Retainer for electrically fired getter

Apparatus is disclosed for retaining an electrically fired getter inside a ring laser gyroscope. A groove is formed in a plate, which is hermetically sealed to a hole in the ring laser gyroscope block. The cylindrically shaped getter is placed longitudinally in the groove such that the getter contacts the groove along only two lines. A mousetrap shaped spring preloads the getter against the groove so as to prevent acceleration forces and vibrations from causing pieces of the getter, heater coil or wire to break loose.

FIELD OF THE INVENTION 
This invention relates to ring laser gyroscopes and more particularly to a 
mechanical retainer for a nonevaporative electrically fired getter to be 
disposed inside the block of such a gyroscope. 
BRIEF DESCRIPTION OF THE PRIOR ART 
A getter comprises a pure sintered metallic alloy, typically consisting of 
titanium and zirconium. The alloy is sintered together with graphite to 
obtain a gas permeable structure. It is considered "nonevaporative" 
because no material is evaporated when it is heated to its activation 
temperature. The getter is disposed within the block of a ring laser 
gyroscope in communication with the optical cavity of the gyroscope. The 
purpose of a getter is to have its sintered metallic alloy material 
combine with unwanted non-noble gas components desorbed from the interior 
of the gyro optical cavity during storage and operation of the ring laser 
gyro. The getter is necessary because evacuation does not accomplish a 
lasting cleanliness of the lasing gas. In order to maintain a long 
lifelength of the ring laser gyroscope, it is important that the optical 
cavity remain filled with only noble gases, during operation of the 
gyroscope. 
A getter is somewhat oxidized during handling It needs to be activated by 
heating it to a high temperature, which may typically be 900.degree. C. 
for one or two minutes. This treatment, called firing, in a vacuum causes 
impurities on the surface of each grain in the getter to diffuse towards 
the interior. It also deliberates hydrogen which may have been absorbed 
due to contact with water vapor. The surface is thereby cleaned and is 
reactivated to absorb more contaminating gas molecules. Absorption is 
usually sufficiently rapid at room temperature, but the getter may be 
heated slightly, for example, to 100.degree.-200.degree. C., if faster 
absorption is desired. 
Since the getter must be located inside the sealed block of the ring laser 
gyroscope and the heater power source is typically outside the block, a 
method must be found for transmitting such power to the inside. Several 
methods are known and are presently in use. In one method, a ring-shaped 
getter is disposed inside the cavity. An RF coil is disposed outside the 
cavity in a coaxial relation with the getter. Electrical RF oscillations 
in the coil generate an oscillating magnetic field, which in turn induces 
RF oscillations in the electrically conductive getter. This current 
flowing around the getter heats it to the desired temperature. The 
disadvantage of this method lies in the fact that the RF coil heats up all 
metal objects in its path. The amount of heating depends on the size and 
shape of the metal object. In some cases springs or other mechanisms used 
to hold the getter in place will reach their annealing temperature and 
thus lose the preload required to hold the getter securely in place. 
Vacuum seals on a ring laser gyroscope are especially prone to failure 
under RF excitation since they are commonly made with metals which have 
low melting points. It can be seen that RF excitation of getters places 
significant restrictions on the design of ring laser gyroscopes. 
The alternate approach to activation of getters lies in heating them via 
electrical heater coils embedded in the getter material. Glass-insulated 
feedthroughs are used to transmit electrical current to the interior of 
the block. A heater coil is disposed inside the getter and is electrically 
connected to the feedthroughs by welding. The heater coil is usually made 
from tungsten or Kanthal and is coated with alumina or magnesia to 
electrically insulate it from the electrically conductive getter body. 
Since power to heat the getter is brought inside the block in the form of 
electrical current, a getter heated in this manner is known as an 
electrically fired getter. 
In the past, electrically fired getters have commonly been supported only 
by their electrical heater wires. The heater wires provided a convenient 
support to keep the hot getter from touching the walls of its container. 
While adequate for some applications this approach is problematic in the 
high stress environments in which ring laser gyroscopes are frequently 
operated. The heater wire and its attachment to the glass-insulated 
feedthroughs are flexible and prone to fracture. Mechanical forces acting 
on the heater can dislodge the alumina or magnesia insulation, thereby 
exposing the heater to the risk of a short circuit. In addition, 
acceleration forces may fracture the tungsten wire element, which can 
become brittle in the high heat of firing. Finally, acceleration forces 
may cause the getter to vibrate due to the compliance of its support 
wires. When the getter and its supports are vibrated at their natural 
frequency the resulting motion can be so violent as to tear the getter 
loose or to shake particles from the sintered getter material. Any 
particle (microscopic of otherwise) broken off the getter can degrade the 
performance of the ring laser gyroscope. 
In order to use an electrically fired getter in the acceleration and 
vibration environments to which ring laser gyroscopes are frequently 
subjected it is necessary to mechanically support the getter such that the 
structure has a natural frequency well above 2 kHz. This support structure 
must contact the getter without acting as a heat sink to the getter while 
it is being activated to 900.degree. C. It also must not lose its preload 
or tension as a result of being in contact with a 900.degree. C. object. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a retainer for an 
electrically fired getter so as to avoid the above-mentioned 
disadvantages. 
It is another object of the present invention to provide a mousetrap shaped 
spring to hold an electrically fired getter against an inside surface of a 
ring laser gyroscope block. 
It is yet another object of the present invention to provide a retainer for 
an electrically fired getter which does not conduct an appreciable amount 
of heat away from the getter during firing. 
It is a further object of the present invention to provide a V-shaped 
groove in an inner surface of a ring laser gyroscope block. 
It is yet a further object of the present invention to provide a 
spring-loaded means for retaining an electrically fired getter against an 
inner wall of a ring laser gyroscope block, which means maintains its 
loading when the getter is fired. 
In accordance with the invention, a cylindrical getter body is preloaded 
against an inside surface of the ring laser gyroscope block by a mousetrap 
shaped spring. The spring is configured such that the section of the 
spring generating the preload will not be annealed or in any way affected 
by the 900.degree. C. temperature of the getter. The preload is constant 
and can be far greater than any acceleration force likely to be present. 
The getter rests longitudinally in a V-shaped groove cut into the wall so 
that the getter physically contacts the wall along only two lines. This 
type of contact in a vaccuum environment has been found to maintain very 
low thermal conductivity between the getter and the groove. This allows 
the getter to be rigidly loaded against the wall without affecting the 
activation of the getter. The mousetrap shaped spring is held against the 
wall by a thin yoke which passes longitudinally through both coils of the 
spring and is welded on both ends to the wall. The entire apparatus, 
including the glass insulated electrical feedthroughs, may be assembled on 
a stepedged disk and mounted in a pluglike fashion in a hole in the 
gyroscope block drilled for that purpose. 
As used herein, the gyroscope cavity includes any volume in communication 
with the lasing gas, including volumes not immediately adjacent to the 
optical path of the gyroscope. The walls of the cavity shall be deemed to 
include any surface which is physically coupled to the gyroscope block and 
is in communication with the cavity, including inward protrusions.

DETAILED DESCRIPTION OF THE INVENTION 
An embodiment of the invention will now be described with reference to the 
accompanying drawings, throughout which like parts are designated by like 
numerals. 
Designated at 10 is a portion of a wall of a triangular ring laser 
gyroscope block. The wall contains a circular hole connecting the interior 
12 of the block with the exterior 14. A step-edged disk 16 hermetically 
seals the hole in a pluglike manner with the larger diameter portion of 
the disk remaining outside the hole and the smaller diameter portion just 
inside. The seal may be accomplished by any suitable method, but an indium 
compression seal as described in commonly assigned U.S. Pat. No. 4,159,075 
is preferred. In this method, the surfaces are cleaned and pressed 
together with an indium wire 18 sandwiched between them to form an 
hermetic seal. 
Designated at 20 is a nonevaporative electrically fired getter for use with 
the present invention. It is cylindrical in shape and can be made from a 
zirconiumtitanium alloy. It is sintered with graphite in order to make it 
porous. Imbedded inside the getter 20 is a heater coil (not shown), which 
may be made from tungsten or Kanthal and coated with alumina or magnesia 
for electrical insulation. The heater coil terminates in axially disposed 
leads 22 and 23 which are welded to glass-insulated feedthroughs 24 and 
25, respectively, and which provide current paths to the exterior 14 of 
the ring laser gyroscope. 
The getter is disposed longitudinally in a V-shaped groove 26 cut into the 
surface of the disk 16 in communication with the interior 12 of the 
gyroscope. It will be recognized that the groove 26 need not have a 
V-shape, and may be formed using raised walls instead of a cutout. This 
arrangement rigidly supports the getter, yet accomodates the natural 
thermal expansion which occurs during firing. Moreover, the portion of the 
getter surface which makes physical contact with the disk 16 is extremely 
small, being limited to only two lines extending the length of the 
cylinder. Heat conduction from the getter into the disk is therefore very 
small and is in fact dominated by the radiative heat transfer. Tests show 
that when a getter supported in this manner is heated to 900.degree. C. in 
a few seconds, the disk 16 remains at approximately 80.degree. C. when 
surrounded by still air. 
The getter 20 is held against the V-groove 26 by a mousetrap shaped 
preloading spring 28. The spring comprises two connected wire coils 30 and 
31 wound in opposite directions on a common axis and longitudinally spaced 
from each other. The portion of the wire which connects the two coils is 
formed into a U-shape, the sides of which extend in a direction 
perpendicular to the winding axis of the coils 30 and 31. At an 
appropriate point on both sides of the "U", the arm is bent approximately 
90 degrees so as to form a proper shape for retaining the getter. The two 
opposite ends of the spring wire also extend in a direction perpendicular 
to the winding axis of the coils 30 and 31. The ends are then bent toward 
each other to form L-shapes. Longitudinally through the center of the 
coils 30 and 31 is disposed a thin yoke 34 which is spot-welded at its two 
ends 36 and 38 to the disk 16. It is not necessary that the welds be at 
the ends of the yoke, as long as there is one on each end of the coil. 
This yoke serves to hold the coils against the disk surface while the 
L-shaped ends and U-shaped center exert force from opposite sides of the 
coils against the disk. The spring 28 can be made from any suitable 
material, such as cold drawn 300 series corrosion resistant steel, and 
should be designed to provide a constant preload on the getter 20 greater 
than the acceleration forces to which it will be subjected. 
One advantage of a mousetrap shaped spring is that it is substantially 
immune from the annealing effect of the heat generated in the getter 
during firing. The only part of the spring which sees the high getter 
temperature is the U-shaped arm, which functions merely to transfer the 
spring force from the coils to the getter of the spring. It can be 
annealed without affecting the clamping energy of the spring. The coils, 
on the other hand, which store most of the energy of the spring, are 
spaced from the getter and thus do not see the high temperature. They are 
also in contact with other metal parts which act as a heat sink to draw 
away heat which does reach the coils. 
The retainer as described above has been built and subjected to firing 
tests and vibration sweep testing at an amplitude of 10 g in three 
mutually perpendicular directions. No particles have broken off either the 
getter or the heater, nor have any resonances been found. 
The invention has been described with respect to a particular embodiment 
thereof, and it will be recognized that many modifications are possible 
without departing from the scope of the invention. For example, the 
mousetrap shape of the spring is not crucial to the invention and may be 
replaced by a spring in a different shape which accomplishes the same 
function. Additionally, the spring may be made of a material different 
from that described above. As another example, the apparatus may be 
mounted on a plate without step-edges, or having a non-disklike shape. It 
may also be mounted directly on an inner surface of the optical cavity. 
Other modifications, too, are possible within the scope of the invention.