Data-driven model-free adaptive fractional-order sliding mode control for the SMA actuator with prescribed performance

In this paper, a novel data-driven model-free adaptive fractional-order sliding mode controller with prescribed performance is proposed for the shape memory alloy (SMA) actuator. Due to the strong asymmetric saturated hysteresis nonlinear characteristics of the SMA actuators, it is not easy to establish an accurate model and develop an effective controller. Therefore, we present a controller without using the model of the SMA actuators. In other words, the proposed controller depends merely on the input/output (I/O) data of the SMA actuators. To obtain the reasonable compensation for hysteresis, enhance the noise robustness of the controller, and reduce the chattering, a fractional-order sliding mode controller with memory characteristics is employed to improve the performance of the controller. In addition, the prescribed performance control (PPC) strategy is introduced in our work to guarantee the tracking errors converge to a sufficiently small boundary and the convergence rate is not less than a predetermined value which are significant and considerable in practical engineering applications of the SMA actuator. Finally, experiments are carried out, and results reveal the effectiveness and success of the proposed controller. Comparisons with the classical Proportional Integral Differential (PID), model-free adaptive control (MFAC), and model-free adaptive sliding mode control (MFAC-SMC) are also performed.

Structural fatigue and fracture of shape memory alloy actuators: Current status and perspectives

Shape Memory Alloy (SMA)-actuators are efficient, simple, and robust alternatives to conventional actuators when a small volume and/or large force and stroke are required. The analysis of their failure response is critical for their design in order to achieve optimum functionality and performance. Here, (i) the existing knowledge base on the fatigue and overload fracture response of SMAs under actuation loading is reviewed regarding the failure micromechanisms, empirical relations for actuation fatigue life prediction, experimental measurements of fracture toughness and fatigue crack growth rates, and numerical investigations of toughness properties and (ii) future developments required to expand the acquired knowledge, enhance the current understanding, and ultimately enable commercial applications of SMA-actuators are discussed.

Force Control for Anthropomorphic Fingers Actuated by Shape Memory Alloy Wires

High accuracy in modelling the behavior of human hand and fingers is obtained using control devices of high biological plausibility. Such devices are typically based on neural networks and are able to control in parallel multiple artificial muscles. This paper presents the structure of an electronic spiking neural network that was implemented to control the force of two opposing fingers of an anthropomorphic hand. In order to increase the level of bio-inspiration, the artificial muscles are implemented using shape memory alloy wires which actuates by contraction as the natural muscles. Moreover, the contraction force of the SMA actuators is directly related to the spiking frequency that is generated by the artificial neurons. The results show that using few excitatory and inhibitory neurons the neural network is able to set and regulate the contraction force of the SMA actuators.

Efficiency Analysis of SMA-Based Actuators: Possibilities of Configuration according to’ the Application

Shape memory alloy (SMA) actuators have recently demonstrated their potential for various applications in fields such as robotics, medicine, aerospace, and automotive. Its features, such as low weight and high force, simplicity, noiseless operation, and low cost compared with other conventional actuator, are only a few advantages of this actuator, which is receiving increasing interest among researchers. However, the use of these actuators is still limited by some of their characteristics: high position error in the cooling stage when the actuator works at frequencies that exceed the necessary cooling time and high electrical energy consumption. Different actuator configurations can help minimize these disadvantages through modifying the length, the number of cables, or the sheath used in the actuator, which modify the characteristics of the complete system. In this work, we developed different configurations of SMA actuators and tested their performance in terms of efficiency and the position error in the cooling stage. The findings demonstrate that over-dimensioned actuators are more energetically efficient and present a faster initial form recovery. The multi-wires actuator configuration produce a better response in terms of position but are less energy efficient. These conclusions allow for the selection of the most appropriate configuration based on the requirements of each particular application.