The skinny on PC-based control systems

May 7, 2007
In this column, Senior Tech Editor Dan Hebert, PE, takes a look at why a process skid manufacturer thinks PC-based control makes sense for its—and other—process applications.
A year ago in this column we looked at how PC-based control systems are infiltrating process plants by stealth through plant machinery ranging from extruders to packaging equipment to inspection systems. Process skids for water treatment, for air handling and for specialized chemical processes like fermentation are also essential elements of your plants. In this column, we will look why process skid manufacturer Custom-Flo thinks PC-based control makes sense for its—and other—process applications.

Custom-Flo builds process skids for water distribution applications such as hot- and chilled-water pump skids. It also supplies chilled and hot water utility-plant controls, thermal storage and casting-plant process systems, including cooling towers, fluid coolers and chillers, associated pumps, heat exchangers and control valves, injection-machine cooling systems and heating and cooling process skids for food processing lines.

The company uses a variety of HMI, controller and I/O systems with PC-based control supplied by Advantech’s eAutomation Group. Custom-Flo vice president Bruce Corso, PE, describes the configuration the company is aiming for. “We want to use an HMI with SCADA software running on Windows CE, control running in a programmable automation controller (PAC), executing IEC 61131-3-based software and I/O integral to the PAC box. The PAC will talk Modbus to the HMI and have a slave Modbus port to talk to other equipment. The HMI software will be web-enabled.”

Corso names the advantages of PC-based control. “Cost and complexity are lower. A lot of the decreased complexity is based on IEC 61131-3 software capabilities. An advantage also exists in the area of interconnectivity. This ranges from OPC connectivity to embedding a C++ program that sends data directly to an SQL database. If the application requires more than just running in a self-contained environment, there will be less complexity with a PC-based platform,” he says, adding that flexibility, capability and interoperability are greater on a PC-based platform. “The same application can be ported to various hardware platforms to serve the needs of a given project with little modification.”

If the right IEC 61131-3 programming software is used, applications can be ported to various products within one vendor’s family or to products of a totally different vendor.

Web connectivity is a big plus for PCs. “Coupling a PC with the right software can offer web-enabled capability without adding more hardware,” he reports.

OPC capability allows a robust set of options for interconnectivity to foreign systems. “Integration of a new system into a legacy system that may consist of several other manufacturers’ systems and data networks is easier with OPC. Connecting a PLC to other vendor systems can be onerous,” observes Corso.

The final benefit, says Corso, is the ability to run other applications concurrently in the PC. 

Corso names two challenges associated with PC-based control: “Customer acceptance is an issue in some cases, and the cost of OPC driver development is a hurdle.”

PC-based controls do have special needs says Mike Berryman, Advantech’s manager of eAutomation and engineering. “All hardware should be industrially rated and capable of surviving heat, vibration and dirty environments. Fanless and spindle-free PC platforms are preferred because fans and rotating media are the leading failure points. I/O systems talk to the PC via popular bus technologies and variations of Ethernet. PC-based control software runs on operating systems ranging from Windows to Linux to even DOS. Most users desire an IEC 61131-3 programming environment with real-time kernel support,” he says.

PCs are most economical for applications with high computational requirements, such as a large number of PID loops or advanced and/or batch control, because PCs provide the most processing power per dollar, says Berryman.  They also offer the most flexibility when integrating control systems into MES and ERP systems.

The main problem with PC-based control, he says, is resisting the temptation to follow the endless upgrade path. “PC hardware vendors provide a new CPU per month and operating system vendors send out patches almost daily. These ‘upgrade opportunities’ can challenge stable and successful PC-based control applications.”