01 January 2003
A New Breed: Is it a PLC or a PC?
It is easy to understand why in the past decade there has been a heightened interest in PC-based control for industrial automation. The PC offers powerful software tools, a floating point processor, plenty of RAM, and a graphical terminal interface.
Many engineers have looked to the PC as the best option when they are working to integrate advanced functionality such as database connectivity, Web-based interaction, analog control and simulation, and communication with third-party devices.
The problem with PC-based control has always been the control. PCs running standard operating sys tems with off-the-shelf hardware are too fragile and temperamental to provide reliable embedded industrial control.
Although some engineers use special industrial computers with hardened hardware and special operating systems, many simply decide not to implement functions that PLCs cannot easily handle.
Other times, when engineers cannot fit all their system functionality into a PLC, they cobble together a system that includes a PLC for the control portion of the code and a PC for the more advanced analog calculations, data logging, and communication.
This is why on the floor of many factories today you will see PLCs being used in conjunction with PCs for data logging, connecting to bar-code scanners, inserting information into databases, and publishing data to the Web.
However, because engineers must incorporate hardware and software from multiple vendors that was not designed to work together, these systems are often difficult to construct, troubleshoot, and maintain.
Instead, engineers can now use products that provide a hybrid mix of the PC and the PLC. This new option for control takes the best features of the PC-the processor, RAM, and powerful software-and mixes it with the reliability and ruggedness of the PLC.
They can rate at up to 50 g of shock and 5 g of vibration for mobile and vibrating environments and feature heavy industrial electromagnetic compatibility ratings for use in electrically noisy environments, as well as a temperature range of -25° to +60°C.
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They incorporate features normally found on the PC such as a floating point processor for custom calculations, an embedded interactive Web server for easy control and monitoring, removable compact flash for data logging, and multiple serial ports for third-party device communication.
Hybrid control is advantageous for engineers who need to implement applications with custom calculations, data logging, external serial device communication, database connectivity, or Web interaction such as semiconductor manufacturing, for instance.
Because some semiconductor chips produce errors at high and low temperatures, part of the manufacturing process monitors the performance of the chips under varying temperatures.
Although it is possible to purchase an off-the-shelf machine to test the chips, these machines are expensive, offer limited temperature profiles, and do not offer the ability to generate reports and view the data throughout the plant.
A hybrid system controls the temperature of the chamber using thermoelectric modules and a cool water supply. It also reads the digital lines from the chip that indicate the internal temperature.
The temperature profile easily updates either through Ethernet or by changing the flash card. To make operators comfortable with the new machine, an interface with an LCD and keypad provides the same temperature and status information as the unit it replaces.
A hybrid system also publishes a Web page so an engineer can both monitor and control the process. It connects over the Ethernet to a database program, which stores the operations information and generates a report.
Engineers in industries ranging from semiconductor to oil and gas are looking to hybrid PC/PLC products as a way to implement more functionality into their control systems.
-Todd Walter, National Instruments