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Pendulum Control Mast Motion Compensation in a Seaway

Executive Summary

Project Background and Objective

Radio frequency antennas are used in a multitude of applications.  However, they are currently limited to land based applications due to the instability of ocean wave motion.  Attempts to fix this problem have resulted in large and expensive systems mounted on naval vessels.  Despite being accurate, these systems are impractical on most naval vessels due to their large size and power consumption.  Additionally, these antennas are fixed to the ship and do not have the versatility to act independently.  With a set of much smaller and less expensive systems mounted on buoys, a much wider network of naval communication can be implemented.

For the purposes of this project, a proof of concept for an actively controlled pendulum system is to be designed.  The requirement for this application is to keep the pendulum on the buoy within 10 degrees of the vertical while the antenna is active and transmitting condition ranging from Sea State 2 to Sea State 5.  This proof of concept and corresponding control algorithm will be completed in only one rotation axis since extrapolating to multiple rotation axes would require additional hardware that is outside the scope of this project.  Still, expanding the control algorithm to the third degree of freedom would only require duplicating the algorithm around another axis of rotation.

Controls

In order to achieve our objective of stabilizing the pendulum motion, we decoupled the motion of the pendulum from the motion of the buoy. This provides passive control so that the pendulum remains stable regardless of the rotation of the buoy. However, the pendulum is still affected by vertical and horizontal displacement of the buoy, so it is necessary to implement active control on the pendulum. After considering various options, we decided to implement a brake that will apply a variable braking force on the pendulum according to a control algorithm.

With such random motion of the seas, properly tuning a control system to the dynamics of the buoy would have been overly complicated for this application. The amount of tolerance in the error allowed gave room for a much slower reaction time of the controller. This means that a reactive method of control, such as PID, could be be used instead of a predictive method based off of the dynamics of the model. Additionally, with the use of a brake instead of a motor, the power consumption dropped considerably, but the control limitations became more apparent. With a braking system, the torque on the pendulum could only be applied in the opposite direction of the velocity. Therefore, a control system was derived based off of the velocity of the pendulum.

Mechanical Structure

In order to test our control system, we had to build a mechanical testing structure to support the swinging pendulum and also to be moved in a way that would simulate sea wave motion. Using Unistrut beams, a frame was built to support two bearings that would allow rotation of the pendulum, and was mounted to act on one end of the shaft.

Summary of Performance Results

Using a dynamic simulation in Working Model, we were able to control an unstable pendulum motion to stay within +/- 10 degrees of vertical.