## page was renamed from How To/Transfer Function Measurements
= Transfer Function Measurements in COMSOL =
In this tutorial, we present a simple method to make a rudimentary transfer function measurement, using Frequency Domain Analysis. This method was discovered on a search for a more direct procedure, and so this tutorial should be modified if such a method is found. The strategy is to physically perturb one portion of the object with an oscillatory drive, and observe the maximum displacement of another portion as frequency is varied.

1) Create or upload your object. We will use a simple steel rectangular bar for this example.

[[http://nodus.ligo.caltech.edu:8080/40m/100726_110926/Screen_shot_2010-07-23_at_2.57.14_PM.png|{{http://nodus.ligo.caltech.edu:8080/40m/100726_110926/Screen_shot_2010-07-23_at_2.57.14_PM.png||width=75%}}]]

2) Under the Solid Mechanics Physics option, add a face load to the surface which needs to be driven. Modify the pressure in each direction as needed. COMSOL understands this as a constant pressure right now, but as we move to FDA, COMSOL will automatically update to an oscillatory pressure.

[[http://nodus.ligo.caltech.edu:8080/40m/100726_111005/Screen_shot_2010-07-23_at_2.57.38_PM.png|{{http://nodus.ligo.caltech.edu:8080/40m/100726_111005/Screen_shot_2010-07-23_at_2.57.38_PM.png||width=75%}}]]

3) Use Prescribed Displacements (or Velocities or Accelerations) to constrain the motion as desired. We will assume that the bar is planted on a plate, and so there is no z-motion of the bottom face.

[[http://nodus.ligo.caltech.edu:8080/40m/100726_111030/Screen_shot_2010-07-23_at_2.57.45_PM.png|{{http://nodus.ligo.caltech.edu:8080/40m/100726_111030/Screen_shot_2010-07-23_at_2.57.45_PM.png||width=75%}}]]

4) Add a Frequency Domain Study, perhaps in addition to an Eigenfrequency Study to cross-check the response.

[[http://nodus.ligo.caltech.edu:8080/40m/100726_111056/Screen_shot_2010-07-23_at_2.58.05_PM.png|{{http://nodus.ligo.caltech.edu:8080/40m/100726_111056/Screen_shot_2010-07-23_at_2.58.05_PM.png||width=75%}}]]

5) Choose a range and step size of frequencies.

[[http://nodus.ligo.caltech.edu:8080/40m/100726_111114/Screen_shot_2010-07-23_at_2.58.18_PM.png|{{http://nodus.ligo.caltech.edu:8080/40m/100726_111114/Screen_shot_2010-07-23_at_2.58.18_PM.png||width=75%}}]]

6) Run the study. Record the maximum displacement seen with each frequency.

[[http://nodus.ligo.caltech.edu:8080/40m/100726_111128/Screen_shot_2010-07-23_at_2.59.02_PM.png|{{http://nodus.ligo.caltech.edu:8080/40m/100726_111128/Screen_shot_2010-07-23_at_2.59.02_PM.png||width=75%}}]]

7) Use Excel, Matlab, or any other plotter to connect these points and produce a transfer function. Evidently, our bar has an eigenmode at approximately 919 Hz.

[[http://nodus.ligo.caltech.edu:8080/40m/100726_111144/Screen_shot_2010-07-23_at_3.00.37_PM.png|{{http://nodus.ligo.caltech.edu:8080/40m/100726_111144/Screen_shot_2010-07-23_at_3.00.37_PM.png||width=75%}}]]