3 Industrial Communication Networks Automation Overview 3 - - PowerPoint PPT Presentation

3 Industrial Communication Networks Automation Overview

EPFL, Spring 2017

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3 Industrial Communication Networks

3.1 Field bus principles 3.2 Field bus operation 3.3 Standard field busses 3.4 Industrial wireless communication

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transducers / actors

Networks in Automation Hierarchy

Hierarchy

Sensor-Actuator Bus

Fieldbus

programmable controllers Control Bus Supervision level Control level Field level Engineering Operator

2

direct I/O microPLCs

Course

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What is a field bus ?

A data network, interconnecting an automation system, characterized by:

• many small data items (process variables) with bounded delay (1ms..1s)
• transmission of non-real-time traffic for commissioning and diagnostics
• harsh environment (temperature, vibrations, EM-disturbances, water, salt,…)
• robust and easy installation by skilled people
• high integrity (no undetected errors) and high availability (redundant layout)
• clock synchronization (milliseconds to microseconds)
• low attachment costs ( € 5.- .. €50 / node)
• moderate data rates (50 kbit/s - 5 Mbit/s), large distance range (10m - 4 km)

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Expectations

• reduce cabling
• increased modularity of plant (each object comes with its computer)
• easy fault location and maintenance
• simplify commissioning (mise en service, IBS = Inbetriebssetzung)
• simplify extension and retrofit
• off-the-shelf standard products to build “Lego”-control systems

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The original idea: save wiring

PLC

But: the number of end-points remains the same ! energy must be supplied to smart devices

field bus COM marshalling bar I/O PLC smart devices tray capacity B e f

• r

e A f t e r dumb devices

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Marshalling (Rangierschiene, Barre de rangement)

The marshalling is the interface between the PLC people and the instrumentation people. The fieldbus replaces the marshalling bar or rather moves it piecewise to the process (intelligent concentrator / wiring)

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Different classes of field busses

poll time, milliseconds

10 100 1000 10,000 10 100 1000 10,000

Sensor Bus Simple devices Low cost Bus powered Short messages (bits) Fixed configuration Not intrinsically safe Twisted pair Max distance 500m Low Speed Fieldbus Process instruments, valves Medium cost Bus-powered (2 wire) Messages: values, status Intrinsically safe Twisted pair (reuse 4-20 mA) Max distance 1200m High Speed Fieldbus PLC, DCS, remote I/O, motors Medium cost Not bus powered Messages: values, status Not intrinsically safe Shielded twisted pair Max distance 800m Data Networks Workstations, robots, PCs Higher cost Not bus powered Long messages (e-mail, files) Not intrinsically safe Coax cable, fiber Max distance miles

One bus type cannot serve all applications and all device types efficiently...

source: ABB

frame size (bytes)

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Fieldbus Application: locomotives and drives

cockpit motors power electronics brakes power line track signals Train Bus diagnosis radio data rate delay medium number of stations 1.5 Mbit/second 1 ms (16 ms for skip/slip control) twisted wire pair, optical fibers (EM disturbances) up to 255 programmable stations, 4096 simple I/O Vehicle Bus cost engineering costs dominate integrity very high (signaling tasks)

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Fieldbus Application: automobile

• Electromechanical wheel brakes
• Redundant Engine Control Units
• Pedal simulator
• Fault-tolerant 2-voltage on-board power supply
• Diagnostic System

Board network ECU Monitoring and Diagnosis Brakes ECU 4 redundant board network 12V und 48V ECU ECU ECU

c

ECU redundant board network ECU

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Networking busses: electricity network control: myriads of protocols

low speed, long distance communication, may use power lines or telephone modems. Problem: diversity of protocols, data format, semantics...

houses substation

Modicom ICCP

control center

Inter-Control Center Protocol IEC 870-6

HV MV LV

High Voltage Medium Voltage Low Voltage

SCADA

FSK, radio, DLC, cable, fiber,...

substation

RTU RTU RTU RTU COM

RTU RTU RTU

Remote Terminal Units

RTU

RTU IEC 870-5 DNP 3.0 Conitel RP 570

control center control center

serial links (telephone)

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Fieldbus over a wide area: example wastewater treatment

Pumps, gates, valves, motors, water level sensors, flow meters, temperature sensors, gas meters (CH4), generators, etc are spread over an area of several km2. Some parts of the plant have to cope with explosives.

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Engineering a fieldbus: consider data density (Example: Power Plants)

 Data transmitted from periphery or from fast controllers to higher level  Slower links to control level through field busses over distances of 1-2 km. The control stations gather data at rates of about 200 kbit/s over distances of 30 m. Acceleration limiter and prime mover: 1 kbit in 5 ms Burner Control: 2 kbit in 10 ms For each 30 m of plant: 200 kbit/s Fast controllers require at least 16 Mbit/s over distances of 2 m The control room computers are interconnected by a bus of at least 10 Mbit/s,

• ver distances of several 100 m.

Field bus planning: estimate data density per unit of length or surface, response time and throughput over each link.

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3 Industrial Communication Networks

3.1 Field bus principles 3.2 Field bus operation 3.3 Standard field busses 3.4 Industrial wireless communication

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Assessment

• What is a field bus ?
• Which of these qualities are required:

1 Gbit/s operation Frequent reconfiguration Plug and play Bound transmission delay Video streaming

• How does a field bus support modularity ?
• Which advantages are expected from a field bus ?

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Objective of the field bus

Distribute process variables to all interested parties:

• source identification: requires a naming scheme
• accurate process value and units
• quality indication: {good, bad, substituted}
• time indication: how long ago was the value produced
• (optional description)

time quality value source description

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Data format

In principle, the bus could transmit the process variable in clear text (even using XML..) However, this is quite expensive and only considered when the communication network

• ffers some 100 Mbit/s and a powerful processor is available to parse the message

More compact ways such as ASN.1 have been used in the past with 10 Mbit/s Ethernet Field busses are slower (50kbit/s ..12 Mbits/s) and thus more compact encodings are used. value length type ASN.1: (TLV) minimum

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Datasets

wheel speed air pressure line voltage time stamp

analog variables binary variables

all door closed lights on heat on air condition on

bit offset

16 32 48 64 66 70

size Field busses devices have a low data rate and transmit always the same variables. It is economical to group variables of a device in the same frame as a dataset. A dataset is treated as a whole for communication and access. A variable is identified within a dataset by its offset and its size Variables may be of different types, types can be mixed.

dataset identifier

dataset

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Dataset extension and quality

To allow later extension, room is left in the datasets for additional variables. Since the type of these future data is unknown, unused fields are filled with '1". To signal that a variable is invalid, the producer overwrites the variable with "0". Since both an "all 1" and an "all 0" word can be a meaningful combination, each variable can be supervised by a check variable, of type ANTIVALENT2: A variable and its check variable are treated indivisibly when reading or writing The check variable may be located anywhere in the same data set.

dataset 1 1 1 1 1

check

1 1 1 1 1 1 1 1 1 1

correct variable error undefined variable value var_offset chk_offset 10 = substituted 00 = network error 01 = ok 11 = data undefined

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PLCs may exchange data, share inputs and outputs allows redundancy and “distributed intelligence” devices talk directly to each other separate bus master from application master !

Hierarchical or peer-to-peer communication

AP

all traffic passes by the master (PLC); adding an alternate master is difficult (it must be both master and slave) input

• utput

input

• utput

PLC PLC PLC PLC PLC central master / slave: hierarchical peer-to-peer: distributed

“slaves” “master” “slaves” “masters” alternate master AP AP AP AP AP

Application

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application processor application processor

Broadcasts

Most variables are read in 1 to 3 different devices Broadcasting messages identified by their source (or contents) increases efficiency. … instances … = variable

application processor

plant image plant image plant image = distributed database Bus refreshes plant image in the background Each station snoops the bus and reads the variables it is interested in.

Each device is subscribed as source or as sink for some process variables

Only one device is source of a certain process variable (otherwise collision) Replicated traffic memories can be considered as "caches" of the plant state (similar to caches in a multiprocessor system), representing part of the plant image. bus plant image

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Transmission principle

The previous operation modes made no assumption, how data are transmitted. The actual network can transmit data

• cyclically (time-driven) or
• n demand (event-driven),
• r a combination of both.

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Cyclic versus Event-Driven transmission

event-driven: send when value change by more than x% of range limit update frequency!, limit resolution cyclic: send value strictly every xx milliseconds nevertheless transmit:

• every xx as “I’m alive” sign
• when data is internally updated
• upon quality change (failure)

misses the peak (Shannon-Nyquist!) always the same, why transmit ? how much resolution?

• coarse (bad accuracy)
• fine (high frequency)

time individual period resolution

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Traffic Memory: implementation

Bus and Application are decoupled by shared memory, the Traffic Memory, (content addressed memory, CAM, also known as communication memory); process variables are directly accessible by application. Ports (holding a dataset)

Application Processor Bus Controller Traffic Memory Associative memory

two pages ensure that read and write can occur at the same time (no semaphores !)

bus

an associative memory decodes the addresses of the subscribed datasets

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Freshness supervision

Applications tolerate an occasional loss of data, but no stale data, which are at best useless and at worst dangerous.  Data must be checked if are up-to-date, independently of a time-stamp (simple devices do not have time-stamping) How: Freshness counter for each port in the traffic memory

• Reset by the bus or the application writing to that port
• Otherwise incremented regularly, either by application processor or bus controller.
• Applications always read the value of the counter before using port data and compare it with

its tolerance level. The freshness supervision is evaluated by each reader independently, some readers may be more tolerant than others. Bus error interrupts in case of severe disturbances are not directed to the application, but to the device management.

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Example of Process Variable API (application programming interface)

Simple access of the application to variables in traffic memory: ap_get (variable_name, variable value, variable_status, variable_freshness) ap_put (variable_name, variable value) Optimize: access by clusters (predefined groups of variables): ap_get (cluster_name) ap_put_cluster (cluster_name) Each cluster is a table containing the names and values of several variables. The clusters can correspond to "segments" in the function block programming.

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Cyclic Data Transmission

address

devices (slaves)

Bus Master plant

Principle: master polls addresses in fixed sequence (poll list)

1 2 3 4 5 6

Poll List

Individual period RTD N polls time [µs] read transfer time [ms]

The duration of each poll is the sum of the transmission time of address and data (bit-rate dependent) and of the reply delay

• f the signals

(independent of bit-rate).

address (i) data (i) address (i+1) 10 µs/km 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 Individual period 44 µs .. 296 µs

Example Execution

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Round-trip delay of master-slave exchange

The round-trip delay limits the extension

• f the bus

master most remote data source repeater repeater closest data sink

access delay propagation delay (t_pd = 6 µs/km)

t_source

distance

t_ms

T_m T_m T_s T_m t_repeat t_repeat (t_repeat < 3 µs) t_repeat

t_sm t_mm

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Cyclic operation characteristics

• 2. The delivery delay (refresh rate) is deterministic and constant.
• 4. No explicit error recovery needed since fresh value will be transmitted in next cycle.

To keep the poll time low, only small data items may be transmitted (< 256 bits)

• 3. The bus is under control of a central master (or distributed time-triggered algorithm).
• 1. Data are transmitted at fixed intervals, whether they changed or not.

Consequence: cycle time limited by product of number of data transmitted and the duration of each poll (e.g. 100 µs / variable x 100 variables => 10 ms)

The bus capacity must be configured beforehand. Determinism gets lost if the cycles are modified at run-time.

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Optimizing Cyclic Operation

Problem: fixed portion of the bus' time used => poll period increases with number of polled items => response time slows down Solution: introduce sub-cycles for less urgent periodic variables length: power of 2 multiple of the base period. Notes: Poll cycles should not be modified at run-time (non-determinism) group with period 1 ms time

4a 8 16 1 4b 64 3

1 ms period (basic period) 2 ms period

2 4a

4 ms period 1 ms 1 ms

1 1 1 2

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Cyclic Transmission with Decoupled Application

The bus master scans the identifiers at its own pace. The bus traffic and the application cycles are asynchronous to each other. Traffic Memory

cyclic algorithms cyclic algorithms cyclic algorithms cyclic algorithms port address

application 1

Ports Ports Ports

application 2 application 4

source port sink port port data sink port cyclic poll

bus controller

bus master application 3 bus

Periodic List Ports

bus controller bus controller bus controller bus controller

Deterministic behavior, at expense of reduced bandwidth and geographical extension.

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Example: delay requirement

Worst-case delay for transmitting all time critical variables is the sum of: Source application cycle time Individual period of the variable on bus Sink application cycle time 8 ms 16 ms 8 ms = 32 ms subscribers application instances

device

publisher application instance bus instance

device device applications

bus

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Event-driven Operation

Detection of an event is an intelligent process:

• Not every change of a variable is an event, even for binary variables.
• Often, a combination of changes builds an event.
• Only the application can decide what is an event, since only the application

programmer knows the meaning of the variables. Events cause transmission only when state changes. Bus load very low on average, but peaks under exceptional situations since transmissions are correlated by process (christmas-tree effect).

• event-

reporting station event- reporting station event- reporting station plant intelligent stations sensors/ actors

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Bus interface for event-driven operation

Application Processor Bus Controller message (circular) queues

bus driver filter application

• Each transmission on bus causes an interrupt.
• Bus controller checks address and stores data in message queues.
• Driver is responsible for removing messages of queue memory and

prevent overflow.

• Filter decides if message can be processed.

interrupt

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Response of Event-driven operation

Interruption of server device at any instant can disrupt a time-critical task. Buffering of events can cause unbounded delays Gateways introduce additional uncertainties Since events can occur anytime on any device, stations communicate by spontaneous transmission, leading to possible collisions Caller Application Called Application Transport software Transport software

interrupt request indication confirm

Bus

time

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Determinism and Medium Access In Busses

Although the moment an event occurs is not predictable, the bus should transmit the event in a finite time to guarantee the reaction delay. Events are necessarily announced spontaneously The time required to transmit the event depends on the medium access (arbitration) procedure of the bus. Medium access control methods are either deterministic or not. Non-deterministic Collision (CSMA/CA) Deterministic Central master, Token-passing (round-robin), Binary bisection (collision with winner)

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Events and Determinism

Deterministic medium access necessary to guarantee delivery time bound but not sufficient since events messages are queued in the devices.

The average delivery time depends on the length of the queues, on the bus traffic and on the processing time at the destination. Often, the applications influence the event delay much more than the bus does. Real-time Control = Measurement + Transmission + Processing + Acting

bus data packets acknowledgements

input and

• utput queues

events producers & consumers

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Events Pros and Cons

In an event-driven control system, there is only a transmission or an operation when an event occurs. Advantages: Drawbacks: Can treat a large number of events – but not all at the same time Supports a large number of stations System idle under steady - state conditions Better use of resources Uses write-only transfers, suitable for LANs with long propagation delays Suitable for standard (interrupt-driven) operating systems (Unix, Windows) Requires intelligent stations (event building) Needs shared access to resources (arbitration) No upper limit to access time if some component is not deterministic Response time difficult to estimate, requires analysis Limited by congestion effects: process correlated events A background cyclic operation is needed to check liveliness

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Summary: Cyclic vs Event-Driven Operation

sending: application writes data into memory receiving: application reads data from memory the bus controller decides when to transmit bus and application are not synchronized

application processor bus controller traffic memory (buffer) decoupled (asynchronous):

sending: application inserts data into queue and triggers transmission, bus controller fetches data from queue receiving: bus controller inserts data into queue and interrupts application to fetch them, application retrieves data

application processor bus controller queues coupled (with interrupts): events (interrupts)

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Mixed Data Traffic

represent the state of the plant represent state changes of the plant

• > Periodic Transmission
• f Process Variables

short and urgent data items

Since variables are refreshed periodically, no retransmission protocol is needed to recover from transmission error.

• > Sporadic Transmission of

Process Variables and Messages infrequent, sometimes long messages reporting events, for:

• System: initialisation, down-loading, ...

Since messages represent state changes, a protocol must recover lost data in case of transmission errors

• Users: set points, diagnostics, status

Process Data Message Data

... motor current, axle speed, operator's commands, emergency stops,... periodic phase periodic phase event sporadic phase

time

basic period basic period sporadic phase

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Mixing Traffic is a configuration issue

Cyclic broadcast of source-addressed variables standard solution for process control. Cyclic transmission takes large share of bus bandwidth and should be reserved for really critical variables. Decision to declare a variable as cyclic or event-driven can be taken late in a project, but cannot be changed on-the-fly in an operating device. Message transmission scheme must exist alongside the cyclic transmission to carry not-critical variables and long messages such as diagnostics or network management An industrial communication system should provide both transmission modes.

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Real-Time communication stack

The real-time communication model uses two stacks, one for time-critical variables and one for messages Logical Link Control time-critical process variables Management Interface time-benign messages Physical Link (Medium Access) Network (connectionless) Transport (connection-oriented) Session Presentation Application

7 6

Remote Procedure Call

5 4 3 2' 1

connectionless connectionless connection-oriented medium access implicit implicit

Logical Link Control

2"

media

common

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Cyclic or Event-driven Operation For Real-time ?

Data are transmitted at fixed intervals, whether they changed or not. Data are only transmitted when they change or upon explicit demand. cyclic operation event-driven operation (aperiodic, demand-driven, sporadic) (periodic, round-robin) Worst Case is normal case Typical Case works most of the time Non-deterministic: delivery time vary widely Deterministic: delivery time is bound All resources are pre-allocated Best use of resources message-oriented bus

• bject-oriented bus

Fieldbus Foundation, MVB, FIP, .. Profibus, CAN, LON, ARCnet

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Time-stamping and synchronisation

In many applications, e.g. disturbance logging and sequence-of-events, the exact sampling time of a variable must be transmitted together with its value. => Devices equipped with clock recording creation time of value (not transmission time). To reconstruct events coming from several devices, clocks must be synchronized. considering transmission delays and failures. bus input input input processing t1 t2 t3 t4 t1 val1

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Example: Phasor information

Phasor transmission over the European grid: a phase error of 0,01 radian is allowed, corresponding to +/- 26 µs in a 60 Hz grid or 31 µs in a 50 Hz grid.

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Time distribution

In master-slave busses, master distributes time as bus frame. Slave can compensate for path delays, time is relative to master In demanding systems, time is distributed over separate lines as relative time, e.g. PPS = one pulse per second, or absolute time (IRIG-B), with accuracy of 1 µs. In data networks, a reference clock (e.g. GPS or atomic clock) distributes the time. A protocol evaluates the path delays to compensate them.

• NTP (Network Time Protocol): about 1 ms is usually achieved.
• PTP (Precision Time Protocol, IEEE 1588), all network devices collaborate to estimate the

delays, an accuracy below 1 µs can be achieved without need for separate cables (but hardware support for time stamping required). (Telecom networks typically do not distribute time, they only distribute frequency)

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NTP (Network Time Protocol) principle

2 ) ( ) (

2 3 1 4

t t t t     

time request time response t1 t2 t3 t4 time request time response t’1 t’2 t’3 t’4 distance time  network delay server network client network delay 

Measures delay end-to-end over the network (one calculation) Problem: asymmetry in the network delays, long network delays

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IEEE 1588 principle (PTP, Precision Time Protocol)

Grand Master Clock

Pdelay-request Pdelay-response

TC TC TC OC OC MC TC OC OC

residence time calculation

peer delay calculation

TC

MC = master clock TC = transparent clock OC = ordinary clock

Two calculations: residence time and peer delay All nodes measure delay to peer TC correct for residence time (HW support)

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IEEE 1588 – 1 step clocks

Sync (contains all  + ) residence time Pdelay_Resp (contains t3 – t2) Pdelay_Req

• rdinary

(slave) clock distance time Sync 1-step transparent clock grand master clock 1-step transparent clock residence time bridge bridge  link delay  t2 t3 Pdelay_Resp t1 t4 Pdelay_Req t2 t3 t1 t4 t2 t3 t1 t4 Sync Pdelay_Resp Pdelay_Req  t5  t5 t6

Grandmaster sends the time spontaneously. Each device computes the path delay to its neighbour and its residence time and corrects the time message before forwarding it residence time calculation

peer delay calculation

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References

To probe further

• http://www.ines.zhaw.ch/fileadmin/user_upload/engineering/_Institute_und_Zentren/INES/IEEE158

8/Dokumente/IEEE_1588_Tutorial_engl_250705.pdf

• http://blog.meinbergglobal.com/2013/11/22/ntp-vs-ptp-network-timing-smackdown/
• http://blog.meinbergglobal.com/2013/09/14/ieee-1588-accurate/

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Networking field busses

Networking field busses is not done through bridges or routers, because normally, transition from one bus to another is associated with:

• data reduction (processing, sum building, alarm building, multiplexing)
• data marshalling (different position in the frames)
• data transformation (different formats on different busses)

Only system management messages could be threaded through from end to end, but due to lack of standardization, data conversion is not avoidable today.

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Assessment

What is the difference between a centralized and a decentralized industrial bus ? What is the principle of source-addressed broadcast ? What is the difference between a time-stamp and a freshness counter ? Why is an associative memory used for source-addressed broadcast ? What are the advantages / disadvantages of event-driven communication ? What are the advantages / disadvantages of cyclic communication ? How is time transmitted ? How are field busses networked ?

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3 Industrial Communication Networks

3.1 Field bus principles 3.2 Field bus operation 3.3 Standard field busses 3.4 Industrial wireless communication

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Different classes of field busses

poll time, milliseconds

10 100 1000 10,000 10 100 1000 10,000

Sensor Bus Simple devices Low cost Bus powered Short messages (bits) Fixed configuration Not intrinsically safe Twisted pair Max distance 500m Low Speed Fieldbus Process instruments, valves Medium cost Bus-powered (2 wire) Messages: values, status Intrinsically safe Twisted pair (reuse 4-20 mA) Max distance 1200m High Speed Fieldbus PLC, DCS, remote I/O, motors Medium cost Not bus powered Messages: values, status Not intrinsically safe Shielded twisted pair Max distance 800m Data Networks Workstations, robots, PCs Higher cost Not bus powered Long messages (e-mail, files) Not intrinsically safe Coax cable, fiber Max distance miles

One bus type cannot serve all applications and all device types efficiently...

source: ABB

frame size (bytes)

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Worldwide most popular field busses

Market shares held by the leading fieldbus and industrial Ethernet systems Source: HMS Industrial Networks, 2016

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Field device: example differential pressure transducer

The device transmits its value by means of a current loop 4..20 mA current loop fluid

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4-20 mA loop - the conventional, analog standard

The transducer limits the current to a value between 4 mA and 20 mA, proportional to the measured value, while 0 mA signals an error (wire break) The voltage drop along the cable and the number of readers induces no error. Simple devices are powered directly by the residual current (4mA), allowing to transmit signal and power through a single pair of wires. 4-20mA is basically a point-to-multipoint communication (one source) The 4-20 mA is the most common analog transmission standard in industry

transducer reader

1

reader

2

i(t) = 0, 4..20 mA

R1 R2 R3

sensor i(t) = f(v) voltage source 10V..24V RL4 conductor resistance RL2 RL3 RL4 RL1 flow

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HART

• Data over 4..20 mA loops

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HART – Principle (1986)

HART (Highway Addressable Remote Transducer) was developed by Fisher-Rosemount to retrofit 4-to-20mA current loop transducers with digital data communication (not for closed-loop communication). HART modulates the 4-20mA current with a low-level frequency-shift-keyed (FSK) sine-wave signal, without affecting the average analogue signal. HART uses low frequencies (1200Hz and 2200 Hz) to deal with poor cabling, its rate is 1200 Bd - but sufficient.

Transmission of device characteristics is normally not real-time critical

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HART - Protocol

Hart communicates point-to-point, under the control of a master, e.g. a hand-held device

preamble start address command bytecount [status] data data checksum 1 1..5 5..20 (xFF) 1 1 [2] (slave response) 0..25 (recommended) 1

Master

Indication

Slave

Request Confirmation Response time-out

Hart frame format (character-oriented, not bit-oriented):

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Universal commands (mandatory):

identification, primary measured variable and unit (floating point format) loop current value (%) = same info as current loop read current and up to four predefined process variables write short polling address sensor serial number instrument manufacturer, model, tag, serial number, descriptor, range limits, …

Common practice (optional) time constants, range, EEPROM control, diagnostics,… total: 44 standard commands, plus user-defined commands Transducer-specific (vendor-defined) calibration data, trimming,…

HART - Commands

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HART - Importance

Practically all 4..20mA devices come equipped with HART today About 40 Mio devices are sold per year. more info: http://www.thehartbook.com/default.asp http://www.hartcomm.org/

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Fieldbus Comparison

Fieldbus BW Max Length Max Data Size Application Max Nodes Notes PROFIBUS (DP and PA) 1.5-12 Mbit/s 31.25 Kbits/s 100 m – 24 km 1900 m 246 bytes Factory Automation Process Automation 127 32 Token passing, master-slave / P2P, operate sensors and actuators (DP), monitor measuring equipment (PA) DeviceNet 250 kBit/s 500 m 8 bytes Factory Automation 64 CSMA/CD, master-slave, multidrop, motors, drives, uses CAN CANopen 10 kBit/s

• 1 Mbit/s

25-1000 m 8 bytes Automobile, Industrial Automation 127 CSMA, Ideal for small data and fast sync, uses CAN

http://www.bierlemartin.de/hengstler/training/fbcomp.htm http://www.pacontrol.com/download/fieldbuscomp.pdf http://www.mtl.de/pdfs/news/open_fieldbus.pdf

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CAN

Mastership multi-master, 12-bit bisection, bit-wise arbitration Link layer control connectionless (command/reply/acknowledgement) Upper layers no transport, no session, implicit presentation

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CAN - Analysis

– interoperability questionable (too many different implementations) – small data size and limited number of registers in the chips. + application layer definition – several incompatible application layers (CanOpen, DeviceNet, SDS) – strongly protected by patents (Bosch) + supported by user organisations ODVA, Honeywell... + application layer profiles – limited product distance x rate (40 m x Mbit/s) – sluggish real-time response (2.5 ms) + bus analyzers and configuration tools available + numerous low cost chips, come free with many embedded controllers – non-deterministic medium access + Market: industrial automation, automobiles

+

• – no standard message services.

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Ethernet Paradigms

switch switch SCADA Fieldbus Ethernet SCADA simple devices PLC PLC PLC Soft-PLC Soft-PLC Soft-PLC Soft-PLC Ethernet costlier field devices Soft-PLC as concentrators Event-driven operation cheap field devices decentralized I/O cyclic operation

Classical Ethernet + Fieldbus Ethernet as Fieldbus This is a different wiring philosophy. The bus must follow the control system structure, not the other way around

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The Ethernet „standards“

IEC SC65C „standardized“ 22 different, uncompatible "Industrial Ethernets“, driven by „market demand“. Compatibility: practically none Overlap: a lot 2 EtherNet/IP (Rockwell. OVDA) 3 Profibus, Profinet (Siemens, PNO) 4 P-NET (Denmark) 6 INTERBUS (Phoenix) 10 Vnet/IP (Yokogawa, Japan) 11 TCnet (Toshiba, Japan) 12 Ethercat (Beckhoff, Baumüller) 13 Powerlink (BR, AMK) 14 EPA (China) 15 Modbus-RTPS (Schneider, IDA) 16 SERCOS (Bosch-Rexroth / Indramat) … In addition to Ethernets standardized in other committees: FF's HSE, (Emerson, E&H, FF) IEC61850 (Substations) ARINC (Airbus, Boeing,..)

Industrial Automation | 2017 68

Ethernet and fieldbus roles

Traditionally, Ethernet is used for the communication among the PLCs and for communication of the PLCs with the supervisory level and with the engineering tools Fieldbus is in charge of the connection with the decentralized I/O and for time-critical communication among the PLCs. Ethernet fieldbus local I/O CPU

Industrial Automation | 2017 69

Future of field busses

Non-time critical busses are being displaced by LANs (Ethernet) and cheap peripheral busses (USB, …) These "cheap" solutions are being adapted to the industrial environment and become a proprietary solution (e.g. Siemens "Industrial Ethernet") The cabling objective of field busses (more than 32 devices over 400 m) is out of reach for cheap peripheral busses such as USB. Fieldbusses tend to live very long (10-20 years), contrarily to office products. There is no real incentive from the control system manufacturers to reduce the fieldbus diversity, since the fieldbus binds customers. The project of a single, interoperable field bus defined by users (Fieldbus Foundation) failed, both in the standardisation and on the market.

Industrial Automation | 2017 70

Fieldbus Selection Criteria

Installed base, devices availability: processors, input/output Interoperability (how likely is it to work with a product from another manufacturer Topology and wiring technology (layout) Connection costs per (input-output) point Response time Deterministic behavior Device and network configuration tools Bus monitor (baseline and application level) tools Integration in development environment Power distribution and galvanic separation (power over bus, potential differences)

Industrial Automation | 2017 71

Assessment

Which is the medium access and the link layer operation of CAN ? What is the wiring philosophy of Industrial Ethernet? Which are the selection criteria for a field bus ? What makes a field bus suited for hard-real-time operation ? How does the market influence the choice of the bus ?

Industrial Automation | 2017 72

3 Industrial Communication Networks

3.1 Field bus principles 3.2 Field bus operation 3.3 Standard field busses 3.4 Industrial wireless communication

Industrial Automation | 2017 73

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)

Industrial Automation | 2017 74

Wireless Landscape

Industrial Automation | 2017 75

Wireless IEEE Numbers

Industrial Automation | 2017 76

Requirements for Industrial Wireless

Wireless Industrial Applications time Non Real

• Hard

Real

• Time

Remote Control Machine Health Monitoring System Configuration Internet Connectivity Control Loops Machine-to-machine communication Events Registration Measurements Media

• Real

S

• f

t t i m e

Industrial Automation | 2017 77

Challenges and Spectrum of Solutions

Wireless Challenges Attenuation Fading Multipath dispersion Interference High Bit Error rate Burst channel errors Application Requirements Reliable delivery Meet deadlines Support message priority

Antenna Redundancy Cooperative diversity ARQ Error Correction Codes Modulation Techniques Transmitter Design

Existing Solutions

Existing Solutions

Industrial Automation | 2017 78

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)

Industrial Automation | 2017 79

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 ??? ??? ???

Industrial Automation | 2017 80

Legal Frequencies

www.fcc.gov

Industrial Automation | 2017 81

Range vs Data Range

1 m 10 m 100 m 1 km 10 km 0 GHz 2 GHz 1GHz 3 GHz 5 GHz 4 GHz 6 GHz 802.11a UWB ZigBee Bluetooth ZigBee 802.11b,g 3G UWB

Industrial Automation | 2017 82

Industrial Example: WirelessHART

• HART (Highway Addressable Remote Transducer) fieldbus protocol
• Supported by 200+ global companies
• Since 2007 Compatible WirelessHART extension

Industrial Automation | 2017 83

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

Industrial Automation | 2017 84

WirelessHART Networking Stack

• Transport layer:
• Quality of Service (QoS): (Command, Process-Data, Normal, Alarm)
• Application layer:
• Standard HART application layer
• Device Description Language
• Timestamping
• Boot-strapping:
• Gateway announcements (network ID and time sync)
• Device sends join request
• Authentication and configuration via network manager

May be replaced by 6TiSCH?

Industrial Automation | 2017 85

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

Industrial Automation | 2017 86

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 in Wireless HART?

Industrial Automation | 2017 87

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