Machine Design

Cell Controllers

In some factories, computers called cell controllers provide coordination among individual workstations and computer-controlled machines. Their duties also may involve supervision of motion controllers, or the collection of manufacturing statistics from such machines.

There is no formal definition of what cell control actually is. A few years ago, most people thought of a cell controller as an all-encompassing computer that watched over an entire plant. Today, a cell controller is often just a small, rugged computer, linked to several PLCs, which doubles as an operator station. This computer frequently downloads different programs to PLCs from its hard disk, eliminating the collections of Mylar tapes normally used to reconfigure processes or assembly lines. It may also act as an information filter that passes manufacturing statistics up to a mainframe. Estimates are that computers based on a PC-compatible format account for the vast majority of cell-controller installations.

Different industries use such controllers for diverse purposes. Suppliers say, for example, that controllers installed in automotive plants generally collect diagnostic information, track parts through assembly lines, and collect quality information in processes such as welding or metal cutting. These data often are used for statistical process-control calculations that are fed back to a larger central computer.

On the other hand, cell controllers in stamping plants and food-processing facilities are likely to combine monitoring tasks with some direct machine control. A typical use is to implement self-tuning by automatically changing process input parameters after an alarm, perhaps eliminating manual adjustments.

Another common role for cell controllers is to serve files. Here, the controller contains a hard disk that holds programs for numerous CNC machines or PLCs. On command, the computer sends a new program out to a given manufacturing machine. The command can come from a machine operator or remotely from a central supervisory computer.

Cell controllers usually employ a passive backplane where cards containing a processor and peripherals are inserted. These machines are adapted to industrial environments with cooling fans, NEMA cabinets, and heat sinking. Such machines can operate in ambient temperatures as high as 60°C. Some are specially designed to work without cooling fans.

Most cell controller vendors provide software that lets the controller communicate with widely used PLCs over networks. This software generally handles communication protocols for local-area networks (LANs) from large PLC vendors. Such software also lets the controller communicate over the LAN with any device that sends ASCII characters.

When cell controllers must talk to less widely used brands of PLCs, however, some custom programming may be needed.

TAGS: Controllers
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