Pathological tremor produces undesired movements that decrease quality of life. Typical treatments such as medications and surgery have varying efficacy, risks, and side effects. Mechanical tremor suppression offers a potential alternative to these treatments: mechanical tremor suppression applies torques about joints to oppose the tremor-producing muscular torque and reduce tremor motion. Typical actuators create bulky exoskeletons that are not suitable for clinical implementations. However, dielectric elastomers enable soft, low-profile actuation that improves the potential for clinical implementation of mechanical tremor suppression. Recent research performs theoretical evaluation of dielectric elastomer-based tremor suppression, demonstrating the ability of dielectric elastomer stack actuators (DESAs) to produce very effective and robust tremor suppression via tremor-active control. A tremor-active approach only actuates to reduce tremor while the human motor system compensates for DESA passive dynamics. This approach leverages the low mechanical impedance of dielectric elastomers to compensate for their low actuation levels. This paper performs benchtop experimental testing of scaled tremor-active suppression using folded DESAs to validate previous theoretical studies. Since DESA manufacturing by folding can only achieve relatively thick layers, the DESAs do not possess the necessary actuation levels to suppress typical tremor amplitudes. Therefore, this paper evaluates a scaled system using a piezoelectric bimorph cantilever beam to represent human motion. Experiments demonstrate the effectiveness of a tremor-active impedance controller for DESA-based tremor suppression. Scaling up to DESAs with higher actuation levels will enable suppression of typical hand tremors.
Recent research proposes dielectric elastomers as actuators for mechanical suppression of pathological tremor. Dielectric elastomers offer several advantages compared to traditional actuators, including decreased weight, smaller profile, reduced rigidity, and better scalability. The similarities between dielectric elastomers and human muscle enable application of the actuators in a bio-inspired approach, where external artificial muscles directly actuate against tremor produced by the underlying human muscle. Two approaches exist for dielectric-elastomerbased tremor suppression: fully-active and tremor-active. In the fully-active approach, the dielectric elastomer actuators must actuate against tremor while also activating to follow the voluntary motion of the joint. In contrast, the tremor-active approach only requires activation against the tremor; the human sensorimotor system compensates for the passive dynamics of the dielectric elastomers. The tremor-active approach is unique to dielectric-elastomer-based tremor suppression since the soft actuators can have mechanical impedances on the same order or less than that of the human body. These two approaches have tradeoffs between actuation and viscoelastic requirements: the tremor-active approach decreases the actuation requirements, but applies limitations to the stiffness and viscoelastic characteristics of the actuator. This paper quantifies the necessary actuator parameters to achieve acceptable tremor suppression performance for each approach. The necessary parameters are normalized by joint parameters to generalize the results for tremor suppression about any joint.
Pathological tremor is an involuntary, rhythmic movement that can inhibit the ability of a person to perform everyday tasks. Recent research explores mechanical means of tremor suppression as an alternative to drugs and surgery. However, traditional control methods also suppress voluntary movements due to the close proximity of tremor frequency and the frequency range of typical voluntary motions. Therefore, the controller must identify and suppress the tremor torque with minimal influence on voluntary movement. In addition to the control design, the actuator plays a critical role in the performance and potential for clinical implementation of a tremor suppression system. Dielectric elastomers offer unique actuation capabilities due to their low stiffness compared to traditional engineering actuators. In particular, dielectric elastomers have similar mechanical properties as human tissue, making them ideal for actuation of the human body. This work applies an adaptive notch filter algorithm for vibration attenuation in a narrow frequency range using dielectric elastomer stack actuators. In this controller, an estimation of the tremor frequency ensures suppression of only the tremor motion. The adaptive filter estimates the tremor torque, and a force controller for the dielectric elastomer tracks the specified force. Simulations show excellent tracking of the desired motion for slower voluntary motions and for slowly varying tremor amplitudes. Even though the controller has diminished tremor suppression in the presence of rapid changes in tremor amplitude, it still offers a significant improvement over the uncontrolled case. Altogether, this work demonstrates the potential for the use of dielectric elastomer actuators in a soft orthosis to suppress pathological tremor.
Pathological tremor results in undesired motion of body parts, with the greatest effect typically occurring in the hands. Since common treatment methods are ineffective in some patients or have risks associated with surgery or side effects, researchers are investigating mechanical means of tremor suppression. This work explores the viability of dielectric elastomers as the actuators in a tremor suppression control system. Dielectric elastomers have many properties similar to human muscle, making them a natural fit for integration into the human biomechanical system. This investigation develops a model of the integrated wrist-actuator system to determine actuator parameters that produce the necessary control authority without significantly affecting voluntary motion. Furthermore, this paper develops a control law for the actuator voltage to increase the effective viscous damping of the system. Simulations show excellent theoretical tremor suppression, demonstrating the potential for dielectric elastomers to suppress tremor while maximizing compatibility between the actuator and the human body.