Nonsmooth control based on finite-time disturbance estimator for the translational oscillator with a rotational actuator system with nonvanishing disturbances

In this article, the stabilization and disturbance estimation of the translational oscillator with a rotational actuator with nonvanishing disturbances are considered. Different from existing methods, a disturbance estimator is designed to eliminate the effects of unexpected external disturbances. As far as we know, this article presents the first finite-time disturbance-estimator-based nonsmooth control scheme for the translational oscillator with a rotational actuator system. Specifically, first, a series of changes of coordinates is made for the model of the translational oscillator with a rotational actuator system. Then, a disturbance estimator is presented to estimate uncertain disturbances and a nonsmooth control scheme is designed to ensure the convergence of all the states. Furthermore, rigorous theoretical analysis is given. Finally, simulation tests are carried out and the obtained results demonstrate that the designed approach exhibits better control performance and stronger robustness than the existing methods.

Experimental Evidence of Generation and Reception by a Transluminal Axisymmetric Shear Wave Elastography Prototype

Experimental evidence on testing a non-ultrasonic-based probe for a new approach in transluminal elastography was presented. The proposed modality generated shear waves by inducing oscillatory rotation on the lumen wall. Detection of the propagated waves was achieved at a set of receivers in mechanical contact with the lumen wall. The excitation element of the probe was an electromagnetic rotational actuator whilst the sensing element was comprised by a uniform anglewise arrangement of four piezoelectric receivers. The prototype was tested in two soft-tissue-mimicking phantoms that contained lumenlike conduits and stiffer inclusions. The shear wave speed of the different components of the phantoms was characterized using shear wave elastography. These values were used to estimate the time-of-flight of the expected reflections. Ultrafast ultrasound imaging, based on Loupas’ algorithm, was used to estimate the displacement field in transversal planes to the lumenlike conduit and to compare against the readouts from the transluminal transmission–reception tests. Experimental observations between ultrafast imaging and the transluminal probe were in good agreement, and reflections due to the stiffer inclusions were detected by the transluminal probe. The obtained experimental evidence provided proof-of-concept for the transluminal elastography probe and encouraged further exploration of clinical applications.

Modeling and Simulation of a Human Knee Exoskeleton's Assistive Strategies and Interaction

Exoskeletons are increasingly used in rehabilitation and daily life in patients with motor disorders after neurological injuries. In this paper, a realistic human knee exoskeleton model based on a physical system was generated, a human–machine system was created in a musculoskeletal modeling software, and human–machine interactions based on different assistive strategies were simulated. The developed human–machine system makes it possible to compute torques, muscle impulse, contact forces, and interactive forces involved in simulated movements. Assistive strategies modeled as a rotational actuator, a simple pendulum model, and a damped pendulum model were applied to the knee exoskeleton during simulated normal and fast gait. We found that the rotational actuator–based assistive controller could reduce the user's required physiological knee extensor torque and muscle impulse by a small amount, which suggests that joint rotational direction should be considered when developing an assistive strategy. Compared to the simple pendulum model, the damped pendulum model based controller made little difference during swing, but further decreased the user's required knee flexor torque during late stance. The trade-off that we identified between interaction forces and physiological torque, of which muscle impulse is the main contributor, should be considered when designing controllers for a physical exoskeleton system. Detailed information at joint and muscle levels provided in this human–machine system can contribute to the controller design optimization of assistive exoskeletons for rehabilitation and movement assistance.

Advanced robust control techniques for the stabilization of translational oscillator with rotational actuator based barge-type OFWT

In recent times, renewable energy demand is rapidly increasing worldwide. Offshore wind energy is one of the alternative solutions to the problems posed by non-renewable energy resources. The kinetic energy of the wind is converted to mechanical energy by using an offshore floating wind turbine (OFWT). The efficiency of the OFWT is dependent upon the vibrational effect induced by the environment. In this paper, for the mitigation of this vibrational effect, a new model of barge-type OFWT is designed by using an active control strategy called translational oscillator with a rotational actuator (TORA). The disturbance observer (DO) based advanced control techniques including robust backstepping sliding mode control (BSMC), backstepping integral sliding mode control (BISMC), backstepping nonsingular terminal sliding mode control (BNTSMC), and a new backstepping integral nonsingular terminal sliding mode control (BINTSMC) technique, are devised for the stabilization of OFWT model. The comparison of these techniques is carried out by using MATLAB/SIMULINK which validates the feasibility and correctness of the proposed OFWT model and control techniques.

Global sliding mode control for the underactuated translational oscillator with rotational actuator system

This article is motivated by the control issues of the translational oscillator with rotational actuator system in the existence of uncertain disturbances. A nonlinear disturbance observer and a global sliding mode control method are proposed for the disturbance estimation and stabilization of the translational oscillator with rotational actuator system. Compared with the existing control methods, uncertain disturbances are estimated by the proposed nonlinear disturbance observer. In addition, the sliding mode control method is continuous and global robustness with respect to disturbances. Specifically, to facilitate the controller design, the dynamics of the translational oscillator with rotational actuator system are rearranged as the cascade form first. Then, a virtual signal is constructed and corresponding error dynamics are derived. Subsequently, a nonlinear disturbance observer and a continuous global sliding mode control method are proposed for the disturbance rejection and stabilization of the translational oscillator with rotational actuator system. Finally, simulation results are provided to verify the effectiveness and robustness of the proposed controller.

Scalable Bi-Directional SMA-Based Rotational Actuator

In industrial applications, rotatory motions and torques are often needed. State-of-the-art actuators are based on either combustion engines, electro-motors, hydraulic, or pneumatic machines. The main disadvantages are the construction space, the high weight, and a large amount of needed peripheral devices. To overcome these limitations, compact and light-weight actuator systems can be built by using shape memory alloys (SMAs), which are known for their superior energy density. In this paper, the development of a scalable bi-directional rotational actuator based on SMA wires is presented. The scalability was based on a modular design, which allowed the actuator to be adapted to various application specifications by customizing the rotational angle and the output torque. On the mechanical side, each module enabled a small rotatory motion, which added up to the total angle of the actuator. The SMA wires were arranged in an agonist-antagonist configuration to provide active rotation in both directions. The presented prototype achieved a total rotation of 100°. The modularity of the mechanical concept is also reflected in the electronics, which is discussed in this paper as well. This consideration allows the electronics to be adapted to the mechanics with minimal changes. As a result, a prototype, including the presented mechanical and electronic design, is reported in this study.

Design of Translational and Rotational Bistable Actuators Based on Dielectric Elastomer

Dielectric elastomer (DE), as a group of electro-active polymers, has been widely used in soft robotics due to its inherent flexibility and large induced deformation. As sustained high voltage is needed to maintain the deformation of DE, it may result in electric breakdown for a long-period actuation. Inspired by the bistable mechanism which has two stable equilibrium positions and can stay at one of them without energy consumption, two bistable dielectric elastomer actuators (DEAs) including a translational actuator and a rotational actuator are proposed. Both the bistable actuators consist of a double conical DEA and a buckling beam and can switch between two stable positions with voltage. In this paper, the analytical models of the bulking beam and the conical DEA are presented first, and then the design method is demonstrated in terms of force equilibrium and moment equilibrium principle. The experiments of the translational bistable DEA and the rotational bistable DEA are conducted, which show that the design method of the bistable DEA is effective.