GB2300395A - Electric servo system for vehicle power steering - Google Patents

Abstract

A power steering system comprises a steering wheel 1 connected to a shaft 2 which has a screw thread 4 engaging an axially-moveable member 5 connected, e.g. through a pin-and slot connection 11, 10 to a shaft 12 connected to the steering rack. A transducer 22 senses the position of the member 5. An electric motor 18 steers the wheels 14, 15 in response to the output from the transducer 22 or in accordance with another desired control function with feedback from a steering position sensor 28. The steering can be controlled independently of the position of the steering wheel 1 within the limits of the pin 11 in the slot 10 and this may be used to simulate skidding as an aid to driver training. When a peg 23 is engaged in a groove in the sleeve 5 the axial movement of the sleeve is limited to movement of the peg against springs and allows power-assisted steering using the limited movement to control the motor 18. With the peg engaged manual, unassisted, steering is also possible. The peg 23 is disengaged from the groove by a solenoid 24.

Description

AN IMPROVED MECHANICAL CONTROL APPLICABLE TO VEHICLE STEERING Servo control devices exist which amplify the force applied by a human controller to a mechanical load. For example the steering of a road vehicle may be provided with an hydraulic cylinder, which exerts a deflecting force upon one or more road wheels proportional to that already derived from the torque exerted upon the steering wheel by the driver.

Such a system normally preserves as exact as possible a mechanical relationship between an input means, such as a steering wheel, and an output means such as a road wheel. There can however be special circumstances requiring a degree of lost motion, whereby said road wheel no longer moves in exact accordance with said steering wheel.

This may for example allow a vehicle driver to experience a simulated skid and learn to correct it. Again, a vehicle travelling in extreme weather conditions or on difficult terrain may perform better if the steering response is briefly accentuated or delayed.

Relatively imprecise methods described in the prior art have involved restricting the power supply to an hydraulic cylinder or driving motor. In my British Patent application number GB2270776A, an improved method for generating lost motion comprises the interposition of a further powered mechanism to form a mechanical link between the original actuating means - such as a steering wheel - and the load. Typically however this method involves considerable mechanical changes to the system.

It is possible in principle to adapt a power assisted steering system known in the prior art, wherein a rotary shaft connecting a steering wheel to a steering rack is in two parts, linked by a spring. A small degree of lost motion is possible between the two ends of said rotary shaft, proportionate to torque, and this motion is transmitted to an electrical potentiometer whereby an electric motor geared to the input shaft of said steering rack is energised through a power amplifier.

In this arrangement the lost motion is generally small because of countervailing motion caused by the said electric motor, and steps are usually taken to ensure that it is mechanically limited should the said electric motor or its power supply fail to operate.

Should however the purpose be deliberately to create a large amount of lost motion, both the said spring and the said potentiometer (or other transducer means) must be capable of correspondingly greater deflection: thus the use of a simple spring device or torque bar may be impracticable. Another problem is that the lost motion, however large, remains torque dependent and hence the positional accuracy suffers.

The purpose of this invention is to create a predeterminable amount of lost motion in a positional servo system, while making the said system easily revert on demand to a form of control which minimises lost motion and maintains fail-safe characteristics.

According to this invention I provide a sliding member which is fitted co-axially to a first driving shaft by a reversibly operable screw thread, and is pinned or splined to slide co-axially upon a second driving shaft in such a manner that rotary motion without concurrent axial movement is possible between said first and second driving shafts. Said sliding member moves axially with respect to both of said driving shafts within preset axial limits.

One of said driving shafts forms a mechanical input to the system, the other of said driving shafts is connected to a mechanical load.

A movable peg, which is spring loaded within restricted mechanical clearances in both the radial and axial directions, engages with said sliding member to restrain its axial movement within a range substantially less than said axial limits. Said movable peg may however be withdrawn by disengagement means causing radial movement of said peg which1 due to the said radial spring loading, can be arranged to occur only with external power applied. The axial position of said sliding member is sensed by transducer means, such an an electrical potentiometer, or inferred by sensing the difference in positioning of the said first and second driving shafts.Control logic and power amplifiers allow a plurality of selectable operating conditions with and without said movable peg engaged, whereby a motor driving said load is energised by a power amplifier actuated by the difference between a demand signal, and a feedback signal derived from said transducer means in required proportions. Thrust and journal bearings are provided to maintain the alignment of said first and second driving shafts and to sustain their axial loading. The said reversibly operable screw thread is characterised as being of sufficiently coarse pitch as to be incapable of locking under applied axial load the forces between mating surface lying within their angle of friction. Such a screw thread may be a ball screw known in the prior art.

Optionally said motor may be energised via said power amplifier only in proportion to the axial deflection of the said sliding member under load, to function as a conventional power-assisted servo system, or used with said movable peg withdrawn and said motor and power amplifier actuated by a finite demand signal balanced against a feedback signal. Both said demand signal and said feedback signal are selectable, and may be indicative of the position of the load or of the sliding member so as appropriately to control the amount of lost motion. It will be seen that the system as described fails safe in the absence of power1 in a manner similar to a conventional power assisted steering system.

A preferred embodiment in terms of converting a vehicle steering system controlled by a steering column and rack, and previously provided.with a torque sensor interposed in the steering column to control an electric steering motor1 will now be described.

A steering wheel 1 is mounted on a shaft 2 which passes part way down a casing 3 and terminates in an external screw thread 4. A control sleeve 5, which constitutes the said sliding member1 screws on shaft 2 with matching screw threads, proportioned so that axial clearance exists to give a required degree of lost motion. This may be the total lock-to-lock steering range of a vehicle.

The angular travel of the steering wheel 1 may be separately limited by a nut 6 mounted on an extension of the screw thread 4, and travelling between fixed stops 7 & 8 which are demountable.

Control sleeve 5 is provided with a slot 10 engaging with a pin 11 mounted in an output shaft 12 so as to transmit torque, while also allowing a limited amount of movement within axial limits.

Output shaft 12 connects further down the casing 3 to a steering rack 13 to which a pair of road wheels 14 & 15 are linked by drag links 16 & 17. An electric motor 18 is coupled through a pinion 19 and gear 20 to the output shaft 12 via a clutch 21 which avoids back-driving the motor. Necessary journal bearings are provided but not shown, a thrust bearing function necessary to the operation of the invention is shown diagrammatically at 30.

Control sleeve 5 is further provided with circumferential grooves into which may pass a linkage to a potentiometer 22 (or other position transducer), and also a movable peg 23 controlled by a solenoid 24 which forms the disengagement means to withdraw it against spring pressure when electrical power is applied.

Preferably, movable peg 23 is provided with a rotary bearing 31 at its tip to minimise friction. The solenoid 24 and the operating coil of the clutch 21 are switchable to electrical power supply 9 by the 4-pole control switch 25, or conveniently from computer logic via relays or amplifiers (not shown). The motor 18 is connected to the output of a balanced power amplifier 26, known in the prior art1 which allows it to be driven in either direction.

As shown, the control switch 25 also selects one or other of the potentiometer outputs together with a demand signal, which may be zero volts corresponding to the centre point of the potentiometer supply, to the input of the balanced power amplifier 26.

The first case to be considered is the furthest clockwise position of control switch 25 which causes no electrical power to be applied to motor, clutch or solenoid. Here the movable peg 23 will be present within the groove of the control sleeve, making a small degree of axial movement possible against spring force, and a mechanical constraint due to movable peg 23 passing through a limited aperture 32 affixed to casing 3 . Very little lost motion can occur and the vehicle steers normally without power assistance.

The second case is with the motor and clutch energised; the control amplifier receives from circuit point 27 a "zero" demand signal to be balanced against the small amount of lost motion registered by the potentiometer 22 in terms of the axial position of the control sleeve 5. In this case the vehicle will operate under power steering given that the spring rate of movable peg 23 and the gain of power amplifier 26 are suitable to generate required torques.

In the third case a further potentiometer 28, which senses the output of the steering rack 13, is connected to the balanced power amplifier 25 in place of potentiometer 22, and the movable peg 23 is withdrawn by solenoid 24. Given a zero demand signal at 27, the road wheels 14 & 15 will be actively maintained by the motor 18 in a straight ahead position, and the steering wheel 1 will be without effect so long as the lost motion falls within the mechanical limit set by the slot 10 and pin 11. This mode of operation would serve to simulate a complete loss of road wheel adhesion, occurring perhaps at a predeterminable pressure applied to the vehicle brakes sensed by a pressure switch 33 to enable solenoid 24.

In a fourth case, the movable peg 23 is again withdrawn and a finite demand signal is introduced at terminal 29. This signal voltage may be set by an instructor, or automatically computed from factors such as the attitude and acceleration of the vehicle, and is balanced against the output of potentiometer 22. In this case there will be a controlled deviation of the vehicle steering from that apparently set by the steering wheel, and the reaction force on the steering wheel will change - thus simulating various types and degrees of skidding.

The functions of control switch 25 may be supplanted by electronic switching or relays, and made responsive to extraneous factors such as position/slippage of road wheels, or to time delays. Demand signals and notional switch positions may be stored and implemented within a computer programme, in order to implement required patterns of instruction or testing, or to modify vehicle handling as may be required under particular operating conditions.

Claims (5)

1. A positional servo system comprising a sliding member which is fitted co-axially to a first driving shaft by a reversibly operable screw thread, and is pinned or splined to slide co-axially upon a second driving shaft in such a manner that rotary motion without concurrent axial movement is possible between said first and second driving shafts while said sliding member moves axially with respect to both of said driving shafts within preset axial limits. One of said driving shafts forms a mechanical input to the system, the other of said driving shafts is connected to a mechanical load. A removable peg engages with said sliding member to restrain its axial movement within a range substantially less than said axial limits. The axial position of said sliding member is sensed by transducer means.A motor driving said load is energised by a power amplifier actuated by the difference between an electrical demand signal, indicative of a required degree of lost motion, and a feedback signal derived from said transducer means. The said reversibly operable screw thread is characterised as being a ball screw, or of sufficiently coarse pitch as to be incapable of locking under applied axial load.

2. Apparatus according to claim 1 wherein the said removable peg is constrained by spring pressure to engage with said sliding member, and withdrawn by disengagement means conditionally upon power capable of operating the said motor being applied to the system.

3. Apparatus according to claim 1 wherein via said electrical demand signal is switchable to be derived from a second sensor indicative of the position of the said mechanical load, so that deflection of the said output shaft driving said mechanical load will be actively cancelled (or amplified according to the polarity of signal connection) by the action of said motor.

4. Apparatus according to claim 1 wherein the said electrical demand signal is a switchable or combined function of parameters which may include not only the position of the said mechanical load but also time, acceleration or speed of a vehicle, the actuation of further mechanism (such as a brake) and slippage of a road wheel: said electrical demand signal being derived by processing the outputs of further sensors, together with means of data storage and transrnission known in the prior art.

5. Apparatus according to claim 1 wherein a clutch interposed between said motor and said mechanical load is released in the event of a fail-safe condition in which full operating torque is transmitted between said driving shafts by the action of said removable peg.