The arrangement of modular VLC interfaces and the transmit/receive properties of the modules achieves 360° coverage.

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SEW-Eurodrive uses Visible Light Communication (VLC) as the communication technology for AGVs. This wireless peer-to-peer communication system for short ranges in the frequency range 400 to 800 THz (750 to 375 nm) offers the required low latency and high reliability and enables communication between cooperating, neighboring AGVs.

As global communication with high throughput is also required in addition to this local communication, the AGVs are equipped with two communication interfaces - an additional WiFi interface is responsible for communication with the infrastructure and for other general purposes. However, this results in another challenge - deciding which packets should be sent to which interface. The aim is to avoid switching interfaces, as this leads to latency peaks. To achieve this, a routing procedure must be implemented that minimizes the number of handovers between the WiFi and VLC networks.

Network optimization through local clusters

VLC routes based on cooperative tasks

Cooperating AGVs use peer-to-peer communication with each other via VLC and WiFi to communicate with the infrastructure.

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Because several communication technologies are available on the FTS, two challenges arise: defining the rules for interface selection and distributing these rules to the network nodes (FTS). The number of communication technology changes must be minimized. VLC should also always be preferred because it frees up resources in the WLAN.

Tests have shown that the forced, unplanned change of communication technology increases latency up to tenfold. It is therefore of great interest to select routes with a long service life. The strategy is to select the VLC routes based on active cooperative tasks to group the cooperating AGVs. This creates VLC clusters.

The routing between WiFi and VLC is implemented using SDN. To avoid high latency times, the number of handovers must be reduced. This is achieved by using a clustering strategy with groups of cooperative AGVs.

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Empirical studies and simulation show that the duration of cooperative tasks is on average one hundred times longer than the connection duration in a peer-to-peer network between AGVs. It is assumed that during the execution of a cooperative task between AGVs, the VLC connections are not interrupted because the line of sight between the vehicles is not interrupted due to the close cooperation. There is no packet loss during the line-of-sight connection. In endurance tests, no packet loss was observed on VLC connections in which the sender and receiver only experienced slight relative movements.

Distribution of routing rules to mobile clients

The authors: Eike Lyczkowski (left) is a member of the Radio and Navigation expert group, Christian Sauer is a member of the innovation project group for navigation and communication technology at SEW-Eurodrive in Bruchsal.

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There are various strategies for distributing routing rules to clients: In the decentralized creation of these rules, a client creates and maintains its own routing tables. Alternatively, routing can be organized and planned by a central unit, which distributes the resulting rules to the clients. This solution is advantageous because the implemented strategy is based on global information. The clustering is implemented with the help of SDN. A central SDN controller obtains information on planned tasks from the AGV fleet management system. This information is converted into routing information (flow table entries) by the controller and transmitted to the SDN switches (i.e. the AGVs).