This paper is concerned with the design and control of actuators for machines and robots physically interacting with humans. According to criteria established in our previous work on mechanical-control co-design for intrinsically safe, yet performant machines, we pursue the Variable Impedance Actuation (VIA) approach, purporting actuators that can control in real-time both the reference position and the mechanical impedance of the moving parts in the machine. In this paper we describe an implementation of such concepts, consisting of a novel electromechanical Variable Stiffness Actuation (VSA) motor. The design and the functioning principle of the VSA are reported, along with the analysis of its dynamic behavior. A novel scheme for feedback control of this device is presented, along with experimental results showing performance and safety of a one-link arm actuated by the VSA motor.
We address the problem of adaptive observer design for nonlinear time-varying systems which can be transformed in the so-called output feedback form (linear in the unmeasured variables). The observer design follows up previous work on adaptive observers for linear systems and has the form of the classical Luenberger observers for linear systems except that the observer gain is time-varying. A specific form of persistency of excitation is imposed to guarantee the convergence of the (state and parameter) estimation errors. As for the output feedback loop, we proceed using a cascade approach, i.e., we impose the appropriate conditions so that the closed loop system has a cascaded structure. Uniform global asymptotic stability may then be concluded based on cascaded systems theory.
The work describes on-going work at the University of Pisa on the field of tele-laboratories and distance learning. In particular, the group is working at the evolution of existing tele-laboratory experiments in the field of robotics and control into learning units of a self-consistent didactic project. The pick-and-place system described has been designed to provide the set-up for robot arm motion planning with specific objectives and evaluation tools.
Traditional control design is based on ideal assumptions concerning the amount, type and accuracy of the information flow that can be circulated across the controller. Unfortunately, real implementation platforms exhibit non-idealities that may substantially invalidate such assumptions. As a result, the systems closed-loop performance can be severely affected and sometimes stability itself is jeopardised. These problems show up with particular strength when multiple feedback loops share a limited pool of computation and communication resources. In this case the designer is confronted with the challenging task of choosing at the same time the control law and the optimal allocation policy for the shared resources (control algortihm/system architecture co-design).
In this paper we consider the generalization of the classical notion of nonholonomy of smooth constraints in analytical mechanics, to a substantially wider set of systems, allowing for discrete and hybrid (mixed continuous and discrete) configurations and transitions. We show that the general notion of nonholonomy can be captured by the definition of two different types of nonholonomicbehaviours, which we call {\em internal}nd {\em external}, respectively. Examples are reported of systems exhibiting either the former only, or the latter only, or both. For some classes of systems, we provide equivalent or sufficient characterizations of such definitions, which allow for practical tests.
In this paper the synthesis and design of a new device for the energization and characterization of Magneto-Rheological Fluids (MRF) for haptic interfaces are presented. Due to the core structure and feeding conditions, only a 3D numerical analysis provides an accurate prediction of the electromagnetic quantities and the rheological behavior of an excited specimen. The design constraints are shown in details and the results in terms of magnetic field inside the fluid and its spatial resolution are discussed.
This paper describes a new generation of actuators for robotic applications, and more generally for machines that are designed to interact with humans. Such actuators, called Variable Impedance Actuators, are designed to achieve fast motion control while guaranteeing safety of human operators in worst-case impact situation. The fundamental innovation is to implement safety by purely mechanical, passive means, to guarantee intrinsic safety, while active control is used to recover performance. The design concept, which is the subject of a patent application, has led to the experimental implementation of a Variable Stiffness Actuator. The effectiveness of the VSA has been recently validated theoretically and experimentally by authors.
Development of a Wearable System Based on Smart Textiles and GPRS Transmission for Remote Multiparametric Monitoring of Cardiac Patients: Preliminary Results of the WEALTHY Project
In this paper we consider the problem of navigating an autonomous robot using primarily vision for localizing the robot, building a map of the environment, and navigating through viapoints to goals. A visual servo scheme is used that can steer the wheeled vehicle among images. Goal images do not necessarily correspond to images physically taken from the desired vehicle posture, as servoing to reconstructed virtual images is possible. A topological image map is constructed to support this, based on images grabbed by on-board cameras, along with a global feature-based metric map, using extended Kalman filter techniques. The method also enables a team of multiple vehicles to merge their information, and to coordinate navigation using each other's images. Realistic assumptions on limited communication bandwidth between agents and available memory storage are taken into account considering informative, memory-safe maps. Simulations and preliminary experimental results on a laboratory setup are reported.
In this paper we pursue an investigation on the role of perceptual flow in the tactile domain, which appears to be a primary source for information such as shape, motion and softness. In this paper, we report on a set of psychophysical experiments involving how humans integrate incoherent tactile flow stimuli. Two experiments are reported, whereby discordant stimuli are conveyed to the subject by two different fingertips, or by two different families of mechanoreceptors in the same finger. Results from the first experiment show that, in these conditions, the tactile and the optic perceptive systems act in a very similar way. In the second experiment the interaction between tactile flow and friction generate an illusory phenomenon peculiar of the tactile system.
The purpose of this document is to provide a compatibility test for mechatronic devices to be used within a diagnostic MR environment. In order to design new devices that can produce tactile stimuli of different nature inside the MRI environment, compatibility tests with several materials and mechatronic devices are reported. Results of these experiments are analyzed in order to evaluate artefacts caused by the presence and actuation of the devices.
In this paper we consider policies for cooperative, decentralized traffic management among a number of autonomous mobile agents. The conflict resolution problem is addressed considering realistic restrictions on possible maneuvers. We formulate this problem as one in Mixed Integer Linear Programming (MILP). The method, which proves successful in a centralized implementation with a large number of cooperating agents, is also extended to a decentralized setting. Conditions for the existence of conflict avoidance maneuvers for a system of 5 autonomous agents with a transitive information structure are provided, along with the explicit policy to be applied by each agent.
In this paper we propose a new approach to motion planning, based on the introduction of a lattice structure in the workspace of the robot, leading to efficient computations of plans for rather complex vehicles, and allowing for the implementation of optimization procedures in a rather straightforward way. The basic idea is the purposeful restriction of the set of possible inputfunctions to the vehicle to a finite set of symbols, or {\em control quanta},which, under suitable conditions, generate a regular lattice of reachable points. Once the lattice is generated and a convenient description computed, standard techniques in integer linear programming can be used to find a plan very efficiently. We also provide a correct and complete algorithm to the problem of finding an optimized plan (with respect e.g. to length minimization) consisting in a sequence of graph searches.
Linear dynamical systems controlled by quantized inputs exhibit phenomena which are typically non-linear, including chaotic behaviours. We consider discrete-time single-input models of the type x(k+1)=Ax(k)+bu(k). The construction of invariant sets for this class of hybrid systems is of utmost importance for the stabilization problem. We first review a technique to construct invariant sets when an arbitrary quantized input set is assigned. We hence study minimality properties for invariant sets when inputs take integer values. There is a relation between a so-called strong minimality property and ergodicity of the closed-loop dynamics, in particular, ergodicity implies strong minimality. A condition ensuring strong minimality is given in terms of the coefficients of the characteristic polynomial of the matrix 'A'. Two examples are presented: the first one shows that strong minimality does not imply ergodicity. The second one shows that our condition for strong minimality is only sufficient: this is done by exhibition of an ergodic dynamics for which our condition is not satisfied.
In this paper we consider the problem of controlling multiple scalar systems through a limited capacity shared channel. Each system is affected by process noise and can be controlled byactuators with values in a {\em fixed}inite set. The control objective is to bound the evolution of the systems in specified sets (controlled invariance). Our goal is to find an optimal allocation of the shared communication resource to the different control activities and to identify correct choices for the design parameters. The paper provides fundamental conceptual tools to attack the design problem in the formal framework of an optimization problem. Namely, we give a feasibility criterion to decide whether a set of design parameters conforms with a control specification (i.e., with the controlled invariance of a specified set for each system). Moreover, we offer the explicit computation of the minimum bit rate necessary for the controlled invariance of a set, which is of utmost importance for solving the optimization problem.
We report results of a pilot study using functional magnetic resonance imaging aimed at determining the neural correlates of tactile flow. We hypothesized that brain response to tactile flow would involve the same cortical areas (V5/MT) that respond to optic flow. Our results showed that V5/MT cortex indeed is activated by tactile flow perception. These findings are consistent with a supramodal organization of brain regions involved in optic and tactile flow processing.
In a previous paper, a device for the characterization of MagnetoRheological Fluids (MRF) has been described. The MRF consist of micro-sized, magnetically active particles dispersed in a carrier medium. When exposed to a magnetic field, MRF change their own consistency turning from fluid to near-solid state responding to the applied field within milliseconds. This interesting property, suggests the possibility to use magnetorheological fluids to mimic the compliance of biological tissues in order to realize a haptic display, such as in surgical training for minimally invasive surgery and/or open surgery simulations [2]. In this scenario the operator could interact with a virtual object which simulate several biological tissues by magnetically tuning the rheological properties of the fluid. In this paper an accurate analysis of an immersive device for the magnetic excitation of the fluid is presented. In particular such analysis is focused on a system of ferromagnetic "pistons", that, properly positioned in the device, can address the magnetic flux in the MRF.
