Note: Descriptions are shown in the official language in which they were submitted.
Page I ~ GCD 82-le
TWO AXIS M~lLTISENSOR
BACKGR~UND OF THE DISCLOSURE
FIELD OF THE INVENTION
The present invention relates to inertial guidance
instrumentation. More particularly, this invention pertains to
multisensors for measuring both the linear acceleration and rate
of rotation of a moving body.
DESCRIPTIOW OF THE PRI3R ART
A number of attempts have been made to utilizé an
inertial mass to detect the rate of rotation of a bodyu
Generally, such attempts have been based upon the coriolis
acceleration experienced by a vibrating or rotating body fixed to
a second body whose rotation is to be sensed. Coriolis
acceleration is described by the following equation:
~ A = 2~ x v;
where: A = coriolis acceleration;
- angular rate of rotating coordinate
system (second body) to be measured; and
v ~ velocity component perpendicular to
the axis of rotation.
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As reEerenced above, the foregoing expresses the basic
principle on which all vibratory gyros as well as spinning wheel
gyros are based; namely, the acceleration experienced by a mass
having a component of velocity perpendicular to the axis of
rotation of the rotating coordinate system to which it is
attached~ The sensing of angular rate with an oscillating
pendulum was first demonstrated by Leon Foucault in the early
1850's. Since then a number of attempts have been made to apply
coriolis acceleration principles tb the design of rate and rate
integrating gyros.
Prominent among the attempts to develop a rate sensing
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p ,~ y ~
gyro in accordance with the foregoing principles have been the
fGllo~ing ~all referred to by trademarlc name): "Gyrotron" of the
Sperry Gyroscope Corporation (1940); the "A5 Gyro" of Royal
Aircraft Establishment, the "Vibrating String Gyro" of ~orth
American Rockwell Corporation (Au~onetics Division, Anaheim,
California); "Viro" of the General Electric Corporation and the
"Sonic Bell Gyro" of General Motors Corporation ~Delco Division).
All of the above-mentioned, with ~he exception of Gyrotron, began
development in the early 1~60's.
In general, the above-narned systems rely upon either a
rotating body or an unconstrained vibrating body to supply the
velocity component v perpendicular to the axis of rotation of the
second body. The acceleration force experienced by such rotating
or vibrating body is then measured in some manner to provide the
coriolis acceleration A. Knowing the coriolis acceleratioh and
the velocity of a force-sensing element~ one can then simply
determine the rate of rotation of the body.
Vibrating bodies offer obvious advantages over rotating
assinblages in terms of mechanical simplicity. In order to
arrange a rotatable inertial instrument having sensitivity to
coriolis acceleration, such as an accelerometer, ball bearings~
slip rings, spin motors and the lilce must be provided~ Further,
a rotational arrangement must be referenced in phase with the
case in which it is mounted to resolve the input angular rate
into two orthogonal sensitive axes.
Present day attempts to measure rotation via the use of
a vibrating inertial sensor have been implemented by means of
open loop vibrating mechanical systems in which the displacement
of an unconstrained vibrating inertial mass upon experiencing
coriolis acceleration generates an electrical signal proportional
to the coriolis force. Such systems operate as tuning forks
wherein the tines ~ibrate at frequency v and are deflected in a
perpendicular plane by an amount ~roportional to A. Such
systems, while less complex mechanically than rotating systems,
have proven to be subject to inaccuracies resulting from the
orthogonal movements required of the vibrating open loop forc~
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Page 3
detecting mechanisms of the "vibrating string" variety.
SUMMARY OF THE INVE~TION
The foregoing and othee disadvantages of the prlor art
are overcome by the present invention that provides apparatus for
sensing both the rate of rotation and the linear acceleration o
a body. Such apparatus includes constrained mass sensors
responsive to linear acceleration along first and second
preselected axes. Means are provided for arranging such sensors
so that the first preselected axis is orthogonal to the second.
Means are further provided for vibrating the sensors. Means is
further provided that is responsive to the coriolis acceleration
forces exerted upon the sensors.
In a further aspect, the present invention provides a
method for sensing the acceleration and the rate of rotation of a
body. Such method includes the step of providing first and
second constrained mass inertial sensors responsive to linear
acceleration and arranging such sensors so that each is
responsive to linear acceleration forces experienced by said body
along orthogonal axes. The sensors are then vibrated at a
preselected frequency and the linear and coriolis acceleration
forces exerted upon the sensors are measured.
The invention will become further apparent from the
following detailed description~ This description is accompanied
by a set of drawing figures including a reference set of
numerals, like numerals of the figures corresponding to like
figures of the written description and like features of the
invention throughout.
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B~IEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a partial Vi~!W, in exploded perspective,
that illustrates the relative arr~ngement of accelerometers in
accordance with ~he invention; and
Figure 2 is a side sectional view of a multisensor in
accordance wi~h the invention.
DE~AILED DESCRIPTION
Turning now to the drawings, Figure 1 is an exploded
pers~ective view of an essential portion of the invention, that
pertaining to the preferred rela/l:ive orientations of the inertial
force-sensing means that comprise the heart of multisensor. The
force sensing means comprises an orthogonal arrangement of upper
and lower accelerome~ers 10 and 1~ respectively. Each
accelerometer is preferably of the force balance type in which a
mass, such as a pendulous mass, is oriented to react to an
acceleration force acting along a predetermined axis, known as
its input axis. Unlike an open loop type of force detection
mechanism, such mass is constrained by the action of reactive
"forcers" so that, rather than effecting a measurable
displacement, the force acting on the mass is a measurable
function of the energy required to enable the forcers to maintain
the null position of the mass as it experiences acceleration
forces. The pickoff sensors, any of a number of conventional
electro-mechanical transducers, produce resultant electrical
signals proportional to ~he force ~acceleration) sensed by the
reacti~e inertial mass within the accelerometer.
While a wide range of inertial acceleration-sensing
instruments may be accomodated and function within the scope of
the inYention, the apparatus as illustrated in Figure 1 utilizes
two A4 MOD IV accelerometers of the pendulous, force balance
type. This accelerometer is in production and presently
available from Litton Systems, Inc. of Beverly Hills, Califor~a.
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ach of the upper and lower accelerometers lO and 12 is shown to
be fixed to a corresponding upper or lower bracket 14, 16
comprising (in the instance oE th? illustrated lower bracket 16)
a central backing member lB sandwiched between two transversely~
oriented flanges 20 and 22. The height of each overall bracket
structure exceeds that the accelerometer fixed to it and each is
mounted so that it extends both below and above such
accelerometer. As will be seen, such arrangement allows the
accelerometers to be mounted within the case of the mul~isensor
in su~h a way that a suspension is effected, minimizing the
possibility of deleterious mechanical feedback between
accelerometer ~n~ case. Holes 24, 26, 2B, 30, 32, and~34 are
provided within the elements of the bracket assemblage for bolts
that secure bracket to accelerometer and to an armature/
diaphragm, disclosed in the following figure.
While the conventional inner workings of the
accelerometers lO and 12 are not shown, input axes 36 and 38
define the orientations of sensitivity to acceleration forces.
Double headed arrows 40 and 42 in~icate the collinear directions
of vibration of the accelerometers while rotation of the body to
which the multisensor case is fixed is measured about the
indicated orthogonal rotation-sensitive axes 44 and 45. Thus,
referring back to the equation for coriolis acceleration, the
system comprised of a multisensor in accordance with the
invention is seen to impose a predetermined vibratory velocity v
upon force-detecting accelerometers lO and 12 along collinear
axes 40 and 42, sense orthogonal rotations ~ about accelerometer
axes 44 and 46 an~ experience coriolis acceleration forces A
along input axes 36 and 380 Additionally, the multisensor system
will, of course, detect non-coriolis induced linear acceleration
forces along the input axes 36 and 38. Such accelerations can be
dis~1nguished from the rate measuring coriolis forces by
appropriate selection of the freauency of vibration of the
accelerometers coupled ~ith convehtional demodulation techniques,
discussod below.
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The functional system ~s illustrated and discussed above
is shown fully implemented in Figure 2, a cross-section of the
case 48 of a multisensor incorporating the teachings of the
invention and including an assemblage within that as shown in the
preceding figure. The instrumentation within the cylindrical
case 48 is essentially orthogo-symmetrical about a horizontal
axis 50; that is, corresponding elements of the instrumentation
abo~e the axis 50 are rotated by ninety degrees from those below
the line. This is shown clearly, of course, in the preceding
figure.
Covers 52 and 54 seal the multisensor. As is seen in
Figure 2~ the bracket 14 securing the upper aecelerometer 10
includes a central backing member 56 joined to transverseley-
oriented flanges 5~ and 60.
Each accelerometer-and-bracket assembly is bolted at top
and bo~tom to a substantially disc-shaped diaphragm/armature
having reinforced center and edge portions separated by a
relatively thin annular diaphragm formed therewith to form
independent double diaphragm suspensions both above and below the
horizontal axis 50. Armature/diaphragms 62 and 64 are bolted to,
and supply the sole support of, the upper bracket-and-
accelerometer assembly while armature/diaphragms 66 and 58
provide the sole support for the lower bracket-and-accelerometer
assembly.
Cylindrical spacers 70 ~nd 72 separate the edges of the
armature/diaphragms, completing a pair of independent vibratory
units within the case 48, the upper vibratory unit comprising
upper accelerometer lO and-bracket assembly sandwiched between
armature/diaphragms 62 and 64 and surrounded by the cylindrical
spacer 70 and the lower vibratory unit comprising lower
accelerometer 12-and-bracket assembly sandwiched between
armature/diaphragms 66 and 68 and surrounded by the cylindrical
spacer 72.
An electromagnet 74 is positioned in the center of the
case 48 by means of an inwardly-extending radia] flange 75 and
cup 78 formed therewith. A conventional acceleration restoring
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amplifier ~0 mounted on thP Elang~ 76 receives pickofE sign~ls
c~eneratecl within th2 accelerometers and, in response, provides
ccritrol signals to forcers within the acceleromcters that act
upon the pendulous mass. Th~ necessary conductors for the
aforesaid are not shown in Figure 2; however, electrical
communication is provided exterior to the multisensor by means, of
upper and lo~er con~uctors 82 and ~4 which are in electrical
communication with the sensing apparatus of the upper and lower
accelerometers 10 and 12 respectively through soldered contact
pads a6 and 88. Each conductor includes six individual
conducitors; one pair of conductors relates to the excitation oF
the light emitting dio~e portion oE the pickoff sensor; another
pair is associated with the oucput of the photodiode portion of
the pickofE; and the third pair provides current to the
accelerometer forcer mechanism~
The electrornagnet 74 drives the upper and lower
double-diaphragm vibratorv units ~efined above by actjvatinS and
deactivating electromagnetic fields which alternately attract and
release the diaphragms 64 and 66. As a consequence of the
driving of the diaphragmsl the vibratory units, including
associated a_celerometers, are ca~sed to oscillate in the
vertical plane. Further, in accordance with the positioning of
the electromagnet 74 between the diaphragms 54 and 66 the two
units, and associated accelerometeLs, vibrate out of phase by 1~0
degrees. By vibrating out of phase, the units, each having
identical resonant feequencies, exert egual and opposite
vibrational forces thereby minimizin~ the vibrational energy
coupled to the case 48 to avoid mounting sensitivities.
The output of each accelerometer is a signal containing
both rate information and linear acceleration (along each
accelerometer's input axis) information. ~he in3ividual
~emodulation of the two ty~es of information is relatively
strai3htforward as a consequence of the differing frequencies of
the rate of rotation and acceleration signals. The output rate
information is modulated at the preselected freguency of
accelerometer vibration while linear acceleration of interestjcan
be expected to be elther constant or to lie within a relatively
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low and predictable frequency ran~e. The frequency of vibration
of the douhle diaphragm suspensions is chosen to be high relative
to system bandwidth requirements to permit the filtering of the
modulated rate signal from the accelerometer output. Angular
rate information is obtained by capacitively coupling the
accelerometer output to a bandpass amplifier cen~ered about the
modulation frequency. The output of the bandpass amplifier is
appl'ed to the input of the demodulator, the reference signal for
the demodulator being chosen to be in phase with the velocity oE
the vibrating unit. The output of the demodulator is then
filtered to provide a d.c, voltage proportional in amplitude to
the angular rate applied with polarity sensitive to direction of
applied angular rate.
Thus it is sQen that there has been provided to the
inertial instrumentation art new and improved apparatus that is
capable of measuring rotation in two orthogonal planes and
acceleration in two orthogonal directions~ By incorporating a
multisensor in accordance with the invention, one is able to
obtain the advantages of a vibratory apparatus in terms of lesser
complexity than that obtainable with a rotary oscillatory
arrangement while avoiding the inherent drawbacks of former
vibrating arrangements.
While the invention has been described ln its presently
preferred embodiment, its scope is not to be so limited. Rather
the invention is intended to encompass that defined in the
fo:Llowing set of claims and all e~uivalents thereof.
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