Climbing with Parallel Robots

Inherently, parallel robots present many advantages to climb in comparison with robots that use serial legs. The availability of a great number of redundant degrees of freedom on the climbing robots with legs does not necessarily increase the ability of those types of machine to progress in a complex workspace. The serial legs mechanisms have a sequential configuration that imposes high torques on the actuators placed on the base. Therefore, the architecture of serial legs of some climbing robots implies a limit on load capability. In contrast with the limitations of the climbing robots with legs, the use of a Gough–Stewart platform as a climbing robot (Stewart, 1965), solves many of these limitations and opens a new field of applications for this type of mechanism. In order to emphasize the great performance of the G–S parallel robot as a climbing robot, it is pertinent to remember that this type of parallel robot is based on a simple mechanical concept that consists of two rings (platforms) linked with six linear actuators joined through universal and spherical joints (this type of structure is also referenced as a 6-UPS parallel robot). These characteristics allow obtaining a mechanical structure of light weight and with high stiffness, which is able to reach high velocities and develop big forces with a very important advantage: the low cost of manufacturing (Lazard, 1992)). The forward kinematic of the G–S platform has been previously analyzed for many authors (Husty, 1994) (Dasgupta, 1994). This paper has been developed in based to references (Almonacid, 2003) (Aracil, 2003) (Aracil, 2006) and (Saltaren 2005) and reflects the state of the art of the researchers made by the authors during the last years. The morphology proposed for the parallel G–S platform as climbing robot is shown in Fig. 1(a). The G–S platform is formed by two rings joined with 6 linear actuators as UPS kinematics chains (where the U degrees of freedom belong to a universal joint, P is a prismatic degree of freedom that belongs to the linear actuator and S is the spherical joint) (Saltaren, 2000). The robot assembly around the tubular structure is carried out through a system of hinges. The holding systems are based on a series of grip devices built in each ring. Those grip devices hold the reference ring firmly attached to the tubular structure while the free ring is displaced by the control system. In Fig. 1(b), we show the climbing robot to work on the outside wall of pipes. In this case the platform of Fig. 1(a) is modified by adding two legs on each one of the external rings of the robot. These legs allow fastening one ring to the pipe while the other ring moves along the structure. The external rings can rotate increasing the working space of the robot. O pe n A cc es s D at ab as e w w w .ite ch on lin e. co m