Single-dof Mechanisms Research Articles

Design of a novel low-speed gait rehabilitation device incorporating a synthesized geared five-bar mechanism

People with difficulty walking due to a stroke or neurophysiological gait deficit require gait rehabilitation. Recent research indicates that the optimal speed range for gait rehabilitation of such patients is less than 0.4 m/s. The design of a novel low-speed gait rehabilitation device based on a single degree of freedom (DOF) geared five-bar mechanism is presented in this paper. For this purpose, a natural gait meta-trajectory is generated using the recently acquired low-speed gait data suitable for human walking. A less-bulky single-DOF geared five-bar mechanism is synthesized and adjusted by applying the accurate and robust shadow robot control (SRC) synthesis method. Therefore, a practical and easily controlled gait rehabilitation device is designed, which can be mounted on any commercial treadmill and enables the patient to walk as close as possible to a normal gait. This device applies to a wide variety of individuals with different physiologies due to the mechanism’s adjustability, provided by the profile blending algorithm method. This method generates a set of continuous and speed-dependent gait trajectories for various individuals. The proposed device is inherently safe and easy to control due to the limited motion range of the single-DOF mechanisms and the use of a single actuator. Furthermore, because it only has two points of contact with the body (at the hip and the ankle), it takes less time for the user to don and doff.

Singularity analysis of spatial single-DOF mechanisms based on the locations of the instantaneous screw axes

The instantaneous kinematics of a mechanism can be fully described through the instantaneous screw axes (ISAs) associated to the relative motions between mechanism's links. Characterizing the instantaneous kinematics of a mechanism includes its singularity analysis, which is the identification of mechanism configurations (singularities) where the instantaneous degrees-of-freedom (DOFs) of the mechanism “somehow” change. In single-DOF mechanisms with time-independent either holonomic or first-order non-holonomic constraints, the locations of the instantaneous screw axes only depend on the mechanism configuration. This property is fully exploited in planar single-DOF mechanisms where all the singularity conditions have been associated to the coincidence of particular instant centers (ICs), thus making the singularity identification possible with a simple analysis of the mechanism configuration. Differently, for single-DOF spatial mechanisms, the geometric conditions on ISAs that identify a singular configuration have not been enunciated for the general case, yet. This paper fills this lack by providing the explicit expressions of all the possible instantaneous input-output relationships, the exhaustive enumeration of the geometric conditions on the ISAs that identify singular configurations and a geometric and analytic technique for the singularity analysis of spatial single-DOF mechanisms, which is the natural extension of the IC-based ones conceived for planar single-DOF mechanisms.

Design and Development of a Mobile Robotic CT System for Intraoperative Use

This paper presents the design of a mobile robotic computed tomography (CT) system for intraoperative use. In particular, this paper presents a novel gantry mechanism and a novel mechanism generating isocentric motions. The developed system mechanically consists of a gantry and a mobile robotic body. The gantry contains the X-ray modules and has an opening mechanism that enables full 360 ${^\circ }$ rotation. The robotic body is capable of one vertical and two isocentric rotational motions used to position the gantry. Unlike existing C-arms and mobile CT scanners, each isocentric motion is achieved using a single-DOF mechanism. With these robotic motions, the mobile robotic CT system can be used intraoperatively in both CT and fluoroscopic imaging modes. The mobile platform enables installation and removal of the device from a surgical bed. The performance of the proposed system was investigated using a laser tracking system. Experimental results show that no significant vibrations or position deviations of the X-ray modules were observed during the CT scanning motion.

Assembly Configurations of Spatial Single-Loop Single-DOF Mechanisms

This paper examines the problem of finding the assembly configurations (ACs), also called circuits, of spatial single-loop single-DOF mechanisms. All six lower pairs are considered: revolute (R), cylindric (C), prismatic (P), helical (H), spheric (S), and planar (E). The RSSR, CSS, RRSC, CCS, and RRSRR mechanisms are covered, along with all variations in which an R joint is replaced with a P or H joint, or an S joint is replaced with an E joint. The RRCRC, RRRCC, and RRRRCR mechanisms are covered, along with all appropriate variations in which an R joint is replaced by a P or H joint. A numerical method is given to find the ACs, and is illustrated with several examples.

Kinematic Synthesis of Planar, Shape-Changing Rigid-Body Mechanisms

Abstract This paper presents a kinematic procedure to synthesize planar mechanisms, composed of rigid links and revolute joints, capable of approximating a shape change defined by a set of curves. These “morphing curves”, referred to as design profiles, differ from each other by a combination of rigid-body displacement and shape change. Design profiles are converted to piecewise linear curves, referred to as target profiles, that can be readily manipulated. In the segmentation phase, the geometry of rigid links that approximate the shapes of corresponding segments from each target profile is determined. In the mechanization phase, these rigid links are joined together at their end points with revolute joints to form a single chain. Dyads are then added to reduce the number of degrees of freedom (DOF’s) to any desired value, typically 1. The approach can be applied to any number of design profiles that can be approximated with any number of rigid links, which can then be used to construct a mechanism with any number of DOF’s. Naturally, greater difficulty is encountered for larger numbers of design profiles and/or links and for more dramatic changes in shape. The procedure is demonstrated with examples of single-DOF mechanisms approximating shape changes between two and three design profiles.

An Automatic Method for Finding the Assembly Configurations of Planar Non-Single-Input-Dyadic Mechanisms

This paper examines the problem of identifying the assembly configurations (ACs), also called circuits, of planar non-single-input-dyadic (NSID) mechanisms. An SID mechanism is a multi-loop mechanism which can be defined by adding one loop at a time such that the mechanism has one degree of freedom (DOF) after each loop is added. An NSID mechanism is any mechanism that does not meet the SID criterion. This includes all multi-DOF mechanisms, and some complex single-DOF mechanisms. An automatic method is presented which allows a computer to determine the ACs of an NSID mechanism. For single-DOF mechanisms, the ACs are represented by curves drawn in a plane represented by two joint variables. For multi-DOF mechanisms, the ACs consist of one or more regions in the plane, which are defined by the curves that bound them. The automated method finds these bounding curves, and then determines which curves belong to the same region, and which regions belong to the same AC. Mechanisms with pin joints and sliding joints are considered.

A centro-based characterization of singularities in the workspace of planar closed-loop manipulators

The paper presents a centro-based characterization of singularities in planar multi-DOF mechanisms with arbitrary kinematic structure. A multi-DOF mechanism is treated as an assembly of several single DOF link sets and hence the terms like affected link set, active link set and active centro have been defined. Criteria for end-effector dependent and end-effector independent singularities of a closed-loop manipulator with given output link have been established in the paper in terms of centroes. Based on the relative disposition of the centroes, the singularities have been classified into five types and their physical implications have been explained. End-effector dependent singularities are shown to lie on the coupler curve of a larger single DOF mechanism. A simple linkage transformation has been proposed for the study of manipulators with physical limits on joint motion under the general framework of close-loop manipulators. Several examples have been worked out to demonstrate the centro-based method of singularity analysis.