Ian Verhappen, P. Eng. is an ISA Fellow, ISA Certified Automation Professional and a member of the Automation Hall of Fame. Ian is a recognized authority on Foundation Fieldbus' industrial communications technologies and process analyzer systems. Verhappen provides consulting services on field level industrial communications, process analytics and heavy oil/oil sands automation. Feedback is always welcome via e-mail at [email protected].
I’m sure readers of this column are familiar with the wireless sensor network standards, WirelessHART and ISA100.11a. However, the Chinese national committee to IEC has also introduced two additional standards: WIA-PA for Process Automation, approved as IEC 62601 in 2011 (which has been mentioned in this column before); and soon-to-be-published WIA-FA for Factory Automation, which is being balloted as IEC/PAS 62948.
IEEE 802.15.4-based WIA-PA standard, as developed by the team led by Professor Yu at the Shenyang Institute of Automation, has the following groundbreaking capabilities:
- Reduced communication traffic and energy consumption via packet aggregation, achieving an average power of 0.63 mW at the routing node whose maximal RF duty cycle reaches 0.18%, in an interference-free environment;
- Adaptive frequency hopping control among the 16 channels approved for the use of the 2.4 GHz spectrum in China and the joint regulation of rate/power, which effectively overcomes expected industrial interferences, resulting in transmission reliability for a 100-node network of at least 99% in all anticipated environments;
- Using multisource-based, high-precision clock synchronization technology that enables real-time closed-loop control by reducing the access time delay from seconds to 10 ms; and,
- Like other wireless sensor network standards, WIA-PA includes 128-bit AES encryption.
Presently, you can only buy WIA-PA and WIA-FA products in China. WIA-FA is, however, the newer standard, and it's the first wireless technology specification developed specifically for factory, high-speed, automatic, control applications. Individual factory sensors have less information, so the network can connect cells that contain multiple data points.
Access to the WIA-FA network is through one of the optionally redundant gateway devices, connected to one or several access devices. Multiple access devices may communicate with the field devices in parallel and form multiple, again optionally redundant, star topologies. Each access device forms a star topology with the field devices. These access devices have the same address and are transparent to the field devices.
WIA-FA network protocol defines the physical layer (PHY), data link layer (DLL) and application layer (AL). DLL communications are based on multiple access devices: time division multiple access (TDMA), frequency division multiple access (FDMA), retransmission and aggregation, while the PHY is based on the IEEE 802.11 physical layer.
WIA-FA DLL is designed to guarantee real-time, reliable and secure communication between WIA-FA field devices and access devices by:
- Adopting a TDMA data-transport mechanism based on the "superframe" concept. Superfames are used to avoid transmission collisions between frames, and ensure reliability and real-time performance of transmission, while supporting frame aggregation/disaggregation, etc.
- DLL management functions include defining device joining, leaving, time synchronization, remote attribute get/set, etc. WIA-FA superframe is a collection of timeslots cyclically repeating at a constant rate. Though the length of a timeslot is configurable, each timeslot is only used for transmitting one frame. A WIA-FA superframe timeslot, together with the radio channels, are assigned to a link with each individual device link specified by a timeslot and a channel. The default superframe consists of beacon timeslots used by a field device to join the network, management timeslots, and data timeslots.
Each WIA-FA network has only one network manager (NM), which resides in the gateway device where it implements the network management function. The WIA-FA network management performs the following functions:
- Allocating the unique 8-bit or 16-bit short address for all devices in the network;
- Constructing and maintaining the redundant star topology;
- Allocating communication resources for communications of WIA-FA devices; and,
- Monitoring the status of the WIA-FA network, including device status and channel condition.
Lastly, the WIA-FA application layer (AL) supports distributed applications for users. The AL comprises user application processes (UAP) and the application sub-layer ASL defines communication services among UAPs on different devices. Each UAP is composed of one or more user application objects (UAO) that interact with industrial processes, while the device management application process (DMAP) is a special UAP.
WIA-FA supports three types of application data transferred between the gateway device and field devices:
- Aperiodical urgent commands such as start and stop commands with data priority RT0;
- Periodical input data (e.g., sensor measurement values, switch status, actuator feedback values), and periodical output data (e.g., actuator setpoints, switch set values) with data priority RT1;
- Aperiodical requests and responses for attribute read-and-write accesses, as well as alarm acknowledgements with data priority NRT (non-real-time);
- Aperiodical alarm reports with data priority RT2; and,
- Periodic management data (priority RT3) for monitoring data and network status messages.
Though perhaps not as well known as the process-based wireless field sensor networks from Fieldcomm group and ISA, the WIA standards developed in China also serve the niches and markets for which they're developed. Recognition of this effort has been made by the IEC; the challenge remains for the market to embrace the appropriate standard based on the application for which it's required or installed.
Homepage image courtesy of Stuart Miles at FreeDigitalPhotos.net
