For a copy of the National Instruments Customer Solution pdf document for this project, viewable through Acrobat Reader, click here.

The Challenge Monitoring, displaying, recording and reporting data being plotted by a multi-pen Chessell strip chart recorder.

The Solution Developing a LabVIEW based SCADA system that uses the NI Modbus server to acquire data from the recorder serial port.

Background A need to improve a critical Temperature and Relative Humidity (RH) monitoring system in a production QA Lab was identified by Union Carbide Corporation, Bound Brook, NJ during September, 1997. RTD probes in several ovens, a density column bath, and room air temperature and humidity sensors were all connected to a Eurotherm Chessell paper strip chart recorder and plotted with a three-color ink cartridge.

Paper jams had become a regular occurrence because of variations in paper consistency and the age of the Chessell instrument. This made it difficult to monitor the inputs without interruption and loss of data. When a critical variable went out-of-spec a significant amount of time could pass before the error was discovered and corrected. Then, a lengthy corrective action report and investigation had to be written. Also, archiving the paper presented a problem because the ink faded over time, making the charts difficult or impossible to read. Sensor data was not easily available for use in printed reports or for plotting on a computer.

System Overview The main objectives for the data acquisition system were: 1) bypass the paper charting system, 2) reduce the number of corrective action reports through improved monitoring, and 3) make the storage and retrieval of data an easier task. To achieve these objectives, we used the Chessell chart recorder (which was designed to communicate via a Modbus serial port) as the sensor conditioning and digitizing component of the system. A 100MHZ, 16MB, 2.0GB desktop computer was also available in the laboratory. National Instruments' Modicon Modbus RTU Industrial Automation (IA) server program was purchased and installed on the computer to support the serial port link to the Chessell Recorder. This server made all of the sensor data available to LabVIEW. Finally, Process Automation Corporation's SCADA Tool Kit For LabVIEW was adapted to perform the following tasks: 1) data retrieval from the IA Server into it's real time data base, 2) alarm monitoring and reporting, 3) automatic data value/time spread sheet report generation, 4) historical data compression and archival, and 4) data trend plotting and printing.

Several variables can be selected one at a time or in groups for viewing in a multi-color, graphical, trend plot. The greatest benefit comes with the ability to view one or more data trends in spans from minutes up to one year on screen in a clear and concise manner. Out-of-spec readings are captured and stored in a separate alarm table which can be imported to Excel or Word. In addition, a tabular report of a user defined time span can be automatically generated for all or some of the variables, as necessary. Archiving can be performed by the user within the program or by using the Windows 95 file manager. Data is automatically compressed into 24 hour files (which are named by date) and stored in directories named after the variable being archived. The compressed files can be retrieved and converted to spread sheet data by the SCADA Tool Kit. The alarm spread sheet file details out-of-spec readings by logging the time a variable went out-of-spec, and when it came back into spec.

Results This upgrade resulted in dramatically improved data acquisition and analysis, made out-of-spec readings easier to identify, and helped to reduce the number of corrective action reports written for the lab. Union Carbide personnel were pleased beyond their initial expectations. This improvement will help Union Carbide meet ever increasing customer demands for quality and reliability.