Lab Team 4 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.

    The table below shows the data we collected while testing our strain gauge. 

     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.

The above photo shows what our oscilloscope displayed when we did not have the "Single" mode on and we tapped our energy harvester.

Hard Tap

Light Tap

    The two photos above were taken with the oscilloscope in "Single Mode". The top photo shows the result of a hard tap, while the bottom photo was taken after a light tap.

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.

This image shows our input and output as read by the oscilloscope in out 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 shows the data we collected. 

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

    The table below shows the data  the output voltage we recorded from the circuit above. 

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

The table below shows the data  the output voltage we recorded from the circuit above. 

When the 1 µF capacitor was replaced with a 100 µF capacitor the peak to peak voltage decreases significantly from 2.6 Volts to 0.16 Volts, while the Mean/RMS voltage increase slightly form 2.8 Volts to 3.3 Volts. 

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:

    2. Briefly explain your results.

When tapping at a rate of 1 flip/second the voltage output increased steadily as the time tapping increased, however; when the tap rate was increased to 4 taps per second the value the increase was not as consistent we believe this is because of difficulty in keeping  our tap strength consistent at the increased rate. 

    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 the diode is removed from the circuit the capacitor fails to accumulate a charge.  The reason for this is that the diode acts as a half-wave rectifier filtering out the negative portion of the sinusoidal voltage wave.  When the diode is removed both positive and negative voltage reach the capacitor with the net result being 0 Volts RMS applied.