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ZigBee wireless mesh networking for process manufacturers27 June 2011Zigbee wireless technology can provide simple, decentralised wireless mesh networks that enable process manufacturers to improve their control and monitoring procedures, and achieve end-to-end visibility of production processes, personnel, logistics and distribution operationsWireless has hitherto been considered by the process sector to be too unreliable as a replacement for traditional point-to-point cable installation, and most wireless networks do not match the security, robustness and reliability of hard-wired telemetry systems.
But wireless technologies are advancing quickly; ZigBee, for example, is a cost-effective, standards-based wireless network designed to operate in electrically noisy industrial environments, and is typically used for connecting sensors in remote control and monitoring applications. The technology is able to support hundreds of devices in star, tree or mesh formations.
ZigBee, like Wi-Fi and Bluetooth, operates over the licence-free IEEE802.15, 2.4GHz frequency band. However, whilst Wi-Fi and Bluetooth have been designed for small networks carrying relatively large packets of information, ZigBee is intended for large networks and lower data rates. A standard one-milliwatt ZigBee chip, for example, has short range (70-100m) and operates at the relatively slow data rate of 250kbit/s.
Another crucial difference that sets ZigBee apart from other wireless systems is that ZigBee/802.15.4 is a full blown telemetry system in its own right, with the ability to provide wireless personal area networking (WPAN) - essentially. digital radio connections between computers and related devices, such as sensors. This kind of network avoids the need for physical data buses such as USB and Ethernet, and offers a number of key features.
These include an acknowledgement that data has been received at its destination; retransmission in the event of failure due to radio collisions or reflections (in a similar manner to TCP/IP networks); validation of message content using data sequence numbering (Frame Check Sequence); network redundancy - such that failure of a node on the network will enable messages to be re- routed via other nodes, and network protocol security encryption.
ZigBee cannot be considered for real time control applications requiring deterministic and fast response times as it has a 10-15ms network latency when routing messages between nodes. However, as ZigBee radio transmissions will propagate up to 100m, it is unlikely that a system will require more than one or two message hops, so the response time is adequate for most remote control and monitoring applications.
In addition, the gateway or network co-ordinator can be configured in ‘beacon mode’ to send and receive high priority messages in known guaranteed time slots called super-frames, when the application demands this level of response.
Peer-to-peer communications
Another major advantage of ZigBee for OEMs is the provision of an intelligent distributed architecture employing peer-to-peer communications protocol, instead of the traditional master-slave relationships that are commonplace in standard PLC networks. This enables a network of intelligent modules to communicate and interact with one another, without the supervisory function of a centralised master controller or PLC.
Whilst a ZigBee network of nodes provides a highly secure, self-healing and robust network, on its own the ZigBee stack is not enough to replace a network of PLCs. What also has to be considered is the control logic, memory and I/O requirement at each node.
Intelligent Distributed Controls’ (IDC’s) ZigBee platform addresses this requirement with a complete system-on-chip (SoC) OEM module - the ZB100, which is used as the basis for all the ZB product range. It integrates the ZigBee communications stack within a high speed processor and provides a C2420 transceiver with 18 configurable general purpose I/O lines for combinations of digital, analogue and serial communication and flash memory. In addition, the device accommodates a wide range of dc supply voltages (4-30Vdc) and provides a unique 16-digit MAC address.
IDC supports its ZigBee product range with complementary software applications, including ‘over-the-air’ programming of data logging, remote control and position tracking. Importantly for users of IDC’s ZigBee range, IDC is not reliant on third-party software providers; the hardware, application firmware, PC- based software and server applications are all designed in-house by the company’s software engineers at its headquarters in Derby.
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