Abstract:
An inertial actuator for active vibration control utilizes a single cylindrical actuator module of the “voice coil” type. A moving armature assembly, including a soft iron shell with a tubular sleeve coaxially surrounding a core with two permanent magnets and corresponding pole plates, is suspended compliantly by a flexure assembly including a multiple-stack array of shaped flexure strips and a pair of end support flexure plates mounted to a base. Two magnetically charged annular air gaps at the pole plates in the moving armature traverse two corresponding annular coils wound in a tubular bobbin that extends as a heat-sinking mounting column attached to the base. This form of inertial actuator provides cost benefits along with improved performance and reliability due to the stability of the moving parts and uniformity of linear motion.

Description:
PRIORITY 
   Benefit is claimed under 35 U.S.C. § 119(e) of pending provisional application 60/551,118 filed Mar. 6, 2004. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of inertial electromagnetic actuators with permanent magnets, especially as related to the field of active vibration control, and discloses an actuator that can be directed beneficially to powered vehicles, e.g. in aeronautic and space travel. 
   BACKGROUND OF THE INVENTION 
   Actuators for active vibration control can be realized utilizing hydraulic, pneumatic or electromagnetic implementation. Electromagnetic actuators commonly utilize moving armatures in configurations whose operating principles are characterized by large variations in magnetic air gap dimensions and consequent large variations in flux density in the air gap which, due to eddy currents and other harmful effects, limit efficiency and available stroke amplitude. 
   However a special category of electromagnetic actuator works on the principle of a loudspeaker voice coil moving in a small air gap of constant dimensions in which the flux density, typically from a permanent magnet, remains substantially constant, providing substantial design advantages. 
   The combination of voice coil type actuator and flat spring flexures has potential for providing the following advantages: 
   1. Maximized volumetric utilization with minimum mass requirements. 
   2. Capability to accommodate multiple actuator modules in parallel, where the stationary coils are mounted on a common base and the armatures containing the magnet structure(s) are connected to a common flexure pack so that high width to depth ratios are possible. 
   3. The active mass ratio is maximized because the shell is included in the armature structure. 
   4. Flexures allow longer stroke than coil springs. 
   5. A greater diversity of materials including composites is possible with flexures rather than coil springs. 
   6. Flexures allow much higher spring rates per unit is volume than coil springs. 
   7. The support flexure element accommodates the dynamic change in length of the working flexure element, locates the inertial mass, and transmits the inertial forces to the base in a minimum of space and minimum mass. 
   8. The coil assembly can benefit from improved thermal bonding to the base and forced cooling can be designed into the system. 
   When a voice coil type actuator is used in a small range of frequencies it is common practice to add springs to the arrangement in order for the armature to operate at or just below resonance. Resonance provides a means to create a force gain and reduces the power input required, but the amplitude of the armature does not change as a function of resonance. The amplitude of the armature is proportional to the output force of the actuator, not the input (coil) force. 
   Higher output force requires a longer actuator to accommodate longer stroke, or an actuator with a large diameter to accommodate additional inertial mass. Lower operating frequency also requires longer stroke, which requires very long springs, adding to the bulk of the actuator and reducing the ratio of active mass to total mass. The increased dimensions and mass of such actuators make them often unsuitable for applications such as helicopters 
   DISCUSSION OF KNOWN ART 
   An example of the use of the voice coil principle in an actuator for active vibration control is disclosed in U.S. Pat. No. 5,231,336 to F. T. van Namen. 
   One known form of inertial actuator for vibration control where a long stroke is required utilizes one or more coil springs as a compliant element in the suspension system. 
   As an alternative, compliance can be provided by the use of substantially flat leaf springs known as flexures or flexure strips, which may be shaped with width and/or thickness variations rather than in a uniform strip form to control the compliance, mass and stress parameters. 
   In  FIGS. 1-3  is shown three views of an actuator utilizing the permanent-magnet/voice-coil principle as has been known and used in conjunction with suspension flexures in actuators for active vibration control in helicopters in a two-module configuration, A non-moving stator portion, intended to be attached to a massive region of a host machine for vibration suppression, typically a motorized vehicle, includes a pair of tubular bobbins  10 A of which the bottom ends are seen in  FIG. 2  located beneath associated actuator modules  10 , coupled directly to the base casting  12 . Each bobbin  10 A carries a pair of annular voice coils located inside the shells of actuator modules  10 , and the lower portion of the bobbins serve to conduct heat from the coils to the base casting  12 , which thus serves as a heat sink. 
   A moving armature portion of the actuator module includes a tubular ferrous shell around each actuator module  10 , two permanent magnets, with circular polepieces, forming a cylindrical core located concentrically inside each actuator module  10 , an end piece  10 B linking the core mechanically and magnetically to the shell sleeve, and an auxiliary mass  14 , connecting the movable armature portion to a vibratable central region of the single overhead stack  16  of flexure strips, which is supported at each end by a vertical flexure  18  extending down and attached to a corresponding end of the base casting  12 . Auxiliary mass  14  is configured with the sloping contour as seen at the top to provide clearance to prevent unwanted contact with the bottom of flexure stack  16  as it bends when deflected upwardly during operational vibration. 
   The contour of each strip in the flexure stack  16  is seen in  FIG. 1  to approximate the shape of a double hourglass, and as seen in  FIG. 2 , each “hourglass” half is located above a corresponding one of the two actuator modules  10 . 
   The vertical flexure plates  18  are also configured in the hourglass shape as seen in  FIG. 3 . Flexure strips and end plates can be varied in quantity and contoured in both width and thickness for desired compliance, mass, reduction of stress, and influence on resonance frequency. 
   OBJECTS OF THE INVENTION 
   It is a primary object of the present invention to provide an improved structure in an inertial electro-magnetic actuator for active vibration control to accomplish increased overall operational efficiency and more uniform motion control in a cost-effective manner. 
   It is a further object to provide improvements in an inertial electromagnetic structure that utilizes a known and proven magnet and coil bobbin configuration. 
   It is a further object to provide an improved flexure system that operates under lower and more uniformly distributed stress. 
   It is further object to minimize the total mass of the inertial actuator, as required for aeronautical use. 
   SUMMARY OF THE INVENTION 
   The foregoing objects have been met in the structure of an inertial actuator by novel overall configuration utilizing only a single actuator module providing the benefits of lower cost along with improved performance and reliability due to better uniformity and stability of moving parts, and by configuring a flexure assembly with multiple stacks of flexure strips, yielding advantages of lower mass and lower, more uniformly distributed stress. 
   From a design viewpoint, a flexure assembly configured as an array containing a multiplicity of stacks of flexure strips provides freedom to select optimal quantities of stacks in the array and strips in the stack for optimum span to width ratios which along with optimal strip shaping can accommodate the most challenging envelope requirements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 ,  2  and  3  are three views of a prior art inertial actuator for active vibration control configured with two actuator modules whose moving armature portions are suspended by a single stack of flexure strips. 
       FIG. 4  is a top view of an inertial actuator in accordance with the present invention configured with a single actuator module whose moving armature portion is suspended by a multiple stack flexure assembly. 
       FIG. 5  is a front view of the subject matter of  FIG. 4  showing the single actuator module. 
       FIG. 6  is a side view of the subject matter of  FIGS. 4 and 5  showing an end support flexure plate. 
       FIG. 7  is a cross-section taken at  7 - 7  of  FIG. 5 . 
       FIG. 8  is a vertical central cross-section of the actuator module of  FIGS. 5-7 . 
       FIG. 9  is a cross-section taken at  9 - 9  of  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   It is to be understood that terms herein indicating direction such as “up”#, “down”, “horizontal” and “vertical” are used in an arbitrary manner for purposes of facilitating descriptions in connection with the drawings showing the base at the bottom, whereas in actual deployment, e.g. on a helicopter, the actuator may be oriented in another designated direction other than the reference horizontal direction of the base shown in the drawings. 
     FIGS. 1 ,  2  and  3  are three views of an inertial actuator for active vibration control of known art as discussed above. 
   In  FIG. 4 , a top view of an actuator in accordance with the present invention, compliant suspension for the moving armature portion of a single actuator module is provided by a multiple array of five stacks  20  of flexure strips as shown. The flexure stacks  20  are clamped down at both ends by clamp bar  22  and across the center by clamp bar  24 . Clamp bars  22  and  24  are tightened down onto the ends of the stacked flexure stacks  20  by bolts  26  extending down through holes in clamp bars  22  and  24  and through holes in the flexure strips of stacks  20 . The outline of a rectangular shell yoke plate  28  appears in the gaps between stacked flexure stacks  20 . A vertical end support flexure  30  is seen at each end. The overall outline is that of base  32  with mounting lugs extending from each end as shown. Base  32  may be fabricated as a casting or machined part, typically of aluminum. 
   In  FIG. 5 , the front view of the inertial actuator of  FIG. 4 , at each end of the array of five flexure stacks  20 , a set of five bolts  26 , traversing clamp bars  22 , threadedly engage a lower clamp bar  22 A which is bolted to the top end of a corresponding vertical end support flexure plate  30  whose lower end is fastened to base  32  via bars  22 B. At the center of the array of stacks  20 , five bolts  26  traverse clamp bar  24  and five corresponding stacks  20 , and threadedly engage shell yoke plate  28  which is attached to the top of tubular shell sleeve  36  forming the outwardly visible portion of the moving armature mass portion of actuator module  34 . The fixed stator mass portion of actuator module  34  includes a tubular bobbin  38 , mainly surrounded by shell sleeve  36 , whose lower portion is seen extending down to base  32  where the bottom end of bobbin  38  is securely fastened. 
   The moving armature mass portion of actuator module  34  includes shell sleeve  36  which is magnetically and mechanically linked at the top end by yoke plate  28  to an internal magnetic core assembly not visible in this view. The moving armature mass portion is suspended by the flexure assembly: an array of five compliant stacks  20  of flexure strips. Mechanical resonance occurs at a frequency determined by the mass of the moving armature portion and the compliance of the flexure assembly of stacks  20 . The resonant frequency can be adjusted by changing the mass of the moving armature portion, e.g. by selecting the thickness of yoke plate  28 , and/or by varying its shape which can be made rectangular as shown ( FIG. 4 ) or circular. Also additional mass may be added either on top of the array of stacks  20  or beneath the array, on top of yoke plate  28 . 
   Under vibration, vertical displacement at the center of each flexure stack  20 , which can range up to a predetermined armature travel limit for which the actuator module is designed, creates an S-bend in each half of the stacks  20  while flexing. The resultant dynamic change in length of the stacks  20  is accommodated by flexing of the two resilient vertical end support flexure plates  30  which locate the inertial mass of the moving armature portion, and which transmit the inertial forces to the base  32  in an overall minimum of space and minimum mass in the inertial actuator. 
   The end plates  18  vibrate in opposite horizontal directions at twice the frequency of the vertical vibration of the array of stacks  20  as they flex, however unwanted horizontal vibration in the array of stacks  20  (and thus in the moving armature mass portion) is avoided by cancellation due to structural balance of the overall flexure assembly. 
   The parameters of end plates  18  are selected for minimal influence on the desired vertical vibration of the moving armature assembly including the array of stacks  20 . 
   The flexure strips in each stack  20  may made from a spring grade of metal or from a composite including materials such as carbon fibers and resin, Thickness of the individual flexure strips in stack  20 , and the quantity of strips in each stack  20 , which typically can range between 6 and 58, and the number of flexure plates  30  at each end, which can be made one (as shown), two or more, are selected to obtain a desired degree of overall compliance, i.e. stiffness. To avoid unwanted noise, it is preferable to prevent contact between the strips (and end plates, if more than one at each end) under vibration by utilizing washers or other spacers in the mountings. 
     FIG. 6  is a side view of the actuator of  FIGS. 4 and 5  showing the special contour of one of the two end support flexure plates  30 , with mirror-image arched regions cut away at each side for optimal compliance and stress distribution. In this view, shell sleeve  36  and yoke plate  28  of the actuator module are seen extending approximately full width of the end support flexure plate  30 . 
   The shape selected for the contour of the cutaway regions of flexure strips  20  and vertical end support flexures  30  is a matter of design choice: basically this contour affects compliance, stress distribution and, to some extent, mass. This contour can be selected from a range of shapes that includes the hour glass shape shown in  FIG. 4  and the elliptical shape shown in  FIG. 6 . Thickness may be varied as well as width, and accordingly, the number of strips per stack  20 , as well as the number of stacks  20  in the overall flexure assembly, to optimize for particular dimensional and operating parameters. 
     FIG. 7  is a horizontal cross-section of actuator module  34 , taken through  7 - 7  of  FIG. 5 , showing the concentric arrangement of tubular soft iron shell sleeve  36  surrounding the tubular support region of bobbin  38  which is configured with an inward bottom flange  38 C by which it is bolted to base  32 , shown in the background, to serve as a heat sink. 
     FIG. 8  is a vertical cross-section through the central axis of actuator module  34  of  FIG. 5 . The movable armature mass portion includes a tubular shell sleeve  36  surrounding a cylindrical core assembly with two magnets  40 A and  40 B, a first poleplate  42  between the magnets and a second poleplate  44  interfacing the bottom side of lower magnet  40 B. The two magnets  40 A and  40 B are arranged in mirror image relationship with regard to magnetic polarity as indicated: NS/SN, i.e. with two like poles interfacing on opposite sides of poleplate  42 . Alternatively they could arranged SN/NS. 
   Soft iron shell yoke plate  28 , attached onto the top end of magnet  40 A and the top end of the shell sleeve  36 , links the core assembly mechanically and magnetically to shell sleeve  36 , providing the magnetic flux path for magnetic flux concentrations in the two annular air gaps at the periphery of circular poleplates  42  and  44 . Centered about these two air gaps are two adjacent “voice” coils  38 A and  38 B each wound into bobbin  38  in corresponding coil winding compartments, each formed by reducing the outer diameter of bobbin  38  so as to leave only a thin inner wall in each compartment. When connected in aiding phase polarity and energized by alternating electric current at the desired frequency of vibration, coils  38 A and  38 B interact with the magnetic flux in the gaps, in accordance with the well known “right hand rule” to vibrate the entire magnetic armature portion in a vertical direction as enabled by flexing of the stacked flexure strips  20  and end support flexures  30  ( FIGS. 4 and 5 ). 
   In order to maximize the efficiency of magnetic circuit it is important to keep the gap between the poleplates  42 , 44  and shell sleeve  36  to a minimum, and to keep the magnetic core structure concentric with the bobbin  38  and to keep the coils  38 A and  38 B centered radially in their magnetically-charged air gaps during their intended reciprocating vertical movement. 
   Coils  40 A and  40 B are dimensioned to provide a required range of vertical travel of the moving armature assembly with a predetermined margin of reserve. The interconnection of coils  40 A and  40 B can be in series or parallel. It is essential for the two coils to be interconnected in proper phase polarity, depending on the direction of each winding, such that the coils co-operate in an additive mode. Furthermore load-sharing of input power and drive forces must be taken into consideration in the design of coils  40 A and  40 B. 
   The configuration shown in  FIG. 8  as a preferred embodiment using two coils  38 A and  38 B, and two magnets  40 A and  40 B, is considered optimal overall; however, the invention could be practiced utilizing the “voice coil” principle with only one magnet and one coil, i.e. with magnet  40 B and coil  38 B eliminated. Alternatively, the tandem arrangement of the two sections shown could be further extended to have three or more sections in which all of the magnets are oriented so as to place like magnetic poles interfacing on opposite sides of each intervening pole plate such as pole plate  42 . Also each coil must be properly connected for proper additive phase polarity as discussed above in connection with the two coils  38 A and  38 B, e.g. for three magnets, the polarity sequences could be NS/SN/NS or the reverse SN/NS/SN. 
     FIG. 9  is a horizontal cross-section of actuator module  34  taken through  9 - 9  of  FIG. 8  showing the concentric arrangement of shell sleeve  36  surrounding coil  38 B which in turn surrounds permanent magnet  40 B. 
   The invention derives benefits from utilizing the voice coil principal in a novel actuator configuration with high force output requirements at low frequencies by using multiple flexure modules along with further advantages from utilizing only a single actuator module, providing further advantages in cost effectiveness and reliability. 
   The flexure stack should be constructed such that the mounting point of the armature moves linearly up and down, parallel to the central axis. The flexure stacks should provide the proper compliance, i.e. spring rate, for the moving armature portion to resonate at a specific frequency, and the maximum operating deflection should be kept within the infinite spring life limit of the material used. For aeronautical use there also is a requirement to keep the total mass of the inertial actuator as low as possible. 
   In the case of lateral accelerations it is possible for the moving armature mass portion to translate in relation to the fixed stator mass portion including bobbin  38  and base  32 , due to the compliance of the vertical end support flexure plates  30 . In order to avoid excessive motion in unwanted directions, a set of guide pins and sleeves may be installed; they should be configured and arranged to not come in contact during normal operation. 
   As alternatives to the flexure configuration described above with two mounting supports, one at each end of the base, the invention could be practiced with three or more mounting support flexure plates arranged in a radial array, with suitable corresponding modification of the shape of the stacked flexure strips, optionally integrated with or adapted to supplant the mounting support flexure plates. 
   The invention may be embodied and practiced in other specific forms without departing from the spirit and essential characteristics thereof. The present 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 by the foregoing description; and all variations, substitutions and changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.