Requirements for transmitting data in the industrial field are high—key parameters include robustness, low energy requirements, data security, and real-time capability for safety-related functions. At the same time, industrial plants are becoming increasingly complex with more and more sensors, control units, and machines communicating with each other.
Because wired systems such as EtherCAT or Profibus can’t be used everywhere, wireless systems are often more suitable, especially for moving equipment or mobile machinery. Many wireless technologies such as Bluetooth or WLAN, however, quickly reach their limits in environments with multiple network partners.
Wireless Industrial Data Transmission
Today, the wireless transmission of industrial data is conducted using radio-based techniques. These include, among others, the WLAN IEEE 802.11n standard with a possible data rate of up to 600 Mb/s or the 802.11ac standard for data rates up to 1300 Mb/s.
Actual net data rates are often much lower in real industrial environments with several network partners and electromagnetic-compatibility (EMC) interferences. In addition, WLAN is viewed as 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 having to be retransmitted. Therefore, safety applications with fast cycle times can’t be reliably operated over WLAN connections.
Further difficulties arise in security protections against eavesdropping breaches. In a radio network, the room serves as a monitoring medium and transmission range is only limited by signal strength. Although they may be weakened, signals can still penetrate machine casings and walls to present problematic issues in terms of privacy as well as security. Without encryption, only one singular device must be in range to access data.
Although different encryption methods are available, most require all hardware to meet modern standards. Older devices within the system can pose a security risk. Li-Fi (light fidelity)—the transmission of data via light—eliminates many of these problems and appears to be a viable alternative for the transmission of industrial data.
What is Li-Fi?
Li-Fi uses visible light communication (VLC) and near-infrared communication (IRC) to transmit data. Because it’s simultaneously used for both communication and lighting purposes, VLC normally uses light-emitting diodes (LEDs) of comfortable white light. IRC, in contrast, typically implements LEDs with supplemental 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 known as transceivers. Transceivers consist of both a receiver and a transmitter able to modulate or demodulate data using a process signal module. Data must be converted from electrical to optical signals in order to transmit. For this purpose, emission intensity is varied with the help of a suitable guidance driver.
In addition to the processing module, the receiver consists of an amplifier and a photodetector. The receiver optic focuses transmitted optical radiation to maximize the signal level. A detector converts the received signal into electrical power, which is then translated into a voltage and strengthened. The signal-processing module demodulates received optical signals.
The distance between the modules and the resulting spot size, also called the field of view, is of particular importance. A narrow field of view typically allows for a higher rate of data to be transmitted over longer distances at a lower bit error rate. To achieve this narrow field of view, however, transceivers must be precisely aligned with one another. Although a larger field of view offers an alternative, a smaller proportion of emitted power hits the receiver. This lower-level signal results in a shorter distance of travel.
Comparing 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 spectrum of light gives it an enormous amount of available bandwidth. The practical data rate is limited only by the optoelectronic components selected for modulation and demodulation.
Using an unregulated spectrum brings additional benefits. Varying radio spectrum regulations among different countries often provide for the costly implementation of machines and systems with integrated wireless data-transmission components. Li-Fi technology eliminates this expenditure.
The system also features real-time capability. Originally developed for computer communication, WLAN offers packet-based, asynchronous data transmission. In contrast, Li-Fi continuously sends data, making it comparable to streaming. Li-Fi can therefore provide for the reliable operation of applications in which data calculation and transmission aren’t allowed to exceed a predetermined time limit.
Li-Fi is also advantageous in that several data links can be built in parallel in spatial multiplexing so that no interferences occur between individual data links. This provides for a secure, disturbance-free industrial environment, eliminating time-consuming identification and rectification of disrupters in the system. In addition, Li-Fi allows for a high density of data-transmission cells, whereby each can respectively access 100% of the available bandwidth. The bandwidth per room volume can therefore be significantly multiplied.
Although the Li-Fi line-of-sight criteria can pose some challenges, it’s extremely advantageous from a data security standpoint. Because they are absorbed by black bodies and reflected by bright bodies, both visible light and infrared radiation are unable to penetrate surrounding walls. Foreign actors outside the walls can no longer access data being transmitted, making Li-Fi an attractive alternative to WLAN.
Li-Fi HotSpot
Figure 2 shows a Li-Fi HotSpot from the Fraunhofer IPMS evaluation kit, which is able to establish an optical data link with a 1-Gb/s data rate at a distance of five meters. The module can be simply integrated into existing systems via a CAT5 cable. Point-to-point connections can be built in half- or full-duplex mode. The HotSpot gets its energy from a 5-V power supply. The module is available for testing and demonstration purposes, and works in the near-infrared area. Emitters are operated according to the Laser Class 1 aspects for eye safety.
2. Here’s a Fraunhofer IPMS Li-Fi HotSpot with optical data link in an industrial setting.
Depending on the application, the size, data rate, transmission distance, and interfaces of the HotSpot can be optimized and further developed according to specific customer requirements. Depending on the ambient conditions, it’s possible to implement distances of up to 30 meters and data rates up to 1 Gb/s. USB 3.0, Ethernet, and Gigabit Ethernet interfaces have already been implemented in industrial applications.
In industry, the Li-Fi HotSpot can typically be applied to point-to-point, point-to-multipoint, or multipoint-to-multipoint connections (Fig. 3). Today, Li-Fi modules help to ensure a secure wireless data link wherever cable, slip rings, and plug-in connectors can’t be used, are affected by constraints, or are pushed to their limits. Whether on mobile cameras transmitting large amounts of data, such as video for process control, mobile transport systems transmitting between the control center and mobile units, or in rotating systems, Li-Fi technology enables robust and wear-free communication.
3. Shown are Li-Fi HotSpot data-transfer variants.
Fraunhofer IPMS at the Optical Networking and Communication Conference and Exhibition
Fraunhofer IPMS scientists will present Li-Fi technology prototypes for optical wireless communication over a 10-meter distance at the 2018 Optical Networking and Communication Conference and Exhibition in San Diego, Calif., from March 13-15. Fraunhofer IPMS will also present the “GigaDock” technology for smaller distances. The real-time capable technology, with bandwidths of up to 12.5 Gb/s, is intended to complement or replace stationary cable connections in highly automated production environments. Visitors to the 2018 OFC are invited to stop by the Fraunhofer IPMS exhibition at Booth 5901.
Alexander Noack, leader of the Optical Wireless Communications Group, has worked 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 microcontroller-based signal processing.
