Abstract:
A valve control unit including three valve units for two brake channels of an electropneumatic brake system is provided. Each valve unit is embodied as a valve modulator device for a braking pressure control circuit including an air inlet valve with a primary solenoid and a bleed valve with a secondary solenoid. Each modulator device armature includes a common solenoid guide arrangement controlled by the magnetic flux of a common magnetic coil. Independent operation of the armatures is achieved whereby flux through the secondary solenoid is weakened by a shunt in the flux circuit which causes a switching delay for the secondary solenoid relative to the primary solenoid. The magnetic resistance of a non-magnetic disc effects switch-off acceleration in the flux circuit with energized secondary solenoid, weakens the field and causes the secondary solenoid to revert to a spring-loaded base state at a higher flux than the primary solenoid.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an improved valve pilot-control unit for a brake-pressure modulator. 
     DE 100 09 116 A1 (hereinafter, “D1”) describes a known valve device for the pilot-control unit of a brake-pressure modulator for an electronic air-brake system (EBS). As shown in FIG. 2 of D1, the pneumatic circuit for the pilot-control unit of a brake-regulating loop of the brake-pressure modulator comprises a 3/2 solenoid valve ( 21 ) as a redundancy valve, a normally open 2/2 solenoid valve ( 22 ) as an air-admission valve and a normally open 2/2 solenoid valve ( 23 ) as a vent valve. As described in DE 42 27 084 A1 (see FIG. 2 therein), the redundancy valve in such circuit can also be used commonly for a second brake-regulating loop. According to this circuit layout, therefore, the pilot-control unit for two brake-regulating loops comprises one 3/2 redundancy valve for both loops and separate 2/2 vent valves and 2/2 air-admission valves for each individual loop. A total of 5 solenoids is provided for actuating the respective armatures of these 5 solenoid valves. 
     FIG. 4 of D1 shows that the solenoid valves according to  FIG. 2  can be constructed with only one sealing seat forming a hermetic seal, at which, while the solenoid is deenergized, an elastomeric insert ( 41 ) provided in the corresponding armature ( 39 ) is urged by the action of the solenoid restoring spring ( 40 ) against a first stroke limiter having a valve-sealing seat ( 31 ) ( 47  for valve  21  of 3/2 type,  43  for valve  23  of 2/2 type, no sealing seat corresponding to  45  for valve  22  of 2/2 type). Besides this sealing seat, the solenoid valves have a second position in which the corresponding armature ( 39 ) is urged by the action of the magnetic force against a second stroke limiter ( 34 ) at which there is formed a detail metal valve seat ( 48  for valve  21  of 3/2 type,  46  for valve  22  of 2/2 type, no sealing seat corresponding to  44  for valve  23  of 2/2 type), which does not seal the unavoidable leaks hermetically but, because of the selected switching system, has no significance for the operation of the brake-pressure modulator. 
     DE-OS 24 03 770 discloses, for a hydraulic ABS solenoid valve, measures for influencing the magnetic forces, in order to obtain three stable and reproducible armature positions as a function of the magnet current, namely, positions for the deenergized condition, the condition for an “exciter stage  1 ” and the condition for a “full exciter stage”. In the deenergized starting position of the solenoid valve, outlet valve ( 12 / 27 ) is closed and inlet valve ( 11 / 28 ) is open; as a result, pressure source ( 3 ) is in communication with brake cylinder ( 2 ) and pressurization takes place in the brake cylinder. During energization corresponding to exciter stage  1 , armature ( 13 ) travels a short distance and closes inlet valve ( 11 / 28 ), thus holding the pressure in brake cylinder ( 2 ). During energization corresponding to the full exciter stage, armature ( 13 ) is pushed up to spacer ring ( 16 ), outlet valve ( 12 / 27 ) opens and brake cylinder ( 2 ) is depressurized. 
     WO 03/053758 (hereinafter, “D2”) describes a brake-pressure modulator for a trailer vehicle, wherein a pilot-control unit containing four valves in the form of one 3/2 “reservoir/venting” selection valve ( 110 ), one 3/2 redundancy valve ( 109 ) and two 2/2 modulator valves ( 106 / 107 ) is used for two different brake-regulating loops. Therefore, the number of valves, and thus also the number of valve magnets, for two brake-regulating loops is reduced to four. However, the pilot-control circuit according to D2 suffers from the disadvantage that it is not possible at any given instant to admit air via one of the two ducts while venting via the other. Instead, at all times, it is only possible to admit air or vent via both ducts simultaneously. Consequently, it is not possible to raise the pressure in one duct and simultaneously lower the pressure in the other duct, as would be highly advantageous for a flexible regulation strategy. A further disadvantage of this solution lies in the series connection of the valves, meaning that the achievable air flow is diminished and the effective nominal width of the pilot-control unit is reduced. 
     DE 35 01 708 A1 describes an electromagnetically actuatable multi-way valve in which two different valves, one of which is an inlet valve ( 9 ,  10 ) that can be actuated via a first armature ( 5 ) and the other of which is an outlet valve ( 23 ,  25 ) that can be actuated via a second armature ( 21 ), can be loaded by only one common coil ( 2 ). Armatures ( 5 ) and ( 21 ) are biased with restoring springs of different dimensions, one a weakly dimensioned restoring spring ( 13 ) for armature ( 5 ) and the other a strongly dimensioned restoring spring ( 17 ) for armature ( 21 ), so that they can be actuated independently of one another by controlling the current in coil ( 2 ). This valve can therefore also be used as a combined air-admission/vent valve in a brake-pressure modulator. As explained below, however, the principle of different design of the restoring springs ( 13 ,  17 ) that underlies this valve for independent actuation leads to difficulties in valve design. The “weak” restoring spring must be able to overcome the gas force acting at inlet ( 9 ,  10 ) and it must therefore be strong enough that the restoring function for inlet valve ( 9 ,  10 ) is assured if the current fails. “Strong” restoring spring ( 17 ) must be strong enough that outlet valve ( 23 ,  25 ) is activated only at much higher magnet current than is the case for inlet valve ( 9 ,  10 ). The strong design of restoring spring ( 17 ) is therefore also limited by the force that the magnet can actually provide. To implement this principle, therefore, it is also necessary to provide a relatively large magnet with the necessary magnetic force. Besides causing higher manufacturing costs, such a magnet must be supplied with greater electrical power, which nevertheless does not lead to a satisfactorily short valve switching time, because of the increase of inertia related to structural size. In addition the choice of larger structural units works against the goal of producing compact devices. 
     SUMMARY OF THE INVENTION 
     Generally speaking, in accordance with the present invention, an improved valve control unit is provided wherein two different brake-regulating ducts can be actuated independently of one another during application of the valve-control unit as a brake-regulating pilot-control unit. The inventive valve control unit overcomes disadvantages associated with conventional valve control units, and offers a compact design that is less costly to manufacture. The inventive valve control unit additionally has the advantage of drawing low current, also resulting in the advantage of favorable heating behavior in the device. A further advantage of the invention is the reduced complexity of contacting and of electrical activation (number of needed end stages as well as associated components). 
     Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification. 
     The present invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described in greater detail hereinafter on the basis of the accompanying drawings, wherein: 
         FIG. 1  is a pneumatic circuit diagram depicting a valve control device according to one embodiment of the present invention having application as a pilot-control unit for an electronic air-brake system (EBS) and capable of making a pilot-control pressure available for two pressure-regulating loops; 
         FIGS. 2 to 8  are diagrams depicting various switched conditions of the valves involved in application of the embodiment of the inventive valve control device as an EBS pilot-control unit; 
         FIG. 9  is a combined sectional diagram of a valve-modulator device in accordance with an embodiment of the present invention including one air-admission valve and one vent valve with one solenoid for both valves and a 3/2 redundancy valve taken along line A-A in  FIGS. 10 ,  12 ,  13  and  14 ; 
         FIG. 10  is an individual sectional diagram of the embodiment of the valve-modulator device depicted in  FIG. 9  taken along line B-B in  FIG. 9 ; 
         FIG. 11  illustrates the effect of a magnetic shunt in the embodiment of the valve-modulator device depicted in  FIG. 10 ; 
         FIG. 12  is an individual sectional diagram of an alternative embodiment of the valve-modulator device in accordance with the present invention taken along line B-B in  FIG. 9 ; 
         FIG. 13  is an individual sectional diagram of another alternative embodiment of the valve-modulator device in accordance with the present invention taken along line B-B in  FIG. 9 ; 
         FIG. 14  is an individual sectional diagram of a further alternative embodiment of the valve-modulator device in accordance with the present invention taken along line B-B in  FIG. 9 ; 
         FIGS. 15   a - d  are circuit diagrams illustrating the magnetic flux loop under various operational states of the embodiment of the valve-modulator device depicted in  FIG. 10 ; and 
         FIG. 16  is a graphical representation of the switching of the air-admission and vent valve of an embodiment of the valve-modulator device as a function of the magnet current flowing through the solenoid in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings where (non-ferromagnetic) parts that are not magnetically conductive are illustrated with cross hatching, so that they can be readily distinguished from singly-hatched magnetic parts, and wherein reference numerals  1 ,  2 ,  4 ,  5 ,  6 ,  17 ,  18 ,  21 ,  22  and  23  are adopted from D2 for identification of devices having like effects,  FIG. 1  shows a pneumatic valve control device ( 1 ) according to one embodiment of the present invention, which in particular is used as a pilot-control unit for an electronic air-brake system. Valve control device ( 1 ) is capable of making a pilot-control pressure available for two different brake-pressure-regulating loops, designated hereinafter as the first and second brake-pressure-regulating loops. 
     In this pilot-control unit, a 3/2 solenoid valve ( 21 ) with two inputs ( 4 ,  5 ) and one output ( 6 ) is used as the redundancy valve, while a first valve-modulator device ( 7 ) is used for the first brake-pressure-regulating loop and a second valve-modulator device ( 7 ′) is used for the second brake-pressure-regulating loop. In 3/2 solenoid valve ( 21 ) constructed as a redundancy valve, a supply pressure is applied at first input ( 4 ) and a redundancy pressure is applied at second input ( 5 ). As is standard in EBS systems, this redundancy pressure is generated by exclusively mechano-pneumatic means; if the pilot-control unit is used in a truck, the redundancy pressure is delivered by the operator-actuated truck brake valve, while if the pilot-control unit is used in a trailer vehicle, the redundancy pressure generated in the truck is transmitted to the trailer via the brake-pressure line (brake hose). 
     At least during application for EBS operation, one pneumatic output ( 8 ) of first valve-modulator device ( 7 ) is in communication with an input ( 17 ) of an air-flow-intensifying relay valve ( 2 ) for the first brake-pressure-regulating loop; in the same way, one pneumatic output ( 8 ′) of second valve-modulator device ( 7 ′) is in communication with an air-flow-intensifying relay valve ( 2 ′) for the second brake-pressure-regulating loop. The outputs ( 18 ,  18 ′) of relay valves ( 2 ,  2 ′) represent the fully-developed brake pressures for the first and second brake-pressure-regulating loops respectively. 
     Because of the cost-effective construction of valve-modulator devices ( 7 ,  7 ′) explained hereinafter, pilot-control unit ( 1 ) can also be used advantageously for applications other than EBS regulation; for example, it can also be used in its basic design as an air-admission/venting device for the left and right air suspension springs of an electronically controlled air suspension system (ECAS). Hereinafter, therefore, the advantageous properties of pilot-control unit ( 1 ) will also be described as regards their general applicability. 
     According to  FIG. 1 , output ( 6 ) of 3/2 solenoid valve ( 21 ) is in pneumatic communication both with first valve-modulator device ( 7 ) for the first pressure-regulating loop and with second valve-modulator device ( 7 ′) for the second pressure-regulating loop. Second valve-modulator device ( 7 ′) is constructed identically to first valve-modulator device ( 7 ), and so it will be sufficient to explain its design hereinafter on the basis of first valve-modulator device ( 7 ), and with reference to drawing  FIGS. 9 to 14 . 
     Each valve-modulator device ( 7 ,  7 ′) includes a first normally open 2/2 solenoid valve ( 22 ,  22 ′) as an air-admission valve and a second normally closed 2/2 solenoid valve ( 23 ,  23 ′) as a vent valve. These valves are in communication with one another via an internal connection ( 9 ,  9 ′). 
     As shown in  FIG. 10 , armatures disposed in a common armature-guide arrangement ( 10 ) for valve-modulator device ( 7 ) and loaded with respective springs ( 22   f ,  23   f ) are provided in the form of a primary armature ( 22   a ) and a secondary armature ( 23   a ) for first 2/2 solenoid valve ( 22 ) and second 2/2 solenoid valve ( 23 ) respectively. Each armature is actuated by a common solenoid ( 11 ) provided for both solenoid valves, and at least secondary armature ( 23   a ) is equipped with an elastomeric insert ( 23   e ). In the embodiment depicted in  FIG. 10 , primary armature ( 22   a ) is also equipped with an elastomeric insert ( 22   e ). Primary armature ( 22   a ) is intended as a switching element for air-admission valve ( 22 ), and secondary armature ( 23   a ) is intended as a switching element for vent valve ( 23 ). 
     While solenoid ( 11 ) is de-energized, both primary armature ( 22   a ) of air-admission valve ( 22 ) and secondary armature ( 23   a ) of vent valve ( 23 ) are in their normal positions defined by spring loading ( 22   f  and  23   f  respectively). 
     If a magnet current I is injected into solenoid ( 11 ) by application of a voltage or current source, a magnetic force acts on both armatures as a function of the magnetic flux Φ flowing through the two armatures as a result of the magnetomotive force:
 
θ= w·I  
 
     (where w is the number of turns). 
     If the magnet current flowing through solenoid ( 11 ) reaches a first magnet current of defined magnitude I 1 , primary armature ( 22   a ) of air-admission valve ( 22 ) is displaced into its switched position determined by the magnetic force, whereas secondary armature ( 23   a ) of vent valve ( 23 ) still remains in spring-loaded normal condition. 
     If the magnet current I flowing through solenoid ( 11 ) reaches a second magnet current of defined magnitude I 2 , which is greater than the first magnet current I 1  by a defined amount, both primary armature ( 22   a ) of air-admission valve ( 22 ) and secondary armature ( 23   a ) of vent valve ( 23 ) are displaced into their switched positions determined by the magnetic force. 
       FIGS. 2 to 8  show the switched conditions of the valves involved in the preferred application of the valve control unit as an EBS pilot-control unit. For clarity, only the functional elements for the first brake-pressure-regulating loop are illustrated, and the solenoids are omitted. 
     In the redundancy operation according to  FIG. 2 , 3/2 solenoid valve ( 21 ), first normally open 2/2 solenoid valve ( 22 ) and second normally closed 2/2 solenoid valve ( 23 ) are in their spring-loaded normal positions, and, so, the redundancy pressure present at first pneumatic input ( 5 ) of 3/2 solenoid valve ( 21 ) is transmitted to output ( 8 ) of valve-modulator device ( 7 ). 
       FIG. 3  shows air admission in EBS operation. During EBS operation, 3/2 solenoid valve ( 21 ) is in switched condition, so that the supply pressure present at second pneumatic input ( 4 ) is active. Second 2/2 solenoid valve ( 23 ) remains in its closed normal position, and first 2/2 solenoid valve ( 22 ) is opened. Armature ( 22   a ) in its illustrated intermediate position indicates the usual “pulsing” during air-admission operation. 
     If a fully-developed pressure is to be held in EBS operation, first 2/2 solenoid valve ( 22 ) moves from the switched position, shown in  FIG. 3 , to closed position (metal-to-metal sealing seat), shown in  FIG. 4 . Thus, an effect of the pressure present at output ( 8 ) is no longer exerted via pilot-control unit ( 1 ). 
     For venting in EBS operation, 2/2 solenoid valve ( 23 ) is moved from the switched position, shown in  FIG. 4 , to open position, shown in  FIG. 5 , usually, also in pulsed manner. 
     For completeness, the valve positions for pure ABS operation are illustrated in  FIGS. 6 to 8 . In ABS operation, the driver&#39;s intent, which is represented by the redundancy pressure, is active, and, so, 3/2 solenoid valve ( 21 ) remains in its spring-loaded normal condition. ABS venting according to  FIG. 6  therefore corresponds to EBS venting according to  FIG. 5  with switched 3/2 solenoid valve ( 21 ). ABS pressure holding according to  FIG. 7  corresponds to EBS pressure holding according to  FIG. 4  with switched 3/2 solenoid valve ( 21 ). And ABS air admission according to  FIG. 8  corresponds to EBS pressure holding according to  FIG. 3  with switched 3/2 solenoid valve ( 21 ). 
     The construction of valve-modulator device ( 7 ) will be discussed hereinafter with reference to  FIG. 10 , which shows a section B-B corresponding to the section direction indicated in  FIG. 9 . 
     A solenoid holder ( 13 ) for common solenoid ( 11 ) is disposed on common armature-guide arrangement ( 10 ). A U-shaped magnet yoke ( 14 ) is provided for generation of a strong magnetic field. 
     On common armature-guide arrangement ( 10 ) there is provided, in the region of primary armature ( 22   a ), a magnetic-field-concentrating yoke bush ( 15 ) of ferromagnetic material. The yoke bush ( 15 ) extends over a certain length region on an armature-guide tube ( 22   r ) provided for the primary armature ( 22   a ). 
     A magnetic-field-concentrating yoke bush ( 16 ) is also provided for secondary armature ( 23   a ). Yoke bush ( 16 ), as shown in  FIG. 10 , extends over a greater length region on armature-guide tube ( 23   r ) provided for secondary armature ( 23   a ) than the length region of yoke bush ( 15 ) of primary armature ( 22   a ). 
     In accordance with the present invention, yoke bush ( 16 ) of secondary armature ( 23   a ), which is longer than yoke bush ( 15 ) of primary armature ( 22   a ), establishes a magnetic shunt connected in parallel with secondary armature ( 23   a ), as explained in greater detail hereinafter. 
     To complete the magnetic loop, there is provided a magnet core ( 12 ), which is disposed immovably between air-admitting 2/2 solenoid valve ( 22 ) and venting 2/2 solenoid valve ( 23 ), and in which internal connection ( 9 ) has the form of a bore. A nonmagnetic disk ( 25 ) of nonmagnetic material is provided in magnet core ( 12 ) at the end thereof directed toward 2/2 solenoid valve ( 23 ). 
     In  FIG. 10  there is also shown a plurality of O-rings, which will not be identified in further detail, and which are used for mutually sealing pressure spaces in valve-modulator device ( 7 ). 
     The construction of pilot-control unit ( 1 ) with 3/2 solenoid valve ( 21 ) and valve-modulator device ( 7 ) is shown in  FIG. 9  as section A-A, which corresponds to the section direction shown in  FIG. 10 . As depicted in  FIG. 9 , armatures ( 21   a ,  22   a ,  23   a ) are identical, and this represents a preferred embodiment. 
     By analogy to DE 101 13 316 A1, these identical armatures ( 21   a ,  22   a ,  23   a ) are advantageously constructed as small armatures with an approximate weight of only about 6 g. The metal body of each armature is completely coated with PTFE plastic and the elastomeric sealing element is attached by simplified vulcanization without coupling agent, although this sealing element is joined interlockingly to the metal body by an undercut. 
     Similarly, it is also advantageous to construct common solenoid ( 11 ) of valve-modulator device ( 7 ) such that it is identical to solenoid ( 27 ) of 3/2 solenoid valve ( 21 ). 
     A comparison of the two valve units ( 21 ,  7 ) reveals the different configurations of the yoke bushes. Yoke bush ( 28 - 1 ), disposed on armature-guide tube ( 21   r ) of 3/2 solenoid valve ( 21 ), is constructed such that it is equal in length to yoke bush ( 15 ) on armature-guide tube ( 22   a ). Yoke bush ( 28 - 2 ), as the counterpart at the lower end of U-shaped magnet yoke ( 29 ) of 3/2 solenoid valve ( 21 ), is identical in length to yoke bush ( 28 - 1 ). Compared with magnet yoke ( 28 - 2 ), however, as explained above, yoke bush ( 16 ) at the lower end of U-shaped magnet yoke ( 14 ) of valve-modulator device ( 7 ), as the counterpart for yoke bush ( 15 ) disposed on armature-guide tube ( 22   r ), is of considerably longer construction. 
     In implementing the magnetic shunt for secondary armature ( 23   a ) according to  FIG. 10 , armature-guide tube ( 23   r ) provided for this armature is made of nonmagnetic material such as, for example, non-rusting steel, as indeed are the other armature tubes ( 22   r  and  21   r ). 
     As illustrated in  FIG. 11 , a magnetic flux Φ 1  is generated in primary armature ( 22   a ) of valve-modulator device ( 7 ) by the magnetomotive force θ, which is generated on the basis of the current I flowing through solenoid ( 11 ) A component Φ N  of this flux is branched off by a magnetic shunt of secondary armature ( 23   a ), so that in this armature there is active a flux Φ 2  that is smaller than Φ 1  by shunt component Φ N :
 
Φ 2 =Φ 1 −Φ N   [1]
 
     For switching to occur, the magnetic-force-determining flux I is controlling for primary armature ( 22   a ) and flux Φ 2  is controlling for secondary armature ( 23   a ) An armature ( 22   a ,  23   a ) changes over from its respective normal condition to its switched condition whenever the magnetic force acting on it exceeds the force of its restoring spring ( 22   f ,  23   f ). 
     For explanation of the magnetic shunt, the three-dimensional magnetic field in  FIG. 11  is illustrated in simplified and schematic form as a magnetic-flux loop ( 3 ), comprising, firstly, a main path ( 19 ), namely the path for the flux Φ 2  that is active in secondary armature ( 23   a ), and, secondly, a shunt path ( 20 ), which forms the magnetic shunt and through which flux Φ N  passes. 
     In magnet core ( 12 ), the flux Φ 1  introduced by primary armature ( 22   a ) is split into fluxes Φ 2  and Φ N  at branch point ( 26 ), where a first common magnetic path with unattenuated flux Φ 1  is followed in magnet core ( 12 ) by a second magnetic path comprising part of the magnetic main path ( 19 ) and having attenuated flux Φ 2 , while a third magnetic path comprising part of magnetic shunt path ( 20 ) and having shunt flux Φ N  is established in magnet core ( 12 ), in parallel with the second magnetic path. 
     The flux Φ N  that is active in shunt path ( 20 ) represents, in accordance with the present invention, the desired cause of the aforesaid switching threshold increase ΔI necessary for switching secondary armature ( 23   a ). To obtain a switching threshold increase ΔI, which is usually predetermined in the valve design of valve-modulator device ( 7 ), the shunt-path flux Φ N  together with the flux Φ 1  can therefore be established by primary armature ( 22   a ) by defining the magnetic resistances involved. 
       FIG. 15   a  shows the equivalent circuit diagram of magnetic flux loop ( 3 ) with main and shunt paths ( 19 ,  20 ) according to  FIG. 11 , with the magnetic-flux-generating magnetomotive force θ, the magnetic fluxes Φ 1 , Φ 2  and Φ N , and with what, for the time being, are generally assumed magnetic resistances R A , R B  and R C . 
     The fluxes can be determined by application of Kirchhoff&#39;s rules to the equivalent circuit diagram according to  FIG. 15   a  The unattenuated flux Φ 1  through primary armature ( 22   a ) is given by: 
                     Φ   1     =     Θ       R   A     +         R   B     ·     R   C           R   B     +     R   C                     (   2   )               
The attenuated flux Φ 2  through secondary armature ( 23   a ) is given by:
 
                     Φ   2     =     Θ           R   A       R   C       ·     (       R   B     +     R   C       )       +     R   B                 (   3   )               
The shunt flux Φ N  is given by:
 
     
       
         
           
             
               
                 
                   
                     Φ 
                     N 
                   
                   = 
                   
                     Θ 
                     
                       
                         
                           
                             R 
                             A 
                           
                           
                             R 
                             B 
                           
                         
                         · 
                         
                           ( 
                           
                             
                               R 
                               B 
                             
                             + 
                             
                               R 
                               C 
                             
                           
                           ) 
                         
                       
                       + 
                       
                         R 
                         C 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     As shown in  FIG. 15   b , the magnetic resistances R A , R B , R C  represent series connections of magnetic resistances of individual mechanical devices according to  FIG. 11  By suitable dimensioning of these devices it is possible, according to equations [2] and [3], to establish the magnetic fluxes Φ 1  and Φ N  necessary for the desired current I 1  for switching primary armature ( 22   a ) and for the switching threshold increase ΔI for switching secondary armature ( 23   a ). 
     According to  FIG. 15   b , the magnetic resistance R A  then represents the series connection of the magnetic resistances of the following mechanical devices according to  FIG. 11  in the common magnetic path through which the flux Φ 1  is passing:
         R 1  (R 1 ) magnetic resistance of U-shaped magnet yoke ( 14 );   R 2  (R 2 ) magnetic resistance of yoke bush ( 15 ) provided for primary armature ( 22   a );   R 3  (R 3 ) magnetic resistance of nonmagnetic armature-guide tube ( 22   r ) for primary armature ( 22   a ) and of the air gap to primary armature ( 22   a );   R 4  (R 4 ) magnetic resistance of primary armature ( 22   a );   R 5  (R 5 ) magnetic resistance of the air gap between primary armature ( 22   a ) and its metal-to-metal sealing seat on magnet core ( 12 );   R 6  (R 6 ) magnetic resistance of the first common magnetic path in magnet core ( 12 ) without field attenuation by magnetic shunt path ( 20 ).       

     Furthermore, the magnetic resistance R B  represents the series connection of the magnetic resistances of the following mechanical devices in magnetic main path ( 19 ) according to  FIG. 11 :
         R 7  (R 7 ) magnetic resistance of the second magnetic path in magnet core ( 12 ), which resistance is attenuated by magnetic shunt path ( 20 );   R 8  (R 8 ) magnetic resistance of the air gap between secondary armature ( 23   a ) and its metal-to-metal sealing seat on magnet core ( 12 );   R 9  (R 9 ) magnetic resistance of secondary armature ( 23   a );   R 10  (R 10 ) magnetic resistance of the air gap to secondary armature ( 23   a ) and of nonmagnetic armature-guide tube ( 23   r ) for secondary armature ( 23   a );   R 11  (R 11 ) magnetic resistance of yoke bush ( 16 ) provided for secondary armature ( 23   a ) along the magnetic path of the bush in radial direction.       

     Finally, the magnetic resistance R C  represents the series connection of the magnetic resistances of the following mechanical devices in magnetic shunt path ( 20 ) according to  FIG. 11 :
         R 12  (R 12 ) magnetic resistance of the third magnetic path in magnet core ( 12 ) for field attenuation by magnetic shunt path ( 20 );   R 13  (R 13 ) magnetic resistance of yoke bush ( 16 ) provided for secondary armature ( 23   a ) along the magnetic path of the bush in axial direction, as part of magnetic shunt path ( 20 ).       

     Resistances R 1  to R 13  explained on the basis of  FIG. 15  are valid for the condition illustrated in  FIG. 11 , in which primary armature ( 22   a ) and secondary armature ( 23   a ) are, respectively, in their normal positions determined by spring loading. When magnet current I increases from zero, the flux Φ 1  in primary armature ( 22   a ) increases in accordance with these resistances until it reaches current I 1 , whereupon primary armature ( 22   a ) switches and bears with its end face directed toward magnet core ( 12 ) to form a metal-to-metal seal therewith. 
     At this instant (see  FIG. 15   c ), the magnetic air-gap resistance (R 5 ) drops practically to zero, and this is associated with a considerable increase of the flux Φ 1 . To prevent secondary armature ( 23   a ) from also switching as soon as the magnet current I 1  is reached, a part Φ N  of this increased flux Φ 1  is branched off via magnetic shunt path ( 20 ), so that the flux Φ 2  remaining in main path ( 19 ) is not yet sufficient to also switch secondary armature ( 23   a ). By means of magnetic shunt path ( 20 ), therefore, the magnetic properties of secondary armature ( 23   a ) are reduced compared with primary armature ( 22   a ) in a very specific manner defined by the division of magnetic flux between resistances R B  and R C . 
     It is only when the magnet current I is increased above the first magnet current I 1  by the value ΔI, so that it reaches the second magnet current with defined magnitude I 2 , that the flux Φ 2  through secondary armature ( 23   a ) is increased to the point that secondary armature ( 23   a ) also changes over to its switched condition (see  FIG. 15   d ). 
     During switching of secondary armature ( 23   a ), the magnetic air-gap resistance (R 8 ) drops practically to zero, and, without further measures, the flux Φ 2  in secondary armature ( 23   a ) would jump abruptly, with the consequence that, to switch secondary armature ( 23   a ) back to its normal condition, such a large decrease of the magnet current would be necessary that it would also cause primary armature ( 22   a ) to switch back, and thus independent actuation of primary and secondary armatures ( 22   a ,  23   a ) would no longer be assured. 
     Such a flux increase during switching of secondary armature ( 23   a ) is prevented by nonmagnetic disk ( 25 ) with its magnetic resistance R 14 , which is disposed in series with the resistance R 6  (see  FIG. 15   d ). 
     However, this resistance is active only when secondary armature ( 23   a ) is switched, since when secondary armature ( 23   a ) is not switched the lines of force in the air gap between secondary armature ( 23   a ) and magnet core ( 12 ) are concentrated at the ferromagnetic surfaces of these units, so that nonmagnetic disk ( 25 ) is affected only by a negligible stray flux. The magnetic resistance R 14  (R 14 ) of nonmagnetic disk ( 25 ) is therefore negligibly small when secondary armature ( 23   a ) is not switched For this case, it will be set equal to zero and not considered further. 
     When the magnetic resistance R 8  of the air gap between secondary armature ( 23   a ) and magnet core ( 12 ) itself drops to zero during switching of secondary armature ( 23   a ), however, the conditions are changed: while secondary armature ( 23   a ) is bearing on magnet core ( 12 ), only one part of the secondary armature ( 23   a ) is in contact with magnet core ( 12 ) via direct iron-to-iron contact with good field transfer, whereas the other part of secondary armature ( 23   a ) is in contact with magnet core ( 12 ) indirectly via the end face of nonmagnetic disk ( 25 ). In the equivalent circuit diagram according to  FIG. 15   d , the magnetic resistance R 14  of nonmagnetic disk ( 25 ) is now active as a series resistance between resistances R 6  and R 7 , whereupon the total resistance of magnetic-flux loop ( 3 ) is artificially increased, thus preventing a sudden increase of the flux Φ 2  through secondary armature ( 23   a ). 
     The switching of primary armature ( 22   a ) and secondary armature ( 23   a ) as a function of the magnet current I is illustrated in  FIG. 16 . As explained, primary armature ( 22   a ) switches when the magnet current I reaches at least the first magnet current I 1 , and it changes over to its normal condition when the magnet current has dropped to at least a third magnet current I 3 . In contrast, secondary armature ( 23   a ) switches at a magnet current of at least the second magnet current I 2  and it returns to its normal condition when the magnet current drops to at least a fourth magnet current I 4 . As shown, the fourth magnet current I 4  is much larger than the third magnet current I 3 , and so the switching ranges for primary and secondary armatures ( 22   a ,  23   a ) are substantially separate from one another. The sharpness of separation for actuation of the two armatures is achieved according to the present invention by the two measures explained above: the magnetic shunt path is controlling for the “retarded” forward switching of the secondary armature (I 2 &gt;I 1 ), and the nonmagnetic disk is controlling for the “advanced” backward switching of the secondary armature (I 4 &gt;I 3 ). It is possible under certain circumstances to make the magnetic shunt flux so high that advanced backward switching of secondary armature ( 23   a ) is also achieved, in which case there is no need to incorporate nonmagnetic disk ( 25 ). 
     In the further embodiments of a magnetic shunt for secondary armature ( 23   a ) depicted in  FIGS. 12 to 14 , the shunt is generated not by changing yoke bush ( 16 ) for secondary armature ( 23   a ), but by changing armature-guide tube ( 23   r ) itself. In these configurations, therefore, yoke bush ( 16 ) for secondary armature ( 23   a ) is made such that it is identical to the “normal” construction ( 15  and  28 - 2 ). 
     In the embodiment depicted in  FIG. 12 , the armature-guide tube is made partly of ferromagnetic material in order to establish the magnetic shunt for secondary armature ( 23   a ). It is composed of a first ferromagnetic part ( 30 ) and a second nonmagnetic part ( 31 ), and the one-piece armature-guide tube ( 23   r ) is formed from the first and second parts. 
     In the embodiment depicted in  FIG. 13 , armature-guide tube ( 23   r ) for secondary armature ( 23   a ) is made completely of ferromagnetic material. 
     Finally, in the embodiment depicted in  FIG. 14 , the actual armature-guide tube ( 23   r ) for secondary armature ( 23   a ) is formed as a hollow-cylindrical extension of magnet core ( 12 ) disposed immovably between primary armature ( 22   a ) and secondary armature ( 23   a ). Spring-holder attachment ( 32 ), which is made of nonmagnetic material and joined to armature-guide tube ( 23   r ), alternatively can also be made of magnetic material. 
     It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the above constructions without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.