GB2030943A

GB2030943A - Power-assisted steering systems

Info

Publication number: GB2030943A
Application number: GB7931062A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1978-09-08
Filing date: 1979-09-07
Publication date: 1980-04-16
Also published as: FR2435379B1; JPS5539892A; DE2839121A1; GB2030943B; FR2435379A1

Abstract

An electro-hydraulic power- assisted steering system is disclosed in which a rotary-angle-dependent sensor responsive to manually applied steering force in used to put into operation a servo system to assist the manual force. The sensor can be in the form of an electric pulse generator 30, Fig. 2, having two probes 31, 32 which produce out of phase signals upon rotation of the steering input shaft 1. Signals from the pulse generator 30 are fed to an evaluation circuit which detects the direction of steering for controlling valves through which pressure fluid is supplied to a hydraulic servo cylinder. The sensor may be an electro-mechanical two- way contact device 35, Fig. 4, in which contacts 36, 37 carried by a movable member 38 are centred by leaf springs 39, 40 secured to a signal ring 41 frictionally engaging a fixed housing. The system is preferably only operative as a parking aid at relatively low vehicle velocities. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electrohydraulic power-assisted steering systems The present invention relates to power-assisted steering systems.

Power-assisted steering systems frequently have torque sensors which are in the form of mechanical-hydraulic control valves or in the form of electrical switches and which are arranged at the input of a steering mechanism. The transmission of signals is usually effected by means of slip-rings in the case of electrical torque sensors in order fully to cover the operating range or range of adjustment which embraces several revolutions of a steering shaft. Owing to the safety requirements, a rotary-angle pick-up of this kind, which is fitted in the steering linkage, has to be of very careful and trouble-free construction, and is therefore expensive.

According to the present invention there is provided an electro-hydraulic power-assisted steering system in which a servo system including a pump, an,accummulatorand a reservoirfora pressure medium, a control valve, and a working cylinder arranged on the steering linkage, has a rotary-angle-dependent sensor responsive to manually applied force for putting the servo system into operation to assist the manual force.

A power-assisted steering system embodying the present invention can have the advantage that a relatively simple pick-up with relatively simple signal evaluation can be used.

In a first embodiment of the invention, the sensor comprises a pulse wheel or disc mounted on the steering shaft, and an electronic pick-up. In a second embodiment, the sensor is a mechanical rotary-angle double contact-maker pick-up which does not require an additional electronic evaluation circuit. By using the "rotary angle" control variable, a desired anthropotechnically favourable transition from the effective range of parking power-assisted steering to the other steering range can be obtained in a particularly advantageous manner without great expense. By way of example, it is conceivable for powerassisted operation in a lower range of vehicle velocity to be cut off only when the steering wheel is not being moved or is approximately in a position for travelling straight ahead. For this solution, the rotary angle has to be evaluated.The embodiment using a pulse disc and pick-up has the advantage that it can be utilised virtually without any appreciable structural intervention in the steering mechanism. Moreover, it is advantageous that, in the case of a rack and pinion steering system, either the steering pinion or the toothed rack can be combined with the pick-up. A further stage of development has the advantage that the rotary-angle pick-up can be connected to an electric direction indicator flasher unit such that, for example, a conventional mechanical return of an actuating lever of the flasher unit can be omitted.

The invention will be further described by way of example with reference to the-accompanying drawings in which: Fig. 1 is a diagram of an electro-hydraulic power-assisted steering system having a sensor, Fig. 2 shows a pulse wheel or disc and pick-up sensor at the input of a steering mechanism, Fig. 3 is a graph showing the time sequence of the signals generated, and Fig. 4 shows an electro-mechanical doublecontact sensor.

An electro-hydraulic power-assisted steering system illustrated in Fig. 1 has a steering linkage comprising a steering shaft 1, a steering mechanism 2, a toothed rack 3 with protective tube and linkages 4 and 5, and a servo system having a pump 6, an accumulator 7, a reservoir 8, a working cylinder 9, and a control valve 10, wherein the latter can be a component part of an hydraulic central control for an anti-lock regulating system for a vehicle braking system. Furthermore, the power-assisted steering system includes an electrical control device 1 The working cylinder 9 can also serve as a conventional steering damper.

A sensor 12 is arranged at the input of the steering mechanism 2 although, alternatively, it could be provided at any other optional location on the steering shaft 1. The sensor 12 has three lead terminals and is connected to the control device 11 by way of three leads 13,14,15.The three leads 13,14,15 are connected in the control device 11 to a switching block or input circuit 1 6 for preparing the "rotary angle" and "direction" signals. Upon rotation of the steering shaft 1 a signal L or R appears at the output of the block 1 6 dependent on the direction of rotation. L and R signals are fed to AND gates 17 and 17' respectively and at the same time are fed to an OR gate 18.The output signal of the OR gate 18 is fed by way of an AND gate 19 to a first timing circuit t1 which, by way of an AND gate 20, actuates a solenoid valve 21 in the valve control unit 10 for a period t1. After this period of time has expired, the actuating signal is blocked at the AND gate 20 by the signal fed, inverted, from the timing circuit t2. Furthermore, the same signal blocks the AND gate 1 9, with the result that one or other of the solenoid valves 21 and 21' is not actuated for the period of time t2.This timed actuation of the solenoid.vaives 21 and 21' is necessary, since, otherwise, in the case of a small steering moment, such as on smooth ice in a parking space, the servo assistance would act up to time when the steering wheel is not longer rotated, and the servo force could be larger than the adjusting force applied. By interrupting the servo force, the servo force is reintroduced at short intervals of time upon further rotation of the steering shaft 1.

It is possible to vary the duty ratio t,/t2, this being particularly advantageous in order to obtain the desired, previously described transition to a vehicle speed range without servo assistance.

Thus, for example, the period of time t, is shortened as the velocity increases, and the period of time t2 is increased. Furthermore, the switching block 16, which selects the rotary angle direction, can control a comparator 22 which supplies a speed signal for enabling or disabling operation of the power-assisted steering as an aid to parking.

By way of this control, the cut-off limit can be increased when the steering wheel is not in the region of its neutral position, or when the steering wheel is moved at the instant at which the cut-off limit is exceeded. In addition to these circuits, the electronic control device 11 has a safety circuit 23 and a circuit 24 for preparing a vehicle velocity signal in response to signals from a rotational speed sensor 25. The purpose of the safety circuit 23 is to detect critical troubles which unintentionally cause the power-assisted steering to occur particularly at higher velocities of the vehicle. By way of example, this can happen when a short-circuit occurs in a power stage in a higher range of velocity, which would lead to permanent activation of the corresponding solenoid valve.If this should occur, the output signal of the power output stage is immediately fed by way of an OR gate 47 to a store whose output signal activates a warning lamp 27 and is at the same time fed to the input of a gate (not shown) and de-activates the solenoid valve. A prerequisite for this is that the velocity signal for a higher velocity must exist at an AND gate 45. It would also be conceivable for a solenoid valve 21 or 21' to have a defect, which solenoid valve, despite the control signal, causes the sensor 12 to supply a corresponding signal for a long period of time, this being caused by the fact that power-assisted action does not take place.In this case, the signal from the power output stage is fed by way of an OR gate and, in parallel with the AND gate 45, by way of a timing circuit 46 having time delay of 0.5 s, and by way of the OR gate 47 to the store which again has the same blocking action as in the last example. A continuous signal from the sensor 12 would be fed to the store by way of an OR gate and again by way of a timing circuit 50 and the OR gate 47.

Fig. 2 shows the sensor 12 arranged at the input of the steering mechanism 2. The steering shaft 1 is shown by hatching. A pulse generator 30 has two feeler pins 31 and 32 which produce two out of phase signals by means of which the input circuit 1 6 is able to evaluate the direction of rotation, The circuit for detecting the direction of rotation is known and is not the subject of the invention. This circuit is frequently used. A further feeler pin or sensor is required when evaluating the neutral position (for travelling straight ahead).

Fig. 3 shows the time characteristic of various signals such as torque Md, signals MWG of the sensor, actuating signal MV for the solenoid valves 21 and 21', and the pressure pin the working cylinder 9. Turning of the steering wheel is associated with a specific increase in torque with respect to time which causes the sensor 12 to transmit pulses to the electronics. The direction of turning is detected by the circuit after the first pulse, this then leading to the above-described actuation of a solenoid valve 21 or 21 ' for the period t,. Actuation of the solenoid valve is interrupted after the expiry of t,. The driver continues to turn the steering wheel during this phase and the rotary angle pulses are again evaluated, thus again leading to the actuation of a solenoid valve 21 or 21'.The driver then subsequently interrupts the movement of the steering shaft 1, thus interrupting actuation of the solenoid valve, since the circuit does not record any movement, that is to say, pulses.

In the example dealt with, the servo force is smaller than the sum of the forces acting upon the steering mechanism. As mentioned initially, cases are also conceivable in which the servo force is larger than the steering forces. In this case, the described control function (timed application of the servo force) interrupts adjustment after the end of actuation.

Fig. 4 shows an electrical-mechanical double sensor 35. This sensor 35 does not require an electrical evaluation circuit Closing of the contacts 36 and 37 results respectively in the signals L and R previously described with reference to Fig. 1. In this example, a driver 38 is connected to the steering shaft and is initially stressed on each side by means of respective leaf springs 39 and 40.

The leaf springs are secured to a signal ring 41 which is connected to the housing (such as the steering mechanism) by frictional force. When in their normal state, the leaf springs 39 and 40, act as return springs and prevent contact-making. On the other hand, turning of the steering shaft results in contact-making, and the servo force is put into operation by way of the electric circuit.

In addition to the forms of the sensors given above, it would be conceivable to construct a sensor as a simple incremental sensor from sliding contacts and a corresponding printed circuit board.

Claims

1. An electro-hydraulic power-assisted steering system, in which a servo system including a pump, an accumulator and a reservoir for a pressure medium, a control valve, and a working cylinder arranged on the steering linkage, has a rotaryangle-dependent sensor responsive to manually applied force for putting the servo system into operation to assist the manual force.

2. A power-assisted steering system as claimed in claim 1, in which the sensor is an electronic rotary angle pick-up and an evaluation circuit is provided for detecting the direction of steering.

3. A power-assisted steering system as claimed in claim 1, in which the sensor is an electromechanical rotary angle pick-up with which is associated a signal ring subjected to friction.

4. A power-assisted steering system as claimed in claim 1 ,2 or 3, in which the sensor is disposed at the input of a steering mechanism.

5. A power-assisted steering system as claimed in any of claims 1 to 4, in which the sensor is fitted in the steering shaft.

6 A power-assisted steering system as claimed in any of claims 1 to 5, including a rotational speed sensor responsive to the vehicle velocity and means associated therewith for varying the range of velocity in which the power-assisted steering system operates.

7. A power-assisted steering system as claimed in any of the claims 1 to 5, in which the powerassisted steering system has a cut-off limit which becomes increasingly effective when increasing vehicle speed.

8. A power-assisted steering system as claimed in any of claims 1 to 7, including a timing circuit whereby operation of the power-assisting servo system can be interrupted and later restored.

9. A power-assisted steering system as claimed in claim 8, in which the time duty ratio of the timing circuit is variable with increasing vehicle velocity.

10. A power-assisted steering system as claimed in any of claims 1 to 9, including a safety circuit.

11. A power-assisted steering system as claimed in any of claims 1 to 10, in which the sensor is also associated with a direction indicator for initiating discontinuance of its operation.

12. A power-assisted steering system as claimed in any of claims 1 to 11, in which the working cylinder arranged on the steering linkage also serves as a steering damper.

13. A power-assisted steering system as claimed in any of claims 1 to 12,0in which the control valve is a component part of a hydraulic central control of an anti-lock regulating system for the vehicle braking system.

1 4. A power-assisted steering system as claimed in any of claims 1 to 13, in which the rotary-angle-dependent sensor is in the form of a pulse disc which is in the form of a printed circuit board and a sliding contact.

1 5. A power-assisted steering system as claimed in claim 14, in which the circuit board is in the form of an optical incremental pick-up.

1 6. An electro-hydraulic power-assisted steering system constructed and arranged and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in Figs. 1 to 3 of the accompanying drawings.

17. An electro-hydraulic power-assisted steering system as claimed in claim 16, but including the sensor constructed and arranged and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in Fig. 4 of the accompanying drawings.