Identification of the dynamic characteristics of nonlinear structures

284
Fig. A3.1.3 Dimension of Component No. 1 (Main Beam)
 
Fig. A3.1.4 Dimension of Component No. 2 (Centre Mass)
Component No. 3 is a uniform beam with dimension (length x width x height) being
(all dimensions are in mm) and Component No. 4 is a rigid mass with
dimension 
as shown in 
2. 
DETAILS OF CONSTANT FORCE CONTROL
In some modal tests, such as for the measurement of structural nonlinearity, it is required
that the amplitude of the excitation force should be kept constant. In cases where the
damping of the structure to be tested is not very light (i.e., if the damping ratio is of the
order of 1% or more), the amplitude of the excitation force can be controlled to be
constant based on a feed-back control loop as shown in 
Shaker
Unit
Force Gauge
If 
If F=F*
If 
increase the generator output V
next frequency point
decrease the generator output V
V
Frequency
Response 
Computer
Analyser
Fig.3.2.1 Excitation Force Amplitude Control
F
F = Actual Force
F* = Target Force

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Appendix 
Details of Experimental Work
285
In the case where the damping of the 
is very low, for the frequency points very
near structural resonance, the response amplitude can become very high and the control of
constant force becomes difficult, then the FRF data at these frequency points should be
left out.