The benefits of two-leg versus three leg power control, in three phase heating applications
It was over 35 years ago that the first DCS systems were installed in glass industry processes. In those early days of automation, there was a clear spilt between DCS and PLC systems, with both types being designed for their specific tasks. DCS took care of the delicate continuous processes like melting and conditioning control.
On the contrary, PLCs took care of digital control. Distributed control was developed to control analogue processes that required high precision, while programmable logic control had to run at high cycle speeds to be able to detect fast events in an instant. PLCs were suited to relatively small dedicated applications, whereas distributed control was suitable for controlling the whole furnace, conditioning and annealing systems and sometimes, even the batch control.
Control failure was much more critical in DCS than PLC systems because of the impact it could have on the most expensive glass manufacturing plant assets. Therefore, redundancy and hot swappable input and output (I/O) boards were a vital requirement, arriving relatively early in DCS systems, compared to late or not at all, in PLCs. When higher processor speeds and larger memory sizes became available at an achievable price, some PLC suppliers tried to enter the DCS market by adding specific functionalities to their product line with sometimes more but unfortunately predominantely less success. The subsequent result was that DCS system suppliers focused on high value businesses like oil, gas and chemical industries.
For the glass manufacturing industry, distributed control became expensive (sometimes prohibitively) and the industry started looking for cheaper solutions. Some suppliers of DCS and PLC control decided to discontinue their DCS offering, in favour of a PLC hardware-based DCS type solution.
As explained in previous articles, it is important to be aware that both types of control were developed to perform completely different tasks. Bringing them together does not necessarily result in ‘the best of both worlds’. Throughout Eurotherm by Schneider Electric history, the focus has been on digitised analogue control and today, the company has over 50 years of experience in this field.
The company’s Programmable Automation Controller (PAC) based systems are still designed predominantly to control continuous processes at high precision, while also at an attractive price level. Using Eurotherm T2750 PAC hardware together with AVEVA System Platform (formerly Wonderware System Platform) enables the business to provide a scalable DCS, which is an ideal fit for the glass industry’s continuous processes. For predominantly digital applications like batch house and cold end processes, the Schneider Electric Modicon product range is utilised. If traditional high end DCS is required, the Schneider Electric EcoStruxure Foxboro Distributed Control System would be recommended.
Some of the concerns coming from customers are understood, including “I do not want to use a mix of hardware”, “I want to reduce the amount of spare parts” and “Our technicians are not able to manage different systems’ software” etc. However, while these arguments may sound valid, in the author’s opinion, they are not. First, it is necessary to understand that both continuous processes and digital processes need a completely different approach in terms of hardware design. DCS systems focus on precision and need to run complex mathematical algorithms and signal filtering sequences. Instead, PLCs focus on speed and the need to run relatively simple strategies as fast as possible.
Another consideration to take into account is that hardware has become relatively cheap and extremely reliable. Modern DCS systems have redundancy built in, most of the time down to the I/O level. In that respect, an immediate replacement of a failing part might not be necessary. Next to that, it should be understood that programming continuous process algorithms requires a completely different skill set compared to programming a logic sequence. The mistake should not be made of assuming that a skilled PLC programmer can do object-oriented programming as well, without specific training and knowledge of continuous process behaviour. Last, but not least, the choice of hardware is perhaps less important than the glass process knowledge of the system supplier.
As well as finding the best ‘fit for purpose’ product line, it is extremely important to focus on the database layer. As explained in previous publications, Eurotherm considers the database to be one of the most important aspects of the whole manufacturing automation plant. The company strongly promotes the use of one data collection, storage and analytics system to cover all processes.
Having multiple databases supplied by different machine manufacturers and automation suppliers leads to inconsistencies in terms of TAG names, time synchronisation and formatting problems. Such a situation makes analytics complex, frustrating for people needing the data for process improvements, does not allow benchmarking and blocks innovation advances. A consistent data acquisition system with a well thought through naming convention can provide a tool for all who are involved in continuous improvement processes, helping to reduce downtime and increase quality.
As a ‘one stop shop’ for control, Eurotherm by Schneider Electric offers comprehensive solutions, covering dedicated hardware for continuous and digital processes, redundant hardware configurations, consistent data storage, standardised operator interfaces, Model Predictive Control (MPC) and analytic tools and most importantly, a global team that understands glass processes.
This article appeared in the January/February 2019, issue 81 of Glass Worldwide.
About the Author:
René Meuleman is Eurotherm Business Leader for Global Glass
Eurotherm Ltd, Worthing, West Sussex, UK
tel: +44 1903 268500