Do Your Homework Before You Go Wireless
What considerations should processors make prior to installing a wireless sensor system?
Paul Richards, CEO, Wireless Sensors
“Wireless devices provide many advantages for plant personnel, such as elimination of wiring, rapid installation, suitability for temporary or mobile applications, and robust operation. However, these attributes cannot be realized without thought given to the installation environment and how the system is to interface with plant equipment.
“The first consideration is around the realities of radio frequency (RF) data transmission, and care should be given to ensure a robust RF path between devices. This problem can be mitigated by proper selection of equipment and careful installation.
“Many different types of RF devices exist and all are not created equal. State-of-the-art devices employ spread-spectrum, frequency-hopping RF technology implemented in mesh network topologies. Spread spectrum and frequency hopping refer to how the signal is transmitted from point to point. The spread spectrum technique spreads the RF energy over a channel to reduce the effects of interference, while frequency hopping allows the device to use multiple frequencies in case it cannot transmit on a given channel due to interference, destructive reflections or null spots. This frequency diversity maximizes the robustness of each individual transmission hop. Even so, the signal cannot always get through, and that is when path diversity or mesh networking comes into play.
“Mesh networks allow the system to use multiple transmission paths, providing a built-in backup in case one path is completely eliminated due to equipment failure, mobile obstructions or other site-specific anomalies. Selecting a system with these attributes provides a basis for a reliable system. Thought should then be given to the proper selection of components and their installation in order to take advantage of these attributes.
“Equipment should be installed such that multiple paths can exist for the signals, eliminating single points of failure. Most current generation wireless sensors operate at the same frequency and similar power levels as Wi-Fi systems, and point-to-point ranges are similar—keeping this in mind as a system is laid out can ensure it is formed in a redundant fashion. Most manufacturers have installation and monitoring tools that allow the installer to see how the system has formed and what paths are available. If these tools indicate a weak path, then additional repeaters or routers should be installed.
“Once the system is configured, it can be implemented in much the same fashion as any conventional wired system, with most manufacturers supporting common communication protocols, such as Modbus TCP, for easy integration into factory floor systems. Most wireless systems are deployed for monitoring rather than control applications, and often customers want to avoid the systems integration complexity associated with factory floor data logging. The flexibility of most wireless systems allows direct connection to LANs and the Internet. Many wireless networks also have the ability to push data to cloud-based servers, which means that only a browser is required to access the data, and makes alarming and reporting available via e-mail to any mobile device. This cloud-based logging completely eliminates any site-based software, as well as the patches, backup and revision control required by locally hosted systems. Since there is no free lunch, this service comes with recurring monthly fees.”
Todd Hansen, Director of Strategic Marketing, Honeywell Sensing & Control
“There are two different wireless sensor systems from which to choose. If redundancy, broader coverage, scalability and higher levels of security are needed, then a mesh network may be the best solution. With a mesh network, the components required include access points located at strategic locations around the facility. The access points also help to reduce the burden on the battery-powered sensors as they need not route messages on behalf of other sensors.
“A two-level mesh network provides the best performance: One level is the mesh formed by the sensors and the second level is the mesh formed by the access points. A dedicated server, an interface software program and a device to enter security codes during setup complete the system. Once set up, the device parameters can be remotely set and monitored.
“For plant installations, the decision to go wireless may be based on the lower installed costs (up to 50 percent less). But from a return-on-investment perspective, the benefits extend to such things as: increased safety—less human intervention and/or manual checks; lower environmental risks—knowing the exact position of a manual valve at any time; the ability to easily add/subtract/relocate sensor components; higher productivity—being able to monitor equipment operation and schedule maintenance; and an ability to apply the sensors on moving equipment where wire or connector integrity is a problem.
“If looking at small subsystems or discrete equipment controls—skids, gate controls, conveyors and safety showers, for example—a point-to-point system may be the best choice. They are less complex and more economical than a mesh network since the sensor/switch and the receiver are all that’s needed. The receiver then directly interfaces into the existing PLC or control system. These point-to-point wireless systems do not have the same level of redundancy as mesh networks, but they are highly reliable and can maintain a high level of security if based on the Advanced Encryption Standard.
“The advantages of a dedicated interface are that it provides a more user-friendly means to manage the network, a better understanding of what’s happening in the network and the means to better control the network—all from one central location or remotely in the field.”
Paul Richards and Todd Hansen offer more best practices for powering wireless devices and avoiding interference on the website at www.chem.info/Wireless-Advice .