Multifunctional and electro-assisted hydrostatic steering system

Abstract

A steering apparatus (100) is described, comprising: at least one hydraulic actuator (105) adapted to vary the orientation of at least one directional member (110) of a vehicle, a manual steering member (135), a first command device (140) adapted to command the hydraulic actuator (105), said first command device (140) comprising a first hydrostatic steering unit (150) hydraulically connectable to the hydraulic actuator (105), a first transmission shaft (145) adapted to mechanically connect the manual steering member (135) with the first hydrostatic steering unit (150), and a first electric motor (200), adapted to apply torsional torques to the first transmission shaft (145), a second command device (215) adapted to command the hydraulic actuator (105), said second command device (215) comprising a second hydrostatic steering unit (225), hydraulically connectable to the hydraulic actuator (105), a second transmission shaft (220) mechanically connected to the second hydrostatic steering unit (225), and a second electric motor (265), adapted to apply torsional torques to the second transmission shaft (220), a valve system (275) adapted to hydraulically connect the first hydrostatic steering unit (150) and / or the second hydrostatic steering unit (225) with the hydraulic actuator (105), and an electronic control system (500) configured to control the first electric motor (200), the second electric motor (265) and the valve system (275).

Description

[0001] MULTIFUNCTIONAL AND ELECTRO-ASSISTED HYDROSTATIC STEERING SYS¬

[0002] TEM

[0003] Technical field

[0004] The present invention relates to a steering apparatus for vehicles, in particular but not necessarily for agricultural vehicles and / or heavy-duty vehicles, such as for example tractors or earthmoving machines. More particularly, the invention relates to a hydrostatic and electro-assisted steering apparatus that allows different modes of operation.

[0005] Prior art

[0006] As is known, a hydrostatic steering apparatus generally comprises a hydraulic actuator, for example a double-acting hydraulic jack, adapted to vary the orientation of at least one directional member of the vehicle, for example a pair of steering wheels, a manual steering member, for example a steering wheel, adapted to be moved by a driver, and a command device adapted to command the hydraulic actuator in response to the movement imparted to the manual steering member.

[0007] The command device usually comprises a hydrostatic steering unit, also known as a “hydroguide”, hydraulically connected to the hydraulic actuator, and a transmission shaft adapted to mechanically connect the hydrostatic steering unit to the manual steering unit. In the event that it is electro-assisted, the command device also comprises an electric motor, which is mechanically connected to the transmission shaft, so as to be able to apply torsional torques to the latter that put it in rotation and / or that contribute to putting it in rotation, assisting or contrasting (as necessary) the torsional forces applied manually by the driver.

[0008] The operation of the electric motor is usually managed by an electronic control and command unit, based on measurement signals from at least two sensors.

[0009] One of these sensors is a sensor adapted to measure the angular position of the rotor of the electric motor with respect to the corresponding stator, which can be advantageously used by the electronic control and command unit, for example, to establish the correct switching instants of the electric voltage on the stator windings.

[0010] The second sensor is instead a sensor adapted to measure the torque that the driver transmits to the drive shaft, which can be used by the electronic control and command unit, for example, to establish the modulus (intensity) and / or the direction of the auxiliary torsional torque that must be generated by the electric motor. A drawback of these conventional steering devices concerns the steering ratio, that is, the ratio between the movement (e.g. rotation) imparted by the driver to the manual steering unit (e.g. steering wheel) and the consequent variation in orientation / inclination of the directional member of the vehicle (e.g. steering wheels).

[0011] To ensure greater driving precision and stability on smooth and high-speed roads, it is generally advisable to have a relatively low steering ratio, while to ensure greater manoeuvrability on uneven and / or low-grip terrain it is generally advisable to have a higher steering ratio.

[0012] Therefore, especially but not exclusively on an agricultural vehicle, a heavy-duty vehicle or any other vehicle intended to move on various surfaces, for example on the road and off-road, it would be advisable to have a steering apparatus capable of operating with a steering ratio with adjustable value depending on the uses.

[0013] Except that, the steering devices outlined above are generally characterized by a substantially constant steering ratio that requires a compromise.

[0014] Another drawback of the conventional steering devices consists in the fact that, by maintaining a mechanical / hydraulic connection between the manual steering member (e.g. the steering wheel) and the directional member of the vehicle (e.g. the steering wheels), they prevent or in any case make it more hard to implement autonomous or remote-controlled driving systems.

[0015] Explanation of the invention

[0016] An object of the present invention is to overcome or at least effectively mitigate the aforementioned drawbacks of the known technique, preferably within the scope of a simple, rational, and relatively inexpensive solution.

[0017] These and other objects are achieved thanks to the characteristics of the invention reported in the independent claims. The dependent claims outline preferred and / or particularly advantageous aspects of the invention which however are not strictly required for the implementation thereof.

[0018] In particular, an embodiment of the present invention makes available a steering apparatus comprising:

[0019] - at least one hydraulic actuator adapted to vary the orientation of at least one directional member of a vehicle, for example of a pair of steering wheels,

[0020] - a manual steering member, for example a steering wheel, - a first command device adapted to command the hydraulic actuator, said first command device comprising a first hydrostatic steering unit (or hydroguide), formed for example by a rotary distributor and metering unit, hydraulically connectable to the hydraulic actuator, a first transmission shaft adapted to mechanically connect the manual steering member with the first hydrostatic steering unit, and a first electric motor (e.g. brushless motor), installed for example coaxially on the first transmission shaft, adapted to apply torsional torques to the first transmission shaft,

[0021] - a second command device adapted to command the hydraulic actuator, said second command device comprising a second hydrostatic steering unit (or hydroguide), formed for example by a rotary distributor and metering unit, hydraulically connectable to the hydraulic actuator, a second transmission shaft mechanically connected to the second hydrostatic steering unit, and a second electric motor (e.g. brushless motor), installed for example coaxially on the second transmission shaft, adapted to apply torsional torques to the second transmission shaft,

[0022] - a valve system, preferably electro-actuated, adapted to hydraulically connect the first hydrostatic steering unit and / or the second hydrostatic steering unit with the hydraulic actuator, and

[0023] - an electronic control system configured to control the first electric motor, the second electric motor and the valve system.

[0024] Thanks to this solution, the steering apparatus is effectively able to operate in different operating modes, thus being more versatile than the known steering apparatuses and overcoming their limits.

[0025] For example, one aspect of the invention provides that the valve system may be controlled to assume at least three operating configurations (alternatively or selectively), of which:

[0026] - a first operating configuration, wherein the hydraulic actuator is hydraulically connected to the first hydrostatic steering unit and is hydraulically separated from the second hydrostatic steering unit,

[0027] - a second operating configuration, wherein the hydraulic actuator is hydraulically connected to both the first hydrostatic steering unit and the second hydrostatic steering unit,

[0028] - a third operating configuration, wherein the hydraulic actuator is hydraulically separated from the first hydrostatic steering unit and is hydraulically connected to the second hydrostatic steering unit.

[0029] In this way, when the valve system is in the first operating configuration, the hydraulic actuator is operated only by the first command device, so that the steering apparatus can operate in substantially the same modes as a conventional hydrostatic and electro-assisted steering apparatus.

[0030] When the valve system is in the second operating configuration, the hydraulic actuator is operated by both the first command device and the second command device.

[0031] With the same displacement imparted to the manual steering member (e.g. with the same rotation imparted to the steering wheel), the second hydrostatic steering unit, driven by the corresponding second electric motor, can therefore be used to vary the amount of operating fluid (e.g. oil) that is sent to the hydraulic actuator, allowing an adjustment of the steering ratio.

[0032] In practice, the electronic control system can be configured to control the second electric motor in such a way as to adjust (e.g. dynamically) the relationship (the ratio / the mathematical function) between the displacement of the manual steering member (e.g. the rotation of the steering wheel) and the (e.g. angular) variation of the orientation of the directional member of the vehicle (e.g. of the steering wheels).

[0033] Finally, when the valve system is in the third operating configuration, the hydraulic actuator is operated only by the second command device, and therefore without any mechan- ical / hydraulic connection with the manual steering member, allowing a simpler implementation of autonomous and / or remote-controlled driving procedures and / or with different manual command than the manual steering member (e.g. Joystick).

[0034] Another advantage of the steering apparatus proposed above consists in that, thanks to the possibility of operating in different operating modes, it can be more easily integrated with advanced driving assistance systems, such as lane-keeping assist, and allows to provide the driver with a more realistic haptic feedback even in the execution of these systems.

[0035] In this regard, one aspect of the invention provides that the electronic control system may be configured to control the first electric motor so that it applies a torsional torque to the first transmission shaft at least when the first hydrostatic steering unit is hydraulically connected to the hydraulic actuator, for example when the valve system is in the first and / or second operating configuration, and possibly also when the first hydrostatic steering unit is hydraulically disconnected from the hydraulic actuator, for example when the valve system is in the third operating configuration.

[0036] In this way, the steering apparatus is advantageously able to provide assistance and / or effective haptic feedback to the driver in all operating modes, possibly also in those in which the steering is commanded exclusively by the second command device.

[0037] Another advantage of the invention consists in the greater operational safety of the steering apparatus which, thanks to the duplication (redundancy) of the command devices, is able to more effectively overcome the failure or malfunction of one of them.

[0038] According to one aspect of the invention, the electronic control system may be configured to control the first electric motor based on signals from a first sensor system.

[0039] This first sensor system may comprise one or more (or all) sensors selected from the group consisting of: an angular position sensor adapted to measure the angular position of a rotor of the first electric motor with respect to a stator thereof, a torque sensor adapted to measure the torsional torque applied manually to the first transmission shaft via the manual steering member, a steering sensor adapted to measure a steering angle of the vehicle, a speed sensor adapted to measure the speed of the vehicle.

[0040] The angular position sensor, which can be for example a so-called Resolver, can be advantageously used by the electronic control system to establish the correct switching instants of the electrical voltage on the stator windings.

[0041] The torque sensor, which may for example be a so-called Torque Angle Sensor (TAS), may be adapted to measure the phase shift between two adjacent segments of the transmission shaft which are mutually connected by a torsion bar.

[0042] In this way, by knowing the angular phase shift and the torsional elastic characteristic of the torsion bar, the electronic control system can in fact advantageously be able to calculate the torsional torque applied by the driver.

[0043] To measure the phase shift, the torque sensor may comprise two elements, of which a sensitive element and a reference element adapted to interact with the sensitive element, each of which is associated with a respective segment of the transmission shaft.

[0044] The torque sensor may be used by the electronic control system to calculate the (assistive or resistive) torsional torque to be applied to the first transmission shaft, and to control the operation of the first electric motor accordingly. In other words, the electronic control system can be configured to determine, based on the signal from the torque sensor, the torsional torque to be applied to the first transmission shaft, and to control the operation (e.g. power supply) of the first electric motor, so that the latter generates and applies the calculated torsional torque to the first transmission shaft.

[0045] In particular, the electronic control system may be configured to implement an adaptive algorithm that adjusts (instant by instant) the torsional torque generated and applied by the first electric motor to the first transmission shaft, based on the torsional torque value measured (instant by instant) by the torque sensor.

[0046] The other sensors can be used to refine and improve the determination of the (assistive or resistive) torsional torque.

[0047] The electronic control system can also be configured to use the torque sensor to know (instant by instant) the actual position of the manual steering member (e.g. steering wheel) in implementing an algorithm to automatically bring said manual steering member to a predetermined position (autocentring).

[0048] According to another aspect of the invention, the electronic control system may be configured to control the second electric motor based on signals from one or more peripheral device (e.g. Joystick) and / or a second sensor system.

[0049] The second sensor system may comprise one or more (or all) sensors selected from the group consisting of: an angular position sensor adapted to measure the angular position of a rotor of the second electric motor with respect to a stator thereof, a steering sensor adapted to measure a steering angle of the vehicle, a speed sensor adapted to measure the speed of the vehicle.

[0050] The angular position sensor, which can be for example a so-called Resolver, can be advantageously used by the electronic control system to establish the correct switching instants of the electrical voltage on the stator windings.

[0051] The steering sensor, which can be a so-called Wheel Angle Sensor (WAS), can instead be used by the electronic control system to perform and / or make possible autonomous and / or remote-controlled driving procedures, which can for example also use a geolocation system (e.g. GPS) of the vehicle.

[0052] Optionally, the second sensor system may also include one or more of the sensors belonging to the first sensor system, such as the mentioned torque sensor (e.g. TAS), for example to implement automatic steering ratio adjustment strategies.

[0053] Another aspect of the invention provides that the electronic control system can be configured to implement a field-oriented control algorithm, namely a Field Oriented Control (FOC) algorithm, to control the first and second electric motors.

[0054] This solution has the advantage of allowing the generation and application of particularly precise torsional torques.

[0055] According to a different aspect of the invention, the electronic control system may be configured to control the valve system based on command signals (e.g. manually generated) from a user interface, i.e. a Human Machine Interface (HMI), such as a dashboard or a touchscreen monitor.

[0056] In this way, the driver can choose the operating mode based on their needs.

[0057] Another aspect of the invention provides that the valve system can comprise:

[0058] - a first valve (e.g. solenoid valve) hydraulically connected to the hydraulic actuator and to both the first and second hydrostatic steering units and switchable between a first position, wherein it places the hydraulic actuator in hydraulic connection with the first hydrostatic steering unit hydraulically separating it from the second hydrostatic steering unit, and a second position, wherein it places the hydraulic actuator in hydraulic connection with the second hydrostatic steering unit hydraulically separating it from the first hydrostatic steering unit, and

[0059] - a second valve (e.g. solenoid valve) hydraulically connected to the hydraulic actuator and at least to the second hydrostatic steering unit and switchable between a first position, wherein it places the hydraulic actuator in hydraulic connection with the second hydrostatic steering unit, and a second position, wherein it hydraulically separates the hydraulic actuator from the second hydrostatic steering unit.

[0060] This aspect of the invention provides a particularly simple and effective solution to enable operation of the steering apparatus in the modes outlined above.

[0061] In this context, the first valve may comprise a spring adapted to hold it in the first position and an electromechanical transducer adapted, when electrically powered, to move it to the second position in contrast with said spring; while the second valve may comprise a spring adapted to hold it in the second position and an electromechanical transducer adapted, when electrically powered, to bring it to the first position in contrast with said spring. In this way, in the event of a malfunction of the electrical system and therefore of a failed power supply to the first and second valves, the latter are automatically set to the positions that correspond to the first operating configuration, ensuring that the steering apparatus can continue to operate safely under the control of the driver through the manual steering unit.

[0062] According to another aspect of the invention, the electronic control system may comprise a first electronic control unit configured to control at least the first electric motor, and a second electronic control unit configured to control at least the second electric motor.

[0063] These first electronic control unit and second electronic control unit may for example be implemented in a configuration selected from the group consisting of:

[0064] - two separate channels within a single electronic control board;

[0065] - two separate electronic control boards housed within a single casing; and

[0066] - two physically separate electronic control units.

[0067] The presence of two control units has the advantage of increasing the safety of the steering apparatus, as it allows to implement protection strategies that can deal with any failures or malfunctions of one of them, using the other.

[0068] Also to increase the safety and reliability of the steering apparatus, the first electronic control unit and the second electronic control unit can then be configured to cross-check data and calculations.

[0069] A further aspect of the invention provides that the hydraulic actuator may comprise a double-acting jack provided with a casing and a plunger which subdivides the inner volume of the casing into two separate operating chambers, each of which is hydraulically connectable by the valve system to the first and / or the second hydrostatic steering unit.

[0070] This embodiment provides a particularly simple and economical solution for making the hydraulic actuator.

[0071] Another embodiment may however provide that the hydraulic actuator comprises a first and a second double-acting jack, each of which is provided with a casing and a plunger which subdivides the inner volume of the casing into two separate operating chambers, the plungers of both the first and the second jack being mechanically joined together, the operating chambers of the first hydraulic jack being hydraulically connectable by the valve system to the first and / or to the second hydrostatic steering unit, the operating chambers of the second hydraulic jack being hydraulically connectable by the valve system to the first and / or to the second hydrostatic steering unit.

[0072] This form of implementation has the advantage of increasing the safety of the steering apparatus, also increasing the separation of the first and second command devices from the hydraulic point of view.

[0073] In general, it is then not excluded that the steering apparatus may comprise, instead of a single hydraulic actuator, a plurality of hydraulic steering actuators, for example of the type indicated above.

[0074] Another embodiment of the invention finally provides a method for operating the steering apparatus outlined above, comprising the steps of:

[0075] - selecting an operating configuration from: o a first operating configuration, wherein the hydraulic actuator is hydraulically connected to the first hydrostatic steering unit and is hydraulically separated from the second hydrostatic steering unit, o a second operating configuration, wherein the hydraulic actuator is hydraulically connected to both the first hydrostatic steering unit and the second hydrostatic steering unit, and o a third operating configuration, wherein the hydraulic actuator is hydraulically separated from the first hydrostatic steering unit and is hydraulically connected to the second hydrostatic steering unit,

[0076] - hydraulically connecting via the valve system the hydraulic actuator to the first and / or second hydrostatic steering unit based on the selected operating configuration,

[0077] - controlling the first and / or second electric motor so as to apply a torsional torque to the first and / or second transmission shaft respectively.

[0078] This embodiment essentially achieves the same effects and advantages outlined above with reference to the steering apparatus.

[0079] According to one aspect of the invention, the method may particularly provide for the step of switching the valve system from the first to the third operating configuration, passing through the second operating configuration, and the step of switching the valve system from the third to the first operating configuration, passing through the second operating configuration.

[0080] In this way it is advantageously possible to implement a smoother transition between operating modes, for example by maintaining consistent tactile feedback to the manual steering member (e.g. the steering wheel) and / or by preserving steering accuracy throughout the transition.

[0081] Obviously all other aspects of the invention outlined above with reference to the steering apparatus are intended to be applicable, mutatis mutandis, also to the operation method, and vice versa.

[0082] For example, the steering apparatus outlined above may provide that the relative electronic control system may be configured to switch the valve system from the first to the third operating configuration, passing through the second operating configuration, and to switch the valve system from the third to the first operating configuration, passing through the second operating configuration

[0083] Brief description of the drawings

[0084] Further features and advantages of the invention will be more apparent after reading the following description provided by way of a non-limiting example, with the aid of the figures illustrated in the attached drawings.

[0085] Figure 1 is a schematic view of a steering apparatus according to an embodiment of the present invention with the hydraulic part highlighted.

[0086] Figure 2 is a schematic view of the steering apparatus of Figure 1 with the electronic part highlighted.

[0087] Figure 3 is a variant of the steering apparatus of Figure 1 .

[0088] Detailed description

[0089] With the aid of the exemplary and non-limiting figures referred to above, a steering apparatus 100 for self-propelled vehicles is described, in particular but not exclusively for agricultural and / or heavy-duty vehicles adapted to move at low speed (typically below 60 km / h), such as for example agricultural tractors or earthmoving machines.

[0090] The steering apparatus 100 generally comprises a hydraulic actuator 105, which is adapted to drive one or more directional members 110 of the vehicle, causing a displacement thereof, typically an angular variation of orientation / inclination.

[0091] In the example illustrated, each directional member 110 is a steering wheel and the hydraulic actuator 105 is adapted to drive each steering wheel in a rotation about a respective steering axis, thereby varying the orientation thereof.

[0092] The hydraulic actuator 105 can be a double-acting hydraulic jack, which is provided with a casing (e.g. a cylinder) 115 and a relative plunger 120 which subdivides the inner volume of the casing 115 into two separate operating chambers 125 and 130.

[0093] The plunger 120 of the hydraulic jack can comprise two opposite rods, which are arranged parallel to the sliding direction of the plunger 120 in the casing 115, for example parallel to the axis of the cylinder, preferably coaxial with the latter.

[0094] Each of these rods protrudes outside the casing 115 and can be mechanically connected to a respective directional member 110, for example to a respective steering wheel, preferably through appropriate levers, so as to transform the axial movement of each rod into a corresponding movement of the respective directional member 110, for example into a rotation of the respective steering wheel about its own steering axis.

[0095] In this way, each displacement of the plunger 120, in one direction or in the opposite direction, corresponds to a change in the position of both directional members 110, for example a change in the steering angle of both steering wheels.

[0096] Although a solution with a single hydraulic actuator 105 has been illustrated, the steering apparatus 100 could comprise a plurality of hydraulic steering actuators 105, for example a plurality of double-acting jacks, each of which could be adapted to drive a respective directional member 110 of the vehicle, for example a respective steering wheel.

[0097] In other embodiments, each double-acting jack could be replaced by any other hydraulic motor, including those of the rotary type, which can be connected to the directional members 110 of the vehicle to cause it to steer in both directions.

[0098] Each double-acting jack could also be replaced by a pair of jacks as will be described below with reference to the variant illustrated in Figure 3.

[0099] The steering apparatus 100 further comprises a manual steering member 135, for example and preferably a steering wheel, which can be manually manoeuvred by a driver of the vehicle.

[0100] In other words, the manual steering member 135 may be manually displaced / moved by the driver of the vehicle, e.g. rotated, to command a variation in the position, e.g. inclina- tion / orientation, of the directional members 110.

[0101] For this purpose, the steering apparatus comprises a first command device 140.

[0102] This first command device 140 comprises a first transmission shaft 145 adapted to mechanically connect the manual steering member 135 (e.g. the steering wheel) to a first hydrostatic steering unit 150, also conventionally called “hydroguide”. The first hydrostatic steering unit 150 is generally adapted to feed an operating fluid, typically an incompressible fluid (e.g. oil), to each hydraulic actuator (motor) 105, so as to cause the actuation thereof, in response to a rotation of the transmission shaft 145 caused by the displacement of the manual steering member 135.

[0103] The operating fluid can be sent, under pressure, to the first hydrostatic steering unit 150 by a pump 155, for example a volumetric pump driven by the motor of the vehicle, which is adapted to withdraw said operating fluid from a tank 160.

[0104] The hydrostatic steering unit 150 may comprise an outer casing 165, which may be provided with an inlet port 170 hydraulically connected to the delivery of the pump 155, a discharge port 175 hydraulically connected to the tank 160, a first connection port 180 hydraulically connectable (as will be detailed below) with the hydraulic actuator 105, for example with the first chamber 125, and a second connection port 185 also hydraulically connectable with the hydraulic actuator 105, but for example with the second chamber 130.

[0105] The hydrostatic steering unit 150 may further comprise a hydraulic distributor 190 and a metering unit 195, both of which may be housed in the outer casing 165.

[0106] The hydraulic distributor 190 and the metering unit 195 (known per se) can both be of the rotary type and, in any case, can both be driven by the transmission shaft 145.

[0107] The hydraulic distributor 190 (here symbolically represented) may for example comprise a rotating group, rotatably received in the outer casing 165, which is provided with two cylinders coaxially inserted into each other.

[0108] A first of these cylinders (the inner one) is adapted to be connected to the transmission shaft 145, so as to be able to rotate on itself around its central axis, driven in rotation by the transmission shaft 145 itself.

[0109] The second of these cylinders (the outer one) is rotatably inserted on the first cylinder, to which it is constrained so as to allow mutual rotations around the common axis for a limited angle in both directions and, subsequently (after covering said angle), so that the two cylinders can rotate together with respect to the outer casing 165.

[0110] Suitable spring means can be provided to normally hold the second cylinder in a predetermined angular position (neutral) with respect to the first cylinder, preferably in a position that is central with respect to the angle of mutual rotation allowed.

[0111] Turning to the metering unit 195, it can be shaped as a sort of volumetric gear pump, for example as the one commonly called “gerotor”.

[0112] In particular, the metering unit 195 can comprise an annular lobe stator and a lobe rotor, placed inside the stator and meshing with it, which is slightly offset with respect to the stator and is adapted to be connected to the transmission shaft 145, so as to be able to be driven in rotation by the latter.

[0113] In the operation of the hydrostatic steering unit 150, as long as the transmission shaft 145 is stationary, the second (outer) cylinder of the distributor 190 is in the already mentioned neutral position with respect to the first (inner) cylinder, as schematically represented in Figure 1 .

[0114] In this neutral position, the pressurized operating fluid is not fed to the hydraulic actuator 105, in the example neither to the first nor to the second operating chamber 125 and 130, which remains inactive.

[0115] For example, when the second cylinder is in the neutral position, the connection ports 180 and 185 may be hydraulically separated from both the inlet port 170 and the discharge port 175.

[0116] At the moment when the transmission shaft 145 is rotated in one of the two directions, the first cylinder of the distributor 190 initially performs a small rotation with respect to the second cylinder, which therefore assumes a first operating position in which pressurized operating fluid (e.g. oil) can be sent to the hydraulic actuator 105, to cause the movement of each directional member 110, for example the steering of each steering wheel, in a first direction.

[0117] For example, when the second cylinder is in the first operating position (to the right of the diagram), the inlet port 170 may be in hydraulic communication, through the metering unit 195, with the first connection port 180, while the second connection port 185 may be in hydraulic communication with the discharge port 175.

[0118] In this way, by continuing to rotate the transmission shaft 145 in the same direction, the metering unit 195 can transfer to the first chamber 125 of the hydraulic actuator 105 an amount of pressurized fluid that is proportional to the completed rotation.

[0119] When the transmission shaft 145 is stopped, the second (outer) cylinder of the distributor 190 is returned to the neutral position, for example by means of the already mentioned spring means, so as to stop sending pressurised fluid to the hydraulic actuator 105, and therefore stop the movement of the directional members 110, for example of the steering wheels, in the configuration reached.

[0120] By reversing the direction of rotation of the transmission shaft 145, the operation of the hydrostatic steering unit 150 is the same, with the only difference that the second (outer) cylinder of the distributor 190 moves to a second operating position with respect to the first (inner) cylinder, in which pressurized operating fluid can still be sent to the hydraulic actuator 105, but to cause the movement of each directional member 110, for example the steering of each steering wheel, in the opposite direction to the previous one.

[0121] For example, when the second cylinder is in the second operating position (to the left of the diagram), the inlet port 170 may be in hydraulic communication, through the metering unit 195, with the second connection port 185, while the first connection port 180 may be in hydraulic communication with the discharge port 175.

[0122] The first command device 140 further comprises an electric motor 200, preferably a permanent magnet electric motor, for example a brushless motor, which is associated with the transmission shaft 145 so as to be adapted to apply torsional torques thereon.

[0123] In particular, the electric motor 200 can comprise a rotor of annular shape, which can be coaxially inserted and rotatably constrained directly to the transmission shaft 145, and a stator, also of annular shape, which coaxially surrounds the rotor.

[0124] The stator can be fixed to a support structure (not shown), for example a casing, to which the transmission shaft 145 can also be rotatably coupled.

[0125] Permanent magnets or rotor electric windings can be directly applied to the rotor, while stator electric windings can be directly applied to the stator, which can be connected to an electrical power source, for example to a vehicle battery and / or an alternator, by means of appropriate electrical connections.

[0126] The first command device 140 may further comprise one or more sensors useful for controlling the electric motor 200.

[0127] These sensors include a torque sensor 205, for example a so-called Torque Angle Sensor (TAS), adapted to measure the torsional torque applied manually by the driver to the transmission shaft 145 through the manual steering member 135 (e.g. the steering wheel).

[0128] To allow the operation of this torque sensor 205, the transmission shaft 145 can comprise (or be formed by) at least two segments, i.e. two axial portions, of which a first segment that can be mechanically connected to the first hydrostatic steering unit 150, and a second segment that can be mechanically connected to the manual steering member 135.

[0129] These two segments can be mechanically connected to each other by means of a torsion bar, which is adapted to allow them at least small relative angular phase shifts.

[0130] In practice, each segment of the transmission shaft 145 can in turn be considered a shaft, coaxial with the shaft defining the other segment, and joined to the latter by means of the torsion bar, so that the first and second segments are mutually integral in rotation, net of the aforesaid angular phase shifts.

[0131] In this regard, it is wished to specify that by “small” angular phase shifts are meant phase shifts which, for the normal torsional torques that are applied to the transmission shaft 145 in the operation of the steering apparatus 100, are nevertheless perceptible and measurable, for example greater than or equal to 0.1 ° sexagesimal.

[0132] At the same time it is preferable that such phase shifts, again for the normal torsional torques that are applied to the transmission shaft 145 in the operation of the steering apparatus 100, are not excessive, for example less than or equal to 10° sexagesimal.

[0133] To allow the angular phase shifts, the torsion bar can be torsionally more yielding with respect to the first and second segments of the transmission shaft 145, e.g., it can have a cross-section with a lower torsional moment of inertia with respect to the cross-sections of the first and the second segment.

[0134] In particular, the torsion bar can be shaped as an elongated body, i.e. as a further shaft, which can be arranged coaxially to the first and second segment, and can have a first axial end fixed (or at least integral in rotation) to the first segment and a second and opposite axial end fixed (or at least integral in rotation) to the second segment.

[0135] In some embodiments, the first and second segment can be axially inserted into each other at least partially, for example by a shape coupling that allows them to rotate mutually freely, and the torsion bar can be enclosed within a cavity obtained, in part, in the first segment and, in part, in the second segment.

[0136] In this context, the torque sensor 205 can be configured to measure the angular phase shift which, due to the effect of the torsion bar, the torsional torque applied by the driver causes between the first and second segments of the transmission shaft 145.

[0137] In this way, by knowing the angular phase shift and the torsional elastic characteristic of the torsion bar, it is possible to calculate the torsional torque applied by the driver.

[0138] To measure the phase shift, the torque sensor 205 may for example comprise two elements, of which a sensitive element and a reference element adapted to interact with the sensitive element, each of which is associated with a respective segment of the transmission shaft 145.

[0139] In addition to the torsional torque, the torque sensor 205 may advantageously also be able to measure the actual position of the manual steering member 135 (e.g. of the steering wheel).

[0140] The sensors of the first command device 140 may further include an angular position sensor 210, for example a Resolver, which is adapted to measure the angular position of the rotor of the first electric motor 200 with respect to the corresponding stator.

[0141] The steering apparatus 100 further comprises a second command device 215.

[0142] This second command device 215 comprises a second transmission shaft 220, which is mechanically connected to a second hydrostatic (or hydroguide) steering unit 225, but preferably separate and independent from the manual steering member 135.

[0143] Like the first hydrostatic steering unit 150, the second hydrostatic steering unit 225 is also generally adapted to feed an operating fluid, typically an incompressible fluid (e.g. oil), to each hydraulic actuator (motor) 105, so as to cause the actuation thereof.

[0144] The operating fluid can be sent, under pressure, to the second hydrostatic steering unit 225 by the same pump 155 which also feeds the first hydrostatic steering unit 150.

[0145] The second hydrostatic steering unit 225 may then similarly comprise an outer casing 230, which may be provided with an inlet port 235 hydraulically connected with the pump delivery 155, a discharge port 240 hydraulically connected to the tank 160, a first connection port 245 hydraulically connectable (as will be detailed below) to the hydraulic actuator 105, for example to the first chamber 125, and a second connection port 250 also hydraulically connectable to the hydraulic actuator 105, but for example to the second chamber 130.

[0146] The second hydrostatic steering unit 225 may further comprise a hydraulic distributor 255 and a metering unit 260, which may both be housed in the outer casing 230.

[0147] The hydraulic distributor 255 and the metering unit 260 (known per se) can both be of the rotary type and, in any case, can both be driven by the transmission shaft 220.

[0148] The hydraulic distributor 255 (here symbolically represented) may for example comprise a rotating group, rotatably received in the outer casing 230, which is provided with two cylinders coaxially inserted into each other. A first of these cylinders (the inner one) is adapted to be connected with the transmission shaft 220, so as to be able to rotate on itself around its central axis, driven in rotation by the transmission shaft 220 itself.

[0149] The second of these cylinders (the outer one) is rotatably inserted on the first cylinder, to which it is constrained so as to allow mutual rotations around the common axis for a limited angle in both directions and, subsequently (after covering said angle), so that the two cylinders can rotate together with respect to the outer casing 230.

[0150] Suitable spring means can be provided to normally hold the second cylinder in a predetermined angular position (neutral) with respect to the first cylinder, preferably in a position that is central with respect to the angle of mutual rotation allowed.

[0151] Turning to the metering unit 260, it can be shaped as a sort of volumetric gear pump, for example as the one commonly called “gerotor”.

[0152] In particular, the metering unit 260 can comprise an annular lobe stator and a lobe rotor, placed inside the stator and meshing with it, which is slightly offset with respect to the stator and is adapted to be connected to the transmission shaft 220, so as to be able to be driven in rotation by the latter.

[0153] In the operation of the second hydrostatic steering unit 225, as long as the transmission shaft 220 is stationary, the second (outer) cylinder of the distributor 255 is in the already mentioned neutral position with respect to the first (inner) cylinder, as schematically represented in Figure 1 .

[0154] In this neutral position, the pressurized operating fluid is not fed to the hydraulic actuator 105, in the example neither to the first nor to the second operating chamber 125 and 130, which remains inactive.

[0155] For example, when the second cylinder is in the neutral position, the connection ports 245 and 250 may be hydraulically separated from both the inlet port 235 and the discharge port 240.

[0156] When the transmission shaft 220 is rotated in one of the two directions, the first cylinder of the distributor 255 initially performs a small rotation with respect to the second cylinder, which therefore assumes a first operating position in which pressurized operating fluid (e.g. oil) can be sent to the hydraulic actuator 105, to cause the movement of each directional member 110, for example the steering of each steering wheel, in a first direction.

[0157] For example, when the second cylinder is in the first operating position (to the right of the diagram), the inlet port 235 may be in hydraulic communication, through the metering unit 260, with the first connection port 245, while the second connection port 250 may be in hydraulic communication with the discharge port 240.

[0158] In this way, by continuing to rotate the transmission shaft 220 in the same direction, the metering unit 260 can transfer to the first chamber 125 of the hydraulic actuator 105 an amount of pressurized fluid that is proportional to the completed rotation.

[0159] When the transmission shaft 220 is stopped, the second (outer) cylinder of the distributor 255 is returned to the neutral position, for example by means of the already mentioned spring means, so as to stop sending pressurised fluid to the hydraulic actuator 105.

[0160] By reversing the direction of rotation of the transmission shaft 220, the operation of the second hydrostatic steering unit 225 is the same, with the only difference that the second (outer) cylinder of the distributor 255 moves to a second operating position with respect to the first (inner) cylinder, in which pressurized operating fluid can still be sent to the hydraulic actuator 105, but to cause the movement of each directional member 110, for example the steering of each steering wheel, in the opposite direction to the previous one. For example, when the second cylinder is in the second operating position (to the left of the diagram), the inlet port 235 may be in hydraulic communication, through the metering unit 260, with the second connection port 250, while the first connection port 245 may be in hydraulic communication with the discharge port 240.

[0161] The second command device 215 further comprises an electric motor 265, preferably a permanent magnet electric motor, for example a brushless motor, which is associated with the transmission shaft 220 so as to be able to apply torsional torques thereon and consequently to rotate it.

[0162] In particular, the electric motor 265 can comprise a rotor, for example of annular shape, which can be coaxially inserted and rotatably constrained directly to the transmission shaft 145, and a stator, also of annular shape, which coaxially surrounds the rotor.

[0163] The stator can be fixed to a support structure (not shown), for example a casing, to which the transmission shaft 220 can also be rotatably coupled.

[0164] Permanent magnets or rotor electric windings can be directly applied to the rotor, while stator electric windings can be directly applied to the stator, which can be connected to an electrical power source, for example to a vehicle battery and / or an alternator, by means of appropriate electrical connections. The second command device 215 may further comprise one or more sensors useful for controlling the electric motor 265.

[0165] These sensors may include an angular position sensor 270, for example a Resolver, which is adapted to measure the angular position of the rotor of the electric motor 265 with respect to the corresponding stator.

[0166] In addition to what has been stated so far, the steering apparatus 100 may also comprise a steering sensor 310, which is configured to measure (directly) a value indicative of a steering angle of the vehicle, in particular with respect to a reference position, for example in which the vehicle on which the steering apparatus 100 is mounted moves along a substantially straight trajectory.

[0167] It is specified that steering angle means, for example, an angle of rotation of the steering wheels with respect to a substantially vertical axis, i.e. with respect to the steering axis. The steering sensor 310, which can be a rotary sensor with a hall or potentiometric effect, can be placed at a pin adapted to define the steering axis of one of the steering wheels with respect to a frame of the vehicle.

[0168] Alternatively, the steering sensor 310 may be an optical or inductive sensor configured to measure the relative position of the plunger 120 relative to the casing 115 of the hydraulic actuator 105.

[0169] The sensors of the steering apparatus 100 may also comprise a speed sensor (not illustrated) configured to measure a value indicative of a speed of (forward or reverse) movement of the vehicle.

[0170] The steering apparatus 100 further comprises a valve system 275, preferably of an electro-actuated type, which is adapted to connect the hydraulic actuator 105 selectively with the first hydrostatic steering unit and / or with the second hydrostatic steering unit.

[0171] In particular, the valve system 275 may be capable of assuming at least three operating configurations (alternatively or selectively).

[0172] In a first operating configuration of the valve system 275, the hydraulic actuator 105 is hydraulically connected to the first hydrostatic steering unit 150, while being hydraulically separated from the second hydrostatic steering unit 225.

[0173] For example, the first connection port 180 of the first hydrostatic steering unit 150 may be hydraulically connected to the first chamber 125 of the hydraulic actuator 105, and the second connection port 185 may be hydraulically connected to the second chamber 130; while the first and second connection ports 245 and 250 of the second hydrostatic steering unit 225 may both be hydraulically separated from the chambers 125 and 130 of the hydraulic actuator 105.

[0174] In a second configuration, the hydraulic actuator 105 is hydraulically connected to both the first hydrostatic steering unit 150 and the second hydrostatic steering unit 225.

[0175] For example, as in the previous configuration, the first connection port 180 of the first hydrostatic steering unit 150 may be hydraulically connected to the first chamber 125 of the hydraulic actuator 105, and the second connection port 185 may be hydraulically connected to the second chamber 130; and at the same time, the first connection port 245 of the second hydrostatic steering unit 225 may be hydraulically connected to the first chamber 125 of the hydraulic actuator 105, and the second connection port 250 may be hydraulically connected to the second chamber 130.

[0176] In a third operating configuration of the valve system 275, the hydraulic actuator 105 is hydraulically separated from the first hydrostatic steering unit 150, while being hydraulically connected to the second hydrostatic steering unit 225.

[0177] For example, the first and second connection ports 180 and 185 of the first hydrostatic steering unit 150 may both be hydraulically separated from the chambers 125 and 130 of the hydraulic actuator 105; while the first connection port 245 of the second hydrostatic steering unit 225 may be hydraulically connected to the first chamber 125 of the hydraulic actuator 105, and the second connection port 250 may be hydraulically connected to the second chamber 130

[0178] To achieve these operating configurations, the valve system may comprise two valves, preferably two solenoid valves, of which a first valve 280 and a second valve 285.

[0179] The first valve 280 is hydraulically connected to the hydraulic actuator 105 and to both the first hydrostatic steering unit 150 and the second hydrostatic steering unit 225.

[0180] It is switchable between a first position (shown in Figure 1 ), wherein it places the hydraulic actuator 105 in hydraulic connection with the first hydrostatic steering unit 150, hydraulically separating it from the second hydrostatic steering unit 225, and a second position (displaced to the right with respect to the figure), wherein it places the hydraulic actuator 105 in hydraulic connection with the second hydrostatic steering unit 225, hydraulically separating it from the first hydrostatic steering unit.

[0181] For example, in the first position, the first valve 280 hydraulically connects the first connection port 180 of the first hydrostatic steering unit 150 to the first chamber 125 of the hydraulic actuator 105, and the second connection port 185 to the second chamber 130; while being closed with respect to the first and second connection ports 245 and 250 of the second hydrostatic steering unit 225.

[0182] In the second position, vice versa, the first valve 280 can hydraulically connect the first connection port 245 of the second hydrostatic steering unit 225 to the first chamber 125 of the hydraulic actuator 105, and the second connection port 250 to the second chamber 130; while it can be closed with respect to the first and second connection ports 180 and 185 of the first hydrostatic steering unit 150.

[0183] The first valve 280 may comprise a spring 290 adapted to hold it normally in the first position and an electromechanical transducer 295 which, when electrically powered, is adapted to move it to the second position in contrast with said spring 290.

[0184] In this way, when it is not energized, i.e. when the electromechanical transducer 295 is not crossed by current, the first valve 280 will be in the first position; while when it is energized, i.e. when the electromechanical transducer 295 is crossed by current, the first valve 280 will be in the first position.

[0185] The second valve 285 is hydraulically connected to the hydraulic actuator 105 and to the second hydrostatic steering unit 225, while it can be hydraulically separated from the first hydrostatic steering unit 150.

[0186] It is switchable between a first position (displaced to the left with respect to Figure 1 ), wherein it places the hydraulic actuator 105 in hydraulic connection with the second hydrostatic steering unit 225, and a second position (illustrated in the figure), in which it hydraulically separates the hydraulic actuator 105 from the second hydrostatic steering unit 225.

[0187] For example, in the first position, the second valve 285 hydraulically connects the first connection port 245 of the second hydrostatic steering unit 225 to the first chamber 125 of the hydraulic actuator 105, and the second connection port 250 to the second chamber 130.

[0188] In the second position, vice versa, the second valve 285 can be closed both with respect to the first and with respect to the second connection port 245 and 250 of the second hydrostatic steering unit 225.

[0189] The second valve 285 may comprise a spring 300 adapted to hold it normally in the second position and an electromechanical transducer 305 which, when electrically powered, is adapted to move it to the first position in contrast with said spring 300.

[0190] In this way, when it is not energized, i.e. when the electromechanical transducer 305 is not crossed by current, the second valve 285 will be in the second position; while when it is energized, i.e. when the electromechanical transducer 305 is crossed by current, the second valve 285 will be in the first position.

[0191] Overall, therefore, when the first valve 280 is in the first position and the second valve 285 is in the second position (as illustrated in Figure 1 ), for example when neither of the two valves 280 and 285 is energized, the valve system 275 is in the first operating configuration, wherein the hydraulic actuator 105 is connected only to the first hydrostatic steering unit 150.

[0192] When the first valve 280 is in the first position and the second valve 285 is also in the first position, for example when the second valve 285 is energized while the first valve 280 remains unenergized, the valve system 275 is in the second operating configuration, wherein the hydraulic actuator 105 is connected to both the first and second hydrostatic steering units 150 and 225.

[0193] When the first valve 280 is in the second position, for example when the first valve 280 is energized, regardless of the position of the second valve 285, the valve system 275 is in the third operating configuration, wherein the hydraulic actuator 105 is connected only to the second hydrostatic steering unit 225.

[0194] As illustrated in Figure 2, the steering apparatus 100 is also equipped with an electronic control system 500, which is overall configured to control the first electric motor 200, the second electric motor 265 and the valve system 275.

[0195] This electronic control system 500 may comprise a first electronic control unit 505 configured to control at least the first electric motor 200, and a second electronic control unit 510 configured to control at least the second electric motor 265.

[0196] In general terms, the first and second electronic control units 505 and 510 can act by varying the power supply electrical parameters respectively of the first and second electric motors 200 and 265, such as current intensity (preferably in the d-q reference frame), voltage and / or switching frequency.

[0197] To allow to the first and second electric motors 200 and 265 to generate and apply precise torsional torques, the first and second electronic control units 505 and 510 may implement field orientation control algorithms, namely a Field Oriented Control (FOC) algorithm, which may involve the use of the angular position sensors 210 and 270 to know the position of the rotor.

[0198] In fact, the correct operation of the first and second electric motors 200 and 265, which can be for example three-phase synchronous permanent magnet electric motors, generally requires that the frequency of the currents of the phases is synchronous with the rotation of the rotor and that the magnetic field generated by the stator windings is always oriented corresponding to the angular position of the rotor.

[0199] Thus, the first and second electronic control units 505 and 510 may be configured to use the angular position sensor 210 and the angular position sensor 270, respectively, to establish the switching instants of the electrical voltage on the stator windings of the first and second electric motors 200 and 265, respectively.

[0200] The first and second electronic control units 505 and 510 may also use a Space Vector Modulation (SVM) technique to achieve efficient actuation of the first electric motor 200 and the second electric motor 265, respectively.

[0201] From a constructional point of view, the first electronic control unit 505 and the second electronic control unit 510 may be implemented in a configuration selected from the group consisting of: two separate channels within a single electronic control board; two separate electronic control boards housed within a single casing; and two physically separate electronic control units.

[0202] The presence of two independent electronic control units 505 and 510 provides the electronic control system 500 with a redundancy that can increase the safety of the steering apparatus 100.

[0203] In the event of a failure of one of the electronic control units 505 or 510, the other can in fact take over the critical functions necessary to maintain control of the steering.

[0204] In addition, the presence of two independent electronic control units 505 and 510 allows an exchange and cross-validation of calculations and commands that improves reliability. However, it is not excluded that, in other embodiments, the first and second electronic control units 505 and 510 can be integrated into a single electronic control unit, i.e. that the electronic control system 500 can comprise a single electronic control unit, for example the first and or second control units 505 and 510, which is configured to control both the first electric motor 200 and the second electric motor 265. The electronic control system 500 may then optionally also comprise a third electronic control unit 515, which is configured to control at least the valve system 275, for example by energizing or de-energizing the first and / or second valves 280 and 285.

[0205] From a constructional point of view, this third electronic control unit 515 can be implemented in a configuration selected from the group consisting of: a separate channel within the single electronic control board in which the channels defining the first electronic control unit 505 and second electronic control unit 510 are also present; a separate electronic control board housed within the same casing in which the electronic control boards defining the first electronic control unit 505 and second electronic control unit 510 are housed; an electronic control unit physically separate from those defining the first electronic control unit 505 and second electronic control unit 510.

[0206] However, it is not excluded that, in some embodiments, the valve system 275 can be controlled, in the same modes as will be described below with reference to the third control unit 515, by at least one of the first and second electronic control units 505 and 510, i.e. that at least one of said first and second electronic control units 505 and 510 can be configured to also control the valve system 275, in the modes that will be detailed below. It is also not excluded that the electronic control system 500 may comprise a single electronic control unit configured to perform the functions (which will be detailed later) of all three electronic control units 505, 505 and 510 mentioned above.

[0207] Given this premise, the third electronic control unit 515 can be configured to allow a driver to select the operating configuration he or she wishes to adopt, for example by choosing between the first, second and third operating configurations outlined above.

[0208] To allow the driver to make this selection, the steering apparatus 100 may comprise a user interface 520, i.e. a Human Machine Interface (HMI), such as a dashboard, a touchscreen monitor or any other device capable of providing input command signals to the third electronic control unit 515.

[0209] Based on the choice made by the driver through the user interface 520, the third electronic control unit 515 can then be configured to command / control the valve system 275, for example to energize or de-energize the first and / or second valves 280 and 285, so as to move it to the chosen configuration.

[0210] It should be pointed out that, in some embodiments and / or in some circumstances, the choice of the operating configuration to be assumed by the valve system 275 could be performed automatically by the third electronic control unit 515, for example based on preset logics involving operating parameters of the steering apparatus 100 and / or of the vehicle on which it is installed.

[0211] It should also be pointed out that, in the event that the valve system 275 must be switched from the first operating configuration to the third operating configuration or, vice versa, must be switched from the third operating configuration to the first operating configuration, the third electronic control unit 515 can be configured, in both cases, to pass through the second operating configuration.

[0212] In this way it is in fact advantageously possible to implement a smoother transition between the operating modes, for example by preserving the precision of the steering throughout the transition.

[0213] When the valve system 275 is in the first operating configuration, the hydraulic actuator 105 is operated only by the first command device 140, whereby the second command device 215 (i.e. the second electric motor 265) can be kept inactive.

[0214] In this first operating configuration, the first electronic control unit 505 can be configured to use the torque sensor 205 to acquire / determine the torsional torque applied by the driver to the first transmission shaft 145 via the manual steering member 135 (e.g. steering wheel).

[0215] On the basis of this torque information, and possibly based on other information provided by other sensors, for example the speed of forward travel of the vehicle and / or the steering angle, the first electronic control unit 505 can be configured to control the operation of the first electric motor 200, preferably the direction and modulus (intensity) of the torsional torque that it generates and applies to the transmission shaft 145.

[0216] For example, the first electronic control unit 505 may be configured to determine, based on the torque information provided by the torque sensor 205, the direction and modulus of the torsional torque that the electric motor 200 must generate and apply to the transmission shaft 145.

[0217] Based on the direction and the torque value thus established, the first electronic control unit 505 can then be configured to supply the electric motor 200 with an electric current such as to obtain therefrom a torsional torque having the desired direction and modulus. In this regard, it should be noted that, depending on the needs, the electric motor 200 can be commanded to generate on the transmission shaft 145 a torque that is oriented in the same direction as that imparted by the driver (assistive), i.e. in the same direction of rotation in which the driver wants to rotate the transmission shaft 145, or in the opposite direction to that imparted by the driver (resistive), i.e. in a direction of rotation opposite to that in which the driver wants to rotate the drive shaft 105.

[0218] For example, the assistive torsional torque generated by the first electric motor 200 may generally be proportional to the torque applied by the driver, but the ratio / proportionality function may decrease with increasing vehicle speed, as measured by, for example, the speed sensor, for better stability; it may vary based on the current steering angle, as measured with, for example, the steering sensor 310; and may vary based on a selection, made by the driver, for example, through the user interface 520, of different levels of assistance (e.g., soft, medium, hard).

[0219] Furthermore, the algorithms and / or logics used by the first electronic control unit 505 to establish the magnitude and direction of the torsional torque, can be designed not only with a view to reducing the effort required of the driver to steer the vehicle but also with a view to providing haptic feedback that makes driving more natural / realistic and therefore more precise and safer.

[0220] Alternatively or in addition, the torque sensor 205 can be used by the first electronic control unit 505 to know instant by instant the actual position of the manual steering member 135 (e.g. of the steering wheel), for example to perform procedures that automatically return it to a predetermined position (autocentring).

[0221] In practice, the first electronic control unit 505 can command the first electric motor 200 to generate a torsional torque that returns the directional members 110 (e.g. the wheels) in a central position.

[0222] This centring torque can be modulated according to the vehicle speed (measured for example with the speed sensor) and the steering angle (measured for example with the steering sensor 310).

[0223] The autocentring function can also be adjusted or disabled as necessary, automatically by the first electronic control unit 505 or by the driver by means of a special manual command provided for example by the user interface 520.

[0224] On the other hand, when the valve system 275 is in the second operating configuration, both command devices 140 and 215 can contribute to operating the hydraulic actuator 105. In this second configuration, the first command device 140 can be controlled by the first electronic control unit 505 in substantially the same modes as described above with reference to the first configuration, which will therefore not be repeated.

[0225] The second electronic control unit 510 can be configured to simultaneously drive the second electric motor 265, causing the rotation of the second transmission shaft 220 and then the actuation of the second hydrostatic steering unit 225, in order to send to the hydraulic actuator 105, for example to the first or second operating chamber 125 or 130, an additional amount of operating fluid (e.g. oil).

[0226] This additional amount of operating fluid, which is added to that sent by the first hydrostatic steering unit 150, means that with the same rotation imparted by the driver to the first transmission shaft 145, through the manual steering member (e.g. steering wheel) 135, the variation in orientation / inclination of the directional members 110 (e.g. wheels) is greater than if the second command device 215 is inactive or isolated.

[0227] In practice, an increase in the steering ratio of the steering apparatus 100 is obtained, i.e. an increase in the ratio between the movement (e.g. the rotation) imparted by the driver to the manual steering member 135 (e.g. the steering wheel) and the angular variation of the orientation of the directional members 110 of the vehicle (e.g. the steering wheels). Accordingly, by appropriately adjusting the amount of operating fluid sent via the second hydrostatic steering unit 225, the second electronic control unit 510 is effectively able to dynamically vary / adjust the steering ratio of the steering apparatus 100, for example based on preset algorithms and / or logics.

[0228] For example, these preset algorithms / logics may for example provide that the second electronic control unit 510 varies the steering ratio based on one or more optional parameters, such as for example the speed and / or the actual steering angle of the vehicle, and / or based on a selection made by the driver, for example via the user interface 520 (e.g. on-road driving mode or off-road driving mode).

[0229] Alternatively or in addition, the second electronic control unit 510 can be configured to control the second electric motor 265 and, consequently, the second hydrostatic steering unit 225, so that the hydraulic actuator 105 causes the directional members 110 (e.g. the wheels) to reach a preset target of the actual steering angle.

[0230] For example, based on the position of the manual steering member 135 (e.g., based on the rotation of the steering wheel), the second electronic control unit 510 can establish a target of the steering angle and can command the second hydrostatic steering unit 225 so that the hydraulic actuator 105 makes the directional members 110 (e.g., the wheels) reach precisely that target.

[0231] In this context, the first electronic control unit 505 can simultaneously continue to control the first electric motor 200 to provide the driver with the correct haptic feedback and appropriate assistance.

[0232] Of course, the action of the first and second electronic control units 505 and 510 can be coordinated to guarantee the driver a smooth and uniform steering sensation.

[0233] When finally the valve system 275 is in the third operating configuration, the hydraulic actuator 105 is operated only by the second command device 215 and therefore without any mechanical / hydraulic connection with (by-wire).

[0234] In this configuration, the second electronic control unit 510 can therefore control the hydraulic actuator 105, regardless of the position of the manual steering member 135 (e.g. steering wheel).

[0235] For example, the second electronic control unit 510 may be configured to control the hydraulic actuator 105 by implementing remotely controlled and / or autonomous driving procedures.

[0236] These autonomous and / or remotely controlled driving procedures can provide, for example, that the steering apparatus 100, i.e. the vehicle on which it is installed, can comprise a geolocation system (e.g. GPS type) connected to a remote electronic control unit.

[0237] Based on the position detected by the geolocation system, the remote electronic control unit can establish the trajectory and / or path to be imparted to the vehicle and can then transmit to the second electronic control unit 510, for example via a wireless communication system 525, the steering angles to be imparted from time to time to the directional members 110 (e.g. to the wheels), so that the vehicle follows said trajectory / path.

[0238] At this point, knowing the actual steering angle, for example by means of the steering sensor 310, the second electronic control unit 510 can operate the second electric motor 265, and therefore the second hydrostatic steering unit 225, so that the hydraulic actuator 105 moves the directional members 110 to the desired orientation.

[0239] Alternatively or additionally, the second electronic control unit 510 may be configured to control the hydraulic actuator 105 based on commands from a Joystick or other peripheral devices different from the manual steering member 135 (e.g. from the steering wheel). At the same time, when the valve system is in the third operating configuration, the first command device 140 (i.e. the first electric motor 200) can be kept inactive, or can be used to simulate a mechanical connection between the manual steering member 135 (e.g. steering wheel) and the directional members 110.

[0240] In other words, the first electronic control unit 505 can be configured so that the first electric motor 200 sets in motion the manual steering member 135, via the first transmission shaft, in a manner corresponding to the variation in orientation / inclination of the directional members 110 imparted by the second command device 215.

[0241] This functionality may be useful to maintain a phasing between the position of the directional members 110 and the position of the manual steering member 135 and / or to provide simulated feedback to the driver based on vehicle dynamics and steering inputs.

[0242] Figure 3 illustrates a variant of the steering apparatus 100, which is similar to the previous one, the description of which is fully recalled, except for the aspects detailed below.

[0243] In this variant, the hydraulic actuator 105 may comprise a first and a second double-acting hydraulic jack, indicated respectively with 105’ and 105”.

[0244] The first hydraulic jack 105’ comprises a casing (e.g. a cylinder) 115’ and a relative plunger 120’ which subdivides the inner volume of the casing 115’ into two separate operating chambers 125’ and 130’.

[0245] The plunger 120’ of the first hydraulic jack 105’ may comprise two opposite rods, which are arranged parallel to the sliding direction of the plunger 120’, for example parallel to the axis of the cylinder, preferably coaxial with the latter, and both protrude outside the casing 115’ (from opposite sides).

[0246] Similarly, the second hydraulic jack 105” comprises a casing (e.g. a cylinder) 115” and a relative plunger 120” which subdivides the inner volume of the casing 115” into two separate operating chambers 125” and 130”.

[0247] The plunger 120” of the second hydraulic jack 105” may comprise two opposite rods, which are arranged parallel to the sliding direction of the plunger 120”, for example parallel to the axis of the cylinder, preferably coaxial with the latter, and both protrude outside the casing 115” (from opposite parts).

[0248] The first hydraulic jack 105’ and the second hydraulic jack 105” are arranged coaxial, i.e. so that the rods of the respective plungers 120’ and 120” are coaxial.

[0249] The rod of the first hydraulic jack 105’ facing the second hydraulic jack 105” is firmly joined to the rod adjacent thereto of the second hydraulic jack 105” itself, in such a way that the plungers 120’ and 120” of the two hydraulic jacks 105’ and 105” are always constrained to move integrally one another, as a single body.

[0250] The remaining rods of the first and second hydraulic jacks 105’ and 105” can each be mechanically connected to a respective directional member 110, for example to a respective steering wheel, preferably by means of suitable levers.

[0251] In this way, an axial displacement of the plungers 120’ and 120”, caused by the first and / or second hydraulic jack 105’ and / or 105”, can be effectively transformed into a corresponding movement of the respective directional member 110, for example into a rotation of the respective steering wheel about its own steering axis.

[0252] Of course, it is not excluded that, in other embodiments, the plungers 120’ and 120” are made directly as a single body and / or that the four operating chambers 125’, 125”, 130’ and 130” are defined within a single casing, instead of in two separate casings 115’ and 115”.

[0253] In any case, the operating chambers 125’, 125”, 130’ and 130” of the first and second hydraulic jacks 105’ and 105” are arranged so that the thrust exerted on the plungers 120’ and 120” by the pressure present in the operating chamber 125’ is concordant with the thrust exerted by the pressure present in the operating chamber 125”, but opposite to the thrust exerted by the pressure in both the remaining operating chambers 130’ and 130”. Via the valve system 275, the operating chambers 125’ and 130’ of the first hydraulic jack 105’ and the operating chambers 125” and 130” of the second hydraulic jack 105” are hydraulically connectable, alternatively, to the first and / or to the second hydrostatic steering unit 150 and 225.

[0254] In particular, the valve system 275 may be capable of assuming at least three operating configurations (alternatively or selectively).

[0255] In a first operating configuration, both hydraulic jacks 105’ and 105” of the hydraulic actuator 105 are hydraulically connected to the first hydrostatic steering unit 150, while they are both hydraulically separated from the second hydrostatic steering unit 225.

[0256] For example, the first connection port 180 of the first hydrostatic steering unit 150 may be hydraulically connected to the first chamber 125’ of the first hydraulic jack 105’ and simultaneously to the first chamber 125” of the second hydraulic jack 105”, and the second connection port 185 may be hydraulically connected to the second chamber 130’ of the first hydraulic jack 105’ and simultaneously to the second chamber 130” of the second hydraulic jack 105”; while the first and second connection ports 245 and 250 of the second hydrostatic steering unit 225 may both be hydraulically separated from the chambers 125’, 125”, 130’ and 130” of the hydraulic actuator 105.

[0257] In a second configuration, one of the two hydraulic jacks 105’ or 105” is hydraulically connected to the first hydrostatic steering unit 150, while the other hydraulic jack 105” or 105’ is hydraulically connected to the second hydrostatic steering unit 225.

[0258] For example, the first connection port 180 of the first hydrostatic steering unit 150 may be hydraulically connected to the first chamber 125’ of the first hydraulic jack 105’, and the second connection port 185 may be hydraulically connected to the second chamber 130’; and at the same time, the first connection port 245 of the second hydrostatic steering unit 225 may be hydraulically connected to the first chamber 125” of the second hydraulic jack 105”, and the second connection port 250 may be hydraulically connected to the second chamber 130”.

[0259] Or, the first connection port 180 of the first hydrostatic steering unit 150 may be hydraulically connected to the first chamber 125” of the second hydraulic jack 105”, and the second connection port 185 may be hydraulically connected to the second chamber 130”; and at the same time, the first connection port 245 of the second hydrostatic steering unit 225 may be hydraulically connected to the first chamber 125’ of the first hydraulic jack 105’, and the second connection port 250 may be hydraulically connected to the second chamber 130’.

[0260] In a third operating configuration of the valve system 275, both hydraulic jacks 105’ and 105” of the hydraulic actuator 105 are hydraulically connected to the second hydrostatic steering unit 225, while they are both hydraulically separated from the first hydrostatic steering unit 150.

[0261] For example, the first and second connection ports 180 and 185 of the first hydrostatic steering unit 150 may both be hydraulically separated from all operating chambers 125’, 125”, 130’ and 130” of the hydraulic actuator 105; while the first connection port 245 of the second hydrostatic steering unit 225 may be hydraulically connected to the first chamber 125’ of the first hydraulic jack 105’ and simultaneously to the first chamber 125” of the second hydraulic jack 105”, and the second connection port 250 may be hydraulically connected to the second chamber 130’ of the first hydraulic jack 105’ and simultaneously to the second chamber 130” of the second hydraulic jack 105”.

[0262] To achieve these operating configurations, the valve system may comprise two valves, preferably two solenoid valves, of which a first valve 280’ and a second valve 285’.

[0263] The first valve 280’ is hydraulically connected to the first hydraulic jack 105’ and to both the first hydrostatic steering unit 150 and the second hydrostatic steering unit 225.

[0264] It is switchable between a first position (shown in Figure 3), wherein it places the first hydraulic jack 105’ in hydraulic connection with the first hydrostatic steering unit 150, hydraulically separating it from the second hydrostatic steering unit 225, and a second position (displaced to the right with respect to the figure), wherein it places the first hydraulic jack 105’ in hydraulic connection with the second hydrostatic steering unit 225, hydraulically separating it from the first hydrostatic steering unit.

[0265] For example, in the first position, the first valve 280’ hydraulically connects the first connection port 180 of the first hydrostatic steering unit 150 to the first chamber 125’ of the first hydraulic jack 105’, and the second connection port 185 to the second chamber 130’; while it is closed with respect to the first and second connection ports 245 and 250 of the second hydrostatic steering unit 225.

[0266] In the second position, vice versa, the first valve 280’ can hydraulically connect the first connection port 245 of the second hydrostatic steering unit 225 to the first chamber 125’ of the first hydraulic jack 105’, and the second connection port 250 to the second chamber 130’; while it can be closed with respect to the first and second connection ports 180 and 185 of the first hydrostatic steering unit 150, possibly putting them in mutual communication.

[0267] The first valve 280’ may comprise a spring 290’ adapted to hold it normally in the first position and an electromechanical transducer 295’ which, when electrically powered, is adapted to move it to the second position in contrast with said spring 290’.

[0268] In this way, when it is not energized, i.e. when the electromechanical transducer 295’ is not crossed by current, the first valve 280’ will be in the first position; while when it is energized, i.e. when the electromechanical transducer 295’ is crossed by current, the first valve 280’ will be in the first position.

[0269] The second valve 285’ is hydraulically connected to the second hydraulic jack 105” and to both the first hydrostatic steering unit 150 and the second hydrostatic steering unit 225. It is switchable between a first position (displaced to the left with respect to Figure 3), wherein it places the second hydraulic jack 105” in hydraulic connection with the second hydrostatic steering unit 225, hydraulically separating it from the first hydrostatic steering unit 150, and a second position (illustrated in the figure), wherein it places the second hydraulic jack 105” in hydraulic connection with the first hydrostatic steering unit 150, hydraulically separating it from the second hydrostatic steering unit 225.

[0270] For example, in the first position, the second valve 285’ hydraulically connects the first connection port 245 of the second hydrostatic steering unit 225 to the first chamber 125” of the second hydraulic jack 105”, and the second connection port 250 to the second chamber 130”; while it is closed with respect to the first and second connection ports 180 and 185 of the first hydrostatic steering unit 150.

[0271] In the second position, vice versa, the second valve 285’ can be closed both with respect to the first and with respect to the second connection port 245 and 250 of the second hydrostatic steering unit 225; while hydraulically connecting the first connection port 180 of the first hydrostatic steering unit 150 to the first chamber 125” of the second hydraulic jack 105”, and the second connection port 185 to the second chamber 130”.

[0272] The second valve 285’ may comprise a spring 300’ adapted to hold it normally in the second position and an electromechanical transducer 305’ which, when electrically powered, is adapted to move it to the first position in contrast with said spring 300’.

[0273] In this way, when it is not energized, i.e. when the electromechanical transducer 305’ is not crossed by current, the second valve 285’ will be in the second position; while when it is energized, i.e. when the electromechanical transducer 305’ is crossed by current, the second valve 285’ will be in the first position.

[0274] Overall, therefore, when the first valve 280’ is in the first position and the second valve 285’ is in the second position (as illustrated in Figure 3), for example when neither of the two valves 280’ and 285’ is energized, the valve system 275 is in the first operating configuration, wherein both the hydraulic jacks 105’ and 105” of the hydraulic actuator 105 are connected only to the first hydrostatic steering unit 150.

[0275] When the first valve 280’ is in the first position and the second valve 285’ is also in the first position, for example when the second valve 285’ is energized while the first valve 280’ remains unenergized, the valve system 275 is in the second operating configuration, wherein the first hydraulic jack 105’ is connected to the first hydrostatic steering unit 150 while the second hydraulic jack 105” is connected to the second hydrostatic steering unit 225.

[0276] The valve system 275 is in the second operating configuration even when the first valve 280’ is in the second position, for example because it is energized, while the second valve 285’ is also in the second position.

[0277] In this case, however, the first hydraulic jack 105’ will be connected to the second hydrostatic steering unit 225 while the second hydraulic jack 105” will be connected to the first hydrostatic steering unit 150.

[0278] When finally the first valve 280’ is in the second position, for example because it is energized, and the second valve 285’ is in the first position, for example because it is also energized, the valve system 275 is in the third operating configuration, wherein both the hydraulic jacks 105’ and 105” of the hydraulic actuator 105 are connected only to the second hydrostatic steering unit 225.

[0279] The electronic control system 500, as well as the modes in which the valve system 275 is moved to the different operating configurations and with which the electric motors 200 and 265 are controlled, can be the same as those already outlined for the first variant, so we omit to repeat the description which is however referred to in full.

[0280] Another difference of this variant consists in the fact that the operating fluid (oil) can be fed to the first and second hydrostatic steering units 150 and 225 through respective pumps 155’ and 155”, and that possibly such operating fluid can be fished and subsequently discharged into respective tanks 160’ and 160”.

[0281] Obviously, a person skilled in the art will be able to make numerous technical-applicative modifications to all that above, without thereby departing from the scope of the invention as hereinbelow claimed.

Claims

CLAIMS1. A steering apparatus (100) comprising:- at least a hydraulic actuator (105) adapted to vary the orientation of at least a directional member (110) of a vehicle,- a manual steering member (135),- a first control device (140) adapted to command the hydraulic actuator (105), said first command device (140) comprising a first hydrostatic steering unit (150) hydraulically connectable to the hydraulic actuator (105), a first transmission shaft (145) adapted to mechanically connect the manual steering member (135) with the first hydrostatic steering unit (150), and a first electric motor (200) adapted to apply torsional torques to the first transmission shaft (145),- a second command device (215) adapted to command the hydraulic actuator (105), said second command device (215) comprising a second hydrostatic steering unit (225), hydraulically connectable to the hydraulic actuator (105), a second transmission shaft (220) mechanically connected to the second hydrostatic steering unit (225), and a second electric motor (265), adapted to apply torsional torques to the second transmission shaft (220),- a valve system (275) adapted to hydraulically connect the first hydrostatic steering unit (150) and / or the second hydrostatic steering unit (225) with the hydraulic actuator (105), and- an electronic control system (500) configured to control the first electric motor (200), the second electric motor (265) and the valve system (275).

2. A steering apparatus (100) according to claim 1 , wherein the electronic control system (500) is configured to control the first electric motor (200) based on signals coming from a first sensor system.

3. A steering apparatus (100) according to claim 2, wherein the first sensor system comprises one or more sensors selected from the group consisting of: an angular position sensor (210) adapted to measure the angular position of a rotor of the first electric motor (200) with respect to a stator thereof, a torque sensor (205) adapted to measure the torsional torque manually applied to the first transmission shaft (145) via the manual steering member (135), a steering sensor (310) adapted to measure a steering angle of the vehicle, a speed sensor adapted to measure the speed of the vehicle.

4. A steering apparatus (100) according to claim 3, wherein the electronic control system (500) is configured to determine, based on the signal from the torque sensor (205), a torsional torque to be applied to the first transmission shaft (145), and to control the operation of the first electric motor (200) so that the latter generates and applies the calculated torsional torque to the first transmission shaft (145).

5. A steering apparatus (100) according to claim 3 or 4, wherein the electronic control system (500) is configured to use the torque sensor (205) to know the actual position of the manual steering member (135) in implementing an algorithm to automatically bring said manual steering member (135) to a predetermined position.

6. A steering apparatus (100) according to any one of the preceding claims, wherein the electronic control system (500) is configured to control the second electric motor (200) based on signals from one or more input peripheral device and / or a second sensor system.

7. A steering apparatus (100) according to claim 6, wherein the second sensor system comprises one or more sensors selected from the group consisting of: an angular position sensor (270) adapted to measure an angular position of a rotor of the second electric motor (265) with respect to a stator thereof, a steering sensor (310) adapted to measure a steering angle of the vehicle, a speed sensor adapted to measure the speed of the vehicle.

8. A steering apparatus according to claim 6 or 7, wherein said input peripheral devices comprise a joystick.

9. A steering apparatus (100) according to any one of the preceding claims, wherein the electronic control system (500) is configured to implement a field-oriented control algorithm to control the first and second electric motors (200, 265).

10. A steering apparatus (100) according to any one of the preceding claims, wherein the electronic control system is configured to implement a self-centring algorithm of the directional member of the vehicle.

11. A steering apparatus (100) according to any one of the preceding claims, wherein the electronic control system (500) is configured to control the second electric motor (2659 so as to regulate the relationship between the displacement of the manual steering member (135) and the variation of the orientation of the directional member of the vehicle (110).

12. A steering apparatus (100) according to any one of the preceding claims, wherein the electronic control system (500) is configured to control the valve system (275) based on commands coming from a user interface (520).

13. A steering apparatus (100) according to any one of the preceding claims, wherein the valve system (275) is controllable in at least three operating configurations, of which:- a first operating configuration, wherein the hydraulic actuator (105) is hydraulically connected to the first hydrostatic steering unit (150) and is hydraulically separated from the second hydrostatic steering unit (225),- a second operating configuration, wherein the hydraulic actuator (105) is hydraulically connected to both the first hydrostatic steering unit (150) and the second hydrostatic steering unit (225),- a third operating configuration, wherein the hydraulic actuator (105) is hydraulically separated from the first hydrostatic steering unit (150) and is hydraulically connected to the second hydrostatic steering unit (225).

14. A steering apparatus (100) according to any one of the preceding claims, wherein the sensor system (275) comprises at least:- a first valve (280, 280') hydraulically connected to the hydraulic actuator (105) and to both the first and second hydrostatic steering units (150, 225) and switchable between a first position, wherein it places the hydraulic actuator (105) in hydraulic connection with the first hydrostatic steering unit (105) hydraulically separating it from the second hydrostatic steering unit (225) and a second position, wherein it places the hydraulic actuator (105) in hydraulic connection with the second hydrostatic steering unit (225) hydraulically separating it from the first hydrostatic steering unit (150), and- a second valve (285, 285') hydraulically connected to the hydraulic actuator (105) and at least to the second hydrostatic steering unit (225) and switchable between a first position, wherein it hydraulically connects the hydraulic actuator (105) to the second hydrostatic steering unit (225), and a second position, wherein it hydraulically separates the hydraulic actuator (105) from the second hydrostatic steering unit (225).

15. A steering apparatus (100) according to claim 14, wherein the first valve (280) comprises a spring (290) adapted to hold it in the first position and an electromechanicaltransducer (295) adapted, when electrically powered, to move it to the second position in contrast with said spring (290), and wherein the second valve (285) comprises a spring (300) adapted to hold it in the second position and an electromechanical transducer (305) adapted, when electrically powered, to bring it to the first position in contrast with said spring (300).

16. A steering apparatus (100) according to any one of the preceding claims, wherein the electronic control system (500) is configured to control the first electric motor (200) so that it applies a torsional torque to the first transmission shaft (145) at least when the first hydrostatic steering unit (150) is hydraulically connected to the hydraulic actuator (105), and preferably also when the first hydrostatic steering unit (150) is hydraulically disconnected from the hydraulic actuator (150).

17. A steering apparatus (100) according to any one of the preceding claims, wherein the electronic control system (500) comprises a first electronic control unit (505) configured to control at least the first electric motor (200), and a second electronic control unit (510) configured to control at least the second electric motor (265).

18. A steering apparatus (100) according to claim 17, wherein the first electronic control unit (505) and the second electronic control unit (510) are implemented in a configuration selected from the group consisting of:- two separate channels within a single electronic control board;- two separate electronic control boards housed within a single casing; and- two physically separate electronic control units.

19. A steering apparatus (100) according to claim 17 or 18, wherein the first electronic control unit (505) and the second electronic control unit (510) are configured to crosscheck data and calculations.

20. A steering apparatus (100) according to any one of the preceding claims, wherein each of said first and second hydrostatic steering units (150, 225) comprise a distributor and a rotary metering unit.

21. A steering apparatus (100) according to any one of the preceding claims, wherein the first electric motor (200) is coaxially installed on the first transmission shaft (145) and the second electric motor (265) is coaxially installed on the second transmission shaft (220).

22. A steering apparatus (100) according to any one of the preceding claims, whereinthe hydraulic actuator (105) comprises a double-acting jack provided with a casing (115) and a plunger (120) which subdivides the inner volume of the casing (115) into two separate operating chambers (125, 130), each of which is hydraulically connectable by the valve system (275) to the first and / or to the second hydrostatic steering unit (150, 225).

23. A steering apparatus (100) according to any one of claims 1 to 21 , wherein the hydraulic actuator (105) comprises a first and a second double-acting jack (105', 105"), each of which is provided with a casing (115', 115") and a plunger (120', 120") which subdivides the inner volume of the casing (115', 115") into two separate operating chambers (125', 130", 125", 130"), the plungers (120', 120") of both the first and second jacks (105', 105') being mechanically joined together, the operating chambers (125', 130') of the first hydraulic jack (105') being hydraulically connectable by the valve system (275) to the first and / or to the second hydrostatic steering unit (150, 225), the operating chambers (125', 130') of the second hydraulic jack (105”) being hydraulically connectable by the valve system (275) to the first and / or to the second hydrostatic steering unit (150, 225).

24. A steering apparatus (100) according to any one of the preceding claims, comprising a plurality of said hydraulic actuators (105).

25. A method for operating a steering apparatus (100) according to any one of the preceding claims, comprising the steps of:- selecting an operating configuration from: o a first operating configuration, wherein the hydraulic actuator (105) is hydraulically connected to the first hydrostatic steering unit (150) and is hydraulically separated from the second hydrostatic steering unit (225), o a second operating configuration, wherein the hydraulic actuator (105) is hydraulically connected to both the first hydrostatic steering unit (150) and the second hydrostatic steering unit (225), and o a third operating configuration, wherein the hydraulic actuator (105) is hydraulically separated from the first hydrostatic steering unit (150) and is hydraulically connected to the second hydrostatic steering unit (225),- hydraulically connecting via the valve system (275) the hydraulic actuator (105) to the first and / or second hydrostatic steering unit (150, 225) based on the selected operating configuration,- controlling the first and / or second electric motor (200, 265) so as to apply atorsional torque to the first and / or second transmission shaft (145, 220) respectively.

26. A method according to claim 25, comprising the step of switching the valve system (275) from the first to the third operating configuration passing through the second oper- ating configuration, and the step of switching the valve system (275) from the third to the first operating configuration passing through the second operating configuration.

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