Crane automation in the slab yard
Slabs produced by the continuous casting lines in the steel shop are further processed into hot rolled coils in the hot strip mill. First, the slabs are stocked temporarily in the slab yard. The slab yard consists of outdoor areas on the one hand, in which slab carriers transport and stock the slabs, and of shops on the other, in which special cranes are used. So as to save costs and improve ergonomics, an important step was taken in 2008: crane 335 was fully automated.
An automated crane offers various advantages compared to manned crane operation. Operating a crane manually may cause back problems because of the vibrations. Moreover, it is a lonely and isolated activity. Automated cranes are also less subject to wear and allow maintenance costs to be reduced.
In the steel business so far, automated cranes only had limited capacity. That is why a team consisting of employees of the hot strip mill, the industrial automation and measuring technique department and the engineering department started working on a tailored solution for ArcelorMittal Gent. Doing so, they faced the additional challenge of using the existing infrastructure (bridge, crane, tong…).
Their efforts resulted in a crane capable of automatically moving a pile of one to seven slabs, which amounts to a maximum weight of 105 tons. They pulled off a difficult feat as the length and the width of the slabs may differ, they may not be piled perfectly on top of each other and may still have a temperature of up to 900°C. To enable the automation process, various systems were used:
- LISA (Logistic Informatisation of the Slabyard Area): a system that automatically takes stock of the slabs and manages them.
- PLATO (Plakkenpark Transport Optimalisatie or Slab Yard Transport optimisation): a programme that automatically generates transport orders. It calculates in a few seconds which subsequent movement operations are the most favourable ones.
- LASE: the laser system LASE consists of 4 laser scanners that are resistant to vibrations and high temperatures. They measure the length, width and depth of the piles and the free space in between them, and transfer these data to a pc via a wireless network. By combining measurements of two lasers at a time, the so-called shadow effect is by-passed..
Cameras are the eyes of the crane operator
- A more complex problem is the swaying of the tongs. The solution is offered by the anti-sway system CEPLUS, consisting of a camera pointed at a reflector on the crane tong. The camera measures the height, skew and swing of the tong and adjusts the crane position so that the tong is not swaying when the crane stops.
The crane automatically moves in all three directions: X (forwards and backwards), Y (left and right) and Z (up and down). Still, all movements are continually monitored from a central dispatching. During the movement process, the crane stops at several moments. On the one hand, there are active waiting positions, where the crane stops in any case and waits for confirmation or readjustments by the crane operator. He checks for instance whether the tong has a good grip on the slab. On the other hand, there are positions in which the crane stops only in case of problems, for instance when the neighbouring pile is too close or when pile measurements prove to be incorrect. In these cases, the crane operator can readjust the position of the crane manually.
The crane operator observes all crane movements from the central dispatching. Five cameras closely monitor the crane movements:
- 4 Pan Tilt cameras, which zoom in on the tong, focus on the travel direction and capture the slabs from different angles. All Pan Tilt cameras are controlled by a PLC (Programmable Logic Controller).
- 1 Mobotix camera, which checks whether the tong is well positioned with regard to the width and length of the pile.
So as to boost the speed of the crane, various optimisations were carried out. For instance, the tong opening is already being adjusted while the crane is still travelling towards the pile. The crane already starts travelling when it rises, and already lowers when it approaches the desired XY position. Furthermore, the crane does not rise to the maximum height, but to a safe height. This height is calculated by LISA and LASE using pile data. These may seem minor elements, but they can accelerate the movement operation by 30%.
Automated cranes require extra safety measures. Indeed, the crane is not manned and cameras focus on specific elements. In order to guarantee the safety in the crane activity zone, the zone has been completely fenced and secured electrically. As soon as a gate is opened, the crane stops. Moreover, the entrance for slab carriers as well as the entrance for trains have been fully secured.