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JUL-AUG 2017

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42 INTECH JULY/AUGUST 2017 WWW.ISA.ORG AUTOMATION BASICS to add a router mounted as near to the instrument as possible and clear of ob - structions. If more than one instrument is in the same difficult location, a single router can service a group. Laying out a network Most networks are designed from two ends, the field and the control room. Field devices must be located accord- ing to their process function, which could easily be in a congested pipe jungle where equipment interferes with clear signal propagation. The final gateway is often placed near the con- trol room, because it is hardwired to the control system. The network must bridge this gap. Creating a sky mesh requires find- ing where it is practical to place rout- ers. Ideally, these should be high off the ground and as close to the individual field devices as possible. Ensuring reli- able communication between the field devices and the nearest sky mesh rout- er may involve a secondary router in between to compensate for signal loss. In most process plants, it is not dif- ficult to find tall structures, such as distillation columns, but they may not be located where they are useful for router placement. Positioning antenna to avoid signal blockage problems as - sociated with such large metallic struc- tures can be tricky. As a rule of thumb, if the router is placed 30 m above the ground, it can reach individual field devices close to ground level up to 50 m away (figure 5). This assumes a few beneficial reflections, balanced against some obstructions from piping. The connection from each field de- vice to the closest router is the most challenging because it often has the most obstructions. Communication between routers and the gateway is easier to visualize and evaluate, since those components are mounted high - er above the process equipment in more open space. Evaluating performance The two most common measures of network performance are bit error rate (BER) and packet error rate (PER). The former uses predetermined bit patterns to check which are received incorrectly, a process requiring dedi - cated software on all the field devices, routers, and gateways. It must be per - formed as a specific test, sending the designated patterns. PER performance measurements, on the other hand, deal with complete packets and can be done without spe- cial tools during normal communica- tion. If a problem is developing, there will be a detectable change in the PER. The most important indicator is determining how often packets get through correctly the first time. Getting the PER as low as possible is the objec- tive, but this can only be done when all radio links are working reliably. A well-designed ISA-100.11a wireless instrumentation network can operate as dependably as wired I/O in most applications. When the communica- tion links connect reliably, latency will be minimized, allowing control room operators and other plant personnel to have all the information they need in a timely manner. n ABOUT THE AUTHOR Shuji Yamamoto is wireless promotion manager in Yokogawa's New Field De - velopment Center. He joined Yokogawa after completing a master's degree in electronic engineering from Shinshu Uni - versity with a specialty in high frequency research. He has had a variety of re - sponsibilities with the company over his career, primarily related to wireless net - working and IIoT. RESOURCES ANSI/ISA-100.11a-2011 (IEC 62734), Wireless Systems for Industrial Automation: Process Control and Related Applications www.isa.org/wireless-systems-for-industrial- automation-118261 Automation Network Selection: A Reference Manual, Third Edition www.isa.org/networkselect Wireless Networks for Industrial Automation, Fourth Edition www.isa.org/wirelessnetworks Wireless Control Foundation: Con- tinuous and Discrete Control for the Process Industry www.isa.org/wirelesscontrol RT: Router I/O: Field instrument Gateway Figure 5. Routers in high positions can reach down to communicate with field devices closer to ground level. The practical area of coverage under favorable conditions is roughly a 90º to 100º cone, with the router as the cone's apex. When planning a wireless network for wireless instru- mentation, following a few basic rules on transmission sight lines and antenna placement can ensure a higher level of success.

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