E-Zine July 2012
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In the figure, the primary and secondary control loops are located within the magnifying glass. The secondary control loop is the flow controller regulating fuel flow and is located in the fuel control valve positioner. The primary controller controls the steam pressure and is located in the steam pressure transmitter.
In this example, all of the control loops are conventional – that is they can be implemented in various ways, such as in single loop pneumatic, electronic, and digital controllers, and distributed control systems. However, in this case, Field Control is implemented with FOUNDATION™ Fieldbus and its function block capability. Field Control located in the field instruments and actuators lowers the cost of the installation by eliminating the need for a multifunction controller for the control loops implemented in the field devices. However, there may be a distributed control system (DCS) installed, perhaps for backup, safety, and display functionality.
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Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control