Monday, March 27, 2017

Week 11

Blog sheet 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.


Flipping strength
Minimum Voltage
Maximum Voltage
Low
 -0.5 V
3 V
High
 -3.8 V
 6.4 V




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.




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.






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
Calculated
Measured
Input
7.07 V
7.18 V
Output
3.54 V
3.59 V

3. Explain how you calculated the rms values. Do calculated and measured values match?

We calculated the RMS input value by Voltage / sqrt(2), and we calculated the RMS output value by dividing the inputs by 2.  Our calculated and measured values are VERY close.  I would say that they match just based on probable random error that will occur in the circuit.    

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



Oscilloscope
DMM
Output Voltage (p-p)
 4.8 V
 4.59 V
Output Voltage (mean)
 6.12 V
 6.03 V




5. Replace the 1 µF capacitor with 100 µF and repeat the previous step. What has changed?


Oscilloscope
DMM
Output Voltage (p-p)
 0 V
 0 V
Output Voltage (mean)
 7.04 V
 6.88 V







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. Fill out the table below:


Tap frequency
Duration
Output voltage
1 flip/second
10 seconds
 744 mV
1 flip/second
20 seconds
 1.26 V
1 flip/second
30 seconds
 1.48 V
4 flips/second
10 seconds
 2.3 V
4 flips/second
20 seconds
 2.86 V
4 flips/second
30 seconds
 3.41 V

2. Briefly explain your results.

It was sort of difficult to make sure that you got consistent taps, and I feel like my finger missed the sensor once or twice when I was tapping... The plot looks linear though.  It looks like your voltage output will be consistent with the number of taps you use but it didn't seem to level off until we got over 3.2 volts.  I think for me, because of my big fingers, someone with a smaller finger or maybe a bigger sensor would create a more consistent output.  

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?

You get very small output levels.  This is because without the diode the voltage will remain in AC, so there is a negative part of the voltage that will cancel out the higher voltage level that would be shown in the output.  The highest voltage level we achieved without the diode was 121 mV. 

4. Write a MATLAB code to plot the date in table of Part C1.






7 comments:

  1. I was really interested to see what values you guys obtained. We had significantly different values for number 1. When we used hard pressure our values were a lot larger than the values that you obtained. We may have just had different perceptions on hard pressure. Did you have any trouble with the set up? We only had a little with oscilloscope.

    ReplyDelete
    Replies
    1. I think it depends in the pressure you gave to it so that why we have different values.
      Thanks for your comment

      Delete
  2. In regards to Scout's comment, I believe you guys made the same mistake as us. As we found out in a later step, you have to change the setting on the oscilloscope to be 10x attenuation. This is because the probes we were using were 10x also. I would ask Dr. Kaya to show you how.

    ReplyDelete
    Replies
    1. I think it depends in the pressure you gave to it so that why we have different values.
      Thanks for your comment

      Delete
  3. This is interesting as well. The last group I saw had a much different output from the tapping than we did. Your group does as well. I'm not suggesting that either one did it wrong, it could have been a faulty piece of electronics. Its interesting though because the different values did not affect your data outcome. Just the same as ours was not impacted.

    ReplyDelete
    Replies
    1. I agree with you about that
      Thanks for your comment

      Delete