Dont Stop the Presses

Any complex control system, or even a simple one for that matter, requires some degree of redundancy to minimize key failure points. Engineers at Tempe, AZ-based Automated Control Technologies (ACT) implemented redundancy in every hardware and software aspect of a paper bundling and delivery system for The New York Daily News, one of the five largest newspapers in the U.S. Since other companies had tried to provide an acceptable system without success, the approach that ACT engineers took is worth a closer look.

According to Ken Reagan, one of ACT's engineers, using coinage, or smaller package bundles of 15, 20, or 25 papers instead of normal bulk bundles of 50 or 75 papers (depending on the thickness of the paper), was a key design aspect of the system. ACT engineers performed a preliminary test of handling coinage to prove that it would work prior to accepting the extensive project. The final design ultimately involved 18 conveyor lines to deliver 1.3 million daily papers. The design specification required no single point of failure, dictating extensive system redundancy.

The process involves analyzing a manifest on a daily basis to determine bulk and coinage requirements for each truck. As each driver checks in, the system's ControlNet routes the proper number of bulk bundles directly from the presses and creates the appropriate coinage for that particular route. Tie lines provide the connection to deliver papers from the presses to the sorter. Finally, the sorter delivers the bundles to the truck.

ACT-developed software analyzes the stops for each delivery route and determines where smaller bundle sizes can be used. This avoids cutting a larger bundle and manual counting of the required number of papers by the driver, saving time and reducing the amount of wasted papers.

For redundancy, there are four programmable logic controllers (PLCs) in the system. Two are for the tie lines and split between a primary tie line. If the PLC for A breaks down the system can still run off of the PLC for B. For each line's PLC there is a back up as well. Hot backup systems automatically switch over in the event of a failure. Rockwell Software^® RSView software manually switches over "warm" backup points, such as the graphics system. The system also emplys 35 Allen-Bradley Panel View^™ operator interface terminals.

"We built in a level of redundancy with PLCs hardware-wise," says Brian Barrett, the PLC and motor control engineer on the project. "The additional redundancy we get is by using the SRM, the Software Redundancy Module, for hot back-up." For a complete plant failure, four PLCs would have to fail.

Smart Motor Control: Allen-Bradley variable-frequency drives from Rockwell Automation provide the motor control for several points in the system.

As an existing Rockwell customer, ACT was familiar with and had confidence in their products. The speed of Rockwell's ControlLogix^® platform and built-in redundancy capability made it an obvious choice. However, the Software Redundancy Modules (SRM) used for this project were serial numbers, 001, 002, and 003. This required extensive communication between Automation and ACT engineers to confirm the architecture and implement it with the leading edge SRM.

There is a number of checks and balances in the system, according to Barrett. For example, when the machine processes a bundle of a specified number of papers, a weight sensor provides feedback confirming the correct amount prior to sending the bundle to the sorter. The bundle is delivered to the truck only if it is within the tolerance of 2½ papers per bundle. The weight of the paper changes on a daily basis depending on the number of pages and inserts in the paper. Based on the page count and weight of page, the system calculates the target weight of the day's completed paper.

To guarantee that data requests are processed within a specified timeframe, ControlNet acts as the backbone in the network. The ControlNet allows communication to each of the DeviceNet controlled tie lines. A ControlNet-to-DeviceNet converter allows the data collection in real time from the tie line, including serial data from stackers and weight data. All the media for ControlNet is redundant so if a cable is disabled, the other cable is automatically used.

While built-in redundancies are extensive, the one area where ACT wanted to avoid duplication was in the programming for the SRMs. The Rockwell Automation SRMs handle the redundancy by design and reduced the code development time for all the potential problem situations. The SRMs critical contribution to the system redundancy provided the final touch to the failsafe design.

Complete Redundancy: In addition to the software redundancy in the SRMs, engineers designed in hardware redundancy in two forms: automatically implemented (hot backup) for the ControlLoqix PLCs, sorter control system and tie lines (not shown), and warm backup (manually implemented) for the graphic system PC.