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. Output voltage with different flipping strength

Figure 1. Oscilloscope output of low strength

Figure 2. Oscilloscope output of high strength

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.

Figure 3. Oscilloscope output of single change

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.

Figure 4. Output of half-wave rectifier

We used 10V Vp as input voltage, and as figure 4 shows, the output voltage close to 10V.

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

Table 2. Effective values of input and output

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

Table 3. Output voltage measured by DMM and oscilloscope

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

Table 4. Output voltage with 100µF capacitor instead of 1µF

When we replaced the capacitor with a larger one we notice that the voltage was more consistent. The peak-peak voltage became zero, and the average voltage increase slightly.

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:

Table 5. Output voltage with different tap frequency 

2.     Briefly explain your results.
     Since every time we applied pressure to the strain gauge in generated a voltage, so we expected the more we pressed it the higher the output voltage would be. This is shown in out table. The output voltage is much higher when we tapped the strain gauge 4 times per second than once per second. We also noticed that the output voltage maxed out at 5.08 V.

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 voltage would not be consistent. The diode only allows positive current to flow the the circuit generating a dc output voltage. Without the Diode the circuit would the capacitor would hardly charge. We created a circuit without the diode and at 4 flips/second for 30 seconds we only measured 480 mV.