• Electromagnetic solenoid valve with variable force motor

    Date: 2011-12-7 9:12:07
    Electromagnetic variable force motors of the subject type include a pole piece having a central structure surrounded by a coil, an armature positioned for motion toward and away from the pole piece, and a spring for urging the armature away from the pole. The pole piece and armature are of ferromagnetic construction so that current in the coil establishes a magnetic field in the pole piece, attracting the armature toward the pole against the force of the spring. Sliding friction between the armature and surrounding structure, only partially reduced by armature guide and bushing structures, results in energy loss and position hysteresis between the armature and pole piece. One exemplary solenoid valve embodying a linear force motor of the described character is disclosed in U.S. Pat. No. 4,579,145.

    It is an object of the present invention to provide an electromagnetic variable force motor which exhibits reduced friction between the armature and surrounding pole piece and housing structures, and thereby obtains both reduced energy loss and reduced hysteresis in armature movement.

    Another object of the present invention is to provide a solenoid valve which embodies an electromagnetic variable force motor of the described character for enhanced precision control of fluids, such as hydraulic fluids in electronically controlled fluid transmissions.

    A further object of the invention is to provide a solenoid valve of the described character which exhibits reduced size and complexity, and which is thus more economical to manufacture than are variable force motor solenoid valves of the prior art.

    In the preferred embodiments of the invention, a housing encloses the pole piece and armature and has journalling surfaces which surround the ball-armature, guiding axial motion thereof while limiting motion transversely of the pole piece axis. The structure which transmits the spring force to the ball-armature is configured so as to limit contact with the surrounding pole piece and housing. There is thus limited contact and friction between the moving elements of the force motor--i.e., the ball-armature and spring force-transmitting element--and the surrounding housing. As a result, hysteresis in ball-armature position versus current is substantially eliminated.

    In accordance with one aspect of the present invention, an electromagnetic variable force motor comprises a pole piece of ferromagnetic construction and an electrical coil coaxially surrounding the pole. Ball size and armature travel are selected in a manner such that the magnetic attractive force is compatible with the desired management characteristic for the flow and pressure requirements. A spring engages the ball-armature and urges the same away from the opposing pole face. That is, magnetic attractive force caused by a given current in the coil and a corresponding reduction in the armature/pole piece air gap is substantially identically balanced by a change in compression and corresponding force in the spring. Most preferably, such armature/pole piece force characteristic and spring rate are both substantially linear, such that displacement of the ball-armature with respect to the pole piece against force of the spring varies substantially linearly with current to the coil.

    A solenoid valve in accordance with the present invention comprises a valve body having a bore with a central axis and fluid passages extending radially or transversely therefrom. A housing of ferromagnetic material is mounted on the valve body, and includes a ferromagnetic pole piece coaxially with the bore and an electrical coil circumferentially surrounding the pole piece. A ball-armature is positioned in coaxial opposition to the pole in engagement with the valve element, and a spring is positioned to engage the ball-armature to urge the same away from the opposing face of the pole piece. The valve element, which is preferably constructed separately from the ball-armature, comprises a valve spool in the preferred embodiments of the invention having axially spaced lands which cooperate with the valve body passages extending from the central bore.

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