Instruction manual

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networks at frequencies less than 500 Hz. Piezoelectric shakers have little or no
useful output at low frequencies.
2.3 Matching networks
The operating guide for the particular matching network used will provide detailed information for
the proper use of matching networks. However, information in this section is presented to aid the
user in understanding and applying matching networks.
While an electromagnetic shaker presents a low, relatively constant, largely resistive impedance,
a piezoelectric shaker is essentially capacitive. This means that the impedance presented by the
piezoshaker will decrease almost linearly with increasing frequency. A piezoshaker creates a
displacement proportional to the voltage input for frequencies below its resonance. Therefore, in
order to get maximum displacement and consequently maximum acceleration levels at low
frequencies, the voltage must be stepped up significantly.
The impedance matching network normally consists of a step-up transformer and an output tap
switching arrangement. A variety of output voltages are then available to maximize the voltage
delivered to the piezoshaker at different frequencies. Due to the capacitive nature of the
piezoelectric shaker, the output impedance of the power amp, and the impedance transformation
of the matching network’s transformer, the voltage at the piezoelectric shaker rolls off at 20dB per
decade after the corner frequency (exactly like an RC filter). As the voltage switch setting on the
matching network is switched from highest to lowest setting, the bandwidth of the system will
increase. In general, piezoshakers can be operated at their lower frequencies with the matching
network switch in its highest voltage position. The relatively low load (high impedance) of the
piezoshaker at low frequencies is not demanding and the power amplifier can maintain the high
voltage swing for maximum shaker output, however as the operating frequency increases, lower
output voltage taps become more efficient. The greatest power requirements for the Power Amp
will occur at the corner frequencies of the matching network and shaker system and at the
resonant frequency of the shaker.
CAUTION: The piezoshakers can be damaged by internal heat build up when run
at high levels for extended periods. This is due to the dielectric loss tangent of the
piezoceramic material that generates heat. When driven at high power levels this
heat buildup will raise the temperature of the piezoceramic to the Curie
temperature where it will depolarize itself. Please contact Meggitt Sensing
Systems for assistance when extended high output testing will be performed.
CAUTION: Do not change switch settings of the matching network when there is a signal present
as this may cause the contacts to arc and destroy the switch.