Hierarchical
Control Instrumentation

Hierarchical approach results in considerable economy of material and labour during installation and also at times of expansion, relocation, operation and maintenance, elaborate Ram Deshpande & Suresh Menon.

Industrial processes range in size and complexity from small specialist requirements to huge multi-location plants. Often, plants that have proved successful at the pilot scale may expand into larger and more complex processes or may be set up in geographically distributed areas but with corporate control at headquarters. Clearly, the architecture of the control system when conceived should be amenable enough to adapt technically and economically to these different requirements.
Hierarchical approach is highly suitable to the address varied needs of a fast-changing industry. It provides a system that is adaptable for fulfilling the techno-economic conditions mentioned above. Hierarchical concept, based on specially designed interchangeable modules, drastically reduces “total cost of ownership” as compared to the traditional method of several field instruments (often of different makes) directly converging on to a common PLC or PC controller.
A mutually compatible system of modular components that can intercommunicate to suit specific process needs (small, medium or large) in a hierarchical configuration is now available. A series of sensors, indicators, transmitters, controllers, pneumatics and actuators are in modular form and are provided with the necessary electronics and “inbuilt intelligence” to make them capable of programming, inter-linking, networking, with autonomous action, indication and diagnosis. Appropriate modules can be fitted and inter-connected at the action point, using a simple and cost-effective effective protocol. This combination of sensor, display, self-controller and actuator becomes a miniature control system at each site. The control person on the spot can programme his process needs using the interface, without overloading the remote PLC programming to communicate the set points. Minor control needs of individual SPMs and equipments are thus met simply, speedily, reliably and also economically.
It is now possible for plants having numerous control points, to be programmed so as to be able to communicate with a common controller by use of a small-scale fieldbus. This results in a minor hierarchy of individual loops (with local control settings) to form a part of a higher level“Control Island”. For levels of Hierarchy refer Figure 1. For more complex requirements having a number of equipments and machinery to automate, such control islands can be treated as individual nodes, which communicate with a PLC or PC using a simple two-wire bus, i.e. AS-i or other fieldbus systems. In this manner, substantial cabling of each device — sensor or transmitter or actuator or valve — direct to main console is replaced by buses.

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