NOV-DEC 2018

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44 INTECH NOVEMBER/DECEMBER 2018 WWW.ISA.ORG Wireless valve control By Shuji Yamamoto I t took many years, but wireless technology has become accepted by skeptical instrument engi- neers worldwide. When ISA100 Wireless instru- mentation first appeared in 2010, engineers were nervous about reliability and noise immunity. Since then, millions of hours of successful operations have been recorded, and wireless is now often the first choice for new plant construction and revamps. One major reason is cost. Wired transmitters re quire a wired infrastructure, which can include a power supply, cabling, conduit, and cable trays to bring the signal to a field junction box or marshal- ling cabinet, along with I/O devices at the control and monitoring system to accept the transmitter's 4–20 mA or fieldbus signal. This can make a wired trans- mitter installation a very expensive and difficult un dertaking. Battery-powered wireless transmitters do not need the wired infrastructure or power sup- ply, so they can be installed quickly and inexpensively. Although few engineers now hesitate to use wireless for monitoring purposes, wireless valve control has not enjoyed such success, often due to latency, along with the safety and reliability required for closed-loop systems. Overcoming latency In a 4–20 mA wired system, the signal is always present. The control system can read a level trans- mitter, determine that a valve needs to close, send the appropriate signal to the valve controller via a 4–20 mA signal, and the loop is closed almost im mediately. Not so with standard wireless. A wireless level transmitter is typically battery powered. To save battery life, the transmitter nor- mally is set to send a wireless signal every second, or less frequently depending on the application. The wireless signal is relayed by other wireless transmitters in a mesh network arrangement until it arrives at a gateway. There, it is added to the plant's network and arrives at the control system. The control system determines that the valve should close and sends the appropriate signal over the network. The network delivers it to a wireless router, which transmits it to the valve controller. Total time elapsed is probably a minimum of 1 or 2 seconds depending on the signal frequency, but it could be much longer—perhaps 10 seconds or more, depending on latency in the type of mesh network. In particular, mesh networks with auto reconstruction of communication paths can sub - stantially delay signal propagation. For a control or safety valve, 10 seconds is entirely too much time. A tank could overflow while the valve awaits a close command. Fortunately, there are ways to address this issue, starting with selecting the right wireless mesh network. Mesh methods Three types of wireless mesh systems are used in the process industries: n Fully automatic: In this system, the path through the mesh is automatically determined and can vary. If, for example, a tractor-trailer parks so as to block a transmitter or gateway, the mesh au tomatically routes around it. It may require routing through three or four additional trans- mitters or repeaters, and it can take time to complete this rerouting. Advantages of an auto- matic system are that instrument engineers do not have to set the mesh route, and the system can automatically compensate for equipment failures and temporary blockages. n Semi-automatic mesh: In this otherwise auto- matic mesh system, engineers can set the mesh path for some transmitters. n Fixed mesh: The mesh path for every transmitter is determined manually. An automatic mesh may be unsuitable for wire- less valve control—and many other real-time con - trol functions—because the latency cannot be de termined beforehand. It frequently varies be- cause of interference or noise on the path of signal propagation. Therefore, fully automated meshes often degrade real-time performance. A semi-automatic or fixed mesh system enables reliable, real-time communication and near instan- taneous discovery of path failure. With a fixed mesh system, the latency for a specific transmitter can be calculated during the design stage. Ideally, for a control valve application, engineers will set

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