Use the SPC Tags dialog box to configure the SPC tag properties. Statistical Process Control (SPC) Tags have the following properties:
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.
Note: Make SPC tag names unique and not identical to any trend tag names. Two tags accessing the same file can result in system errors which may include lost or corrupted trend/SPC data.
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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 the SPC tag runs in.
If the Cluster Name is not set, then CitectSCADA considers this SPC tag to run on every defined cluster.
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:
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Expression |
PT104 |
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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:
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Expression |
PT104/COUNTER |
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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:
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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.
Sample Period
The sampling period of the data, in hh:mm:ss (hours:minutes:seconds). CitectSCADA 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), CitectSCADA 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 the associated trend files before you run the new runtime system.
Type
The type of SPC trend:
Note: SPC does not support Periodic-Event trends, which is a combination of the properties of Periodic and Event trends. In addition, sif you use event-based SPC tags, your display screen refresh rate may be slower than desired or practical for your application.
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.
Comment
Any useful comment.
File Name
The file where the data is to be stored. You need to specify the full path or use path substitution.
When CitectSCADA 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. (CitectSCADA uses a separate file for each SPC tag.)
By default, CitectSCADA stores the file in the [DATA] directory on the hard disk where you installed CitectSCADA. The default name of the file is the SPC tag name. However, you can specify an alternate file name like this:
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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 CitectSCADA. Where <Name> is the SPC Tag Name.
Note: If you use the File Name property, ensure 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.
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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: Do not use 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.
Storage Method
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.
Note: If you edit this property in an existing project, you need to delete the associated trend files - before you run the new runtime system.
No. Files
The number of history files stored on your hard disk (for this tag).
If you do not specify the number of files, 2 history files are stored on your hard disk. The maximum number of files you can specify is 270.
Note: If you edit this property in an existing project, delete the associated SPC trend files - before you run the new runtime system.
Time
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).
Note: If you edit this property in an existing project, delete the associated SPC trend files before you run the new runtime system.
Period
The period of the history file, in hh:mm:ss (hours:minutes:seconds). Alternatively, you can:
If you do not specify a period, the period defaults to Sunday (weekly).
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UNINTENDED EQUIPMENT OPERATION If you edit an SPC tag's Period property, 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. |
Note: If you edit this property in an existing project, delete the associated SPC trend files before you run the new runtime system.
Subgroup Size
The size of each subgroup. The default value for this value is 5. Valid values are 1 - 25 inclusive.
Standard 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 CitectSCADA. 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.
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 CitectSCADA. 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.
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 CitectSCADA. 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.
Extended forms fields
The following fields are implemented with extended forms (press F2).
Privilege
The privilege necessary by an operator to display the SPC data on an SPC page.
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.
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.
