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Distributed networks bring control closer

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SYSTEM architectures founded on programmable logic controllers (PLCs) have long been the preferred method of delivering plant-wide control. Over 30 years ago, electronic logic controllers – the forerunners of today’s PLC – first proved their superiority in providing speed, reliability and diagnostics against legacy relay logic. Since then, PLCs have continued to offer manufacturers fresh and more viable means of controlling and monitoring their plants.

In recent years, improvements in process control and networking technology have led to operators adopting more distributed approaches to control, in place of traditional centralised PLC control architectures. Distributed control systems can dramatically reduce installation and maintenance costs, and offer improved installation flexibility and troubleshooting capabilities, when compared with traditional centralised control architectures.

Smaller controller formats are the essential element of more distributed control architectures. Each “small” controller is made responsible for a machine module or production element – such as filling or packaging – of the complete manufacturing process. These, in turn, are networked together and usually overseen by a larger supervisory controller that manages the total operation.

The demands of industry are driving plant floor devices themselves to be increasingly produced with built-in intelligence – small controllers that are dedicated to unique and often specific types of control routines and processes.

These localised controllers are underpinning the next stage in the distributed control topology, bringing control even closer to plant floor machinery and affording system designers unprecedented flexibility. The advent of such small PLCs, and advancements in distributed control networks and communication technologies, has provided system designers with an exciting new palette of flexible solutions to select from.

To fully appreciate the benefits of this next stage in decentralised production control, it is important to understand the origins of modern control systems.

Centralised monsters

WHILE the relay-based control systems of three decades ago offered foolproof maintenance and inherent localisation of failures – usually one relay panel per machine – the effectiveness of such control systems was always limited. This was due primarily to their inability to store or record process information and lack of system expandability.

Occupying large amounts of panel space, relay-based control systems required labour-intensive installation, commissioning and maintenance, and not surprisingly, were increasingly replaced by the centralised electronic logic controller in the early 1970s: the PLC.

The earliest PLCs were centralised monsters, often requiring large cubicles to house processors and numerous I/O points, all wired back to racks forming the hub of plant control cabling. These were connected to a solitary PLC, which controlled the plant. Operator consoles with rows of switches, push buttons and meters were located at a central location on the factory floor.

While more modern centralised PLCs offer reduced size and much increased processing power when compared with relay-based systems, they still require labour-intensive I/O wiring and programming.

In a pure centralised PLC architecture without redundancy, a controller failure can lead to a temporary production shutdown.

This loss of productivity is unacceptable in today’s competitive environment, especially when the implementation of a distributed control system can localise the impact of this fault, minimising downtime.

Distributing processing power

SYSTEM designers are increasingly opting for distributed control system architectures, where a number of “decentralised” PLCs are distributed throughout the plant.

Located in close proximity to the associated process on the factory floor, or within the production module itself, these decentralised PLCs are connected to a supervisory PLC via a fieldbus communications cable. The fieldbus connection allows high-speed communication between the central PLC and decentralised PLCs on the distributed network.

The processing of real-time control algorithms is minimised due to reduced burden of the supervisory PLC. Such networks are made viable by the proliferation of proprietary and non-proprietary fieldbus communications. These provide simplified control structures, improved diagnostic capabilities and offer a more efficient means of data transfer than legacy hardwired I/O cabling.

Delivering decision making closer to the machine, a distributed control system improves reaction to high-speed production events when compared with its centralised counterpart, communicating with the main processor only for control and diagnostics on high-level issues.

In the event of a main processor failure, or a failure of a decentralised control element, a distributed control system can permit a predetermined and predictable response. Depending on the failure mode, the bulk of the production process may continue unabated while localising the faulty area, initiate sequential shutdown of machine module(s), or execute a complete but controlled production shutdown.

Decentralised architectures permit a dramatic reduction in labour and material costs associated with wiring. Cable runs are shortened in a distributed control system, as field sensor and actuator connections are localised to the distributed I/O block, while communication between machine-based PLC and supervisory PLC is achieved through a single bus connection.

The next stage in the evolution – using distributed PLCs – leverages this advantage even further. Distributing the processing power to remote-mounted controllers ultimately reduces the functionality requirements (i.e. size, speed and capacity) of the main PLC, and provides users with unprecedented system design flexibility.

Integrated intelligence

THE introduction of communication networks has seen control move toward distributed PLC units. Users now have the choice of multiple-vendor solutions that work within the distributed control architecture.

Controllers that combine both conventional sequential PLC control and “motion control” all in a single compact platform, provide system designers with entirely new levels of development flexibility, efficiency and overall system cost-savings.

SEW-Eurodrive’s new PLC/motion controller series, the Movi-PLC, is an ideal case-in-point. It permits new levels of control system decentralisation, along with seamless connectivity between the electronic drive controller and the PLC.

Specifically designed to support members of the SEW-Eurodrive drive inverter set, the Movi-PLC product family boasts integrated PLC control and “motion control”. Housed in its own enclosure or slotting into the drive system’s “option slot”, the slim-line controller streamlines total system development, reducing wiring between PLC and electronic drive, resulting in saving of panel space.

Often, many inverter-driven control applications require a dedicated PLC system and involve time-consuming drive-to-PLC interfacing, plus additional programming and panel space allocation for the dedicated drive/motion PLC.

Control code development was all bespoke - a laborious process undertaken machine-by-machine and drive-by-drive. The universal Movi-PLC completely reverses this process. It simplifies the need for cumbersome “motion” programming, as it is provided with a wide selection of pre-written and tested “function blocks” – specifically designed for immediate use with the SEW-Eurodrive electronic drive family. Similarly, where multiple drive controllers need to be addressed, the Movi-PLC allows connectivity and simultaneous control of up to 12 individual drives.

Distributed controllers, such as the Movi-PLC, also deliver increased security to OEMs and system integrators. A centralised control system typically provides a single point from which one person can gain access to the entire control algorithm – an effective “open door” to OEM’s intellectual property.

Relinquishing complete access to one or a few pieces of hardware within such systems can be considered an unnecessary risk. By contrast, the Movi-PLC allows the OEM not only to program and test the entire machine module before it is shipped as part of a larger skid-mounted package, but to lock down intellectual property – a major deterrent to competitors targeting reverse-engineering strategies and eliminating the “fiddle factor” from production floor personnel.

Pick the programming

PROGRAMMING a centralised PLC involves writing lists of code in a variety of graphical and textual programming languages. This process can be time-consuming and costly. The vendor-specific programming languages used in industry today can mean that dedicated programming technicians are often required, further inflating system development costs.

This increasing number and complexity of control system software and systems has resulted in the standardisation of controller programming by the International Electrotechnical Commission (IEC). First published in 1993, the IEC 61131-3 standard defines five PLC programming language standards.

“Ladder Logic”, “Function Blocks Diagram” and “Sequential Function Chart” graphical programming languages, along with “Structured Text” and “Instruction List” textual programming languages represent industry’s most used protocols. The standard also defines both graphical and textual sequential function chart elements to organise programs for sequential and parallel control processing.

Standardising PLC programming languages minimise programming and training costs that are potentially incurred by users at the system-commissioning stage. More advanced controllers such as the Movi-PLC are designed in accordance with IEC 61131-3 programming standard. This saves time in system development, as an expert with a “vendor-specific” programming language is no longer required.

OEMs are able to leverage the engineering knowledge of programmers who are able to write programs in an “industry standard” programming language.

The Movi-PLC takes this standardisation and program modularity further, by providing a library of standard function and motion blocks usable in a number of programming languages. The pre-written and tested motion function blocks are compliant with the PLC Open “motion control” standard further reducing machine control programming effort.

The flexible future

WHILE the lack of a single network communication standard somewhat limits the proliferation of distributed control architectures, the continued implementation of system architectures founded on industrial Ethernet is fast providing, what appears to be, a universal distributed network solution.

Nevertheless, the reality of today’s industrial control landscape demands a breed of controllers that support multiple fieldbus protocols including Profibus, Modbus and DeviceNet.

A decentralised controller must come equipped with multiple communication interfaces – this is an essential in providing the ideal solution to industry’s current circumstance.

Looking to the future, Ethernet-based control networks truly represent the “next step” in industrial communication either as the sole communication network, or integrated with other fieldbus networks.

Communication networks based on Ethernet technology offer additional benefits in machine commissioning, process control and status feedback. Due to the ease of installation and availability of connection hardware, Ethernet-based communication networks represent a natural progression in industrial communications. Some PLC manufactures recognise the need for flexibility and provide modern PLCs with integrated Ethernet-based interface, plus a variety of alternative fieldbus interfaces.

IEC-61131-3- and PLC-OPEN-compliant, all members of the Movi-PLC family support a wide range of external communications. These include Ethernet protocols, Profibus interface, CANopen interfaces, RS 485 ports, and a selection of pluggable I/O.

As a result, the Movi-PLC can be incorporated into a broad array of system architectures: as a stand-alone controller supporting multiple connected drives, as a slave controller working in response to a supervisory PLC, or directly linked to a local SEW-Eurodrive drive operating panel (DOP) operator interface, for local control and monitoring.

When used within a distributed control system, such powerful communications functionality can allow manufacturers to interrogate their production processes from the other side of the plant.

Next generation control

POWERFUL small PLCs, coupled with advances in industrial control system networking, are underpinning the evolution of the next-generation of distributed control architectures.

The marriage of conventional PLC control and motion control, all in a single highly granular controller, will deliver a multitude of operational and financial benefits to OEM, system integrators and manufacturing-floor personnel.

Such highly distributed control elements are destined to form the backbone of tomorrow’s control solutions. Distributed control networks are the way of the future in the majority of production/manufacturing applications. The essential element in advancing such levels of distributed intelligence is the fast-evolving motion-enabled, small PLCs--processing power and control connectivity in slimmest and most elegant point form.

* Commentary by Darren Klonowski, strategic marketing and product manager, SEW Eurodrive

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