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E-Zine June 2012FOUNDATION™ Fieldbus allows control loops to be located in the field instruments and final control elements. The function blocks for processing process variables and for performing control functions are modeled after those used in distributed control systems, and may be used in much the same ways. When the control loop is closed completely using only function blocks resident in the field devices, no control calculations are required in the multifunction control processor of a typical control system. However, not all control loops are appropriate for execution in field devices. For example, loops requiring significant data processing capability, such as a BTU controller requiring steam table look-ups, may require implementation in a multifunction control processor. Field Control algorithms are generally designed to implement feedback, feedforward, and cascade control loops. Complex interactive control loops are often are inappropriate for Field Control. Model reference controls and most multivariable matrix controllers are usually too computationally complex for Field Control as well. Field Control function blocks can be used to implement feedback loop controls. When cascade (master/slave) loop control is used, the master (primary) loop typically is typically located in its process variable transmitter. The slave (secondary) controller is typically located in its final control element, usually a control valve positioner. Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control ISSN 1538-5280 |
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