Suppression of Upper Limb Tremor
The first project in this category is orthotic suppression of upper limb tremor that is often seen in patients with Parkinson, multiple sclerosis, stroke, head trauma, or essential tremor. The tremor manifests with visible and inmost cases debilitating shaking of the extremities and can be particularly impairing to fine motor skills, such as in writing, typing, manual labor, or playing an instrument. Current treatments for tremor include a collection of prescription drugs that may cause severe side effects (nausea, ataxia, confusion, hallucinations), and in non-responsive cases, stereotactic thalamotomy and deep brain stimulation that can lead to permanent complications (paresthesia, dysarthria, speech impediment, and headache) in 9-23% of patients.
Given these drawbacks, the goal of this research project is to assess the kinematics and dynamical characteristics of tremor and develop and an orthotic device that would be functionally and cosmetically acceptable for everyday use. The project requires new, innovative, and highly efficient actuators, active tunable dampers, and advanced control algorithms that function flawlessly together.
Materials and Methods
Magnetorheological (MR) dampers excel at creating energy efficient variable damping forces and have the additional advantages of rapid response time and high-fidelity control. The ideal damper would have an operational range comparable to the forces imposed by the tremor, yet still provide minimal resistance to voluntary motion. Because of the mechanical impedance due to the relatively high viscosity of most MR fluids, this presents a challenge.
The MR dampers developed in this project were specifically designed for application to a medical orthosis for the mechanical suppression of pathological tremor in the human forearm. Dampers of the general design proposed here would be mounted above the dorsal and radial surfaces of the forearm, with the moving shafts connected to the hand using articulated linkages and an orthotic glove.
The properties of the dampers under dynamic magnetic fields are evaluated in an experimental setup designed to produce constant velocity of the piston. For the purpose of analyzing cyclic motion of the dampers, a second experimental setup was designed to convert the rotation of a brushless DC motor to sinusoidal translation of the piston of the damper via a linear bearing.
| A CAD representation of the orthosis design.
Multiphysics Finite Element Model
COMSOL Multiphysics (COMSOL Inc., Los Angeles, CA) is used for multiphysics finite element analysis of the magnetic field created in the piston/cylinder gap, and the MR fluid flow velocity and its dynamic viscosity during harmonic cycling.
| FE model of the magnetic field
|| FE analysis of the MR fluid flow due to harmonic cycling of the piston
MRD Simulation Results Video:
Bingham Plastic Flow
The MR fluid flow is modeled with the Navier-Stokes equations, assuming incompressibility and the viscosity variation described by an yield stress dependent upon the magnetic field intensity. The Dirichlet boundary conditions assume no slip and no penetration at the rigid boundaries.
In the simulation, the motion of the piston head is specified by a sine wave function of 4 Hz. Stress in the axial direction is integrated over each moving surface to calculate the total resistance force of the damper. Fairly good agreement is seen between the FEM and experimental data, though there is a phase difference, due to in imperfect sinusoidal signal being applied to the damper in the experimental data.
| Experimental setup for damper characterization under cyclic motion
This work was partially supported by a startup grant form Bobby B. Lyle School of Engineering, SMU, Dallas, TX.