The invention relates to a means of increasing the tractability, stability, and manoeuvrability and safety of four-wheeled vehicles and agricultural implements, while at the same time minimising the damage inflicted on the ground traversed.

Presently available four-wheeled vehicles and agricultural implements tend to be

S imperfect compromises between two extremes. At one end of the spectrum there are two wheel drive implements with a differential and two steerable wheels which are free to rotate at any appropriate speed. The advantage of this system is that (providing traction is not lost) it inflicts minimum damage on the ground traversed. The disadvantage of this system is that it tends to lose

10 traction prematurely, especially when working on hills where weight is transferred from one or both of the driving wheels. At the other end of the spectrum are skid-steer implements where two left hand wheels are driven independently of two right hand wheels. In this case all wheels are non steerable and always point straight ahead. The advantage of this system is that

15 traction is maximised. The disadvantage is that when the vehicle turns, the wheels move sideways across the ground and scuff, or at a horizontal speed that does not match their speed of rotation or both.

The objective of the present invention is to combine the tractive advantages of independent but positive drive to the driving wheels with the non scuff

10 advantages of steerable wheels. The present invention comprises a four- wheeled vehicle having two sets of drive wheels of substantially the same diameter where the left-hand set of drive wheels is independently and positively rotated at one particular velocity about horizontal axes and the right-hand set of drive wheels is independently and positively rotated at another particular 5 velocity. The velocities may be the same or different. The velocities are related to the appropriate steering angles of the steerable wheels by a control system such that skidding and scuffing are substantially avoided. The drive wheels may also be steerable wheels. For the purposes of the specification, skidding and scuffing are defined as relative movement between the contact

3 _j 0 patch of the tyre and the ground in a direction parallel and perpendicular to the rolling direction of the wheel respectively. Independent and positive driving of the wheels means that the velocity of one set of driving wheels is not affected by the velocity of another set of driving wheels such as that which occurs with a differential connection. The steering angle of a steerable wheel is the angle

-1,5 between the rolling direction of the wheel and the straight ahead position. Anti¬ clockwise rotation is considered positive. The steering angles of all steerable

wheels are not the same, and thus a particular steering angle is appropriate to a particular steerable wheel. To change their respective rolling directions, the steerable wheels can be turned about a vertical axis through their rotational centre. Both two wheel drive and four wheel drive variants are described below.

5 In the four-wheel drive variant of the invention, the two left-hand wheels are driven independently of the two right hand wheels. However, the driving wheels are also turned about their vertical axes to the appropriate steering angles to minimise scuffing. This is achieved by a control system which relates the appropriate steering angles of each of the driving wheels to the velocities of

^• O the left hand and right hand sets of driving wheels.

In this case all four-wheels contribute to the tractive effort while skidding will only occur when the ratio of the rotational velocities of the left hand and right hand sets of wheels is above or below certain critical values described later. However, as this skidding will be in the rolling direction it will not tend to

IS displace the tyres (which will normally operate with a low pressure) from the wheel rims.

Note that the front and rear wheels are turned about their vertical axes at equal and opposite steering angles. This ensures that the two left-hand drive wheels move along the same circular path with a single radius of curvature while the

2.0 two right hand drive wheels move along the same circular path with a single radius of curvature which will be different from that of the left-hand drive wheels when the vehicle is turning. Both circular paths have the same centre. If the angles were not equal and opposite it would be necessary to drive the four driving wheels at four separate velocities to avoid "wind up" and S subsequent skidding of one left-hand wheel relative to the other or one right hand drive wheel relative to the other.

In the two-wheel drive variant of the invention, the two non-steerable driving wheels which point straight ahead are driven independently while the two free¬ wheeling steerable wheels are turned about their vertical axes to the appropriate 30 steering angles to minimise scuffing by means of a control system which relates the appropriate steering angles to the velocities of the two driving wheels. The two driving wheels may be located at the front or the rear of the vehicle.

The control system which relates the appropriate steering angles of the steerable wheels to the velocities of the drive wheels can have three basic forms: the appropriate steering angles can be deduced from the drive wheel velocities set by the driver; the drive wheel velocities can be deduced from the ς steering angles set by the driver; or the steering angles and drive wheel velocities can be deduced from driver controls which select the radius of a turn and the vehicle speed.

Four means of driver control are possible.

In most agricultural implements the speed of the engine is governed to a set 10 speed by the driver so that the velocities of the left and right -hand wheels are determined by the ratio of the rotational velocity of the left-hand wheel and the right-hand wheel to the rotational velocity of the engine. In a preferred embodiment of the invention the left and right-hand drive ratios will be continuously variable from a maximum in a forward direction to a maximum in a 15 reverse direction.

In the first means of driver control the left and right-hand drive ratios are controlled by left and right-hand levers. However, this system has the disadvantage that it would be possible for an inexperienced driver to set the rotational velocities of the driven wheels to inappropriate values. It will be 2.0 shown below that for the four-wheel drive variant there are ratios of the velocities of the left-hand and right-hand sets of wheels which should be avoided because when these ratios pertain it is not possible to avoid scuffing. In the case of the two-wheel drive variant of the invention, if the rotational velocity of the left or right-hand drive wheel it is set to zero, the non-rotating 25 drive wheel would be forced to rotate about its vertical axis causing scuffing, albeit minor.

Another disadvantage of the lever system described above is that inexperienced drivers may have difficulty exercising fine control of the implement's path.

With the second means of driver control both the above problems can be 30 overcome by the use of a single velocity control lever to control the average velocity of the left-hand and right-hand drive wheels and a steering wheel to control the difference in velocity between the left-hand and right-hand drive wheels. In this case the control system would first deduce the appropriate

steering angles for the steerable wheels from the average velocity of the drive wheels and the difference in velocity of the drive wheels and turn the steerable wheels to these angles.

With the second means both skidding and scuffing are substantially eliminated as it would not be possible for the driver inadvertently to set to zero the speed of either the left or right-hand drive wheels of the two-wheel drive variant nor select an inappropriate velocity ratio with the four-wheel drive variant.

The third means of driver control is similar to the second means but is achieved by an alternative method.

In the third means of driver control the steering angles of the steerable wheels are set by a steering wheel so that the path of all wheels have the same centre of curvature. The left-hand and right-hand drive wheels are then independently and positively driven at appropriate velocities to avoid scuffing and skidding. One control system for achieving this is by two continuously variable drive systems linked to the steering mechanism in such a way as to achieve the appropriate velocity differential. These continuously variable drives could be either hydrostatic or friction drives and could also be used to vary the overall gearing of the implement. Another method is to use a single hydraulic pump to drive left-hand and right-hand hydraulic motors where the relative velocities of the latter are controlled by a flow proportioning valve linked to the steering mechanism.

^* A fourth means of driver control of the four wheel drive variant of the invention utilises a rotatable joy stick. This increases the manoeuvrability of the vehicle by allowing independent translation and rotation of the vehicle. In this system ■ of driver control the direction of translation of the vehicle is determined by the direction of displacement of the joy stick from its null position, whereas the rotation of the vehicle is determined by the degree of rotation of the joy stick.

Various conventional electrical, electronic, hydraulic and mechanical methods or combinations thereof can be used to achieve these means of driver control.

The driveability of the four-wheel drive variant can be further improved if each of the four driven wheels (which are rotatable about a normally vertical axis) are mounted at the lower end of four normally vertical double-acting hydraulic rams.

This arrangement of the driven wheels allows the following operational advantages to be achieved with the aid of appropriate systems conventionally available.

Firstly, the ground clearance can be varied by the driver by extending or contracting all the hydraulic rams. A high ground clearance would enable the implement to straddle crops or other obstacles. Alternatively, a low ground clearance could be used on hills were maximum stability is required.

Secondly, on uneven ground most of the weight of the implement would be carried on two diametrically opposite wheels. Although this would not cause a 0 total loss of traction due to the positive nature of the drive system, it would be desirable to spread the load more evenly to maximise tractive effort (especially on slippery ground). To this end the ram supporting the lowest unweighted wheel could be extended automatically until all four-wheels carried approximately the same load.

'5 Thirdly, on steep ground the inexperienced driver is often intimidated by the inclination of the cockpit of the implement, even though the latter may be perfectly stable. To overcome this psychological problem the hydraulic rams on the lowest corners of the implement could be automatically extended while those on the higher side of the implement could be automatically contracted. By 0 this means it would be possible to reduce the angle of inclination of the cockpit by say 1 5°. This would significantly reduce the psychological barrier to operations on steep hills.

Fourthly, the speed of implements over uneven ground is often limited by their lack of a suspension system which allows high vertical acceleration to be 25 experienced by the implement and its driver. To overcome this difficulty the four double-acting hydraulic rams could be used as the basis of an active suspension system programmed to minimise vertical acceleration of the cockpit, thus allowing the implement to be driven more rapidly over rough ground.

The four hydraulic rams could fulfil all of the above four functions simultaneously ^* 0 or the driver could select each function. Thus it would be the decision of the driver to decide which of the functions was more important in the prevailing circumstances. For example, when working over crops the driver could select the maximum ground clearance. On steep hills the driver could select the self-

levelling function. On slippery ground the driver could select the load equalisation function. When traversing fairly flat but bumpy ground the driver could select the active suspension function.

In order that the invention may be more clearly understood, several preferred 5 embodiments thereof will now be described with reference to the accompanying drawings, wherein:

Figure 1 is a plan view of a four-wheel drive variant of the invention with two levers as driver controls.

Figure 2 is a plan view of the two-wheel drive variant of the invention IO with a steering wheel and single velocity control lever as driver controls .

Figure 3 is a series of schematic plan views of a four wheel drive variant of the invention which is controlled by means of a rotatable joy stick. These views 3(a) to 3 (h) show the relationship between the position and rotation of the joy stick the centre 0 and radius of curvature r of the IS vehicles path, the steering angles of the drive wheels Φi, Φ ₂ , ₃ and Φ ₄ and their relative angular velocities <»•, to ω ₄ . Note that Φ ₁ to Φ and ω, to ω ₄ are as defined with respect to Figure 1 .

These diagrams depict the case where the front and rear tracks are the same and the left and right wheel bases are also the same. The front and rear tracks 2.0 of the four-wheel drive variant must be the same, whereas the front and rear tracks of the two-wheel drive variant can be different. It is also assumed that the rolling diameters of all the drive wheels are the same. The rolling diameters of the free-wheeling steerable wheels of the two-wheel drive variant can be the same as that of the drive wheels or different.

25 Note that steering angles measured in a clockwise direction are deemed to be negative whereas steering angles measured in an anti-clockwise direction are deemed to be positive.

In the interests of clarity electric wiring and hydraulic piping have not been shown in the diagrams.

In the four-wheel drive variant depicted in Figure 1 an internal combustion engine 1 drives a right-hand variable displacement hydraulic pump 2 which in turn drives hydraulic motors 3 and 4 mounted in the steerable front and rear right-hand wheels 5 and 6 respectively. The internal combustion engine also drives a left-hand variable displacement hydraulic pump 7 which in turn drives hydraulic motors 8 and 9 mounted in the front and rear left-hand wheels 10 and 11 respectively.

The rotational velocity of the right-hand wheels ω, is controlled by the position of the right-hand lever 12 which changes the stroke of the right-hand variable displacement pump so that its output can be varied from a maximum flow in one direction through zero flow to a maximum flow in the reverse direction. As a consequence the speed of both right-hand hydraulic motors can be varied from a maximum in the forward direction to a maximum in the reverse direction.

The rotational velocity of the left-hand wheels ω. is controlled by the position of the left-hand lever 13 in an analogous way.

In order to minimise scuffing all the driving wheels are turned to their appropriate steering angles by a control system 14 which relates the appropriate steering angles to the rotational velocities by the following equations:

^{= ~} ~\ ^{~ tan l} 2[Γ3L(/lI - Y) + E] ^J

_ι a(l - Y) and φ ₂ = -φ ₄ = tan [ _2E ]

where E = ~b ± -sjb ² - (1 - Y)[b ² + X(l - Y)]

a'

X = -

where Φ _{1 f} Φ ₂ , Φ ₃ , and Φ ₄ are the angles of the right-hand front wheel, left-hand front wheel, right-hand rear wheel and left-hand rear wheel respectively and a and b are the wheel base and track respectively. The steering of the driving

wheels is achieved by servo motors 15, 16, 17 and 18. The above equations only apply when the following inequation is satisfied:

2 a where X = ^— 4

5 In the two-wheel drive variant depicted in Fig 2 an internal combustion engine

19 drives a right-hand variable displacement pump 20 which in turn drives a hydraulic motor 21 which drives the rear right-hand non steerable drive wheel 22. The internal combustion engine 19 also drives a left-hand variable displacement pump 23 which in turn drives a hydraulic motor 24 which drives

10 the rear left-hand non-steerable drive wheel 25.

The path of the two-wheel drive variant can be controlled by right and left-hand levers as for the four-wheel drive variant.

Existing two-wheel drive implements with independent drives incorporate free¬ wheeling wheels on the front which are also free to rotate about a vertical axis

'5 and are commonly referred to as castors. Although there is no problem with scuffing for this configuration a more serious problem arises when the implement traverses a slope because the castors cannot exert a restoring torque to stop the front of the implement moving down the slope. This restoring torque must be provided by the drive wheels which causes these to lose traction

2.0 prematurely.

The above-mentioned problem is overcome in the present invention by employing an "intelligent castor" system wherein the appropriate steering angle for the two free-wheeling steerable wheels is obtained by means of a control system.

In the two-wheel drive variant depicted in Fig. 2, the appropriate angles for the ^• 2-5 right and left free-wheeling steerable front wheels are given by the equations:

where Φ ₅ and Φ _β are the steering angles of the right-hand front wheel 26 and the left-hand front wheel 27 respectively and a and b are the wheel base and track respectively. The steering of the front wheels is achieved by servo motors 28 and 29 respectively.

For both the four-wheel drive and two-wheel drive variants of the invention the right-hand and left-hand levers can be replaced with a steering wheel 30 and a single speed control lever 31.

)D In this case the velocity control lever 31 determines the average drive wheel velocity ω, where ω = (ω- _| + c-2)/2 and the steering wheel 30 determines the relative velocity difference Δω/ω between the left-hand and right-hand drive wheels where

= l(ω _l - ω ₂ ) l(ω +ω ₂ ) ω

IS When using the above driver control means appropriate steering angles for the right-hand wheels are given by

and the appropriate steering angles for the left-hand wheels are given by

0 where

AO

An appropriate relationship between the steering wheel angle ψ and — is ω given by

Aω b b ψ ω r K, - ψK.

where r is the average radius of curvature of the left-hand and right-hand sets of drive wheels and K-j and «2 are constants which determine the steering characteristics.

When using the above driver control means for the two-wheel drive variant the appropriate steering angles for the free-wheeling steerable wheels 26 and 27 are given by:

0 Φ 5 = tan [a Δω /b(ω + Δω/2)l

and φ ₅ = tan ^"1 [aΔω/b(ω-Δω/2)l

However it will be appreciated that all means of driver control so far described can be applied to both the four-wheel drive and two-wheel drive variants, and a combination of driver control means could be incorporated in a single vehicle.

5 The operation of the fourth means of driver control using a rotatable joy stick will now be described with reference to Fig. 3.

Fig. 3 (a) depicts the default position where the joy stick is at its null position and is unrotated. The angular velocity of all wheels is zero as is the rotation of the wheels from the "straight ahead " position.

O Figure 3. (b) shows that when the joy stick is pushed forward, but not rotated, all wheels point straight ahead and have the same angular velocity which is proportional to how far the joy stick is pushed forward.

Figure 3. (c) shows that when the joy stick is pushed side ways, but not rotated, all wheels point 90° to the "straight ahead" direction and have the same angular

•2.S velocity which is again proportional to how far the joy stick is pushed side ways.

Figure 3. (d) shows that when the joy stick is pushed both forward and sideways at an angle θ to the "straight ahead' direction all the wheels point in

this direction. All wheels will have the same angular velocity which will be proportional to how far the joy stick has been displaced from its null position.

Figure 3. (e) shows that when the joy stick is rotated but not displaced the wheels are rotated until their rolling direction is tangential to the circle passing 5 through their vertical axes of rotation. The angular velocity of the wheels will be proportional to the amount of rotation of the joy stick.

Figure 3.(f) shows that when the joystick is pushed forward and rotated to the left the vehicle turns to the left about a centre located to the left of the vehicle. The radius of curvature is proportional to the ratio of the forward displacement J O of the joy stick to its rotation. This radius of curvature determines the angle of the wheels and the angular velocity of the wheels is selected to avoid scuffing.

Figure 3. (g) shows that when the joy stick is pushed sideways and rotated to the left the vehicle turns about centre located behind the vehicle. Once again the radius of curvature is determined by the ratio of the sideways displacement IS of the joy stick to its rotation.

Figure 3. (h) shows the general case where the joy stick is displaced both forward and sideways and also rotated, in this case the average angle of the wheels is determined by the position of the joy stick and the radius of curvature is determined by its rotation. Once again the radius of curvature is proportional ■2.0 to the ratio of the translation of the joy stick to its rotation.

This last case has the disadvantage that in order to avoid scuffing the four wheels would have to be driven at four different speeds. In all the preceding cases since only one or two wheel speeds are required only two variable speed drives are required to drive the four wheels. In this last case four variable speed 25 drives would be required. The control logic would be that the centre of curvature is determined by the position and rotation of the joy stick and the four wheels are turned to be at right angles to the lines joining their vertical axes of rotation and the centre of curvature. The angular velocity of the wheel would then be set to be proportional to their radii of curvature.

0 It is unlikley that the advantages of the motion depicted in Figure 3.(h) are worth the extra complexity of having to have 4 independent variable speed drives.

There are a number of conventional methods for obtaining independent and positive rotation of the driving wheels of the invention. As mentioned previously a combination of hydraulic motors and pumps can be used as well as friction drives. Independent electric or internal combustion motors can also be used to power each drive wheel.

The control system which relates the appropriate steering angles to the rotational velocities at the drive wheels can be implemented in a number of ways. As mentioned previously, mechanical linkages and hydraulic components can be used to relate the appropriate steering angles to the driving wheel velocities. Electrical and electronic methods can also be used. The method used needs only to emulate the equations described above. A preferred embodiment of the control system incorporates a microprocessor which is programmed with an algorithm based on the above equations and appropriate to the variant of the invention used. The algorithm may involve the direct use of the equations using the tan ^"1 function or involve the use of a "look-up" table based on the results of these equations. With the microprocessor-based control system, the drive wheel velocities could be inputs to the algorithm and the steering angles would be outputs, or vice versa. The inputs and outputs could be analog or digital in form. Velocity inputs could be derived from the driver control settings or directly from the wheels. A combination of input sources could be used involving feedback arrangements. Steering angle outputs can be conditioned by conventional electronic means to control actuators which turn the steerable wheels. The actuators may be electric motors or hydraulic rams for instance.

An advantage of the micro-processor based control system is the increased range of steering angles possible compared to mechanical systems and so increased vehicle manoeuvrability is possible.