David Case

Design and Characterization of a Small-Scale Magnetorheological Damper for Tremor Suppression

This paper explores the design methodology and effectiveness of small-scale magnetorheological dampers (MRDs) in applications that require variable damping. Previously, applications of MRD have been chiefly limited to vehicle shock absorbers and seismic vibration attenuators. There has been recent biomedical interest in active-damping technology, however, particularly in the field of rehabilitation robotics. The topic at hand is the feasibility of developing MRDs that would be functionally and cosmetically adequate for actuation of an upper limb tremor suppression orthosis. A Bingham plastic model is used to determine MRDs functional characteristics, and experimental data are presented to validate the mathematical model. The feasibility of applying the developed small-scale MRDs to attenuation of tremorous motion is explored.

Force and Stiffness Backstepping-Sliding Mode Controller for Pneumatic Cylinders

In most applications that involve human-robot interactions, such as prosthetics, orthotics, rehabilitation, and locomotion, compliant actuators with variable stiffness can be used to improve safety and comfort of the device. Another advantage of the stiffness control is minimizing the energy consumption by adjusting the stiffness of the actuator to the natural stiffness of the controlled system. This paper introduces a new backstepping-sliding mode force-stiffness controller for pneumatic cylinders. The global ultimate-bounded stability of the closed-loop system was proven by the Lyapunov method. Based on a detailed mathematical model of the pneumatic system that includes the dynamics of the valves, the algorithm was proven able to track the desired force and stiffness independently without chattering. Validating experiments using a real-time platform were performed for a pneumatic cylinder suitable for wearable robotics applications. The performance of the proposed algorithm was compared with the performance of a previously reported pneumatic force-stiffness sliding mode controller.

Dynamical Modeling and Experimental Study of a Small-Scale Magnetorheological Damper

This paper introduces a multiphysics finite-element dynamic model for a small-scale magnetorheological (MR) damper. The model includes the analysis of the magnetic flux lines, field intensity, non-Newtonian fluid flow, and evaluation of the resistance force under prescribed motion and standard electrical test signals. A new regularized viscosity definition, which improves model solvability, is employed to describe the quasi-Bingham plastic behavior of the MR fluid. Extensive model validation was performed through comparison with the analytic model presented in the previous work and with the data from experimental testing. This model is intended to be used in the optimization of the MR dampers employed in the development of an upper limb orthosis, for tremor attenuation in patients suffering from pathological tremor.

Robust Controller for Tremor Suppression at Musculoskeletal Level in Human Wrist

Tremor is a rhythmical and involuntary oscillatory movement of a body part and it is one of the most common movement disorders. Orthotic devices have been under investigation as a noninvasive tremor suppression alternative to medication or surgery. The challenge in musculoskeletal tremor suppression is estimating and attenuating the tremor motion without impeding the patients intentional motion. In this research a robust tremor suppression algorithm was derived for patients with pathological tremor in the upper limbs. First the motion in the tremor frequency range is estimated using a high-pass filter. Then, by applying the backstepping method the appropriate amount of torque is calculated to drive the output of the estimator toward zero. This is equivalent to an estimation of the tremor torque. It is shown that the arm/orthotic device control system is stable and the algorithm is robust despite inherent uncertainties in the open-loop human arm joint model. A human arm joint simulator, capable of emulating tremorous motion of a human arm joint was used to evaluate the proposed suppression algorithm experimentally for two types of tremor, Parkinson and essential. Experimental results show 30-42 dB (97.5-99.2%) suppression of tremor with minimal effect on the intentional motion.

Adaptive Suppression of Severe Pathological Tremor by Torque Estimation Method

Tremor is a rhythmical and involuntary oscillatory movement of a body part. In addition to social embarrassment, tremor can be debilitating for daily activities. Recently, wearable active exoskeletons emerged as a noninvasive tremor suppression alternative to medication or surgery. The challenge in musculoskeletal tremor suppression is identifying and attenuating the tremor motion without adding resistance to the patients intentional motion. In this research, an adaptive tremor suppression algorithm was designed to estimate the tremor fundamental frequency and calculate the proper suppressive force to be applied by the orthosis to the patients arm. Stability of the closed-loop system and robustness against the parametric uncertainties were analyzed. An experimental setup was designed and developed to emulate the dynamics of a human wrist with intentional and tremor motion. A pneumatic cylinder and a sliding mode integral controller was used to apply orthotic suppressive force. The algorithm was implemented with an NI cRIO real-time controller and tested using clinical data from ten patients with severe pathological tremor. Experimental results showed tracking of the tremor frequency with less than 3-s response time, and an average 34.5 dB (98.1%) and 11.8 dB (74.3%) reduction of tremor amplitude at the fundamental and second-harmonic frequencies, respectively. The average resistance force to the intentional motion was 0.7 N and the average position error was 2.08% (0.18 dB). The results were compared with passive tremor suppression using a tunable magnetorheological damper.

A Lumped-Parameter Model for Adaptive Dynamic MR Damper Control

The dynamic behavior of a small-scale magnetorheological damper intended for use in a tremor-suppression orthosis is characterized through experimental analysis and mathematical modeling. The combined frequency response of both the electromagnetic coil and the fluid particles is modeled by a third-order transfer function. The output of this function is an effective current that, combined with piston position and velocity, is empirically related to the resistance force of the damper. The derived model demonstrates high-fidelity to experimental testing of the damper under variable piston velocity and applied current within the expected frequency range of pathological tremor. The model is, thus, deemed suitable for use in a control algorithm for the mechanical suppression of tremor via magnetorheological damping.

Design of robust nonlinear force and stiffness controller for pneumatic actuators

This paper introduces a new backstepping-sliding mode controller designed specifically for pneumatic actuators. Based on a detailed mathematical model of the pneumatic system that included the dynamics of the valves, the algorithm was proven able to track the desired force and stiffness independently without chattering. The global Lyapunov asymptotic stability of the pressure tracking for each chamber was analyzed. Numerical simulations and validating experiments using a real-time platform were performed for a pneumatic actuator suitable for wearable robotics applications.

ACTIVE TREMOR ESTIMATION AND SUPPRESSION IN HUMAN ELBOW JOINT

A new control algorithm was developed for tremor estimation and suppression in a second order linear model of the human elbow joint. An adaptive method developed to estimate a simple harmonic disturbance was generalized for tremorous motion with spectral composition similar to the clinical reports for action tremor. Numerical simulations showed the ability of proposed controller to reduce tremor amplitude without generating significant resistance against voluntary motion of the arm. The designed algorithm can be used in an upper-limb orthosis to suppress debilitating tremorous motion of the arm.Copyright

Multiphysics modeling of magnetorheological dampers

The dynamics of a small scale magnetorheological damper were modeled and analyzed using multiphysics commercial finite element software to couple the electromagnetic field distribution with the non-Newtonian fluid flow. The magnetic flux lines and field intensity generated within the damper and cyclic fluid flow in the damper under harmonic motion were simulated with the AC/DC and CFD physics modules of COMSOL Multiphysics, respectively. Coupling of the physics is achieved through a modified Bingham plastic definition, relating the fluid’s dynamic viscosity to the intensity of the induced magnetic field. Good agreement is confirmed between simulation results and experimentally observed resistance forces in the damper. This study was conducted to determine the feasibility of utilizing magnetorheological dampers in a medical orthosis for pathological tremor attenuation. The implemented models are thus dimensioned on a relatively small scale. The method used, however, is not specific to the damper’s size or geometry and can be extended to larger-scale devices with little or no complication.

Design and Development of a Human Arm Joint Simulator for Evaluation of Active Assistive Devices Control Algorithms

A system capable of simulating the dynamic behavior of a single DOF joint of the human arm was developed. The inertia, stiffness and damping produced by the passive tissue, as well as the muscle torque and its variable stiffness and damping specific to human shoulder, elbow, wrist, and finger joints can be accurately reproduced. The torque created by external loads and by active assistive devices for people with disabilities, can be emulated. This allows the system to be used for safety and performance evaluation of control algorithms implemented in active orthotic, prosthetic, and rehabilitation devices in a safe and highly reproducible environment, before testing on actual human subjects.Copyright