PCB Piezotronics ICP RHM240A01 Installation And Operating Manual page 9

5.0 POLARITY
Extension of the mounting area of an ICP strain sensor
produces a positive-going voltage output. The retraction of the
mounting area produces a negative-going voltage output.
6.0 LOW-FREQUENCY MONITORING
Strain sensors used for applications in short term, steady-state
monitoring, such as sensor calibration, or short term, quasi-
static testing should be powered by signal conditioners that
operate in DC-coupled mode. PCB Series 484 Signal
Conditioner operates in either AC or DC-coupled mode and
may be supplied with gain 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, 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 strain sensor by static methods.
In order to take full advantage of the long DTC built into the
strain 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 time constant in the system. For this reason, the signal
conditioner, as well as the readout device, must be DC
coupled.
Figure 7.1 Characteristic Discharge Time Constant Curve
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ICP QUARTZ STRAIN SENSOR OPERATION MANUAL
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 7.1, 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
7.2, is this: for the first 10% of the DTC, the signal lost is
approximately proportional to the time elapsed.
Figure 7.2 Step Function Response
For example, a sensor with a 150-second DTC loses
approximately 1% of its output level the first 1.5 seconds (1%
of 150) after the application of a steady state strain within the
measuring range. In this case, the output reading must be taken
within 1.5 seconds of the strain application for 1% 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 strain sensor. This
avoids compromising the sensor DTC.
8.0 CALIBRATION
Strain sensors are calibrated relative to a strain gage reference
sensor. A calibration certificate is supplied with each strain
sensor providing its relative voltage sensitivity (mV/). A
calibration must be performed once strain sensors are installed
in the specific equipment being measured. This is necessary so
that a direct comparison of relative data can be made thereby
allowing the user to set control limits and properly monitor a
specific event as well as the entire process.
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