Kyoko Shibata

Novel approach to ambulatory assessment of human segmental orientation on a wearable sensor system

A new method using a double-sensor difference based algorithm for analyzing human segment rotational angles in two directions for segmental orientation analysis in the three-dimensional (3D) space was presented. A wearable sensor system based only on triaxial accelerometers was developed to obtain the pitch and yaw angles of thigh segment with an accelerometer approximating translational acceleration of the hip joint and two accelerometers measuring the actual accelerations on the thigh. To evaluate the method, the system was first tested on a 2 degrees of freedom mechanical arm assembled out of rigid segments and encoders. Then, to estimate the human segmental orientation, the wearable sensor system was tested on the thighs of eight volunteer subjects, who walked in a straight forward line in the work space of an optical motion analysis system at three self-selected speeds: slow, normal and fast. In the experiment, the subject was assumed to walk in a straight forward way with very little trunk sway, skin artifacts and no significant internal/external rotation of the leg. The root mean square (RMS) errors of the thigh segment orientation measurement were between 2.4 degrees and 4.9 degrees during normal gait that had a 45 degrees flexion/extension range of motion. Measurement error was observed to increase with increasing walking speed probably because of the result of increased trunk sway, axial rotation and skin artifacts. The results show that, without integration and switching between different sensors, using only one kind of sensor, the wearable sensor system is suitable for ambulatory analysis of normal gait orientation of thigh and shank in two directions of the segment-fixed local coordinate system in 3D space. It can then be applied to assess spatio-temporal gait parameters and monitoring the gait function of patients in clinical settings.

A Wearable Ground Reaction Force Sensor System and Its Application to the Measurement of Extrinsic Gait Variability

Wearable sensors for gait analysis are attracting wide interest. In this paper, a wearable ground reaction force (GRF) sensor system and its application to measure extrinsic gait variability are presented. To validate the GRF and centre of pressure (CoP) measurements of the sensor system and examine the effectiveness of the proposed method for gait analysis, we conducted an experimental study on seven volunteer subjects. Based on the assessment of the influence of the sensor system on natural gait, we found that no significant differences were found for almost all measured gait parameters (p-values < 0.05). As for measurement accuracy, the root mean square (RMS) differences for the two transverse components and the vertical component of the GRF were 7.2% ± 0.8% and 9.0% ± 1% of the maximum of each transverse component and 1.5% ± 0.9% of the maximum vertical component of GRF, respectively. The RMS distance between both CoP measurements was 1.4% ± 0.2% of the length of the shoe. The area of CoP distribution on the foot-plate and the average coefficient of variation of the triaxial GRF, are the introduced parameters for analysing extrinsic gait variability. Based on a statistical analysis of the results of the tests with subjects wearing the sensor system, we found that the proposed parameters changed according to walking speed and turning (p-values < 0.05).

A wearable force plate system for the continuous measurement of triaxial ground reaction force in biomechanical applications

The ambulatory measurement of ground reaction force (GRF) and human motion under free-living conditions is convenient, inexpensive and never restricted to gait analysis in a laboratory environment and is therefore much desired by researchers and clinical doctors in biomedical applications. A wearable force plate system was developed by integrating small triaxial force sensors and three-dimensional (3D) inertial sensors for estimating dynamic triaxial GRF in biomechanical applications. The system, in comparison to existent systems, is characterized by being lightweight, thin and easy-to-wear. A six-axial force sensor (Nitta Co., Japan) was used as a verification measurement device to validate the static accuracy of the developed force plate. To evaluate the precision during dynamic gait measurements, we compared the measurements of the triaxial GRF and the center of pressure (CoP) by using the developed system with the reference measurements made using a stationary force plate and an optical motion analysis system. The root mean square (RMS) differences of the two transverse components (x- and y-axes) and the vertical component (z-axis) of the GRF were 4.3 ± 0.9 N, 6.0 ± 1.3 N and 12.1 ± 1.1 N, respectively, corresponding to 5.1 ± 1.1% and 6.5 ± 1% of the maximum of each transverse component and 1.3 ± 0.2% of the maximum vertical component of GRF. The RMS distance between the two systems CoP traces was 3.2 ± 0.8 mm, corresponding to 1.2 ± 0.3% of the length of the shoe. Moreover, based on the results of the assessment of the influence of the system on natural gait, we found that gait was almost never affected. Therefore, the wearable system as an alternative device can be a potential solution for measuring CoP and triaxial GRF in non-laboratory environments.

A Mobile Force Plate and Three-Dimensional Motion Analysis System for Three-Dimensional Gait Assessment

In order to implement an unobstructed assessment of three-dimensional (3-D) gait, we developed a mobile force plate and 3-D motion analysis system (M3D) to measure triaxial ground reaction forces (GRF) and 3-D orientations of feet. Calibration and test experiments were conducted to characterize the sensor developed. To test the accuracy of the new measurement system, validation experiments by using the reference measurements of a commercially available measurement system were performed in a gait laboratory, where a stationary force plate, a motion capture system based on high-speed cameras and a motion track system of XSENS were adopted to analyze human movements. Experimental results supported the proposal that the developed system can be used to measure triaxial GRF and orientations with an acceptable precision during successive walking gait.

A Small and Low-Cost 3-D Tactile Sensor for a Wearable Force Plate

In this paper, a new 3-D tactile sensor is proposed for measuring triaxial ground reaction force (GRF) distribution. A pressure-sensitive electric conductive rubber (PSECR) and compact pectinate circuits were used to design the sensing cells of the sensor, making it possible to implement a low-cost and compact system without a complex 3-D structure. Moreover, to tailor the application for measuring human GRF, we adopted the use of elastic rubber as the contact interface of the sensor in order to realize a comfortable human-sensor interface. Calibration and test experiments were conducted to characterize the developed sensor, and a small triaxial force sensor (Tec Gihan, Japan) as well as a six-axial force sensor (Nitta Corporation, Japan) were used as verification measurement devices. Coupling effect tests were performed to calculate cross-sensitivity of the sensor. The experimental results of repeatability, nonlinearity, hysteresis, and dynamic tests indicate that the sensor is feasible for implementing 3-D tactile measurement.

Ambulatory Estimation of Knee-Joint Kinematics in Anatomical Coordinate System Using Accelerometers and Magnetometers

Knee-joint kinematics analysis using an optimal sensor set and a reliable algorithm would be useful in the gait analysis. An original approach for ambulatory estimation of knee-joint angles in anatomical coordinate system is presented, which is composed of a physical-sensor-difference-based algorithm and virtual-sensor-difference-based algorithm. To test the approach, a wearable monitoring system composed of accelerometers and magnetometers was developed and evaluated on lower limb. The flexion/extension (f/e), abduction/adduction (a/a), and inversion/extension (i/e) rotation angles of the knee joint in the anatomical joint coordinate system were estimated. In this method, since there is no integration of angular acceleration or angular velocity, the result is not distorted by offset and drift. The three knee-joint angles within the anatomical coordinate system are independent of the orders, which must be considered when Euler angles are used. Besides, since there are no physical sensors implanted in the knee joint based on the virtual-sensor-difference-based algorithm, it is feasible to analyze knee-joint kinematics with less numbers and types of sensors than those mentioned in some others methods. Compared with results from the reference system, the developed wearable sensor system is available to do gait analysis with fewer sensors and high degree of accuracy.

A six-dimension parallel force sensor for human dynamics analysis

In this paper, parallel support principle of force sensor is discussed, and a six-dimension parallel force sensor is presented for human dynamics analysis. The force sensor with parallel support mechanism was designed to measure six-axis reaction forces during human walking. Finite element method was adopted to optimize mechanism dimension of the force sensor. Sensitivity of force sensor was improved by distributing strain gages on the maximum strain positions. A three-direction drag mechanism was designed for calibrating load cells in the parallel force sensor, and method of least-squares was used to calculate calibration coefficients.

Measurement of human lower limb orientations and ground reaction forces using wearable sensor systems

This paper presents a study on quantitative dynamics analysis of human lower limb using developed wearable sensor systems that can measure reaction force and detect the following gait phases: initial contact, loading response, mid stance, terminal stance, pre-swing, initial swing, mid swing and terminal swing. Since conventional camera-based motion analysis system and reaction force plate system require costly devices, vast space as well as time-consuming calibration experiments, the wearable sensor-based system is much cheaper. Gyroscopes and two-axis accelerometers are incorporated in this wearable sensor system. The former are attached on the surface of the foot, shank and thigh to measure the angular velocity of each segment, and the latter are used to measure inclination of the attached leg segment (shank) in every single human motion cycle for recalibration. Ground reaction forces during human walking are synchronously measured using a wearable force sensor integrated in a shoes mechanism. Finally, experiment has been performed to compare the measurement results from the wearable sensor system with the data obtained from an optical motion analysis system and a force plate. The results showed that the measurement of human lower limb orientations and reaction forces for human dynamics analysis could be reliably implemented using the wearable sensor systems.

Imitation Control for Biped Robot Using Wearable Motion Sensor

In conventional imitation control, optical tracking devices have been widely adopted to capture human motion and control robots in a laboratory environment. Wearable sensors are attracting extensive interest in the development of a lower-cost human-robot control system without constraints from stationary motion analysis devices. We propose an ambulatory human motion analysis system based on small inertial sensors to measure body segment orientations in real time. A new imitation control method was developed and applied to a biped robot using data of human joint angles obtained from a wearable sensor system. An experimental study was carried out to verify the method of synchronous imitation control for a biped robot. By comparing the results obtained from direct imitation control with an improved method based on a training algorithm, which includes a personal motion pattern, we found that the accuracy of imitation control was markedly improved and the tri-axial average errors of x-y- and z-moving displacements related to leg length were 12%, 8% and 4%, respectively. Experimental results support the feasibility of the proposed control method.

Ambulatory measurement and analysis of the lower limb 3D posture using wearable sensor system

An original approach for ambulatory measurement and analysis of lower limb 3D gait posture was presented, and a wearable sensor system was developed according to the approach. To explicate the lower limb posture, thigh orientation angles were calculated based on a virtual sensor at the hip joint and double analog inertial sensors (MAG3) on the thigh; Knee joint angle in sagittal plane was calculated with combination of angular accelerations and angular velocities measured by two MAG3 on the thigh and shank on the basis of the virtual-sensor based algorithm. The developed wearable sensor system was evaluated on the lower limb. Without integration of angular acceleration or angular velocity for the thigh orientation angles and the knee joint angle, the calculated result was not distorted by offset and drift. Using virtual sensors at the hip joint and the knee joint were more simple, practical and effective than fixing physical sensors at these joints. Compared with the result from the reference system, the measured result with the developed wearable sensor system was feasible to do gait analysis for the patients in the daily life, and the method can also be used in other conditions such as measuring rigid segment posture with less sensors and high degree of accuracy.