DrDAQ is a versatile instrument that connects to the USB port of any PC. Using the supplied PicoScope software it can be used as an oscilloscope, spectrum analyzer and signal generator.
1. Brief description
A plastic optical fiber is attached to a (cantilever) beam to monitor its deflection. The change in the light intensity of the optical fiber is monitored using a light-dependent resistor (LDR) and a basic voltage divider circuit. The output of the LDR is continuously measured by the ADC-16 and this simple system is able to provide real-time beam deflection monitoring with a PC.
2. What the experiment is trying to teach
The experiment highlights the potential use of an optical fiber as a sensor for monitoring, in real time, the deflection of a structure. Students will also gain practical experience in building up a basic electrical circuit based on simple devices such a light-dependent resistor (LDR), reading resistor values, principle of voltage dividers and Ohm’s law.
3. Prior knowledge required
Students need to have an idea of how an optical fiber transmits light through its core and the principle of total internal reflection (TIR) and the effect of excessive bending on the light transmission in these light-guiding media. Secondly, students should have some appreciation of the function of a light-dependent resistor, the principles of voltage dividers and Ohm’s law.
4. Target group
This experiment is suitable for students taking Advanced Physics and serves as a simple introduction to the practical use of optical, electronic and optoelectronic devices.
Safety warning: Although it is generally safe to work with most low-power LEDs, for safety reasons, do not look directly into the LED when illuminated. A soldering iron should only be used under supervision.
Figure 1 shows the experimental setup for the experiment.
The experimental arrangements for both the three-point bend and tensile tests are shown in Figure 4. A standard voltage supply was used to power the light emitting diode The detector and data acquisition system consisted of a light-dependent resistor (LDR) and a low-cost commercial data acquisition system from Pico Technology which automatically records voltage changes across the LDR as the light intensity varies. The data acquisition system offers up to a 16-bit resolution analog to digital conversion (ADC) with up to 8 input channels. The resolution of the ADC system allows for the detection of voltage changes as small as 40 mV in electrical signal. A sampling rate of 2 data samples per second will be sufficient for the purpose of the experiment. The data from the optical fiber were automatically collected by the computer and displayed graphically in real time.
A schematic of the beam with the bonded optical fiber sensor is shown in Figure 2. Details will be given in the following section.
When the experiment is properly set up, the beam should flex about its centre with a load applied at the centre of the supports, as shown in Figure 3, where W is the applied load, dctr is the central deflection.
Dynamic monitoring using optical fiber sensors:
An important aspect in monitoring real-life engineering structures involve performing dynamic analyses of vibrating structures. Optical fiber sensors based on the above system represent a cost effective method to monitor a dynamic system. As a further study, the beam used in the above experiment may be subjected to a continuous deflection using a cam attached to a motor, providing a constant rate of deflection to the beam while monitoring the output signal of the optical fiber. Other types of ADC supplied by Pico Technology Ltd may be more appropriate for dynamic monitoring. Contact Pico Technology for advice.