  Semi-active vibration reduction techniques are defined as techniques in which
controlled actions do not operate directly on the system's degrees of freedom
(like in the case of active vibration control) but on the system's parameters,
e.g. mass or stiffness. In literature, semi-active systems are claimed to
involve substantially less energy than active. Cyclic variation in the
stiffness of a structural system was addressed in several past contributions as
an effective semi-active vibration reduction method. The presented applications
range from step-wise stiffness variations on a simple spring-mass system to
continuous stiffness changes on rotor blades under aerodynamic loads. Beyond
these diversities, a general statement about the potential of the method on the
basis of this published work is made difficult by the complexity and different
nature of the physical devices used to modify the stiffness of the considered
systems. Those devices mostly introduce spurious effects (loads, damping or
friction) which superpose to the vibration suppression part related to the
stiffness variation. In particular, a considerable portion of the vibration
suppression can result from negative work performed on the system by the device
in charge of the stiffness variation, which would de facto classify the
approach as active vibration suppression. This work focuses on theoretical
considerations on the truly semi-active mechanism of vibration reduction by
cyclic stiffness variation, which - as shown on simple spring-mass systems - is
related to transfer of energy within the system from lower to higher vibration
modes. We will use for this vibration reduction mechanism the term stiffness
modulation to distinguish it from undifferentiated methods, for which the
designations "variable stiffness" or "switched stiffness" is used. In most
cases, the modulation-related, semi-active component of the vibration reduction
coexists with a spurious (active, in the sense discussed above) contribution
due to the work performed by the stiffness variation interface as well as with
the passive contribution of dissipative effects or devices. We will show that
the spurious active component can be diminished compared to the other
contributions in dependence of the stiffness characteristic.
