4
ICP
3-COMPONENT FORCE LINK OPERATION MANUAL
features or
a
zero "clamped" output often necessary in
repetitive, positive polarity pulse train applications.
If you wish to learn more about ICP® sensors, consult
PCB's General Signal Conditioning Guide (G-000lE), a
brochure outlining the technical specifics associated
with piezoelectric sensors. This brochure is available
from
PCB by request, free of charge.
7.0
DISCHARGE TIME CONSTANT
The discharge time constant (DTC) of the entire
transduction system from sensor to readout must be
considered when attempting to calibrate an ICP® force
sensor by static methods. In order to take full advantage
of the long DTC built into the force sensor, it is best to
DC couple from the sensor to the readout device.
Several dual-mode PCB
signal
conditioners (e.g., Series
484) use direct coupling techniques to decouple the
output signal from the sensor bias voltage. With the
output of the signal conditioner coupled to
a
DC
readout, such as a digital voltmeter
(DVM)
or
oscilloscope, the time constant of the sensor is not
compromised by AC coupling elsewhere
in
the system.
When DC coupling to a system, it is important to DC
couple the entire system and not just from the sensor to
the signal conditioner. The
system
time constant is
determined by the shortest t ime constant in the system.
For this reason, the signal conditioner, as well as the
readout device, must be DC coupled.
The discharge time constant represents the decay rate of
an input signal. One DTC represents the amount of time
taken for the signal to decay to 37% of the initial peak
value. As illustrated in
Figure 4,
this is an exponential
decay. Approximately five DTC intervals are needed
for a peak signal to naturally decay back to zero
.
The rule of thumb for signal discharge, as outlined in
Figure 4,
is this: for the first 10% of the DTC, the signal
lost is approximately proportional to the time elapsed.
Step Function Response
For example, a sensor with a 500-second DTC loses
approximately 1
%
of its output level the first five
seconds (I% of 500) after the application of a steady
state force within the measuring range. In this case, the
output reading must be taken within five seconds of the
force application for I% accuracy. If it is impossible to
avoid AC coupling somewhere in the sensing system, try
to keep the coupling DTC at least an order of magnitude
longer than the DTC of the force sensor. This avoids
compromising the sensor DTC.
8.0 CALIBRATION
A NIST (National Institute of Standards and
Technology) traceable calibration graph is supplied with
each force sensor certifying its voltage sensitivity
(m V
/lb).
Calibration procedures follow accepted
guidelines as recommended by ANSI (American
National Standards Institute), ISA (Instrument Society
of America), and ISO (International Organization for
Standardization). These standards provide the
establishment and management of complete calibration
systems, thus controlling the accuracy of a
sensor's
specifications by controlling measuring and test
equipment accuracy. PCB is A2LA accredited for
technical competence in the field of calibration, meeting
the requirements of ISO/IEC 17025-1999 and
ANSI/NCSL 2540-1-1994.
