Optimal positioning of a load suspended from a station-keeping helicopter
Controlling the position and attitude of a helicopter hovering in the presence of atmospheric turbulence is a difficult task which demands considerable pilot work-load which becomes even more difficult'when a load is suspended from the helicopter, because the oscillations of the load aggravate the situation. Tasks that require a suspended load to be kept fixed relative to a point in space, while the helicopter remains at hover, are extremely difficult to achieve. Several load-positioning systems exist but provide inadequate solutions to the problem. A brief account of such systems and their limitations is given before describing the automatic hovering control system proposed in this thesis. It causes appropriate motion of the helicopter to achieve the desired stationarity of the load. The techniques of modern control theory were employed to design this optimal controller. Digital simulation was used for testing the response of the resulting optimal system. The mathematical model of two connected rigid bodies moving in space (representing the helicopter and suspended load) is described in detail. Several combinations of cable length-load weight were chosen and in each case the response of the closed-loop system was investigated. It was found that considerable reduction of the oscillations of the load can be achieved when suitable cable arrangements are used. The use of winch control of lateral displacement of the load also improves the lateral response of the entire system. An augmented mathematical model was used which included both the dynamics of the control actuators and the models representing atmospheric turbulence and sensor noise. Since many of the state variables of the system cannot be physically measured, it is obvious that only limited information on the state of the system would be available for processing by such a controller. Therefore two solutions to the problem were considered: (i) the use of a state estimator to provide to the controller the lost feedback information; , and (ii) the use of an output regulator which takes into account the fact that limited feedback information is available. The responses of the closed-loop systems using each of these solutions were investigated and compared. The numerical problems encountered in this design are analysed and some means .of overcoming them are suggested. Finally, the best combination of cable arrangement and controller is described with reference to several important factors such as system simplicity and performance.