H∞ control of a twin lift helicopter system

Two H^∞ controller designs are presented for a twin lift helicopter system (TLHS). The TLHS configuration consists of two US-60A Sikorsky Blackhawk helicopters jointly lifting a heavy payload. The first design presented considers the case in which the tethers connecting each helicopter to the load are equal in length, and the second considers the case in which the lengths are unequal. Both designs are based on a seven-degree-of-freedom model linearized about hover. The primary objective of each controller is to minimize the control action and pitching motion required to stabilize the helicopters as they perform elementary maneuvers. A simulation of a typical TLHS command scenario is used to evaluate the stability robustness characteristics of the resulting two feedback systems with respect to structured parametric uncertainty