Valerio Salvucci

Force Maximization of Biarticularly Actuated Manipulators Using Infinity Norm

There is a rising interest in biologically inspired manipulators equipped with biarticular actuators-actuators that span two joints-for solving the known limitations of conventional systems. In contrast with kinematic redundancy, actuator redundancy resulting from the presence of biarticular actuators has the added advantages of bringing more stability, reducing the inertia of the robot links, and decreasing the nonlinearity of the end effector force as a function of force direction. In this paper, the advantage of the infinity norm optimization criteria on a robot designed under the actuator redundancy paradigm is investigated. A closed form solution based on the infinity norm for a manipulator with mono- and biarticular actuators is derived. The proposed infinity norm-based approach is compared with the conventional method based on pseudoinverse matrix by both calculation and experiment. Under the same actuator limitations, the maximum end effector force produced with the proposed method is significantly greater than the one produced by the conventional method. The proposed closed form solution is suitable for redundant systems with three inputs and two outputs, bringing the advantage of an higher maximum output without the need for iterative algorithms.

Comparing Approaches for Actuator Redundancy Resolution in Biarticularly-Actuated Robot Arms

Biarticular actuators-actuators spanning two joints-play a fundamental role in robot arm designs based on the human musculoskeletal actuation structure. Unlike kinematic redundancy, actuator redundancy resulting from biarticular actuation brings advantages such as increased stability, reduced link inertia, and decreased nonlinearity of the end-effector force with respect to the force direction. The way the actuator redundancy is resolved is a fundamental problem, as it strongly characterizes robot arms performance. In this study, the six most significant actuation redundancy resolution approaches in the literature-1-norm, 2-norm, infinity-norm, phase different control (PDC), nonlinear hase different control (NLPDC), and linear programming (LP)-are analyzed with respect to their design, and experimentally compared with each other using BiWi, a biarticularly actuated and wire-driven robot arm. In addition, an integrated control framework to resolve actuator redundancy maximizing end-effector force and simultaneously minimizing the necessary input torques is proposed.

BiWi: Bi-articularly actuated and wire driven robot arm

Recently, there has been increasing attention on animal inspired robot arms equipped with bi-articular actuators.

Experimental verification of infinity norm approach for force maximization of manipulators driven by bi-articular actuators

Recently, there has been increasing attention on animal inspired manipulators equipped with bi-articular actuators. Bi-articular actuated manipulators usually present more actuators than joints, resulting in actuator redundancy. In order to resolve this torque load sharing problem, in this paper a new approach based on infinity norm is proposed and successfully implemented on BiWi, Bi-articularly actuated and Wire driven manipulator. The infinity norm model maximizes the force at the end effector given the maximum actuator joint torques. Therefore, it is an approach to optimize actuators design for bi-articularly actuated manipulators. The infinity norm approach is based only on linear functions for actuator redundancy resolution.

Infinity norm approach for precise force control of manipulators driven by bi-articular actuators

In recent years there has been increasing interest in manipulators presenting animal muskuloskeletal characteristics such as bi-articular actuators. Manipulators driven by bi-articular actuators usually have more actuators than joints, presenting therefore actuator redundancy. In this paper a new approach based on ∞-norm to resolve actuators redundancy is proposed. The proposed method is compared with the Phase Different Control (PDC) approach, which is based on human muscle activation level patterns. It is shown that the infinity norm approach produces no error in output force, while the PDC approach produces non-zero error. Moreover, in this paper, a PDC approach with non-linear model that eliminates error in output force is also proposed. However the PDC approach with non-linear model is more complex than the proposed infinity norm approach.

Superconducting Fault Current Limiter Design Using Parallel-Connected YBCO Thin Films

Superconducting fault current limiters (SFCLs) are able to reduce fault currents to an acceptable value, reducing potential mechanical and thermal damage to power system apparatus and allowing more flexibility in power system design and operation. The device can also help avoid replacing circuit breakers whose capacity has been exceeded. Due to limitations in current YBCO thin film manufacturing processes, it is not easy to obtain one large thin film that satisfies the specifications for high voltage and large current applications. The combination of standardized thin films has merit to reduce costs and maintain device quality, and it is necessary to connect these thin films in different series and parallel configurations in order to meet these specifications. In this paper, the design of a resistive type SFCL using parallel-connected YBCO thin films is discussed, including the role of a parallel resistor and the influence of individual thin film characteristics, based on both theory and experimental results.

Development of simplified statics of robot manipulator and optimized muscle torque distribution based on the statics

Statics for a two-link manipulator is re-derived based on the biarticular muscle coordinate in this paper. Torques of two joint-motors to generate a certain force at the endeffector can be calculated in a simple trigonometric function form. A H infinity optimization algorithm is applied to the redundancy problem and minimizes the necessary torque for each muscle. This algorithm can generate novel torque patterns whose peak values are minimized and the maximum torque of the actuators can be set small. The suggested statics provides not only insight to the robot design, but also some interpretation to our bodys muscle activation strategy.

Non-linear phase different control for precise output force of bi-articularly actuated manipulators

Bi-articular actuators – actuator spanning two joints – play fundamental role in robot arms designed under the human musculoskeletal actuation paradigm. Unlike kinematic redundancy, actuator redundancy resulting from bi-articular actuation brings advantages such as increasing stability, reducing links inertia, and decreasing non-linearity of the end-effector force with respect to the force direction. The traditional phase different control (PDC) resolves actuator redundancy on the basis of a linearized model derived from measured human muscle activity. Such linear model produces a non-zero error in calculation between a desired output force and necessary inputs. In this paper, the non-linear phase different control (NLPDC) is proposed to resolve actuator redundancy with no error. The maximum end-effector force of BiWi, bi-articularly actuated, and wire-driven arm, is measured using both PDC and NLPDC. When the robot arm moves towards singular configurations, the measured error in output force remains within the modeling error if using NLPDC, while such error increases significantly for PDC. Furthermore, unlike PDC, the proposed NLPDC allows design of joint stiffness and torque independently, reduction of necessary total muscle input force, and precise calculation of maximum output force.

Disturbance rejection improvement in non-redundant robot arms using bi-articular actuators

Recently, bio-inspired robotic arms equipped with bi-articular muscles, which are actuators that produce torque in two consecutive joints, have been raising interest. Usually bi-articular actuators are present as a redundancy in actuation, which results in advantages such as dramatical increase in range of end effector impedance which can be achieved without feedback, and the ability to produce a homogeneous maximum output force at the end effector. These advantages however are due to the use of bi-articular actuators in addition to the traditional mono-articular ones. Therefore, drawbacks as design complexity and cost are present. In this paper, the role of bi-articular actuators for robot arm that do not present actuator redundancy is investigated. It is shown that for jumping/walking robots, in static conditions the maximum force in the direction parallel to ground is bigger in the configuration with bi-articular actuators. In dynamic conditions, this results in a greater capability of disturbance rejection to forces directed horizontally respect to the ground. As a result, the presence of bi-articular actuators improves the balance capability of jumping/walking robots.

Mathematical and Experimental Verification of Efficient Force Transmission by Biarticular Muscle Actuator

Abstract This paper proves that the biarticular muscle structure which is common in most of animals improves the force generation efficiency in a two-link manipulator. To this end, the statics of two configurations – two joint-actuators configuration and one joint-actuator and one biarticular actuator configuration – are compared using two types of Jacobian. Moreover, the statics are simplified to clarify this difference and an analytical solution about the efficiency of the biarticular muscle structure is proposed. Finally, experimental results verify this advantage of the biarticular muscle structure.