Optimization of Sensor Placement Combinations and Classification Thresholds for the Accelerometer-Based Activity Recognition
Binary decision tree classification and usage of multiple accelerometers have been suggested to be simple and accurate method for the recognition of daily activities. However, the determination of classification thresholds and sensor placement combinations have been empirical and subjective. The purpose of this study was to find optimal set of sensors and classification thresholds in a systematic way considering both accuracy and decision margin of binary decisions. Sixteen healthy men participated in this study. Wireless tri-axial accelerometers were attached on eight body locations i.e., forehead, arm, chest, waist, wrist, thigh, shank, and ankle. Subjects performed three static activities (standing, sitting, and lying) and two dynamic activities (walking and running). Binary tree classification scheme with four binary decisions was constructed using signal magnitude area, variance of vertical acceleration, and tilt angle as classification features. Optimal thresholds and decision margin for every sensor location were calculated for each binary decision. Then, all sensor combinations (with 1–3 sensors) were evaluated in terms of total accuracy and overall decision margin to derive optimal ones. Three of four binary decisions (static vs. dynamic, walking vs. running, and upright posture vs. lying) showed 100% accuracy at multiple sensor locations. The classification of standing versus sitting was rather difficult, where the best accuracy was 98.75% at two sensor locations (wrist and thigh). The optimal single sensor location was the wrist (accuracy: 98.15%, margin: 22.8%). Optimal combination of two sensor locations was arm and thigh (accuracy: 99.5%, margin: 209.3%) and that of three sensors was arm, thigh, and ankle (accuracy: 99.5%, margin: 214.8%). Wrist would be the best option for single sensor recognition. If accuracy and decision margin are of critical interest, two sensors at arm and thigh are recommended. Three sensors are not recommended because of minor improvement.
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Document Type: Research Article
Publication date: January 1, 2018
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