OP-2.2Calibration Techniques for Indigenously Developed GRF Measurement System in comparison to Standard Reference Tool

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OP-2.2Calibration Techniques for Indigenously Developed GRF Measurement System in comparison to Standard Reference Tool Powered By Docstoc
					 Calibration Techniques for Indigenously Developed
    GRF Measurement System in comparison to
              Standard Reference Tool
                Venkateswarlu Gaddam*, Gautam Sharma, Neelesh Kumar, Amod Kumar, S.K.Mahna*
                                 Central Scientific Instruments Organisation, (CSIR) Chandigarh
                                                   NIT Kurukshetra, Kurukshetra*

Abstract: Force Plate (FP) is used for measurement of Ground          available standard Kistler FP is shown in Fig 1 (a). Data was
Reaction Force (GRF). The developed FP is divided into three          acquired while walking a person on the Kistler FP and it was
main sections; Force Plate (FP), Data Acquisition system (DAQ-        observed that the shape of a curve is similar to M which is
NI) and LabVIEW software. It consists of four potentiometer           shown in Fig 2 (a).
based force sensing devices sandwiched between two plates (top
                                                                         The developed FP (Fig 1(b)) incorporates linear
and bottom plates). DAQ-NI is used as an interfacing device
between FP and computer. The purpose of this work is to design        potentiometer as the sensing device with LabVIEW software
and develop a low cost FP, which can be widely used to measure        for data analysis [3]. Static and Dynamic calibration were
vertical ground reaction force (VGRF) of human body while             conducted for the determination of sensitivity and linearity of
walking. To obtain precise and accurate measurement of the            the system [2]. An M shape curve from the Developed FP was
VGRF, linear potentiometer is utilized as the main sensing            observed during the procedure of dynamic calibration as
device. We used springs for supporting the rods in between the        shown in Fig 2(b).
plates, which work as the elastic elements for free movement of
upper plate in vertical direction. Data collected from the FP is
send to a computer through DAQ-NI for further processing and
data analysis with LabVIEW software. Both static and dynamic
calibration methods were conducted with the help of developed
FP. For the static calibration, known loads were placed on the FP
for determination of sensitivity and linearity. Whereas in the
dynamic calibration, data was acquired while walking a person
on the developed FP and the same person’s data was acquired by
Kistler FP, which is the available standard reference tool. Both
the readings were compared with each other, then the results                    (a)   Kistler FP                  (b)      developed FP
(output voltage, sensitivity) were found that, the output voltage
of the developed FP is nearly double and sensitivity is three times                                  Fig 1: force plates
that of Kistler FP.

Keywords— Ground Reaction Force, Force Plate, Calibration.

                     I. INTRODUCTION

   The most common force acting on the body is the GRF,
which is equal in magnitude and opposite in direction to the
force that the body exerts on the supporting surface through
the foot [1]. The GRF essentially is the vector summation of
the three reaction forces (Fx, Fy and Fz) resulting from the              (a)   Output from BioWare              (b)    output from LabVIEW
interaction between the foot and the ground [1]. But, the study
                                                                                                   Fig 2: VGRF curves of a person
focuses on the vertical component of GRF (i.e. Fz), which is
considered to be prominent amongst the three components of
the GRF and has importance in the gait analysis, clinical
                                                                                 II. MATERIALS AND METHODS
assessment etc. Force Plates are commonly used in
biomechanics laboratories to measure GRF in the human and
                                                                      A. Force Plate Design:
animal locomotion. Force plates being used across the world
                                                                               Force plate consists of four potentiometers
are manufactured by AMTI, Bertec and Kistler International.
                                                                      sandwiched between two brass plates as shown in Fig 1(b).
We are using Kistler FP (Kistler Instrumente AG, type 9286B,
                                                                      Each brass plate has a thickness of 4mm.The plates are
Switzerland) as a standard reference tool in our Gait
                                                                      supported by six pillars which are further arranged by six
laboratory. This FP was developed with piezoelectric sensors
                                                                      springs. Because of the elasticity property of the springs, it
as sensing devices and uses BioWare software (Version
                                                                      gets compressed when a person applies certain amount of, Type 2812A-04) to analyse the acquired data. The
force on the FP, and consequently it comes back to its original     the resultant signal is in the form of voltage. Initially, no
position when the force is released. The height and inner           output signal was obtained below 300N. As the loads were
diameter of each spring is 70 mm and 18.5 mm respectively.          increased from 301N to 1416N, the output signal i.e. voltage
When the two plates were attached by bolting the four sensors       also increased with increasing load. Data were collected from
to the bottom plate, the overall height becomes 95 mm as            the developed FP, and then the corresponding graph was
shown in Fig 3(b). Springs and potentiometers have a major          plotted between output voltage and applied load. A small
role during the time of calibration work. Potentiometers are        degree of non-linearity was seen between input and output
sensing devices which give an output voltage signal when            readings. During the calibration process it was found that one
some amount of force is applied in vertical direction of the        of the springs was bent, which consequently affects the
sensor.                                                             accurate readings. Even after changing springs non-linearity
                                                                    occurred in the graph plotted. Because of these flaws we were
                                                                    skeptic about the performance of the springs, therefore second
                                                                    approach was applied.

                                                                               In the second approach, fixed loads were applied at
                                                                    each corner of the plate. For this, another block diagram was
                                                                    developed with the help of LabVIEW software to analyse
                                                                    output voltage from individual sensors and their resultant.
                                                                    Output from each of the individual sensors and their resultant
    (a)Top view                             (b) front view
                                                                    are obtained in separate panel to see contribution of the each
                                                                    sensor. A load of 330N was applied at first corner of the plate
                  Fig3.Mechanical drawing in AUTO CAD               (i.e. first potentiometer). All readings of potentiometers
                                                                    including the resultant of all sensors were observed. It was
B. Experimental setup:                                              found that the first potentiometer gives higher value of voltage
        The experimental set-up consists of three main parts        compared to the rest of the potentiometers. The second and
as shown in fig 4: the developed FP, DAQ-NI and computer.           fourth potentiometer give low voltage values compared to first
The DAQ-NI is externally power supplied through an adapter.         and resultant shows an average value. However the third
Each of the four potentiometers draws a voltage of 5V from          sensor, which is located diagonal to the first sensor, does not
the DAQ and their output are connected to the respective            give any response. This is due to the reason that the upper
connector locations of DAQ. The output of DAQ-NI was                plate gets displaced from its original position, thus losing
connected to the computer through a standard USB 2.0 cable,         contact with the sensor. In order to remove this problem of
which has LabVIEW software as the analysing tool.                   plate displacement, clamps were fixed on each of the four
                                                                    sides of the FP.
                                                                              Similarly, the same load was applied at each corner
                                                                    of the FP. From the data obtained it was observed that the
                                                                    sensor where load was applied gives more output voltage
                                                                    compared to the remaining sensors. Even after applying the
                                                                    second approach desired results i.e. linearity and sensitivity
                                                                    were not obtained properly. To get rid of this problem, grids
                                                                    were drown on butter paper and pasted over the FP. After that
                                                                    known weights were placed over the symmetrical points of the
                                                                    developed grids, which help in finding the active region over
                                                                    the FP.

          Fig 4: Complete Experimental set up of Developed FP       E. Dynamic Calibration Method:
                                                                              Dynamic calibration is the main heart of this
C. Calibration work:                                                development of FP. In this mode of calibration, readings were
          Calibration is the validation of specific measurement     taken from the developed FP and then from available standard
techniques and equipment. Simply, calibration is a                  reference tool, i.e. Kistler FP. After acquiring sufficient
comparison between measurements-one of known magnitude              amount of readings, data from both FPs were compared with
or set with one device and another measurement made in a            each other.
similar way as possible with a second device. The purpose of                  In the first approach, readings were collected for a
calibration is to check if the developed FP gives the same          total number of four healthy human subjects from the
measurement readings as the Kistler FP, thereby checking the        developed FP, wherein a person was asked to walk on the
repeatability, sensitivity, linearity of VGRF for the Developed     force plate with their normal walking speed. For acquiring the
FP. For this task, calibration work is classified into two types,   data from the FP, a software program was developed in
Static calibration and Dynamic calibration.                         LabVIEW as shown in Fig.5. The resultant output signal is an
                                                                    M-shape curve (also known as Dual-Dump) which is used to
D. Static Calibration Method:                                       measure maximum and minimum peaks of VGRF. Four more
          The method of static calibration was again                normal-walking trails were performed for the same subject
performed using two approaches. In the first approach, fixed        and the average of all voltage values was calculated.
loads were applied at the centre of the FP to get linearity, and    Similarly, the data was acquired for three more subjects.

                                                                                         Fig 7.Dynamic calibration curve

      Fig 5: Developed LabVIEW program for VGRF measurement

         In the second approach, developed FP and Kistler FP
were arranged along the walkway, and then data was acquired
simultaneously from both the FPs. On comparing the readings
from the two FPs it was found that the output voltage of the
developed FP was nearly double (Fig 8) and sensitivity was
three times that of Kistler FP as shown in Table 1. Graphs
were plotted for both the data sets of force plates (Fig.8) and
better output (linearity and sensitivity) was obtained with this
                                                                                       Fig8. Comparison of both the plates
         Range of          Sensitivity of        Sensitivity of
        Weight (kg)         Kistler FP           Developed FP
                             (mv/N)                 (mv/N)
                                                                               IV. CONCLUSIONS AND DISCUSSION
       55-66            12.22                  41.11
                                                                             In the field of Biomechanics, force plate is regarded
       66-75            6.667                  28.88
                                                                   as one of the most important devices to evaluate various gait
         Table 1: Sensitivity comparison of both plates            parameters. The research study aims at the development of a
                                                                   force plate for clinical as well as Gait Laboratory use. The
                                                                   present study, which involved the evaluation of the VGRF
                           III. RESULTS                            (M-shape curve), showed that the shape of GRF is similar as
                                                                   seen on Bioware software of the Kistler FP. Such a force plate
          The static calibration curve drawn between known         is used to establish a relationship between force and load. As
applied load and output voltage is shown in Fig 6. It is           of now, this research work has been done with the aim of
observed that a small degree of non-linearity occurred due to      determining the relationship between voltage and load. The
imbalance of applied loads and springs. Dynamic calibration        future work focuses on the conversion of an output voltage
curve was obtained by plotting the data of subjects with           into force.
respect to output voltage (Fig 7). The curve thus obtained was               The developed FP can also be used for measurement
not perfectly linear but a fair amount of sensitivity was          of symmetry index. For the accurate measurement of
observed as compared with Kistler FP. A curve depicting the        symmetry between the lower limbs two force plates are
integration of both the FPs i.e. Developed and Kistler was also    needed. For this reason another force plate is being developed
drawn (Fig.8). The output voltage of Developed FP was found        on the same lines. This developed FP is inexpensive as
out to be double than that of Kistler FP.                          compared with Kistler FP. Therefore further research studies
                                                                   involving the use of force plates will prove to be economical.
                                                                             A set of calibration experiments were conducted to
                                                                   evaluate the overall performance of the developed force plate.
                                                                   Better results were found for the developed FP as compared to
                                                                   Kistler force plate from the perspective of sensitivity,
                                                                   linearity, output voltage and shape of curve.

                                                                                              V. REFERENCE

                                                                    [1]. Senanayake S.M.N.A .(2004) Walking, Running and Kicking using
                                                                   Body Mounted Sensors. Proceedings of the 2004 IEEE. Conference on
                                                                   Robotics, Automation and Mechatronics Singapore. Pg 1141-1146.
                                                                   [2]. Hynd D.,Hughes S C.,Evins D J.( 2000) The development of a long, dual-
                                                                   platform triaxial walkway for the measurement of forces and temporal-spatial
                      Fig 6.Static calibration curve
data in the clinical assessment of gait. Proc Instn Engrs vol 214 Part H: pg
[3]. Liu T., Inoue Y., Shibata K.(2009) 3D Force Sensor Designed Using
Pressure Sensitive Electric Conductive Rubber. Journal of system design and
dynamics, VOL 3 No 3 pg 282-295.
 [4]. Roland E.S., Hulla M.L., Stover S.M., (2005) Design and demonstration
of dynamometric horseshoe for measurinh ground reaction loads of horses
during racing conditions. Journal of Biomechanics 38 : pg 2102-2112

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