Add an SPC Tag

Equipment

The name of the equipment associated with the SPC tag. Select a name from the drop-down list of existing equipment definitions, or enter a name.

There is a limit of 254 characters across the Equipment and Item Name fields, including any separating periods (.).

Item Name

The name of the item with which the SPC tag is associated.

Items form part of an equipment hierarchy. They can be used to associate tags, alarms and trends with a particular attribute of a physical piece of equipment (see Items).

There is a limit of 254 characters across the Equipment and Item Name, including any separating periods (.).

Note: When defining an item name, avoid using the reserved words. If you use any of these, an error message will display when you compile your project.

SPC Tag Name

The name assigned to the SPC data. If you are logging a variable, use the same name for the SPC tag that you used for the variable tag.

Cluster Name

The name of the cluster that runs the SPC tag. This field needs to be defined if your project has more than one cluster.

You can leave this field blank in a multi-cluster system if cluster replication is enabled (see the parameter [General]ClusterReplication).

Type

The type of SPC trend:

• TRN_PERIODIC

• TRN_EVENT

  Note: SPC does not support Periodic-Event trends, which is a combination of the properties of Periodic and Event trends. In addition, if you use event-based SPC tags, your display screen refresh rate may be slower than desired or practical for your application.

Expression

The logged value of the SPC tag. Enter a value of 254 characters or less. You can log individual variables by using a Variable Tag, for example:

Expression: PT104

Comment: Logs the Variable Tag PT104

The value of the process variable PT104 is logged. Variable PT104 has to be defined as a variable tag. You can also log any Cicode expression or function, for example:

Expression: PT104/COUNTER

Comment: Logs Variable Tag PT104 divided by the Variable Tag COUNTER

Trigger

The Cicode expression (or variable tag) that triggers data logging. Enter a value of 254 characters or less. For example:

Trigger: PT104 500

In this example, logging occurs when the value of the variable tag (PT104) falls below 500.

For a periodic SPC trend, data is logged only while the value of the trigger is TRUE. In the above example, data is logged continuously while the value of PT104 remains less than 500. Logging ceases when the value rises to (or above) 500. Logging does not occur again until the value of PT104 falls below 500.

You do not have to specify a trigger for a periodic SPC trend. If you do not specify a trigger for a periodic SPC trend, then logging will occur continuously.

For an event SPC trend, data is logged once when the value of the trigger changes from FALSE to TRUE. In the above example, one sample is logged when the value of PT104 first becomes less than 500. Another sample is not logged until the value of PT104 rises to (or above 500) and again falls below 500.

Subgroup Size

The size of each subgroup. The default value for this value is 5. Valid values are 1 - 25 inclusive.

Sample Period

The sampling period of the data, in hh:mm:ss (hours:minutes:seconds). Plant SCADA checks the Trigger each sample period. If the Trigger is TRUE (or has just changed from FALSE to TRUE, in the case of event SPC trends), Plant SCADA will log the value of the Expression.

Examples

Sample Period: 30 - Logs data every 30 seconds

Sample Period: 10:00 - Logs data every 10 minutes

Sample Period: 10:00:00 - Logs data every 10 hours

Sample Period: 2:30:00 - Logs data every 2 and a half hours

This property is optional. If you do not specify a sample period, the sampling period will default to 10 seconds.

Note: If you edit this property in an existing project, delete or move or move the associated trend files before you run the new runtime system.

Eng Units

The engineering units of the variable/expression being logged. The engineering units are used by the SPC trend scales and SPC trend cursor displays.

Format

The format of the variable/expression being logged. The format is used by the SPC trend scales and SPC trend cursor displays.

This property is optional. If you do not specify a format, the format defaults to ####.#.

Deadband

The value that SPC Tag needs to return to before the SPC data becomes inactive.

Lower Spec Limit

The Lower Specification Limit (LSL). This value is used as the lower limit to determine process capability. When used in conjunction with the USL it provides a tolerance for your process.

If you are unfamiliar with process capability and capability indices, ask for expert opinion. Rather than leave this blank (at least) attempt an estimate. Enter a value that you think is the lowest acceptable value of this tag. If you leave this field blank only your capability analysis will be affected.

Upper Spec Limit

The Upper Specification Limit (USL). This value is used as the upper limit to determine process capability. When used in conjunction with the LSL it provides a tolerance for your process.

If you are unfamiliar with Process Capability and capability indices, ask for expert opinion. Rather than leave this blank (at least) attempt an estimate - Enter a value that you think is the highest acceptable value of this tag. If you do leave this field blank only your capability analysis will be affected.

x Double Bar

The calculation override for process mean (X double bar). If a value is specified here it will be used in every SPC calculation, instead of the value calculated by Plant SCADA. This will affect the calculation of control limits which are normally a function of the collected samples of data.

You should not use this field unless you are experienced in SPC.

Range

The calculation override for process range (R bar). If a value is specified here it will be used in every SPC calculation, instead of the value calculated by Plant SCADA. This will affect the calculation of control limits which are normally a function of the collected samples of data.

You should not use this field unless you are experienced in SPC.

St Deviation

The calculation override for process standard deviation (s bar). If a value is specified here it will be used in every SPC calculation, instead of the value calculated by Plant SCADA.This will affect the calculation of control limits which are normally a function of the collected samples of data.

Do not use this field unless you are experienced in SPC.

Comment

Any useful comment (254 characters).

File Name

The file where the data is to be stored. You need to specify the complete path or use path substitution (see Use a Path Substitution).

When Plant SCADA collects data from your plant floor, it stores the data in a file on the hard disk of your computer. When it subsequently uses the data to display an SPC trend, it reads the data from this file. (Plant SCADA uses a separate file for each SPC tag.)

The File Name property is optional. If you do not specify a file name, the file name defaults to [DATA]:<Name> on the hard disk where you installed Plant SCADA. Where <Name> is the SPC Tag Name.

You can also specify an alternate file name like this:

File Name: [DATA]:TANK131

Where [DATA] specifies the disk and path for the data. Use path substitution to make your project more 'portable'.

Be aware that you will need to manually configure the access control list for a new folder location so that it matches the permissions that were applied to the default location during installation. See Configure Directory Security for Modified Folder Locations.

Note:

• If you use the File Name property, confirm that no other SPC tags or trend tags use the same file name. A compile error message (E2172) will be raised if duplicate file names are detected.

• Duplicated file names can occur if your path substitutions are not configured correctly, for example, if [TREND1] and [TREND2] incorrectly point to the same location. If this occurs, an error will be logged to the syslog.dat file, and a hardware error will be raised.

• If a path with invalid characters (such as '*' or '?') is detected at runtime, an error will be logged to the syslog.dat file, and a hardware error will be raised.

• A file extension should not be used when specifying a file name. If you edit this property (change the file name or path) in an existing project, existing trend data is ignored.

• You can no longer store trend files in the 'Bin', 'Runtime', 'Backup' or 'User' directories, or any sub-directories of these. If this is attempted, an error will be logged to the syslog.dat file, and a hardware error will be raised. If you have existing Version 3.xx or 4.xx projects that use these directories to store trend files, the path for these will have to be changed to the 'Data' directory.

No. Files

Note: If you edit this property in an existing project, delete associated SPC trend files before placing the system back in service.

The number of history files stored on your hard disk (for this tag).

By default, two history 2 history files are stored on your hard disk. The maximum number of files you can specify is 270.

Period

Note: If you edit this property in an existing project, delete associated SPC trend files before placing the system back in service.

The period of the history file, in hh:mm:ss (hours:minutes:seconds). Alternatively, you can:

• Specify a weekly period by entering the day of the week on which to start the history file, for example Monday, Tuesday, Wednesday, etc.

• Specify a monthly period by entering the day of the month on which to start the history file, for example 1st, 2nd, 3rd, 4th, 5th, etc.

• Specify a yearly period by entering the day and the month on which to start the history file, for example 1st January, 25th February, etc. The day and month needs to be separated by a space.

  If you do not specify a period, the period defaults to Sunday (weekly).

Time

Note: If you edit this property in an existing project, delete associated SPC trend files before placing the system back in service.

The time of day to synchronize the beginning of the history file, in hh:mm:ss (hours:minutes:seconds). If you do not specify a time, the file is synchronized at 0:00:00 (i.e. midnight).

Storage Method

Note: If you edit this property in an existing project, delete associated SPC trend files before placing the system back in service.

Select either Scaled or Floating Point as the storage method for the SPC data (64 characters). The key difference between these two options is that Scaled is a two-byte data storage method, whereas Floating Point uses eight bytes.

Floating Point storage has a dramatically expanded data range in comparison to Scaled storage, allowing values to have far greater resolution. However, you need to consider that it also uses a lot more disk space. Scaled should be used where compatibility with pre-V5.31 trend history files is necessary.

If you do not specify a storage method, it is set to Scaled by default.

Area

The area to which the SPC data belongs. Only users with access to this area (and any necessary privileges) will be able to display the SPC data on an SPC page. For example, if you enter Area 1 here, operators need to have access to Area 1 (plus any necessary privileges) to display the SPC data.

Privilege

The privilege necessary by an operator to display the SPC data on an SPC page.

Historize

This field enables you to automatically historize and publish the specified SPC tag in CitectHistorian.

If you set this field to "TRUE", the variable will be included in an automated configuration process within the Historian environment. If you set the field to "FALSE" (or leave it blank), the variable will not be included.

Project

The project in which the SPC tag is configured.