Wireless automation for process control

Several different wireless technologies have evolved to support flexible links for industrial networking and process control applications, with a range of capabilities from hard real-time to low-cost.

There is a range of unlicensed frequencies available, varying from the sub-1GHz variants in different parts of the world - 915MHz band for the Americas, 868MHz in Europe and 920MHz in Japan - to the unified 2.4GHz band. This provides greater volume, lower costs and more opportunities for integration of the RF transceiver with a low-cost microcontroller core.

For applications that require a hard real-time capability such as monitoring critical chemical or nuclear processes, a low latency solution is vital. Wireless versions of fieldbus protocols such as WirelessHART provide a way to deliver that real-time performance in a more flexible manner with wireless links. While it is based on the popular unlicensed band at 2.4GHz used by other technologies - notably WiFi and ZigBee - it also includes security and interference protection that makes sure every packet of information is delivered within a certain time.

Within the 2.4GHz global unlicensed band ZigBee is emerging as a robust yet flexible industrial protocol, allowing mesh networks to develop so that data can hop from one node to the next. ZigBee has also now added support for IPv6 so that these nodes can be accessed directly from the Internet. While this requirement has not seen huge demand in industrial applications, the use of remote monitoring and wider networking is growing and will be a key requirement for systems in the next few decades.

 

ZigBee IPv6

After several years of discussion, a version of the ZigBee mesh networking standard that supports the new IPv6 Internet protocol has been launched. IPv6 extends the address space of today’s IPv4 protocol, which has essentially run out of numbers to identify all the devices on the Internet.

Moving ZigBee to support IPv6 requires changes to software in the network layer, and, as it is also designed to work with the ZigBee Smart Energy protocol (version 2, SE2) at different frequencies, can need changes to the MAC and PHY layers in a device. Revision 0.7 of the SE2 is being discussed in the industry and takes ZigBee into other frequency bands, not just the unlicensed 2.4GHz band. This supports smart power grid, power and water utility applications as well as industrial machine-to-machine (M2M) applications such as process control.

ZigBee IP is the first open standard for an IPv6-based full wireless mesh networking solution and provides seamless Internet connections. The key is that, unlike ZigBee today, ZigBee IP networks do not need gateway devices to connect to the Internet and so will work directly with other Ethernet and Wi-Fi devices. The fact that this will also work with the sub-1GHz bands around the world opens this up as a global opportunity for process control equipment developers.

The specification adds network and security layers and an application framework to the existing IEEE 802.15.4 standard to provide a more scalable architecture but with the same proven, end-to-end security using TLS1.2 protocol, link layer frame security used in ZigBee. This is based on AES-128-CCM algorithm and support for public key infrastructure using standard X.509 v3 certificates and ECC-256 cipher suite.

A key change is that ZigBee IP continues to support global operation in the 2.4GHz frequency band but now also supports regional operation. Raw data throughput rates of 250Kbs can be achieved at 2.4GHz with 16 channels, 40Kbs at 915MHz through 10 channels and 20Kbs at 868MHz on one channel. Transmission distances range from 50 to 200 metres, depending on power output and environmental characteristics.

RS Zigbee IP.pngBecause there is no need for a gateway on the network, ZigBee IP had to support the development of discovery mechanisms with full application confirmation. Devices can be added anywhere in the network, and paired with devices perhaps on other networks, so the discovery, authentication and pairing layers are significantly more complex

Interference will be an increasingly important challenge with IPv6-based systems that are more spread out and working with other networks in the 2.4GHz band such as Bluetooth and WiFi. As a result ZigBee IP products have access to 16 separate, 5MHz channels in the 2.4GHz band so that band management can be used to avoid interference. This can either be done manually when a device is added to the network, or, as is more likely, using adaptive software to monitor the quality of a link and hop to a new band if necessary. This will also help keep the power consumption low, as higher power will not be needed in the link to overcome the interference.

More importantly several of these do not overlap with US and European versions of WiFi and so these bands can be used carefully to avoid likely sources of interference. ZigBee already incorporates an IEEE 802.15.4 CSMA-CA mechanism that reduces the probability of interfering with other users, plus ZigBee uses automatic retransmission of data to ensure network robustness. Because the duty cycle of a ZigBee product is usually extremely low, relatively few packet data units are transmitted, reducing the likelihood of an unsuccessful transmission and making the protocol more robust. Additionally, the ZigBee IP protocol suite contains mechanisms that allow an operational network to move to a different channel.

The 802.11 WiFi standard (a,b,g,n) is cost effective but there can be issues of interference from other networks. While it lacks real time support it can be well suited to monitoring applications such as streaming a video feed from a remote camera.

 

WirelessHART

WirelessHART enables users to quickly and easily gain the benefits of wireless technology while maintaining compatibility with existing HART devices, tools and systems. The wireless version includes several features to provide built-in 99.9% end-to-end reliability in all industrial environments such as channel hopping to avoid interference and provide coexistence with other wireless networks. Clear Channel Assessments test for available channels, while blacklisting avoids frequently used channels, optimizing bandwidth and radio time. For the time critical links to the interface, time synchronization provides on-time messaging and the self-healing network topology means any breaks or failures do not impact the data transfer.

Systems can be developed using flexible 2.4GHz transceivers as the RF transceiver.

 

Security

Security is also a key consideration for an industrial network, and WirelessHART employs the latest security techniques to provide the highest levels of protection available via 128bit AES encryption and a unique encryption key for each message, as well as device authentication.

Each device in the mesh network can serve as a router for messages from other devices. In other words, a device doesn't have to communicate directly to a gateway, but just forward its message to the next closest device. This extends the range of the network and provides redundant communication routes to increase reliability.

 

Conclusion

The range of wireless frequencies and protocols available for process automation, coupled with the different performance requirements down to hard real-time responses, has created a fragmented industry. The emerging ZigBee IP technology opens up all the regional frequency bands alongside the 2.4GHz band and gives the flexibility to connect equipment up in new ways. This requires higher security levels, and the new protocol combines higher performance with more flexibility and the promise of an effective cost reduction programme as devices ship in ever-higher volumes.