Intelligent Oscillation Controller
Scott Mcleod: Electrical Engineering Class of 2009
Brett Pihl: Mechanical Engineering Class of 2008
Sandeep Prabhu: Mechanical Engineering Class of 2008
The overall goal of this project is to create a system that induces a forcing function upon a basic, spring, mass, wall system to achieve an arbitrary periodic acceleration profile on the mass. An accelerometer is mounted upon the mass in the system. A PIC microprocessor records this acceleration data as well as controls a speaker (with the help of a DAC) that provides the external force to the system. This PIC communicates with MATLAB via Serial RS-232 communication. MATLAB processes this data and sends back a control signal for the speaker. After several iterations the actual mass acceleration profile begins to match the chosen profile it is told to "learn."
The basic mechanical system for this device is a simple one, but must be assembled with precision. The major components are the speaker, linear ball bearings on a precision rod, and a spring. First, the speaker is mounted perpendicular to the ground. This must then be attached via a rod to the mass. We tried several approaches, but what seemed to be the best solution was to epoxy about a one inch section of PVC pipe to the center of the speaker. The diameter of the pipe we used matched the diameter of the junction in the speaker where the cone turns from concave to convex. This pipe also had two tapped holes running along the length of the section for a plate to attach to. The plate attached to this PVC also had a threaded hole in the center for the rod that attaches to the mass. The forces exerted by the speaker are small enough (our measurements show only about 3 Newtons) that only about an 8 ounce mass is needed. The bearing we used didn't require any additional mass to satisfy this constraint. The other end of the rod simply attached to the block which we machined and mounted atop the bearing. A piece of sheet metal was screwed into the other side of the block with spacers. This piece of metal is used to anchor spring to the mass, and allows the spring to easily be removed. A similar piece of sheet metal is attached to the wall on the opposite side of the spring. However, this sheet has a vertical slot about an eighth of an inch wide cut from the bottom. This allows the coil of the spring to slide up further on the plate, thereby creating a more solid connection.
It is important to design each component with all other components in mind at the same time. Mainly, making sure that the linear slide is level, and the rod attached to the speaker is centered, level, and in line with the mount on the mass and spring on the opposite side of the mass. This will help to ensure that all motion in the system is one dimensional.
The above was not the first iteration of our mechanical design. We originally had a homemade linear slide. However, we found the lack of precision resulted in unreliable bode plots of the system due to loose tolerances of the design creating side-to-side motion. The first iteration also had the rod epoxied directly to the mass and speaker. During initial testing the connection between the speaker and rod actually severed.
This current design is advantageous because it is modular. The threaded rod allows for minor distance changes to ensure the spring attached to the wall is at its natural rest length. To attach it the plate is detached from the PVC on the speaker and screwed onto the rod. With the spring detached, the other end of the rod is screwed into the threaded hole on the mass. Finally, the plate is then screwed into the PVC through the threaded holes. The non-permanent spring attachment also allows for springs with different k-values to be added to the system. If the spring is longer or shorter then desired, a simple change in rod length is all that is needed to incorporate the new spring into the system.
A common question regarding this project is its applications to the real world. In the Northwestern University LIMS lab, a similar type of undertaking is being researched, but on a much grander scale. This same type of oscillation control is being done for 6 dimensions (X, Y, Z, Roll, Pitch, Yaw). However, the microprocessors used in this type of control are extremely expensive, and this one dimensional test of a learning system provides a possibly cheaper solution. The six dimensional control system has possible real-world applications in product assembly.