applications in process industry - washington university …lu/cse521s/slides/wirelesshart.pdf ·...
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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.
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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 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
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.
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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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