ISA’s SP100 Industrial Wireless standardization committee continues its heavy schedule of teleconferences, email debates and frequent face-to-face meetings. The process control suppliers, meanwhile, continue to introduce wireless transmitters and system products at a rapid pace without any claims of standardization. End-user demand for wireless products is rapidly increasing, but end-user participation in the work of SP100 remains sparse.
In September 2006, at the invitation of SP100, suppliers and other interested parties submitted presentations and supporting white papers proposing their wireless solutions—in-whole or in-part. At the time there was a separation between simple data acquisition applications (SP100.14) and more complex systemwide use of wireless for control networks (SP100.11). Some 25 presentations were submitted, along with 15 white papers. When the issues settled, members of the SP100 committee realized that there were few technical differences between the submitted proposals! In the general meeting following the ISA Expo in Houston, the committee unanimously agreed to end the division between the two types of applications and merge the effort into a single wireless solution for the industrial plant and shop floor.
The standard being developed is now labeled SP100.11a, which will define the OSI layer specifications (e.g. PHY, DLL, etc), security specifications, and management (including network and device configuration) specifications for wireless devices serving application classes 1 (closed loop control) through 5 (data acquisition) and, optionally, class 0 (safety) for fixed, portable and moving devices. The standard is to focus on performance needs for periodic monitoring and process control where latencies on the order of 100 ms can be tolerated with optional behavior for shorter latency.
The standard is to provide consideration for the following:
- Low-energy consumption devices
- Scalability to address both small and large installations
- Wireless infrastructure
- Interfaces to and interoperability with legacy infrastructure and applications
- Security and network management requirements in a functionally scalable manner
- Robustness in the presence of interference found in harsh industrial environments
- Coexistence with other wireless devices anticipated in the industrial work space, such as IEEE 802.11x (Wi-Fi), 802.16x (WiMedia), ZigBee, cell phones, Bluetooth and other relevant standards
- Interoperability of SP100 devices
Suppliers working on the standard have now coalesced into two groups, each advocating a similar field network, but differing approaches to control system integration. The WNSIA (Wireless Networks for Secure Industrial Applications) group includes Honeywell, Adaptive Instruments, 3eTechnologies International, Endress+Hauser, Flowserve, Omnex and Yokogawa. WNSIA proposes a field network centered about a set of field access points to reduce the number of “hops” necessary to get data from sensors to controllers. Its rationale is to minimize latency for feedback control connections. Additionally, WNSIA also proposes a fully integrated protocol linking the field network to legacy control systems through a gateway device.
The Collaborative Initiative (CI) group is centered about a very complete network architecture proposed by Dust Networks with backing from members Emerson, Invensys (Foxboro), Apprion, Certicom, Sensicast, Advanced Industrial Networks, Siemens, General Electric, Software Technology Group, NewTrax, Machine Talker, Oak Ridge National Laboratory and Texas Instruments/Chipcon. The CI/Dust proposal calls for a self-healing wireless mesh field network using channel-hopping. The mesh network provides greater distances at low power consumption and alternate routing in case of obstructions and interference sources.
Physical Layer and MAC
While the committee acknowledges that radio technology will change rapidly over the next few years, a complete standard must include at least one radio. The debate over the use of a licensed radio frequency for industrial automation has been shelved for two reasons:
- A licensed radio frequency band would require custom radios that would probably cost too much for this narrow market, and
- The time and effort to secure a single worldwide frequency for industrial automation use is both time-consuming and out of the scope of the SP100 committee.
The type of radio selected depends on the operating frequency (band) and is the subject of the Physical Layer subcommittee. The Media Access Control (MAC) sublayer is usually defined in conjunction with the Physical Layer, since it must provide the firmware interface for the radio.
Both groups have proposed networks based on the use of IEEE 802.15.4 (ZigBee) radios at 868/915 MHz or 2.4 GHz with frequency- (channel-) hopping on top of the base radio’s DSSS (Direct Sequence Spread Spectrum) protocol. These radios are designed to use very little power and be included on a battery-powered, solar cell or energy-scavenging device. Working groups are now meeting to decide on the specific physical layer and medium access sublayer protocols to be included in the standard. The committee is also looking at 802.15.4a radios recently approved by the IEEE 802.15.4 committee that use either of two forms of Ultrawideband (UWB) protocol:
- Chirp-OFDM (Orthogonal Frequency Division Multiplexing) operating in the 2.4 GHz band, or
- Impulse Radio operating in the unlicensed UWB 3.5 to 10 GHz band.
These radios are expected to become very popular in many applications and will be very low- cost as well as have very low power consumption.