NOV-DEC 2018

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INTECH NOVEMBER/DECEMBER 2018 45 AUTOMATION BASICS paths so the critical components in real-time con- trol (the sensor, such as a level or pressure transmit- ter, and the valve controller, for example) will have direct, line-of-sight paths to gateways, eliminating any "hops" and minimizing latency. Although some applications require lower or higher performance, the ISA100 Wireless stan - dard aims to ensure systems can transmit com- mand signals within 1 second. This is because it was primarily developed under the principle that 1-second latency covers a reasonable range of ap - plications without involving painful trade-offs or asking the user to apply unproven control con - cepts. Maintaining and supporting these existing operating principles is one of the key benefits of using ISA100 Wireless technology. One-second transmission times and reliable communication Safety operations are already exploiting the ability of ISA100 Wireless to provide 1-second transmission times, with safety systems set up in numerous refin- eries and process plants using ISA100 Wireless net- works. For example, an ISA100 Wireless adapter can be installed on an on-off valve (figure 1), so the control system can shut a valve in a minimum of 2 seconds, but the total response time will depend on the publica- tion period of the control signal. For example, the de- vice shown in figure 1 accepts signals once every 2 sec- onds, but other devices may introduce longer delays. A monitoring example is a PROFIsafe system using ISA100 Wireless to connect a gas detector to a safety programmable logic controller. In gas detection, sen- sor data must be quickly transmitted to a safety sys- tem when hazardous gases are detected. Using the PROFIsafe protocol over ISA100 Wireless ensures cor- rect message sequencing, message content, device address, and parameterization. The implementation relies on the low latency, reliability, error handling, and security intrinsic to ISA100 Wireless to meet SIL 2 standards. In these kinds of applications, response speed is critical, but must also be accompanied by a high level of reliability. Organizations can achieve this by using ISA100 Wireless's automatic retransmission feature. The packet arrival rate is an indicator of the cer- tainty of data transmission. A packet arrival rate of 90 percent means packets will arrive nine times in 10 transmissions. The calculation of the packet arrival rate with retransmissions is shown below. n In the case of one actual transmission + one re transmission: (1 – 0.1 • 0.1) = data arrival rate 99 percent n In the case of one actual transmission + three retransmissions: (1 – 0.1 • 0.1 • 0.1 • 0.1) = data arrival rate 99.99 percent As the calculations show, when the num - ber of retransmissions increases, the reliabil - ity of communication rapidly increases. Doubling reliability The ISA100 Wireless Duo-cast system is a redundant fixed mesh (figure 2) that pro- vides extremely high reliability by delivering data to the control- ler in a predetermined time, even if the wire- less communication and the wireless access point have a single fail- ure. Essentially, critical measurements are sent directly to two gate- ways simultaneously, ensuring that one of them will get through. This has the reliability needed for both wire- less valve monitoring and control and for other critical real-time control applications. With its 1-second latency time and reli - ability from automatic retransmission and Duo-cast, a well-de - signed ISA100 Wireless instrumentation net - work can operate as dependably as wired I/O in most critical applications, includ - ing valve control. n ABOUT THE AUTHOR Shuji Yamamoto (Shuuji.Yamamoto@jp.yokogawa. com) is the wireless promotion manager in Yok- ogawa's New Field Development Center. He joined Yokogawa after completing a master's degree in electronic engineering from Shinshu University with a specialty in high frequency research. He has had a variety of responsibilities with the company over his career, all primarily related to wireless net- working and Industrial Internet of Things. Figure 1. With proper project application engineering considering system dynamics, a simple ISA100 Wireless adapter allows this on/off valve to be reliably and quickly controlled. Figure 2. The ISA100 Wireless Duo-cast function gives a critical wireless transmitter a redundant path to a gateway.

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