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.