The perfect mix High-level plant automation system provides efficient workflow. Andreas Diegner. A major process engineering project has been carried out at the Sto coatings plant in Stühlingen. Advanced control technology has been used to create a fully integrated manufacturing system which provides rapid, accurate production with complete batch repeatability and tracking at every stage of the process. In order to be able to offer both large batches and also better individual customer service in terms of small batches, Sto AG, one of the largest European manufacturers of facade elements, plasters and paints, set out to develop completely new production structures and capacities at its headquarters in Stühlingen, Germany. The result of the project has been an architecturally and technologically advanced manufacturing plant with a fully automated, just-in-time production capability. Vertical operation in the silo tower The most conspicuous element of the new plant is the silo tower, which is more than 50 metres high and provides storage for 62 silo containers of the silicate fillers necessary for a wide range of products. Here, the industrial process operates vertically. Incoming raw materials are first transported to the top of the silos and are moved downwards through several levels, while they are weighed, mixed and filled. The batch sizes in this area are between two and six-and-a-half tonnes; larger than those in the so-called service factory, where the process is primarily horizontal and small amounts ranging from just 2.5 kg to two tonnes are proportioned, filled, mixed and delivered very quickly on a 'just in time' basis. This significantly reduces inventory and warehousing. High-end controllers at the heart of the concept The automation systems developed for the plant have been crucial to its efficient operation and allow future expansion to be easily implemented. From the intake of raw materials, the core processes of proportioning, mixing and filling, through to final shipping in customer-specific containers, everything is taken care of by advanced control technology, with bus systems providing vertically and horizontally integrated communication. At one particularly time-critical point in the process, a high-performance combination of high-end controller and communications processor implements short cycles to maximise productivity. From the very start, the automation was designed on a fully modular, decentralised model and it was therefore capable of being expanded flexibly in order to accommodate all 14 mixing lines which would make up the final configuration. Four of these lines are now in operation, and work on another started even before the first project stage was completed. Switches are used to link the automation operations to the higher-level plant instrumentation and control system, where more than 100 individual automation components and the operational phases required for the processing of close to 1500 product recipes are drawn up, managed, and forwarded to the master controllers in the central control room via an Industrial Ethernet electronic communications network. From here, visualisation and batch servers ("Fujitsu Siemens" brand) supply a wide range of clients in the factory. Plant availability is maximised by measures which include a CO 2 extinguishing system, an uninterruptible power supply for controllers, servers and switches as well as partial redundancy in the form of additional computers which are held on a cold standby basis. Four programmable controllers are at the heart of the production facility. They share the tasks of dry loading the silos, dry proportioning from the silos, liquid loading of the tanks and liquid proportioning of pastes, binders and additives from the tanks, and then finally share control of the mixing. Weighing system aids rapid production Texturing sands and fillers are individually forwarded over two proportioning scales from the silos to a distribution system and from there moved onwards in dry holding tanks to the mixing lines. At the same time, the liquid components are proportioned out into movable tanks, forwarded to the mixers at precisely the right moment, and mixed in accordance with the recipe into a homogeneous product. The entire manufacturing control system is shown in schematic form in Figure 1. During the last two steps in particular, a very large number of phase parameters must be forwarded from the plant management level to the controllers, which then process them and forward them to subordinate sub-controllers as well as to the process I/O (input/output electronic communication system). In total, around 250 distributed I/O stations on several storeys of the building have been installed. All of these are linked, together with proportional valves, valve islands, frequency converters, weighing modules and operator panels by Profibus DP, while the entire measuring technology communicates with the control centre over a separate dedicated Profibus PA line. (These two forms of the internationally standardised electronic communication system refer to 'Profibus for Decentralised Peripherals'. and 'Profibus for Process Automation', respectively.) To transfer the operational phases, which can comprise as many as 22,000 data points, as swiftly as possible from the plant management level to the controllers and process them, automation engineers, suppliers and users worked together to optimise the original concept during the course of the project, which lasted for more than two years. Most advanced CPUs available were used As processor technology advanced, the "Simatic S7-400 CPU 416-2" which was the most powerful PLC available when the project began and which was already in use at some of the customer's other plants, was replaced by the most powerful CPU 417-4, which became available, in combination with the advanced communications processor "CP 443-1 Advanced", which resulted in a significant reduction in communication time as well as cycle times - a considerable factor in plants that run three shifts. Information on the proportioned amounts, including dribbling, cumulative weighing values and any instances of operator intervention, are written back to the controllers, forwarded to the higher-level plant management system and archived. This makes it possible to exactly reproduce the composition of each individual batch even years later, a capability which is increasingly demanded by major customers and is expected from a manufacturer certified to DIN EN ISO 9001. Workflow is automated throughout the system
The prepared plasters and paints are transferred from the mixers either automatically, in buckets (Figure 2), or manually using a swivel-mounted filling hose, into large containers on special stacker trucks for weighing, labelling and palletising. A modular control concept was also implemented for these tasks, which were downstream of process engineering and more closely production-related. Unstacking of the buckets, filling and labelling are controlled by dedicated controllers precisely matched in performance to the tasks at hand, while a different controller meets all palletising requirements efficiently. Transport of the full pallets from the mixing lines to the high-bay warehouse is again handled by a dedicated controller, which communicates with the line controllers over a Profibus-DP-DP coupler. Movable panels According to one of the line managers, the portable panel shown in Figure 3 proved to be exceptionally practical. These panels can be plugged into and removed from the Profibus network during operation, and a 10 metre long flexible cable and multiple connection boxes along the transport path give the operator the necessary freedom of movement. Sto is currently using two of these ergonomic devices, even the simplest version of which has a dead man's switch on the underside and both touch-screen and key operation. These units provide efficient operator control and monitoring while ensuring optimum insight into the process. To save time, the appropriate on-screen forms can simply be acquired from the configuration data for the central operator panel in the control cabinet. An effective team effort The entire automation design was based upon Siemens' Simatic range of process control equipment. Austrian company Doubrava GmbH & Co. KG acted as the general contractor for the planning and implementation of the industrial outfitting of the factory. RK Prozesstechnik GmbH & Co. KG, Lauchringen, Germany, which has an established relationship with Sto, was largely responsible for the automation engineering design and equipment connection. Results at a glance - A major process engineering project has been carried out at the Sto coatings plant in Stühlingen, Germany. - The system has been designed on a fully decentralised and modular basis in order to allow for expansion from the initial four production lines to a planned total of 14. - The monitoring system provides rapid, accurate production with complete batch repeatability and tracking at every stage of the process. - Some of the most advanced control technology available today has been employed, with Profibus and Industrial Ethernet communications. - Mobile operator panels are used to provide communication with the centralised systems from any point in the plant. The author: -> Andreas Diegner holds a diploma in engineering and is a Marketing Manager for Siemens Automation and Drives in Nuremberg, Germany.
Figure 1: Schematic diagram showing the production sequence from silo tower to filling off.
Quelle/Publication: European Coatings Journal 10/2006 Ausgabe/Issue: 44 Seite/Page: Figure 2: "It's all in the bucket" - The last step in the complex production process is automated filling.
Quelle/Publication: European Coatings Journal 10/2006 Ausgabe/Issue: 44 Seite/Page: Figure 3: The transport of full and empty pallets is controlled via two mobile hand-held electronic panels which can be plugged in and disconnected without interrupting operations.