From GPS to DGPS in the slab yard


In the continuous caster, steel slabs are produced with an average length of 10 m, a thickness of 22 cm and a width of 1.5 mm. The slabs weigh approximately 22 tonnes and are stored in the slab yard before they are further processed in the hot strip mill.

How are the slabs stored?

A part of the slabs are stored temporarily in the shops, where storage bays have been created. Special cranes are used to transport the slabs within the shops.

Laser detection and position monitoring by means of a fixed rack in the crane's guidance system enable the destination of each slab to be determined precisely.


The storage capacity in the shops is not always sufficient. Slabs are therefore also stored in neighbouring outdoor yards. Slab carriers are employed for transport to and from the yards.
The use of these large, four-wheel vehicles, however, requires special storage arrangements.

In order to deposit slabs, the vehicle travels on large wheels between the storage bays. Adjacent storage bays must therefore be separated by a gap of at least 4 metres.

An optimised GPS system appeared to be a suitable means to position the slabs in the outdoor yards precisely and to monitor their exact location. Since the equipment is exposed to the elements at all times (***), it must be particularly robust.

(***): The weather is not the only factor here. The equipment is located on board these large vehicles, which travel on uneven terrain. The shocks cause damaging vibrations and loosen connections. This is an important aspect, reflected in the requirements placed on the equipment.


GPS

The Global Position System consists of 31 satellites orbiting the earth in six paths at an altitude of 20 km. This results in at least four satellites being above the horizon at any given point on the earth's surface. Each satellite transmits coded radio signals that can be used by a GPS receiver to calculate the latitude, longitude and elevation to an accuracy of approximately 10 m. This accuracy is insufficient to distinct two adjacent storage bays. GPS is thus inadequate to steer the slab carriers.


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Therefore, the accuracy of GPS was improved to 1 m and in fact to a few centimetres by correcting the satellite signals from the GPS receiver on the vehicle by means of position information obtained from a fixed GPS transmitter serving as a reference station. This differential GPS (DGPS) solution delivers an accuracy of 0.5 m.

The slab carriers were fitted with a GPS system, and the GPS aerial was fixed above the tong in order to ensure optimum reception of the satellite signals.


Recording slab manipulation

Each slab carrier is equipped with a PC, which is connected in turn to the GPS receiver. An application on the PC shows the driver his location at all times. The position of the slab carrier is used to record the storage points of the slabs deposited by the carrier. All changes in the storages points are stored on the PC.


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This is an example of what the slab carrier driver can see on the screen

  1.  A plan view of the storage yard is displayed. The storage bays on which the driver is to deposit slabs are indicated in blue.
  2. When driving into a storage bay, the driver records the number of slabs he has actually deposited.
  3. The green dot indicates the position of his slab carrier.
  4. The XY coordinates and the name of the storage bay are displayed on the left of the screen.

The PC on the slab carrier is connected to a central computer by means of a wireless network. Each slab manipulation is signalled immediately to the central computer managing the stock over the network by the PC on the slab carrier. The position of each slab is known at all times, so that when the slab is to be rolled, its location can be found immediately and it can be transferred quickly to the hot strip mill.




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