Enhanced Hardware Design Of Force Platform Full Paper V1

Enhanced Hardware Design of Force Platform S.M.N.A. Senanayake, Senior Member, IEEE , M. M. Danushka R. Marasinghe Student Member, M Chandrapal, M. H. Tung, IEEE, N. J. M. Kumar Student Member, IEEE, Yulius ,D. Gouwanda

Abstract— Force platform is a platform used to quantify the ground reaction forces exerted on it. It is widely for gait analysis, rehabilitation, anomalies detection, etc. This paper presents the design and construction of two similar platforms. The design focuses on improvement of the existing force platform developed at Monash University Sunway Campus. Each platform has a dimension of 480mm x 540mm x 15mm and consists of 144 force sensitive resistor (FSR). These two force platforms can be combined into one to provide a larger movement area for test subjects. Furthermore, several experiments are conducted to monitor the force distribution on human feet in several activities.

I. INTRODUCTION

T

HE subject of biomechanics is growing as athletes turn to science and technology to improve their performances. Force platform is one of the technologies used by athletes to evaluate their performances in various sports events such as running and jumping. Other than sports, force platforms are also used in various fields such as gait analysis, orthopedics, prosthetics, anomalies detection, and other general industrial uses. By analyzing the ground reaction forces and the movement pattern of an athlete on the platform, it is possible to determine the appropriate flooring, footwear and supports needed for a particular type of sport. Besides that, these data may prevent crucial injuries and help to improve the quality of a particular type of sport. Due to its vast applications, Srinivasan et al, developed a pressures sensitive floor that consists of numerous sensor mats capable of independently gathering and transmitting pressure information [1]. Paradiso et al, designed and developed a magic carpet that uses piezoelectric wires as its sensing elements [2]. In Monash University Sunway Campus, force platforms were constructed using Force Sensitive Resistor (FSR) as its sensing elements [3], [4]. Manuscript received November 12, 2007. This work was supported in part by Monash University Sunway Campus, Malaysia D. Gouwanda is with the Monash University Sunway Campus, Bandar Sunway 46150, Malaysia (e-mail: [email protected]). S. M. A. Senanayake is with Monash University Sunway Campus, Bandar Sunway 46150, Malaysia (e-mail: [email protected]) M. Chandrapal is with the Monash University Sunway Campus, Bandar Sunway 46150, Malaysia (e-mail: [email protected]). M. H. Tung is with the Monash University Sunway Campus, Bandar Sunway 46150, Malaysia (e-mail: [email protected]). M. M. D. R. Marasinghe is with the Monash University Sunway Campus, Bandar Sunway 46150, Malaysia (e-mail: [email protected]). N. J. M. Kumar is with the Monash University Sunway Campus, Bandar Sunway 46150, Malaysia (e-mail: [email protected]) Yulius is with the Monash University Sunway Campus, Bandar Sunway 46150, Malaysia (e-mail: [email protected]).

These force platforms are able to provide quantitative and qualitative analysis on force exerted by a human on the floor during walking, running, and jumping. They are able to provide valuable information on how the ground reaction forces are distributed over the human feet during walking, running and jumping. Despite its merits in analyzing the ground reaction forces of human feet during various activities, previous designs possess several drawbacks in their hardware architectures. By referring to the shortcomings of previous designs, a newly improvised force platform system is presented in this paper. In addition, this system will have better features in term of the flexibility, portability, rigidity as well as its sensing capability. II. DRAWBACKS IN PREVIOUS DESIGNS As mentioned earlier, previous force platforms possess several limitations in their hardware designs. One of the limitations is its base material. In previous designs, yoga mat and wooden board are used as base materials. Yoga mat is too soft and does not imitate the real-life flooring hence lead to less reliable readings [3]. While wooden board is harder than yoga mat, but it is fragile. Intensive activities performed on top of the force platforms may lead to the cracking of the platforms [4]. Another drawback of previous designs is the use of individual wires to connect the sensing elements. This caused the board to be extremely bulky and introduced unnecessary complications [3], [4] . Lastly, the final drawback of previous designs is the design layout of the signal conditioning circuit. In the first design, conventional breadboard and wires are used to connect various components in the circuit [3]. This design, as shown in Fig. 1, is very hard to troubleshoot and the error ratio is significantly high. Thereby, to improve the aesthetic of the circuit and ease future troubleshooting, customized Printed Circuit Boards (PCB) are made in the second design [4]. However, this design is not viable as it introduced enormous jumper in the circuitry.

Fig. 1. One of the signal conditioning circuits developed in previous work [3]

III. ENHANCED HARDWARE ARCHITECTURE In order to improve the previous design, SolidWorks is implemented in the earliest stage of the design process. SolidWorks is a 3D Computer Aided Design (CAD) software that employs a parametric, feature-based approach to create models and assemblies. Before any fabrications and assemblies, a 3D model of the complete force platforms is formed initially. Through this initial design, errors in the design stage are reduced to minimum. A complete design of the force platform is shown in Fig. 2

Fig. 3. A 3D model of the second layer.

C. Third Layer As mentioned earlier, previous designs utilized individual wires to connect each sensor. Even though it was simpler, this method was error prone and difficult to troubleshoot. For this reason, in the latest design, a customized Printed Circuit Board (PCB) is made as the third layer. This layer provides the connection between the sensors and the signal conditioning circuit. In addition, a soft padding has been placed on the bottom to protect electrical circuitry.

Fig. 4. The third layer of the force platform before etching process Fig. 2. A 3D model of the complete force platform

Through several corrections in the design stage, the final model of the force platform consists of four layers where each layer has its own properties and purposes. A. Top Layer This layer is made of acrylics board that has dimension of 540 mm x 480 mm with thickness of 3 mm. On this layer there are 144 holes with 20mm diameter drilled through the acrylic board to expose the sensitive surface of the sensor. Exposing the sensitive surfaces enable the sensor to come into direct contact with the applied force. Additionally, there are also 54, M4 countersunk screw holes that spread evenly across the layers to hinder the whole platform from sagging or bulging. B. Second Layer The second layer of the platform is designed to house the FSR sensors. Therefore, there are 144 holes with 12 mm diameters drilled to place FSR connectors. In housing the sensor, there are two aspects needed to be considered carefully: firstly the FSR’s have to be in alignment with the holes in the layer above, secondly the FSR connectors have to match with the copper clad board in the layer below. To accommodate these considerations, similar to previous layer, there are 54 holes with 4 mm diameter spread evenly across it.

D. Bottom Layer The bottom layer functions as a cover to hold all the layers together and ensure the stability and rigidity of the platform. It is fashioned from the same material as the top layer and the second layer: an acrylic board with a dimension of 40cm x 54cm and thickness of 3mm. Furthermore, this layer has an additional soft padding layer attached to the bottom side. The main purpose of this padding is to prevent the board from sliding while a subject is performing his activities on the force platform.

Fig. 5. A 3D model of the bottom layer

V. LABJACK AND DATA ACQUISITION SOFTWARE

Fig. 6. Completed force platform

IV. SIGNAL CONDITIONING CIRCUIT The signal conditioning circuit is mainly responsible for collecting and transmitting the signals received from the sensing elements to data acquisition device, LabJack. It contains numerous operational amplifiers and multiplexers. In this circuit, row and column selection method is introduced to switch readings among the 144 sensing elements. A row selector switches the analog signals received from the sensing elements in row manner. Afterward, six outputs from the row selection circuits are multiplexed by using a single multiplexer, which is called a column selector. A general overview of this circuit is shown in Fig. 7.

LabJack is a USB/Ethernet based measurement and automation devices. It has 14 analog inputs (12- to 16-bit), 2 analog outputs (12-bit), 23 digital I/O, 2 counters, and 6 timers. 8 of the digital I/O can be configured to be up to 6 timers and 2 counters. In this work, LabJack is used to sample the analog signals from the sensing elements and to transmit them to a workstation i.e. computer for further processing. In the workstation, LabJack is coupled with LabVIEW to provide quantitative and qualitative analysis of human gait. LabVIEW is a graphical programming language that uses icons instead of lines of text to create applications. Using LabVIEW, an interactive and user friendly program is developed. This program prompts the user to key in relevant information such as subject’s weight and height, number of samples, and the activity to be performed i.e. static, walking, running or jumping. Subsequently, it scans the force platforms, samples the analog signals received and display them interactively in the software interface. VI. ACTIVITIES ON THE FORCE PLATFORMS Activities performed on the force platforms include static (initial position of the subject), walking, running and jumping. The platform is capable of analyzing the force intensity, ground reaction forces and instantaneous (also maximum) force during walking, running and jumping. The experiment results, which are acquired in real time, enable close monitoring of test subject’s gait. Additionally, results obtained from the previous experiment can be review interactively in the following session. VII. CONCLUSION

Fig. 7. An overview of the signal conditioning circuitry

Finally based on the electrical design, customized PCBs are made. PCB developed in this work is considered as one of the major improvement. In this design, the total amounts of electrical jumpers and connectors have been reduced significantly. Furthermore, the circuit board layout has been fully optimized to ease future maintenance and troubleshooting procedures. These improvements have considerably reduced the overall cost required to produce the signal conditioning circuits too.

Enhanced hardware architecture of the force platform has been successfully developed. It solved the limitations of the previous design and improved the rigidity, flexibility, portability and sensing capability. Furthermore, the design layout of the signal conditioning circuit has been fully optimized to reduce the overall cost and ease the future troubleshooting procedures. Finally, several experiments were conducted to monitor and record force distribution on human feet in static, walking, running and jumping. Furthermore, the results are satisfactory and have proven that the overall system is considerably reliable and accurate. REFERENCES [1]

[2]

Fig. 8. Signal conditioning circuit

P. Srinivasan, G. Qian, D. Birchfield, A. Kidané, “Design of a pressure sensitive floor for multimodal sensing”, Proceedings of the International conference on Non-visual & Multimodal Visualization, 2005 J. Paradiso, C. Abler, KY. Hsiao, M. Reynolds, “The magic carpet: physical sensing for immersive environment”, Proc. of the Conference on Human Factors in Computing Systems, pp. 277-278, 1997.

[3]

[4]

S. M. A. Senanayake, D. Looi, M. Liew, K. Wong and J. W. Hee, “Smart floor design for human gait analysis”, The Impact of Technology on Sports, F, K. Fuss, A. Subic and S. Ujihashi, Ed. London: Taylor & Francis, 2007, pp. 187-192 B. H. Khoo, S. M. A. Senanayake, D. Gouwanda and B. D. Wilson, “A portable vertical jump analysis system”, The Impact of Technology on Sports, F, K. Fuss, A. Subic and S. Ujihashi, Ed. London: Taylor & Francis, 2007, pp. 637-642