Applications in Process Industry Ø  Enhance safety, optimize process, protect environment

q  Detect leaks before they lead to environmental problems.

q  Monitor the status of manually operated valves.

q  Monitor safety relief valves to detect venting to avoid accidents.

Ø Health, Safety, and the Environment (HSE) regulations.

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Process Control Ø  Feedback control loop controls a physical process.

q  Example: control temperature by manipulating heat supply.

Ø Centralized vs. peer-to-peer control.

2  

Actuator!

Monitor!

Reference!

Controlled"variable"-

error" sample"

Manipulated"variable"

change"

Process!

Controller! control"input"

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Why Wireless Ø Cost reduction: wiring is economically infeasible

Ø  Easier installation: inaccessible locations

Ø  Easier maintenance q  Wired networks cannot handle severe heat or exposure of chemicals q  A wireless infrastructure can remain in place for many years.

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Challenges in Wireless Ø  Strict timing requirement

Ø High security concerns

Ø  Reliable communication despite wireless deficiencies

Ø  Plant environments are inherently unreliable

q  Interference, obstacles, power failures, lightening, storms…

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Wireless Technologies

Ø   Existing standards fail in industrial environments q  ZigBee: static channel

q  Bluetooth: quasi-static star network

Ø  WirelessHART q  For process measurement and control applications

q  First open and interoperable wireless standard to address the critical needs of real-world industrial applications

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History Ø HART (Highway Addressable Remote Transducer Protocol)

q  Most widely used field communication protocol

q  30 million devices worldwide

Ø WirelessHART released in Sep 2007 (as a part of HART 7) q  Adds wireless capabilities to the HART protocol while maintaining

compatibility with existing devices, commands and tools.

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Wireless for Process Automation Ø World-wide adoption of wireless in process industries

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1.5+  billion  hours    opera2ng  experience  

100,000s  of  smart  wireless  field  devices  

10,000s  of  wireless    field  networks   Courtesy:  Emerson  Process  Management  

Offshore   Onshore  

Killer  App  of  Sensor  Networks!  

WirelessHART Use Cases Ø  Improved control of plant steam supply by detecting “cool

spots” in cross plant steam lines

Ø  Reducing risk of overfilling tanks by adding redundant level measurements (in oil and petroleum refineries)

Ø Monitor and control safety valves

Ø Monitor and control pressure and temperature of process fluids and gases

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What is special? Ø  Reliable: 99.9%

Ø  Secure

Ø  Self-organizing, self-healing

Ø  Interoperable

Ø  Supports both star and mesh topologies

Ø  Built-in time synchronization

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Network Architecture

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Network Manager Ø Centralized brain

Ø Manages the network and its devices q  Routing, scheduling

q  User/administrator interacts with the Network Manager

q  Generates network management packets to devices

Ø  Redundant Network Managers supported (only one active)

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Field Devices Ø  Sensor/actuator/both

Ø Connected to the process or plant equipment

Ø Combines wireless communication with traditional HART field device capabilities

Ø May be line or battery-powered

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WirelessHART Adapter

Ø  Enables communication with a non-native device through a WirelessHART Network. .

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Gateway Ø One gateway can support up to 80 devices

Ø A Gateway provides q  One or more Access Points providing the physical connection into the

WirelessHART network q  One or more Host Interfaces connecting the Gateway to backbone

networks (e.g., the plant automation network) q  A connection to the Network Manager

q  Buffering and local storage for publishing data, event notification, and common commands

q  Time synchronization sourcing

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Other Devices Ø Handheld devices

q  Portable applications used to configure, maintain or control plant assets.

q  Typically belong to networks of different standards

Ø  Plant Automation Network

q  Connects client applications to the gateway

Ø  Security Manager

q  Industry standard AES-128 ciphers/keys

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WirelessHART PHY Ø Adopts IEEE 802.15.4

q  Same 16 mutually orthogonal channels

q  Operates in the 2.4GHz ISM band

q  Data rate of up to 250 Kbps

Ø  Radio transceivers

q  Omni-directional

q  Half-duplex q  100 meters LOS @ 0 dB

q  Time to switch between channels: 0.192 ms

q  Radio turn-on time: 4 ms

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How to achieve reliability? Ø  Time diversity

Ø Channel diversity q  Channel hopping

q  Channel blacklisting

Ø  Route diversity q  Graph routing

Ø  Power Diversity

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TDMA Data Link Layer Ø  10 ms time slot

q  Transmission starts at a specified time after the beginning of a slot •  Source & destination set channel •  Allows receiver to begin listening

q  Enough time for transmission + ACK

Ø  Superframe: a series of time slots defining the communication

schedule of a set of devices

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Time Synchronization Ø Gateway is the root source of time

Ø When a device receives a packet •  Δt = time of arrival – expected arrival time

•  sends Δt to the sender via ACK

Ø  Sender adjusts time

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Shared vs. Dedicated Time Slots Ø A time slot may be shared or dedicated

Ø Dedicated slot: only one sender sends to a receiver

Ø  Shared slot: multiple senders attempt to send to a receiver

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Shared Time Slots Ø  Devices contest for access using a contention-based scheme.

q  Behave similar to Slotted Aloha

q  Use collision-avoidance (backoff).

Ø Using shared links may be desirable when q  Throughput requirements of devices are low

q  Traffic is irregular or comes in bursts

Ø  May reduce latency since devices do not need to wait for dedicated slot q  True only when chances of collisions are low

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Channel Hopping Ø  Enhances reliability

q  Avoid interferers

q  Reduce multi-path fading effects

Ø  Blacklisting restricts hopping to some channels

Ø  Each device has a channel map (logical to physical)

Ø ActiveChannel = (ChannelOffset + ASN) % #Channel

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Routing Ø  WirelessHART supports both Graph and Source routing

Ø  Graph routing: provides redundant paths

Ø  Routing graphs q  Uplink graph: upstream communication q  Downlink graph: Downstream communication

q  Broadcast graph

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Scheduling

Ø  Slots and channel assignment q  Each receiver uses a separate channel for reception

q  A transmission is followed by a retransmission on the same link on a dedicated slot, then again on another link on a shared slot

Ø  Each network contains exactly one overall schedule that is created and managed by the Network Manager.

Ø  The schedule is organized into superframes

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Superframe

25  

Superframe

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Superframe

Ø All devices must support multiple superframes

Ø At least one superframe is always enabled while additional superframes can be enabled or disabled

Ø  Slot sizes and the superframe length are fixed and form a network cycle with a fixed repetition rate

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Data Link Protocol Data Unit (DLPDU)

Ø  Five DLPDU types: q  Data

q  Keep-Alive (periodic)

q  Advertise (periodic) q  Disconnect

q  ACK

  Ø Devices receiving a packet with an unknown packet type must

not acknowledge the packet and shall immediately discard it.

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Network Initialization Ø  WirelessHART Network automatically starts up and self-organize.

Ø  Before a network can form, a Network Manager and a Gateway must exist.

Ø  The Network Manager activates the first superframe. This establishes the system epoch – ASN 0.

Ø  Once the Network Access Point starts to advertise, devices can begin to join the network.

Ø  As devices join, the network forms.

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Network Maintenance

Ø Advertise and Keep-Alive DLPDUs assist in building and maintaining the device's neighbor list

Ø A Keep-Alive must be transmitted to the neighbor if Last Time Communicated > Keep Alive Interval.

Ø Keep-Alive transmissions are repeated until a new DLPDU is received from the neighbor

Ø Keep-Alive no more often than once per 30 seconds (if temperature varies 2º C per minute or less).

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Network Maintenance Ø  Path failures are reported to the Network Manager when

devices lose connectivity to neighbors.

Ø After the Path Fail Interval lapses, a Path-Down Alarm is generated (by both the sender and the receiver).

Ø As each device's Health Report Timer lapses, the devices generate health reports, which include indications of any problems the device is having with a neighbor.

Ø Default period of each devices health report is 15 minutes.

31  

Network Maintenance Ø Devices continue trying to reestablish communication until

the links between them are deleted by the Network Manager.

Ø  It is common for broken paths to be restored after a temporary environmental effect passes.

Ø  If the disruption persists, additional Path-Down Alarms will be generated when the Path Fail Interval lapses again.

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Best Practices Ø  Each field device should have at least three neighbors

q  The 3rd neighbor will act as a backup if one of the two primary paths is obstructed or unavailable.

Ø Devices (antenna) mounted >0.5m from any vertical surface.

Ø Devices mounted >1.5m off the ground.

Ø  25% of the network devices should have a direct connection to the gateway in large networks.

 

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