The various control acronyms that take us through
the evolution of process automation include: direct digital
control (DDC), supervisory computer control (SCC) and
advanced process control (APC). DDC marked the beginning of
the age of the digital computer in the process control world.
SCC evolved out of the early DDC projects in response to the
lack of tangible benefits realized from the early DDC project
s. APC came of age with the
introduction of commercial multivariable predictive control
(MPC) products. Those of us who are veteran practitioners
appreciate that APC has a history and would agree that it is
a history worth recounting.
APC, like most (if not all) engineering exercises, has been,
and will always be, a blend of art and science. As a specific
engineering discipline, APC can be considered a subset of the
general topic of process automation. As illustrated in Fig. 1,
the art and science of process automation can be defined in
terms of four paired concepts: function vs. form, technique vs.
technology, solutions vs. tools, and
people vs. machines. The logical union of the art and science
shown in Fig. 1 represents the active universe of
the practice of process automation.
Fig. 1. The art and
science of process automation.
Art and science.
The art and science of process automation are about function
in contrast to form, with function being what is to be done vs.
the form of where it will be done. For process automation, the
form is the computing device. Technique, as opposed to
technology, draws the distinction between methods used vs. how
those methods are implemented. In this case, technology is the
basic software within the computing device. Solutions imply
problems solved using the tools on hand. Consider tools as a
collection of pre-programmed software functions. And, finally,
it is about people as opposed to machines. Machines, in this
context, are illustrative of the processes within the scope of
the automation task vs. those individuals, the people, who are
The general concept of process automation can be extended to
the specific concept of APC with the use of the traditional
hierarchy of the control pyramid shown in Fig. 2. A few words
of clarification for this version of the pyramid: It shows a
clear distinction between Level 1 and all those above it. Level
1 regulatory control is that portion of the control system
required for the safe and stable operation of the process under
normal operating conditions. In practice, this includes all of
the single-loop controllers, as well as the safety-instrumented
systems essential for safe and stable process operation.
Level 2 differs from Level 3 only in the number of variables
included within the scope of the higher-level regulatory
controllers. Level 4 differs from all those below it in terms
of optimization being done in steady-state vs. the dynamics of
Levels 1, 2 and 3 control. While the distinction may be
somewhat a matter of semantics, it remains a useful one for
purposes of discussion.
Finally, the arrows in Fig. 2 are meant to reflect the flow
of information between the process and the APC system while
Levels 2, 3 and 4 together comprise the concept of APC as
presented in this discussion.
Fig. 2. Process control
For the Silver Jubilee issue (January 1970) of
Instrumentation Technology, the readers
overwhelmingly selected the digital computer as the most
significant development during the previous quarter-century.
Furthermore, virtually all the experts saw the role of the
digital computer expanding in all aspects of process
In the preface to his book, Automation: Its Purpose and
Future, published in 1955, Dr. Magnus Pyke observed
scientists do not, as a rule, distinguish themselves as
commentators on general affairs. Indeed, the more distinguished
they become as scientists the more they tend to restrict their
thinking to their own concentrated field. This modern attitude
of scientists arises in part from the fact that science has
become so technical that its practitioners spend all their
intellectual efforts in mastering the facts and techniques of
their particular branch.2
In describing the new style of industrial work,
Dr. Pyke goes on to point out that, early on, machines were
developed to do away with the necessity of mechanical
work. Electronic computers, however, do away with the necessity
for mental effort. In recognizing some potential
limitations on that score, he points out that computers
are only machine tools and their usefulness depends on
the skill and accuracy of the people using them.
With the arrival of the digital computer, the new
style of process control became known as direct digital
control, or DDC. In DDC, the computer calculates the values of
the manipulated variables (like valve positions) directly from
the values of the set points, controlled variables and other
measurements on the process. The decisions of the computer are
applied directly to the process. The early efforts to implement
DDC reflected a time when the basic control system was analog,
be it electronic or pneumatic. Thus, there was a clear
distinction between the analog and digital parts of the
The first DDC systems were installed on a commercial basis
in the mid-1960s. One of the earliest systems to be publicized
was installed and used in the startup of a 500 million pound
per year ethylene plant at the Phillips Petroleum refinery in Sweeny,
Texas.3 Since this was a first-time installation of
a DDC system, Phillips decided to retain conventional
electronic analog control on critical loops and use DDC on all
others. Critical loops were defined as those loops that would
be difficult to control manually if the DDC system should fail.
Of the 180 total control loops, two-thirds (120) were put on
The initial investment in DDC systems was justified on the
basis of providing a lower cost alternative to conventional
analog controllers. As the Phillips experience showed,
this justification proved to be a myth. The lower cost of the
analog controllers was offset by the higher cost for backup
capability in case of DDC system failure. While intangible
benefits were recognized, other more tangible sources of
benefits were needed to justify the investment. Phillips found
that the most promising area for additional benefits lies in
the implementation of supervisory computer control, or SCC.
While not included in the original project scope, SCC was implemented
after the unit startup was completed.
The DDC system at the Phillips refinery was judged to have been a
technical success from the beginning. It was available for
plant start-up and was, in fact, essential to plant startup
since initially the system was provided without analog backup.
It failed, however, on the economics used for the initial
justification of the system. The DDC system was made an
economic success after the fact with the addition of SCC
functions using the DDC system as a base.
SCC was born out of the need to provide tangible benefits
for the use of the digital computer within the process control
hierarchy. As demonstrated by Philips success, the
efficacy of SCC was established early on for not only new
plants but also as an add-in technology for existing plants.
The success of SCC through the 1970s and into the early 1980s
is well documented. Both the art and science developed rapidly
as computers became more powerful and interfaces to a variety
of instrument systems were developed. The introduction of
real-time database software, along with support of higher-level
programming languages, became powerful tools in the hands of
the practitioner. The creativity of the engineer was put to
good use implementing solutions for a variety of control
problems that could not be addressed in the analog control
While SCC progressed rapidly, the Phillips project team gave
a word of caution. This is a direct quote from the project
team: Implementation of supervisory and optimizing
control has not progressed as rapidly as technology would
permit; and, as a result benefits have been delayed. This is
due, in part, to the time required to build up operations
confidence in the system. Is this perhaps a lesson we are
still learning today?
The next big thing in SCC came along with the commercial
introduction of the multivariable predictive controller (MPC).
IDCOM and DMC are two of the more notable that achieved early
commercial and technical success. It was during this time,
through the marketing campaigns of the MPC suppliers, that the
moniker of APC came into general use. And again, many
successful applications of MPC have been noted in the
While MPC achieved demonstrated success as an integral part
of the APC system, the experience with optimization was less
than stellar. Mr. Latour gave an early optimistic view of
online optimization in this magazine in 1979, along with a
number of applications that showed promise.4,5 With
a few exceptions, the concept has never quite lived up to its
Things have come a long way in terms of the new style of
process control. The overall efficacy of APC is well
established and has been thoroughly documented. At the same
time, cases where expectations were not met and systems failed
to deliver a sustained rate of return are also well known. An
assessment of the state of APC was given by Mr. Wang in the
July 2011 issue of this magazine.6 It is
disheartening to note that he estimated that more than 50% of
APC applications are in off mode or do not work at
all. He goes on to say that only about 10% are fully working.
There is a plethora of opinions as to the reasons for this; two
of the more impassioned are cited here.7,8 What I
find particularly interesting about these gentlemens
views is that they are as much about the business of APC as
about the technology of APC.
I think everyone would agree that much has changed in the
evolution from DDC to APC. We now work within a totally digital
world with ever more powerful machines with limitless
capability to, as Mr. Pyke pointed out, do away with the
necessity for mental effort.
Further overlap and blending.
Keen observers of the stock market say the trend is
your friend. To me that suggests that it is important to
not only understand where you are at but to also appreciate how
you got there. I submit that a better appreciation of the blend
of art and science, and more importantly how it has changed
over time, will give us insight into where we might be going.
To that end, an informal and unscientific survey was conducted
asking a number of veteran APC practitioners to complete an
exercise comprised of a series of questions. The questions
1. What is the relative contribution of the art vs. the
science of APC?
2. At what level in the control hierarchy has the efficacy
of APC been demonstrated?
3. How has this blend changed over time?
In terms of Fig. 1, consider the answer to the first
question being described by the relative sizes of the two
circles. The second question would define the degree of overlap
of the two circles, with the higher up on the control hierarchy
as defined in Fig. 2, the greater would be the extent of the
As might be expected, the opinions expressed in the survey
were as varied as the number of people who participated.
However, there was some degree of consensus concerning a trend
and the reasons behind that trend.
The results of the survey suggest that, in the beginning, it
was all about the science (maybe a 10/90 split between art and
science). Since DDC had never been done before, the engineers
were making it up as they went along. As the Phillips project showed, they did a pretty
good job. By 1985, the split had shifted to about 50/50 and the
overlap was growing as MPC began achieving its early success.
Today, the split is estimated at 90/10, a complete reversal
from the beginning, with the overlap about the same as it was
For me, the contemporary view gleaned from the survey
seemed, at first, counterintuitive. The science has certainly
improved with the evolution of more powerful commercial
toolkits for implementing the MPC and optimization components
of the APC system. But, paradoxically, as more of what used to
be the art is now captured in the science, the problems that
can be solved and the techniques required to define the
solution have become ever more complicated, requiring ever more
creativity to be successful. The majority of the reasons for
this current view are related to the people (in terms of the
skills of the APC practitioner), the motivations of operations
personnel and the support of management. These are all issues
that have been in play since the beginning but have now become
the determinate factor in the success or failure of APC.
As a group, the survey participants remain optimistic about
the future. While much has been accomplished and much remains
to be done, the journey continues to be an interesting one. In
contemplating the future, a quote by Charles Lyell from 1863
seems to be apropos. He wrote: The rate of progress in
the arts and science proceeds in a geometrical ratio as
knowledge increases, and so, when we carry our retrospect into
the past, we must be prepared to find the signs of retardation
augmenting in a like geometrical ratio.
The optimist believes that the overlap of the circles will
continue to grow. The realist may view the future as Mr. Lyell
sees it, with the overlap staying about the same. There will
always be the pessimists among us who view the whole thing
falling apart. Personally, I choose to remain an optimist.
1 Smith, C.L., Digital Computer Process
Control, Intext Publishers, New York, 1972.
2 Pyke, M., Automation: Its Purpose &
Future, The Scientific Book, London, 1955.
3 Parsons, J.R., M.W. Oglesby, and D.L. Smith,
Performance of a direct digital control system, ISA
Conference, Philadelphia, Pennsylvania, October 1970.
4 Latour, P.R., Online computer optimization
1: What it is and where to do it, Hydrocarbon Processing, June
5 Latour, P.R., Online computer optimization
2: Benefits and implementation, Hydrocarbon
Processing, July 1979.
6 Wang, J., What is the outlook for advanced
control engineering? Hydrocarbon Processing, July
7 Latour, P.R., Demise and keys to the rise of
process control, Hydrocarbon Processing, March
8 Friedman, Y.Z., Has the advanced process
control industry completely collapsed? Hydrocarbon Processing, January
Michael Delaney is a chemical
engineering professional with over 30 years of
experience in the field of process automation and
control. He has practiced the art and science of APC
with Standard Oil of Ohio, Setpoint, Honeywell,
Pavilion Technologies, Mustang Engineering and BPEC
Consulting. He is currently a senior consulting
engineer with ProSys, Inc.