Moored floating platforms are subject to external disturbances such as waves, wind, and currents that cause them to drift. There are certain applications, such as diving support, drilling pipe-laying, and tanking between ships in which precise positioning of moored platforms is very important (Muñoz-Mansilla, 2011). Figure P1.4 illustrates a tethered platform in which side thrusters are used for positioning. A control
system is to be designed in which the objective is to minimize the drift, Y, and an angular deviation from the vertical axes, ϕ (not shown). The disturbances acting on the system’s outputs are the force, F, and the torque, M, caused by the external environment. In this problem, the plant will have one input, the force delivered by the thrusters (Fu) and two outputs, Y and ϕ. Note also that this is a disturbance attenuation problem, so there is no command input. Draw a block diagram of the system indicating the disturbances F and M, the control signal Fu, and the outputs Y and ϕ. Your diagram should also have blocks for a controller, the one-input two-output plant, and a block indicating how the disturbances affect each of the outputs.
