When PLCs Aren't Enough

PC-Based Systems Can Offer Distinct Advantages

A decision to use a web server in a PC-Based system--instead of communicating over a local area network--makes data sharing with multiple PCs virtually free.

I've spent years designing, commissioning, and supporting control systems on a variety of machines and systems. I have come to understand that there is no single solution to every problem.

For many machine builders and their customers, PLCs are the only solution. PLCs are reliable, easy-to-use and maintain, and ladder logic programming is easily learned and understood by controls engineers and maintenance personnel.

The movement toward open systems, however, shows there are choices that can provide benefits unavailable in the PLC world. This is due in part to the nature of open technology. A PC gives you the choice of many types of network interface cards (NIC), digital-analog converter cards, I/O cards, serial ports and other add-in cards to tailor performance. You are not limited to certain manufacturers or to specific part numbers.

 I can purchase 10-MB, 100-MB or 1-GB NICs with a choice of Cat5, coaxial or fiber. This lets me configure a system for performance (1-GB fiber) or price (10-MB Cat5). This choice applies to virtually every PC component as well.

I've found that PCs are more effective where the equipment being controlled uses the same or similar processes in different projects. While PLCs are getting better at code re-use, it is still much easier with PCs. This flexibility allows a programmer to more easily develop very robust code, complete with diagnostics, data storage, and information screens to assist with commissioning and troubleshooting.

In the material handling industry, many elements of a system, such as sorters and mergers, form standard building blocks that are often repeated on subsequent systems, just deployed in different ways. Given the right software and a PC with enough memory and hard drive space, one can create a generic sorter program that's usable on any project in every possible configuration.

The sorter is configured by setting code parameters that define its operation. The end result is that I never have to program the most complicated parts of each system. The code is proven, startup is quicker, and I can build in features that I otherwise would never have time to develop specifically for a given project.

In a material handling system, where product can be processed at a rate of 200 cartons/min. in configurations that can span up to 500,000 sq.ft. in a single building, it might be minutes before an operator sees a problem. Although basic troubleshooting can be accomplished by looking at the state of the system and at the input and output devices, it is much more effective to look at the status of the last 1,000 cartons as they passed through the system. This root-cause analysis provided by the advanced diagnostics built into the controls software is the only effective method of preventing problems from reoccurring.

One of the most overlooked assets of a PC-based controls system is a monitor. On small systems, a fully featured human-machine interface (HMI) can be costly. On larger systems, the cost of HMI development may not be prohibitive, but the cost of placing HMI stations throughout a large building for the operators might prove costly.

The inherent networking capability of PCs makes it easy to inexpensively propagate information to as many workstation screens as needed. In addition, using a web server in a PC-based system,instead of a local area network,makes data sharing with multiple PCs virtually free.

In material handling, moving data can be more demanding than moving material. That's because a control system must monitor carton movement through many zones, facing a decision point each time a carton passes a bar code or RFID scanner.

Modern systems might support 30 or more scanners and the movement of well over 100,000 cartons/day. A material-handling system server must be able to hold and pass along large amounts of information very quickly. Cartons won't divert at the proper locations if the data processing is too slow. A PC-based system that can store the information on a local hard drive has a clear advantage in data storage and decision-making speed vs. a system where decisions are made remotely and passed via high-speed or serial messaging.

Gary Cash is vice president of controls engineering for FKI Logistex Automation Division, a designer and supplier of material handling systems for distribution center operations. He can be reached at gary.cash@fkilogistex.com

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