Week 11: 03/28/2016 – 04/02/2016

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: Strain gauge characteristics

Flipping Strength	Minimum Voltage	Maximum Voltage
Low	-100mV	400mV
High	-1.20V	1.40V

Picture 1: This is the output with a low pressure tap.

Picture 2: This is the output with a high pressure tap

   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.

Picture 3: A single tap of our strain gauge measured.

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.

Picture 4: This circuit is for the half wave rectifier.

Picture 5: The input to our half-wave rectifier circuit.

Picture 6: The output to our half-wave rectifier circuit.

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

The table below has our calculations for the Half Wave Rectifier circuit. To calculate the input and  we divided the voltage by 2squareroot2. To calculate the output we divided the voltage by squareroot2. Ideally the diode does not use any of the voltage so it is able to be 100% efficient. When we measured the results, we did not have the ideal efficiency.

Table 2: Half Wave Rectifier Voltages 

Effective (rms) Values	Calculated	Measured
Input	3.53V	3.64V
Output	3.53 V	2.99V

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

Picture 7: This is the half wave rectifier with a capacitor connected in parallel at the end,

Picture 8: This is a picture of our output voltage from the Oscilloscope.

Table 3: This table displays our readings for the circuit shown in Picture 7.

	Oscilloscope
Output Voltage (p-p)	4.08V
Output Voltage (mean)	1.67V

4.   Replace the 1 μF capacitor with 47 μF and repeat the previous step. What has changed?

We did not have a 100 μF capacitor so we had to use a 47 μF capacitor for this part.

Picture 9: The oscilloscope readings of the previous circuit with a larger capacitor.

Table 4: This table displays our readings as shown in Picture 9.

	Oscilloscope
Output Voltage (p-p)	320 mV
Output Voltage (mean)	3.11V

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:

Picture 10: This picture is the real life model of our constructed circuit.

Table 5: This table displays our data for tapping the strain gauge at different frequencies for different amounts of time.

Tap Frequency	Duration	Output Voltage
1 Tap/Second	10 Seconds	390 mV
1 Tap/Second	20 Seconds	650 mV
1 Tap/Second	30 Seconds	1.13 V
4 Tap/Second	10 Seconds	1.65 V
4 Tap/Second	20 Seconds	1.86 V
4 Tap/Second	30 Seconds	2.20 V

2.    Briefly explain your results.

We found that the longer and more frequently that you tap the strain gauge the higher output you have. However, the voltage had a maximum at roughly 2.20 Volts because of the size of the capacitor. Once it reached this voltage it began to discharge and then had to be recharged.

   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?

When you do not use the diode in the circuit you cannot obtain higher voltage levels. Without the diode the electricity will be in AC and not DC and there will be a negative part to the wave that will not allow as large of an output. A photo of out results is below.

Picture 11: This is the output when there is no diode in the circuit.