GB2301568A

GB2301568A - Hydraulic power-assisted steering system

Info

Publication number: GB2301568A
Application number: GB9611132A
Authority: GB
Inventors: Bernd Langkamp
Original assignee: Daimler Benz AG; Mercedes Benz AG
Current assignee: Daimler Benz AG
Priority date: 1995-05-31
Filing date: 1996-05-29
Publication date: 1996-12-11
Anticipated expiration: 2016-05-29
Also published as: DE19519875C1; FR2734778A1; GB9611132D0; FR2734778B1; GB2301568B

Abstract

The steering system includes the servomotor and the hydraulic pump (12). The drive train (4,5) between the servomotor (6) and the steering unit contains the dynamometer (19). The hydraulic medium flow to the servomotor is regulated in such a way, that an actual power value determined by the dynamometer corresponds to a nominal value which is dependant on the manual force applied to e.g. the steering wheel. A switch valve (9) is located between the motor and pump. In neutral position, the valve connects both sides of the motor to each other and/or to a reservoir (13), to leave the motor passively movable. In two positions of the valve, one motor side is connected to the pressure side of the pump, while the other motor side is connected to the pump intake side, or the reservoir.

Description

1 Hydraulic servocontrol for a power steering system for motor vehicles

2301568 The invention relates to a hydraulic servocontrol for a power steering system for motor vehicles, with a steering wheel, a control assembly comprising vehicle guiding wheels positively coupled thereto, a hydraulic servomotor drive-connected to the control assembly, and a hydraulic pump adapted to be connected to the servomotor and to be controlled in dependence on a manual force on the steering wheel and capable of being determined by means of a measured-value transmitter or in dependence on a manual force moment.

Such a power steering for motor vehicles is the subject of DE 3,435,544 Al. This describes a rack-and-pinion steering which works with hydraulic power assistance and in which the rack is drive-connected to a doubleacting piston/cylinder unit which serves as a servomotor and the two sides of which are connected to a gear pump reversible in feed direction and having a controllable delivery. The said pump is controlled in terms of feed direction and delivery in dependence on a measured-value transmitter which is arranged on the steering column between the steering wheel and the pinion of the rack-and-pinion steering and which reacts to the torque transmitted between the steering wheel and pinion and/or to the transmitted rotational stroke.

German Offenlegungsschrift 2,915,890 discloses a steering device which is provided for motor vehicles and in which a steering wheel can be rotated against resistance. Depending on the resistance and the direction of rotation of the steering wheel, the delivery of a pump and a switching valve arranged on the pressure side of the pump are controlled, via which switching valve, in each case one side of a double-acting piston/cylinder unit serving as a servomotor can be connected to the pressure side of the abovementioned pump and the other side can be connected to 2 a relatively pressureless hydraulic reservoir. In a middle position of the swit=hing valve, the pressure side of the pump is connected directly to the reservoir, whilst the piston/cylinder unit is blocked hydraulically. The piston/cylinder unit then actuates the guiding wheels of the vehicles analogously to the rotational movement of the steering wheel.

Moreover, in present-day motor vehicles, it is customary to have hydraulic power steering, in which a slide valve, which has two throttle stages leading from the pressure side of a hydraulic pump to a relatively pressureless reservoir, is adjusted in dependence on the actuating force detectable on the steering wheel. These throttle stages each possess an inlet and an outlet throttle, between which a motor connection branches off, the said motor connection being connected in each case to one side of a hydraulic servomotor capable of being driven in two directions. When the slide valve is displaced in one direction, the inlet throttle of one throttle stage is opened increasingly, with simultaneously increasing throttling of the associated outlet throttle, whilst the throttles of the other throttle stage are adjusted in the opposite direction in each case. A finely controllable pressure difference can thereby be generated in one direction or the other between the motor connections, in order, by means of the servomotor, to generate a correspondingly controllable boost force in one direction or the other.

These last-mentioned power steering systems are, admittedly, distinguished by a good control behaviour. Nevertheless, the outlay in terms of construction is comparatively high, especially when there is additionally provided a reaction control, by means of which the manual force is controlled analogously to the boost force generated by the servomotor.

The servocontrols mentioned in the first paragraph are distinguished by a comparatively low outlay in terms of 3 construction. However, there is no possibility of truly sensitive handling.

The present invention seeks to provide an especially cost-effective servocontrol which can be handled sensitively.

According to the present invention there is provided a hydraulic servocontrol for a power steering system for motor vehicles, with a steering wheel, a control assembly comprising vehicle guiding wheels positively coupled thereto, a hydraulic servomotor drive-connected to the control assembly, and a hydraulic pump adapted to be connected to the servomotor and to be controlled in dependence on a manual force on the steering wheel and capable of being determined by means of a measuredvalue transmitter or in dependence on a manual force moment, wherein a dynamometer is arranged in the drive transmission between the servomotor and control assembly, and the inflow of hydraulic medium to the servomotor is controllable in such a way that a f orce actual value determined by the dynamometer corresponds to a desired value dependent on the manual force.

The invention is based on the general idea of predetermining a desired value for the force of the servomotor in dependence on the manual force detectable on the steering wheel and of regulating this servomotor by controlling the inflow of the hydraulic medium according to the desired/actual value comparison.

Such regulation can be achieved at relatively little outlay. It is then guaranteed, at the same time, that the manual f orce changes analogously to the boost force, whilst basically any functional relations between the manual force and boost force can be predetermined.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawing, in which:- Figure 1 shows a basic representation of a power steering according to the invention for motor vehicles in 4 the neutral position, Figure 2 shows a corresponding representation when the steering is actuated in one direction, and Figure 3 shows a diagram which reproduces possible functional relations between the manual force and boost force.

In the example represented, the steering is designed as rack-and-pinion steering. The rotational movement of a steering wheel 1 is transmitted via a steering shaft 2 to a pinion 3 which meshes with a rack 4. The rack 4 is positively coupled via track rods 5 to the guiding wheels (not shown) of the vehicle for the purpose of adjusting the guidance of the latter.

Moreover, a non-toothed part of the rack forms a piston rod of a doubleacting piston/cylinder unit 6, the piston working spaces 7 and 8 of which are connected to one another via a 4/3-way valve 9 in the neutral position, shown in Fig. 1, of this valve 9, so that the piston 10 of the unit 6 and consequently the rack 4 remain passively movable. In the other two positions of the valve 9, in each case one of the piston working spaces 7 and 8 is connected to the pressure side P of a pump 12 driven by electric motor 11, whilst the respectively other piston working space is connected to the suction side S of the pump 12 or to a hydraulic reservoir 13 adjacent to the pump 12.

The valve 9 is tensioned into the neutral position of Figure 1 by springs 14 and is brought by electromagnets 15 and 16 selectively into one switching position or the other and held in the respective switching position, until the electromagnet 15 or 16 assigned to this switching position is de-energized.

An electronic control unit 17 serves for controlling the electromagnets 15 and 16, said control unit also controlling the power of the electric motor 11 and therefore the delivery of the pump 12.

On the input side, the control unit 17 is connected to a first dynamometer 18, the signals of which represent the torque transmitted between the steering wheel I and the pinion 3. For this purpose, the steering shaft 2 can, f or example, be of two-part design, the two parts being connected to one another via a piezoelectric element, by means of which an electrical voltage analogous to the transmitted torque can then be generated, the said voltage being fed to the control unit 10 as an input signal.

Furthermore, the control unit is connected at the input side to a dynamometer 19 which, once again, can be formed essentially by a piezoelectric element. Consequently, the control unit 17 "knows" the steering forces transmitted between the rack 4 or the piston/cylinder unit 6 and the guiding wheels of the vehicle. Dynamometers 19 are in this embodiment assigned to both track rods 5.

The system represented functions as follows:

When the steering wheel 1 is actuated, the dynamometer 18 generates a signal which represents the manual force and which is different depending on the direction of rotation of the steering wheel 1. In response to this signal, the control unit 17 energizes one of the electromagnets 15 and 16 or the other, so that the valve 9 is brought out of the neutral position of Figure 1 into a switching position, for example the switching position of Figure 2.

Simultaneously, the electric motor 11 is switched on or brought to increased power, so that the pump 12 conveys hydraulic medium correspondingly. The piston/cylinder unit 6 consequently generates a boost force assisting the steering movement of the steering wheel 1.

The actual value of this boost force is recorded by means of the dynamometer 19 or respectively the dynamometers 19, the control unit 17 then regulating the electric motor 11 in such a way that a predetermined relation occurs between the manual force recorded by the dynamometer 18 and the boost force recorded by the dynamometer 19.

For example, according to Figure 3, it is possible 6 to design this relation according to the curve K,, that is to say a high boost force F is achieved in one direction or the other even when the manual force moment M is relatively low.

If appropriate, the control unit 17 can take into account additional parameters, such as, for example, positions of a randomly actuable switch or the travelling speed of the vehicle, and vary the functional relation between the manual force moment M and the boost force F, for example according to the curve K2. In this case, a higher boost f orce F is achieved only in the case of a higher manual force moment M.

The ratios of the curve K, are advantageous in the case of a very low travelling speed, for example in the manoeuvring mode. The ratios of the curve K2 are expedient at a higher travelling speed, as a rule the regulating behaviour of the control unit 17 being varied continuously with an increasing travelling speed. The curves K, and K2 represent only the ratios at constant speeds, on the one hand very low speed and on the other hand higher speed.

The regulation of the boost f orce F by means of the control unit 17 is carried out at a comparatively low outlay, since, for this purpose, only change-overs of the valve 9 and a power control of the electric motor 11 have to be carried out.

The dynamometers 18 and 19 can also be of a simple type.

Altogether, therefore, a servocontrol with predeterminable reaction control is achieved at a low outlay in terms of construction.

Contrary to the representation shown in Figure 1, the valve 9 can also be designed as a proportional valve, as represented symbolically in Figure 2.

Moreover, the valve 9 can also be controlled pneumatically.

7 claims 1. A hydraulic servocontrol for a power steering system for motor vehicles, with a steering wheel, a control assembly comprising vehicle guiding wheels positively coupled thereto, a hydraulic servomotor drive-connected to the control assembly, and a hydraulic pump adapted to be connected to the servomotor and to be controlled in dependence on a manual force on the steering wheel and capable of being determined by means of a measured-value transmitter or in dependence on a manual force moment, wherein a dynamometer is arranged in the drive transmission between the servomotor and control assembly, and the inflow of hydraulic medium to the servomotor is controllable in such a way that a f orce actual value determined by the dynamometer corresponds to a desired value dependent on the manual force.

Claims

2. A servocontrol according to Claim 1, wherein a switching valve is

arranged between the pump and the servomotor and is capable of being driven in two directions, which switching valve, in a neutral position, connects the two sides of the servomotor to one another and/or to a reservoir and consequently causes the servomotor to be passively movable, and wherein, in two switching positions of the valve, in each case one side of the servomotor is connected to the pressure side of the pump, with the other side of the servomotor being simultaneously connected to the suction side of the pump or to the reservoir.

3. A servocontrol according to Claim 1 or 2, wherein the valve is biassed into its neutral position by spring means.

4. A servocontrol according to any one of Claims 1 to 3, wherein the valve comprises a proportional valve.

8 5. A hydraulic servocontrol for a power steering system for motor vehicles, substantially as described herein with reference to, and as illustrated in, the accompanying drawings.