TECHNICAL FIELD OF THE INVENTION

The invention relates to systems for drive train control for vehicles. The invention is particularly directed to the feature of controlling a complementary propulsion unit for a heavy road vehicle.

BACKGROUND OF THE INVENTION

For heavy road vehicles, it is known that there is sometimes a desire for providing driving force on several wheel pairs such that the vehicle for example is provided with a driving force on a rear pair of wheels as well as on front pair of wheels. In many cases, it is desirable to be able to control the traction of the vehicle such that one or several wheel pairs may be connected or disconnected from the power source depending on the traction force demand. The propulsion unit may be the same for all the driving wheels or be a combination of different power sources, e.g. a vehicle provided with a mechanical drivetrain connected to an internal combustion engine and a hydraulic power source connected to hydraulic motors. A vehicle provided with such a combination of mechanical drivetrain and hydraulic motors is for example

disclosed in EP 1 886 861.

Even though the known vehicles provided with traction forces on both rear wheels and front wheels provide an improved traction of the vehicles compared to those vehicles provided with traction on only rear or front wheels, there is still a desire for a further improved traction control of such a vehicle.

DESCRIPTION OF THE INVENTION

An object of the invention is to provide a compact control system for a heavy road vehicle provided with traction on both rear wheels and front wheels. The invention is particularly directed to a vehicle which is provided with a mechanical drive train for traction of a pair of wheels, e.g. a pair of rear wheels, and a hydraulic propulsion system for traction of another pair of wheels, e.g. a pair of front wheels. The heavy vehicle may be of a heavy load carrying kind and is particularly useful for trucks which in their duty frequently are used in rough conditions, e.g. timber loading trucks which may be used on small provisional roads or tracks in the forest where the path may be loose or muddy and additional traction force is desired. The vehicle may also be another kind of heavy road vehicle which frequently starts and stops during its working time, e.g. a public bus used at least occasionally in city traffic. Hence, the

mechanical/hydraulic hybrid drive system is suitable for vehicles to be used as goods or passenger carriers which demands to provide a comfortable and efficient propulsion when traveling at creep speed, e.g. in a frequently "stop and go"-situation as well as when traveling at higher speeds over longer distances. In order to provide the desired traction under

circumstances with poor traction conditions shall both systems also be possible to use simultaneously.

A heavy vehicle of the kind described above shall thus preferably be adapted to run smoothly on normal roads at a relatively high velocity, e.g. up to 90 km/h, while also assure traction at low speeds on unpaved, provisional roads. In order to function in a desirable way under different conditions as exemplified above, the complementary drive, i.e. the hydraulically propelled front wheels, shall be able to be

disconnected when driving at high speeds and being able to provide an additional traction force when desired, normally at relatively low speeds. In general, there is no need for using the complementary hydraulic

propulsion units above 30 km/h even though it may be advantageous to use them sometimes up to 50 km/h. The vehicle may also be provided with a creep drive

function at low velocities when only the hydraulic drive is used.

In order to facilitate the control of the vehicle traction, the vehicle has been provided with a traction regulator including a control button or control lever having at least two positions, position N and position H, for selecting which traction mode that shall be used. If the traction regulator is set in position N is normal driving mode selected intended for long distance travel on roads and if the traction regulator is set position H is a creep drive mode selected in which only the hydraulic drive is used. In the normal mode (N) is the mechanical traction system active, either alone or with the aid from the hydraulic traction system

depending on driving parameters and manually changeable settings as will be described later. The selection of the H position, i.e. the creep drive mode, will also cause a change of an already existing control button, control lever or the like control device in the driver cabin to be used for controlling the hydraulic drive unit. This may for example be an upshift/down shift button, a cruise control device having the normal function of increasing/decreasing the set speed, accelerator, volume control for the radio or any other device having the possibility to indicate an increase (+) function or decrease (-) function. Such control buttons or levers may thus be used for indicating a desired forward movement of the vehicle, using the increase (+) function to provide and control a forward movement or acceleration, and a desired backward or reverse movement of the vehicle by using the decrease (-) function in order to provide a deceleration or reverse movement. Hence, any existing control device which is suitably located in the driver cabin and in intuitive way may provide for indicating and

controlling reverse and forward movement of the vehicle may thus be used.

In an alternative design of the traction regulator is the traction regulator designed to include two H positions, position HF and position HR, for the creep drive mode such that the HF position is defining a forward creep drive mode and the HR position is defining a reverse creep mode. In this case may the same control devices as described before be used but now will they only be used to control the velocity without being able to be used to change travel

direction of the vehicle. In addition may other control devices, e.g. an accelerator, which not have the possibility to be used in a simple way for defining direction of the travel, be used as a speed control device . In still an alternative design of the traction

regulator is the traction regulator designed to include two N positions, position NM and position NH, for the normal traveling mode such that the NM position is defining a mechanical normal mode in which only the mechanical drive unit is used and the NH position is defining a hybrid normal mode in which the hydraulic drive unit will be used as an additional traction aid which automatically is activated according to certain criteria, e.g. when driving below a certain speed for a specific time, when there is a slip detected on the mechanically driven wheels and the speed is below a certain limit or when starting the vehicle as a start aid until a certain time limit has passed or the speed of the vehicle has reached a certain limit.

The traction regulator may for example be used in a load carrying truck provided with one or several rear axles whereof at least one axle is drivingly connected by a mechanical drivetrain to an internal combustion engine. The mechanically driven rear axles are usually provided with a differential function and the

mechanical drivetrain connecting the internal

combustion engine with the driven axles is normally connected via a stepped gearbox.

The load carrying truck may also have one or several front axles provided with steerable wheels whereof at least one pair of wheels have been provided with a hydraulic motor each. The motors may for example be hub motors rotating with the wheels and thus having a rotating housing. The hydraulic motors are in general preferred to be able to be decoupled such that the hydraulic traction only is used when desired, e.g. at low speeds on loose or muddy ground. The hydraulic motors may have a variable or fixed displacement.

The hydraulic motors forms part of a hydraulic system and are powered by a hydraulic pump. The hydraulic pump is preferably a pump having a variable displacement which can be controlled to deliver the hydraulic liquid at a desired flow rate or pressure. The pump is most likely driven by the same internal combustion engine which is used as a power source for the mechanical drive of the rear wheels and may be connected to a Power Take-Off (PTO) on the engine.

To include a control system for a hydraulic drive which may deliver a specific torque to each wheel should the control unit take into account steering wheel angle and wheel base of the vehicle in order to provide the right torque from the hydraulic motors. When using a creep drive mode which only includes propulsion from the hydraulic system, the pump may be controlled by changing the engine speed or to change the displacement of a variable displacement pump. In case the pump is a reversible variable displacement pump may the direction of travel be changed by reversing the displacement pump. In case the pump not is reversible may the flow direction of the hydraulic liquid be changed by control valves .

The traction regulator is connected to a control unit which receives input signals from the traction

regulator. The control unit is further connected to the hydraulic and mechanical traction systems to control which systems that shall be used. The same or another control unit is preferably connected to further sensors or status indicators relevant for the control of the traction systems, e.g. speed sensors, slip detecting sensors, steering angle sensors or torque sensors, in order to be provided with relevant data for sending output signals to the hydraulic and/or mechanical traction system. These input signals are used by the control unit in an algorithm in order to compute an output signal which will control the work of the pump to deliver a desired hydraulic flow or pressure.

The traction regulator is preferably designed such that a mode may not be changed from the first, normal mode (N, NM, NH) to the second, hydraulic mode (H, HF, HR) unless the vehicle is moving below a certain speed or is at standstill. It may also be a compulsory condition that the gear shift is set to neutral, alternatively that the change of mode from normal to hydraulic automatically changes the gear shift position to neutral .

The control unit is further connected to a control device which normally is used for the control of some other function in the vehicle, e.g. an up/down-shift button or lever, accelerator or volume control.

Depending on the set control status of the traction regulator will the function of this control device change such that its normal function is replaced for control of the hydraulic motors when a hydraulic mode is selected. A purpose of the arrangement is to avoid an additional control device to be installed and thus provide a simplified control panel in the driver cabin. A

suitable control device which is easy accessible may b( used to control the creep drive, e.g. some control device comprised in the steering wheel or the speed control which usually is used for controlling the mechanical traction system during normal mode driving. It could of course also be possible that two control devices changes their function, e.g. could the gear selector be used for changing between forward and reverse while the accelerator is used for controlling the speed.

The control of the speed in the hydraulic creep drive mode hydraulic pump may be made by changing the

displacement volume of the pump. The control strategy may be to control the pump to deliver a desired flow. Another way of controlling the speed is to directly influence the engine by the accelerator and the

accelerator will thus in this case maintain its normal function in that the engine speed is controlled by the accelerator. However, in this case will there also be a control of the flow from the hydraulic pump, e.g. by changing the displacement volume of the pump to be proportional to the engine speed. The control in this case is preferably made such that there is no hydraulic drive at idle speed (no displacement volume) and the speed will increase with increased accelerator pressure (increased displacement volume) . The control by the accelerator may be set such that below a lower limit, preferably close to but above the idle speed, is the displacement zero and the displacement volume will thus increase proportional to the engine speed until the maximum displacement volume is reached at another, upper engine speed limit. Suitable engine speed limits for the engines used for these kind of vehicles, i.e. heavy road vehicles, may be a lower limit somewhere between 500 and 800 rpm, i.e. the limit where the displacement angle will change from zero to provide a flow, and the displacement angle will then increase (proportionally) until the maximum displacement angle is reached at an upper limit somewhere between 1000 and 1400 rpm where after the displacement is at maximum for engine speeds above this limit.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 Describes a schematic view of a heavy load

carrying vehicle provided with an auxiliary hydraulic traction on the front wheels and a control regulator

Fig. 2 Describes different embodiments of the control regulator

Fig. 3 Describes a schematic figure of an axial cross sectional view of a hydraulic motor DETAILED DESCRIPTION OF THE DRAWINGS

In figure 1 is shown a schematic view of a traction system 1 for a heavy road vehicle. The traction system 1 comprises a first mechanical propulsion system 12 and a second hydraulic propulsion system 13. The first mechanical propulsion system 12 comprises a pair of traction wheels 2a, 2b which are located on a rear, driven axle 3 which is powered by an internal

combustion engine (ICE) 4. The rear, driven axle 3 is connected to the ICE 4 via a gearbox 5. The gear box 5 may be a stepped gear box and the mechanical drive train may comprise a Double Clutch Transmission (DCT) in order to reduce the time for a change of gear. The second, hydraulic propulsion system 13 further

comprises a second pair of traction wheels 6a, 6b, the front wheels, which are driven by a pair of hydraulic motors 8a, 8b which are connected to and powered by a hydraulic pump 7. The traction system 1 also comprises a control unit 9 which is connected to the ICE 4, the gear box 5 and the hydraulic pump 7. Even though it is not necessary for the control unit 9 to be connected to the ICE 4 and gear box 5, it is considered to be beneficial for providing a desired control of the hydraulic propulsion system 13. The control unit could of course also be connected to other parts of the propulsion systems 12, 13, e.g. it may be connected to the hydraulic motors 8a, 8b in order to send output signals to control valves in the motors 8a, 8b. The control unit is also connected to speed sensors 10a, 10b, 10c for indicating the speed of the front driven wheels 6a, 6b, the rear driven wheels 2a, 2b and a pair of rear, non-driven wheels 11a, lib. To provide separate speed sensors for all wheel axles is not essential for the basic function of the invention but may be useful for traction control when using when using both propulsion systems 12, 13. Speed sensors could be replaced for or used together with further sensors used for control of the propulsion units. The control unit 9 is further connected to a traction regulator 14 which is used by the driver for selecting between different drive modes. Different embodiments of the regulator are described in figure 2. The mode selection of the traction regulator 14 will influence the function of a control device 15 such that it will change its function depending on the selected drive mode .

In figure 2a is described a first embodiment of the traction regulator 14a in which the traction regulator may change between a normal mode N and a hydraulic mode H. The normal mode N is what the driver usually selects when driving on a road or in normal traffic situations. The hydraulic mode H is selected when the driver is driving at slow speed which may include frequent start and stop cycles and are in general terms often referred to as creep drive. In this mode is only the hydraulic drive activated. In addition to decouple the mechanical drive will the setting of the traction regulator 14a in the hydraulic mode (H) also change the function of the control device 15 from its normal function to control the hydraulic propulsion system 13 (see figure 1) . The control device may for example be a gear selector which function is changed from shifting gears up and down by pressing a shift up button (+) or shift down button (-) (or moving a gear selector lever in different

directions) to accelerate the vehicle in a forward direction (+) or reverse direction (-) . The control device may for example function such that the vehicle is accelerated in the desired direction while the button (or lever) is indicating (+) or (-) and kept at constant speed when the button is released (or lever in neutral position) . In this case, the control device will provide a function similar to a cruise control with the option that it will be possible to reduce and reverse the speed. Alternatively, the hydraulic propulsion unit 13 may be controlled to continue to drive in the forward direction as long as the (+) function is selected and stop driving when the (+) function no longer is pushed and thus decelerate to zero speed when the control device is released. The function for driving in the rearward direction is the analogue. Alternatively, more than one control device may be used for controlling the hydraulic (H) creep drive mode. The gear selector (or another control device suitable for indicating (+) for forward and (-) for reverse) could be used for indicating the direction of drive while another control device (e.g. an

accelerator) could be used for controlling the speed.

In figure 2b is another embodiment of the invention shown and the traction regulator 14b is provided with one normal mode (N) while the hydraulic mode has been divided into two sub-modes, a first sub-mode (HF) to be selected for hydraulic creep drive forward and a second sub-mode (HR) to be selected for hydraulic reverse creep drive. In this case will there only be necessary for the control device which changes its function to only control the speed of the vehicle and not the direction since direction may be selected by the traction regulator. Hence, the above described control devices in the first embodiment may be used also in this case but the will not provide for the selection of direction but only speed control.

In figure 2c is shown still another embodiment of the traction regulator 14c disclosed in which there are two normal sub-modes which may be selected, a first normal sub-mode (NM) in which only the mechanical drivetrain is used for propulsion of the vehicle and a second sub- mode (NH) in which the hydraulic propulsion is

automatically activated to aid the mechanical

drivetrain in certain driving situations, e.g. when the vehicle is driving below a certain speed, a slip is detected on the powered mechanical wheels, at take-off or other situations where an additional torque may be desired. There could of course be further driving modes to be selected, e.g. an economy mode or hill-climbing mode. However, in this embodiment is there only one hydraulic mode included and this mode will function in all essential aspects in the same way as disclosed in the first embodiment shown in figure 2a.

In figure 2d is a fourth embodiment of the traction regulator 14d shown in which there are two normal sub- modes (NM, NH) and two hydraulic (HF, HR) sub-modes for the hydraulic creep drive. These sub-modes have been explained for the embodiments shown in figure 2b and 2c and will function in this embodiment essentially the same way as disclosed in association with these

figures .

Above has been briefly described a few different examples of the invention. It is thus possible to select other control devices to change function

depending on the lay out of the control panel in the specific vehicle. Likewise, the exact way the control device function in order to control the speed of the hydraulically driven motors may vary as long as the function is changed from the normal function of the control device to control the speed of the vehicle by controlling the hydraulic system and motors.

In fig 3 is shown a schematic figure of an axial cross sectional view of a hydraulic motor 8 suitable for the system disclosed in figure 1. The hydraulic motor 8 comprises an outer cam ring 20 having an essentially hexagonic shape provided with rounded edges 21 and rounded, inwardly raised portions 22 in between the edges 21. The cam ring 20 is rotating with a wheel connected to the hydraulic motor 8. The cam ring 20 is further divided in direction fields 23, 24 which are defined by the peak of the raised portions 22 and the edges 21. A direction field 23 which extends from an edge 21 to a peak of the raised portion 22 in the clockwise direction corresponds to a clockwise

rotational field 23 and such a field 23 will be thus be referred to as a forward rotational field hereinafter. A direction field 24 which extends from an edge 21 to a peak of the raised portion 22 in the counterclockwise direction corresponds to a counterclockwise rotational field 24 and such a field will be thus be referred to as a reverse rotational field 24 hereinafter. The hydraulic motor further includes a central distributor plate 25 also rotating with the wheel and provided with forward channels 26 and reverse channels 27. The channels 26, 27 have 6 openings each which are adapted to fit in and connect hydraulically with hydraulic pistons 28, in this case eight pistons, which are located symmetrically around the rotational centre of the motor 8 on a fixed cylinder block 29. The forward and reverse channels 26, 27 are designed such that the forward channels 26 are located in the same circle sectors as the forward rotational fields 23 of the cam ring 20 and the reverse channels 27 are located in the came circle sectors as the reverse rotational fields 24 for delivering hydraulic liquid to the pistons 28. When either the forward channel 26 or the reverse channel 27 is pressurized, the camring 20 and a wheel attached thereto will move correspondingly to provide a forward motion or a reverse motion of a vehicle. In the figure, it is shown that two pistons 28 d, h (upper left and lower right pistons) are fitted with and hydraulically connected with openings of the forward channel 26 and ready to receive pressurized hydraulic liquid from the hydraulic system. If the hydraulic liquid in the forward channel 26 is pressurized, the upper left piston 28g and the lower right piston 28d will be pushed outwards and cause a clockwise (forward) motion of the cam ring 20 and a wheel attached to the cam ring 20. As the cam ring 20 and the distributor plate 25 rotates, the connection between the pressurized pistons 28 d, g (upper left and lower right pistons) will be disconnected and depressurized such that the pistons 28 d, h may easily be returned into the fixed cylinder block 29. While the cam ring 20 is moving in the forward direction, the left piston 28f and right piston 28c will become hydraulically connected to the forward channel 26 and these pistons 28c, f will be pushed outwards and provide for a continuing forward motion of the camring 20. This procedure will thus continue for the cylinders 28 until the forward channel 26 is depressurized. If a reverse motion is desired instead, the reverse channel 27 is pressurized instead and a reverse motion of the cam ring 20 and an attached wheel is achieved. In order to decouple the hydraulic engine, the space between the cam ring 20 and the cylinder block 29, usually the space in the hydraulic motor defined by a motor housing, could be pressurized such that the pistons will be pushed into the cylinder block 29 and a wheel connected to the hydraulic motor 8 may rotate more or less freely. Hence, the hydraulic motors may be decoupled in an efficient way such that there are small losses due to additional friction from the hydraulic motor when decoupled. Since the hydraulic engines usually not are intended to be used for

propulsion of the load carrying truck when travelling above 30 km/h on a road or highway, it is important that the mounting of the hydraulic motors to the vehicle not will contribute with a significant

additional rolling resistance when decoupled.

Even though it is exemplified above to have six cams on the cam ring, the number of cams could be different, e.g. 9 or 10. Likewise, the number of pistons needs not to be 8 but could be 10 or 12 for example.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims, as interpreted by the description and drawings.