Manufacturing AUTOMATION

Features Factory
PLC makeover


March 15, 2007
By Michael Codrington

Topics

Many owners of highly automated systems encounter issues when it comes to the maintenance and support of their legacy equipment. In the world of industrial automation, such issues typically include lost production, difficulty in the acquisition of spare parts, and the need to train staff on these older systems.

PLCs, often at the core of an automated system, represent a key concern as they age because of the costs associated with parts and training. But migrating to new PLC platforms involves a great deal of complexity to successfully implement. Allen-Bradley PLC-3s were commonly deployed in control systems right up to the 1990s and, while the PLC-3s are fairly sturdy workhorses, by now it’s time for many of these systems to be replaced.

An issue with Allen-Bradley PLC-3 migration is that the code cannot readily be converted using automation. Conversion services are available that produce logic that may run on other Allen-Bradley PLCs, but the resultant code requires a great deal of work to debug it and manually create the portions that do not automatically convert.

Giffels, a consulting firm that provides control system engineering expertise, has successfully carried out migration projects to upgrade PLC-3s to both PLC-5 and ControlLogix platforms. Recently, two processors at a major automotive manufacturer were converted to PLC-5s, and three PLC-3s at the Liquor Control Board of Ontario (LCBO) were replaced with ControlLogix platforms. The automotive implementation included the addition of two HMI stations for supervisory control, and the LCBO implementation included necessary revisions to code in supervisory computers.

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Automotive
A large automotive manufacture in the greater Toronto area (GTA) uses PLCs to control and monitor its production facilities. The manufacturer was using PLC-3s to control two preparation processes within a paint shop, with a hard-wired panel serving as the main operator interface. The company decided it was time to update its PLCs because it was becoming costly to replace parts. It also wanted to increase the amount of process information available to operators and plant-wide supervisory systems.

The switchover was not without challenges. The automaker had a heavy mix of analogue I/O, heavy proportional-integral-derivative (PID) – the logic that is used to maintain a variable at a certain rate – and a high number of PLC-3 rack I/O channels, which required rewiring to suit the fewer available I/O channels in the PLC-5 line. The relatively small footprint to mount replacement hardware, and the few windows available for testing, were additional challenges.

The team decided not to employ automated conversion utilities because the heavy degree of analogue code indicated it would not convert well. However, Giffels developed software to automatically generate PLC code for a significant amount of the discrete I/O, thereby reducing human error and debugging time, as well as ensuring consistent code throughout the PLCs.

Extensive effort was expended to ensure that all hardware and software conformed to corporate standards, and that personnel at the conversion site and other locations were familiar with the hardware deployed, and with the layout and design of all code.

Once the replacement PLCs and HMIs were in place, the old PLC-3s and the operator interface panel were removed. Building on this new system, additional instrumentation was installed and enhanced functionality programmed into the system to improve process control and monitoring. New control schemes were later added to minimize material and energy usage.

These improvements reduced raw material costs and improved the final paint quality on the vehicles.

Liquor Control Board of Ontario
Driven by similar legacy issues, the LCBO’s Durham Regional Logistics Facility in Whitby, Ont., initiated a project to replace PLC-3s controlling an extensive conveyance system. Most of the wines and spirits consumed in the GTA are routed through this facility, with tens of thousands of cases shipped to numerous LCBO outlets in the GTA each day.

The LCBO turned to Giffels to replace the PLC-3s and a Pyramid Integrator, and to make necessary modifications to a Digital Equipment Corporation (DEC) supervisory computer.

Allen-Bradley ControlLogix, using Ethernet and remote I/O communications, was selected as the replacement platform. The project was extensive in scope, ranging from preliminary design through to detailed design, implementation, testing, documentation and training. A staged approach was employed to minimize risk.

Many factors had to be considered when designing and implementing the new system. It had to be compatible with the existing operating system, software and hardware, and have the capacity for future expansion. Early stage testing was required to provide simulation of I/O and messaging, as well as normal/abnormal processing conditions. During implementation testing, the system had to include rapid fall-back procedures that would allow users to revert to the old systems. The LCBO also wanted a transparent system so that the revised system looked identical on the users’ screens. Other requirements included secure network access to the PLCs; careful attention in the handling of subroutines and indirect addressing, which had been used heavily in the original programming; and extremely high annual uptime requirements. Lastly, the new system had to be something that could be implemented without impacting production throughput.

Solution
While the PLC-3-based control systems for the LCBO were sophisticated and complex, many of their components stemmed from the 1980s and, as such, significant innovation was required. During the ControlLogix implementation, an intermediate PLC-5 relaying data “packets” was deployed to temporarily bridge legacy software with the latest hardware. In addition, the team deployed designs that avoided “re-addressing” I/O. As a result, the LCBO did not have to be concerned about changes to the labelling of field devices and wiring.

The PLC-5 was configured with proxy files to mimic the PLC-3 data structures, and it served to translate the PCCC format – the old Allen-Bradley communication protocol – to the latest CIP protocol, and vice versa. An extensive data packet communication scheme was required for positive verification of data arrival at final destination. Coding within the DEC OpenVMS machine (in c language) was approached in such a way as to minimize changes and risk. PLC addresses in existing code were re-directed to and from PLC-5 locations, making changes transparent to the calling routines, which had initiated communication to the PLCs.

Giffels used hardware to permit live switching of I/O, enabling rapid switching to and from the new platform for testing purposes. The team also created code to emulate obsolete program calls. They used automation, where possible, for generating code and for transferring data between old and new systems, thereby providing the highest levels of accuracy.

The LCBO has since upgraded the supervisory computer to be compatible with ControlLogix, and the PLC-5 will soon be eliminated.

Results
In both applications, Giffels developed detailed plans and worked extensively to deliver solutions that met the automaker’s and the LCBO’s implementation requirements, as well as their cost, operational efficiency and reliability objectives.

There was no negative impact on production. Each facility operated, uninterrupted, on a five-day, 24-hour basis during implementation. Since the migration, key benefits include the provision of additional process data and alarm conditions to operators, and increased availability of process data to supervisory systems for reporting and historical analysis purposes. The team was able to preserve functionality, including interaction with controlling/indicating I/O, operator interfaces and related computer systems, thereby providing smooth, reliable operation of the systems. The team also provided improved, secure access to the PLCs using commercial, off-the-shelf technology, facilitating more rapid and mobile system troubleshooting by support personnel. The clients now have an enhanced programming and troubleshooting environment in the RSLogix programming suite, which they use to carry out further optimization and to improve throughput.

For both the automaker and the LCBO, maintenance, spare parts and downtime costs have been reduced by eliminating the need for expensive, obsolete hardware. As well, a platform has been provided in each case that is consistent with each company’s long-term migration plans.

Additional benefits are found in the enhanced development and troubleshooting environment provided by more powerful development software, and in the elimination of the need to maintain specially trained skill sets associated with the older PLC-3 and Pyramid Integrator platforms.


Michael Codrington is a senior engineer, control systems, with Giffels Associates Limited, based in Toronto.