Jeff A. Christensen
Manager, PC Control
GE Fanuc Automation
Charlottesville, Va.
Control-systems designers have long relied on industrially hardened programmable logic controllers for factory-floor applications. They deliver reliable, real-time control in a robust package. However, as desktop personal computers advance and their costs continually decline, they look more attractive for a broad range of control applications. The changes are making it a tougher to decide between PCs and PLCs.
Although there can be many reasons to choose one control system over another, selection usually centers around throughput and scan time. Throughput is influenced by factors such as processor speed, type of I/O structure, I/O interface, efficiency of application code, and operator interfaces. By varying these components, throughput can increase or decrease.
Scan time, measured in milliseconds, is determined by
Tps = Tbs+Tis+Tle,
where Tps = predicted scan time, Tbs = base scan time, Tis = scan-impact time, and Tle = estimated logic execution time.
Base scan is the sweep time of the processor when it contains no program, no attached programmer, no configuration, and no other modules are in the system. This contributed time factor is unique to the specific type of processor. By comparison, scan-impact time is programmer communication scan time, I/O scan time, intelligent module scan time (I/O interface), and I/O interrupt scan time, while estimated logic execution is the time required to execute the logic portion of the program which consists of various instructions.
The factors of the scan-impact time that have the greatest affect on overall system throughput are usually the I/O and the intelligent modules. Depending on the I/O structure (local rack based or distributed) contributing factors to the I/O scan often vary but at least include module filter times, input sampling time, and bus-scan time. However, when comparing PC and PLC-based control systems, the I/O structure is usually the same. The factor that varies between systems becomes the intelligent module or I/O interface that connects the I/O network to the processor backplane.
PCs and PLCs offer a wide variety of I/O and communication interfaces for various types of LANs. Synchronous and asynchronous scan are examples of the variations among I/O and communication interfaces. A synchronous interface starts the I/O-device update only when the processor communicates with the interface. An asynchronous interface, on the other hand, scans the I/O devices on a continuous basis, independent of processor communication with the interface board. The comparison table represents the difference in throughput of PLCs and PCs when their interfaces are changed to accommodate the various types of I/O. The application code, number, and type of I/O are the same for the PC and PLC within each test. The factor that varies in these examples is the interface required to incorporate the specific I/O into the PLC and PC systems.
By varying the type of interface, in this case asynchronous versus synchronous, the throughput varies as well. When used with a PC-based solution, the asynchronous interface decreases throughput time, which is less than the PLC-based system. Additionally, the PC-based system uses about 50% of the PC’s processing capabilities, leaving the remaining resources for other tasks such as human-machine interfaces, spreadsheets, database manipulations, and hardware interactions. A benefit of the PC-based system is the ability to increase or decrease the load on the processor, which can improve or diminish throughput. PLC-based systems do not have this capability.
The remaining variable in the equation for predicted sweep is the estimated logic execution. This factor also varies with the type of processor in the system as well as the complexity or types of instructions used within the application code. Instruction execution speed essentially remains constant if the processor is unchanged. The execution-speed table highlights the various types of processors including a PC processor, a midsize PLC processor, and a high-end PLC processor. Running under various instructions and execution speeds, the PC processor routinely falls between high-end and medium-sized PLC processors.
When determining control-system speed, accuracy, and repeatability, consider key factors such as I/O architecture, processor capabilities, and interface connections. By varying these factors, throughput time for PLC and PC systems can increase or decrease. These data show that a PC can provide faster throughput than a PLC and that system configuration is the key factor.
In addition to speed, accuracy, and repeatability, several other factors are critical to the selection process. Consider, for example, application requirements such as networking, data handling, human-machine interfaces, and I/O configurations. While PCs and PLCs are both proven means of control, one method may be a better fit for a particular application than the other. Also consider a hybrid architecture, one that combines PCs and PLCs to exploit the strengths of both.