IAP Example-Excursion Counter

Do you want to count the number of times a limit is exceeded in a time period? Then this is the page for you.

Application

Consider an SO₂ monitoring station. An alarm is required if the threshold limit is exceeded for more than one third of a six minute sampling period. Thus if in the six minute period the amount of SO₂ is greater than the threshold limit for a total of two minutes generate an alarm.

Implementation

To implement this scheme the following TDC entities are required (PM/APM/HPM assumed):

Tag name

Entity type

Purpose

AI101

Analog Input

SO₂ signal

AQ101

Reg PV-Totaliser

Counts the number of times the limit is exceeded during the sampling period

AU101

Logic

Totaliser control

Tag name	Entity type	Purpose
AI101	Analog Input	SO₂ signal
AQ101	Reg PV-Totaliser	Counts the number of times the limit is exceeded during the sampling period
AU101	Logic	Totaliser control

AI101 Configuration

This is configured as a conventional analog input.

The PV high alarm limit should be set to the required threshold limit and the priority should be set to NOACTION or JOURNAL. This will stop the operator being alerted to temporary threshold exceeding.

AQ101 Configuration

Configure a regulatory PV entity with an input connection to AI101.PVHIFL . The range should be set to 360 seconds (6 minutes) and the PV high alarm limit should be 120 seconds with an appropriate priority.

AU101 Configuration

A logic slot should be configured that implements the following logic function:

The purpose of the logic is to generate an ON signal once every six minutes.

Initially logic block 1 will be on since the inputs will be ON and OFF (note the second input is inverted). 360s later logic block 2 will be on. This causes logic block 1 to go OFF but only on the next scan. On the next scan the process is repeated again.

Logic block 2 is connected to the totaliser reset flag. When logic block 2 is ON the totaliser will reset.

Alternatives

The only real alternative for this is to use CL which converts a configurable solution into one requiring a programmer.