On the mechanical design, control approach and sensorization solution of planar cable driven parallel robots
- Autores: Guillermo Rubio Gómez
- Directores de la Tesis: Fernando J. Castillo García (dir. tes.)
- Lectura: En la Universidad de Castilla-La Mancha ( España ) en 2021
- Idioma: inglés
- Tribunal Calificador de la Tesis: Pierluigi Rea (presid.), Francisco Moya Fernández (secret.), Concepción Alicia Monje Micharet (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencias y Tecnologías aplicadas a la Ingeniería Industrial por la Universidad de Castilla-La Mancha
- Materias:
- Enlaces
- Tesis en acceso abierto en: RUIdeRA
- Resumen
Cable driven parallel robots are a special kind of parallel manipulators in which the end-effector is connected to a fixed frame by means of several cables instead of rigid links. The end-effector pose can be controlled by controlling the length of these cables.
This kind of robots offers several advantages compared to the conventional rigid ones. One of the main advantages is their scalability and thus the possibility of covering large workspaces, as long cables are easy to wind. Additionally, they present a high power to weight relation which results in high dynamic capabilities.
On the other hand, they present also several drawbacks as complex dynamics because cables are elastic elements that provide an unidirectional force transmission, they can pull but not push, therefore, positive cable tension must be guaranteed at all times. In practice, positiveness is not the only requirement for cable forces, they have to be kept between a minimum and a maximum limit for the feasible robot operation. In this sense, the feasible workspace of a Cable Driven Parallel Robot is defined as the set of platform poses where the robot end-effector can be balanced by means of forces within the feasible limits, under a certain externally applied wrench. Moreover, in the case of over-constrained cable driven parallel robots, i.e. with more cables than degrees of freedom, there are infinity possible cable force distributions that can balance the end-effector, this supposes an additional problem as one has to chose one among all the possible cable force distributions. This has been one of the major open problems in the field and extensive research has been carried out to propose methods to find feasible solutions to this mathematical problem. Additionally, the cable direction changes as the end-effector moves along the workspace yielding to a strong change in the load to be moved by each motor, which in turn results in an important reduction of the robot workspace and the necessity of complex control strategies to effectively reach the full workspace.
Due to these drawbacks, the range of real-world applications of cable driven parallel robots is still limited. This thesis focuses on the feasible application of cable driven parallel robots to both vertical and horizontal, large, planar workspaces. For these specific workspace characteristics, suspended cable robots, i.e. cable robots in which all cables act against the gravity direction are specially well suited, however, the conventional schemes require extremely high power for the motorization system if a large workspace has to be reached. To solve this problem, in first place, new mechanical designs are proposed to substantially increase the feasible workspace of such robots with reasonable tension limits. On the other hand, the proposed mechanical designs bring new challenges from the dynamical control point of view.
In the case of vertical workspaces, the addition of a passive carriage which can freely slide along a guide attached to the top of the frame is proposed, which significantly increase the feasible workspace of the robot, for the same motorization system compared to the conventional one. On the other hand, this new design reduces the stiffness of the robot in the horizontal direction, which can cause undesired vibrations and low robustness from real operation environment perturbations as wind. To solve this problem, a specific control approach is proposed and tested in a robot prototype.
In the case of large, planar, horizontal workspaces, where occupancy of the space perpendicular to the working plane has to be reduced, the proposed solution consist of employing a close cable loop along with passive carriages in two opposite frame borders. This new design can significantly reduce the required power, even for very large workspaces. As drawback, undesired oscillations appear on the direction perpendicular to the carriages. To solve this problem, a specific control approach has been proposed as well. The performance of this control approach and the overall robot accuracy have been experimentally assessed in another robot prototype.
Finally, for any kind of CDPR, measuring tension is an critical matter, it is required for security reasons to avoid the possibility of cables break or actuators overload. Additionally some robots' control systems rely on these measurements as feedback. State of the art on systems to measure cable tension shows that they present important restrictions in terms of measuring range, precision and repeatability. In this sense, a novel device for measuring cable tension, specifically designed to be applied in CDPR, has been proposed and experimentally validated.
