The Role of Subsystems in Industrial ControlApril 25, 2008 By: JJ Thiara
Automation professionals favor complete end-to-end solutions. But to meet the unique requirements of specific applications, engineers turn to tailored subsystems, which optimize the overall system, reduce commissioning time, and cut costs.
A Question of Perspective
Like the human body, an industrial control system can be broken down into a series of processes, each performed by functionally related components working together to form a complex whole. A layman may walk onto a plant floor and see complete systems at work, but the automation professional takes a step closer to see the individual components that make up the big picture and evaluates how each fits into the process and addresses the unique requirements of the application.
The development of industrial control systems is driven by the needs of large industry segments known as vertical markets (e.g., material handling and food and beverage industries). As you'll see, complete solutions for these markets are essentially a series of specialized components, or subsystems, each geared toward an individual process.
A Case in Point
Material handling OEMs strive to provide conveyors that maximize throughput, and they continue to search for control solutions that achieve this goal simply and cost-effectively. Because different conveyors require varying levels of industrial control, a single solution may not include all of the desired functionality for one installation but might be overkill for another–albeit similar–application. As a result, solutions for specific areas have been developed that can be integrated to create an overall warehouse solution; put simply, a full warehouse control solution, comprising a set of complete subsystems. This is most evident in the control systems of accumulation conveyors.
An accumulation conveyor is one that controls the flow of the materials on it to optimize throughput while preventing collisions between the packages being conveyed. The control system achieves this by driving and/or stopping multiple independent sections called zones. Zone control logic dictates how each zone behaves, based on feedback from the other zones.
Traditionally, zone control systems consisted of standard photoelectric sensors hardwired to a PLC, where the necessary logic functions were carried out. But complete solutions have evolved from the traditional approach and now consist of specialized components that allow the zone control logic to be moved out onto the plant floor while allowing for simple plug-and-play installation and interoperability.
Accumulation conveyor zone control solutions can be classified as either basic or complex. Basic zone control manages only accumulation lanes, have no merge or divert sections, and do not perform information gathering. Complex zone control solutions include merge and divert conveyor sections and gather information. Basic and complex zone solutions can be applied as needed for each conveyor section, working together to form a complete control system for the entire conveyor.
In the grand scheme, interacting conveyor control subsystems form the foundation of a material-handling control system that runs the plant floor. This complete plant floor architecture is a combination of conveyor zone control, centralized controllers (e.g., PLCs, PCs), and networks (device- and control-level), all of which governs and optimizes operations. The modular nature of the subsystem approach speeds the development cycle for OEMs and allows plants to get their conveyor systems up and running faster, saving valuable commissioning time.
Exercising Complete Control
Complete solutions, optimized with well-chosen subsystems, not only address the challenges associated with a specific application or industry but also make it easier and less expensive for the user to deploy the solutions. Driven by the needs of larger markets, the solutions ultimately benefit every market to which the technologies are applied by setting the benchmark for improving industry processes while reducing total cost of ownership.
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