GB2395469A

GB2395469A - Controlled planetary steering transmission with hydraulic rack assistance

Info

Publication number: GB2395469A
Application number: GB0323005A
Authority: GB
Inventors: Johann Merz
Original assignee: ZF Lenksysteme GmbH
Current assignee: Robert Bosch Automotive Steering GmbH
Priority date: 2002-10-02
Filing date: 2003-10-01
Publication date: 2004-05-26
Anticipated expiration: 2023-10-01
Also published as: DE10245975A1; FR2848519A1; GB0323005D0; GB2395469B; FR2848519B1

Abstract

A steering system comprises a hydraulic assistance computing unit 13, controlling a valve 2 in response to signals including sensed input torque, and a steering wheel 9 delivering the input torque to a portion of a steering column 10, from which rotation is transmitted via epicyclic gearing 16 and a pinion 11 to a rack 8. The epicyclic gearing 16 comprises a ring gear 19 which may be braked and whose angle of rotation is monitored. The epicyclic gearing 16 may comprise a fixed ring (19a, fig 3) engaging a first set of planet gears (18a, fig 3) driving a second set of planet gears (18b, fig 3), which engage a movable ring gear (19b, fig 3), via sun gears (17a, 17b, fig 3). A ring gear braking element 20 may comprise a motor (22, fig 2), which can provide a controlled reaction force. If intervention is required to deflect vehicle wheels independently of a driver, braking is released so that the steering wheel 9 does not snatch. The system typically comprises a torque angle sensor 12, an accumulator 4 and a pump 1 which is operated when necessary by an engine or motor 14 via a clutch 15.

Description

- 1 Power steering system with hydraulic power assistance Description

5 The invention relates to a power steering system with hydraulic power assistance, in particular for motor vehicles, comprising a steering wheel, which is provided with a steering column and is workingly connected by means of a steering pinion to a toothed rack, wherein a pressure medium pump is provided for generating a hydraulic pressure for the power steering system. Such a power steering system is 10 known from D12 198 29 531 Al.

lihe generic specification discloses a power steering system with hydraulic power

assistance for a motor vehicle, wherein a steering spindle or a steering column provided with a steering wheel is connected to an input element of a steering 15 gear. An output element of the steering gear is connected to the wheels of the motor vehicle that are to be steered. In accordance with a torque present in the steering spindle/input element region two working chambers of a servomotor of a power assistance device are loaded by a servo-pump with pressure medium via a steering valve. The servo-pump and/or the pressure medium pump is in said 20 case driven by a combustion engine.

According to the generic specification two sensors for detecting an angle of

rotation and a torque are disposed on the steering spindle and on the output element of the steering gear respectively. Both the magnitude and the direction 25 of the hydraulic power assistance as well as the resetting of the wheels to be steered are controlled by means of a common, electronically controlled electromagnetic valve. For detecting the steering force a torque sensor is provided. 30 The generic power steering system is a further development of the previously known power steering systems of the type known e.g. from EP O 440 638 B1.

- 2 I3y virtue of the generic power steering system, power assistance is possible by simple means. 1'hc steering precision is moreover improved.

A drawback of the generic specification is that a driver-indcpcudent steering

s intervention, which may be needed e.g. to stabilize the vehicle, is not possible.

From practice superimposed steering systems are known, in which a planetary gear is disposed in the steering column. In said case, the steering wheel (input) engages into the sun wheel. The steering pinion (output) is workingly connected lo to the coupling rod and/or the planet carrier of the planet wheels. The ring gear of the planetary gear is blocked or rotated to the right and/or left by means of an electric motor. By means of said operation the phase position of input and output is adjusted in a known manner in accordance with preset strategies. In such superimposed gears a mechanically controlled servo-valve is used and the Is hydraulic supply of the power steering is effected via a system with an open hydraulic ccntre.

Drawbacks of the known superimposed gears are the overall size, the weight and the energy demand necessary for the use of the relatively powerful electric ho motor. The result is correspondingly high costs.

The underlying object of the present invention is further to improve the known power steering systems with hydraulic power assistance so that, in particular, a driver-independent intervention for controlling the vehicle wheels is possible in a as simple, economical and reliable manner.

According to the invention said object is achieved by means of the features of claim 1.

do By virtue of the solution according to the invention interventions controlled by a computing unit for deflection of the vehicle wheels are possible in a simple and advantageous manner, independently of the driver. Up till now, such

- 3 intcrventions have not been possible owing to the rigid connection between the steering pinion (which is workingly conuectecl to the toothed rack) and the steering wheel, because a deflection of the vehicle wheels controlled by the computing unit (ECU) would inevitably lead to a movement of the steering s wheel that was possibly not controllable by the driver. Up till now, this problem could be solved only by means of so- called steer-by-wire steering systems, in which the steering wheel and/or a steering handle does not have a rigid connection to the toothed rack. In a disadvantageous manner, because of the necessary redundant systems steer-by-wire steering systems are very costly and lo therefore still do not present an alternative. By virtue of the solution according to the invention the advantage of steer-by-wire steering (namely that the computing unit, where necessary, may intervene in a controlling manner) may be realized economically through the use of a planetary gear. 'the computing unit may control the pressure medium pump for generating a hydraulic pressure for Is the power steering system independently of the steering wheel of the driver and hence may generate a suitable pressure for deflection of the toothed rack. In a normal situation, i.e. where no intervention in the vehicle dynamics is necessary, the computing unit controls the hydraulic power steering system in accordance with the parameters that are detected by a steering-side torque-angle sensor and 20 hence reflect the steering request of the driver.

The solution according to the invention does however make it possible for the servo-power and the displacement direction of the toothed rack to be implemented (also) independently of the steering movement since it is 25 electrohydraulically controlled. The steering valve for said purpose is preferably designed as an electromagnetic valve. The connection between the steering wheel and the toothed rack and/or the steering pinion is effected by means of a planetary gear. In said case, preferably the steering wheel is connected to the sun wheel and the steering pinion is connected by the coupling rod (planet carrier) to do the planet wheels. A braking element may engage in a controlled manner into the ring gear of the planetary gear. Said braking element is in said case preferably designed in such a way that the ring gear may both rotate unbraked

- 4 and be completely blockable. Because of the known mode of operation of the planetary gear it is possible to prevent abrupt steering locks of the vehicle wheels that are effected in emergency situations or the like by the computing unit from being transmitted only in a damped manner or not at all via the sun wheel to the s steering wheel. For said purpose, all that is required is a corresponding control of the ring gear of the planetary gear by means of the braking element. The braking element is in said case preferably controlled by the computing unit.

The planetary gear is preferably designed as a known double planet set, wherein lo the braking element engages into the movable ring gear.

According to the invention a steering-angle sensor is moreover provided for detecting the angle of rotation of the ring gear. Through measurement of the angle of rotation of the ring gear relative to the steering angle of the steering-

s wheel-side torque-angle sensor the assignment of the superimposed angle is effected. A re-centring may be effected by setting the two angles to zero, wherein the braking element is separated from the ring gear and the toothed rack is pushed 20 into the intended position through corresponding activation of the electromagnetic valve.

The solution according to the invention advantageously makes it possible to effect driver-independent steering interventions in an economical manner. An Is electric motor is not needed for this purpose. The apparatus according to the invention may be realized in a compact, lightweight manner.

According to the invention it may be provided that for the hydraulic supply of the power steering system a system with a closed hydraulic centre is used. In do addition to the fact that said system is highly dynamic, a low energy demand combined with a high output may be realized.

- 5 In a dcvclopmcnt of the invention it may be provided that the braking element comprises an electric motor for generating a reaction force upon the steering handle. s In contrast to the electric motor that is known in respect of the previously known superimposed gear, the electric motor provided in a development of the solution according to the invention may be of relatively small dimensions.

Unlike the known electric motor that is provided and/or dimensioned for rotation of the ring gear and hence for deflection of the steering valve, the 10 electric motor provided in the present case is merely required optionally to convey a pleasant steering sensation to the driver. I'his may be effected in a simple manner in that the electric motor allows the ring gear to run ahead or trail. Control of the electric motor may be effected in dependence upon the parameters detected by the torque-angle sensor and/or the steering-angle sensor.

In a further alternative development it may equally be provided that the electric motor is used to brake or block the ring gear and hence is designed, e.g. provided with a spur gear as a braking element.

20 In a development of the invention it may further be provided that the electric motor is provided as a redundant system for deflection of the vehicle wheels, in the event of failure of other participating elements, e.g. the pressure medium pump. As Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:-

FIGURI! 1 shows a diagrammatic view of a power steering system according to the invention having an electromagnetic valve with a closed hydraulic centre; FIGURE, 2 shows a diagrammatic view of a steering wheel having a planetary 30 gear disposed in a steering column and having a braking element engaging into the ring gear of the planetary gear; and

- G FIGUR1S 3 shows a construction of the planetary gear as a double planet set.

As power steering systems with hydraulic power assistance are already common knowlcdgc, with rcfcrcocc being made to DE 198 29 531 Al, only the features of s relevance to the invention arc described in detail.

The mode of operation of planetary gears is likewise common knowledge, reference being made merely by way of example to Dubbel, Mechanical Engineering Pocketbook, 19'h edition, G 142 H. Figure 1 shows a power steering system for motor vehicles comprising a pressure medium pump 1, an electromagnetic valve 2, which in the illustrated embodiment is designed as a proportional valve, a pressure medium reservoir 3 and a hydraulic accumulator 4.

As Figure 1 reveals, the hydraulic power steering system illustrated in the cmbodimcot is designed with a closed hydraulic centre (accumulator system).

The hydraulic accumulator 4 may, in an advantageous compact arrangement, be integrated in the pressure medium pump 1.

In an alternative, non-illustrated form of construction the electromagnetic valve 2 may alternatively be designed with an open hydraulic centre (continuous flow system). Depending on the application, corresponding advantages result in a known manner from the two possible constructions.

The pressure medium pump 1 controls a pressure medium flow to two working chambers 5a, 5b. Said working chambers 5a, 5b may be formed in a known manner by a housing 6, in which is disposed a working piston 7, which separates the working chambers 5a, 5b from one another. 'I'he working piston 7 is 30 connected to a toothed rack 8. A steering request presented by the driver by means of a steering wheel 9 in said case acts via a steering column 10 (and/or a

- 7 stccring spindle) upon a steering pinion 11, which is workingly connected to the toothed rack 8.

The steering request prcscoted by the driver by means of the steering wheel 9 is s detected by a stccring-whecl-side torque-angle sensor 12 and transmitted to a computing unit 13. 'lihe computing unit 13 for control of the electromagnetic valve 2 also takes into account and/or processes the servo oil pressure to the working chambers 5a, 5b and further parameters. The electromagnetic valve 2 is therefore activated on the basis of the detected steering torque, the servo oil 10 pressure and further parameters. The fact that the servo oil pressure is also detected and taken into account results in an advantageous steering feel. In the case of the electromagnetic valve 2 with a closed hydraulic centre that is illustrated in the embodiment, the pump pressure may be different from the servo oil pressure to the working chambers 5a, 5b. 'I'hc control of the 15 electromagnetic valve 2 by means of suitable signals of the computing unit 13 as well as the detection of the servo oil pressure is indicated in Figure 1 by two dashed lines. 'he connection and/or the signal transmission of the torque-angle sensor 12 to the computing unit 13 is likewise symbolized by a dashed line in Figure 1.

lthe computing unit 13 is supplied with additional parameters by means of a non illustrated CAN bus. For control of the electromagnetic valve 2 the computing unit 13 may process additional parameters, such as the driving speed, the steering speed, acceleration values, the gear speed, the load condition, the engine speed, 25 ESP signals, ABS signals, the steering angle as well as driver-specific settings.

On the basis of these parameters and the signals of the torque-angle sensor 12, which may advantageously take the form of a piezoelectric element, the computing unit 13 controls the electromagnetic valve 2.

30 On the basis of the signals of the computing unit 13 the electromagnetic valve 2 controls the pressure medium flow to the working chambers 5a, 5b in such a way

- 8 that the steering request presented by the driver by means of the steering wheel 9 is hydraulically assisted.

l'he pressure medium pump 1 is driven by means of a motor, which in the s embodiment is designed as electric motor 14. According to the invention, however, the electric motor 14 may alternatively take the form of a combustion engine. In said case, it is advantageous when the capacity of the pressure medium pump 1 is variable in dependence upon the output required for the hydraulic assistance. A clutch 15 is disposed between the pressure medium 10 pump 1 and the electric motor 14. The pressure medium pump 1 is thereby separable from the electric motor 14.

As tests have revealed, (,()2 emissions and the fuel consumption are thereby reduced. This is due to the fact that in driving situations where no steering Is movement is required, i.e. where no hydraulic assistance is needed, the pressure medium pump 1 is separable from the electric motor 14.

The clutch 15 may be controlled by the computing unit 3 and for said purpose may correspond therewith (see dashed connecting line in Figure 1).

Designing the clutch 15 as an electromagnetic clutch has proved particularly suitable. 'lihe clutch 15 may therefore react quickly and dynamically to variations. By means of the hydraulic accumulator 4, which is provided in the case of electromagnetic valves 2 with a closed centre, a specific steering 25 movement may be guaranteed even in the open state of the clutch 15. A combination of the clutch 15 with the combustion engine of the motor vehicle has proved particularly suitable in terms of the energy advantages. It is thereby possible to dispense with an electric motor 14 of the type illustrated in the embodiment. The clutch 15 may be controlled on the basis of the pressure values in the hydraulic accumulator 4.

- 9 The resetting of the deflected vehicle wheels into the driving straight-ahead position may be effected and/or assisted by means of an additional logic. The previously mentioned parameters processed by the computing unit 13 may be s used here for control purposes.

So that the deflection of the vehicle wheels may be effected by the computing unit 13 independently of the driver, as is meaningful particularly for emergency interventions, according to the invention a planetary gear 16 is provided in the 10 steering column 10.

The planetary gear 16 is basically of a known construction and is shown in detail in Figure 2. Fig. 3 shows a preferred development of the planetary gear 1G as a double planet set. The planetary gear 16 according to Fig. 2 comprises a sun Is wheel 17, which is workingly connected to the steering wheel 9 and/or into which the steering wheel 9 engages as an input. 'l'he planetary gear 16 moreover comprises planet wheels 18, which are workingly connected to the steering pinion 11 by a coupling rod (planet carrier), which is not shown in detail. This means that the output (steering pinion 11) is formed by the coupling rod of the 20 planet wheels 18. The planetary gear 16 moreover in a conventional manner comprises a ring gear 19.

The ring gear 19 may be blocked by means of a braking element 20. By separating the braking element 20 from the ring gear 19 the ring gear 19 may Us rotate again. As Fig. 1 and Fig. 2 basically reveal, the planetary gear 16 is provided with a steering-angle sensor 21 for detecting the angle of rotation of the ring gear 19. Through measurement of the angle of rotation at the ring gear 19 relative to the steering angle at the steering-wheel-side torque-angle sensor 12, assignment of the superimposed angle is effected. The steering-angle sensor 21 30 in said case corresponds with the computing unit 13.

- 10 A re-centring is possible by setting the two angles (of the torqueangle sensor 12 and of the steering-angle sensor 21) to zero, wherein the braking element 20 is separated and the toothed rack 8 is pushed into the corresponding position through suitable activation of the electromagnetic valve 2.

The braking element 20 may engage into the ring gear 19, e.g. through closure of a clutch that is not shown in detail. For said purpose, the braking element 20 may be designed e.g. as a friction brake. As regards the construction of the braking element a large number of possibilities are obvious to the person skilled 10 in the art.

The braking action of the braking element 20 upon the ring gear 19 is controlled by the computing unit 13. The braking action of the braking element 20 upon the ring gear 19 may be metered between a total standstill and unbraked rotation 15 of the ring gear 19.

When it is necessary for the computing unit 13 to intervene, the braking element 20 may be positioned by a corresponding control command of the computing unit 13 in such a way that the driver senses only a slight reaction, which is So controllable by means of the normal manual steering force. The deflection of the vehicle wheels brought about by the computing unit 13 gives rise to a change of position of the toothed rack 8 and hence of the steering pinion 11 and of the coupling rod connected to the steering pinion 11 and/or of the planet wheels 18.

In said case, the braking element 20 partially or even totally clears the ring gear Us 19, so that the planet wheels roll on the sun wheel 17. The sun wheel 17 and hence also the steering wheel 9 therefore move, depending on the position of the braking element 20, only in a metered manner or are completely stationary. The solution according to the invention consequently makes it possible for the computing unit 13 to intervene in a corrective and stabilizing manner without the 30 steering wheel 9 being snatched away from the driver and/or the driver losing control over the vehicle.

- 11 Figure 2 shows a development of the braking element 20 with an electric motor 22 for generating a reaction force upon the steering wheel 9. In said case, it may be provided that the electric motor 22 functions as braking clcmcot 20 and, in addition to generating a reaction force upon the steering wheel 9, also effects the s braking and/or blocking of the ring gear 19. The braking element 20 is therefore additionally designed as an actuator and/or as a controller. For the purpose of engagement into the ring gear 19 the electric motor 22 designed as braking element 20 may be provided e.g. with a spur gear 23. Alternatively, however, any desired gear wheel may be used. The electric motor 22 may engage in a braking lo manner into the ring gear 19 e.g. through closure of a non-illustrated clutch.

rl'he reaction force may be generated in alternative developments also without electric motor 22.

The planetary gear 16 according to Fig. 2 enables a geared through-drive from 15 the steering wheel 9 and from the sun wheel 17 to the steering pinion 11 and to the coupling rod of the planet wheels 18 respectively. As is common knowledge in the case of planetary gears 16, said throughdrive, depending on its construction, may be differently geared.

So Fig. 3 shows a development of the planetary gear 16 as a double planet set, which enables a 1:1 through-drive between steering wheel 9 and steering pinion 11 (and/or steering gear). Here, in a conventional manner one ring gear is designed as fixed ring gear 19a and the other ring gear as movable ring gear 19b.

To avoid unwanted movements of the steering wheel 9, caused by movements 2s introduced on the part of the steering pinion 11, the movable ring gear 19b is partially or completely cleared by the braking element 20 so that the planet wheels 18b roll on the sun wheel 17b. The steering-wheel- side set of the planetary gear 16 likewise comprises planet wheels 18a and a sun wheel 17a.

30 The planet wheels 1 8a and 1 8b are disposed in each case on a not specifically designated planet carrier. As the principle of a planetary gear 16 designed as a

- 12 doublc planet set is common knowledge, a detailed description thereof is not

provided here.

Various control options of the planetary gear 16 are in said case obvious to the s person skilled in the art, given knowledge of the solution according to the Inventlon. Fig. 3 shows a development of the diagrammatically illustrated braking element 20 as worm gear 24, which optionally corresponds with a non-illustrated electric 10 motor.

Claims

1. A power steering system with hydraulic power assistance for motor vehicles, the system comprising: s (a) a steering wheel, which is provided with a steering column and is workingly connected by means of a steering pinion to a toothed rack, (b) a pressure medium pump for generating a hydraulic pressure for the power steering system, (c) a computing unit, which for control of an electromagnetic valve processes 10 signals from a steering-side torque-angle sensor and additional parameters, (d) a planetary gear disposed in the steering column, wherein a braking element engages in a controlled manner into a ring gear of the planetary gear, and (e) a steering-angle sensor for detecting the angle of rotation of the ring gear.

2. A power steering system according to claim 1, characterized in that the steering wheel is workingly connected to a sun wheel and the steering pinion is workingly connected to a coupling rod of the planet wheels of the planetary gear.

3. A power steering system according to claim 1, characterized in that the planetary gear is designed as a double planet set, wherein the braking element engages at the movable ring gear.

as

4. A power steering system according to claim 1, 2 or 3, characterized in that the braking element engages into the ring gear through closure of a clutch.

5. A power steering system according to one of claims 1 to 4, characterized in that the braking element is designed as a friction brake.

6. A power steering system according to one of claims 1 to 5, characterized in that the braking element comprises an electric motor for

- 14 generating a reaction force upon the steering wheel.

7. A power steering system according to one of claims 1 to 6, characterized in that the braking action of the braking element upon the ring s gear is controllable by means of the computing unit.

8. A power steering system according to claim 7, characterized in that the braking action of the braking element upon the ring gear is meterable between a total standstill and unbraked rotation of the ring gear.

9. A power steering system according to one of claims 1 to 8, characterized in that the electromagnetic valve is a valve with a closed hydraulic centre. lS

10. A power steering system according to claims 1 to 9, characterized in that the electromagnetic valve is designed as a proportional valve.

11. A power steering system according to one of claims 1 to 10, To characterized in that the computing unit processes signals of the steering-angle sensor for detecting the angle of rotation of the ring gear.

12. A power steering system according to one of claims I to 11, characterized in that the computing unit for control of the electromagnetic valve Us processes additional parameters such as driving speed, steering speed, acceleration values, gear speed, load condition, engine speed, ESP signals, ABS signals, steering angles and driverspecific settings.