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English Cascade Historian™
Real-time historical data storage
The Cascade Historian is an event driven data storage program used to maintain persistent time-sequence data sets derived from process data.
Features and Benefits
In addition to providing storage the Historian also offers a historical query facility suitable for generating graphs and tabular output for export to other analysis programs. Other programs can also send data to the Cascade Historian by linking the API library and making the appropriate function calls.
The data storage mechanism within the Cascade Historian will maintain any number of simultaneous time histories, one for each configured process point.
The data for each point is maintained in one or more files, allowing for removal of stale data or offline archival of old data. These files are either numbered in increasing numerical order, or dated. When using dated files, the Cascade Historian automatically begins logging to a new file at midnight.
Data logging by event
The Cascade Historian records data by event. If no data change event occurs for a point, no data is written to disk. This saves disk space when the process point is idle, and captures even very short duration events when the point is changing. This is both more efficient and more accurate than a sampling historian. It is possible to place both a time and value deadband on each point to reduce storage for points whose values are constantly changing by insignificant amounts. The Cascade Historian will not reorder out of sequence data.
Data is stored on disk in fixed length binary records to minimize disk space, with a time resolution of nanoseconds. These files can be easily read by any custom program as well as by the Cascade Historian.
Any process may request data from the Cascade Historian. The Cascade Historian maintains a configurable in-memory cache for each point being recorded so that queries of recent data will not require disk access. If the request cannot be satisfied from the in-memory cache, then the disk files related to the point will be consulted for the data to satisfy the request. The requests can take one of the following forms:
- Raw Data - Simply returns all recorded events for the data point, reported vs. time.
- Periodic Data - Performs linear interpolation on the data to produce a data set at an even time interval. This produces the same result that a sampling historian would produce.
- Relative Interpolation - Performs interpolation of one data point against another to produce a Y vs. X data set. Interpolation is performed on Y to produce (X,Y) pairs at the times of known values of X.
The Cascade Historian has been built to allow quick addition of other forms of historical query.
Configuration
he Cascade Historian is configured through an ASCII configuration file. The file consists of a set of commands, and may contain comments. Any configuration command that is available in the configuration file may also be transmitted to the running Cascade Historian at any time. This allows the control system to reprogram the historian based on control events or on-line user configuration. The configuration command set consists of:
| add: | add a new point value manually |
| bufsize: | set the size of the in-memory cache for a point |
| deadband: | set a value or percentage deadband for a point |
| deletehistory: | permanently stop recording data for a point |
| disable: | temporarily stop recording data for a point |
| enable: | restart recording data for a point |
| filebase: | change the root file name for a point |
| flush: | write any cached data to disk |
| history: | add a new point to the historian for recording |
| unregister: | stop accepting value changes from the Cascade DataHub |
| register: | begin accepting value changes from the Cascade DataHub |
In addition to the data queries, the following commands can be transmitted at run-time to the Cascade Historian to request information:
| describe: | query configuration information for a point |
| earliest: | query the earliest known value of a point |
| latest: | query the latest known value of a point |
| historylength: | query the number of values recorded for a point between a given start and end time |
| historycount: | query the number of points configured in the Cascade Historian matching a given pattern |
| version: | query the Cascade Historian version |
The Cascade Historian is designed to run optimally with the Cascade DataHub, but this is not a requirement for proper operation. It is possible for any task to transmit information to the Cascade Historian as it becomes available.
Options
The Cascade Historian can be supplied as a complete package with graphical trend displays and configuration screens to determine log file parameters. We can also supply the Cascade Historian without a graphical interface so that it may be easily integrated into existing applications. Cogent can work with you to integrate the Historian into your current control application or we can provide you with a complete turnkey application including field device drivers and complete historical file backup facilities.