Construction of a Vibrating Structure for Demonstration of Vibration Measurement and FFT Analysis

Many Mechanical Engineering Technology (MET) programs include a Basic Instrumentation class structured to give students hands-on experiences with measurement and analysis systems. As part of a module on vibrations and the Fast Fourier Transform (FFT), a lab was developed to create a three-tiered vibrating structure. This test apparatus consists of relatively inexpensive components with threaded rods for the columns, plywood for the tiers, and an adjustable orbital sander for the vibration source. The vibration source is mounted to the top tier, and its rotational speed is adjusted by a controller made for common power tools. Different vibrational modes for the structure can be observed by varying the speed of the vibration source. In one mode the entire structure sways back-and-forth with the top tier showing the largest displacement. In a second mode the center tier is vibrating while the upper and lower tiers remain nearly motionless. One or more accelerometers may be attached to the tier having the largest displacement for a specific mode. The accelerometer output is measured using either an oscilloscope or a computer data acquisition system (DAS). A second test may be performed using a weight on the center tier to demonstrate a change in natural frequency with the added mass to the system. It is also possible to adjust the natural frequency by adjusting the position of the center tier relative to the upper and lower tiers. Including the oscilloscope in the required measurements gives the student exposure to a second measurement system as well as allowing them to double-check the results of their analysis of the data from the DAS. Analysis of the data is performed outside of the lab using MathCAD or Excel and builds off a previous tutorial lab on frequency analysis methods based on Fast Fourier Transform (FFT) methods. The students are required to use the FFT results to identify the resonant frequencies as part of their lab report. Introduction: A three-tiered structure serves as an excellent platform for students to conduct accelerometer vibration measurements and analyze the acquired data with the ubiquitous Fast Fourier Transform (FFT). While the concept behind the structure was originally developed at the University of Pittsburgh at Johnstown , the current laboratory procedure has expanded the concept by simplifying the structure, adding in an inexpensive adjustable vibration source that allows for the exploration of multiple modes, and expanding the mounting possibilities of the accelerometer(s). In conducting this lab, students gain experience with the mounting and use of accelerometers, learn about different vibrational modes and how they are changed by mass and shape, get familiar with current data acquisition systems, and learn how to utilize an FFT to analyze data. Each of these skills are likely to be required of MET students post-graduation. P ge 24317.2 Construction of the Structure: Cost and flexibility were prioritized in the design of the three-tiered structure. While the utilized dimensions are arbitrary, the discussed systems (Figure 1) consists of three tiers of 3⁄4” thick square wooden plates that are 11 7/8” on a side, 4 threaded rods of size 5/16”-18 that range in length from 24” to 36”, and the associated fastening hardware. Figure 1: Completed Assembly Construction of the structure begins with cutting the four threaded rods matching lengths. The location of the center platform is then established by positioning nuts and washers at the same position along the threaded rod. It is recommended that two nuts be used in combination at each point so that the second nut acts as a jam nut as shown in Figure 2. When vibrating, the nuts will frequently loosen causing undesirable secondary vibrations in the system.