Electromagnetic valve actuator for a valve of an engine

An electromagnetic valve actuator for a cylinder valve, including a pair of electromagnets including magnetic cores and coils serially connected and wound around the magnetic cores in the same direction. An armature is moveable against biasing forces of springs in one direction to open the cylinder valve and in the opposite direction to close the cylinder valve. A permanent magnet is prevented from being undesirably influenced by an opposing magnetic field relative to the permanent magnet.

BACKGROUND OF THE INVENTION
 The present invention relates to electromagnetic valve actuators which may
 be used for actuating a cylinder valve, for example, of an internal
 combustion engine of vehicles, by mainly using an electromagnetic force.
 Such electromagnetic valve actuators have been disclosed in U.S. Pat. Nos.
 5,799,630 and 4,779,582. The former of the conventional techniques
 includes a disk-like armature fixed to an intake valve of an engine, and
 valve-closing and valve-opening electromagnets that attract the armature
 for moving the intake valve to the closed and full open positions. There
 are provided a valve-closing spring for biasing the armature in such a
 direction as to move the intake valve toward the closed position and a
 valve-opening spring for biasing the armature in such a direction as to
 move the intake valve toward the full open position. Each electromagnet is
 connected to an electronic control unit that controls an energizing
 current for the electromagnet depending on operating conditions of the
 engine. The intake valve is operated to move to the closed and full open
 positions and held therein by association of the spring forces of the
 springs and the attractive forces of the electromagnets alternately
 energized. The latter of the conventional techniques includes a housing
 made of a magnetic material, an armature connected with an intake valve of
 an engine and moveably disposed within the housing, and a pair of
 compressed springs biasing the armature for retaining the valve in a
 neutral position between closed and full open positions of the valve. The
 armature has an H-shape and includes a sleeve portion extending along the
 center axis of the armature. A pair of electromagnets are disposed in such
 a manner that the armature is interposed therebetween. An annular
 permanent magnet is provided for holding the armature in the respective
 closed and full open position. The electromagnets include upper and lower
 cores having lower and upper faces opposed to the sleeve portion of the
 armature. The electromagnets include upper and lower coils that are wound
 around the cores and disposed on upper and lower faces of the permanent
 magnet, respectively. When the valve is placed in the respective closed
 and full open position, each coil is activated with a current therethrough
 to cancel the magnetic field of the permanent magnetic pole and allow the
 spring to move the valve member toward the other position. Thus, the
 motion of the valve is shifted by alternate energization of the coils.
 However, in the actuator described in the former, upon the valve being
 moved between the closed and full open positions, the electromagnets are
 alternately activated with a current to attract the armature against the
 spring force of the springs. The valve is held in the closed or full open
 position by continuous energization of the electromagnet. This causes an
 increased consumption of electrical energy, resulting in undesirable
 increase in engine load and fuel consumption. In the actuator of the
 latter, the coils of the electromagnets are not connected in series and
 independently cooperate with the corresponding core to generate an
 opposing magnetic field relative to the magnetic field of the permanent
 magnet upon being energized for the cancellation of the magnetic field of
 the permanent magnet. The magnetic circuit is formed in which the magnetic
 flux passes through the core, the housing, the north pole of the permanent
 magnet and the south pole thereof, and the armature and returns to the
 core. The magnetic flux of the electromagnet thus passes through the
 permanent magnet in the direction reverse to the magnetic flux of the
 permanent magnet. Therefore, the permanent magnet is influenced by the
 opposing magnetic field relative to the permanent magnet and thus tends to
 be demagnetized. This will lead to considerable reduction of the
 durability of the permanent magnet. Further, since resistance in the
 magnetic circuit will be increased due to the passage of the magnetic flux
 through the permanent magnet in the reverse direction, the electric energy
 consumption required for the cancellation of the magnetic field of the
 permanent magnet will become greater.
 SUMMARY OF THE INVENTION
 The present invention contemplates solving the above-mentioned problems of
 the conventional actuator.
 It is an object of the present invention to provide an electromagnetic
 valve actuator capable of reducing electric energy consumption of the
 electromagnets and preventing a permanent magnet from being demagnetized
 due to the influence of the opposing magnetic field, serving for
 increasing the durability of the permanent magnet.
 According to one aspect of the present invention, there is provided an
 apparatus for actuating a cylinder valve of an engine, the cylinder valve
 having a closed position, a full open position and a neutral position
 between the closed and full open positions, the apparatus comprising:
 an armature moveable in a direction of an axis, said armature including a
 sleeve portion extending in the axial direction and a disk portion
 connected with an inner periphery of the sleeve portion and adapted to be
 fixed to the cylinder valve;
 a pair of springs biasing the armature toward a valve-neutral position
 corresponding to the neutral position of the cylinder valve;
 a pair of electromagnets attracting the armature for moving the cylinder
 valve to the closed and full open positions, said electromagnets being
 disposed in an axially opposed relation to the armature, said
 electromagnets including a pair of axially spaced magnetic cores; and
 a permanent magnet retaining the armature for holding the cylinder valve in
 the closed and full open positions;
 wherein the sleeve portion of the armature cooperates with the permanent
 magnet to define a first air gap radially extending therebetween and
 cooperates with each of the magnetic cores to define a second air gap
 radially extending therebetween, and the disk portion of the armature
 cooperates with each of the magnetic cores to define a third air gap
 axially extending therebetween and variable with the axial motion of the
 armature.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIGS. 1, 2A and 2B illustrate the embodiment of an electromagnetic valve
 actuator according to the present invention, which is applied to an intake
 valve of an engine and may also be used with an exhaust valve of the
 engine.
 Referring now to FIG. 1, the actuator includes an electromagnetically
 actuating mechanism 24 for actuating an intake valve 23 of a vehicle
 engine, a permanent magnet 32 retaining the intake valve 23 in a closed
 position thereof and a full open position thereof, and a valve-closing
 spring 25 and a valve-opening spring 26 that are adapted for biasing the
 intake valve 23 toward a neutral position between the closed and full open
 positions. FIG. 1 shows the intake valve 23 placed in the neutral
 position. The intake valve 23 is so configured as to open and close an
 open end of an intake port 22 formed in a cylinder head 21. The open end
 of the intake port 22 is exposed to a combustion chamber. The intake valve
 23 includes a valve head 23a engageable with an annular valve seat 22a
 provided at the open end of the intake port 22. The intake valve 23 is
 engaged with the valve seat 22a in the closed position and disengaged
 therefrom in the full open position. The intake valve 23 also includes a
 valve stem 23b formed integrally with the valve head 23a and extending
 upwardly from the center of an upper surface of the valve head 23a. The
 valve stem 23b is slidably moved within a slide hole 21a formed in the
 cylinder head 21.
 The electromagnetically actuating mechanism 24 includes a generally
 cylindrical housing 28 fixed to the cylinder head 21 through fastening
 bolts 27, an armature 29 disposed within the housing 28 so as to be
 moveable in a direction of a center axis X, and a pair of valve-closing
 and valve-opening electromagnets 30 and 31 attracting the armature 29 for
 moving the intake valve 23 to the closed and full open positions. The
 valve-closing electromagnet 30 and the valve-opening electromagnet 31 are
 disposed in an axially opposed and spaced relation to the armature 29.
 The housing 28 includes a pair of generally cylindrical lower and upper
 housing halves 33 and 34 made of a magnetic material. The lower and upper
 housing halves 33 and 34 are connected together at opposed outer
 peripheral flanges thereof by using fastening bolts 35. The lower and
 upper housing halves 33 and 34 have substantially same structure. The
 lower housing half 33 includes a bottom wall and an inner sleeve 33a
 extending upwardly from a central portion of the bottom wall. The inner
 sleeve 33a has an upper radial flange 33b extending radially outwardly
 from an upper end portion of the inner sleeve 33a. The inner sleeve 33a
 with the upper radial flange 33b forms a reverse L-shape shown in FIG. 1
 and cooperates with the bottom wall to define a cylindrical bore 33c. The
 upper housing half 34 includes a top wall and an inner sleeve 34a
 extending downwardly from a central portion of the top wall. The inner
 sleeve 34a has a lower radial flange 34b extending radially outwardly from
 a lower and portion of the inner sleeve 34a. The inner sleeve 34a with the
 lower radial flange 34b forms an L-shape shown in FIG. 1 and cooperates
 with the top wall to define a cylindrical bore 34c substantially axially
 aligned with the bore 33c. Through the bores 33c and 34c, an upper portion
 of the valve stem 23b is received moveably in the axial direction. A cover
 35 is disposed on the top wall to close the bore 34c.
 The permanent magnet 32 is secured to an inner circumferential surface of a
 middle portion of the housing 28 in which the lower and upper housing
 halves 33 and 34 are connected together. The permanent magnet 32 is
 arranged in a radially outwardly spaced relation to the inner sleeves 33a
 and 34a of the lower and upper housing halves 33 and 34. There is a
 suitable radial space between the permanent magnet 32 and the inner
 sleeves 33a and 34a, in which a portion of the armature 29 is disposed as
 explained later. The permanent magnet 32 has a cylindrical shape and a
 north magnetic pole N at an inner circumferential portion thereof and a
 south magnetic pole S at an outer circumferential portion thereof. The
 cylindrical permanent magnet 32 is increased in an axial length, i.e., in
 an inner circumferential area opposed to the armature 29, so as to
 sufficiently attract the armature 29. In this embodiment, the axial length
 of the permanent magnet 32 is greater than an entire axial length of the
 armature 29.
 The armature 29 is disposed coaxially with the intake valve 23 and moveable
 together therewith upwardly and downwardly along the center axis X. The
 armature 29 has an H-shaped cross section shown in FIG. 1. The armature 29
 includes a disk portion 29a and a sleeve portion 29b connected with an
 outer incumferential periphery of the disk portion 29a and integrally
 formed with the disk portion 29a. The disk portion 29a is fixed to a
 threaded upper end of the valve stem 23b by a nut 36 for the unitary
 motion with the intake valve 23. The disk portion 29a extends in a
 direction perpendicular to the center axis X and is disposed within an
 axial space S defined between the radial flange 34b of the inner sleeve
 34a of the upper housing half 34 and the radial flange 33b of the inner
 sleeve 33a of the lower housing half 33. The disk portion 29a has an upper
 end face opposed to a lower axial end face 34d of the radial flange 34b
 with an axial air gap 44a and a lower end face opposed to an upper axial
 end face 33d of the radial flange 33b with an axial air gap 44b. The axial
 air gaps 44a and 44b are variable as the armature 29 moves along the
 center axis X, as explained in detail later. The sleeve portion 29b
 extends from the junction with the disk portion 29a in two opposing axial
 directions. The sleeve portion 29b is disposed in the radial space between
 the permanent magnet 32 and the radial flanges 33b and 34b of the inner
 sleeves 33a and 34a. The sleeve portion 29b has an outer circumferential
 surface opposed to an inner circumferential surface 32a of the permanent
 magnet 32 with a slight radial air gap 42. The outer circumferential
 surface of the sleeve portion 29b is entirely effective to be attracted by
 the permanent magnet 32 in the valve-neutral position, shown in FIG. 1, of
 the armature 29. The sleeve portion 29b has an inner circumferential
 surface 29c opposed to outer circumferential surfaces of the radial
 flanges 33b and 34b with radial air gaps 43. The radial air gaps 43 are
 disposed on the upper and lower sides of the disk portion 29a,
 respectively. Preferably, the radial air gaps 43 may be set at such a
 large value as to effectively reduce leakage of the magnetic flux of the
 electromagnets 30 and 31.
 The valve-closing electromagnet 30 includes a magnetic core formed by the
 inner sleeve 34a of the upper housing half 34 and a coil 30a wound around
 an outer circumferential surface of the magnetic core. The magnetic core
 includes opposed pole piece portions formed by the lower and upper end
 portions of the inner sleeve 34a, respectively. The valve-opening
 electromagnet 31 includes a magnetic core formed by the inner sleeve 33a
 of the lower housing half 33 and a coil 31a wound around an outer
 circumferential surface of the magnetic core. The magnetic core includes
 opposed pole piece portions formed by the upper and lower end portions of
 the inner sleeve 33a, respectively. The coils 30a and 31a are connected in
 series and turned around the corresponding magnetic cores 34a and 33a in a
 same direction. One terminal end 37a of the coil 30a is connected with a
 terminal end 37b of the coil 31a. The other terminal ends 38a and 38b of
 the respective coils 30a and 31a are connected to a power source 40 and a
 controller 41 via an amplifier 39.
 The controller 41 is programmed to determine an operating condition of the
 engine depending on signal outputs from detectors and develops a control
 signal for activating the coils 30a and 31a with an electric current. The
 detectors include a crank angle sensor 50 detecting the number of engine
 revolution and a temperature sensor 52 detecting temperatures of the
 electromagnets 30 and 31, and also may include an airflow meter. The
 controller 41 may be constituted by a microcomputer including
 microprocessor unit (MPU), input ports, output ports, read-only memory
 (ROM) for storing the control program, random access memory (RAM) for
 temporary data storage, and a conventional data bus.
 The valve-closing spring 25 is installed in a compressed state within the
 bore 33c of the inner sleeve 33a of the lower housing half 33 and biases
 the armature 29 upwardly. Specifically, the valve-closing spring 25 has a
 lower end portion supported on an upper face of the cylinder head 21 and
 an upper end portion supported on a central portion of the lower end face
 of the disk portion 29a of the armature 29. The valve-opening spring 26 is
 installed in a compressed state within the bore 34c of the inner sleeve
 34a of the upper housing half 34 and biases the armature 29 downwardly.
 Specifically, the valve-opening spring 26 has a lower end portion
 supported on a central portion of the upper end face of the disk portion
 29a and an upper end portion supported on a rearside face of the cover 35.
 Setting loads of the valve-closing and valve-opening springs 25 and 26 are
 the same. The valve-closing and valve-opening springs 25 and 26 associate
 with each other to hold the armature 29 in a valve-neutral position, shown
 in FIG. 1, corresponding to the neutral position of the valve 23 when the
 coils 30a and 31a of the electromagnets 30 and 31 are not activated with
 an electric current.
 An operation of the electromagnetic valve actuator will be explained
 hereinafter.
 When the engine is stopped and the coils 30a and 31a of the valve-closing
 and valve-opening electromagnets 30 and 31 are not activated with an
 electric current, the armature 29 is placed in the valve-neutral position
 shown in FIG. 1. In this condition, the upper axial air gap 44a between
 the disk portion 29a of the armature 29 and the radial flange 34b of the
 inner sleeve 34a of the upper housing half 34 is equal to the lower axial
 air gap 44b between the disk portion 29a and the radial flange 33b of the
 inner sleeve 33a of the lower housing half 33. Densities of the magnetic
 fluxes of the permanent magnet 32 respectively extending toward the
 electromagnets 30 and 31 are equivalent.
 Next, the engine starts and the coils 30a and 31a of the electromagnets 30
 and 31 are activated with an electric current in such a direction that a
 south magnetic pole S is generated at the lower end portion of the inner
 sleeve 34a of the upper housing half 34 and a north magnetic pole N is
 generated at the upper end portion of the inner sleeve 33a of the lower
 housing half 33. Namely, the lower end portion with the radial flange 34b,
 of the inner sleeve 34a acts as the south magnetic pole piece portion S of
 the electromagnet 30 and the upper end portion with the radial flange 33b,
 of the inner sleeve 33a acts as the north magnetic pole piece portion N of
 the electromagnet 31. Thus, the lower pole piece portion of the
 electromagnet 30 and the upper pole piece portion of the electromagnet 31
 have the opposing polarities S and N upon activating the
 serially-connected coils 30a and 31a wound in the same direction. In this
 condition, the density of the magnetic flux extending from the magnetic
 pole N of the permanent magnet 32 through the disk portion 29a of the
 armature 29 toward the S pole piece portion of the electromagnet 30 is
 larger, while the density of the magnetic flux extending from the magnetic
 pole N of the permanent magnet 32 through the disk portion 29a of the
 armature 29 toward the N pole piece portion of the electromagnet 31 is
 smaller. The armature 29 is attracted toward the S pole piece portion of
 the electromagnet 30 by the larger flux density. The armature 29 is then
 moved from the valve-neutral position to the valve-closing position
 against the spring force of the spring 26. As the armature 29 moves from
 the valve-neutral position toward the valve-closing position, the axial
 air gap 44a on the electromagnet 30 side becomes smaller while the axial
 air gap 44b on the electromagnet 31 side becomes greater. The intake valve
 23 is upwardly moved with the armature 29 from the neutral position and
 placed in the closed position shown in FIG. 2A with the engagement of the
 valve head 23a with the valve seat 22a. The coils 30a and 31a are then
 instantly de-energized. Even in this condition where the coils 30a and 31a
 are de-energized, the intake valve 23 can be retained in the closed
 position by the attraction of the permanent magnet 32 relative to the
 armature 29. In the closed position of the intake valve 23, there is
 generated a magnetic flux circuit as indicated by arrow in FIG. 2A.
 Although only the right half of the magnetic flux circuit is shown in FIG.
 2A for simple illustration, the left half thereof is similar to the right
 half. In the magnetic flux circuit, the magnetic flux extending from the
 magnetic pole N of the permanent magnet 32 passes through the radial air
 gap 42, the disk portion 29a of the armature 29, the smaller axial air gap
 44a on the electromagnet 30 side, the lower end portion of the magnetic
 core 34a of the electromagnet 30 and the top wall and outer
 circumferential wall of the upper housing half 34, and enters the magnetic
 pole S of the permanent magnet 32.
 Subsequently, for moving the intake valve 23 from the closed position to
 the full open position, the coils 30a and 31a are activated with a reverse
 electric current flowing in a direction opposite to the above-described
 direction. By the activation of the coils 30a and 31a with the reverse
 electric current, the magnetic pole N is generated at the lower end
 portion of the inner sleeve 34a of the upper housing half 34 and the
 magnetic pole S is generated at the upper end portion of the inner sleeve
 33a of the lower housing half 33. Namely, conversely to the
 above-explained case of energization for moving the intake valve 23 to the
 closed position, the lower pole piece portion of the electromagnet 30 has
 the magnetic pole N and the upper pole piece portion of the electromagnet
 31 has the magnetic pole S. The density of the magnetic flux extending
 from the magnetic pole N of the permanent magnet 32 toward the S pole
 piece portion of the electromagnet 31 becomes larger, while the density of
 the magnetic flux extending from the magnetic pole N of the permanent
 magnet 32 toward the N pole piece portion of the electromagnet 30 becomes
 smaller. In this state, there is generated a magnetic flux circuit in
 which the magnetic flux extending from the magnetic pole N of the
 permanent magnet 32 passes through the radial air gap 42, the disk portion
 29a of the armature 29, the axial air gap 44b on the electromagnet 31
 side, the S pole piece portion of the electromagnet 31, the bottom wall
 and the outer circumferential wall of the lower housing half 33 and enters
 the magnetic pole S of the permanent magnet 32. Substantially no or less
 amount of the magnetic flux passes through the permanent magnet 32 in a
 direction opposed to the magnetic flux of the permanent magnet 32. Thus,
 the permanent magnet 32 is prevented from being influenced by an undesired
 opposing magnetic field relative thereto which causes demagnetization
 thereof, upon energizing the coils 30a and 31a in the reverse direction.
 The armature 29 is attracted toward the S pole piece portion of the
 electromagnet 31. The armature 29 is moved toward the valve-neutral
 position with the assistance of the spring force of the spring 26 and then
 attractively moved to the valve-opening position, shown in FIG. 2B,
 against the spring force of the spring 25. Upon the motion of the armature
 29 toward the valve-opening position, the axial air gap 44b on the
 electromagnet 31 side becomes smaller, while the axial air gap 44a on the
 electromagnet 30 side becomes greater. The variable axial air gap 44a and
 44b are set in such a manner as to be smaller than the radial air gap 43
 when the armature 29 is placed in the respective valve-closing and
 valve-opening positions as shown in FIGS. 2A and 2B. The intake valve 23
 is downwardly moved with the armature 29 through the neutral position to
 the full open position in the disengagement of the vale head 23a from the
 valve seat 22a. The coils 30a and 31a are instantly de-energized. Even in
 this state, the intake valve 23 can be held in the full open position by
 the attraction of the permanent magnet 32 relative to the armature 29. In
 the full open position of the intake valve 23, there is generated a
 magnetic flux circuit indicated by arrow in FIG. 2B, in which the magnetic
 flux extending from the magnetic pole N of the permanent magnet 32 passes
 through the radial air gap 42, the disk portion 29a of the armature 29,
 the smaller axial air gap 44b on the electromagnet 31 side, the upper end
 portion of the magnetic core 33a of the electromagnet 31, the bottom wall
 and the outer circumferential wall of the lower housing half 33, and
 enters the magnetic pole S of the permanent magnet 32.
 FIG. 3 illustrates characteristic curves of the permanent magnet 32, the
 electromagnets 30 and 31 and the springs 25 and 26, which are exhibited
 upon shifting the intake valve 23 between the closed and full open
 positions. In FIG. 3, the permanent magnet 32 creates the attraction Fm as
 indicated by curves 100, exerted on the armature 29 against the spring
 forces 112 and 110 of the springs 26 and 25. When the intake valve 23 is
 in the respective closed and full open positions, the attraction Fm of the
 permanent magnet 32 overcomes the combined spring force Fs, as indicated
 by line 114, of the springs 25 and 26. When the coils 30a and 31a of the
 electromagnets 30 and 31 are activated with the reverse electric current
 for shifting the intake valve 32 between the closed and full open
 positions, the repulsion FR, as indicated by curve 102, of the armature 29
 is generated. Namely, in the case of activation of the coils 30a and 31a
 with the reverse current for shifting the intake valve 32 from one of the
 closed and full open positions to the other thereof, the combined force of
 the combined spring force Fs and the repulsion FR of the armature 29
 overcomes the attraction Fm of the permanent magnet 32 to eliminate the
 retention of the armature 29 by the permanent magnet 32. The intake valve
 23 is thus urged to move from one of the closed and full open positions
 toward the other thereof.
 Referring now to FIG. 4, a relationship between the activation of the coils
 30a and 31a of the electromagnets 30 and 31 and the closing and opening
 motion of the intake valve 23 is explained. When activating the coils 30a
 and 31a with a coil current C1 shown in FIG. 4, for shifting the intake
 valve 23 from the closed position to the full open position, the intake
 valve 23 is moved from the closed position to the full open position owing
 to the spring force of the spring 26 and the attractive force of the
 electromagnet 31. Immediately after that, the energization of the coils
 30a and 31a is stopped but the intake valve 23 is retained in the full
 open position as indicated by valve lift curve in FIG. 4, by the
 attraction of the permanent magnet 32. Likewise, when activating the coils
 30a and 31a with a coil current C2 shown in FIG. 4, the intake valve 23 is
 moved from the full open position to the closed position in a manner
 reverse to that described above.
 With the arrangement of the permanent magnet 32, it is not necessary to
 continuously supply an electric current to the coils 30a and 31a of the
 electromagnets 30 and 31 in order to attractively hold the armature 29 in
 the valve-closing and valve-opening positions. This also serves for
 reducing the electric power consumption.
 Further, when the direction of the energization of the electromagnets 30
 and 31 is reversed for moving the intake valve 23 from one of the closed
 position and the full open position to the other thereof, the armature 29
 is attracted by the magnetic field of one of the electromagnets 30 and 31
 which is the same as the magnetic field of the permanent magnet 32.
 Namely, the magnetic flux of the one of the electromagnets 30 and 31 is
 substantially prevented from passing through the permanent magnet 32 in
 the direction opposed to the direction of the magnetic flux of the
 permanent magnet 32. Thus, the permanent magnet 32 can be prevented from
 being influenced by the undesired opposing magnetic field relative to the
 magnetic field thereof and thus be effectively avoided from being
 demagnetized. This results in improving the durability of the permanent
 magnet 32.
 Furthermore, since, upon the energization of the electromagnets 30 and 31
 in the reverse direction, the magnetic flux is substantially prevented
 from passing through the permanent magnet 32 in the direction opposed to
 the magnetic flux of the permanent magnet 32, the reluctance in the
 magnetic flux circuit formed thereupon can be reduced. This causes
 reduction of the electric current supplied to the coils 30a and 31a
 required upon the energization thereof in the reverse direction. This can
 contemplate to reduction in power consumption.
 Further, since the coils 30a and 31a of the electromagnets 30 and 31 are
 connected in series and wound in the same direction, the attractive force
 of one of the electromagnets 30 and 31 is exerted on the armature 29 with
 the assistance of the spring force of one of the springs 25 and 26 which
 is associated with the one of the electromagnets 30 and 31 upon the
 energization for shifting the intake valve 23 between the closed and open
 positions. This can improve the response motion of the armature 29.
 Although the invention has been described above by reference to a certain
 embodiment of the invention, the invention is not limited to the
 embodiment described above. Modifications and variations of the embodiment
 described above will occur to those skilled in the art, in light of the
 above teachings. The scope of the invention is defined with reference to
 the following claims.