Sensor window compliant mounting assembly

A sensor window compliant mounting assembly (42), such as used in a missile (20) to protect a sensor (40), has a truncated hemispherical sensor dome window (52) and a dome mount housing (60). The dome mount housing (60) includes a hollow tube (62) with an opening (64), an internal circumferential thread (66) adjacent to the opening (64), and an integral bezel retainer ring (68) extending circumferentially around the opening (64). The bezel retainer ring (68) engages and retains the exterior surface (56) of the dome base (54). A spanner nut (72) is threadably engaged to the internal circumferential thread (66) of the dome mount housing (60), and a partially compressed fiber metal washer (74) reacts between the spanner nut (72) and the lower surface (58) of the dome base (54) to bias the sensor dome window (52) toward the bezel retainer ring (68).

BACKGROUND OF THE INVENTION 
This invention relates to the mounting of sensor windows used to protect 
sensors, and, more particularly, to a compliant mounting assembly that 
permits the mounting and window to be heated to elevated temperature 
without loss of mounting and protective integrity. 
Various types of optical, electro-optical, infrared, acoustic, and radar 
sensors are used in missiles, aircraft, and other applications to sense 
the environment and especially to search for targets. The sensor itself is 
usually rather delicate and must be protected from aerodynamic forces, 
dirt, heat, and other external agents that could damage it. A protective 
sensor window that is transparent to the energy sensed by the sensor is 
placed over the sensor to protect it. The sensor window is held in place 
by a window mounting assembly. 
In one application, an infrared sensor is protected by a 
infrared-transparent ceramic window in the shape of a truncated dome that 
fits over the sensor and is held in place in a metallic mounting. In 
conventional practice, an adhesive is used to bond the protective window 
to the mounting assembly. However, in some cases the sensor system is to 
be used in an aerodynamic environment where it may be heated to 
temperatures as high as 1200.degree. F. At such temperatures, conventional 
adhesives soften and are incapable of holding the protective window in 
place. Moreover, the differences in thermal expansion coefficients between 
the ceramic window and the metallic mounting may cause the window to 
become loose in the mounting so that hot gas could penetrate to the sensor 
and damage it. Tests using such a conventional approach have led to 
mounting-related failures. 
There is a need for an improved sensor window assembly for a wide variety 
of sensor systems. The sensor window assembly should withstand elevated 
temperatures without loss of mounting integrity and protective function. 
The present invention fulfills this need, and further provides related 
advantages. 
SUMMARY OF THE INVENTION 
The present invention provides a sensor window mounting assembly that 
securely retains the sensor window in place. The assembly may be heated to 
elevated temperatures without loss of structural or protective functions. 
The assembly of the invention is fully compatible with existing sensor 
designs and mechanical constraints imposed by the surrounding structure. 
In accordance with the invention, a sensor window compliant mounting 
assembly comprises a sensor window having a window base with an exterior 
surface and a lower surface and a window mount housing. The window mount 
housing includes a hollow tube with an opening at a mounting end thereof, 
an internal circumferential thread adjacent to the opening, and a bezel 
retainer ring extending circumferentially around the opening of the window 
mount housing at the mounting end and having an interior surface. The 
bezel retainer ring is sized to engage the exterior surface of the window 
base with the interior surface of the bezel retainer ring and thereby 
retain the window base therein. A spanner nut is threadably engaged to the 
internal circumferential thread of the window mount housing. There is 
further a means for biasing the window toward the bezel retainer ring of 
the window mount housing, the means for biasing reacting between the 
spanner nut and the lower surface of the window base. 
In a preferred embodiment, a sensor window compliant mounting assembly 
comprises a truncated hemispherical sensor dome having a dome base with an 
exterior surface and a lower surface and a dome mount housing. The dome 
mount housing includes a hollow tube with an opening at a mounting end 
thereof, an internal circumferential thread adjacent to the opening, and 
an integral bezel retainer ring extending circumferentially around the 
opening of the dome mount housing at the mounting end and having an 
interior surface. The bezel retainer ring is sized to engage the exterior 
surface of the dome base with the interior surface of the bezel retainer 
ring and to retain the dome base therein. A sealant is placed between the 
exterior surface of the dome base and the interior surface of the bezel 
retainer ring. A spanner nut is threadably engaged to the internal 
circumferential thread of the dome mount housing. A partially compressed 
fiber metal washer reacts between the spanner nut and the lower surface of 
the dome base, and a gasket is placed between the fiber metal washer and 
the lower surface of the dome base. 
In one embodiment, a missile comprises a missile body a sensor in the 
missile body, a sensor window through which the sensor faces, and a sensor 
window compliant mounting assembly as set forth herein. 
An important feature of the mounting assembly is the means for biasing that 
compliantly forces the window base into the bezel retainer ring. This 
means for biasing is preferably a sintered fiber metal washer, but which 
may be a spring or other type of compliant device. The means for biasing 
is desirably partially compressed during assembly of the mount. This 
arrangement holds the window securely to the bezel retainer ring of the 
mount housing, yet permits a small amount of relative movement responsive 
to the different thermal expansion and contraction of the window and the 
housing. This permissible relative movement allows the retention of 
structural and sealing integrity under aerodynamic forces and heating at 
temperatures far above those possible with the conventional mounting 
approach. 
The present invention thus provides a advance in the art of sensor systems, 
and particularly in regard to a mounting assembly that permits use of the 
sensor system over a wider range of conditions that heretofore possible. 
Other features and advantages of the present invention will be apparent 
from the following more detailed description of the preferred embodiment, 
taken in conjunction with the accompanying drawings, which illustrate, by 
way of example, the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a missile 20 in which a sensor system and compliant 
mounting assembly may be used. The missile 20 includes a body 22, an 
engine 24 in a tail 26 of the body 22 of the missile 20, and aerodynamic 
control surfaces 28 mounted to the sides of the body 22 of the missile 20. 
The control surfaces 28 are controllable by motors 30. 
A sensor system 32 is mounted in a nose 34 of the body 22 of the missile 
20. Another sensor system 36 is mounted on a side 38 of the body 22 of the 
missile 20. In practice, there is usually only a single sensor system 32 
or 36 in any one missile 20. A nose-mounted sensor system 32 is typically 
present in most such missiles, and a side-mounted sensor system 36 may be 
present for particular applications. Both types of sensor systems 32 and 
36 are shown here for illustrative purposes. 
Each sensor system 32 and 36 includes a sensor 40 mounted within a mounting 
assembly 42. An output signal from the sensor 40 is conveyed to a control 
unit 44, which processes the signal. The control unit utilizes this and 
other information to generate and send commands to the control surface 
motors 30 and the engine 24. This brief description of a missile 20 
presents only a general outline of the environment in which the sensor 
system and mounting assembly may be used. Most missiles include many other 
features and the described features may be placed differently than shown 
here. 
FIG. 2 illustrates a sensor window compliant mounting assembly 50 for the 
side-facing sensor system 36, and FIG. 3 illustrates an enlarged detail. 
The corresponding mounting assembly for the forward-facing sensor system 
32 is similar in relevant respects. 
The mounting assembly 50 includes a protective window for the sensor. In 
the illustrated case, the protective window is a truncated hemispherical 
sensor dome 52 made of a ceramic material such as sapphire that is 
transparent to infrared radiation. The sensor dome 52 includes a region 
termed the dome base 54. The dome base 54 has an exterior surface 56 on 
the outside of the dome 52 and a lower surface 58 defining the truncation 
of the hemisphere. 
The mounting assembly 50 further includes a dome mount housing 60 that 
includes a hollow tube 62. The hollow tube 62 may be straight, as for a 
forward-facing sensor system, or may be straight in part and have a 
slightly angled portion, as for the illustrated side-facing sensor system. 
The hollow tube has an opening 64 at an end adjacent to the dome base 54. 
The hollow tube 62 is of sufficiently large diameter that the sensor dome 
52 may be pushed through the interior of the hollow tube 62, and to the 
opening 64. The hollow tube 62 is also internally threaded with an 
internal thread 66 adjacent to the opening 64 and the sensor dome 52. The 
utilization of the internal thread 66 will be discussed in more detail 
subsequently. 
Adjacent to the opening 64 and dome base 54, and outwardly from the 
internal thread 66, the hollow tube 62 is formed into an bezel retainer 
ring 68 which in this case is integral with the tube 62. The bezel 
retainer ring 68 is an inwardly tapered end portion of the hollow tube 62. 
The bezel ring 68 has an interior surface 70. The bezel retainer ring 68 
is diametrally sized such that the exterior surface 56 of the dome base 54 
slidably engages to the interior surface 70 of the bezel retainer ring 68 
when the dome base is pushed through the interior of the hollow tube 62. 
The sensor dome 52 is thereby retained against outward movement from the 
hollow tube 62 by the engagement between the exterior surface 56 of the 
dome base 54 and the interior surface 70 of the bezel retainer ring 68. In 
this preferred embodiment, the bezel retainer ring 68 is integral with the 
hollow tube 62. Alternatively, the bezel retainer ring could be provided 
as a separate piece that is attachable to the end of the hollow tube 62. 
A spanner nut 72 has an external thread 73 that matches the internal thread 
66 of the hollow tube 62. The spanner nut 72 is threadably engageable to 
the internal thread 66 of the hollow tube 62. 
A means for biasing the sensor dome 52 toward the bezel retainer ring 68 is 
provided at a location such that the means for biasing reacts between the 
spanner nut. 72 and the lower surface 58 of the dome base 54. In the 
preferred embodiment, the means for biasing is a sintered fiber metal 
washer 74. Such a fiber metal washer 74 is made by forming a felt of small 
metallic fibers in a loose array and sintering the felt of metallic fibers 
together, so that each metal fiber acts as a spring. Collectively, the 
fibers impart a spring-like compliancy to the washer. Such fiber metal 
washers are known in the art, and are available commercially from 
Brunswick Technetics, Delano, Fla. Alternatively, other means for biasing 
such as Belleville, wave, or slotted spring washers, or garter springs, 
can be used. 
Optionally but preferably, a gasket 76 of a material such as a polyimide 
material can be placed between the fiber metal washer 74 and the lower 
surface 58 of the dome base 54. Since the sensor dome 52 is preferably a 
ceramic material, the soft, nonmetallic gasket 76 prevents scratching or 
other damage to the sensor dome 52 by the fiber metal washer 74, which 
could lead to premature failure of the sensor dome 52. The presence and 
use of the gasket 76 is within the scope of the statement that the fiber 
metal washer 74 mechanically reacts against the lower surface 58 of the 
dome base 54, because when the gasket 76 is present the mechanical 
reaction still occurs through the intermediate gasket 76. 
A thin sealant layer 78 is located between the interior surface 70 of the 
bezel retainer ring 68 and the exterior surface 56 of the dome base 54. 
The sealant is preferably a viscous sealant that withstands the 
temperature to which the sensor system and mounting assembly are 
subjected. The sealant need not provide mechanical strength, only a 
sealing action. A preferred sealant layer 78 is about, 0.002 inches of RTV 
silicone or a polysulfide material. The RTV silicon sealant is available 
commercially from General Electric Co., Waterford, N.Y. as Type 630 
sealant. 
To assemble the mounting assembly 50, the parts as described are first 
provided. The interior surface 70 of the bezel retainer ring 68 and/or the 
exterior surface 56 of the dome base 54 are provided with a coating of the 
sealant material that is to become the sealant layer 78 at the completion 
of assembly. The sensor dome 52 is inserted dome-end first into the hollow 
tube 62 until the dome end protrudes through the opening 64. The sensor 
dome 52 cannot pass through the opening 64, inasmuch as the exterior 
surface 56 of the dome base 54 slidably engages the interior surface 70 of 
the bezel retainer ring 68. 
The gasket 76, when used, is inserted into the hollow tube 62 and placed 
against the lower surface 58 of the dome base 54. The fiber metal washer 
74 or other means for biasing is inserted into the hollow tube 62 and 
placed against the lower surface 58 of the dome base 54, or against the 
gasket 76 when used. The spanner nut 72 is inserted into the hollow tube 
62 and threadably engaged to the internal threads 66. The spanner nut 72 
is tightened slightly, so that the fiber metal washer 74 or other means 
for biasing is preloaded and partially compressed during ambient 
temperature assembly. The spanner nut 72 may optionally be locked into 
place. 
The preloaded fiber metal washer 74 or other means for biasing holds the 
dome base 54 firmly against the bezel retainer ring 68. The retention is 
completely mechanical. There is no dependence for retention upon an 
adhesive that could weaken and fail during service. 
The retention of the sensor dome 52 is compliant and resistant to failure 
during heating or cooling as a result in the difference in thermal 
expansion of the sensor dome 52 and the other components of the assembly 
50. In an environment where the sensor dome 52 enlarges relative to the 
hollow tube 62, the fiber metal washer 74 compresses further. In an 
environment where the hollow tube 62 enlarges relative to the sensor dome 
52, the fiber metal washer 74 expands to retain a tight joint between the 
bezel retainer ring 68 and the dome base 54. Because the heating and the 
temperature of the mounting assembly 50 are not uniform during a typical 
aerodynamic heating cycle, the actual temperature distribution is not 
readily predicted. The approach of the invention retains a tight, 
compliant seal of the sensor dome 52 to the hollow tube 62 regardless of 
the temperature distribution within a normal operating range. In most 
cases, a slight leakage is acceptable. 
Although a particular embodiment of the invention has been described in 
detail for purposes of illustration, various modifications may be made 
without departing from the spirit and scope of the invention. Accordingly, 
the invention is not to be limited except as by the appended claims.