Wrist Rehabilitation

This work was completed in collaboration with Derek Bissell and Dr. David Reinkensmeyer of the Biorobotics Laboratory at UC Irvine.  To view Derek Bissell’s Master’s Thesis, click here. The goal of this research was to design a passive device capable of cancelling out the excess stiffness in the wrist of stroke patients that suffer from spasticity. This excess stiffness is also known as tone and is characterized by a curve that is dependent on wrist angle.

Fig. from Mirbagheri and Settle

The curve above is integrated in order to find a torque profile that is capable of cancelling tone.

By placing a linear torsional spring on one ground pivot of a six-bar linkage, the second ground pivot will act like a nonlinear torsional spring.

If linkage dimensions can be found that create the correct relationship between angles φ and ψ, then the desired torque profile will be present at the joint measured by φ. Finding these linkage dimensions is possible through the synthesis of a six-bar function generator. This methodology makes linkage synthesis for a specified torque profile equivalent to linkage synthesis for function generation. The desired torque profile (labelled “Polynomial”) and the linkage generated profile (labelled “Linkage Profile”) appear below.

A video of the prototype appears below.

Current work on this project includes the development of a portable tone-cancelling wrist brace for everyday functionality. The device is passive and features a single linear torsional spring capable of achieving a complex torque profile at the wrist joint through the motion of a six-bar linkage. The torque profile cancels out the excess stiffness in the stroke patient’s wrist similar to the tested design above.

M. M. Mirbagheri and K. Settle, 2010. “Neuromuscular properties of different spastic human joints vary systematically,” 32nd Annual International Conference of the IEEE EMBS, Buenos Aires, Argentina.

Humanoid Gait

The motion of a human walking gait was observed and recorded by analyzing videos. From these videos, three periodic joint angle functions were documented at the hip, knee, and ankle, which are shown in the figures below.

Knee Function

Ankle Function


A Stephenson II function generator was designed to replicate each of these joint angle functions. Information on designing Stephenson function generators is found here and in [1]. The mechanism generated functions appear in the figures above as well. The three linkages appear here:

Hip Function Generator

Knee Function Generator

Ankle Function Generator

These function generators were designed into the assembly of a machine that replicates the human’s walking gait.  All three function generators were packaged into a compact motor-drive unit.  The function generator motions are passed to the distal joints by parallelogram linkages.

The dynamic simulation below propels the body forward by friction forces at the foot. The hip assembly was permitted to bob up and down, but fore-aft and lateral rotations were artificially constrained so that the machine does not tip over. A spring was added at the end of the foot to allow toe flex. The machine shows promise for walking with low-powered controls systems, perhaps integrating onto the human body in order to take advantage of the human being’s balance and control while providing powered locomotion.

[1] M. Plecnik and J. M. McCarthy, 2015. “Computational Design of Stephenson II Function Generators for 11 Accuracy Points,” Accepted for publication in the Journal of Mechanisms and Robotics, May 2015.