Steering system for motor vehicles

A steering system for motor vehicles, has a steering handle, for example steering handwheel, capable of being operated by the driver. A steering actuating drive provides for the steering adjustment of steerable vehicle wheels. A steering-angle desired-value encoder is capable of being operated by the steering handle, and a steering-angle actual-value encoder operates via the steerable vehicle wheels. A regulating and control arrangement operates the steering actuating drive as a function of a comparison of the desired and actual values of the steering angle. The risk of a collision between a vehicle equipped with the steering system and an obstacle is reduced, and the steering system is suitable for steer-by-wire operation by equipping the steering system with collision prevention apparatus which determine steering angles (collision steering angles) making a collision possible between the motor vehicle and an obstacle. The regulating and control arrangement hinders or prevents the setting of a desired steering angle coinciding with one of the collision angles.

BACKGROUND OF THE INVENTION
 This application claims the priority of German application 198 31 071.4,
 filed in Germany on Jul. 10, 1998, the disclosure of which is expressly
 incorporated by reference herein.
 The invention relates to a steering system for motor vehicles, a steering
 handle, for example, steering handwheel, capable of being operated by the
 driver, a steering actuating drive for the steering adjustment of
 steerable vehicle wheels, with a steering-angle desired-value encoder
 capable of being operated by the steering handle, a steering-angle
 actual-value encoder capable of being operated by the steerable vehicle
 wheels, and a regulating and control arrangement operating the steering
 actuating drive as a function of a comparison of the desired and actual
 values of the steering angle.
 A steering system of the foregoing type may be designed, in particular, for
 steer-by-wire operation, in which, where appropriate, a mechanical
 steering column, customary in conventional steering systems, can be
 replaced by another mechanical system or else by a hydraulic system, in
 particular a hydrostatic system.
 Car manufacturers are making great efforts, based on various concepts, to
 increase vehicle safety and, consequently, the safety of vehicle
 occupants. According to one of these concepts, measures are indicated, by
 means of which, for example, the risk of collision of the vehicle with an
 obstacle is reduced. Collisions of this kind may occur, for example, when,
 if a first vehicle changes lane, a second vehicle is in the so-called
 "blind spot" of the first vehicle. Collisions when the vehicle is being
 manoeuvred are less serious, but are all the more frequent. Also, the fear
 of collisions, for example when the vehicle is being parked, often means
 that, when parking spaces are tight, drivers often do not even attempt to
 park the vehicle.
 Electronic parking aids are known for manoeuvring the vehicle. These aids
 generate an acoustic signal, and warn the driver, on the basis of
 range-measuring sensor technology, when a critical distance between the
 vehicle and an obstacle is reached. At higher speeds, parking aids of this
 type are unsuitable for avoiding collisions, for example during a change
 of lane.
 EP 0,738,647 A2 discloses a conventional power steering system with a rigid
 steering column which has a servomotor for assisting a manual steering
 force exerted by the driver. Moreover, the known power steering system has
 means for deciding whether there is the possibility of a collision between
 the vehicle and an obstacle detected in the steering direction. When the
 risk of a collision is detected in the known power steering system, there
 is a reduction in the power assistance, so that a collision course can be
 controlled only by increased manual steering force. At the same time, in
 order to avoid a collision, the known power steering system may work as
 sluggishly as a conventional power steering system in which the power
 assistance has failed.
 The collision avoidance principle used in the known power steering system
 can be applied, however, only to a steering system having a rigid steering
 column. It is not possible to use it in a steering system which works by
 steer-by-wire operation or in a steering system with an open steering
 column, that is to say a steering system without a rigid steering column,
 for example a steering system with a hydraulic rod, a steering column with
 couplings, planetary assemblies or other gearings which interrupt the
 rigid drive-through from the steering wheel to the steered wheels. This is
 because, in steering systems of this type without a rigid coupling between
 the steering handle and the steered wheels, such a reduction in the power
 assistance would not be noticed on the steering handwheel by the driver.
 Thus, if lateral wheel forces act on the steered wheels, changes in
 direction of travel which can no longer be controlled by the driver may
 occur.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide a steering system in which
 a risk of collision between a vehicle equipped with it and an obstacle is
 reduced, while, at the same time, the steering system is to continue to be
 suitable for steer-by-wire operation.
 This problem has been solved according to the present invention by
 providing a steering system comprising a steering handle arranged to be
 operated by a vehicle driver, a steering actuating drive operatively
 arranged for steering adjustment of steerable vehicle wheels, a
 steering-angle desired-value encoder operatively associated with the
 steering handle; a steering-angle actual-value encoder operatively
 associated with the steerable vehicle wheels; a regulating and control
 arrangement arranged to operate the steering actuating drive as a function
 of a comparison of desired and actual values of the steering angle; and a
 collision prevention apparatus, configured to determine steering angles
 defining a collision path between the motor vehicle and an obstacle,
 wherein the regulating and control arrangement is operatively arranged to
 hinder or prevent the setting of a desired steering angle coinciding with
 one of the collision steering angles.
 The invention is based on the general recognition of acting on the steering
 system such that the driver is hindered or even prevented from setting a
 desired steering angle coinciding with a collision steering angle. With
 the aid of this measure, faulty steering manoeuvres leading to collisions
 are intercepted or reduced as early as on the desired-value side of the
 steering system, and the actual-value side of the steering system can
 remain unchanged.
 In particular, the steering actuating drive can transmit the same high
 stabilizing actuating forces to the steerable vehicle wheels. When the
 setting of a desired steering angle coinciding with one of the collision
 angles is hindered or prevented, it follows from the unchanged dependence
 of the actual steering angle on the desired steering angle that an actual
 steering angle coinciding with one of the collision steering angles also
 can be set only with difficulty, or not at all. Thereby, in actual fact, a
 collision course of the vehicle with a detected obstacle is impeded or
 avoided.
 Since the actual-value side of the steering system is unchanged, for
 example, lateral forces acting on the steerable vehicle wheels cannot
 cause any uncontrolled steering manoeuvre. The steering system according
 to the invention is therefore particularly suitable for steer-by-wire
 operation.
 Moreover, the invention provides that the regulating and control
 arrangement is configured to operate the steering actuating drive to
 prevent an actual steering angle from coinciding with one of the collision
 steering angles.
 The invention is also based on the recognition, particularly in a steering
 system working by steer-by-wire operation, that the dependence of the
 actual steering angle on the desired steering angle is modified, to the
 effect that an actual steering angle predetermined by a desired steering
 angle is set only as long as the set actual steering angle does not
 correspond to one of the detected collision steering angles.
 Desired steering angles exceeding the desired steering angle are then
 ignored. This measure also results in the effective prevention of a
 collision between the vehicle and a detected obstacle. Since, in this
 system too, the actuating forces for steering the steerable vehicle wheels
 are unchanged, uncontrolled steering movements cannot occur, in this
 embodiment either, in the event of lateral forces acting on the steerable
 vehicle wheels, as a result of which this steering system, too, can be
 used with steer-by-wire operation.
 While this solution gives the steering system or the vehicle priority over
 the driver's wishes, the above-mentioned solution adopts a different
 approach and leaves ultimate control over the vehicle to the driver.
 Particularly in a development of the first-mentioned embodiment of the
 steering system according to the invention, countersteering apparatus can
 be provided. Thereby, the steering handle capable of being operated by the
 driver can be subjected to a countersteering force which counteracts the
 steering force exerted on the steering handle by the driver. The
 countersteering apparatus contemplated by the present invention can hinder
 the setting of a desired steering angle coinciding with one of the
 collision angles, as long as the countersteering force generated by the
 steering handle is lower than the steering force exerted on the said
 steering handle by the driver, it can even prevent such setting, if the
 countersteering force generated by the steering handle is equal to or
 higher than the steering force exerted by the driver.
 In order to exert the countersteering force, the countersteering apparatus
 may be provided with a brake and/or with an electric motor. Each of these
 components is coupled to the steering handle or to a steering train
 connected to the steering handle in order to generate the corresponding
 countersteering forces on the steering handle.
 Preferably, in the case of a steering system working by steer-by-wire
 operation, a manual force controller can be provided in order to improve
 the driver's driving sensation. The manual-force controller is operated by
 the regulating and control arrangement for simulating lateral forces
 acting on the steering wheels and is coupled to the steering handle, in
 particular via a steering train, in order to transmit reduced simulated
 wheel forces to the steering handle. Consequently, the driver is given the
 sensation of the current road condition and the vehicle driving state.
 A manual-force controller of the foregoing type can serve, in the steering
 system according to the present invention, as countersteering apparatus
 and generate the desired countersteering forces. In such a steering
 system, therefore, the collision prevention measures according to the
 invention can be implemented without any appreciable constructional
 outlay.
 In advantageous embodiments of the steering system according to the
 invention, acoustic and/or optical and/or haptic apparatuses which are
 operated by the regulating and control arrangement can be provided to
 generate an acoustic or optical or haptic signal for the driver in
 response to an attempt to set a steering angle coinciding with a collision
 angle. In addition, with the aid of a warning signal of this kind, the
 driver's attention can be attracted and a corresponding reaction brought
 about.
 In an embodiment which generates a haptic signal in order to warn the
 driver, this haptic warning signal may take the form of an oscillation or
 vibration which is perceptible on the steering handle, in particular on
 the steering handwheel. In this embodiment, the haptic signal may be
 generated, for example, by the above-mentioned manual-force controller or
 the above-mentioned countersteering apparatus. These oscillations or
 vibrations may be superposed on the driver's steering movement, while,
 according to an expedient embodiment, these oscillations and vibrations
 superposed on the steering angle at the steering handwheel are filtered
 out when the steering actuating drive is operated as a function of the
 desired steering angle, so that these oscillations and vibrations are not
 transmitted to the steerable vehicle wheels.
 Whereas the above-mentioned embodiments referred essentially to the
 situation where the driver intends to steer the vehicle in the direction
 which will probably result in a collision, the following embodiments also
 take into account driving situations in which the vehicle is already on a
 collision course at the moment when an obstacle is detected.
 According to an advantageous embodiment of the steering system according to
 the present invention, if the actual steering angle coincides with one of
 the collision angles the regulating and control arrangement can carry out
 a steering action by operating the steering actuating drive
 correspondingly, in order to set a steering angle not coinciding with one
 of the collision steering angles. This steering action is capable of
 taking place independently of the desired steering angle. Thereby,
 collisions can be effectively prevented, particularly when the vehicle is
 being manoeuvred.
 In an expedient embodiment of the above-mentioned invention, the steering
 action independent of the desired steering angle can be carried out as a
 function of the current driving state of the vehicle, and corresponding
 apparatus for detecting the current driving state is provided. For
 example, a steering action of this kind is expedient during slow vehicle
 movements, in order to prevent a collision with an obstacle located at a
 short distance from the vehicle. In contrast to this, the regulating and
 control arrangement may be programmed in such a way that a steering action
 does not take place at higher speeds.
 In a further advantageous embodiment of the steering system according to
 the invention, the regulating and control arrangement can carry out a
 braking action and/or an engine action and/or a gearing action and/or a
 suspension action as a function of the current driving state. For example,
 a vehicle which has begun to swerve at relatively high speed can be
 stabilized considerably better by a controlled action on the engine power,
 the suspension and the metered individual-wheel braking of the vehicle
 than by controlled steering action.
 Other objects, advantages and novel features of the present invention will
 become apparent from the following detailed description of the invention
 when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS
 A steering system according to the invention is shown in the sole figure
 and has a steering handle, e.g. a steering handwheel 201, on which a
 vehicle driver acts in order to introduce a steering force manually into
 the steering system. The steering handwheel 1 is rotationally fixed to a
 steering train 2 which, at its end facing away from the steering handwheel
 1, meshes via a pinion 37 with a steering rod 3. This steering rod 3
 serves for steering steerable vehicle wheels 4 of a steerable axle of the
 vehicle.
 A hydraulic coupling 5 is arranged in the steering train 2 so that, when
 the steering system is operating normally, the coupling 5 is open and
 consequently uncouples the steering handwheel 1 from the steering rod 3.
 The steering system illustrated, when operating normally, therefore works
 on the steer-by-wire principle.
 For the steering system to work by steer-by-wire operation, a
 steering-angle desired-value encoder 6, which is capable of being actuated
 by the steering handwheel 1, is arranged in the steering train 2. The
 steering-angle desired-value encoder 6 generates signal values which are
 correlated with the desired steering angle required in each case. These
 signal values are transmitted via corresponding signal lines 7 to a
 central regulating and control arrangement 8 of the steering system. This
 regulating and control arrangement 8 is preferably computer-assisted and
 contains memories as well as programs, filed in these memories, for
 operating the steering system.
 A steering-angle actual-value encoder 9 is arranged at a suitable point,
 here at that end of the steering train 2 which is assigned to the steering
 rod 3, to detect the current steering angle of the vehicle wheels 4 and
 communicate a signal value correlated therewith to the regulating and
 control arrangement 8 via a corresponding signal line 10.
 By a comparison of the desired and actual values of the steering angle, the
 regulating and control arrangement 8 determines a control or regulating
 signal which is transmitted via a corresponding control line 11 to a
 proportional control valve 12. As a result, the control valve 12 switched
 in a suitable way. The sole figure illustrates the initial position of the
 control valve 12 which the latter assumes when, for example, the supply of
 current to the steering system is interrupted.
 The control valve 12 connects a pressure line 13 so that hydraulic medium
 is sucked out of a hydraulic-medium reservoir 15 via a hydraulic-medium
 pump 14, to connections 16, 17 of a steering actuating drive 18. Depending
 on the desired steering direction, one or the other connection 16, 17 is
 subjected to more pressure than the other. In the illustrated embodiment,
 the steering actuating drive 18 is configured as a double-acting
 piston/cylinder assembly having a piston 19 coupled to the steering rod 3.
 Thereby, an adjusting movement of the piston 19 brings about a steering
 movement of the vehicle wheels 4 via the steering rod 3.
 In the steering actuating drive 18, two chambers 38, 39 are separated from
 one another by the piston 19. One of these chambers communicates with the
 connection 16, and the other chamber communicates with the connection 17.
 The pressure difference between the chambers 38, 39 causes movement of the
 piston. That connection 16 or 17 of the steering actuating drive 18 which
 is connected in each case to the chamber 38 or 39 having the lower
 pressure is connected, via the control valve 12, to a return line 20 which
 returns the hydraulic medium displaced in the steering actuating drive 18
 into the hydraulic-medium reservoir 15.
 A safety valve 21 is arranged between the connections 16, 17 of the
 steering actuating drive 18 and, in an emergency, short-circuits the
 connections 16, 17 in order to prevent the steering from being locked as a
 result of a hydraulic shut-off of the steering actuating drive 18. In the
 illustrated embodiment, the safety valve 21 is in the emergency position.
 When the steering system is operating normally, however, the safety valve
 21 assumes its other position, in which the connections 16, 17 are not
 connected to one another via the safety valve 21. Even here, the safety
 valve 21 automatically assumes its emergency position such as, for
 example, the occurrence of a power failure.
 The connections 16, 17 communicate with pressure sensors 22, 23 which
 measure the current hydraulic-medium pressure in the respective
 connections 16, 17 and, consequently, in the respective chambers 38, 39 of
 the steering actuating drive 18 which communicate therewith. Signal values
 correlated with the pressure values are transmitted to the regulating and
 control arrangement 8 via corresponding signal lines 24, 25.
 In addition to the above-described control signal for operating the control
 valve 12 and, consequently, for operating the steering actuating drive 18,
 the regulating and control arrangement 8 also sends a further control
 signal, via a corresponding control line 26, to a manual-force controller
 27. The latter is configured as, for example, an electric motor and is
 coupled to the steering train 2 via a gearing such as, for example, a disc
 28. So that the driver operating the steering handwheel, is given the
 sensation of lateral forces on the vehicle wheels 4 and therefore the
 sensation of the vehicle driving behavior, the manual-force controller 27
 simulates a counterforce which counteracts the driver's steering force and
 which is transmitted to the steering handwheel 1 via the disc 28 and the
 steering train 2. This measure gives the driver the impression of steering
 against resistance, as in a conventional steering system.
 Furthermore, the driving sensation or the road sensation can be improved
 even further via the manual-force controller 27. If lateral forces acting
 on the vehicle wheels 4 due, for example to road unevenness, generate
 pressure fluctuations in the chambers 38, 39 of the steering actuating
 drive 18 and consequently at the respective connections 16, 17, these
 pressure fluctuations are detected by the pressure sensors 22, 23 and are
 transmitted via the signal lines 24 and 25 to the regulating and control
 arrangement 8. There, corresponding control signals for the manual-force
 controller 27 are calculated, so that the lateral forces triggered by, for
 example, ground unevenness and acting on the vehicle wheels 4 can also be
 simulated on the steering handwheel 1. These lateral forces can be
 perceived at the handwheel 1 by the driver, even though, when the steering
 system is working by steer-by-wire operation, the steering handwheel 1 is
 per se uncoupled from the movements of the vehicle wheels 4 due to the
 open coupling 5.
 In order to activate the steer-by-wire operation of the steering system,
 the coupling 5 is subjected to a hydraulic pressure generated by a second
 hydraulic-medium pump 29 which otherwise serves to, for example, generate
 a hydraulic pressure for a levelling device of the vehicle. Here, when an
 accumulator charging valve 31 is appropriately switched and a coupling
 switching valve 32 is appropriately switched, the delivery side of the
 hydraulic-medium pump 29 is hydraulically connected to the coupling 5 via
 a pressure line 30. When a corresponding hydraulic switching pressure
 prevails at the coupling 5, the latter opens and thereby uncouples the
 steering rod 3 from the steering handle 1.
 In an emergency such as, for example when the electrics of the steering
 system break down, the coupling switching valve 32 assumes the position
 illustrated in FIG. 1. The coupling 5 is then connected to the
 hydraulic-medium reservoir 15, so that the switching pressure in the
 coupling 5 breaks down. At the same time, the coupling 5 closes and, via
 the steering train 2, connects the steering rod 3 to the steering handle
 1, so that the steering forces exerted on the steering handle 1 can be
 transmitted directly to the steering rod 3.
 The accumulator charging valve 31 is also configured to be fail-safe. In
 order to charge a pressure accumulator 40 of the coupling system, the
 accumulator charging valve 31 is adjusted to the right from the
 illustrated mid-position. In order to charge a corresponding pressure
 accumulator (not illustrated) of the levelling device, the accumulator
 valve 31 is adjusted correspondingly to the left. In an emergency, the
 accumulator valve 31 can be switched to a currentless position, so that it
 automatically assumes the illustrated mid-position, in which the switching
 pressure in the coupling 5 breaks down.
 The steering system according to the present invention has, moreover,
 apparatus 33 for preventing a collision of the vehicle with an obstacle.
 This apparatus can comprise, for example, distance sensors and the like
 for detecting the position of an obstacle in relation to the external
 dimensions of the vehicle. In the illustrated embodiment, the collision
 prevention apparatus 33 transmit the relative coordinates of a detected
 obstacle in relation to the vehicle to the regulating and control
 arrangement 8 via a corresponding signal line 34.
 Moreover, via corresponding signal inputs 35, the regulating and control
 arrangement 8 has available the necessary information on the current
 driving state of the vehicle, such as, for example, its speed. With the
 current actual steering angle, the regulating and control arrangement 8
 can then calculate, from the determined relative position of the obstacle,
 a range of steering angles which may lead to a collision between the
 vehicle and the obstacle if they are set at the vehicle wheels 4. The
 functioning of the collision prevention apparatus according to the present
 invention will now be explained. A first vehicle equipped with the
 steering system according to the present invention travels in the
 right-hand lane of a road which has two lanes in the direction of travel.
 A second vehicle in the left-hand lane approaches the first vehicle at
 insignificantly higher speed until it disappears in the blind spot of the
 first vehicle travelling in the right-hand lane. The driver of the first
 vehicle then wishes to overtake a third vehicle travelling in front of
 him, and at the same time overlooks the second vehicle located in the
 blind spot and attempts to begin changing lanes.
 Even while the second vehicle has been approaching, however, the distance
 sensors of the collision prevention apparatus 33 have detected it as a
 potential obstacle and determined corresponding collision steering angles.
 These collision steering angles are constantly updated for every change in
 the relative position between the first and second vehicle.
 In order to prevent a collision between the vehicles, the regulating and
 control arrangement 8, at the latest when the attempt is made to steer
 towards the second vehicle, generates a countersteering force via the
 manual-force controller 27 and hinders the driver from setting a collision
 steering angle on the steering handwheel 1.
 In another embodiment which adopts a different design approach, the
 counterforce generated by the manual-force controller 27 can be so high
 that the steering handwheel 1 cannot be rotated at all in the direction in
 which the regulating and control arrangement 8 expects a collision. In
 both approaches, therefore, the setting of a desired steering angle at
 which there is risk of a collision is prevented or hindered.
 It is also within the contemplation of the present invention to allow the
 steering handwheel 1 to rotate in the direction in which there is a risk
 of collision. The regulating and control arrangement 8, is, however, then
 not converting this rotation into a corresponding operating signal for the
 steering actuating drive 18. In contrast to the above-described
 embodiments, a desired steering angle at which there is the risk of
 collision may be set, but is not converted into a critical actual steering
 angle by the regulating and control arrangement 8.
 The steering system according to the invention can be further improved with
 the aid of additional measures. For example, the regulating and control
 arrangement 8 can evaluate the available signal values in order to
 establish whether the vehicle is to be maneuvered. In this case, for
 example, other, in particular shorter distances between the vehicle and a
 detected obstacle, may be considered critical, than is the case, for
 example, when the vehicle speed is high. Moreover, there can be provision,
 when a specific distance limit value is reached, for activating a vehicle
 brake via a corresponding signal line 36, in order to stop the vehicle
 even before a collision.
 The regulating and control arrangement 8 can also be configured to detect,
 for example, swerving of the vehicle and carry out a braking action on
 individual wheels as well as an action on the engine power, an action on
 the suspension system and an action on the gearing of the vehicle to
 regain control over the vehicle which has begun to swerve, for example on
 black ice.
 It is clear that in situations which have such complexity, the regulating
 and control arrangement 8 of the steering system cooperates, or is at
 least partially integrated into, an on-board computer of the vehicle.
 Moreover, in advantageous embodiments, in addition to hindering the
 operation of the steering handle 1 in the "wrong" direction, a vibration
 can be transmitted to the steering handwheel 1 via the manual-force
 controller 27, so that the driver receives a haptic warning signal.
 Furthermore, a signal lamp or a horn can be activated to warn the driver
 and/or other road users.
 Due to mechanical uncoupling between the steering handle 1 and the
 steerable vehicle wheels 4 in a steer-by-wire system, this system may also
 be designed in such a way that it executes steering maneuvers
 automatically. For example, parking in relatively narrow parking spaces
 could take place automatically thereby. There may also be provision for
 the steering system to execute an avoidance maneuver automatically, with
 correspondingly comprehensive range-measuring or distance-measuring
 instruments then being necessary. Instead of carrying out the steering
 maneuvers automatically, the system can urgently recommend a steering
 maneuver by, for example acoustic signal messages.
 On a so-called emergency level of the steer-by-wire system, i.e., when the
 steering handle 1 is coupled to the steerable vehicle wheels 4 via the
 mechanical steering tain 2, with the coupling 5 closed, the steering
 actuating drive 8 can serve as a servomotor. On this emergency level,
 therefore conventional power steering is available to a greater or lesser
 extent. Similarly, the manual-force controller 27 can also be used as a
 servomotor for assisting the forces exerted on the steering handle 1 by
 the driver.
 For collision prevention, the servomotors (steering actuating drive 18 or
 manual-force controller 27), working alternately or jointly, can then
 assist to a lesser extent, or even block, the driver's steering movements
 in a direction which will probably lead to a collision with an obstacle.
 The setting of a collision angle is thereby hindered or prevented.
 Furthermore, on this emergency level, the servomotors 18 or 27 can be
 activated via the regulating and control arrangement 8, so that
 countersteering forces counteracting the driver's steering force are
 transmitted to the steering train 2.
 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.