Loredana Zollo

An experimental study on compliance control for a redundant personal robot arm

Abstract Human–robot interaction represents a critical factor in the design of personal robots as well as in the implementation of robot behavior and control. This work investigates and proposes solutions to the problem of controlling an anthropomorphic robot arm for personal assistance, by dealing with the peculiarities of its design, i.e. the mechanics of its cable-actuated, intrinsically compliant structure, and by emphasizing its potential in applications of physical and functional interaction with the environment and with human users. To satisfy the requirements of increasing the safety in the interaction and the robot functionality in tasks performed in cooperation with humans, three solutions are developed and tested for the considered personal robot. The initial idea is aimed at developing an efficient as well as computational convenient interaction control strategy, i.e. a compliance control scheme in the Cartesian space. The analysis of its limited performance suggests two further control strategies, i.e. a compliance control scheme in the joint space and an impedance–compliance control scheme. Their compared analysis points out that all the three solutions can safely operate in the human environment, but from a functional point of view only the last two schemes can effectively control the personal robot arm in its whole workspace. The paper describes the mechanics of the considered robot arm, with special regard to its anthropomorphism and cable-actuation and presents in details the three control schemes. They are critically evaluated through the experimental results achieved in tasks of physical and functional interaction with the environment and with human users. The impedance–compliance controller emerges as the more appropriate to the addressed application as well as to the peculiar cable-actuated structure.

BioMechatronic Design and Control of an Anthropomorphic Artificial Hand for Prosthetics and Robotic Applications

The paper proposes a biomechatronic approach to the design of an anthropomorphic artificial hand. The hand is conceived to be applied to prosthetics and biomedical robotics; hence, anthropomorphism is a fundamental requirement to be addressed both in the physical aspect and in the functional behavior. As regards the hand mechanics, a cable-driven underactuation is proposed in order to enlighten the structure, allow anthropomorphic self-adaptation to the object to be grasped, and simplify the control. Two simple PD control systems are formulated and evaluated in a common task of grasping a cylindrical object. The reference input for the control is derived from data on human subjects performing the same task and extracted by the literature. The paper reports simulation results about the comparison with the human case when both control systems are used to close the finger, so to derive specific indications for the improvement of the hand design

A bio-inspired approach for regulating visco-elastic properties of a robot arm

Neurophysiological studies show that humans possess the capability of generating appropriate motor behaviors to different uncertain environmental conditions by combining a forward action, produced by the internal forward dynamic model, and a feedback control, realising the transformation from sensory information to motor commands. To this regard, a control system based on the combination of a feedforward and a feedback control loop has been developed in order to provide a robot arm with human-like adaptation capabilities. The work analyses the role of biological coactivation in the mechanism of adjustable visco-elastic arm properties and proposes a function for the evaluation of the robot arm coactivation based on the measure of the position error and the interaction force. The coactivation function is used to update the proportional and derivative parameters of the feedback controller and, consequently, the arm visco-elasticity in unpredictable environmental conditions. Finally, experimental results on the evolution of the coactivation in the adaptation and de-adaptation phases are provided in the last section of the paper.

Regulation with on-line gravity compensation for robots with elastic joints

In this paper a PD control with on-line gravity compensation is proposed for robot manipulators with elastic joints. The work extends the existing PD control with constant gravity compensation, where only the gravity torque needed at the desired configuration is used throughout motion. The control law requires measuring only position and velocity on the motor side of the elastic joints, and the on-line compensation scheme estimates the actual gravity torque using a biased measure of the motor position. It is proved via a Lyapunov argument that the control law globally stabilizes the desired robot configuration. Experimental results on an 8-d.o.f. robot manipulator with elastic joints show that this control scheme improves the transient behavior with respect to a PD controller with constant gravity compensation. In addition, it can be usefully applied in combination with a point-to-point interpolating trajectory leading to a reduction of final steady-state errors due to static friction and/or uncertainty in the gravity compensation.

Functional compliance in the control of a personal robot

The research in the field of advanced robotics is turning its attention more and more to man and his assistance, by developing systems such as service robots, personal robots, and even humanoid robots. Interaction control of such robot manipulators is of paramount importance for an effective execution of manipulation and tracking and, over all, for a safe and effective interaction with the humans. The paper concerns the problem of the control of an 8 degree of freedom anthropomorphic arm named DEXTER, mounted on the mobile platform of the MOVAID System, a robotic system for household personal assistance. The goal is to realize a compliant control for this manipulator in tasks of assistance to disabled and elderly people. On the basis of the control theory applied to industrial robotics, a specific compliant control solution has been developed for the DEXTER peculiar mechanical structure and actuation system, which cause a coupled joint configuration. The solution provides the capability of regulating the robot compliance according to the level of stiffness of the interaction environment. The paper describes the theoretical model of the control system, the implementation on the MOVAID platform and the experimental results in the execution of a set of demonstration tasks.

An anthropomorphic robotic platform for experimental validation of biologically-inspired sensory-motor co-ordination in grasping

The aim of the work is the integration of an anthropomorphic robotic platform, starting from a neurophysiological model of grasping, in order to provide tools for an experimental validation of the model and also to provide new anthropomorphic solutions for robot grasping. The resulting robotic system is composed of an anthropomorphic arm/hand system and visual and tactile sensors. Grasp planning, control and learning have been achieved by a neural approach, inspired by a model of the inter-connections among the brain areas involved in grasping, as formulated by neurophysiologists. After a description of objectives and specifications for the robotic platform, the system is illustrated and experimental results are reported. Finally, the results are discussed as a starting point for current activities, involving the development of novel human-like robotic components and the implementation of more sophisticated learning schemes.

Robust pose estimation algorithm for wrist motion tracking

The wrist plays a fundamental role in reaching and grasping actions, i.e. it guides the hand to the grasp position and adjusts its orientation on the basis of the grasping type and task. This paper proposes a novel, low-cost method for wrist pose estimation by using the Asus Xtion Pro Live motion sensing device and a robust marker-based tracking approach based on Unscented Kalman Filter (UKF). The hand palm kinematic model is also considered. The applicability of the approach to evaluate some interesting kinematics parameters, such as position, orientation, Range Of Motion, angular and linear velocity and trajectory has been proved. In particular, since the nature of the paper is to present a novel approach for wrist pose estimation, only initial validation for wrist kinematic measurement will be reported.

Experimental validation of functional compliance in an anthropomorphic personal robot

The development of humanoid robots and their application as personal robots introduce problems related to the interaction of robots with humans and with environments specifically designed for human beings. In tasks where human-robot cooperation is required, compliance of the robot arm can provide a helpful solution, not only to ensure safety, but especially to increase the robot functionality and usability. The concept of functional compliance is illustrated and an experimental validation is reported, where the functionality of an anthropomorphic robot arm is comparatively assessed with and without compliant control. The paper describes the implementation of a compliant control scheme on an anthropomorphic robotic manipulator, illustrates in detail the methodology adopted for the experimental comparative validation and reports the results obtained in the execution of a sample task by a set of potential users, showing the increased performance in the case of compliant control. Indications are also provided on the improvement of acceptability, which is also affected by the enhanced performance.

An impedance-compliance control for a cable-actuated robot

A research work on the interaction control of a cable-actuated robot arm, the Dexter arm, is presented in this paper. Firstly, general considerations on the cable-actuated structures and their application potential are provided and then the Dexter structure peculiarities are accurately analyzed in order to develop proper control solutions. Starting from the analysis of the limitations of the compliance control schemes in Cartesian space and in joint space, previously implemented and experimentally validated on the Dexter arm, a novel control strategy, named impedance-compliance controller, is developed. The proposed control strategy tries to combine the benefits of a compliance control scheme in Cartesian space with the benefits of an impedance control scheme in the operational space by compensating the dynamics of the sole proximal joints. The impedance-compliance controller is capable to achieve accurate smooth motions while guaranteeing functional control of the whole structure, even though a greater computational complexity is required The last section of the paper, dedicated to the experimental results, points out the differences with the previously experimented control solutions anti provides some proofs of the increased Dexter functionality.

Bio-Inspired Interaction Control of Robotic Machines for Motor Therapy

The idea of robot-aided motor therapy was first introduced in the early 1990s (Khalili & Zomlefer, 1988), (Hogan et al., 1992), and is gaining an increasing popularity. Many different platforms have been developed worldwide (Krebs et al., 1998), (Colombo et al., 2000), (Lum et al., 1999), (Reinkensmeyer et al., 2000a), (Kiguchi & Fukuda, 2004), (Costa et al., 2004), and some commercial systems appeared on the market sustained by encouraging, though still limited results of clinical trials. The design and the production of devices for application in robot-aided motor therapy requires a multidisciplinary group, composed of neuroscientists, physiatrists, therapists and bioengineers, that strictly collaborate for the definition of the functional specifications of the machine, so that neurophysiological requirements are fulfilled and the artificial system and the natural system are integrated as functional and less invasive as possible. A crucial design challenge in robot-aided rehabilitation is interaction control between the robotic machine, the patient, and the therapist. That is because the machine has to permanently operate in a constrained motion, where a direct mechanical coupling always exists between the patient (or the limb involved in the robotic treatment) and the machine (Reinkensmeyer et al., 2000b). Tight physical interaction between the robot and the human body is not occasional, like in many other industrial or service robotic applications, but it is an intrinsic functional requirement; moreover, the working environment can be regarded as partially unstructured, since interaction conditions between the robot and the patient can notably vary depending on the residual motor capabilities of patients and their unpredictable reactions to therapeutic stimuli. The design and development of interaction control for rehabilitation robotic machines can resort to a wide range of control strategies derived from industrial robotics for managing human-machine physical interaction (Siciliano & Villani, 1999), (Gorinevsky et al., 1997), (Salisbury, 1980), (Kazerooni et al., 1986), (Zollo et al., 2003). Take for example impedance control (Hogan, 1985): it is successfully used in motor therapy (Krebs et al., 1998) as it allows finely regulating the mechanical impedance of robots interacting with unstructured environments. It is basically thought for interaction in the Cartesian space and, consequently, it is especially applicable to ‘operational machines’, where only the motion of the robot end-effector in the operational space (and not that of the robot joints in the joint space) is equivalent to that of the natural effector of the human limb hand or foot.