WO2013119147A1

WO2013119147A1 - Power steering system

Info

Publication number: WO2013119147A1
Application number: PCT/SE2012/000014
Authority: WO
Inventors: Sten Ragnhult; Henrik LINDBĂCK; Gustav NEANDER
Original assignee: Volvo Lastvagnar AB
Current assignee: Volvo Truck Corp
Priority date: 2012-02-07
Filing date: 2012-02-07
Publication date: 2013-08-15
Anticipated expiration: 2014-08-07

Description

POWER STEERING SYSTEM

FIELD OF INVENTION

The invention relates to a power steering system for a vehicle and the control of such a system. The invention is in particular directed to a power steering system comprising a hydraulic steering gear for providing an additional steering torque to the steerable wheels in dependence on the steering torque applied to the steering column and an electrical assistance unit for providing an additional torque to the steering column in dependence of the signals from a control unit.

BACKGROUND ART

Power steering systems are today present in a lot of vehicles in order to provide an additional torque to the steering system. The systems are mainly present in order to allow the driver to steer a vehicle with less torque applied to the steering wheel than if no steering aid system were present. In these cases is it common to use some kind of proportional aid which add a torque to the steering system proportional to the steering torque from the steering wheel. In more sophisticated systems may the steering aid be dependent on further parameters, e.g. the speed of the vehicle and / or the steering angle. Steering aid system may also be used to control the steering feeling for the driver and thus apply a certain torque depending on specific driving conditions.

Power steering systems may be powered by different power sources, e.g. by a hydraulic steering gear or an electrical motor working on a steering column or a steering system. In some cases may a power steering system comprise both electric and hydraulic powered steering aid units. These units will thus cooperate in order to provide the desired properties of the steering feeling. On such a system using both a hydraulic steering gear and an electric motor in power steering system is disclosed in US 5,080,186. In this patent is disclosed the use of an electric motor in order to control a rotary slide valve for the hydraulic steering gear system.

However, even though the above described power steering system may provide for a desired help in many cases, a modified power steering system is needed which may be controlled more efficient and accurately during certain circumstances. SUMMARY OF INVENTION

The invention is related to a power steering system which is used in an ordinary steering arrangement for a vehicle comprising a steering wheel which is mechanically connected to steerable wheels via a steering column. The power steering system comprises a hydraulic steering gear mechanically connected to the steerable wheels. This hydraulic steering gear is usually designed to influence the steered wheels or steering axle directly. However, the hydraulic steering gear may provide a steering torque at essentially any part of the steering system and could for example influence the steering column. The additional steering torque to the steerable wheels from the hydraulic steering gear is usually applied in dependence of the mechanically applied steering torque to an input shaft of the hydraulic steering gear. The steering torque provided by the hydraulic steering gear is usually proportional to the torque applied to the steering column. This steering column torque is usually provided by at least manually turning the steering wheel but could also be applied by other means as will be described below. The steering system further includes an electrical assistance unit comprising an electric motor. The electric motor is mechanically connected to the steering column in order to provide a turning torque to the steering column. The steering system further comprises a torque sensor for sensing the applied torque to the steering column and an angle sensor for sensing the steering angle of the steering column. The torque and angle sensors are connected to a control unit which receives input signals from the torque sensor and angle sensor. The control unit is further connected to the electric motor used for controlling the torque applied to the steering column by the electric motor.

In order to provide the desired controllability of the power steering system and be able to use the electric motor in an efficient way is the electrical assistance unit, comprising the electric motor, located such that it provides a torque on the steering column in between said hydraulic steering gear and said steering wheel, i.e. the steering column is connected to the input shaft of the steering gear at a location which is more remote from the steering wheel than where the electric motor of the assistance unit is engaged with the steering column.

In order to provide an efficient steering system which may control the steering accurately and respond quickly to the input signals, may the electric motor be arranged around said steering column such that the steering column forms part of the rotor of the electric motor. The electric motor is preferably arranged such that the steering column is concentrically located around the steering column. The motor may thus be influencing the steering column without the need of any gear or power transmitting connection which may cause additional delay in the providing of a torque from the electric motor to the steering column. The steering system may be designed such that the hydraulic steering gear is mechanically influenced by both the turning torque applied to the steering column by the manually applied turning force to the steering wheel and the automatically applied turning force by the electrical assistance unit such that a change of torque in the steering column mechanically causes a change in the hydraulic flow. Hence, the resulting steering aid from the hydraulic steering gear is the resulting force from both the manually applied steering torque from the steering wheel and the automatically applied torque from the electric motor.

The hydraulic steering gear may be constructed to include a torsion bar, which is influenced by the resulting turning torque of the steering column, such that a turning torque on the steering column in a first rotational direction results in an increased pressure of a hydraulic fluid in a first chamber relative a pressure of the hydraulic fluid in a second chamber such that a steering torque aid is provided in said first direction. In an analogous way, a turning torque on the steering column in a second rotational direction, opposite said first rotational direction, results in an increased pressure of a hydraulic fluid in the second chamber relative a pressure of the hydraulic fluid in the first chamber such that a steering torque aid is provided in said second rotational direction. Hence, the hydraulic steering gear is designed to provide an additional steering force in the direction on which there is a turning torque on the steering column.

Above it was explained that it is possible to influence the steering gear directly by the torque on the steering column by mechanically influencing valves or opening which by pressure differences causes a steering action. It is also possible to provide the hydraulic steering gear with electromechanical valves which are influenced by the turning torque applied to the steering column. The steering wheel column could be provided with some kind of torque sensors, e.g. piezoelctric elements, which may indicate the steering torque. The sensed torque may be used to send signals proportional to the resulting force from the manually applied turning force from the steering wheel and the automatically applied turning force by the electrical assistance unit such that a change of torque in the steering column causes the valves to change its positions such that there will be a change of steering torque aid from the hydraulic steering gear. In such an arrangement it will be possible to adjust the hydraulic steering aid to be different for different driving situations for the same torque, e.g. in dependence of the speed, ground surface or steering angle. However, since there is an electrical assistance unit present in the power steering system, such adjustments may be made by the electrical assistance unit as well. The torque provided by the electric motor in the electrical assistance unit may be adapted to provide a torque in dependence of the applied steering torque and steering angle. The steering torque aid from the electrical assistance unit may depend on further parameters such as vehicle speed, the direction of travel, the load of the vehicle, ground surface properties, inclination, yaw rate and/or lateral acceleration for example.

The torque sensor for sensing the applied steering torque may be a turning torque indicating torsion bar. This sensor will thus sense the applied turning from the turning of the steering wheel. This torsion bar may be a part of the steering column, i.e. the torsion bar will allow a certain play when performing a turning action such that there will be a lag in the turning action of the steerable wheels. The maximum allowable play may be in the interval of 5 to 10 degrees. If there is a turning motion which should cause a torque which twists the torsion bar to the limit, there will be a fixed stop and the remaining torque will be absorbed by a stiff connection in the steering column when the steering torque is above the maximum torque limit value. There could of course also be other kind of torque sensors used. The turning torque indicating torsion bar may be connected to the steering column at a portion forming part of the electric assistance unit and/or at a location in between the electric assistance unit and the steering wheel. Hence, the torque indicating torsion bar is in this case located at a portion of the steering column which is between the engagement of the electric motor on the steering column and the steering wheel connection to the steering column. Even though it is also possible to locate the torque indicating torsion bar between the engagement of the electric motor on the steering column and the connection of the steering column to the input shaft of the steering gear, the location of the torsion bar above the electric motor, i.e. the torsion bar is located closer to the steering wheel than the electric motor, provides certain benefits when using the electrical assistance unit to provide a torque on the steering column for filtering undesired influences from fluctuations or dislocations of the wheels to be felt in the steering wheel as will be discussed further below. The turning torque indicating torsion bar may be designed to include and comprises an integrated angle sensor so as to form a torque angle sensing torsion bar in order to provide a compact sensor system for deciding the torque and steering angle.

As briefly mentioned above, the power steering system described herein may be used for filtering undesired steering torques, originating from deflections or vibrations of the steered wheels, from being transported to the steering wheel and cause an unpleasant feeling for the driver. This may be achieved by using the electrical assistance unit to provide a counterforce which counteracts an undesired turning of the steering wheel and thus the steering column. This feature will also help in keeping the vehicle right on the desired track and counteract deviations of the vehicle from the desired traveling direction. In order to be able to control the electric assistance unit to counteract undesired influences from the road on the steering system is it necessary for the control unit to be able to detect if the torque on the steering system is induced by the a driver, e.g. by turning the steering wheel, or if it originates from an undesired turning of the wheels from the road. This could for example be achieved by measuring the turning or angle of the steering column above and below the turning torque indicating torsion bar, i.e. at a first, upper location between the torsion bar and the steering wheel and at a second, lower location between the input shaft to the steering gear and the turning torque indicating torsion bar. These angle sensors could also be present in the torsion bar, preferably at the upper and lower end of the torsion bar. Hence, the torque sensor may be configured to sense a difference between a torque on the steering column induced by turning of the steering wheel and a torque induced by dislocation of the wheels so as to cause a turning torque on the steering column. This could for example be achieved by recognizing which of these angle sensors that indicates a turning first or which sensor which is turned most. The origin of the turning force is there after decided to be the steering wheel if the upper angle sensor is turned first or most and the turning force is decided to originate from dislocation of the wheels if the lower angle sensor is turned first or most. The origin of the turning torque could also be decided based on the turning frequency, e.g. very quick and oscillating turns is most likely originating from the wheels (e.g. driving on a cobble stone road) or a very quick, strong steering impulse turning could be the result of a turning force from hitting a stone by the wheels. Hence, the control unit may be connected to said torque sensor and angle sensor in order to receive input signals which may be used for indicating the magnitude and origin of the turning force. This input could be used by the control unit if it is programmed to interpret the signals to send a control signal to said electrical assistance unit to provide a counter force which counteracts the initial turning force by providing a turning force in the opposite direction of the same magnitude as the initial force if it is decided the initial force is originating from a dislocation of the turning wheels. In case a reliable recognition of the origin of the steering force could be established by frequency analysis or turning torque patterns could it be possible to use only the torsion bar for deciding the origin of the turning torque.

In general, the electric motor comprised in the electrical assistance unit need not to be very powerful. If the motor is used for efficiently counteract small and quick oscillating turning torques from the wheels, these forces are rather small and the need for a strong assistance from the electric motor is not needed. In the cases there is a desire for a stronger force to be active, the turning torque from the electric motor will get assisted by the steering gear such that there will be an amplification of the turning torque in order to achieve the desired total counteracting torque. Since a smaller motor is easier to fit in the steering system and a rather low steering force is needed in general in order to provide enough torque, the maximum power of the electric motor may be lower than 50 Nm, e.g. within the range 15 - 50 Nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Discloses a schematic view of a power steering system

FIG. 2 Discloses a schematic view of an electrical assistance unit

DETAILED DESCRIPTION OF THE DRAWINGS

In figure 1 is schematically described a power steering system 1 according to the invention. The power steering system 1 comprises a steering wheel 2 mechanically connected to a steering column 3. Located on the steering column 3 is an electrical assistance unit 4 which is mechanically connected to the steering column 3. The electrical assistance unit 4 is connected to a control unit 5 in order to recive control input through an output wire 6 from the control unit 5 and to send input signals from various sensors (not shown in this figure) through an input wire 7 to the control unit to be used for deciding the control output to the electrical assistance unit. In order to simplify the figure, the inputs to the control unit 5 is shown to be only incoming from the electrical assistance unit but there may be several other inputs from various relevant sensors in the steering system 1 or other sensors indicating driving or ambient conditions in order to control the electrical assistance unit. The steering column 3 is connected at its end to an input shaft 8 of a hydraulic steering gear 9. When the steering column 3 is turned will the turning movement be transmitted to the steering gear 9 and steering torque is reinforced by controlling the hydraulic pressure from a hydraulic liquid pressure reservoir 10 in a first chamber 11 or second chamber 12. Depending on the turning direction and / or torque direction will the pressure in the first chamber 11 change relative the pressure in the second chamber 12 such that a torque is added in the turning direction. The steering torque is transmitted from the steering gear 9 by a drop arm 13 to a steering arm 14 connected to a first steerable wheel 15 such that the first wheel 15 turns. The first wheel 15 is mechanically connected to a link arrangement 16 such that the steering force is transmitted to a second steerable wheel 17 located on the same axle 18. The steering gear 9 and the transmission of the turning force to the steerable wheels 15, 17 have only been briefly explained since this kind of arrangement is known in this field and is herein merely described for the sake of understanding the invention. The invention will work for other hydraulic steering gear arrangements as well which are designed to provide an additional steering torque in the turning or torque direction. As will be described more in detail in figure 2, the invention is directed to the inclusion of an electrical assistance unit 4 in a power steering system comprising a hydraulic steering gear 9 and how this electrical assistance unit 4 is integrated in the power steering system 1 to cooperate with the hydraulic steering gear 4. As indicated above, the electrical assistance unit 4 is located on the steering wheel column 3 in between the steering wheel 2 and the input shaft 8 of the steering gear 9. By placing the electrical assistance motor 4 at this location is it possible to control the torque provided by the electric assistance unit 4 accurately and allowing it to provide a controlled torque for both providing additional torque to the steering column 3 in order to aid in a steering action initiated by the steering wheel 2 or to be used to counteract undesired torques originating from the dislocation of the wheels 15, 17.

In figure 2 is shown an enlarged view of the part of the steering column 3 comprising the electrical assistance unit 4. As shown in this figure is the electrical assistance unit 4 enclosing the steering column 3. The electrical assistance 4 includes an electric motor 19 having a rotor 20 and a stator 21. The steering column 3 is divided in an upper part 22 which is fixedly connected to the steering wheel 2 (see figure 1) and a lower part 23 which is fixedly connected to the input shaft 8 of the steering gear 9 (see figure 1). In between the upper part 22 and lower part 23 of the steering column 3 is a torsion bar 24 located. The torsion bar 24 is further provided with a first angle sensor 25 close to its upper end, i.e. at where the torsion bar 24 is connected to the upper steering column part 22, and a second angle sensor 26 close to its lower end, i.e. where the torsion bar 24 is connected to the lower steering column part 23. By this arrangement will it thus be possible to detect the degree of torsion of the torsion bar 24 by comparing the angle difference between the first angle sensor 25 and the second angle sensor 26. Hence, the torsion bar 24 will together with the first angle sensor 25 and the second angle sensor 26 form a torque sensor. These angle sensors 25, 26 need thus not indicate the absolute angle but only the relative angle between the sensors in order to decide the magnitude of the twisting force and function as a torque sensor. However, in order to decide in which direction the twisting force is working is it necessary to sense in which direction the upper part of the torsion bar 24, comprising the first, upper angle sensor 25, is twisted relative the lower part of the torsion bar, comprising the second, lower sensor 25. By using angle sensors which indicate the absolute angle may this problem be easily solved by comparing the absolute angles from the first and second angle sensor 25, 26. Another way of deciding the direction of torque could be to compare the angular velocity between the angular sensors 25, 26. Using angular velocity or acceleration of the sensors could also be used for deciding the origin of the turning force. The force should thus be decided to origin from the side which has the highest angular acceleration or velocity, i.e the force is decided to originate from the wheels if the angular acceleration or velocity is indicated higher from the second, lower angle sensor 26 than from the first, upper sensor 25 and if the first, upper sensor 25 indicates a higher angular speed or velocity than the second, lower sensor 26 is the turning force decided to originate from the steering wheel. If the magnitude, turning direction and origin of the torque is decided, an appropriate control signal may be sent to the electrical assistance unit 4. The electrical assistance unit 4 may thus be helpful in both preventing or decreasing a torque from the wheels sensed in the steering wheel as well as providing an additional steering torque to the steering column in order to help in a steering action or creating a desired steering wheel feeling when turning.

The arrangement described above only exemplifies one way of realizing the invention. It is obvious that the angle sensors may be located at other locations, e.g one angle sensor at the steering wheel and another in the steering gear or at the steered wheels. It is neither important the kind of sensors used for this power steering system and other sensors for detecting the magnitude and direction of the torque and/or the steering angle may be used. Likewise, any kind of sensors or sensor system which may detect the origin of the force may be used instead of the sensor system described herein.

It shall also be noted that the above location of the electric motor provides a compact system which may be used without the feature of adapting the system to be able to counteract undesired turning forces from the wheels. The electrical assistance unit could thus be suitable as an amplifier of the steering force from the steering wheel and be used art low velocities when ranging or parking a vehicle provided with the system or serve as a backup steering aid system which is active in case the hydraulic gear stop functioning.

Claims

Ιθ" CLAIMS

1. A power steering system ( 1 ) comprising:

• A steering wheel (2) mechanically connected to steerable wheels (15, 17) via a steering column (3)

• A hydraulic steering gear (9) mechanically connected to the steerable wheels (15, 17) whereby said hydraulic steering gear (9) is adapted to provide an additional steering torque to the steerable wheels (15, 17) in dependence of the mechanically applied steering torque to an input shaft (8) of the hydraulic steering gear (9).

• A torque sensor (24, 25, 26) for sensing the applied torque to the steering

column (3)

• An angle sensor (25, 26) for sensing the steering angle of the steering column (3)

· An electrical assistance unit (4) comprising an electric motor (19) mechanically connected to the steering column (3) in order to provide a turning torque to the steering column (3),

• A control unit (5) connected to receive input signals from said torque sensor (24, 25, 26) and angle sensor (25, 26), said control unit (5) also connected to said electrical assistance unit (4) in order to control the torque applied to the steering column (3) by the electrical assistance unit (4)

characterized in that

• said electrical assistance unit (4) is located such that it provides a torque on the steering column (3) in between said hydraulic steering gear and said steering wheel (2)

2. A power steering system (1) according to claim 1 characterized in that said electric motor (19) is arranged concentrically around said steering column (3) such that the steering column (3) forms part of the rotor (20) of the electric motor.

3. A power steering system (1) according to claim 1 or 2 characterised in that said

hydraulic steering gear (9) is mechanically influenced by the turning torque applied to the steering column (3) by the manually applied turning force to the steering wheel (2) and the automatically applied turning force by the electrical assistance unit (4) such that a change of torque in the steering column (3) mechanically causes a steering torque amplification in the same direction as the applied steering torque.

4. A power steering system (1) according to claim 3 characterized in that a torsion bar in the hydraulic steering gear (9) is influenced by the resulting turning torque such that a turning torque on the steering column (3) in a first rotational direction results in an increased pressure of a hydraulic fluid in a first chamber relative a pressure of the hydraulic fluid in a second chamber such that a steering torque aid is provided in said first direction and a turning torque on the steering column (3) in a second rotational direction, opposite said first rotational direction, results in an increased pressure of a hydraulic fluid in the second chamber relative a pressure of the hydraulic fluid in the first chamber such that a steering torque aid is provided in said second rotational direction.

5. A power steering system (1) according to claim 3 characterized in that said hydraulic steering gear (9) comprises electromechanical valves which are influenced by the turning torque applied to the steering column (3) by the manually applied turning force to the steering wheel (2) and the automatically applied turning force by the electrical assistance unit (4) such that a change of torque in the steering column (3) causes the valves to change its positions such that there will be a change of steering torque aid from the hydraulic steering gear (9).

6. A power steering system (1) according to any previous claim characterised in that said electrical steering assistance unit (4) is adapted to provide a torque in dependence of the applied steering torque and steering angle.

7. A power steering system (1) according to claim 6 characterized in that said electrical steering assistance unit (4) is adapted to provide a torque in dependence of the vehicle speed.

8. A power steering system (1) according to any previous claim characterised in that said torque sensor (24, 25, 26) for sensing the applied steering torque from the turning of the steering wheel (2) is a turning torque indicating torsion bar.

9. A power steering system (1) according to claim 8 characterised in that said turning torque indicating torsion bar is connected to the steering column (3) at a portion forming part of the electric assistance unit and/or at a location in between the electric assistance unit (4) and the steering wheel (2).

10. A power steering system (1) according to claim 8 or 9 characterised in that said

turning torque indicating torsion bar comprises an integrated angle sensor (25, 26) so as to form a torque angle sensing torsion bar.

11. A power steering system (1) according to any previous claim characterized in that said control system(5) is configured to sense a difference between a torque on the steering column (3) induced by turning of the steering wheel (2) and a torque induced by dislocation of the wheels (15, 17) so as to provide a turning torque on the steering column (3).

12. A power steering system (1) according to claim 11 characterized in that said control unit (5) is connected to said torque sensor (24, 25, 26) and receives input signals indicating the magnitude and origin of the turning force, said control unit (5) programmed to send a control signal to said electrical assistance unit (4) to provide a counter force of the same magnitude as a force originating from a dislocation of the turning wheels (15, 17) and working in the opposite direction to the turning force from the wheels (15, 17).

13. A power steering system (1) according to claim 11 or 12 characterized in that said torque sensor (24, 25, 26) comprises a first and a second angle sensor (25, 26) located at different places along the length of the steering column (3) whereby the origin of the turning force is decided by detecting which angle sensor (25, 26) that is first or most influenced by the turning force.

14. A power steering system (1) according to any previous claim said electric motor (19) comprised in the electrical assistance unit (4) has a maximum power of 50 Nm, preferably within the range of 15 - 50 Nm.