BACKGROUND OF THE INVENTION At the present time, electromagnetic solenoids are widely used as inexpensive and efficient electromechanical actuators. Examples of their use include valve actuation, door latching, and many other applications where two positions, namely on and off, are suitable.

Heretofore, however, solenoids have not been suitable for applications requiring controlled positioning or proportional motion because of the highly nonlinear behavior of conventional solenoids. As an alternative,"voice coil"actuators using a strong permanent magnet and a moving coil have been used. Another alternative is to use a rotary actuator, such as an electric motor, driving a lead-screw to achieve linear motiorx Voicecoil actuators, howearer, do not produce adequate force in a given size unit, particularly compared to a solenoid, and lead-screw actuators are generally best used in low speed situations. Both are significantly more costly than solenoids.

Solenoids operate by use of magnetic force on a movable member made of ferromagnetic material, commonly steeL The force of attraction is proportional to the square of the flux density, 2 The flux density, in turn, is a function both of the current flowing in the coil and the position of the moving member.

Current flowing in the coil produces a magnetic field in the ferromagnetic body, moving member, and in the air gap between them. Because the magnetic permeability li of the ferromagnetic structure is much greater than that of air (typically 1000 times or

more), the resulting flux density depends strongly on the length of the air gap, and therefore on the position of the moving member. This can be expressed as F = B2 A, where F is force, B is flux density in the air gap, and A is the cross-sectional area of the air gap.

B-bLNI where N is the number of turns in the coil, I is the current leff flowing in the coil and Ieff is the total effective magnetic length of the magnetic circuit, including the length of the flux path through ferromagnetic material and the length of the air gap. Since the ferromagnetic material has a value for u times the permeability of air, leff is approximately liron + ulair. Combining terms and rearranging, effect of the radial air gap surrounding the left- hand end of the movable member has been omitted here because, since it does not vary with motion of the member, its effect can be accounted for as a constant addition to liron.

This nonlinear behavior produces a series of curves, when graphed as displacement versus current, compared to a straight line for the force of a spring as a function of displacement that acts in a direction counteracting the magnetic force, so as to pull the solenoid "open"under no-current conditions. This is a description of conventional solenoid operation.

The only stable points of operation are the intersections of the linear curve of the spring and the magnetic force curves for various

currents. If the magnetic force is greater, the member is pulled toward the closed position. If the spring force is greater, the member is pulled toward the open position.

Various prior art attempts to use Hall effect sensors have been proposed, though none relate to linear control of solenoids. Stupak U. S Patent No. 4,665,348 discloses a controlled force reluctance actuator in which the current in the solenoid is controlled by a signal representative of the flux density in the magnetic circuit of a variable reluctance actuator. Stupak teaches that the flux density in the magnetic circuit and controls the current by taking the output of a Hall effect sensor to a control circuit to maintain substantially constant flux density.

Gennesseaux U. S Patent No. 5,621,293 discloses a variable reluctance linear motor in which a lineariser is used, as described with respect to Figs. 5a, 5b and 5c. Pailthorp U. S Patent No.

4,656, 400 describes a solenoid where the field is varied by position sensing. Banick et al. U. S Patent No. 5,032,812 also uses a sensor to indicate the position of the core with respect to a plugnut Lovett et al. U. S Patent No. 6, 225, 767 discloses general matrix equations for the purpose of providing a desired force. None of these references relate to control of the operation of solenoids at intermediate positions.

It would be of great advantage in the art if a solenoid could be developed that would permit it to function or operate between open and closed positions.

It would be another great advance in the art if such a solenoid could operate such that the position of the movable member could be controlled from fully open to fully closed, stopping at any position there between.

A further advantage would be achieved if such a solenoid could be made to produce a force linearly proportional to a control voltage, independent of position.

Other advantages will appear hereinafter.

SUMMARY OF THE INVENTION It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner.

Specifically, the present invention provides for control of solenoids over the entire range of the moving member.

The value of flux density in the air gap is sensed, preferably with a magneto restrictive element such as a Hall effect semiconductor magnetic field sensor, to produce a voltage proportional to the intensity of the magnetic flux. The output of the flux sensor is squared and that value is subtracted from a command signal, with the results then amplified, and converted to current such as with a voltage-to- current converter.

The solenoid coil is then driven with the resulting current It is necessary that the resulting current be unipolar, because solenoids without permanent magnets can only attract The command signal should have a bias applied to it or a bias may be added within the control loop. Any means for producing a unipolar signal is suitable, and the use of a bias voltage is quite effective and simple.

The solenoid may also include a biasing means, such as a coil spring or the like, which restrains the moving member, in which case the position of the moving member is then directly proportional to the command voltage, since spring extension or compression is directly proportional to force.

The squaring function may be done with a digital microcomputer, for example, or with a readily-available and inexpensive analog multiplier integrated circuit

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the invention, reference is hereby made to the drawings, in which : FIG. 1 is a schematic, sectioned view of a typical solenoid for use with the present invention ; FIG. 2 is a graphical representation of displacement of the moving member of the solenoid of Fig. 1 by various currents, along with a linear expression of the opposing force of a spring used therewith; FIG. 3 is a schematic diagram showing the electronic circuitry of the implementation of the present invention for use with the solenoid of Fig. 1 with the spring included; and FIG. 4 is a graphical representation of displacement of the moving member of the solenoid of Fig. 1 by various forces produced according to the present invention, along with a linear expression of the opposing force of a spring used therewith.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A typical solenoid device is shown in Fig. 1 as 10, generally, and includes a ferrous material 11 and coil 13, which operates in a conventional manner to move moving member 15. These devices have, as explained above, many end uses where simple, inexpensive on and off operations are needed. The present invention, as shown in Fig. 1, further includes a flux sensor 17 located proximate the plunger stop 19. The other end 16 of moving member 15 is schematically shown for example purposes as abutting stop 21 and includes spring 23 which acts as an opposing force against which moving member is placed by operation of current in coil 13.

Spring 23 pulls the solenoid moving member 15 to an open position when there is no current in coil 13. Shown in Fig. 2 is the displacement of a typical solenoid under the force of spring 23, which produces a linear movement Of course, other biasing means may be used, such as, for example and not by way of limitation, elastomeric member, leaf springs, and the like. A solenoid operates when current passes through coil 13, generating magnetic force on moving member 15. Fig. 2 also illustrates nonlinear displacement forces for three arbitrary currents, identified as current 1, current 2, and current 3.

The only stable points of operation are the intersections of the spring curve and the magnetic forces, for those are the only places where the forces are in balance. If the magnetic force is greater, then the moving member 15 is pulled toward the closed position. As shown in Fig. 2, the current must be raised to the level of current 3 to get member 15 to move closer than 0.25 units from the closed position.

When that condition exists, moving member 15 moves all the way closed, against stop 19, because the difference between magnetic force and spring force grows faster than the spring force, with respect to displacement Even when stop 19 prevents member 15 from moving to a displacement less than 0. 05 units, for example, the magnetic force is still much greater than the spring force. If current 1 or current 2 is then applied rather than current 3, for example, the spring will pull member 15 to the right in Fig. 2 until the spring curve intersects a current curve indicating a stable position. This movement is highly hysteretic, and is known as"snap action" behavior.

In accordance with this invention, sensor 17 senses the instantaneous value of flux density in the air gap 25 between the ferromagnetic material 11 and the moving member 15. The preferred sensor 17 is a Hall effect semiconductor magnetic field sensor. Such sensors are inexpensive and available from Microswitch, Microns, Panasonic and other manufacturers. Hall effect sensors produce a voltage proportional to the intensity of magnetic flux flowing through them They are available in very small packages, such as the Microns HAL-400, which is 1.5 millimeters thick in the direction of flux being measured, and is 2. 5 x 4.5 mm in the other dimensions.

Turning now to Fig. 3, operation of the present invention is shown. Sensor 17 produces an output 31, which is in turn squared, or multiplied by itself, by multiplier 33. The output 35 of multiplier 33 enters a subtracter 37 which subtracts output 35 from command voltage 39 to produce a net output 41, which output 41 is amplified in

amplifier 43 and converted to current in voltage-to-current converter 45 to produce the current 47 for coil 13. Various electronic devices exist for squaring or multiplying voltages, such as digital microcomputers and analog multiplier integrated circuits.

Because of the accurate conversion of a command voltage input to a force output, the present invention can be used to make a solenoid actuator useful as a vibratory transducer with faithful reproduction of the command signal, replacing more expensive and large devices such as voicecoil transducers which have strong and expensive permanent magnets. In the vibratory embodiment, the control voltage would be a sinusoidal waveform, such that vc = Vmax sin wt In Fig. 3, a biasing voltage 49 is added to net output 41 by adder 51 to insure that the current 47 is unipolar. Fig. 3 produces a voltage Vb + vmax sin (wt), with the Vb being the bias. The force on moving member 15 is directly proportional to the magnitude of the command signal regardless of the instant position of said member. When, as in Fig. 1, the moving member 15 is restrained by spring 23, the position of the moving member 15 is directly proportional to the command voltage since spring extension (or compression) is directly proportional to force.

The present invention, as exemplified in Figs. 1 and 3, comprises a linear device with constant transfer function (force out per volt in), which is suitable for inclusion in other control systems.

For example, an outer loop could be comprised of a summer

subtracting the outputs of a position sensor from a command signal, the results of this summer then being amplified and used as the command voltage for this system. Useful applications of this embodiment would include poppet valves, dampers, and the like. One example is an electrically modulated expansion valve for refrigeration.

If the solenoid of this invention is used to drive a mass, then this device, along with suitable vibration sensors and control electronics, could be used for active cancellation of vibrations as in a laundry appliance or airborne, marine, optical or transportation situations where externally induced vibrations must be dealt with.

The present invention is useful as a self-driven oscillator for vibratory applications, using a suitably conditioned output of a velocity sensor fed back as the command voltage. Such a system would vibrate with a pure sine wave at the natural resonant frequency of the mass and spring that are being driven. The device would self-adjust to changes in load as in a vibratory feeder.

A spring restrained solenoid, as shown in Figs. 1 and 3 for example, where the force is independent of displacement, has a linear operation. As noted in Fig. 4, in contrast to the nonlinear curves of Fig. 2, forces such as force 1, force 2 and force 3 vary with control signal input but not with displacement. As seen in Fig. 4, the displacement changes linearly in response to control signal input, seeking the single position where the spring restraining force balances the commanded force. It is completely free of the on/off behavior of conventional spring-loaded solenoids in Fig. 2

It is contemplated that the Hall effect sensor or other magnetic field sensor may be co-packaged with a monolithic integrated circuit comprising the electronic control circuitry described herein. In this embodiment, a generic solenoid controller device would be produced.

This generic solenoid controller device could be imbedded in the solenoid actuator as part of its manufacture. A generic design is possible since solenoids operate in the same general magnitude of magnetic flux level, that level being determined to some extent by the magnetic properties of the ferromagnetic material The power drive circuitry 45 in Fig. 3 may still be external for power dissipation considerations and to accommodate the voltage and current requirements of a specific application.

Because the preferred embodiment of this invention employs a silicon Hall-effect sensor and other signal processing circuitry (either linear or digital) amenable to placement in silicon, a generic silicon chip can be produced. Such a chip, when embedded in a solenoid actuator design, would provide force proportional to a control voltage.

A given chip design could be used to control solenoids of widely varying sizes, ratings and designs.

While particular embodiments of the present invention have been illustrated and described, it is not intended to limit the invention, except as defined by the following claims.