Stewart platform

Abstract

We present an application associated with the so-called Stewart’s platform, which is a robot of closed cinematic chain. This is one of the most important examples of totally parallel manipulator, understanding as such a robot that possess two bodies, one fixed and the other mobile, which are connected between them by several arms. Typically each arm is controlled by an actuator. Stewart’s platform has, therefore, a parallel configuration of six degrees of freedom composed of two rigid bodies connected by six prismatic actuators. The biggest rigid body is named the base, and the mobile body is called the mobile platform. Here, the goal is to design a robust control to stabilize Stewart’s platform with three degrees of freedom around a wished position when we do not have complete information with regard to the initial conditions and the permanent disturbance that affect this platform. The specific application consists in an aerostatic balloon easy to manipulate that is mooring to earth by a cable of approximately 400 m of length. This balloon is connected to the base platform and a video camera is fixed to the mobile platform to keep under surveillance a specific area of 20 km2 approximately. Since the platform basis is over the mobile one, we will name this platform inverted Stewart’s platform. Due to the type of application, the platform is permanently under the action of the force of the wind. Therefore, we will work with the wind’s acceleration as our permanent disturbance. Another characteristic of our implementation is that we have only output (not state) information available. In this situation the implementation of an OISM control seems to be useful.