Alberto Trevisani

Cable-Direct-Driven Robot (CDDR) with Passive SCARA Support: Theory and Simulation

This article presents a new planar translational cable-direct-driven robot (CDDR) with actuation redundancy and supported against loading normal to the motion plane with a passive planar two-degree-of-freedom SCARA-type (Selective Compliance Assembly Robot Arm) serial manipulator. This allows the robot to resist cable sag without being supported on the motion plane. The proposed robot architecture may assure high payload-to-weight ratio, resistance to forces normal to the plane of motion, and a potentially large workspace. Another benefit is that the passive SCARA has structure to provide end-effector moment resistance, which is not possible with many proposed translational CDDRs. Moreover, the passive robot can also serve as an independent Cartesian metrology system. This article derives the kinematics and dynamics models for the proposed hybrid serial/parallel architecture. Additionally it proposes a dynamic Cartesian controller always ensuring positive cable tensions while minimizing the sum of all the torques exerted by the actuators. Simulation examples are also presented to demonstrate the novel CDDR concept, dynamics, and controller.

Nomenclature for renal replacement therapy in acute kidney injury: Basic principles

This article reports the conclusions of a consensus expert conference on the basic principles and nomenclature of renal replacement therapy (RRT) currently utilized to manage acute kidney injury (AKI). This multidisciplinary consensus conference discusses common definitions, components, techniques, and operations of the machines and platforms used to deliver extracorporeal therapies, utilizing a “machine-centric” rather than a “patient-centric” approach. We provide a detailed description of the performance characteristics of membranes, filters, transmembrane transport of solutes and fluid, flows, and methods of measurement of delivered treatment, focusing on continuous renal replacement therapies (CRRT) which are utilized in the management of critically ill patients with AKI. This is a consensus report on nomenclature harmonization for principles of extracorporeal renal replacement therapies. Devices and operations are classified and defined in detail to serve as guidelines for future use of terminology in papers and research.

Feedback control of flexible four-bar linkages: a numerical and experimental investigation

Abstract A control scheme for a four-bar linkage with all the links flexible is proposed and tested both numerically and experimentally. The control strategy consists in selecting a reduced number of measurable variables through which performing position and vibration are controlled independently. The controlled variables are the crank angle and the link curvatures, which provide an adequate description of the temporal evolution of the mechanism position and vibrational phenomena. Position control is performed through a proportional integral and differential (PID)-like regulator while proportional controllers are employed to damp the fundamental components of the link oscillations. A force of gravity compensator is introduced to increase the control system performances and appropriate devices are proposed to avoid coupling effects among the controlled variables. The control scheme is first tested and tuned in simulation, where the dynamic behaviour of the flexible linkage is reproduced through a fully coupled non-linear model based on the finite element theory. The performances of the control scheme are assessed by studying the step response of the closed-loop system. The numerical results attained prove that the proposed control scheme achieves efficient positioning and vibration suppression performances. The experimental validation of the control scheme is carried out on an instrumented prototype of the flexible four-bar linkage. Experimental recordings are in good agreement with the numerical results therefore confirming both the effectiveness of the control scheme and the accuracy of the dynamic model.

Delayed Reference Control of a Two-Mass Elastic System

An innovative non-time-based control scheme for path tracking and vibration control of a two-mass system is introduced in this paper. The basic idea of the scheme, called delayed reference control (DRC), is to make the path reference of the system be a function of an action reference parameter which depends both on time and a variable which plays the role of a time delay. By suitably computing the value of the delay on the basis of the vibration measured, such vibration can be actively suppressed while an independent position regulator ensures an accurate tracking of the desired path. The DRC scheme is therefore suitable for those applications, in particular in the robotic field, where a pre-defined path through space must be followed precisely while the time taken to carry out the task is not a primary concern. In this paper the stability of the system is investigated, and numerical results are provided to assess the performance of the proposed method, compared to that of an optimal linear quadratic regulator.

A Constrained Convex Approach to Modal Design Optimization of Vibrating Systems

This paper introduces a general and flexible design method for the inverse modal optimization of undamped vibrating systems, i.e., for the computation of mass and stiffness linear modifications ensuring the desired system eigenstructure. The technique is suitable for the design of new systems or the optimization of the existing ones and can handle several design requirements and constraints. Paramount strengths of the method are its capability to modify an arbitrary number of parameters and assigned vibration modes, as well as the possibility of dealing with mass and stiffness matrices with arbitrary topologies. To this purpose, the modification problem is formulated as a constrained inverse eigenvalue problem and then solved within the frame of convex optimization. The effectiveness of the method is assessed by applying it to two different test cases. In particular, the second investigation deals with a meaningful mechanical design application: the optimization of a system recalling an industrial vibratory feeder. The results highlight the generality of the method and its capability to ensure the achievement of the prescribed eigenstructure.

Delayed Reference Control Applied to Flexible Link Mechanisms: A Scheme for Effective and Stable Control

This paper extends the use of delayed reference controllers to the simultaneous motion and vibration control of flexible link mechanisms. Vibration damping is achieved by introducing an “equivalent damping force” into the system through the computation of a suitable delayed time. The delayed time, which is based on the measured vibrations, is then employed in the trajectory planner to set the reference input. The stability of the controller is discussed and its effectiveness is proved by applying it to a four-bar planar linkage with flexible links.

PLANNING OF DYNAMICALLY FEASIBLE TRAJECTORIES FOR TRANSLATIONAL, PLANAR, AND UNDERCONSTRAINED CABLE-DRIVEN ROBOTS

Extensively studied since the early nineties, cable-driven robots have attracted the growing interest of the industrial and scientific community due to their desirable and peculiar attributes. In particular, underconstrained and planar cable robots can find application in several fields, and specifically, in the packaging industry. The planning of dynamically feasible trajectories (i.e., trajectories along which cable slackness and excessive tensions are avoided) is particularly challenging when dealing with such a topology of cable robots, which rely on gravity to maintain their cables in tension. This paper, after stressing the current relevance of cable robots, presents an extension and a generalization of a model-based method developed to translate typical cable tension bilateral bounds into intuitive limits on the velocity and acceleration of the robot end effector along a prescribed path. Such a new formulation of the method is based on a parametric expression of cable tensions. The computed kinematic limits can then be incorporated into any trajectory planning algorithm. The method is developed with reference to a hybrid multi-body cable robot topology which can be functionally advantageous but worsen the problem of keeping feasible tensions in the cables both in static and dynamic conditions. The definition of statically feasible workspace is also introduced to identify the positions where static equilibrium can be maintained with feasible tensions. Finally, some aspects related to the practical implementation of the method are discussed.

Design of Delayed Reference Controllers for Simultaneous Path Tracking and Active Vibration Damping of Multi-Degree-of-Freedom Linear Systems

This paper presents an innovative non-time-based control strategy for the simultaneous path tracking and active vibration damping of multi-degree-of-freedom linear systems. The fundamental difference between the proposed scheme, and those that have already appeared in literature, is that it allows keeping the coordinated motion of different actuators. In other words, the use of such a scheme guarantees that a desired path in space is accurately followed while suppressing the undesired vibrational effects. The control strategy consists in actively suppressing the undesired vibrations by suitably changing the path reference for the system. Such a result can be achieved by making the path reference for the system be a function of an action reference parameter rather than just the time. The reference parameter depends on time and on a variable which plays the role of a time delay. The latter is computed on the fly, on the basis of the measured vibration. A cascade control structure is obtained, where no specific position controllers or actuators are to be employed in the inner loop.The theory developed is demonstrated through numerical simulations by applying it to the vibration and path tracking control of a four-degrees-of-freedom two-mass system.

Experimental Validation of a Trajectory Planning Approach Avoiding Cable Slackness and Excessive Tension in Underconstrained Translational Planar Cable-Driven Robots

The objective of this paper is providing the first experimental evidence of theeffectivenessofanoff-linetrajectoryplanningapproachdevelopedtoensurepos- itive and bounded cable tensions in under constrained planar two-degree-of-freedom translational cable robots. The hybrid (serial/parallel) topology of the investigated robot is general enough to ensure wide applicability of the proposed trajectory plan- ning method, which translates the usual bilateral tensile cable force constraints into kinematic constraints on the velocity and acceleration of the robot tool center point along the desired path. Kinematic constraints are computed making use of the robot dynamic model and can then be incorporated in any trajectory planning algorithm. In this work a smooth trajectory planning algorithm based on quintic polynomials is adopted. The experimental setup is presented and the results obtained by applying the method to two sample paths are discussed.

A Non-Time Based Controller for Load Swing Damping and Path-Tracking in Robotic Cranes

This paper proposes the use of the non-time based control strategy named Delayed Reference Control (DRC) to the control of industrial robotic cranes. Such a control scheme has been developed to achieve two relevant objectives in the control of autonomous operated cranes: the active damping of undesired load swing, and the accurate tracking of the planned path through space, with the preservation of the coordinated Cartesian motion of the crane. A paramount advantage of the proposed scheme over traditional ones is its ease of implementation on industrial devices: it can be implemented by just adding an outer control loop (incorporating path planning) to standard position controllers. Experimental performance assessment of the proposed control strategy is provided by applying the DRC to the control of the oscillation of a cable-suspended load moved by a parallel robot mimicking a robotic crane. In order to implement the DRC scheme on such an industrial robot it has been just necessary to manage path planning and the DRC algorithm on a separate real-time hardware computing the delay in the execution of the desired trajectory suitable to reduce load swing. Load swing has been detected by processing the images from two off-the-shelf cameras with a dedicated vision system. No customization of the robot industrial controller has been necessary.