AU2008202820A1 - Improved gantry tractor - Google Patents

Description

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IMPROVED GANTRY TRACTOR Z TECHNICAL

FIELD

The invention relates to a means of increasing the tractability and manoeuvrability of gantry tractors while at the same time minimising soil compaction and damage to the ground traversed.

00 In order to increase productivity ground engaging implements are becoming wider and the draw bar force required to move them through the ground is becoming larger. This draw bar force can Obe provided by a heavy and powerful traditional tractor. As these heavy tractors cause 00 compaction of the soil traversed, steps are generally taken to reduce this compaction problem.

0 One way of reducing the compaction pressure is to replace wheeled tractors with tracked tractors.

Another is to always drive the tractor wheels or tracks along the same path so that the soil compaction is limited to this path.

Gantry tractors have significant advantages over traditional tractors if they utilize many driven wheels. In this case the ground engaging tools are located between the plurality of both front and rear driven wheels of the gantry tractor. In general all wheels will be driven. In this case the force required to move the tools through the ground is distributed between many driven wheels, so the downward force required on each driven wheel will be reduced. This vertical force is mainly provided by the weight of the gantry tractor. This vertical force can be increased if necessary by addition of ballast. Because the downward force required on individual wheels will be reduced, less soil compaction will occur.

A disadvantage of gantry tractors is that they are unwieldy and difficult to transport between paddocks. Manoeuvrability can be improved by turning all the wheels through 90 degrees so they can be moved parallel to their long axis. Ideally the gantry tractor should be transported along narrow aisles of un-worked ground adjacent to fences. However it will be impossible to drive the gantry tractor though narrow gateways in the fence without driving on to the worked area.

A gantry tractor is described below that is capable of "snaking" through gateways. The essential feature of this gantry tractor is that it consists of two or more four wheel modules which are rotatably joined at hitch points. When the gantry tractor is not being transported the modules are also latched together by struts to form a rigid truss.

The invention describes means of ensuring that all modules follow the path taken by the lead module. In general both the steering effect of the angles of the wheels (both driven and un-driven) 00 and the steering effect of positively driving the individual wheel speeds will be used to positively steer each module. The two systems will be cooperatively redundant if each acting alone would nproduce the same centre of curvature. In some circumstances one steering system will be much IND more effective than the other. In such circumstances only one steering system may be activated.

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BACKGROUND AND PRIOR ART 00

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SThere are two basic methods of manoeuvring a wheeled vehicle. One method is to turn one or 00 more steerable wheels. The other method is to drive one or more left hand wheels independently of one or more right hand wheels. In general these two steering systems will conflict with one cN' another when each tries to achieve a different centre of curvature for the path of the vehicle. This conflict causes a braking effect, which results in fuel wastage, scuffing of the ground traversed and associated tyre wear.

The traditional method of avoiding conflict between the two basic steering systems is to disable one system so that it cannot conflict with the remaining system. For example in a traditional road vehicle, the steering effect of driving the drive wheels at the same speed is eliminated by incorporating a differential into the drive train to the driving wheels. Conversely in a zero turn radius vehicle which is steered by driving the left hand drive wheel independently of the right hand drive wheel, the steering effect of one or more non driven wheels is eliminated by rendering the latter free to turn to any angle. In other words, they are turned into castors. When only one steering system is operative we have a non-redundant steering system.

The Problems to be solved Unfortunately, making one steering system compliant with the other leads to stability and traction problems when the vehicle is operated in difficult conditions. If the sideways, forwards or backwards force on the vehicle increases and/or the coefficient of friction between the tyres and the ground decreases, the system used to manoeuvre the vehicle will eventually fail. For example, the differential becomes the Achilles' Heel of the traditional tractor when working on steep terrain, and especially in slippery conditions. In this environment weight is transferred from the uphill drive wheel making it liable to spinning. Although the stability of the traditional tractor can be improved by the use of a limited slip differential or a lockable differential, it is somewhat illogical to provide a differential in the first instance along with a subsidiary system which either impedes its operation, or stops it altogether.

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,K Similarly it can be seen that the Achilles' heel of the zero turn radius vehicle when traversing a steep slope are the non-driven castors. Because these castors cannot exert any sideways force on their end of the vehicle, the tendency for this end to swing down the hill can only be prevented ID by the two drive wheels applying opposing forces to the vehicle even though they may be driven at the same speed. As the steepness of the slope traversed increases, the uphill drive wheel eventually loses traction and the front of the vehicle swings down the hill. In short, the grip of the drive wheels on the ground is exhausted by the drive wheels fighting against each other in 00 providing the torque necessary to stop the castored end of the vehicle swinging down the hill.

00 A method of overcoming the problems of traction and stability is to allow both steering systems to operate, but to allow one steering system to dominate the other. In this case the stability and traction problems are reduced at the expense of the introduction of a scuffing problem on turning.

For example the elimination of the differential from the rear axle of four wheeled motor bikes improves traction at the expense of introducing a scuffing problem.

A more extreme example of conflict between the two basic methods of manoeuvring a vehicle occurs in skid steer vehicles (both wheeled and tracked). In this case the dominant steering system is the independent drive to the right hand and left hand drive wheels or tracks. The second enabled but dominated steering system is the wheel or track angle which is usually fixed at zero degrees and tends to drive the vehicle straight ahead. The conflict between the two steering systems causes the vehicle to take a path which is a compromise between the paths that would be produced by each system alone. This method of manoeuvring causes extreme scuffing with associated ground damage, fuel wastage and tyre or track wear. If both the wheel angle steering system and the driven-wheel steering system are operative, we will generally have a conflicting redundant steering system.

In traditional vehicles, rotation and translation are generally linked. Translation of the vehicle along a curved path usually involves rotation, and rotation of the vehicle always involves translation. As a consequence, rotation and translation in a confined space can be a problem.

Vehicles steered by independently driving the left and right hand wheels have improved manoeuvrability since they can be made to rotate about their own centre. This is pure rotation without translation). Manoeuvrability can be further increased by allowing translation in any direction without the need for rotation. This pure translation is sometimes referred to as crab steering.

The Solution proposed previously 00

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(Ki The essential feature of the invention previously proposed by Spark (Australian Provisional SApplication PR 0473 (03-10-2000) and Patent Cooperation Treaty Application PCT/AU01/01247 n (03-10-2001)) is that both basic systems of manoeuvring a vehicle are to be used in unison so that they both try to produce the same centre of curvature for the path of the vehicle. In this case we have a cooperative redundant steering system. With both systems reinforcing each other it will be possible to effectively manoeuvre the vehicle in much more difficult conditions than if only one

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N system was used with the other system either disabled or dominated. Furthermore any centre of 00 curvature can be selected by the driver, which further improves the manoeuvrability of the

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Sprevious invention. This enables the invented vehicle to execute either pure rotation or pure 00 translation or any combination of translation and rotation.

N(N The preferred means of driver control of the four wheel steering/four wheel drive variant of the previously proposed invention is by means of a rotatable joystick. This maximises the manoeuvrability of the vehicle by allowing independent translation and rotation of the vehicle. In this means of driver control, the direction of translation of the vehicle is determined by the direction of displacement of the joystick, whereas the rotation of the vehicle is determined by the degree of rotation of the joystick. The amount of displacement of the joystick determines the root mean square of the four wheel speeds. Pure translation occurs when the joystick is displaced but not rotated. Pure rotation occurs when the joystick is twisted as far as it will go.

Alternatively, two separate devices could be used for driver control. One joystick could be used to determine the radius of curvature of the path of the vehicle and the root mean square wheel speed, and the second joystick could be used to determine the direction of the centre of curvature.

Alternatively, a joystick, steering wheel, knob or lever could be used to determine the radius of curvature of the path of the vehicle, and a separate joystick could be used to determine the direction of the centre of curvature of the path of the vehicle and the root mean square wheel speed.

DRAWINGS

In order that prior art and the present invention may be more clearly understood, some preferred embodiments thereof will now be described with reference to the accompanying drawings.

Although a four wheel steering/four wheel drive vehicle will be described, it will be appreciated that the principles invoked can be applied to any vehicle with more than one wheel.

00 Figure l(a) is a plan view of the general case of a four wheel steering/four wheel drive module of n the gantry tractor.

\O Fig l(b) shows alternative means of driver control.

Figure 2 shows a gantry tractor made by latching four modules together.

Figure 3 shows the unlatched gantry tractor manoeuvring around obstacles.

O Figure 4 shows the relationship between the desired path of the module and its centre of 00 curvature.

Figure 5 shows an alternative path through a gate.

00 Figure 6 shows one means of latching the modules together.

0 Figure 7 shows a second means of latching the modules together.

Some relevant embodiments of previous inventions In the four wheel steering/four wheel drive variant of the invention depicted in Figure 1, an internal combustion engine 1 drives two right hand variable displacement hydraulic pumps 2 and 3 which in turn drive hydraulic motors 4 and 5 mounted in the steerable front and rear right hand wheels respectively. The internal combustion engine 1 also drives left hand variable displacement pumps 8 and 9 which in turn drive hydraulic motors 10 and 11 which are mounted in the steerable front and rear left hand wheels 12 and 13 respectively The effective angles of the wheels 6, 12, 7 and 13 are shown as 1, 2, 0 3 and 0 4 respectively.

The effective rotational speed of the wheels 6, 12, 7 and 13 are col, 02, w 3 and C04 respectively.

The driver controls the vehicle by selecting the radius of curvature of the vehicle's path and the sense of rotation by rotating the joystick 14. If the joystick 14 is not turned the radius of curvature of the path of the vehicle will be infinity and the vehicle will move in a straight line parallel to the direction of displacement of the joystick 14. If the joystick 14 is twisted as far as it will go in a clockwise direction, the radius of curvature of the path of the vehicle will be zero and the vehicle will rotate clockwise about its own centre. Between these two extremes the radius of curvature of the path of the vehicle is given by: Rt ct(90 R cot(90 0 0/Om8, R y)/ t 00

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Where t is the track of the vehicle, 0 is the rotation of the joystick and 08, is the maximum g rotation of the joystick 14.

IC If the driver displaces the rotatable joystick 14 at an angle y to the straight ahead position, the direction of the of curvature of the path of the vehicle will by at right angles to the direction of 0 joystick displacement and R x and RY will be given by the following equations: 00

R

x R/(tan 2 1) 1 2 R cos tp 00 0 and Ry R tan y /(tan 2 1 2 R sin p The driver selects the direction of the centre of curvature by displacing the joystick 24 at right angles to this direction. The centre of curvature of the path of the vehicle is now specified by the two components R x and R He selects the root mean square of the four wheel speeds by the amount of displacement of the joystick 14.

The control system then rotates the four drive wheels to the following angles: tan 0 x -t/2) tan 2 (b/2-Ry)/(R x +t/2) tan 3 (b/2+Ry)/(R x -t/2) tan 4 (b/2+Ry)/(R x +t/2) Where b is the wheel base of the vehicle, Ry is the displacement of the centre of curvature forward of the centre of the vehicle and R x is the displacement of the centre of curvature to the right of the centre of the vehicle.

The amount of displacement of the joystick d determines the root mean square of the four wheel speeds (RMSWS) according to the equation: RMSWS Kd (w 2

W)

2 2 where K is an appropriate constant.

The individual wheel speeds are given by the equations: o, KdR 1

RMSR

0 2 KdR 2

/RMSR

0 3 KdR /RMSR 0 4 KdR 4

RMSR]

where R 2 (b/2-R) 2 +(Rx 2 where R2 (b/2-R) 2

+(R

x +t/2) 2 where R 2 (b/2+Ry) 2 -t/2) 2 where R 2 (b/2+R 2

+(R

x +t/2) 2 And RMSR is the root mean square radius, which is given by: RMSR +R +R2 +R) 1 2 /2 (R +R +t 2 /4+b 2 /4) 1 2 Note that when the rotation of the joystick 0 is a maximum the radius of curvature will be zero and the direction of the displacement d of the joystick 14 will be immaterial. It will be natural for the driver to push the joystick 14 forward in this case to commence forward rotation.

If the above equations for wheel angles and wheel speeds are satisfied then the two basic methods of steering the vehicle will reinforce each other. Such a vehicle would combine the traction and stability of skid steer vehicles with the non scuffing advantages of traditional road vehicles. However the vehicle described above has much greater manoeuvrability since it is capable of both pure rotation and pure translation (in any direction).

The general embodiment of the previous inventions The general embodiment of the invention is shown in Fig Alternative means of driver control are shown in Fig The preferred means of driver control is by means of a rotatable joystick 14.

Alternatively, one joystick 15 could be used to determine the radius of curvature of the path of the vehicle and the root mean square wheel speed, and a second joystick 16 could be used to determine the direction of the centre of curvature.

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Alternatively a steering wheel 17 (or steering knob or lever) could be used to determine the radius of curvature of the path of the vehicle and the root mean square wheel speed, and a second ;joystick 18 could be used to determine the direction of the centre of curvature.

IND

The system used to control the wheel angles may work as follows:

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00 The angle of a particular wheel will be measured. An on board computer will calculate (or Sapproximate from a look up table) the correct angle from the driver's inputs of 0 and V. If an 00 error exists between the actual angle and the desired angle an actuator will be energised so as to eliminate this error. The on board computer will adjust the angles of all the other steerable wheels before repeating the cycle.

A similar system will be used to control the wheel speeds. The wheel speed of a particular wheel will be measured. The on board computer will calculate (or approximate from a look up table) the correct wheel speed from the driver's inputs of V and d (the latter determining the root mean square wheel speed). If an error exists between the actual speed and the desired speed the drive to the wheel be adjusted so as to eliminate the error. The on board computer will adjust the speed of all other wheel speeds before repeating the cycle.

In large vehicles the actuators used to turn the wheels could be rotary hydraulic actuators.

Alternatively double acting cylinders connected to rack and pinions could be used. In this case the engine 1 would also drive an auxiliary hydraulic pump (not shown in Figure 1) which would drive the actuators via control valves activated by the on board computer.

In large vehicles the wheels could be driven by in built hydraulic motors which are powered by variable displacement hydraulic pumps. These pumps are driven by an internal combustion engine, which is governed to run at a constant speed. The speed of the wheels is controlled by varying the displacement of the pumps from a maximum flow in one direction to zero to maximum flow in the reverse direction. This allows the speed of the wheels to be varied from maximum forward to zero to maximum in reverse. The on board computer is used to alter the displacement of the pumps to produce the desired wheel speeds.

Application of the Concept of Cooperative Redundancy of two Steering Systems to Gantry Tractors consisting of Two or more Hitched Modules 00

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In order to increase their productivity agricultural implements are becoming wider and wider.

These implements require heavier and more powerful tractors to pull them. As these heavy ntractors tend to compact the soil under their wheels, there has been a move to controlled traffic IND farming, where the tractor wheels always move on the same path, thus reducing the area of the field compacted to a series of narrow strips.

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Gantry tractors have several advantages over traditional tractors. The essential feature of a 00 gantry tractor is that it is slightly wider than the implements it "pulls". These implements are Slocated between the front and rear wheels of the gantry tractor. Generally many pairs of wheels OO are used in order to distribute the weight between all wheels and the tractive force between the driving wheels. Usually all wheels are driving wheels.

The main disadvantages of gantry tractors are as follows: 1. They are clearly ungainly and difficult to manoeuvre. In order to keep them on the correct path some form of automatic guidance system is almost inevitable.

2. At the end of a pass (usually a straight line) the gantry tractor must be shifted sideways to face fresh ground. Since it is not generally feasible to rotate the gantry tractor 180 degrees, the implements themselves must be effectively rotated (or reversed).

3. The biggest problem is transporting the gantry tractor from one field to another along narrow lanes of compacted soil. These unproductive lanes are generally adjacent to fences. The ideal gantry tractor would be able to move parallel to fence lines and "snake" through gates in these fence lines.

The application of computer integration of the steering system and the driving system of an advanced gantry tractor will now be described. The essential feature of this gantry tractor is that is consists of a series of four wheel modules that are hitched together. In working configuration, the modules are also latched together to form a single rigid frame. Although four modules are shown in Figure 2, any number of modules can be employed.

All wheels can be driven independently at any desired speed between maximum forward and maximum reverse. All wheels can be independently turned between +95 degrees and degrees.

Translation errors are defined as the perpendicular distance between a reference point on the tractor and the desired path. Rotation errors are defined as difference between the actual heading 00

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of the tractor and the desired heading, where the latter is defined as the heading of the point on the desired path which is closest to the reference point.

IN Working mode: Figure 2 shows the proposed gantry tractor is working mode. In this mode the modules are latched together to form a rigid trusswThe position of the two uncoupled hitch points 19 at either 00 end will be monitored continuously by means of a geographic information system or some other Spositioning system. The orientation (or heading) of the gantry can be monitored by some form of 00 compass, or it can be deduced from the position of the two said hitch points. The operating procedure is as follows: 1. The gantry will start from the non-worked unploughed) compacted lane probably adjacent to a fence. The implements will already be disengaged from the ground.

2. The gantry will be positioned so that the translation error is zero. This can be done with the wheels at 90 degrees. The rotation error will be eliminated by turning the wheels 20 to 0 degrees, and driving the wheels at the opposite ends of the gantry in opposite directions until the rotation error is zero. The speed of the intervening wheels 21 will be a linear interpolation between the speed of the end wheels. It is also possible to eliminate the translation and rotation errors simultaneously using the techniques described in the parent application Australian Application 2004203063 (for conventional tractors).

3. The gantry will be driven forward onto the ground to be worked and the implements 22 engaged with the soil.

4. The gantry will be driven forward to work the soil. Translation and rotation errors will be continuously monitored. Translation errors can best be corrected by turning all wheels in unison and crab steering the gantry along a circular path that is tangential to the desired path.

Since all wheel angles are identical, all wheels will be driven at the same speed. Small rotation errors are less important, but these can be corrected by speeding up the lagging wheels. If the speed up of the leading wheels is zero, the speed up of the intervening wheels will be proportional to their distance from the leading wheels. Ideally the wheels should be turned so their centre of curvature is the same as that caused by the wheel speed differences. However if the rotation errors are small the scuffing caused by not turning the wheels will be negligible.

Side Shift Mode 00

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5. At the end of the pass the implements 22 will be disengaged from the ground and the gantry driven on to the non-worked but compacted lane. The implements 22 are then rotated (or ;reversed) 6. All wheels are turned 90 degrees and the gantry driven sideways until the translation error relative to the new desired path is zero.

7. Steps 2 to 7 are now repeated until the field is completely worked.

00 If steps 5 and 6 are combined, say by driving and turning all wheels in unison so they move along

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Sa circular arc which is tangential to the desired side shift path, the problem of turning stationary 0 wheels through 90 degrees is avoided.

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N Transport Mode Figure 3 shows a gantry tractor passing through a gate. This is the most difficult manoeuvre. U turns and right angle turns (also shown in Figure 3 are easier to achieve. The control strategy to be used is as follows: 1. The gantry tractor is moved along a lane adjacent to a fence. Translation errors can best be corrected by turning all the wheels in unison. Rotation errors can best be corrected by turning the leading and trailing wheels slightly in opposite directions. The wheels in between will be turned by amounts proportional to their distance along the gantry (by a process of linear interpolation).

2. Prior to the gantry approaching the gateway, the desired path of the reference points the GPS sensors) must be stored in the on board computer. In this case the reference points will be all the hitch points of the four-wheel modules. In this example the path of the hitch points through to gate will be circular arcs. In principle any path could be used, including straight lines. The disadvantage of the latter is they require sudden changes in the radius of curvature of the centre of the modules.

3. As each module approaches the gateway it is unlatched from the following module so that the modules can articulate around the hitch points. The leading module will have already been unlatched.

4. When the front hitch point 23 of the first module reaches the start of the desired circular arc the desired radius of curvature of the path of the centre of the module Ro begins to reduce from infinity. The centre of rotation of the module is given by the intersection of the normal to the path of the front hitch point and the normal to the path of the rear hitch point. See Figure 4.

Once the speed of the centre of the module has been selected, the angular velocity of the module about its instant centre of curvature can be calculated. The speed of the centre of the module can be expressed as the speed of a notional castor located at the centre of the module. The relation between (o and the root mean square wheel speed RMSWS is given by the equation: oo= RMSWS/(1 (t 2 /4R 0 2 b 2

/AR

0 2 11 2 RMSWS/(1 (t 2 +b 2 12 /2Ro) 6. The individual effective wheei angles of the module are given by the equations: tan 1 (b12 Ry)/(Rx t/ 2) tan 0 2 (b/2-Ry)/(Rx +t12) tan 0 3 (b/2+Ry)/(Rx -t12) and tan0b 4 (b/2±Ry)/(Rx ±t/2) The individual effective wheel speeds are given by the equations: (o (o 0

RI/R

0 (0 cooR 2

IR

0 C03 -cooR 3

IR

0 (04 o 0

R

4

IR

0 where R2=(b/2-Ry) 2 2 where R 2 (b/2-RY 2 +t/2) 2 whr 2 where R 2 (b/2+R Y) 2 2 where 0 oo KdR 0 RMSR where R 0

RX

2

R

and RMSR (R X 2 y2+ t 2 1A b 2 /4) 1 2 (R 0 2 t 2 /4 +b2/12 00 0 Note that as the front or rear hitch points move along a circular path both the centre of curvature of the path of the module and the radius of curvature of the path of the module will change ;constantly.

8. The velocity of the rear hitch point can be calculated from the angular velocity K of the (N module and its radius of curvature R 0 This must be identical to the velocity of the front hitch of the second module. The centre of curvature, radius of curvature and velocity of the second module can now be deduced.

00 9. Steps 4 to 8 are repeated until the appropriate wheel angles and wheel speeds for all (-i modules are calculated.

(N 10. The control system implements the above wheel speeds and angles.

00 11. The calculations are repeated as the gantry tractor snakes its way through the gate.

If translation or rotation errors are detected they can de corrected by means of the strategies outlined in the parent application for vehicles with independent four wheel steering. However errors in the path of the modules must be corrected in a cooperative fashion so that there is no conflict between modules. Conflict is avoided by calculating the corrective path required by the first module. The desired path and velocity of the first rear hitch point becomes the desired path and velocity for the second front hitch point. Any furthercorrection of the second module must be achieved by adjusting the path of its rear hitch point. To avoid instability (such as oscillation) the errors in the positions of the front and rear hitch points should become zero at the same time. The errors of the following modules must be corrected in a similar cooperative fashion.

Figure 5 shows a slightly more complicated path through a gate, which allows narrower gates to be negotiated. In this case the modules turn away from the fence slightly before they turn through the gate at a steeper angle.

A strategy for controlling the path of the articulated gantry tractor is now as follows: 1. The articulated gantry tractor is lined up at the beginning of the path so that all the hitch points lie on the correct position as indicated by means of a geographic positioning system (or some other navigation system). The driver selects the speed of the centre of the lead module.

2. The on board computer calculates required linear and angular velocities of the first module.

The computer then calculates the correct individual wheel speeds and wheel angles. It also calculates the correct rear hitch angle.

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00 0 3. The on board computer calculates the desired linear and angular velocities for the second module to keep it on the desired trajectory. The computer then calculates the correct ;individual wheel speeds and wheel angles and the rear hitch angle for the second module.

n (N 4. Steps 2 and 3 are repeated for all modules in the train.

The control computer then proceeds to implement all individual wheel speeds and wheel 00 angles and hitch angles.

0 6. Errors detected in the path of the modules should be corrected in the cooperative manner 0 Sdescribed above in order to avoid conflict between the modules. Most errors will be due to small amounts of skidding of the driven wheels and scuffing of all wheels (where wheel spinning is regarded as negative skidding).

In working and side shift mode the modules are latched together by means of struts 24 to form a rigid truss. In transport mode the modules are unlatched by effectively disconnecting one end of the struts 24 that latch the modules together. Figure 6 shows one method of disconnecting one end of the strut 24 by lifting it vertically so that a vertical pin 25 on the end of the strut 24 by lifting it so that the vertical pin 25 on the end of the strut no longer engages a vertical hole 26 at the corner of the adjacent module. Figure 7 shows an alternative method of unlatching the modules where a pin 26 latching one end of the strut 27 to a rotatable sleeve 28 attached to the corner of the adjacent module is withdrawn. The other end of the strut is connected to the first module by means of a vertical hinge 29.

Key Definitions and Arguments: Cooperative redundancy between two steering systems exists if the steering effect of both steering systems is identical.

Conflicting redundancy between two steering systems exists if the steering effect of each steering system is different.

A non-redundant steering system exists if only one steering system influences the path of the vehicle.

The steering effect of a steering system is the centre of curvature if the path of the vehicle that would result if the steering system was non-redundant (that is acting alone).

00 The Cooperative Redundant Steering/(Drive) System can also be referred to as the Computer Integrated Steering/Drive System. Whereas the first term emphasises what the system is the ;Zsecond term emphasises how it is achieved.

SnC COC is abbreviation for centre of curvature of the path of the vehicle ROC is abbreviation for radius of curvature of the path of the vehicle RMSWS is abbreviation for root mean square wheel speed 00 RMSR is abbreviation for root mean square radius SThe track is defined as the distance between the centres of the contact patches of the left and right hand wheels, where "ti" is the track of the inner wheels.

00 SThe wheelbase is the distance between the front and rear axles.

Translation error "TE" is the distance of the reference point on the vehicle from the desired path (measured perpendicular to the desired path) where errors to the right of the desired path are considered positive.

Rotation error "RE" is the rotation of the vehicle relative to the direction (or heading) of the desired path, where clockwise rotation s considered positive. If the desired path is curved the relevant direction is the direction (or heading) of the desired path at a point nearest to the vehicle reference point.

Claims (4)

1. A gantry tractor consisting of two or more modules hitched together so that each module can rotate relative to its neighbour or neighbours about a substantially vertical axis through the hitch points, where each module has four wheels all or some of which will be driven where all wheels can rotate about a substantially vertical axis plus or minus an angle greater than 00 degrees, where the modules can be latched together with struts which connect the front left (N corner of one module with the front right corner of the neighbouring module and the rear left 0 corner of the first mentioned module with the rear right corner of the second mentioned module, so that when all the modules are latched together they form a rigid truss in the horizontal plane.

2. A gantry tractor according to claim 1 where the modules are latched together where all wheels are turned through 90 degrees to allow the gantry tractor to move in a direction parallel to a straight line through all the hitch points.

3. A gantry tractor according to claim 1 where the modules are latched together where all the wheels are oriented substantially at right angles to the long axis of the truss where the path of the tractor when its tools are engaged with the ground is controlled by continuously monitoring the location of two or more hitch points and correcting the translation error relative to the desired path by turning all the wheels through a small angle and driving the tractor forward until the translation error is eliminated, where the translation error is the deviation of the reference point of the vehicle from the desired path.

4. A gantry tractor according to claim 1 where the modules are latched together where all the wheels are oriented substantially at right angles to the long axis of the truss where the path of the tractor when its tools are engaged with the ground is controlled by continuously monitoring the location of two or more hitch points and correcting the rotation error relative to the desired path by speeding up the lagging wheels where the amount of speed up is proportional to the lateral distance of each wheel from the leading pair of wheels, where the rotation error is the deviation of the heading of the vehicle from the desired heading. A gantry tractor according to claim 1 which can be manoeuvred along a curved path by unlatching the modules so they can rotate relative to each other about their common hitch points and then controlling the angle of all wheels and the speed of all driven wheels so that all wheels follow the desired path. 00 17 F 6. A gantry tractor according to claim 5 where the desired trajectory of the leading and trailing ;hitch points of each module is converted to a desired centre of curvature and rate of rotation IND about this centre for each module. The instantaneous wheel speeds and wheel angles can then be calculated and implemented by an appropriate control system. 00 00