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Using wireless sensors in industrial applications

THE last few years, wired industrial sensors have become smarter and smaller. At the same time another key development has taken place: A growing interest and utilisation of wireless sensor in industrial applications.

Scenario 1 — As an instrumentation maintenance supervisor in a mine, mineral processing or industrial plant you were asked by management to monitor a number of process tempera ture parameters for a period of two weeks and report the results to a metal lurgical engineer. Following the comple tion of the first set of tests you were asked to repeat the tests in three other locations around the plant utilising the same hardware but add an alarm output

Scenario 2 — While running the above tasks you receive an urgent new request from management to modify the current monitoring system and add flow and pressure parameters to your existing monitoring system. To monitor flow and pressure parameters you must add another monitoring node a short distance away from the current location. The monitoring period for flow and pressure is scheduled for 72 hours only

Scenario 3 — As a maintenance tech nician you were asked to monitor the plant’s production line electric motor’s ‘health’ by attaching temperature, ac current and vibration sensors to a number of motors scattered around the production area. You will be required to report of any imminent sign of motor breakdown by generating an alarm

To meet such ever-changing monitoring requirements, you will need to install sensors, lay cables, install a data logger device in each location and appoint an employee to oversee these tests. You then need to ensure the downloading of data logger files on a daily basis, transfer the logged data to a PC, analyse the tests results and report to management

Points to consider

• How much time will it take to install the various sensors?

• Can sensors be installed ad-hoc?

• What is the cost of digging a cable trench?

• What is the cost of installing a cable tray?

• Should I install the required cable above or below ground?

• What is the cost in man-hours required to transfer data from the tests site?

• What is the cost of shifting to a new location in order to replicate the original tests?

Many such scenarios are being addressed worldwide day in and day out by instrumentation technicians, maintenance supervisors, process engineers and plant managers.

Enter the world of wireless sensors. Wireless data transmission between sites is nothing new in industrial applications. Being able to transmit data directly from multiple wireless sensors using mesh network technology and central moni toring industrial systems is a relatively new approach. Successful integration between wireless sensors and Supervisory Control and Data Acquisition (SCADA) or PLC systems proved so far that using such technology can effectively meet the dynamic requirements of today’s indus trial control systems.

What industries can make use of wireless sensors? The short answer is: almost all industries.

Wireless sensors can be easily incor porated into numerous applications. Many heating and cooling HVAC appli cations can wireless temperature and humidity sensors. Industrial furnaces may use RTD or thermocouple wireless temperature sensors.

Pulse counting applications such as water and energy metering are good candidates for battery operated wireless networked sensors. Monitoring of liquid level in tanks or grain level in silos is another group of applications that can benefit from this technology. Food processing and pharmaceutical plants belong to a growing group of wireless sensors users. Water quality and waste water monitoring in remote locations are good candidates for wireless sensors

One of the more important aspects of using wireless sensors is the ability to transfer data between a network of wire less nodes to SCADA stations, data loggers and PLC control systems. Popular control systems are already installed on-site using their own serial or Ethernet cables communicating using various communications protocols.

Since these sensors use advanced communications technology known as wireless mesh networks, a gateway device will be needed in order to close the gap between the wireless network on one side and the existing wired network on the other. One of the more popular communications protocols to be used in the gateway is Modbus RTU. Modbus RTU is being used by literally hundreds of instrumentation and control systems suppliers. Other communications protocols to be aware of are: Modbus TCP, OPC, Fieldbus, Profibus, DF1 and more.

It is important to understand that wireless sensors can be powered by both external power sources and by internal batteries. Battery power provides the user with the flexibility to relocate sensors from site to site generally simpli fying the installation. When wireless sensor nodes are powered by their own internal batteries you will need to be aware of a potential slower rate of data update at the central monitoring station.

Mesh networks basics

Wireless mesh networks are designed to provide high bandwidth communica tions channels between wireless sensor nodes over a specific coverage area. Wireless nodes cooperatively and auto matically make data forwarding deci sions based on their “knowledge” of the existence of other network members. A wireless node normally needs only transmit to its closest neighbour. If a node drops out due to hardware failure or other reasons, its neighbour should automatically be able to find another route to transfer data to the next node.

ZigBee is one implementation of wireless networks. It is a low cost, low data rate and low power wireless mesh network proprietary standard. The advantage of low power consumption is translated into long operating life using smaller batteries. ZigBee mesh network adds high reliability to network communications enabling it to poten tially be used in numerous industrial control systems.

Issues for consideration

• Wireless sensors eliminate the need for cables, cable trenches or cable trays

• Wireless sensors can be easily moved from point A to point B within minutes

• A network of wireless sensors is quite flexible. New nodes can be added at any time without disturbing existing network installation or commu nications strategy

• Standard communications interface between wireless sensors network and existing SCADA and PLC systems is essential

• Always perform a wireless site survey to identify the optimum location for your wireless sensors

• Wireless sensors may reach communi cations distance of 500m or better when installed in open space line of sight and about 30 to 100m or better inside buildings

• Wireless sensors can operate for extended periods powered by very small batteries

• When powered by batteries data is transferred only when required or based on a programmed sleep cycle

• When powered by batteries, data transfer between wireless sensors and the central monitoring station may be delayed by a few seconds in order to conserve battery power through a SLEEP/ WAKE-UP cycle

Wireless sensors can be deployed almost anywhere at a far lower overall cost than wired sensors. These devices running the latest in microprocessors and embedded wireless technology will become more acceptable and more widely used as users gain confidence in the technology.

Now that wireless sensors comply with industry standards for radio communications such as IEEE 802.15.4, ZigBee, and WirelessHART, wireless sensors are more acceptable and will become the future of communication and control networks in industrial envi ronment. End-user adoption of this technology in manufacturing and process control plants, mines, power generation plants, maintenance instru mentation, and medical devices is already on the increase.

[Motti Gill is managing director of Conlab.]

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