Modeling and Control of Joint-Actuated Buoys
Open AccessOur motivation is the practical problem of aiming a payload on a small floating buoy at an arbitrary point in the sky. The buoy payload might be a camera, a directional antenna, or some other sensor with a narrow field of view. Such a sensor could, for example, perform some meteorological measurements of the air column above the ocean surface, or provide directional communications with a satellite in a remote sensing application.Specifically, we develop a control law which solves the pointing and stabilization problem for a particular two-body buoy configuration. The buoy's mechanical design is central to the development as well. The buoy configuration considered here is a long cylindrical buoy with a driven two axis universal joint near the middle. The buoy is ballasted to float upright and the joint is used to stabilize the buoy such that the payload can be pointed at any point within 35 degrees of the vertical.A numerical model and simulation framework are developed to experiment with control strategies for this system. The dynamics and kinematics of the two-body buoy problem are derived as a self-contained system. The external forces and moments modeled by the simulation are buoyancy, gravity, and drag. Additionally, the effects of regular and irregular surface waves are also modeled. Several experiments are conducted to inform and validate the numerical model of the buoy dynamics.Both a sliding mode control law with feed forward (SMC+FF) and a proportional integral and derivative control law with feed forward (PID+FF) seem to be able to control the payload well. The feed forward term allows the SMC or PID elements of the control law to handle only the disturbance rejection needs.The buoy system's simulated performance in regular and irregular seas is mixed. If there is significant wave energy at the buoy's resonant frequency, the buoy becomes uncontrollable. This is not a function of the control law, but rather of the passive characteristics of the buoy's structure.Additionally the effects of system latency, control rate, initial condition, actuator acceleration limits, and payload position with respect to the vertical are all compared and characterized.An alternate joint design of an ``elevation over azimuth'' joint was considered and discarded. An alternate method of solving for the system dynamics in the simulation using constraint equations was implemented and evaluated, as well.
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