Magnetostrictive linear motor

The magnetostrictive rod of a linear motor is made of twinned single crys Terfenol-D having its crystalline axis aligned with the rod axis to produce non-uniform transverse magnetostriction during axial magnetostrictive elongation of the rod in response to a triggering magnetic field. Slide bearing forces applied through spring biased support plates are limited to flat surfaces formed on the rod perpendicular to a transverse axis along which the rod is magnetostrictively contracted.

BACKGROUND OF THE INVENTION 
This invention relates generally to motors or actuators through which 
electrical energy is converted into mechanical energy by magnetostriction. 
Linear motors based on electrostriction operating principles are well known 
and are similar in operation to magnetostrictive types of linear motors. 
In a magnetostrictive linear motor, an active element is movable in one 
direction of motion relative to a container associated with its stator. 
The movable element is tightly held in the container while the motor is 
deenergized. The stator also has already associated therewith an 
electromagnetically energized coil for generating a magnetic field that is 
oriented in the direction of motion imparted to the movable element. In 
continuous motion types of motors, having relatively small strokes, the 
movable element is maintained clamped to its stator at one axial location 
in the direction of motion. In motors producing discontinuous motion, the 
movable element is either clamped to the stator at two or more axial 
locations or the clamping location is changed during operation. 
The use of polycrystalline Terfenol-D as a most desirable magnetostriction 
material has already been proposed for the movable element of 
magnetostrictive linear motors. Such active element material is 
magnetostrictively elongated by a localized magnetic field swept along the 
direction of motion as the element shrinks from the inner wall surface of 
its stator support lube by contracting uniformly in directions transverse 
to its motion. The movable element clamped to the stator support tube in 
the static condition of such motors is under randomly applied radial 
stress in multi-axis directions. 
Various problems are inherent in the latter type of linear motor already 
known in the art. First, the polycrystalline Terfenol-D material must be 
ground into a near perfect circular cross-section in order to establish a 
uniform tight fit within the support tube. Second, any wear between the 
outer cylindrical surface of the movable element and the inner wall 
surface of the support tube renders the motor inoperative. Thirdly, motor 
performance is limited by its pre-stressed condition. 
It is therefore an important object of the present invention to provide a 
linear motor of the type having the desirable properties of a crystalline 
Terfenol-D magnetostrictive element, but avoids the aforementioned 
problems associated therewith and meets the high levels of performance 
necessary for high power micropositioners, injection valves, micrometering 
valves and other devices relying on accurate linear motion. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, twinned single Terfenol-D is 
selected as the magnetostrictive material for the active longitudinal 
element of a linear motor with its [112] crystalline axis substantially 
aligned with the element axis in the direction of motion to improve linear 
magnetostriction under an applied magnetic field of lowered energy. The 
properties of the selected magnetostrictive material are such as to render 
magnetostriction perpendicular to the motion axis non-uniform establishing 
a preferential axis of maximum contraction extending at right angles 
through flat surfaces on the movable element to which slide bearing 
clamping forces are restrictively applied for reduced wear. The slide 
bearing forces are applied through support plates of a clamping device to 
the flat surfaces of the movable element on opposing sides thereof while 
the other two opposing sides remain unencumbered. The bias of prestressed 
springs are exerted on the flat slide bearing surfaces to regulate 
continued application of the clamping forces as the movable element 
contracts transverse to the direction of elongation in response to a 
relatively low triggering magnetic field applied by means of an 
electromagnetically energized coil. An optimum bias field is maintained by 
means of such coil in the static condition of the motor. A significant 
reduction in heat and conservation of energy is thereby realized through 
the linear motor arrangement of the present invention in addition to 
meeting the high performance demands aforementioned.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Studies have established that the compound Tb.sub.x Dy.sub.1 -x Fe.sub.y, 
referred to as Terfenol-D, possesses an unusually large magnetostrictive 
strain. A large positive magnetostrictive strain or expansive deformation 
for Terfenol-D at room temperature has been found to be along the [112] 
crystalline axis of single crystal material. Single crystal Terfenol-D 
material may be formed into axially elongated rods by growth which occurs 
in the direction of its crystalline axis perpendicular to parallel 
dendrite sheets. The single crystals of Terfenol-D thus contain parallel 
sets of twin boundaries parallel to the [112] crystalline axis by reason 
of which the crystals are referred to as twinned single crystals. 
Large jumps in magnetostriction, referred to as "burst effects", have been 
observed in twinned single Terfenol-D crystals subjected to magnetic 
fields parallel to the crystalline axis. Because of such magnetostriction 
jumping phenomenon, only a moderate triggering magnetic field 
(superimposed on a static bias field) is required to transfer energy 
between magnetic and mechanical states. By appropriate magnetic heat 
treatment of the twinned single crystals, the magnetostrictive strain 
along the crystalline axis is effectively increased under relatively low 
applied compressive stress. 
In regard to magnetostrictive strain transverse to the crystalline axis 
during the aforementioned burst effects characterized by jumps in the 
axially orientated magnetostriction, almost simultaneous moment rotation 
occurs causing a jump in negative magnetostrictive deformation in the form 
of contraction of the Terfenol-D in a preferential transverse direction 
perpendicular to the crystalline axis while opposite expansion occurs in 
another transverse direction. In accordance with the present invention the 
direction in which the transverse negative magnetostriction occurs 
coincides with the direction in which clamping forces are exerted to 
resist motion of the magnetostrictive element parallel to interfacing 
contact planes thereon. 
FIGS. 1 and 2 illustrate by way of example a linear motor, generally 
referred to by reference number 10, constructed in accordance with an 
embodiment of the present invention having an axially elongated active rod 
element 12. The rod element is made of single crystal type Terfenol-D 
material having a chemical composition Tb.sub.03 Dy.sub.07 Fe.sub.109 and 
is in slide bearing contact with a stator assembly, generally referred to 
by reference number 14. The stator assembly includes an 
electromagnetically energized solenoid coil 16 operatively positioned 
about the rod 12 within a container cavity 18 formed in a stator housing 
20. 
The [112] crystalline axis of the twinned single crystal Terfenol-D rod 12 
is substantially coincident (within 2 degrees) with the longitudinal rod 
axis 22 while the magnetic field generating coil 16 is axially aligned 
therewith to magnetostrictively cause axial elongation or strain of the 
rod 12 along its axis 22 while it is under transverse compressive bearing 
stress. The magnetostrictive behavior of the rod 12 in the axial direction 
is accompanied by moment rotation about axis 22 causing magnetostriction 
transverse to axis 22. Such transverse magnetostriction varies between 
maximum negative magnetostriction along preferential axis 24 and maximum 
positive magnetostriction angularly spaced from axis 24 along expansion 
axis 26 as shown in FIG. 2. In accordance with the present invention, the 
accompanying compressive stress is established in the rod 12 by the 
clamping forces transmitted along axis 24 through slide bearing support 
plates 28 at the locations of maximum negative transverse 
magnetostriction. As shown in FIG. 2, such support plates 28 are flat or 
planar shaped and in sliding contact with planar surfaces 30 on opposing 
sides of the rod 12, which is rectangular in cross-section. The slide 
bearing type of clamping forces transmitted to surfaces 30 of the rod will 
be regulated by pre-stressed coil springs 32 mounted by the stator housing 
20 in engagement with the support plates 28 adjacent the opposite 
longitudinal ends thereof, for example, as shown in FIG. 1. 
The magnetostrictive jumping phenomenon or burst effect, hereinbefore 
referred to, occurs with respect to the single crystal Terfenol-D rod 12 
as evidenced by the magnetostrictive strain curves 34 and 36 in the graph 
of FIG. 3 when the magnetization field exceeds a relatively low trigger 
level of 100 Oe, for example, at a temperature of 20.degree. C. and while 
the rod is under compressive stress. Thus, the curves 34 and 36 
demonstrate that as the compressive stress increases from 7.6 to 18.9 MPa, 
a larger external field must be applied to produce expansion work against 
load. Curves 38 in FIG. 4, on the other hand, characterize the onset of 
the magnetostrictive state of the rod under conditions of a compressive 
stress of 7.6 MPa and magnetization fields between 250 and 2000 Oe. Such 
magnetostrictive states effectively extend beyond a peak value at 
0.degree. C. to 60.degree. C. as depicted by the curves in FIG. 4. FIG. 5 
graphically illustrates that because of the magnetization jumping or burst 
effect under the conditions indicated, only a moderate triggering value of 
the magnetic field superimposed on a static bias field, is required to 
transfer energy between magnetic and mechanical states. Thus, if an 
optimum bias magnetic field is introduced, the ratio of mechanical to 
magnetic energy reflected by curve 40 may be increased to that reflected 
by curve 42 at 20.degree. C., for example, as shown in FIG. 5. Large 
amounts of energy may accordingly be transferred from the internal 
magnetic state stored in the active rod element 12 to the external 
mechanical state by a small applied triggering magnetic field. 
With reference to the foregoing description, the motor 10 in its static 
condition has a magnetic bias field established through the coil 16 and 
maintained therein while the rod 12 is restrictively gripped or clamped on 
opposing sides thereof between the support plates 28 with the other 
opposing sides unencumbered as shown in FIG. 2. As a triggering magnetic 
field is applied, the rod 12 is magnetostrictively elongated in the axial 
direction of axis 22 as shown in FIG. 1 while maximum negative transverse 
contraction occurs along axis 24 as shown in FIG. 2. Such transverse 
contraction of the rod will accordingly reduce the slide bearing clamping 
forces exerted thereon through the spring biased support plates 28, 
regulated by the pre-stressed condition of the coil springs 32. Positive 
transverse magnetostriction along expansion axis 26 as shown in FIG. 2 
simultaneously occurs but does not affect the axial elongation of the rod 
since slide bearing contact therewith is limited to surfaces 30 
perpendicular to the axis 24 and parallel to axis 26. The initial static 
compressive stress established in the rod under the bias of springs 32 is 
accordingly reduced during motor operation to controllably prolong the 
burst effect until axial magnetostriction of the rod is completed to 
execute the desired motor stroke with accuracy. 
The selection of twinned single crystal type Terfenol-D as the 
magnetostrictive material for the active element of the motor was found to 
exhibit a 20% improvement over a polycrystalline type of Terfenol-D in its 
linear magnetostriction as well to enable lowering of the applied magnetic 
field to only 300 Oersteds for example. Also, the properties of the 
twinned signal crystal Terfenol-D accounts for the non-uniform nature of 
magnetostriction perpendicular to the direction of motion along the rod 
axis 22 and the availability of maximum negative magnetostriction along 
axis 24 perpendicular to the flat slide bearing surfaces 30 of the rod 
along which the rod shrinks from the spring biased support plates 28 to 
reduce wear and regulate continued application of slide bearing forces 
during motor operation. Since the opposing sides of the rod 12 
perpendicular to the axis 26 remain unencumbered as aforementioned, 
additional longitudinal prestressing means along along axis 22 including 
stress bolts (not shown), may be provided according to other contemplated 
embodiments for successful motor operation under certain external load 
conditions. 
Numerous other modifications and variations of the present invention are 
possible in light of the foregoing teachings. It is therefore to be 
understood that within the scope of the appended claims the invention may 
be practiced otherwise than as specifically described.