EE3901/EE5901 Sensor Technologies

Practical 2 — Strain gauges and instrumentation amplifiers

Introduction

You will need the following equipment:

• Texas Instruments INA826 instrumentation amplifier on a JCU-made breakout board (click for schematic).
• Strain gauge test rig (having 6 strain gauges in various positions on each side of a cantilever beam, and a vibrating motor to apply a periodic load).
• Strain gauges for temperature compensation (being 2 strain gauges on a metal disk).
• Breadboard, power supply, oscilloscope, and access to standard electronics components and tools.

Your task

1. Inspect the test rig to familiarise yourself with the layout of the strain gauges.
2. Design a Wheatstone bridge circuit to measure the strain in the cantilevered beam. Essentially this means choosing which strain gauges go in which arms of the bridge. Some hints:
   • Multiple designs are possible. You should try to maximise the sensitivity of the bridge circuit.
   • You must choose a design that provides temperature compensation (by incorporating unstressed gauges into your design in an appropriate way). Recall that you analysed temperature compensation effects in tutorial questions, so you may wish to refer back to those questions as you complete this practical.
3. Calculate the output voltage by analysing your bridge circuit. You can assume the following parameters:
   $$\begin{align} \text{Gauge factor: }G & =2.1\\ \text{Strain: }\epsilon & =\pm100 \times 10^{-6}\\ \text{Supply voltage: }V_{s} & =5\ \text{V}\\ \text{Strain gauge nominal resistance: }R_{0} & =120\ \text{Ω}. \end{align}$$
4. Design an amplifier circuit using the supplied instrumentation amplifier to amplify the voltage produced by the Wheatstone bridge. You will need to refer to the datasheet to determine how to set the gain of the amplifier.
5. Build your circuit on a breadboard using the supplied breakout board and use an oscilloscope to monitor the output voltage from your amplifier. Press down on the cantilever beam and you should see a change in voltage.
6. Generate a periodic mechanical load using the vibrating motor and measure the resulting output waveform.

Assessment criteria

You will be awarded a binary mark (1 or 0) based upon completion of the task. To receive a mark of 1, you must:

1. Show a sketch of your chosen bridge layout.
2. Show your calculations predicting the output voltage across the bridge under the conditions of Eq. (1)-(4).
3. Explain to your tutor how you set the required gain on the instrumentation amplifier.
4. Show the working circuit with the output voltage displayed on an oscilloscope screen.

If you are not able to complete the task within the scheduled practical session, you have 7 days to complete the above items and submit evidence (photos, etc) by email to your lecturer.