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Industrial Ethernet Book Issue 71 / 39
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A look at wireless technologies for industrial applications

Wireless communication has been used in industry for over 30 years. Among the first applications was in the wireless control of cranes in warehouses, where proprietary radios achieved flexible control of moving devices. During the past decade, standardised radio technologies like Wireless LAN (IEEE802.11), IEEE 802.15.4 and Bluetooth technology (IEEE802.15.1) have become the dominating technologies for industrial use. Mats Andersson, CTO connectBlue AB, Sweden looks at the most popular wireless technologies.

MODERN INDUSTRIAL PLANTS comprise many varied devices interconnected in different ways. Such devices include simple data collection units (I/Os) without built-in intelligence, more intelligent devices (sensors with built-in intelligence, single-loop controllers or programmable controllers) and supervisory systems (used as HMI, for data logging and supervisory control). All are interconnected using different communication protocols and media types that - in some cases - can be replaced by wireless technologies to achieve a number of advantages.

No one technology

No single wireless technology offers all the features and strengths that fit the various industrial application requirements, so standardised wireless technologies, such as Wireless LAN, Bluetooth and IEEE 802.15.4 (as well as a number of proprietary technologies) are all used in practise.

The main requirements could either be high data throughput, or robustness, or low power (the latter especially for battery operated devices). Wireless LAN is often used for production planning and data acquisition, as well as applications where rapid roaming is required. Bluetooth is used for HMI, programming and service/maintenance, in addition to real-time control tasks. Other technologies, such as IEEE 802.15.4 (ZigBee, WirelessHart etc.) and Bluetooth low energy, have become increasingly used for sensors, actuators and a multitude of other small devices that need to be interconnected.

The adoption of wireless communication in an industrial environment is typically a gradual process. The initial requirements include the creation of islands of wirelessly enabled devices connected to an existing infrastructure/wired network.

The wired network may be a standard IP-based network like Profinet, or an industrial fieldbus network, such as Modbus TCP, Devicenet, Controlnet or Interbus-S.

Use of wireless communication

The use of wireless communication can be divided into the following:

• Serial cable replacement;

• Ethernet cable replacement and Ethernet infrastructure;

• Seamless roaming;

• Fieldbus cable replacement;

• App in a smart phone or other mobile device;

• Wireless sensors and actuators network;

Serial cable replacement. Many of today¡¯s industrial devices still use traditional serial interfaces (UART, RS232, RS422 or RS485) to connect to configuration or programming tools. These tools are typically connected ad-hoc when a reconfiguration or reprogramming is needed and the tools normally operate on a standard PC. The tools typically use an application dependent or device specific communication protocol to communicate with the device. All these abilities make them good candidates for wireless connections.

Fig. 1: PC-based programming tool. This is shown connected to a PLC as a serial cable replacement.

Figure 1 shows a PC-based programming tool connected to a PLC with a serial cable replacement, while Figure 2 shows an RS422/485 multi-drop serial port communication replacement. Figure 3 shows a serial to WLAN infrastructure.

Fig. 2: RS422/485 multi-drop serial port communication replacement.

Fig. 3: Serial to Wireless LAN infrastructure.

There are two ways in which a serial cable replacement can be created:

• An external wireless adapter connected to an external serial port of the industrial device. The wireless adapter emulates a serial port and transfers the data over the air;

• A built-in wireless adapter connected internally to the device electronics;

The serial cable replacement solution would either use Bluetooth technology or Wireless LAN, both of which are standard in most modern laptops. For an ad-hoc connection, Bluetooth is the most suitable, while Wireless LAN often is best suited for connections through an Ethernet infrastructure.

Ethernet cable replacement and infrastructure. The use of Ethernet based communication in industrial communication is increasing dramatically. The most used protocols (Modbus TCP, Profinet, and Industrial IP) transfer messages on an Intranet using Ethernet.

Ethernet cable replacements are most commonly used in industrial applications where there are mobile, rotating and temporary installations with either a need for replacing the Ethernet cable with a robust and maintenance-free wireless connection, or a need to connect to a Wireless LAN infrastructure. For easy set up and wireless configuration, a transparent Ethernet to a wireless adapter or a gateway is typically used.

Commonly used scenarios include a simple point-to-point Ethernet cable replacement based on two wireless Ethernet port adapters (Fig. 4). For the wireless connection, Wireless LAN would be used for applications requiring high band width, and Bluetooth technology would be used when robust data transfer and/or high system density is needed.

Fig. 4: Simple point-to-point Ethernet cable replacement. This is based on two wireless Ethernet port adapters.

A second scenario (Fig. 5) envisages connecting devices such as PLCs and HMI panels to an existing wireless infrastructure, which is typically a Wireless LAN network. Whether or not the devices have Ethernet ports or serial port interfaces there are industrial adapters available that can via a standard Wireless LAN access point (AP) transfer transparent data over the Wireless LAN network to the wired network backbone.

Fig. 5: Connecting devices to an existing network. Industrial adapters can transfer transparent data over the Wireless LAN network to the wired network backbone via a standard Wireless LAN AP.

Seamless roaming. Roaming is used in data communication where there are moving devices, such as AGVs, or where the device¡¯s data communication path changes from one AP to another. In such cases, the existing connection performance is affected by the roaming procedure since a scan for new wireless networks is required. Then, the established wireless connection must be terminated before a new connection can be up and running.

In standard solutions, the device (client) scans for an available AP, then connects to it. To maintain the communication connection with the control system, the client stays connected to an AP as long as possible. When the radio signal gets too weak, the client starts to scan for new APs that can offer a better radio signal. When such an AP is found and selected, the client starts a connection procedure to the new AP. This handover phase can take anything from 50ms to several seconds until the communication connection is fully up and running again (see Figure 6).

Fig. 6: Seamless roaming. In this case, it has been provided by connectBlue, which uses standard APs with two or more wireless clients that cooperate on each moving device.

Seamless roaming solutions can provide handover time delays within a few milliseconds. Some of these need APs with supplier specific solutions, but there are others that use standard APs with two or more wireless clients that cooperate on each moving device (again, see Fig. 6). An advantage of the latter, where several independent devices are used, is that roaming can be combined with redundancy.

Fieldbus cable replacement. Well-known fieldbuses (Profibus, CANBus, DeviceNet, InterBus-S, etc.) have a large installed base and a wide range of available products and devices. The large existing installed base is the main reason why fieldbuses are installed in increasing numbers. However, there are tougher timing requirements when replacing a fieldbus with a wireless link than there are when replacing a serial port based device with a wireless link. There is a large variety of fieldbuses that are either standard or vendor specific. An example of such a device is a CAN/ Bluetooth adapter (Fig. 7).

Fig. 7: A CAN / Bluetooth adapter. A wide range of fieldbuses are either standard or vendor specific.

App in mobile devices. Mobile devices such as iPhone/iPad, Android, Win CE, and LINUX devices are very widely used in everyday life. By installing a tailored ¡®app¡¯, such mobile devices can become powerful and cost efficient tools for industrial applications. Being a third-party software program developed specifically for a smart phone or a mobile device for industrial applications, an app can be designed to gather certain data, and to perform specific tasks - such as to act as an HMI panel or a remote control.

The wireless communication between the mobile device and the industrial device typically uses a Wireless LAN TCP/IP based network connection or Bluetooth technology through the Bluetooth Serial Port Profile (SPP). The new Bluetooth low energy technology will be suitable for such applications.

Wireless sensors and actuators. The uptake of wireless communication to sensors and actuators is forecast to grow fast because of the increasing need to keep better track of energy usage, controlling devices, and utilities. Also, many sensors and actuators are not yet part of a network and a large portion of these will be battery powered in order to lower installation cost.

Sensors and actuators may be of different types. Some have significant built-in intelligence; others are simple I/O devices serving as low-end interfaces to the process equipment. Depending on the requirements, the choice of wireless technology and its accompanying implementation strategy may differ. The software for a simpler sensor or actuator may be implemented directly in the CPU of the wireless chipset.

A more demanding device is a vibration sensor situated on a moving axis. To support a complete wireless solution, this application needs an alternative power solution in the form of a battery or other source. Some wireless technologies are better suited for low power modes than others (Table 1). The wireless interface for such devices can be achieved by integrating a simple wireless module into the sensor/actuator, or by using a more advanced wireless module with a built-in CPU capacity to handle the sensor/actuator functionality.

Table 1:This provides a quick overview of the differences between the wireless technologies offered in industrial applications.

Industrial requirements. Industry poses high demands on the wireless communication, including providing reliability and robustness, advanced security features, the need for similar configuration and operation as commonly used automation tools, real-time and deterministic behaviours may be required, as well as an elevated temperature range. All these requirements are supported slightly differently by the various wireless technologies. In the segments below, the differences are explained in more detail (see also Table 1):

Bluetooth technology (IEEE 802.15.1) is well suited for wireless integration of automation devices in serial, fieldbus and industrial Ethernet networks. Bluetooth is specified for devices with high demands on small footprint, low power consumption and cost-efficiency.

Bluetooth provides a range of 10m but can - with a long-range module - cover 200 - 400m in free line-of-sight. It is cyclic and provides fast transmission of smaller data packages. The maximum data throughput is 780 kbit/s gross (up to ~700 kbit/s net). With Bluetooth v2.1+EDR (Enhanced Data Rate), the data throughput is 2.1 Mbit/s gross (~1.5 Mbit/s net). The latency is 5 - 10 ms, and security features include 128-bit encryption that offers protection against data eavesdropping. The high system density allows several wireless devices to be connected in the same radio environment and operate together. Additional features include Adaptive Frequency Hopping (AFH), Forward Error Correction (FEC), narrow frequency channels, and low sensitivity to reflections / multi-pathing.

Bluetooth v4.0 low energy technology is well suited for sensors, actuators and other small devices that require extremely low power consumption. It offers the following features: high numbers of communication nodes with limited latency requirements; very low power consumption; robustness equal to Classic Bluetooth; good real-time features (if a small number of nodes are connected); and a very short wake-up / connection time.

Wireless LAN (IEEE 802.11) is well suited for monitoring, configuring and data acquisition, but can also be used for time critical control.

Further, the built-in roaming functionality is useful in factory automation applications with moving devices. Implementing Wireless LAN in these types of applications often requires customised solutions, such as tailored or proprietary roaming software, as well as frequency planning and specific installation means. An example is the use of expensive leakage-cables. With such tailoring, stable latency and low roaming hand-over delays can be achieved.

Wireless LAN typically has a range of 200m (up to 400 - 500m in free line-of-sight) in the 2.4GHz band and some 50m in the 5GHz band (802.11a)- free line of sight up to 150m. However, obstacles and interference could lower the range substantially. The data throughput is 11 to 54 Mbit/s gross (~5 to 25 Mbit/s net) for IEEE 802.11b/g and 300 Mbit/s gross (~70 Mbit/s net for IEEE 802.11n). Security models include WEP, WPA, WPA2, TPIK and PSK EAP. As IEEE 802.11a operates on the 5GHz band, it allows 19 extra non-overlapping channels in addition to the three non-overlapping channels in the 2.4GHz band.

ZigBee, WirelessHART and ISA SP-100 are all used in industrial applications and all are based on IEEE 802.15.4. The low power consumption makes it well-suited for battery operated devices. The technologies are mostly used in applications such as energy monitoring, process and building automation. The mesh network functionality makes it capable to cover wide areas when there are no requirements on low latency.

IEEE 802.15.4 provides for low power consumption, a short wake-up/connection time, a high number of communication nodes, a gross data throughput of 20 - 250 kBit/s over a range (excluding mesh functionality) of 75m. It allows for the automatic building of mesh-networks, and there are alternative radio possibilities on the 868MHz and 915MHz bands. Security features include 128-Bit encryption. See Table 1.

Co-existence. As more than one wireless technology is often used in parallel, there could potentially be disturbances which are not allowed in an industrial application. Therefore, it is important to optimise co-existence of various wireless technologies to achieve disturbance-free operation. All of today¡¯s most used wireless technologies operate in the 2.4GHz band (Fig. 8) and they address potential disturbances in the following manner:

• Wireless LAN has three non-overlapping channels with a 22MHz bandwidth, and uses Direct-Sequence Spread Spectrum (DSSS). DSSS ensures that the transmitted signal takes up more bandwidth than the information signal that is being modulated, so the wireless communication link becomes less vulnerable to disturbances;

• Bluetooth technology has 79 channels with a bandwidth of 1MHz and combines this with Adaptive Frequency Hopping (AFH) to avoid interferences. AFH monitors the bit-rate and when disturbances (such as when another wireless technology occupies the link) are found, Bluetooth stops using the occupied channels. The occupied channel is monitored in the background and as soon as it is free, it can be used again. Bluetooth low energy technology also uses AFH, but only uses 40 x 2MHz wide channels;

• IEEE 802.15.4 has 11 channels with a bandwidth of 5MHz and is using Direct-Sequence Spread Spectrum (DSSS);

Too crowded?

The 2.4 GHz frequency band is very crowded - especially so for wireless LAN, which is well-established throughout offices on to production planning.

To achieve disturbance-free communication, the wireless LAN must not be peturbed. One solution is to use the 5GHz band (IEEE 802.11 a). However, even though this is currently increasing in popularity for many industrial applications, there is a large installed base of IEEE 802.11b/g systems. Where wireless LAN and IEEE802.15.4 are used in parallel, co-existence can be implemented by making room for some IEEE 802.15.4 channels in-between the three WLAN channels. This allows WLAN and IEEE 802.15.4 to work reliably in parallel.

When performing a service discovery or establishing a device connection, Bluetooth activities can disturb a WLAN network. To ensure smooth operation in parallel with other wireless technologies, ConnectBlue has developed its Low Emission Mode, which allows co-existence during service discovery and connection set-up. It combines limited output power with optimisation of service discovery and connection set-up parameters. It achieves this without jeopardising Bluetooth specification or interoperability between Bluetooth enabled products, which is what is required.

Source: Industrial Ethernet Book Issue 71 / 39
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