Automation Direct DL05 User Manual page 100

PV Rate-of-Change Alarm
An excellent way to get an early warning of a process fault is to monitor the rate-of-change of
the PV. Most batch processes have large masses and slowly-changing PV values. A relatively
fast-changing PV will result from a broken signal wire for either the PV or control output, a
SP value error, or other causes. If the operator responds to a PV Rate-of-Change Alarm
quickly and effectively, the PV absolute value will not reach the point where the material in
process would be ruined.
The DL05 loop controller provides a programmable PV Rate-of-Change Alarm, as shown
below. The rate-of-change is specified in PV units change per loop sample time. This value is
programmed into the loop table location V+21.
PV slope OK
PV
rate-of-change alarm
Sample time
As an example, suppose the PV is the temperature for your process, and you want an alarm
whenever the temperature changes faster than 15 degrees/minute. The PV counts per degree
and the loop sample rate must be known. Then, suppose the PV value (in V+03 location)
represents 10 counts per degree, and the loop sample rate is 2 seconds. Use the formula below
to convert our engineering units to counts/sample period:
Alarm Rate-of-Change =
From the calculation result, you would program the value 5 in the loop table for the rate-of-
change. The PV Rate-of-Change Alarm can be independently enabled and disabled from the
other PV alarms, using bit 14 of the PID Mode 1 Setting V+00 word.
The alarm hysteresis feature (discussed next) does not affect the Rate-of-Change Alarm.
PV slope excessive
Sample time
15 degrees 10 counts / degree
X
1 minute 30 loop samples / min.
DL05 Micro PLC User Manual, 6th Edition, Rev. A
Chapter 8: PID Loop Operation
Loop Table
V+21 XXXX PV Rate-of-Change Alarm
PID Mode and Alarm Status V+06
Bit 15 14 13 12 11 10 9 8 7 6
PV Rate-of-Change Alarm
150
= = 5 counts / sample period
30
1
2
3
4
5
6
7
5 4 3 2 1 0
8
9
10
11
12
13
14
A
B
C
D
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