Re: [foxboro] 4 to 20 mA output tight closure conditions for valves
- From: <D.Moras@xxxxxxxxxxxxx>
- To: <foxboro@xxxxxxxxxxxxx>
- Date: Mon, 8 Jul 2013 03:21:18 +0000
Hi,
Does anyone know if there is a Foxboro 4 to 20mA output option that causes the
mA value to be forced below 4mA (simulated tight shut off condition) when the
output of a controller is at 0%. I have seen this as a standard option in
other systems for some valves that don't tend to pinch off the best when at
zero command (4mA), instead they stay partially open. I'm wondering if
something can be done in FOX to compensate
Setting the LSCO1 parameter of the AOUT block to force the DCS output to drive
below 4mA when the output command is at 0%, may do it but I looks messy. It
looks like changing the LSCO1 parameter to a positive value will cause the RAWC
value to drive the A/D converter safely below 4mA. See example below. Has
anyone actually tried this.
SCO = 3: Inverse Linear (12800 to 64000)
Analogue Output 4 to 20 mA
RAWC = ((OUT - LSCO1) * 51200/(HSCO1 - LSCO1)) + 12800
Signal conditioning setting (SCO) = 3, = 4 to 20mA
LSCO1
5.5
LSCO1
5.5
HSCO1
100
HSCO1
100
51200
Range of integer count
51200
Range of integer count
12800
Offset bias for integer count
12800
Offset bias for integer count
OUT
0
OUT
100
RAWC
9820.10582
RAWC
64000
RAWC@4mA<mailto:RAWC@4mA>
12800
RAWC@4mA<mailto:RAWC@4mA>
12800
0% output mA
3.068783069
100% output mA
20
Any views or a neater way to engineer this functionality?
Regards,
David
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