Abstract:
A materials testing device including a frictionless suspension that operates without sliding contact between an armature assembly and the suspension. The disclosure describes materials testing device that includes a moving magnet linear motor and a flexural suspension which is mounted using a thin layer of acrylic adhesive.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     The invention relates to materials testing systems, and more particularly to frictionless suspending in materials testing systems. 
     It is an important object of the invention to provide a materials testing system having an improved suspension. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the invention, a materials testing device for applying force to a test specimen includes a linear motor which includes an armature assembly which is mechanically coupleable to the test specimen, and a stator assembly. The materials testing device further includes a suspension for supporting the armature and for controlling the motion of the armature relative to the stator and to the test specimen, the suspension being arranged and constructed to operate without sliding contact between the armature assembly and the suspension. 
     In another aspect of the invention, a materials testing device includes a core of low reluctance magnetic material having two mutually opposing faces and an air gap separating the mutually opposing faces. A coil is wound around the core. A permanent magnet assembly is positioned in and substantially fills the air gap in noncontacting relationship with the core. The materials testing device further includes a frictionless flexural suspension structure for supporting the permanent magnet assemblPy and for controlling the direction of motion of the permanent magnet assembly. 
     In still another aspect of the invention, an electromechanical actuator assembly includes a structure of low reluctance material, a coil wound on the structure, the structure having substantially planar opposing faces. An air gap is between the opposing faces. A movable permanent magnet assembly having regions of opposite polarity is disposed in and substantially fills the air gap. The actuator assembly is characterized by three axes, a first of the axes perpendicular to the opposing faces, a second of the axes perpendicular to the first axis and between the regions of opposite polarity, a third of the axes perpendicular to the first axis and the second axis. A substantially frictionless suspension assembly supports the permanent magnet assembly. The suspension assembly has different stiffnesses along each of the three axes. The suspension is stiffest along the first axis. 
     Other features, objects, and advantages will become apparent from the following detailed description, which refers to the following drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of a materials testing system according to the invention; 
     FIG. 2 is an isometric view of a linear motor assembly according to the invention; 
     FIG. 3 is an isometric view of the core portion of the linear motor assembly of FIG. 2; 
     FIG. 4 is an isometric view of the core portion of FIG. 3 and a magnet assembly, positioned according to the invention; 
     FIG. 5 is a cross-sectional view of the core portion and magnet assembly of FIG. 4; 
     FIGS. 6 a  and  6   b  are isometric views of selected elements of the motor assembly of FIG. 2; and 
     FIG. 7 is a cross-sectional view of the interface of selected elements of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     With reference now to the drawings and more particularly to FIG. 1, there is shown a materials testing system according to the invention. Linear motor  10  includes a stator assembly  12  and an armature  14 . Frictionless flexural suspension element  16  supports and controls the motion of armature  14  without any sliding contact between moving elements and stationary elements. Armature  14  is mechanically coupled to test fixture  18 . In operation, armature  14  moves along an axis indicated by arrow  17  and applies a force to test specimen  18 , either inducing motion or mechanical stress, or both, in test specimen  18 . 
     Linear motor  10 , stator assembly  12 , armature  14 , and frictionless flexural suspension system  16  will be shown in more detail in subsequent figures and described in corresponding sections of the disclosure. The mechanical attachment between linear armature  14  and test specimen  18  may be conventional. The configuration of the test specimen is dependent on the specific materials test to be performed. Test specimen  18  may also be a test jig or fixture. 
     Referring now to FIG. 2, there is shown a linear motor  10  with a frictionless flexural suspension element  16  according to the invention. Stator assembly  12  includes a frame  11  to which core portion  13  is mechanically attached. Frame  11  serves as an element which provides convenient coupling of core portion  13  and other elements of linear motor  10 . Other embodiments of stator assembly  12  may not require frame  11 . Frictionless flexural suspension system  16  holds armature  14  in position relative to other linear motor elements and controls the motion of armature  14  and may exert a restorative force along the direction of motion of armature  14 . The direction of motion of armature  14  will be discussed below. 
     Referring to FIG. 3, there is shown core portion  13 . Core portion  13  is made of a low reluctance material such as iron, and may be shaped with an outer portion  18 ’. Protruding inwardly from opposite sides of outer portion  18 ’ may be two central core portions  20  and  22 , which terminate in opposing faces  24 ,  26  respectively, separated by air gap  28 . Coils  30 ,  32  are wrapped around central core portion  13 . 
     Referring to FIG. 4, disposed in air gap  28  is permanent magnet assembly  34  which forms a portion of armature  14 . Magnet sections  36  and  38  are discussed in the discussion of FIG.  5 . 
     Referring to FIG. 5, there is shown a cross section of the assembly of FIG. 3, taken along line  5 — 5  of FIG.  4 . Magnet assembly  34  is divided into two section  36 ,  38  with alternating poles, disposed in and substantially filling air gap  28 , and in noncontacting relation with faces  24 ,  26  of central core portions  20  and  22 . Stator assembly  12  of FIG. 1, coils  30 , and permanent magnet assembly  34  are elements of a permanent magnet transducer typically in accordance with U.S. Pat. No. 5,216,723, which also shows other topologies for core portion  13 . In addition to the topologies shown in U.S. Pat. No. 5,216,723, other types of linear motors, such as described in U.S. Pat. No. 5,661,446, or tubular moving magnet linear motors can be used. 
     Referring again to FIG. 4, when electrical current flows through coils  30 , permanent magnet assembly  34  moves in the direction of axis  40  (hereinafter the “motion axis”). When permanent magnet assembly  34  is centered between opposing faces  24  and  26 , the magnetic forces in the direction of axis  41  between permanent magnet assembly  34  and the opposing faces balance, keeping the magnet assembly centered. However, if the magnet assembly becomes uncentered, the magnetic forces between the magnet assembly and opposing faces becomes unbalanced, and the magnet assembly is urged toward one of the opposing faces  24 ,  26  of FIG. 3, causing magnet assembly  34  to contact, or “crash” into, one or both of opposing faces  24 ,  26  of FIG.  3 . This force, perpendicular to the intended motion of the permanent magnet assembly  34 , and toward one of the opposing faces  24 ,  26  is referred to a “crashing force.” The axis  41  perpendicular to the axis of motion and running between the opposing faces is referred to as the “crashing axis.” In addition to an uncentered situation, other causes of crashing behavior may include misalignment of the suspension holding the permanent magnet assembly  34  and lack of parallelism between permanent magnet assembly  34  and the opposing faces  24 ,  26 , and nonuniformity in the geometry of magnet assembly  34 . For increased efficiency, however, it is desirable for permanent magnet assembly  34  to substantially fill air gap  28 , and to remain in gap  28 . There is minimal force acting along axis  43 , which is perpendicular to axis of motion  40  and crashing axis  41 . The parameters, therefore, for suspension element  16  are different along the three axes  40 ,  41 , and  43 . Along axis of motion  40 , the suspension should allow motion but limit the range of motion along axis of motion  40  so that armature  14  remains in air gap  28 . Along crashing axis  41 , suspension element  16  should preferably be able to resist crashing forces. Along axis  43 , the suspension element should limit motion, but does not need to resist strong forces. 
     Referring to FIGS. 6 a  and  6   b,  there is shown the frame  11 , frictionless flexural suspension system  16 , and armature  14 . For clarity, core portion  13  is not shown. Frictionless flexural suspension system  16  includes two flexure components  46 ,  48 . Referring to FIG. 6 a,  armature  14  includes a carrier tray  44  for magnet sections  34 ,  36 . Carrier tray  44  is attachable to flexure component  48  (and to flexure component  46 , not shown). Flexure components  46 ,  48  may be formed of a band having a planar portion  50 , with the ends  52  of the band bent so that they are approximately perpendicular to the plane of the planar portion. The ends  52  may be attached to frame  11  by multiple rivets through rivet holes  66 . The rivets “sandwich” flexure components  46 ,  48  between pressure plate  15  of FIG. 6 b  and frame  11 . The flexure components  46 ,  48  are oriented such that the plane of the band is perpendicular to motion axis  40  of FIGS. 3 and 4 of armature  14  and such that the band extends in a direction parallel to the crashing axis  41  of FIGS. 3 and 4, as shown in FIG. 6 b.    
     Flexure components  46  and  48  control motion along all three axes  40 ,  41 ,  43  of FIG.  4 . Flexure components  46 ,  48  flex to allow motion along axis  40 . The flexing results in a restorative force along the axis of motion, to keep magnet sections  34 ,  36  in air gap  28 . Flexure components  46 ,  48  are very rigid along crashing axis  41  to overcome crashing forces and prevent crashing behavior. Flexure components  46 , 48  are sufficiently rigid along axis  43 , perpendicular to axis of motion  40  and crashing axis  41  to keep armature  14  in air gap  28 . 
     If one or more of the causes of crashing behavior noted above is present, the crashing force is present even if there is no current flowing through the coil. Even if the magnet assembly is perfectly symmetric and centered in the gap, this only represents a “metastable” equilibrium position, analogous to a ball resting on the top of a very sharp, steep hill. Any external disturbance would send the ball to one side or another. In this ball analogy, gravity is the forcing function. In the case of the linear motor, the magnetic field is the forcing function. To center the magnet assembly in the gap against crashing forces, the magnet assembly is attached to a suspension system according to the invention as one method of achieving this result. 
     One method of estimating the centering force necessary is to assume that the total external mmf drop of the magnet is in the air gap (that is the central core portions are infinitely permeable). For a magnetic perturbation of 5% (interpreted as a +2.5% field variance at the top surface from the mean value and a −2.5% variance from the mean at the bottom surface, a magnetic pressure can be calculated by P m =B 2 /2 μ o , where μ o  is 1.25×10 −6 . A nominal value of 1.0 T can be assigned to B. An approximate pressure is thus 4×10 5  N/m 2  (60 psi). The differential pressure would be about (4 B*ΔB)/2 μ o  or a resulting pressure of (4*0.025*1.)/2.5×10 −6 , or 0.4×10 5  N/m 2 . For the motor sizes on the order of sq. in. the (estimated) side force can be as high as 6 lbs. Tests on actual motors yielded crashing force measurements on the order of 0-5 lbs (0-22 N) for a geometrically centered magnet. By carefully displacing the magnets within the center of the air gap it was possible to determine that the magnetic-induced negative stiffness was on the order of 1,000 to 4,000 lbs/in, depending on the size of motor built (the bigger the motor the higher the crashing instability). 
     A suspension system that fulfills these requirements includes two flexure components  46 ,  48 , of stainless steel such as Sandvik 7C27Mo2, of dimensions shown in FIG. 6 b  and having a thickness of 0.33 mm (0.012 inches). Other materials, such as other metals or molded or themoformed composites may be suitable. 
     Referring now to FIG. 7, there is shown a cross section of the interface of one end of flexure component  46  and frame  11 . Flexure component  46  is “sandwiched” between pressure plate  15  and frame  11 . On the interfacing surfaces of frame  11  and flexure component  46  is a compliant layer  60  approximately 25-75μ thick of acrylic adhesive film having a modulus of elasticity in the range of 0.1 ms. Similarly, on the interfacing surfaces of flexure component  46  and pressure plate  15  is a similar compliant layer  60 ′. Rivet  42  holds the sandwich configuration in place, and applies force in the direction indicated by arrows  62 , normal to the interfacing surfaces and to the layer of acrylic adhesive, thereby preloading compliant layers  60  and  60 ′ and assisting in keeping the compliant layers in place. Rivet  42  is inserted in a manner such that it completely fills opening  64  in flexure component  46 , thereby preventing any motion in the direction indicated by arrow  66 . The other end of flexure component  46 , end both ends of flexure component  48  are both coupled to frame  11  in the manner shown in FIG.  7 . 
     Motion of the armature causes the flexure component  46  to deflect to positions such as indicated by  46 ′ and  46 ″. When flexure component  46  deflects toward frame  11  as in position  46 ′, flexure component  46  is urged toward frame  11 . Compliant layer  60  compresses so that it deforms from undeformed boundary  82  to deformed boundary  84 , thereby preventing contact between frame  11  and flexure component  46 . The preventing of contact between frame  11  and flexure component  46  greatly reduces the occurrence of fretting, which can be a significant source of wear and eventual failure of suspension systems that experience repetitive contact of elements made of dissimilar materials such as stainless steel flexure component  46  and frame  11 , which may be made of a metal such as aluminum or a molded plastic. 
     When flexure component  46  is deflected to position  46 ″, compliant layer  60 ′ behaves in a manner similar to the compliant layer  60  as described in the above paragraph. 
     The invention may be practiced using other types of suspensions that operate without any sliding contact, such as air bearings, liquid bearings, and magnetic bearings. In an exemplary embodiment, flexure bearings are used because they are simpler and less expensive than other types of nonsliding contact bearings. 
     Suspension systems employing air bearings typically have a separate pump and associated “plumbing,” fittings, and other components. Suspension systems employing fluid bearings are effective with fluid in the gap. Fluid bearings can remain indefinitely in the gap without the pumps or other components typically used with air bearings if the surface tension is sufficient to withstand the crashing force between magnet structure  34  and core faces  24 ,  26 . The fluid pressure differential relative to ambient pressure is 
     ΔP=γ/R, where 
     γ=fluid surface tension N/m, 
     R=fluid radius of the meniscus. (m). 
     The highest load from the magnet can be estimated to be 90 N (20 lbs) for the geometries presented, representative for the case in which the magnet is displaced off-center and rests against the pressure developed in the fluid layer. This results in a pressure, P˜90N/(9.6×10 −4 )=9×10 4  N/m 2 . For a typical gap of 0.001′ between the magnet structure and the core face, the required meniscus radius is one-half that clearance or 0.0005″ (1.3×10 −5  m). Using mercury (the fluid with exceptionally high surface tension) with a surface tension of 465 dyn/cm (0.465 N/m) results in a pressure differential, ΔP, of 0.465/(0.0005/39) , or 3.6×10 4  N/m 2 . This is very close to the pressure required to maintain separation of the magnet structure from the core faces. The stiffness of such a bearing against crashing loads is about                  k     d                 c       =                2          γ        (     w   *   h     )       /     g   2           ,                 =                2   *   0.465   *       (     3.2   ×     10     -   2       *   1.5   ×     10     -   2         )     /       (     1.3   ×     10     -   5         )     2           ,               =                2.6                 N        /          µ   .                                    
     Other lubricants (oils, films) have surface tensions less than one-tenth that of mercury, and the gap is preferably reduced by at least a factor of 10. The damping forces (drag) would increase as the gap gets smaller, since F drag =velocity*μA/g. For velocity˜1 m/s, and the case of oil, (μ f , fluid viscosity of 100 cP, or 0.1 N−s/m 2 , surface tension 32 dyn/cm or 0.03 N/m), the drag force is 1*(0.1)*(3.2×10 −2 *1.5×10 −2 )/2.5×10 −6 , resulting 15 N. The damping factor, would be 15 N−s/m. 
     So a suspension system employing a fluid film approach must have a small gap, but the size of the gap must be very precise, and the amount of precision required increases as the gap gets smaller. This represents a significant manufacturing challenge which makes materials testing system employing the invention cheaper and more reliable than materials testing systems employing fluid bearings. 
     It is evident that those skilled in the art may now make numerous modifications of and departures from the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed and limited only by the spirit and scope of the appended claims. Other embodiments are within the claims.