A new conveyor system to transport Europallets, steel pallets, etc., from the stores to the assembly area had to be installed within the three-week works shutdown. The time frame had to be strictly adhered to, otherwise production could not re-start on time.
The design principle is based on PROFIBUS-DP and was implemented by installing an overhead rail system. The functionality of an overhead rail system is defined through five modules - points, control of fire protection door and lifting mechanism, curve safety and positioning sections - together with control of the straight sections. A module was developed for each of the five functions. In addition to the distributed I/O of the PLC, the module contains the power supply, emergency stop function, manual operation and power unit. Power is supplied to the modules from a busbar trunking system along the track. An interface for connecting the modules is located every 50 cm along the busbar. In addition, there is a single ring circuit for the emergency stop circuit and the PROFIBUS system connecting the distributed I/O stations to the PLC controllers.
The large number of I/O modules means that several CPUs are required to control the system. In view of the widespread nature of the system, the CPUs are also located in the individual areas and connected using PROFIBUS-DP. The CPUs also communicate via the PROFIBUS with a warehouse management computer, which issues the instructions to the overhead rail wagons.
The individual modules could be tested as stand-alone units. As they are all independent, they then simply needed to be installed on site and connected to the PROFIBUS-DP. This enabled the tight three week installation schedule to be met.
There is another advantage in using the distributed version of PROFIBUS-DP. Extensions, such as an additional points system, would not need expensive re-wiring and the distributed nature of the system means there would be no space problems in the central control cubicle.