Rockwell Automation Publication 1444-UM001D-EN-P - June 2018 433
CIP Objects Appendix A
CDE (Complementary Differential Expansion) Measurements
The two channels are configured with opposite sense and with suitable
individual offsets such that at the nominal cross-over point their individual
measurements are zero (DC Measurement Object instance attributes). While
the 'normal' sense probe returns a negative displacement value then it is the lead
probe for the CDE measurement, otherwise the 'counter' sense probe is used.
The implementation includes protection against one probe failure (the CDE is
not based on a probe in TX Fail if the other probe of the pair is TX OK). The
implementation also incorporates a progressive changeover between probes.
This changeover is incorporated to avoid a sudden jump in the measurement
value around the cross-over point. It is applied automatically over ±15% of the
offset of the normal sense probe, about the changeover point. The following
graphic illustrates the operation of these features where the yellow highlights
indicate the single channel providing the CDE data.
In the example with both probes OK, there are three CDE results shown that
are based on both probe results.
• The first as the normal sense probe approaches its limit, is still weighted
towards the data from that probe.
• The second, close to the cross-over point is nearly equally weighted.
• The third as the counter sense probe is taking over, is now weighted
towards the data from that probe.
Final CDE value can be separately adjusted with the overall axial offset,
attribute 18.
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