Page 237 - Mechanical Engineers' Handbook (Volume 2)

226 Data Acquisition and Display Systems

can facilitate research, for example, on what conditions lead to the best yield of a particular
product.
The ease of performing this operation is a particular advantage of relational databases.
There are some disadvantages, including overhead due to the access methods, extra space
requirements due to the creation of indexes, and costs and complexity associated with the
database management system. Indexes may add as much or more than 100% to the size of
a database. Old data must be managed and removed as with any other storage system. This
typically is via an automated program, since the structure is not as simple as just looking
for the file creation date of a file.

5.4 Using Third-Party Data Acquisition Systems
When data storage is fairly simple, it is not hard to store data in the above-mentioned
methods, but when one is using sophisticated methods of data compression, it is recom-
mended that systems that have robust implementations of those methods be used rather than
attempting to reinvent the wheel. They can be quite complex to implement reliably. Transfer
of data to those systems can occur through a variety of methods, including creation of files
that are captured by the other systems, insertion of data into standard interfaces such as OPC
or message buses, or insertion into database tables which are monitored by the other systems
(often ODBC links). Third-party systems often have software development kit (SDK) inter-
faces that allow the engineer with some programming skills to store data directly into the
system.
Process historians are optimized for storing time-based data. A technique used to provide
some relational capability to the data is the following:
• Store related data at exactly the same time stamp (time stored in the database for
when the data elements were collected).
• Treat data stored with the same time stamp as being part of the same record.
• Select a set of these ‘‘records’’ based on a time range.
• Search a variable for some attribute value (e.g., having some value or range of values).
• Provide to the display system the values of some related variable in the same record
having the same time stamp as the desired attribute variables.
This is functionally the same as performing a relational database query on a set of records
in a table, with criteria based on values in some columns.

6 DATA DISPLAY AND REPORTING

There are a variety of ways to reference and display data acquired from a sensor and stored
in suitable media. The current value can be inspected, values can be stored for inspection
later, values can be trended, alarm conditions can be detected and reported, or some output
back to the process can be performed.

6.1 Current-Value Inspection
Often, one wants to see the data as they are being collected. This can be of critical importance
in experiments which are hard or costly to repeat, allowing the researcher to react to situa-
tions as they occur. As it is collected, each data item will be called the current value for that