Adjustable Linkages Research Articles

Design parameters and torque profile modification of a spring-assisted hand-opening exoskeleton module.

There is a growing demand for functional rehabilitation orthotics that can effectively assist in patient recovery from motor impairments after stroke. The hand in particular is a complex system that has proven difficult to mimic with current exoskeleton technologies. This paper presents data-driven design parameters to increase the functionality and improve the assistance profile of the ArmAssist-2.0 hand module. Improvements from the previous model include adjustable linkages to fulfill the largest population of users, new joint locations to more accurately represent biomechanics of the hand, and a more impairment-appropriate torque profile to assist in hand opening, adjustable through interchangeable springs. In most passive hand orthoses, assistance forces tend to decrease as the hand and thumb extend, opposite the needs of a typical patient hand. This project utilizes a variable spring moment arm about the revolute axes to match common patient impairment more accurately. The revised assistance profile for the hand maintains a nearly linear relationship. Results conclude that the final assembled device fits comfortably in the hand with noticeable improvements in joint locations, adjustability, and the force profile for the metacarpophalangeal (MCP) joint. An issue arises with the extension of the proximal interphalangeal (PIP) joint due to the nature of rapidly changing moment arms and multiple springs in series. The issue and possible solutions are discussed.

A Method for Adjustable Planar and Spherical Four-Bar Linkage Synthesis

This paper describes a new method to synthesize adjustable four-bar linkages, both in planar and spherical form. This method uses fixed ground pivots and an adjustable length for input and output links. A new application of Burmester curves for adjustable linkages is introduced, and a numerical example is discussed. This paper also compares a conventional synthesis method (nonadjustable linkage) to the new method. Nonadjustable four-bar linkages provide limited solutions for five-position synthesis. Adjustable linkages generate one infinity of solution choices. This paper also shows that the nonadjustable solutions are special cases of adjustable solutions. This new method can be extended to six position synthesis, with adjustable ground pivots locations.

Analysis and optimal synthesis of adjustable linkages for path generation

Adjustable linkages can generate different paths with a simple adjustment of the fixed pivot position of the driven side-link. Limited work has been done in the area of synthesis of four-bar linkages for continuous path generation, especially of adjustable four-bar linkages. There are two kinds of adjustable four-bar linkages with fully rotatable driving side-links, adjustable crank-rocker and adjustable drag-link linkages. The path flexibility of the two adjustable linkages is analysed. The optimal synthesis model is set up based on the link-length structural error of the driven side-link introduced in this paper, which can effectively reflect the overall difference between the desired and the generated paths, can avoid the difficulty of selecting corresponding comparison points on the two paths, and can be calculated conveniently. A modified genetic algorithm is employed to obtain the global optimal solution. The results of two optimal synthesis examples verify the effectiveness of the proposed method.

Adjustable Straight-Line Linkages—Possible Legged-Vehicle Applications

The stimulus for this paper is the expressed need in proposals for legged vehicles both to propel the vehicle in rectilinear translation parallel to the mean ground-profile beneath it and to allow the chassis-height to be adjustable. Energy considerations point toward having a single propulsion-actuator on each leg-linkage rather than adopting the less economical option of simultaneous actuation at two or more serially connected leg-joints. Of the linkages whose kinematics are here studied some relate more closely to legged vehicles than others do; the paper should be viewed in a general rather than a specific context. Exact and approximate straight-line planar linkages are covered. Derivatives of Richard Roberts’s linkage are put forward as those that are the most promising adjustable linkages whose joints are all turning pairs.

Technology Transfer in the Design of Adjustable Linkages

When the input crank is in line with the fixed link, the circling-point curve degenerates into a circle and straight line. This characteristic applied to a four-bar linkage with symmetrical coupler curves can be utilized to design linkage mechanisms adjustable for variable circular arcs. The design technique is simple and should stimulate the designer’s interest.