GB2188295A - Power-assisted steering devices - Google Patents

Abstract

A vehicle, in particular a lift truck, with a steering shaft (1) to which is fixed a disk (2) carrying marks (3) that act in cooperation with a sensor (4) to produce electrical control signals that are converted in electrical circuits (5, 6) to control the hydraulic flow to a proportional-acting steering valve device (8) from which fluid flow is direction to a steering cylinder (14). <IMAGE>

Description

SPECIFICATION Power-assisted steering devices The invention relates to power-assisted steering devices incorporating a steering wheel on a steering shaft, and means to detect the turning movement of the steering wheel and generate an electrical signal which controls currentflowing through an electric motor of a power assistance device in a vehicle, for example an industrial truck such as a fork-lift truck.

In one known steering device ofthis kind used for vehicles driven by electric batteries, the partwhich detects the turning movement is a tacho-generator coupled to the steering shaftforturning movement, the output ofwhich is connected via an amplifierto an electric motor whose shaft is in turn connected to the steering shaft, so that torque generated by this electric motor assists any turning ofthe steering shaft caused by the steering wheel, as described in the German Patent Specification No DE-OS 2546 330.

For such steering devices, there are in general two types of system, firstly this previously known system in which an electric motor generates torque to assist the turning movement of the steering shaft, and secondly systems in which the electric motor drives a pump to generate hydraulic power assistance, which is controlled by means of a proportional-acting steering pump and steering valve, the flow controlled by this valve being fed to steering cylinders.

In one known steering device ofthis latter kind the torque occurring in the steering shaft is measured, and this measurement signal is fed to a control device which controls the currentflowing through an electricmotorwhich inturndrivesthepumpwhich delivers hydraulic fluid to the proportional-acting steering valve, as described in the German Patent Specification No DE-OS 29 15890.

In a device of the sortfirst described, in which the torque ofthe electric motor is applied to the steering shaft to assistthe turning movement, it would be expedient to measure the torque acting between the steering wheel and steering column, asthistorque is to be reduced bythetorque generated by the electric motor.

In a steering device in which the flow of pressure medium generated by a pump is fed to the steering cylinder, the required volume of liquid delivered by the pump becomes greaterthefurtherthe steering wheel isturned, i.e. thefurtherthe piston in the steering cylinder is to be moved. This means that with a device of this kind it is expedient to measure theturning movementand notthetorque in the steering shaft, as occurs in the known device.

One object ofthe present invention is to provide an inexpensive power-assisted steering device with expediently matched parts.

The present invention consists in a power-assisted steering device having a steering wheel on a steering shaft for a vehicle, in which detector means are provided to assess any turning movement of the steering shaft and generate an electrical signal which controls current flowing through an electric motor of a steering power assistance device, said detector means comprising a remote sensor which works in contact-free manner to sense marks on a disk connected to the steering shaft.

The proposed solution in accordance with the invention can be used to advantage in many vehicles fitted with power-assisted steering, and essentially exhibits a partwhich detectstheturning movement and turning speed, working without any wear-producing contact, to generate an electrical signal which controls the currentflowing through the electric motor ofthe power-assisted steering device, and thus governing the speed of rotation of the electric motor such that the currentflowing through the electric motor or the flow of pressure medium generated by the pump matches the steering power assitance requirement at any particular moment.

Advantageously, when used for a vehicle fitted with hydraulic power-assisted steering, said steering shaft is connected to a proportional-acting steering pump connected to a steering control valve connected to a power steering pump that is driven by said electric motor and selectively feeds hydraulic pressure medium to the steering cylindervia a pair of pipes.

In this way, the known system functions are combined with one another in an expedient manner, i.e. such that a power-assisted steering device in which flow delivered by a pump is fed to a steering cylinder controlled by the steering device, the flow delivered by the pump is independent of the speed of rotation ofthe steering wheel, and both the extent and speed of the turning movement are detected by the sensorthatworks in contact-free manner, and thusfreeofwearorfaults.

Preferably, said pump is designed and connected in such a way that it not only feeds the power-assisted steering device, but also further load devices.

The invention will now be described with reference to the drawings, in which Figure l is a simplified block-schematic diagram illustrating the operational arrangement of one exemplary embodiment in accordance with the invention; Figure2 similarly shows further details ofthe electrical circuit in one alternative exemplary embodiment constructed in accordance with the invention; and Figure 3 shows details of yet another exemplary embodiment.

In the embodimentshown in Figure 1, a steering shaft 1 is connected to a disk 2 which exhibits black and white marks 3 around its circumference.

Opposite these, a sleeve (not shown) which goes around the steering shaft carries an optical sensor 4, from which a lead 9 runs to an electronic signal preparation stage 5, whose output is taken via a further lead 9a to an electronic signal amplifier 6, and thence via a lead 9bto an electric motor24,which drives a power steering pump 7. The power steering pump7takes in fluid from a reservoir 11 through a pipe 10, and delivers the fluid through a pipe 1 2to a steering control device 8, which contains a proportional-acting steering pump, and is connected via a pair of pipes 13 to opposite ends of a steering cylinder 14.

The manner of operation is as follows. When the manual steering wheel (not illustrated) isturned in any arbitrary manner, the steering shaft 1 and the disk 2 are also turned, with the result that the marks 3 trigger pulses in the sensor 4, which are carried via the leads 9 to the signal preparation stage 5, and the prepared signal then fed to the amplifier 6, from which the outputcurrent is fed to the power steering electric motor 24 which drives the power steering pump 7, and the speed of rotation of the electric motor 24, and thus the speed at which the power steering pump 7 is driven, is proportional to the speed at which the steering shaft 1 is turned, so that the hydraulic flow delivered through the pipe 12 is proportional to the flow required to pass along one of the pipes 13 to an end ofthe steering cylinder 14.

If the relationship between the electrical current flowing in the lead 9b and the speed of rotation is altered by a change in the viscosity of the oil, this onlychangesthesteering ratio, and the principle remains operatively effective. The frequency ofthe pulses emitted by the sensor4 is always proportional to the speed of rotation of the steering shaft 1. A monostable multivibrator is activated by means of an alternating-current amplifier and a rectifier in one preferred circuit arrangement, or in another exemplary embodiment the signal can be converted into an analogue signal by means of a low-pass fiiter, and processed further in an amplifier.

In the alternative exemplary embodiment ofwhich further details are shown in Figure 2, the components 1 to4 are the same as in Figure 1. In this case, a lead 15 runs from the sensor 4to an alternating-current amplifier 16, whose output is connected via a lead 17totransmitan amplified pulseforeverypulsefedin by the lead 1 5. The frequencyfa of the pulses in the lead 17 is still the same asthatofthe pulses in the lead 15. The lead 17 feeds a rectifier 18 whose output is connected via a lead 19 which transmits positive-going pulses at double the frequency of the pulses flowing through the leads 15 and 17.The lead 19 runsto a monostable flip-flop 20, the output of which is in the form of rectangular pulses connected via a lead 21 to an output stage pulse amplifier 22, from which the current passes along a lead 23 to a power steering electric motor 24 which drives the pump 7 (not shown). The electronic assemblies 16,18 and 20 together form a signal-amplifying stage 5. The configuration of the signals is such that a constant train of pulses ofwidth tkand period tflowsthrough the lead 21 at the maximum angularvelocity ofthe turning movement ofthe steering shaft 1 occurring in operation, instead of individual pulses.

In the further exemplary embodiment shown in Figure 3,the components 1 to21 are the same as described with reference to the embodiment shown in Figure 2, but in this case the lead 21 is taken to a frequency/voltage converter 28, from which a lead 25 runs to an output stage 26, from which the current flows along a lead 27 to the power steering electric motor 24.

When it comes to the design of the mechanical parts, it must be remembered that it is necessary to define a turning movement at the manual steering wheel, preferably in terms of speed and, where appropriate, direction, in addition to magnitude, even at the start of a movement, i.e. while the electric motor to be powered by the controlled current is still stationary. If there is any free motion between the manual steering wheel and the next part actuated by the latter, for example because the manual steering wheel has to be turned a certain distance in orderto operate a proportional-acting steering valve, a check must be made to establish whether this free motion is suitable and adequate to achieve the required function.In other cases or even as a matter of principle, it may also be expedient to position an elastic element between the steering shaft, to which the manual steering wheel is fixed and on which the disk2 bearing the marks 3 is connected, either directly or possibly through an intermediate gearing system, and that partto which the turning movements or torquves are to be transmitted from the steering shaft 1, the elasticity of said elastic element being sufficientto make it possible for at least one and preferably a plurality of marks 3 to be moved past the sensor 4through the action of a manual force on the manual steering wheel, even when the electric motor 24 is stationary. The elasticity of conventional tyres can be sufficient if a mechanical link exists between the steering shaft and steerable wheels, for example.

In order to guarantee smooth progressive starting ofthe electric motor 24, it is recommended thatthe electronic control system 16,18,20 and 22 or 26 be so formed that the pulses coming from the control system are timed. The electric motor 24 is then connected through a free-running diode, known per se. Such timing with individual pulses offers the further advantage that adjustment of the pulse duty ratio makes it possible to use one and the same electric motorforvehicles working on different operating voltages.For example, if the electric motor 24 is designed for a vehicle working on an operating voltage of 40 volts and the same steering device is nowto be usedforavehicleworking on an operating voltage of 80 volts, the pulse duty ratio can simply be controlled so that the operating voltage of 80 volts is fed to the electric motor with a pulse duty ratio of 1:1, thus achieving the effect of a 40 volt operating voltage in practice.

The invention can be utilised in any form of vehicle, but is particularly suited to industrial vehicles, such as fork-lift trucks.

The signals from the disk 2 can be recognised by an opto-electronic reflex coupler working with normal light, orwith a laser beam, or by inductive electronic or capacitive electronic or magnetic electronic means. In particular a laser opto-electronic reflex coupler or a magnetic electronic signal recognition system designed like a magnetic tape sound head allows a very high density of signals on the signal disk, so that even small movements are readily recognised and produce an adequate control adjustment.This is important in power-assisted steering devices with a proportional-acting pump, in that a valve-switching function first takes place in the proportional-acting pump when turning movement ofthe steering wheel begins, i.e. the steering wheel has to be turned through a certain angle before the hydraulic steering power assistance becomes effective. When the markings are sufficiently dense, this small movement is sufficient to set the electric motor running and thus produce an adequate delivery from the servo pump.

However, a device constructed in accordance with the invention has one more advantage over the known devices, in which the torque in the steering shaft is measured, inthat even when the steerable wheels of the vehicle reach full lock, known devices will still allow a torque, to be generated by hand at the steering wheel, even a very high torque, with the resultthatthe current flowing through the electric motor control led by the torque is the maxim u m possible, whereas in fact no more hydraulicfluid is required, as the piston in the steering cylinder has reached its end position and cannot move any further.With a device constructed in accordance with the invention in contrast, the steering wheel cannotbeturnedanyfurtherandasa resultthe sensor detecting the turning movement stops emitting the signal, so ceasing to drive current to flow th rough the electric motor. Consequently, significant advantages are obtained through the fact that in embodiments in accordance with the invention the power-assisted steering is controlled contingent upon the turning speed of the steering wheel. Any leakage losses are also offset by maintaining a minimum flow.

The principle ofthe solution proposed in accordance with the invention can also be used in purely mechanical steering systems, in which a hydraulic power assistance device is also provided, such as in the case of cars in which a cylinder is disposed parallel with the steering rack, and is subjected to the action of pressure such that the force at the piston acts in the same direction as the force generated by the torque at the steering wheel, with the result that less force is required at the steering wheel, depending on the force applied hydraulically.

In the case offorklifttrucks it is known forthe steering shaft to be connected to the steerable wheel or steerable wheels by means of a drag element (chain). In this case, and above all with pivoting bogie steering sysstems with a single wheel pivotable about a vertical axis, or two wheels disposed close together and pivotable about a vertical axis, the chain has to transmit all the force thatcan be applied atthe steering wheel, andthe chain sprocket on the steering shaft is generally of a smallerdiameterthanthe manual steering wheel.

Consequentlytheforces in the chain are greaterthan the manual forces, with the result that the chain has to be designed to handlefairlylargeforces.

However, when hydraulic power assistance is provided in accordance with the present invention, it acts on the component which is turned by the chain and on which the steerablewheel ofthevehicleis mounted, so that the chain merely needs to transmit a force sufficient to define the actual movement, while the main partoftheforce is applied to the componentforthesteerablewheels bythe power-assisted steering.

The turning movement signal-generating device, working without contact, can be formed of a pulse-generating disk disposed on the steering shaft, for example a toothed wheel, the individual projectionsorteeth of which cooperate with a magnetic sensor. The arrangement is even simpler when a disk is provided bearing black and white marks which are appropriately illuminated and moved past an optical sensor which triggers a pulse every time such a mark moves past, the sensor being constituted buy a photodiode for example.

The disk bearing these marks can be a disk provided on the vehicle steering shaft for other purposes,for example a slip ring for a sliding contact to transmit the electrical signal from a horn switch on the steering wheel to the electrically-operated horn, for example.

Whereas, with a device of the known kind described above, with an electric motor acting on the steering shaft, in addition to detecting the absolute magnitude of the turning speed it is also necessary todetectthedirectionofrotation oftheforce, sothat the torque generated by the electric motor always acts on the steering shaft in the correct direction, but with most ofthe embodiments described it is only necessary to detect the absolute value for the turning movement, while the conversion into one or other direction is effected by the proportional-acting steering valve, which is activated via the steering proportioning pump operated from the hand wheel.

To reducethe number of auxiliary pumps required on a vehicle it is known forthe pump to be driven at a constant speed, orto be driven at the speed atwhich a primary energy source driving the vehicle, such as a Diesel engine, is running at any moment, and for the delivery of this pump to be fed to a priority valve which always feeds a certain flow or at least a predetermined minimumflowtothesteering system, and only feeds the excess flow to other load devices that are to be driven from this pump. Such devices are relatively expensive.Therefore, in accordance with one expedient embodiment of the invention,thesignal emanating from the part detecting the turning movement controls an electrical valve which is connected to the pump delivery line and controls the flow to the steering device so that the steering device always receives the flow that is actually required, no surplus flow has to be carried off in the steering device, and moreover the full flow not used by the steering device can actually pass to other load units.

Whereas with the known device, in which the torque in the steering shaft is measured, this torque first has to be converted into a signal for the electrical current which passes through the motor, with a device constructed in accordance with the invention pulses are generated directly which can be used for pulse control for the electric motor. Because of the contact-free pick-up a device constructed in accordance with the invention works with freedom from wear and is very inexpensive, particularlywhen a disk already present on the steering shaft is used forthe marks, and the sensor is located in an opening already provided in the steering shaft sleeve for the leads emanating from the sliding contacts.

With the embodiment of the invention using a magnetically-controlled directional three-position, four-portvalve, the proportional-acting steering pump required so far is eliminated, and in its place the simple electromagnetically-actuated control valve is provided and the flow of fluid governed by this valve determines the speed of movement of the piston in the steering cylinder, and the piston moves as long as this valve is open. This means that when the steering wheel is turned, the valve is opened, and opened wider the greater the speed atwhichthe steering wheel is turned, and is automatically closed again when the steering wheel is stationary. A second sensor is necessary with this device, so that the turning direction can be detected in addition to the absolute value for the turning speed, and the valve can be opened in a particular direction depending on the direction in which the steering wheel is turned.

Claims (17)

1. A power-assisted steering device having a steering wheel on a steering shaft for a vehicle, in which detector means are provided to assess any turning movement of the steering shaft and generate an electrical signal which controls currentflowing through an electric motor of a steering power assistance device, said detector means comprising a remote sensorwhich works in contact-free manner to sense marks on a disk connected to the steering shaft.

2. Asteering device as claimed in Claim 1, for a vehicle fitted with hydraulic power-assisted steering, in which said steering shaft is connected to a proportional-acting steering pump connected to a steering control valve connected to a power steering pump that is driven by said electric motor and selectivelyfeeds hydraulic pressure medium to the steering cylindervia a pair of pipes.

3. A steering device as claimed in Claim 2, in which said power steering pump has its output also connected to a load device, in addition to the power-assisted steering device,the delivery pipe being connected to a proportional-acting valve controlled electricaliy bythe signal generated bythe turning movement detector means, from which a pipe runs to the proportional-acting steering valve.

4. Asteering device as claimed in any preceding Claim, in which said remote sensor is an electronic reflex coupling transducerfeeding a signal-generating system.

5. Asteering device as claimed in Claim 4, in which said remote sensor is responsive to light signals, and black and white marks are provided on said disk.

6. Asteering device as claimed in Claim 4 or Claim 5, in which said sensor is responsive to a laser beam reflected by said marks on said disk.

7. A steering device as claimed in Claim 4, in which said sensor is responsive to inductive, capacitive or magnetic marks on said disk.

8. Asteering device as claimed in anyoneof Claims 4to 7, in which said disk is provided to act as a slip ring.

9. A steering device as claimed in any one of Claims 2 to 8, in which said sensor feeds an electronic signal preparation system whose output is fed to an electronic signal amplifying device supplying the electic motor which drives said power steering pump.

10. Asteering device as claimed in Claim 9, in which said detector means feeds a signal preparation device that consists of an alternating-current amplifier, a rectifier, and a pulse converter connected to the latter.

11. A steering device as claimed in any one of Claims 1 to 5, in which said sensorfeedsan alternating-current amplifier whose output is taken via a rectifierto a pulseconverterthatfeedsa frequency/voltage converter and an output from the latter is fed to an amplifier which runs a direct-current electric motor.

12. Asteering device substantially as described with reference to Figure 1 or Figure 2 or Figure 3.

13. Avehicle having a steering device as claimed in any preceding Claim.

14. Aforklifttruckas claimed in Claim 13.

15. A vehicle fitted with a hydraulic power-assisted steering device as claimed in Claim 2 or Claim 3, in which the pump ofthe hydraulic power-assisted steering device is connected to a magnetically-controlled three-position/four-port directional control valve, the two outlets of which are connected to said steering cylinder, said directional control valve being controlled electromagnetically by electromagnets connected to the signal preparation means under the control of the sensor, and an additional sensor being provided to determine the direction of rotation ofthe steering shaft and operate an electrical switch-over device controlling the actuation of one of the two magnets ofthe directional control valve.

16. Asteering device as claimed in Claim 1, when used in a steering system having a permanent mechanical link between a manual steering wheel and steerablevehiclewheels, a power-assisted steering device or power-steering cylinder being subjectto the action of a pressure medium and being disposed parallel with one ofthe displaceable parts of the steering device, whereby the action on the pressure chamber can be controlled by means of said sensor.

17. Asteering device as claimed in any one of Claims 1 to 11, in which said disk provided with said marks and cooperating with said sensor is rigidly connected to a manual steering wheel, and an elastic element is connected between the mechanical system which comprises the manual steering wheel with the disk and that partto which the forces generated atthe manual steering wheel areto be transmitted.