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The demands on data transmission in industry are high. Key parameters include robustness, low power requirements, data security, and real-time capacity for security-related functions. At the same time, industrial installations are becoming more and more complex with more and more sensors, control units and machines communicating with each other.
Since wired systems such as EtherCAT or Profibus cannot be used everywhere, wireless systems are often more suitable, especially for mobile equipment or mobile machines. However, many wireless technologies such as Bluetooth or WLAN quickly reach their limits in environments with multiple network partners.
Wireless industrial data transmission
Today, wireless transmission of industrial data is achieved using radio techniques. These include, among others, the IEEE 802.11n WLAN standard with a possible data rate of up to 600 Mb / s or the 802.11ac standard for data rates of up to 1300 Mb / s.
Actual net data rates are often much lower in actual industrial environments with multiple network partners and electromagnetic compatibility (EMC) interference. In addition, WLAN is considered an inappropriate communication channel for real-time applications with security requirements. System susceptibility often results in packet loss and high latency times due to data retransmission. Therefore, security applications with fast cycle times cannot be operated reliably over WLAN connections.
Further difficulties arise in the security protections against eavesdropping. In a radio network, the room serves as a monitoring medium and the transmission range is only limited by signal strength. Although they can be weakened, signals can still penetrate machine enclosures and walls and present problematic privacy and security issues. Without encryption, only one device needs to be within range to access the data.
While different encryption methods are available, most require all hardware to meet modern standards. Older devices in the system may pose a security risk. Li-Fi (light Fidelity) – the transmission of data through light – eliminates many of these problems and appears to be a viable alternative for industrial data transmission.
What is Li-Fi?
Li-Fi uses visible light communication (VLC) and near infrared (IRC) communication to transmit data. Because it is used simultaneously for communication and lighting purposes, VLC normally uses light emitting diodes (LEDs) of comfortable white light. IRC, on the other hand, typically implements LEDs with additional laser diodes. (Fig. 1).
1. At VLC, the frequencies of light used for transmission are between 380 and 780 nm. For IR, the frequencies are between 780 and 3 µm.
Optical transmission occurs between at least two transmitting / receiving units called transceivers. Transceivers consist of both a receiver and a transmitter capable of modulating or demodulating data using a process signal module. Data must be converted from electrical signals to optical signals in order to be transmitted. For this purpose, the emission intensity is changed with the help of a suitable guide conductor.
In addition to the processing module, the receiver consists of an amplifier and a photodetector. The optics of the receiver focus the transmitted optical radiation to maximize the signal level. A detector converts the received signal into electrical energy, which is then translated into voltage and boosted. The signal processing module demodulates the received optical signals.
Of particular importance is the distance between the modules and the size of the resulting spot, also called field of view. A narrow field of view generally allows a higher data rate to be transmitted over longer distances at a lower bit error rate. To achieve this narrow field of view, however, the transceivers must be precisely aligned with each other. Although a wider field of view offers an alternative, a smaller proportion of the emitted power reaches the receiver. This lower level signal results in a shorter travel distance.
Compare WLAN and Li-Fi
With a higher net data rate of up to 1 Gb / s, Li-Fi HotSpot has a major advantage over common standards such as WLAN. Li-Fi’s use of the globally unregulated light spectrum gives it a huge amount of available bandwidth. The practical data rate is limited only by the optoelectronic components selected for modulation and demodulation.
There are additional advantages to using unregulated spectrum. Different radio spectrum regulations in different countries often provide for the costly implementation of machines and systems with integrated wireless data transmission components. Li-Fi technology eliminates this expense.
The system also has real-time capability. Originally developed for computer communication, WLAN provides asynchronous packet data transmission. In contrast, Li-Fi sends data continuously, making it comparable to streaming. Li-Fi can therefore ensure the reliable operation of applications in which the calculation and transmission of data should not exceed a predetermined time limit.
Li-Fi is also advantageous in that multiple data links can be built in parallel in spatial multiplexing so that no interference occurs between the individual data links. This provides a secure and disruption-free industrial environment, eliminating the tedious identification and rectification of disruptors in the system. In addition, Li-Fi enables a high density of data transmission cells, each of which can respectively access 100% of the available bandwidth. The bandwidth per room volume can therefore be increased considerably.
While the Li-Fi line-of-sight criteria can pose some challenges, they are extremely beneficial from a data security perspective. Because they are absorbed by black bodies and reflected by bright bodies, visible light and infrared radiation are unable to penetrate surrounding walls. Foreign players outside the walls can no longer access the transmitted data, making Li-Fi an attractive alternative to WLAN.
Li-Fi access point
Figure 2 shows a Li-Fi access point from the Fraunhofer IPMS evaluation kit, which is capable of establishing an optical data link with a data rate of 1 Gb / s at a distance of five meters. The module can be easily integrated into existing systems via CAT5 cable. Point-to-point connections can be established in half-duplex or full-duplex mode. The HotSpot derives its power from a 5 V supply. The module is available for test and demonstration purposes and operates in the near infrared range. The transmitters operate under laser class 1 aspects for eye safety.
2. This is a Fraunhofer IPMS Li-Fi access point with optical data link in an industrial environment.
Depending on the application, the size, data rate, transmission distance and interfaces of the HotSpot can be optimized and developed according to specific customer requirements. Depending on the ambient conditions, it is possible to implement distances of up to 30 meters and data rates of up to 1 Gb / s. USB 3.0, Ethernet and Gigabit Ethernet interfaces have already been implemented in industrial applications.
In industry, Li-Fi HotSpot can generally be applied to point-to-point, point-to-multipoint or multipoint-to-multipoint connections. (Fig. 3). Today, Li-Fi modules help ensure a secure wireless data link anywhere cables, slip rings and plug-in connectors cannot be used, are affected by stress, or are pushed to their limits. Whether on mobile cameras transmitting large amounts of data, such as videos for process control, mobile transport systems transmitting between the control center and mobile units, or in rotating systems, Li-technology. Fi enables robust and wear-free communication.
3. HotSpot Li-Fi data transfer variants are shown.
Fraunhofer IPMS at Optical Networking and Communication Conference and Exhibition
Scientists from Fraunhofer IPMS will present prototypes of Li-Fi technology for wireless optical communication over a distance of 10 meters at the Optical Networks and Communication Conference and Exhibition 2018 in San Diego, Calif., March 13-15. Fraunhofer IPMS will also present “GigaDock†technology for smaller distances. The technology capable of operating in real time, with bandwidths of up to 12.5 Gb / s, is intended to supplement or replace fixed cable connections in highly automated production environments. Visitors to OFC 2018 are invited to stop by the Fraunhofer IPMS exhibition at stand 5901.
Alexander Noack, Head of Optical Wireless Communications Group, has been working at the Fraunhofer Institute for Photonic Microsystems (IPMS) since 2011. Noack studied Electrical Engineering at the Dresden University of Technology in Germany and received his PhD in 2016 in the field of microcontrollers. based on signal processing.
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