This claims the benefit of German Application No. 10 2004 061 955.7, filed Dec. 23, 2004 and hereby incorporated by reference herein. 
     BACKGROUND 
     The present invention relates to a solenoid valve device, in particular a hydraulic valve device, having a solenoid element which is movable from a non-energized retracted state to an energized extended state, the stroke of the solenoid element from its retracted state to its extended state in the range at an essentially constant magnetic force being referred to as a constant force working stroke and in the related art being normally reduced, via stroke limitation, by an idle stroke in which the magnetic force is not constant, but decreases over the stroke. 
     In known electrohydraulic proportional pressure valves, the working stroke of the solenoid element is limited to the range having a constant force so that a pressure function of the entire valve, proportional to an input signal, for example, the solenoid current, may be represented over the entire pressure range. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a solenoid valve device, in particular a hydraulic valve device, manufacturable in a cost-effective manner and requiring less installation space than conventional solenoid valve devices. 
     A solenoid valve device, in particular a hydraulic valve device, for example, an electrohydraulic proportional pressure valve, has a solenoid element which is movable from a non-energized retracted state to an energized extended state, the stroke of the solenoid element from its retracted state to its extended state in the range at an essentially constant magnetic force being referred to as a constant force working stroke and in the related art being normally reduced, via stroke limitation, by an idle stroke in which the magnetic force is not constant, but decreases over the stroke. According to the present invention, the stroke limitation of the solenoid element is varied in such a way that both the constant force working stroke and the idle stroke are utilized as the working stroke of the solenoid element. The solenoid element is preferably a proportional solenoid, which includes an armature having a plunger and a coil and is used to displace a valve piston when the coil is energized. When the working stroke of the solenoid element is extended by the idle stroke, greater piston strokes and thus opening cross sections may be implemented in the solenoid valve device to reduce the back pressure. By using the idle stroke, the stroke work in the operation of the solenoid valve device may be reduced. The disadvantage of a non-constant and reduced magnetic force in the idle stroke compared to the conventional design is consciously taken into account according to the present invention. 
     A preferred exemplary embodiment of the solenoid valve device may be characterized in that a rest point of the solenoid element in the non-energized state is designed in such a way that a stroke from the rest point to a minimum working point, initiated by energizing the solenoid element, is ensured for all operating states (volume flows and viscosities). This design ensures, for example, reliable entry into the constant force range (proportional range) and regular operation of the solenoid valve device for small strokes. 
     Another preferred exemplary embodiment of the solenoid valve device may be characterized in that a starting current of the solenoid element during switch-on of the solenoid valve device is higher than the current when the minimum working point is reached, but lower than a maximum admissible current. This ensures that the solenoid element is moved reliably from its rest position defined by the rest point to the minimum working point and thus to the beginning of the working force range. 
     Another preferred exemplary embodiment of the solenoid valve device may be characterized in that the minimum working point is located in the range of the constant force working stroke of the solenoid element. The minimum working point is preferably, but not necessarily, located on a constant force branch of the characteristic curve of the solenoid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages, features and details of the present invention result from the description that follows, in which different exemplary embodiments are individually described with reference to the drawing. The features mentioned in the claims and in the description may be essential to the invention per se or in any desired combination. 
         FIG. 1  shows a Cartesian coordinate diagram in which the magnetic force is plotted against the solenoid stroke at constant current in the form of a characteristic curve of a proportional solenoid; 
         FIG. 2  shows a characteristic curve as in  FIG. 1  for an extended working stroke; and 
         FIG. 3  shows a Cartesian coordinate system in which the pressure force of a pressure limiting valve and the magnetic force are plotted against the path of a valve piston, i.e., the working stroke of the solenoid at different electric currents. 
         FIG. 4  shows an embodiment of a solenoid valve device in the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Within the scope of the present invention, ideas to reduce the cost and the installation space of valve solenoids of electrohydraulic pressure valves have been applied. In particular, approaches for stabilizing the rocking motion in the chassis hydraulics have been sought for keeping the required stroke work of the solenoid as low as possible. At operating temperatures, in general small paths are sufficient to set the required pressures at the control edges of a pressure valve. However, at high volume flows and low oil temperatures (high oil viscosity), very small back pressures are implemented, which makes it necessary for the valve to provide large opening cross-sections. This requirement is directly related to large valve strokes. In a conventional design, large valve strokes result in considerably more expensive and larger solenoids. 
       FIG. 1  shows magnetic force F plotted against solenoid stroke H at a constant current in the form of a force/path characteristic curve of a proportional solenoid.  FIG. 4  shows the solenoid  10  preferably has an armature having a plunger  20 , the plunger  20  being able to displace a valve piston  30 . U.S. Pat. Nos. 6,637,282 and 6,955,101 for example describe and depict solenoids and are hereby incorporated by reference herein. The fully retracted state of the solenoid plunger is labeled  1 . The fully extended state of the solenoid plunger is labeled  2 . A stroke limitation of conventional solenoid valves ensures that not the full stroke of the solenoid, but only a constant force working stroke KH in which the magnetic force F is constant, is used for the solenoid working stroke. An idle stroke LH, in which the magnetic force F decreases over stroke H, is not used for the working stroke in conventional solenoid valves. 
     The stroke work, which determines the cost and the installation space, is given in  FIG. 1  by the area under characteristic curve  4  in constant force stroke KH. 
     According to the present invention, the working stroke is extended by idle stroke LH, and greater piston paths and thus greater opening cross sections may be implemented in the valve  30  (shown in  FIG. 4 ) to reduce the back pressure. 
       FIG. 2  shows a force/path characteristic curve of a solenoid valve  30  according to the present invention, where both constant force stroke KH and idle stroke LH (see  FIG. 1 ) are used for working stroke AH. The fully retracted state of the solenoid plunger  20  (shown in  FIG. 4 ) is labeled  11 . The fully extended state of the solenoid plunger  20  (shown in  FIG. 4 ) is labeled  12 . A minimum working point, for example, for the pressure limitation function, is labeled  13 . A maximum working point is labeled  14 . A rest point is labeled  16 . The advantage of the characteristic curve illustrated in  FIG. 2  is the reduction of stroke work by the use of the decreasing characteristic curve branch as indicated by shaded area  18 . A disadvantage is a magnetic force F which, in the extended work range, is reduced in comparison with the conventional design and is not constant. 
       FIG. 3  shows pressure force P and magnetic force F of a solenoid valve  10  (shown in  FIG. 4 ) according to the present invention plotted against the stroke, i.e., path W of a slider or valve piston  30  (shown in  FIG. 4 ). The path is also referred to as a slider path. The solenoid valve  10  (shown in  FIG. 4 ) is, for example, a pressure-limiting valve.  FIG. 3  shows different solenoid characteristic curves  27 ,  28 ,  29  for different electric currents  21 ,  22 ,  23 . The intensity of the electric currents decreases from  21  to  23 . In addition,  FIG. 3  shows a valve characteristic curve  31  for a constant volume flow and increasing pressure. The minimum working point for the pressure limitation function is labeled  33  in  FIG. 3 . The maximum working point for the pressure limitation function is labeled  34  in  FIG. 3 . In  FIG. 3 , the rest point is labeled  36 . 
       FIG. 3  shows that the supposed disadvantage of reduced and non-constant magnetic force results in no impairment of the valve function. In the non-energized state, rest point  36  on valve characteristic line  31  is such that for all operating states (volume flows and viscosities) a path from rest point  36  to minimum working point  33  of the solenoid valve  10  (shown in  FIG. 4 ) according to the present invention, initiated by energizing the solenoid  10  (shown in  FIG. 4 ) is ensured, as indicated by arrows  38 ,  39 . Starting current  22  is higher during switch-on than current  21  when minimum working point  33 , i.e., the minimum pressure to be regulated, is reached, but lower than maximum admissible current  23 . The range of idle stroke LH (see  FIG. 1 ) is indicated in  FIG. 3  by two dashed lines  41 ,  42 , between which magnetic force F decreases. Minimum working point  33  is preferably located in the range of solenoid characteristic curves  27  through  29  in which the magnetic force F is constant. 
     The present invention provides a proportional valve, in particular a pressure-limiting valve, having an extended working range, which utilizes the non-constant range of the characteristic curve of a proportional solenoid. This makes the use of large and expensive solenoids unnecessary. The present invention also provide a method for altering a solenoid working range by altering a stroke limitation, for example using a controller for the solenoid. 
     LIST OF REFERENCE NUMERALS 
     
         
           1 . retracted state 
           2 . extended state 
           4 . characteristic curve 
           10 . solenoid valve device 
           11 . retracted state 
           12 . extended state 
           13 . minimum working point 
           14 . maximum working point 
           16 . rest point 
           18 . shaded area 
           20 . plunger 
           21 . current 
           22 . current 
           23 . current 
           27 . solenoid characteristic curve 
           28 . solenoid characteristic curve 
           29 . solenoid characteristic curve 
           30 . valve piston 
           31 . valve characteristic curve 
           33 . minimum working point 
           34 . maximum working point 
           36 . rest point 
           38 . arrow 
           39 . arrow 
           41 . dashed line 
           42 . dashed line
A solenoid valve device, in particular a hydraulic valve device, has a solenoid element which is movable from a non-energized retracted state into an energized extended state, the working stroke of the solenoid element from its retracted state into its extended state including a constant magnetic force working stroke and an idle stroke in which the magnetic force is not constant, but decreases over the stroke.