Adjustable shape memory metal actuator

A shape memory metal actuator includes an actuator element that is secured at one end to an end fitting which is in turn secured to an overload protection spring. An adjustable screw is mounted to a frame and is positioned such that the overload protection spring biases the end fitting into contact with the adjustment screw. The end fitting lifts off of the adjustment screw when excessive stretching forces are applied to the actuator element. The adjustment screw can be moved to vary the rest position of the end fitting, thereby allowing manual compensation for cyclic creep of the actuator element.

BACKGROUND OF THE INVENTION 
The present invention relates to an improved shape memory metal (SMM) 
actuator which can readily be adjusted to compensate for cyclic creep. 
SMM actuators typically utilize an actuator element made of a shape memory 
metal such as a nickeltitanium alloy. Such materials are characterized by 
a transition temperature at which they transform from a martensitic state 
to an austenitic state. During this transformation, the SMM actuator 
element can do work. It is well known that as SMM actuator elements are 
cycled in temperature the effective length of the SMM element gradually 
increases, both when in the austenitic state and when in the martensitic 
state. Such a gradual increase in length is typically referred to as 
cyclic creep. If not corrected, cyclic creep can adversely degrade the 
effective stroke of the SMM actuator. In addition, it is widely recognized 
that SMM actuator elements should be protected from excessive stresses if 
the life of the actuator element is not to be unduly shortened. U.S. Pat. 
No. 4,490,975 addresses problems related to both cyclic creep and overload 
protection for SMM actuator elements. 
SUMMARY OF THE INVENTION 
This invention is directed to an improved SMM actuator which allows manual 
adjustment for cyclic creep in a remarkably simple and straightforward 
manner. 
According to this invention, a shape metal memory actuator is provided 
which comprises a frame and a shape metal memory metal actuator element 
having first and second ends and defining a direction of motion. Means are 
provided for connecting the first end of the actuator element to an 
actuator terminal which is movable with respect to the frame. An 
adjustment member is adjustably positioned with respect to the frame along 
the direction in motion and defines a stop surface. An overload protection 
spring is mounted between the frame and the second end of the actuator 
element and is oriented to bias the second end to a rest position 
determined by the stop surface. The overload protection spring biases the 
second end with a selected force which allows the second end to pull away 
from the stop surface when excessive forces are applied to the actuator 
terminal, thereby protecting the actuator element from excessive forces. 
The adjustment member and the overload protection spring cooperate to 
permit adjustment of the rest position of the second end of the actuator 
element, thereby allowing manual compensation for cyclic creep. 
The preferred embodiment described below is a simple, inexpensive actuator 
which provides both overload protection to the shape memory metal element 
as well as provision for manual adjustment to compensate for cyclic creep. 
Without such compensation for cyclic creep, the life of a shape memory 
metal actuator is often limited by the gradual reduction in stroke caused 
by cyclic creep. The preferred embodiment described below, by allowing 
manual compensation for cyclic creep, allows the stroke of the actuator to 
be maintained even in the face of cyclic creep, thereby extending the life 
of the actuator. 
The invention itself, together with further objects and attendant 
advantages, will best be understood by reference to the following detailed 
description, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT 
Turning now to the drawings, reference numeral 10 refers to the presently 
preferred embodiment of the actuator of this invention. For purposes of 
illustration, the actuator 10 is shown connected to a louvered lamp. 
As shown in FIGS. 1-3, the actuator 10 is mounted within a lamp housing 12 
which defines a lens opening 14. Several spaced louvers 16 are mounted to 
extend across the lens opening 14, and the louvers 16 are linked together 
by a cross bar 18. The cross bar 18 is in turn linked to a bell crank 20. 
The housing 20, louvers 16 and linkage 18,20 of this preferred embodiment 
are identical to those described in greater detail in a copending U.S. 
patent application Ser. No. 703,895, filed Feb. 21, 1985, entitled 
Automatic Takeup and Overload Protection Device for Shape Memory Metal 
Actuator and assigned to the assignee of this invention. This application 
is hereby incorporated by reference for its detailed description of the 
louver linkage. 
The actuator 10 includes an actuator element 30 which in this embodiment is 
a straight length of an SMM wire such as Nitinol. This actuator element 30 
is connected at one end to a first end fitting 32 which defines an eye 36 
shaped to mount to the bell crank 20. The first end fitting 32 serves to 
mount an electrical lead 34 as well as a rigid metal tube 38 which is 
concentric with the actuator element 30. A return spring 40 is mounted 
around the exterior of the tube 38 to bias the tube 38 to the right as 
shown in FIG. 2, thereby biasing the louvers 16 to the closed position. 
The above-identified U.S. Patent application should be referenced for 
further details of the structure and operation of the actuator element 30 
and the return spring 40. 
As shown in FIGS. 4-7, the other end of the actuator element 30 is secured 
to a second end fitting 42 which defines a relatively narrow shank 44 and 
a relatively broader head 46. The head 46 defines two V-shaped grooves 48. 
The actuator element 30 is secured to the first and second end fittings 
32,42 by respective clamps 50. In this embodiment, the clamps 50 each 
define central openings 52 and are formed of a material such diecast zinc. 
The clamps 50 are crimped in place as shown in FIG. 8 in order to hold the 
first and second end fittings 32, 42 to respective ends of the actuator 
element 30. 
An overload protection spring 60 which in this embodiment is a leaf spring 
60 defines a fixed end 62 fixedly mounted to the housing 12. An electrical 
lead 64 is secured to this fixed end 62. The spring 60 also defines a free 
end 66 which defines a slot 68 sized to receive the shank 44. When 
assembled as shown in FIG. 4, the shank 44 is received in the slot 68 and 
the free end 66 is received in the groove 48 to engage the second end 
fitting 42 and therefore the actuator element 30 to the free end 66 of the 
overload protection spring 60. 
An adjustment screw 80 is threadedly mounted to the housing 12. This 
adjustment screw 80 defines a slotted head 84, which is accessible from 
the exterior of the housing 12, and a stop surface 82. The adjustment 
screw 80 is positioned such that the overload protection spring 60 biases 
the head 46 of the second end fitting 42 into contact with the stop 
surface 82. Thus, the stop surface 82 defines the rest position of the 
second end fitting 42. 
FIG. 6 shows an enlarged view of one of the ends of the actuator element 30 
prior to attachment to the clamp 50. Preferably, the two ends of the 
actuator 30 are identical. As shown in this figure, the actuator element 
30 defines a cold formed region 90 next to an adjacent region 92 of the 
actuator element 30. The extreme end 94 of the actuator element 30 is 
undeformed and retains the original shape. In this preferred embodiment, 
the undeformed shape of the actuator element 30 is a wire having a 
diameter of 0.5 mm. Thus, both the extreme end region 94 and the adjacent 
region 92 have a diameter of 0.5 mm. The cold formed region 90 has a 
thickness of 0.2 mm. The junction between the cold formed region 90 and 
the adjacent region 92 defines a transition zone 96 which is smoothly 
radiused with a radius of curvature of 0.2 mm. In this embodiment, the 
clamp 50 is 3 mm in length, the cold formed region 90 is 5.5 mm in length, 
and therefore approximately 2.5 mm of the cold formed region 90 extends 
out of one end of the clamp 50 (FIG. 8). For this reason, the transition 
zone 96 is spaced from the clamp 50 by 2.5 mm, about five times the 
diameter of the adjacent region 92. In this embodiment, the cold formed 
region 90 is formed in a single pressing operation with a hydraulic press. 
However, other cold forming methods are believed to be suitable as well. 
SMM is transformed into the much harder and stronger austenitic state when 
it is severely cold worked. In particular, if an SMM wire is deformed to 
about 40% of its original diameter, the flattened portion is extremely 
strong and hard. In fact, in the presently preferred embodiment tungsten 
carbide anvils are used to cold work the SMM wire. When Nitinol is cold 
worked, high speed tool steel is indented by the SMM wire. 
Since the cold formed region 90 is fully austenitic throughout the 
temperature cycling of the actuator element 30, it is of no consequence 
that the cold formed region 90 remains cool due to the heat sink 
characteristics of the end fitting 42. Because the cold formed region 90 
remains in the austenitic state it retains sufficient strength and does 
not tend to elongate and break. Because the transition zone 96 is spaced 
from the clamp 50 by about five times the diameter of the actuator element 
30, the transition zone 96 is heated above the transition temperature of 
the SMM wire during temperature cycling of the actuator element 30, 
thereby reducing the tendency of the actuator element 30 to fracture at 
the transition zone 96. 
The transition zone 96 is provided with a gentle radius between the cold 
formed region 90 and the adjacent region 92 in order to minimize any 
increase in the average stress. The extreme end 94 is allowed to remain in 
the original round form so as to form an anchor point that will resist any 
tendency of the actuator element 30 to slip out of the clamp 50. 
The overload protection spring 60 cooperates with the adjustment screw 80 
to protect the actuator element 30 from excessive stresses and to provide 
ready adjustment of the rest position of the second end fitting 42. In 
this way, overall elongation of the actuator element 30 due to cyclic 
creep can be compensated for. In this preferred embodiment, the overload 
spring 60 biases the second fitting 42 against the stop surface 82 with an 
installed spring force of about 700 grams and a spring rate of about 180 
grams/mm. In the event applied forces on the actuator element 30 exceed 
700 grams, the overload spring 60 flexes, allowing the second end fitting 
42 to move away from the adjustment screw 80. In this way, the actuator 
element 30 is protected from excessive stretching forces. 
In effect, the adjustable screw 80 defines an adjustable rest position for 
the second end fitting 42 of the actuator element 30. When the length of 
the actuator element 30 increases during use due to cyclic creep, the 
adjustment screw 80 can be moved to take up the increased length of the 
actuator element 30. In this way, the effective stroke of the actuator 
element 30 is preserved, even in the face of cyclic creep. 
Of course, it should be understood that a wide range of changes and 
modifications can be made to the preferred embodiment described above. For 
example, the actuator 10 can be used in a wide variety of settings in 
addition to the louvered lamp shown. Furthermore, the adjustment member 
and overload protection spring can be adapted for use in a wide variety of 
actuators, and other methods for attaching fittings to the end of the 
actuator element (e.g. adhesives such as epoxy or low temperature solder) 
can be used. In addition, it is not critical that the cold formed end 
portion of the actuator element be used in all embodiments. It is 
therefore intended that the foregoing detailed description be regarded as 
illustrative rather than limiting, and that it be understood that it is 
the following claims, including all equivalents, which are intended to 
define the scope of this invention.