Abstract:
A steering system for a motor vehicle comprising at least one steered wheel, an actuator motor and an actuator, wherein the steering movement initiated by the driver of the vehicle and the movement initiated by the actuator motor are superimposed by the actuator to generate a movement of the steered wheel. The actuator motor is activated for movement by a manipulated variable that depends at least on the steering movement initiated by the driver. Significant for the invention is that the manipulated variable is limited in magnitude. The difference between the manipulated variable and a restricted manipulated variable in parallel through an integrated stage and is superimposed upon the manipulated variable to prevent the vibration of a released steering wheel.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a steering system for a motor vehicle. 
     2. Related Art 
     A steering system is known from DE-OS 40 31 316 (corresponding to U.S. Pat. No. 5,205,371) and, as far as relevant for understanding the present invention, illustrated with the aid of FIGS. 1 and 2. The steering movements, or steering wheel angles δ L , applied in such a steering system by the driver through the steering wheel 11, are in the actuator 12, superimposed with the movements of the actuator motor 13, or motor angle δ M . The overall movement thus created, for instance the angle δ L  &#39;, is transmitted via the steering gear 14 or steering linkage 16, respectively, to the steered wheels 15a and 15b for adjustment of the steering angle δ V . The actuator motor 13, may be configured as an electric motor. The functional principle of such power steering system is that the steering can be made very indirect by the gear ratio i U   of the actuator 12, thereby achieving smaller steering wheel moments M L . Very large steering wheel angles δ L  caused thereby are avoided by superimposition of suitable actuator motor angles δ M  allowing adjustment of initial angles δ L  &#39; required with steering angles of usual magnitude, according to the relationship δ L  &#39;=δ L  /i U  +δ M . The motor angle δ M  necessary for steering assist or its set value, respectively, is determined from the steering wheel angle δ L . The steering wheel in such power steering systems tends to vibrate in the released state, especially at standstill of the vehicle, so that audible vibrations of the steering wheel may occur with sufficiently strong stimulation. 
     The objective of the present invention consists in controlling, or regulating, the power steering system to the effect that no appreciable vibrations will be audible. 
     SUMMARY OF THE INVENTION 
     As mentioned above, the invention is based on a steering system for a motor vehicle comprising at least one steered wheel, an actuator motor and an actuator, wherein the steering movement initiated by the driver of the vehicle and the movement initiated by the actuator motor are superimposed by the actuator to generate the steering movement of the steered wheel. To initiate actuator motor movement, the actuator motor is activated by a manipulated variable that depends at least on the steering movement initiated by the driver. Essential for the invention is that said manipulated variable is limited in its magnitude. 
     The limitation of the manipulated variable (generally a motor current limitation) represents the major cause of the vibration tendency of the steering wheel, that is, the fact that the motor current can assume only certain values and therefore, as the case may be, cannot reach its set value. This cause is compensated for by a specific extension of the controller according to the invention. 
     The core of the invention consists in reproducing the limitation of the manipulated variable for the actuator motor in the controller and feeding the difference between an ideal and a restricted manipulated variable back in integrated fashion, and as the case may be, amplified. This brings about, while the limitation is effective, the integration of an offset and its addition to the ideal manipulated variable, the result of which is that the ideal manipulated variable is able to escape the limitation more quickly. The aforementioned vibration tendency of the released steering wheel is effectively avoided thereby. 
     One provision, specifically, is superimposing the difference between the restricted manipulated variable and the ideal manipulated variable in additive fashion upon the restricted manipulated variable by way of an integrating member. 
     As long as the limitation is not effective, the inventional offset remains constant, so that an offset, once created, leads to subsequent permanent control deviations. To avoid such permanent control deviations, care is taken in the following embodiments of the invention that the offset tends toward zero while the limitation is not effective. To realize this idea, essentially five options are named in the following for alternative or combined application. 
     The first option consists in the negative feedback of the value formed in the integrating member, i.e., of the offset, to the integrator input. That is, the difference or the amplified difference is reduced by the difference passed through the integrating member or by the amplified difference passed through the integrating member. A provision may be that said negative feedback of the integrator occurs with the amplification factor α&gt;0. This embodiment has the advantage that this measure for avoiding the permanent control deviation influences in the case of a small amplification factor α the effect of the inventional avoidance of steering wheel vibrations only insignificantly. 
     A further option for avoiding the permanent control deviations consists in performing a negative feedback of the integrator, as the case may be, with the amplification factor α&gt;0, only whenever the limitation is ineffective. This results in a reduced offset with a first-order delay behavior. 
     A further option for elimination of the permanent control deviation consists in ramping the difference passed through the integrating member, prior to the superimposition of the unlimited control signal, at a specific preset speed to the value zero, and at that, whenever the magnitude of the manipulated variable is not restricted, that is, whenever the manipulated variable limitation is ineffective. 
     Another option of avoiding the aforementioned permanent control deviation consists in passing the difference routed through the integrating member prior to superimposition with the unrestricted manipulated variable signal through a delay component with a second-order delay behavior when the magnitude of the manipulated variable is not restricted, that is, whenever the manipulated variable limitation is ineffective. 
     A last option that is provided according to the invention for elimination of the permanent control deviation is passing the difference channeled through the integrating member or, as the case may be, the amplified difference passed via the integrating member through another amplifier stage and superimposing it after such amplification of the difference between the unrestricted and the restricted manipulated variable signal. This embodiment, notably with a small amplification factor α&gt;0, results as well in an only insignificant influencing of the inventional avoidance of the steering wheel vibrations of the released steering wheel. Further favorable embodiments derive from the subclaims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 show schematically the steering system on which the invention in the exemplary embodiment is based. FIG. 3 shows in its parts a and b a block diagram of the steering system; for one, in the case of a steering wheel gripped by the driver (FIG. 3a) and in the case of a released steering wheel (FIG. 3b). FIG. 4 shows with the aid of a block diagram in exemplary fashion the control or regulation, respectively, while FIG. 5a depicts with the aid of a block diagram the inventional control strategy in exemplary manner. FIGS. 5b and 5c show improvements of the inventional control strategy. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is described hereinafter with the aid of an exemplary embodiment. 
     As briefly mentioned above, FIGS. 1 and 2 show a steering system according to the prior art, on which the present exemplary embodiment is based. 
     FIG. 3a shows the basic structure of the control and regulation of the power steering system in the state of the gripped steering wheel, whereas FIG. 3b deals with the situation in the state of the released steering wheel. To that end, block 33 combines schematically the actuator 12, and the actuator motor 13. Block 32 signifies the active component consisting of the actuator and actuator motor 33 and the manipulated variable restriction 34, which actually is a property of the actuator motor 33. Fed to the control, or regulator, 31 proper, on its input end (FIGS. 3a, b) are the steering wheel angle δ L  and the additional steering intervention δ korr . The additional controller input δ korr  serves the superimposition of corrective dynamic steering interventions. Envisaged here, e.g., are steering shares that depend on the state of travel (vehicle speed, yaw movements or instabilities cause by side winds). Furthermore, as regards the corrective steering intervention δ korr , reference is made to the description of the vehicle controller 44 (FIG. 4). 
     Depending on the above steering angle signals and feedbacks, the control signal u is by means of the control, or regulator, 31 fed back to the active component 32. Named as feedback quantities, e.g., are the actual angular position of the actuator motor 13, and the momentary actual value for the motor current. In addition to feeding the steering wheel angle δ L  to the control or regulator, respectively, 31, the steering wheel angle is fed also, directly, to the actuator 12. The active component superimposes the steering wheel angle δ L  initiated by the steering wheel and the steering angle δ M  initiated by the actuator motor, or electric motor 13, respectively, by means of the actuator 12. On the output of active component 32 or actuator 12, respectively, is then the steering movement δ L  &#39; after superimposition. Overall, it follows thus from FIG. 3a to activate the actuator motor 13, depending on the steering wheel angle and, as the case may be, depending on a corrective dynamic steering intervention. 
     FIG. 3b shows the power steering system with the steering wheel released. The state of the released steering wheel is characterized in that the steering wheel angle δ L  is an output quantity of the active component 32 and is not preset by the driver, creating in addition to the controller feedback a further closed-loop active circuit. This circuit tends to vibrations attributable for the most to a manipulated variable restriction, that is, to the fact that the motor current I that powers the actuator can assume only limited values and therefore, as the case may be, cannot reach its set value. Therefore, the invention proposes to use a control that makes allowance for the manipulated variable restriction. 
     FIG. 4 shows the arrangement of the control and regulator which in the normal operation, i.e., with the steering wheel gripped, results in the intended operation of the servo system. To that end, the steering angle δ L  preset by the steering wheel and sensed (sensor 28 in FIG. 2) and, optionally, the linear velocity V X  of the vehicle are fed to the power steering control 41. The corrective steering intervention δ korr  is determined in the vehicle regulator 44, depending on the signals of the sensors 26 (FIG. 2). For that purpose, e.g., the previously mentioned vehicle linear velocity V X , the yaw angle velocity ω and/or the transverse acceleration a y  of the vehicle can be sensed. Aside from the corrective steering intervention δ korr , the set value δ MLH  for the motor angle is determined, as mentioned, with the aid of the power steering control from the steering wheel angle δ L . Determined, for one, by the power steering control and, for another, by the vehicle control, the set values are at the connection point 45 added to the set motor angle δ M ,soll. The motor angle δ M  is updated to its set value δ M ,soll with the aid of a position controller 42. To that end, the momentary actual value δ M ,ist of the motor angle is fed to the position controller 42 as input signal. The output signal I soll  of position controller 42 is the set value for the current controller 43. For current control, the set value of the motor current is at point 46 compared with the momentary actual value I ist  of the motor current, and the difference is fed to the current controller 43 to form control signal u of the actuator motor 13. The current controller 43 features a manipulated variable restriction, since the motor current, naturally, is limited. This leads to the restricted manipulated variable I soll . Shown in FIG. 4, this structure is characterized, in addition to its simplicity, in that it contains no dynamic controller shares, notably no I-shares. 
     To reduce the vibration tendency with the steering wheel released, due to the manipulated variable restriction 34, an extension of the PD position controller 42 is proposed, such as shown in FIG. 5a. The PD position controller 42 itself features the P-amplification K p  (unit 51) and is able to weigh the set/actual value (δ M ,soll, δ M ,ist) derivations with different amplification factors K Ds , or K Di  (units 52 and 53). The proportionally amplified set value is now linked at the connection point 55 with the mentioned, differently weighted time derivations, forming the ideal manipulated variable I id  (ideal motor current). This ideal manipulated variable I id , however, must be limited in the manipulated variable restricting unit 56 to the restricted manipulated variable I soll  which represents the set current value I soll  of the motor or current controller 43, respectively. 
     The core of the invention now consists in reproducing the current limitation 34 of the motor in the controller as unit 56 and feeding the difference between the restricted manipulated variable I soll  and the ideal manipulated variable I id , respectively, back in integrating fashion with the amplification κ. To that end, the difference Δ between the restricted and the unrestricted (ideal) manipulated variable is formed in the connection point 59. Upon passage through the amplifier stage 57 (amplification by the value κ), the difference Δ V  thus amplified is passed to the integrator 58, obtaining the offset x. This offset x is added to the (unrestricted) ideal manipulated variable I id . While the restriction 56 is effective, an offset x is thereby integrated and added to the ideal manipulated variable I id , the effect of the offset being that the manipulated variable escapes the restriction more quickly. 
     As long as the restriction 56 is not effective, the offset x remains constant, so that, once created, an offset x leads then to a permanent control deviation. To avoid such permanent control deviation, care must be taken to have the offset tend toward zero while the restriction 56 is not effective. Available therefor are several options, which will be addressed hereafter: 
     1. In FIG. 5b, the offset x is in addition to the circuit shown in FIG. 5a fed back before the integrating stage 58 by way of amplifier stage 59. Achieved thereby is a permanent negative feedback of the integrator 58 with the amplification factor α&gt;0; that is, the integrator is substituted by a first-order delay member. The offset x tends thus at disappearing difference Δ toward zero. If the offset is not zero, the offset x does no longer grow without restriction, but this measure influences at a small amplification factor α the effect of the inventional avoidance of steering wheel vibrations only insignificantly. 
     2. In FIG. 5c, the controller according to FIG. 5a is modified to the effect that the offset x is by way of the amplifier stage 59&#39; utilized for difference formation between the restricted and the unrestricted manipulated variable signal. Furthermore, the offset signal amplified by the amplifier stage 59&#39; is subtracted from the original offset signal x. The modification of the controller as illustrated in FIG. 5c accomplishes the effect that offset x tends toward zero with a first-order delay behavior only when the difference Δ disappears; that is, the restriction is not effective. With the restriction effective, the offset can grow without restriction. With small amplification factors α&gt;0, only insignificant influences on the inventional avoidance of steering wheel vibrations occur as well. 
     3. For avoiding the mentioned permanent control deviations, a negative feedback of the integrator 58 (FIG. 5b) can be provided for only when the restriction 56 is ineffective. That is, when the (unrestricted) ideal manipulated variable I id  equals the restricted manipulated variable I soll . This results in a reduction of the offset x with a delay behavior of first order. 
     4. A further option for suppression of the permanent control deviation consists in ramping the offset x back to the value 0 with a specific, preset velocity while the restriction 56 is not effective. 
     5. A reduction of the offset x with a delay behavior of second or higher order may as well be provided for while the restriction 56 is not effective. 
     The above third, fourth and fifth measures do not influence the effect of the inventional steering wheel vibration suppression. With these measures it is possible, additionally, to commence with the reduction of the offset x not immediately upon termination of the effectiveness of the restriction 56, but only a certain time thereafter. 
     When the maximum value of the physical manipulated variable restriction 34, or 56, is not constant, the inventional steering wheel vibration suppression can be improved by reproducing said variable maximum value also in the controller. Specifically, this is the case whenever the maximum motor current depends on the motor speed. 
     A further variant of the inventional steering wheel vibration suppression consists in forgoing the reproduction of the restriction in the controller and, instead of the restricted manipulated variable I soll  according to FIG. 5a, 5b and 5c, utilizing a measurement of the real manipulated variable, in the present exemplary embodiment of I ist  in FIG. 4. A prerequisite is that no large dynamic delays exist between the ideal set value and the measured restricted value. 
     In addition to the inventional system, the behavior of the power steering system at released steering wheel can be improved further still by a suitable entering of the angular velocity dδ L  /dt or dδ L  &#39;/dt at the free input δ korr . These entries may be effective as well only temporarily. 
     In summary, it can be said regarding the invention that a control is concerned here that allows for the current limitation in order to prevent vibrations of the released steering wheel. Proposed to that end is an extension of a PD position controller, that is, of a static controller, for making allowance for the manipulated variable restriction. This inventional extension of the controller does not affect the regular function of the power steering system with the steering wheel gripped. The invention provides a simple controller for analog or digital realization. Inventionally extended, the PD position controller can be used also in other cases in which a manipulated variable restriction causes vibration problems. Presented were different options for taking the offset created by the inventional steering wheel vibration suppression back to zero. This enables a sensitive adaptation of the inventional extension to the respective requirements. 
     The vibration tendency of the steering wheel is given at the same extent when using a cascade control instead of a PD motor position servomechanism (without the inventional steering wheel vibration suppression).