I don’t think I would bet the farm on a PC controlling a large plant, even if you found a decent real-time operating system. I designed a system about sixteen years ago to control a six degree-of-freedom space simulator and based the operator interface on a ‘386 PC running a real-time Unix kernel with X-Windows. The customer really wanted Microsoft Windows but it wasn’t stable enough or real-time capable. Maybe today I would consider Windows Embedded, but only if someone twisted my arm real hard.
All the I/O (several hundred points but not the thousands you need) was monitored by the PC, and the PC calculated the driving functions for the hydraulic actuators in real time, sending the results in analog form via D/A converters to the servo valves. The valves could have accepted digital position commands through an RS-485 distributed serial connection, but this would not have been fast enough. We needed the actuators to move in synchronism, so all the D/A outputs were updated simultaneously. I kept the PC out of the position control loops too, using analog position and velocity sensors on each hydraulic actuator as negative feedback to the D/A position commands. The analog position signals were also digitized by high-resolution A/D converters for the operator display and recording purposes. The A/Ds were fast enough to handle real-time digital feedback control. The PC wasn’t. Today you can buy a PLC with PID instructions built in, so I would probably go that way now.
Today’s PCs are about five hundred times faster than what I had to work with in 1990. It is probably possible to “upgrade” this system to “all digital” control using a dual-core Pentium, but what real-time OS would you rely on? On the other hand, PLCs are made for this kind of task.
For a refrigeration plant, I think a distributed network of PLCs would be appropriate. You can still use one or more PCs as monitoring stations and to send commands for setpoints. But I think I would ask the vendor for an embedded PC, not connected to any external network, even through a firewall, to supervise the whole plant. And leave the real-time controls to the PLCs. Make sure you have a really good PLC programming team on board too.Which is better for controlling an Industrial refrigeration plant, a PC computer or a PLC?
Simple answer is PLC. A PC is not stable enough. However PC's can be used for SCADA so utilizing the reliability of a PLC and the flexibility of a PC.
plc, easier programming and cheaper
which is better to drive to the store, a moped or a tandem axle, 6X6 monster truck? they both will do the same thing, only one is more feasable. a pc has more than enough computing power to handle the operation of the equipment, but im not aware of a mainstream interface for adapting the computer to the equipment. a PLC (programmable logic controller) is a device that is a self contained computer that actually has the line load connections built right onto it in the form of a terminal block. I dont see the need to replace a plc with a pc. if you have 100 pieces of equipment and each one has its own plc and one plc goes bad, all your other equipment will still run. if you have 100 pieces of equipment connected to one pc and the pc goes out, what happens? redundancy is safe.
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