Add an SPC Tag

System Model > Statistical Process Control (SPC) > Using Statistical Process Control > Add an SPC Tag

SPC tags specify data that is to be collected for use in SPC operations. Once data is defined it can be dynamically analyzed (as SPC graphs and alarms) at runtime.

SPC tags are similar to trend tags. Like trends, Citect SCADA can collect and store any amount of SPC data. The only restriction on the amount of data that you can store is the size of the hard disk on your computer. (Citect SCADA uses an efficient data storage method to optimize the use of storage space on your computer's hard disk.) For long-term storage, you can archive the data to disk or tape (without disrupting your runtime system). You can also log data at regular intervals (periodic trend), or only when an event occurs (event trend), in the same manner as Trend Tags.

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.

When you define an SPC tag you will want to be sure to fill in the upper specification limit (USL) and lower specification limit (LSL) if you intend to perform a capability analysis. These values have to accurately represent the users requirements, and the target value lie midway between the two. If these fields are left blank the capability analysis will be meaningless.

To add an SPC tag:

On the System Model activity, select SPC.
Add a row to the Grid Editor.
Type the required information in each column, or in the fields in the Property Grid.

For a description of the properties, see below.

Click Save.

Statistical Process Control (SPC) Properties

Note: If an SPC tag was generated by the Equipment Editor, a number of fields on the properties form will be shaded. To configure these fields, you will need to use Equipment Editor (see Using Equipment Editor to Generate Tags).

Equipment Properties

Field	Description
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 entering an Item Name, there are a number of reserved words that you need to avoid. These reserved words relate to syntax that is used for tag extensions. They are: • field • valid • override • overridemode • controlmode • status • v • vt • q • qt • t If you use one of these reserved words, an error message will display when you compile your project.

Field

Description

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 entering an Item Name, there are a number of reserved words that you need to avoid. These reserved words relate to syntax that is used for tag extensions. They are:
• field
• valid
• override
• overridemode
• controlmode
• status
• v
• vt
• q
• qt
• t
If you use one of these reserved words, an error message will display when you compile your project.

General Properties

Field

Description

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.

UNINTENDED EQUIPMENT OPERATION

If using the File Name property, ensure that the SPC tag uses a unique name. Two tags accessing the same file can result in system errors which may include lost or corrupted trend/SPC data.

Failure to follow these instructions can result in death, serious injury, or equipment damage.

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

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). Citect 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), Citect SCADA will log the value of the Expression.

Examples

Sample Period	Comment
30	Logs data every 30 seconds
10:00	Logs data every 10 minutes
10:00:00	Logs data every 10 hours
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 Tagneeds 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 Citect 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 Citect 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 Citect 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).

Storage Properties

Field

Description

File Name

The file where the data is to be stored. You need to specify the complete path or use path substitution.

When Citect 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. (Citect SCADA uses a separate file for each SPC tag.)

By default, Citect SCADA stores the file in the [DATA] directory on the hard disk where you installed Citect SCADA. The default name of the file is the SPC tag name. However, you can 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'.

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 Citect SCADA. Where <Name> is the SPC Tag Name.

Note: If you use the File Name property, confirm that no other SPC tags or trend tags use the same filename. Two tags accessing the same file can result in system errors which may include lost or corrupted trend/SPC data.

UNINTENDED EQUIPMENT OPERATION

If using the File Name property, ensure that the SPC tag uses a unique name. Two tags accessing the same file can result in system errors which may include lost or corrupted trend/SPC data.

Failure to follow these instructions can result in death, serious injury, or equipment damage.

Note: Avoid using a file extension when specifying a file name. If you edit this property (change the file name or path) in an existing project, existing SPC data is ignored.

No. Files

UNINTENDED EQUIPMENT OPERATION

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

Failure to follow these instructions can result in death, serious injury, or equipment damage.

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

UNINTENDED EQUIPMENT OPERATION

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

Failure to follow these instructions can result in death, serious injury, or equipment damage.

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

UNINTENDED EQUIPMENT OPERATION

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

Failure to follow these instructions can result in death, serious injury, or equipment damage.

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

UNINTENDED EQUIPMENT OPERATION

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

Failure to follow these instructions can result in death, serious injury, or equipment damage.

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.

Security Properties

Field	Description
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.

Historian Properties

Field	Description
Historize	This field enables you to automatically historizeAn object ID associated with every tag in a project that uniquely identifies the tag for use by tag-based drivers, automatically generated at compile. It is used instead of the actual address of the register (which is what most other drivers use to read from and write to I/O devices). and publish the specified variable tag in Schneider Electric's Historian application. 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.

Field

Description

Historize

This field enables you to automatically historizeAn object ID associated with every tag in a project that uniquely identifies the tag for use by tag-based drivers, automatically generated at compile. It is used instead of the actual address of the register (which is what most other drivers use to read from and write to I/O devices). and publish the specified variable tag in Schneider Electric's Historian application.

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 Properties

Property	Description
Project	The project in which the SPC tag is configured.