Selecting the right wireless sensor network

Selecting a wireless sensor network that can meet the demands of a process manufacturing application is critical.

In 2009, NAMUR, the international association and standards body for automation technology users in the process industry, produced the NAMUR NE124 recommendations, which define requirements for wireless standards technologies, the technology employed and solutions from different manufacturers to ensure the necessary functions for sustainable use to be practical in the process industry is provided.

Probably the most important requirement for data networks is reliability. Although a figure is not stated in NAMUR NE124, greater than 99% reliability is needed to ensure data is available when required. Most wireless technologies do not provide a reliability figure, probably because it is much lower than 99%. The reason for this is plants present challenging environments for wireless, with permanent steel infrastructure, such as furnaces and storage tanks, and heavy vehicles and shipping containers that change position, all preventing radio transmission.

Most wireless sensor network (WSN) technologies use a star topology, with point-to-point communications and a clear “line-of-sight” required between the gateway and every sensor. In a typical plant, this is problematic, requiring additional gateways that add cost. Some WSN technologies use a star topology, but with a backbone to create many sub-networks, each with an intermediate backbone router. The drawback here is that the backbone network and power may need to be installed in a hazardous area, increasing project complexity and cost.

WirelessHART uses a mesh topology, whereby a sensor without line of sight with the gateway communicates with a neighboring sensor, which then relays the communication to the next sensor and so on until it reaches the gateway. This enables data to be routed around metal obstacles. If there is a weak spot in the mesh topology, a repeater is used, with no need to install additional gateways, a network backbone or power cabling. Sensors communicate to many neighboring sensors and this forms a mesh of connections, with multiple paths from each sensor to the gateway. Data path redundancy is the key to maximizing reliability.

WirelessHART is also self-organizing with no need for manual configuration of communication paths between sensors. The mesh network forms automatically. Moreover, the self-organizing mesh adapts dynamically. If a sensor is moved, runs out of battery or is obstructed, the system dynamically chooses an alternative path to route data. As a result, WirelessHART achieves 99% reliability. NAMUR NE124 does not specify the need for a specific range from a sensor to gateway. What matters is the depth; the maximum number of hops in one communication path. WirelessHART can perform seven hops or more. This allows it to reach sensors installed deeper inside plant units than other technologies can. As a result, this gives WirelessHART the longest effective range and greatest coverage inside a plant.

Real-time performance

Wireless sensors are mostly used in applications that are not time-critical – such as to help eliminate manual data collection. For these applications, an update period of one minute, one hour or even one day are fine, but for other use cases an update period of one second may be required. Many WSN technologies only support update periods of one minute or longer, which means they are unsuitable for these applications. WirelessHART offers real-time capability, with update periods of one second for applicable sensor types.

WirelessHART helps to maximize battery life with time-synchronized communication. The sensor sleeps conserving battery, wakes up briefly to make a measurement, then goes back to sleep again. As a result, it is possible to achieve a battery life of between 5 and 10 years for sensors, helping lower total cost of ownership (TCO).

Wireless sensor security

WSN technologies all have similar security mechanisms, but they differ in terms of ease of commissioning sensors. For several WSN technologies, configuring the security credentials is very time-consuming and involves multiple steps and special tools. Even for sensors using the same technology, the tools and procedures differ from one sensor manufacturer to the next. For WirelessHART, commissioning is simplified using HART field communicators or HART modems. Instrument technicians will be very familiar with these tools used for existing 4-20 mA/HART instruments.

Multi-vendor interoperability

There are WSN technologies that look open, licensed to several manufacturers, but in fact are not an IEEE or IEC standard, but owned by a single company. Proprietary technologies always have associated risk. WirelessHART is an international standard, IEC 62591. Standardization dramatically reduces risk, which is why instrumentation and control engineers specify IEC standards for all electrical and electronic equipment including WSNs.

Users also need to monitor the self-diagnostics in the sensors. Most WSN technologies do not have a common backhaul network application protocol for sensor configuration and advanced diagnostic data. As a result, sensors cannot be integrated into a plant’s existing intelligent device management (IDM) software. WirelessHART solves this with the HART-IP application protocol over Ethernet.

HART-IP is used for the backhaul network to bring sensor configuration and advanced diagnostics data into the plant’s existing IDM software. HART-IP supports electronic device description language (EDDL) and field device integration (FDI). This means that all types of WirelessHART sensors can be managed from the same software used for the plant’s existing wired devices. Even IDM software, based on FDT/DTM (field device tool/device type manager), can be used to manage WirelessHART sensors.

Portable tools

Several WSN technologies have no common method of bench or field configuration and calibration. Even for the same technology, manufacturers have different ways of configuring their sensors. Managing a variety of portable tools in such a heterogenous environment causes delays. A direct interface for a portable calibrator is not supported. Some sensors cannot be calibrated or “trimmed” to correct for long-term drift.

To achieve the desired operational improvements, wireless sensors and applications cannot be isolated islands of automation and must integrate with the existing automation systems. Because many WSN technologies don’t define a standard application protocol with a common message format or data types, the wireless gateway does not convert sensor measurement data to Modbus or OPC and cannot be easily integrated. This makes it hard to put that data to good use. Some WSN solutions are for a single use-case, which creates the situation where you have multiple heterogenous systems with multiple software to monitor each application.

WirelessHART uses the standard HART application protocol, which has a common message format and common data types for all WirelessHART sensors, regardless of manufacturer and sensor type. This enables WirelessHART gateways to convert standard data, from a mix of sensors from multiple vendors, to common industrial protocols, such as Modbus, EtherNet/IP and OPC. As a result, data can be integrated with existing systems like the plant historian or control system, without custom programming or scripting. It is then available to produce analytics, dashboards, reports and notifications.

Some WSN technologies only transmit measurement and status data, no detailed diagnostics like a vibration spectrum or waveform is available, or requires scripting to be decoded. WirelessHART can provide advanced diagnostics communicated by request-response as a non-real-time “second channel.” This enables advanced diagnostics like a vibration spectrum or waveform to be centrally analyzed.

Jonas Berge is senior director of applied technology at Emerson. This article originally appeared on Control Engineering Europe’s website. Edited by Chris Vavra, web content manager, Control Engineering, CFE Media and Technology, [email protected].

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Keywords: wireless sensors, wirelessHART, process manufacturing

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