A post on The Great Kanduski: Best Practices in Industrial Networking blog last week caught my eye. The post, Wireless Wasted with Thumbs opens:
The most common implementation of Industrial Wireless, especially WirelessHART is to install a “thumb” on the ‘spare’ outlet of a conventional HART transmitter so that the HART information can be accessed wirelessly practically anywhere. In my mind this is a step back because most control systems support HART communications directly via their AI and AO cards.
I thought I’d check with our wireless team who developed a Smart Wireless THUM adapter, based on the IEC 62591 (WirelessHART) standard, and announced its availability last year. Emerson’s Micah Naasz offered some thoughts that I wanted to share.
His first point was that there are still quite a number of automation systems in operation that do not have HART modems on their analog input and analog output I/O. If the system does already support HART I/O and the HART devices are already wired, then the THUM adapter may not be a good application. An exception may be in gathering data from multivariable devices. Some automation systems cannot accept the extra process variables from these multivariable devices. A WirelessHART THUM adapter and gateway can provide this additional information back to the automation system.
If process manufacturers need to access the diagnostic information to improve their operations and their automation system does not support HART I/O, the choice is either to wire HART multiplexers or use WirelessHART THUM adapters. Difficulties in distances, cable infrastructure, hazardous locations, etc. may prove too large an economic hurdle to justify the project. Micah notes an example application where the plant might have power, but not control wiring. Radar gauging on tank farms is an example and THUM adapters allow gathering of this level data in an efficient manner.
In many facilities, control can be improved by getting feedback on valve position. A gateway and THUM adapter mounted on valve position monitor units can provide this without any wiring changes. For instrumentation maintenance, HART devices that do not currently provide their diagnostics to the asset management or control system can be added so that initial maintenance assessments can be performed remotely from the device.
In a closing point:
I suppose you could also make a case that if you wanted to access the HART data without installing the “HART Stripper” system which requires lifting and landing every wire that might be valid as well though hardly an economic reason to install a complete wireless infrastructure.
Infrastructure for a WirelessHART installation scales from a single WirelessHART gateway connected natively or via serial MODBUS, MODBUS TCP, or OPC and a WirelessHART device. In a 2008 post, WirelessHART Installation Best Practices, I shared ways to scale your wireless infrastructure to maintain high communications reliability.