U.S. patent number 3,652,969 [Application Number 04/828,243] was granted by the patent office on 1972-03-28 for method and apparatus for stabilizing and employing temperature sensitive materials exhibiting martensitic transitions.
This patent grant is currently assigned to Robertshaw Controls Company. Invention is credited to Donald W. Carey, James R. Willson.
United States Patent |
3,652,969 |
Willson , et al. |
March 28, 1972 |
METHOD AND APPARATUS FOR STABILIZING AND EMPLOYING TEMPERATURE
SENSITIVE MATERIALS EXHIBITING MARTENSITIC TRANSITIONS
Abstract
A method and apparatus for stabilizing and employing temperature
sensitive material exhibiting martensitic transitions for use in
control and work performing devices. The method includes subjecting
the martensitic-transition material to a greater unit stress than
the material would be required to work against in its application
to thereby stretch the material beyond its expected deflection, and
subsequently completing a number of temperature cycles while the
material is in such overstressed condition, through which it is
heated to a point above its transition temperature and cooled back
to its annealed temperature. After treatment the material operates
through complete work cycles with no loss of dimension stability.
In one embodiment, the apparatus defines a temperature sensitive
switch including a load to apply an increased stress to the
material during the initial stabilization temperature-cycling
period and a reduced work-load stress during periods of in-service
operation.
Inventors: |
Willson; James R. (Garden
Grove, CA), Carey; Donald W. (Anaheim, CA) |
Assignee: |
Robertshaw Controls Company
(Richmond, VA)
|
Family
ID: |
25251248 |
Appl.
No.: |
04/828,243 |
Filed: |
May 27, 1969 |
Current U.S.
Class: |
337/140; 148/563;
337/393 |
Current CPC
Class: |
C22F
1/006 (20130101); F03G 7/06 (20130101); H01H
61/0107 (20130101); H01H 37/323 (20130101) |
Current International
Class: |
C22F
1/00 (20060101); F03G 7/06 (20060101); H01H
37/00 (20060101); H01H 61/01 (20060101); H01H
37/32 (20060101); H01H 61/00 (20060101); C21d ();
H01h 037/50 (); H01h 071/18 () |
Field of
Search: |
;337/123,140,382,393
;148/11.5,131 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gilheany; Bernard A.
Assistant Examiner: Morgan; Dewitt M.
Claims
What is claimed is:
1. An accurate temperature responsive control device
comprising:
means for sensing temperature;
moveable control means coupled to said temperature sensing means
for performing a control function in response to actuation by said
temperature sensing means; and
means coupled to said temperature sensing means for applying a load
thereto having a particular value;
said temperature sensing means being a temperature sensitive
material exhibiting a martensitic transition at a critical
temperature, which has been deformed by a force greater than the
particular value of said load means and subsequently
temperature-cycled through said critical temperature in a positive
and then a negative direction repetitively in succession to
complete a plurality of complete cycles, heating of said material
above its critical temperature thereafter moving said temperature
sensitive means from its rest position resulting in movement of
said control means to effectuate said control function, and cooling
of said material below its critical temperature thereafter allowing
said temperature sensitive means to return precisely to its rest
position under the influence of said load means.
2. The invention as recited in claim 1 wherein said plurality of
complete cycles is at least three.
3. The invention as recited in claim 2 wherein said temperature
sensitive material comprises an alloy comprising 53.5-56.5 percent
nickel by weight, the remainder being essentially titanium.
4. The invention as recited in claim 2 wherein said temperature
sensitive material comprises an alloy comprising 55 percent nickel
by weight, the remainder being essentially titanium.
5. The invention as recited in claim 4, wherein said control
function comprises electrical switching.
6. An accurate temperature responsive control device,
comprising:
means for sensing temperature comprising a temperature sensitive
material exhibiting a martensitic transition at a critical
temperature;
moveable control means coupled to said temperature sensing means
for performing a control function in response to actuation by said
temperature sensing means;
means coupled to said temperature sensing means for applying a load
thereto;
stabilizer means coupled to said temperature sensing means and said
load means for selectively applying an additional stabilization
load to said temperature sensing means; and
means coupled to said temperature sensing means for
temperature-cycling said temperature sensitive material through
said critical temperature in a positive and then a negative
direction repetitively in succession to complete a plurality of
complete cycles after said additional stabilization load has been
applied thereto by said stabilizer means to thereby temperature
stabilize said material for subsequent operation without said
additional stabilization load;
heating of said material above its critical temperature thereafter
moving said temperature sensing means from its rest position
resulting in movement of said control means to effectuate said
control function, and cooling of said material below its critical
temperature thereafter allowing said temperature sensing means to
return precisely to its rest position under the influence of said
load means.
7. The invention as recited in claim 6, wherein said plurality of
complete cycles is at least three.
8. The invention as recited in claim 7 wherein said temperature
sensitive material comprises an alloy comprising 53.5-56.5 percent
nickel by weight, the remainder being essentially titanium.
9. The invention as recited in claim 7 wherein said temperature
sensitive material comprises an alloy comprising 55 percent nickel
by weight, the remainder being essentially titanium.
10. The invention as recited in claim 9 wherein said control
function comprises electrical switching.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to the stabilization of
temperature sensitive materials and, more particularly, to a method
and apparatus for stabilizing temperature sensitive materials
exhibiting martensitic transitions for use in control and work
performing devices.
Many diversified applications in the systems control art, to
mention but one, require a simple, yet efficient heat sensitive
element for converting thermal energy into mechanical energy. One
of the most obvious applications for such an element is the
conventional thermostat used extensively in the control of home and
office heating and cooling systems as well as a number of small
home appliances. Heretofore, what was considered to be the most
effective element for the direct conversion of heat into mechanical
energy was the bimetallic couple wherein two metals having
dissimilar degrees of thermal expansion are bonded together. While
such devices have generally served the purpose, they have not
proven entirely satisfactory under all conditions of operation.
Some of the more obvious reasons for these limitations are the
limited mechanical deflection per degree temperature change, the
inefficient thermo-mechanical energy conversion, and the difficulty
of manufacture and standardization.
With recent developments in metallurgy, specifically in the study
of thermally sensitive materials which exhibit martensitic
transitions, research efforts have been directed toward seeking a
better thermo-mechanical conversion element. At this point, while a
detailed theoretical explanation of martensitic transition type
materials is unnecessary for the purpose of disclosing the present
invention, a brief discussion thereof will be described for the
sake of clarity. Certain nickel-titanium alloys, for example,
containing approximately 53.5-56.5 percent nickel with the
remainder being essentially titanium, have been found to undergo a
temperature dependent martensitic transition at a particular
critical temperature, this temperature being a function of the
alloy composition. This transition is produced by applying a load
to the material which is sufficiently great to produce a greater
deflection below its critical temperature than would normally be
expected. The structural deformation this produced causes a
molecular change which is accompanied by the liberation of heat.
Graphically it has been found that such a structural transition
follows a curve of decreasing modulus of elasticity as well as a
curve of decreasing modulus of torsion as the temperature
decreases. If the material under stress is now heated to a point
above its critical temperature, it will move in a direction
opposite to the direction in which it has been deformed with the
capability to perform useful work. It is important to note,
however, that the curves of increasing modulus of elasticity and
torsion with increasing temperature are different than the curves
observed during the decreasing temperature transition; and, more
importantly, the cyclic transition produces certain changes in the
physical properties of the material which cause it to take a set
after each cycle preventing it from returning precisely to its
original position. This periodically increasing offset has,
heretofore, proven to be a major inhibiting factor in the
development of an acceptable commercial device using a
thermo-mechanical element of the type discussed above in the place
of the conventional bimetallic element.
OBJECTS OF THE INVENTION
It is, therefore, an object of the present invention to provide a
method and apparatus for stabilizing temperature sensitive
materials exhibiting martensitic transitions for use in control
devices.
Another object of this invention is to provide a stabilized
thermo-mechanical conversion element having a closed
temperature-deflection loop.
This invention has the further object in the provision of a control
device having improved operational characteristics.
An additional object of the present invention is the provision of a
work performing device having stabilization means integrally
incorporated therein.
An advantage of the invention is the provision of a simple and
efficient temperature sensing control device.
An additional advantage of the present invention is the provision
of a stabilizing process and apparatus permitting the use of
materials heretofore impractical for performing control and work
functions where closed-loop temperature-deflection cycles are
contemplated.
SUMMARY OF THE INVENTION
The present invention is summarized in that a method for
stabilizing a temperature sensitive material exhibiting a
martensitic transition at a critical temperature to perform work
upon a load having a particular value includes the steps of
applying a force to the material having a value greater than the
particular value of the load, and temperature-cycling the material
through the critical temperature in a positive and then a negative
direction repetitively in succession to complete a plurality of
complete cycles.
In addition, the present invention includes apparatus for
stabilizing and utilizing such a temperature sensitive material as
a control or work performing device.
The inventive concept as well as other objects and advantages of
the present invention will become more fully apparent from the
following detailed description of the preferred embodiments of the
invention when considered in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a stabilizing apparatus for stabilizing a temperature
sensitive material for subsequent use in a control device;
FIG. 2 shows a temperature sensing control device utilizing a
pre-stabilized thermal sensor;
FIG. 3 shows a temperature sensing control device including an
integral stabilizing means;
FIG. 4 shows a stabilization temperature-elongation curve for the
material to be used in the apparatus of FIGS. 1, 2 and 3; and
FIG. 5 shows the closed-loop temperature-elongation curves produced
after stabilization by the method and apparatus of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is shown a simplified apparatus
operating according to the method of the present invention to
pre-stabilize a thermally sensitive material exhibiting a
martensitic transition at a critical temperature for subsequent
cyclic use in control devices. The structure consists of a
cantilevered beam 10 mounted to wall 12 so as to support a load 14
for vertical movement, as shown by the arrow. The load is hung from
the beam 10 by a single drawn piece of martensitic transition type
wire 16 firmly attached by connectors 18. A radiant heat source 20,
the temperature of which can be manually adjusted, is located
adjacent the wire 16 within heating proximity thereto.
Before going into the details of operation of the device, it is
important to note first that other suitable heat sources may be
employed such as direct internal heating by a current flowing
therethrough, or the like; and second, the thermal sensor 16 may be
shaped and mounted in any number of various ways, for example, as a
cantilevered beam or a coiled spring, depending upon the
operational characteristics desired and the contemplated
application of the heat sensing material.
In describing the operation of the device of FIG. 1, reference will
be made to the curves illustrated in FIGS. 4 and 5. It is further
pointed out that the description, below, of the operation of the
device of FIG. 1 will serve also to outline the method of the
present invention.
FIG. 4 shows a curve illustrating the elongation characteristics of
a nickel-titanium wire cycled through its critical temperature a
number of times. The apparatus of FIG. 1 can be utilized to produce
the above-mentioned curves, and, in one experiment, a load of
40,000 pounds per square inch was employed. At temperature T, the
load applied to the nickel-titanium wire produces an elongation,
measured vertically, of value E.sub.1. As the temperature produced
by source 20 is decreased, the material follows segment A.sub.1 of
the curve which illustrates the rapid increase of elongation
produced by the load when the material passes through its critical
temperature. As the temperature is then increased through the
critical temperature in a positive going direction, the material
follows segment A.sub.2 which shows how the alloy tends to return
to its original position. This characteristic shape-memory action
exhibited by materials such as nickel-titanium is primarily due to
the aforementioned martensitic transitions which take place at the
critical temperature. As explained above, due to certain molecular
changes which take place in the structure of the material when
temperature-cycled under load, the material does not return
precisely to its original elongation E.sub.1 but decreases only to
point E.sub.2. As the material is temperature-cycled again through
its critical temperature, segments B.sub.1 and B.sub.2 of the curve
are followed showing a further offset since elongation point
E.sub.3 is the shortest length the material will then reach.
Additional cycle C.sub.1 -C.sub.2 produces similar results, as
expected.
Thus, with a load of 40,000 pounds per square inch, the
nickel-titanium alloy wire when utilized in the apparatus of FIG. 1
will exhibit martensitic transitions during temperature-cycling
through its critical temperature but at the end of each cycle will
not return precisely to its starting point. If, according to the
present invention, the wire, which was temperature-stabilized at
40,000 pounds per square inch, is temperature-cycled with a reduced
load of 20,000 pounds per square inch, for example, the curves
illustrated in FIG. 5 will be produced. As can be seen, the curves
form closed-loops since the elongation of the wire at the end of
each cycle is precisely the same as at the beginning thereof. Thus,
by temperature-cycling a temperature sensitive alloy of the type
referred to above at a load greater than the load to be utilized in
the contemplated control device, the material becomes cyclically
stabilized and exhibits closed-loop operation required in most heat
sensing electrical and mechanical control units.
Referring now to FIG. 2, wherein similar numerals are used to refer
to similar components utilized in FIG. 1, there is illustrated an
electrical single-pole double-throw temperature sensitive switch
22. The device employs a temperature sensitive element 16 which has
been previously temperature stabilized by cycling at an increased
load in apparatus of the type shown in FIG. 1. The mid-point of the
wire is coupled to the moveable bar 24, which forms the switchable
contact of the electrical switch. The bar 24 is in turn connected
to a spring load 26 which is less than the load 14 utilized during
the pre-stabilizing temperature-cycling process performed by the
apparatus of FIG. 1. This assures accurate closed-loop operation as
explained with reference to FIG. 5. The two fixed contacts 28 and
30 of the switch 22 are shown affixed to a base or frame member
32.
One typical application of the apparatus shown in FIG. 2 is a
conventional thermostat for a home or office heating system. In
this application, radiant heat source 20 schematically illustrates
the radiant ambient heat produced by the room or area in which the
thermostat is mounted and for which the thermostat is designed to
monitor. As the temperature of the room decreases below the
critical temperature of the wire sensor 16, thereby indicating a
need for heat, the wire is allowed to be stretched by load spring
26 which then moves bar 24 away from contact 28 toward contact 30
completing an electrical current path from contact 30 to the
contact on bar 24 to thereby initiate operation of the heating unit
used (not shown). Furthermore, as the temperature of the room
subsequently increases, the temperature sensing wire 16 returns to
its initial position, due to its inherent shape-memory, against the
force produced by spring 26 to thereby move bar 24 away from
contact 30 back to its original position in physical contact with
contact point 28. As the temperature of the room fluctuates, the
device will continue to cycle indefinitely in the same manner, the
wire sensing material 16 remaining in its stabilized condition
having once been pre-stabilized according to the principles of the
present invention.
In FIG. 3, there is shown a more refined switching apparatus
combining the desired operational characteristics of the alloy
under present discussion with the stabilization process of the
present invention. With this apparatus, an unstabilized
nickel-titanium wire, or the like, can be immediately installed in
place as element 16 without requiring pre-cycling in a separate
unit. The switch shown in FIG. 3 is basically similar to the unit
in FIG. 2 with the exception of a threadably engageable load 34
attached to the lower end of spring 26. In operation an
unstabilized sensor wire 16 is placed in the switch as shown and
the load 34 is threadably removed from its mounting bore 36 in
frame 32, as illustrated. In addition, since the additional load
will cause a greater than normal elongation, contact 30 is bent
slightly to the dotted position shown in FIG. 3 during this
stabilization period. Since load 34 is now applying an additional
force to the wire 16 over that applied by spring 26 above, the
device is ready for pre-cycling to stabilize the wire sensor for
subsequent closed-loop operation without the additional load 34. To
accomplish the pre-cycling heating of element 16, a controllable
electrical power source 38 is shown coupled to the wire whereupon
heat will be internally generated therein at the desired times. The
unit is then temperature-cycled through a number of complete
cycles, as typified by the curves of FIG. 4. After this is
completed, load 34 is threadably mounted to frame 32 and the
contact end of contact 30 is bent back to its operative position,
whereupon a decreased force will be applied to the element 16 which
then will provide closed-loop operation as exemplified by the
curves of FIG. 5. It is noted that radiant heat source 20 and
electrical power supply 38 are both provided to apply heat to the
temperature sensitive nickel-titanium element 16; however, other
diverse heat sources can be utilized depending upon the particular
installation.
In summary, there is shown and described a method and apparatus for
stabilizing and employing temperature sensitive materials
exhibiting martensitic-transitions at various critical
temperatures, such as nickel-titanium, or the like, for use in
control and work performing devices where accurate
temperature-elongation closed-loop operation is required. Thus,
control devices utilizing nickel-titanium, for example, as the
temperature sensing element having new, improved, and desired
characteristics are made feasible for many diverse and commercially
important applications.
Inasmuch as the present invention is subject to many variations,
modifications and changes in detail, it is intended that all matter
contained in the foregoing description or shown in the accompanying
drawing shall be interpreted as illustrative and not in a limiting
sense.
* * * * *