The trend system simultaneously supports data collection in two separate modes.  The first mode is real-time trending; in this mode the trend system records the latest trend data into a combination of memory-based and disk-based circular buffers.  As a new point is recorded the oldest point is removed from the circular buffer, so that at any given time only the last n hours, minutes and seconds-worth of data is stored on the system (n is a configurable parameter).

In addition, the trend system can record historical trend data sets and store them as files on disk for later review. The historical trend system is driven by triggers, which are definitions of events and the data that should be recorded when the event is detected.  The historical trend system and triggers are defined in more detail on the Historical Trend Triggers section of this manual.

The following host processes comprise the Trend System:

• TrendMgr – the trend manager process

• TrendGsm – the data acquisition process acquiring data from global shared memory regions

• TrendSvr – the server process that services client requests for trend data

• TrendTrig – the server process that manages historical trend data acquisition

The following screens are available as part of the Trend System:

• TrendX – trend display and analysis

• TriggerMgr – trigger configuration and activation

• TrendFileMgr – historical trend file manager

A historical trend trigger is the definition of an event along with a list of data to be recorded when that event is triggered.  An event is triggered when a set of variables meet some prescribed conditions.  In an aluminum smelting plant, for instance, a trigger could be defined to capture data during an anode effect.  The trigger could watch for when the pot resistance gets too high, and when it does record a set of data including the pot resistance, volts, and line amps.  Once this trigger is configured in the historical trend system, all anode effect events would be captured to individual historical trend files on disk for later review.  

Once a trigger has been activated, the historical trend trigger system monitors each of the trigger conditions and continuously evaluates each of them to determine when they have all been met. During this time the trigger is Waiting for trigger event detection.  If this occurs, and all trigger conditions are met, then data collection begins and the trend system begins to write data to the file.  This will continue until the trigger conditions are no longer all met.  At this point the data file is closed and left on the disk for later review.

Several options are available to enhance data collection:

• A pre-trigger time period may be configured so that when a trigger event is detected the trend system will actually go backwards in time to retrieve the data leading up to the event and save this in the historical trend file before the data for the event itself.  In many cases this information is crucial for determining the cause of a given event.

• A post-trigger time period may be configured so that once the trigger conditions are no longer satisfied the trend system will continue to record data for a period of time and append it to the historical trend file after the data for the event itself.  In many cases this information is useful for determining how the control system reacts after a given event.

• A maximum trend time period may be specified to keep file sizes small and manageable.  This value limits the maximum amount of time of historical data that can be stored in a single file; if the event lasts longer than this amount of time then the data recorded for that event will span multiple files.  This option is useful for continuously recording data (i.e. the event is defined as always triggered) and generating files of finite size.

Triggers can be created, modified, activated, and deactivated using the TriggerMgr screen.

Historical trigger files are stored on the host as ASCII text files.  Following is an example file: