Other platforms and operating systems are supported through a source code toolkit. "We provide a toolkit with example source code that can be used to port the control engine, as well as options like alarm, event, trend, web, etc., to any operating system or any platform," says Jim Desrosiers, global sales manager for ICS Triplex ISaGRAF. "Our customers can stick these engines anywhere they want to use them including a PC, a drive, or network equipment."
Open Automation and Control finds the ISaGRAF/Windows CE platform to be a better fit for process control than a PLC. "The openness of IEC 61131 and PC-based control engines lead to better solutions than PLCs. PLC I/O is not normally sufficient for serious process applications," says Turnbull.
PC-based control engines can be used with I/O from any number of vendors. Multiple communication ports allow the simultaneous use of different types of I/O, also from different vendors.
"Good algorithms and a regular execution time are essential in process control, and PLC vendors are not normally good in these areas. The IEC 61131 function block diagram language works well for many process applications," adds Turnbull. The simultaneous and interactive use of any of the five IEC 61131 languages is a key attribute of PC-based control with IEC 61131.
Open Automation and Control considered desktop PCs and industrial PCs for its applications, but both options were rejected. "Generally, a PC is not rugged enough. It could be mounted in a suitable enclosure, but this is costly and large. The TECLA CE-based controller is very compact and rugged in all aspects," concludes Turnbull.
HyRadix (www.hyradix.com) makes gas processing equipment that produces a hydrogen-rich gas from natural gas or LPG. This gas is used in fuel cells and in industrial hydrogen applications. The Des Plaines, Ill., company uses PC Worx programming software from Phoenix Contact (www.phoenixcon.com/automation).
PC Worx is an IEC 61131 programming software package designed for use with PC-based logic controllers as well as with Phoenix's Interbus controllers. HyRadix uses an embedded CE platform containing an HMI as its target control engine.
Control of analog loops in the HyRadix system is via a combination of conventional PID loops as well as other complex control algorithms. The system includes state control and process fault handling logic, and there are also significant requirements for data logging on prototype and field test units. Field test units have a requirement for remote access to enable configuration from anywhere in the world.
Based on these requirements, the best solution was PC-based control. "A Windows CE-based system with an integral HMI (HMI screens developed in Microsoft embedded Visual Basic) became a more attractive solution than a conventional PLC," says Janus Kubinski, a senior controls engineer with HyRadix. "The integration of a PLC controller with an HMI panel addresses our size and cost requirements, and the Windows CE environment allows for limited but adequate data acquisition and interconnectivity."
PCs Keep the Water Running
Automated Control Systems (www.automation-software.com), Vancouver, Wash., uses shoebox industrial PCs to control five of the largest hydroelectric facilities in the U.S. The company claims these power plants represent about 11% of the total U.S. hydroelectric capacity. ACS is installing PC-based control in seven more plants representing another 6% of hydroelectric capacity.
ACS uses more than 250 SoftPLC controllers (www.softplc.com) in these applications. Each is a Pentium-based 133-266 MHz system with 32 MB Ram and 20-32 MB of solid state flash disk. Some of the SoftPLCs have additional communication ports to allow integration of power meters, governors, and excitation systems. The systems use over 30,000 points of MTL I/O (www.mtl-inst.com) along with 96 SCADA nodes from Intellution (www.gefanucautomation.com).
"PC-based control lets us use best-of-breed components such as I/O that meets strict IEEE surge-withstand criteria," says Daniel Perrier, PE, ACS president. "We also added functionality to the relay ladder logic in the form of a Modbus Master Communication program. Even when PLCs offer Modbus communications, the nature of the proprietary hardware makes adding additional communication ports expensive or impossible."
Additional requirements included keeping the SoftPLC clock synchronized to a satellite clock for certain top-of-the-hour reporting requirements. This extension to the SoftPLC instruction set was done in Java. "The SoftPLC gives us rock-solid real-time performance and the ability to have an almost limitless programming ability in terms of functionality and flexibility," adds Perrier.
Another utility using PC-based control is water and wastewater treatment in Rock Hill, S.C. The city uses an AutomationDirect (www.automationdirect.com) system consisting of a PLC backplane, a WinPLC industrial PC that plugs into the backplane, and local and remote PLC I/O.
The wastewater system consists of 18 racks of I/O (15 with WinPLC processors), one main server running Entivity's Think and Do software, and seven PC workstations. The system has 1,600 digital inputs, 500 digital outputs, and 300 analog inputs and outputs.
The water treatment plant consists of 21 racks of I/O (18 with WinPLC processors), one main server, and three PC workstations. This system has 1,056 digital inputs, 500 digital outputs, and 352 analog inputs and outputs.
PC-based control offered two key features for these applications: flow chart programming and distributed control. "Our water plant filter tables have 30-40 pages of flow chart control, which in ladder logic would convert to several thousand lines of code. Troubleshooting is straightforward with the flow charts but would be very unwieldy with ladder logic," says Jonathan White, maintenance superintendent with Rock Hill.
"Most of our processes are spread over plant sites that can be up to 40 acres. In the water and wastewater business it makes sense to use distributed process control that is monitored by a master or server-based HMI system as opposed to a central controller," adds White.
If your application requires functionality, as outlined in Table I, then PC-based control might be a good fit. The operating systems are robust, and the newer form factors are not only rugged but are also easier to integrate into industrial control systems than desktop PCs.