Students at Cooper Union Use LabVIEW and NI CompactDAQ to Study Mechanical Vibrations

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"LabVIEW software and NI CompactDAQ provided Cooper Union students with a small, simple, and affordable system for making vibration measurements in the lab and field. "

- Melody Baglione, The Cooper Union for the Advancement of Science and Art

The Challenge:
Finding versatile, portable, and affordable instrumentation for students who need to take hands-on measurements for a variety of projects in the lab and in real-world environments for mechanical engineering undergraduate and advanced mechanical vibration courses.

The Solution:
Using NI LabVIEW software and the NI Sound and Vibration Measurement Suite with NI USB dynamic signal analyzers (DSAs) with signal conditioning for accelerometers, students study the dynamic characteristics of structures in real-world projects involving mechanical vibration and modal analysis ranging from classic cantilever beam analyses to subway vibration studies.

Author(s):
Melody Baglione - The Cooper Union for the Advancement of Science and Art

Subway Vibration Project

Using the NI USB-9234 DSA with a PCB triaxial accelerometer controlled by LabVIEW on a notebook computer, students measured acceleration at various locations on the No. 6 train of the New York City subway to observe the effects of subway vibrations on humans. Peak magnitudes occurred at approximately 0.6 and 21 Hz. A literature study by the students revealed vibration frequencies between approximately 2 and 8 Hz can cause immediate and long-term harm to the human body and nervous system, the peak at 0.6 Hz could be harmful to passengers. Students found the average steady-state magnitudes occurring at 0.6 and 21 Hz were on the order of 0.01 g, whereas literature cites values harmful to humans on the order of 0.2 g.

As a result, the students concluded that subway vibrations will unlikely adversely affect passengers. Based on their preliminary findings, they also determined that location within the subway car has little or no effect on vibrations felt by passengers. The project gave the students the opportunity to learn digital signal processing, the ability to see the implications of collecting random vibration data, and the chance to interact with a NYC Metropolitian Transportation Authority (MTA) engineer who confirmed the consistency of their findings.

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Figure 1: Students measured acceleration at various locations on the No. 6 train of the New York City subway.

Impact Modal Analysis of Two 20 in. Zildjian Ride Cymbals Project

Students performed impact modal analysis on two 20 in. cymbals in an attempt to gain a better understanding of the natural frequencies of the cymbals’ vibrations. They used two different support structures in the analysis – a PVC structure with elastic bands and a conventional cymbal stand. The student used the data collected to derive four frequency response function rows, one for each combination of cymbal and support structure.

The input was an impulse, delivered and tracked via a PCB 086C03 impact hammer with a hard plastic tip; the output was an acceleration signal, measured via a PCB A352C65 piezoelectric accelerometer. The students used LabVIEW software for data acquisition and analysis. Data was acquired with an NI 9234 DSA module and an NI USB-9162 high-speed USB carrier. The experimental results showed that the fundamental frequencies for the K Ride and the K Constantinople cymbals were 31 and 28 Hz, respectively. The students examined the difference in response using each support structure and analyzed the spectral content for each of the cymbals up to 300Hz .

Figure 2: Impact Modal Analysis of Two 20” Zildjian Ride Cymbals Project

The Effects of a Mass Damper on the Forced Response of a Structural Model Project

Students examined the effect of a mass damper on the forced response of a structural building model.

The students constructed a structural scale model of a three-story building and excited the model harmonically using a Modal Shop electrodynamic shaker and a signal generated from an NI 9263 analog output module in an NI CompactDAQ chassis.

They measured the response of the model using accelerometers attached to each floor and a USB-9234 dynamic signal acquisition module in the same chassis. The building was excited without the damper from 1 to 20 Hz and displacement data was collected at 1 Hz intervals. After analyzing the experimental data, students discovered that the tuned mass absorber may have only added an unevenly distributed mass, changing some of the natural frequencies.

Figure 3: The Effects of a Mass Damper on the Forced Response of a Structural Model Project

Modal Analysis of a Carbon Fiber Closeout Panel on the Formula SAE Race Car Project

A carbon fiber/Kevlar composite body panel for the Formula SAE racecar was modally tested to determine how anisotropic materials behave under modal testing. Analysis of an aluminum cantilever beam and a Plexiglas sheet were used to supplement this experiment. In the course of the experiment, several observations were made about the nature of anisotropic composite materials and mounting schemes in relation to their vibration and modal properties, especially concerning reciprocity and nonlinearity.

Figure 4: Modal Analysis of a Carbon Fiber Closeout Panel on the Formula SAE Race Car Project

 Modal Shaker Testing of a Cantilever Beam Project

Modal testing was performed on an aluminum cantilever beam to determine modal frequencies and mode shapes. A Modal Shop modal shaker is used to excite the structure. Several different excitation signals are used and the results are compared to determine the best choice of excitation. Excitations used include: periodic random, swept sine, burst random, and burst swept sine (burst chirp).

Figure 5: Modal Shaker Testing of Cantilever beam project

Modal Analysis of an Aluminum and Wooden Baseball Bat Project

 Students performed modal analysis on a wooden and an aluminum baseball bat to determine differences in performance. They chose a free/free mounting condition for two bats of the same length and conducted modal analysis on three points along the length of the bats.

They used an accelerometer and an impact hammer with a hard plastic tip attachment in conjunction with LabVIEW to obtain the frequency response function (FRF) matrices of the bats. As students acquired data, key experimental techniques were developed to obtain very clean results. The results indicated linearity of the bats and the imaginary parts of the FRF matrices showed mode shapes. They also postprocessed the data using a modal analysis software package to obtain visualizations of the first and second order bending modes.

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Figure 6: Impact Testing of an Aluminum Bat on a Platform

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Figure 7: Wooden Bat FRF with Coherence Overlaid

Overall, LabVIEW software and NI CompactDAQ provided Cooper Union students with a small, simple, and affordable system for making vibration measurements in the lab and field.

Author Information:
Melody Baglione
The Cooper Union for the Advancement of Science and Art
41 Cooper Square
New York, NY, NY 10003
United States
Tel: 212-353-4295
Fax: (212) 353-4327
melody@cooper.edu

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