Review:

Distributed Computer Control System For Industrial

Process Control

Andrianda Utama 1 Magister Teknik informatika, Universitas bina Darma Palembang,

South Sumatera, Indonesia

Andrianda.utama@tech-center.com

In a distributed computer control system in which remote stations are interconnected by a communications link, the remote stations take turns having

supervisory control over the communications link. Each remote is numbered in

sequence in a predetermined succession order and takes its turn having control

of the communications link in accordance with this succession order. When a

remote station has completed its turn of having supervisory control, it transmits a first control message over the communications link. The next remote station

in the succession order then takes control over the communication link by

transmitting a second control message over the communications link. Each

remote station has two variable timers which are set in response to said first

control message and said second control message, respectively, and have time out intervals depending upon the offset of the receiving remote station from the

remote station which transmitted the control message. If and when any of these

variable timers times out in a given remote station, then that remote station will

assume control of the communications link by transmitting the second control

message on the communications link.

1 Introduction

Distributed computer systems rely on data communication networks for exchanging data.

Within a defined geographical area, e.g. an office, a control station or a manufacturing plant,

local area networks (LAN) are used to interconnect components, i.e. communication entities, of the system. To enable remote access to the distributed computer system, typically, the local

area networks are connected to the world wide web of the Internet and/or interconnected

through communication links over publicly accessible territories and/or systems. To protect the

distributed computer system from intrusion, the local area network is delimited by a perimeter

consisting of firewalls and/or virtual private network (VPN) terminators. The perimeter, defined by VPN terminators, firewalls and/or other intrusion detection systems (IDS), protects

the distributed computer system from malicious data traffic originating from sources external to

the local area network.

Conventionally, encrypted communications either required the termination of the encrypted

connection at the perimeter or giving up content-_based traffic control in firewalls or intrusion detection systems, because encrypted traffic cannot be looked into. Consequently, security of

data communication is limited to the perimeter and there is thus a risk that malicious data

content is introduced into the distributed computer system, e.g. from or introduced through one

of its components, and exchanged between its components via the local area network Particularly in distributed control systems, intrusion of malicious data into the system can be of

catastrophic consequences.

2. CONTROL STATION

2.1. Workstation / Single Board Computer

In view of the above, it is a broad, overall, object of the present invention, among others, to

provide a distributed single board computer industrial control system which does not have the

programming drawbacks normally associated with systems of this type. It is another object of the present invention to provide a distributed single board computer

industrial control system in which each of the computers has a common master program and in

which each computer can be easily adapted to a particular application without

modification of the master program.

It is still another object of the present invention to provide a distributed single board computer industrial control system in which each computer has a common master program and in which a

worker, relatively unskilled in programming, can easily adapt each computer to a particular

application.

In accordance with the present invention, an industrial control system having a plurality of

controlled devices is provided with a single board computer connected to each device through an input/output interface. Each computer is controlled by a master or composite program that

includes program steps adapted to achieve control and monitoring of all possible logic 'control

functions and command sequences that exist within the system.

A user alterable interconnection device (UAID) allows only those portions of the master

program that relate to the logic control functions and/or control sequences of a particular controlled device to be operatively connected with the input/output ports associated with that

device. As the single board computer continuously and successively loops through the master

program, only those program steps that relate to the logic control functions and/ or control

sequences for the controlled device are operative, as determined by the UAID, to effect control

and monitoring of the controlled device.

FIG. 1 is a schematic representation of the overall organization of an industrial control system

in accordance with the present invention in which a plurality of controlled devices are

connected to an associated single board computer through an input/output interface

1.

FIGS. 2-3 are examples of logic diagrams which maybe implemented by single board

computers of the system

FIGS. 12a and 12b depict a flow chart illustrating the program employed in each of the single

board computers to control the output devices in accordance with the logic functions selected by the user of the system.

2.2. Programmable Logic Control (PLC)

The concept of distributed industrial control systems, which have both a hardware and software

component, is known in the art. Distributed control has been used since companies began

installing programmable logic controllers (“PLCs”) to manage independent parts of a factory

poor. PLCs are used in industrial control systems to provide coordinated control of equipment,

devices, and processes. PLCs generally comprise a central processing unit (“CPU”) and a

plurality of input/output (“I/O”) modules having I/O connection terminals. PLCs are ordinarily connected to various sensors, switches, or measurement devices that provide inputs to the PLC

and to relays or other forms of output to control the field equipment or other controlled

elements. As control technology evolved, the idea of islands of programmable controllers was

discarded in favor of larger, centralized controllers. Industry is now moving back to a

decentralized approach in which small, intelligent controllers gather data locally and share it across a network. The move back to a decentralized approach may be explained by describing

the disadvantages of using a centralized control system. First, a centralized control system has

lower flexibility and scalability. The maximum number of I/O modules and therefore I/O

connections that can be controlled is determined during design by the model of controller used.

Second, because a single processor (CPU) and a given amount of memory are used for the entire system, any future additional device, or any change in the system’s con?guration, must

consider these limitations. Alternatively, an oversized central controller must be chosen in

advance. Third, the wiring requirements for a centralized system, wherein every device must be

wired to a central controller, are extensive. Although the use of remote I/O modules can reduce

wiring requirements, adding more I/O modules to an existing controller does not add processing power and memory, which are usually fixed.

Alternatively, a centralized system could be used wherein a single controller controls all the

devices in the system. This would allow every condition to be set in one control program, but

the wiring requirements in such a system would be much more extensive, and, importantly, the

system would suffer from the lack of flexibility and scalability. Were an existing device to need functional expansion, or were any additional device to need control in the future, it would

require additional I/O modules to be added to the controller (if technically possible) and would

require additional control programming as well, which would increase the workload for the

fixed central processor and memory.

For these and other reasons, today’s automation world is moving towards distribution of control rather than building centralized systems. In addition, the industry is making use of small

(including “micro” and “nano”) PLCs and exploiting the latest improvements in

communications and network technologies which have allowed companies to migrate from a

centralized structure to a true distributed architecture. Such improvements include embedded networking capabilities (especially Ethernet, TCP/IP based solutions), new powerful CPUs that

make controllers faster and stronger, enhanced security features for decentralized architectures,

the physical downsizing of controllers, and reduced pricing.

FIG. 4 is a high-level ?oW diagram of a control program implementation method in accordance With the present invention.

FIG. 5 is a How diagram of the main part of a control program implementation method in accordance With the present invention.

FIG. 6 is a relational diagram shoWing programmable logic controllers in a network configuration in accordance with the present invention.

FIG. 7 is a table shoWing a control statement Weighting matrix in accordance With the present

invention.

3. DATA LINK COMMUNICATION

The present invention relates to control systems of the type having a plurality of remotely located process control units connected together through a communications link and, more

particularly, to a control system in which each of the remote units sequentially assumes

supervisory communication control of the communication link and in which high reliability

information transfer is achieved between remotes.

Many system type industrial installations, for example, those related to industrial process-type manufacturing and electrical power generation, employ a large number of physically distributed

controlled-devices and associated sensors for effecting coordinated operation of the overall

system. In the past, coordinated control of the various devices has been achieved by manual

operation and various types of semi-automatic and automatic control systems including

electromagnetic relay systems, hardwired solid-state logic systems, and various types of computer control systems. The computer system have included central systems in which the

various sensors and controlled devices are connected to a central computer; distributed control

systems in which a remotely located computer is connected to each of the controlled devices

and to one another; and hybrid combinations of the central and distributed systems.

The successful functioning of the control system is vital to any industrial process, and, accordingly. Distributed systems have generally been preferred over central systems because

the failure of one of the remotely located control computers generally does not cause a system

wide failure as in the case of the failure of the central computer in the central system.

However, in many distributed computer systems, one of the remotes or a specially designed

control unit generally handles supervisory communication control of the communication buss and, for these systems, failure of the communication buss supervisor can lead to a system-wide

failure.

In many industrial control systems, the various communication busses that extend between the

remotely located computer process control units are exposed to high electrical noise

environments. Accordingly, the information transferred over the communication buss can be subjected to error-inducing interference because of the harsh electrical environment. In view of

this, a control system must have a means for detecting errors within the transmitted information

in order to provide high reliability data transmission between remotes.

FIG. 1 is a schematic diagram of an exemplary process control system including a plurality of remote process control units (remotes) connected to a common dual-channel communications

link;

FIG. 2 is a schematic block diagram of an exemplary remote process control unit of the type

shown in FIG.1;

FIG. 3 is a schematic block diagram of a communication controller employed in the remote

station shown in FIG. 2;

FIG. 5 illustrates the format of an exemplary or illustrative information block for transferring

informationbetween remotes;

FIG. 5A illustrates the format of a header frame ofthe information block shown in FIG. 5;

FIG. 58 illustrates the format for a data/informationframe of the information block shown in

FIG. 5;

FIG. 5C illustrates the format for an acknowledgement block (ACK) for acknowledging

successful receipt of an information block;

FIG. 5D illustrates the format for a non-acknowledgement block (NAK) for indicating the

unsuccessfultransmission of an information block between remotes;

FIG. 6 illustrates, in pictorial form, two identical data blocks having the format shown in FIG. 5 successively transmitted on each communication channel of the communication link illustrated

in FIG. 1;

FIG. 7 is a flow diagram summary of the manner in which a source and a destination remote

effect communications with one another;

FIG. 8A is a partial flow diagram illustrating in detail the manner in which a source and a

destination remote communicate and validate information transferred between one another.

FIG. 8B is a partial flow diagram which completes the flow diagram of FIG. 8A and illustrates in detail the manner in which a source and a destination remote communicate and validate

information transferred between one another;

FIG. 9 is a legend illustrating the manner in the flow diagrams of FIG. 8A and FIG. 8B are to

be read;

FIGS. 10A through 10F are exemplary tables illustrating the manner in which supervisory

control of the communication link is transferred from remote to remote.

4. NETWORK ARCHITECTURE

To aid in the understanding of the prior art and the problems associated therewith. it may be

helpful to provide a brief overview of distributed computing environments. As used herein. a

computing environment consists of a plurality of computers connected by a network which

allows the computers to communicate and pass information and/or data between themselves.

The network may range from a local environment. such as a local area network (LAN). to a very large and expansive network. such as a wide area networks (WAN). and many other

distributed network systems. The computer environment also includes the various operating

systems. storage mediums. and other processing resources which reside on these computers and

that are interconnected and accessible via the network through various software products.

Further. a collection of information service systems and application systems generally embodied within software products. are also considered to be part of the computing

environment. The information service systems and application systems used within any given

computing environment range from commercially available software applications

(e.g.spreadsheets. word processors. databases) to custom developed software products tailored

for a specific use within a designated computing environment. A dominant architecture that is receiving widespread use in many current distributed computing environments is known as the

client-server architecture. Client-server architecture is a hierarchical architecture for distributed

computing environments that is generally divided into two layers.

One layer within the client-server architecture includes most of the application systems.

Application systems include fourth generation languages. computer aided software engineering tools. programming languages and their support tools. and various other commercially available

software products. This first layer typically represents the client layer.

A second layer within the client-server architecture includes most of the information service

systems. The information service systems are software products such as database management

systems and data repositories. specialized data access methods. application servers. and any number of service based monolithic software systems. This second layer represents the server

layer.

Include the introduction of a third or middle layer. This third layer typically includes software

products designed to provide various infrastructure or interfacing services between other

components of the distributed computing environment such as between an application system and an information service system. Such software products are classified as middleware

products or systems. A recent term for the hierarchical architecture that utilizes middleware

systems is a three-tier system or multi-tier system architecture. Where the middleware system

consists of software products. applications and services that had previously existed either in the

client layer or server layer. Three-tier or multi-tier layers are well known in the art.

5. PROCESS CONTROL

The present invention relates to digital distributed process control systems which have

controllers or control stations at each of a number of distributed locations with each controller

controlling a plurality of control loops. More particularly, this invention relates to a method and apparatus for carrying out a required control strategy for the loops at any one location with a

maximum of flexibility in the type of strategy to be executed while at the same time

minimizing the cost per loop.

The control of complex industrial processes has evolved from the use of a large number of

simple single loop controllers, which either perform without central direction or, alternatively, are directed by a central computer, toward the use of distributed systems. In distributed

systems, widely spaced control stations are connected for communication with one another and,

if desired, with a host computer. Each of the stations usually is capable of controlling a large

number of loops and is microprocessor based with the host computer being employed for

complex computing, control, and storage functions beyond the capability of the stations. The individual stations of distributed control systems typically execute control on a number of

loops by either of two general approaches. The first is the use of time slots during which are

executed selected library algorithms which determine the functional relationship between

measured variables (controller inputs) and controlled variables (controller outputs) of the

process loops. The second is the use of user-entered programs to determine those functional relationships.

With the time slot approach a fixed number of slots is established for each scan period during

which the controller inputs the measured variables and supplies the control signals to the

control elements of the loops. Each slot can be used to execute any one of a number of

common algorithms stored as firmware in a library of I algorithms. The output for each of the slots can alternatively be used as a control output to an associated loop or as an input to another

slot where supplementary processing of the control signal can be carried out before the signal is used for control of a loop. This approach has some severe limitations, however, when it is

desired to apply it to the extremely diverse combinations of control strategies which may be

required in an industrial environment. Thus, for example, where the library of algorithms

includes a standard PID algorithm to provide proportional, integral, and derivative functions as

well as a summing algorithm, a multiplying algorithm, and a full range of algorithms for logic functions, it will be evident that, while the PID algorithm may make efficient use of the time

for one slot, the less complex algorithms may not. Thus, the execution of non-standard control

strategies which require a number of summers and multipliers or a number of logic functions,

will not use the limited number of slots efficiently.

FIG. 1 illustrates a distributed process control system of the type for which this invention is

useful.

FIG. 2 illustrates an optical-electrical interface of the type which can be used in the

arrangement of FIG. 1.

FIG. 3 is a block diagram showing a controller of the system.

In accordance with the present invention there is provided a method and means for carrying out

the control of a process using a plurality of control loops in a digital distributed process control

system. This method includes providing a database having a plurality of memory cells for

storing numerical and boolean values for use in the control of the loops and providing in

firmware a library of frequently used control algorithms which can be sequentially executed to produce an algorithm output which is a predetermined function of selected algorithm inputs

obtained from the database.

There is a scanning of a plurality of time slots during each of which the station controller is

assigned to produce an algorithm output in accordance with a predetermined function of the

algorithm inputs associated with that slot, as determined by the algorithm selected for that slot. Concurrently established sequential programs are executed to provide other control strategies.

These programs are run to provide a program output to the database having a predetermined

functional relationship to program inputs from the database. Each algorithm output and

program output is used as either a controller output or as a value to be stored in said data base

for use as an algorithm input for another slot or a program input for another program. The controller input to the database is used as a program input or as an algorithm input, whereby the

control elements of the process are controlled in accordance with the desired strategy as

established by the algorithms associated with the time slots and by the sequential programs.

6. DISTRIBUTED CONTROL SYSTEM

With the development of commercially available digital computers industrial process control

has been typically handled through centralized direct digital control systems. These systems

included a main frame computer which is programmed to control processes. From time to time, the program would be modified to change the control of processes and to

accept new process control functions or parameters. As a result, the use of computerized direct

digital control of processes was expanded in order to control more of the total process than

could previously been achieved with prior conventional analog process controllers. However, a

number of major fundamental problems existed with such a main frame, direct digital control system. As with any complex piece of equipment, maintenance was difficult and required

personnel with specialized training. As the process control became more sophisticated, it

became impractical to maintain in-house service personnel for the computerized system.

Therefore, users had to rely upon vendors for maintenance and repair support. Moreover, in a

continuous process, the failure of the central computer could have catastrophic effects because control of a substantial portion if not all of the 35 overall process would be interrupted. In some

areas of technology, such as glass forming, reliance upon a single main frame central

processing control system became impractical because of the potential of these catastrophic

failures.

FIG. 1 is a drawing showing the hierarchical interconnections of the various components of the

distributed control system.

6.1. Distributed Computer Control System

More recently, distributed computer control systems have been developed in which a single

board computer is connected to each control device with each single board computer having a

specific program dedicated to the particular control device. However, a number of 45

drawbacks have been associated with the distributed computer control systems. In order to conserve memory and reduce operational time, the single board computers are generally

programmed in a low-level, assembly-type language. As a result, many system users are

reluctant to employ distributed single board computers because of the difficulty of obtaining or

training personnel to program the computers. Moreover, because maintenance personnel are

typically not trained in programming, a large inventory of single board computers are necessary in order to properly repair a system malfunction by replacing the microprocessor which is

down. In addition, should there be a desire to upgrade or modify the program in any given

microprocessor, such modification or upgrading will often require the modification of the

overall system control program.

Thus, cost effective retrofitting of existing industrial process control systems is not feasible. Johnson et al. disclosed in U.S. Pat. No. 4,253,148 a system of distributed control of a process

which was designed to overcome the difficulty of having a large inventory of preprogrammed

microprocessors for each device to be controlled. Thus, in the Johnson et al. system a master or

composite program was developed which was capable of controlling all possible logic

functions and command sequences for all of the devices to be controlled. This program was downloaded into each of a plurality of microprocessors. Because each microprocessor was

designed to control a specific device, a user alterable interconnection device was provided in

each microprocessor in the form of a programmable read only memory (PROM). The PROM in

each microprocessor allowed only those portions of the master program which related to the

logic control functions and control sequences of a particular control device to be connected to

the input/output ports associated with the device. Thus, as each microprocessor continuously looped through the master program, only those program steps which related to the control

functions and sequences for a control device would be operative. This system had the drawback

that a sometimes massive program had to be downloaded into each microprocessor. In addition,

the time constraints on such a system which must loop through the entire master program is

such that some processes cannot be adequately controlled. Moreover, because the master control program is stored in each microprocessor, storage space for other data such as process

control data and operational parameters is limited.

It, therefore, is an object of the present invention to provide an improved distributive control,

microprocessor based process control system, with interchangeable components whose

programming is transparent to the operators.

FIG. 1 depicts a plurality of networked digital data processors for use in practicing the

invention;

FIG. 2 depicts objects for controlling a process in a system according to the invention;

6.2. Distributed Computer Security System

This invention relates to a method and apparatus for controlling access by users to applications

programs in 10 a distributed computer system. A framework for security in a distributed

computer system has been proposed by the European Computer Manufacturers’ Association

(ECMA) and is described in the following references. 15l) ECMA TR/46 “Security in Open Systems-a Security Framework” July 1988

2) ECMA standard ECMA/ 138 December 1989

3) “Network Access Control Development”, COMPACS 90 Conference, London, March 1990

The ECMA security framework permits a user to be authenticated to the system, and to obtain

as a result a data package referred to as a privilege attribute certificate (PAC) which represents a certified collection of access rights. When the user wishes to access a target application, the

user presents the PAC to that application as evidence of the user’s access rights.

An advantage of this approach is that the user does not need to be authenticated separately to

individual applications—the authentication procedure is performed once only, to obtain the

PAC. The PAC can then be used several times to access different applications. The object of the present invention is to build on this idea of using PACs, to provide an

improved method ofaccess control.

FIG. 1 is a schematic diagram of a distributed computer system embodying the invention.

Computer security systems are often based on the basic access control model, which provides a

foundation of secrecy and integrity security procedures. To do its work, the reference monitor needs a trust worthy way to know the access control rule and the source of the request. Usually

the access control rule is attached to the object; such a rule is called an access control list or

ACL. For each operation, it specifies asset of authorized principals, and the reference monitor

grants a request if its principal is trusted at least as much as one of the authorized principals for

the requested operation. It should be understood that operation of the reference monitor is separated and distinct from

other security issues, such as whether a requestor is who he/ she/ it claims to be. That type of

security is typically provided by using encryption and digital signature techniques, as will be

understood by those skilled in the art. The present invention is directed at systems and methods

for governing access to objects in distributed computer system that allow for “compound principals”. In summary, the present invention is a security system governing access to objects

in a distributed computer system. Each object has an access control list having a list of entries.

Each access conrol list entry can represent either a simple principal or a compound principal.

The set of allowed compounds principals is limited to a predefined set of allowed combination

of simple principals, roles, delegations and conjunctions in accordance with a defined hierarchical ordering of the conjunction, delegation and role portions of each compound

principal.

1. Compound Principal Method

The distributed computing system is provided with a naming service having a secure

membership table that contains a list of assumptions. Each assumption specifies

either one principal as being stronger than another specified principal, or specifies

one role as being stronger than another specified role. These assumptions reduce the

number of entries needed in an access control list by allowing an entry to state the weakest principals and

roles that are authorized to access an object, with all stronger principals and roles

being included by way of the assumptions listed in the membership table.

The reference checking process, typically handled by a reference monitor found at

each node of the distributed system, grants an access request if the requestor is stronger than any one of the entries in the access control list for the resource

requested. Furthermore, one entry is stronger than another entry if for each of the

conjuncts in the latter entry there is a stronger conjunct in the former. Additional rules

used by the reference monitor the reference checking process govern the processes of

comparing conjuncts in a requestor principal with the conjuncts in an access control list entry and of using assumptions to compare the relative strengths of

principals and roles. The present invention provides a framework for making practical

use of compound principals in distributed computer systems.

FIG. 1 is a block diagram a distributed computer system with a trusted naming

service for storing secure data shared by the members of the system.

FIG. 2 is a block diagram of one node of the distributed computer system shown in FIG. 1.

FIG. 3 is a block diagram representing an access control list.

FIG. 4 schematically depicts the elements of one entry in an access control list.

FIG. 5 is a block diagram representing a membership table, which contains a list of assumptions.

FIG. 6 is a flow chart of the process performed by a node’s reference monitor to determine whether or not to grant a request for access to a specified object.

2. Locally Cached Authentication Credentials Method

In general, in most prior art systems authenticating each request by a requester requires digitally signing the request, as well as an exchange of information called

“credentials” between the requester and the server to enable the server to authenticate

the digital signature on the request. The authentication process can impose significant

overhead on the operation of distributed computer systems, especially when the

number of requests transmitted between nodes is high. In summary, the present invention is a security system governing access to objects in a distributed com puter

system. The computer at each node of the distributed system has a trusted computing base that includes an authentication agent for authenticating requests received from

principals at other nodes in the system.

Requests are transmitted to servers as messages that include a first identifier (called

an Auth ID) provided by the requester and a second identifier provided (called the sub

channel value) by the authentication agent of the requester node. Each server process has an associated local cache that identifies requesters whose previous request

messages have been authenticated.

When a request is received, the server checks the request’s first and second identifiers

against the entries in its local cache. If there is a match, then the request is known to

be authentic, without having to obtain authentication credentials from the requester’s node, because the authentication agents guarantee authenticity of such request

messages.

If the identifier in a request message does not match any of the entries in the server’s

local cache, then the server node’s authentication agent is called to obtain

authentication credentials from the requester’s node to authenticate the request message. Upon receiving the required credentials from the requester node’s

authentication agent, the principal identifier of the requester and the received

credentials are stored in a local cache by the server node’s authentication agent. The

server process also stores a record in its local cache indicating that request messages

from the specified requester are known to be authentic, thereby expediting the process of authenticating received requests.

A further optimization is that the server process local cache is used to store a list of

the object access control list entries previously satisfied by each requester, thereby

enabling the server process to expedite granting access to previously accessed objects.

FIG. 1 is a block diagram a distributed computer system with a trusted naming

service for storing secure data shared by the members of the system.

FIGS. 2 and 3 are block diagrams of one node of the distributed computer system

shown in FIG. 1.

FIG. 4 is a block diagram of two computers, one having a requester process that is

requesting access to a server process in the second computer.

FIGS. 5A and 5B schematically depict an Authentication ID table and Channel Assignment Table maintained by authentication agents in the preferred embodiment

of the present invention.

FIG. 6 schematically represents a data packet.

FIG. 7 schematically depicts a “local cache" of au thentication data maintained by

authentication agents in the preferred embodiment of the present invention.

FIG. 9 is a block diagram representing an access control list.

FIG. 10 is a ?ow chart of the authentication process performed by the authentication agents associated with a requester and a server.

7. CONCLUSION

Distributed Computer Control System for Industrial Process Control has been conceived and partially implemented. DCS is well suited to solving some very awkward Industrial Process Control problem encounter by conventional computers in real-time control

applications. for controlling an industrial process includes a plurality of remotely located

process control units (remotes) each coupled to an associated input/output device(s) and

adapted to communicate with one another through a dual channel communications link.

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