Souheil-Antoine Tahan

Synthesis of Differentially Driven Planar Cable Parallel Manipulators

In this paper, the idea of using cable differentials in the architecture of planar cable-driven parallel robots is introduced. Cable differentials are a type of mechanisms with several outputs driven by a single input. Using them in cable parallel manipulators can decrease their cost and control complexity. However, due to their kinematic constrains, cable differentials cannot be arbitrarily used in the design of these manipulators. Thus, a synthesis method is proposed to tackle this issue. First, the general requirements and characteristics of differentially driven planar cable mechanisms are reviewed. Then, the advantages of using these differentials instead of typically actuated cables are shown through a comparison between differentially actuated planar cable robots and fully actuated ones. The results reveal that with the same number of actuators, using differentials may lead to larger workspaces and improved kinetostatic properties. Subsequently, the systematic synthesis of differentially driven planar cable mechanisms is presented. For this, a method to find the different arrangements of q cables in a differential is proposed. Then, valid arrangements with 2, 3, and 4 cables are investigated. Finally, several differential actuation schemes are considered and all possible differentials with q=2, 3, and 4 cables are found.

Analysis and Optimization of a New Differentially Driven Cable Parallel Robot

In this paper, a new 3-DOF differentially-actuated cable parallel robot is proposed. This mechanism has a hybrid architecture and is driven by a prismatic actuator and three cable differentials. Through this design, the idea of using differentials in the structure of a spatial cable robot is investigated and their differences with architectures using individually-actuated cables are specified. Considering their particular properties, the kinematic analysis of the robot and the relationship between the actuation forces and the external wrench are presented. Then, two indices are defined to evaluate the wrench closure and wrench feasible workspaces of the robot. Using these indices, the robot is subsequently optimized. Finally, the performance of the optimized differentially-driven robot is compared with fully-actuated mechanisms. The results show that through a proper design methodology, the robot can have a larger workspace and better performance using differentials than the fully-driven cable robots using the same number of actuators.

On the performance of nondestructive testing methods in the hydroelectric turbine industry

Welded joints of turbine runners are one of the most critical parts of Francis turbines due to the presence of welding discontinuity and high stress. Because of thermal cycles, solidification, cooling distortion and residual stresses, welded joints always include discontinuities of different types and sizes. Some specific parameters will limit welding flaw dimensions in some or all direction based on the joint geometry, material and welding procedure. If discontinuities of critical size remain undetected, fatigue cracks might initiate and propagate in these zones because of dynamic in-service stresses leading to high repair costs and long down times. Therefore, reliable NDT methods and good knowledge of the probability of occurrence of welding flaws is important for fatigue life estimations. Every NDT method has its weaknesses; therefore, even after meticulous inspections it is likely for some discontinuities of critical sizes to remain in the welded joint. Our objective is to clarify the probability of detection and occurrence of different types of welding flaws in hydroelectric turbine runners. Furthermore, an overview of current nondestructive inspection methods and their capability in characterizing flaw dimensions will be discussed. Finally, advanced NDT techniques, for the characterization of welded joints integrity, will be proposed.

Curvature Estimation for Metrology of Non-Rigid Parts

At the end of the manufacturing process, engineers need to know if a manufactured part fits its computer-aided design (CAD) model and how is the amplitude of inherent variation of manufacturing process. Non-rigid parts, at free state condition, may have a significant different form than their CAD model due to gravity loads; residual stresses induced distortion and/or assembly load. Today, a complicated and expensive specialized fixture is needed to conform these parts. To tackle the above challenges, we present in this paper a new approach for metrology of fixtureless non-rigid parts. This approach combines the curvature properties of manufactured parts with the extreme value statistic test as identification method to distinguish profile deviation due to the manufacturing process from part’s deformation due to the flexibility of the part and to determine whether the tolerance fits the CAD model or no. This approach is tested on simulated typical industrial sheet metal giving satisfying results in terms of percentage of errors in defect area and in peak profile deviation estimated.Copyright