Omnidirectional liquid filled accelerometer design with liquid and housing temperature compensatio

Geipel, D. H.

Hawley, W. W.

Longyear, W. D.

English

NASA Technical Reports Server

REPLY TO 
AfTN OF: GP 
NATIONA~ AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON, D.C. 20546 
July 6, 1971 
MEMORANDUM 
FROM : 
K S ~ / ~ c i e n t i f  i c B Technical Information Division . , t  
Attn: M i s s  Winnie M. Morgan 
~ ~ / ~ f f i c e  qf Assis tant  General 
Counsel f o r  Patent Matters 
SUBJECT: Announcement of NASA-Owned 
U.S. Patents  i n  STAR 
I n  accordance with t i e  procedures contained i n  t he  Code GP 
t o  Code US1 memoranddm on t h i s  subject ,  dated June 8, 1970, 
t he  attached NASA-oded U.S . patent  is being forwarded f o r  
abs t rac t ing  and annoincement i n  NASA STAR. 
The following in£ o d t i o n  is provided : 
U.S. Patent NO. 1 
corporate source 
Supplementary 
Corporate Source 
: Ford Motor Company 
NASA Patent  C a s e  NO. : /4 7/27 
Gayle Parker 
Enclosure : 
Copy of Patent  
https://ntrs.nasa.gov/search.jsp?R=19710020789 2020-03-17T03:11:50+00:00Z
Sept. 6, 1966 W. W. HAWLEY ETAL 3,270,565 
OMNIDIRECTIONAL ACCELERATION DEVICE 
Filed Dec. 26, 1962 2 Sheets-Sheet 1 
Sept. 6, 1966 W. W. HAWLEY ETAL 3,270,565 
OMNIDIRECTIONAL ACCELERATION DEVICE 
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3,276,565 United States Patent Office Patented Sept. 6, 1966 
1 
3,270,565 
OMNIDIRECTIONAL ACCELERATION DEVICE 
Wilbur W. Hawley, Santa Ana, David H. Geipel, Fuller- 
ton, and Willis D. Longyear, Newport Beach, Calif.; 
granted to National Aeronautics and Space Admin- 
istration under the provisions of 42 U.S.C. 2457(d) 
Filed Der. 26,1962, Ser. No. 247,136 
3 Claims. (Cl. 73497) 
This invention relates to liquid filled accelerometers 
and more particularly to an accelerometer for measuring 
the magnitude of acceleration forces independent of 
direction. 
The recent advent of space travel and the need to in- 
vestigate planetary surfaces has created the need for a 
variety of testing and measurement devices. One such 
device required is a means for measuring acceleration 
forces which are omnidirectional in nature. The device 
must be capable of measuring the magnitude of accelera- 
tion forces independent of direction and, in addition, 
possess the qualities of extensive linear range of sensi- 
tivity, workable output signal, ruggedness, and extreme 
reliability. 
Directional accelerometers are well known in the art 
and are available in almost any degree of simplicity, ac- 
curacy, and reliability. However, such accelerometers 
all have the property of being designed to measure the 
component of acceleration in a specific direction. The 
measured acceleration in general is a function of the 
magnitude of the applied acceleration vector with respect 
to the direction of the sensitive axis of the instrument. 
In order to provide an omnidirectional sensing capability 
several unidirectional accelerometers responsive to ac- 
celeration forces acting in different directional axes must 
be combined. Basically, omnidirectional accelerometer 
devices in the art have combined the vector signals from 
a plurality of accelerometers and presented the combined 
output signals to summing circuits for producing a single 
output. Such accelerometers are by their nature more 
complicated and offer much to be desired in reliability, 
simplicity, and accuracy. Accordingly, it is an object 
of this invention to provide an accelerometer sensitive 
to acceleration forces in all directions. 
According to a principal aspect of the accelerometer 
of this invention, a liquid filled spherical housing is pro- 
vided in which the changes in static pressure of the liquid 
are directly proportional to the acceleration force on the 
housing. The magnitude of the static pressure changes 
of the liquid at the center of the housing is independent 
of the direction of the acceleration forces. By measuring 
the static pressure of the liquid in the spherical housing, 
the magnitude of acceleration forces in any direction can 
be obtained. 
According to another aspect of the invention a liquid 
filled accelerometer is provided in which a chamber in 
a spherical housing is filled with a liquid. The static 
pressure of the liquid is measured at the center of the 
housing providing an output directly proportional to the 
acceleration forces in any direction on the housing. 
Means are provided in the chamber for compensating for 
temperature changes in the housing and the liquid. 
It is therefore another object of this invention to pro- 
vide an omnidirectional accelerometer device. 
It is a still further object of this invention to provide a 
liquid filled omnidirectional accelerometer. 
It is a further object of this invention to provide a 
liquid filled spherical housing for measuring accelera- 
tion forces from any direction. 
Other objects of the invention will become apparent 
from the following description read in conjunction with 
the accompanying drawings in which: 
FIG. 1 is a view of an accelerometer illustrating one 
embodiment of the invention, 
2 
FIG. 2 is an end view of another embodiment of the 
accelerometer of the invention, 
FIG. 3 is an end view of the accelerometer device of 
FIG. 2 particularly illustrating the temperature compen- 
FIG. 4 is an end view of another aspect of the in- 
vention, and 
FIG. 5 is a schematic diagram illustrating the pressure 
gradient in the liquid of the accelerometer device of 
The accelerometer of this invention determines the mag- 
nitude of acceleration forces independent of direction 
and includes a spherical housing having a spherical cham- 
ber filled with liquid subject to acceleration forces. 
15 Means are provided for measuring the static pressure of 
the liquid in response to acceleration forces on the hous- 
ing with the pressure of the liquid at the center of the 
housing being directly proportional to the magnitude of 
the acceleration forces. According to a secondary as- 
20 pect of the invention, means are provided for compen- 
sating for changes in thermal expansion of the housing 
and liquid of the accelerometer device. 
Referring now to the drawings, and in particular to 
FIG. 1, there is illustrated one embodiment of the inven- 
25 tion. A spherical housing 11 has included therein a 
spherical chamber filled with a liquid 13. A pressure 
sensing device 14 is attached to the housing 11 and 
extends into the chamber to measure the pressure of the 
liquid 13 at the center 16 of the housing 11. The pres- 
30 sure of the liquid at the center 16 varies in direct pro- 
portion to the magnitude of an acceleration force applied 
to the housing 11 and is independent of the direction of 
the force. The accelerometer is therefore omidirectional. 
During acceleration of the device of FIG. 1, a static 
35 pressure at any point in the liquid 13 is proportional to 
the acceleration a, with the density of the liquid p ,  and 
the distance h of that point away from the effective free 
surface of the liquid remaining constant. The pressure 
measuring point is located at the center 16 of the cham- 
40 ber having a radius r. The pressure is then proportional 
to the acceleration force according to the equation: 
ps=apr. Thus, it may be seen that the pressure is inde- 
pendent of the direction of the application of the ac- 
celeration a. The housing 11 may be constructed of any 
45 solid material and in one embodiment is 'comprised of 
hard steel. The liquid 13 is desirably selected from 
liquids of high density characteristics such as mercury. 
The pressure sensor 14 may be selected from a number 
of highly reliable and accurate pressure sensing devices 
50 and is preferably of a type such as a strain gauge hav- 
ing high frequency sensing characteristics. 
Referring now to FIGS. 2 and 3, there is illustrated 
another aspect of the invention in which a spherical hous- 
ing 21 has a spherical chamber filled with a liquid 23 
55 and a pressure sensor 24 rigidly attached to the housing 
21. The sensor 24 has its pressure sensing point at the 
center 16 of the spherical chamber and housing. The 
chamber of the housing 21 has also included therein a 
sphere 27 smaller than the ,chamber and attached to the 
60 housing 21. The sphere 27 is selected from a material 
such as Invar having a coefficient of thermal expansion 
so that the expansion rates of the housing 21, the liquid 
23, and the sphere 27 are balanced. In this manner, tem- 
perature changes which cause the liquid 23, the housing 
65 21, and the sphere 27 to expand are compensated for. 
In this manner, the pressure on the liquid is independent 
of the temperature of the device. As illustrated in FIG. 
3, an end view of the device of FIG. 2 the sphere 27 
has a plurality of slots 29 extending from the outer 
70 diameter radially inward with a center cavity 30 com- 
municating with the outer portion of the chamber having 
the liquid 23. The liquid 23 communicates with the 
5 sating means therefor, 
10 FIG. 4. 
3,270,966 
3 
center point 16 to enable the pressure sensor 24 to meas- 
ure the static pressure of the liquid 23. In this manner 
the sphere 27 reduces the movement of the liquid 23 in 
the chamber in addition to providing temperature com- 
pensation. The sphere 27 is rigidly attached at a plu- 
rality of points to the housing 21 to prevent any move- 
ment of the liquid 23 during operation. 
As has been noted with respect to the embodiment of 
FIG. 1, the pressure at the center point of the accelerome- 
ter is independent of the direction of application of ac- 
celeration forces and is directly proportional to the mag- 
nitude of such forces. A further aspect of the invention 
is illustrated in FIG. 4 in which a pair of transducers 31 
and 32 are oppositely disposed and attached to a housing 
33. A spherical chamber 34 within the housing 33 is 
filled with liquid. The pressure on the liquid is directly 
proportional to the acceleration forces from any direc- 
tion on the housing 33. The oppositely disposed pressure 
sensors 31 and 32 measure the pressure at points 35 and 
36 in the spherical chamber. The points 35 and 36 are 
equal distances from the center 16 of the spherical cham- 
ber. The sum of the pressures at points 35 and 36 dur- 
ing acceleration from any direction on the housing 33 
will always be twice the value of the pressure at the 
center 16, which in turn is directly proportional to the 
acceleration force on the housing 33. 
In FIG. 5,  a schematic diagram of the pressure gradi- 
ent on the liquid 23 caused by acceleration forces, an ac- 
celeration on the housing 33 in the direction as shown 
by the arrow 41 creates a pressure gradient. The pres- 
wre at the point 35 is indicated by the line 42, the pres- 
sure at the center point 16 is indicated by the line 43, 
and the pressure at the point 36 is indicated by the line 
44. The 5um of the lines 42 and 44 is equal to twice 
the value of the line 43 which is directly proportional to 
the acceleration force on the housing 33. The provi- 
sion of a pair of pressure sensors in the embodiment of 
FIG. 4, assures that the shape of the spherical chamber 
will be maintained with accuracy and reliability without 
the necessity of placing a pressure sensor at the center 
to measure the pressure directly at the center point 16. 
The system for a temperature compensation as dis- 
closed in the embodiment of FIG. 2 minimizes any liquid 
movement within the chamber. When the thermal ex- 
pansion rate of the liquid in the chamber is added to the 
very low expansion rate of the compensator the sum 
exactly equals the expansion rate of the housing mate- 
rial, thus providing no motion of the liquid in the cham- 
ber. The reduction in volume of the liquid in the cham- 
ber due to the compensating element does not increase 
any errors caused by the housing and pressure sensor 
diaphragm deflections. An alternative way of compen- 
sating for temperature would be to provide orifice means 
leading from the output of the spherical chamber to an 
accumulator for compensating for changes in volume of 
liquid due to temperature changes. 
The single pressure sensor embodiment as illustrated 
in' FIGS. 1 and 2, for example, has the advantage of 
eliminating the necessity for summing the output of the 
pair of pressure sensors in the device of FIG. 4. The 
single pressure transistor embodiment of FIG. 2 also 
prevents output errors which may be caused by inter- 
action of two pressure sensor diaphragms during some 
impact conditions. 
The omnidirectional accelerometer of the device of this 
invention is completely insensitive to direction of accel- 
eration and has a broad range of sensitivity and frequency 
response. The device is particularly applicable to high 
impact conditions such as space applications wherein the 
sphere is dropped on a surface to measure acceleration at 
imoact. The tme of liquid utilized in the device may be 
a n i  liquid or flkd of high density and may be selected 
from liquids such as mercury having the characteristic of 
high coefficient of expansion and high density. 
LO 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 
70 
I 
4 
Although the invention has been described and 3- 
lustrated in detail, it is to be clearly understood that the 
same is by way of illustration and example only and is 
not taken by way of limitation, the spirit and scope of 
this invention being limited only by the terms of the ap- 
pended claims. 
We claim: 
1. An accelerometer for determining the magnitude of 
acceleration forces from all directions comprising, 
a spherical housing responsive to omnidirectional ac- 
celeration forces, 
a spherical chamber within said housing filled with a 
liquid, 
a pair of oppositely disposed pressure sensors attached 
to said housing and extending into said chamber for 
measuring the static pressure of said liquid at op- 
posite ends of said chamber, whereby the average 
pressure at  said pair of pressure sensors is linearly 
proportional to the magnitude of said acceleration 
forces, and 
a sphere in said chamber smaller than said chamber and 
attached to said housing for compensating for 
changes in volumetric expansion of said liquid and 
said housing caused by changes in temperature. 
2. An accelerometer for determining the magnitude of 
a spherical housing responsive to omnidirectional ac- 
celeration forces, 
said hcusing having a spherical chamber filled with a 
liquid, 
a sphere in said chamber smaller than said chamber, 
concentric with said chamber, and attached to said 
housing, the coefficient of thermal expansion of said 
sphere being predetermined to provide temperature 
compensation for said housing and said liquid, 
passages in said sphere communicating with the center 
of said sphere, 
a pressure sensor in the center of said sphere in com- 
munication with said passages for measuring the 
static pressure at the center of said sphere, 
whereby the pressure at the center of said sphere as 
measured by said pressure sensor is linearly propor- 
tional to the magnitude of said acceleration forces. 
3. An accelerometer device comprising: a substantially 
spherical housing defining a substantially spherical cham- 
ber filled with liquid; substantially spherical means in said 
chamber smaller than said chamber, concentric therewith, 
and attached to said housing, the coefficient of thermal 
expansion of said spherical means being predetermined 
to provide temperature compensation for said housing and 
said liquid, said spherical means having passages extend- 
ing to its surface and communicating with its center; and 
pressure sensing means disposed and adapted to measure 
pressure changes of said liquid substantially at the com- 
mon center of said spherical means and spherical cham- 
ber, the pressure changes at the center of said chamber 
being a function of the magnitude of acceleration forces 
applied to said device. 
5 
acceleration forces from all directions comprising, 
References Cited by the Examiner 
UNITED STATES PATENTS 
2,037,949 4/1936 Tate _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  73-406 
2,650,991 9/1953 Ketchledge _ _ _ _ _ _ _ _ _ _  73-516 
2,728,868 12/19-55 Peterson _ _ _ _ _ _ _ _ _ _ _ _  73-515 
2,761,043 8/1956 Larson _ _ _ _ _ _ _ _ _ _ _ _  73-393 X 
2,832,581 4/1958 Youngs _ _ _ _ _ _ _ _ _ _ _ _ _  73-516 
RICHARD C. QUEISSER, Primary Examiner. 
JOSEPH P. STRIZAK, Examiner. 
J. J. GILL, Assistant Examiner. 


Omnidirectional acceleration device Patent

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