Accelerometer with mounting/coupling structure for an electronics assembly

A mounting/coupling structure for use in an accelerometer to mount the electronics assembly with respect to the case, and to electrically couple the electronics assembly to the acceleration sensitive structure. The mounting/coupling structure may be positioned such that it provides gas damping for the paddle, and may also include means for holding the acceleration sensitive structure in a fixed position with respect to the case.

FIELD OF THE INVENTION 
The present invention relates to accelerometers in which an acceleration 
sensitive structure and an associated electronic assembly are mounted 
within a common case. 
BACKGROUND OF THE INVENTION 
A prior art accelerometer with high performance potential is described in 
U.S. Pat. No. 3,702,073. The accelerometer comprises three primary 
components, a reed, and upper and lower stators or magnetic circuits 
between which the reed is supported. The reed includes a movable paddle 
that is suspended via flexures to an outer annular support ring, such that 
the paddle can pivot with respect to the support ring. The paddle, 
flexures and support ring are commonly provided as a unitary structure 
composed of fused quartz. 
Both upper and lower surfaces of the paddle include capacitor plates and 
force rebalancing coils. Each force rebalancing coil is positioned on the 
paddle such that the central axis of the coil is normal to the top and 
bottom surfaces of the paddle, and parallel to the sensing axis of the 
accelerometer. A plurality of mounting pads are formed at spaced-apart 
positions around the upper and lower surfaces of the annular support ring. 
These mounting pads mate with inwardly facing surfaces of the upper and 
lower stators when the accelerometer is assembled. 
Each stator is generally cylindrical, and has a bore provided in its 
inwardly facing surface. Contained within the bore is a permanent magnet. 
The bore and permanent magnet are configured such that an associated one 
of the force balancing coils mounted on the paddle fits within the bore, 
with the permanent magnet being positioned within the cylindrical core of 
the coil. Current flowing through the coil therefore produces a magnetic 
field that interacts with the permanent magnet to produce a force on the 
paddle. Also provided on the inwardly facing surfaces of the stators are 
capacitor plates configured to form capacitors with the capacitor plates 
on the top and bottom surface of the paddle. Thus movement of the paddle 
with respect to the upper and lower stators results in a differential 
capacitance change. 
In operation, the accelerometer is affixed to an object whose acceleration 
is to be measured. Acceleration of the object along the sensing axis 
results in pendulous, rotational displacement of the paddle with respect 
to the support ring and the stators. The resulting differential 
capacitance change caused by this displacement is sensed by a feedback 
circuit. In response, the feedback circuit produces a current that, when 
applied to the force balancing coils, tends to return the paddle to its 
neutral position. The magnitude of the current required to maintain the 
paddle in its neutral position provides a measure of the acceleration 
along the sensing axis. 
Size is a very important constraint for many accelerometer applications, 
such as small air-launched missiles, and a number of attempts have 
therefore been made to reduce accelerometer size. For example, the size of 
a force rebalance accelerometer can be reduced by using only a single 
magnetic circuit, rather than the two magnetic circuits that are commonly 
used in opposition. This modification reduces the number of parts, but by 
less than half because parts must be present to fulfill all the original 
functions, i.e., to supply magnetic flux, to provide an acceleration 
sensitive structure including a proof mass, to servo the proof mass to its 
null position, and to hold a proof mass assembly in its proper 
relationship to the magnetic circuit, case, etc. On the other hand, using 
only a single magnetic circuit increases the scale factor by a factor 
greater than 2, and also reduces linearity. 
SUMMARY OF THE INVENTION 
In light of the considerations set forth above, there is a significant need 
for design improvements for accelerometers that are capable of reducing 
the overall size of the accelerometer, without tradeoffs in terms of 
assembly costs or performace. The present invention provides such an 
improvement in the form of a mounting/coupling structure for use in an 
accelerometer to perform a plurality of functions. These functions include 
the mounting of the electronics assembly with respect to the case, and 
electrically coupling the electronics assembly to the acceleration 
sensitive structure. The present invention thereby permits a reduction of 
the size of the accelerometer, without reducing performance or adding to 
the manufacturing costs. In a preferred embodiment, the present invention 
is usable in an accelerometer that includes an acceleration sensitive 
structure and an electronics assembly, both mounted within a case. The 
acceleration sensitive structure includes a paddle, a support, and means 
for supporting the paddle with respect to the support such that the paddle 
has a degree of freedom along the sensing axis. The acceleration sensitive 
structure also includes sensing means for sensing movement of the paddle 
with respect to the support. The electronics assembly includes a sensing 
circuit that when electrically coupled to the sensing means, produces a 
signal indicative of paddle movement, and therefore of acceleration. 
The invention provides a mounting/coupling structure positioned on a first 
side of the acceleration sensitive structure, the mounting/coupling 
structure including means for mounting the electronics assembly with 
respect to the case, and means for electrically coupling the sensing means 
to the sensing circuit. Preferably, the mounting/coupling structure also 
includes means for holding the acceleration sensitive structure in a fixed 
position with respect to the case, and is preferably positioned such that 
it provides gas damping for the paddle. In an accelerometer that includes 
force rebalancing means, the mounting/coupling structure also preferably 
includes means for electrically coupling the electronic assembly to the 
force rebalancing means.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1-3 illustrate an accelerometer that includes a mounting/coupling 
structure in accordance with the present invention. The accelerometer 10 
measures acceleration along sensing axis SA, and includes stator 12, reed 
14, plate 16, and electronics assembly 18, all mounted within case 20 
formed by mounting flange 21 and cap 22. Reed 14 is held between plate 16 
and stator 12, and has coil 24 positioned on its upper surface. Stator 12 
in turn bears against cap 22 via positioning ring 26 and spring washer 28. 
The stator comprises excitation ring 30, magnet 32 and pole piece 34. The 
stator is shaped so that coil 24 occupies a comparatively narrow gap 
between pole piece 34 and excitation ring 30, to provide the force 
rebalancing function well known to those skilled in the art. Plate 16 is 
held against reed 14 by inner shoulder 36 of mounting flange 21, and the 
mounting flange and cap 22 are interconnected by welding or by any other 
suitable process to form case 20. Plate 16 is formed from an electrically 
insulating material, preferably a ceramic. The plate includes means, 
further described below, for electrically interconnecting electronics 
assembly 18 with reed 14. Means (not shown) are also provided for coupling 
the electronics assembly to an electrical connector on the outer surface 
of mounting flange 21. 
Reed 14 is shown in greater detail in FIGS. 2 and 3. The reed has an 
overall disk-like shape, and includes annular support ring 40 and paddle 
42 connected to one another via flexures 44 and 46 that are positioned on 
opposite sides of opening 45. For most of its perimeter, paddle 42 is 
separated from support ring 40 by gap 48. Three raised mounting pads 50a, 
51a, and 52a are located at approximately equally spaced positions around 
the upper surface of support ring 40, and three similar mounting pads 50b, 
and 52b (see FIG. 5) are located immediately beneath the pads 50a-52a on 
the lower surface of the support ring. In the assembled accelerometer, the 
upper mounting pads 50a-52a contact stator 12, while lower mounting pads 
50b-52b contact plate 16. 
Paddle 42 is mounted via flexures 44 and 46 such that the paddle can pivot 
with respect to support ring 40 about hinge axis HA that passes through 
the flexures and that is horizontal and parallel to the plane of the 
drawing in FIG. 2. Coil 24 is mounted on the upper surface of paddle 42, 
such that the outer edge of the coil is approximately coextensive with the 
outer edge of the paddle, except adjacent flexures 44 and 46 where the 
coil overhangs the flexures and opening 45. Capacitor plate 60 is 
positioned on the upper surface of paddle 42 within the area bounded by 
coil 24, and forms a capacitor with the adjacent surface of pole piece 34, 
or with a second capacitor plate located on the lower surface of the pole 
piece. The capacitor forms a portion of a pickoff circuit for detecting 
movement of the paddle from its null position. In the illustrated 
embodiment, a second capacitor is formed between capacitor plate 62 (FIG. 
5) on the lower surface of paddle 42 and capacitor plate 64 (FIG. 6) on 
the upper surface of plate 16. 
A portion of support ring 40 adjacent to the flexures is divided slot 70 
into inner ring 72 and outer ring 74. Mounting pads 50a and 50b are 
positioned on outer ring 74 only, and the flexures are connected to inner 
ring 72. As a result of this arrangement, stress coupled into reed 14 via 
the mounting pads is isolated from the flexures. The split support ring 
approach allows mounting pads 50a and 50b to be located near the flexures, 
on outer ring 74, without creating direct mechanical coupling of the 
mounting pad to the flexure area of the support ring. In addition, the 
evenly spaced distribution of the mounting pads about the circumference of 
the support ring permits the center of preload force to be located almost 
anywhere within the diameter of the proof mass assembly. The preferred 
location is on centerline 76, to provide equal mounting pad loading. This 
low sensitivity with respect to the exact position of the center of 
preload force allows the use of low-cost preload techniques, such as 
spring washer 28 shown in FIG. 1. 
Plate 16 performs a number of interreleated functions in the the 
illustrated accelerometer. First, as shown in FIG. 1, the plate provides a 
mounting structure for electronics assembly 18, and also provides a 
mechanical ground for reed 14 and stator 12. In addition, as described 
below, the plate provides electrical connections between the electronics 
assembly and the reed, and provides a damping surface for movement of 
paddle 42. In addition, the plate may also provide a portion of the 
pickoff circuit for detecting proof mass movement. 
FIGS. 4-6 detail the electrical connections between electronics assembly 18 
and reed 14. FIG. 4 is a view similar to that of FIG. 2, and shows paddle 
42 suspended from support ring 40 by flexures 44 and 46, support ring 40 
including slot 70 and mounting pads 50a-52a. The upper surface of the reed 
includes metal trace 80 that is connected to capacitor plate 60 on the 
upper paddle surface. From the capacitor plate, trace 80 crosses flexure 
44, travels about one-third of the way around support ring 40 between slot 
70 and gap 48, to a position past mounting pad 51a. Trace 80 then extends 
across the outside edge of support ring 40, and forms contact 86 on 
mounting pad 51b on the lower surface of reed 14, as shown in FIG. 5. 
Referring now to FIG. 6, in the assembled accelerometer, contact 86 on 
mounting pad 51b on the lower reed surface makes physical and electrical 
contact with contact 90 on the upper surface of plate 16. Contact 90 is 
connected to trace 92 that extends for a short distance around the upper 
surface of plate 16, and then passes to the lower surface of the plate via 
slot 94 to thereby complete the electrical connection between capacitor 
plate 60 and electronics assembly 18. 
As shown in FIGS. 5 and 6, the lower surface of paddle 42 includes second 
capacitor plate 62 that combines with capacitor plate 64 on the upper 
surface of plate 16 to form a second capacitor for use in the pickoff 
detection system. Capacitotr plate is connected to trace 100 crosses 
flexure 44, extends about one-third the way around support ring 40 between 
gap 48 and slot 70, and forms contact 102 on mounting pad 51b. In the 
assembled accelerometer, contact 102 makes physical and electrical contact 
with contact 104 on the upper surface of plate 16. Contact 104 coupled to 
the electronics assembly via trace 106 that passes through slot 108. 
Referring to FIG. 5, coil 24 on the upper surface of paddle 42 is connected 
to a pair of wires 120 and 122 that extend from the upper to the lower 
paddle surface via opening 45. On the lower paddle surface, wires 120 and 
122 are connected to traces 124 and 126, respectively. The upper surface 
of plate 16 includes recess 128, to provide space for wires 120 and 122 on 
the adjacent paddle. Trace 124 extends across flexure 46 to support ring 
40, and then extends about one-third the way around the support ring 
between gap 48 and slot 70, and forms contact 130 on mounting pad 52b. In 
the assembled accelerometer, contact 130 makes physical and electrical 
contact with contact 132 on the upper surface of plate 16. Contact 132 is 
connected to the electronics assembly by trace 134 via slot 136. 
Trace 126 (FIG. 5) crosses from the lower to the upper paddle surface (FIG. 
4), crosses flexure 46, and then extends about one-third the way around 
supporting ring 40. At the end of slot 70, trace 126 extends through the 
support ring, to form contact 140 on mounting pad 52b. Contact 140 makes 
physical and electrical contact with contact 142 on the upper surface of 
plate 16. Contact 142 is connected to the electronics assembly by trace 
144 via slot 146. 
Referring to FIG. 6, the electronics assembly includes an electrical ground 
that is connected via trace 150 to contact 152, trace 150 passing through 
slot 154 in plate 16. Trace 150 is also connected to capacitor plate 64, 
to thereby ground one plate of the capacitor formed between the plate and 
reed. Contact 152 makes physical and electrical contact with contact 156 
on the lower surface of support ring 40 (FIG. 5). Contact 156 is in turn 
connected to contact 158 on the upper surface of support ring 40 (FIG. 4), 
by a metallic connection (not shown) that extends about the outer edge of 
the support ring. Contact 158 in turn makes physical and electrical 
contact to stator 12, such that the stator and its capacitor plate are 
also electrically grounded. 
FIG. 7 illustrates the mounting/coupling struts of the present invention in 
a second type of accelerometer. Accelerometer 210 shown in FIG. 7 includes 
upper plate 212, reed 214, lower plate 216 and electronics assembly 218, 
all mounted within case 220 formed by mounting flange 221 and cap 222. 
Reed 214 is held between the upper and lower plates. The assembly 
comprising the plates and the reed is held between shoulder 234 of 
mounting flange 221, and insulation cap 226 that bears against cap 222 
through spring washer 228. Thus the overall geometry of this embodiment is 
similar to that shown in FIGS. 1-3, with upper plate 212 generally 
replacing stator 12. 
Reed 214 comprises paddle 242 connected to support 240 via one or more 
flexures 244. Force sensing transducers 250 and 252 are connected between 
support 240 and paddle 242, and measure acceleration in a manner well 
known to those skilled in the art. 
As in the embodiments of FIGS. 1-3, lower plate 216 serves to support the 
reed, to provide a damping surface for the motion of pendulum 242, and 
also to provide electrical connections between forced transducers 250 and 
252 on reed 214 and the electronic assembly. The electrical connections 
may be formed in a manner essentially identical to that shown in the prior 
embodiment, with the electrical connections from the reed to the lower 
plate being made by electrical contacts on the lower surface of support 
240, and on the upper surface of lower plate 216. 
FIG. 8 shows the mounting/coupling structure of the present invention in 
use in a third type of accelerometer. Accelerometer 310 is generally 
similar to accelerometer 210 shown in FIG. 7, except that reed 314 is 
configured as shown in U.S. Pat. No. 4,872,342. Once again, lower plate 
316 serves as a mechanical ground for the reed and electronics assembly 
318, provides a damping surface for movement of pendulum 342, provides 
electrical connections between the reed and the electronics assembly, and 
contacts case 320 at shoulder 334 to position the reed within the case. 
While the preferred embodiments of the invention have been illustrated and 
described, variations will be apparent to those skilled in the art. 
Accordingly, the scope of the invention is to be determined by reference 
to the following claims.