Automation solutions with decentralised I/Os combine the advantages of low installation and cabling overheads with maximum modularity in the field. Their flexibility, simple handling and high availability bring significant efficiency enhancements and cost reductions - particularly when they are implemented with an IP20 solution. Distributed automation means cost-effective shifting of the I/Os into the process zone. (The extent to which this is possible depends on the nature of the process environment, of course.)
Components and systems with an IP20 protection rating have proven to be a suitable choice for these applications, even though IP67 technologies naturally have their advantages - but also their disadvantages, combined with the fact that the cost factor often makes the latter a less viable option in the larger scheme of things.
Finely-granulated I/O configurations
In addition to its IP67 products, Mitsubishi Electric has thus developed Melsec ST (slice-type terminal) series with an IP20 protection rating and modules that are just 12,6 mm wide. The I/O system can be configured precisely for the needs of any application by plugging together the required number of 'slices' in the head station.
The costs of the slices are optimised with a granularity of two for digital signals and one for analog values; so-called block modules with 16 inputs or outputs can also be used to achieve a further significant cost reduction compared to the slice versions.
On the first level the product portfolio includes digital outputs in transistor (with short-circuit protection) and relay versions, current and voltage analog inputs and also current and voltage analog outputs. The design of the I/O modules is very simple, which has many advantages for the availability of the system. The base modules are available in two versions with either screw or spring terminals. Power distribution modules allow users to connect through their power lines without additional terminals and a contact on the top-hat rail automatically connects the PE potential to the terminal. The second level component is the electronics module, which is simply plugged into the base module.
This separation from the terminals with plug-in modules makes it possible to have 'permanent wiring', ie, there is no need to disconnect and reconnect the wiring when electronics modules are replaced, thus virtually eliminating the possibility of wiring errors. A coding element ensures that not more than one module of the same type can ever be plugged into the base module, a feature that prevents errors which could otherwise cause unnecessary malfunctions and expensive downtime.
Sharing expertise
Mitsubishi Electric developed the base modules in collaboration with terminal technology experts Weidmüller. The objective was a product with enhanced quality standards that would deliver greater system availability. New contact designs were developed and the existing contacts were all gold-plated to meet the more exacting specifications. Compliance with the necessary terminal standards and practical accessories like contact bridges for the cable shields were integrated into the system right from the start.
The electronics, which were developed entirely in Japan by Mitsubishi Electric, were designed to the same high standards. Here the brief included diagnostics functions on every module and via the bus, suppressor circuits for the I/Os as standard equipment, exacting tolerances and a long service life. The same attention was paid to the extensive accessory equipment, such as the equipment identification labelling and colour coding systems, which round off the product line.
The 'head' of the system
The head station plays a special role in the system. At the moment it is available as a Profibus DP slave module, but CC Link and realtime Ethernet versions are also being developed. Like the other components, the head station was designed for maximum system availability and easy servicing. Electronics modules may be changed while the system is running, without having to deactivate the other components - bus traffic continues and the functions of the other modules are not impaired. This hot swapping can be carried out by staff members without special experience, and without a laptop or other advanced tools. One only has to select the module to be swapped with pushbuttons on the head station.
The Melsec ST can be integrated in any Profibus network with a GSD file. To make parameter configuration and engineering easier a new configuration module was developed for Mitsubishi's GX Configurator DP Profibus software package. The new GX Configurator ST module provides invaluable configuration aids, documentation functions and plausibility checks that help to identify and eliminate errors before the configuration is actually tested in the production system. In online mode the software provides a Profibus logging function and Profibus diagnostics through the head station.
Practical benefits
The top design priorities for this I/O concept included universality and cost-effectiveness for smaller applications, combined with integrated product capabilities going well beyond what one normally expects from standard components.
The Melsec ST series is ready for applications in heavy-duty environments. The newly developed contact technology provides a high resistance to vibration, offering reliable contact performance, among other things thanks to the gold plating. The practical benefits of these functions become more apparent when used in process applications.
The series has already proved itself in pilot projects in applications where maximum availability is essential - systems like sewage treatment plants where the process cannot be 'stopped' for servicing. With the Melsec ST it is still possible to remove and replace remote electronics modules without impacting the bus node or the functioning of the other I/O components.
This also applies for applications where thermal factors play a key role and starting up and shutting down the production process takes a long time. A leading manufacturer in the motor industry is already using the Melsec ST series in such a process, because of its great reliability and the improved system availability achieved. The line looks even more attractive when one examines the maintenance, servicing and repairs performance.
For some customers efficient diagnostics and easy handling in the event of malfunctions are decisive criteria. Digital modules can be replaced by ordinary trained service personnel with a manual, eliminating the significantly greater overheads incurred by programming staff using a portable PC or programming unit.
Circuit Breaker Industries
Circuit Breaker Industries, a company wholly owned by Reunert, is an authorised distributor of the Mitsubishi Electric range of automation products. This range includes programmable logic controllers (PLCs), human machine interfaces (HMIs), variable speed drives (VSDs), servo drives and Mitsubishi motor control equipment (ie, magnetic contactors, medium and high voltage vacuum contactors, thermal overload relays and electronic motor protection relays). A qualified team of sales engineers is able to advise customers and supply back-up support and service. All products carry a 12-month warranty.
For more information contact Shaun Loesch, Circuit Breaker Industries, 011 928 2000, sloesch@cbi.co.za, www.cbi.co.za
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