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Description  |
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BACKGROUND OF INVENTION
(a) Field of the Invention
The invention relates to a novel knee laxity evaluator (KLE) system.
The invention also relates to a motion module/digitizer combination which
can be used in the KLE, or which can be used independently or in other
systems. More specifically, the invention relates to such a combination
which can measure, in three dimensional space, and relative to the
position of a first point or body or co-ordinate system, position or
motion of a second point or body, as well as position or motion of the
second point or body relative to a third, fourth, fifth . . . nth points,
or positions of the second body, or combinations thereof.
(b) Description of Prior Art
Currently, the practice of measuring knee laxity is limited to a subjective
evaluation by a physician of relative displacements at the knee. Through
such an examination, damage to ligaments could be ascertained as a
function of excess laxity or joint movement during passive loading by the
physician. The limitations of this technique are: (a) a high level of
subjectivity; (b) no quantitative or reproducible results; (c) no
knowledge of applied forces; and (d) there are complicated motions which
cannot be evaluated by human feel alone and hence there is important
information being lost.
In accordance with the present invention, a KLE includes a motion module,
that is, a module for measuring, in three dimensional space, movement of a
point or body relative to a fixed point or body. Modules of this type are
known in the art as is illustrated, for example, in U.S. Pat. No.
3,944,798, Eaton, Mar. 16, 1976, U.S. Pat. No. 4,057,806, Furnadjiev et
al, Nov. 8, 1977, and U.S. Pat. No. 4,205,308, Haley et al, May 27, 1980.
Electrical and electronic digitizers are also known in the art. For
example, a two dimensional digitizer is illustrated in U.S. Pat. No.
3,956,588, Whetstone et at, May 11, 1976.
However, there are no teachings in the art for combining the first systems,
usually referred to as motion modules, and digitizers, whereby it is
possible to measure the position or motion of a second point or body
relative to the position of a first point or body and also relative to
third, fourth, fifth . . . nth points or positions of the second body or
combinations thereof.
SUMMARY OF INVENTION
It is therefore an object of the invention to provide a novel knee laxity
evaluator system.
It is a further object of the invention to provide a knee laxity evaluator
system which uses a motion module.
It is a still further object of the invention to provide a combined
electronic motion module/digitizer combination which can be used in the
KLE or which can be used independently or in other systems.
It is a still further object of the invention to provide a novel
dynanometer which can be used in the KLE or which can be used
independently.
It is a more specific object of the invention to provide such a combination
which will measure, in three dimensional space, and relative to the
position of a first point or body, position or motion of a second point or
body as well as position or motion of the second point or body relative to
the third, fourth, fifth . . . nth points, or positions of the second body
or combinations thereof.
In accordance with a particular embodiment of the invention, a knee laxity
evaluator comprises an instrumented seat for seating a patient and
restraint means for restraining a portion of the patient to the
instrumented seat whereby to measure forces applied to the patient at an
unrestrained part thereof. Motion module means measure the motion of the
unrestrained part of the patient relative to the restrained part thereof,
and processor means analyze outputs of the instrumented seat and the
motion module means and provide indications of applied force and motion of
the unrestrained part relative to the restrained part.
From a different aspect, and in accordance with the invention, a motion
module/digitizer combination comprises an elongated member having a first
end and a second end and comprising a first link arm and a second link arm
and means movably connecting the first link arm to the second link arm so
as to permit translational motion between the first end and the second
end, the means for connecting being disposed intermediate the first and
second ends, the means also including first transducer means for measuring
the translational motion. First mounting means are provided at the first
end for mounting the first end at a first point, body or co-ordinate
system and second mounting means are provided at the second end for
mounting the second end at a second point or body. Second transducer means
are provided at the first end for measuring three dimensional rotational
motion of the first link arm relative to the first point or body and a
third transducer means is provided at the second end for measuring three
dimensional rotational motion of the second link arm relative to the
second point or body. In accordance with the invention, a portion of the
second link arm is detachable from the remainder of the second link arm
and the second mounting means and means are connectable to the remainder
of the second link arm. Whereby, the combination of the first link arm,
the remainder of the second link arm, and the means connectable, can be
used to measure the position in space of third, fourth, fifth . . . nth
points or combinations thereof. And whereby, when the remainder of the
link arm is reattached to the portion of the second link arm and the
second mounting means, measurements can be performed to determine the
position in space of the second point or body relative to the position of
the first point or body or the position in space of the second point or
body relative to the third, fourth, fifth . . . nth points or positions of
the second body or combinations thereof.
From a still different aspect, and in accordance with the invention, there
is provided a dynanometer for determining the magnitude and direction of
an applied force. The dynanometer includes three spaced beams and support
means on each of the beams. Platform means are disposed on and supported
by the support means for receiving and being deflected by the applied
force. The deflection of the platform is transmitted to the beams to cause
deflection of the beams. Means are provided for measuring the deflection
of the beams in two directions thereof. In accordance with the invention,
the beams are arranged such that no two redundant directions of deflection
of the beams are permissible.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be better understood by an examination of the following
description, together with the accompanying drawings, in which:
FIG. 1 is a three dimensional view of a patient, with KLE attached, being
examined by a physician;
FIG. 2 is a side view of the patient;
FIG. 3 is a more detailed side view of the patient;
FIG. 4 is a front view of the leg showing motion/module digitizer
attachment;
FIG. 5 is a rear view of the leg showing the electrogoniometer attachment;
FIG. 6 is a front view of an electronic motion module/digitizer combination
in accordance with the invention;
FIG. 7 is a side view of the combination;
FIG. 8 illustrates examples of inserts;
FIG. 9 is a flow chart of software for processing the electrical outputs of
the combination to achieve the desired results;
FIG. 10 illustrates a dynanometer in accordance with a further aspect of
the invention; and
FIG. 11 illustrates in greater detail one of the beams of the invention
dynanometer.
DESCRIPTION OF PREFERRED EMBODIMENTS
Turning first to FIGS. 1 to 5, there is illustrated a patient 1 having the
KLE attached and being examined by a physician 3. The KLE system includes
a thigh restraint means 7, an instrumented seat 9 and a motion module 11.
The motion module is connected at one end 13 to the instrumented seat
which, as will be seen below, constitutes a fixed point or body. The other
end 15 is connected to a second point or body. The purpose of the
instrument is to determine the movement of the second point or body
relative to the first point or body in three dimensional space.
The instrumented seat may be mounted on an examining table 17 and consists
of a dynanometer 19 which measures applied forces. Instruments for
measuring force are described in MEASUREMENT SYSTEMS: APPLICATION AND
DESIGN by E. O. Doebelin, McGraw Hill, pps 333-350. The instrumented seat
may also have an adjustable seat back arrangement 18 as is well known in
the art.
The thigh restraint 7 comprises two or more pairs of off-set straps, which
are fastened to each other by fastening means 8, and which displace soft
tissue and may also provide a torquing of the tissue about the femur in
order to minimize movement of the femur relative to the seat.
The lower leg attachment, illustrated best in FIGS. 4 and 5, comprises a
strap member 21, for example a velcro strap on which is carried the
attachment 15a for the end 15 of the motion module. The lower leg
attachment functions by referencing the motion module to three bony
prominences of the lower leg, namely, the tibial crest 23 and the medial
and lateral malleoli, 25 and 27 respectively. Rollers 29 align themselves
to the bony contours of the tibial crest, and balls in malleolar cups 31
do the same at the malleoli. These balls and rollers allow the skin to
move between the attachment of the bone so that the attachment will move
only with the bone which is important in attaining a true bone position
measurement.
The system also includes a microprocessor based monitor 33 which receives
outputs from the dynanometer and the motion module. Thus, the KLE is
capable of sensing and measuring applied loads and displacements existing
during the use of all standard knee evaluation techniques. In addition,
the present KLE is designed to minimize the effects of soft tissue while
still permitting the physician to hold, palpate and manipulate the joint
as in normal procedures while the KLE provides accurate applied force and
tibial-femoral motion readings in displayed and printed form.
The dynanometer force plate measures forces and moments in basic directions
and permits the physician to know exactly to what levels the knee is being
stressed. This is important when measuring laxity since the amount of
relative bone motion depends on the stress applied. Knowledge of the
forces is of utmost importance to the objective interpretation of joint
laxity.
The motion module measures the true three dimensional position of the tibia
relative to the seat, and hence, as the thigh is restrained, to the femur.
The motion module is an electromechanical device which functions on the
principle that at least six measurements are required to totally define
the position of an object in space as will be further discussed below. It
will consist of a means capable of measuring six degree of freedom, three
dimensional motion of one point or body relative to another point or body
and preferably comprises a unique arrangement of electronic components
capable of measuring rotational or translational displacements. A specific
module is described below in association with FIGS. 6 to 8. Generally
speaking, the two points or bodies between which relative motions are
being measured are connected by a single rigid telescopic arm, or a single
arm having a joint between its two ends.
The force measurement on the dynanometer is accomplished through the
principles of opposite and equal reaction forces. The forces applied to
the knee of the patient are reacted to by the femur and thigh which are in
turn transmitted to the dynanometer. In as much as the forces are of a
different arrangement in the dynanometer as compared to the knee,
knowledge of the relative position of the relative position of the knee
and the dynanometer, provided by the motion module, permits a theoretical
interpretation of the forces and their representation in the co-ordinate
system of the knee.
In operation, a patient is seated in the instrumented seat and the thigh of
the leg of interest is restrained as shown in FIG. 1. The lower leg
attachment is then mounted on the same leg as illustrated in the drawings,
and the motion module is connected between the seat and the lower leg
attachments. The physician can then twist the lower leg, and he will
receive outputs indicating relative displacement as well as force applied.
Turning now to FIGS. 6 to 8, there is illustrated a particular motion
module/digitizer combination which can be used in the KLE environment.
However, as also mentioned, the combination can be used in other systems
or it can be used independently. For example, it could be used in
association with machine tools and other mechanical systems where it is
necessary to be able to measure displacement of a first point or body
relative to a second point or body.
To measure the motion of a body in three dimensional space, six unique
measurements are required relating to the six degrees of freedom of motion
in three dimensional space. The measurements can constitute six unique
rotational measurements or six unique translational measurements or
combinations thereof, i.e., four rotational and two translational, etc.
The combination in accordance with the invention takes five unique
measurements of rotational motion and one measurement of translational
motion.
Referring now to FIGS. 6 and 7, the combination includes an elongated
member 101 having a first end 103 and a second end 105. The elongated
member comprises a first link arm 107 and a second link arm 109. The link
arms 107 and 109 are joined together at 111 to permit relative
translational movement as between 103 and 105 and to measure this
translational movement. In the embodiments illustrated, the link arms are
connected for pivotal motion whereby to permit relative translational
motion of 103 and 105, and a rotary transducer means is used to measure
this translational motion as will be discussed below.
As will be obvious, other means could be used for so connecting arms 107
and 109. For example, one of the arms could include a sleeve for overlying
the other arm and for permitting movement of the other arm into and out of
the sleeve. A translational transducer means could be included in the
sleeve for measuring the translational motion.
Examples of rotary transducer means which can be used are resistive
potentiometers, variable inductance transformers, syncro resolvers,
inductance potentiometers and variable reluctance transducers. Examples of
translational transducers which could be used are dial indicators,
resistive potentiometers, variable inductance transformers, capacitance
transducers, piezoelectric transducers, ionization transducers and optical
transducers.
In describing the illustrated embodiment, rotary and translational
potentiometers, respectively, are utilized. Accordingly, these will be
henceforth referred to. However, it is to be understood that such
translational and rotary potentiometers could be replaced by respective
ones of the above-mentioned transducers.
Disposed at the end 103 is a first rotary potentiometer 113 which is
disposed in line with the arm 103 and rotatable about an axis at right
angles to the arm 103. A second rotary potentiometer 115 is disposed at
right angles to the potentiometer 113 and is rotatable about an axis at
right angles to the axis of the potentiometer 113. Potentiometer 115 is
mounted on mounting block 117 for mounting the arrangement at one end
thereof.
Disposed at second end 105 is a third rotary potentiometer 119 which is in
line with the second link arm 109 and which rotates about an axis at right
angles to the second link arm 109. A fourth rotary potentiometer 121 is
disposed at right angles to potentiometer 119 and is rotatable about an
axis at right angles to the axis of potentiometer 119. A fifth rotary
potentiometer 123 is also disposed at right angles to potentiometer 119
and is rotatable about an axis at right angles to the axis of
potentiometer 119. Potentiometer 123 is also at right angles to
potentiometer 121 and its axis of rotation is also at right angles to the
axis of potentiometer 121.
Potentiometer 123 is connected to mounting block 25 for mounting the
arrangement at a second point.
In the illustrated embodiment, arms 107 and 109 are connected at 111 by a
sixth rotary potentiometer 127 which is in line with both arms 107 and 109
and whose axis of rotation is at right angles to both arms 107 and 109.
The arrangement as thus far described can measure the motion in three
dimensional space of end 105 relative to end 103 or vice-versa and is
referred to as motion module. In accordance with the invention, there is
provided the potential for digitizing the positions of third, fourth,
fifth . . . nth points or bodies (henceforth, the use of the term points
will be used and understood to refer to points or bodies) in three
dimensional space, or combinations thereof, and of then measuring the
motion or position of one of the points 103 and 105 relative to the
position of the other point or relative to the third, fourth, fifth . . .
nth points, or positions of the second body, or combinations thereof. This
potential is achieved by making one of the link arms disconnectable from
its respective mounting block and reconnectable again thereto. In the
illustrated embodiment, link arm 109 is disconnectable from mounting block
125. Specifically, the protrusion 129 which extends from potentiometer 119
is insertable into a receptacle 131. The protrusion is also removable from
the receptacle, and other inserts, such as those illustrated in FIGS. 8A,
8B, etc. can be inserted into the receptacle for digitizing the positions
of other points in space.
For an understanding as to how the combination operates, we will take the
intersection of the axes of potentiometers 113 and 115 as the global
origin 0. Thus, potentiometers 113, 115 and 127 define a spherical
co-ordinate system about 0. Specifically, potentiometers 113 and 115
provide the conventional angles .theta. and .phi. respectively, while the
potentiometer 127, combined with 107 and 109, provide the length of the
vector R. (Knowing the length of 107 and 109, and knowing the angle
therebetween, it is quite easy to determine the length of the vector R).
Point B is defined as the intersection of the axes of potentiometers 119,
121 and 123 and is considered the origin of the "moving body" co-ordinate
system. In distinction thereto, 0 is considered the origin of a "fixed"
body or co-ordinate system. Specifically, mounting block 125 would be
mounted on a moving body. Mounting block 117 would be mounted on the fixed
body or co-ordinate system, and the measurement of the movement of 105
relative to 103 would define the motion of the moving body relative to the
fixed body or co-ordinate system.
The final description of the moving body motion is contained in the three
finite rotations provided by the potentiometers 119, 121 and 123.
To illustrate how the combination is used as a digitizer, the protrusion
119 is removed from the receptacle 131, and one of the digitizer tips
illustrated in FIG. 8 is inserted into the receptacle in place of the
protrusion 119. The tip is then pointed at points of interest, namely, a
third, fourth, fifth . . . nth points above-mentioned, and a reading is
taken of the three dimensional position in space of these points.
As will be understood, conductive leads from the potentiometers will be
brought to a connecting board, which could be disposed on the mounting
blocks 117, so that the electrical signals developed at the potentiometers
can be brought to a processing means such as the processing means
illustrated schematically at 133 in FIG. 1. It will, of course, be
necessary to provide DC power to the potentiometer to measure the changing
resistance thereof, as well known in the art, and this DC power could also
be provided from the processing means 133.
The potentiometers will provide the data for determining the extent and
direction of the motion of point 105. In order to determine the direction
and extent, the data must be processed. Preferably, the data is processed
by computer means. A flow chart for controlling such a computer is
illustrated in FIG. 8.
Three basic subroutines are employed in digitization, two of which are
illustrated in the flow chart. The DIGMAT (digitization transformation
matrix) and DIGIT (digitization) are shown in the flow chart while the
NEWTIP (support routine for user defined tip) must be provided by the user
and takes into account the dimensions and shape of the user supplied tip.
While the user must write a program employing the subroutines in a manner
appropriate to his specific application, in all cases the following
procedure must be used.
The protrusion 129 is removed from the receptacle 131, and one of a variety
of tips is inserted in the receptacle. The mounting blocks 117 must be
firmly mounted at a position which both permits easy access to most points
of interest and is also appropriate for any subsequent motion measurement
using both upper and lower components of the motion module. A position of
interest is then pointed at with the tip.
The physical characteristics are inputted into the computer memory, and a
code is then presented to the computer to let it know which of the tips is
being used.
Upon pointing at the position with the tip, the program must be activated
either through a remote switch or a keyboard entry. The control program
will then scan the signals in the potentiometer, and then, in sequence,
call the subroutine DIGMAT, which uses as input the voltage values of
potentiometers 113, 115 and 127, as well as the voltage of the power
supply. DIGMAT outputs to transformation matrices which are used in the
subroutine digit which is the next subroutine to be called. DIGIT actually
computes the position of DTIP in the global coordinate system using as
input the output of DIGMAT and DTIP coordinates in potentiometer 127
coordinate system.
An output is then provided of the points in the global coordinate system,
that is, relative to the point 0.
This procedure is repeated until all of the points of interest have been
digitized. The TIP is then removed from the receptacle 131 and the
protrusion 129 is again inserted in the receptacle. The subroutine LOCTRN,
which computes the coordinates of the digitized points in the local
coordinate system (that is, with the point B as an origin) is then called.
These points are then outputted to the GLOTRN subroutine which will be
discussed below.
In the meantime, the mounting block 125 would have been attached to a point
of interest. Displacements of this point are performed, and the
potentiometer signals are once again scanned. This data is communicated to
the computer and the subroutine DISMAT is called. DISMAT computes the
contents of the transformation matrix describing the body in three
dimensional space. The subroutine GLOTRN is then called and outputs new
positions of those points previously digitized on the body or analytically
generated points, in the global system. This procedure continues as the
point of interest moves through different positions.
The following are the technical specifications of the subroutines:
SUBROUTINE DIGMAT (DVOL, DT12, DT3)
Description
This subroutine computes matrix DT12 as well as matrix DT3 which locates
the position of potentiometer-113 and potentiometer-127 coordinate
systems, respectively.
These two matrices are strictly inputs to subroutines DIGIT and NEWTIP, and
have no significance to the user.
Input
DVOL (4), voltages of potentionmeters-115, 113 and 127, and the power
supply, respectively. (Note 1)
Output
DT12 (3,3) and DT3 (3,3) are the abovementioned matrices. (Note 1)
Note
1--all the variable names starting with D in each subroutine, are double
precision.
SUBROUTINE DIGIT (DT12, DT3, DTIP, DPNTRF)
Description
This subroutine computes the coordinates of the digitizer tip with respect
to the global coordinate system.
Input
DT12 (3,3), and DT3 (3,3) locate the position of potentiometer-113 and
potentiometer-127 coordinate systems, respectively. (Refer to subroutine
DIGMAT). (Note 3)
DTIP (3) are the coordinates of the tip in use with respect to the
potentiometer-127 coordinate system. (Note 1, 2 and 3)
Output
DPNTRF (3) are the coordinates of the tip with respect to the global
coordinate system. (Note 2 and 3)
Notes
1--The coordinates of the digitizer tips are provided as part of the
Digitizer Unit. For the coordinates of User tip, use subroutine NEWTIP.
(Refer to subroutine NEWTIP)
2--In all coordinate arrays 1, 2 and 3 are X, Y and Z coordinates,
respectively. (e.g. DTIP (1)=X coordinate)
3--All variable names starting with D are in Double Precision.
SUBROUTINE NEWTIP (DT12, DT3, DPNTRF, DTIP)
Description
The main purpose of this subroutine is to define the coordinates of any
user-designed tip with respect to potentiometer-127 coordinate system
without independently measuring the tip dimensions. In order to find the
tip constants, first mount tip number 1 (see 8b) and touch a point (Note
1). Then mount the new tip and touch the same point. Through the software
the coordinates of the point are computed by tip number 1 and are used to
compute the constants for the new tip. (Refer to the Control Program Flow
Chart).
Notes
1--For best results, use a point within 6 and 8 inches from the base of the
digitizer.
Input
DT12 (3,3), and DT3 (3,3) locate the position of potentiometer-113 and
potentiometer-127 coordinate systems, respectively. (Note 1)
DPNTRF (3): coordinates of the digitized point by tip number 1 with respect
to global coordinate system. (Note 1 and 2)
Output
DTIP(3): coordinates of the tip with respect to potentiometer-127
coordinate system, or better known as the new tip constants. (Note 1 and
2)
Notes
1--All variable names starting with D are double precision.
2--In the coordinate system arrays 1, 2 and 3 are X, Y and Z coordinates,
respectively.
SUBROUTINE DISMAT (DVOL, DMAT2)
Description
DISMAT computes the position of the local coordinate system with respect to
the global coordinate system. The local coordinate system is in line with
indicated edges of upper mounting block.
Input
DVOL(7) voltage readings of potentiometers-113, 127 115, 119, 121, 123 and
the power supply line, respectively. (Note 1)
Output
DMAT2 (4,3) consists of:
DMAT2 (4,1), DMAT2 (4,2) and DMAT2 (4,3) are the coordinates of point B in
global coordinate system.
DMAT2 (3,3) defines the position of the local coordinates system with
respect to the global coordinate system. (Note 1)
DMAT 2 (4,3) is input only to subroutines LOCTRN and GLOTRN, and has no
significance to the user.
Note
1--All variable names starting with D are double precision.
SUBROUTINE LOCTRN (DMAT2, DPOINT, DPNTLC, N)
Description
LOCTRN computes the coordinates of the digitized points in local coordinate
system.
These coordinates are constant as long as the upper mounting block is fixed
to the bone or some other chosen mounting base.
If the upper mounting block is shifted these coordinates should be computed
again by calling subroutine LOCTRN. (Refer to the Control Program Flow
Chart)
Input
DMAT2 (4,3), from subroutine DISMAT (refer to subroutine DISMAT). (Note 1)
N is number of points; integer
DPOINT (3,N): coordinates of the digitized and analytical points in the
global coordinate system. (Note 1 and 2)
Output
DPNTLC (3,N) coordinates of the points with respect to the local coordinate
system. (Note 1 and 2)
Note
1--All variable names starting with D are double precision.
2--In the coordinates arrays 1, 2 and 3 are X, Y and Z coordinates,
respectively.
SUBROUTINE GLOTRN (DAMT2, DPNTLC, DPNTGL, N)
Description
GLOTRN computes the new coordinates of the points in the global coordinate
system.
Input
DMAT 2 (4,3), from subroutine DISMAT (refer to subroutine DISMAT). (Note 1)
N number of points; integer.
DPNTLC (3,N): coordinates of the points in local coordinate system. (Note 1
and 2)
Output
DPNTGL (3,N) new coordinates of the points in global coordinate system.
(Note 1 and 2)
Notes
1--All variable names starting with D are double precision.
2--In the coordinate arrays 1, 2 and 3 are X, Y and Z coordinates,
respectively.
Although reference was made to dynanometers above, in accordance with a
further aspect of the invention, there is provided a novel dynanometer,
comprising a triple beam support system, illustrated in FIGS. 10 and 11
hereof. As seen in these Figures, the dynanometer comprises a supporting
frame 201 which, in the illustrated embodiment, comprises a four walled
structure. Disposed centrally of one wall is a first spherical or
rectangular beam 203. Second and third rectangular or spherical beams 205
and 207 are disposed in the corners opposite the wall of the 203 beam.
As seen in FIG. 11, each beam comprises a vertical support portion 209 and
horizontal deflection member 211. The vertical and horizontal members are
joined by spherical/linear bearings 213. The bearings are the key to the
proper operation of this triple beam system since they release all moments
at the beams permitting the moments to be measured at various beams
through their respective bending deflections, rather than being lost as
axial beam compression or pure moments.
The beams are arranged such that the longitudinal axis of each deflection
member 211 is the perpendicular bisector of a respective side of an
equilateral triangle. This particular arrangement is convenient for
subsequent analyses.
Supported at the top surfaces of the vertical members is a platform 215.
The platform can be of any convenient shape, so long as it is supported by
the top surfaces of the vertical members. In the illustrated embodiment,
the platform 215 forms an equilateral triangle, and a beam is disposed at
each corner of the triangle. The horizontal member of the beam is
perpendicular to the side of the triangle opposite its corner.
Forces are measured by their application through the platform 215. These
forces result in the deflection of the beams. The deflections are measured
as an indicator of the forces.
Although in the illustrated embodiment the beams are at the corners of an
equilateral triangle, any arrangement of three beams where there are no
two redundant directions of deflections are permissible for two reasons:
(a) Such an arrangement will provide a rigid mechanical mechanism; and
(b) The minimum of six non-redundant forces required for the solution of
the equilibrium equations will be measured.
Devices, illustrated schematically at 217 and 219 with respect to beam 203,
221, 223 with respect to beam 207 and 225, 227 with respect to beam 205
are used to measure the deflection of the beams. The devices can comprise
conventional displacement transducer devices and they would be mounted
onto the rigid support frame and in such a manner that deflections in only
the two planes of interest are measured for each beam. The resulting force
measurements would be the minimum required to fully define all the
external forces and moments acting on the platform. The necessary
formulations for equilibrium of rigid body are the subject of numerous
engineering text.
Conventional displacement transducers which can comprise the items 217 to
227 are:
Capacitance gauge
Linear variable differential transformer (LVDT)
Hall effect detector
Reflected/interrupted light intensity
Rectilinear potentiometer.
With the set-up as illustrated, the deflections for each beam will be
resolved in two directions through each beam. One direction is parallel to
the axis of the vertical member and consists of the forces labelled F2,
F4, F6, and the other direction is perpendicular to the first direction as
illustrated by the arrows labelled F1, F3 and F5. The devices 217, 221 and
227 measure the forces in the directions F2, F4 and F6, while the devices
217, 223 and 225 measure the forces in the direction F1, F3 and F5.
When a force is applied to the platform 215, depending on the magnitude of
the force and the direction of the application thereof, the beams 203, 205
and 207 will be deflected by different amounts. The magnitudes of
deflection are resolved in the two directions as above-described, and the
magnitudes in the respective directions are measured by the devices 217 to
227. Using this technology and well-known mathematical vector
transformations, the force applied at the platform can be calculated.
Although particular embodiments have been described, this was for the
purpose of illustrating but not limiting the invention. Various
modifications, which will come readily to the mind of one skilled in the
art, are within the scope of the invention as defined in the appended
claims.
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