Cédric Baradat

Effects of non-negligible cable mass on the static behavior of large workspace cable-driven parallel mechanisms

Cable-driven robots are currently extensively studied. Generally, for this type of manipulators, cables are considered to be massless and inextensible. But for large working volume applications, their mass cannot be neglected. Based on a well-known model which describes the profile of a cable under the action of its own weight, the inverse and forward kinematics of minimally constrained cable-driven manipulators can be numerically computed. This paper studies the effects of taking cable mass into account by comparison to classical massless cable model. It highlights the real effects of such a model on cable lengths to reach a given position. The effects on cable tensions are also studied.

Simplified static analysis of large-dimension parallel cable-driven robots

This paper introduces a new simplified static analysis of parallel robots driven by inextensible cables of non-negligible mass. It is based on a known hefty cable static modeling which seems to have been overlooked in previous works on parallel cable-driven robots. This cable modeling is obtained from a well-known sagging cable modeling, known as the catenary, by assuming that cable sag is relatively small. The use of the catenary has been shown to lead to a non-linear set of equations describing the kinetostatic behavior of parallel robots driven by cables of non-negligible mass. On the contrary, the proposed simplified static analysis yields a linear relationship between (components of) the forces in the cables and the external wrench applied to the robot mobile platform. As a consequence, by means of the simplified static analysis, useful wrench-based analysis and design techniques devised for parallel robots driven by massless cables can now be extended to cases in which cable mass is to be accounted for.

Geometry Selection of a Redundantly Actuated Cable-Suspended Parallel Robot

This paper is dedicated to the geometry selection of a redundantly actuated cable-suspended parallel robot intended to manipulate heavy payloads over a wide workspace. Cable-suspended refers here to cable-driven parallel robots in a crane-like setting, where all the cable drawing points are located on top of the base frame, gravity being used to keep the cables taut. Geometry selection consists of determining the relative positions of the cable drawing points on the base frame and of the cable attachment points on the mobile platform together with the cable arrangement between these two sets of points. An original performance index is introduced. It is defined as the maximum acceptable distance between the mobile platform geometric center and the center of mass of the set consisting of the platform and a payload. This performance index is of particular interest in heavy payload handling applications. Used within a two-phase geometry selection strategy, it yields a new cable-suspended robot geometry having a very large workspace to footprint ratio and able to handle heavy payloads. A large-dimension redundantly actuated cable-suspended robot was built in order to demonstrate these capabilities.

On the determination of cable characteristics for large dimension cable-driven parallel mechanisms

Generally, the cables of a parallel cable-driven robot are considered to be massless and inextensible. These two characteristics cannot be neglected anymore for large dimension mechanisms in order to obtain good positioning accuracy. A well-known model which describes the profile of a cable under the action of its own weight allows us to take mass and elasticity into account. When designing a robot, and choosing actuator and cable characteristics, a calculation of maximal tension has to be done. However, because cable mass has a significant effect on cable tensions, a model including cable mass has to be included in the design step. This paper proposes two methods to determine the appropriate cable and hence the maximal tensions in the cables. Applied to a large dimension robot, taking cable mass into account is proved to be necessary in comparison with an equivalent method based on the massless cable modeling. In this paper, only moving platform static equilibria are considered (slow enough motions).

Above 40g acceleration for pick-and-place with a new 2-dof PKM

This paper introduces a new two-degree-of-freedom parallel manipulator producing two translations in the vertical plane. One drawback of existing robots built to realize those dof is their lack of rigidity along the transversal axis, another one being their limited ability to provide very high acceleration. Indeed, these architectures cannot be lightweight and stiff at the same time. The proposed architecture is a spatial mechanism which guarantees a good stiffness along the transversal axis. This parallel architecture is composed by two actuated kinematic chains, and two passive chains built in the transversal plane. The key feature of this robot comes from the passive chains which are coupled for creating a kinematic constraint: the platform stays in one plane. A stiffness analysis shows that the robot can be lighter and stiffer than a classical 2 dof mechanism. A prototype of this robot is presented and preliminary tests show that accelerations above 40 g can be achieved while keeping a low tracking error.

A Reconfigurable Robot for Cable-Driven Parallel Robotic Research and Industrial Scenario Proofing

Picturing the interest of research institutions and industrial actors, the list of research and demonstration parallel cable-driven robot prototypes is growing by the day. LIRMM and Tecnalia have decided to put knowledge in common in order to develop novel concepts for cable-driven parallel robotics and demonstrate its capabilities in industrial tasks. We have developed together a reconfigurable cable robot for this purpose. The robot main characteristics, e.g. footprint, mobile platform geometry and drawing point layout can be modified at will, making it particularly suitable for studying in good conditions new configurations or novel control laws, as well as any scenario suggested by our partners. The present paper first provides an overview of the robot. Afterwards, a more specific view on the different components and the capabilities of reconfiguration are presented, as well as examples of layouts meant for various research and industrial projects.

Kinetostatic Analysis of Cable-Driven Parallel Robots with Consideration of Sagging and Pulleys

Cable-driven parallel robots manipulating heavy payloads typically use cables having non-negligible diameters and mass. The associated cable guiding pulleys may then have non-negligible radii whose influence on the robot kinematics can hardly be neglected. This chapter focuses on the output pulleys from which the cables extend to the robot mobile platform. A kinetostatic analysis of cable-driven parallel robots considering both cable mass and output pulleys is presented.

A novel (3T-1R) redundant parallel mechanism with large operational workspace and rotational capability

This paper presents a novel 4 dofs (3T-1R(1)) parallel redundant mechanism, with its complete study regarding inverse and direct geometric models (IGM and DGM), as well as singularity and workspace analysis. The robot is capable of performing a half-turn about the z axis (a complete turn would be theoretically possible if it were not for possible unavoidable inter-collisions in the practical case), and having all of its prismatic actuators along one direction, enables it to have an independent x motion - only limited by the stroke of the prismatic actuators. The mechanism is characterized by elevated dynamical capabilities having its actuators at base. Moreover, the performance of the robot is evaluated considering isotropy in velocity and forces.

Locomotion approach of REMORA: A reonfigurable mobile robot for manufacturing Applications

This paper presents the locomotion approach of a novel quadruped robot which is able to carry various effectors for achieving manufacturing tasks in large workspaces. Equipped with lockers on some of the passive joints and clamping devices at the end of its limbs, this quadruped uses eight actuators for achieving manufacturing tasks as well as locomotion tasks. In the following sections, we first present the proposed robot and its two working modes. Then, the locking strategy of the robot is formulated as an optimization problem. Also, a practical method for managing the limbs swinging movement is addressed. At last, the presented approach is applied on two concrete examples. Possessing a low degree of kinematic redundancy, the proposed quadruped shows a reasonable locomotion capacity which allows it to achieve locomotion with respect to some extra constrains in its workspaces.

Simulation and Control with XDE and Matlab/Simulink of a Cable-Driven Parallel Robot (CoGiRo)

We present in this paper the process allowing to create a cable-driven parallel robot (CDPR) simulation within the XDE software environment in C\({+}{+}\) language. The elementary classes constituting a CDPR are shown with their constructor specificities. The winches, the pulleys, the cable fastenings and the platform are presented. The parameterization of elements such as the cable material characteristics, structure and size are detailed. An interface between the XDE cable-driven parallel robot simulator and a Matlab/Simulink controller have been developed. Inputs and outputs are exchanged between the controller and the simulated cable-driven robot, exactly as it is done with a physical robot.