Simulation of Process Control with WirelessHART Networks Subject toClock DriftMauro De Biasi, Carlo Snickars, Krister Landern¨as, and Alf IsakssonABB AB, Corporate ResearchSE-721 78 V¨aster˚as, Sweden{mauro.d.biasi,carlo.snickars, krister.landernas, alf.isaksson}@se.abb.comAbstract— This work describes simulation of wireless controlusing the WirelessHart standard. Specifically issues concerningclock drift between controllers and wireless network are ad-dressed. The simulations have been done using an extension ofthe Simulink package TrueTime.I. INTRODUCTIONWireless technologies are becoming more and more im-portant both in public and in industrial environments. Theapparent benefit of wireless communication is to removethe restriction of being attached to expensive and messycables. The advantages given by wireless technology areseveral. First of all, it permits to carry the capability of wirednetworks to areas that cables cannot reach. Consideringindustrial plants, wireless technologies can significantly facil-itate deployment and reconfiguration by eliminating the needfor installing and maintaining cabling, reducing both costand time. However, due to the lack of maturity and failureto provide real-time performance no wireless technologyhas been widely adopted for process automation. This maychange with the introduction of WirelessHART.WirelessHART is an optional HART Physical Layer thatprovides a low cost, relatively low speed (e.g. comparedto IEEE 802.11g), wireless connection. It adopts the IEEE802.15.4 physichal layer and it works in the 2.4GHz ISMradio band using 15 different channels. The communicationbetween the devices is performed using Time Division Mul-tiple Access (TDMA) with time slots of 10 ms. A seriesof time slots form a superframe which can be of arbitrarylength. WirelessHART also enables channel hopping to avoidinterferences and reduces multi-path fading effects. One ormore sources and one or more destination devices may bescheduled to communicate in a given slot. The slot may bededicated to communication from a single source device ora slot may support shared communication. In this last case,the MAC protocol used is CSMA/CA.This work addresses the problem of clock drift betweencontroller and the WirelessHART network that occurs whenno synchronization exists between the two. Our theories havebeen verified using an extension to the simulation environ-ment TrueTime [7]. This extension has added the possibilityto simulate the newly released standard WirelessHART [1].The rest of this paper is organized as follows: Section IIis a general introduction to the WirelessHart protocol. ThenSection III gives a brief introduction of TrueTime. In sectionIV the controller and the network is described in some detail,and in Section V some compensation methods are descried.This is followed by simulation examples in Section VI andsome conclusions in the last section.II. WIRELESSHART NETWORK DESCRIPTIONThe Structure of a WirelessHART network is shown inFig. 1. All communications of the WirelessHART Networkpass through the gateway. Consequently, the gateway mustroute packets to the specified destination (network Device,host application, or network manager). The gateway usesstandard HART commands to communicate with networkdevices and host applications. The plant automation networkcould be a TCP-based network, a remote IO system, or abus such as PROFIBUS DP. The Network Manager createsan initial superframe and configures the Gateway. A detaileddescription of the components of a wirelessHART networkisgivenin[2]and[3].Fig. 1. The structure of a WirelessHART network.A. MAC Protocol Descrip tionThe main tasks of the Medium Access Control (MAC)protocol are:• slot synchronization;• identification of devices that need to access the medium;• propagation of messages received from the NetworkLayer;Annual IEEE International Computer Software and Applications Conference0730-3157/08 $25.00 © 2008 IEEEDOI 1357Annual IEEE International Computer Software and Applications Conference0730-3157/08 $25.00 © 2008 IEEEDOI 10.1109/COMPSAC.2008.1631357Annual IEEE International Computer Software and Applications Conference0730-3157/08 $25.00 © 2008 IEEEDOI 10.1109/COMPSAC.2008.1631355Authorized licensed use limited to: Univ of Calif Berkeley. Downloaded on January 18, 2010 at 16:55 from IEEE Xplore. Restrictions apply.• to listen for packets being propagated from neighborsWirelessHART uses Time Division Multiple Access(TDMA) and channel hopping to control access to thenetwork. TDMA is a widely used Medium Access Controltechnique that provides collision free, deterministic commu-nications. It uses time slots where communications betweendevices occur. A series of time slots form a TDMA super-frame (see Figure 2).Fig. 2. The SuperFrame structure [1]All devices must support multiple superframes. Slot sizesand the superframe length (in number of slots) are fixedand form a network cycle with a fixed repetition rate.Superframes are repeated continuously. For successful andefficient TDMA communications, synchronization of clocksbetween devices in the network is critical [8]. Consequently,tolerances on time keeping and time synchronization mech-anisms are specified to ensure network-wide device clocksynchronization. It is imperative that devices know whenthe start of a slot occurs. Within the slot, transmissionof the source message starts at a specified time after thebeginning of a slot. This short time delay allows the sourceand destination to set their frequency channel and allows thereceiver to begin listening on the specified channel. Sincethere is a tolerance on clocks, the receiver must start to listenbefore the ideal transmission start time and continue listeningafter that ideal time. Once the transmission is completeand the destination device indicates, by transmitting anACK, whether it received the source device data-link packet(DLPDU) successfully or with a specific class of detectederrors. Communicating devices are assigned to a superframe,slot, and channel offset. This forms a communications linkbetween communicating devices.III. DESCRIPTION OF TRUETIMEThis section describes the use of the originalMatlab/Simulink-based simulator TRUETIME [7], whichpermits to design networked control systems simulatingreal-time kernels, network transmissions (using wiredor wireless networks), and continuous plant dynamics.TrueTime is constituted by a six blocks library and by acollection of C++ functions