An apparatus was developed to accurately measure components of force along three mutually perpendicular axes, torque, and the center of pressure imposed by the foot of a subject walking over its surface. The data obtained were used to supplement high-speed motion picture and electromyographic (EMG) data for in-depth studies of normal or abnormal human gait. Significant features of the design (in particular, the mechanisms used to support the loadcell transducers) are described. Results of the development program and typical data obtained with the device are presented and discussed

Hagy, J. L.

Keller, C. W.

Musil, L. M.

English

NASA Technical Reports Server

1 r ' I
i
i ° _,'" - s
I
i i
i t a i
0
(. N 6-!91,ql
E
19, LOADCELL SUPPORTS FOR A DYNAMIC FORCE PI.A_'E
By C. W. Keller and L. M. Musil
Lockheed Missiles and Space Company, Inc.
Sunnyvale, California
and
By John L. Hagy
Shriners Hospital for Crippled Children
San Francisco, California
SUMMARY
• n aT_paratus was developed to accurately measure components of force
along three, mutually perpendicular axes, torque, and the center of pressure
imposed by t.e foot of a subject walking over its surface. The data obtained
are used to supplement high-speed motion picture and electromyographic (EMG)
data _or in-deptl" studies of normal or abnormal human gait. Significant fea-
tures of the design--in particular, the mechanisms used to support the load-
ceil transducers--are described. Results of the development program and
typical data obtained with the device are presented and discussed.
INTRODUC TION
Since 1965, a group of orthopedic surgeons, medical researchers,
and a.-rospace engineers have worked together* to develop and perfect the
equipment and techniques needed to thoroughly characterize and quantitize
human gait parameters. Early efforts by Sutherland and Hagy (Ref. 1) were
focuJed on the use of high-speed motio_ picture cameras and a multi-chan-
nel EMG system to acquire data on angular motions and neuro-muscular
activities of the lower extremities during walking. As these efforts progressed,
the need for force, torque, and center of pressure measurements which were
correlated in time with the motion picture and EMG data, became increasing-
ly obvious. Consequently, in 1971, a device designated as the DTnamic Force
Platet was conceived and built. With this de _.ce, precise measurements of
*The work des'c'ribed'herein was sponsored by the Shriners Hospital for
Crippled Children, San Francisco, California
tU. S. Patent pending. Development models are currently in operation
at Shriners Hospital, San Francisco, California; the Mayo Clinic,
Rochester, Mlrmesot:; and Children's Health Center, San Diego, Cal-
ifornia
1976012084-261
https://ntrs.nasa.gov/search.jsp?R=19760012103 2020-03-20T01:56:44+00:00Z
t t
, 1
0
(,
vertical force, forward-aft shear force, medial-latercl shear force, torque,
E and center of pressure location are obtained as tractions of lhe percent of
walk cycle (time) while the °,:oject's foot is in contact with the plate.
During the design and development of the force plate system, the
most critical problem encotmtered was that of devising loadcell support
mechanisms which were suitably rigid, yet which permitted the six degrees
of freedom needed to obtain the desired measurements. These supports
were required to provide reialively high load capability, minimum misalign-
ment, and negligible friction. Three different support mechanism concepts
were develcped and evaluated during tbe program. In the chronological
order of their development, these were: (i) sliding friction supports.
(Z)ball-joint/conical pivot supr-.__s, and (3) spherical segment rocker sup-
ports. The latter were found to be superior in terms of accuracy, repeat-
ability, low-frictiox,, low-noise, and high-load capabilities. Moreover,
installation and calibration procedures were significantly simplified with
this design.
SYSTEM DESCRIPTION
The Dynamic Force Plate is show_ in the photograph and cut-away
perspective drawing of Figs. I and 2, respectively. Application of current
aerospace instrumentation technology to the design of the system resulted
in selection of piezoelectric quartz crystal Ioadcell transduc,_rs to obtain
the necessary measvrements. Originally developed for tl,e adverse environ-
ments and demanding re.quirements of unique aerospace a.pp!_cation.s, these
rugged 1oadcells extend heretofore unavailable measurement capah;.l;_tle_
to the medical community. Compared to more conventional strain gage load.-
cells, the piezoelectric devices offer the following advantages: (I) passive
operation (i.e., no active excitation power supply is required), (2) awider
load range for a given load rating, (3) a higher sensiti_,ity due to higher out-
put voltage, (4) ahigher frequency response, (5) ahigher structural rigi- _.
dity, (6) awider operating temperature range, and(7) a smaller physical
size e.avelope.
The particular loadcell transducers selected provide up to ± 10-volt ,
; output signa!s over a load range from approximately 0.09N to Z. ZkN (0.02
to 500 lbf) with a frequency response from near DC (static force) to approx-
imately 200 hz':'. Once installed, they require little or no mai_c:-,,_nce or
recalibration. Output signals are generated only in response to changes in
the applied load; consequently, they are summed electronically using a
charge amplifier in order to determine the total net force applied at any
particular instant in time.
•",'-Th_ v-'_. ue given is representative of the total sy_t,.-m and depends primarily
upon the mass and rigidity of the plate; the upper 1.imit of response for the
transducer alone is 5000 hz.
266
I
1976012084-262
t !
, I
Q l
l
I
t !0
(,
I
Development models feature use of a transparent plexiglas plate in
order to permit photographing of the footprint pressure patterns from be-
neath the walk-way surface as shown in Fig. 3. However, high-load, high-
frequency-response models now being plalmed for use in large animal ga:t
studies will require metal or composite plates in order to accommodate
- vertical forces of up to 17.8 kN (4000 lbf) and to provide system frequency
responses of up to I000 hz or higher.
The Dynamic Force Plate system, as demonstrated by the existing
developmental models, is eminently suited to automatic data acquisition
and modern, high-speed computer reduction, analysis, and display tech-
niques. In the installation at Shriners Hospital, for example, force, torque,
and cente_ of pressure data are recorded, processed, and plotted in en-
gineering units within approximately 7 minutes after any given walk cycle.
This is _ccomplished using an Electronic Processors, Inc. general purpos.e
EP!-118 minicomputer which is connected directly to the force plate system
(Ref. z).
LOADCELL SUPPORTS
Fig. 4 shows a closeup view of one corner of the force plate assembly.
One of tl-e two loadcclls provided to measure forward-aft shear and torque,
and the single !o._dcell used _o determine medial-lateral shear, can be seen
L as installed within the l_-adframe. This photograph, taken looking down on
the device with the cove_ plate removed, actua _y shows the load cells
assembled with the original sliding friction supports. However, it is also
representativ..,of the installation for the ball-joint/conical pivot supports
and the spherical _egment rocker supports developed later. Details of each
of these support mechanism concepts are presented and discussed in the
f_llowing paragraphs.
Sliding Friction Suppo r _s
Exploded views of the shear/torque force link assembly (on the left)
and the compression spring assembly (on the right) are shown in Fig. 5 for
: the sliding frick'on support concept. The force ].inkassembly shovm is
typical for an)"one of the thre_- shear/torque loadcells and for any one o_ the
four vertical loadcells, although detail dimensions do vary for the shear/
torque and the vertical force applications. Each of the three shear/torque
loadcell assemblies is pre-loaded in compression b_, one of the three spring
assemblies provided. Th_s permits measurement of load in both directions
using a single 1oadcell transducer at each location. The output is zeroed
electronically after the preload is adjusted to the desired value.
With this conceph all force link (loadcel_) and compression spring
assemblies are mounted rigidly to the load frame through the threaded
mounting studs. In or:ler to load any given transducer, the pla_ must be
Ires to deflect in the direction of the applied load. This requires sliding of
the micro_eal bearing surfaces normal to the loadcell axis (see detail shown
in Fig. 5) for all of the system transducers mounted perpendicular to the
267
1976012034-263
?
t
t
,°
i"I
(,
applied load axis. A potential error in measuring the applied load is thus
( introduced due to the frictional reactions imposed by the transverse-axis
loadcells. A secondary source of error also results from the side load
imposed on any given loadceli transdu_.-_r due lo tt_c sliding friction.
During the development program, an analysis was performed to pre-
dict the magnitude of potential errors due to friction. A coefficient of static
friction of 0.0Z was assumed in the analysis based or_ a data sheet supplied
by the Microseal Corporation for the appropriate materials and surface
finish. Results of the analysis indicated that an error in a measured shear
load of approximately 17.8N (4.0 Ibf) could occur due to frictional resistance
of the vertical force 1oadcells with a 90.7-kg (Z00-1bm) subject on the plate.
This magnitude of error was considered tc be unacceptable, and the design
of the 1oadcell supports was modified. Experimental results obtained with
the sliding friction support concept showed that the errors in measured
vertical force were not significant, but that those observed for shear loads
were indeed high compared to the range of the expected loads.
Ball-Joint/Conical Pivot Supports
The loadcell support concept shown in Fig. 6 differs from the sliding
friction support concept in that uach loadcell assembly can rotate in order to
achie'¢e transverse-axis deflections. An adapter with a conical pivot is used
in conjunction with a ball-joint swivel to provide the necessary rotational
freedom.
Experimental results obtained with this system showed a marked
improvement in accuracy. Errors due to internal friction within the mech-
aJaisms were reduced to negligible values. However, it was found that the
initial calibration of the system was q_ te difficult due to a lack of axial
rigidity within the neoprene washers used to all gn and cushion the swivel
ball joint. Moreover, the maximum load capability of the system was
limited by- yielding of the conical pivot due to its extremely small contact
al.ea.
Spherical Segment Rocker Supports
The final loadcell support system concept that was evaluated durin$
tl'_ development program is shown in Fig. 7. Using this concept, rota-
tional freedom is provided by spherical segment rockers without sacrific-
ing axial load capability. In addition, the difficulty encountered previously
in calibrating the system due to the lack of axial rigidity was eliminated.
In order to better provide the geometry envelope needed for the rocker
segments, conical disc washer springs were used in lieu of the coiled com-
pression springs.
k
268
1976012084-264
! •
E
t I J
J
{,
E TYPIC,\]. F'()Ii"t' _';..:' ;'2 "A !,'_ ()IVI't'UFS
2he data shov,'n i}-. " ,_-. ,-. ' I", " ' .', :'c c}!}t,tlI_.'d _{} illus'rale the
general capability of fl_c I, .,-,_,. ,.., _ ,.....-;_cz_. 1,..c v ;dk:_ with lke
right foot of one nor:_al s,,D,c.. .c:', .... ..' ._. u ..._._,,- "]-,c loft c pit.'t: system
currently in operalion at S]..ri,.,..P.. ;:,, _ .', 2,,, C:-_pplcd Children, San
Francisco. Since this ;t;,-_,. :_,. ',. i- ,,_,;.:. ,,..i _Ji__., 'I} _,n-linc t(} an EPI-II8
.... "'_ :d qa,.};inc-piol_edminicomputer, the dala ",w',-t "{, ,",.,',, ,'_ .... .: ,, a:
automatically.
In Figs. 8 ti_l',}t._] _, .' ', .,, , ',,' ...... ] *a o_:_,:.nc.d :t'.. ;'.. nominal
cadence of 50 walk Lyclcs p-., :, ',, '. _, :,_ _. "_ed ;:,_ f,:m _i,}P. of per-
cent of walk cycle (time). . ..... i" ,It :. _.,._ .... d ,_s tk{. :,:riod _£ _ime
which elapses between c,ms,,, ._'_.'c : ..... ! <'.'::,-5 ,}f "t ;)a:"tc',.2ar fo{,_ I'u this
case the right foot) :_nd ;s, .I ,.:- ,,:-, .i _,,_o,;t_ .}£ ..talc:ice. As
shown in the figures, _,}:e ::.:'_': ] :., ..,_ - ..2:, ,. • .:'z :.' u "_cr_en: of ',ke ;valk
cycle, toe off occurs (Iyp_,,.ii,) ' .._ ...... , .,_,d _I,c final hoe! ._Irike
occurs at 100 pert_en_. Vc: ":, , . ; ,". , -,'' - ,_-'.. sh,,;,.r .{)*," .,:. ;±nd med-
ial-lateral shear force _,,::_,, ._,._. ,, .- ,'td -:_ i::_s. -: _;.l-o,_}_ 10,
respectively, in terms of ','_c , _ . ,.n' .,: ' ,,d, ,,,.__'i-,_ k_".,:i l_):'ct: "_alues
can be deternxined simpl_ b', _' ,_ ,,' "_ ., :', ,_n_ ,,r body v',,ight ;'aiues
by the actual body we:c},' v.! • " ' '., :,, ,.,r _; .... .,"_,_lar
subject. Torque ,.al,._(.a ,,_',. ' -. ... " _ ;£ , ,::'. '_-) _.: J']F. II.
Inspect.on of _iic {1_' ; ,,, . - "i r,,.,,. ] t :, cais the
remarkable repca_abiti* T- ,_f '.,,-".,,- , ,,': .. ,;-_c_._ ,_{ ':t_ P,ded bv
the foot of a normal s_:b_,_ I ', '}.,. ' :,,', _, :'::,: ._:,, , css-
ire walk cycles. 5t,tdzcs , ,_ . ", "- :_.-..',,'. " ., :_ ._.n,_ivsis
Laboratory have shov._ t.,.-," ",,. : ,' .... ,, _,.:, :" :'i, q_,_e
similar for differon_ ]m,:_.; :} , .. .. -i,. ,, ." ,;' , },..,,"t('evis:ics
can be distinguished bv , ........ ,'. , ,: ,,: • ,i. r,.-' _,_l,.icc:,;, and
even by compario_ th,_s{. {}1..,:_,,,' ,,' "' , : .... " ,_:M _:_r 'l-,, ,'i_}_ £oo', of
a given subject. M._re_,',',.P, _;_,._,i . ,,' , ,t .' ,,,,-, ':.,_-.-,.. },,,_: n,_ed betxveen
normal average plots (_. ,: .... _,.., -.,.:..- ,: '., ,_ ,', , for ,, ,_.i,rcs{:nIa_ive
number of norr',al suhjec'.<) .,_ {: l, ,. ,. ,... -, . .,,;-_.:-_# _}l{,'_._ {)b',:i],, d for an
abnormal subject. It _s _t:is ,. ,.,, t,,.i ,_,, ! .... 2, _ _, ,]_d ...... "..,it, i, dis'inguish
and compare spe.zific delail,_ ,,( _i .... ,,-, _ I _,I :,}m,,r'._,,,! :_...._i_ {ia'a, provided
by the Dynamic Force P1;.t_{: , ,,'._'_..;_, 't,i _, ._i •' "._'cs its ,,re;,, _ ,Ja_ue as a
diagnostic and evahtat_ve t_c,1.
Another charac_eris_i,. ,i.,t _ '""t"" ,,i 't,,: ])yn:,.t.dc For, e t_la_c system
is the center of pressure (C}})-lil:,{ , 1;; ,,},.y. t'it_. 12 sI,,_}x,.,s CP-Tirne his-
tories for the 5 normal-subj(,_ _, v,'_llf. { \,, ]cs 4is_ ,t._sed above. In Ibis graph,
the X and Y coordinate valt.t's refer l_} _l_c i}o._iti,{m of the CP on the surface
of the force plate with respecl to a fixed .,'cft-,r,,ncc point (X=(}, Y-9) at one
corner of the plate, l'er,:ent of x_,tIR , 5.{ l** (li;_,) values arc sl_OWll ill the
circles 2or each walk ,yclo. ,_il,, ,. 11.... ; ,.:,_1 3" coordinaIes ,_rc _ ompuicd
from measured vertical force ,.'al_a,,_, ,',]M s',nt ,' {by d,:fini_ion} the vertical
force values are zero .xt {} ;tnd (,[>.1}erc,.n ', ,,t _hc w,lk , ":'¢It?, thc CP-Time
histories shown range fron_ :Q}pr,)xi_,_,dclv [ It} a:q}roxim;dc, ly 59 percent of
the walk cycle.
OPJG_A,LPAGE IS
OFPOORoALrrg
I 269
1976012084-265
CONCLUDING REMARKS
The application of aerospace instrumenta+ion technology to the meas-
urement of body forces imposed on a walk-way d,.,"iugnormal or abnormal
gait has resulted [n the development of a unique p_edical research apparatus.
Using the spherical segment rocker su,_port zr:echanism concept developed
during the program, the accuracy' and repeatab-_lityof the data obtained are
significantly enhanced. Sin_ilar force plaie designs can be developed and
used to obtain reliable gait da,a for other"human or animal studies where
force measurement capabilities ,.[up to 17._ kN (4000 Ibf) are required.
REFERENCES
I. Sutherland, David H., M.D. , and Hagy, John L. : Measurement of
Gait Movements from Motion P'cture Film. The Journal of Bone
and loin, Surgery, Vol. 54-A, No. 4, pp. 787-797, 3une, 1972.
Z. Co_nputer Applications; Medi<al Eieclronics and Equipment News,
August, 1973.
%
iL
27u '-
i
1976012084-266

f0 "
(,
i l 'J
° i
' I ) " ir
I
O.
e
!
• J. ,
t
J i
" I
I. J i
0
(--, I
1976012084-270
I I ! , i
" ! Ii
% I kp
I
;
0 i- ,
0
1
r , t
%
4 o
276
v
1976012084-272

1 !
i ._. • i i '! " " _ I !
0 I
(
278
] 9760] 2084-274
((q'l--'Ul) ]noMoJ. IYN'd]J.X2 (q'i-'Ul) _rlogO.I. "iVNII]J,NI
._ _ _, o .
- _
- _u I_ k\ I -r'
" U__ I" I.\W _ o
_ m 'N
.
- _ -_I =
2 "i -J '_X• I .. I . ,_ I l I I I 0
('aN) ]rl_)11O.l,'IyN_I]J.X'.J (raN) ano_o_ IYNII|LNI
i 279
_2
I I
] 9760 32084-275
I I I I__ I -I,, ,I -f_
("0) 3.LVNI(31100:)X
280
i
1976012084-276


Loadcell supports for a dynamic force plate

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server