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.

Table 1.1: Strain gauge characteristics

Flipping strength	Minimum Voltage	Maximum Voltage
Low	- 1.36 V	3.52V
High	-1.44 V	5.92 V

Photo 1.1 Low Flipping strength graph. 

Photo 1.2 High Flipping strength graph.

Table 1.2: Strain gauge characteristics

Tapping  strength	Minimum Voltage	Maximum Voltage
Low	-4 V	29.6 V
High	-12.2 V	56.8 V

Photo 1.1 Low Tapping strength graph.


Photo 1.2 High Tapping strength graph.

  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.

Table 2.1: Strain gauge characteristics

Flipping strength	Minimum Voltage	Maximum Voltage
Low	- 1.28 V	10.4 V
High	-1.4 V	16.4 V

Photo 2.1 Low Flipping strength graph.

Photo 2.2 High Flipping strength graph.

Table 2.2: Strain gauge characteristics

Tapping  strength	Minimum Voltage	Maximum Voltage
Low	-2.8 V	1.8 V
High	-12 V	8 V

Photo 2.3 Low Tapping strength graph.

Photo 2.4 High Tapping strength 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.

Photo 1.1 Outputs Reading. 

  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	3.54 V	3.689 V
Output	2.5 V	2.6 V

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

(Peak to Peak Voltage)/(2*sqrt(2))

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

	Oscilloscope	DMM
Output Voltage (p-p)	2.16 V	1.78 V
Output Voltage (mean)	3.01 V	0.63 V

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

	Oscilloscope	DMM
Output Voltage (p-p)	.08 V	.028 V
Output Voltage (mean)	3.38 V	.011 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	.65 V
1 flip/second	20 seconds	1.44 V
1 flip /second	30 seconds	3.46 V
4 flips/second	10 seconds	5.4   V
4 flips/second	20 seconds	6.5   V
4 flips/second	30 seconds	8.6  V

  2.     Briefly explain your results.
As we tapped at 1time/sec the voltage would climb very slowly, but as we tapped 4times/sec the voltage would climb very rapidly and partially though the 30 second cycle it started to drop and then climbed rapidly through the rest of the cycle up to a much higher voltage than the 1 tap/second.

  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?

If we leave the diode out of the circuit the output of the circuit would look more like a sine wave because you would see the negative and positive values from the capacitor. So the voltage would oscillate between negative and positive.

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

clear all;
close all;

x = [10 10 20 20 30 30];
y1 = [.65 5.4 1.44 6.4 3.46 8.6];

plot (x, y1) 
grid on

xlabel('Time (s)')

ylabel('Vout (v)')

Graph 4.1 Voltage output of tapping strain gauge