EXACT METHODS FOR DETERMINING THE KINEMATICS OF A STEWART PLATFORM USING ADDITIONAL DISPLACEMENT SENSORS

The Stewart platform (SP) is a parallel closed-kinematic chain robotic mechanism that is capable of providing high structural and positional rigidity. Because of its unique capability, the platforms have been e mployed in many control engineering applications such as simulator sha kers, robotic manipulators, etc. However, a main problem often found i n the implementation of a real-time controller for the platform is the lack of an efficient algorithm for solving its highly nonlinear forwa rd kinematic transformation (FKT), where one seeks to find the transla tional and orientational altitudes of the moveable platform from knowi ng the lengths of the platform linkages. This article describes two ne w direct and exact methods for computing the translational and rotatio nal displacements of an SP by employing extra translational displaceme nt sensors (TDSs), in addition to the existing TDSs for the six links of the SP. The key for the approach lies in knowing where to employ th e TDSs for determining positional vectors of strategic platform locati ons. By taking advantage of a tetrahedral geometry, closed-form soluti ons for the FKT can then be derived and directly evaluated. The new me thods produce accurate solutions with only minimal computation necessa ry. The advantages and disadvantages of the proposed methods are discu ssed and compared to an existing method. The exact methods are being i nvestigated for an on-line implementation of a nonlinear adaptive cont rol system and redundancy scheme for a 25-ton Stewart platform-based C rew Station/Turret Motion Base Simulator (CS/TMBS) at the U.S. Army Ta nk-Automotive Command (TACOM).