EPFL, Spring 2017 3 Industrial Communication Networks

3 Industrial Communication NetworksAutomation Overview

EPFL, Spring 2017

Industrial Automation | 2017 2

3 Industrial Communication Networks

3.1 Field bus principles

3.2 Field bus operation

3.3 Standard field busses

3.4 Industrial wireless communication


transducers / actors

Networks in Automation Hierarchy

Hierarchy

Sensor-Actuator Bus

Fieldbus

programmable

controllers

Control Bus

Supervision level

Control level

Field level

EngineeringOperator

2

direct I/O microPLCs

Course


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)


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


The original idea: save wiring

PLC

But: the number of end-points remains the same !

energy must be supplied to smart devices

field bus

CO

M

marshalling

barI/O

PLC

smart devices

tray

capacity B

e

f

o

r

e

A

f

t

e

r

dumb devices


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)


Different classes of field busses

poll time, milliseconds

10

100

1000

10,000

10 100 1000 10,000

Sensor BusSimple devicesLow costBus powered Short messages (bits)Fixed configuration Not intrinsically safeTwisted pairMax distance 500m

Low Speed FieldbusProcess instruments, valvesMedium costBus-powered (2 wire)Messages: values, statusIntrinsically safeTwisted pair (reuse 4-20 mA)Max distance 1200m

High Speed FieldbusPLC, DCS, remote I/O, motorsMedium costNot bus poweredMessages: values, statusNot intrinsically safeShielded twisted pairMax distance 800m

Data NetworksWorkstations, robots, PCsHigher costNot bus powered Long messages (e-mail, files)Not intrinsically safeCoax cable, fiberMax distance miles

PV 6000

SP 6000

Honeywell

AUTO

1

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

source: ABB

frame size(bytes)


Fieldbus Application: locomotives and drives

cockpit

motorspower electronicsbrakes

power line

track signals

Train Busdiagnosis

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)


Fieldbus Application: automobile

- Electromechanical wheel brakes

- Redundant Engine Control Units

- Pedal simulator

- Fault-tolerant 2-voltage on-board power supply

- Diagnostic System

Board networkECU

Monitoringand

Diagnosis

BrakesECU

4

redundantboard network

12V und 48VECU

ECU

ECU

c

ECU

redundant

board network

ECU


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

ICCPcontrol

center

Inter-Control Center Protocol

IEC 870-6HV

MV

LV

HighVoltage

Medium

Voltage

LowVoltage

SCADA

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

substation

RTU

RTU RTU

RTU

COM

RTU RTU RTU Remote Terminal UnitsRTU

RTU

IEC 870-5 DNP 3.0 Conitel RP 570

control

center

control

center

serial links (telephone)


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.


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,

over distances of several 100 m.

Field bus planning: estimate data density per unit of length or

surface, response time and throughput over each link.








Assessment

• What is a field bus ?

• Which of these qualities are required: 1 Gbit/s operationFrequent reconfigurationPlug and playBound transmission delayVideo streaming

• How does a field bus support modularity ?

•Which advantages are expected from a field bus ?


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)

timequalityvaluesource description


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

offers 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.

valuelengthtypeASN.1: (TLV)

minimum


Datasets

wheelspeed

airpressure

linevoltage

time

stamp

analog variables binary variables

all door closedlights on

heat on

air condition on

bit offset

16 32 480 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


Dataset extension and qualityTo 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

0 1 0 1 1 1 0 0 0 1

check

0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1

0 0

1 1

correct variable

error

undefined

variable value

var_offset

chk_offset

10 = substituted

00 = network error01 = ok

11 = data undefined


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 output

input output

PLC

PLC PLC PLC

PLC

central master / slave: hierarchical

peer-to-peer: distributed

“slaves”

“master”

“slaves”

“masters”

alternate

master AP

APAPAP

AP Application


applicationprocessor


BroadcastsMost variables are read in 1 to 3 different devices

Broadcasting messages identified by their source (or contents) increases efficiency.

…

instances

…

=

variable


plantimage

plantimage

plantimage

=

distributeddatabase

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

plantimage


Transmission principle

The previous operation modes made no assumption, how data are transmitted.

The actual network can transmit data

• cyclically (time-driven) or

• on demand (event-driven),

• or a combination of both.


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

reso

lutio

n


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


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.


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.


Cyclic Data Transmissionaddress

devices

(slaves)

BusMaster

plant

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

1 2 3 4 5 6

PollList

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 of 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


Round-trip delay of master-slave exchange

The round-trip

delay limits the extension

of the bus

master most remote data sourcerepeaterrepeater

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


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.


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 643

1 ms period(basic period)

2 ms period

2 4a

4 ms period

1 ms 1 ms

1 11 2


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.

TrafficMemory

cyclic algorithms

cyclic algorithms

cyclic algorithms

cyclic algorithms

port address

application1

Ports Ports Ports

application2

application4

sourceport

sinkport

port data

sinkport

cyclic poll

bus controller

busmaster

application3

bus

PeriodicList

Ports

bus controller

bus controller

bus controller

bus controller

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


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

publisherapplication instance

bus instance

device device

applications

bus


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

intelligentstations

sensors/actors


Bus interface for event-driven operation

ApplicationProcessor

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


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

CallerApplication

CalledApplication

Transportsoftware

Transportsoftware

interrupt

request

indication

confirm

Bus

time


Determinism and Medium Access In Busses

Although the moment an event occurs is not predictable, the busshould 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)


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 output queues

events producers

& consumers


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


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)


Mixed Data Traffic

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

-> Periodic Transmission

of 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 longmessages 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


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.


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

Application7

6

Remote Procedure Call 5

4

3

2'

1

connectionless

connectionless

connection-oriented

medium access

implicitimplicit

Logical Link Control2"

mediacommon


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 widelyDeterministic: delivery time is bound

All resources are pre-allocated Best use of resources

message-oriented busobject-oriented bus

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


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


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.


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)


NTP (Network Time Protocol) principle

2

)()( 2314 tttt

time request

time response

t1

t2

t3

t4

time request

time response

t’1

t’2

t’3t’4

distance

time

network

delay

servernetworkclient

network delay

Measures delay end-to-end over the network (one calculation)

Problem: asymmetry in the network delays, long network delays


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

calculationpeer 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)


IEEE 1588 – 1 step clocks

Sync

(contains all + )

residence

time

Pdelay_Resp

(contains t3 – t2)

Pdelay_Req

ordinary

(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_Reqt2

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


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/

http://www.ines.zhaw.ch/fileadmin/user_upload/engineering/_Institute_und_Zentren/INES/IEEE1588/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/


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.


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 ?








Different classes of field busses

poll time, milliseconds

10

100

1000

10,000

10 100 1000 10,000

Sensor BusSimple devicesLow costBus powered Short messages (bits)Fixed configuration Not intrinsically safeTwisted pairMax distance 500m

Low Speed FieldbusProcess instruments, valvesMedium costBus-powered (2 wire)Messages: values, statusIntrinsically safeTwisted pair (reuse 4-20 mA)Max distance 1200m

High Speed FieldbusPLC, DCS, remote I/O, motorsMedium costNot bus poweredMessages: values, statusNot intrinsically safeShielded twisted pairMax distance 800m

Data NetworksWorkstations, robots, PCsHigher costNot bus powered Long messages (e-mail, files)Not intrinsically safeCoax cable, fiberMax distance miles

PV 6000

SP 6000

Honeywell

AUTO

1

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

source: ABB

frame size(bytes)


Worldwide most popular field busses

Market shares held by the leading fieldbus and industrial Ethernet systems

Source: HMS Industrial Networks, 2016


Field device: example differential pressure transducer

The device transmits its value by means of a current loop

4..20 mA current loop

fluid


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 reader1

reader2

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

R1 R2 R3

sensor

i(t) = f(v)

voltage

source

10V..24V

RL4 conductor resistance

RL2 RL3 RL4RL1

flow


HART

Data over 4..20 mA loops


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


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..55..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):


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


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/


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)

DeviceNet250

kBit/s500 m 8 bytes

Factory

Automation64

CSMA/CD, master-slave,

multidrop, motors, drives, uses

CAN

CANopen

10 kBit/s

-

1 Mbit/s

25-1000 m 8 bytes

Automobile,

Industrial

Automation

127CSMA, 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



http://www.mtl.de/pdfs/news/open_fieldbus.pdf


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


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.


Ethernet Paradigms

switch

switch

SCADA

Fieldbus

Ethernet

SCADA

simple

devices

PLC PLC PLC

Soft-PLC Soft-PLCSoft-PLCSoft-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


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,..)


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


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 environmentand 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.


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)


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 ?








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)


Wireless Landscape


Wireless IEEE Numbers


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

-R

eal

Soft

time


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 SolutionsExisting Solutions


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)


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 ??? ??? ???


Legal Frequencies

www.fcc.gov


Range vs Data 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


Industrial Example: WirelessHART

HART (Highway Addressable Remote Transducer) fieldbus protocol

Supported by 200+ global companies

Since 2007 Compatible WirelessHART extension


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


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?

https://datatracker.ietf.org/wg/6tisch/charter/


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


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?


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

Documents

EPFL, Spring 2017 3 Industrial Communication Networks