TECHNICAL FIELD 
     The present invention relates to a method and system of controlling an electromechanical axial setting device, particularly suited for friction couplings. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method of returning an electromechanical axial setting device, wherein the axial setting device includes the following: two setting rings centered on a common axis, one of which is axially held, with the other one being axially displaceably mounted, and one of which is rotationally fixedly held in a housing, with the other one being rotatingly drivable. The two setting rings, on their respective end faces facing one another, each include an identical plurality of circumferentially extending grooves. The grooves, in a plan view of the end faces, have depths which rise in the same circumferential direction. Pairs of grooves in the two setting rings each accommodate a ball. The rotatingly drivable setting ring is connected to an electric motor in respect of drive, and the axially displaceable setting ring is loaded by pressure springs towards the axially held setting ring. When applying a positive voltage to the electric motor, the axial setting device moves into an advanced position, and when disconnecting the voltage from the electric motor, the axial setting device returns into a starting position. 
     Furthermore, the invention relates to an electromechanical axial setting device for carrying out the inventive method. 
     Axial setting devices of the foregoing type have a simple and compact design and comprise short reaction times, such as they are required, for example, in friction couplings in locking differentials. The use of such setting devices is described in DE 39 20 861 C2, DE 39 15 959 C2, DE 39 09 112 C2, DE 38 15 225 C2 and DE 100 33 482.2. In these publications, it is mentioned several times that to make locking differentials comprising such setting devices compatible with vehicles provided with ABS systems and/or ESP systems, it must be possible for such axial setting devices to be returned quickly. Such a return motion is achieved by return springs. When in the form of spiral springs, they rotate the rotated setting ring backwards directly and thus allow the axially displaced setting ring to return. When in the form of axial springs, with groove assemblies without self-inhibition, they push back the axially displaced setting ring and thus rotate the rotated setting ring backwards. 
     Axial setting devices of the foregoing type, especially those wherein reinforced return springs or voltage reversing circuits effect a rapid return for the purpose of achieving a rapid disconnection of the friction coupling, at the completion of the return motion, experience a hard jerk due to the balls hitting the groove ends of the ball grooves of the setting ring. This jerk is so pronounced that in a vehicle it is regarded as an unacceptable adverse effect on the comfort conditions in the vehicle. Furthermore, if the driver is unprepared for such a jerk, it can make the driver feel insecure and cause him to regard the jerk as damage to the vehicle. 
     It is therefore necessary to achieve a rapid return motion and a dampened stopping behavior. It would therefore be desirable to provide a new method of returning an electromechanical axial setting device, especially for friction couplings. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved method and system for controlling an electromechanical axial setting device. The present invention provides that, during the return motion, shortly before the starting position is reached, the electric motor is short-circuited for the purpose of generating a braking moment. A further method in accordance with the invention provides that, during the return motion, shortly before the starting position is reached, a positive voltage is briefly applied to the electric motor for the purpose of generating a braking moment. At the end of the returning process, shortly before the balls reach the end stops, the means described here thus offer an electric braking method which can be achieved with very few additional switching mechanisms, while the basic mechanical configuration can remain unchanged. 
     A device in accordance with the invention includes a rotational position sensor which is arranged at one of the rotating parts, e.g. at the first setting ring and which, shortly before the balls reach the end stops in the ball grooves, initiates the respective switching operation, i.e. it either initiates short-circuiting or applies a positive voltage. In one embodiment it is also possible to replace the non-contact sensor by switching contacts which first, shortly before the end stops are reached, effect short-circuiting or the application of voltage and which, when the balls reach the end stops in the ball grooves, disconnect the electric motor. The means for dampening the stopping effect and for braking the returning motion of the electric motor constitute much simpler means than would be required by mechanical brakes and/or damping devices. 
     To accelerate the return motion, a negative voltage may be applied to the electric motor for returning purposes. For accelerated control cycles, this method can be improved further by the following steps. For returning purposes, first a negative voltage is applied to the electric motor and when the idling speed of the electric motor has been reached, the electric motor is disconnected from the voltage. 
     By way of the control strategy described herein it is possible to save a great deal of time as compared to the simple passive return motion initiated by spring force in that, especially during the acceleration phase, the spring force is supported by the drive effected by the electric motor. Surprisingly, time is also saved as compared to a permanently applied negative voltage during the return motion, the effect of which permanently applied negative voltage is counteracted by the induced counter voltage if the idling speed is exceeded. 
     Another embodiment includes a voltage reversing circuit for the electric motor and a motor speed recording device for the electric motor which are logically connected to one another via the idling speed of the electric motor in such a way that the voltage reversing circuit is disconnected if, in the course of the device being returned, the idling speed of the electric motor is reached. 
     To be able to determine the point in time at which the negative voltage at the electric motor is interrupted, it is possible to use direct speed monitoring means. However, if the dynamic behavior of the axial setting device is known, a simple time switch can be used for limiting, in terms of time, the connection time of the negative voltage at the electric motor. 
     In a preferred embodiment, use is made of a permanently excited direct current motor. 
     Other advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention. 
     In the drawings: 
     FIG. 1 shows an inventive axial setting device in a first axial view. 
     FIG. 2 shows the device according to FIG. 1 in a section along line A—A. 
     FIG. 3 shows the device according to FIG. 1 in an axial counter view relative to FIG.  1 . 
     FIG. 4 shows the device according to FIG. 1 in a section along line B—B. 
     FIG. 5 shows the device in the illustration according to FIG. 4, but in a plan view. 
     FIG. 6 shows a rotatingly drivable setting ring of the device in the form of a detail in an axial view of the grooves. 
     FIG. 7 shows the setting ring according to FIG. 6 in a sectional view along line A—A. 
     FIG. 8 shows a cylindrical section through a groove in an enlarged view. 
     FIG. 9 shows the detail X of FIG.  7 . 
     FIG. 10 shows the detail Y of FIG.  7 . 
     FIG. 11 shows the setting ring according to FIG. 6 in a perspective view. 
     FIG. 12 shows the second setting ring of the device according to FIG. 1 in a plan view of the end face comprising the grooves. 
     FIG. 13 shows the setting ring according to FIG. 12 in a sectional view along line A—A. 
     FIG. 14 shows a cylindrical section through a groove in an enlarged view. 
     FIG. 15 shows the section B—B of FIG.  12 . 
     FIG. 16 shows the setting ring according to FIG. 12 in a perspective inclined view. 
     FIG. 17 shows an inventive setting device used together with a differential gear in a partial section. 
     FIG. 18 shows an assembly according to FIG. 17 in an axial view. 
     FIG. 19 shows different characteristics of an optimised disconnection strategy. 
     FIG. 20 shows the torque curve for the inventive disconnection strategy in connection with prior art strategies. 
     FIG. 21 shows the motor speed as a function of time for the inventive disconnection strategy according to FIG. 20 as compared to strategies according to the state of the art. 
     FIG. 22 shows the characteristic curves for the torque and engine speed as a function of time for an inventive braking strategy. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 to  5  will largely be described jointly. By way of a first flange plate  12 , an electric motor is bolted on to a bearing block  13  which forms a second flange plate  14 . In the bearing block  13 , there is supported an extension of the motor shaft  15  which carries a first pinion  16 . In the bearing block  13 , there is also supported an ancillary shaft  17  which carries a further pinion  18  which engages the pinion  16  and which, for the purpose of forming a reduction stage, carries a further pinion  19 . Via the bearing flange  14  the bearing block  13  is secured, for example, to a drive housing. A bearing sleeve  21  whose axis extends parallel to the motor axis is shown to include a flange plate  22  which is rotatably supported in the drive housing. A first setting ring  23  is supported on the bearing sleeve  21  by way of a radial bearing  24 . The setting ring  23  includes a tooth segment  25 . The pinion  19  of the ancillary shaft  17  engages the tooth segment  25  of the first setting ring  23 . In parallel to the first setting ring  23 , there is arranged a further setting ring  26  which, by way of a holding lug  27 , in a rotationally fast way can engage the drive housing. Between the setting rings  23 ,  26 , there is arranged a plurality of balls  29  which are held in a cage  28  and by way of which the second setting ring  26  is centered on the first setting ring  23 . The first setting ring  23  is supported via an axial bearing  31  on a disc  32  which is secured by a securing ring  33  on the bearing sleeve  21 . The second setting ring is supported via an axial bearing  34  on a pressure plate  35  which is held by plate spring packages  36  in the flange plate  22 . The pressure plate  35  simultaneously acts on pressure pins  37  which penetrate the flange plate  22 . Ball grooves in the setting rings  23 ,  26  holding the balls  29  are provided in the form of ramps rising across the circumference in opposite directions. The electric motor is shown to have cable connections  38 ,  39 . By driving the electric motor  11 , the tooth segment  25  and thus the first setting ring  23  are rotated relative to the second setting ring  26  which, via the holding lug  27 , engages the drive housing and which, as a result, is axially displaced against the returning force of the plate springs  36  and in consequence, loads the pressure pins  37 . Further details as regards the functioning of the setting rings can be obtained from the description of the following drawings. 
     A voltage reversing circuit  49  and a motor speed recording device  48  for the electric motor  11  are logically connected to each other as a function of the idling speed of the electric motor  11  in such a way that the voltage reversing circuit  49  is disconnected if, in the course of the device being returned, the idling speed of the electric motor is reached. The voltage reversing circuit applies either on positive or negative voltage to the electric motor  11  as desired. The voltage reversing circuit  49  also acts as a short circuit switching assembly when short circuiting of the electric motor  11  is desired. Likewise, the motor speed recording device  48  operates as a rotational position sensor for indicating a position of the setting ring. 
     FIGS. 6 to  11 , again, will be described jointly below. The first setting ring  23  with the tooth segment  25 , in its end face, has five ball grooves  41  which are circumferentially distributed at a pitch angle of 72° and which each span a circumferential length of 58°. As can be seen in FIG. 8, the ball grooves, across the ring circumference, have a helical angle of 1.5° and thus a variable depth between two stops  42  and  43  for the balls  29 . In the sectional view of the ball groove, the ball is shown in its two stopping positions in a dash-dotted line. 
     FIGS. 12 to  16  will also be described jointly below. In its end face, the second setting ring  26  includes five ball grooves  44  which are circumferentially distributed at a pitch angle of 72° and span a circumferential length of 58°. The holding lug  27  with the guiding groove  47  is clearly drawn. As can be seen in FIG. 14, the ball grooves have a variable depth across the circumference due to a helical angle of 1.5° and include two stops  45 ,  46  for the balls  29 . One ball is shown in dash-dotted lines in its two stopping positions. 
     The rising gradients of the ball grooves  44  in the second setting ring  26  rise in the same direction as the gradients of the ball grooves  41  in the first setting ring  23 . As the setting rings  23 ,  26  are mounted in such a way that their end faces containing the ball grooves  41 ,  44  face one another, a relative rotation of the two setting rings relative to one another causes a ball  29  to roll so as to rise simultaneously in both ball grooves  41 ,  44  or so as to descend simultaneously. The cage holds the balls in the ball grooves in positions which correspond to one another. A relative rotation of the two setting rings  23 ,  26  relative to one another in a first direction thus pushes the two setting rings  23 ,  26  apart, whereas a relative rotation in the opposite direction allows said two setting rings  23 ,  26  to approach one another. The former is achieved entirely by driving the electric motor; the latter is achieved in particular by the returning force of the plate springs  36 . 
     FIGS. 17 and 18, will be described jointly below. A setting device of the above-mentioned type can be mounted in a differential drive  51 . In this case, the bearing sleeve  21 ′ is integral with a differential carrier  52  which is rotatably supported in the differential drive via rolling contact bearings  53 ,  54 . In the differential carrier  52 , there are supported two axle shafts  55 ,  56  which carry bevel gears  57 ,  58  which engage differential bevel gears  59 ,  60 . A friction coupling  61  includes first friction plates  63  which are connected in a rotationally fast way to a sleeve  62  which is secured to an axle shaft  55 , as well as two friction plates  64  which are connected to the differential carrier  52  in a rotationally fast way. The friction coupling  61  is arranged between an axially displaceable pressure plate  65  and a supporting member  66  fixed in the differential carrier  52 . The pressure plate  65  can be loaded directly by the pressure pins  37 ′ which are displaced when the first setting ring  23 ′ rotates relative to the second setting ring  26 ′. The second setting ring  26 ′ is held in a rotationally fast way by pins  67 ,  68  which engage holding lugs  27  and which are secured in the differential drive housing  51 . By rotating the setting ring  23  in a first direction, the friction coupling  61  is closed, so that the differential drive develops a locking effect, whereas by returning the setting ring  23 , the friction coupling  61  is disconnected, so that the differential drive again becomes an open differential. 
     FIG. 19 shows the above-mentioned process of disconnecting a friction coupling with reference to various characteristics on a time axis. These characteristics are the torque transmitted by the friction coupling (torque transmitted), the current in the electric motor (current) and its rotor&#39;s rotational speed (rotational speed rotor). In the negative time range, the setting device is shown in the outermost advanced position. Starting from time 0.0, the setting device is returned as quickly as possible into the starting position, avoiding any stopping jerks. In the negative time range, the current is characterised by pulse width modulation with rectangular jumps between 0 and approximately 25 A. The torque transmittable by the friction coupling is constant at approximately 2000 Nm. The rotational speed of the electric motor fluctuates with the frequency of the pulse width modulation approximately around 0. 
     At the time 0.0, the electric motor is subjected to negative current (active disconnection), as a result of which the engine speed increases to returning negative values, with the transmittable torque decreasing due to the friction coupling being opened. After approximately 0.01 seconds, the current is switched off, so that at the time of 0.015 seconds, the current in the electric motor becomes 0. The disconnection of the current (passive disconnection) has been chosen to be such that, approximately after the elapsed time, the motor reaches its nominal speed of approximately 200 radian/sec., so that thereafter, due to the effect of the plate springs, the speed can continue to increase in returning. The transmittable torque continues to decrease up to a time of 0.095 seconds. At that point in time, the electric motor is short-circuited, so that, within the shortest possible time, a short-circuit current reaches a value of approximately 45 A. As a result, the speed of the motor, up to the time of 0.14 seconds, is again braked to 0, as a result of which only slight overshooting occurs. The transmittable torque had already previously reached the value 0. As a result of the braking process initiated by the electric motor, the balls stop against the groove stops in a completely impact-free way. 
     FIG. 20 shows the torque curve (transmitted torque) of the friction coupling as a function of time as a result of the application of negative current (active “optimum”) being interrupted in accordance with the invention, as compared to a permanent application of negative current (active “standard”) and the return motion taking place entirely as a result of spring force (passive). The return motion under spring force is referred to as being “passive”—it can be seen that such spring force effects the slowest decrease in transmittable torque. “Active standard” refers to the permanent application of negative current to the electric motor which, due to an internal voltage being induced, from approximately 0.04 seconds onwards, leads to a clear reduction in the transmittable torque, whereas “active optimum” indicates the application of negative current which, after the idling speed has been reached, is interrupted after approximately 0.04 seconds, which results in the quickest possible decrease in the transmittable torque. 
     For the three types of disconnection described in connection with FIG. 20, FIG. 21 shows the curves of the returning speeds of the electric motor (rotational speed rotor) as a function of time. The spread, in terms of time, of the increases in rotational speed is responsible for the above-mentioned torque decreases, with the speed 0 occurring when the balls reach the end stops in the ball grooves. In the case of the permanent application of negative current (active standard), this takes place at the latest point in time and especially much later than in the case of the free (passive) return motion, whereas the short application of negative current (active optimum) in accordance with the inventive method leads to the balls reaching the stops in the quickest way, which is characterised by the zero crossing of the speed curve accompanied by a related overshoot. The overshoot indicates the return motion of the motor as a result of jerk-like spring-back at the end stops in the ball grooves. 
     FIG. 22 shows two processes which are similar to those shown in FIGS. 20 and 21, i.e. the illustration of the transmittable torque (transmitted torque) and the illustration of the motor speed in the case of the return motion (rotational speed rotor) and which refer to a further advantageous control process which takes place directly prior to the balls reaching the end stops in the ball grooves. 
     The curves marked “active” refer to the torque and speed curves resulting from the short application of negative current in accordance with the process type “active optimum” according to FIGS. 20 and 21. The “active” curves show the steep decrease in the returning negative motor speed to 0 and the overshoot to a clearly positive speed when the balls hit the stops in the ball grooves hard. The curves “active with short circuit” refers to the torque and speed curves under the influence of electric motor short-circuiting shortly before the balls hit the stops. The short-circuiting ensures early braking of the motor, so that the speed is returned to 0 in a practically impact-free way. The undesirable hard impact of the balls against the ends of the ball grooves is thus avoided. 
     From the foregoing, it can be seen that there has been brought to the art a new and improved electromechanical torque control system and method which includes the elimination of stopping noise. While the invention has been described in connection with one or more embodiments, it should be understood that the invention is not limited to those embodiments. Thus, the invention covers all alternatives, modifications, and equivalents as may be included in the spirit and scope of the appended claims.
A method of returning an electromechanical axial setting device, particularly suited for friction couplings. The axial setting device includes two setting rings centered on a common axis, one of which is axially held, with the other one being axially displaceably mounted, and one of which is rotationally fixedly held in a housing so as to be prevented from rotating, with the other one being rotatingly drivable. On their respective end faces facing one another, the two setting rings each include an identical multitude of circumferentially extending grooves. Each of the grooves, in a plan view of the end faces, define a depth which rises in the circumferential direction. Pairs of grooves in the setting rings each accommodate a ball. The rotatingly drivable setting ring is connected to an electric motor in respect of drive. The axially displaceable setting ring is loaded by pressure springs towards the axially held setting ring. When applying a positive voltage to the electric motor, the axial setting device moves into an advanced position and when the voltage is disconnected from the electric motor, the axial setting device is returned to a starting position.