Case Studies



Executive Summary:

Kredit Automation was contracted to retrofit the PLC and HMI on a line that bundles tubes coming off the fabrication line. (Tubes are made by roll forming cut-to-length steel sections into a tube and welding the seam. Various chemical treatment and coating steps including intense UV irradiation result in tubes fit for various harsh conditions and duty.) This project was done to replace aging equipment for which reliability and parts availability and a closed proprietary HMI were increasing concerns. Those goals were accomplished and the customer was further pleased by the usability of the new program for the line in the other mill.

Project Details

Tube bundling sounds mundane but when a single line is responsible for handling different tube sizes and producing bundles of varying geometry (and doing so at a consistently rapid rate) the control challenges are more subtle than they first appear. There is more I/O—several hundred points—than one might imagine. A fairly large and fast PLC is needed to run this line.

The customer was concerned that the PLC (an older model from a reputable European company) was aging and that software and spare parts were becoming an increasing concern and risk. The operator interface was of particular concern because it was non-COTS and was a complete black box. They had just one spare and the spare didn’t have the very latest program. There was no way to duplicate the HMI or upgrade the program in the spare. The decision the customer had to make was between converting the program to the European company’s latest PLC and creating a completely new HMI or switching to a Rockwell (Allen-Bradley) PLC and HMI. While the former approach would have been feasible, the Rockwell direction was chosen by this customer. Their plant was standardized on the SLC-500 family so the Ethernet-based SLC-5 was used along with FactoryTalk View SE for the HMI.

Constraints regarding the geometry of the bundles (e.g. square vs. hexagonal) and other matters were analyzed and it was realized that these constraints could be effectively handled with classic database design. Microsoft Access was used (part of the Microsoft Office product). Changes and modifications could be accomplished much more easily using Access than having to modify PLC code and HMI screens. The figure below shows how the table relationship tool in Access provides an intuitive visual design tool that enables these relationships to be set up to the meet the requirements. Trying to do this in PLC code would be much more complex and hard to modify. And since it is Microsoft Office it is a COTS component.

Because both FactoryTalk SE and Access use VBA as their macro language, Visual Basic code was used to set up a system whereby the HMI screens and Access forms were part of one unified HMI, both of which communicate with the PLC. The database was a recipe and order system that was simple and integrated into the control system. It is emphasized that this system can be duplicated simply by installing FactoryTalk and Office…no customization of the PC whatsoever is required. Just loading the software and placing the application files on the new computer will allow it to run out of the box.

The PLC program was structured and commented to make it amenable to being modified and upgraded. An upgrade to the Logix family is planned for next year. Modular design techniques—a KAC specialty—made it possible to reuse large parts of this program on another tube line in the plant. The lines were different enough that an ordinary program would have required extensive work and probably ended up a kludge but by carefully structuring the program it was possible to reuse modules as-is and create another clean program for the other mill.

Because the old prints and documentation were incomplete and in a foreign language, and because the program was in an arcane assembly language format with no tags or descriptors, KAC applied an innovative technique to study the behavior of the program. First the I/O was compiled and translated to a point where a decent I/O list existed. Then the I/O cards were videotaped at high speed while the line ran. The patterns and behavior of the LED lights were studied to ascertain basic things about the sequences. Then the program was written using this insight. While this was very tedious and time consuming, it allowed the project to move forward.
As is standard practice in these types of projects, KAC omitted any obsolete or unused aspects or features or equipment from the line when designing the new program and operator interface screens. Only the equipment and process steps that would be used going forward were included. While this was most of the original system, it did allow some trimming and cleanup to take place from the standpoint of the customer. In particular, the HMI (including the integrated database entry and selection forms) were much better suited for their needs than the old HMI, not to mention larger, clearer and more user-friendly. The customer gained a COTS system with a new, well-structured, highly commented program and a dramatically improved operator interface.

Case Studies

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