WIRELESS COMMUNICATION

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WIRELESS COMMUNICATION   -   Applicability & Implementation of ZIGBEE in Industrial Automation

  B.OBULIRAJ
B.E CSE

 

            The wireless communication technologies are rapidly spreading to many new areas, such as automation, data acquisition, Home Area Network (HAN) and monitoring systems. Various communication protocols employed to perform the above applications are X-10, Bluetooth, Wi-Fi and Zigbee. While other wireless technologies are concerned with exchanging large amount of data (high data rate), Zigbee is the wireless standard that supports low data rate for monitoring and control.

            Zigbee is the software layer based on IEEE 802.15.4 standard which was developed by IEEE and Zigbee Alliance. Though X-10 is simple its low speed, low reliability and lack of security force to alternate wireless technologies like blue tooth and Zigbee, which overcomes the draw backs of X-10. The increased cost & High Power consumption of Bluetooth makes Zigbee as a promising technology for implementing HAN and Industrial automation.

            This paper describes the need of deploying Zigbee network, which is an emerging wireless protocol that offers flexible, secured, reliable, inexpensive and ultra low power consuming communication link that promotes long life for devices with non rechargeable batteries. The main concern for manufactures of industrial control and HAN are robustness and security. Zigbee address both.

1. Introduction

ZigBee is a new low rate wireless network standard designed for automation and control network. The standard is aiming to be a low-cost, low-power solution for systems consisting of unsupervised groups of devices in houses, factories and offices. Expected applications for the ZigBee are building automation, security systems, remote control, remote meter reading and computer peripherals. The ZigBee standard utilizes IEEE 802.15.4 standard as radio layer (MAC and physical layer). Three radio bands are defined as follows

• Global use: ISM 2.4 GHz band with 16 channels and data rate of 250 kb/s;
• USA and Australia: 915 MHz band with 10 channels and data rate of 40 kb/s;
• Europe: 868 MHz band with single channel and data rate of 20 kb/s.

The defined channels are numbered 0 (868 MHz), 1 to 10 (915MHz) and 11 to 26 (2.4 GHz). An IEEE 802.15.4 packet has maximum length of 127 bytes including header and 16-bit checksum (CRC), where the payload is up to 104 bytes. Some channels have duty cycle restriction or recommendation to achieve minimum conflicts among different ZigBee networks.

2. Comparing Zigbee with other existing standards

Of the few attempts to establish a standard for home networking that would control various home appliances, the X-10 protocol is one of the oldest. It was introduced in 1978 for Home Control System. It uses power line wiring to send and receive commands. The downside of its simplicity is slow speed, low reliability, and lack of security. The effective data transfer rate is 60bps, too slow for any meaningful data communication between nodes.

            Wi-Fi and Bluetooth are the alternative standards suitable for wireless networking. ZigBee is supposed to do what Wi-Fi or Bluetooth are not doing-two-way communication between multiple devices over simple networks using very less power and at very low cost. It uses the free 2.4 GHz band and the IEEE-defined 802.15.4 standard. And, unlike many wireless licensed technologies it is an open standard. ZigBee typically transfers a few bytes of sensor readings between devices, it requires very low bandwidth, and low power. In fact, the low power proposition gives it an edge over Bluetooth. The comparison of various wireless standards are given in Table 2.1.

Comparison of Wireless Standards
Market Name	ZigBee	Bluetooth	Wi-Fi
Standard	802.15.4	802.15.1	802.11b
Application Focus	Monitoring & Control	Cable Replacement	Web, Email, Video
System Resources	4Kb - 32 Kb	250Kb+	1Mb+
Battery Life (Days)	100-1,000+	7-Jan	.5-5
Network Size	Unlimited (264)	7	32
Bandwidth (Kb/s)	20-250	720	11,000+
Transmission Range (Meters)	1-100+	1-10+	1-100
Success Metrics	Reliability, Power, Cost	Cost, Convenience	Speed, Flexibility

Table 2.1 Comparison of Wireless Standards

3. ZigBee Network: A network coordinator manages the Zigee network.  The coordinator starts the network, takes    care of the structure and controls the joining and leaving of the devices in the network. If a device (orphan) intends to join an existing network it has to start network association procedure.

First the device sends an authentication requested that is answered by the coordinator within a predefined time. If the device intends to rejoin a network, it has to start orphan notification procedure. To leave the network, the device issues a disassociation request.

3.1 Network Configurations

The IEEE 802.15.4 standard employs the long 64-bit address (IEEE) and a short 16-bit addresses. The short address supports over 65 535 nodes per network. The long address should be unique globally, the short address is assigned by the network coordinator when a device joins the network and is unique within the given network. The short address of a network coordinator is 0x00. The network identificator (PAN ID) is a 16-bit number that is used to distinguish between overlaying networks. There can be 250 nodes per network (depending on the profile) and many networks located in the same area. To join a network the device has to know the PAN ID of the network it intends to associate. The IEEE 802.15.4 MAC enables network association and disassociation.

There is an optional superframe structure with beacons for time synchronization, and a guaranteed time slot (GTS) mechanism for high priority communications. The synchronization of the devices within a beacon enabled network is performed by listening to the beacons transmitted by the coordinator. This enables the devices to sleep for long periods, as the beacons can be set between 15 ms and approximately 4 minutes and significantly help conserving power. In case of a network without beacons, the devices periodically poll the coordinator for data. The period of the polling may be set individually for each device. The medium access method to the channels is carrier sense multiple access with collision avoidance (CSMACA). The ZigBee uses the direct sequence spread spectrum (DSSS) modulation.

.3.2 Network topology

ZigBee supports either a single-hop star topology constructed with one coordinator in the center and the end devices.eg: RFD devices – not capable of routing. The structures of various Zigbee network topologies are shown in Fig 3.2.1. (The devices in the star topology can only communicate via the network coordinator. The star topology is necessary for RFD devices, as they are not capable of routing. )The ZigBee also supports a mesh topology which is shown in Fig 3.2.1(.The mesh networking is one of the key features of the ZigBee technology. At present most of the ZigBee stacks are preliminary and the support for multihop topologies is limited, but the base) mesh functionality is usually supported. The last possible ZigBee topology is tree, which is a multiple star topology with one central node that is the ZigBee network coordinator(. The preferred topologies are the mesh and star. Mesh topology enables flexible network configuration and provides redundancy in the available routes)

Star topology	Mesh topology

3.4 Power Consumption

ZigBee is designed for applications that need to transmit small amounts of data while being battery powered so the architecture of the protocols and the hardware is optimized for low power consumption of the end devices. The only disadvantage is that the coordinator has to be able to store all the data in its buffers, which might lead to large RAM needs.

4. Zigbee Stack architecture

The ZigBee stack forms the upper layers of the

IEEE802.15.4 PHY and MAC sub-layer specifications.

 It realizes the network layer (NWK) and in the application layer provides application support sub-layer (APS)

and the ZigBee device object (ZDO).

4.1 Application layer

The application layer carries the code of the individual custom application. According to the ZigBee specification this code is written into the ZigBee device object and the function of the device is specified. The application support sublayer (APS) forms the low level of the application layer.

4.2 Network layer

The network layer (NWK) handles the network level of the communication. It is managing the network structure and handles routing and security functions for the relayed messages. The ZigBee network is a dynamical network and the network layer needs to maintain the information about the nodes within the network

4.3 The MAC layer

The MAC (Medium Access Control) is controlling access to the shared radio channel. It generates and recognizes the addresses, verifies the frame check sequences. The MAC layer is also responsible for the scheduling of the frame transmissions in the non-beacon mode using a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) method

4.4 Physical layer

The lowest layer of the protocol stack – the physical layer is defined by the IEEE 802.15.4 standard and is implemented in the silicon. The physical layer is encoding the bits to be sent and decoding of the bits received. Some of the information available on the physical layer is provided to the MAC layer

Fig 4.4.1. OQPSK Modulation

The process of the raw data translation into the modulated signal is shown in the Fig.4.4.1. The OQPSK modulation at the 2.4 GHz is the chipping rate 2 million chips per second, where 32 chips represent the 4 bits of the original data. Two chipping codes are being transmitted at a time, which yields for the raw data rate of 250 kbps.

5.Deploying Zigbee in Automation

5.1 Conventional Automation Network

Regardless of specific application needs, industrial automation networks consist of PLCs (programmable logic controllers) that communicate with remote sensors to gather data regarding such variables as pressure, temperature, vibration, sound, and strain. When the application includes a control function, PLCs act on that data by issuing commands that orchestrate processes among such actuators as relays, motors, solenoids, and valves. An example is shown in Fig 5.1.1

Fig 5.1.1. Schematic of automation networks

The success of this automation depends on reliable communications between PLCs and sensors/actuators. The communication links have traditionally been hard-wired, with data transmitted over cable.

5.2. Design overview

Figures given below illustrate the overall design.  It can communicate wirelessly with a PC through the ZigBee protocol to receive instructions and send sensor data. Temperature sensing is used as an example application. Temperature sensor is interfaced with PIC Micro controller as shown in the figure  XBee Transceiver is interfaced  as shown in the figure

Block Diagram of Zigbee 

Figure 5.2.1 Interfacing of Temperature with PIC

Receiver Unit is shown in the figure 5.3.3 with PIC Micro controller

Figure 5.2.2 Interfacing of XBee Sensor               Transceiver with PIC

Figure 5.2.3 Block Diagram of Zigbee Receiver Unit

5.3. IMPLEMENTATION DETAILS

5.3.1 COMMUNICATION

Two XBee modules were used, one on the PC side (through a serial port) and one on the robot. The adoption of the ZigBee technology makes it straightforward to establish communications.

A wireless link is always half duplex. However, our application can transmit and receive at the same time via a serial link to the UART at your end of the interface. This module is made possible by two software buffers. There is a transmit buffer and a receive buffer, and each buffer provides a temporary parking place for 100 bytes.

Like WiFi and Bluetooth, Zigbee is a standard for wireless data transmission. However, it differs from the first two standards in that it is purely intended to be used for Industrial applications. The XBee module works via a conventional serial TTL Link. Its job is to form the data into packets and send it to another XBee module or another node that complies with the Zigbee standard.

There are three separate sections in Transceiver based XBee Module. They are

1.PowerSupply            2.Transceiver   3.Indicator Portion

Fig 5.4.1.1 Schematic Diagram of a Transceiver        

based XBee Module

5.4.2 POWER SUPPLY:

The supply voltage can lie anywhere in the range of 8-20VDC, and it

does not have to be well filtered or regulated. The first component the supply voltage encounters is D2, which protect the circuit against reverse polarity connection. It is followed by 7805, which provides regulated 5V Supply voltage and an adjustable regulator LM317 generates 3.3V supply voltage for the XBee module. Although 5V may seem rather low as a input voltage     for 3.3V regulator,    it does not present a problem with this arrangement, and it has a advantage that very little heat is dissipated in the regulator.

5.3.3 TRANSCEIVER

The DB9 connector is connected to one of the serial ports of a PC Via a standard serial interface extension cable. Be sure to use 1- to- 1 cable instead of crossover (null-modem) cable. The serial port of the computer work with symmetrical voltages and negative logic

Logic level	PC	XBee	TTL
0	+12V	0V	0V
1	-12V	+3.3V	+5V

Table 5.4.3.1 Various logic levels of voltages

Now a 12V appears on the transmit data (Tx) line 2N3904 can handle. This voltage causes transistor to be cutoff, so the normal logic 1 at a level of 3.3V appears on the receive data line of the XBee module (Pin3) thanks to pull up resistor R3. The XBee when the PC puts a logic 1 on the data out line.

The purpose ofthe protection diode D1 is to reduce the hazardous voltage to a value of -0.6V which the module (which operates exclusively from 3.3V) also transmit signal to PC with the voltage of 3.3V

5.4.4 INDICATOR PORTION

Now let us look at Pin 6 of XBee module, which is the pulse width modulation ( PWM ) Pin. The XBee module uses this pin to indicate the strength of the most recently received signal.

RESULTS, DISCUSSIONS & CONCLUSION

Factors	Conventional Method	Zigbee
Erection Cost	Very High	Less
Electrical Burnouts	Possibility is High	No Possibility
Safety	Less	More
Maintenance	Difficult	Easy

CONCLUSION: There are several kinds of wireless technologies; the main difference being their range. Some offer connectivity over an area as large as your desktop whilst others can cover a medium-sized office space.Our most familiar wireless network, the mobile phone, covers whole continents.Wireless technology can offer businesses more flexible and inexpensive ways to send and receive data.The four key benefits of wireless technology are:Increased efficiency - improved communications leads to faster transfer of information within businesses and between partners/customers. You are rarely out of touch - you don't need to carry cables or adaptors in order to access office networks. Greater flexibility and mobility for users - office-based wireless workers can be networked without sitting at dedicated PCs. Reduced costs - relative to 'wired', wireless networks are, in most cases, cheaper to install and maintain. You can find out more about the specific benefits that different wireless solutions offer you by looking at the different wireless options.

References

[1].Zigbee technology based design,IEEE       

computer society 2006

[2]. Implementing the Maxstream Xbee Zigbee module to work with PIC microcontroller.

 [3]. Deploying Zigbee in industrial automation by Tim Cutler, Industrial embedded system resource guide 2005

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[5]. Wireless sensor and data transmission needs and technologies for patient monitoring in the operating room and intensive care unit, M. Paksuniemi, H. Sorvoja, E. Alasaarela, R. Myllylä Proceedings of the 2005 IEEE ,Engineering in Medicine and Biology 27th Annual  Conference, Shanghai, China, September 1-4, 2005

 [6]. Users make a Beeline for Zigbee Sensor Technology, David Geer, Industry Trends,       December 2005 , Published by the IEEE Computer Society

 [7]  Willig, A.; Matheus, K.; Wolisz, A., “Wireless Technology in Industrial Networks”,  Proceedings of the IEEE, Vol. 93, N. 6, June 2005, pp. 1130-1151.