ABBIndustrial Networks Connecting Controllers via OPC

available parts, also for testing purposes.

1.3 Goals

The goals of this Master’s Thesis are stated as follows:

• Setup and evaluation of a test environment

• Setup of test systems

• Theoretical and practical evaluation of the test systems concerning performance, availability and reliability.

• Identification of improvements and different approaches

• Comparison with alternatives

As a starting point for the performance requirements, the current implementation was taken. The corresponding quantity and type of variables are displayed in

Table 1.1 with 32-bit floating point values (floats) as analog in- and outputs and

1-bit boolean values as so-called status and command bits. In the current configuration with AC450 and AC160, all variables are written to the AF100 fieldbus with a

cycle time of 256 milliseconds. Therefore we determined the minimum requirement

for round-trip times from one controller to the other to exactly this time.

In agreement with the advisors, instead of elaborating the optional extension

stated in the task description (Appendix C), we spent more time on trying out a

second PROFIBUS approach and the theoretical derivation of a redundancy concept.

1.4 Structure

For the reader’s convenience this Master’s Thesis is structured thematically starting

with an overview of components and terms (2) in the next chapter. The following

chapters inform about the test system setup (3), the evaluations that were made

(4) and finally the results (5). In a subsequent chapter the subject redundancy

is treated (6) before the thesis comes to an end with the conclusion and outlook

(7). Additional information as well as a CD-ROM containing more detailed data is

located in the appendix of this thesis.

Chapter 2

Components and Terms

In this chapter, hardware and software parts as well as terms used for our test

system and evaluations will be described. Some additional devices and programs

concerning redundancy are introduced not until the chapter according. Information

on the version numbers can be found in Appendix B.

2.1 Basic Terms

• Performance, in this thesis, refers to the capability of a communication

component in means of speed and throughput.

• Availability is the term for the probability that a system will perform its

specified functions when used under stated conditions. A common mathematical definition of operational availability is Ao = MT BF/(MT BF + MDT),

whereas MTBF is the “mean time between failure” and MDT the “mean down

time” [2]. However, in this thesis, availability is used in a more general manner, since the basis for mathematical operations is not available.

• Reliability means the probability of a device remaining failure free during a

specified time interval, e.g. the maintenance interval: R = e

λt

• Redundancy is the implementation of extra components in addition to the

ones needed for normal operation. Thus, redundancy normally increases reliability and availability.

2.2 OPC

OPC, originally short for “OLE for Process Control”, is an open, standardized

software communication interface specification launched in 1996 by a task force of

different automation companies, later forming the OPC Foundation. As the former

name indicates, OPC is an adaption of Microsoft’s Object Linking and Embedding

OLE1

to the process control business, which used to be highly proprietary at that

point of time. Thus it was almost impossible to efficiently combine products of

different vendors. By providing so-called OPC servers with their devices, buses and

software, vendors open their products to any OPC compliant client able to connect

to the server for data exchange. Usually, an OPC server can handle several clients

at once, while these clients—e.g. visualization or calculation applications—can connect to different servers in order to obtain their needed information

Over the years, the OPC Foundation has been adding eight additional specifications to the original one, therefore the name OPC was freed from its original

meaning and is now used as an umbrella term [3]. Some important specifications

are quickly explained in the following:

• DA (Data Access) is the original and most widely used standard of OPC.

Its purpose is the cyclic polling of real time data, for example for visualization

purposes.

1

http://msdn.microsoft.com/archive/default.asp?url=/archive/en-us/dnarolegen/

html/msdn_aboutole.asp

• HDA (Historical Data Access), in contrary, specifies the access to already

stored data.

• AE (Alarms and Events) describes the non-cyclic, event-based exchange

of alarms and events.

• Data eXchange is a specification from 2002 which regulates the direct communication between two OPC servers.

For this Master’s Thesis it was made use both of the DA specification for the

main purpose of communication as well as the AE specification in order to display

and log round-trip times. Unfortunately, the promising Data eXchange specification

is almost inexistent in practice and could therefore not be used in our thesis.

The underlying technique to exchange data is the component object model

COM of Microsoft Windows, therefore OPC can only run on Windows operating

systems [4]. A new generation of OPC specifications recently published is called