Marc Vedrines

Decentralized Robust Control Strategies with Model Based Feedforward for ElasticWebWinding Systems

Demand for improved performance under a wide variety of dynamic conditions and web materials are placing additional emphasis on developing new advanced control strategies. This paper presents decentralized Hinfin controllers with model based feedforward for web winding systems which provide improved web tension and velocity regulation in the presence of uncertainties. First, mathematical models of fundamental elements in a web process line are presented. A new state space model is developed which enables calculation of the phenomenological model feedforward signals and helps in the synthesis of decentralized Hinfin controllers around the set points given by the reference signals. Different Hinfin control strategies with additive feedforward have been validated on a nonlinear simulator identified on a 3-motor winding test bench

Definition and Computation of Tensegrity Mechanism Workspace

Tensegrity mechanisms using linear springs as tensioned elements constitute an interesting class of mechanisms. When considered as manipulators, their workspace remains however to be defined in a generic way. In this article, we introduce a workspace definition and at the same time a computation method, based on the estimation of the workspace boundaries. The method is implemented using a continuation method. As an example, the workspace assessment of a two degrees of freedom (DOF) planar tensegrity mechanism is presented. [DOI: 10.1115/1.4029809]

Moving Web Tension Determination by Out of Plane Vibrations Measurements Using a Laser

The tension measurement is a key point in the operation of web systems handling thin materials such as textile, paper, polymer or metal. An experimental procedure is proposed to compute the web tension using a contactless sensor, which can be implemented on existing industrial web handling machines. A laser sensor measures the position of the web during the process and identifies out of plane vibrations. Using a sliding FFT, those vibrations can be separated into forced vibrations created by the process and machines, and free vibrations. It is then possible to compute the web tension as the frequencies of free vibrations are directly related to web tension. A test bench has been set up to represent a classical web handling systems and measurements have been made to identify free and forced vibrations. The procedure to compute web tension from measurements is then detailed.

Modelling and H∞ low order control of web handling systems with a pendulum dancer

Abstract The plant considered in this paper is an unwinding - winding system with pendulum dancer mechanism for elastic webs. First, a non linear mathematical model of the web process line is presented. A state space model is deduced which helps in the synthesis of the H ∞ controllers around the set points given by the reference signals. Industrials systems typically use decentralized PI controllers. In this paper, performances of low order H ∞ controllers for such systems are analyzed.

FIXED – ORDER AND STRUCTURE H∞ CONTROL WITH MODEL BASED FEEDFORWARD FOR ELASTIC WEB WINDING SYSTEMS

Abstract In web transport systems, the main concern is to control independently speed and tension in spite of perturbations such as radius variations and changes of setting point. This paper presents the application of nonsmooth nonconvex optimization techniques to design centralized fixed order and decentralized fixed order and fixed structure H ∞ controllers with model based feedforward for web winding systems. The approach provides improved web tension and velocity regulation. First, mathematical models of fundamental elements in a web process line are presented. A state space model is developed which enables calculation of the phenomenological model feedforward signals and helps in the synthesis of H ∞ controllers around the set points given by the reference signals. The H ∞ control strategies with additive feedforward have been validated on a nonlinear simulator identified on a 3-motor winding test bench.

Design and evaluation of a novel variable stiffness spherical joint with application to MR-compatible robot design

In this paper, the design of a new variable stiffness spherical joint for MR-compatible robotics is presented. It is based on the use of prestressed cable-driven mechanisms in singular configurations to provide large stiffness variation ranges, including zero stiffness configuration as required by the medical context. An original implementation is proposed, with a prestress adjustment system using pneumatic energy and taking advantage of multimaterial additive manufacturing. The proposed component combines compactness, MR-compatibility and is lightweight. The system is evaluated on a dedicated experimental setup with validation of the expected behavior, with in particular a very large achievable range of stiffnesses. The approach is effective for the design of such device and constitutes a novel solution for the design of variable stiffness devices with complex motions.

Design Optimization Using Genetic Algorithms of Web Handling Systems: The Case of the Pendulum Dancer Mechanism

Web systems handling elastic webs are very common in industry. The plant considered in this paper is an industrial winding system with pendulum dancer mechanism. Such a studied dancer moves along an arc of circle. Its utility is double: it imposes the web tension (in steady state operation) and filters the variations of tension mechanically. It is actuated by an air jack with adjustable pressure and has a stiffness and viscous dynamic behavior. Pendulum dancers have been rarely presented and studied in publications. The non-linear model built in Matlab software environment is used as a simulator. Moreover, the state space model useful for modern controller computing can be found thanks to linearization around an operating point. In order to improve the unwinder-winder control performances, the mechanical parameters of the pendulum dancer have to be optimized. The more constant the dynamic behavior over a large frequency band will be, the better the performance of a linear time invariant controller will be obtained. This is the main objective in desensitizing the dancer. The optimization is achieved by using genetic algorithms. The obtained results are discussed and the benefits of this design optimization for industry applications are presented.Copyright

Parallel Singularities for the Design of Softening Springs Using Compliant Mechanisms

In this paper, the design of nonlinear softening springs using compliant mechanisms is investigated. The use of compliant structures is of great interest, because of the resulting absence of backlash and friction. We demonstrate that the existence of parallel singularities is a necessary condition for the architecture of a compliant softening spring. From this result, two original arrangements of softening springs are derived, with the introduction of traction and torsion softening springs. A synthesis is performed and the traction spring is numerically and experimentally assessed. As nonlinearity can also be obtained from material properties, the interest of using additive manufacturing with multi-material capability is investigated. Rubber-like materials exhibit a hyper-elastic behavior. Their integration in the proposed compliant architecture is shown to be of interest to customize the geometry of a softening spring according to the designer requirements.Copyright

Towards the Control of Tensegrity Mechanisms for Variable Stiffness Applications: A Case Study

Cable-driven tensegrity mechanisms can be considered to control at the same time position and stiffness. Adequate control solutions have however not been proposed yet. This paper focuses on the development of an original control method using a tension distribution algorithm adapted from related work on cable-driven mechanisms. The algorithm is being used to modify the mechanism configuration together with its stiffness through the level of prestress in the system, which constitutes a step towards the exploitation of such mechanisms for variable stiffness applications. Simulations show encouraging results on the stiffness variation capacity of the presented mechanism.

Influence of Spring Characteristics on the Behavior of Tensegrity Mechanisms

There is today a growing interest for tensegrity mechanisms. Their analysis is however challenging because of their self-stress state. The most popular tensegrity mechanisms use linear springs as tensioned elements. Their synthesis for given user requirements is an open issue. In this article, we propose as a first step to better understand the influence of the spring characteristics, that constitute important design parameters. The influence of spring free length is in particular assessed, considering two planar tensegrity mechanisms. Impact of the spring selection on the workspace, the stiffness and the actuation requirements is observed. The simulation results outline that using nonzero free length springs can be of interest, and conclusions are given on further steps towards a synthesis method.