BRUEL & KJAER 4812 - Introduction

1. INTRODUCTION
Until recently, all electrodynamic vibration exciters, or shakers as they are of ten called,
looked like the one shown in the sketch in Fig.1.1a. A drive coil, carrying an alternating
current. was suspended in a gap in a concentric magnetic field. The drive coil was sus-
pended by upper and lower flexures, which allowed it to move up and down in the gap
and kept its motion rectilinear. Because the drive coil was built into the shaker, the char-
acteristics of a particular shaker were fixed. It could be a heavy-duty type, a light-weight
type, or a large displacement type .
With the advent of the B & K Exciter Systems, it is now possible to have as manyas five
different shakers i n one system. The difference is that the B & K exciters have set both
sets of flexures up above the drive coil (Fig.1 .1 b) .
Upper Flexure ---,
low er Fle xu re --_,
;::;il~~--- Field Coil
Lower Flexure
la) Ib)
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Fig. 1. 1. Construction principles of Vibration Exciters
The flexures and drive coil therefore form a Head which can be lifted off the Body con-
taining the magnetic field and exchanged for another Head. Hence, specialized Heads can
be
built which all fit the same Body The V System of Exciters has five such specialized
Heads and two bodies. Each Head can be used on either the electromagnetic Exciter Body
4801 or the Permanent Magnet Exciter Body 4805. The force range available is from
155t0445N.
There are three square table Heads, varying from a light-weight, high acceleration type to
a large heavy-duty one. There is also a Head with a single, centrally located mounting
point for structural excitation, and a Calibration Head with a built-in reference acceler-
ometer.
For the electromagnetic Exciter Body 4801 , a choice of two different types of base are
also available, under the designations 4801 T and 4801 S. The 4801 T is a spring su-
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