The scope of this work is to show the applicability of the Twisted String Actuators (TSAs) for lightweight, wearable and assistive robotic applications. To this aim, we have developed a novel surface electromyography (sEMG)-driven soft ExoSuit using the TSAs to perform both single and dual-arm elbow assistive applications. The proposed ExoSuit, with an overall weight of 1650 g, uses a pair of TSAs mounted in the back of the user, connected via tendons to the user's forearms to actuate each arm independently for supporting external loads. We confirm this new light-weight and customizable wearable solution via multiple user studies based on the biceps and tricep' sEMG measurements. We demonstrate that user's muscles can automatically activate and regulate the TSAs and compensate for the user's effort: by using our controller based on a Double Threshold Strategy (DTS) with a standard PID regulator, we report that the system was able to limit the biceps' sEMG activity under an arbitrary target threshold, compensating a muscular activity equal to 220% (related to a single arm 3 kg load) and 110% (related to a dual arm 4 kg load) of the threshold value itself. Moreover, the triceps' sEMG signal detects the external load and, depending on the threshold, returns the system to the initial state where it requires no assistance from the ExoSuit. The experimental results show the proposed ExoSuit's capabilities in both single and dualarm load compensation tasks. Therefore, the applicability of the TSAs is experimentally demonstrated for a real-case assistive device, fostering future studies and developments of this kind of actuation strategy for wearable robotic systems.

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Vibrations are present in many mechanical structures and machines, and are often associated with their elastic parts. Characterizing these vibrations, i.e., obtaining their frequencies, amplitudes and phases, is of most interest in many applications ranging from the maintenance of civil structures to motion control.This article presents a method for the on-line and reliable identification of the defining parameters of two unknown sinusoidal signals through the use of their measured sum in the presence of noise and an offset. It is based on the algebraic derivative approach, defined in the frequency domain, which yields exact calculation formulae for the unknown parameters of the signal, i.e., the amplitudes, phases and frequencies of the two sinusoids and the value of the constant term. The on-line estimation is performed in a time interval which is only a fraction of the first full cycle of the slower component of the measured signal. This feature allows the algorithm to be used to monitor time varying parameters in these vibration signals.This algorithm has been used in experiments with a flexible beam, which is a representative platform of a vibrating mechatronic system. It estimated all the vibration signal parameters quickly and accurately, proved to be insensitive to high frequency noises, and accurately tracked the time variations of the signal parameters.

Abstract.This paper presents a controller design to compensate the effects of time delay in a flexure-based piezoelectric stack driven nanopositioner. The effects of the time delay in flexure nanopositioners is illustrated and identified by means of experimentally obtaining the frequency response of the system. Moreover, a theoretical model which takes into account the dependence between the sampling time and the delay introduced is proposed. The proposed control design methodology not only accommodates for time delay but also ensures the robust stability and allows its application to systems with a larger delay than other schemes proposed previously. Limitations and future work are discussed.

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