Abstract:
A positioning device for converting a rotary motion into a linear motion includes a drive unit configured to generate a torque, a drive shaft on which an eccentric is arranged, a coupling element comprising a slot, an output shaft arranged at the eccentric. The output shaft is configured to move in the slot of the coupling element. An adjusting element is connected with the coupling element. The adjusting element is supported so as to be linearly movable with the coupling element. The slot comprises a guide path configured to cooperate with the eccentric which comprises an angle with a plane perpendicular to a direction of movement of the adjusting element.

Description:
CROSS REFERENCE TO PRIOR APPLICATIONS 
       [0001]    This application is a U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/EP2010/066613, filed on Nov. 2, 2010 and which claims benefit to German Patent Application No. 10 2009 053 428.8, filed on Nov. 19, 2009. The International Application was published in German on May 26, 2011 as WO 2011/061051 A1 under PCT Article 21(2). 
     
    
     FIELD 
       [0002]    The present invention provides a positioning device for converting a rotary motion into a linear motion comprising a drive unit generating a torque, a drive shaft on which an eccentric is arranged, an output shaft arranged at the eccentric and movable in a slot of a coupling member, and an adjusting element connected with the coupling element and supported so that the adjusting element is adapted to be moved linearly together with the coupling element. 
       BACKGROUND 
       [0003]    Such positioning devices are used in particular to drive exhaust gas recirculation valves, but they may also be used in waste gate valves, butterfly valves or as VNT actuators. 
         [0004]    Various valves with positioning devices or similar positioning devices are known, wherein an electric motor serves as the drive unit whose drive shaft is coupled with eccentrics of various types, the motion of the eccentrics being converted, via different coupling mechanisms, into a linear motion of a valve rod serving as an adjusting element. 
         [0005]    EP 1 319 879 A1 describes a valve driven by an electric motor, wherein an output shaft is arranged eccentrically with respect to a drive shaft, a roller being provided rotatably on the output shaft and traveling in a slot of a coupling element. The roller is spring-biased in one direction. The traveling path of the coupling element slot provided for the roller is perpendicular to the direction of movement of the coupling element. The development of the force-stroke curve of this element is thus fixed. 
         [0006]    DE 102 21 711 A1 describes a similar valve wherein two eccentrics are coupled with each other. In this design, the slot that serves as a traveling path for a ball bearing is also designed as a straight line that extends perpendicularly to the direction of movement. It is again not possible to provide special required force-stroke curves during the actuation of the valve with sufficient variability. 
         [0007]    A valve driven by an electric motor is also described in EP 1 378 655 A2, wherein a rotating member comprises two opposite slots in which a rod is guided which in turn is connected with a valve rod. The slot may here be configured as a defined curve. With this design, it is possible to set a defined effort for the adjustment of the valve as a direct function of the stroke. The required structural space is rather large, as is the number of components needed. 
       SUMMARY 
       [0008]    An aspect of the present invention is to provide a positioning device which allows the selection of a force-stroke curve or a rotational angle-stroke curve for specific applications and which at the same time requires as little space as possible. 
         [0009]    In an embodiment, the present invention provides a positioning device for converting a rotary motion into a linear motion which includes a drive unit configured to generate a torque, a drive shaft on which an eccentric is arranged, a coupling element comprising a slot, an output shaft arranged at the eccentric. The output shaft is configured to move in the slot of the coupling element. An adjusting element is connected with the coupling element. The adjusting element is supported so as to be linearly movable with the coupling element. The slot comprises a guide path configured to cooperate with the eccentric which comprises an angle with a plane perpendicular to a direction of movement of the adjusting element. Due of the previously unknown interaction of an eccentric drive and a slot curve path, it is possible for the first time, and to a much larger extent than before, to set force-stroke curves rotational angle-stroke curves that allow for an adjustment of such a positioning device to a number of different applications. The structural space is thereby not larger than with other known eccentric drives. With the same total stroke, the length of the eccentric may be chosen to be even smaller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention is described in greater detail below on the basis of embodiments and of the drawings in which: 
           [0011]      FIG. 1  shows a perspective view of the coupling device and the eccentric of a positioning device according to prior art; 
           [0012]      FIGS. 2   a ) and  b ) shows the coupling device and the eccentric of a positioning device of the present invention at the respective end positions; 
           [0013]      FIG. 3  shows the function of linear stroke over an angle of rotation for a positioning device with a coupling device of  FIG. 2  in graphic representation; and 
           [0014]      FIG. 4  shows the function of force over stroke for a positioning device with a coupling device of  FIG. 2  in a graphic representation, compared with the corresponding function of a plane slot. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In an embodiment of the present invention, the slot can, for example, describe a curve with a varying pitch. This provides additional possibilities for the adjustment of force-stroke curves to specific applications. 
         [0016]    In an embodiment of the present invention, a roller or a bearing can, for example, be arranged on the output shaft, which travels in the slot so that friction between the slot or its traveling path and the outer path of the rolling body, i.e. the bearing or the roller in the present instance, is minimized. 
         [0017]    In an embodiment of the present invention, the initial position of the rotation for the actuation of the adjusting element can, for example, be a position which, seen in the direction of rotation, is situated before a dead center existing for the axial movement of the output shaft, which dead center is passed during the rotational movement to the end position. It thus becomes possible to realize short strokes of a valve with rather large actuating angles, which allows for an exact proportioning in the sensitive adjustment range shortly after leaving the closed position. 
         [0018]    In an embodiment of the present invention, a first portion of the slot to be traveled by the output shaft can, for example, have an upward slope with respect to the plane perpendicular to the direction of movement of the adjusting element, while a second portion to be traveled can, for example, have a downward slope. With such a design, a further adaptation of the relationship between the angle of rotation and the resulting stroke can be obtained as well as an adaptation to a desired force-stroke characteristic which may lead, for example, to a largely constant effort for the adjustment in the first portion. At the same time, such an adaptation allows for an additional reduction outside an upstream transmission. 
         [0019]    In an embodiment of the present invention, the upward slope in the first portion to be traveled can, for example, be steeper than the upward slope of a rolling line of the output shaft when traveling through the portion from a first end position to the top dead center, seen with respect to the axial movement of the output shaft. It is thereby provided that a stroke occurs in this adjustment region. 
         [0020]    The can thus be kept constant in a significant region about at least one of the two end stops. The available adjustment force thereby becomes independent of tolerances that could occur, for example, as a result of a thermal expansion of the valve rod. Such an almost constant force curve in the region of the closed position for about 15-25% of the full stroke is required in particular in case of the application of the positioning device as an actuator of a waste gate valve, because of the prevailing gas pressure forces at the gate. 
         [0021]    A positioning device is thus provided whose coupling device, in combination with the eccentric, leads to the possibility of a selectable force-stroke setting by appropriately adjusting the selected rotational angle range with respect to the slot. The present positioning device also allows an adjustment between the angle of rotation and the stroke for a better proportioning. The required structural space is at the same time kept very small. 
         [0022]    An embodiment of the positioning device of the present invention is illustrated in the drawings and will hereinafter be described. 
         [0023]      FIG. 1  illustrates a detail of a positioning device corresponding to the prior art. The part here illustrated is the part of the positioning device essential to the present invention. 
         [0024]    As is known per se, the positioning device comprises a non-illustrated rotary drive unit such as, for example, an electric motor, which drives a drive shaft  2 . On the end of the drive shaft  2  opposite the drive unit, an eccentric  4  is provided in a manner secured against rotation. At the end of the eccentric  4  remote from the drive shaft  2 , an output shaft  6  is provided that extends parallel to the drive shaft  2  so that the output shaft  6  rotates in a circular manner about the drive shaft  2  when the drive shaft  2  is rotated. 
         [0025]    A ball bearing  8  is arranged at the end of the output shaft  6  opposite the eccentric  4 , the inner race thereof being fastened on the output shaft  6 . An outer race  10  of the ball bearing  8  moves in a slot  12  of a coupling element  14  to which an adjusting element  16  in the form of a valve rod of a globe valve not illustrated in detail herein. The valve rod is supported in a housing in a manner known per se so that it can only perform a linear stroke movement with the coupling element. In the coupling device  14  illustrated, the slot  12  is an opening limited in height by two limiting walls  17 ,  18  whose mutual distance substantially corresponds to the circumference of the ball bearing  8  and whose width is determined by the length of the eccentric  4  and by the adjustment angle thereof. The limiting walls  17 ,  18  that serve as the guide path  20  of the ball bearing  8  are designed as straight planes that extend perpendicularly to the direction of movement of the valve rod  8  when the drive unit is operated. 
         [0026]    In comparison with the above, the slot  12  according to the present invention, or the resulting guide track  20 , illustrated in  FIG. 2  is designed as a curve. A curve in the sense of the present application is thus a line that is not necessarily completely linear. 
         [0027]    This curve is designed so that a positioning device of this type is suitable, for example, to drive a waste gate valve. With such a valve, it is desired that, when leaving the closed position of the valve, the valve force remains approximately constant over a certain opening range. 
         [0028]    In  FIG. 2 , the eccentric  4  is only indicated as a connecting line between the fulcrum of the eccentric  4  and the pivot point  6  of the bearing  8  or a roller.  FIG. 2   a ) illustrates the coupling element  14  in a position in which the adjusting element  16  is in a first end position that is defined, for example, by correspondingly formed stops for upstream gears or other movable parts. In this position, the bearing  8  is located at the left end of the slot  12  below a dead center  24  of the eccentric  4  that is the top dead center with respect to the axial movement of the pivot point  6 . 
         [0029]    If, hereafter, the drive unit is operated clockwise and the eccentric is thereby rotated clockwise, the bearing  8  is rotated at a constant distance around the rotational axis of the drive shaft  2  and rolls along the guide path  20  of the slot  12  that is only movable in the vertical direction. A first part of the slot  12  travelled by the rolling of the bearing  8  has a slope  22  with respect to a plane vertical to the actuation direction of the adjusting element, which slope is steeper than the respective circular arc traveled by the bearing. Despite the upward movement of the be aring, this causes a downward movement of the slot  12  and thus of the coupling element  14  and the adjusting element  16 . The part of the guide path  20  following after the top dead center  24  has been passed first has a lesser upward slope  22  that eventually passes into a downward slope in a second part  26 .  FIG. 2   b ) illustrates the second end position reached after both parts  22 ,  26  have been passed. 
         [0030]    The stroke resulting from this movement is plotted in  FIG. 3  over the rotational angle. It is evident that the resulting graph  28  is rather flat in the first part and is significantly steeper as the stroke becomes larger. Such a curve is advantageous, since it is drastically facilitates volume flow control, because in the part just after opening a small change in the stroke results in a rather significant change in the volume flow, whereas, with a rather large stroke, a change in stroke only results in rather small changes in the volume flow. 
         [0031]    In  FIG. 4  the graph  30  illustrates the force-stroke curve of a positioning device according to  FIG. 2 , whereas the dotted graph  32  represents the force-stroke curve of a positioning device with a guide path perpendicular to the movement direction of the adjusting element as illustrated in  FIG. 1 . In a range of up to about 4 mm of stroke, corresponding to about 25% of the total stroke, the valve force to be applied by a positioning device of  FIG. 2 , contrary to the valve force to be applied by a positioning device of  FIG. 1 , only changes slightly with the stroke. 
         [0032]    Besides this advantage of setting a desired force curve, it is also possible to realize additional reductions and transmissions with respect to the entire adjustment range by using the slot to increase or reduce the adjusting path of the adjusting element relative to the length of stroke of the output shaft. 
         [0033]    Additional structural space can be saved in this manner. The positioning device of the present invention allows adjusting both force-stroke curves and rotational angle-stroke curves depending on the respective application, so that such a positioning device can be used in many different applications, basically without resulting in a larger required structural space as compared with known positioning devices. 
         [0034]    The scope of protection of the claims is not restricted to the embodiment described, different forms of the guide path may be desired depending on the application, in order to achieve the advantageous force-stroke curve. It is also possible to design such a positioning device to act in the opposite direction.