Executive Summary

The main objective of the work performed in project was to design, build, test, and document an electro-mechanical control system that maintains a radio frequency antenna within an orientation +/- 10-deg from vertical.  The control system was required to function under sea state 5 conditions with the wave heights ranging from 0.1m to 4m with periods of 3 to 16.5 seconds and needed to run on less than 25 amp-hours per day over the course of 4 days.

To meet these requirements, a pendulum test stand was built for the purposes of proving the implementation of a braking control system on the pendulum.  For the purposes of this prototype, weights were used to represent the antenna so that a proof of concept test could be performed.  The mechanical pendulum apparatus was used for the purposes of keeping an antenna oriented in a fixed direction while the pendulum was oriented vertically.  The pendulum is a passive system for control of the antenna orientation.  However, it was found when run in a ocean wave simulator, Orcaflex, the pendulum reached its natural frequency and became unstable.  Thus, the idea of implementing active control in addition to the passive system was analyzed.  Through comparison of design difficulty and power requirements, a method of variable damping via an electromagnetic brake was decided upon.  The key advantages of this design were that the system required less power than most other active systems, the braking force could be controlled electronically, no gearing was required, and the brake was easily mountable to the structure.

The other main components considered for the final design were the supporting structure as well as the pivoting axel of the pendulum.  First of all, the material decided upon for the structure was Unistrut because of its versatility and availability.  This offered substantial resistance to compression caused by the pendulum loading.  Additionally, the configuration decided upon resembled an A-frame as shown in Figure 1. 

Figure 1. Overview of the entire system with an A-frame structure and a weighted  pendulum

This assembly offered the proper support to the vertical loading as well as provided resistance to rocking parallel to the motion of the pendulum arm.  The cross straps were instrumental in securing the structure from swaying perpendicular to the pendulum motion as well.

            The second main area of interest was the axel and the other components closely related to it.  This consisted of the bearings, pivot shaft, keyway, and pendulum arm.  These mechanisms were required to support the load and rotate smoothly without adding a significant amount of damping to the system.  The pivot shaft was made from hardened 4140 steel to give it the appropriate stiffness.  Into this shaft, was machined a keyway that connected the rod with an aluminum block insert placed inside of the steel pendulum arm as seen in Figure 2.  The motion is transferred from the rod to the keyway and from the keyway to the aluminum insert and in turn the pendulum arm.

Figure 2. Close up view of the pendulum's rotational axis along with some key components

The control system, consisting of an electromechanical brake, a motor driver, and an Arduino Nano microcontroller, was used to limit the motion of the pendulum in unstable conditions.  Since the application of a pendulum was meant to stabilize the system passively during most normal conditions, the brake only needs to be activated when the system starts to reach its natural frequency.  The Arduino microcontroller runs a Proportional Derivative (PD) control loop that actuates the brake proportionally to the pendulum rotational velocity and position.  The Arduino polls accelerometers and gyroscope sensors mounted to both the antenna pendulum and main buoy body.  It then generates scaled actuation output signals according to this inputted data and controls the braking force applied to the pivot shaft to slow the pendulum rotation.