Siemens Add 7 AddFEM, Manual

Page: 33 / 148

Mode of Operation AddFEM
3-6
AddFEM
C79000–G8076–C900–03
Behavior if one of the two sensors fails
If one of the two signals fails, the value of the speed of the failed channel is redu-
ced step–by–step to 0, as in single–channel detection (refer to Chapter 4, Techni-
cal Data. The sign is frozen with the state which it had before the fault occurred.
This ensures that speed recognition is still possible on one channel. However,
changes in the sense of rotation are no longer recognized.
Recognition of a wire break can be carried out by, for example, comparing the two
Ǡ
channels for equality of values. However, when the two changes are checked for
equality of values take into account that natural differences between the two chan-
nels can arise through inaccuracies in the speed sensor (tooth flanks) as well as
through acceleration processes (refer to the following chapter). In view of the multi-
tude of possible applications, channel–specific wire–break recognition is therefore
not carried out on the module, but rather has to be configured plant–specifically on
the host system.
Information on updating and possible speed differences between the two chan-
nels
Module–internal calculation of the frequency is always coupled to a signal change
(for the measurement of the period of oscillation) and is therefore carried out at
different moments at the 2 ”phase–offset” sensors.
When there are changes in the speed this therefore results in slight deviations in
the displays of Channels 1 and 2, since one of the two channels always (alternati-
vely) supplies the more recent value. The precision and relevance can thus be in-
creased in particular at lower speeds if the values of both channels are determi-
ned.
The speed difference (
n
f) in the two channels arising through the acceleration pro-
cesses which has to be taken into account at the minimum when the two channels
are monitored, lies in the following range
n
f =
p
(f
2
+ a) – f (for frequencies 125 Hz) (a = acceleration, f = frequency)
n
f = a
<
2msec (for frequencies > 125 Hz)
Remark: Simplified formulae for sensors with recognition of the sense of rotation at
which the two channels operate with a phase offset of 90
.
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