Beckhoff EP3356-0022 Documentation page 70

Commissioning/Configuration
Measurement principle of delta-sigma (ΔΣ) converter
The measurement principle employed in the EL/EP3356, with real sampling in MHz range, shifts
aliasing effects into a very high frequency range, so that normally no such effects are to be ex-
pected in the KHz range.
Averager
In order to make use of the high data rates of the Analog-to-Digital converter (ADC) even with slow cycle
times, a mean value filter is connected after the ADC. This determines the sliding mean value of the last 4
measured values. This function can be deactivated for each mode via the CoE object "Mode X enable
averager".
Software filter
The EL/EP3356 is equipped with a digital software filter which, depending on its settings, can adopt the
characteristics of a Finite Impulse Response filter (FIR filter), or an Infinite Impulse Response filter (IIR filter).
The filter is activated by default as 50 Hz-FIR.
In the respective measuring mode the filter can be activated (0x8000:01, 0x8000:02) [} 104] and
parameterized (0x8000:11, 0x8000:12) [} 104] (the EL3356-0000 supports only mode 0).
FIR 50/60 Hz
• The filter performs a notch filter function and determines the conversion time of the terminal/box. The
higher the filter frequency, the faster the conversion time. A 50 Hz and a 60 Hz filter are available.
Notch filter means that the filter has zeros (notches) in the frequency response at the filter frequency
and multiples thereof, i.e. it attenuates the amplitude at these frequencies. The FIR filter operates as a
non-recursive filter.
PDO filter
• The filter behaves like the 50/60 Hz FIR filter described above. However, the filter frequency can be
adjusted here in 0.1 Hz steps by means of an output data object. The filter frequency range extends
from 0.1 Hz to 200 Hz and can be reparameterised during operation. To do this the PDO 0x1601
("RMB filter frequency") must be displayed in the process data and the entry "PDO filter frequency"
must be selected in the object 0x8000:11. This function allows the EL/EP3356 to suppress interference
of a known frequency in the measuring signal. A typical application, for example, is a silo that is filled
and weighed by a driven screw conveyor. The rotary speed of the screw conveyor is known and can be
adopted into the object as a frequency. Thus mechanical oscillations can be removed from the
measuring signal.
Fig. 67: Notch characteristic/amplitude curve and step response of the FIR filter
IIR-Filter 1 to 8
• The filter with IIR characteristics is a discrete time, linear, time invariant filter that can be set to eight
levels (level 1 = weak recursive filter, up to level 8 = strong recursive filter). The IIR can be understood
to be a moving average value calculation after a low-pass filter.
70 Version: 1.1.0 EP3356-0022