Electromagnetic actuator having a pneumatic dampening element

Information

  • Patent Grant
  • 6279524
  • Patent Number
    6,279,524
  • Date Filed
    Wednesday, February 9, 2000
    24 years ago
  • Date Issued
    Tuesday, August 28, 2001
    23 years ago
Abstract
An electromagnetic actuator for operating a driven member includes an electromagnet having a yoke provided with a pole face and a coil supported by the yoke and adapted to be supplied with electric current to produce an electromagnetic force; and an armature connectable to the driven member and movable in a path of motion into a first position and a second position spaced from the first position. The armature is moved by the electromagnetic forces into the second position in contact with the pole face. A resetting spring is coupled to the armature and exerts a spring force thereon for urging the armature away from the second position toward the first position. The actuator further includes a dampening assembly which has a piston affixed to the armature and a dampening cylinder provided in the yoke and having an open end oriented toward the armature and being in an axial alignment with the piston. In the first position of the armature the piston is axially spaced from the dampening cylinder. The piston enters the dampening cylinder solely during a terminal portion of the path of motion of the armature during displacement thereof toward the second position.
Description




CROSS REFERENCE TO RELATED APPLICATION




This application claims the priority of German Application No. 199 05 176.3 filed Feb. 9, 1999, which is incorporated herein by reference.




BACKGROUND OF THE INVENTION




In an electromagnetic actuator for operating a driven member at least one electromagnet is provided which has a yoke body and with which a back-and-forth movable armature plate is associated. When the electromagnet is energized, the armature plate is moved against the force of a resetting spring from a first switching position into a second switching position in which the armature plate engages the pole face of the yoke body. If the electromagnet is de-energized, the armature plate is returned to its first switching position under the effect of the resetting force of the resetting spring.




For certain applications, for example, for operating an engine valve of a piston-type internal-combustion engine where the engine valve constitutes the driven member, two spaced electromagnets are provided whose pole faces are oriented towards one another and between which the armature plate, attached to a guide bar, is movable back and forth against the force of resetting springs. When the electromagnets are de-energized, the armature plate assumes a mid position between the two electromagnets. If the electromagnets are alternatingly energized, the armature plate moves from a first switching position which is defined by its engagement with the pole face of one electromagnet, into its second switching position after de-energization of the last-named, momentarily holding electromagnet and energization of the other, momentarily capturing electromagnet. The second switching position of the armature plate is defined by its engagement with the pole face of the other, capturing electromagnet. By means of a suitable control of current supply in an alternating manner, the armature plate and thus the driven member may again be moved into the first switching position.




In the earlier-noted mode of application for operating an engine valve in a piston engine not only a high switching frequency is obtained but, based on the relatively large stroke of the engine valve, the armature has an oscillating path (stroke) of significant length. Despite the resetting force of the resetting spring arranged at the capturing electromagnet and acting on the armature plate during its approach to the pole face of the capturing electromagnet, the net attracting force on the armature plate increases exponentially, since, as the distance between the armature plate and the pole face of the capturing electromagnet decreases, the electromagnetic force increases exponentially, while the opposing spring force increases only linearly. As a result, the armature plate is increasingly accelerated until it impacts on the pole face of the capturing electromagnet. The resulting high impact velocity leads to a significant noise generation and may also lead to disadvantageous rebounding phenomena.




Conventional pneumatic dampening arrangements for braking the armature, such as disclosed, for example, in published European Patent Application 0 870 906, not only require a significant structural height; also, they are not capable of delivering the required dampening force with acceptable frictional values.




By means of controlling the current supply of the momentary capturing electromagnet, it is feasible to change the magnetic force during the approaching phase of the armature plate in such a manner that the armature plate arrives into contact with the pole face with a reduced speed. It is, to be sure, in principle possible to change, by controlling the current supply of the capturing electromagnet, the magnetic force in such a manner that the armature plate arrives with a “zero” impact velocity in contact with the pole face of the capturing electromagnet. Because of stochastic oscillations in the system, however, a reliable, secure capturing and holding of the armature plate is not feasible. Thus, for reasons of operational reliability the current supply of the capturing electromagnet must be set in such a manner that the armature plate arrives in contact with the pole face with a reduced, but significant residual speed. While rebound phenomena are in this manner prevented, noise generation nevertheless persists. By means of the above-described reduction of the impact velocity, however, significantly reduced dampening forces are required.




SUMMARY OF THE INVENTION




It is an object of the invention to provide an improved electromagnetic actuator which has a pneumatic dampening element of compact construction.




This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the electromagnetic actuator for operating a driven member includes an electromagnet having a yoke provided with a pole face and a coil supported by the yoke and adapted to be supplied with electric current to produce an electromagnetic force; and an armature connectable to the driven member and movable in a path of motion into a first position and a second position spaced from the first position. The armature is moved by the electromagnetic forces into the second position in contact with the pole face. A resetting spring is coupled to the armature and exerts a spring force thereon for urging the armature away from the second position toward the first position. The actuator further includes a dampening assembly which has a piston affixed to the armature and a dampening cylinder provided in the yoke and having an open end oriented toward the armature and being in an axial alignment with the piston. In the first position of the armature the piston is axially spaced from the dampening cylinder. The piston enters the dampening cylinder solely during a terminal portion of the path of motion of the armature during displacement thereof toward the second position.




Since in case of a suitable control of the current supply of the capturing electromagnet the impact velocity is reduced, a very compact dampening element may be provided which may be integrated directly into the electromagnetic actuator without adversely affecting the course of the magnetic field lines of the electromagnet. It is a further advantage of the invention that such a dampening element may be utilized in armature plates with different circumferential contour without changing the construction; that is, the same dampening element may be used for an armature plate whether its contour is, for example, circular or polygonal.




According to an advantageous feature of the invention, the armature plate is coupled with a guide bar which, in turn, is guided in a bore of the yoke body and further, the dampening cylinder is arranged in the region of the bore and surrounds the guide bar. Such an arrangement has the advantage that the dampening element is situated in the region of the pole face of the yoke body in which the course of the field lines is, in any event, distorted by the preferably non-magnetic guide bar so that an arrangement of the dampening cylinder in the pole face, on the one hand, and the associated dampening piston on the face of the armature plate, on the other hand, does not lead to additionally disturbing the magnetic flux. The piston body is formed by an annular collar surrounding the guide bar in the zone of the plane of the armature plate. Expediently, the terminal peripheral edge of the dampening piston oriented towards the dampening cylinder is rounded. As a result, the dampening force exerted by the air in the annular chamber of the dampening cylinder on the dampening piston does not increase abruptly as the dampening piston enters the dampening cylinder; rather, by virtue of the rounded piston edge the pressure increases progressively because of the progressively narrowing air gap.




According to a further advantageous feature of the invention, the dampening cylinder is arranged in a cavity of the yoke body. Such a cavity may be, for example, an enlargement of the guide bore for the guide bar. The enlargement is dimensioned dependent on the size of the dampening cylinder.




Since usually the yoke of such an electromagnet is formed of a lamina stack made of thin sheet metal plates, and thus the yoke cavity itself cannot be directly used as a cylinder, according to an advantageous feature of the invention, the dampening cylinder is formed by an annular body made of a plastic material, expediently polytetrafluoroethylene. It is further expedient to arrange the annular body with a radial clearance in the cavity of the yoke. As a result, in conjunction with a rounded edge of the dampening piston an automatic centering of the dampening cylinder is obtained. Such an automatic centering compensates not only for manufacturing inaccuracies but also for a changing clearance between the guide bar and the guide in the yoke. It is an advantage of using a plastic material, particularly PTFE for the dampening cylinder, in conjunction with the small radial clearance for the annular body forming the dampening cylinder, that a low-friction dampening element is obtained. Thus, the delay at the beginning of the armature motion upon de-energization of the holding electromagnet is practically not affected by the frictional forces generated between the dampening piston and the wall of the dampening cylinder. By virtue of the small radial clearance of the annular body in the socket of the yoke and also by virtue of the minimal annular clearance between the guiding component for the guide bar on the one hand and the guide bar itself on the other hand, the dampening cylinder has a minimal leakage space. Consequently, during the last phase of the penetration of the dampening piston into the dampening cylinder no reverse spring effect can appear; rather, the air present in the progressively diminishing cylinder chamber may escape in a throttled manner.




According to a further advantageous feature of the invention the annular body is in contact with an elastic intermediate layer which, however thin, reduces the sound transmission in the device.




According to a further advantageous feature of the invention a guide sleeve is arranged in the yoke for guiding the guide bar. Further, that end of the guide sleeve which is oriented towards the pole face is formed as a socket for the dampening cylinder. By virtue of such a construction, the dampening cylinder which, in any event, has very small dimensions, may be integrated in the guide sleeve whereby a compact, easily installable structural component is obtained. Also, no angular deviations from the alignment between the axis of the dampening cylinder and the axis of the guide sleeve can occur.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is an axial sectional view of an electromagnetic actuator incorporating the invention.





FIG. 2

is an enlarged axial sectional detail of the construction shown in FIG.


1


.











DESCRIPTION OF THE PREFERRED EMBODIMENT




The electromagnetic actuator illustrated in

FIG. 1

is composed essentially of two electromagnets


1


and


2


each having a respective laminated yoke


3


and


4


in which respective coils


5


and


6


are arranged. The coils


5


and


6


are coupled to a non-illustrated controllable current supply.




The yokes


3


and


4


each have a bore


7


in which a respective guide sleeve


8


is secured, for example, by gluing. In each guide sleeve


8


one or two glide sleeves


9


, made, for example, by a self-lubricating sintered body, are secured by press fit.




The respective pole faces


10


.


1


,


10


.


2


of the two spaced electromagnets


1


and


2


are oriented toward one another. In the space between the pole faces


10


.


1


,


10


.


2


of the two electromagnets


1


and


2


a plate-shaped armature


11


is arranged which is fixedly connected with a guide bar


12


which, in turn, is guided in the guide sleeve


8


of the electromagnet


2


. In the electromagnet


1


a separate guide bar


13


is arranged which is supported at one end on the guide bar


12


and, at its other end, on a resetting spring


15


with the intermediary of a spring seat disk


14


.




The free end


16


of the guide bar


12


contacts an end of a driven member


17


, for example, the stem of an engine valve with which a resetting spring


18


is associated. Both resetting springs


15


and


18


are so designed that in case the electromagnets


1


and


2


are in a de-energized state, the armature


11


assumes a position in the middle between the two pole faces


10


.


1


,


10


.


2


.




The two electromagnets


1


and


2


are supplied alternatingly with current from a controlled current supply, causing the armature


11


to alternatingly travel to the pole face


10


.


1


of the electromagnet


1


and the pole face


10


.


2


of the electromagnet


2


. Further, upon energizing the solenoid


5


or


6


with a holding current the armature


11


is maintained at the respective pole face


10


.


1


,


10


.


2


for a predetermined period.




Thus, in each instance during the approach of the armature


11


to the pole face of the energized, that is, the capturing electromagnet, the magnetic force affecting the armature


11


, in case of a non-regulated current supply, exponentially increases, while the force of the associated resetting spring increases only linearly. Thus, the armature


11


is moved towards the respective pole face


10


.


1


or


10


.


2


with increasing speed so that it impacts on the pole face with a relatively high speed. By a suitable reduction of the current supply to the coil of the capturing electromagnet, as the armature


11


approaches the pole face, the magnetic force exerted on the armature


11


is reduced in such a manner that by the moment the armature contacts the pole face, the magnetic force only slightly exceeds the respective resetting force of the associated resetting spring. As a result, the armature


11


arrives into contact with the pole face with a reduced speed.




To further reduce the armature impact on the respective pole face


10


.


1


or


10


.


2


, according to the invention a dampening assembly


19


is provided which is essentially composed of a piston


20


affixed to the guide bar


16


and respective dampening cylinders


21


provided in the respective yokes


3


and


4


. The piston


20


enters into the respective dampening cylinder


21


shortly before the armature


11


arrives into contact with the respective pole face


10


.


1


or


10


.


2


. The air cushion present during the introduction of the piston


20


into the respective cylinder


21


may be caused to decay by a throttle gap defined between the circumference of the piston


20


and the guide bar


7


on one hand and the inner face of the glide sleeve


9


on the other hand, whereby the armature


11


arrives softly into contact with the respective pole face


10


.


1


or


10


.


2


.





FIG. 2

illustrates an embodiment of the dampening assembly


19


in greater detail. In the region of its connection with the armature


11


the guide bar


12


has a collar


22


of a greater diameter than that of the guide bar


12


proper. The collar


22


is so structured that it axially projects beyond the armature


11


on both sides and forms the dampening piston


20


. The peripheral edge


24


at both ends of the dampening piston


20


is rounded.




The upper end of the guide sleeve


8


provided, for example, in the yoke


4


of the electromagnet


2


is provided with a socket


25


in which a ring


26


is inserted. The ring


26


is made of a wear-resistant plastic material having a low surface friction, such as PTFE. The ring


26


is seated on the bottom of the socket with the interposition of a thin elastic intermediate layer


27


. The ring


26


is immobilized in the socket


25


by an annular securing member


28


having a radially inwardly extending rim


28


.


1


. The inner diameter of the ring


26


is only slightly greater than the outer diameter of the dampening piston


20


and thus constitutes the dampening cylinder


21


. The ring


26


is supported in the socket


25


by the securing member


28


in such a manner that the ring


26


has a slight radial play. Thus, the dampening cylinder


21


formed by the ring


26


may be self-centered relative to the dampening piston


20


.




As the dampening piston


20


enters the dampening cylinder


21


, first an open annular clearance is obtained because of the rounding of the edge


24


. The clearance narrows as the dampening piston


20


further penetrates into the dampening cylinder


21


until the cylindrical outer wall of the dampening piston


20


and the cylindrical wall of the dampening cylinder


21


arrive in a practically direct contact with one another. The clearance between the dampening piston


20


and the dampening cylinder


21


is so designed that the dampening piston


20


is moved along the wall of the dampening cylinder


21


with minimal friction.




As soon as the dampening piston


20


has penetrated into the dampening cylinder


21


, in the closed cylinder chamber first an air cushion is obtained which, however, due to the minimal leakage present, for example, between the sleeve


9


and the guide bar


12


, decays so that the armature


11


eventually arrives into contact with the pole face of the yoke body.




It is to be understood that the invention is not limited to the illustrated and described embodiment of an actuator having two electromagnets. Rather, the invention may find application in actuators which have a sole electromagnet and in which the armature


11


is maintained by a resetting spring in a first switching position in the deenergized state of the electromagnet and upon energizing the electromagnet it moves in the direction of the pole face of the electromagnet against the force of the resetting spring and after contacting the pole face, the armature is maintained in its defined second switching position as long as the electromagnet is in an energized state.




By way of an example of dimensions, in case of a diameter of the piston


20


of, for example, 10 mm, the piston


20


projects beyond the related surface of the armature


11


by approximately 3 mm. The inner diameter of the dampening cylinder


21


defined by the ring


26


is slightly greater than the diameter of the piston body


20


and has, for example, an outer diameter of approximately 14 mm. In case of a total depth of approximately 3 mm, the ring


26


has a thickness of approximately 1.6 mm so that for the intermediate layer


27


an elastic material having only a thickness of 0.4 mm may be provided. The elastic intermediate layer


27


is, for example, of NBR having a hardness of


70


shore. The thickness of the retaining rim


28


.


1


of the securing member


28


extending over the ring


26


has then a thickness of, for example, 1 mm. These dimensional relationships result in a small overall structural volume of the dampening assembly


19


according to the invention.




It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.



Claims
  • 1. An electromagnetic actuator for operating a driven member, comprising(a) an electromagnet having (1) a yoke provided with a pole face and a passage; and (2) a coil supported by said yoke and adapted to be supplied with electric current to produce an electromagnetic force; (b) an armature connectable to the driven member and movable in a path of motion into a first position and a second position spaced from said first position; said armature being moved by the electromagnetic forces into said second position in contact with said pole face; (c) a resetting spring coupled to said armature and exerting a spring force thereon for urging said armature away from said second position toward said first position; (d) a dampening assembly including (1) a piston affixed to said armature; and (2) a dampening cylinder provided in said yoke and having an open end oriented toward said armature and being in an axial alignment with said piston; in said first position of said armature said piston being axially spaced from said dampening cylinder; said piston entering said dampening cylinder solely during a terminal portion of said path of motion during displacement of said armature toward said second position; said passage of said yoke being in axial alignment and in communication with said dampening cylinder; (e) a guide bar affixed to said armature and passing through said passage; and (f) a sleeve positioned in said passage; said guide bar passing through said sleeve; said sleeve having an end portion situated at said pole face; and said end portion forming a socket accommodating said dampening cylinder.
  • 2. The electromagnetic actuator as defined in claim 1, wherein said piston has an end oriented toward said dampening cylinder and further wherein said end of said piston has a rounded peripheral edge.
  • 3. An electromagnetic actuator for operating a driven member, comprising(a) an electromagnet having (1) a yoke provided with a pole face and a socket; and (2) a coil supported by said yoke and adapted to be supplied with electric current to produce an electromagnetic force; (b) an armature connectable to the driven member and movable in a path of motion into a first position and a second position spaced from said first position; said armature being moved by the electromagnetic forces into said second position in contact with said pole face; (c) a resetting spring coupled to said armature and exerting a spring force thereon for urging said armature away from said second position toward said first position; (d) a dampening assembly including (1) a piston affixed to said armature; and (2) a ring disposed in said socket and having an inner space constituting a dampening cylinder; said dampening cylinder having an open end oriented toward said armature and being in an axial alignment with said piston; in said first position of said armature said piston being axially spaced from said dampening cylinder; said piston entering said dampening cylinder solely during a terminal portion of said path of motion during displacement of said armature toward said second position.
  • 4. The electromagnetic actuator as defined in claim 3, further comprising a radial clearance between said ring and said socket.
  • 5. The electromagnetic actuator as defined in claim 3, wherein said socket has a bottom; further comprising an elastic layer positioned between said ring and said bottom.
  • 6. The electromagnetic actuator as defined in claim 3, wherein said ring is plastic.
Priority Claims (1)
Number Date Country Kind
199 05 176 Feb 1999 DE
US Referenced Citations (4)
Number Name Date Kind
4515343 Pischinger et al. May 1985
4867111 Schneider et al. Sep 1989
5223812 Kreuter Jun 1993
5943988 Burger et al. Aug 1999