Industrial Automation
Automation Industrielle
Industrielle Automation
3
Field busses
3.4
Industrial Wireless
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
Events Registration
Measurements
Media
n
- ti
al
Re
Remote Control
Machine Health Monitoring
System Configuration
Internet Connectivity
me
Soft Real - time
No
Wireless Industrial
Applications
Hard
e
m
i
Real - T
Control Loops
Machine-to-machine communication
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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
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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
Attenuation
Fading
Multipath dispersion
Interference
High Bit Error rate
Burst channel errors
Existing
Solutions
Existing
Solutions
Antenna Redundancy
Cooperative diversity
ARQ
Application Requirements
Reliable delivery
Meet deadlines
Support message priority
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Error Correction Codes
Modulation Techniques
Transmitter Design
Field bus wireless 3.4 - 9
Reliability for wireless channel
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)
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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
10 km
3G
1 km
100 m
802.11a
802.11b,g
10 m
ZigBee
Bluetooth
ZigBee
UWB
1m
0 GHz
1GHz
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UWB
2 GHz
3 GHz
4 GHz
5 GHz
6 GHz
Field bus wireless 3.4 - 13
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 without
performance 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
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