Controls

The main objective of this project was to provide a proof of concept prototype for control of a pendulum in open ocean conditions.  In order to achieve this control, we decoupled the motion of the pendulum from the motion of the buoy so that the pendulum would provide passive control. In addition, we added a brake that would stop the motion of the pendulum when it became unstable. An accelerometer and gyroscope sensor was mounted on the system, and the velocity and acceleration from the sensors were then used in the control algorithm. This is summarized in the figure below.

Figure 1: Flow Chart of Controls

Using Working Model, we created a simulation of the system, and then applied control to test the effects of our control algorithm. We first simulated the motion of a buoy on a series of sine waves, and then simulated the motion of a pendulum connected to the buoy. The pendulum was decoupled from the rotation of the buoy but was affected by movement in the x and y directions. We then applied proportional control on the position of the pendulum to simulate the effect of our brake. The uncontrolled and controlled motion of the pendulum is shown in the figure below.

Figure 2: Control method simulation from Working Model

            This simulation showed that our control algorithm was capable of controlling an unstable pendulum motion to be within our desired parameters and was a feasible control algorithm.

Mechanical Testing Structure

The mechanical testing structure consisted mainly of the frame and pendulum assemblies. The designs of both are summarized below.

In designing our mechanical structure, the major assumptions that were made were: 

• • The testing setup would not be subject to extended testing at sea, so corrosion of materials is not a factor.

  • The bearings on the pivot shaft experience minimal lateral forces.

The final frame design used Unistrut beams with brackets to connect beams together. The sidewalls were designed with triangular truss elements to support against vertical forces, and straps are added across the structure for lateral stability.

Figure 3: Final Frame Design in SolidWorks

The pendulum component consists of two separate elements, the pivot shaft and the swinging pendulum arm, shown in the figure below.

Figure 4: Final Frame Design in SolidWorks

The pivot shaft is subject to forces from the braking torque and the from the weight of the pendulum beam. Because the electromagnetic brake we originally intended to use requires a shaft with a diameter of 40 mm, we used a 40 mm hardened steel shaft for this part.

For the swinging arm, we used a 1.2 m (4 ft) long steel square beam to get a stiffer arm that would not bend when swinging with 50 kg (115 lbs) attached to it. From analysis and simulation, we found that the beam was able to withstand the forces without bending, and weights could be loaded onto the beam with a horizontal rod.