Fig. 9 Principle Of Operation - Beckhoff EL3692 Documentation

Product overview
Measuring principle of the terminal
The terminal actively applies a current to the test specimen. This current simultaneously flows through
internal reference resistances. The actual resistance is determined from the proportion of the measured
voltages. The terminal measures ratiometrically, i.e. fluctuations in the supply voltage and/or the measuring
current have no influence on the accuracy.
Fig. 9: Principle of operation
The figure illustrates the following relationship between the known resistance R
resistance R :
x
R / R = U / U
x ref x ref
R = R * U / U
x ref x ref
The two channels of the EL3692 are measured alternately. To compensate capacitive processes the
conversion time depends on the measuring range: the larger measuring range the longer the conversion. To
measure a resistance at short intervals, the EL3692 should be used in single-channel mode. For measuring
inductive/capacitive loads it should be noted that transition processes influence the result. A delay time can
be specified in the channel settings in the CoE. The measurement and updating of the process data will only
take place once this delay time (in [ms]) has elapsed (CoE 0x80n0:05 and :31 [} 132]). This delay time is
particularly significant in Autorange mode: If it is too small, the measuring range may not change in
Autorange mode because the respective measuring range limits are not reached. The automatic control in
the EL3692 will try to optimize the waiting times, although manual adaptation may still be required.
The measuring frequency is determined from the filter setting for each channel. The lower the filter setting,
the more measurements are carried out and averaged, and the more reliable is therefore the measurement.
The respective resistance is measured with 24 bit resolution including sign. It is represented as 32-bit
process data.
Each of the 9 measuring ranges extends over one decade, e.g. 1 .. 10 Ω and uses a typical measuring
current, see Table. In Autorange mode the terminal determines the measuring range itself, or the user can
specify it via process data or CoE. The measuring ranges of the two channels can be used independently.
In Autorange mode each measuring range is extended 10% beyond the respective full-scale value.
Measuring range changeover takes place in this range.
For more detailed information on settings and operating modes please refer to the sections on Process data
[} 111], Operational characteristics, Diagnostics [} 106] and Specific data [} 115].
12 Version: 2.1
and the unknown
ref
EL3692