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Vibration monitoring with optical fibers

IntroductionSuitable ages 16+ KS5

This experiment uses inexpensive plastic optical fibers for measuring the vibration response of a cantilever beam with a mass attached to the free end. The modulation of the light passing along the optical fiber is monitored using a light detector (e.g. 

Educational data loggers

photodiode / phototransistor). The output of this photodetector is continuously measured by the ADC-12 data logger, and this simple system is able to provide real-time dynamic monitoring with a PC.

What the experiment is trying to teach

The experiment highlights the potential use of an optical fibre as a sensor for monitoring, in real time, the dynamic response of a structure. Such a system uses what is known as the fibredyne technique where the flexing of the fiber produces a modulation of the carrier beam. This technique is also used to measure the flow rate of a liquid passing down a tube, across which such an optical fiber has been stretched. The fiber is set into vibration by the vortices that it sheds. Measurement of the modulation of the light passing through it can be used to determine the flow rate. Vehicle speeds and weights can also be determined by this technique through the use of an optical fiber embedded in a rubber mat over which vehicles pass.

Equipment required

  • ADC-12 or any other suitable data logger
  • Light detector (photodiode/phototransistor)
  • Red light emitting diode (LED)
  • 2 x 1.5V batteries in battery holder (to power LED)
  • 10 ohm, 0.25 W resistor (to drop down the voltage to that required by the LED)
  • Optical fiber connectors (SMA 905 multimode type connectors)
  • Plastic optical fibers — 1 metre, step-index type, material: PMMA
  • (Optical fibers and connectors are available from www.fos-ta.com. )
  • A flexible beam (e.g. a metal, wooden or plastic ruler)
  • G-clamp
  • Fast-curing adhesive
  • A voltage/signal amplifier may also be required

Experiment setup

  1. Attach the optical fiber sensor to the beam (e.g. ruler) using a suitable adhesive, such as superglue, and wait for it to dry. Ensure that the bond is secure and that the sensor does not de-bond when the beam is flexed.
  2. Connect the optical connectors to each end of the optical fiber and couple them to the LED and light detector respectively, ensuring that they are securely coupled to minimize any unwanted changes in the voltage signal due to losses in the coupling and to environmental noise. Connect the light detector's output to the ADC-12 (or similar).
  3. Attach one end of the beam to the side of a table and secure it using a G-clamp.
  4. Attach a small (e.g. 50 g) mass at the free end of the beam using some Blu-Tack.

Carrying out the experiment

  1. Check that PicoScope shows that there is voltage output reading from the light detector.
  2. Adjust the scale of the y-axis of the graph in PicoScope to obtain an optimum display. If you have a signal amplifier, you can also adjust the voltage level sent to the ADC-12. Typical timebase setting is 500 ms/div and 0.1 volt/div.
  3. Apply a small deflection to the beam, release and observe the change in the voltage level as indicated by PicoScope. What happens to the signal? Can you determine the frequency of the vibration (i.e. number of cycles per second)?
  4. First, record the vibration frequency when the beam is not carrying any masses. Vibrate the beam by applying a small deflection at the end and then releasing. Measure the period T, the time it takes for 1 cycle of vibration. Configure the oscilloscope to trigger-mode with -10% delay in order to capture sufficient vibration trace for analysis. Alternatively, you may press the start/stop button of the PicoScope to freeze the display once you are satisfied with the vibration trace. Calculate the frequency of the vibration by taken the reciprocal of the period — 1/T. Use the cursor function in your PicoScope to aid the measurement of the period of vibration over a few cycles.
  5. Now try with a different mass on the end of the ruler and observe the changes in the vibrating frequency.

Figure 1 below shows the experimental set-up to measure the vibration response of the cantilever beam.

experiment layout

Figure 1: experiment layout

Questions and discussion of results

Plot a graph of Frequency along the y-axis against Attached Mass along the x-axis. What conclusion can you draw from the results?

(You should see an inverse relationship between the Attached Mass and the Free Vibrating Frequency of the ruler.)

Further study

Try the experiment using a stiffer or less-stiff ruler. How does the stiffness affect the free vibration frequency? (answer)

Use the same technique, as outlined in this experiment, to measure water flow down a pipe.

Credits, comments and further info

A complete educational fibre sensor kit based on this experiment can be purchased from www.zugophotonics.com

This experiment was written by Dr. Kevin S C Kuang, Department of Civil Engineering, National University of Singapore.

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