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Experiment to measure the deflection of a cantilever beam

Discussion of results

The following examples are results that I have previously obtained using a deflection machine to provide the deflection at a constant rate. While the changes in the optical signal/voltage signal were monitored continuously by PicoLog. The results are shown in Figure 4.

The test was repeated several times to assess the repeatability of the optical fibre output. The results of the repeatability test are shown in Figure 5.

voltgae input for data logger

Figure4: Graph showing the propertional reduction in light intensity/voltage signal when the deflection machine deflects the beam.

multiple input data logger

Figure5: Plot of six sensors against strain to assess signal repeatibility and to determine the strain gauge factor.

I have also performed a continuous loading and unloading test as a means of testing the responsiveness of the optical fibre as a beam deflection sensor. The results are very predictable and hence show that the sensor has the potential to be used a sensor for monitoring the deflection of beams.

measuring beam deflection using a data logger

Figure 6: Plot of the loading and unloading of the beam.

Answers to questions

  1. When the beam is deflected, the signal should decrease
  2. When the deflection is removed, the signal should return to the initial reading.
  3. Working principle of circuitry.
    • The resistance of the LDR increases when dark and vice versa. Hence when the optical fibre is bent, more light is loss thereby increasing the resistance of the LDR. By Ohm’s law, the current flowing through the circuit will decrease and thus decreases the voltage signal across the 1k Ohm resistor. This voltage signal change will be monitored by the ADC and displayed on the PC.
    • Conversely when the deflection is removed, the light intensity from the optical fibre increases. The light output that falls on the LDR increases thereby increasing the resistance of the LDR. By Ohm’s law, the current flowing through the circuit will increase and thus increases the voltage signal across the 1 kilohm resistor.
  4. When the deflection is upwards, the optical fibre signal should increase although due to the difference in the response of the optical fibre to direction of deflection, the amount of signal increase for a given deflection may not be the same.

Teachers' notes

  • For safety reasons, ensure that the students do not look directly into the LED when it is illuminated.
  • Ensure the students are supervised when using the soldering iron to build the circuitry.
  • The expected length of the experiment including setup time is approximately 2 hours. 1 hour for setup and 1 hour for experiment.

Credits, comments and further info

This experiment was written by Dr. Kevin S C Kuang, Materials Science and Engineering, University of Liverpool.


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