Output feedback control with a nonlinear observer based forward kinematics solution of a Stewart platform

In the control issue of the 6-DOF moving platform, positions and some rotation angles of the platform can not be measured with accelerometers and tilt sensors. Without full state feedback, the inverse kinematics must be applied into the control scheme to convert the desired platform position and orientation to the leg lengths. The actual control goal becomes to control individual leg lengths. To achieve the output feedback control, the forward kinematics solution is solved by using a nonlinear observer designed to estimate the system states including 3-axes translations and rotations. The conventional forward kinematics solutions using Newton-Raphson and other elimination-based methods have too much computational burden or are too complex to take out in the real time applications. In this paper, the nonlinear observer and a sliding mode controller are used to control the six states of the platform directly. The stability of whole system is verified to ensure the control errors would converge.