28 March 2010
Often we think of RFID for keeping track of automobiles or containers that travel though the manufacturing process. But the technology is readily adaptable to many other kinds of solutions. In this steel mill example, RFID keeps track of the position of a huge ladle as it carries hundreds of tons of molten steel on an overhead crane system.
HKM (Hüttenwerke Krupp Mannesmann) produces preliminary products used in steel production in Duisburg – slabs and billets (round steel bars).
It is hot at the iron and steel works company Hüttenwerke Krupp Mannesmann GmbH (HKM) in Duisburg, Germany. As one of the leading steel manufacturers in Europe, the company delivers preliminary products used in steel processing, so-called slabs or billets (round steel bars), to its partners, such as Thyssen Krupp Steel AG. In order to manufacture these products, melted metal must be transported, mixed, and processed.
The raw materials are transported in steel tubs, called pig iron ladles, through the huge warehouse and production halls. Depending on the ladle, between 90 and 425 tons have to be moved in the process — a task that can only be performed by a giant crane.
Overall, HKM has about 50 giant cranes in use, ten of which are used for the transport of liquid materials high above the hall floors. If something goes wrong, under the rough and dusty conditions on the steel formation, the best case scenario results in a lot of lost time; in the worst case scenario, it is damaged caused by the hot liquid material or the bulky raw iron ladles and the related costs.
CALIBRATING USING RFID
HKM relies on high temperature solutions. With the Turck BL ident RFID system and special sensors with an expanded temperature range up to 100 ?C, the hot ladles can be transported for hundreds of metres within the production halls precisely to the centimetre. The tracking avoids expensive errors during transport or when filling with raw materials.
Sensitive lifters: these foundry ladles travel hundreds of metres carrying molten steel and stop precisely to within a centimetre of the destination, thanks to RFID.
Another crane, called the BS6L crane, transports the emptied ladles to the filling area where they are cleaned and then reheated, if required, in order to be able to hold liquid raw material again.
About 16 meters above the floor, sitting in a small cockpit, the crane operator controls the machine with the help of the monitor. There are no longer any positioners on the ground; the crane's movements and positions are visualised and displayed on the operator’s monitor. Visualisation helps him position the crane when picking up and setting down loads, which requires a high level of precision from the system.
ROTARY POSITION TRANSDUCER PRONE TO SLACK
In order to move the crane in a controlled manner over the approximately 400 meter-long distance between the pick-up and filling location, a rotary position transducer is installed on a rotor disk which records the movements of the crane and forwards them to the main control unit.
This approach is not without its problems, because the travel sensors detect the position of the crane only from the actual rotation of its huge rotor disks. And because the steel wheels of the crane run on steel rails, a certain amount of slack cannot be ruled out. This results in an inaccuracy that the travel sensors cannot compensate for.
After several hundred metres of travel, the ‘missing’ centimetres can lead to serious errors at the pick-up and drop-off areas. The situation makes it impossible for the crane operator to accurately pick up or set down the ladles.
Reference points: The RFID tags are mounted on specified positions of the crane's travel path.
This is where the RFID system comes into play: A Q80 read-write head is mounted near a crane rotor disk and records the signals of the transponder mounted at specific points on the rails, thus allowing millimetre-precise location coordination with the travel sensors in the main control unit — regardless of slack. Using the known positions of the tag, the control unit can calibrate the signal of the rotary position transducer and therefore reliably prevent any influence on possible slack. The tags are especially designed for use in high temperature environments up to 210 °C.
HKM has selected a control system with low installation and maintenance costs that always checks the signals of the travel sensor and, if necessary, can update them in the control unit.
The system not only prevents serious accidents, but also makes efficient supply chain management possible. ‘In the spirit of complete material monitoring, it is important for us to know precisely where the crane puts its load down so that we can determine where each of the individually numbered ladles are located,’ says Wilhelm Leurs, a technician in the crane maintenance department at HKM.
The system works so reliably that the HKM is planning on equipping 30 of its giant cranes that are used to transport raw and end products with a similar RFID tracking solution. ‘Even if we do not completely take advantage of the comfort of the BL ident system, this solution is unbeatable for our control purposes,’ summarises Mr. Leurs.
SENSORS INSTEAD OF POSITIONERS
In addition to the RFID solution, Turck's inductive sensors have also proven reliable for use in this tough iron and steel works environment. Two special sensors that meet the IP68 protection class are mounted on each of the massive lifting hooks, which the crane uses to lift and transport the foundry ladles, weighing several tons. These sensors record whether the lifting bolts on the foundry ladles lie exactly and safely in the laminated hooks.
An RFID read/write head mounted near a crane rotor disk records the signals from the RFID tags mounted on the rail.
Here, too, from his cockpit 18 meters above the floor, the crane operator has difficulty detecting whether the ladle is correctly placed on the hook. The use of two sensors placed at a 90° angle guarantees that the bolts are reliably placed across the entire surface of the laminated hooks.
For Mr. Leurs, the heat, shock, and vibration-resistant S 100 sensors from the BI15-K series turned out to be essential and reliable control tools.
“We have never had problems with the Turck sensors and will therefore equip all cranes that transport liquid raw materials with these sensors,” he adds.
Due to their high measured switching distances of up to 15 millimeters, the special sensors are suitable for use under the tough conditions in steel production where slag and fine metal dust get into the robust machines and can hinder or distort sensor readings.
—the author, Matthias Schick, is a Turck sales specialist in Mülheim
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