Abstract:
An electromagnetic actuator composed of: an armature movable along a linear path between first and second end positions; electromagnets positioned and operative for selectively moving the armature to either one of the first and second end positions; and a mechanical holding element operative in response to movement of the armature to either one of the end positions for holding the armature in the end position to which the armature has moved until an appropriate one of the electromagnets is operated to move the armature to the other one of the end positions.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to electromagnetic valve actuators in which the displacement of valves is controlled by energizing actuator electromagnets with currents having suitable waveforms, or pulse patterns. 
     Actuators of the type here under consideration may be used, for example, in place of conventional mechanical valve lifters for actuating automotive engine cylinder intake and exhaust valves. However, such valves may be employed in other types of power systems or fluid flow systems. 
     Known electromagnetic valve actuators include an armature shaft which contacts a valve stem to a valve head, bias springs which act on the armature shaft to urge it towards an intermediate position and electromagnets that are individually energizable to move the armature shaft, and thus the valve head, to either one of two end positions. These two end positions correspond, respectively, to a closed position in which the valve head mates with a valve seat and an opened position in which the valve head is spaced from the valve seat. 
     An example of such an electromagnetic valve actuator is disclosed in commonly owned U.S. Pat. No. 5,782,454, the entire disclosure of which is incorporated herein by reference. 
     In the operation of such a valve actuator, the valve head will remain in one of its end positions as long as one of the electromagnets is producing a magnetic field sufficient to hold the armature in that position against the force of the bias springs. Therefore, a significant current must be supplied to the energized electromagnet for as long as the valve head is to be maintained in the opened or closed position. As a result, a considerable amount of electrical power would be consumed in the operation of one of these valve actuators. In systems employing a plurality of valves, which may be up to 48 valves in some automotive engines, the current consumption level is proportionally higher. 
     As a general rule, it is preferable that movement of a valve between its opened and closed position occur in the shortest time possible. The speed of movement for a given assembly depends on the level of current supplied to the electromagnetic being energized, which in turn determines the acceleration experienced by the valve and actuator components which move as a unit with the valve. 
     However, the higher the valve displacement speed, the greater the impact associated with arrival of the valve at its end position and the greater the likelihood that the valve will experience some bounce at the end of its movement. These factors adversely affect the performance of the associated engine. 
     BRIEF SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to reduce the electrical energy required by such a valve actuator. 
     A more specific object of the invention is to eliminate the need for supplying a holding current, or to reduce the level of such holding current, during periods when the valve head is to remain in either one of its end positions. 
     Another specific object of the invention is to provide a mechanism which mechanically clamps the armature in either one of its end positions until a new armature movement is required. 
     A further object of the invention is to produce a controlled braking force that reduces the landing velocity of the actuator armature shaft at the valve head end positions, to thereby reduce bounce, noise and component wear. 
     The above and other objects of the invention are achieved by an electromagnetic actuator comprising an armature that includes an armature shaft movable along a linear path between first and second end positions; electromagnet means positioned and operative for selectively moving the armature to either one of the first and second end positions; and mechanical holding means operative in response to movement of the armature to either one of the end positions for contacting and holding the armature in the end position to which the armature has moved until the electromagnet means are operated to move the armature to the other one of the end positions. The holding means according to the invention may be in the form of either a clamping device which produces a frictional holding force, or a latching device which can engage a formation on the armature shaft in an interlocking manner. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is an elevational, cross-sectional view of a first embodiment of an electromagnetic valve actuator equipped with a clamping device according to the present invention. 
     FIG. 2 is a cross-sectional detail view of one embodiment of a clamping device according to the invention. 
     FIG. 3 is an exploded, cross-sectional view of the clamping device of FIG.  2 . 
     FIGS. 4,  5  and  6  are axial end views of three components of the clamping device of FIGS. 2 and 3. 
     FIG. 7 is a side view of another component of the device of FIGS. 2 and 3. 
     FIG. 8A is a side elevational detail view of a modified form of construction of one of the components of the embodiment of FIGS. 1-7. 
     FIG. 8B is a top plan view of the component shown in FIG.  8 A. 
     FIG. 9 is a top plan view of a second embodiment of a device according to the invention, with a top portion of the actuator housing removed. 
     FIG. 10 is a view similar to that of FIG. 9 showing another embodiment of a device according to the invention. 
     FIG. 11 is a view similar to that of FIG. 1 showing an embodiment of a latching device according to the invention. 
     FIG. 12 is a detail view, partially in cross section, of a component of the device of FIG.  11 . 
     FIG. 13 is a longitudinal, cross-sectional view of a further component of the device in FIG.  11 . 
     FIG. 14 is an end view of the component of FIG.  13 . 
     FIG. 15 is a longitudinal, cross-sectional view of another component of the device of FIG.  11 . 
     FIG. 16 is an end view of the component of FIG.  15 . 
     FIG. 17 is a view similar to that of FIG. 1 showing another embodiment of a clamping device according to the invention. 
     FIG. 18 is a longitudinal, cross-sectional view of one component of the device of FIG.  17 . 
     FIG. 19 is a plan view of a primary component of a further embodiment of a clamping device according to the invention. 
     FIG. 20 is a side elevational view of the device which incorporates the component of FIG.  19 . 
     FIG. 21A is an exploded detail view showing components of the device of FIG.  20 . 
     FIG. 21 is a longitudinal, cross-sectional view of a further embodiment of a clamping device according to the invention. 
     FIG. 22 is a longitudinal, cross-sectional detail view of a portion of the device of FIG.  21 . 
     FIGS. 23 and 24 are axial end views of two components of the device of FIG.  21 . 
     FIG. 25 is a detail view, in the direction of arrow  285  of FIG.  24 . 
     FIG. 26 is a plan view of further components of the device shown in FIG.  21 . 
     FIG. 27 is a rear detail view of one of the components of the device shown in FIG.  21 . 
     FIG. 28 is a cross-sectional view taken along the line B-B′ of FIG.  27 . 
     FIGS. 29 and 30 are side elevational views of two of the components of the device shown in FIG. 21, which components are also shown in FIGS.  27  and  28 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an electromagnetic valve actuator  10  equipped with a clamping mechanism according to one embodiment of the invention. Electromagnetic valve actuator  10  includes a lower electromagnet  12  and an upper electromagnet  13 , each including a coil. An armature shaft  14  extends through passages in electromagnets  12  and is fixed to, or integral with, an armature  20  made of a magnetizable material. Armature  20  is in the form of a disc located between electromagnets  12  and  13 . Armature shaft  14  is coupled to a valve stem  15  via a mechanical or hydraulic coupling  16 . Coupling  16  serves to compensate for thermal growth experienced by valve stem  15 . Examples of such couplings are disclosed in pending U.S. application Ser. No. 09/146,738, filed on Sep. 3, 1998, the disclosure of which application is incorporated herein by reference. Such couplings are presently employed in motorcycle and racing car engines and would be used in actuators according to the invention installed in automotive vehicle and aircraft engines, although they may not be required in actuators associated with air valves, etc. 
     Valve stem  15  extends through a passage, or valve guide, in a cylinder head  22  and the lower end of valve stem  15  carries a valve head  24  associated with a valve seat  25 . Armature shaft  14  and valve stem  15  extend along a central axis  18  which also defines a linear path along which armature shaft  14  and valve stem  15  move as a unit between closed and open positions of valve head  24 . 
     Actuator  10  also includes two biasing springs  26  and  28  which bias armature shaft  14 , valve stem  15  and armature  20  in a neutral position, at least approximately between the closed and open positions of valve head  24 . Further details of the actuator structure will be found in U.S. Pat. No. 5,782,454, cited above. 
     When lower electromagnet  12  is energized, armature  20  is pulled downwardly along central axis  18  to move valve head  24  to its open position. On the other hand, when the upper electromagnet  13  is energized, armature  20  is moved upwardly along central axis  18  to move valve head  24  to its closed position. Operation of valve  10 , as described thus far, is described in detail in U.S. Pat. Nos. 5,222,714 and 5,355,108, the disclosures of which are incorporated herein by reference. 
     The structural elements described thus far correspond to elements shown in FIG. 1 of U.S. Pat. No. 5,782,454. In order to maintain valve head  24  in either one of its end positions in this prior art device, the energized electromagnet must be continuously supplied with a level of current sufficient to reliably overcome the biasing force of springs  26  and  28 . 
     According to the present invention, the valve actuator  10  described thus far is supplemented by a mechanical clamping assembly  30  that acts to hold armature shaft  14 , valve stem  15 , armature  20  and valve head  24  in either end position by a mechanical clamping action that will be maintained passively until an energizing current is supplied to an electromagnet to release the remaining force imposed on the armature and to urge armature  20  and valve head  24  toward their other end position. 
     One preferred embodiment of such a clamping assembly is shown in greater detail in FIGS. 2-7 which are, respectively, a partial cross-sectional view of the clamping assembly in its assembled state, a partial cross-sectional exploded view showing the components of the clamping assembly separated from one another, three axial end views of components of the clamping assembly and a detail view of another one of the components of the clamping assembly. 
     The clamping assembly embodiment shown in FIGS. 2-7 is composed of two end plates  34  and  36 , a rotary bearing  38 , roller pins  40 , return spring units  42  (not shown in FIG. 2) for biasing the roller pins  40  in radially outward directions and an input member, or linear camshaft,  44 . Armature shaft  14  extends through end plates  34  and  36  and rotary bearing  38 . Member  44  is guided in bores in end plates  34  and  36  and is fixed to armature  20 . Therefore, member  44  will move in unison with armature shaft  14 , valve stem  15 , armature  20  and valve head  24  parallel to central axis  18 . 
     Rotary bearing  38  is held between end plates  34  and  36  via two sets of ball bearings  48  that allow rotary bearing  38  to pivot relative to end plates  34  and  36  with the minimum achievable drag and wear. End plates  34  and  36  are fixed in position in actuator  10 , as by suitable bolts secured in upper electromagnet  13 , so that end plates  34  and  36  are prevented from rotating about central axis  18 . The inner circumference of rotary bearing  38  is provided with a plurality of camming surfaces  50 , one for each pin  40 . 
     Pins  40  extend through a central bore in rotary bearing  38  and the ends of pins  40  have reduced cross sections and are supported in recesses  51  formed in end plates  34 ,  36 . Each recess  51  has basically a circular cross section and is dimensioned to hold its associated pin  40  in a defined position. However, each recess  51  is preferably slightly elongated in the radial direction, by several thousandths of an inch, to permit limited movement of its associated pin in the radial direction. In addition, each end plate  34 ,  36  is provided with an annular recess  52  that receives a respective one of spring units  42 . 
     Each spring unit  42  is a one-piece element that includes a circular outer support ring  42   a  which carries an axially projecting rim  42   b,  several radially inwardly projecting spokes, an inner ring composed of a plurality of radially deformable spring elements  42   c  and a plurality of radially outwardly projecting tabs  42   d.  Each spring unit  42  is seated in the recess  52  of a respective end plate  34 ,  36 , with rim  42   b  being located in a circular groove  53   a  and tabs  42   d  being located in slots  53   b  of the associated end plate. The engagement of tabs  42   d  in slots  53   b  prevents rotation of each unit  42  relative to its respective end plate and engages one end of each pin  40  to bias pins  40  radially outwardly against camming surfaces  50  and away from armature shaft  14 . 
     Each end of each pin  40  is inserted between ring  42   a  and a respective spring element  42   c  of a respective spring unit  42  so that the respective spring element  42   c  resiliently biases its associated pin  40  away from armature shaft  14 . 
     The illustrated embodiment is provided with four pins  40 , only one of which is shown in FIG.  6 . 
     Rotary bearing  38  carries a rotation pin  54  that engages in a camming slot  56  provided in input member  44 . As shown in FIG. 7, slot  56  has a linear central portion and is curved at its ends. When valve head  24  approaches either one of its end positions, a respective curved end of camming slot  56  engages rotation pin  54 , causing rotary bearing  38  to pivot through a small angle, generally in the range of 5° to 10°, in the direction of arrow  58  in FIG. 6 about central axis  18 . 
     As rotary bearing  38  pivots, camming surfaces  50  move relative to pins  40  in the clockwise direction with respect to the view of FIG. 6, pressing pins  40  against armature shaft  14 . This clamps armature shaft  14  in place, preventing movement of armature shaft  14  along central axis  18  and therefore holding valve head  24  in either its open or closed position. 
     According to an exemplary preferred embodiment of the invention, pins  40  are dimensioned so that when rotary bearing  38  is in its rest position, shown in FIG. 6, so that armature shaft  14  is free to move parallel to axis  18 , there will be a clearance of about 0.001″ to 0.01″ between pins  40  and armature shaft  14 . Presently, the preferred clearance is of the order of 0.003″ and it is preferred that camming surfaces  50  be given a slope that will cause pins  40  to come into contact with armature shaft  14  after a rotation of rotary bearing  38  through an angle of 5° to 10°, corresponding to a circumferential displacement of camming surfaces by a distance of approximately 0.02″ to 0.03″. 
     If armature shaft  14  has a diameter of 0.30″, which is a typical dimension for automobile engine valve stems, rotary bearing  38  may have an outer diameter of the order of 0.93″and a thickness of the order of 0.30″, each of end plates  36  and  38  may have an outer diameter of the order of 1.5″, and the assembly of end plates  34  and  36  and rotary bearing  38  may have a thickness of 0.45″. 
     During prolonged use of a valve actuator equipped with the above-described clamping device, the engaging surfaces of rotation pin  54  and camming slot  56  will be subjected to wear, resulting in a progressive reduction in the clamping force created between pins  40  and armature shaft  14 . In addition, temperature variations experienced by the clamping device will adversely effect the clamping action. In order to minimize these effects, rotation pin  54  can be replaced by a flexible beam member, as shown in FIGS. 8A and 8B. This member includes a flexible beam  70  having a camming element  72  at its outer extremity. Beam  70  and camming element  72  are dimensioned so that when the clamping device is initially placed into use, beam  70  will be resiliently flexed by a small amount whenever rotary bearing  38  has been rotated to its clamping position. As the engaging surfaces of camming slot  56  and camming element  72  experience wear, the degree of flexing will diminish, but rotary bearing  38  will continue to be rotated sufficiently to securely clamp pins  40  against armature shaft  14 . This flexing of beam  70  will also help to prevent changes in the extent of pivoting movement of rotary bearing  38  as a result of temperature variations. 
     In existing electromagnetic valve actuator assemblies, the combined force level produced by springs  26  and  28  when armature  20  is in one of its end positions is in the range of about 170 to 200 lbs. The rotary clamping element of the above-described embodiment has the capability of applying a clamping force sufficient to oppose this spring force when both electromagnets are de-energized. However, tests have shown that a lower clamping force level of the order of about 70 lbs would be desirable and this clamping force level will allow the holding current to the energized electromagnet to be reduced by a satisfactory amount from the level required to displace armature  20  to the selected end position. In this case, if the current to the energized electromagnet is completely cut off, the net force produced by springs  26  and  28  will begin to move armature  20  away from its end position. 
     According to further embodiments of the invention, the mechanical actuation of the clamping devices can be replaced by various types of electrical actuation, including actuation by a piezoelectric element as shown in FIG.  9  and actuation by an auxiliary electromagnet, as shown in FIG.  10 . 
     Each of FIGS. 9 and 10 shows two actuators disposed side-by-side for operating two valve heads associated with one engine cylinder. 
     Referring to FIG. 9, rotation pin  54  of each clamping device is engaged by one end of a lever  80  that is mounted to pivot about a horizontal axis  82 . Lever  80  is engaged, at a point between its ends, by a piezoelectric driver  84  that is connected to receive a suitable drive voltage (connection not shown). When such a voltage is applied to driver  84 , it expands horizontally against lever  80  in order to pivot rotary bearing  38  into a clamping position. 
     In the embodiment shown in FIG. 10, rotation pin  54  of each clamping device is engaged by one end of a lever  90  whose other end is pivoted to rotate about a horizontal axis  92 . Lever  90  is associated with an auxiliary electromagnet  94  which, when energized by a suitable drive current (connection not shown) attracts lever  90  in a manner to displace rotation pin  54  and thus rotate rotary bearing  38  into its clamping position. 
     In the embodiments illustrated in FIGS. 1-8, neither electromagnet  12  or  13  need be provided with energizing current during the periods when valve head  24  is being held in either one of its end positions. Alternatively, a reduced level of energizing current may be supplied to that one of electromagnets  12  and  13  which last moved valve head  24  to its present end position. 
     In the case of the embodiments illustrated in FIGS. 9 and 10, both electromagnets  12  and  13  may be de-energized and the energy consumed by piezoelectric driver  84  or electromagnet  94  to effect clamping of armature shaft will be substantially lower than the electrical energy that would be consumed by one of electromagnets  12  and  13  to hold valve head  24  in its end position if the actuator were not provided with a clamping device according to the invention. 
     FIG. 11 is a cross-sectional view showing a portion of an electromagnetic valve actuator equipped with a further embodiment of a clamping or latching assembly according to the invention. The assembly according to this embodiment includes a support member  102  containing a clamping electromagnetic, a clamping armature  104  and a clamping or latch mechanism  106 . 
     Support member  102  is fixed to a spacer block  108  forming a component of the electromagnetic valve actuator housing. Support member  102  has a central bore through which both armature  104  and armature shaft  14  extend. Armature  104  is free to move, parallel to central axis  18 , relative to both support member  102  and shaft  14 . 
     Mechanism  106  is constructed and mounted to be urged against shaft  14  under the influence of biasing springs, as will be described in greater detail below. 
     Support member  102  is secured to spacer block  108  by means of threaded bolts or machine screws, one of which is shown at  112 . Similarly, mechanism  106  is secured to block  108  by at least one machine screw  114 . 
     Mechanism  106  may be provided with frictional breaking and/or clamping surfaces, or may, as illustrated in FIG. 11, have inwardly projecting latching elements that will engage in an annular recess  116  in the outer surface of shaft  14 . In embodiments which employ latching, the clamping assembly establishes a well defined end position for shaft  14 . 
     In the illustrated embodiment, recess  116  is located to cause mechanism  106  to latch shaft  14  in its upper end position, in which the associated valve is in its closed state. However, in further accordance with the invention, shaft  14  could be provided with a further recess  116 ′, shown in broken lines, which would be engaged by the latch elements of mechanism  106  when shaft  14  is in its lower end position, corresponding to the full, open position of the valve. 
     Mechanism  106  is moved to an unclamping, or unlatching, position in response to downward movement of armature  104 , which occurs when the clamping electromagnet carried by support member  102  is energized. At this time, armature  104  is attracted to the clamping electromagnet, causing camming surfaces at the lower end of armature  104  to engage inwardly facing surfaces of mechanism  106 . Further downward movement of armature  104  effects radial separation of the elements of mechanism  106  by an amount sufficient to allow free movement of shaft  14  along central axis  18 . 
     One embodiment of mechanism  106  is shown in greater detail in FIG. 12, which is a top plan view. Mechanism  106  includes an end support  122  via which mechanism  106  is secured to block  108 , as already described. 
     Mechanism  106  further includes two machine screws  124  each engaging in a threaded bore (not shown) in end support  122  and extending through passages in two half sliders  128 . Each screw  124  further extends through two cylindrical compression springs  130  which constitute the biasing springs that urge half sliders  128  together. 
     Each half slider  128  is provided with a recess  132 . The two recesses  132  of the two half sliders  128  cooperate to define a passage for shaft  14 . Recesses  132  may also define braking or clamping surfaces for shaft  14 , in which case half sliders  128  may be made of a type of material utilized for brake pads. 
     Alternatively, each recess  132  may be provided, as shown, with a projection defining a latch element  134  that will engage in recess  116 , and recess  116 ′ if provided, when shaft  14  is in one or two defined positions. 
     An embodiment of armature  104  is illustrated in FIGS. 13,  14 ,  15  and  16 . Armature  104  is composed of an armature body  140  and an armature cap  142 . Armature body  140  is shown in FIGS. 13 and 14, FIG. 13 being a cross-sectional view and FIG. 14 being an end view in the direction of arrow  143  in FIG.  13 . Armature body  140  has a disc portion  144  which is acted on by the clamping electromagnet in support member  102  and a tubular portion  146  having a through bore. Tubular portion  146  is dimensioned to receive shaft  14 , as shown in FIG. 11, and to slide with respect to support member  102 . In addition, tubular portion  146  is provided with a male screw thread  147  at the end remote from disc portion  144 . 
     Referring to FIGS. 15 and 16, armature cap  142  is constructed and dimensioned to be screwed onto the end of tubular portion  146  that is remote from disc portion  144 , armature cap  142  being provided with a female screw thread  147 &#39;that mates with screw thread  147  on tubular portion  146 . When armature cap  142  is assembled to armature body  140 , a frustoconical surface  148  on armature cap  142  will be directed away from disc portion  144  and will provide the camming surface which acts on half-sliders  128 . 
     Referring to FIG. 16, which is an end view in the direction of arrow  150  of FIG. 16, the outer periphery  152  of armature cap  142  is provided with two flat surfaces for engagement of armature cap  142  by a wrench. 
     FIGS. 17 and 18 are elevational, cross-sectional views of, respectively, a further embodiment of a clamping assembly according to the invention and one component of that assembly. This embodiment is structurally similar to the embodiment of FIGS. 11-16 in that the assembly includes the same support member  102  and spacer block  108 . In addition, this embodiment further includes an armature  160  which has the same general form as armature  104 , but differs with respect to the configuration of the lower end of its tubular portion  166 , this end being remote from disc portion  144 . In addition, the assembly of FIGS. 17 and 18 does not have a clamping or latch mechanism of the type employed in the embodiment of FIGS. 11-16, but is provided, at the same location, with a washer  170  having an inner wall which tapers downwardly, toward electromagnet  13 . 
     As shown most clearly in FIG. 18, the lower end of tubular portion  166  has an outer wall  174  that tapers downwardly and the lower end of tubular portion  166  is provided with two or more slots  176  which are preferably coextensive with tapered outer wall  174 , in the longitudinal direction of armature  160 . Tapered outer wall  174  is dimensioned to mate with the tapered inner surface of washer  170  so that when armature  160  is pulled downwardly upon actuation of the electromagnet in support member  102 , a camming action will occur between washer  170 , which is held securely between support member  102  and electromagnet  13 , causing the lower end of tubular portion  166  to be pressed against armature shaft  14  and to perform a braking and clamping operation. 
     In the embodiments of FIGS. 11-18, the clamping assemblies are dimensioned to assure that a gap will remain between disc portion  144  and the upper surface of support member  102  and its associated clamping electromagnet when that electromagnet has been energized. This will help to reduce both wear on the armature and armature-to-electromagnet contact noise. 
     Another embodiment of a clamping mechanism according to the invention is illustrated in FIGS. 19 and 20 which are, respectively, a plan view and a side elevational view. 
     This embodiment includes a circular component  200  made of piezoelectric material, component  200  being secured to upper electromagnet  13  by means of three bolts  202  and three spring washers  204 . Neither washers  204  nor bolts  202  are shown in FIG.  19 . 
     Component  200  includes a rigid outer ring  210 , an array of radially extending spokes  212  distributed at uniform intervals about the circumference of ring  210  and an inner ring composed of a plurality of arcuate parts  214 . Parts  214  are spaced apart around the periphery of the inner ring and are separated from one another by radial slots  216 . In addition, parts  214  are provided with bores  218  for the passage of bolts  202 Bores  218  are made slightly larger in diameter than the shanks of bolts  204  in order to permit radial movement of parts  214 . 
     The outer surface  220  of ring  210  is an anode surface and the inner surface of ring  210  is a cathode surface, each of these surfaces being coated with a nickel or silver layer, or substrate, via which an excitation voltage can be applied to component  200 , and more specifically to ring  210 . 
     The operating mechanism of this embodiment is based on the principle of piezoelectric ring expansion/contraction in the “transverse ring mode” direction. A voltage applied between the conductive coatings on surfaces  220  and  222  will, depending on the polarity of the voltage, cause radial expansion or contraction of ring  210 . When the polarity of the voltage causes expansion, ring  210  will expand radially such that inner and outer surfaces  220  and  222  both move radially away from one another. This, inner surface  222  moves radially inwardly, or contracts. This produces inward radial movement of spokes  212  and inner ring parts  214  inwardly to clamp the armature shaft. If desired, a coating or layer of a material having good braking characteristics can be deposited in the inner surfaces of inner ring parts  214 . Since the operation of this component is based on expansion and contraction of outer ring  210 , spokes  212  and inner ring parts  214  need not be made of piezoelectric material. However, manufacture is simplified if component  200  is a one-piece, homogeneous body, which requires that the entire component be made of piezoelectric material. 
     As shown in FIG. 20, component  200  is shaped so that inner ring parts  214  are longer, in the axial direction, than outer ring  210  or spokes  212 . The greater length of inner ring parts  214  provides improved support against the electromagnet or cylinder head for the fairly fragile piezoelectric material. 
     Isolation pads and/or an elastomer coating surrounding the mechanism (not shown) can be added to provide damping effects that will reduce impact forces and mechanical noise, prevent moisture and oil contamination, and provide electrical isolation for the system. 
     Because thermal growth will occur, the required tolerances are tight. The thermal expansion coefficients of the piezoelectric, brake and armature shaft materials must be closely coordinated. The required rapid response time, of the order of 10 microseconds, of the piezoelectric material will make possible the creation of multiple braking profiles, for example by pulsing the amplitude of the voltage applied to the piezoelectric material. A braking profile is a particular excitation voltage time variation pattern that will determine the timing and braking rate of a particular braking operation. Excitation voltage levels for the form of construction disclosed herein will range between 200 and 2500 volts depending on the amount of expansion or contraction required. The accompanying current level will be minimal, typically between 1 and 5 mA. A microprocessor control system can be programmed to adjust the excitation voltage as a function of temperature and the expansion coefficients of the component materials. 
     In order to assure that inner ring parts  214  can experience the required radial movements in response to radial expansions and contractions of outer ring  210 , parts  214  must be secured to electromagnet  13  by a suitable holding, or clamping, force. This is achieved, in the case of the disclosed embodiment, by the use of securing element units that apply accurately defined clamping forces to inner ring parts  214 . One such securing element unit is shown in FIG.  20 A. The component shown in FIGS. 19 and 20 will be provided with three of these units. 
     The unit shown in FIG. 20A includes bolt  202  in the form of a shoulder bolt, associated with spring washer  204 . Bolt  202  has a shank that is provided at its lower end with a radially extending abutment surface  230 . Electromagnet  13  is provided with a blind bore having a threaded portion for receiving a mating threaded portion of bolt  202 , the blind bore additionally having a recess  232  for receiving a lower portion of the shank of bolt  202 . Recess  232  has a base  234  on which abutment surface  230  will rest when bolt  202  is fully installed. The length of the shank of bolt  202  and the depth of recess  232  are selected to assure that when abutment surface  230  rests on base  234 , washer  204  will be pressed against the upper surface of an associated inner ring part  214  with a clamping force that will hold component  200  securely in place and allow the necessary radial movements of inner ring parts  214 . 
     The clamping device shown in FIGS. 19 and 20 could be employed in the actuator shown in FIG. 1 in place of clamping device  30  an its associated components. 
     Further embodiments of mechanical clamping devices according to the invention can be constructed to achieve a clamping action in response to a small angular rotation of the armature shaft when the valve reaches either one of its end positions. One embodiment of a device of this type is illustrated in FIGS. 21-30. 
     FIG. 21 is a cross-sectional view showing electromagnets  12  and  13  and a portion of armature shaft  14  associated with one valve actuator. In this embodiment, armature shaft  14  forms a unit with an armature  250  that is movable, as in the previously described embodiments, with armature shaft  14 , along central axis  18  in a space between electromagnets  12  and  13 . 
     In this embodiment, electromagnet  13  is provided with an enlarged central bore and a support tube  252  is fixed in that central bore. The inner surface of tube  252  is dimensioned to permit shaft  14  to slide easily therein and will be provided, if necessary, with guide bushings, as would the smaller diameter bore in electromagnet  12 , in accordance with the usual practice in the art. 
     The upper end of tube  252  is provided with a blind bore  254  to receive components of a clamping device according to this embodiment of the invention. This clamping device additionally includes two linear cams, one of which,  256 , is visible in FIG.  21  and the other of which,  257 , is shown in FIGS.  26  and  27 - 29 . Each of these cams, including cam  256 , is held in position between electromagnets  12  and  13  by a side plate  258  that will be secured to the actuator housing, as by machine screws  259 , depicted in FIG.  26 . Each cam  256 ,  257  is held in a respective recess in plate  258 , these recesses being to prevent movement of cams  256 ,  257  parallel to central axis  18  and perpendicular to the plane of FIG.  21 . 
     As will be explained in greater detail below, when armature  250  is displaced toward either one of its end positions, armature  250  is pivoted, along with shaft  14 , through a small angle about axis  18  by the action of linear cams  256  and  257  (FIGS. 26-30) and the rotation of shaft  14  causes cylindrical pins  276  (FIGS. 24 and 25) housed in bore  254  to be clamped against shaft  14 . The resulting level of clamping force causes shaft  14  and armature  250  to be retained in the end position while a reduced level of current is being applied to that one of electromagnets  12  and  13  that was energized to move armature  250  to its current end position. Suitable relations among mechanical clamping force, biasing spring force and electromagnet hold current are described earlier herein in connection with the embodiment shown in FIGS. 1-8 and that discussion is equally applicable to the embodiment presently being described. Subsequent movement of armature  250  to its opposite end position is effected by energizing the appropriate electromagnet  12  or  13  to produce a magnetic attracting force sufficient to overcome the mechanical clamping force. 
     FIG. 22 is a detail view of the portion of the structure of FIG. 21 that is enclosed by a circle. This view shows the upper end of tube  252  and illustrates two of the components of the clamping device which are installed in blind bore  254 . These components include a cylindrical race  260  and a pin retainer  262 . Race  260  is fixed, as by a force fit or cementing, in blind bore  254  and pin retainer  262  is held in place in race  260  by means of inwardly directed flanges at both ends of race  260 . Retainer  262  holds a series of clamping pins  276 , which are not illustrated in FIG.  22 . 
     FIG. 23 is a cross-sectional detail view in the direction of plane A—A of FIG. 22, showing one form of construction of race  260 . This race has a profiled inner surface composed of a series of recesses  270  separated by lands  272 . The inner surface of race  260  also has a series of grooves  274  that will be engaged by pin retainer  262  to prevent it from rotating relative to race  260 . 
     Also shown in FIG. 23 in broken lines are two positions for one of pins  276 . In position  276 - 1 , pin  276  is fully seated in an associated recess  270  and contacts shaft  14  with a light bearing force that allows axial movement of shaft  14 . In position  276 - 2 , pin  276  has been moved circumferentially and radially inwardly by a small amount to apply a clamping force to shaft  14 . As will be explained in greater detail below, this movement of pin  276  is produced by a small angular rotation of shaft  14 . Because shaft  14  is always in contact with pins  276 , rotation of shaft  14  causes pins  276  to roll against shaft  14  and the inner surface of race  260 , resulting in the movement between positions  276 - 1  and  276 - 2 . In the embodiment illustrated, nine such pins are provided, each pin being associated with a respective recess  270 . 
     FIG. 24 is a cross-sectional view of pin retainer  262 , also taken in plane A—A of FIG. 22, but drawn to a slightly larger scale than is FIG.  23 . 
     Pin retainer  262  may be formed from an initially flat strip of high strength steel or high temperature plastic having portions which are cut and bent to form outwardly projecting tabs  280 . These tabs are cut out in such a way that a continuous portion  282  of the sheet remains along each edge thereof. This sheet is then bent into a circular shape, which is the shape shown in FIG. 24, with the two ends of the sheet meeting at an abutment plane  284 . When retainer  262  is installed in race  260 , tabs  280  will engage in grooves  274  in race  260 . 
     Retainer  262  is further provided with a plurality of curved leaf type compression springs  286 , each spring  286  being fastened at its midpoint to a respective tab  280 , as by spot welding or staking, so that both ends  288  of each spring  286  project circumferentially away from its associated tab  280 . 
     FIG. 24 further shows two pins  276  each in one of the positions  276 - 1  and  276 - 2 , which are the same positions as those identified by corresponding numerals in FIG.  23 . Each pin  276  is urged toward the position  276 - 1  by the action of its associated spring  286 . When in position  276 - 1 , each pin  276  is seated in an associated recess  270  and contacts shaft  14  with a low contact force. Upon rotation of shaft  14 , in the counterclockwise direction with respect to the plane of FIG. 24, pins  276  will roll in contact with shaft  14  so as to be urged against their respective leaf springs  286  and to move out of the associated recess  270 . This causes each pin to move into the position  276 - 2  and to apply a clamping force to shaft  214 . FIG. 24 additionally shows, in broken lines, at pin position  276 - 2 , deflection of the ends  288  of the associated spring  286 . 
     When shaft  14  rotates back in the clockwise direction to its original position, pins  14  again rotate with shaft  14  back into recesses  270 . 
     FIG. 25 is a detail view in a plane parallel to central axis  18  showing a portion of retainer  262 . 
     FIG. 26 is a top view taken in a plane perpendicular to central axis  18 , illustrating the portion of the clamping device that is associated with armature  250 . It will be noted that, in this embodiment, armature  250  has a rectangular outline but it will be understood, from the following description, that armature  250  can have other forms that will enable it to interact in the required manner with cams  256  and  257 . Side piece  258 , screws  259  and cams  256  and  257  are shown in exploded form in order to more clearly illustrate the form of the individual components. Each recess in side piece  258  houses two compression springs  290  that extend into blind bores formed in the rear side of each of cams  256  and  257 . Springs  290  maintain a spacing between cams  256  and  257  and the bottoms of the recesses in side piece  258  so that cams  256  and  257  have some freedom of movement toward and away from the bottoms of the recesses. In the assembled device, side piece  258  will be fastened to actuator housing  292  in order to hold cams  256  and  257  in place, while permitting those cams to undergo some movement perpendicular to the central axis  18 . 
     A rear view of cams  256  and  257  is shown in FIG. 27, from which the form of the blind bores in the cams can be seen. FIG. 28 is a cross-sectional view along the plane B—B of FIG.  27  and also illustrates one of the blind bores. 
     FIGS. 29 and 30 are side elevational views illustrating the camming surfaces of cams  257  and  256 , respectively. As shown in FIG. 29, the camming surface of cam  257  that faces armature  250  has, at a midpoint of its length, a recess  294 , while the camming surface of cam  256  has, at a corresponding point, a projection  296 . 
     Referring again to FIGS. 21 and 26, as armature  20  is displaced parallel to central axis  18  from one of its end positions, one end of a side edge of armature  250  will be acted on by cam  256 , while the other end of that side edge will be acted on by cam  257 . Movement of armature  250  over projection  296  will tend to increase the force applied by armature  250  to cam  256 , thus increasing the opposing force produced by the springs  290  associated with cam  256 , and simultaneously movement of armature  250  over recess  294  will tend to reduce the force applied by armature  250  to cam  257 , thus tending to reduce the force produced by the springs  290  associated with cam  257 . Therefore, armature  250  will be pivoted clockwise, with respect to the viewing direction of FIGS. 23,  24  and  26 , about central axis  18  by an amount determined by the spring rates of all springs  290 , which spring rates are preferably identical for all springs  219 . Pivotal movement of armature  250  in this sense tends to release the clamping forces applied by pins  276  to armature shaft  14 . 
     As armature  250  moves parallel to central axis  18  in either direction away from recess  294  and projection  296 , the opposing forces produced by the springs  290  associated with cam  256  will decrease and the opposing forces produced by the springs  290  associated with cam  257  will increase, causing armature  250  to undergo a small degree of counterclockwise pivotal movement about central axis  18 . Because shaft  14  is in contact with pins  276 , this pivotal movement will act to displace pins  276  from the position  276 - 1  to the position  276 - 2  shown in FIGS. 23 and 24, creating a clamping action between pins  276  and shaft  14 . 
     Thus, the angular position of armature  250  when in contact with recess  294  and projection  296  is set to correspond to position  276 - 1  of pins  276 , while the angular position assumed by armature  250  when not in contact with recess  294  and projection  296  corresponds to the position  276 - 2  of pins  276 . 
     It will be seen that in the embodiment illustrated in FIGS. 21-30, armature shaft  14  will be automatically held in either end position with a force sufficient to allow the holding current applied to either one of electromagnets  12  and  13  to be reduces and will remain in that position until the electromagnet which is then holding armature  250  is de-energized. 
     In a preferred form of construction of the embodiment of FIGS. 21-30, race  260 , pins  276  and shaft  14  will all be made of materials having essentially the same coefficient of thermal expansion. This embodiment will be installed in an actuator having the form shown in FIG. 1, in place of clamping mechanism  30  and its associated components. 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.