Temperature compensated positioning device

The temperature compensated positioning device consists of a stationary member, a movable member, a compensating material, and support material between each member. Each member is attached to two surfaces which must be held in a precise dimensional relationship with respect to each other automatically over a wide temperature range.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to apparatus for maintaining a desired dimensional 
relationship between two surfaces when that apparatus is exposed to an 
environment consisting of wide variations in temperature. 
2. Description of Prior Art 
The ability to achieve and maintain a desired dimensional relationship 
between two surfaces has previously been achieved by electro-mechanical 
means, by thermal stabilization, and by temperature compensating 
structures. 
The electro-mechanical methods require the use of temperature sensing 
devices, associated sensing and drive circuits and motors to move the 
position of one surface as a temperature change is sensed. The drawbacks 
involved with these methods are the cost, complexity, size, and weight 
penalties imposed by the motors, control circuitry, and associated 
structural mounts involved. 
The thermal stabilization methods require that provisions such as thermal 
blankets and heaters be incorporated to essentially maintain an isothermal 
environment. In most cases where even a minimal temperature range is to be 
encountered, the power required and other problems with maintaining the 
ability to operate at high temperatures (limiting allowable insulation) 
make this approach infeasible. 
The use of temperature compensating structures has permitted a limited 
amount of temperature compensation movement to be achieved in specific 
designs by varying the structural materials used in the housing containing 
the surfaces desired to be separated. Strength requirements of the housing 
structure involved normally limit the material selection to a narrow range 
of strong materials (e.g., stainless steel, invar, or aluminum) and each 
application must be highly individualized. In addition to the cost 
involved in each specialized housing design, the limited range of 
temperature related movement achievable normally results in less than 
optimum results. 
Accordingly, it is an object of the present invention to provide a 
temperature compensating positioning device that has a combination of 
characteristics which will enable it to overcome the aforementioned 
disadvantages of the prior art devices and which make it particularly well 
suited to the requirements of avionic equipment. Such avionic equipment 
requirements which this novel invention can meet include small size, and 
weight, operability over a wide temperature range, and high reliability. 
SUMMARY OF THE INVENTION 
The foregoing objects are achieved according to this invention through the 
utilization of a movable member positioned within a stationary member in a 
slip fit fashion. A temperature compensating material is placed along the 
axial length of the members and is firmly affixed at its axial extremeties 
to a protruding lip of each of the members. 
A supporting means is located between each of the members so that the 
movable member, can be approximately centered and supported within the 
stationary member. Such supporting means also facilitate the axial 
movement of the movable member as the compensating material expands or 
contracts as the temperature increases or decreases.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A cutaway perspective view of the temperature compensating positioning 
device is shown in FIG. 1. Movable member 2 has a major portion of its 
axial length contained within the axial length of stationary member 4. 
Movable member 2 and stationary member 4 are securely attached to the two 
surfaces respectively, whose dimensional relationship is to be controlled 
over temperature. The members are assembled together in a slip fit fashion 
with a support means 6, such as O rings, used to provide radial support 
and axial centering. It should be mentioned that while the cross section 
of members 2 and 4 and material 8 are shown to be circular in FIG. 1, 
their cross section could be of any geometrical shape. The invention is 
not intended to be limited to circular cross sections for those elements. 
The axial ends of temperature compensating material 8 are firmly attached 
to the protruding lips of each movable and stationary member. In this 
manner the compensating material 8 establishes the predetermined axial 
dimensional relationship at room temperature between the two surfaces to 
be controlled. 
As the temperature environment changes the axial length dimension of 
compensating material 8 will change as a function of its length and 
coefficient of thermal expansion. The change in the length dimension can 
be predetermined by the following relationship: 
EQU 1c=a"1 (1) 
where, 
1c=temperature induced movement (inches/.degree. C.) 
a"=coefficient of thermal expansion (inches/inch/.degree. C.) 
1=length of compensator material (inches) 
Therefore, by changing the nominal length and the composition of the 
material used for compensating material 8, a wide range of predictable 
dimensional change characteristics over a temperature range can be 
achieved with this same basic device. 
The instant invention has been conceived so that the compensating material 
8 is not required to carry any structural loads. This is accomplished by 
locating the compensating material contiguous to the axial length of 
stationary member 4. Such a structure allows the designer great latitude 
in the selection of a wide range of materials that would be suitable e.g., 
metals and plastics. In the event that a desired material does not have 
sufficient strength characteristics to allow the end attachment of its 
ends to the movable member, a device such as a spring could be used to 
apply an axial force to the movable member to insure that both members 
remain in close contact with the compensating material. 
The invention may be more fully appreciated with the delineation of its 
operation in a practical application. One such application is illustrated 
in FIG. 2 where the instant invention is utilized in an imaging system. In 
such a system, the instant invention can provide a simple, inexpensive, 
lightweight, and compact method of maintaining focus of a lens on a fixed 
image plane throughout a wide temperature range. 
In the configuration shown in FIG. 2 stationary member 4 is securely 
fastened to a first surface, in this case an imaging plane surface 10. The 
movable member is securely fastened to a second surface, in this case 
optical lens surface 12. At the normal operating temperature of the system 
(e.g., room temperature) the desired image will be in focus at imaging 
plane surface 10. Changes in temperature will affect the optical equipment 
situated to the right of lens surface 12 in FIG. 2 in such a fashion as to 
cause the focal length of the lens located at lens surface 12 to change 
(as shown by the dashed lines). This will create an out of focus condition 
at imaging plane surface 10. For a particular lens system one can 
predetermine the amount of change in the focal length for an anticipated 
change in temperature. 
For example, it may be determined that the distance between lens surface 12 
and imaging plane surface 10 will vary by 0.00052 inches per degree 
centrigrade from a specified nominal room temperature value over an 
anticipated temperature exposure range. The length of a suitable 
compensating material can be determined by using the relationship 
expressed in equation 1. 
By selecting a variety of coefficients of thermal expansion from a text 
such as Marks' Mechanical Engineer's Handbook, McGraw Hill Publishing 
Company, one can determine what type of compensating material to utilize. 
For this example, a high density polyethylene with a coefficient of 
thermal expansion of 0.000198 inches/inch/.degree. C. is found to be very 
practical because a length (1=1c/a"=0.00052/0.000198=2.626 inches) of only 
2.626 inches would be sufficient. 
FIG. 3 represents the application of the instant invention in a system in 
which it is desired to maintain two objects a fixed distance apart. Object 
16 is securely fastened by way of housing 14 to stationary member 4. The 
second object, object 18 is securely fastened to movable member 2. The 
distance between objects 16 and 18 will vary as the temperature changes 
because the structural members between them, primarily housing 14, will 
expand and contract as the temperature increases or decreases. Once the 
anticipated temperature range has been determined and knowing the 
composition and length of housing 14 between object 16 and stationary 
member 4, equation 1 can be used to calculate the amount of temperature 
induced movement one can expect between objects 16 and 18. Equation 1 can 
then be used again to calculate the length of a suitable compensating 
material which can conteract any expansion or contraction of housing 14 so 
that objects 16 and 18 can be maintained at an equal distance over the 
operational range of temperatures. 
The instant invention can also be utilized in a system where it is desired 
to change the distance between two surfaces or objects in a predetermined 
manner. By using equation 1 and knowing the distances that it is desired 
to move the two objects, one can determine the composition of the 
compensating material which should be used. Merely by changing the 
temperature of the compensating material by a thermal blanket or any type 
of heating or cooling device the distance between the two objects can then 
be varied in a predetermined manner a thermal blanket 20 is shown in FIG. 
3, although its exact location relative to the compensating material is 
not critical. 
While a preferred embodiment of the invention has been shown and described, 
various other embodiments and modifications thereof will become apparent 
to persons skilled in the art, and will fall within the scope of 
inventions as defined in the following claims.