed in WO 96/15656 cannot readily fill these requirements.

Ideally, the steering effect should be continuous from full left lock to full right lock in both the longitudinal and transverse directions.

The suspension of the vehicles of the type disclosed in WO 96/15656 have an hydraulic suspension fitted directly to the wheels which requires control in order to keep the elongate section substantially in the same orientation relative to the vertical when the vehicle is traversing rough terrain. This system of controlling the relative position of the elongate section to the ground is unsatisfactory because it requires sensors, a central processor and a hydraulic pressure source. In the system disclosed the height of the elongate section relative to the wheels can vary.

Previously, in four wheel steer mode it has been hard to keep the wheels of the rear axle synchronized with those of the front axle. The most common technique for implementing four wheel steer is to connect the hydraulic steering actuator of the rear wheels in series with the hydraulic actuator of the front wheels. On machines with selectable two wheel steer or four wheel steer, the engagement and disengagement is electrically interlocked to ensure that this only occurs when both axles are in the straight ahead position. The problem with this approach is that hydraulic leakage across the pistons within the steering system cylinders results in misalignment of the two axles. The other way in which alignment is ensured is to use a fly by wire system in which the direction of the wheels is monitored and adjusted accordingly. The major disadvantage associated with the latter technique is that the system cannot be used on public roads when the rear axle must be mechanically locked to satisfy the safety regulations.

Hydraulically powered vehicles can be difficult to drive on roads since traditionally the flow of hydraulic fluid to the hydraulic motors driving the wheels is controlled by a hand control. To brake such a vehicle it is necessary to reduce the hydraulic fluid flow rate using the hand control.

Conventionally, ancillary vehicles in agriculture used to collect, grain, straw etc. from a harvesting vehicle must be driven by an operator. This is labour intensive.

According to a first aspect of the present invention there is provided a steering system suitable for a vehicle which is drivable and steerable in a first mode in which the vehicle moves substantially in its longitudinal direction and in a second mode in which the vehicle moves substantially transverse to its longitudinal

direction, said steering system comprising : two pairs of wheels, each wheel being independently steerably mounted and the mounts of the wheels of each pair of wheels being connected together by a telescopic link with a hydraulically actuated mechanism to lock the telescope link such that the telescopic links are locked in said first mode and at least one of said telescopic links is allowed to float in said second mode.

In this way it is possible in road mode for the wheels to be steered under the direct control of the vehicle driver and computer controlled steering is not necessary so that the vehicle driver can maintain control in the event of power failure.

Furthermore, in the road mode, the wheels are kept rigidly aligned.

The Ackermann steering effect may be evident in both the road mode (with rigid interconnection of the steerable wheels) and field mode (when all steerable wheels are independent).

The tie bar and actuator may comprise a hydraulic piston and cylinder arrangement pressurised to extend the tie bar and to hold it extended, when the vehicle is operating in the road mode.

Possibly the wheels are steered by hydraulic actuators.

Suitably, if the wheels are steered by hydraulic actuators, when in road mode, the hydraulic pressure applied to the hydraulic actuator of the telescopic link of a pair of wheels is at least the highest pressure applied to the hydraulic actuators of that pair of wheels such that the force applied to move the wheels is always less than the force applied by the hydraulic actuator of the telescopic link.

In this way it is not necessary for the operator of the vehicle to increase the hydraulic pressure of the hydraulic actuator before driving in the longitudinal direction. Transfer from field to road mode can be accomplished without the driver needing to leave his cab.

Optimally, the hydraulic pressure applied to the hydraulic actuator of the telescopic links is held by non-return valves.

Conveniently each telescopic link is connected to the wheel mounts in a position between the centre of the vehicle and the axis of driving rotation of the

wheels which the telescopic link joins. This allows the wheels of each pair of wheels to rotate in different directions to change between road and field modes.

According to a second aspect the present invention there is further provided a four wheel steer vehicle comprising a pair of wheels at each end of the vehicle, each wheel being independently steerable and mounted such that it can undergo steering rotation in a direction such that a steering deflection angle decreases from a position of maximum lock towards the other wheel of its pair of wheels before increasing by at least 90° before no longer being possible, said steering deflection angle being defined as the acute angle measured between (i) the line joining that wheel and the other wheel on the same side of the vehicle, and (ii) a line pointing in the same direction as that wheel when said wheel is in a position of maximum lock towards the other wheel of its pair of wheels.

If desired, the steering rotation may be as much as 260°.

In this way, when the vehicle changes from moving in the longitudinal direction to moving in the transverse direction (from road mode to field mode) the wheels at each end of the vehicle are turned inwards toward one another (rather than both put on to full lock in the same direction as is the case in WO 96/15656). This arrangement allows several different types of steering to take place including two wheel steer and four wheel steer in the road mode, four wheel steer about a point including the centre of the vehicle, road mode crab steer, field mode crab steer as well as rotation about one wheel and rotation about a point between two wheels at one end, etc.

In a preferred embodiment, partial four wheel steer is implemented in the road mode. This means that to turn the vehicle in a given direction, the front wheels turn in that direction and the rear wheels turn a corresponding smaller amount in the opposite direction.

According to a third aspect of the present invention, there is provided a method of synchronizing the wheels of front and rear axles in a four wheel steer vehicle, in which, during a turn, the front and rear wheels steer in opposite directions, said method comprising:

detecting if the wheels of the front axle are pointing left or right of straight ahead ; detecting if the wheels of the rear axle are pointing straight ahead or left or right of straight ahead ; enabling steering rotation of the wheels of the rear axle rightwards only if the wheels of the rear axle are pointing left of straight ahead or if the wheels of the front axle are pointing left of straight ahead ; and enabling steering rotation of the wheels of the rear axle lefnvards only if the wheels of the rear axle are pointing right of straight ahead or if the wheels of the front axle are pointing right of straight ahead ; such that the wheels of the front and rear axles synchronise when turning the front wheels through the straight ahead position if the rear steering angle is leading the front steering angle.

This method has the advantage that it can be implemented without fly by wire control and can be mechanically or hydraulically implemented.

According to a four aspect of the present invention there is further provided a vehicle comprising a pair of wheels at each end, each of said wheels having an associated steering actuator which includes a piston having a thread formed on its cylindrical surface and a corresponding hydraulic cylinder in which the piston lies and having a thread formed on its inside surface, such that the piston rotates as the hydraulic pressure in said cylinder is increased or decreased, the wheel being mounted to the piston such that the steering angle of the wheel changes with the rotation of the piston.

In this way it is possible to arrange a system which allows rotation of wheels by at least 180° since the wheels can be conveniently steered from above.

According to a fifth aspect of the present invention there is further provided a hydraulically driven vehicle comprising ; speed control and brake pedals ; a variable displacement hydraulic pump; a hydraulic motor driven by the hydraulic flow generated by the hydraulic pump ; and

a control system which: upon a change in the amount of depression of the speed control pedal, progressively changes the hydraulic flow supplied by the hydraulic pump until the magnitude of the hydraulic flow is a predetermined proportion of the amount of depression of the speed control pedal ; and increases the rate of decrease in the hydraulic flow in proportion to the amount of depression of the brake pedal.

Such a vehicle has the advantage that it can be driven in a similar way to an ordinary combustion engine driven car. This increases the safety of the vehicle.

According to a sixth aspect of the present invention there is provided a method of guiding a self-propelled slave vehicle relative to a master vehicle comprising the steps of : attaching a telescopic link pivotally at one end to said slave vehicle and pivotally at the other end to said master vehicle ; detecting the length of said telescopic link ; detecting a first angle between the telescopic link and the master vehicle and a second angle between the telescopic link and the slave vehicle ; determining the relative positions of said vehicles using said lengths and said first and second angles ; and steering and controlling the power to the wheels of the slave vehicle such that the slave vehicle remains within a predetermined relative position to the master vehicle.

According to a seventh aspect of the present invention there is provided a method of guiding a self-propelled slave vehicle relative to a master vehicle comprising the steps of : attaching a first telescopic link pivotally at one end to a first point on said slave vehicle and pivotally at the other end to a second point on said master vehicle attaching a second telescopic link pivotally at one end to a third point on said slave vehicle and pivotally at the other end to a fourth point on said master vehicle ; said first and third points being spaced apart and said second and fourth points being spaced apart.

detecting the values of at least three of : the length of the first telescopic link ; the length of the second telescopic link ; the angle between the first telescopic link and one of the vehicles; and the angle between the second telescopic link and one of the vehicles ; using said at least three values to determine the relative positions of said vehicles ; and steering and controlling the power to the wheels of a slave vehicle such that the slave vehicle remains within a predetermined relative position to the master vehicle.

The advantage of the sixth and seventh aspects of the present invention is that the slave vehicle does not need an operator in order to accurately follow the master vehicle.

It is also an object of the present invention to provide a method and apparatus for guiding the vehicle in a field for carrying out agricultural operations.

Preferably the guiding system will be capable of allowing the vehicle to be worked without an operator.

According to an eighth aspect of the present invention there is provided a vehicle according to any one of the first to seventh aspects, further comprising two global positioning system receiving antennae at spaced locations on the vehicle and a signal processing unit responsive to the respective signals to determine the position of the vehicle.

According to a ninth aspect of the present invention there is provided a method of controlling the operation of a gantry agricultural vehicle in field working mode, comprising sensing the position of each end of an implement carrier of the gantry vehicle using a real time kinematic-differential global positioning system antenna mounted near each end of said implement carrier at a respective end of the gantry vehicle, comparing the positions of the two ends of the implement carrier in order to define the position of the mid-point of the implement carrier, and comparing the positions of the two end points of the implement carrier for determining the orientation of the implement carrier ; said implement carrier being movable

longitudinally relative to the gantry and adapted to carry individual implements for working on crop rows over which the gantry is passing, said method further comprising the steps of :- sensing the position of the implement carrier with respect to crop rows to maintain centring of implements carried by the carrier over the respective crop rows, sensing any deviation of the implement head (s) from the respective crop row, comecting the error of the implement head (s) relative to the crop row (s) by longitudinal displacement of the implement carrier relative to the gantry, and then centring the implement carrier relative to the gantry by steering the vehicle back to a position relative to the implement carrier at which the implement carrier is at or near the centre of its range of movement relative to the gantry while simultaneously moving the implement carrier in the opposite direction relative to the gantry to maintain the position of the implement head over the line of crops being processed.

Further objects and advantages will become apparent from the following description, given by way of example only, with reference to the accompanying drawings in which : FIGURE 1 is a side, front and end view of the vehicle of the present invention ; FIGURE 2 is a schematic of the various steering modes in road mode ; FIGURE 3 is a schematic of the various positions of the wheels in field mode ; FIGURE 4 is a schematic elevational view of the piston steering mechanism of the present invention.

FIGURE 5 is a schematic diagram of the arrangement of a preferred embodiment of the method of actuating the steering ; FIGURE 6 is a schematic elevation view of a preferred embodiment of the steering mechanism of the present invention ; FIGURE 7 is a schematic graph of the variation in speed control pedal displacement, brake pedal displacement and the swash plate angle with time; FIGURE 8 is a schematic diagram of the guidance system of the present invention ; and

FIGURE 9 is a plan view of the vehicle in operation in a field.

The vehicle comprises an elongate middle section 10. Respective pairs of wheels 12 are connected to each end of the elongate section 10.

The wheels are mounted such that the vehicle can move in the longitudinal direction with respect to the elongate section 10 and also in a direction transverse to the elongate section. When the vehicle travels in the longitudinal direction, this is termed'road mode'and when the vehicle travels in the transverse direction this is termed'field mode'. The elongate section 10 comprises an implement carrier.

Vehicles such as this are used in farming and are advantageous over tractors and more traditional farm vehicles in that fewer passes are required in order to plough, sow or harvest an entire field. In this regard, implements such as ploughs, fertilizer spreaders etc., can be attached to the elongate middle section.

The vehicle is designed to be driven along roads in road mode to the fields where the vehicle is driven in field mode. As can be seen from Figure 1, the cab 14 for the driver is situated at one end of the elongate section and can rotate such that the driver always faces substantially in the direction of travel.

Figure 1 also shows that there is a receiving antenna 23 on the cab 14 and a further receiving antenna 24 on the motor housing at the far end of the vehicle.

These two antennae 23 and 24 are receivers for global positioning system (GPS) signals in order to allow the position of the vehicle to be determined, for controlling the operation of the vehicle or for guiding an operator in doing so, so as to use the same track for each pass along a given line of crops or set of lines of crops.

A preferred form of GPS system for use with the vehicle is a real time kinematic-differential GPS (RTK-DGPS) system which can give an accuracy of + or-2cms with an update rate of 5 per second during operation. Preferably the signals from the two antenna 23 and 24 are passed to a twin channel DGPS receiver which ensures that for each update processing of the signal from one antenna there will be a simultaneous update processing of the signal from the other antenna, thereby ensuring that there will be no difference in time between the receipt of a GPS signal on one antenna (23) as compared with that on the other antenna (24).

The steering system of the vehicle will now be described in greater detail. As can be seen from Figure 2, each of the four wheels of the vehicle are steerable. In Figure 2 the diagram labelled (A) shows the vehicle wheels in the straight ahead condition longitudinally of the elongate section 10 (road mode), (B) shows the vehicle in a left turn in two wheel steer mode, (C) shows the vehicle in four wheel steer mode and (D) shows the vehicle in crab steer mode whereby the vehicle moves diagonally with respect to the elongate section 10.

Figure 3 shows the positions of the wheels in field mode. In Figure 3 the diagram labelled (A) shows the vehicle moving transversely to the elongate section 10 (field mode) in a straight line, (B) shows the vehicle rotating about a single wheel, (C) shows the vehicle rotating about a point just outside the area covered by the vehicle, (D) shows the vehicle set up to rotate about the centre of the vehicle and (E) shows the vehicle set up in field mode for crab steer.

It should be noted from Figures 2 and 3 that the wheels of each pair of wheels are joined by telescopic link 15.

The telescopic link 15 is fitted with an hydraulically actuated mechanism such that the telescopic link 15 can be locked in the fully extended position.

Conveniently the telescopic link 15 and the hydraulically actuated mechanism may be combined as a hydraulic tie bar 15. The remaining description is given by describing the hydraulic tie bar 15 but is equally applicable to the combination of the telescopic link and hydraulically actuated mechanism to lock it.

The hydraulic tie bar 15 is rotatably attached to part of a steering arm 17 which rotates as the steering angle a of the wheels change. The mountings of the tie bar 15 to the steering arm 17 are arranged so that when the vehicle is in the straight ahead road mode condition (Figure 2a) the tie bar is situated on the vehicle side of the wheel pivots 25 to ensure that all field mode and road mode steering conditions can be met.

As can be seen from Figure 2, in road mode the hydraulic cylinder 20 of the hydraulic tie bar 15 is always fully extended. This can be achieved by virtue of the steering oil pressure (at the highest applied pressure to any steering actuator (not shown in Figures 2 and 3)) and this pressure is held by non-return valves to prevent

pressure loss in the event of failure of the hydraulic pressurization. It will be immediately obvious that there are other methods of ensuring that the hydraulic cylinders 20 remain fully extended to keep the tie bar extended. For example a bolt could be inserted into the cylinder before driving in road mode, or the cylinders could be pre-pressurised before driving in road mode.

In road mode if the wheels are steerable by the vehicle driver mechanically or hydraulically, the vehicle is always steerable even in the event of hydraulic pressure loss. Furthermore with such a system the control of the wheels is not under so-called "fly by wire"wherein the wheels are driven under the command of a computer. In the preferred embodiment of the present invention the wheels are steered hydraulically. More preferably, in road mode, the wheels are steered directly by inputs from the driver.

It should be noted that both pairs of wheels are set up in a similar way.

As can be seen from Figure 3, in field mode, the hydraulic piston 20 is allowed to float to allow each wheel to be fully independently steered.

When changing the setup of the vehicle from road mode to field mode, the wheels at the end of the vehicle are turned in towards one another (rather than both put on to full lock in the same direction). The result of this is that the total travel from full left lock in the road condition to full right lock in the field condition is substantially 180°. In Figure 2b the angle a, the steering deflection angle, is the acute angle measured between (i) the line Ll joining that wheel and the other wheel on the same side of the vehicle, and (ii) the line L2 pointing in the same direction as that wheel, when the half of that wheel furthest from the other pair of wheels is closest to the other wheel of its pair of wheels. The wheels are mounted such that the steering deflection angle a decreases from a maximum value before increasing when the front right hand side wheel is turned from full left lock in the road mode to full right lock in the field mode. The increase is at least 90° but less than 180° and preferably less than 135°. This arrangement means that all of the required steering possibilities are available to the driver of the vehicle and also allows the hydraulic tie bar to be fully extended in all required road mode manoeuvring conditions.

When transferring between road mode and field mode the vehicle must be stopped because the wheels will need to be rotated away from each other which is impossible when the vehicle is moving.

It should be noted that in Figures 2 and 3, the actuation means by which the wheels are steered are not shown. The hydraulic tie bar is not used to steer the wheel, rather its only function is to keep the steering mechanically secure in the road mode.

The geometry of the wheels is such that the Ackerman steering effect is evident when the vehicle is both in the road mode (with rigid interconnection of the steerable wheels) and the field mode (when the wheels are independently steerable).

In practice this means that at full lock the steering deflection angle (X, of the inside wheel is-54° whereas that (al) of the outside wheel is +45°. This is however vehicle dependant.

In a prefen ed embodiment, in road mode, partial four wheel steer is always implemented. This system works by providing front and rear hydraulic steering actuators with hydraulic cylinders of different bore diameters connected in series. If the bore diameter of the rear steering cylinder is greater than that of the front steering cylinder then the steering system can be arranged such that during a turn in one direction, the front wheels turn in that direction and the rear wheels turn by a correspondingly smaller angle in the opposite direction.

In four wheel steer mode misalignment of the front and rear wheels often occurs. To overcome this problem two cam valves are fitted to each axle. The cam valves signal whether the steering deflection is to the left or right of straight ahead or is straight ahead (0 steering deflection angle). The front and rear axle steering actuators are in hydraulic series and pilot valves are disposed in the circuit. The pilot valves are controlled by the cam valves and only allow rear steering movement to the left if the rear steering deflection angle is to the right of straight ahead or if the front steering angle is to the right of straight ahead, and only allow rear steering movement to the right if the rear steering deflection angle is to the left of straight ahead or if the front steering angle is to the left of straight ahead. In this way, during a turn to the right, if the steering deflection angle of the front axle decreases, but not enough for

the steering deflection angle to be 0, the deflection of rear axle can also change accordingly by moving to the left because the rear wheel is to the right of straight ahead. However, if the steering deflection angle of the front axle decreases further, and the steering deflection angle of the rear axle reaches 0 before the front axle (the wheels of the rear axle lead the wheels of the front axle), the rear axle cannot turn further left until the steering deflection angle of the front axle is left of straight ahead.

This re-synchronization occurs on every transition of the wheels of the front axle through the straight ahead position only if the rear steering angle leads the front steering angle. The hydraulics of the system are arranged such that this is the case.

This resynchronization technique introduces a deadband in which, for small angles of steering deflection angle at the start of a turn off straight ahead, the wheels of the front axle steer whilst the wheels of the rear axle remain in the straight ahead position. This is a result of lag introduced by the cam valves at small angles. In fact this is advantageous because it reduces initial sideways movement of the rear of the vehicle towards the outside of a turn. The size of the deadband may be varied by changing the profile of the cams but is preferably set to be +/-3° to +/-5° of front axle steering deflection.

The above described method for axle synchronization can be implemented fully automatically and continuously such that realignment occurs on every turn through the straight ahead direction. Alternatively, the misalignment can be continuously measured and if it reaches a predetermined amount this can be indicated to the operator. Realignment then takes place using the above described method, under the command of the vehicle operator. A further alternative would be for the realignment to take place automatically only when the sensed misalignment reaches a predetermined amount.

Figure 4 shows the way in which the wheels can be steered. The wheel 12 is connected to a piston 71 by linkage 85. The axis of the wheel is arranged to be perpendicular to the longitudinal direction of the piston 71 and the axle of the wheel lies directly under the centre of the piston 71. The piston 71 sits in an hydraulic cylinder 72 which is attached to the vehicle. The cylinder is mounted substantially vertically plane. The piston 71 has a thread 80 formed on its outer surface. The

thread of the piston 80 mates with a corresponding thread 75 on the inside of hydraulic cylinder 72. The wheel becomes steerable by applying hydraulic pressure at the hydraulic inlet 70. When the piston 71 extends from or retracts into the hydraulic cylinder 72 under the applied hydraulic pressure the threads 75 and 80 interact to rotate the piston and thereby steer the wheel 12.

There are other ways of steering the wheels such as attaching hydraulic cylinders to the steering arm 17 of the wheels 12. However the system shown in Figure 6 allows the overall height of the vehicle to be minimized. This is because of the difficulty of providing a hydraulic tie bar 15 as well as steering actuators on the arm 17 with the wheel is still capable of bring rotated through about 180 °.

Figure 5 shows a further possible embodiment of the hydraulic steering actuators and tie bar. In the Figure, a pair of wheels 12 is depicted. The wheels are pivoted directly above their centre at points 200. Plates 205 pivot around points 200 and are rigidly connected to the wheels to transmit any rotation to the wheels 12.

Hydraulic tie bar 15 is connected to the underside of plates 205. The connection points 207 of the hydraulic tie bar 15 to plates 205 are, when the wheels are in a straight ahead position, on the inside of the wheels. This is exaggerated in Figure 6.

In this way, Ackerman steering can be implemented by ensuring that the geometry is such that the two lines 245 drawn through points 200 and points 207 for each wheel, cross in the centre of the rear axle.

Actuators 210 connected to the underside of plate 205 are used in the road mode for steering rotation of plates 205. They are positioned such that in road mode they have good mechanical advantage for turning the wheels. In field mode, actuators 220 are used to steer the wheels. They are connected to the upper sides of the respective plates 205 such that actuators 210 used in the road mode do not interfere with actuators 220 used in the field mode when the wheel is rotated. This is further illustrated in Figure 6. For maximum mechanical advantage in each mode, the angle between the pivot point of plate 205 and the two attachment points of actuators 201 and 220 should be about 90°.

Preferably the vehicle is a hydraulically driven vehicle. Most agricultural machinery is of this type and the engine driving the variable displacement hydraulic

pump, (usually a swash plate pump) is generally utilised at constant speed. The hydraulic motors which drive the wheels of the vehicle are supplied with hydraulic fluid from the hydraulic pump driven by the engine. In the case of a swash plate pump the flow rate from the hydraulic pump is varied by changing the deflection angle of the swash plate. At zero deflection angle the displacement of the hydraulic pump is zero and no hydraulic fluid flows. Normally the required flow rate is selected by a hand control. It is advantageous that in road mode the vehicle of the present invention may be driven in a manner similar to that of a car with a speed control pedal and a brake pedal. In order to achieve this, the displacement of or the hydraulic pressure generated by the pedals is measured and this information is passed to a control system. The control system is designed to vary the angle of the swash plate such that the vehicle drives like a car with an automatic transmission and powered by a combustion engine.

The function of the control system is shown schematically in Figure 7. Upon a change in the amount of displacement (depression) of the speed control pedal the angle of the swash plates is progressively changed such that the flow of hydraulic fluid increases gradually with time. The increase continues until attainment of a predetermined ratio between the amount of displacement of the speed control pedal and the hydraulic flow. This behaviour is to simulate a car accelerating when the accelerator pedal is depressed. If the displacement of the speed control pedal is decreased, the control system will gradually decrease the swash plate angle such that the hydraulic flow slowly decreases until the predetermined ratio between the displacement of the speed control pedal and the flow rate is achieved. If the speed control pedal is released to the normal position (i. e. the foot is removed from the pedals) the swash plate angle is caused to decrease slowly such that the vehicle slows down progressively. If pressure is applied to the brake pedal the rate of decrease of swash plate angle increases to simulate braking. If emergency pressure is applied to the brake pedal the swash plate angle is returned to zero as quickly as possible.

The angle of the swash plate is controlled by two valves is in series. The first valve is a directional valve and the second valve is a proportional valve. The fluid which flows through the proportional valve then subsequently enters the directional

valve. The valves are arranged such that if either of the valves fails, the position of the swash plate may still be returned to zero deflection by the operation of the still functioning valve. A further safety feature can be included by ensuring that the valves are arranged such that, if power to the valves fails, the angle of the swash plate will be returned to zero. It is also possible to connect drum brakes on the driving wheels of the vehicle with the brake pedal such that if a hydraulic hose ruptures, the vehicle can still be brought to a halt under control.

In some applications, it is necessary to drive vehicles of this type in close proximity to each other. For example, during harvesting, one machine can be used to collect the crop and another machine can be used to transport the collected crop. It is advantageous if the second vehicle collecting the crop is self-propelled and steerable such that when the vehicle is full it can be driven away independently of the first vehicle. Figure 8 (a) shows one possibility for determining the relative position of one vehicle to the other vehicle. This involves pivotally connecting a telescopic link 100 between the two vehicles. The link is allowed to extend and retract and to pivot at both the connection to the leading (master) vehicle and to the trailing (slave) vehicle. Three parameters are required in order to determine the relative position of the slave vehicle to the master vehicle namely the length L of the telescopic link, the angle a of the telescopic link 100 to the master vehicle and the angle ß of the telescopic link 100 to the slave vehicle. With this information, a control system can steer the wheels of the slave vehicle and control the power delivered to those wheels such that the slave vehicle follows the master vehicle within a predetermined range of relative positions.

A second alternative is shown in Figure 8 (b) wherein two telescopic links 101 and 102 are used. Six possible variables can be measured. These are the lengths L, and L2 of the telescopic links, the angle al, a2 of the telescopic links to the master vehicle and the angles pi and ß2 of the telescopic links to the slave vehicle. With any three of these variables it is possible to work out the relative position of the slave vehicle to the master vehicle and for the control system to take corresponding action.

Figure 9 shows a plan view of the vehicle in the field with the cab 14 at the lefthand end of the middle section 10 of the vehicle and the motor housing at the

opposite end, with the antenna 23 and 24 shown in plan view. Within a range of less than 2 miles is a ground station represented by an antenna 25.

Global positioning (GPS) depends on a system of geostationary satellites emitting signals which can be processed by a receiving station in order to determine the horizontal position of the receiving station relative to the ground, and preferably also the height above a chosen reference datum, for example mean sea level.

However, it is well known that (i) GPS includes an error component which was originally deliberately included in order to allow correction only by military users so that their accuracy was better than that of a civilian user, and additionally (ii) there will in any case be errors resulting from atmospheric effects, but these will not vary over the short distance between the two antennae on the vehicle or within the short distance between the vehicle and the ground station.

By incorporating the ground station antenna 25 within the same geographic location as the area of operation of the vehicle it is possible for the ground station, at a fixed terrestrial position, to determine the difference between the actual ground position and the ground position determined from the received GPS signals so as to calculate the local error. The ground station then transmits a signal to the local user, in this case the GPS unit on the vehicle, to indicate to the vehicle the local geographic error of the GPS network. This allows the ground position determined by the GPS unit on the vehicle to give a much higher accuracy than is the case with a free standing GPS receiver, particularly when the deliberately superimposed error is removed. The RTK-DGPS system preferred for the present application allowed a very high degree of accuracy as compared with a conventional civilian GPS system, for example plus or minus 2 cm, and by increasing the frequency of the update rate to 5 updates per second it is possible to determine with considerable accuracy the actual position and the rate of change of position of the GPS receiver.

Furthermore, by incorporating a single twin channel receiver on the vehicle it is possible to ensure strict synchronisation of the sampling of the GPS signals to allow updating of the last measured position.

In the present vehicle, which may have a span of 10 metres for the middle section 10 between the two end units, determining the position from two separate

GPS antennae at opposite ends of the implement carrier of the vehicle allows determination of not only the position of the implement carrier but also determination of the orientation of the implement carrier by comparing the positions of the two ends and defining the line between them as the angle of orientation of the implement carrier. This orientation determination is only effective if it is possible to know, at any given instant, the simultaneous positions of the two ends of the implement carrier. Where random sampling for updating purposes occurs there will be an error resulting from possible lack of synchronisation of the sampling of the position of one end with respect to the sampling position of the other end. Even with an updating rate of 5 samples per second there will still be an error which can be eliminated if only the instant of sampling can be synchronised between the two ends of the implement carrier. This would allow an accuracy of heading determination to plus or minus 0. 1 °, whereas conventional marine heading sensors have an accuracy of plus or minus 0. 5 °.

By having the two RTK-DGPS antennae no more than 10 metres apart it is effectively the case that the errors of the two signals, resulting from either geographic location or the nearby presence of buildings and/or trees, is identical for the two ends. This, together with the ability to correct for the geographic error by reference to the position error of the fixed position ground station 25, allows very high degree of accuracy to be achieved.

By positioning the receiving antennae on the implement carrier, it is possible to improve the response of the implement-positioning function of the vehicle where the implement carrier is mounted for movement longitudinally of the middle section 10.

Where the vehicle which is shown in Figure 1 is in field mode (the upper views of Figure 1) and is found to have the implement positioned laterally displaced with respect to the line or lines of crops being handled, it is possible to correct the situation by steering the vehicle back to bring the implement over the line of the crops. However, the response is far more rapid if the implement carrier is initially moved longitudinally of the middle section 10 to bring the implement (s) back to the correct position with respect to the line of crops, and then the vehicle is crab steered

(as illustrated in the lower view of Figure 3) to bring the implement carrier back towards the centre of its range of movement relative to the vehicle, while maintaining the or each implement centred over the respective row of crops by simultaneously bringing the implement carrier back in the direction opposite to that in which the middle section 10 moves during crab steering. Thus the implement (s) will always be maintained over the crops, and be rapidly returned to that position in the event of an inadvertent creeping off line, and the less rapid response of the vehicle in crab steering field mode is able to restore the implement carrier to the centre of its movement range in order to be ready for the next need for a corrective movement.

Ideally the antennae are always mounted at a standard height above ground.

This mounting at a standardised height enables the RTK-DGPS unit to be able to calculate inclination of the implement carrier with respect to the horizontal by measuring the sholtening of the horizontal distance between the two antennae and relating this to shortening of the horizontal projection of the implement carrier when the vehicle is inclined by having one end unit below the other. The direction of inclination can be identified by determining the differential heights of the two antennae in such a foreshortened (inclined) configuration. Using a second method, namely assessing the differential heights of the two antennae 10 metres apart, the accuracy of inclination measurement can be determined to within plus or minus 0.10 metre length of the middle section 10.

Where the vehicle is operating over inclined ground and the middle section 10 has an inclination of 24%, for two antennae 23 and 24 placed 2 metres above ground level there will be uncorrected lateral position errors of typically 50 cm.

However, when the inclination is known, and can be used to apply a correction to the position value, this error of plus or minus 50 cm can be reduced to plus or minus 0. 8 cm, given the accuracy of determination of the inclination value given above.

Where the control system relies on the measured position to provide the required feedback to allow the vehicle to return gradually to its desired location relative to the implement, there will nevertheless be some degree of en-or in the correcting loop, giving rise to possible overshooting. However, when the ground

speed is known it is possible to incorporate this as an"inner loop"control function which will considerably improve the response of accuracy of the position control.

The ultimate aim for the vehicle according to the present invention is that it should be capable of driverless operation. In that event it will be necessary to build in independent control mechanisms to override the vehicle in the event of a parameter exceeding the permitted error tolerance. This independent signal could be (i) the ground speed, as determined by any system independent of the GPS unit (e. g. wheel speed sensors, ground sensing radar or a vision-based system).

It is furthermore highly advantageous to know the ground speed of the two ends of the vehicle independently of one another since the rate of operation of the implement or the rate of application of fertilizer and/or herbicide by way of dispensers mounted on the implement carrier will need to be controlled in response to the ground speed ; where the two ends of a 10 metre long vehicle are moving at different speeds there will be a need for differential control of the rate of operation and/or dispensing at each implement head along the implement carrier in order to allow for the difference in ground speed at each station along the middle section 10, for example when traversing a curved path. This determination of the different ground speeds at the ends of the vehicle can be achieved by the RTK-DGPS system, or measured by any other suitable means.

Although in the above description it has been indicated that the positioning will be determined solely by use of RTK-DGPS facilities, it is of course possible to incorporate an independent positioning system, for example a panoramic lens camera having a horizontal image plane and viewing the 360° horizon pattern in order to give an indication of rough position. Another possibility would be to have ground sensing radar on the vehicle.

When the system is sufficiently refined to allow for a reliable driverless operation, it is equally envisaged that the main harvesting vehicle can be serviced by auxiliary feeder vehicles, all entirely by driverless operation, such that while the harvesting vehicle makes a succession of non-stop traverses along the crop line and executes a field turn at each end of the crop line before traversing the next adjacent crop line, the servicing vehicles may approach that vehicle, either (i) by catching it

up from behind or (ii) by positioning themselves in front, and progressively matching their position and speed until they are in cooperation with it for transferring the harvested crop from the harvesting vehicle to the feeder vehicle, or for replenishing the fertilizer/herbicide reservoir or replenishing any other supplies on the primary working vehicle from the servicing vehicle.

In operation there will be two separate grids of presentation of position :- (a) Firstly there will be a geographic grid related to the DGPS fixed grid with axes X and Y parallel with geographic latitude and longitude.

(b) Secondly there will also be a field grid which is strictly related to the direction of the line of crops, and the spatial location of the GPS receiving antenna will thus be the point of intersection of the implement carrier axis with the particular crop line being followed. The field grid has axes R and S (and possibly Z) with the orientation of the R axis constantly updated to remain parallel to the crop row.