Wireless LAN has by now become well established in industrial plants. It complements existing wired infrastructure, especially where laying cables is difficult or impossible, or is used with mobile devices – across a variety of industries.
Cutting the cable brings many advantages. In applications that require trailing cables and slip rings, wireless links provides vastly more freedom while eliminating the down time that comes when these moving parts must be repaired. Wireless technologies also make it possible to use notebooks, smart phones or tablets to collect diagnostic data, which often simplifies life for operators and technicians.
Another significant benefit is that wireless links free companies to build their infrastructure the way they want it, not the way that fits a cabling scheme. Wireless connections can be created with much less effort than laying new cable. Reliability is now solid enough that safety applications can be handled without wired connections.
However, users need to understand that Wi-Fi in the office and industrial grade wireless aren’t identical. There are differences between the Wireless LAN (WLAN) version that users know from home or the office and the version used in harsh industrial environments (IWLAN). Industrial requirements such as determinism and real-time communication were not even considered during the development of the ubiquitous IEEE 802.11 standard, which is the basis of WLAN.
Optimized, industry-specific extensions (iFeatures) are needed to enable WLAN to withstand harsh industrial environments. If a wireless solution for an application with real-time requirements is desired, for example, based on Profinet IO, Siemens offers the iPCF (industrial Point Coordination Function) extension for the Scalance W700 portfolio. That ensures that each Profinet IO device can respond within its cycle time on the Profinet IO controller. Using a polling method, the access point cyclically queries each client module in its radio cell at very short intervals, and the transmission of information that is not time-critical is postponed until there are free cycles.
Siemens developed many wireless architectures, which were all optimized for specific application types:
- iPCF (industrial Point Coordination Function)
- iPCF-MC (industrial Point Coordination Function–Management Channel)
- iREF (industrial Range Extension Function)
In larger networks, there are multiple access points and consequently multiple radio cells between which client modules switch via a transition process called roaming. No communication takes place during roaming. To ensure that real-time messages are transmitted in time, including during roaming, the time response has been further optimized.
Moreover, if the application allows the definition of the sequence in which the various client modules roam between different radio cells, roaming times can be significantly less than 50 ms. With rapid roaming and polling, it is also possible to transfer safety-relevant information wirelessly via PROFIsafe in addition to Profinet IO – for example, in applications with fixed routes, such as a suspended monorail or rail-guided vehicles. The RCoax radiating cable ensures that the radio-frequency field is actually located where it is needed and with the appropriate signal strength and signal quality.
Applications with freely movable wireless network devices, such as an automated guided transport system or a Simatic Mobile Panel 277F IWLAN, can make use of the iPCF-MC (industrial Point Coordination Function–Management Channel) extension, which also functions according to the polling method. iPCF-MC requires the use of an access point with two radio interfaces, known as a dual access point.
One of the interfaces is used for exchanging data between the client module and the access point, and the second is used to send (broadcast) administrative messages. The second radio interface of each access point transmits on the same radio channel – called the management channel – and is set up so that the client modules can simultaneously receive administrative information from all access points within range, as soon as they are not participating in a data exchange. The wireless network node determines the best access point to connect to, based on the information received.
Optimizing radio conditions
Operators also have additional requirements for IWLAN, such as minimizing the number of access points along the route and avoiding overlapping radio channels. Using an industry-specific extension, the iREF (industrial Range Extension Function), it’s possible to cover three different radio areas within a cell with three connected antennas. Consequently, the area served by an access point becomes larger. Using iREF, only the antenna with which the relevant WLAN client can best be reached is used.
This reduces interference with neighboring access points, resulting in higher data throughput for the entire system. In addition, selective transmission on the right antenna provides higher transmitting power, resulting in increased range.
Hardware suitable for industry
The industrial environment requires not only specific enhancements to the software; the hardware, too, must be adapted to the environment. Outdoors, the devices must have a suitable protection class (IP65) to protect them against wind and weather. The devices should also not be accessible to the public or operable by anyone – especially with regard to possible vandalism such as cable pulling or sabotage. Indoors, the devices should be able to withstand great temperature fluctuations (–40°C to +70°C), as condensed water can accumulate in these situations and cause damage.
That’s why versions for harsh environmental conditions (Scalance W788-2 M12 EEC) with conformal coating are also available for Scalance W700. Due to the limited space, a compact design is important for installation in the control cabinet. All Scalance W devices for the control cabinet can be easily mounted either on a standard mounting rail or an S7-300/S7-1500 mounting rail.
Operators do not need to worry about reliability or fail-safe functioning when using IWLAN in industrial environments, as IWLAN is far superior to home or office WLAN in this regard. Wireless now offers the performance needed for all applications, from simple diagnostics to safety applications like emergency stops.
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