Modeling and Control of a Pneumatically Driven Stewart Platform

Electrically driven Stewart platforms are used in the field of machine tooling and robotics, where very accurate positions have to be reached associated with heavy loads. In this paper we present a pneumatically driven Stewart platform powered by fluidic air muscles. Due to the elasticity of the muscles and air as driving medium, the robot is predestined for applications where compliance plays a major role. Compliant behavior is necessary for direct contact with humans. Fitness is an area, where this contact is given and a fast movement is needed for the body workout. Another field of application are simulators for computer games or 6D cinemas. To realize the six degrees of freedom (x, y, z, a; b ; g ) for the Tool Center Point (TCP) there are six fluidic muscles. Due to the fact that the muscles are only able to pull on the platform, there is a spring in the middle that applies a compressive force to the moving part of the robot. The spring is a non modified spiral spring which is commonly used for the suspension of a passenger car. As a result of the kinematical model (inverse kinematics, forward kinematics) the workspace is optimized. To dimension and test the dynamical behavior, a Matlab/Simulink model is derived. This is done by applying the Projection Equation, a synthetical method for obtaining the equations of motions for multi body systems. Based on the dynamical model we develop a control concept in a cascaded structure (pressure control, linearization, position control). A laboratory setup is used to validate the simulation model. Both, simulations as well as experimental results demonstrate the success of the proposed concept.