!
!
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The!automatic!stand ‐by! function!avoids!overshoots!following!t emporary!process !interruptions!(i.e.!if!the! pressure!goes! to!
zero)!that!may!ca use!the!controller s!integral!component!to!saturat e.!When!the!process!restarts,!a!saturated!output!is!likely!to!
cause!an!exce ssiv e!and!potentially!dangerous!oversh oot!(starting!the!motor!at!full!speed).!This !fea ture!is!not!active!while!in!
ma nual!control!mode.!
!
The!parameters!are:!
1. Pre ssure!Stand‐by!Threshold!
Automatic!stand‐by!pressur e!threshold!va lue! to!s witch!c ontroller!into!the!aut omatic!stand‐by!s equence!(in!physi cal!
units!from!0!t o!15!%! of!full!scale!or !OFF).!I nput!excursions!of![SP±!Threshold]!start!the!automatic!stand‐by!feature.!
If!set!to!OFF,!the! automatic! stand‐by!feature!is!di sabled.!
2. Pre ssure!Stand‐by!Recovery!Time!
The!maximum!of!time!(from!1!to!60!seconds)!the!automatic!stand‐by!sequen ce!is!allowed!t o!continue.!If!the!exc ursi on!
lasts!longer!than!this!time,!manual !mode!with!0%!controller!output!is!applied!(emergency!stop).!
3. Pre ssure!Stand‐by!Active!Limit!
An!active!power!l imitation!value.!It!limits!power!to!the!“known!good!steady!state!power”!required!±!t he!Pres sure!Stand ‐
by!Active!Limit!value.!This!impr oves!sa fety!for!a!v ery!sensitive!react ing! pressure!processes!by!avoiding!larg e!deflections.!
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The!automatic!stand ‐by! function!is!activated!by!setting!the! Pressure!Stand‐by!Threshold!to!a!value!other!than!OFF.!The!
unfilte red!controller!input!is!monitore d!(not#the#slower#filtered#display#va lue),!and!when!it!leaves!the!ban d!above!or !bel ow!
the!setpoi nt!s et!by!the!“Pressur e!St and‐by!Threshold ”!parameter,!the!output!is!immediately!set!to! the!steady!state!value!
sto red!when!the!process!was!first!stab le
(1.!see!Finding ! the!Steady!State!P ower!below)
.!
!
If!the!input!recov ers!within!the!“Pressure!stand‐by!recovery!time”,!the!contro ller!waits!for!two!and!half!time!the!integral!
value!( 2.5!*!Ti)!and!then !returns!to!norma l!“running”!conditions.!
!
If!the!process!input!does!not!recov er,!the!output!remains!at!the!steady!state!value!until!the!Pressure!s tand‐by!recovery!time!
has!elapsed,!at!w hich!time!the!controller!is!switched!to!manual!mode!at!0%!power.!
!
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[7869a( $>>95 $7398"(3J9(*49%%649(A37#5GU"(0J49%J'85($%(9L&99595Y(
Changing!the!setpoint!while!the!Automatic!st and‐by!is!active!will!cancel!the!stand‐b y!sequence.!It!will!no t!be!able!to!activate!
until!a!new!steady!st ate! value!has!been!found.!
!
**The!old!model!ATC880!monitor!the! input!a nd!output!the!fea ture!only!began!if !the!threshold!was!passe d!and!the!output!
was!at!saturation.!ATC990!monitors!the!input!level,!but!not!output!saturation!because!the!process!could!see!heavy!
disturbances!before!the!output!is!saturated.!
!
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