Electrodynamic shaker armature
displacement is limited only by the axial length of the armature coil and
the physical limitations of the armature suspension system. Since most shakers
are provided with an adequate axial coil length to maintain relatively linear force
generation at low frequencies, the primary limitation is that of physical
interference of the suspension components. Since shakers have an available
operating displacement window, it is most common to rate and discuss vibration
test displacement in peak to peak terms. For this reason, most engineering
equations of motion involving vibration testing will use displacement in peak
to peak units(sometimes called "double amplitude displacement").
The rated displacement of electrodynamic
shakers is usually the maximum relative displacement available between the
armature and the shaker body/suspension. When considering the suitability
of a shaker for a given test, it is important to consider the various factors
that may reduce the available test article absolute displacement.
Since the same force that is applied
to the armature coil is also applied to the shaker body, the shaker body
is also accelerated and has a displacement definable by the normal equations
of motion. This body motion can have the exact opposite phase relative to
the armature motion and therefore, must share the available relative (rated)
shaker armature displacement with the armature and test article. In other
words, the test article displacement added to the shaker body displacement
must be less than the rated shaker displacement.
Another factor reducing the available
displacement is the natural deflection of the armature suspension when a
test article and fixture are placed on a shaker in the vertical shaker orientation.
The weight of this added load offsets the armature downward and therefore
reduces the available downward armature displacement. This reduction of the available
stroke on one end of symmetrical alternating vibration reduces the allowable
peak to peak displacement by double the amount of the deflection.
For most test articles, the shaker
body weight is significantly heavier than the test article, fixture and
armature and its displacement motion can be ignored. In that case, the required
displacement equation found in the Systems Engineering section applies: |