The integration of robots into human environments is advancing rapidly, driven by the demand for systems that combine robot accuracy and repeatability with human flexibility and adaptability. In this context, human-centered manipulation applications must address uncertainties arising from human, robotic, and environmental factors. As a result, effective robotic manipulation requires both accurate pre-grasping motions and precise grip strength control, especially in tasks where robotic devices are remotely controlled to grip objects with fine, desired, and adjustable grip force. The present work tackles the challenges posed by uncertainties and non-ideal conditions in surface electromyography (sEMG)-driven human-in-the-loop (HITL) robot hand control applications. In this regard, a novel probabilistic shared autonomy framework for fine grip strength regulation is introduced, leveraging Hidden Markov Models (HMMs) applied to tactile data to encode the HITL grasping action into proper, probabilistically consistent phases. These phases are then exploited to modulate the level of shared autonomy between the human operator and the robot hand, enabling precise control over grip strength. The presented shared autonomy framework was evaluated under multiple experimental conditions, testing grip force regulation performance with a group of 10+10 intact-limb participants and a participant with amputation in both static (fixed hand-object configuration) and dynamic (pick-and-place and recipe preparation) grasping tasks, with differentiated goals inspired by real-world requirements. Moreover, to explore generalizability, experiments were conducted with both anthropomorphic and industrial robotic hands, properly equipped with tactile sensors. Experimental outcomes are supported by statistical analysis and show, for the considered sample, the effectiveness of the proposed shared autonomy control architecture in achieving fine, smooth, and controllable grip strength regulation with respect to the baseline case in absence of our approach.

Trajectory learning is one of the key components of robot Programming by Demonstration approaches, which in many cases, especially in industrial practice, aim at defining complex manipulation patterns. In order to enhance these methods, which are generally based on a physical interaction between the user and the robot, guided along the desired path, an additional input channel is considered in this article. The hand stiffness, that the operator continuously modulates during the demonstration, is estimated from the forearm surface electromyography and translated into a request for a higher or lower accuracy level. Then, a constrained optimization problem is built (and solved) in the framework of smoothing B-splines to obtain a minimum curvature trajectory approximating, in this manner, the taught path within the precision imposed by the user. Experimental tests in different applicative scenarios, involving both position and orientation, prove the benefits of the proposed approach in terms of the intuitiveness of the programming procedure for the human operator and characteristics of the final motion.

In this article, the variety of approaches proposed in the literature to address the problem of mapping human to robot hand motions are summarized and discussed. We particularly attempt to organize under macrocategories the great quantity of presented methods that are often difficult to be seen from a general point of view due to different fields of application, specific use of algorithms, terminology, and declared goals of the mappings. First, a brief historical overview is reported, in order to provide a look on the emergence of the human to robot hand mapping problem as a both conceptual and analytical challenge that is still open nowadays. Thereafter, the survey mainly focuses on a classification of modern mapping methods under the following six categories: direct joint, direct Cartesian, task-oriented, dimensionality reduction based, pose recognition based, and hybrid mappings. For each of these categories, the general view that associates the related reported studies is provided, and representative references are highlighted. Finally, a concluding discussion along with the authors' point of view regarding future desirable trends are reported.

This paper reports the development of a manipulation system for electric wires, implemented by means of a commercial gripper installed on an industrial manipulator and equipped with cameras and suitably designed tactile sensors. The purpose of this system is the execution of wire insertion on commercial electromechanical components. The synergy between computer vision and tactile sensing is necessary because, in a real environment, the tight spaces very often prevent the possibility to use the vision system, also when the same task is performed by a human being. A novel technique to speed up the generation of training data sets for convolutional neural networks (CNNs) is proposed. Therefore, this technique is used to train a CNN in order to detect small objects (such as wire terminals). Moreover, aiming to prevent faults during the task and to interact with the environment safely, several machine learning approaches are used to produce an affordable output from the tactile sensor. The proposed approach shows how a cheap sensor embedded with suitable intelligence can provide information comparable to a more expensive force sensor.

In this paper, the problem of suppressing sloshing dynamics in liquid handling robotic systems has been faced by designing a dynamic filter that starting from the desired motion of the liquid container calculates the complete position/orientation trajectory for the robot end-effector. Specifically, a design philosophy mixing a filtering technique that suppresses the frequency contributions of the reference motion that may cause liquid oscillations and an active compensation of lateral accelerations by a proper container re-orientation has been adopted. In principle, the latter contribution requires the knowledge of acceleration of the reference trajectory, but because of the use of an harmonic smoother that performs a shaping of the original motion, it is possible to obtain the value of the acceleration in runtime. In this way, the proposed methods can be applied also to reference motions that are not known in advance, e.g. commands directly provided by a human operator. This possibility has been demonstrated by means of a number of experimental tests in which the user teleoperates the robot carrying the container with the liquid by simply moving in the free space its hand, whose 3D position is detected by a motion capture system.

The gap between human waiters and state-of-the-art robot systems that try to serve something to drink is often embarrassing, with the former able to manipulate glasses and trays or glasses on trays with incredible dexterity and the latter that move at incredible slowness. In this video, we want to show that robots can do it better by moving a bottle or a tankard full of beer that are simply placed on a flat steel plate connected the flange of a robot manipulator. The robot tracks the trajectory defined by a human operator that moves its hand in the 3D space, with a motion capture system that acquires in real time the position. A feed-forward controller, placed between the user and the robot and based on the combination of a smoother and proper orientation compensation, counteracts the lateral accelerations and suppress sloshing phenomena of the liquids. Eventually a camera mounted on the robot arm provides a visual feedback to the operator with monitoring purposes. The challenge for the operator was to drop the carried object. will the feed-forward control be robust enough to avoid this event, even at high speed? Watch the video and find out!

This paper presents a new feedforward controller based on a continuous-time finite impulse response filter, designed to minimize the vibrations that usually affect robot manipulators with elastic joints. In particular, Variable Stiffness Joints (VSJ) robots are considered, since they are usually characterized by a very low level of damping which makes the problem of the oscillations quite important. The proposed approach allows to simplify the overall control structure of VSJ robots, which is based on a decentralized control of each servomotor, imposing the desired position and the desired stiffness at each joint, and on a novel feedforward control, filtering the reference signals. After analyzing some of the filter properties and the method for the parameters choice, experimental results on a VSJ robot demonstrate the importance of the proposed filtering action for minimizing vibrations and oscillations.

This paper presents a feed-forward approach to reduce sloshing dynamics in liquid handling robotic systems. According to our solution, the dynamics of a liquid into an open vessel manipulated by a robot can be described by means of a spherical pendulum mechanical model. By doing this, the sloshing problem can be addressed as a vibration suppression problem for a second order system. More in details, the pendulum model is utilized to tune an exponential filter which shapes the reference trajectory for the robot, thus achieving a sloshing-free motion of the liquid inside the vessel.

In this paper, an ongoing work for verifying the behavior of a twisted string actuator in contact with a sliding surface or guided through a sheath is presented. The twisted string actuation system is particularly suitable for very compact and light-weight robotic devices, like artificial limbs and exoskeletons, since it allows the implementation of powerful tendon-based driving systems, based on small-size DC motors characterized by high speed, low torque and very limited inertia. One of the major limitations of this actuation system is by now related to the fact that the string should not be in contact with any obstacle, because this contact will alter the twisting angle propagation along the string and, eventually, completely stop the string twisting. This design constraint imposes a straight path between the motor and the linear load attached to the other string end. After the presentation of the basic properties of the twisted string actuation system, the model of the twisted string in contact with a sliding surface is discussed. The behavior of the system has been then experimentally verified and discussed. A preliminary evaluation of control strategies for compensating the side effects generated by the contact of the twisted string with the sliding surface is also presented.

In this paper, an hybrid position-position and position-velocity teleoperation control scheme for a generic mobile manipulator is presented and discussed. The mobile manipulator is composed by a mobile platform and a 5 dof arm, and the proposed control scheme allows the simultaneous control of both the devices by means of a single haptic device characterized by an open kinematic chain and not specifically designed for mobile manipulators teleoperation (e.g. a Phantom Omni). The proposed teleoperation controller overcomes the mismatch of the control signals to be sent to the arm (position) and to the mobile platform (velocity) through a proper partition of the master device workspace. Tests have been performed both by simulation and with a real setup. The setup is composed by a 6 dof Phantom Omni haptic device acting as master, and a single-arm Kuka youBot omnidirectional manipulator acting as slave. Experimental results related to a pick and place task, performed on the real setup and involving the motion of both the arm and the platform are reported and commented.

In this paper, an ongoing work for the implementation of a variable stiffness joint actuated by a couple of twisted string actuators in antagonistic configuration is reported. The twisted string actuation system is particularly suitable for very compact and light-weight robotic devices, like artificial limbs, exoskeletons and robotic hands, since it renders a very low apparent inertia at the load side, allowing the implementation of powerful tendon-based driving systems, using as actuators small-size DC motors characterized by high speed, low torque and very limited inertia. The basic properties of the twisted string actuation system are firstly presented, and the way how they are exploited for the implementation of a variable stiffness joint is discussed. A simple control algorithm for controlling the joint stiffness and position simultaneously is discussed, and a the feedback linearization of the device is taken into account and validated in simulation.

In this work, we deal with the problem of planning a manipulation task for a robotic system composed of at least one dexterous arm and a dexterous multi-fingered hand. The goal of the local planner is to include both, the arm and the hand, in the execution of the task in a coordinated way. This is achieved by using the workspace of the hand which is computed offline. During the online planning, the current in-hand manipulation capability is evaluated taking advantage of the dimensions of the hand workspace and considering the task itself. Dynamic weights enable the computation of the instantaneous contributions of the two subsystems on the motion of the manipulated object. The method is evaluated in simulation as well as in several experiments on the real robot.

In this paper, a novel repetitive control scheme is presented and discussed. The general framework is the control of repetitive tasks of robotic systems or, more in general, of automatic machines. The key idea of the proposed scheme consists in modifying the reference trajectory provided to the plant in order to compensate for external loads or unmodelled dynamics that cyclically affect it. By exploiting the dynamic filters for the B-spline trajectory planning, it has been possible to integrate the trajectory generation within a repetitive control scheme able to modify in real-time the reference signal with the aims of nullify interpolation errors. Experimental results obtained controlling two joints of a standard industrial manipulator are reported, showing the effectiveness of the proposed method.

In this paper, the implementation of a variable stiffness joint actuated by a couple of twisted string actuators in antagonistic configuration is presented. The twisted string actuation system is particularly suitable for very compact and light-weight robotic devices, like artificial limbs and exoskeletons, since it renders a very low apparent inertia at the load side, allowing the implementation of powerful tendon-based driving systems, using small-size DC motors characterized by high speed, low torque and very limited inertia. After the presentation of the basic properties of the twisted string actuation system, the way how they are used for the implementation of a variable stiffness joint is discussed. A simple PID-based motor-side algorithm for controlling simultaneously both the joint stiffness and position is discussed, then the identification of the system parameters is performed on an experimental setup for verifying the proposed model and control approach.

In this paper, the design and experimental evaluation of a cable driven robotic gripper for underwater applications is presented. The gripper has three fingers and is characterised by a large workspace if compared with other similar devices reported in literature. Its kinematic configuration allows to execute both parallel and precision grasps on objects with very different dimensions. The gripper has 8 degrees of freedom actuated by only three motors by means of a suitable coupling of the joints obtained through the cable transmission. Moreover, in order to facilitate the execution of complex tasks, special force/torque sensors are mounted on the fingertips. The paper reports the main specifications deriving from the particular tasks in which the gripper is involved, and illustrates the proposed design solutions. Results obtained from real underwater experiments are provided as well, in order to demonstrate the capabilities of the gripper.

In this paper, a novel force/torque sensor is presented. The sensor is based on optoelectronic components and therefore its design is relatively simple and reliable. The sensor design make it suitable for the integration in different robotic systems, such as e.g. the fingers of robotic hands. The basic principle and the design of the sensor are described in this paper, along with a specific prototype implemented for underwater applications. Experimental data are presented and discussed to illustrate the main features of the proposed sensor, and its use as an intrinsic tactile sensor is evaluated.

We present an automated solution for the acquisition, processing and classification of electroencephalography (EEG) signals in order to remotely control a remotely located robotic hand executing communicative gestures. The Brain-Computer Interface (BCI) was implemented using the Steady State Visual Evoked Potential (SSVEP) approach, a low-latency and low-noise method for reading multiple non-time-locked states from EEG signals. As EEG sensor, the low-cost commercial Emotiv EPOC headset was used to acquire signals from the parietal and occipital lobes. The data processing chain is implemented in OpenViBE, a dedicated software platform for designing, testing and applying Brain-Computer Interfaces. Recorded commands were communicated to an external server through a Virtual Reality Peripheral Network (VRPN) interface. During the training phase, the user controlled a local simulation of a dexterous robot hand, allowing for a safe environment in which to train. After training, the user's commands were used to remotely control a real dexterous robot hand located in Bologna (Italy) from Plymouth (UK). We report on the robustness, accuracy and latency of the setup.

In this paper, the advantages of CACSD (Computer Aided Control System Design) tools for integrating a robotic system in a packaging machine are illustrated. Beside the mechanical integration of the robot into the machine architecture, it is necessary a functional integration, that requires a precise synchronization with the other parts of the system. In the proposed application, a robot with a parallel kinematics is used for pick-and-place tasks between two conveyor belts. It is therefore necessary a proper motion planning which allows to synchronize the grasp and release phases with the conveyor belts, avoiding obstacles and guaranteeing the compliance with bounds on velocity, acceleration and limits in the workspace. A trajectory composed by quintic polynomials has been considered and a specific tool has been designed in the Matlab environment, which allows to modify the parameters of the trajectory and to analyze the obtained motion profiles from both the kinematic and dynamic point of view.

In this paper, a novel technique for online generating trajectories in the 3-D space is presented. The trajectory planner is based on cubic B-splines. However, while the definition of B-splines requires the solution of a global problem that involves the entire set of via-points to be interpolated/approximated, and therefore it is not suitable for online implementation, the proposed generator is able to approximate spline functions with the prescribed precision on the basis of local computations, which only need the knowledge of a very limited number of via-points. FIR filters are the foundation of this result. As a matter of fact the planner is composed by a first FIR filter for the computation of the control points from the sequence of desired via-points, followed by a chain of moving average filters. Therefore, the generator combines the characteristics of B-spline trajectories (smoothness and minimum curvature) and those of FIR filters (simple structure and computational efficiency). Moreover, besides standard cubic curves, the so-called smoothing B-splines have been considered for online trajectory generation. This allows to find a tradeoff between the possibility of exactly crossing the given via-points and the smoothness of the resulting trajectory. A simple teleoperation task with a Puma 560 industrial manipulator has been arranged for experimentally validating the proposed method.

In this paper, a method to derive the three predominant synergies and their temporal weights for planning grasps of the UB Hand IV (University of Bologna Hand, version IV) is proposed. The method adopted to define the postural synergies from experiments is based on the kinematic structure of the robotic hand and on the taxonomy of the grasps of common objects. The control strategy, exploiting postural synergies, that drives the hand during reach to grasp is further described. During prehension the hand moves continuously in a configuration space of highly reduced dimensionality with respect to its degrees of freedom. The experiments confirm that the UB Hand IV works efficiently in a synergy based framework for grasp planning and prehension control. It is shown that the introduction of the third predominant synergy significantly improves the grasping synthesis and performance, especially for the adduction/abduction motion of the thumb.

In this paper, the control of a redundant robotic manipulator with variable stiffness actuation is addressed. The problem of controlling simultaneously the end-effector position and stiffness exploiting the robot redundancy for the optimization of the robot configuration is considered, and the relation between the manipulator redundancy and the selection of both the joint and end-effector stiffness is discussed. The controller is configured as a cascade system that allows the decoupling of the actuators dynamics from the arm dynamics and the consequent reduction of the order of the manipulator dynamic model. Only the actuator and joint positions are needed by the controller, introducing in this way a significant simplification with respect to previously proposed state feedback techniques. The effectiveness of the proposed approach is verified by simulations of a 3-DOF planar manipulator.

This video presents the evolution of the robotic hands, called UB Hands (University of Bologna Hands), developed at the Laboratory of Automation and Robotics of the University of Bologna during more than 25 years of research in this field. Starting from the UB Hand I, the first robot hand prototype developed in our labs, the different design solutions and philosophies that have been followed toward the innovative UB Hand IV, also called DEXMART Hand, recently developed within the DEXMART project are presented. Remarkable characteristics of the UB Hands are also the whole hand manipulation capabilities, the ability of reconstructing the contact forces over the whole hand surface as in the case of the UB Hand II, and the presence of force/tactile sensors as in the UB Hand IV. Moreover, the use of soft covers for the emulation of the human tissue characteristics has been studied, and the adoption of innovative design concepts based on compliant structures has been introduced in the UB Hand III and IV.

In this paper, we deal with several aspects related to the control of tendon-based actuation systems for robotic devices. In particular, the problems that are considered in this paper are related to the modeling, identification, and control of tendons sliding on curved pathways, subject to friction and viscoelastic effects. Tendons made in polymeric materials are considered, and therefore, hysteresis in the transmission system characteristic must be taken into account as an additional nonlinear effect because of the plasticity and creep phenomena typical of these materials. With the aim of reproducing these behaviors, a viscoelastic model is used to model the tendon compliance. Particular attention has been given to the friction effects arising from the interaction between the tendon pathway and the tendon itself. This phenomenon has been characterized by means of a LuGre-like dynamic friction model to consider the effects that cannot be reproduced by employing a static friction model. A specific setup able to measure the tendon's tension in different points along its path has been designed in order to verify the tension distribution and identify the proper parameters. Finally, a simple control strategy for the compensation of these nonlinear effects and the control of the force that is applied by the tendon to the load is proposed and experimentally verified.

This paper presents the latest results in the development of the low-level controller of the robotic hand UBH-IV (University of Bologna Hand, version IV). In particular, the friction effects acting at joint level have been rendered by means of a LuGre-like model and a procedure for the identification of the friction model parameters is described. With the aim of providing an online estimation of the effects due to the interaction of the robotic hand with the environment, a controller able to evaluate the overall external torque acting on the finger joints and to discern between friction and torques generated by the external interaction force without using direct measures of the contact forces is proposed. The identification and control tests are carried over on an experimental setup composed by a single finger phalanx, manufactured with the same material and techniques of the hand itself.

Joint stiffness plays an important role in both safety and control performance, particularly in human-friendly robots using artificial pneumatic muscles. Due to the limited control bandwidth of pneumatic muscles, stiffness characteristics and their effects on safety in the frequency domain should be taken into account. This paper introduces the concept of instantaneous stiffness and validates its model with the Stanford Safety Robot (S2ρ. The potential effects of instantaneous stiffness on safety is explored through experimental comparison of peak impact accelerations under various impact conditions. Instantaneous stiffness demonstrates different effects on the impact acceleration depending on impact velocity and controller gain. Finally, the paper discusses the stiffness characteristics as a guideline for design and control to improve the robot safety while maintaining the control performance.

In this paper, the postural synergies configuration subspace given by the fundamental eigengrasps of the UB Hand IV (University of Bologna Hand, version IV) is derived through experiments. This study is based on the kinematic structure of the robotic hand and on the taxonomy of the grasps of common objects. Experimental results show that it is possible to obtain grasp synthesis for a large set of objects both in the case of precision or power grasps by using only a very limited set of dominant eigengrasps. The tasks here presented are planned with an initial hold of the hand followed by reach and grasp phases, that are unique for each object/grasp combination, during which the robotic hand posture evolves continuously within a subset of the hand configuration space given by the two predominant eigenpostures. The paper reports the method adopted to define from experiments the postural synergies for the UB Hand IV and the results of the grasp tasks performed adopting the defined synergies.

In this paper, the relation between B-splines and FIR (Finite Impulse Response) filters is demonstrated and exploited to design a digital filter for trajectory planning, combining the very simple structure and computational efficiency of FIR filters with the flexibility of splines. In particular, the trajectory generator consists of two main elements. The former is devoted to the solution of an optimization problem that, given a set of points to be interpolated (or approximated), provides the control points defining the spline. The latter, a cascade of moving average filters, gives the trajectory profile at each sampling time on the basis of such points. The proposed method has been applied to several robotic and industrial applications, and in this paper two case studies are reported as examples: an industrial robot performing a welding operation and a mobile robot moving in an environment with obstacles. With respect to these tasks, the main features of the trajectory generator are shown: the possibility of planning trajectories with high degree of smoothness (continuity of the derivatives), the possibility of easily changing the duration of the trajectory (and therefore the velocity, acceleration, jerk, etc. of the trajectory) maintaining the same geometric path, the possibility of locally modifying the pre-planned path.

This paper reports the modeling activities related to the development of an innovative tendon-driven robotic finger, designed as the fundamental element of a new biologically-inspired artificial hand. The finger is realized in plastic material by means of 3D-printing, a production process that allows a remarkable simplification of the mechanical design. Through 3D-printing, we were able to easily implement solutions that could be very difficult, if not impossible, to obtain with conventional manufacturing. A detailed simulation model of the robotic finger has been developed with the aim not only of designing and testing suitable control strategies for the finger, but also of investigating the benefits and the flaws of particular design solutions. As a matter of fact, this approach to design and realization of robotic fingers, that fulfills the requirements in terms of compactness, integration and simplified assembly, has a significant drawback in frictional phenomena on both tendons and joints. For this reason, an adapted LuGre friction model is proposed in order to simulate and study the finger behavior.

In this paper, the characterization of the force distribution along a tendon sliding on a curved pathway, subject to friction and visco-elastic effects, is investigated. In order to have a better understanding of the system behavior, a specific setup able to measure tension forces in different points along the tendon's path has been built. Experimental data collected by measuring the tendon tension forces during both the pulling and the release phase are presented, and theoretical models reproducing the tendon behavior with increasing fidelity are proposed. In particular, the friction arising from the interaction between the tendon pathway and the tendon itself is characterized by means of a LuGre-like dynamic friction model. The introduction of a dynamic friction model allows to reproduce in simulation some effects arising during experimental activities that cannot be reproduced employing an equivalent static friction model. Moreover, the adoption of tendons made by polymeric fibers introduces hysteresis in the tendon transmission characteristic due to the plasticity and creep phenomena typical of these materials. With the aim of reproducing this behavior, a visco-elastic model is used for modeling the tendon compliance.

Tendon-based transmission systems present many positive aspects and greatly simplify the mechanical design of small robotic devices, such as robotic fingers. On the other hand, they introduce several nonlinear effects that must be properly considered by the control algorithms to achieve a suitable performance level in the regulation of the finger joint torques. In this paper, the model of the tendons-based driving system and of the nonlinear effects arising from the use of sliding paths instead of pulleys for the tendon routing are discussed, and control algorithms aiming at compensating these nonlinearities are presented. Both models and control algorithms have been validated by experiments. In particular, in order to gain a better insight on the force distribution along the tendon, an experimental setup for the measurement of the tension in some intermediate points has been developed. After the identification of the tendon characteristics, a suitable control law for the compensation of the nonlinear effects due to the friction acting on the transmission system has been applied. The proposed compensation scheme is based on a sliding-mode controller with boundary layer, where the boundary threshold is modulated as a function of the desired tendon tension.

In this paper, a systematic procedure for the definition of the dynamical model in port-Hamiltonian form of mechanical systems is presented as the result of the power-conserving interconnection of a set of basic components (rigid bodies, flexible links, and kinematic pairs). Since rigid bodies and flexible links are described within the port-Hamiltonian formalism, their interconnection is possible once a proper relation between the power-conjugated port variables is deduced. These relations are the analogous of the Kirchhoff laws of circuit theory. From the analysis of a set of oriented graphs that describe the topology of the mechanism, an automatic procedure for deriving the dynamical model of a mechanical system is illustrated. The final model is a mixed port-Hamiltonian system, because of the presence of a finite-dimensional subsystem (modeling the rigid bodies) and an infinite-dimensional one (describing the flexible links). Besides facilitating the deduction of the dynamical equations, it is shown how the intrinsic modularity of this approach also simplifies the simulation phase.

The time domain passivity framework is attracting interest as a method for granting stability in both telerobotics and haptic contexts; this paper employs this approach in order to introduce a novel concept, the Bilateral Energy Transfer for haptic telepresence. Loosely speaking, the Bilateral Energy Transfer is the straightforward transfer of energy between the two opposite sides of a teleoperation network, the master and slave robots. In an ideal telepresence scenario master and slave robots behave as rigid connected masses [1], and their power exchange is lossless; conversely, realistic scenarios include sources of energy leaks, i.e. elements that modify the power flows in the network. Moreover, if energy leaks have an active nature, they become source of instability for the system. This work isolates two sources of instability normally present in a teleoperation system, i.e. the delayed communication channel and robot velocity estimation based on digital position acquisition. These energy leaks are counterbalanced by two independent controllers, whose design is based on energetic consideration, and whose employment allows to achieve the Bilateral Energy Transfer. The presented arguments are sustained by simulations and experiments.

Physical human-robot interaction requires the development of safe and dependable robots. This involves the mechanical design of lightweight and compliant manipulators and the definition of motion control laws that allow to combine compliant behavior in reaction to possible collisions, while preserving accuracy and performance of rigid robots in free space. In this framework, great attention has been given to robots manipulators with relevant elasticity at the joints/transmissions. While the modeling and control of robots with elastic joints of finite but constant stiffness is a well- established topic, few results are available for the case of robot structures with variable joint stiffness -mostly limited to the 1-dof case. We present here a basic control study for a general class of multi-dof manipulators with variable joint stiffness, taking into account different possible modalities for changing the joint stiffness on the fly by an additional set of commands. It is shown that nonlinear control laws, based either on static or dynamic state feedback, are able to exactly linearize the closed- loop equations and allow to simultaneously impose a desired behavior to the robot motion and to the joint stiffness in an decoupled way. Illustrative simulations results are presented.

In robotics, the manipulation ofa prioriunknown objects involves several steps and problems that must be carefully considered and solved by proper planning and control algorithms. For example, once suitable contact points have been computed, the control system should be able to track them in the approach phase, i.e., while the relative position/orientation of the object and the gripper of the robotic system change due to the approaching movement of the robot toward the object. This correspondence paper proposes a practical method for the tracking of grasp points in image space that is based on transferring previously computed grasp points from an initial image to subsequent ones and on the analysis of the new grasp configuration. Three different options are proposed for this transference. Experimental results show the interesting practical performance of the general procedure.

In this paper, two problems related to the simulation of virtual environments for haptic systems are considered. The first problem is how to simulate, in discrete time and with low computational effort, dynamic systems in order to preserve their passivity properties. As a matter of fact, simulation of complex systems in real time may lead to undesired effects, like unstable behaviours of the haptic interface, if proper care is not given to the definition of the simulation algorithm. An algorithm is presented here able to maintain the passivity properties of the physical (simulated) system with a reduced computational complexity. The second problem discussed in this paper is the interconnection of algorithms running at different frequencies, i.e., the control algorithm of the haptic interface (running typically at high frequency) and the algorithm simulating the virtual environment (running at lower frequency). A proper software interface, able to connect these two algorithms in an energetic-consistent manner, is presented and discussed. The general framework of both these techniques is the passivity theory and the so-called port-Hamiltonian formalism.

In this paper, the dynamic model of a robot with antagonistic actuated joints is presented, and the problem of full linearization via static state feedback is analyzed. The use of transmission elements with nonlinear relation between the displacement and the actuated force allows to control both the position and the stiffness of each joint. The main advantage of this actuation modality is that the achieved stiffness becomes a mechanical characteristic of the system and it is not the result of an immediate control action as in the classical impedance control scheme (Davison, 2003). Different examples of implementation of this kind of devices are known in literature, even if limited to one single joint (Kjita et al., 2003; Laumond and Kineocam, 2006; Mansard and Chaumette, 2004 and 2006) and the application of antagonistic actuated kinematic chains in the field of robotic hand design is under investigation (Stasse et al., 2006). After a brief review of the dependence of the properties of antagonistic actuation on the transmission elements characteristics, a scheme for simultaneous stiffness-position control of the linearized system is presented. Finally, simulation results of a two-link antagonistic actuated arm are reported and discussed.

In this paper, a simple way to model flexible robotic links is presented. This is different from classical approaches and from the Euler-Bernoulli or Timoshenko theory, in that the proposed model is able to describe large deflections in 3D space and does not rely on any finite-dimensional approximation (e.g., modal approximation). The model has been formulated within the port Hamiltonian formalism because intuitive considerations on the geometric behavior of the elastic link naturally define a Stokes-Dirac structure, the kernel of a port Hamiltonian system. Moreover, port Hamiltonian systems can be easily interconnected, thus allowing the description of complex systems as a composition of parts in an object-oriented way. By combining rigid bodies, springs, dampers, joints and, finally, flexible links, it is virtually possible to model and mathematically describe whatever complex mechanical structure formed by beams. In order to demonstrate the dynamical properties of the model and how complex mechanisms can be obtained by port interconnection, simulations of 1-DoF and 2-DoF serial manipulators and of a 2-DoF flexible closed kinematic chain are presented.

In this paper, the port Hamiltonian model of a manipulator is presented as the result of the power-conserving interconnection of a set of main components (rigid bodies, flexible links and kinematic pairs). Since rigid bodies and flexible links are described within the port Hamiltonian formalism, their interconnection is possible once a proper relation between the power conjugated port variables is deduced. These relations are the analogous of the Kirchoff laws of circuit theory. The final model is a mixed port Hamiltonian system because of the presence of a finite dimensional subsystem modelling the rigid bodies and of an infinite dimensional one describing the flexible links. The intrinsic modularity of the approach simplifies the model deduction and simulation, while the Hamiltonian description suggests the development of energy-based controllers

In this paper, the tendon-sheath driving system for a robotic hand is presented and its force transmission characteristics are analyzed. The use of tendon-based transmission permits to reduce the size and the complexity of the actuation chain in many mechanical devices. A simple static model that describes the tendon-sheath driving system is presented, and its behavior is compared both with simulative results, obtained with a lumped parameters model of the tendon, and with experimental results. Different static and dynamic friction models are used in the simulations and the related results are compared to highlight some phenomena that are not visible from the static model of the tendon. A simple force control algorithm with feedforward friction compensation based on the static friction model is also presented

The first part of this paper describes the development of a humanoid robot hand based on an endoskeleton made of rigid links connected with elastic hinges, actuated by sheath routed tendons and covered by continuous compliant pulps. The project is called UB Hand 3 (University of Bologna Hand, 3rd version) and aims to reduce the mechanical complexity of robotic end effectors yet maintaining full anthropomorphic aspect and a good level of dexterity. In the second part this paper focuses on the early experiences of the UB Hand 3 in performing manipulation tasks.

In this work the static and dynamic characterization of viscoelastic pads for robotic hands is performed. A quasi-linear model, developed to describe the behavior of human hand pads and, more generally, of biological tissues, is adopted in order to overcome the problems tied to classical (linear) models, often used in the robotic field. Through experimental tests, the values of the parameters of this model have been found for two different materials (a polyurethane gel and a silicon rubber) which show a behavior similar to that of human pads and seem very suitable for robotic applications. Finally, the model has been extended with the use of digital filters, making the identification process very straightforward.

In this paper, the problem of online estimation of the mechanical impedance during the contact of a robotic system with an unknown environment is considered. This problem is of great interest when controlling a robot in an unstructured and unknown environment, such as in telemanipulation tasks, since it can be easily shown that the exploitation of the knowledge of the mechanical properties of the environment can greatly improve the performance of the robotic system. In particular, a single-point contact is considered, and the (nonlinear) Hunt-Crossley model is taken into account, instead of the classical (linear) Kelvin-Voigt model. Indeed, the former achieves a better physical consistency and also allows describing the behavior of soft materials. Finally, the online estimation algorithm is described and experimental results are presented and discussed.

The visual tracking of grasp points is an essential operation for the execution of an approaching movement of a robot arm to an object: the grasp points are used as features for the definition of the control law. This work describes a strategy for tracking grasps on planar objects based on the use of a homography. In particular, the homography is used for transferring (translating) a grasp from a view of an object to a second one, providing in this way a correspondence in the second view. The grasp tracking procedure combines the search of a new grasp with the translation and the evaluation of that grasp between object views. Results of the proposed grasp tracking strategy are shown in case of grasps computed for a two-finger and a three-finger gripper.

The paper describes work in progress at the University of Bologna concerning the design of a new anthropomorphic robot hand. The hand is based on the modular assembly of articulated fingers that adopt an original configuration of their structure, made with rigid links connected by elastic hinges that are coaxially crossed by flexible tendons. This innovative design is suitable to host distributed sensory equipment and continuous compliant cover, allowing a high level of anthropomorphism together with great structural simplification, reliability enhancement and cost reduction. Furthermore, the proposed solution is very flexible, as it can be adapted to many different hand configurations and is not dependent on a particular type of actuation, being compatible with future availability of any kind of artificial muscles.

In this paper, the problem of the on-line estimation of the mechanical impedance during the contact of a robotic system with an unknown environment is considered. Indeed, the knowledge of the mechanical properties could allow to improve the interaction between robotic devices and unstructured and unknown environments, e.g. in telemanipulation tasks. A single-point contact is considered and the (nonlinear) Hunt-Crossley model is taken into account and its better physical consistency in describing the behavior of soft materials is discussed in comparison with the classical (linear) Kelvin-Voigt model. Finally, the on-line estimation algorithm is described and experimental results presented.

In this paper, the dynamics of a piezoelectric material is presented within the new framework of multi-variable distributed port Hamiltonian systems. This class of infinite dimensional system is quite general, thus allowing the description of several physical phenomena, such as heat conduction, elasticity, electromagnetism and, of course, piezoelectricity. The key point is the generalization of the notion of finite dimensional Dirac structure in order to deal with an infinite dimensional space of power variables. In this way, the dynamics of the system results from the interconnection of a proper set of elements, each of them characterized by a particular energetic behavior, while the interaction with the environment is described in terms of mechanical and electrical boundary ports.

In this paper, a novel design approach for the development of robot hands is presented. This approach, that can be considered alternative to the "classical" one, takes into consideration compliant structures instead of rigid ones. Compliance effects, which were considered in the past as a "defect" to be mechanically eliminated, can be viceversa regarded as desired features and can be properly controlled in order to achieve desired properties from the robotic device. In particular, this is true for robot hands, where the mechanical complexity of "classical" design solutions has always originated complicated structures, often with low reliability and high costs. In this paper, an alternative solution to the design of dexterous robot hand is illustrated, considering a "mechatronic approach" for the integration of the mechanical structure, the sensory and electronic system, the control and the actuation part. Moreover, the preliminary experimental activity on a first prototype is reported and discussed. The results obtained so far, considering also reliability, costs and development time, are very encouraging, and allows to foresee a wider diffusion of dexterous hands for robotic applications.

In this work, a cartesian impedance controller purposely designed for dexterous manipulation is described. Based on the main features of the DLR Hand II, concerning kinematic structure and sensory equipment of fingers, this control strategy allows to overcome the main problems encountered in fine manipulation, namely: effects of the friction (and unmodeled dynamics) on robot performances and occurrence of singularity conditions. The achieved control scheme bas been experimentally validated by testing it on a finger of the DLR Hand.

In advanced robotic applications where a physical interaction of the robot with the environment takes place, the measurement of interaction forces is of basic importance for a safe execution of the desired tasks. In the paper, a 'stress' sensor is presented and its basic characteristics illustrated and discussed. Among its positive features, one may find the very compact size, good electrical properties, and the fact that it can be easily 'constructed' directly on the mechanical part by deposition of the sensing elements directly on the mechanical structure. These are very interesting properties for its use in advanced robotic systems.

In this paper we consider the problem of stabilizing in a desired configuration a mobile manipulator; only the arm's joint displacement information and the measures provided by the camera mounted on the end-effector are used to stabilize the system. In particular, no knowledge about the position and orientation of the mobile base is supposed to be available. An hybrid control algorithm, based on the concatenation of a sensor-based feedback control and an open-loop strategy, is proposed. A 3-DOF planar manipulator mounted on a mobile base, modelled as an unicycle, is considered as a case study, and simulation results are reported in order to demonstrate the capabilities of the proposed control algorithm.

In this paper, we study the interconnection of two robots, which are modeled as port-controlled Hamiltonian systems through a transmission line with time delay. There will be no analysis of the time delay, but its presence justifies the use of scattering variables to preserve passivity. The contributions of the paper are twofold: first, a geometrical, multidimensional, power-consistent exposition of telemanipulation of intrinsically passive controlled physical systems, with a clarification on impedance matching, and second, a system theoretic condition for the adaptation of a general port-controlled Hamiltonian system with dissipation (port-Hamiltonian system) to a transmission line.

In space applications, it is conceivable that part of the robotic activities could involve the grasp and/or manipulation of free-floating objects in the absence of gravity. In this case, synchronous application of contacts seems to represent a basic feature in order to efficiently grasp the floating items. In this sense, an additional difficulty is that objects may have irregular shape and/or be non well positioned in the gripper workspace. These difficulties cannot be handled in a simple way with standard 2-jaw grippers, with one (or two) degrees of freedom. In the paper, an activity for designing and experimenting with a gripper for this type of operations is reported, and the first laboratory results are presented and discussed. The main features of the gripper are its kinematic configuration (3 fingers with 3 dof) and its sensorial equipment, features that improve the dexterity of this device if compared to more classical devices.

In this paper, an extension of the so-called intrinsic passive control (IPC) is illustrated, showing that an improvement of performances can be achieved by considering different types of energy-storing elements, i.e. "springs", in the IPC. In particular, two new "springs" are introduced: a 'variable rest length' spring and a 'variable stiffness' spring, that are properly defined in order to maintain the passivity of the IPC and to improve its performances in given situations. Simulations of the resulting control, applied to a defective system and to a simple telemanipulation device, are presented and discussed.

The VIDET project is aimed at investigating the possibility of developing a wearable robotic system for helping the mobility of visually impaired persons. The basic idea involves the conversion of real-time depth data gathered through stereo-vision into a virtual, "bas-relief" model perceivable by means of a haptic interface. In this paper we describe the real-time stereo system, review the basic principles of the two main haptic devices developed so far, and present new experimental results concerning extraction of depth data by the stereo system and haptic perception of the virtual model recovered from stereo-data.

A haptic interface designed for exploration of unknown environments by a visually impaired person is presented. The main feature of this interface is that it is based on a single actuated wire, and therefore is a defective device, with minimum three actuators for 30 environments. The defectiveness of the device is partially compensated by using proper control strategies. Experiments on a laboratory setup are presented and discussed.

Manipulability measures are well known tools for the analysis of the capability of a mechanical device of performing in the velocity or force domain. They have been firstly introduced for single kinematic chains, assuming them to have full controllability of each joint and considering the kinematics and the statics of the manipulator. Then, significant extensions have been proposed, including the dynamic case, the presence of multiple arms, different motion capabilities of the robotic device(s). Only a few studies have concerned the study of performance indices for devices actuated by wires (or tendons), and, in practice, nobody has considered in this context (wire actuation) the case of defective systems. In this paper, this case is explicitly addressed taking into consideration as applicative example the WireMan, a haptic interface for helping mobility of visually impaired people.

Two results concerning the control of force-reflecting teleoperation systems with time-delays are presented. The first result consists of a general framework for stability analysis of such systems. This analysis is performed on the basis of methods developed for linear time-delay systems, here extended and applied to the case of teleoperation schemes. The analysis shows whether the overall system is asymptotically stable independently of time-delay, or determines the maximum time-delay which guarantees asymptotic stability. The second issue discussed in the paper is the design of a control law able, under proper conditions, to stabilize the time-delay teleoperator. Examples of application of the stability criteria are discussed and experimental results of the control law are given.

The theory of force and dynamic manipulability for general systems of multiple co-operating robot manipulators is developed. Manipulability analysis refers to the study of the performance of the system regarding to the mechanical transformation of inputs (forces and torques at actuated joints) into outputs (forces and torques exchanged with the environment or accelerations of a reference member), in relation to different configurations of the system and different directions in the input and output spaces. For this purpose, the concept of manipulability ellipsoids for single robot arms is generalized so as to encompass multi-limb co-operating systems with general kinematic structure.

A portable haptic interface based on tensioned wires is presented and some control strategies are discussed. In this work defective configurations with 3 and 1 tensioned wires are considered, illustrating how in each case a proper control strategy may render to the operator a clear sensation of the interaction with the virtual environment. In this manner, simple, robust, light, portable devices can be designed and realized, allowing their use in a wide range of applications. In particular, the haptic device considered has been designed to be used by visually impaired people.

Two techniques for slip detection with a rubber-based tactile matrix sensor are presented. The described results have been obtained within a common research activity between the Robotics and Automation Laboratory of the University of Bologna and the Control Laboratory of the Delft University of Technology. The first technique is based on a frequency analysis of the position of the center of force distribution. The main idea is that before a slip situation actually occurs, it is possible to detect a micro movement of the object due to the elasticity of the rubber. By implementing a FFT of the center of distribution and by testing the frequency components, the slip is identified before it practically occurs. The second approach is based on the principle that the normal forces measured by the tactile sensor fluctuate with a certain frequency during slip because of the rubber elasticity. This fluctuation is due to a 'catch and snap back' effect, which is present when an object is slipping over the sensor's surface. By testing the frequency domain of the normal force a slip condition can be detected. A comparison of the two approaches is presented and discussed on the basis of experimental results obtained in both laboratories.

In this paper, the analysis of the differential kinematics and manipulability measures of robotic systems comprised of multiple cooperating limbs is considered. The goals of this study can be articulated in four points: 1) to enumerate the degrees of freedom of the manipulation system; 2) to describe analytically all possible first-order differential motions of the system at a given configuration; 3) to evaluate in the velocity domain the functionality of a manipulation system, with respect to the task it is required to perform; and 4) to calculate the bounds for the velocities achievable by the system, given bounds on the capabilities of joint actuators. The assumptions made on the robotic system are quite general, so that many complex devices (e.g., dextrous hands, legged vehicles, whole-arm manipulators, etc.) can be dealt with in a unified and convenient framework.>

This paper demonstrates fundamental problems with dexterity measures found throughout the robotics literature and offers a methodology for correcting those problems. Measures of robot dexterity derived from eigenvalues, eigenvectors, similarity transformations, singular-value decompositions and the Moore-Penrose inverse of the manipulator Jacobian do not have invariant physical meaning. The paper presents manipulability ellipsoids and manipulability screw-subspaces for both redundant and nonredundant manipulators.>

In this paper, the problem of the analysis of the force systems acting in static conditions in a generic manipulation device is considered. By generic manipulation device we mean a manipulation system composed by one or more manipulators, each of them with one or more links in contact with an object. Different contact models may be taken into account, for example complete constraint, soft-finger, hard-finger, and so on. Two are the main goals of the analysis: to understand which sets of forces, belonging to the joint torque space, the contact force space, and the external wrench space, are present or applicable to the system, and in which manner forces in these sets can be obtained. In fact, the answer to these two questions, crucial in problems grasp optimization and control, is not trivial in the case that some or all of the manipulators in the system have limited mobility. In this paper, a technique is proposed aiming to solve these problems, and some examples are presented to illustrate the method.>

The manipulability ellipsoids are used in robotics as a measure of manipulators' performances achievable during task execution. The definition of these geometrical entities is based on the Jacobian of the manipulator, and the physical meaning given to the ellipsoids is related to the capability of the mechanism to apply forces with the end effector or to move the tool in some directions of the task-space. P. Chiacchio et al. (ibid., vol.7, p.678-685, 1991) have extended this concept to the case of cooperating manipulators. Nevertheless, some questions are still open, both with respect to the basic definition and use of manipulability ellipsoids. The commenter shows by simple examples that the use of manipulability ellipsoids for multiarm systems gives misleading results, and he disagrees with the above authors' definition of ellipsoids. The authors defend their work.>

The coordinator of the movements of multiple robot arms manipulating a common object is considered. In order to provide a general framework for the study of such cooperating systems as common industrial arms, multifingered hands, and legged vehicles, the analysis does not rely on the assumption of full mobility for each cooperating arm, which is otherwise common in related literature. The aim of the present work is to provide a systematic method to characterize the mobility and differential kinematics of general cooperating systems. The proposed analysis and algorithms provide an insight into the structure of the input (joint)-output (task) relationship of such systems.>

In this paper, the main design features of the University of Bologna robotic hand version II (UB Hand 11), currently at the set-up phase, are pre- sented. In particular, a detailed description is given of the adopted solutions with respect to hand-arm me- chanical integration, sensorial equipment, and control architecture and algorithms.