June 22, 2008
June 22, 2008
June 25, 2008
13.594.1 - 13.594.9
Exper imentally Deter mining Mass Moment Pr oper ties
A project is presented that requires students to experimentally determine the mass moment properties for a three-dimensional test specimen. The project begins by attaching the test specimen (a composite structure made from small wooden blocks) to a pendulum platform of known mass properties. The platform is equipped with an accelerometer to allow dynamic motion tracking. Multiple swing tests are performed and the dynamic response of the loaded pendulum is recorded using a National Instruments data acquisition system. Using an oscillator model and Fourier analysis, the students calculate both the damping ratio and the damped natural frequency of the loaded platform. Armed with these results and the known pendulum geometry, the students back calculate the mass moment properties of the test specimen. The students also estimate the inertia properties using composite body methods and CAD software to allow comparison of the experimental results. An error analysis of discrepancies between the three methods is required. The project has been found to effectively demonstrate the significance of mass properties in determining the dynamics of a system. Furthermore, the project is a challenging experimental assignment that requires mature experimental techniques; as the results are highly sensitive to inaccuracies in measuring the dynamic response.
Introduction and Motivation
A theory course in dynamics is typically included at the sophomore level of most mechanical engineering programs1,2. It is common for the content of such a course to begin with particle dynamics. For such analyses the student need only know the mass of the particle(s) involved; usually given in the problem. The course content progresses to rigid body dynamics. With the introduction of rigid bodies, an understanding of the distribution of the mass within the body is required. Typically, however, that said distribution is again given or is readily determined. As a result, students tend to view the inertial properties of a body as a trivial input to a mathematical problem; similar to the length of a connector or the mass of a component. In reality, determining the inertia properties of a real structure can be quite challenging.
The project presented herein demonstrates the link between the inertia properties and the dynamic response of the rigid body; in as much as the dynamic response can actually be used to determine unknown inertia. The project is part of a junior-level course entitled Experimental Mechanics taught at Oklahoma Christian University. The course focuses on experimental investigations directly related to the theory learned in Strength of Materials and Dynamics (both prerequisites to this course).
The students are assigned the simple objective of determining the inertia properties of a rigid body analytically, computationally, and experimentally. Figure 1 shows a typical rigid body sample used within the course. Each sample is fabricated from wooden building blocks and no two samples are identically configured.
Newberry, B. (2008, June), Experimentally Determining Mass Moment Properties Paper presented at 2008 Annual Conference & Exposition, Pittsburgh, Pennsylvania. 10.18260/1-2--4227
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