Abstract
The motion of a rigid body can be represented by the instantaneous screw axis (ISA, also known as the helical axis). Recently, an invariant representation of motion based on the ISA, namely, the screw axis invariant descriptor (SAID), was proposed in the literature. The SAID consists of six scalar features that are independent from the coordinate system chosen to represent the motion. This method proved its usefulness in robotics; however, a high sensitivity to noise was observed. This paper aims to explore the performance of inertial sensors for the estimation of the ISA and the SAID for a simple experimental setup based on a hinge joint. The free swing motion of the mechanical hinge was concurrently recorded by a marker-based optoelectronic system (OS) and two magnetic inertial measurement units (MIMUs). The ISA estimated by the MIMU was more precise, while the OS was more accurate. The mean angular error was ≈2.2° for the OS and was ≈4.4° for the MIMU, while the mean standard deviation was ≈2.3° for the OS and was ≈0.2° for the MIMU. The SAID features based on angular velocity were better estimated by the MIMU, while the features based on translational velocity were better estimated by the OS. Therefore, a combination of both measurements systems is recommended to accurately estimate the complete SAID.
Highlights
The estimation of the instantaneous screw axis (ISA), known as twist axis, helical axis or axis of rotation [1], plays a notable role in the biomechanical analysis of human joints
Discussion computed by the optoelectronic system (OS) and magnetic inertial measurement units (MIMUs)
This study comparatively examined two measurement systems, the OS and the MIMU, for the calculations of the ISA and the screw axis invariant descriptor (SAID) of a simple motion task based on a mechanical hinge setup
Summary
The estimation of the instantaneous screw axis (ISA), known as twist axis, helical axis or axis of rotation [1], plays a notable role in the biomechanical analysis of human joints. The general screw displacement for a rigid body can be formulated in terms of points and lines representing some finite displacements by means of screw displacement pairs [5]. Such a representation for motion was widely adopted in the fields of robotics and motion analysis as it represents a convenient way to describe and manipulate trajectories and the relative motion between rigid bodies, such as the consecutive segments of a robot. Sensors 2020, 20, 49 manipulator [6,7,8] Another example is a recent study where the ISA representation was exploited to determine the twist and the wrench acting on a vehicle suspension system [9]
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