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system, a short pause in the
"Power
On" position gives the field time
enough to build-up to its final value. This delay in switch-on will prevent
large and possibly damaging surges being transmitted to the object under
test.
If the amplifier is switched from "Off" to "Power On" when the AMPLI-
FIER GAl N is not at "Reset", the CURRENT trip circuitry will be trigger-
ed. This prevents transients being transmitted to the exciter load if the input
to the amplifier is still connected.
4.5.2. Displacement
The front panel control DISPLACEMENT LIMIT is used to pre-set the
peak-to-peak level of shaker table displacement at which the DISPLACE-
MENT circuitry trips. The control is continuously variable and can be set
anywhere between 0.1-2.0 inches (2.5-50 mm).
With sine wave excitation the wide band performance of electro-dynamic
shakers can be summarized by the asymptotic plot of acceleration against
frequency (Fig.4.7.a). At low frequencies (up to approx. 20Hz), accelerat-
ion is limited by the available table displacement between end-stops. The
maximum attainable velocity is determined by the maximum amplifier out-
put voltage and dynamic loss effects in the mounts and other suspension
parts. The velocity limitation usually falls within the freqeuency range
20-200 Hz. Above about 200Hz the maximum acceleration available
depends on the driving coil current and the moving mass. The regions of
constant velocity and constant displacement are shown in F igs.4. 7 .b and
4.7.c.
Since the amplifier output voltage is proportional to the table velocity
over the frequency range where full displacement is attainable (and since
velocity is the first derivative of displacement), integrating the output volt-
age will give a signal proportional to the table <;Jisplacement. The constant of
proportionality (termed the "Head Constant") depends on the dimensions
of the driving coil, number of turns, field flux density, etc. -and is electric-
ally set-up using the HEAD CONSTANT control. The numerical value of the
Head Constant is stamped on each Exciter Head of the B & K shaker
system.
The
logarithmic plot of displacement against frequency (Fig.4. 7.c) shows
that displacement falls off as frequency increases. The gain/frequency
characteristic of the displacement sensing circuitry is arranged to match the
37
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