Steering system for non-track-bound motor vehicles

A steering system of a non-track-bound motor vehicle having steered vehicle wheels which are operatively connected to a steering handle in normal operation only via an electronic regulating arrangement which checks itself continuously for correct operation. In a manner dependent on a desired-value transmitter actuated by the steering handle and on an actual-value transmitter actuated by way of the vehicle wheels, this regulating arrangement actuates an actuating drive for the steering displacement of the steered vehicle wheels. If the regulating arrangement malfunctions, a positive coupling between the steering handle and the steered vehicle wheels is automatically activated. This also occurs if a characteristic variable correlated with the desired/actual value deviation of the steering angle is outside tolerance thresholds.

BACKGROUND OF THE INVENTION
 This application claims the priority of 198 39 951.0-21, filed Sep. 2,
 1998, the disclosure of which is expressly incorporated by reference
 herein.
 The present invention relates to a steering system for non-track-bound
 motor vehicles, with a steering handle, e.g. steering wheel, actuated by
 the driver, a steering actuating drive for steering displacement of
 steerable vehicle wheels, a desired steering-angle transmitter which can
 be actuated by the steering handle, and an actual steering-angle
 transmitter which can be actuated by the steerable vehicle wheels. A
 regulating arrangement controls the steering actuating drive as a function
 of a comparison between the desired values and actual values of the
 steering angle and continuously checks itself and sensors interacting with
 it for malfunctions. A mechanical or hydraulic positive coupling is
 arranged between the steering handle and the steerable vehicle wheels and,
 when the regulating arrangement is operating correctly, is opened or
 remains open (inactive normal state) and, if the regulating arrangement is
 defective, is automatically closed (active special state).
 Known steering systems which operate on the "steer-by-wire" concept are
 fundamentally familiar and are being developed for future motor vehicles.
 These systems offer the fundamental advantage that, at least as regards
 the regulating arrangement and the associated sensors, they are suitable
 for a very wide variety of vehicles without design modifications. On one
 hand, virtually any transmission ratio between the actuating displacement
 of the steering handle and the change in the steering angle of the steered
 vehicle wheels can be achieved by appropriate programming. There is
 moreover the possibility of connecting the regulating arrangement to
 additional sensors in order to automatically allow for or eliminate
 parameters to be specified, e.g. the effects of side winds.
 In order to be able to guarantee the required degree of safety in the event
 of system faults, provision is made to automatically activate an operating
 mode for abnormal operation or emergency operation if a fault occurs in
 the regulating arrangement or signals that are to be evaluated by the
 regulating arrangement fail. In this operating mode, a positive coupling
 is provided between the steering handle and the steered vehicle wheels.
 The steering system thus operates in principle in the manner of a
 conventional steering system, although the mechanical steering column
 customary in conventional steering systems can be replaced, if required,
 by other mechanical systems or by hydraulic systems, in particular
 hydrostatic systems.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide advantageous measures as
 regards the operating reliability of a steering system of the
 above-mentioned type.
 According to the invention, this object has been achieved by providing that
 the regulating arrangement activates the special state, i.e. active
 positive coupling between the steering handle and the steered vehicle
 wheels, if
 (i) a deviation which exceeds a first threshold value occurs between the
 desired value and the actual value of the steering angle and/or
 (ii) a deviation which exceeds a second threshold value is maintained
 between the desired value and the actual value of the steering angle for a
 time to be specified and/or
 (iii) the time average of the desired/actual value deviation, of the
 steering angle exceeds a third threshold value during a second time period
 to be specified.
 The invention is based on the recognition of the advantages of monitoring
 not only the correct operation of the regulating arrangement and the
 associated sensors but also characteristic variables correlated with the
 desired/actual value deviation of the steering angle. In this way, a
 switch is made to the special mode a sufficiently long time before a
 critical state in the case of creeping functional impairments of the
 steering system. It is advantageous here that whatever the functional
 impairments of the actuating drive, e.g. increasing stiffness due to wear
 or dirt, they cannot impair the safety of the steering system.
 According to a preferred embodiment of the invention, the monitored
 characteristic variables of the desired/actual value deviation of the
 steering angle are varied as a function of parameters to be specified.
 For example, the permissible maximum value of the desired/actual value
 deviation, i.e. the abovementioned first threshold value, can be reduced
 as the speed of travel of the vehicle increases and/or as the actuating
 forces of the actuating unit decrease. This ensures that the steering
 movements of the steered vehicle wheels are monitored more strictly at a
 high speed of travel.
 Similarly, the second and/or third threshold value can be varied, and the
 growth with time or speed of a reduction in at desired/actual value
 deviation can thus be monitored in a corresponding parameter-dependent
 manner.

DETAILED DESCRIPTION OF THE DRAWINGS
 In the embodiment illustrated in FIG. 1, a motor vehicle has steerable
 front wheels 1 which are coupled to one another by way of track rods 2 and
 a rod 3 to allow steering. The rod 3 forms the piston rod of two
 piston-cylinder units 4, 5 which are arranged parallel to one another and
 are each double-acting units.
 The piston-cylinder unit 4 is coupled via two hydraulic lines 6, 7 to the
 two piston working spaces of a double-acting piston-cylinder unit 8, the
 pistons of which are positively coupled mechanically to a steering wheel
 9. The pistons of the unit 8 are displaced to the right and left when the
 steering wheel is turned clockwise and counterclockwise, respectively.
 When the pistons of the piston-cylinder unit 8 are displaced, the steering
 wheel is turned accordingly.
 The steering wheel 9 is furthermore connected in terms of drive to a
 non-self-locking electric motor 10 which, with the motor shaft held fast,
 can operate as a pure force generator whose purpose will be explained
 below.
 A normally closed shut-off valve 11 is arranged between the hydraulic lines
 6, 7 and can be switched out of the closed position shown into its open
 position against the force of a return spring by energizing its actuating
 magnet and automatically moved into the closed position shown or held in
 this position by the return spring when the electric current acting on the
 actuating magnet is switched off.
 The piston-cylinder unit 5 is connected by hydraulic lines 12, 13 to two
 connections of a control valve 14, which is connected by two further
 connections to a relatively unpressurized hydraulic reservoir 15 and a
 hydraulic pressure source, in the embodiment shown a hydraulic accumulator
 16 and pumps 17, 18. The accumulator 16 can be recharged by the pump 17
 and a further pump 18. Both pumps 17, 18 are protected by non-return
 valves 19 against backflow from the delivery side to their inlet side and
 connected on the inlet side to the reservoir 15. The pump 17 is driven by
 an electric motor 20. The pump 18 can be connected to the engine 22 of the
 motor vehicle by a clutch 21.
 A normally open shut-off valve 23 arranged between the hydraulic line 12,
 13 and can be moved out of the open position shown into its closed
 position or held in this closed position against the force of a return
 spring by electrical energization of its actuating magnet.
 An electronic regulating and control arrangement 24 is connected on the
 input side to a transmitter 25 for the actual value of the steering angle
 of the front wheels 1. This transmitter 25 can, for example, interact with
 the rod 3, which executes an actuating stroke analogous to the steering
 angle when the wheels 1 are turned.
 The input side of the regulating and control arrangement 24 is furthermore
 connected to a transmitter 26 for the desired value of the steering angle,
 this transmitter being actuated by the steering wheel 9. The input side of
 the regulating and control arrangement 24 is moreover connected to a
 torque sensor 27 which records the forces or moments transmitted between
 the steering wheel 9 and the electric motor 10.
 Finally, the input side of the regulating and control arrangement 24 is
 connected to a multiplicity of pressure sensors 28, 29 and 30. The signals
 of these sensors represent the hydraulic pressures in the hydraulic lines
 6, 7 and 12, 13 and the pressure at the pressure inlet of the control
 valve 14 respectively.
 On the output side, the regulating and control arrangement 24 is connected
 to the actuating magnets of the shut-off valves 11 and 23 and of the
 control valve 14. The output of the regulating and control arrangement
 also controls the electric motors 10, 20 and the clutch 21.
 In normal operation, the regulating and control arrangement 24 moves and
 holds the changeover valves 11, 23 by energizing the actuating magnets
 assigned to the abovementioned valves 11, 23. Accordingly, the
 piston-cylinder unit 4 is hydraulically decoupled from the piston-cylinder
 unit 8 and from the steering wheel 9. The pressure difference between the
 two piston working spaces of piston-cylinder unit 5 is controlled by
 actuation of the control valve 14, more specifically in the manner
 described below.
 Via the transmitter 25, the regulating and control arrangement 24 records
 the actual value of the steering angle of the front wheels 1. Via the
 transmitter 26 actuated by the steering wheel, the regulating and control
 arrangement 24 receives the desired value of the steering angle. The
 actuating magnets of the control valve 14 are then controlled in
 accordance with a desired/actual value comparison carried out by the
 regulating and control arrangement 24.
 If there is no desired/actual value deviation, the control valve 14 remains
 in the illustrated central position, in which the piston-cylinder unit 5
 is switched hydraulically to the free-running mode and connected to the
 reservoir 15, while the accumulator 16, which is recharged continuously as
 required by the pump 17 or 18 as a function of the signal from the
 pressure sensor 30, is shut off from piston-cylinder unit 5.
 If a desired/actual value deviation occurs, the control valve 14 is
 displaced to the right or left out of the central position shown,
 depending on the direction of the desired/actual value deviation. As a
 result, one piston working space of the piston-cylinder unit 5 is
 connected to the delivery port of the control valve 14 and the other
 piston working space of the unit 5 is connected controllably to the
 reservoir 15 and a controllable pressure difference takes effect at the
 piston-cylinder unit 5. Consequently, the result that the piston-cylinder
 unit 5 produces an actuating force in a direction determined by the
 direction of the desired/actual value deviation of the steering angle. In
 this way, a desired/actual value deviation of the steering value is
 eliminated quickly and the front wheels 1 follow the turning of the
 steering wheel 9.
 From the signals of the pressure sensors 29 and/or from the electrical
 voltages and current intensities at the actuating magnets of the control
 valve 14, the regulating and control arrangement 24 can directly or
 indirectly determine the pressure difference acting at the piston-cylinder
 unit 5. The extent of the pressure difference is correlated with the
 forces or moments transmitted between the steered wheels 1 and the
 piston-cylinder unit 5. In correlation with these forces, the regulating
 and control arrangement 24 determines a desired value for a hand force
 that can be felt at the steering wheel 9 and sets it by corresponding
 activation of the electric motor 10. The torque sensor 27 recording the
 forces or moments acts between the electric motor 10 and the steering
 wheel 9 and hence the actual value of the hand force. Consequently, the
 engine 10 is regulated as a function of a desired/actual value comparison
 for the hand forces. In this way, the driver is provided with haptic
 feedback at the steering wheel 9 on the forces acting between the steered
 vehicle wheels 1 and the piston-cylinder unit 5.
 The regulating and control arrangement 24 monitors itself continuously for
 correct operation. In addition, the signals from the transmitters and
 sensors 25 to 30 connected to the input side of the regulating and control
 arrangement 24 are checked continuously for plausibility. If a system
 fault is detected, the actuating magnet of the shut-off valve 11 is
 de-energized, with the result that the shut-off valve 11 switches to the
 closed position shown in FIG. 1 and the piston-cylinder units 4 and 8 and
 hence the steerable front wheels 1 and the steering wheel 9 are positively
 coupled to one another hydraulically.
 Because the regulating and control arrangement 24 carries out a continuous
 comparison between the desired value recorded by the sensor 26 and the
 actual value of the steering angle, recorded by the sensor 25, for the
 purpose of controlling the control valve 14, it is also achievable, with
 appropriate programming or configuration of the regulating and control
 arrangement 24, to record characteristic values for the desired/actual
 value deviation, as explained at the outset.
 If these characteristic variables indicate a system fault, the steering
 wheel 9 and the steerable front wheels 1 are positively coupled to one
 another.
 This occurs, for example, if the desired/actual value deviation exceeds a
 first threshold value. This can be specified in a parameter-dependent
 manner and, for example, be reduced if the actuating forces of the
 piston-cylinder unit 5 recorded by the pressure sensor 29 fall or assume
 vanishingly small values. The regulating and control arrangement 24 can
 furthermore be coupled directly or indirectly to a measuring element (not
 shown) for the speed of travel of the vehicle, allowing the abovementioned
 threshold value also to be lowered when the speed of travel increases.
 Consequently, the steered wheels 1 are ensured to follow in a particularly
 accurate manner the actuating displacements of the steering wheel 9
 brought about by the driver at a relatively high speed of travel or in the
 case of small externally acting steering forces, e.g. those during normal
 straight-ahead travel.
 In addition or as an alternative, provision can be made to activate the
 positive coupling between the steered wheels 1 and the steering wheel 9 if
 a deviation exceeding a second threshold value between the desired value
 and the actual value of the steering angle is maintained for a
 predetermined time. The time or the second threshold value can be reduced
 with increasing speed of travel and/or decreasing actuating forces of the
 piston-cylinder unit 5. If the value specified for the second threshold is
 sufficiently small, the desired/actual value deviation is ensured lie
 within a narrow tolerance band in the long term.
 Finally, in addition or as an alternative, provision can be made to switch
 on the positive coupling if the time average of the desired/actual value
 deviation exceeds a third threshold during a predetermined second time.
 This approach allows only a limited growth in the desired/actual value
 deviation and a rapid reduction in a previous growth of the desired/actual
 value deviation.
 The embodiment illustrated in FIG. 2 differs from the arrangement shown in
 FIG. 1 essentially in that piston-cylinder unit 6 has been replaced by a
 hydrostatic reversible pump 31. Thereby, the hydraulic medium can be
 exchanged between hydraulic lines 6, 7. In drive terms, this pump 31 is
 positively coupled, on one hand, to the electric motor 10 and, on the
 other hand, to the steering wheel 9.
 In addition, another reversible and, preferably, likewise hydrostatic pump
 32 can connect hydraulic lines 12, 13. This pump is driven by an electric
 motor 33 actuated by the regulating and control arrangement 24.
 The embodiment in FIG. 2 operates substantially the same as the embodiment
 shown in FIG. 1. That is, when the changeover value 11 assumes its closed
 position, the hydrostatic pump 31 and piston-cylinder unit 4 and hence the
 steerable vehicle wheels 1 and the steering wheel 9 are positively coupled
 to one another.
 This positive coupling is performed if the regulating and control
 arrangement 24 detects a malfunction in the steering system. If, in such a
 situation, there is a malfunction in parts of the hydraulic system of
 piston-cylinder unit 5 or this seems possible, the power supply to the
 magnet of the changeover valve 23 is automatically switched off, with the
 result that the changeover valve 23 switches to the open position shown in
 FIG. 2 and piston-cylinder unit 5 is switched hydraulically to the
 free-running mode in all circumstances.
 In normal operation, i.e. when the regulating and control arrangement 24
 does not detect any malfunctions, the changeover valves 11, 23 are held in
 positions by the regulating and control arrangement 24. The electric motor
 33 is then operated by the regulating and control arrangement in one or
 other direction with a higher or lower actuating force as a function of a
 desired/actual value comparison of the steering angle, i.e. as a function
 of the difference between the signals of transmitters 25 and 26, with the
 result that piston-cylinder unit 5 transmits a corresponding actuating
 force to the steered wheels 1.
 Otherwise, the desired/actual value deviation of the steering angle is
 again monitored in the same way as was described above for the embodiment
 in FIG. 1. As soon as characteristic variables of the desired/actual value
 deviation are outside predetermined tolerance thresholds, the steering
 wheel 9 and the steered wheels 1 are positively coupled to one another.
 The embodiment shown in FIG. 3 differs from the embodiments described above
 first in that a mechanical direct drive between the steering wheel 9 and
 the steered vehicle wheels 1 is provided for emergencies.
 In the embodiment shown, the rod 3 is for this purpose arranged as a rack
 which meshes with a pinion 40 that, for its part, is connected
 mechanically to the steering wheel 9 via a clutch 41 spring-loaded in the
 closing direction and an adjoining shaft 42. The clutch 41 can be
 disengaged against the force of its closing spring by an actuating motor
 43 in order to disengage the mechanical direct drive between the steered
 vehicle wheels and the steering wheel 9. In drive terms, the shaft 42 is
 furthermore connected (in a non-self-locking manner) to the
 non-self-locking electric motor 10. Respective torque sensors 44, 45 are
 arranged between the steering wheel 9 and the shaft 42 and between the
 electric motor 10 and the shaft 42.
 For the purpose of determining the actual steering-angle value, the pinion
 40 is connected to the transmitter 25, which is configured as an angle
 sensor. The shaft 42 is connected at the steering wheel 9 to the
 transmitter 26, which is likewise configured as an angle sensor and,
 during normal operation, serves as a desired steering-angle transmitter.
 Otherwise, the rod 3 again forms the piston rod of piston-cylinder unit 5,
 which once again can be actuated in accordance with the arrangement shown
 in FIG. 1 or 2.
 In normal operation, the actuating motor 43 of the clutch 41 is
 continuously excited by the regulating and control arrangement 24 to hold
 the clutch 41 open against the force of its closing spring. The regulating
 and control arrangement 24 also actuates piston-cylinder unit 5 or the
 elements controlling this unit 5 such that the actuating forces produced
 by the unit 5 eliminate any difference between the desired steering-angle
 value supplied by transmitter 26 and the actual steering-angle value
 supplied by transmitter 25. In this connection, attention is drawn to the
 statements relating to FIGS. 1 and 2.
 In the event of any system faults, the actuating motor 43 is switched off
 by the regulating and control arrangement 24. As a result, the clutch 41
 engages and the steering wheel 9, and steered vehicle wheels 1 are
 positively coupled to one another.
 In this third embodiment also, characteristic variables of the
 desired/actual value deviation of the steering angle are once again
 monitored in order, if necessary, to positively couple the steering wheel
 9 and the steered wheels 1 to one another as explained for the embodiments
 in FIGS. 1 and 2.
 As a deviation from the embodiments shown in the drawing, provision can
 also be made to arrange the torque sensors 27 in FIGS. 1 and 2 or 44 in
 FIG. 3 on the steering-wheel side of unit, 8, the pump 31 or electric
 motor 10 and of the sensor 45. This configuration can be advantageous
 because the signals of sensors 27, 44 are then affected less by friction
 that could be caused by unit 8, the pump 31 or the electric motor 10.
 The foregoing disclosure has been set forth merely to illustrate the
 invention and is not intended to be limiting. Since modifications of the
 disclosed embodiments incorporating the spirit and substance of the
 invention may occur to persons skilled in the art, the invention should be
 construed to include everything within the scope of the appended claims
 and equivalents thereof.