Michael Sfakiotakis

This paper addresses the design, development, control, and experimental evaluation of a soft robot arm whose actuation is inspired by the muscular structure of the octopus arm, one of the most agile biological manipulators. The robot arm is made of soft silicone and thus possesses enhanced compliance, which is beneficial in a variety of applications where the arm may come into contact with delicate features of its environment. The arm is composed of three elongated segments arranged in series, each one of which contains several pneumatically actuated chambers embedded in its silicone body, which may induce various types of deformations of the segment. By combining the segment deformations, and by imitating the antagonistic muscle group functionality of the octopus, the robot arm can bend in various directions, increase or decrease its length, as well as twist around its central axis. This is one of the few octopus-inspired soft robotic arms where twisting is replicated in its motion characteristics, thus greatly expanding the arm’s potential applications. We present the design process and the development steps of the soft arm, where the molding of two-part silicone of low hardness in 3d-printed molds is employed. In addition, we present the control methodology and the experimental evaluation of both a standalone segment and the entire three-segment arm. This experimental evaluation involves model-free closed-loop control schemes, exploiting visual feedback from a pair of external cameras in order to reconstruct in real time the shape of the soft arm and the pose of its tip.

We present the design, kinematic modeling, and control of a wearable mechatronic thumb device, which is intended to seamlessly integrate with the human hand. The device exhibits lightweight construction, portability, robustness, and safety during use. Initially, we describe the mechanical design of the thumb and an accompanying custom-made data glove that is utilized as a user interface. The kinematic analysis of the mechatronic thumb in conjunction with the human hand is then derived. We subsequently propose intuitive control methods to effectively oppose and coordinate the mechatronic thumb with the remaining fingers, so as to facilitate tasks such as envelope or precision tip grasping, as well as in-hand object manipulation. The potential of the proposed mechatronic thumb to compensate for thumb loss or disability is demonstrated through a series of experiments involving stable grasp and in-hand manipulation tasks of various objects. Grasp quality measures are utilized to assess the experimental results, focusing on the cooperative grasp of the designed mechatronic thumb and the other fingers, in comparison to a non-impaired human hand.

We first prove that stabilizability is an invariant of the static output feedback (SOF) equivalence class. Hence, necessary and sufficient conditions for it to occur have an expression in terms of SOF invariants. Then we revisit the classical stabilizability theory for scalar systems and reexamine it from the invariant point of view. We bring to light the deeper relationships of the methods of Hermite, Routh-Hurwitz, Lyapunov, Nyquist and Evans reposing on their invariant properties. Specifically, we show that stabilizability can be expressed in terms of a) “critical gains” defined as the real zeros of Hermite’s Bezoutian; b) “critical frequencies” defined as the natural frequencies of real axis intersections in the Nyquist plot and of imaginary axis intersections in the Evans plot; c) “critical polynomials” defined as closed loop polynomials when assigning critical frequencies by critical gains; d) “critical intervals”, defined as intervals limited by successive critical gains. Our first result is that the signature of Hermite’s Bezoutian is constant within critical intervals. A very simple algebraic stabilizability criterion, based on this result, is then derived. We establish the validity of this criterion with Routh, Hurwitz and Lyapunov methods. We prove also that the winding number of the Nyquist plot around points of the real axis, is constant within critical intervals. We establish the relationship between the above signatures and winding numbers with the critical frequencies. Our most important result, from the theoretical point of view, is the correlation of the stabilizability within critical intervals with the stability of critical polynomials at their limits. Our findings constitute a decisive step towards the analytical solution for the persisting problem of static output feedback stabilizability of linear time invariant multivariable systems.