Wireless mesh networking promises end-to-end process visibility

17 May 2011

More commonly recognised for its applications in building automation projects, CEE was interested to find out more about the capabilities of the ZigBee wireless mesh network in the process manufacturing environment.

The technology defined by the ZigBee specification is intended to be a simple and less expensive networking solution than other WPANs, such as Bluetooth and is mainly targeted at radio-frequency applications that require low data rates, long battery life, and secure networking.

The 802.15.4-based system is designed for remote control and sensors, which tend to be many in number but only require small packets of data, and in the main, have extremely low power requirements for long life. The power and flexibility inherent in the technology is exemplified by its ability to support hundreds of devices in star, tree or mesh formations.
IDC, a UK-based automation and tracking specialist, is offering a range of ZigBee-based wireless solutions designed to provide simple decentralised wireless mesh networks. In the process manufacturing environment this is said to enable an improvement in control and monitoring procedures, achieving end-to-end visibility of production processes, personnel, logistics and distribution operations.

Wireless has been considered by many as an unreliable replacement for traditional fixed cables in process environments. Many wireless networks do not offer the security, robustness and reliability needed for replacing hard-wired telemetry systems.

Overcoming barriers
However, IDC believes that ZigBee has the potential to overcome these barriers; having been designed to address the need for a cost-effective, standards-based wireless network, which is able to operate in electrically noisy industrial environments; and, typically, for connecting sensors in remote control and monitoring applications.

In a similar way to Wi-Fi and Bluetooth, ZigBee operates over the licence-free IEEE802.15, 2.4GHz frequency band. However, while 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, evidenced by the standard one-milliwatt ZigBee chip, which is short range (70-100 mm) and relatively slow, only 250 Kbits/sec.

ZigBee/802.15.4 is also a full-blown telemetry system in its own right, with the ability to provide wireless personal area networking (WPAN) i.e. digital radio connections between computers and devices such as sensors. This would eliminate the need for a physical data-bus such as USB and Ethernet cables. It also benefits from an acknowledgement that data has been received at its destination and retransmission in the event of failure due to radio collisions or reflections - in a similar was to TCP/IP networks.

Other benefits include validation of message content using data sequence numbering and 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.

Although ZigBee cannot be considered as a solution for real-time control applications that require deterministic and fast response times - it has a 10 – 15ms network latency when routing messages between nodes within the network. 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, making the system response time adequate for use in 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, for more time critical applications.

Another advantage of ZigBee, for OEMs, is that the technology provides an intelligent distributed architecture employing a peer-to-peer communication protocol, in place of the more traditional master-slave relationship found in standard PLC networks. As a result, ZigBee offers the ability for a network of intelligent modules to communicate and interact with each other, with no requirement for a centralised master controller or PLC.
While a ZigBee network of nodes provides a secure, self-healing robust network, on its own the ZigBee stack it 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 for its own application. IDC has addressed this requirement with an OEM module that provides a complete System-on-Chip (SoC) solution.

The module is used as the basis for the company’s ZB product range. It integrates the ZigBee communications stack within a high-speed processor and provides a transceiver with 18 configurable general purpose I/O lines for combinations of digital, analogue and serial communication and flash memory. In addition, the unit accommodates a range of DC supply voltages (4-30vDC) and provides a 16- digit MAC address.

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