PLC graphical languages speed process design and startup

PLC graphical languages speed process design and startup

A solvent extraction, drying, and calcining operation at Unocal Molycorp’s Lanthanide mine in Mountain Pass, Calif., has been rebuilt and expanded to produce high-purity neodymium oxide more efficiently. The rebuilt solvent extraction phase, along with entirely new drying and calcining facilities, is now in operation.

A 1988-era PLC (programmable logic controller), which continues to serve various refining operations at the mine, had insufficient capacity to handle the neodymium stream’s new equipment and revised operations. To supplement the old control system, Molycorp installed three Modicon TSX Quantum automation controllers from Schneider Automation Inc. Two of these new controllers are using the new Modicon Concept 2.0 graphical software package based on the IEC- 1131 standard. Linked by a Modicon Modbus Plus high-performance peer LAN, the three PLCs will control all aspects of neodymium oxide production. They will also exchange data with a site-wide FisherRosemount RS3 distributed control system. Eventually, all PLC tasks at the mine – about 3,700 points in total – will be converted to perhaps a dozen networked PLCs accessible from a single point.

Opportunity to try graphical programming

“As an experiment, we elected to program two of the PLCs using the Windows-based Concept package,” said Dave Bhame, electrical superintendent for Molycorp.

“I programmed the first controller myself using DOS-based Modicon Modsoft to meet a very short deadline. For the two remaining PLCs, I saw a perfect opportunity to put PLCs in place at the mine that used a Microsoft Windows-based editor so the electrical department could gain experience in that area,” Bhame said. “The controls industry seems to be standardizing on Windows. Graphical programming should be easier to troubleshoot, especially fault trapping.

“Kilborn International, Inc., prepared the Concept program for the solvent extraction PLC in six weeks, and we found fewer bugs in startup than we’ve normally experienced with a PLC programmed conventionally,” he said. “Drying/calcining operations, now in startup and handled by the other Concept-programmed PLC, are proceeding smoothly as well.”

The two PLCs total about 700 points, both discrete and analog. Scan times are in the range of 100 ms.

Ladder logic coupled with function blocks

“I’ve programmed in ladder logic so long that I find it difficult to think in computer languages anymore,” said Doug Engel, control systems engineer at Kilborn. “Because ladder creates a virtual electrical system of wired devices, programs will probably look no different a thousand years from now. A contact always looks like a contact in an electrical diagram.

“For the Molycorp project, Concept provided an efficient way to write IEC ladder as the base logic while inserting IEC function block diagrams — written in ladder or other languages – for complex or repetitive tasks,” Engel said.

Although a library of standard FBD blocks is included with the Concept package, Engel generally prepared his own derived blocks. These custom blocks provide the neodymium facility with a PLC control system combining the simplicity of ladder logic with much of the power and versatility of a DCS.

The solvent extraction Quantum PLC, as an example, has the following customderived blocks:

Mining industry motor start (100 copies)

Cascaded motor group start (7)

Valve control (12)

Fault trap alarm (30)

Analog hi-hi to lo-lo alarm (26)

16-bit PID input/output scaling (34)

Runtime totalizer (100)

Flow totalizer (12)

Copyright PRIMEDIA Intertec Aug 1998

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