Abstract:
An electrically actuated engine valve provides an armature having one or more teeth extending outward from the armature along the actuation axis to be received by corresponding sockets in the cores of opposed electromagnets. The teeth do not restrain the movement of the armature but in approaching the cores provides a magnetic flux path that produces a more constant force of attraction during actuation of the valve. This enables the valves to overcome initial opposing forces such as caused by pressure on the valve heads to which the armature is attached and provides a path of inductive coupling between the opposed coils that can reveal armature position providing a method of accurately controlling armature seating speed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates to actuators for moving the intake and exhaust valves of internal combustion engines, and specifically to an electronically actuable engine valve providing improved force characteristics and a signal indicating the valve position. 
     Electrically actuable valves, in contrast to valves actuated mechanically by cams and the like, allow a computer-based engine controller to easily vary the timing of the valve opening and closing during different phases of engine operation. 
     One type of actuator for such a valve provides a flat plate armature which moves back and forth between two electromagnets. The armature is attached to a valve stem of a valve. 
     When the electromagnets are unpowered, the armature is held in equipoise between the two electromagnets by two opposing springs. Prior to operation, the armature is drawn against one of the electromagnets by an “initialization” current in the retaining electromagnet. The spring between the armature and the retaining electromagnet is compressed while the opposing spring is stretched. Once the armature is drawn fully toward the receiving electromagnet, the initialization current is reduced to a “holding” level sufficient to hold the armature against the electromagnet until the next transition is initiated. 
     A change of valve state from open to closed or vice versa, is effected by interrupting the holding current. When this occurs, the energy stored in the opposed compressed and stretched springs accelerates the armature off of the releasing electromagnet toward the new receiving electromagnet. When the armature reaches the receiving electromagnet, that electromagnet is energized with the “holding” current to retain the armature in position against its surface. 
     In a frictionless system, the armature reaches a maximum velocity at the midpoint between the two electromagnets (assuming equal spring forces) and just reaches the receiving electromagnet with zero velocity. In a physically realizable system in which friction causes some of the stored energy of the springs to be lost as heat, the armature will not reach the receiving electromagnet unless the energy lost to friction is replaced. This is accomplished by creating a “capture” current in the receiving electromagnet prior to the armature contacting that electromagnet. 
     The capture current must be of sufficient magnitude to overcome the opposing forces resisting movement of the armature, however, it is equally important that the capture current be limited to prevent damage to the armature, electromagnet, or valve and to limit impact noise. If the capture current is turned on too soon (or is too great in magnitude), the armature may be accelerated into the electromagnet (and the valve into its seat) at excessive velocity. Conversely, the armature may not be captured by the receiving electromagnet and the valve may not close if the capture current is turned on too late or is too low in magnitude. 
     Accurate control of the capture current is facilitated if the position and velocity of the armature as it approaches the receiving electromagnet can be measured. Because the force between the electromagnet and armature varies rapidly with distance, sensors for measuring armature distance must be very accurate. Small measurement errors in distance can produce large errors in the calculated force applied to the armature, upsetting correct armature control. 
     Unfortunately, position sensors that are sufficiently accurate for this purpose and yet robust enough to survive in the environment of an internal combustion engine are expensive and therefore impractical. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides inter-engaging teeth and sockets on opposing armature and electromagnet core faces to reduce the variation in the force of attraction between the armature and a core. As the force profile of the actuator becomes more linear, the demands on the position sensor are reduced and peak initialization and capture currents are reduced. In addition, the teeth and socket structure creates a mutual inductance between opposed electromagnets that may be measured to derive armature position. 
     Specifically, then, the present invention provides an electrically actuable engine valve having a first and second coil wound about a common actuation axis and spaced apart by an actuation distance. A first and second core, incorporating the first and second electromagnets, respectively, present opposed core faces across the actuation distance. A valve having a valve head sized to cover a valve seat of an internal combustion engine is attached to a valve stem, the latter supported by valve stem supports holding the valve stem aligned with the actuation axis for movement along the actuation axis. An armature plate extends in a plane perpendicular to the axis and attaches to the valve stem for movement therewith along the actuation axis. At least one spring is attached to the armature plate to bias the armature plate to a neutral position between the core faces. The armature plate and at least one given core have a mating tooth and socket extending parallel to the actuation axis, the tooth and socket sized to provide a more linear relationship in the attractive force between the given core and the armature as a function of separation distance between the given core and the armature for a constant coil current. 
     Thus it is one object of the invention to provide a more linear attractive force between the armature and the core as a function of distance thereby providing better initialization of the armature position and improved control of armature position and speed. 
     The tooth may be on the armature plate and the socket on the core or the socket may be on the armature plate and the tooth on the core. 
     Thus it is an advantage of the invention that it provides flexibility in design as may be necessary to minimize armature weight or maximize armature flux path. 
     The tooth and the socket may be on only one side of the armature and a corresponding surface of the core. 
     Thus it is another object of the invention to provide different force profiles for the two sets of coils, which will be suitable for the actuation of an exhaust valve since the exhaust gases provide additional resisting force on the opening of the valve. 
     The armature plate may include a plurality of slots extending along the actuation axis. 
     Thus it is another object of the invention to reduce induced eddy currents in the armature plate such as cause resistive losses. 
     The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a phantom, fragmentary perspective view of a cylinder head and its valve assembly showing location of the electromagnet actuator of the present invention; 
     FIG. 2 is a cross-section of the electromechanical actuator of FIG. 1 taken along lines  2 — 2  showing an armature attached to a valve stem and positioned between two coils; 
     FIG. 3 is a graph depicting force of attraction between an actuated coil and the armature of the present invention for a prior art planar armature and for the tooth and socket armature of the present invention showing the greater linearity of force as a function of distance for the latter; 
     FIGS. 4 a  and  4   b  are fragmentary cross-sections similar to FIG. 2 showing alternative embodiments of the tooth and socket design of the present invention; 
     FIG. 5 is a fragmentary cross section similar to that of FIG. 2 showing an asymmetric armature plate useful for exhaust valves; 
     FIG. 6 a  is a fragmentary perspective view of a rectangular version of the armature plate of FIG. 2 showing parallel surface slots to reduce eddy current flow; 
     FIG. 6 b  is a fragmentary perspective view of a circular version of the armature plate of FIG. 2 showing radial surface slots to reduce eddy current flow; 
     FIG. 7 is a block diagram of a controller useful for use with the actuator of FIG. 1 for alternately driving one coil and reading induced voltage in the second opposed coil for armature control; and 
     FIG. 8 is a simplified graph of voltage versus armature distance from the unpowered coil showing the decreasing amplitude of coupled voltage to the unpowered coil as the armature moves toward the driven coil when the coil that is driven is driven with an alternating current such as produced by a hysteretic controller. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, an electromagnetically actuated valve  10  suitable for use with the present invention provides a coil assembly  12  fitting around a valve stem  14 , the latter which may move freely along its axis. The valve stem  14  extends downward from the coil assembly  12  into a piston cylinder  16  where it terminates at a valve head  18 . Generally, power applied via leads  20  of the coil assembly  12  will move the valve head  18  toward or away from a valve seat  22  within the cylinder so as to provide for the intake of air and fuel or recirculated exhaust gas, or exhaust of exhaust gas depending on the engine and valve type. 
     Referring now to FIG. 2, the coil assembly  12  provides two toroidal coils  24  and  26  of helically wound electrical wire. The coils  24  and  26  are spaced apart coaxially along the valve stem  14  and fit within cores  28  and  30 , respectively, which provide for the concentration of magnetic flux at opposed open faces  32  when the coils  24  and  26  are energized. 
     Between the open faces  32  of the cores  28  and  30  is a disk-shaped armature plate  34  attached to the valve stem  14 . The armature plate  34  may be a solid soft iron plate for easy manufacturing and high magnetic attraction. The surface of the armature plate  34  extends perpendicularly to the axis of the valve stem  14 . The space between the open faces  32  is sufficient so that the valve stem  14  may move by its normal range along actuation axis  36  before the armature plate  34  is stopped against either the open faces  32  of core  28  or core  30 . 
     Helical compression springs  38  extend outward from the cores  28  and  30  away from the armature plate  34  about the valve stem  14  to be constrained by collars  39  on the valve stem  14 . Absent the application of current to either of coils  24  and  26 , the springs  38  bias the armature plate  34  to a point approximately midway between the cores  28  and  30 . 
     Referring now to FIG. 3 for prior art valves similar to that of FIG. 2 but having a planar armature plate  34 , the function  40 , relating force of attraction between the armature plate  34  and an energized one of the cores  28  or  30  to distance between the armature plate  34  and that cores  28  or  30  for a constant current through the cores  28  or  30 , varies abruptly as a function of distance, the force decreasing rapidly in the first few millimeters of separation. This rapid fall-off in force with distance makes it extremely hard to produce sufficient force to initially attract the armature plate  34  to one of the cores  28  and  30 . Further the non-linearity makes control of the velocity of the armature plate difficult. 
     The present invention, in contrast, provides a more nearly linear function  42  relating force to distance between the armature plate  34  and the cores  28  or  30  of an energized one of coils  24  or  26 . This function  42  is much more constant providing greater forces at greater distances between the armature plate  34  and cores  28  or  30  and less variation in force for a given current as may provide greater precision to control the armature velocity. 
     Referring again to FIG. 2, the greater linearity of force provided by the present invention results from the use of one or more teeth  44  extending along the actuation axis  36  out from the broad surfaces of the armature plate  34  toward corresponding open faces  32  of the cores  28  and  30 . The cores  28  and  30  have sockets  46  corresponding to the teeth  44  to interfit with the teeth  44  as the armature plate  34  moves toward either of the respective open faces  32 . Importantly, the sockets  46  are in the cores  28  and  30  and the coils  24  and  26  are not affected and remained encased in cores  28  and  30 . 
     Referring now to FIGS. 2 and 6 a , teeth  44  extending outward along the actuation axis from a base  50  of the armature plate  34 , the base  50  being generally a portion of the armature plate  34  aligned with the coils  24  or  26 . The teeth are generally trapezoidal in cross section, having sloped walls  53  terminating at a plateau tip  52 . 
     The height of the teeth  44  from base  50  to tip  52  substantially equals the separation between the base  50  and the opposing portion of the core  28  or  30  when the armature plate  34  is in a neutral position biased by the springs  38  with no energizing either of the coils  24  and  26 . Thus the tips  52  of the teeth  44  nearly engage their corresponding sockets  46  prior to powering of either coil  24  or  26 . Other heights may also be selected among those that render the force function  40  more linear. Generally however, the height of the teeth  44  will be considerable and at least half the distance between the base  50  and the portion of the open faces  32 , which it abuts. Precise shaping of the teeth  44  and sockets  46  may be determined with commercially available finite elements magnetic device modeling programs. 
     Referring still to FIG. 6 a , the broad surfaces of the armature plate  34  may be scored with a plurality of longitudinal slots  54  extending into the surface of the armature plate  34  along the actuation axis  36  to break the path of eddy current flows which may tend to run cyclically around the surface of the armature plate  34  dissipating energy as resistive heating. These slots  54  may be filled with an electrically insulating material or left open and may run at a variety of orientations around the surface generally across to the expected path of such eddy currents. Reduction of eddy current losses is particularly important because of the high electromagnetic transience necessary for the operation of a valve of this kind. Winding structure and wire geometry with reduced proximity loss and eddy current loss may also be used. 
     Referring now to FIG. 6 b , in an alternative embodiment the armature plate  34  is disk shaped and has multiple annular teeth  44  which correspond with multiple sockets  46  on the cores  28  and  30  (not shown). In this case the slots  54  extend radially. 
     Referring now to FIGS. 4 a  and  4   b , alternative versions of the teeth  44  and sockets  46  may be provided. In FIG. 4 a , teeth  44   a  may be positioned symmetrically about the valve stem  14  centered on the windings of a coil (e.g.  24 ). The plateau tips  52   a  of the teeth  44   a  are as wide as the windings of the coil  24  and the sloped walls  53   a  cover the remainder of the open face  32   a  leaving minimal base  50  close to the valve stem  14 . Sockets  46   a  are formed in the open face  32  of the core  28  and hold each of the windings of the coil  24  at their deepest portions and are shaped corresponding to the teeth  44   a.    
     Referring to FIG. 4 b , in an alternative embodiment, the teeth  44   b  have hemicircular cross-sections (similar to the teeth of FIG. 6 b ) as opposed to the trapezoidal cross sections. As in the embodiment of FIG. 2, the teeth  44   b  flank the windings of the coil  24  so that the windings are positioned in between sockets  46   b . In each of the embodiments of FIGS. 4 a  and  4   b , no change in the basic dimensions of the windings of coils  24  and  26  is required and they remain encased in the cores  28  and  30 . 
     Referring now to FIG. 5, the armature plate  34  may be asymmetric across a bisecting plane perpendicular to the valve stem  14  with the teeth  44  extending on only one side of the armature plate  34  toward sockets  46  on only one core  30  and wherein the opposite side of the armature plate  34  and its opposing core  28  is planar. In this way, armature mass is reduced and fabrication simplified while improved actuation force is provided toward core  30  which may preferably be the lower core of cores  28  and  30  allowing improved opening, for example, of an exhaust valve where exhaust back pressures resist that opening and greater initial forces are required. 
     Referring now to FIG. 7, the operation of the teeth  44  and sockets  46  such as provide an interdigitation of the armature plate  34  and the cores  28  and  30  promotes a mutual inductance between an activated one of the coils  24  and the other inactivated coil  26  or visa versa. This mutual inductance is dependent on the position of the armature plate  34  with respect to those coils  24  and  26  and the armature plate  34 , which serves as a magnetic pathway between these two coils  24  and  26 . Accordingly, a measure of the mutual inductance may be used to determine the position of the armature plate  34 . 
     During activation of one coil ( 24  for example) by means of a switching amplifier  60  producing a fluctuating magnetic field, an induced current will be detectable in the other coil  26  dependent on the proximity of the armature plate  34  to that coil  26 . The switching amplifier  60  may be a hysteretic amplifier switching current to the coil  24  on and off in a varying duty cycle to control the average current to a predetermined amount dictated by a s control signal  62  indicating that the valve should be opened or closed. 
     Both of the coils  24  and  26  are connected to a mutual inductance calculator  68  receiving a measure of drive current through the coil  24  and induced voltage across the coil  26  to deduce a measure of mutual inductance. This measure may be provided to a look-up table  70  to be related to an armature position according to empirically derived table entries. The armature position is provided to a controller (not shown) which uses it to allow sophisticated control of the valve operation. 
     Generally, as shown in FIG. 8, the mutual inductance calculator  68  will see a voltage curve  70  following a decreasing envelope  72  as the armature plate  34  moves toward the activated coil  24 . This envelope  72  may be compared by the mutual inductance calculator  68  to the current output to the coil  24  by the switching amplifier  60  (which is also varying according to the signal  62  received by the switching amplifier  60  to control armature plate velocity) and the relationship between current and voltage is used to deduce the mutual inductance and hence the position of the armature plate  34 . This position is used by a valve controller (not shown) to control armature plate velocity. Depending on that velocity, the drive current to coil  24  may be increased or decreased to provide for a soft seating of the valve. 
     Generally the position signal will be used to decrease the current drive as the armature plate  34  approaches the respective coil so that the armature plate and electromagnet will contact at zero velocity. Subsequent to that time, a holding current less than the capture current used to draw the armature plate  34  in is used to hold the armature plate  34  in position making use of the far greater forces that exist when the electromagnet armature plate contacts. Other more complex control strategies may be enabled by this system. 
     Upon seating of the valve and a contacting of the armature plate  34  against the core  28 , a holding current is maintained as is understood in the art until the time when the valve state is to be changed and switching amplifier  60  output to coil  24  is turned off. The valve controller (not shown) will then provide a signal to the switching amplifier  60  to connect to coil  26  via an internal commutator formed of solid state switches as is understood in the art. Now the process is reversed with coil  24  serving to provide a position measurement of the armature plate  34  as it is drawn to coil  26 . 
     The teeth  44  and sockets  46  provide improved inductive coupling between the two coils  24  and  26  thus rendering this technique practical and provide increased linearity of the forces exerted on the armature plate  34  by the respective coils  24  and  26  rendering improved control of the armature motion possible. 
     The above description has been that of a preferred embodiment of the present invention, it will occur to those that practice the art that many modifications may be made without departing from the spirit and scope of the invention. For example, it will be understood that an auxiliary coil may also be used for the purpose of measuring mutual inductance or other magnetic sensing means. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.