Automationdirect.com DL05 - PV Rate-Of-Change Alarm

PID Loop Operation
Maintenance
8–55
PID Loop Operation
The PV Deviation Alarms are reported in
the two bits in 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 easy to
do. Additionally, you can monitor PID
alarms using DirectSOFT.
PID Mode and Alarm Status V+06
013456789101112131415 2Bit
Red Deviation
Yellow Deviation
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.
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.
One powerful 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.
Loop Table
V+21 PV Rate-of-Change AlarmXXXX
PV
PV slope OK
Sample time
PV slope excessive
rate-of-change alarm
Sample time
PID Mode and Alarm Status V+06
013456789101112131415 2Bit
PV Rate of
Change Alarm
As an example, suppose the PV is temperature for our process, and we want an
alarm when the temperature changes faster than 15 degrees / minute. We must
know PV counts per degree and the loop sample rate. Then, suppose the PV value
(in V+03 location) represents 10 counts per degree, and the loop sample rate is 2
seconds. We will use the formula below to convert our engineering units to counts /
sample period:
15 degrees
Alarm Rate-of-Change =
1 minute
X
10 counts / degree
30 loop samples / min.
=
150
30
= 5 counts / sample period
From the calculation result, we 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 Rate-of-Change
Alarm