Wednesday, November 17, 2010

PLC Hybrid Controllers

Skid mounted process equipment, machine, and robot builders often need discrete, analog and motion control functionality for the systems they build. This meant figuring out how to integrate several task-specific controllers to get the job done until recently. The means toward that end have changed, there are hybrid controllers now available that perform all those control functions within one integrated platform.

It defines hybrid controllers as capable of at least two of three general types of control: analog (process), discrete (on/off), and motion. Hybrid controller differ from special purpose controllers in that key only perform one type of control. Special purpose controllers typically include small PLCs (discrete), motion only controllers, and multi loop PID controllers (analog).

Many machine builders recall a time not long ago when all controllers were special purpose. PLCs were not invented until well into the 1960s, and vendor spent decades perfecting and refining on/off control with PLCs. It was not until the 1990s that most PLC vendors began to add motion control and process control functions to PLCs.

Single and multi loop controllers have been used as long as loop controllers, but the addition of other control functions is a fairly recent phenomenon. Motion control vendor’s first added discrete control, and some motion controllers now can also execute analog process control functions.

Virtually every PLC vendor offers hybrid control technology capable of analog, discrete and motion control. Most loop control suppliers offer products that also can perform discrete control, but motion is not addressed with this class of products. Motion controllers that perform both motion control and discrete control are now common and it is also possible to find motion controllers with some level of analog process control capability. Controller selection is now driven by the specific characteristics of each application, not by the products characteristics.


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