Industrial Automation
Automation Industrielle
Industrielle Automation
4
Application Layer Protocols
4.2
Open Process Control (OPC)
OPC: Open Process Control
Source: http://home.hit.no/~hansha/documents/lab/Lab%20Work/opc.htm
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OPC 4.2 - 2
OPC: Open Process Control
Manufacturer-independent application programming interface (API) for automation
- To implement clients which can access plant data coming from remote devices
(PLCs, field bus devices, real-time databases) easily
- Set of commands collected in a software library (DLL) to access OPC servers
OPC clients
- read and write process variables, read alarms and events and acknowledge alarms,
and retrieve historical data from data bases according to several criteria
- Implemented on automation platforms (e.g. ABB Ax800), which may act themselves
as an OPC server to publish their data, events and historical data.
OPC server
- Supplied by manufacturer of automation devices supplies
- Communicates with its devices through a proprietary protocol
- Manages several devices of the same type, several servers can run in parallel and
each server can be accessed by several clients in the same network
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OPC 4.2 - 3
OPC Overview
4.2.1 OPC Overview
Usage and specifications
Clients and Servers: configuration
4.2.2 OPC Data Access
Objects, Types and properties
Communication model
4.2.3 OPC Alarms and Events Specification
Events
Alarm Conditions
4.2.4 OPC Historical Data Specification
Overview
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OPC 4.2 - 4
4.2.1 What is OPC ?
OPC (formerly: "OLE1 for Process Control", now: "Open Process Control") is
an industry standard set up by the OPC Foundation (http://www.opcfoundation.org/)
specifying the software interface (objects, methods) to a server that
collects data produced by field devices and programmable logic controllers.
API
covered by the
OPC standard
application
(OPC client)
OPC server
X
OPC server
(simulator)
servers
OPC server
Y
:
Ethernet
Field bus
PLCs Brand X
PLCs Brand Y
Sensors/Actors
1) OLE (Object Linking and Embedding) is a Microsoft technology for connecting software components.
It has since been extended by the COM / DCOM technology. It corresponds to Java Beans.
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OPC 4.2 - 5
Before OPC
visualization
history
data base
MasterBus
MMS driver
ABB PLCs
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XWAY
driver
Profinet
driver
Télémécanique PLCs
Siemens PLCs
OPC 4.2 - 6
With OPC
Operator
Historian
(Information
Manager)
application software is
written independently from
the type of controller
the drivers still exist,
but the clients do not
see them anymore
ABB AC800M
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ABB
OPC server
Schneider
OPC server
Siemens
OPC server
MMS
XWAY
ProfiNet
Télémécanique TSX
Siemens S7
OPC 4.2 - 7
Importance of OPC
• Greatest improvement in automation since IEC 61131.
• More than 150 vendors offer OPC servers to connect their PLCs,
field bus devices, displays and visualization systems.
• Used for data exchange between applications and for accessing databases
• DLL for Automation Interface (VB,..) and Custom applications (C++,..)
• Three major components:
1) OPC - DA = Data Access (widespread, mature)
2) OPC - AE = Alarms and Events
3) OPC - HDA = Historical Data Access
… and some profiles* (batch,…)
* A “profile” is a subset or a specialization of a standard to form a stricter standard, adapted
to an application. E.g. 100 Mbit/s, full-duplex, fibre-optical Ethernet is a profile of IEEE 802.3.
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Specification 1: OPC DA for Data Access
Process variables describe the plant's state, they are generated by the sensors or
calculated in the programmable logic controllers (PLCs).
Process variables can be sent upon a change, on demand or when a given time elapsed.
The OPC DA (Data Access) specification addresses collecting Process Variables.
The main clients of OPC DA are visualization and (soft-) control.
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Specification 2: OPC AE for Alarms and Events
Events are changes in the process that need to be logged, such as "production start"
Alarms are abnormal states in the process that require attention, such as "low oil pressure"
OPC AE (Alarms and Events) specifies how alarms and events are subscribed, under
which conditions they are filtered and sent with their associated messages.
The main clients of OPC AE are the Alarms and Event loggers.
determine the exact time of change
(time stamping)
categorize by priorities
log for further use
acknowledge alarms
(events are not acknowledged)
link to clear text explanation
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OPC 4.2 - 10
Specification 3: HDA for Historical Data Access
Historical Data are process states and events such as: process variables, operator actions,
recorded alarms,... that are stored as logs in a long-term storage for later analysis.
OPC HDA (Historical Data Access) specifies how historical data are retrieved from the logs
in the long-term storage, filtered and aggregated (e.g. compute averages, peaks).
The main client of OPC HDA are Trend Displays and Historians.
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Beyond Microsoft: OPC UA
In a move to get more independence from Microsoft and use web technology,
a new specification called " Unified Architecture" (formerly. OPC XML) that uses web
services for all kinds of transactions: query, read, write, subscribe,...
The classical OPC DA, AE and HDA are implemented with XML / SOAP / WSDL
this allows encryption and authentication of process data.
OPC UA does not only standardize the interfaces, but also the transmitted data.
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Client and Servers
4.2.1 OPC Overview
Usage and specifications
Clients and Servers: configuration and communication
4.2.2 OPC Data Access
Objects, Types and properties
Communication model
4.2.3 OPC Alarms and Events Specification
Events
Alarm Conditions
Automation Interface
4.2.4 OPC Historical Data Specification
Overview
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Server(s) and Client(s) in the same node
node
client application
(OPC client)
OPC server
devices
client application
(OPC client)
OPC server
devices devices
OPC server
devices
Clients and servers run as parallel processes
The OPC specification defines the interface between client and server in the form
of objects and methods.
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OPC for internal communication: ABB’s SCADA (800xA) as example
ABB's Operator Workplace (800xA) is at the same time OPC server and OPC client.
Software components (agents) within AIP expose their properties as OPC objects.
Internal (within the PC) and external communication (between PCs) takes place over
OPC.
800xA
aspects
aspects
functions
Asset
Optimizer
Enterprise
Historian
OPC server
Windows
PC
process
data base
OPC
connections
OPC client
ABB
OPC server
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Schneider
OPC server
Siemens
OPC server
OPC 4.2 - 15
Direct and Fieldbus access
direct connection
fieldbus connection
client application
(OPC client)
client application
(OPC client)
(local)
OPC server
(local)
OPC server
FB Manager
fieldbus
I/O devices
The OPC server is running
all the time, even if no
client is present
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proprietary
protocol
can also
be a pointto-point
link
fieldbus
fieldbus
FB agent
FB agent
PLC
PLC
OPC 4.2 - 16
Accessing a server in another node
client application
(OPC client)
stub
DCOM
TCP/IP
DCOM
Limitation:
does not work over firewalls.
Solution:
Tunneller
OPC UA
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TCP/IP
TCP/IP
DCOM
DCOM
OPC
server
OPC server
FB Manager
fieldbus
OPC 4.2 - 17
Assessment Overview
• What is the objective of OPC ?
• On which technology does OPC rely ?
• What is an OPC Server ?
• What do the main OPC specifications describe ?
• How can an OPC Server access data on another machine? And
how does it work for OPC clients?
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OPC Data Access
4.2.1 OPC Overview
Usage and specifications
Clients and Servers: configuration
4.2.2 OPC Data Access
Objects, Types and properties
Communication model
4.2.3 OPC Alarms and Events Specification
Events
Alarm Conditions
Automation Interface
4.2.4 OPC Historical Data Specification
Overview
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OPC 4.2 - 19
OPC DA: Item properties
The process data are represented by three dynamic properties of an item:
1. value:
numerical or text
2. time-stamp:
time at which this data was transmitted from the PLC to the server
UTC, not local time.
3. quality:
validity of the reading (not readable, dubious data, o.k.)
optional static properties
description:
a text string describing the use and of the variable (optional)
engineering unit: the unit in which the variable is expressed (optional)
ID
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V
Q
T
D
U
OPC 4.2 - 20
OPC DA: Objects as viewed by the OPC server
An OPC server is structured as a directory with root, branches and leaves (items)
Process Line 1
Tag Name
Controller 1
Controller 2
Controller_3.Prog_1
Controller_3.Prog_2
TAG
TAG
TAG
Level_1
Level_2
An item is identified by its
"fully qualified ItemID",
e.g.
Ramp4
"Process_Line_1.Controller_2.Level_2"
Cell 1
Machine 2
Branches may contain other branches and items
The structure may also be flat instead of hierarchical
This structure is defined during engineering of the attached devices and sensor/actors.
(Intelligent servers can configure themselves by reading the attached devices)
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OPC DA: Objects as viewed by the OPC client
A client builds its own hierarchy, using the server’s hierarchical view.
Items in the server are defined by the programmer of the PLC
A full-fledged PLC may export some 10’000 items, a client needs only a subset.
A client builds groups, populating them with items it is interested in.
Items of a group are expected to have similar real-time requirements
Groups are not hierarchical, but flat.
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OPC DA: Mapping items to groups
Each client structures its items by groups,
independently from the server.
clients
Client1
Initially, the client browses the server structure to
check if the items it is interested in exist.
GroupX
A client registers its groups and items at the server.
The server keeps the structure of all its clients.
Server root
Client2
GroupZ
Item1 Item2 Item3 Item1 Item2
server
Area 1
TAG
Temperature
TAG
Heat_On
TAG
Level
Area 2
TAG
Empty_Valve
Area 51
TAG
Fill_Valve
Oven_1
Tank_1
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OPC DA: Communication Model
4.2.1 OPC Common
Overview: usage and specifications
Clients and Servers: configuration
4.2.2 OPC Data Access
Objects, Types and properties
Communication model
4.2.3 OPC Alarms and Events Specification
Events
Alarm Conditions
Automation Interface
4.2.4 OPC Historical Data Specification
Overview
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OPC 4.2 - 24
4.2.2 OPC DA: Example of access to a variable
controller program
OPC application
Reactor_1.Program2
ReadItem
("OPC:Reactor1:
Program2.MotorSpeed")
Value: 112
OPC
server
load
symbol
table
MW%1003
MotorSpeed
MW%1004
Temperature
…
….
symbols
code
Get () 192.162.0.2, MW%1003)
Return (MW%1003, 112)
Network
Reactor_1
Marker: MW%1003
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OPC DA: Read Communication Models (per group)
synchronous
client
myGroup.SynchRead()
asynchronous
server
client
Call
myGroup.AsyncRead()
Reply
server
Call
Reply
myGroup_AsyncReadComplete()
client
myGroup.IsSubscribed
on change
("subscription-based")
server
Subscribe
Notify
myGroup_DataChange()
Notify
myGroup_DataChange()
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OPC DA: Write Communication Models (per group)
client
myGroup.SynchWrite()
server
Call
client
myGroup.AsyncWrite()
Reply
server
Call
Reply
myGroup_AsyncWriteComplete()
The OPC interface accesses only groups, not individual items.
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OPC 4.2 - 27
OPC DA: communication paradigm
OPC DA works according to the “shared memory” paradigm.
This means that a newer value overwrites the older one, no queues or history are kept.
The server does not guarantee that different clients see the same snapshot of the plant.
The server does not guarantee that all changes to variables are registered,
changes may be missed if the polling period is too long.
OPC DA Client
OPC DA Client
OPC DA Server
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OPC DA: Assessment
1. How does the OPC server know a) where to fetch an item? b) which items belong
to which group?
2. What are the DA the read and write operations ?
3. Is communication done by items, by groups or by collection of groups ? Why?
4. Can a change of an OPC variable be notified as an event, or shall the client poll ?
5. What are the implications of the shared memory paradigm for the application
developer?
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OPC Alarms and Events
4.2.1 OPC Overview
Usage and specifications
Clients and Servers: configuration
4.2.2 OPC Data Access
Objects, Types and properties
Communication model
4.2.3 OPC Alarms and Events Specification
Events
Alarm Conditions
4.2.4 OPC Historical Data Specification
Overview
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OPC 4.2 - 30
Alarms and Events: Purpose
The controllers (PLC) generate events in response to changes in the plant variables.
together with their precise time of occurrence, type, severity and associated message for
the human operator.
An OPC AE server registers these events and makes them available to several clients
Alarms are described as state machines and may require acknowledgement.
The OPC Alarms & Events Interface gives access to the AE server, allowing to:
- browse the OPC AE Server for predefined events.
- enable or disable alarms and events
- subscribe to alarms and events of interest
- receive the event and alarm notifications with the associated attributes
- acknowledge alarms
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AE: Definitions
An event is a general change of state that is relevant to the OPC server.
An event signal a change:
1) in the field device ("production started")
2) in the OPC server ("alarm acknowledged")
3) in the application ("operator action")
An alarm indicates a state of the process that requires attention and is relevant to the OPC
server. An alarm is represented by an alarm condition, which is a state machine determining
if the alarm has been enabled, triggered or acknowledged. An alarm rises several events.
An event or an alarm does not transmit process values, but boolean information indicating a
change of state, its originator, the time of its occurrence and a message intended for a
human operator.
Alarms and events may not get lost
(contrarily to OPC DA, which does not guarantee completeness)
Alarms and event are precisely time-stamped by their source,
(contrarily to process variables, which are time-stamped by the receiving OPC server).
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AE: communication paradigm
OPC AE works according to the “message passing” paradigm, contrarily to OPC DA, that
works according to the "shared memory" paradigm.
This means that an event is kept in a queue until all clients have read it (or timed out).
The AE server guarantees that different clients will see all events in the same sequence.
OPC AE Client
OPC AE
Client
OPC AE
Server
12:34 23.114
12:34 32.334
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AE: Displaying Alarms and Events
Alarms and events are usually displayed differently on an operator screen.
- Events are displayed in an event list that can become quite long (typically 1000 entries),
entries are not cleared when the source of the event returns to normal
- Alarms are displayed in a short list (typically 50 alarms)
appearance changes when the alarm is acknowledged,
an alarm line is cleared when the alarm signal is cleared (but remains in the log).
Ack
checkbox
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AE: Events
4.2.1 OPC Overview
Usage and specifications
Clients and Servers: configuration
4.2.2 OPC Data Access
Objects, Types and properties
Communication model
4.2.3 OPC Alarms and Events Specification
Events
Alarm Conditions
4.2.4 OPC Historical Data Specification
Overview
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AE: Events kinds
OPC AE defines three kinds of events:
1.
simple: process control system related events (change of a boolean variable)
2.
condition-related: notifies a change of an alarm condition (CLEARED, ACKNOWLEDGED),
(see later)
3.
tracking-related: origin outside of the process (e.g. operator intervention)
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AE: Event- identification
An event is identified by
- its source (the object that generates the event. e.g. Tank1) and
- the event name (which can be the same as in another object, e.g. HiLevelCond)
HiLevelCond HiLevelCond
event
event
LoLevelCond LoLevelCond
event
Tank1
event
event name
Tank2
event signal (boolean expression)
is an external signal to be used ?(boolean)
signal name for external signal (20 characters)
name of the source (30 characters)
message (60 characters)
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AE: Events - Notification
Tank1LevelHigh_SimpleEvent
(source, timestamp, message,
severity, category)
AE Client
specified communication
COM/DCOM
OPC AE
Server
queue
unspecified communication
network, fieldbus or
internal bus
event notification
message
timestamp
Controller
Event
FB
Tank1
Plant
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Level Switch
OPC 4.2 - 38
AE: Events - Time Stamp
There are three places where events can be time-stamped:
- at the device that originally produced the data (external event - low-level event)
allowing Sequence-Of-Events with a high accuracy, down to microseconds
- at the controller, (internal event) using the controller's clock to time-stamp messages
giving accuracy not greater than the period of the tasks, about 1 ms.
- at the OPC Server, when an event message arrives (tracking events)
not more accurate than DA, about 10 ms)
The OPC server can be configured to register the time stamp at the instant of the
event transition (positive or negative) and the instant of the acknowledgement.
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AE: Alarm conditions
4.2.1 OPC Common
Overview: usage and specifications
Clients and Servers: configuration
4.2.2 OPC Data Access
Objects, Types and properties
Communication model
4.2.3 OPC Alarms and Events Specification
Events
Alarm Conditions
4.2.4 OPC Historical Data Specification
Overview
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OPC 4.2 - 40
AE: Alarms - Condition Definition
An (alarm) condition is described in a named state machine
The condition state is defined by three variables:
• Enabled:
the condition is allowed to send event notifications
• Active:
the alarm signal is true
• Acknowledged:
the alarm has been acknowledged
Alarm signal
(e.g. FIC101.PV > 100 AND FIC101.PV < 150)
Acknowledgement signal
(a positive transition of a boolean variable)
Condition
Condition state
Enable (positive transition)
Disable (positive transition)
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AE: Alarms - Condition states and acknowledgement
event notification
alarm signal
acknowledgement
condition state
Inactive
Acked
Active
Active
Unacked Acked
Inactive
Acked
Active
Inactive
Unacked Unacked
Inactive
Unacked
Inactive
Acked
alarm_signal 
Enabled
Inactive
Acked
condition state
transitions
(here: always
enabled)
Ack 
alarm_signal 
alarm_signal 
Enabled
Inactive
Unacked
Enabled
Active
Unacked
Ack 
Enabled
Active
Acked
alarm_signal 
An event is generated each time the alarm signal changes state, or is acknowledged
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AE: Alarms - Acknowledgement
An alarm condition becomes active when the PLC produces an alarm signal describing
an abnormal state defined by the application (e.g. the level of the tank is too high).
The operator is expected to acknowledge this condition (client ack)
Alternatively, a local operator may use a button or HMI that the PLC reads (field ack)
event notification
Tank1Level_ConditionEvent
AE Client
client ack (acknowledger ID)
COM / DCOM
OPC AE
Server
Network, field bus,
or internal bus
message
LevelHigh
Condition
timestamp
controller
Alarm Signal
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AckButton
(field ack)
OPC 4.2 - 43
AE: Summary alarms and events
Event
Alarm
AE Client
AE Client
COM / DCOM
OPC AE
Server
event notification (source, timestamp, message)
OPC AE
Server
alarm notification (source, timestamp, message,
condition, subcondition, severity, type)
message
timestamp
Event
FB
message
controller
event
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Condition
controller
alarm ack
OPC 4.2 - 44
OPC A&E: Assessment
1. What is the difference between Alarms and Events?
2. Where are Alarms and Events time stamped?
3. How does the “message passing” paradigm influence the OPC client application
developer?
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OPC Common Overview
4.2.1 OPC Overview
Usage and specifications
Clients and Servers: configuration
4.2.2 OPC Data Access
Objects, Types and properties
Communication model
4.2.3 OPC Alarms and Events Specification
Events
Alarm Conditions
4.2.4 OPC Historical Data Specification
Overview
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Historian Example
GE Fanuc/Intellution iHistorian (iFix...)
Questions to the historian:
What was the value of FIC101 last week ?
What was the flow average during October ?
Which were the daily averages in October ?
What was the total flow in each month ?
How much fuel did we use for the batch ?
Give the answers in form of tables,
pie diagrams, spreadsheet, reports
Features:
Unlimited Point Collection
Sub-Second Data Collection Rates
Enhanced Data Compression
True Thin Client Administration
Fault Tolerant Architecture
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Data collection, archiving and retrieval
Report generation
Computations (e.g. VBScript)
Secure access (FDA 21 CFR 11)
20'000 actions/ s,
Up to 100'000 data points
OPC 4.2 - 47
HDA: Historical Data Access
HDA Clients
e.g. Trend Analysis
e.g. Event Logger
independent
processes
OPC HDA
Server
hidden
calculations
history
database
raw and
ordered data
collector
proprietary
data acquisition
OPC DA Server
Field device
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Field device
OPC 4.2 - 48
HDA: Purpose
An OPC HDA server gives access to a historical data base (logs) in which data from the
process have been collected and time-stamped, possibly through an OPC DA interface.
The OPC HDA interface clients, such as trend analysis, product tracking or data mining,
that require ordered access these data logs.
The OPC HDA interface allows to:
- browse the historical data base
- retrieve data through proper filtering, e.g. by date range, by identity, by property
- build aggregates over the retrieved data, such as average, minimum, maximum.
- enter new entries, correct entries or remove entries
- enter / delete annotations in the history data base
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HDA: Raw log
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By definition, values are registered when they change.
(even if data are acquired by periodic polling)
Data in the historical database are identified by their
• itemID (here, represented by their name),
• value, (of the respective type)
• quality (good, stale, bad), and
• timestamp (UTC).
OPC 4.2 - 50
HDA: How to reduce raw data (even before HDA comes into play)
Data sent to the OPC DA server on change, collector records data in a circular buffer log
Since storage capacity is limited, data are reduced by:
1. if a variable is received more often than the log's minimal storage interval (e.g. 1s), the log keeps
the latest of all values of the interval
2. if (and only if) a received variable changed by more than the "exception deviation" (e.g. 5%,
analog values only), it is entered into the log.
3. if a variable changed by less than the exception deviation, it may be forced into the log after the
log's maximum storage interval (e.g. 4 s) elapsed.
min time
max time
max time
max time
max
exception
deviation
min
time
= process value (event)
= log entry
If the time scale of the log is smaller than that of the trend display in this case, values have
to be interpolated to be displayed correctly
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OPC 4.2 - 51
HDA Application: Trend Display
Parameters:
log: how were data sampled
• time scale (with possible offset, zoom, pan)
• amplitude scale (low range, high range, scale units)
• style: smoothed, stepped, filled (several ways to display the same data)
• extrapolate: how to display values not received (e.g. because they did not yet change)
Industrial Automation
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OPC 4.2 - 52
HDA: Hierarchical logs
long-term log
1 hour_forever
 5.2 MB / Year
1
2
3 27 28 29 30
september octobre
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
october
1 mn_7 days log
 6 MB
d-7
1s_24 hours log:
50 traces @ 12 B
 52 MB
23
24
1
2
3
d-6
4
5
d-5
6
7
8
d-4
9
10
d-3
11
12
13
A hierarchical log is built on the data contained in the parent log.
To reduce the log size, several aggregations can be applied:
- record only maximum, minimum, average over a period, etc...
Industrial Automation
2013
d-2
14
15
16
yesterday
17
18
19
today
20
21
22
Actual data
OPC 4.2 - 53
Toy Examples: Logging Temperature
http://home.hit.no/~hansha/documents/subjects/IA6209/project/IA6209%20Project%20Work%20-%20Weather%20System.pdf
Industrial Automation
2013
OPC 4.2 - 54
Real-Life Examples and Exercise
Integration of redundant GE Mark V turbine controllers at a power plant
The controllers were going to tie into the plant’s main DCS, an
ABB Advant control system. Process Portal B and MicroSCADA
client applications were used to visualize process data from the
turbine controllers.
Exercise: Draw schema of system and describe which OPC specifications
are used and how.
Wind Generation
AES Wind Generation manages 7 different wind farms across the
United States, five of which comprise more than 500 turbines,
with a total generating capacity of over 700 MW. There are six
different turbine models, from four different manufacturers.
Exercise: Explain which OPC concepts are useful in this context
and how they can be applied to support a SCADA system
http://www.matrikonopc.com/portal/downloads.aspx?dID=132 and
https://www.matrikonopc.com/portal/downloads/case_studies/AESWindGeneration_OPC.pdf
Industrial Automation
2013
OPC 4.2 - 55
To probe further….
OPC Foundation:
Specifications http://www.opcfoundation.org
SoftwareToolbox
Examples in Visual Basic
http://www.softwaretoolbox.com/Tech_Support/TechExpertiseCenter/OPC/opc.html
The Code Project
OPC and .NET
http://www.codeproject.com/useritems/opcdotnet.asp
Matrikon
Free client and server:
http://www.matrikon.com
WinTech
Toolkit for an OPC server
http://www.win-tech.com/html/opcstk.htm
NewAge Automation
Toolkit for an OPC server
http://www.newageautomation.com
Industrial Automation
2013
OPC 4.2 - 56