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.

Table 1: Strain gauge characteristics

Flipping strength	Minimum Voltage	Maximum Voltage
Low	-380mV	4V
High	1V	35V

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	3.536V	3.535V
Output	1.5916V	1.696V

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

	Oscilloscope	DMM
Output Voltage (p-p)	.848V	Not Possible
Output Voltage (mean)	1.72V	Not Possible

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

Since no 100 µF capacitor was available, we were provided with a 470 µF capacitor.  This is a much larger capacitor and means that we would not see the cap discharge at 1KHz.  In order to fix this we had to turn the frequency down.  In this case we used 6.3 Hz. 

	Oscilloscope	DMM
Output Voltage (p-p)	920mv	Not Possible
Output Voltage (mean)	5.67V	Not Possible

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	5.64 V
1 flip/second	20 seconds	5.8 V
1 flip /second	30 seconds	6.5 V
4 flips/second	10 seconds	5.72 V
4 flips/second	20 seconds	5.98 V
4 flips/second 10 flips/sec	30 seconds 10 sec	7.3 V 10.4 V

2.       Briefly explain your results.

The flips on the strain gauge greatly affected the amount the capacitor was charging. With the more flips per second the faster the voltage went up across the capacitor. We even added a higher flip rate just to observe what will happen and the voltage was greatly increased. 

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?

Without the diode it doesnt work as well because the circuit is not a half wave rectifier anymore. The diode is needed for the rectifier circuit.