Custom Optoelectronic Force Sensor Based Ground Reaction Force (GRF) Measurement System for Providing Absolute Force Junghoon Park1, Sangjoon J. Kim1, Youngjin Na1 and Jung Kim1 1
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 34141, Korea (Tel : +82-42-350-3271; E-mail: junghoon.park, sangjoon.j.kim, youngjin.na,
[email protected]) Abstract – We propose an optoelectronic force sensor based insole type ground reaction force (GRF) measurement system, which can be used to measure the absolute GRFs produced by the user. Although force sensitivity resistors (FSR) has been widely used to measure the GRFs, they provide only the relative weight of the user. Accurate measurement of GRFs is essential to control robotic devices, such as lower-limb exoskeleton and prosthesis and to make a diagnosis for gait abnormality. To validate the performance of the developed insole type GRF measurement system, we measured GRFs during level ground walking and compared the results with FSRs. For measured GRFs, the GRF data from our system was identical compared with that from a force plate, but the GRF data from FSRs was saturated due to capacity of FSR (~450N). Our insole type measurement system provided the absolute GRFs in walking. Keywords – ground reaction force (GRF), optoelectronic force sensor, force sensitivity resistor (FSR) 1. Introduction Insole type ground reaction force (GRF) measurement systems have been developed for monitoring a gait pattern and controlling a lower-limb exoskeleton and prosthesis. In clinical research, gait abnormality can be diagnosed by analyzing the measured GRF data using insole type force sensors [1], [2]. The terrain information and gait phases were identified using the GRFs for the control of robotic devices, such as lower-limb exoskeleton and prosthesis [3]. The most widely used sensor for measuring FRFs is the piezo-resistive type force sensor. F-scan is the existing products using piezo-resistive type force sensor for various research filed. They utilized force sensitivity resistor (FSR) for developing insole GRF measurement system. In addition, Howell et al. developed a low-cost, wearable, and wireless insole-based gait analysis system that provides kinetic measurements of gait using low-cost force sensitive resistors [4]. Rueterbories et al. used FSR to investigate the gait events for the stroke patients [5]. However, the drift of FSR during a repetitive loading and the capacity of FSR are not enough to cover the gait analysis for human walking [6]. In order to utilize GRF for gait abnormality analysis and robotic control, the absolute GRF data is essential. For example, the increase of the weight by an external load has to be compensate to reduce
Fig. 1. (a) Developed optoelectronic based force sensor, (b) force sensitivity resistors(FSR), (c) data aquisition process of two kinds of sensors, and (d) the proposed GRF measurement system. a load for the user who is a wearing lower-limb exoskeleton. In this study, we developed an optoelectronic force sensor and an insole-type GRF measurement system with four developed sensors. To increase the capacity of the developed sensor, we installed springs inside the sensor. Performance was validated using a step response test and a dynamic walking test and compared with that of FSRs. 2. Materials and Methods We developed a uniaxial force sensor using an optical distance sensor, which is used to measure the relative distance between two aluminum plates. The gap between the two plates are changed depending on the force loaded on the upper plate of the sensor. To bear high loads, belleville springs are installed in the sensor as shown in Figure 1 (a). The springs and structure of the sensor was set to cover up to a maximum force capacity of 900N. FSR (Flexiforce, A201, Tekscan, USA), which has been widely used for measuring GRF, was chosen for comparing the performance of the proposed system (Figure 1 (b)). The maximum capacity of the FSR is 450 N. Four FSRs were used to measure the GRFs during level ground walking with the proposed sensors. Sensor location was selected based on the structure of human foot
Fig. 2. (a) Treadmill-type force plate test with the developed GRF measurement system (b) experiments setup for the heel strike impact test skeleton (calcaneus, metatarsal, metatarsus, and phalange) as shown in Figure 1 (c) The locations of developed sensor and FSR were identical for four locations in the proposed system. All signals in this experiments were acquired by a microcontroller (Jarduino, JCnet, Korea) with 400 Hz sampling rate (Figure 1 (d)). The raw force data acquired using both the developed force sensor and FSR were low-pass filtered at a cut-off frequency of 5 Hz. 3. Experiments and Results 3.1 Treadmill Level Ground Walking Test Figure 2 (a) shows the experimental setting of the level ground walking test on a treadmill equipped with a force plate. A male subject weighing 71kg wearing the developed insole-type GRF measurement system was asked to walk at 4 km/h for 30 minutes on a treadmill. A total of 150 steps were extracted. Figure 3 (a) shows the GRF data acquired from the force plate and the summation of the four force data measured using the developed GRF measurement system. The normalized root mean square error (NRMSE) between the force plate data and the measured data was approximately 14.68±4.75 %.
Fig. 3. (a) GRF comparision between force plate and developed sensor system (b) GRF of force sensors at calcaneus, metatarsal, metatarsus, and phalange (c) GRF comparision between force sensor and FSR at calcaneus 3.2 Heel Strike Impact Test Figure 2 (b) shows the experimental setting for the heel strike impact test. The developed custom force sensor and FSR were overlapped and fixed to the insole of a flexible shoe. The locations of the sensors were as shown in the Figure 1 (c) A male subject weighing 69 kg was asked to walk at a self-selected speed. A total of 15 steps were acquired for data analysis. Figure 3 (b) shows the force data measured at the calcaneus, metatarsal, metatarsus, and phalange and the summation of the four for the developed custom force sensor. Figure 3 (c) and (d) are the measured force data of the developed custom force sensor and FSR located at the heel of the insole-type GRF measurement system. Figure 3 (c) is the filtered average data of the 15 steps and Figure 3 (d) is the data of a single
step. Both in Figure (c) and Figure (d) clipping was observed when a larger load the maximum capacity was applied to the FSR. 4. Conclusion We proposed a custom force sensor for measuring GRF, which has higher force capacity than FSR. By applying the belleville springs to the force sensor, we were able to increase the range of force measurement. Unlike other kinds of force sensors, the developed force sensor does not require external amplifiers. This makes it easier to install the force sensor to the sole of shoes and measure GRF. For future works, we will install the developed force sensor to a lower limb exoskeleton to investigate various terrains and to extract human intention. Moreover, we will develop an algorithm to compensate the weight when user carries higher loads. Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and future Planning (No. 2015-002966).
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