Wireless SafetyBUS p link co-ordinates tandem AGVs

During the assembly of the Boeing 787 'Dreamliner' aircraft, preassembled parts of the fuselage, as well as parts of the wings, which are sometimes 10-20m in length and several meters in diameter, are mounted on carriers, which, in turn, are moved by AGVs (automated guided vehicles). Due to the sheer size of the parts, some carriers need two AGVs to operate in tandem. That means the AGVs need synchronous control and operation.

The risk associated with a possible loss of the payload requires a classification according to SIL 3 (per IEC 61508) or Safety Category 4 (per EN 954-1). An adequate safety design of the application is therefore essential.

Initially production requires eight AGVs, and it must be possible to group these in any pairings. Moreover, as the AGVs are constructed symmetrically, an additional requirement is that the AGVs can be coupled both at the back and the front. There are therefore possible options for each of the AGV pairs.

For future expansion and supplementary needs, more AGVs may be introduced. Of course, the AGVs must not interfere with each other during operation.

In terms of safety, all of the original eight AGVs are identical. Each AGV is equipped with four laser scanners (safety category 4) that are mounted on the four sides. These scanners are programmed with 16 warning and 16 safety zones. The laser scanners and the additional safety electronics, such as emergency stop switches on the sides and the drive control, are integrated using a PSS 3006 (Pilz programmable safety system - or safety PLC). The whole set communicates via SafetyBUS p.

Safe wireless coupling

The AGVs are controlled via safe radio remote control, which provides the control signals via I/O modules and SafetyBUS p to the safety controller. The particular challenge in this application is the safe wireless coupling of the AGVs under the various operation modes.

First, the AGVs must communicate in tandem operation mode with high availability; second, they must not be disturbed by other AGV pairs or interfere with them; third, they must be able to couple on the front or the back.

The first approach recommended using radio links for the communication. Unfortunately the bandwidth problem makes that quite difficult. There is simply no public radio band that would support the coupling of eight or more vehicles in a production hall over a safe radio system. On top of that, there are no mechanisms available for a safe and reliable ad hoc network structure within automation. For these reasons a safe coupling with an optical link using the BWF system from Hokuyo has been selected.

Hokuyo's BWF systems create safe IR-optical links between the two SafetyBUS p networks within the two coupled AGVs. The AGVs set up their optical link only while they are operating in tandem mode. This way, interference with other AGVs is practically impossible. In addition, the same optical band may be used for each of the connections in all AGVs. A complex management of frequencies or devices IDs is therefore not necessary. That is especially helpful with respect to future expansions with additional AGVs.

The complex safety application to couple multiple, dynamically changing and mobile devices in variable configurations has been solved in an elegant manner. In particular, the use of safe IR-optical bus coupling modules has been proven to be very practical, as all devices may use identical designs and there is no need for radio network planning. Because optical links are used, side effects caused by expansion of the system or errors due to unknown interferers are completely eliminated. In addition, interference with the safe radio remote control is impossible. For the user, this realisation is straightforward, reliable and expandable.