FR3058101A1 - DRIVING ASSISTANCE SYSTEM FOR AN ALL-TERRAIN VEHICLE - Google Patents

Abstract

The present invention relates to an assistance system for driving a secondary axle (23, 24) fitted to a vehicle comprising a main axle, comprising a pump (200) driven by a drive such as a heat engine of the vehicle, a hydraulic engine machine (130, 140) fitted to each drive wheel (23, 24) of the secondary axle, and a control unit (290), characterized in that it comprises control means (290, 292 ) adapted to automatically control the thrust applied by the driving wheels of the secondary axle (23, 24) according to the terrain configuration, including the amplitude and the direction of the slope of the terrain.

Description

FIELD OF THE INVENTION

The present invention relates to the field of hydrostatic transmissions for machines powered by motors fitted to the wheels or axles.

Such motors are generally called "wheel motors". In particular, such motors can incorporate the bearing function, including the bearings and a wheel shaft. The rotor of these motors rotates at the speed of the wheels. Such motors can also incorporate static or dynamic brakes. These motors are particularly advantageous for being placed on straddle machines without an axle, that is to say with broken axles or independent wheels to have hydrostatic traction on steered wheels, or for machines with variable height.

More specifically, the present invention relates to vehicles of the type illustrated in FIGS. 1 and 2 attached, comprising at least two motorized axles, namely a main axle and a secondary axle, and a control unit, for example a central unit formed by a computer, which controls the motorization of at least one axle.

PRIOR ART

Different types of gear are known of the type illustrated in FIGS. 1 and 2. Generally according to the prior art, a main central unit drives the motorization of at least one axle, preferably all of the axles, to distribute the torque correctly over the set of driving wheels of the machine. Such a main central unit is controlled, or driven, for driving the machine, and for this can be interfaced with a driver, or even a robot or an external remote control. The machine is controlled or driven in that it receives orders to carry out a trajectory at a certain speed.

FIG. 1 diagrammatically illustrates a known machine of the aforementioned type comprising a motorized main axle P and a motorized secondary axle S placed on a common chassis C, supplied by a pump Po controlled by the central unit ECU.

FIG. 2 schematically illustrates a known machine of the aforementioned type comprising a motorized main axle P placed on a tractor vehicle VT and a motorized secondary axle S placed on a towed vehicle or trailer R, powered by a pump Po controlled by the central unit ECU .

Hydrostatic transmission systems are known comprising a power source, for example a heat engine or another alternative power source, at least one pump actuated by the power source and at least one wheel or axle motor powered by the pump, for the motorization of a machine.

The present invention relates to this type of hydrostatic transmission system.

The combination of a pump actuated by a power source, and a wheel or axle motor constitutes a hydrostatic transmission. In a known manner, the reduction ratio of the transmission can be varied by varying the displacement of the pump, and possibly of the motors. The circuit can also be controlled in pressure by the displacement control of the pump. In the usual way, machines with hydrostatic transmission are controlled by flow or pressure via a displacement control of the pump. For machines fitted with a variable displacement pump, the machine can be stopped by zeroing the displacement of the pump.

In a manner known per se, the drive of the pump (s) by the power source, in particular by a heat engine, is carried out directly or indirectly, that is to say that the pump shaft can be linked to the shaft of the power source, or through other interposed members.

In a hydrostatic transmission of a mobile machine, the distribution of the torque transmitted to each wheel of the vehicle is proportional to the state of pressure at the terminals of the engine, and to its active displacement.

When using mobile machinery, the operating conditions can vary widely. This can be linked to the variation in load on each wheel, on the ground itself, flat or sloping (uphill, downhill, slope ...), with the consequence of a risk of slipping of one or more wheels of the gear.

On certain mobile machines, one can have a part of the machine which has its main transmission, and other parts corresponding to load-bearing wheels which have no transmission. For example, one can have load-bearing axles on a machine, or trailers, or trailed implements. On these machines, we are interested in placing a secondary hydrostatic transmission, or assistance, on these load-bearing wheels.

As indicated above with regard to FIGS. 1 and 2, certain manufacturers offer the driver to adapt the distribution of the torque to the different driving wheels of the machine, by acting on a selector located at the driving position, at the level of the central unit. ECU, and requiring manual control. The driver adjusts the thrust he wishes on the secondary part of the machine which is assisted. On very simple systems, the assisted part provides a constant thrust, and the driver activates or neutralizes the assistance depending on the terrain.

In this type of ergonomics, the driver's intervention is necessary to anticipate a skating situation and avoid uneasy driving conditions for an inexperienced driver and also to adjust the desired torque to the power wheels, depending in particular on the machine goes up or down a slope.

The provision of a selector at the driving position, however, creates many problems in practice because the user must drive the vehicle correctly, often in difficult conditions, while controlling the selector.

(see page 10) Besides, it is not necessary to have an electronic entity ECU in the literal sense. In a broader manner according to the state of the art, the part of the machine corresponding to the axles P and

S, which are piloted must simply be managed by the driver. This may be a mechanical transmission, or a hydraulic transmission with mechanical control, or a hydraulic transmission managed by an ECU central unit.

In the following description and in the claims, the expression “control unit” will be used to designate both a central unit formed by a computer or electronic entity and a command operated by a driver without a central unit, whether directly or via a robot, remote control or any other means.

There is shown in Figure 3 attached, schematically, a hydraulic system comprising a hydrostatic transmission thus known to those skilled in the art.

We see in Figure 3 attached an open type hydraulic circuit, that is to say in which a hydraulic pump 19 for supplying the circuit sucks oil into a reservoir 13 and the oil return at the outlet of the supply circuit is returned to the tank (as opposed to so-called closed circuits in which generally a booster pump is required to maintain a minimum pressure in the closed circuit, which comprises a forward branch and a return branch between the pump main and the hydraulic receiver of the circuit, the closed loop does not pass through the tank, while the booster pump draws from the tank to maintain a minimum pressure in the closed loop).

The present invention can moreover be applied both to an open type hydraulic circuit and to a closed type hydraulic circuit.

More precisely still, one can see in the attached FIG. 3 a tractor vehicle referenced schematically 10 and a trailer referenced schematically 20.

The towing vehicle 10 comprises a power source or main engine 12, for example a heat engine, a clutch 14 associated with the engine 12 and a gearbox 16 connected to the clutch outlet and interposed between the clutch 14 and a driving axle 18.

Also seen in Figure 3, a PTO 17 driven directly or indirectly by the main motor 12 and placed for this purpose, either on an output of the engine 12, or at the output of the gearbox 16, and a hydraulic machine forming a pump 19 driven by the PTO 17.

The towing vehicle 10 further comprises a reservoir 13, a suction duct 11 which connects the reservoir 13 to the inlet of the pump 19, a supply duct 15 connected between the outlet of the pump 19 and a connector 30 intended to be connected to a complementary connector element 31 provided on the trailer 20 and a return duct 34 placed between a connector 32 intended to be connected to a complementary connector element 33 provided on the trailer 20 and the tank 13. The towing vehicle 10 can also include a filter 36 placed on the return duct 34.

The trailer 20 comprises a secondary axle 22 comprising two wheels 23, 24 associated with respective hydraulic machines

130, 140 forming motors.

In FIG. 3, the conduits, mounted in parallel, for supplying the motors 130, 140 are referenced 131, 141, the return conduits, mounted in parallel, for the two motors 130, 140 are referenced 132, 142, and referenced 133, 143 of the conduits which lead to the casing of the motors 130, 140.

The connections defined between connectors 31, 33 and the conduits

131, 132, 133, 141, 142, 143 mentioned above are controlled by a set of components comprising hydraulic valves grouped in FIG. 1 under the reference 50.

As indicated above, a selector 40 with manual control is made available to the user, on the tractor vehicle 10, at the operator's position to control the components 50 on demand.

As indicated previously, although having rendered great services, the systems of the type illustrated in FIG. 3 have the disadvantage of obliging the driver of the vehicle to maintain his attention both on the control of the selector 40 and on the driving. of the craft. In practice, these obligations lead to complicated situations for an inexperienced driver. In particular, it is not pleasant, and difficult for an inexperienced driver that the assistance gives effects contrary to driving, in particular that the secondary part of the machine which is assisted pushes the main part of the machine, by example in the descents.

PROBLEM POSED TO SOLVE

In this context, the present invention aims to improve the state of the art by proposing new means which allow automatic displacement control on a machine, without requiring the intervention of the driver. In other words, one of the problems to be solved within the framework of the invention is, for an assisted secondary axle, which is not the main axle of a vehicle, to automatically control the thrust torque, in particular according to the slope of the ground on which the vehicle moves.

BASIS OF THE INVENTION

This object is achieved in the context of the present invention by means of a system for assisting the drive of a secondary axle fitted to a vehicle also comprising a main axle, which system comprises a pump driven by a drive such as a motor. thermal system of the vehicle, a hydraulic machine forming an engine equipping each driving wheel of the secondary axle, and a control unit, characterized in that the main axle is a driven driving axle and the secondary axle is a motorized follower axle, and that it comprises suitable control means for automatically controlling and modulating the thrust applied by the drive wheels of the secondary axle according to the configuration of the terrain, in particular the amplitude and the direction of the slope of the terrain, so that the driving force generated by the thrust applied by the driving wheels of the secondary axle, combined with the driving force induced by the slope of te drive on which the vehicle travels, does not disturb the driving force generated by the main driven axle.

In the context of the present invention, the term “does not disturb” means that the motive force generated by the thrust applied by the drive wheels of the secondary axle is modulated, taking into account the motor force induced by the slope of the terrain. , whether it is an ascent or a descent, so that the sum of the motive force generated by the thrust applied by the drive wheels of the secondary axle and the motive force induced by the slope of the terrain, does not lead to an antagonistic component greater than that of the driving force generated by the driving main axle and controlled.

It should be noted that the secondary vehicle is always follower of the master vehicle, and that the speed of the wheels is only linked by the ground. The secondary system seeks to define a thrust torque, which means that if the wheels of the secondary vehicle do not touch the ground, their speed may be different from those of the master vehicle.

According to other advantageous but non-limiting characteristics of the invention:

- the system includes a control unit which controls the main drive axle and a central unit which controls the secondary axle.

- the system comprises two central units, one which controls the main driving axle and the other which controls the secondary axle.

- the secondary axle is placed on a towed vehicle or trailer, while the main axle is placed on a tractor vehicle,

- the system includes an engagement (clutch) or short-circuit valve of the engine driving the drive wheels of the secondary axle,

- the control means adapted to automatically control and modulate the thrust applied by the drive wheels of the secondary axle according to the configuration of the terrain are also adapted to operate this control and this modulation according to the load on the secondary axle and the configuration of the secondary element of the vehicle carried by the secondary axle,

- the wheel motors fitted to the secondary axle are convertible wheel motors,

- the wheel motors perform a bearing function of the wheel.

The present invention also relates to a method for assisting the drive of a secondary axle fitted to a vehicle also comprising a main axle, using a system comprising a pump driven by a drive such as a heat engine of the vehicle, a hydraulic machine forming an engine fitted to each drive wheel of the secondary axle, if necessary an engagement (clutch) or short-circuit valve of the engine driving the drive wheels of the secondary axle and a control unit , characterized in that the main axle is a driven drive axle and the secondary axle is a powered follower axle, and that it comprises steps consisting in automatically controlling and modulating the thrust applied by the drive wheels of the axle secondary according to the configuration of the ground, in particular the amplitude and the direction of the slope of the ground, so that the motive force generated by the thrust has applied by the drive wheels of the secondary axle, combined with the engine force induced by the slope of the terrain on which the vehicle is traveling, does not disturb the driving force generated by the main drive axle.

The present invention also relates to vehicles equipped with the system previously defined and / or implementing the method according to the present invention.

QUICK DESCRIPTION OF THE FIGURES

Other characteristics, aims and advantages of the present invention will appear on reading the detailed description which follows, and with reference to the appended drawings, given by way of nonlimiting examples and in which:

FIGS. 1 and 2 previously described schematically represent machines known from the state of the art comprising a central unit which controls jointly a main axle and a secondary axle,

FIG. 3 previously described represents a hydraulic circuit known from the state of the art,

- Figures 4 and 5 attached schematically represent vehicles according to the invention, respectively of the tractor and towed assembly type for Figure 3, with a single chassis according to Figure 4, comprising a control unit for controlling the main axle and a electronic central unit for driving the secondary axle, for example two separate central units for driving the main axle and the secondary axle respectively,

FIG. 6 represents a schematic view of an exemplary embodiment of a hydraulic circuit according to the present invention,

- Figures 7, 8, 9 and 10 respectively represent the configurations of the circuit in forward traction on an increasing slope, retained forward on a decreasing slope, retained reverse on an increasing slope in the forward direction and reverse traction on a increasing slope in the forward direction,

FIG. 11 schematically represents a summary of the four configurations corresponding to FIGS. 7 to 10,

FIG. 12 represents a view similar to FIG. 11 in the case of taking into account an additional state of braking of the vehicle,

FIGS. 13 and 14 schematically illustrate variants of FIGS. 11 and 12 within the framework of a variant of the invention incorporating a proportional pressure control function,

FIG. 15 schematically represents a hysteresis function taken into account in the context of a variant of the invention,

FIG. 16 schematically represents the pressure control curve according to a variant of the invention,

FIG. 17 represents a variant of FIG. 14 in the context of pressure control as a function of the angle of the slope of the terrain on which the vehicle is moving,

FIG. 18 schematically illustrates the transition phase between two pressure configurations in accordance with the present invention, and

- Figure 19 shows schematically a cross-sectional view of a hydraulic machine with radial pistons preferably used in the context of the present invention.

GENERAL DESCRIPTION

There is shown schematically in Figures 4 and 5 appended vehicles according to the present invention, in which a main axle P and a secondary axle S are controlled by respective central units ECU1 and ECU2 made up of computers or electronic entities. The ECU1 central unit which controls the main axle P receives its orders from a driver, or even a robot or a remote control. It is however not necessary in the context of the present invention to have an electronic entity ECU1 in the proper sense. More broadly, the part of the machine corresponding to the axle P, which is controlled, must simply be managed by the driver. This can be a mechanical transmission, or a hydraulic transmission with mechanical control, or a hydraulic transmission managed by an ECU electronic central unit. In general, it is observed that the part of the machine corresponding to the axle P forms an assembly driven directly by the driver.

(see page 4) In the following description and in the claims, the expression “control unit” will be used to designate both the set of potential control means of the main axle within the framework of the present invention. , including in particular both a central unit formed by a computer or electronic entity and a command operated by a driver without a central unit, whether directly by a driver or pilot, or via a robot, a remote control or any other way.

The ECU2 central unit which controls the secondary axle S has its own control based on information specific to the configuration of the terrain, in particular the amplitude and the direction of the slope of the terrain, or even to the load on the secondary axle or to the configuration of the element carried by the secondary axle. In the broad sense, the ECU 2 is any means of control for controlling the secondary axle S, whatever its form. It can be any form of automatic, electronic, electric, or mechanical piloting.

FIG. 4 schematically illustrates a machine comprising a motorized main axle P placed on a tractor vehicle VT powered by a pump Pol controlled by a control unit, for example the main central unit ECU1, and a motorized secondary axle S placed on a vehicle towed or trailer R, powered by an associated auxiliary pump Po2 controlled by the secondary central unit ECU2. In FIG. 4, the possibility of using a control unit to drive the main axle P, and not an electronic central unit ECU1, is shown diagrammatically by the arrow directly connecting the symbol of a driver and the pump Pol.

FIG. 5 schematically illustrates a machine comprising a motorized main axle P and a motorized secondary axle S placed on a common chassis C, the motorized main axle P being powered by a pump Pol controlled by the main central unit ECU1 and the axle secondary motorized S being supplied by an associated auxiliary pump Po2 controlled by the secondary central unit ECU2. In FIG. 5, the possibility of using a control unit to drive the main axle P, and not an electronic central unit ECU1, is also shown diagrammatically by the arrow directly connecting the symbol of a driver and the pump Pol.

The orders applied respectively to the ECU1 and ECU2 central units are a priori independent and different. The driver controls and modulates the speed of the vehicle via the main central unit ECU1, by acting on the main axle P, while the secondary central unit ECU2 can modulate the thrust generated by the secondary axle S without direct link with the main ECU1 central unit. Typically the secondary central unit ECU2 controls the thrust, depending on whether the machine is moving on an ascending or descending slope.

In the following description and in the claims, the term “thrust” will be used to generate both a positive force with respect to the direction of movement of the vehicle corresponding to traction and a negative force with respect to the direction of movement of the corresponding vehicle withholding.

Thus the main central unit ECU1 essentially manages the speed of the machine, while the secondary central unit ECU2 essentially manages a torque, via a pressure or a slip speed difference at the wheels. We can therefore at certain times have a control to decrease the speed, because the driver wants to slow down on the main part of the machine, while ECU2 pushes the secondary part of the machine, because it is still on a slope rising, and this does not disrupt driving. However, with the same orders from the driver, on a descending slope, ECU2 would decide to carry out a restraint so as not to disturb the driving.

Furthermore, in the following description and in the claims, the part of the vehicle carried by the master axle P will be called the “vehicle master element” and the part of the vehicle carried by the secondary axle S. The vehicle master element constitutes a tractor element, while the secondary vehicle element constitutes a towed element, in the case of a vehicle in two elements, tractor on the one hand and towed element or trailer on the other hand. On the other hand, the vehicle master element and the secondary vehicle element are combined when the main axle P and the secondary axle S are placed on a common chassis.

EXAMPLE OF EMBODIMENT OF THE INVENTION

Subsequently and in order to simplify, the invention will essentially be described with reference to a configuration of a combined type machine, comprising a tractor vehicle and a towed vehicle or trailer. The invention is however not limited to this particular configuration and also applies to machine configurations comprising the main drive and driven axle and the secondary follower axle on a common chassis.

It can be seen in FIG. 6, a hydraulic system adapted to be installed on a vehicle trailer, for example the towed element of a soil working tool, such as a plow with hydraulic wheel-motor, or such that a towed potato harvester with hydraulic wheel motors.

The system shown in FIG. 6 is adapted to be mechanically driven by the PTO 17 of the tractor element not shown in FIG. 6.

The hydrostatic transmission shown in FIG. 6 is of the closed circuit type. It could be, if necessary, of the open type, as indicated previously.

We find in Figure 6:

- two supporting wheels 23, 24 of the secondary axle S, in this case the towed element,

- Two hydraulic machines forming motor 130, 140 connected to the respective shafts of the wheels 23, 24. If necessary, the wheels 23, 24 could be connected by a common axle. However, preferably, in the context of the invention, the wheels 23, 24 are placed on a towed trailer of the straddle type machine without axle, that is to say that the wheels 23, 24 are independent and are not connected by a common axle, although they are coaxial when traveling in a straight line.

- Supply conduits 131, 141, return conduits 132, 142 and conduits 133, 143 connected to the crankcase.

flow dividers 139, 149 respectively associated with supply lines 131, 141 and return lines 132, 142,

brakes 135, 145, for example of the drum brake type,

actuators 136, 146 of the aforementioned brakes 135, 145,

a temperature sensor 134 associated with the flow divider 149, a pressure sensor 144 associated with the flow divider 139.

a flow divider 147 supplied by the line coming from the braking circuit of the tractor, leading to the actuators 136, 146,

a pressure sensor 148 associated with the flow divider 147,

- a hydraulic machine 200 forming a pump,

a mechanical switching box 202 interposed between the power take-off 17 and the input of the pump 200, making it possible to engage or not the drive of the pump 200, and having, if necessary, an auxiliary gearbox function,

a booster pump 210 associated with the pump 200,

- a pressure limiter 213 associated with the booster pump 210.

- an oil tank 220,

a line 212 by which the booster pump 210 pumps into the tank 220 via a filter 214,

- a block 230 having an oil cooler function. The cooling block 230 is known per se and does not constitute an essential element of the invention. It will therefore not be described in more detail below.

- A control valve 240. The control valve 240 is according to Figure 6 a three-way valve, three positions. Its structure and operation are known to those skilled in the art and will therefore not be described in more detail below.

a valve 250 for engaging / disengaging or clutching / declutching the assistance. The engagement / disengagement valve 250 is a two-way valve, two positions according to FIG. 6. Its structure and its operation are known to those skilled in the art and will therefore not be described in more detail below.

a feed line, in high pressure in forward direction, referenced 204 which connects the outlet of the pump 200 to the control valve 240 and a return or low pressure line in forward direction referenced 206 which connects the control valve 240 at the inlet of pump 200,

a supply line 242 which connects the high pressure supply line 204 to the flow divider 139 in advance, and therefore to the supply inputs 131, 141 of the motors 130, 140, by means of the control valve 240

a return line 244 connected by the control valve 240 to the return line 206 and which communicates with the flow divider 149 and therefore the outputs 132, 142 of the motors 130, 140.

- A pressure relief assembly 260 placed between the high pressure 204 and low pressure lines 206. The structure and operation of such a pressure relief assembly 260 are known to those skilled in the art and will therefore not be described further in detail later,

- A double-acting cylinder 270 serving to control the pump 200 by controlling the displacement of said pump 200 with variable displacement. Typically and in a manner known per se, the actuator 266 controls the position of a plate controlling the variable displacement of the pump 200.

a pressure reducer assembly 275 comprising more precisely two pressure reducers 276, 277 communicating respectively with the two chambers of the double-acting cylinder 270 to ensure piloting in proportional pressure of the cylinder 270 allowing piloting of the displacement of the pump 200 to displacement variable.

- pressure limiters 278, 279 associated respectively with proportional pressure reducers 276, 277. The proportional pressure reducers 276, 277 regulate a level of thrust proportional to the level of the motors 130, 140,

a switching assembly 280 comprising two on / off switching valves 282, 284 associated respectively with the two chambers of the double-acting cylinder 270 to ensure the switching of the system between a traction position and a retained position according to the slope of the terrain on which the vehicle,

- a central control unit 290,

a man / machine interface 292 connected to the central unit 290 and allowing the user to configure various operating parameters of the system,

a vehicle speed sensor 294, and

- A tilt sensor 296, or an inclinometer, adapted to measure the slope on which the vehicle is moving.

The precise structure of the circuit shown in FIG. 6 will not be described in detail below since a certain number of these components are known per se to those skilled in the art.

The means 275, 280 as well as the double-acting cylinder 270 adapted to control the displacement of the variable pump 200 are in particular known per se to those skilled in the art and described for example in documents EP 2551524, FR 3011288 and W02014 / 016304.

MEANS AND VARIANTS CONFORMING TO THE INVENTION

STRUCTURE OF HYDRAULIC MACHINES

Preferably, the hydraulic machines forming the motors 130, 140, as well as where appropriate the pump 200, are radial piston motors of the type illustrated in FIG. 19 appended.

Preferably, the hydraulic machines forming motors 130, 140 are more precisely retractable piston motors.

The structure of such motors 130, 140 is well known per se to those skilled in the art and will therefore not be described in detail below.

POCLAIN HYDRAULICS itself manufactures and markets many radial piston engines of the type shown in Figure 19.

It will be recalled, however, that preferably, the radial piston hydraulic motors of the type illustrated in the attached FIG. 19 include in a casing 110:

a multi-lobe cam 112 preferably formed on the internal surface of an element of the casing,

a cylinder block 114 mounted with relative rotation in the casing 110,

a shaft 116 linked in rotation to the cylinder block 114,

pistons 118 guided in radial sliding in the respective cylinders 117 of the cylinder block and bearing on the lobes of the cam 112, preferably by means of rollers 115.

Radial piston engines of the type illustrated in FIG. 19 appended also include (not shown in FIG. 19) a distributor adapted to successively apply a pressurized fluid in the cylinders 117 and consequently on the pistons 118, so that the successive support of the pistons 118, via the rollers 115, on the lobes of the cam 112, causes the relative rotation of the cylinder block 114 and of the elements which are linked to it with respect to the casing 110.

We will now describe various characteristics, optional for some, specific to the control process according to the present invention.

ABSENCE OF POSITIVE DOWNHILL

Firstly, the control means integrated into the central unit 290 in accordance with the present invention are preferably adapted so as not to apply positive thrust or traction in descent on the secondary axle constituted by the wheels 23, 24, c ' that is, when the vehicle is traveling on a negative or descending slope, but on the contrary apply restraint on this secondary axle. In practice, such a retention can be operated by applying a high pressure not to the inlet of the motors 130, 140, but to their outlet.

VALUE. DIFFERENT GROWTH BETWEEN THE FLAT AND A SLOPE

To this end, the control means 290 are preferably adapted to define a first thrust value when the vehicle is traveling on a flat surface and a second thrust value different from the first value when the vehicle is traveling on a slope.

TORQUE OR SPEED CONTROL

According to the invention, the thrust defined by the motors 130, 140 is determined:

either by a torque control at the wheels 23, 24, that is to say at hydraulic pressure, defined by a pressure control of the pump 200,

- Either by a speed control at the wheels 23, 24, defined by a control in the displacement position of the pump 200 to which the speed of the secondary wheels 23, 24 is linked, by defining a speed difference, positive or negative, between the wheels 23, 24 of the secondary axle and a speed reference representative of the speed of the vehicle master element, that is to say of the main driving part of the vehicle, for example from a measurement the speed of the main axle or the wheels of the main axle, or of a ground radar, a GPS datum or a speed datum measured on a carrier wheel of the master element of the vehicle, or any other means equivalent.

PUSH DEFINITION PARAMETERS

As indicated above, according to the invention, the thrust applied by the drive wheels 23, 24 of the secondary axle is automatically controlled as a function of:

- the load on the secondary axle,

the configuration of the secondary element of the vehicle carried by this secondary axle, and

- the configuration of the terrain, in particular the amplitude and the direction of the slope of the terrain on which the vehicle is moving.

The load represents the state of charge of the secondary element of the vehicle, that is to say in the case of a towed vehicle, if the machine constituting the towed element is more or less loaded (the load represents example the filled or empty state of the trailer or a sprayer tank). The load value can be entered by the user via the man-machine interface, or given by load sensors, or level for liquid loads.

The configuration of the machine corresponds to its implementation: for example spray boom deployed or not, special tool in place or not, size of the wheels.

The configuration of the terrain relative to the secondary element of the vehicle corresponds to:

- on the one hand to the slope: the configuration can be defined from a map and / or a GPS position of the machine, or by a sensor, in particular a sensor chosen from the group comprising a tilt sensor (inclinometer) or a force sensor on the drawbar or a coupling angle sensor or by the indication of an inertial unit or any other equivalent means.

- on the other hand to machine data: for example a turn sensor (angle of the steered wheels for example, or drawbar angle in another angle than previously), or by an indication of an inertial unit or by the change of speed, or whether a gear is engaged or not (two different treatments, you can choose to push during gear change or not) or an indication of the speed of the master wheels or of the tractor to define a target speed of the secondary wheels . It should be noted that the target speed does not allow the behavior of the abutment of the machine to be defined. To carry out a traction or a restraint, it is necessary to calculate a positive or negative delta speed of the wheels of the abutment, compared to the target value. This speed delta, or slip, is directly linked to the torque exerted by the wheels. A negative slip determines a hold, a positive slip determines a push. If the wheels of the abutment only repeated the target speed, they would exert no torque, and the machine would be coasting.

VEHICLE SPEED DEPENDENCE

The transmission of the secondary axle can be disengaged by the central unit 290, in particular when running and from a certain speed. In particular, it is possible to deactivate the wheel motors 130, 140 which are cam motors which can be deactivated when the vehicle speed exceeds a predetermined threshold.

TAKING INTO ACCOUNT OF PARAMETERS OTHER THAN THE SLOPE

Preferably, in the context of the present invention, the control operated by the central unit 290 essentially makes an evolution of thrust as a function of the slope on which the vehicle is moving. As a function of the acquisitions of slope parameters coming from the sensor 296, which are compared to a threshold, the central unit 290 defines the thrust of the secondary axle 23, 24, in particular by defining whether it is necessary to control in traction or restraint (traction when the vehicle is moving on a flat or uphill grade, restraint when the vehicle is traveling downhill).

The thrust defined by the central unit 290 can also be modulated as a function of the other parameters (for example and without limitation load, deployment of accessories such as spray bars, bends, etc.).

Thus in the context of the invention, the thrust value defined by the central unit 290 as a function of the inclination detected by the inclinometer 296 can be modulated by thresholds or proportionally, as a function of the other operating parameters of the vehicle. For example and without limitation, the thrust defined by the central unit 290 can be multiplied by a factor depending on the load or the configuration of the tool fitted to the trailer, or even for example depending on the amplitude of a turn followed by the vehicle.

DIFFERENT MODES OF EMBODIMENT

We will now define different specific embodiments in accordance with the present invention.

FIRST EMBODIMENT

In the context of this first embodiment, the central unit 290 performs a comparison of the slope on which the vehicle is moving with respect to an inclination threshold. The central unit 290 is in this case provided for simply defining a positive thrust or traction. This mode is the simplest.

It corresponds to two possible states: weak traction on the flat and strong traction uphill.

At the start, it is assumed that the vehicle is moving horizontally on a flat surface.

The central unit 290 controls the pump 200 for the low traction value applied to the flat ground.

The central processing unit 290 analyzes the acquisition of the inclination obtained from the inclinometer 296.

It carries out a comparison with a preset threshold value.

When the inclination provided by the inclinometer 296 is greater than the threshold, the central unit controls the pump 200 in strong traction defined for a rise.

In this case, the central processing unit 290 is programmed to target two target values of pressure or speed difference, for example a first pressure, of weak traction, for displacement on flat ground and a second pressure, of strong traction, during the climb.

SECOND EMBODIMENT

In the context of this second embodiment, the central processing unit 290 performs a comparison of the slope on which the vehicle is traveling with respect to a negative tilt threshold corresponding to a descending slope. The central unit 290 defines in this case two possible states: traction when the vehicle is moving on a flat or uphill slope and a restraint when the vehicle is descending on a negative slope.

As long as the vehicle is moving on flat ground or uphill, the central unit 290 applies the predefined traction.

When the central processing unit 290 detects via the inclinometer 296 a negative slope greater than a threshold value, it imposes a restraint at the level of the motors 130, 140.

In this case, the central processing unit 290 is programmed to target two target values of pressure or speed difference, for example a first traction pressure, for movement on flat or uphill ground and a second holding pressure, when a descent.

THIRD EMBODIMENT

According to the third embodiment, the central processing unit 290 performs a comparison with respect to two inclination thresholds which define three states: a weak traction on the flat, a strong traction when climbing and a restraint when descending.

At the start, we consider that the towed machine is horizontal. The central unit 290 imposes the traction which corresponds to piloting on flat ground.

The central processing unit acquires the inclination via the inclinometer 296 and compares with a first threshold value.

When the central processing unit 290 detects that the inclination measured by the inclinometer 296 is greater than the upward slope threshold, it controls the pump 200 with a view to the defined traction when climbing.

When the central unit detects, on the contrary, that the inclination measured by the inclinometer 296 is less than the second descending slope threshold, which means that the vehicle is traveling on a negative slope, the central unit 290 imposes a restraint at the level motors 130, 140.

This third embodiment is defined by four dials which correspond to the representations illustrated in FIGS. 7, 8, 9 and 10 respectively and summarized in FIG. 11.

In FIG. 11, possible states of constant pressure have been illustrated in situations of forward travel on a downward slope or reverse gear on an upward slope. As a variant, these states can implement a pressure evolution.

In the third embodiment, the central processing unit 290 is programmed to target three target values of pressure or speed difference: for example a first pressure on the flat, a second pressure on the rise and a third negative pressure on the descent.

FOURTH EMBODIMENT

According to this fourth embodiment, the central unit 290 imposes a thrust at the level of the motors 130, 140 proportional to the slope, positive or negative. According to this fourth embodiment, the central unit 290 thus imposes a traction or a retention proportional to the slope on which the vehicle is moving.

The traction and the proportional restraint define a pressure or a speed difference for the secondary transmission formed by the motors 130, 140.

PROPORTIONAL PUSH AND STEERING ON THE SLOPE

As a preferred example, the control means integrated into the central unit 290 are adapted to apply via the wheels 23, 24 thanks to the motors 130, 140, a thrust proportional to the slope.

Schematically illustrated in FIGS. 13 and 14 and in FIG. 17, piloting proportional to the slope.

It can be seen in these figures 13, 14 and 17 that the regulation curve defining the pressure applied to the motors 130, 140, and controlled by the central unit 290, depends on the value of the slope on which the vehicle is moving.

TAKING INTO ACCOUNT OF THE BRAKING STATE

According to a variant, the central unit 290 is adapted to take into account braking of the master axle of the vehicle.

According to this optional characteristic of the invention, the central unit 290 defines a specific control of the motors 130, 140 when the vehicle is in braking mode.

The “brakes applied” parameter can be taken into account for the control of the secondary axle composed of the wheels 23, 24, as illustrated in FIGS. 12 and 14.

Compared to the methods previously described with reference to FIGS. 7 to 10 and FIG. 11, only the steering on the flat is modified. In this case, we pass to a restraint mode with a negative restraint thrust value.

This provision is important insofar as if the towing machine or the master axle is braked, this can change the traction priorities of the secondary axle placed on the towed part or trailer.

If the brake is activated, the assistance value is modified on the plate by the central unit. Typically in this case, the central processing unit 290 will choose the selection of the light retained state “0” which will be defined subsequently. The fact that the braking information is used to modify the assistance values does not mean that the abutment is directly controlled by the driver. The ECU2 always decides whether to apply traction or restraint depending on the terrain configuration. In particular, braking of the main part of the machine does not generate a repetition of the braking order on the secondary part via the ECU 2.

As can be seen in FIGS. 12 and 14, the value of the pressure applied to the motors 130 and 140 can then be kept constant whatever the speed of the vehicle.

Figures 12 and 14 have in fact illustrated possible states of constant pressure in situations of forward travel on downward slope or reverse on upward slope, as well as on the flat. As a variant, these states can implement a pressure evolution.

On the other hand if the vehicle is not braked, on the flat the central unit 290 will choose the selection of the state “0” full power.

The same choice is made in reverse.

As a variant, when the control means 290, 292 detect a braking mode of the master axle of the vehicle, they maintain the previous state, for example traction or restraint, then modifying if necessary the corresponding value of the thrust, traction or restraint, to guarantee for example a hill start or to ensure a restraint when starting downhill.

VALUE. DIFFERENT PUSH OR RETENTION DEPENDING ON THE SLOPE

The flat hold value may be different from the downhill hold, in the same way that the flat pull may be different from the slope pull.

TARGET PRESSURE OR SPEED GAP VALUES

Preferably within the framework of the invention, the central unit 290 is adapted to define 4 target values of pressure on the motors 130, 140 or of speed difference between the wheels 23, 24 of the towed element and the wheels of the towing vehicle: for example a first target value of pressure or speed difference in traction when traveling on a flat, a second target value of pressure or speed difference in traction when traveling uphill , a third target value of negative pressure or holding speed deviation when moving on the flat accompanied by braking and a fourth target value of negative pressure or holding speed deviation when moving downhill.

HYSTERESE

As illustrated in FIG. 15, the central processing unit 290 can use slope thresholds defined with hysteresis, to avoid jolts on the vehicle. For example, the central unit can control by switching from flat mode to climb mode from a slope greater than or equal to 5 °, but only starting from climb mode to flat mode again for a slope less than or equal to 4 °.

As can be seen in FIG. 15, such a hysteresis can be applied both to the threshold of upward slope and to the threshold of downward slope.

LOCINOMETER LOCATION AND CALIBRATION

The inclinometer 296 can be fixed on the towing vehicle or on the trailer, that is to say the towed element.

In the context of the invention the inclinometer 296 is preferably placed on the towed element, when the secondary axle is placed on a towed element. Thus it is ensured that the central unit ECU2 does indeed receive reliable information on the slope on which the secondary axle moves, even if the towing vehicle is, depending on the advance, placed on a different slope.

Preferably according to the invention, there is provided a process for calibrating the inclinometer 196 to define which output value of the inclinometer corresponds to a movement of the vehicle on the flat.

To correct the mounting of the inclinometer and / or the crushing of the wheels under load and / or the angle of the hitch and / or the height of the tractor and / or the effect of a change of tire mounting and / or the mounting of such and such a tool and / or a dependency according to the type of tractor used to tow the coupled machine, according to the invention there is preferably also provided a calibration process which consists in:

- 1 / Place the configured, harnessed, equipped and loaded machine on a flat floor,

- 2 / Proceed to a step of acquiring the output value of the inclinometer 296, and

- 3 / Save this value as the characteristic value of the flat machine.

The man / machine interface 292 is an interface adapted to activate the system, and which for a towed tool, such as spray bars, is preferably placed in the cabin of the towing vehicle. This man / machine interface 292 is suitable for defining the machine parameters, in particular load, type of tool, desired power, as well as in particular for initializing the inclinometer and activating the assistance.

BASIC STRUCTURE OF THE CYLINDER CONTROL MEANS

As indicated previously, the displacement of the displacement of the pump 200 is operated by the double-acting cylinder 270 itself piloted by a pressure reducing assembly 275, pressure limiting valves 278, 279 and a switching assembly 280.

The two switching valves 282, 284 which form the switching assembly 280 and constitute pump solenoid valves, regulate the direction of flow.

The two pressure reducers 276, 277 which form the pressure reducer assembly 275, regulate the pressure to be reached and thus operate a pressure control.

All of these means make it possible to do the four quadrants of traction: front, rear, traction and restraint.

DEFINITION OF THRUST OR POWER LIMIT FUNCTION

FIG. 16 illustrates a typical curve used in the context of the present invention by the central unit 296 to ensure the definition of the pressure applied to the motors 130, 140 by the aforementioned means 275, as a function of the speed.

This regulation curve defines a maximum power applied to the motors 130, 140.

Preferably the pressure is constant and maximum under a speed threshold, then is defined by the relation (P denoting the power): P = (6OO.PWR..q) / Speed, which is adapted to apply a constant power to the motors 130, 140, and the pressure then drops to reach a zero value when the speed exceeds a maximum threshold.

The corresponding algorithm performs a power limitation, to preserve the life of the motors 130, 140 as well as limit the heating of the oil.

Depending on the pressure and the speed, the curved part of the diagram illustrated in Figure 16 represents a constant power (P = f (flow, pressure)). Thus the central unit 296 limits the power. In other words, the electronics of the central unit 296 limit the pressure applied to the motors 130, 140 when the speed of the vehicle increases.

The piloting envelope illustrated in FIG. 16 is therefore defined by:

the maximum pressure given by the pump 200 (function of u maximum for the pump 200 or the motors 130, 140) defined by the pump control cylinder 270 and / or by the software integrated in the central unit 290,

- the maximum speed for the motors 130, 140, defined by the maximum displacement of the pump 200, and

- the curved part illustrated in FIG. 16 which corresponds to a calculated maximum power (defined by the software integrated in the central unit 290).

The regulation illustrated in FIG. 16 corresponds to a maximum thrust in the context of the application targeted by the present invention.

PLURALITY OF STATES

According to an optional feature of the invention, the central unit 290 offers the driver's choice of assistance in several states, for example in 3 states: a first state without thrust generated by the motors 130, 140, a second state of maximum thrust generated by the motors 130, 140 and a third state of modulated thrust.

Concretely, the aforementioned choices can be offered to the driver, within the framework of an application of the invention, in the form of an invitation to choose via the interface 292 between three selections "0", "1" or "2".

The selection "0" corresponds to a slight push preferably corresponding to a fixed pressure applied to the motors 130, 140 in the case of a pressure control. Alternatively it may be a slight pull or a slight restraint. The pressure applied to the motors 130 and 140 can also be just adapted to overcome the inertia and the friction of the motors 130, 140 and then correspond to zero thrust or zero restraint. This selection "0" can correspond in this case to an electronic freewheeling form.

Selection "1" corresponds to maximum traction power. This selection “1” corresponds to the control according to the maximum power curve illustrated in FIG. 16. It is chosen only in the event of a climb.

Selection "2" corresponds to a thrust modulated according to the slope. It is chosen downhill only to operate restraint.

FIG. 13 illustrates a choice offered to the driver via the interface 292 according to the following methods:

- in forward gear, the driver can choose from the aforementioned selections, the “0” or “1” mode uphill, the “0” mode on the flat, the modulated “2” mode or the slightly restrained “0” mode, or even the "2" retained mode downhill.

- in reverse, the driver can choose from the aforementioned selections, the "0" slight restraint mode on uphill, the "1" full power mode on the flat, the "1" full power mode downhill.

Alternatively, the interface 292 may only offer the driver control in the form of a choice limited to the above-mentioned "0" or "2" states. In this context the selection “0” can be a weak push or freewheeling and the selection “2” can be a push proportional to the slope, positive or negative (traction or restraint).

MODULATED TRACTION

To achieve the modulated traction, the central unit 290 applies a corrective coefficient linked to the inclination data coming from the inclinometer 296. Thus the central unit 290 applies a fraction of the maximum available power, which depends on the slope measured. As can be seen in FIG. 17, the central unit in this case defines a 3-dimensional regulation envelope. The 3 input parameters correspond to the torque (or pressure), the speed and the incline.

For several inclinations, the central unit 290 defines coefficients a modulating the applied pressure. For example for a slope at -7 °, the central unit 290 can give a thrust of 70% of the maximum power curve.

In FIG. 17, the curve admitting a pressure of 350 bars is the maximum power curve. The second curve is a curve corrected for the coefficient and obtained for the slope value of the moment corresponding to a slope of the terrain measured.

TRANSITION

In the event of a transition from the flat or ascent corresponding to a traction, to a descent corresponding to a restraint (which restraint can be operated at full power), the central unit 290 preferably uses an algorithm which performs a smooth transition, making a progression, for example linear, between the state before the change applied for the flat or the climb and the state after the change corresponding to a descent, over a time range t, as illustrated in FIG. 18. This arrangement is applies generally when the inclination data changes suddenly, whether in the flat direction or ascent to a descent or vice versa in the descent direction to a flat or ascent.

FIG. 18 illustrates the transition applied within the framework of the invention during a rapid passage requested between a traction regime and a restraint regime. Typically the central unit 290 then requires that the pump 200 by rotating at the same speed and in the same direction, proceeds from a positive pressure to a negative pressure. This transition from one pressure to another with progressiveness over a time range t, is effected for example over 2 seconds.

The central unit 209 defines a maximum speed of pressure change. This parameter can be entered by the user via the man-machine interface 292. For example, by entering the "2 seconds" parameter, the driver forces the system to calculate the speed of change of displacement of pump 200 to go from one full positive power to full negative power in 2 seconds. Thus for a given speed (by the wheels) and a given instantaneous pressure, the central unit 290 defines a speed of change.

ADVANTAGES OF THE INVENTION

The present invention makes it possible to have an assisted secondary axle, which does not disturb the driving required by the master axle controlled by the driver.

It allows you to drive faster safely downhill, knowing that the secondary axle holds and does not risk turning the machine over.

APPLICATIONS

The present invention applies to all machines, in particular trailers or towed tools which must be assisted.

It generally applies to any driving axle of a machine that one wishes to assist.

As indicated previously, the present invention applies in particular to a hydraulic transmission machine comprising an axle controlled by the driver, which is the master axle, and another secondary axle which is automatically follower according to the invention. The master axle can be of hydraulic transmission, directly controlled by the driver.

The invention applies to a machine which has a mechanical transmission on an axle, and an assisted axle, for example a tractor (assisted front axle, mechanical rear axle), a harvester, a truck, a self-propelled machine.

The invention applies to a trailer or a trailed wheeled implement, for example a wheeled plow or a potato harvester trailed behind a tractor.

Of course the present invention is not limited to the embodiments which have just been described, but extends to all variants in accordance with its spirit.

In particular, the present invention is not limited to the means described above and illustrated in the appended figures for composing the pressure reducing assembly 275, the pressure limiting devices 278, 279 and the switching assembly 280, but extends to all functionally equivalent means.

The present invention preferably applies, but is not limited to, assistance circuits incorporating hydraulic machines, forming a pump or motor, of the radial piston type.

The present invention preferably applies, but is not limited to, towed vehicles, in particular agricultural towed vehicles.

According to other advantageous features of the invention:

the central unit 290 uses the signal from a service brake pressure sensor and controls the system so as not to push the vehicle, in particular the tractor or the towed element, when braking is detected,

- the central unit 290 uses the signal from an oil temperature sensor and it controls the system to operate a disengagement of the assistance in the event of a temperature above a threshold. In this case, preferably, information is made available to the driver before reaching this cut-off threshold (for example in the form of a flashing of a lamp signaling the presence of a fault). This arrangement can be used to directly control the cooling device and, if necessary, to remove the thermostatic probe at the input of the refrigerant module.

- the central unit 290 uses the signal from a speed sensor in the wheels and controls the system to authorize dynamic engagement regardless of the direction of travel. If necessary, the central unit also uses the signal from this wheel speed sensor to avoid engaging the assistance at too high a speed. The central unit 290 can, for example, allow dog clutching up to 8 km / h (working speed in difficult terrain). It can cause the assistance to be cut in the event of overspeed (identical number of revolutions at the wheel regardless of the tires fitted). It can also support a speed sensor option to detect the direction of travel compared to a drawbar sensor.

- Within the framework of the invention, it is also possible to envisage eliminating the pressure sensor on the high pressure supply branch in forward gear since such a sensor is redundant with the speed measurement and in this sense is not not indispensable.

Claims (27)

1. A system for assisting the drive of a secondary axle (23, 24) fitted to a vehicle also comprising a main axle, which system comprises a pump (200) driven by a drive such as a heat engine of the vehicle, a hydraulic motor-forming machine (130, 140) equipping each driving wheel (23, 24) of the secondary axle and a steering unit (290), characterized in that the main axle is a driven driving axle and the secondary axle is a motorized follower axle and that it comprises control means (290, 292) adapted to automatically control and modulate the thrust applied by the drive wheels of the secondary axle (23, 24) according to the configuration of the terrain, including the amplitude and direction of the terrain slope, so that the driving force generated by the thrust applied by the drive wheels of the secondary axle, combined with the driving force induced by the slope of the terrain on which circulates the vehicle, does not disturb the driving force generated by the main driven axle. 2. System according to claim 1, characterized in that it comprises a control unit which controls the main driving axle (P) and a central unit (ECU2) which controls the secondary axle (S). 3. System according to one of claims 1 or 2, characterized in that the secondary axle is placed on a towed vehicle or trailer, while the main axle is placed on a tractor vehicle. 4. System according to one of claims 1 to 3, characterized in that the control means adapted to automatically control and modulate the thrust applied by the drive wheels of the secondary axle according to the configuration of the terrain are also suitable to operate this control and this modulation according to the load on the secondary axle and the configuration of the element carried by the secondary axle. 5. System according to one of claims 1 to 4, characterized in that it comprises a man / machine interface (292) connected to a central unit (290) and allowing the user to configure different operating parameters of the system. 6. System according to one of claims 1 to 4, characterized in that it comprises a tilt sensor (296) for measuring the slope on which the vehicle is moving. 7. System according to one of claims 1 to 6, characterized in that the control means (290, 292) are adapted so as not to apply downward thrust on the secondary axle (23, 24), when the vehicle is traveling on a negative or decreasing slope. 8. System according to one of claims 1 to 7, characterized in that the control means (290, 292) are adapted to apply restraint to the secondary axle (23, 24), when the vehicle is moving on a negative or decreasing slope. 9. System according to one of claims 1 to 8, characterized in that the control means (290, 292) are adapted to define either a torque control on the wheels (23, 24), by a pressure control of the pump (200), or a speed control at the wheels (23, 24), defined by a control in the displacement position of the pump (200) to which the speed of the secondary wheels (23, 24) is linked, in defining a speed difference, positive or negative, between the wheels (23, 24) of the secondary axle and a speed reference representative of the driven machine which corresponds to the master element of the vehicle. 10. System according to one of claims 1 to 9, characterized in that the control means adapted to automatically control and modulate the thrust applied by the drive wheels of the secondary axle according to the configuration of the terrain are also suitable to operate this control and this modulation as a function of the load on the secondary axle and that the load represents the state of charge of the secondary element of the vehicle, that is to say if the secondary element of the vehicle is more or less loaded, for example the filled or empty state of a trailer such as a towed implement. 11. System according to one of claims 1 to 10, characterized in that the control means adapted to automatically control and modulate the thrust applied by the drive wheels of the secondary axle according to the configuration of the terrain are also suitable to operate this control and this modulation according to the configuration of the secondary vehicle element and the configuration of the secondary vehicle element corresponds: - on the one hand to the slope, according to a parameter chosen for example in the group comprising: a configuration defined from a cartography and / or a GPS position of the machine, or by a sensor, in particular a chosen sensor in the group comprising an inclination sensor (296) or a force sensor on the drawbar or a coupling angle sensor or by the indication of an inertial unit, - on the other hand to machine data, according to a parameter chosen for example from the group comprising data coming from a turn sensor, from an inertial unit, from a speed change, the fact that a speed is engaged or not or a speed indication representative of the speed of the vehicle master element, to define a target speed of the secondary wheels. 12. System according to one of claims 1 to 11, characterized in that the control means (290, 292) are adapted to define a change in thrust as a function of the slope on which the vehicle is moving, for example defining a first thrust value when the vehicle is traveling on a flat surface and a second thrust value different from the first value when the vehicle is traveling on a slope. 13. System according to one of claims 1 to 12, characterized in that the control means (290, 292) are adapted to define a change in thrust modulated as a function of at least one parameter other than the slope, by example the deployment of accessories such as spray bars. 14. System according to one of claims 1 to 13, characterized in that the control means (290, 292) are adapted to compare the slope on which the vehicle is moving relative to a negative tilt threshold corresponding to a descending slope and define two possible states: traction when the vehicle is moving on a flat or uphill slope and restraint when the vehicle is descending on a negative slope. 15. System according to one of claims 1 to 14, characterized in that the control means (290, 292) are adapted to compare the slope on which the vehicle is moving with respect to two inclination thresholds and define three Possible states: weak traction on the flat, strong traction uphill and restraint downhill. 16. System according to one of claims 1 to 14, characterized in that the control means (290, 292) are adapted to impose a thrust at the motors (130, 140) proportional to the slope, positive or negative , and thus impose a traction or restraint proportional to the slope on which the vehicle is moving. 17. System according to one of claims 1 to 16, characterized in that the control means (290, 292) are adapted to take into account braking of the master axle of the vehicle. 18. System according to claim 17, characterized in that the control means (290, 292) are adapted to impose a restraint mode with a constant restraint value when braking of the master axle of the vehicle is detected. 19. System according to one of claims 1 to 17, characterized in that when the control means (290, 292) detect a braking mode of the master axle of the vehicle, they maintain the previous state, for example traction or restraint, then modifying if necessary the corresponding value of the thrust, traction or restraint, in particular to ensure a hill start or a restraint when starting downhill. 20. System according to one of claims 1 to 19, characterized in that the control means (290, 292) are adapted to define 4 target values of pressure on the motors (130, 140) or of speed difference between the wheels (23, 24) of the secondary element of the vehicle and the wheels of the master element of the vehicle: a first target value of pressure or speed difference in traction when moving on the flat, a second target value of pressure or of speed difference in traction during an uphill movement, a third target value of negative pressure or speed difference of restraint during movement on the flat accompanied by braking and a fourth target value of negative pressure or holding speed difference when traveling downhill. 21. System according to one of claims 1 to 20, characterized in that the control means (290, 292) are adapted to exploit the slope thresholds defined with hysteresis, to avoid jolts on the vehicle. 22. System according to one of claims 1 to 21, characterized in that the control means (290, 292) implement a calibration process of an inclinometer (196) to define at which output value of the inclinometer corresponds to a movement of the vehicle on the flat. 23. System according to one of claims 1 to 22, characterized in that the control means (290, 292) are adapted to define a constant and maximum pressure on the motors (130, 140) below a speed threshold, a constant power over a speed range, and a pressure which drops to reach a zero value when the speed exceeds a maximum threshold. 24. System according to one of claims 1 to 23, characterized in that the control means (290, 292) are adapted to offer the driver's choice of assistance according to several states, for example to offer the driver's choice of assistance according to 3 states: a first state without thrust generated by the motors (130, 140), a second state of maximum thrust generated by the motors (130, 140) and a third state of modulated thrust. 25. System according to one of claims 1 to 24, characterized in that the control means (290, 292) are adapted to achieve a smooth transition, by making a progression, for example linear, between the state before a change applied for a dish or an ascent and the state after the change corresponding to a descent, or vice versa, over a predetermined time range t. 26. A method of assisting in the driving of a secondary axle fitted to a vehicle comprising a main axle, using a system comprising a pump (200) driven by a drive such as a heat engine of the vehicle, a hydraulic machine (130, 140) forming an engine equipping each driving wheel of the secondary axle, and a steering unit, characterized in that the main axle is a driven driving axle and the secondary axle is a powered follower axle and that it comprises steps consisting in automatically controlling the thrust applied by the drive wheels of the secondary axle according to the configuration of the terrain, in particular the amplitude and the direction of the slope of the terrain, so that the driving force generated by the thrust applied by the drive wheels of the secondary axle, combined with the engine force induced by the slope of the terrain on which the vehicle travels, does not disturb the driving force ge born by the main driven axle. 27. Vehicle equipped with a system according to one of claims 1 to 25 and / or implementing the method according to claim 26.