Abstract:
A method for actuating a “steer-by-wire” steering drive mechanism by means of at least two triggerable control motors and gear units downstream of them, by which a gear element, connected to the wheels, can be driven simultaneously, is characterized in that the two control motors can be triggered independently of one another in such a way that by them both, drive forces in both the same direction and opposite directions and of equal or different amounts oriented in both the same and in opposite directions can be generated simultaneously.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for actuating a “steer-by-wire” steering drive mechanism by means of at least two triggerable control motors and gear units downstream of them, by which a gear element, connected to the wheels, can be driven simultaneously. 
     2. Description of the Prior Art 
     In steer-by-wire steering drive mechanisms, either a so-called mechanical fallback plane or redundant control motors are needed. The mechanical fallback plane can be achieved for instance by a direct transmission of force from the steering wheel to the wheels to be steered if the control motors should fail. In the case of redundant control motors, some of these motors can take on the task of the others in emergencies. A steer-by-wire drive mechanism that has redundant control motors is taught for instance by U.S. Pat. No. 4,741,409, which describes single-wheel control. 
     As in all steering systems, even in such steer-by-wire steering drive mechanisms any steering play that occurs is problematic. 
     The object of the invention is therefore to refine a generic method for actuating a “steer-by-wire” steering drive mechanism in such a way that any steering play that may occur is maximally avoided, while producing as little irritating noise as possible. 
     SUMMARY OF THE INVENTION 
     In a method for actuating a steer-by-wire steering drive mechanism of the type defined at the outset, the above object is attained in that the two control motors can be triggered independently of one another in such away that by them both, drive forces in both the same direction and opposite directions and of equal or different amounts oriented in both the same and in opposite directions can be generated simultaneously. 
     Because of the capability of triggering the two control motors independently of one another in such a way that they simultaneously generate drive forces in the same direction or in particular in opposite directions of equal or different amounts, it is made possible and in an especially advantageous way that on the one hand the play is compensated for continuously by the generation of oppositely oriented drive forces. On the other hand, even high steering angle forces can be generated by drive forces in the same direction, which are required in particular in extreme situations. 
     The control motors are triggered such that they generate forces or drive moments in the same direction, if a customarily high steering force has to be exerted, then the two control motors are preferably triggered. 
     To achieve the most optimal possible compensation for the steering play, it is advantageously provided that the motors are triggered such that in normal operation, a steering angle in one direction is brought about only by actuation of one control motor, while simultaneously with this an actuation of the other control motor in the opposite direction is effected to generate a defined braking moment or a defined braking force. In this way, the entire steering drive mechanism is tensed somewhat, which highly effectively prevents any steering play. 
     To enable adapting the braking moment or braking force to different steering requirements, resulting in particular from different driving or road conditions and the like, it is advantageously provided that the magnitude of the braking moment or of the braking force is variable. 
     In another highly advantageous embodiment, the braking moment or the braking force is adjusted such that over all the steering angles, a constant difference in the drive forces generated by the two control motors ensues. Because of this difference in drive forces, which represents a tensing force, continuous tensing of the steering drive mechanism is made possible during normal operation. 
     To improve this tensing still further and in particular also to prevent play during very rapid adjusting motions, in an advantageous embodiment it is provided that superimposed on the forces generated by the two control motors, each of the forces having an opposite sign, is a force component that is proportional to the differential rpm of the control motors. As a result, differential vibrations of the motors, which can arise from elasticities in components of the motors and of the gear units, are in particular damped highly effectively. 
     Purely in principle, the most various motors can be used as the control motors. Hydraulic motors, for instance, are conceivable. Advantageously, electric motors are used as the control motors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects and advantages of the invention will be apparent from the detailed description contained herein below, taken in conjunction with the drawings, in which: 
     FIG. 1, schematic illustrating, a steer-by-wire steering drive mechanism in which the method of the invention is employed; 
     FIG. 2, is, a force graph schematically illustrating a first exemplary embodiment of the method of the invention; 
     FIG. 3, is, a force graph schematically illustrating a further exemplary embodiment of the method of the invention; and 
     FIG. 4, is, a force graph schematically illustrating a yet another exemplary embodiment of the method of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A steer-by-wire steering drive mechanism shown schematically in FIG. 1 includes two control motors M 1 , M 2 , for instance electric motors, each of which is followed by a gear unit G 1 , G 2 , which by way of pinions R 1 , R 2  simultaneously drive a gear element, connected to the wheels RA 1 , RA 2  and taking the form of a rack Z that causes steering motions at the wheels RA 1 , RA 2 . The two motors M 1 , M 2  can be triggered by a control circuit S. 
     Methods for eliminating any steering play that might occur will now be described in conjunction with FIGS. 2-4. 
     In FIGS. 2-4, the forces F 1 , F 2  generated by the control motors M 1  and M 2  respectively are each plotted over the rack force FZ. 
     As seen from FIG. 2, if the rack force FZ is positive, then the control motor M 1  is triggered such that by way of the gear unit G 1  downstream of it and the pinion R 1  actuated by this gear unit, a rack force FZ on the rack Z is generated that is substantially proportional to the drive force F 1 . Simultaneously to this, the control motor M 2  is triggered such that via the gear unit G 2  downstream of it and the pinion R 2 , a constant braking force −FB on the rack is generated. It should be noted that the motor MI exerts a force on the rack Z in one direction, for instance to the left in FIG. 1, while conversely the control motor M 2  generates a force in the opposite direction, for instance to the right in FIG.  1 . Upon a steering angle to the left, in this way, by the control motor M 1  via the gear unit G 1  and pinion R 1 , a leftward-oriented drive force F 1  on the rack Z is generated, while simultaneously a rightward-oriented braking force FB is generated by the drive motor M 2 . Conversely, upon a steering angle to the right, a drive force F 2  to the right is generated on the rack Z by the drive motor M 2  and the gear unit G 2  downstream of it as well as the pinion R 2 , while simultaneously a braking force FB to the left is generated by the drive motor M 1 . 
     As a result, defined tensing forces are generated in the steering drive mechanism, namely a rightward-oriented tensing force in the event of a steering angle to the left and a leftward-oriented tensing force in the event of a steering angle to the right. Because of these tensing forces, it is attained that the gear wheels of the gear units and the pinions R 1 , R 2  all always rest on the same tooth flanks of the rack Z, and as a result plays have no effect. In the example shown in FIG. 1, the pinion R 1  downstream of the control motor M 1  always rests on the left tooth flanks of the rack Z, while conversely the pinion R 2  downstream of the control motor M 2  always rests on its right tooth flanks. 
     If a rack force in the positive direction is for instance required, for instance to accomplish a steering motion to the left, then the control motor M 1  and the gear unit G 1  downstream of it furnishes a motor force F 1 =FZ+FB via the pinion R 1 , while conversely M 2  furnishes a force F 2 =−FB, as FIG. 2 shows. It will be understood that each control motor M 1 , M 2  has to be able to generate forces great enough that the maximum required rack force FZmax is attainable; that is, the applicable relationship must be: Fmax&gt;FZmax+FB; Fmax is the maximum force of one control motor M 1 , M 2 . 
     Then, if for instance when steering while stopped (under poor road conditions) or in other circumstances the individual control motors M 1 , M 2  cannot generate high enough forces to cause a steering angle, then the actuation of the steering drive mechanism can be done in the way explained below in conjunction with FIG.  3 . As FIG. 3 shows, the braking force FB can be varied, and in the case shown in FIG. 3 reduced, if the drive force of the driving control motors M 1 , M 2  is insufficient (range I). If no braking force at all is available any longer (points II), then the control motor M 1  or M 2  that performs braking in the normal operating state can also be triggered in such a way that it generates a drive force in the same direction as the other motor that does the driving in a normal operating state. In that case, however, any steering play that may be present will become operative. This case occurs only in exceptional situations. 
     In yet another exemplary embodiment, explained in conjunction with FIG. 4, the two motors M 1 , M 2  are triggered in such a way that by way of all the steering angles, a constant difference between the drive forces generated by the two control motors M 1 , M 2  is established. This differential force, functioning as a tensing force FV, compensates for any steering play that may be present in the normal operating state. As described above, in these cases as well, when the rack forces are extraordinarily high, both motors M 1 , M 2  are used for driving, but with a change of sign of 1 motor force at a time (see points III), any steering plays that are present become operative. 
     The tensions described above can be further improved by superimposing an additional component on the motor forces F 1 , F 2 ; this component depends on the difference in the rotary speeds of the motors M 1 , M 2  and damps the differential motion. For a proportional imposition, the following equations then apply: 
     
       
           F   1   ′=F   1   +K ×(ω 2 −ω 1 ), 
       
     
     
       
           F   2   ′=F   2   −K ×(ω 2 −ω 1 ) 
       
     
     in which F 1 ′, F 2 ′ represent the forces generated by the motors M 1 , M 2 ; F 1 , F 2  is the force component of the motors M 1 , M 2  that results from the characteristic tension curve; K is an amplification factor; and ω 1 , ω 2  are the rotary speeds of the motors M 1 , M 2 . 
     The addition of such force components prevents plays from becoming operative even during rapid adjusting motions. Furthermore, differential vibrations of the motors, which can occur because of elasticities in the gear unit components, shafts and the like, are damped. 
     If one motor fails, tensing is no longer possible. In that case, the triggering of the motor that is still functioning can be done by way of an emergency program without tension. 
     The especially great advantage of the method described above is considered to be in particular that in designing the steering drive mechanisms, it is no longer necessary to provide for freedom from play. As a result, greater tolerances are allowable in production, and thus the production costs are reduced considerably. By triggering of the motors M 1 , M 2 , the characteristic tension curves can be selected in such a way, and modified via parameters, that either the play does not become operative under any circumstances (tension represented by the characteristic curve of FIG.  2 ), or a play becomes operative only briefly under extremely high steering forces (see FIG. 3, FIG.  4 ). Electric motors with a preferential direction of rotation can be used; the preferential direction of rotation corresponds to the usual drive direction of that particular motor. As a result, either greater motor forces can be generated, or smaller motors can be used. 
     The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments are thereof possible within the spirit and scope of the invention, the latter being defined by the appended claims.