Yokogawa EJX930A - Manual (Page 126)

<Appendix 5. PID Block>
A5-8
IM 01C25T02-01E
A5.15 Manual Fallback
Manual fallback denotes an action in which a
PID block changes mode to Man and suspends
the control action. Manual fallback takes place
automatically as a means of abnormality handling
when the following condition is met:
• IN.status is Bad except when the control action
bypass is on.
To enable the manual fallback action to take place
when the above condition is met, Target to Manual if
BAD IN must be specified beforehand in STATUS_
OPTS.
The table below shows the options in STATUS_
OPTS.
Options in
STATUS_OPTS
Description
IFS if BAD IN Sets the sub-status component of
OUT.status to IFS if IN.status is Bad
except when PID control bypass is
on.
IFS if BAD CAS
IN
Sets the sub-status component of
OUT.status to IFS if CAS_IN.status
is Bad.
Use Uncertain
as Good
Does not regard IN as being in Bad
status when IN.status is Uncertain
(to prevent mode transitions from
being affected when it is Uncertain).
Target to Manual
if BAD IN
Automatically changes the value of
MODE_BLK.target to MAN when IN
falls into Bad status.
Target to next
permitted mode
if BAD CAS IN
Automatically changes the value of
MODE_BLK.target to Auto (or to
Man if Auto is not set in Permitted)
when CAS_IN falls into Bad status.
A5.16 Auto Fallback
Auto fallback denotes an action in which a PID
block changes mode from Cas to Auto and
continues automatic PID control with the user-set
setpoint. Auto fallback takes place automatically
when the following condition is met:
• IN.status (data status of IN) is Bad except when
the control action bypass is on.
To enable the manual fallback action to take place
when the above condition is met:
• Target to next permitted mode if BAD CAS
IN must be previously specified in STATUS_
OPTS.
- AND -
• Auto must be previously set in MODE_BLK.
permitted.
A5.17 Mode Shedding upon
Computer Failure
When the data status of RCAS_IN or ROUT_IN,
which is the setting received from a computer as
the setpoint SP, falls to Bad while the PID block
is running in the RCas or ROut mode, the mode
shedding occurs in accordance with the settings in
SHED_OPT.
If the RCAS_IN data is not renewed within the time
specified by SHED_RCAS in resource block, the
data status of RCAS_IN falls to Bad.
A5.17.1 SHED_OPT
The SHED_OPT setting stipulates the
specifications of mode shedding as shown below.
Only one can be set.
Available
Setting for
SHED_OPT
Actions upon Computer Failure
Normal shed,
normal return
Sets MODE_BLK.actual to Cas *1 , and
leaves MODE_BLK.target unchanged.
Normal shed,
no return
Sets both MODE_BLK.actual and
MODE_BLK.target to Cas *1 .
Shed to Auto,
normal return
Sets MODE_BLK.actual to Auto *2 , and
leaves MODE_BLK.target unchanged.
Shed to Auto,
no return
Sets both MODE_BLK.actual and
MODE_BLK.target to Auto *2 .
Shed to
Manual,
normal return
Sets MODE_BLK.actual to Man, and
leaves MODE_BLK.target unchanged.
Shed to
Manual, no
return
Sets both MODE_BLK.actual and
MODE_BLK.target to Man.
Shed to
retained
target, normal
return
If Cas is in MODE_BLK.target, sets
MODE_BLK.actual to Cas *1 , and
leaves MODE_BLK.target unchanged.
If Cas is not set in MODE_BLK.target,
sets MODE_BLK.actual to Auto *2 , and
leaves MODE_BLK.target unchanged.
Shed to
retained
target, no
return
If Cas is set in MODE_BLK.target, sets
both MODE_BLK.actual and MODE_
BLK.target to Cas *1 .
If Cas is not set in MODE_BLK.target,
sets MODE_BLK.actual to Auto *2 , and
MODE_BLK.target to Cas.
*1 The modes to which a PID block can transfer are limited
to those set in MODE_BLK.permitted, and the priority
levels of modes are as shown below. In fact, if Normal
shed, normal return is set for SHED_OPT, detection of a
computer failure causes MODE_BLK.actual to change
to Cas, Auto, or MAN, whichever is set in MODE_BLK.
permitted and has the lowest priority level.
FA0505.ai
Higher priority
level
Lower priority
level
ROut RCas Cas Auto Man
*2 Only when Auto is set as permitted mode.