Week 11: Strain Gauges

Part A: Strain Gauges

Strain gauges are used to measure the strain or stress levels on the materials. Alternatively, pressure on the strain gauge causes a generated voltage and it can be used as an energy harvester. You will be given either the flapping or tapping type gauge. When you test the circle buzzer type gauge, you will lay it flat on the table and tap on it. If it is the long rectangle one, you will flap the piece to generate voltage.

1. Connect the oscilloscope probes to the strain gauge. Record the peak voltage values (positive and negative) by flipping/tapping the gauge with low and high pressure. Make sure to set the oscilloscope horizontal and vertical scales appropriately so you can read the values. DO NOT USE the measure tool of the oscilloscope. Adjust your oscilloscope so you can read the values from the screen. Fill out Table 1 and provide photos of the oscilloscope.

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Table 1.

     The minimum and maximum values with a soft and hard tap.

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Image 1.
     The strain gauge’s initial voltage from the tap and its response.

2. Press the “Single” button below the Autoscale button on the oscilloscope. This mode will allow you to capture a single change at the output. Adjust your time and amplitude scales so you have the best resolution for your signal when you flip/tap your strain gauge. Provide a photo of the oscilloscope graph.

     The photo above also works for this.

Part B: Half-Wave Rectifiers

1. Construct the following half-wave rectifier. Measure the input and the output using the oscilloscope and provide a snapshot of the outputs.
Image 2. (top left) this is the input wave (top right) is the circuit (bottom) is the output.

2. Calculate the effective voltage of the input and output and compare the values with the measured ones by completing the following table. Effective (rms) values

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Table 2.
     The root-mean-square values for the input to and output from the half-wave rectifier.

3. Construct the following circuit and record the output voltage using both DMM and the oscilloscope.

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Table 3.

     The peak-to-peak and the root-mean-square values as measured by the oscilloscope and the digital multimeter when a 1-microfarad capacitor is used. Notice on the rightmost column the notes are actually switched, this was an error.

4. Replace the 1 μF capacitor with 100 μF and repeat the previous step. What has changed?
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Table 4.
    The peak-to-peak and the root-mean-square values as measured by the oscilloscope and the digital multimeter when a 100-microfarad capacitor is used. Notice on the rightmost column the notes are actually switched, this was an error.

Part C: Energy Harvesters

1. Construct the half-wave rectifier circuit without the resistor but with the 1 μF capacitor. Instead of the function generator, use the strain gauge. Discharge the capacitor every time you start a new measurement. Flip/tap your strain gauge and observe the output voltage.

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Table 5.

     The maximum voltage we acquired with varying tap frequencies and duration of tapping.

2. Briefly explain your results.

     Our group noticed that faster taps produced a higher voltage than slower taps for a longer interval. Because the gauge works similarly to an impulse function, we think that the more taps in a shorter duration would give a higher output voltage. However, this may be logarithmic as it seems over longer a longer duration of taps does not seem to change the voltage much.

3. If we do not use the diode in the circuit (i.e. using only strain gauge to charge the capacitor), what would you observe at the output? Why?

The gauge also creates a negative voltage after being pressed. The positive voltage spike, although large, happens for such a small amount of time, that the time that the voltage spends in the negative area can cancel it out. It would be like taking the integral of a sine wave over an entire period rather than over half of a period.

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9 thoughts on “Week 11: Strain Gauges

  1. Overall good job on your lab. From what I’ve seen, you guys were one of the few groups who put down values for measuring the output peak to peak and mean with the DMM. Your explanation about the tap frequency was well said.

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    1. Upon review we actually did not get the peak to peak values from the DMM directly; we actually multiplied the mean by sqrt(2)*2 to get the value. We didn’t realize that “impossible” was the answer Dr. Kaya was looking for, whoops. Thanks for your feedback!

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  2. Seems you got much larger voltage values than we did. I think i will have to revisit what we did in Part C question 1 to see why our results differed. Everything looks rather good.

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    1. We noticed that without VERY hard taps the capacitor discharged too quickly to output a very high voltage. We imagine that a larger capacitor may help, although if you want it to charge quickly hard taps are the way to go.

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  3. Hey! It looks like you guys were the only one’s I’ve seen so far to include any pictures of actual breadboards. Good going!

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  4. I’m confused about number 2 in part B. How did you calculate the output RMS voltage? If you are using an input voltage with a max value of 10 V, I believe the output RMS value should be closer to about 3.2 V (Using the formula Vmax/pi). Everything else looks good, and very clear explanations. Nice job!

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  5. After seeing the results you guys got for part C, i think my group may want to go back and see what went wrong. We got very small voltages! Looks like you guys got the correct results. Blog looks good, pictures and info are great as always.

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