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Chapter 2
FRF Frequency Response Function Extension Module
Newton's Law and
Apparent Mass
Setting Up an FRF
Measurement
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Newton's Law states that force equals mass x acceleration:
F = ma
Rearranging this gives:
m = F/a
The units for this are N/m/s2. The definition of a Newton:
N = kg x m/s2
So we can see that:
N/m/s2 = kg
When setting up an FRF measurement, first set up the test hardware before you
configure the parameters for the FRF Module.
Figure 1 - Example FRF Measurement Setup
B
A
1 Accelerometer connected to connector B
2 Hardware being tested
3 Modally tuned hammer fitted with correct tip connected to connector A
4 Foam support to allow free movement
Test Hardware Setup
When setting an FRF test, you need this equipment:
• Accelerometer
• Hammer
• Object whose FRF measurement you are collecting
• Support for the object that provides free movement when the object is
struck with the hammer, for example, as a foam support
Rockwell Automation Publication 1441-UM003A-EN-P - May 2011
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32159-M
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