Metrohm viva 1.0 (process analysis) - Manual (Page 853)

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7 Devices
viva 1.0 (for Process analysis)
￭￭￭￭￭￭￭￭ 841
Example
The aim in the given switching example is to actually remain between 7
and 17 mA. To accomplish this, the following values must be entered as
the lower and upper limit:
(7 - 4)/0.8 = 3.75 mA
(17 - 4)/0.8 = 16.25 mA
ARW only with I may not then be selected in this case.
The controller starts running at the value 0 after the parameters have been
set. Particularly in cases of asymmetrical value ranges such as are found in
the example, this can be extremely awkward, because it first has to run
for a while before it is able at all to get outside the negative limitation. For
this reason, it is expedient in such situations to use a different (here:
larger) offset and/or slope and correspondingly adjusted limits to achieve a
situation in which the controller is already in a useful status at the time of
switch-on status and can begin controlling immediately. The question of
which values are expedient here is, however, dependent on the concrete
application, and the associated set of parameters can be determined with
the method presented in the example.
If the output value exits the value range of the 16-bit terminals, then it will
automatically be limited to this range. In the case of the above-mentioned
12-bit terminals, this mechanism does not engage correctly. If values fall
below the lower limit, limitation will be correct; if the range is exceeded,
then the value range will, however, be mirrored once again. This leads to
large discontinuities in the output value and to non-functioning control in
the event of incorrect limitation by the controller. The parameterization of
the limitation is, for that reason, to be particularly observed with these ter-
minals.
7.10.8.1.6 Properties - PID controller
Dialog window Configuration ▶ Devices ▶ [Edit] ▶ Properties... ▶ Properties - 'De-
vice type' - 'Device name' ▶ Controller ▶ [Edit] ▶ Properties - 'Controller type'
Control variable
Selection of the controlled variable from all analog inputs or selection of a
calculated value.
Selection 'Port designation' | calculated value
'Port designation'
All analog input ports can be selected.
0% Mapping
Measured value to be used when the Low level (0 %) is applied as input
value.