A Two Axis Skin Friction Balance Nulling Circuit Using An Optimal Observer

A two-axis skin friction balance developed at the Langley Research Center employs a state-variable feedback nulling circuit developed via multivariable pole-placement theory. Because of the inherently noisy wind tunnel environment where the balance operates, a less noise-sensitive controller is needed. This paper investigates the application of optimal-observer-based control laws derived using both pole-placement techniques and optimal linear regulator theory. Although a pole-placement-derived control law with optimal observer provides fast response and excellent noise rejection, its observer requires an electrical input proportional to the unknown skin friction force, which is unavailable. To overcome this limitation, an optimal linear regulator design is proposed which estimates the unknown force input and provides excellent noise rejection, at the expense, however, of significantly slower transient response.