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Analysis of a Wearable Joint Flexion Sensor Using Anatomical Tissue Models
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Real-time motion capture of the human arm in the home environment has many usecases, such as video game and therapy applications. The required tracking can be based onoff-the-shelf Inertial Measurement Units (IMUs) with integrated three-axis accelerometers, gyroscopes,and magnetometers. However, this usually requires a homogeneous magnetic field to correctfor orientation drift, which is often not available inside buildings. In this paper, RPMC (RestPose Magnetometer-based drift Correction), a novel method that is robust to long term drift inenvironments with inhomogeneous magnetic fields, is presented. The sensor orientation is estimatedby integrating the angular velocity measured by the gyroscope and correcting drift around the pitchand roll axes with the acceleration information. This commonly leads to short term drift aroundthe gravitational axis. Here, during the calibration phase, the local magnetic field direction for eachsensor, and its orientation relative to the inertial frame, are recorded in a rest pose. It is assumed thatarm movements in free space are exhausting and require regular rest. A set of rules is used to detectwhen the user has returned to the rest pose, to then correct for the drift that has occurred with themagnetometer. Optical validations demonstrated accurate (root mean square error RMS = 6.1), lowlatency (61 ms) tracking of the user’s wrist orientation, in real time, for a full hour of arm movements.The reduction in error relative to three alternative methods implemented for comparison was between82.5% and 90.7% for the same movement and environment. Therefore, the proposed arm trackingmethod allows for the correction of orientation drift in an inhomogeneous magnetic field by exploitingthe user’s need for frequent rest.
Objective:
Sport research often requires human motion capture of an athlete. It can, however, be labour-intensive and difficult to select the right system, while manufacturers report on specifications which are determined in set-ups that largely differ from sport research in terms of volume, environment and motion. The aim of this review is to assist researchers in the selection of a suitable motion capture system for their experimental set-up for sport applications. An open online platform is initiated, to support (sport)researchers in the selection of a system and to enable them to contribute and update the overview.
Design:
systematic review; Method: Electronic searches in Scopus, Web of Science and Google Scholar were performed, and the reference lists of the screened articles were scrutinised to determine human motion capture systems used in academically published studies on sport analysis.
Results:
An overview of 17 human motion capture systems is provided, reporting the general specifications given by the manufacturer (weight and size of the sensors, maximum capture volume, environmental feasibilities), and calibration specifications as determined in peer-reviewed studies. The accuracy of each system is plotted against the measurement range.
Conclusion:
The overview and chart can assist researchers in the selection of a suitable measurement system. To increase the robustness of the database and to keep up with technological developments, we encourage researchers to perform an accuracy test prior to their experiment and to add to the chart and the system overview (online, open access).
Flexible sensors that can be integrated into clothing to measure everyday functional performance is an emerging concept. It aims to improve the patient's quality of life by obtaining rich, real-life data sets. One clinical area of interest is the use of these sensors to accurately measure knee motion in, e.g., osteoarthritic patients. Currently, various methods are used to formally calculate joint motion outside of the laboratory and they include electrogoniometers and inertial measurement units. The use of these technologies, however, tends to be restricted, since they are often bulky and obtrusive. This directly influences their clinical utility, as patients and clinicians can be reluctant to adopt them. The goal of this paper is to present the development process of a patient centered, clinically driven design for an attachable clothing sensor (ACS) system that can be used to assess knee motion. A pilot study using 10 volunteers was conducted to determine the relationship between the ACS system and a gold standard apparatus. The comparison yielded an average root mean square error of ~ 1°, a mean absolute error of ~ 3°, and coefficient of determination above (R2) 0.99 between the two systems. These initial results show potential of the ACS in terms of unobtrusive long-term monitoring.
This paper describes a feasibility study for a self-contained, wearable full-body motion capture system based on time-of-flight measurements that provide absolute distances between points on the body. Our motivation for the system is to allow an e-textile garment to sense its own shape using only body-worn sensors, thereby enabling it to dynamically adapt its sensing and processing elements to the user's current pose. Furthermore, a garment that can sense its own shape would enable an untethered and self-contained motion capture system. We explore the potential accuracy of the system via simulation driven by motion data from several users performing various activities, including effects such as the number and placement of sensors on the torso, shadowing of signal transmission by the body, and sensor directionality. We conclude that the system is feasible, albeit with an accuracy that is at least an order of magnitude less than state-of-the-art laboratory systems, and that its accuracy will depend heavily upon the transmission properties of the sensors.
Objective:
We introduce and validate a new class of wearable coils that seamlessly monitor joint flexion in the individual's natural environment while overcoming shortcomings in state-of-the-art.
Methods:
Our approach relies on Faraday's law of induction and employs wrap-around transmit and receive coils that get angularly misaligned as the joint flexes.
Results:
Simulation and in vitro measurement results for both copper and e-thread coils are in excellent agreement. As a proof-of-concept, a cylindrical arm model is considered and feasibility of monitoring the 0°-130° range of motion is confirmed. The operation frequency of 34 MHz is identified as optimal, bringing forward reduced power requirements, enhanced angular resolution, and extreme robustness to tissue dielectric property variations. Performance benchmarking versus state-of-the-art inertial measurement units shows equivalent or superior performance, particularly for flexion angles greater than 20°. Design guidelines and safety considerations are also explored.
Conclusion:
Contrary to "gold-standard" camera-based motion capture, the reported approach is not restricted to contrived environments. Concurrently, it does not suffer from integration drift (unlike inertial measurement units), it does not require line-of-sight (unlike time-of-flight sensors), and it does not restrict natural joint movement (unlike bending sensors).
Significance:
The reported approach is envisioned to be seamlessly integrated into garments and, eventually, redefine the way joint flexion is monitored at present. This promises unprecedented opportunities for rehabilitation, sports, gestural interaction, and more.