He who weighs, wins
30 November 2008
How modern weighing technology increased productivity at a grain mill in Mannheim.
Dosing unit with electronic system and inlet beak
Kampffmeyer is a leading supplier of basic raw materials for the food industry, refined milled grain products, and other convenience goods for the baking profession. The Hamburg-based company is active throughout Europe and boasts a closely integrated network of independent mills.
Fig. 1 - Dosing unit with electronic system and inlet beak
Kampffmeyer's Hildebrandmühle in Mannheim is the most modern durum mill in Europe and produces semolina for all purposes, using a unique hygienic and technological concept. It has a grinding capacity of 200,000 tons/year. [Semolina comes from the Italian for ‘semi-milled.’ It is the coarse grains of ground durum wheat.]
WHAT IS DURUM WHEAT?
The Mediterranean, along with the United States and Canada, are the growing regions for durum. It is a hard, glassy, and protein-rich grain, and is especially suited to the production of pasta. The grain is particularly useful for dense products, and has an intensive yellow colour and high boiling tolerance. Growing areas with a sufficiently warm and dry climate are necessary to ensure these characteristics during maturation. Durum wheat flour is less useful for baking, as the dough is rich in gluten, making it difficult to become soft.
OBSOLETE DOSING UNITS
The emptying of 23 grain silos at the Hildebrandmühlen in Mannheim was previously controlled by flow metering and dosing units. However, because the dosing electronics were obsolete and spare parts procurement became increasingly expensive and difficult, it was decided to replace them with modern Siwarex weighing processors.
This new technology was to be optimised for the demands of production and the weighing electronics were to be connected to the existing Simatic S7 PLC using Profibus DP. All menus in the existing Simatic WinCC SCADA system also needed to be adjusted.
The proportioning scale permits the continuous dosing of free-flowing grain. The current flowing amounts are shown in tons per hour and added up in kilograms.
The dosing system consists of a control unit (control cabinet) and a dosing unit (beneath the silo). The dosing unit consists of an inlet beak, a pusher, a deflector plate, a force sensor with four strain gauges (interconnected to form a complete Wheatstone bridge), add-on electronic systems, and a valve group.
Deflector plate with force sensor
Fig 2 - Deflector plate with force sensor
Together, these components form a control loop, with the control variable being the product flow volume.
The product flows over the deflector plate and produces a deflecting force. This is converted into an electric signal in the force sensor, which is amplified in the electronic systems. This actual value is proportional to the flow rate. The value is transferred to the control unit. The control unit specifies the desired flow volume – the nominal value. This nominal value is compared to the actual value by the controller. An actuating signal for the pusher is generated based on this. The valve group transforms the electric signal into a pneumatic signal, and the pusher is actuated via a diaphragm.
NEW WEIGHING SYSTEMS
The control functions of the control units integrated in the control cabinets were integrated into the program of the Simatic S7-400. The electronic systems at the dosing units were replaced by Siwarex electronic weighing systems.
The Siwarex electronic weighing systems were built into a Simatic ET 200M the modular I/O station in the new control cabinet in the central control room and connected to the Simatic S7 via Profibus DP. The force sensor remained in use. The strain gauges (Wheatstone bridge) of the force sensor were each connected using a special six-core cable and an expansion box.
The Siwarex weighing module supplies the weighing cells with power and converts the measurement signal into a weight value. During the weighing procedure, the Siwarex weighing module monitors and controls the weighing cell signals and states. The optimised Simatic-internal data exchange enables the direct evaluation of the signals and states in the PLC program. The Siwarex module can easily be adapted to changes in the system technology thanks to the PLC's influence on the weighing process.
Previously, the dosing units could only be controlled as a group: There were three groups with six proportioning scales each and one group with five proportioning scales. With the new solution, all Siwarex scales can also be controlled individually and any groups can be formed, allowing the silo management to become far more flexible. For the operation and adjustment of the flow meter dosing units using Siwarex, new clearly arranged menus were added to Simatic WinCC.
The application software required modifications to the PLC, and the integration of the Siwarex components were implemented in several steps so that the ongoing operations of the mill would be affected to the smallest extent possible.
—Dipl.-Ing. (FH) Volker Weiss, Siemens AG Industry Automation www.siemens.de/siwarex
Fig. 3 - Valve group
*** SIWAREX SELLING POINTS ***
• Simpler operation thanks to clearer menus and faceplates in WinCC
• Better control over emptying of silos
• Greater precision in cost accounting
• More flexible silo management due to individual control
• Greater availability and reduction of spare parts costs through use of modern electronic weighing systems
Click here to read the article "Weighing, control, and operator control in one package"
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