Automation Direct DL05 User Manual page 99

Chapter 8: PID Loop Operation
If the process remains out of control for some time, the PV will eventually cross one of the
1
outer alarm thresholds, named High-high alarm and Low-low alarm. Their threshold values
are programmed using the loop table registers listed above. A High-high or Low-low alarm
2
indicates a serious condition exists, and needs the immediate attention of the operator.
The PV Absolute Value Alarms are reported in the four bits in
3
the PID Mode and Alarm Status word in the loop table, as
shown to the right. We highly recommend using ladder logic
to monitor these bits. The bit-of-word instructions make this
4
easy to do. Additionally, you can monitor PID alarms using
DirectSOFT 5.
5
PV Deviation Alarms
The PV Deviation Alarms monitor the PV deviation with respect to the SP value. The
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deviation alarm has two programmable thresholds, and each threshold is applied equally
above and below the current SP value. In the figure below, the smaller deviation alarm is
called the "Yellow Deviation", indicating a cautionary condition for the loop. The larger
7
deviation alarm is called the "Red Deviation", indicating a strong error condition for the loop.
The threshold values use the loop parameter table locations V+17 and V+20 as shown.
8
9
SP
10
11
The thresholds define zones, which fluctuate with the SP value. The green zone which
surrounds the SP value represents a safe (no alarm) condition. The yellow zones lie outside
the green zone, and the red zones are beyond those.
12
The PV Deviation Alarms are reported in the two bits in the
PID Mode and Alarm Status word in the loop table, as
13
shown to the right. We highly recommend using ladder logic
to monitor these bits. The bit-of-word instructions make this
14
easy to do. Additionally, you can monitor PID alarms using
DirectSOFT 5.
A
The PV Deviation Alarm can be independently enabled and disabled from the other PV
alarms, using bit 13 of the PID Mode 1 Setting V+00 word.
B Remember the alarm hysteresis feature works in conjunction with both the deviation and
absolute value alarms, and is discussed at the end of this section.
C
D
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DL05 Micro PLC User Manual, 6th Edition, Rev. A
Red Deviation Alarm
Yellow Deviation Alarm
Yellow Deviation Alarm
Red Deviation Alarm
PID Mode and Alarm Status V+06
Bit 15 14
High-high Alarm
High Alarm
Low Alarm
Low-low Alarm
Red
Yellow
V+17 XXXX
Green
V+20 XXXX
Yellow
Red
PID Mode and Alarm Status V+06
Bit 15
Red Deviation
Yellow Deviation
13 12 11 10 9 8 7 6 5 4 3 2 1 0
Loop Table
Yellow Deviation Alarm
Red Deviation Alarm
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0