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PAPER ON :
ZIGBEE IN
INDUSTRIAL AUTOMATION NETWORKS
AUTHORS:
- J.MUTHUKRISHNAN
9790165041
B.E.,ELECTRONICS & INSTRUMENTATION
III YEAR
- A.DHINESH KUMAR
9551166169
B.E.,ELECTRONICS & INSTRUMENTATION
III YEAR
ZIGBEE IN
INDUSTRIAL AUTOMATION NETWORKS
ABSTRACT:
The promise of ZigBee is now starting to be realized in industrial automation networks; as with any other new technology, ZigBee users must be sufficiently educated in the technology to be able to deploy it successfully. Much of what has been written and said about ZigBee has been focused around home automation, especially lighting control and building automation. However, what is needed for a residential deployment is very different from what is required for an industrial deployment. This paper provides an overview of ZigBee from the perspective of industrial automation including when ZigBee is appropriate and when ZigBee is not appropriate. The paper then discusses the components needed to add ZigBee networks to existing industrial automation networks. Special attention is given to the topic of gateways and options for integrating the ZigBee network with existing industrial automation application software. A methodology is also presented that can be used to design ZigBee networks for situations commonly found on the factory floor. The paper concludes with an overview of the nearterm advancements expected in the marketplace in relation to ZigBee.
INTRODUCTION:
Companies are increasingly relying on wireless networks in applications that monitor factory floorconditions and control automated systems. The move to wireless is fueled by the fact that wiring runsare expensive, in terms of both initial runs and reconfiguration, as well as by ongoing performanceimprovements and cost reductions in industrial wireless technology. ZigBee, a new low-cost,standards-based, wireless mesh networking technology created by the ZigBee Alliance, holdstremendous promise for advancing wireless further in industrial settings, and products supporting theZigBee standard are just beginning to enter the market. This paper examines the unique needs ofwireless industrial automation, the ZigBee components that come together to address those needs andtheir interoperation, and the promise ZigBee holds for industrial automation as it gains a foothold as amainstream technology.
WHY ZIGBEE IN INDUSTRIAL AUTOMATION?
ZigBee is designed to support easily deployed, redundant path, low-cost systems for integrating eventhe simplest end devices into wireless automation. While its potential usefulness extends to a widerange of applications that include home as well as industrial, it offers three inherent characteristics thatare especially desirable in industrial settings:
Low cost: The typical ZigBee radio costs far less than most sensors and signal conditioners, adding about $30 to the cost of any device incorporating one. This provides an economic justification for automating even the simplest of sensors, extending wireless networking’s reach throughout the factory floor.
Mesh networking: Traditional wireless networks are point-to-multipoint, with a programmable logic controller (PLC) communicating directly with various devices on the factory floor. Challenges arise when there is a barrier in the line of sight between a PLC and a given end device. ZigBee introduces a mesh networking technology, in which some or all of the end devices with which a PLC communicates also function as routers that can communicate with each other. The ability to automatically route communications around barriers as a self-healing network eliminates line-of-sight concerns.
Multi-source products: As an open standard, ZigBee provides customers with the ability to choose vendors as needed. ZigBee Alliance working groups define interoperability profiles to
whichZigBee-certified devices must adhere, organized by class. A ZigBee-certified modem will communicate with any other ZigBee-certified modem in its class, promoting competition and the ability for end users to choose the most effective device for each particular networknode, regardless of manufacturer.
ZIGBEE IN INDUSTRIAL AUTOMATION:
MONITORING VS. CONTROL
It’s important in discussing industrial automation to make a distinction between control applications and monitoring applications. Monitoring applications are those that report such conditions as fluid level, pressure, and temperature for the purposes of recording and of issuing alarms when a condition has exceeded a specific acceptable level. Control applications are those that direct automation devices to take action, which can be as simple as opening or closing a valve or as complex as performing real- The distinction between monitoring and controlling is worth noting because ZigBee, as a mesh networking technology, introduces latency when communication is handed off from one routing device to another.
INDUSTRIAL AUTOMATION COMPONENTS:
The typical industrial automation system consists of one or more Programmable Logic Controllers(PLC) plus sensors and actuators working in concert to monitor and/or control a discrete or continuousprocess.
The PLC is the brain of an automation system, programmed to orchestrate network activities.Controllers run ladder logic programs and communicate with end devices to receive information or toinstruct them to perform an action.PLCs receive information from sensors, which are end devices that sense and report such conditions astemperature, sound, pressure, strain, and vibration. Sensors are the primary end devices in monitoringapplications and provide the inputs upon which control decisions are based.
PLCs send instructions to actuators, which are such electromechanical devices as valves, motors,solenoids, and relays that perform actions (which may depend on the state of sensor inputs). Actuatorsare the primary end devices in control applications, and a number of them may work in closecoordination to carry out a complex task.The human-machine interface (HMI) to these systems is provided by software programs that keep awatch on sensor inputs and PLC outputs, and that generate alarms when process variables exceedpreset levels.
WIRED INDUSTRIAL AUTOMATION NETWORKS:
In a wired industrial automation network, electrical cabling is run between the PLC and each of thesensors and actuators with which it communicates. By far the most popular communications protocolfor this link is Modbus, which is an openly published communications bus designed specifically foruse with PLCs and supported by the vast majority of industrial electronic devices. The PLC alsotypically has a wired connection to the computer system running the monitoring application to providethe human-machine interface, with Ethernet as the networking technology for that link.
ZIGBEE WIRELESS INDUSTRIAL AUTOMATION:
In a ZigBee industrial automation network, ZigBee devices provide an over-the air network interface between the PLC and the sensors and actuators with which it communicates.ZigBee sensor modems provide a combination multichannel signal conditioner and ZigBee module for sensors. Individual versions of ZigBee sensor modems can support various input types, such as 4-20mA and full-wave bridge, and can provide 4-20mA transmitters, digital I/O, and excitation voltages.
ZigBeeModbus modems connect to actuators, and combine the function of a serial output device with a ZigBee module. Modbus commands issued by the PLC are transported over the ZigBee network and relayed to the I/O module.
ZigBeeModbus gateways provide the takeout point for the ZigBee network, mapping Modbus addresses to ZigBee addresses and formatting responses from ZigBee remote devices into the Modbus protocol. These mappings prevent the Master PLC from having to be ZigBee-aware, and preserve the functionality of existing PLCs, sensors, actuators, and monitoring programs. The application functions exactly as before, except that wireless links have replaced wired connections.
ZigBeeModbus address assignments: The mapping of Modbus commands to ZigBee addresses at theZigBeeModbus gateway is a straightforward process. Modbus assigns a single address to each device in the network, as does ZigBee. The device can be an I/O block that has many sensors and actuator, or an individual sensor or actuator. The only operation required is to convert that address from Modbus conventions to ZigBee conventions, which is performed in the Modbus gateway’s controller software.
I/O mapping:Modbus uses a standard set of registers to read and write input and output values. Thus, the various I/O of the ZigBee sensor modem must be mapped to the standard Modbus registers to allow standard Modbus commands to be used. In addition to the device address resolution, the Modbus gateway also translates the Modbus register specified in the Modbus command to the appropriate I/O of the ZigBee sensor modem. The mapping used by the ZN-2401 ZigBee Sensor Modem is provided in the table below.
Modbus commands: All inputs and outputs at ZigBee sensor modems and Modbus modems are
expressed as properly formatted Modbus commands, which includes fields for Modbus address, function code, and register address, plus a fourth field for number of registers to read for inputs or for ON of OFF values for outputs. For example, to read the analog-to-digital channel 0 input of a ZigBee sensor modem which has been assigned Modbus address 4, the following Modbus command would be sent to the ZigBeeModbus gateway.
The following Modbus command would be sent to turn digital output 1 ON at a ZigBee sensor modem
ZIGBEE MESH NETWORKING:
ZigBeeModbus modems and sensor modems can operate as routers (while still providing I/O), capableof communicating with other routers, gateways, or end devices. The presence of routers is what makesa ZigBee network self-healing. Should a router or end device become unable to communicate directlywith the coordinator or primary router due to a dropped link, it will automatically establish an RF linkwith another router and transmit its data back to the gateway through this second router.
ZIGBEE WIRELESS DISTRIBUTED CONTROL NETWORKING:
A second common industrial automation architecture is Distributed Control System (DCS), in whichmultiple PLCs provide local process control. This provides easy segmentation of control functions,especially when there is infrequent interactionbetween process functions. There is still the need forcentral monitoring if not supervisory control of the system. The figure below shows data flow betweenDCS components, including various “languages” used. (For a simplified DCS network, see Figure 5.).
Each HMI application uses its own language to communicate between the application clientworkstation and application server. This language is proprietary to each application vendor and, intruth, there is very little need for third parties to be knowledgeable about this language.
Between the HMI application server and the industrial automation hardware is where things getinteresting. The HMI vendor typically provides industrial automation hardware too. Thus the interfacebetween the application and the hardware is written for the specific vendor’s products. Users, however,demand the flexibility to mix and match HMI applications and hardware from multiple vendors. Toavoid having towrite drivers for each HMI/hardware combination, the industry formed the OPCFoundation to develop a standard interface for HMI applications and industrial automation hardware.This interface is implemented using a server-based software component called an OPC server.TheOPC server abstracts the specifics of the HMI application and the hardware, allowing products
thatsupport OPC to be used in combination. The OPC server allows the ZigBee Ethernet gateway tocommunicate seamlessly with any HMI application that supports OPC.
The OPC server communicates with the ZigBee Ethernet gateway. While in theory, thiscommunication could use the ZigBee command set used by the ZigBee module in the gateway, thatcommand set isnot optimized for communications over an Ethernet network. Therefore, anintermediate or “meta” language was developed to communicate with the gateway over an Ethernetconnection. The OPC server knows and uses this meta language to communicate with the gateway.
The ZigBee gateway communicates with remote devices using the ZigBee command language. Thegateway accepts the meta language from the OPC server encapsulated in TCP/IP packets, removes theTCP/IP packet, and translates the meta language commands into the ZigBee command language. Fordata coming from the ZigBee endpoint devices, the gateway formats the data into the meta languageformat and encapsulatesthe message in a TCP/IP datagram. The datagram is then sent to the OPCserver over the Ethernet network.
INDUSTRIAL AUTOMATION – ZIGBEE FUTURE:
As ZigBee is a very new standard, the most useful ZigBee products available today are designed tointroduce ZigBee benefits to existing networks. The ZigBee gateways, sensor modems, and Modbusmodems discussed so far do exactly that, preserving existing investments while replacing expensiveand cumbersome wiring with
low-cost and flexible wireless connections.
As ZigBee becomes a mainstream technology, we will see the introduction of off-the-shelf ZigBeeenabledsensors, actuators, and PLCs. Rather than needing modems attached to these devices, they willcontain ZigBee radios themselves to enable cost-effective, ready-to-use devices for creating greenfieldZigBee networks.
As this happens, ZigBee gateways will continue to be key components, providing interfaces to getZigBee data off the ZigBee network and onto wired networks that host human-machine interfaceapplications and other applications that make use of the ZigBee data. However, the function of thesegateways will change as applications become ZigBee aware, streamlining the mapping process.
GROWTH OF ZIGBEE WIRELESS TECHNOLOGY :
CONCLUSIONS:
As a very well-designed standard created with specific applications in mind, ZigBee has much to offerindustrial automation applications. Low-cost deployment and redeployment, mesh networking to coverentire industrial plants and factories, and an open standard with multiple vendors make ZigBeeespecially adept at addressing the needs of industrial automation applications. As the industrialcommunications equipment industry continues its embrace of ZigBee, we will see ZigBee devicesoptimized to meet the unique needs of industrial automation. This will mean products that functionwell in settings with high RF noise floor, extreme temperatures, and rough handling.
Yet even as the industry evolves toward this optimization, there will for many years be a need forZigBee products that address existing industrial automation networks and existing applications, and thecurrent crop of solutions – ZigBee modules, sensor modems, and gateways – will likely be keys toimplementing ZigBee networks for some time to come.
REFERENCE:
- ZigBee Takes It Easy: Short-range wireless "ZigBee" networks are ready to unwire your house, Technology Review article by Eric S. Brown, August 19, 2004
- Ember.com
- Zigbeenetworks.com