Industrial Control using Wireless SensorNetworks

Kamran Khakpour M.H Shenassa RFControl Co. K.N.Toosi University of technology

Research and Development Department Department of Control Engineeringkkhakpour@gmail.com shenassa@eetd.kntu.ac.ir

Abstract Wireless sensor networks have been an activeresearch topic during the past few years. Researchershave conducted extensive studies on various aspects ofwireless sensor networks: information dissemination [4],energy-efficient routing [1] and security [8], etc. Inaddition, wireless sensor networks have beensuccessfully applied to environmental and wildlifehabitat monitoring. However, real time control overwireless network systems still is a problem and nobodycan be confident enough about it; but the are somesporadic events in control systems such as alarms andevents that can be less critical than periodic loops so thatdelay may have no effect on them. The main purpose ofthis paper is to show the performance of the Wirelesssensor networks using ZigBee standard in industrialcontrol systems. This standard has been used specificallyfor loom control and its performance has been surveyed.

Keywords: wireless, industrial control, sensor networks

I. Introduction

As its name implies, wireless sensor networks are mostlyused for sensing, that is, collecting data and feeding thedata to a central processing point. Several industrialorganizations, such as WINA [11] and ZigBee [12], havebeen advocating the application of wireless technologiesin industrial control. For example, ZigBee SpecificationV1.0 was ratified in late 2004 and ZigBee compliantproducts are readily available on the market [13].Industrial process control has real-time requirements fordeterministic data transfers. Current sensor networks bytheir nature have failed to meet this need.A traditional process control system is shown in Figure(1) [2]. As illustrated in this figure, devices are connectedto controllers via some types of buses, such as Fieldbus[9] and Profibus [10]. Usually, each device is either a

sensor or an actuator. A sensor collects certain statusinformation of a process and feeds it to a controller.

Based on the readings from sensors, the controllerdetermines whether the actuators should act to maintainsome physical property of the process, e.g., the flow in apipe must be kept at a fixed speed. In this case, the taskson sensors, controllers and actuators comprise a controlloop. The control loop is executed periodically tomaintain the constant flow in the pipe. There can bemultiple control tasks running on a controller. Users caninteract with controllers via workstations which areconnected to the corporate network. In a wireless processcontrol system, the buses between controllers and devicesare replaced by wireless networks. Although the benefitof wireless process control is very attractive, manytechnical issues have to be resolved to make wirelessprocess control feasible:· Transient Interferences: wireless communication is

sensitive to interferences. We need to providedeterministic and timely data delivery in case oftemporary wireless link failures.

· Power Efficiency: Energy is always a problem forbattery-powered devices. The control system should

Figure (1) traditional process control system

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not fail if one sensor or actuator is depleted ofenergy.

· Security: Some sensor readings are sensitive andneed to be protected from eavesdropping. Also, theactuator should be shielded from intruders to avoidunintended actions.

Real time control over wireless network systems still is aproblem and nobody can be confident enough about it;but the are some sporadic events in control systems suchas alarms and events that can be less critical than periodicloops so that delay may have no effect on them. Themain purpose of this paper is to show the performance ofthe Wireless sensor networks using ZigBee standard inindustrial control systems. This standard has been usedspecifically for loom control and its performance hasbeen surveyed. For this specific application othertechnology’s performance has been measured by theauthor before in [3]. It has been shown that Bluetooth hassome weakness to be used for real time control thusanother special developed wireless network has beenused [3]. In this paper, however, operators in the loomhall are also equipped with wireless transmitter and areparts of wireless network in order to monitoring theirperformance as well. So, the size of network is biggerthan what it was in before study [3]. By this approach,attendance of each operator on his dedicated machinewill be recorded and then by analyzing recorded datamany other useful data will be resulted. There are 120looms in the hall and every three machine has beendedicated to one operator. So totally we have 160 nodesthat 40 nodes change their position regarding to initialposition.

II. Wireless Sensor Networks

A. What is ZigBee?

ZigBee is a wireless technology that builds upon theInstitute of Electrical and Electronics Engineers (IEEE)802.15.4 standard [12]. This IEEE standard defines ashort range, low-power, low data rate wireless interfacespecifically designed for small devices that have limitedpower, CPU and memory resources. ZigBee is managedby the ZigBee Alliance, a group of over 170 companiescreating ZigBee related semiconductors, developmenttools and products. The Alliance specifies the ZigBeearchitecture (ZigBee network, security, application, andprofile definitions) and provides conformance andinteroperability testing for ZigBee products. The Alliancealso promotes ZigBee as a brand and manages the futuretechnology direction. Version 1.0 of the ZigBeespecification was ratified in December, 2004. TheZigBee Alliance is putting a certification program intoplace and expects to certify a number of OEM productsas ZigBee-compliant during the next 6- 12 months. Atpresent there are no ZigBee-certified products, only

certified development tools and platforms. Initialproducts that opt for ZigBee certification will beconsumer products targeting home automation andsecurity.What does the development of ZigBee mean formanufacturers? To the OEM, ZigBee is essentially anapplication framework for small, low-power wirelessdevices. ZigBee is more than a communication protocol,thus the term “ZigBee stack” is misleading in the sensethat the word "stack” makes one think of a protocol. Infact ZigBee is as much an application framework as it isa network protocol. Many of the design decisions that ledto ZigBee are best understood in this light.

B. Sensor Network Overview

The ZigBee network layer supports three networktopologies: star, tree, and mesh [12]. Each of thesenetwork types has particular properties that may make itmore or less suitable for a various manufacturingapplications. Star networks are the simplest type ofnetwork. They consist only of end devices and acoordinator. The disadvantage of the star configuration isthat the network can extend to only two hops (enddevice, coordinator, end device). This limits thecoverage of a single network. Another disadvantage isthat a star network requires reliable links between all enddevices and the ZigBee coordinator. There is no pathredundancy. Advocates of mesh networks tend to ridiculethe star configuration, but most wireless sensor networksin service today are star networks.

Figure (2) ZigBee Network Topology Model

ZigBee 2nd network topology, the tree network, can alsobe beacon oriented. Unlike a star network a tree networkcan allow multi-hop communication and thus can extendthe network coverage. One weakness of tree networks isthat they require highly reliable tree links [5]. Especiallylinks close to the root of the tree must be highly reliable.Failure of a single such link can cause many nodes tolose communication. Also the latencies of tree networkswill be significantly longer than those of star networks.Mesh networks allow multi-hop communication and alsosupport multiple paths between network nodes forincreased reliability. Routers on mesh networks are ableto discover and characterize the alternative routes

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available to them and choose the best path. Meshnetworks can extend widely. The drawbacks of ZigBeemesh networks are that router nodes in the must bepowered on at all times, and router nodes must also becapable of storing route information and discoveringroutes. This adds cost and complexity to each node in thenetwork. Latencies in mesh networks are difficult toestimate so this topology is not well-suited forapplications that require guaranteed or low latency [5].Many factors determine the complexity of the sensornetwork. Power management requirements may placerestrictions on communication speed, modulationtechnique, and medium access mechanisms. Applicationsunconcerned with power management will often useindustry standard communications mechanisms and theInternet to move data to the user. An urban trafficmonitoring system using video monitors as the sensors isan example of such a system. Sensor networks exhibitseveral unique and challenging characteristics. Asoutlined in [6], small physical size and low powerconsumption are primary attributes of the sensing"nodes". The small physical size made possible byimproved hardware design must be complimented withefficient software to ensure reasonable processing,storage, and communication overhead. Robust operationis also required as operation will be largely unattended.Application reliability can be enhanced by tolerance ofindividual sensor node failure, as long as sufficientnumbers and density of nodes are present. Some of thecharacteristics of sensor networks and other systems (ad-hoc networks or single-sensor systems) are:· The number of sensor (nodes) in a sensor network

may be significantly larger (by orders of magnitude)than ad-hoc or traditional single-sensor systems.

· Individual sensors/nodes may be prone to failures.· The communication "topology" of a sensor network

frequently changes.· Sensor nodes often communicate using a broadcast

paradigm. Ad-hoc and single-sensor systems moreoften use point-to-point communications.

· It is assumed that sensor nodes are limited in termsof power availability, computational capacity andmemory.

· Depending on the application sensor nodes may ormay not have a form of global identification. Globalidentification comes at the price of increasedoverhead.

III. Case Study

Weaving machinery monitoring and control systemdisplays real time state of weaving machines status andgives information about the number of picks and warp,weft and mechanical halts and stops of weaving machine.In addition, it displays the machine operator attendance

on the halted machine. The wireless technology omitsexistence cabling and has given high flexibility to thesystems. The system operates independent of physicalelements, which means we do not have to change thesetting when displacing the machines. Wirelesstechnology not only removes cabling but also has veryhigh safety against noises existing in weaving factories.In this study Chipcon CC1010 and TI’s CC2431 locationengine have been used. Locationing allows finding theposition of nodes whose where-about is unknown orsubject to change, in particular, those in an ad-hocwireless network. For example, an operator may changeits location because of some reasons. In this manner, theplaces where operators have been visited can berecorded.There are 120 looms in the hall and every three machinehas been dedicated to one operator. So totally we have160 nodes that 40 nodes change their position regardingto initial position. The device gets sample every second.Thus, there is eight samples totally every second to besent through the network. The main idea is to measurethe network performance but in addition there is a needfor loom monitoring and control that has been done bythis project.

Figure (2) Loom monitoring WSN

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Figure (3) Loom monitoring WSN Internal PCB

In this implementation, operators are equipped withRFID tag that stores their IDs. Different categorizedoperators have different ID. In this manner centralcomputer can recognize them with their department. Forexample, maintenance operators and machine operatorscan be recognized by central computer and a specificreport can be produced by central computer. Each enddevice has a RFID reader that reads RFID data and sendsit to central computer through wireless sensor network.As maintenance operator attendance beside haltedmachine is desired for recording and reporting by centralcomputer, this category of operator is equipped with onlyRFID. In this approach, only loom machinery reads theID of its operator and sends it to central computer. Somealarms have been used to announce related operator forhalted machine. This alarms are kind of vibrator that isinside of a wearable device. This device also can read theRFID tag of the operator.

Figure (4) Graphical view of monitoring software

IV. Experimental Result

In this experiment ZigBee standard with meshnetworking have been used for loom monitoring andcontrol of some sporadic events and networkperformance has been measured.Signal is interfered with a lot of influencing effects andits strength varies in different places. The theory andmeasured RSSI has been shown in figure 5.

Figure (5) CC1010 RSSI

Delay performance studies for ZigBee were conductedusing echo. Central computer sends a packet to the nodeand the node echoes back the packet. The time differencebetween transmissions and reception of the packet is theplainly delay. The maximum delay that was recorded bythe software is 32ms and the minimum is 6.02ms.Comparing to implementation in [3] by the authors, it hasbeen shown clearly that ZigBee has a better performancein low SNR environment.

Figure (6) SNR for three different wireless technology[7]

In addition to measuring wireless network performance,loom's data have been sent through the air and stored in acomputer for reporting and analyzing. Very good resulthas been obtained from the system that can help the hallmanager in controlling the production and operatorsmanual and faults of machine. All of this informationfinally will lead to high efficiency and good products. Inthis new implementation, maintenance groupperformance also can be recorded, while it did not exist

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in the old version of loom monitoring implemented bythe authors.

V. Conclusion

The design and implementation of a wireless sensornetwork targeted for loom monitoring systems andpredictive maintenance was discussed and presented.Tests were carried out to determine system performance.The experimental results show that a near real-timesystem can be set up with the SSNs.ZigBee with its excellent low-power capability providesan excellent alternative for Bluetooth and Wi-Fi in termsof power and performance. Although, ZigBee is notabsolutely real time like any other existencecommunication technologies in real world, comparing toBluetooth and Wi-Fi, it can be more reliable and robustdue to high performance in low SNR environment andthat can be very useful for textile and other industry thatdoes not need hard real time control.

References

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[2] D. Chen, M. Nixon, T. Aneweer, R. Shepard, and A.K. Mok, Middleware for Wireless Process ControlSystems. Architectures for Cooperative Embedded Real-Time Systems Workshop, December 2004

[3] Shenassa M.H., Khakpour K., "Loom DataMonitoring Using Wireless Technology", IEEEConference on Information & CommunicationTechnologies, ICTTA April 24 - 28, 2006, Volume:2, Page(s): 2704- 2708

[4] C. Intanagonwiwat, R. Govindan and D. Estrin,Directed Diffusion: A Scalable and RobustCommunication Paradigm for Sensor Networks, The 6thAnnual International Conference on Mobile Computingand Networks (MobiCom 2000), Boston,MA, August 2000

[5] ARC Strategies, www.ARCweb.com

[6] J. M. Kahn, R. H. Katz, and K. S. J. Pister, "NextCentury Challenges: Mobile Networking for "SmartDust". In International Conference on Mobile Computingand Networking (MOBICOM), pages271-278, 1999.

[7] ZigBee and IEEE 802.15.4: A brief introduction GeirE. Øien, NTNU, 2006

[8] A. Perrig, R. Szewczyk, V. Wen, D. Culler, and J. D.Tygar. SPINS: Security Protocols for Sensor Networks.Mobile Computing and Networking, 2001

[9] Foundation Fieldbus standard, http://www.Fieldbus.org

[10] Profibus standard, http://www.profibus.org/

[11]WINA, http://www.wina.org/

[12] ZigBee Alliance, http://www.zigbee.org/

[13] Chipcon Products, http://www.chipcon.com/

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