Beckhoff EP3356-0022, Documentation

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Commissioning/Configuration
EP3356-0022 30 Version: 1.4
5.3 Basic function principles
The measuring functions of the EP3356 can be described as follows:
• The EP3356-0022 Analog Input Box are used to acquire the supply voltage to a load cell as a
reference voltage, and the differential voltage that is proportional to the force acting on the cell.
• It is compulsory to connect a full bridge. If there is only a quarter or half bridge available, you have to
add external auxiliary bridges. In this case you have to modify the nominal characteristic value.
• The reference and differential voltages are measured simultaneously
• Since the two voltages are measured at the same time, there is basically no need for a high-precision
reference voltage with respect to the level. On changing the reference voltage, the differential voltage
across the full bridge changes by the same degree. Therefore a stabilized reference voltage should be
used that is subject to only low fluctuations (e.g. the EL95xx supply terminal)
• The change of the quotient of the differential and reference voltages corresponds to the relative force
acting on the load cell.
• The quotient is converted into a weight and is output as process data.
• The data processing is subject to the following filtering procedures:
◦ the analog converter (ADC) integrates over 76 samples
◦ calculation of mean values in the averager (if activated)
◦ software filter IIR/FIR (if activated)
• The EP3356 has an automatic compensation/self-calibration function. Default state: self-calibration
activated, execution every 3 minutes
◦ errors in the analog input stages (temperature drift, long-term drift etc.) are checked by regular
automatic calibration, and compensated to bring the measurement within the permitted tolerance
range.
◦ the automatic function can be deactivated or activated in a controlled manner
• The EP3356 can also be used as a 2-channel analog input box for voltage measurement [
}
51] .
• The EP3356 has a timestamp function that can be activated through Distributed Clocks. The filter
functionality is not available in Distributed Clocks mode.
General notes
• The measuring range should always be used as widely as possible in order to achieve a high
measuring accuracy. Please consider the max. input voltage (see technical data).
Fig. 19: Max. input voltages
• Parallel operation of load cells [
}
16] is possible with the EP3356. Please note:
◦ Load cells approved and calibrated by the load cell manufacturer for parallel operation should be
used. The nominal characteristic values [mV/V], zero offset [mV/V] and impedance [Ω, ohm] are
then usually adjusted accordingly.