Contents
Schematic of OPC Server.
Figure 1.
Introduction
COM/OLE applications are widely used in Industrial Automation and Process Control domains.
Since not many programmers are aware of these technologies, I thought to share my experience in OPC
and most developers would love to expand their knowledge about COM/OLE application areas.
This is my first ever article on Code Project, so if you feel anything needs to be improved, drop me a line.
Your suggestions and comments are most welcome.
The goal of article: Give simple & understandable overview of OPC.
More detailed information and specifications can be downloaded from OPC Foundation and websites
mentioned in Resources section.
So first, about the above diagram...
Theory about Figure 1:
On left-bottom is a hardware device. It could be a
PLC (programmable logic controller), a temperature sensor, roller belt controller, or just
about any industrial device. Central part is the OPC server. Right side is the client that wants
to operate the device. The OPC Server gives out standard COM interfaces so that any client
aware of these OPC Interfaces can interface with server and operate the device
- without programming device driver for that hardware. Hardware can be
as varied as OPC server can handle and clients can still read and write from this hardware.
Now back to our OPC course. This article gives an overview of OPC and covers
following topics. Source code is not available, since most of server code is specific to hardware.
And client would still need a server to connect to. But yes, if you want to experiment with OPC,
check out the Resources section below.
History of OPC
In 1996, a group of companies Rockwell Software, Intellution, et. al. came together
to form a consortium that defined a standard for process control. SCADA and similar
standardization efforts were being made at that time in process control and automation
industry. This group created an initial draft that was based on Microsoft's
Object Linking and Embedding (OLE). It was called OLE for Process
Control (OPC). OPC Foundation
publishes specifications and manages everything that goes into OPC technology.
Today more than 300 companies are part of OPC Foundation. Microsoft, GE, Siemens, Rockwell,
ABB, Ford, Honeywell and more are members that actively contribute and consume OPC technology.
In 2005, OPC Foundation released OPC Unified Architecture draft. There are predecessor
drafts released before - OPC HDA, OPC DX, OPC AE, OPC Commands, OPC Security. We will briefly
study them in this article.
By now, you must have a question in your mind. Let me answer it below...
Why do we need OPC? its Benefits
I was also a first timer to OPC (coming from varied experience in DevTools, eLearning, CAD/PLM,
Multimedia), and had no clue as to why do we need it. I found that learning it through a case study
was very helpful, so I am going to do the same for you. To understand the need for standardization in process control industry, let me give you
an actual case study performed by a giant organization in Petrochemicals business.
Refer to figure 2. It shows the devices and software application that were in action on this
particular petrochemical plant.
Figure 2.
PLC, Vibration sensor, and Calculation engine work on different communication protocols
- TSAA,
Modbus, and DDE. So the software applications that need data from these devices also need to be aware
of TSAA, Modbus, and DDE protocol. Additionally each device needs to serve all the three applications.
The process historian, GUI application providing human machine interface and the machine condition
monitor all communicating with all devices results in a complexity. This deployment took the company
around 10 days. And its cost was approximately US $ 50,000.
Now, someone like me :) came to them and said, "hey guys, why don't we think smart and implement
as in Figure 3?". Observe the reduced complexity. Now all software applications talk only to the OPC
Servers. And all devices also respond only to the OPC server. OPC Server provides the Black Box
for software applications around hardware.
Figure 3.
The new implementation took just 2 days to deploy and cost incurred was approximately US $10,000.
They could have eliminated 2 OPC servers as well and implemented device drivers right inside a single
OPC server. This is a tradeoff and off the shelf OPC servers may have standard protocols support
like DDE, Modbus and TSAA.
As you observed correctly, the benefits are:
- Reduced load on device.
- Scalability of system has increased.
- OPC server provides caching of data.
- Client applications need not know hardware protocol details.
- Increased life for device (it need not serve multiple clients anymore).
- Interoperability (Unix/Linux and Windows - both platforms are supported by OPC)
- Standardization
Now that you have some insight into OPC benefits and history, let's take one step ahead to understand
OPC terminology and concepts.
OPC Concepts & Terminology
OPC specification defines a consistent terminology to identify various entities involved. The best
analogy to OPC way of data representation is that of a file system hierarchy. OPC calls it as OPC
Address Space. Within this address space every entity can be uniquely identified by its "full-path".
For example, refer to figure 4.
Figure 4.
OPC Address Space is hierarchical. It starts with a root.
Then there are logical
folders and
sub-folders.
Leaf nodes are
OPC Items or Tags.
Generally your sensors and actuators form the leaf nodes. This is an important point to understand
and we will spend some time on it. For example, you have a device that can sense temperature, Pressure, and Humidity. Then
these attributes are generally mapped as leaf nodes under some folder-subfolder hierarchy. We will see
what that hierarchy can be in a moment. If you have an actuator (say, Turn_ON_Boiler), then that
also becomes a leaf node. You can write to
Turn_ON_Boiler values of 1 or 0 for example, and
necessary action will be taken - of course we just assumed that your hardware control mechanism
behaves this way. You can read from
Temperature to know sensed value.
Hope we are all clear till this point. Now let's understand this "logical" folder stuff. The
logical folders and sub-folders are used to represent state of your Industrial Automation system.
This configuration can actually map to some physical configuration or even geographical setup.
Let's spend some time to understand this one as well. For example, in this diagram we have an ethernet
communication channel to which a wireless meter is connected. And within wireless meter, there are
two classes "Average" and "Channel1". These folders and sub-folders are very effective means of
grouping your process control parameters and values.
This address space is "published" to all clients of OPC server. The OPC clients can be simple
spreadsheets like Excel, process historians that keep record of data coming from various industrial
plant entities. Every client can create a logical entity called as group
inside OPC server. Each OPC Group can contain a set of OPC Items.
Synchronous & Asynchronous OPC Item reading
OPC specification supports synchronous as well
as asynchronous reading and writing on hardware devices. Since not all hardware device operations
are fast-enough, the asynchronous mechanism of reading and writing to a device comes very handy.
COM Interfaces & Component Categories
Well, so now suppose you wish to write your first OPC client. If you were developing a client that
runs on local machine as that of server, and if you were aware of all OPC Server ProgIDs, then we
would not have any worries. But since that is not possible, we need an enumeration mechanism. Following
is an enumeration mechanism.
OPC Component Categories
Enumeration issue is addressed by requiring OPC servers to implement
component categories. For example, all servers that are OPC DA 1.0
(more about this in later sections) compliant implement COM category
with CatID {63D5F430-CFE4-11d1-B2C8-0060083BA1FB}. Similarly the
servers compliant with OPC Data Access 2.0 implement category with
CatID {63D5F432-CFE4-11d1-B2C8-0060083BA1FB}.
So OPC Foundation distributes a Windows
Service called as OPCEnum.
Enumerating OPC Servers
OPCEnum.exe runs as a Windows Service and provides enumeration
functionality. Using OPCEnum, the client applications can query
available OPC Servers on local as well as remote networked machines.
Now let's look at server component interfaces.
Figure 5.
There are various COM interfaces that OPC Server must support according the OPC Specification compliance required.
These basic interfaces have specific functionality.
IOPCBrowse This interface has methods that allow browsing OPC Address Space. Client can query level
by level folders and subfolders till the leaf nodes. The hierarchy looks like shown in
figure 4.
IOPCItemIO Interface is used for performing OPC Item read and write operations.
Now let's understand how OPC Client interacts with the OPC Server. Important steps involved are summarized
in figure 6, below.
Figure 6.
I know you must still be wondering about this
OPC Group term. Let's spend
some time understanding it. Every client when opens a session with OPC Server, needs to create a logical
group of items that it wants to read or write. Client can maintain multiple groups. Each group is given
a unique name and has following attributes:
- Update rate [milliseconds] at which the values of items are read.
- Active or In-active Flag [Boolean] only if group is active that the items are processed.
- Deadband [%] variations inside this band are considered NULL.
- IO Mode [Synchronous/ASynchronous] all operations performed are either sync or async.
- Time Bias [time] local time band with respect to GMT.
The OPC Group component supports following interfaces.
Figure 7.
As you may have identified these IOPCSyncIO etc interfaces that are relevant for synchronous
and asynchronous OPC Item read and write operations. OPC Group Components also support connection point container
for events that are used for client notifications.
At least you have some idea by now that OPC is just another COM application in action - just that it is a standard
meaning all OPC Clients and OPC Servers are bound by design contract.
OPC Foundation provides compliance test applications. Using these suites one can execute certain number
of test cases on OPC Server and determine their compliance to particular OPC Specifications. There are several
OPC specifications available (explained in next section). A typical compliance report looks like as shown
in figures 8 & 9.
Figure 8.
Figure 9.
OPC Compliance Test Suite also supports testing for performance whereby the OPC Server in question is loaded
with multiple items read and write requests. These determine time required per item operation and also passed
and failed test cases. Once compliance is achieved, the report can then be uploaded to OPC Foundation web
site for public reference. So that customers buying OPC Server from a particular vendor can make sure that
the server is compliant to a particular specification and may meet their integration need with other software
packages used on the plant.
OPC Foundation is responsible for maintaining and releasing new specifications. It is a body made up of
industry experts, companies that produce OPC products and organizations that consume OPC products. So far OPC
Foundation has released following specifications.
- OPC-DA (Data Access)
Provides access to real-time data. We can query most recent values of temperature, pressure, density,
acceleration, and other types of process control data from OPC-DA server.
- OPC-HDA (Historical Data Access)
Used to retrieve historical process data for analysis. This data is typically stored in archives,
databases or remote telemetry systems.
- OPC-AE (Alarms & Events)
OPC AE servers are used to exchange and acknowledge process alarms and events.
- OPC-DX (Data eXchange)
Defines how one OPC server exchanges data with other OPC servers.
- OPC-XML (XML Data Access)
Defines schema and data representation format based upon XML standard. Makes it possible to share
& manipulate process control data across all operating systems � Windows, Unix, Solaris, etc.
- There were several efforts put onto - OPC Security, OPC Batch, OPC Commands, OPC for ERP.
Most recently OPC Foundation has released a draft for OPC Unified Architecture
that attempts to provide even more integration of process control data and software with other industrial
software systems like ERP.
There are plenty of resources available on the web about OPC.
Hope you enjoyed knowing a bit of OPC in this article. If you have any questions or suggestions, please post
them here or drop me a line.
srt@Suchit-Tiwari.Org
or
Suchit.Tiwari@Ge.com Your suggestions and comments are most welcome.
