No abstract

Magneto-Inertial Measurement Unit sensors (MIMU) display high potential for the quantitative evaluation of upper limb kinematics, as they allow monitoring ambulatory measurements. The sensor-to-segment calibration step, consisting of establishing the relation between MIMU sensors and human segments, plays an important role in the global accuracy of joint angles. The aim of this study was to compare sensor-to-segment calibrations for the MIMU-based estimation of wrist, elbow, and shoulder joint angles, by examining trueness (“close to the reference”) and precision (reproducibility) validity criteria. Ten subjects performed five sessions with three different operators. Three classes of calibrations were studied: segment axes equal to technical MIMU axes (TECH), segment axes generated during a static pose (STATIC), and those generated during functional movements (FUNCT). The calibrations were compared during the maximal uniaxial movements of each joint, plus an extra multi-joint movement. Generally, joint angles presented good trueness and very good precision in the range 5°–10°. Only small discrepancy between calibrations was highlighted, with the exception of a few cases. The very good overall accuracy (trueness and precision) of MIMU-based joint angle data seems to be more dependent on the level of rigor of the experimental procedure (operator training) than on the choice of calibration itself.

This paper proposes a technique to calculate the coefficient of friction for fingertip-object interface. Twelve subjects (6 males and 6 females) participated in two experiments. During the first experiment (the "imposed displacement" method) a 3D force sensor was moved horizontally while the subjects applied a specified normal force (4N, 8N, 12N) on the surface of a sensor covered with different materials (sandpaper, cotton, rayon, polyester, and silk).The normal force and the tangential force (i.e. the force due to the sensor motion) were recorded. The coefficient of friction (µ d ) was calculated as the ratio between the tangential force and the normal force. In the second experiment (the "beginning slip" method), a small instrumented object was gripped between the index finger and the thumb, held stationary in the air and then allowed to drop. The weight (200g, 500g and 1000g) and the surface (sandpaper, cotton, rayon, polyester and silk) in contact with the digits varied across trials. The same sensor as in the first experiment was used to record the normal force (in a horizontal direction) and the tangential force (in the vertical direction). The slip force (i.e. the minimal normal force or grip force necessary to prevent slipping) was estimated as the force at the moment when the object just began to slip. The coefficient of friction was calculated as the ratio between the tangential force and the slip force. The results show that: (1) the "imposed displacement" method is reliable; (2) except sandpaper, for all other materials the coefficient of friction did not depend of the normal force; (3) the skin-sandpaper coefficient of friction was the highest µ d = 0.96±0.09 (for 4N normal force) and the skin-rayon rayon coefficient of friction was the smallest µ d = 0.36±0.10; (4) no significant difference between the coefficients of friction determined with the "imposed displacement" method and the "beginning slip" method was observed. We view the "imposed displacement" technique as having an advantage as compared to the beginning slip method, which is more cumbersome (e.g. dropped object should be protected from impacts) and prone to subjective errors due to uncertainty in determining the instance of the slip initiation (i.e. impeding sliding).

No abstract

No abstract