3 Field busses3.4 Industrial Wireless

Industrial AutomationAutomation IndustrielleIndustrielle Automation

Field bus wireless 3.4 - 2Industrial Automation 2013

Motivation for Industrial Wireless

• Reduced installation and reconfiguration costs

• Easy access to machines (diagnostic or reprogramming)

• Improved factory floor coverage

• Eliminates damage of cabling

• Globally accepted standards(mass production)

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Wireless Landscape

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Wireless IEEE Numbers

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Requirements for Industrial Wireless

Wireless Industrial Applications

Real

Remote ControlMachine Health Monitoring

System ConfigurationInternet Connectivity

Control LoopsMachine-to-machine communication

Events RegistrationMeasurements

Media

-R

eal

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Wireless for Non Real-Time Applications

• Remote Control:

– Used for remote control of overhead cranes

– High security requirements

– Long code words to initiate remote control action

• Machine health monitoring:

– Accurate information about status of a process

– Local on demand access: PDA or laptop that connects to sensors or actuators

– Control room: access point / gateway

6

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Wireless for Soft Real-Time Applications

Measurements:– For physical process, timestamp values – Ability to reconstruct course of events – Requires clock synchronization; precision dictated by granularity of

measurement– E.g. geological or industrial sensors collecting data and

transmitting them to base station or control room

Media:– Delay and loss rate constraints for user comfort – E.g. voice and video transfer

Control loops:– Slow or non-critical operations– Low sample rate– Not affected by a few samples being lost– Delay constraint based on comfort demands– E.g. heat control and ventilation system

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Wireless Hard Real-Time Applications

• Late transmission cannot be tolerated

• E.g. control loops

Assumes fault-free communication channel

Wireless:– Error probability cannot be neglected – Sporadic and bursty errors

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Challenges and Spectrum of Solutions

Wireless Challenges

AttenuationFadingMultipath dispersionInterferenceHigh Bit Error rateBurst channel errors

Application Requirements

Reliable deliveryMeet deadlinesSupport message priority

Antenna Redundancy

Cooperative diversity

ARQ

Error Correction Codes

Modulation Techniques

Transmitter Design

Existing SolutionsExisting Solutions

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Radio wave interferes with surrounding environment creating

multiple waves at receiver antenna, they are delayed with respect

to each other. Concurrent transmissions cause interference too.

=> Bursts of errors

•Forward Error Correction (FEC):

Encoding redundancy to overcome error bursts

•Automated Repeat ReQuest (ARQ):

Retransmit entire packets when receiver cannot decode the packet

(acknowledgements)

Reliability for wireless channel

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Deadline Dependent Coding

Uses FEC and ARQ to improve Bit Error Rate:

–Re-transmissions before deadline

–Different coding rate depending on remaining time to deadline

–Tradeoff between throughput and how much redundancy is needed

–Additional processing such as majority voting

–Decoder keeps information for future use (efficiency)

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Existing protocols- comparison

Feature 802.11 Bluetooth Zigbee / 802.15.4

Interference from other devices

-- Avoided using frequency hopping

Dynamic channel selection possible

Optimized for Multimedia, TCP/IP and high data rate applications

Cable replacement technology for portable and fixed electronic devices.

Low power low cost networking in residential and industrial environment.

Energy Consumption High Low (Large packets over small networks)

Least (Small packets over large networks)

Voice support/Security Yes/Yes Yes/Yes No/Yes

Type of Network / Channel Access

Mobile / CSMA/CA and polling

Mobile & Static / Polling

Mostly static with infrequently used devices / CSMA and slotted CSMA/CA

Bit error rate High Low Low

Real Time deadlines ??? ??? ???

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Range

1 m

10 m

100 m

1 km

10 km

0 GHz 2 GHz1GHz 3 GHz 5 GHz4 GHz 6 GHz

802.11a

UWB

ZigBee BluetoothZigBee

802.11b,g

3G

UWB

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Legal Frequencies

www.fcc.gov

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Industrial Example: WirelessHART

• HART (Highway Addressable Remote Transducer) fieldbus protocol

• Supported by 200+ global companies

• Since 2007 Compatible WirelessHART extension

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WirelessHART Networking Stack

• PHY:

– 2,4 GHz Industrial, Scientific, and Medical Band (ISM-Band)

– Transmission power 0 - 10 dBm

– 250 kbit/s data rate

• MAC:

– TDMA (10ms slots, static roles)

– Collision and interference avoidance:Channel hopping and black lists

• Network layer:

– Routing (graph/source routing)

– Redundant paths

– Sessions and broadcast encryption (AES)

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WirelessHART Networking Stack

• Transport layer:

– Segmentation, flatten network

– Quality of Service (QoS): (Command, Process-Data, Normal, Alarm)

• Application layer:

– Standard HART application layer

– Device Description Language

– Smart Data Publishing (lazy)

– Timestamping

– Events

– Command aggregation

• Boot-strapping:

– Gateway announcements (network ID and time sync)

– Device sends join request

– Authentication and configuration via network manager

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Design Industrial Wireless Network

• Existing wireless in plant; frequencies used?

• Can the new system co-exist with existing?

• How close are you to potential interferences?

• What are uptime and availability requirements?

• Can system handle multiple hardware failures withoutperformance degradation?

• What about energy source for wireless devices?

• Require deterministic power consumption to ensure predictable maintenance.

• Power management fitting alerting requirements and battery replacement goals

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Assessment

• Why is a different wireless system deployed in a factory than at home?

• What are the challenges of the wireless medium and how are they tackled?

• How can UWB offer both a costly and high bandwidth and a cheaper and high bandwidth services?

• Which methods are used to cope with the crowded ISM band?

• Why do we need bootstrapping?

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References

• Wireless Communication in Industrial Networks, Kavitha Balasubramanian, Cpre 458/558: Real-Time Systems, www.class.ee.iastate.edu/cpre458/cpre558.F00/notes/rt-lan7.ppt

• WirelessHART, Christian Hildebrand, www.tu-cottbus.de/systeme, http://systems.ihp-microelectronics.com/uploads/downloads/2008_Seminar_EDS_Hildebrand.pdf

• WirelessHARTTM Expanding the Possibilities, Wally Pratt HART Communication Foundation, www.isa.org/wsummit/.../RHelsonISA-Wireless-Summit-7-23-07.ppt

• Industrial Wireless Systems, Peter Fuhr, ISA, www.isa.org/Presentations_EXPO06/FUHR_IndustrialWirelessPresentation_EXPO06.ppt