You are going to measure the resonant frequency of a bar using strain gages.  Here is what you need to do.

• Clamp the bar with pre-attached strain gages to the table in lab.  This may work best at the corners of the table, but it can be done on the side of the table.
• Be sure that the bar is perpendicular to the edge of the table.
• Be sure that the strain gages are as close as possible to the table.
• Be sure that the C-clamp is as tight as possible.  You may have to retighten the C-clamp after you try to "twang" the bar.
• Wire the strain gages as a voltage divider.  Here is the circuit for a voltage divider.  The two resistors shown in the circuit are the strain gages.  (Click here to learn about strain gages.)  It doesn't matter which strain gage is the top or the bottom.
• When the bar flexes one gage will stretch - increasing its resistance, and the other gage will compress - decreasing its resistance.  Since resistances change in opposite directions, they both contribute to a change in the same direction for the output voltage.

• You will be applying 12 volts to the circuit, but do not connect the power supply yet.
• Connect the output of the voltage divider to the oscilloscope.  Set the scope as follows.
• 100 ms/div on the time scale.
• 10 mv/div on the vertical (voltage) scale.
• Set the channel for an AC measurement.  You have to do this because the output of the voltage divider is nominally 6v with a 12v supply.  However, superposed on that is a very small signal due to the oscillations when you twang the bar.  When the scope is set to AC, it puts in a high pass filter that blocks the nominal 6v, and only lets through the relatively rapid vibration signals.
• You may want to experiment with a low-pass filter at the output of the voltage divider since there will be considerable noise in the signal coming out of the voltage divider.
• Now, set the power supply to 12 v.  Do not connect it.  Just set it to 12v.  Then, turn it off, and connect to your voltage divider.  After it is connected to your voltage divider turn the power supply back on and do the following.
• Twang the bar at the same time as you hit the run button on the scope.
• Hit the stop button to keep the data.
• Do over again until you have "good" data on the scope, then turn off the power supply.
• Now, use Benchlink to "grab" the data into a data file.
• Bring up the data in Excel or Matlab to be sure that it looks OK.
• Now you are ready to do the frequency analysis, and you can examine the spectrum of the signal.
• Doing that, determine the resonant frequency of the bar.

In your write-up, address these issues.  (And, do your reports as a lab group.)

• Show how you calculated the resonant frequency and give the calculated value of the resonant frequency.
• Tell what you expect the resonant frequency to do (increase or decrease) if the bar is lengthened and otherwise stays the same.