To meet the growing demands from industry for ever more flexible process control solutions, leading UK weighing specialists, Procon Engineering have developed a powerful Intelligent Weigh Transmitter with DeviceNet communications. The GS200DN Intelligent Weight Transmitter enables the client to install weighing systems which provide Digital movement of data between the instrument and PLC in real time. A single DeviceNet node can sustain over 1000 transactions per second and therefore the high speed communications through the DeviceNet fieldbus provide a far more flexible and accurate control of weighing functions within the process.
Procon have installed a number of their DeviceNet based weighing systems at sites around the UK for leading building materials manufacturer, Marley Building Materials Limited The building materials manufacturing industry is particularly prone to dramatic changes in demands, a situation which requires companies to be highly efficient and flexible in order to meet changing circumstances.
Companies like Marley Building Materials are seeing increased pressures from tighter demands on product quality and consistency from their customers on one hand, while on the other they realise that any plant must be capable of being adaptable. If plant requires relocating then it must reinstalled and made operational in as short a time as possible. Unfortunately, the trends in overall manpower reduction result in lower maintenance, with the need for engineers to be multi-disciplined. This means that any problems must be quickly identified, ideally before shut down occurs.
MBM's electrical engineering manager , Compton John highlights some of the problems they had experienced in the past with their control systems: " 10 years ago, we invested in large multi-parallel I/O PLCs. Structuring industrial control systems into 'societies' was beyond our budget and systems were simply interlocked by relays with the added hazard of stray voltages between control panels. These expensive PLCs suffered from being slow, difficult to expand and had limited networking capabilities. To compound matters further, fault finding was extremely tedious.
The advent of small micro-modular PLCs with on line programming facilities led to the redistribution of I/O from the larger PLCs. Unfortunately, this methodology inherited the same problems as before in that it was still predominantly parallel I/O based. The momentum of 'industrial real time serial network' technologies from 1994 onwards meant the best of both worlds was possible. The reduced cost of the micro-modular PLC and the industrial real time serial control networks give significant benefits, including flexibility and ease of expansion, speed of commissioning, reduced and simplified wiring, excellent diagnostics, digital information interchange all combining to give tangible asset value."
Weighing is at the heart of MBM's production and using DeviceNet based instrumentation has realised significant benefits. The system not only has excellent diagnostics but higher speed communications provide higher accuracy and repeatability together with real time process batching and control. This results in less material wastage and better product quality. In a typical installation for making roofing tiles different types and grades of sand are blended and conveyed to the main mixer via the weigh conveyor. The required amounts of cement and colour pigments for a particular product are then added to the mixing vessel. GS200DN weight controllers transmit weight data from the main mixer , weigh conveyor and storage vessels back to a DeviceNet Scanner Card fitted in the main PLC.
As Compton John confirms: " Feedback from the GS200DN controllers is providing valuable information. For instance in the past with the standard 4-20mA output, we could not determine whether a particular weigh bin, which showed a zero reading, really was on zero or sitting at a negative value . Negative weight values can be a relatively common occurrence when the lid of the bin or the exhaust sock has been left off after maintenance. We discouraged the use of the auto tare facility which would mask the unseen build up of material that can occur within the vessel. The DeviceNet system allows us to initiate an alarm if the vessel weight goes below zero. Another problem, now overcome, relates to when the weight value of the vessel should be read. Originally we had to install settling timer to try to make sure the vessel was at rest before taking the weighing. DeviceNet can now tell the PLC when the vessel is no longer in motion. If this reports that the vessel is in motion outside the official weighing or discharge cycle, an alarm is activated, indicating that the vessel may have a faulty or leaking fill or discharge valve. The use of industrial real time serial networks within MBM has given us clear savings in both production and engineering time and costs. It simplifies installation, diagnostics and maintenance and is now written into our electrical code of practice and specifications."
As a foot note, MBM have been pleasantly surprised to find that at their Beenham plant one added benefit of this tighter control on their production process is that they are getting noticeably fewer breakages of tiles during handling and packing. This is attributed to the higher consistent quality now achieved through the use of the Procon weighing system. .