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 Reaction Force (GRF). The developedFP is divided into three main sections; Force Plate (FP), Data Acquisition system (DAQ-NI) and LabVIEW software. It consists of four potentiometer based force sensing devices sandwiched between two plates (top and bottom plates). DAQ-NI is used as an interfacing devicebetween FP and computer. The purpose of this work is to design and develop a low cost FP, which can be widely used to measure vertical ground reaction force (VGRF) of human body while walking.To obtain precise and accurate measurement of the VGRF, linearpotentiometer is utilized as the main sensing device. We used springs for supporting the rods in between the 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 methodswere 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 FPand 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 (output voltage, sensitivity) were found that, the output voltage of the developed FP is nearly double and sensitivity is three times 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 interaction between the foot and the ground[1].But,the study focuses on the vertical component of GRF (i.e. Fz), which is considered to be prominent amongst the three components ofthe GRF and has importance in thegait analysis, clinical assessment etc. Force Plates are commonly used in biomechanics laboratories to measure GRF in the humanand animal locomotion. Force plates being used across the world are manufactured by AMTI, Bertec and Kistler International. We are using Kistler FP (Kistler Instrumente AG, type 9286B, Switzerland) as a standard reference toolin our Gait laboratory.This FP was developed with piezoelectric sensors as sensing devices and uses BioWare software (Version 4.0.1.2, Type 2812A-04) to analyse the acquired data. Theavailable standard Kistler FP is shown in Fig 1 (a).Data wasacquired while walking a person on the Kistler FP and it was observed that the shape of a curve is similar to M which is shown in Fig 2 (a).
Thedeveloped FP (Fig 1(b)) incorporates linear potentiometer as the sensing device with LabVIEW software for data analysis[3]. Static and Dynamic calibration were conducted for the determination of sensitivity and linearity of the system[2]. An M shape curve from theDeveloped FP was observed during the procedure of dynamic calibration as shown in Fig 2(b).
(a)Kistler FP (b) developed FP
Fig 1: force plates
(a)Output from BioWare (b) output from LabVIEW
Fig 2: VGRF curves of a person
II. MATERIALS AND METHODS
A. Force Plate Design:
Force plate consists of four potentiometers sandwiched between two brass plates as shown in Fig 1(b). Each brass plate has a thickness of 4mm.The plates are supported by six pillars which are furtherarranged bysix springs. Because of the elasticity property of the springs, it gets compressed when a person applies certain amount of force on the FP, and consequently it comes back to its original position when the force is released. The height and inner diameter of each spring is 70 mm and 18.5 mm respectively. When the two plates were attached by bolting the four sensors
to the bottom plate, the overall height becomes 95 mm as shown in Fig3(b).Springs andpotentiometershave a major role during the time of calibrationwork. Potentiometers are sensing devices which give an output voltage signal when some amount of force is applied in vertical direction of the sensor.
(a)Top view (b) front view
Fig3.Mechanical drawing in AUTO CAD
B. Experimental setup:
The experimental set-up consists of three main parts as shown in fig 4: the developed FP, DAQ-NI and computer. The DAQ-NI is externally power supplied through an adapter. Each of the four potentiometers draws a voltage of 5V from the DAQ and their output are connected to the respective connector locations of DAQ. The output of DAQ-NI was connected to the computer through a standard USB 2.0 cable, which has LabVIEW software as the analysing tool.
Fig 4: Complete Experimental set up of Developed FP
C. Calibration work:
Calibration is the validation of specific measurement techniques and equipment. Simply, calibration is a comparison between measurements-one of known magnitude or set with one device and another measurement made in a similar way as possible with a second device. The purpose of calibration is to check if the developed FP gives the same measurement readings as the Kistler FP, thereby checking the repeatability, sensitivity, linearity of VGRF for the Developed FP. For this task, calibration work is classified into two types, Static calibration and Dynamic calibration.
D. Static Calibration Method:
The method of static calibration was again performed using two approaches. In the first approach, fixed loads were applied at the centre of the FP to get linearity, and the resultant signal is in the form of voltage. Initially, nooutput signal was obtained below 300N. As the loads were increased from 301N to 1416N, the output signal i.e. voltage also increased with increasing load. Data were collected from the developed FP, and then the corresponding graph wasplotted between output voltage and applied load. A small degree of non-linearity was seen between input and output readings. During the calibration process it was found that one of the springs was bent, which consequently affects the accuratereadings. Even after changing springs non-linearity occurred in the graph plotted. Because of these flawswe were skeptic about the performance of the springs, therefore second approach was applied.
In the second approach, fixed loads wereapplied 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 are obtained in separate panel to see contribution of the each sensor. A load of 330N was applied at first corner of the plate (i.e. first potentiometer). All readings of potentiometers including the resultant of all sensors were observed. It was found that the first potentiometer gives higher value of voltage compared to the rest of the potentiometers. The second and fourth potentiometer give low voltage values compared to first and resultant shows anaverage value. However the third sensor, which is located diagonal to the first sensor, does not give any response. This is due to the reason that the upper plate gets displaced from its original position, thus losing contact with the sensor. In order to remove this problem of 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 thedeveloped grids, which help in finding the active region over the FP.
E. Dynamic Calibration Method:
Dynamic calibration is the main heart of this development of FP. In this mode of calibration, readings were taken from the developed FP and then from available standard reference tool, i.e. Kistler FP. After acquiring sufficient amount of readings, data from both FPs were compared with each other.
In the first approach, readings were collected for a total numberoffour healthy human subjects from the developed FP, wherein a personwas asked to walk on the force plate with theirnormal walking speed. For acquiring the data from the FP, a software program was developed in LabVIEW as shown in Fig.5.The resultant output signal is an M-shape curve (also known as Dual-Dump) which is used to measure maximum and minimum peaks of VGRF. Four more normal-walking trails were performed for the same subject and the average of all voltage values was calculated. Similarly, the data was acquired for three more subjects.
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 approach.
Range of Weight (kg) / Sensitivity of Kistler FP (mv/N) / Sensitivity of Developed FP(mv/N)
55-66 / 12.22 / 41.11
66-75 / 6.667 / 28.88
Table 1: Sensitivity comparison of both plates
III. RESULTS
The static calibration curve drawn between known applied load and output voltage is shown in Fig 6. It is observed that a small degree of non-linearity occurred due to imbalance of applied loads and springs. Dynamic calibration curve was obtained by plotting the data of subjects with respect to output voltage (Fig 7).The curve thus obtained was not perfectly linear but a fair amount of sensitivity was observed as compared with Kistler FP. A curve depicting the integration of both the FPs i.e. Developed and Kistler was also drawn (Fig.8). The output voltage of Developed FP was found out to be double than that of Kistler FP.
Fig 6.Static calibration curve
.
Fig 7.Dynamic calibration curve
Fig8. Comparison of both the plates
IV. CONCLUSIONS AND DISCUSSION
In the field of Biomechanics, force plate is regarded as one of the most important devices to evaluate various gait 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 (M-shape curve), showed that the shape of GRF is similar as seen on Bioware software of the Kistler FP. Such a force plate is used to establish a relationship between force and load. As of now, this research work has been done with the aim of determining the relationship between voltage and load. The future work focuses on the conversion of an output voltage into force.
The developed FP can also be used for measurement of symmetry index. For the accurate measurement of symmetry between the lower limbs two force plates are needed. For this reason another force plate is being developed on the same lines. This developed FP is inexpensive as 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
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[2]. Hynd D.,Hughes S C.,Evins D J.( 2000) The development of a long, dual-platform triaxial walkway for the measurement of forces andtemporal-spatial data in the clinical assessment of gait. Proc Instn Engrs vol 214 Part H: pg 193-201.
[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