Strain Gauge Based Displacement Measurement - Ben Madara¶
Problem:
There are several displacement measurement methods. Currently, I utilize a linear variable differential transformer to measure the compaction and dilation of my experimental samples under load. This LVDT is housed inside of a high pressure vessel. LVDT is quite large for a crowded space as it requires two additional steel blocks to be fit into the set up. Additionally, the necessity of the steel mounting blocks means the measurement is not directly on the sample. This means the displacement measured is also dependent on any tilting of the mounting and loading blocks.
Solution:
I built a strain gauge based system to measure the horizontal displacement. The design was a prototype to be used outside of the pressure vessel that fits onto the 10x10 blocks in the biax. The larger size was for simplicity of electrical connection and the use of already owned strain gauges. The system works by measuring the change in resistance between four strain gauges laid out in a diamond shape by means of a Wheatstone bridge. The base material for the diamond shape bracket needs an elastic yield point higher than 30 MPa, so I used an aluminum alloy (McMaster-Carr 5052). The bracket frame was designed in Onshape. The design was first 3d printed to ensure sizing and functionality of the parts. Then a water just was used to cut the bracket pieces out of a ¼” aluminum plate. Once the pieces were in hand I was able to apply the four strain gauges, one to each of the aluminum bracket pieces. With the strain gauges super glued in place the connections were then soldered to complete a Wheatstone bridge. By using an external power source and multimeter the bracket was then bench tested. Upon a successful bench test the Wheatstone bridge was then connected to an amplifier circuit built on an Arduino Uno. The signal was amplified by the Texas Instruments INA128 amplifier. The Arduino was programmed to print and plot the voltage coming out of the Wheatstone bridge. The prototype successfully recorded strain and fit onto the 10x10 blocks for use in the biax. For future development of this project the first step would be to get higher quality strain gauges. Additionally, another amplifier and filtering of the data would improve the signal to noise ratio in the measurements.