Application Notes - Beckhoff EP3356-0022 Documentation

Commissioning/Configuration
6.6

Application notes

Symmetric reference potential
The EL/EP3356 measures the two voltages U und U independently of each other and without
supply bridge
electrical reference to a supply voltage. The measuring accuracy can be increased still further if an internal
coupling is employed to prevent the internal measuring circuits from drifting apart. To this end the EL/
EP3356 has an internal switch that is closed in the default state and establishes a potential reference
between the internal electronics and the bridge point.
Fig. 73: Internal switch for increasing the measuring accuracy
If several strain gauges are supplied by the same power supply and equalising currents flow that falsify the
measurement, the switch can be opened via CoE 0x8000:05 [} 104]. Alternatively, electrically isolated strain
gauge supplies are to be installed.
Open-circuit recognition
The EL/EP3356 has no express open-circuit recognition facility. If one of the bridge wires is broken,
however, the voltage measured there generally moves towards the final value, thus displaying an error in the
status word. Over/underrange of the supply voltage is likewise indicated.
Input freeze
If the terminal/box is placed in the freeze state by InputFreeze in the control word, no further analog
measured values are relayed to the internal filter. This function is usable, for example, if a filling surge is
expected from the application that would unnecessarily overdrive the filters due to the force load. This would
result in a certain amount of time elapsing until the filter had settled again. The user himself must determine
a sensible InputFreeze time for his filling procedure.
For clarification: temporal control of the InputFreeze and the decision regarding its use must be realised by
the user in the PLC, they are not components of the EL/EP3356.
In the following example (recorded with Scope2) impulses on a 15 kg load cell are recorded; the filter is wide
open at IIR1 so that steep edges occur in the signal.
EP3356-0022 Version: 1.1.0 77