Understanding the Software Options

Operator
interface
node
Transmitters
and
final control
elements
Regulatory
control
node
Fieldbus
Operator
interface
node
Transmitters
and
final control
elements
Ethernet(s)
Those familiar with standards in
the computer industry were not at all
surprised by the standard that eventually
appeared for Fieldbus. The ISA
(Instrument Society of America) took
the lead to develop a standard, one
complication being that the standard
had to be a world-wide standard and
not just a U.S. standard.
But in addition, the approach effectively
envisioned the standard driving
the technology — again, trying to swim
upstream. Everyone was on board until
the deliberations started to converge to a
single architecture, but then the wheels
started to come off. In order to get a
standard for Fieldbus, five networking
architectures were incorporated into
the standard. So much for the notion
that a standard should provide one —
and only one — way to do something.
Ever wonder why the various control
system suppliers use different units
for PID tuning coefficients? Want a
standard? Following the practices in
the computer industry, the standard
would be as follows:
1. The proportional mode tuning parameter
can be controller gain, proportional
gain, or proportional band.
2. The integral mode tuning parameter
can be reset time, reset rate, or
reset gain.
3. The derivative mode tuning parameter
can be derivative time or derivative
gain.
4. The units for time may be seconds,
minutes, hours, or fortnights.
The benefits? Specification writers
could demand compliance, suppliers
could promise compliance, and the
ISA could make money selling copies
of the standard — clearly a win-winwin
proposition.
Customary architecture
Figure 1 presents a generic block diagram
for process control systems. The
central component is a communica-
Special
function
node
Transmitters
and
final control
elements
Regulatory
control
node
Fieldbus
Special
function
node
Transmitters
and
final control
elements
FIGURE 1.
Architecture
of a typical
process
control system
tions network that enables information
to be quickly and reliably transferred
between nodes. Each node consists of
a computer whose function falls into
one of the following categories:
Regulatory control node. In a continuous
plant, this is provided by PID
function blocks complemented by various
algorithms. In a batch plant, the
simple and repetitive sequences (pulling
vacuum, transferring a material,
and so on) are implemented in the
regulatory control node. Except for a
few LEDs that indicate power, run/
halt, fault status, and so on, a regulatory
control node conveys no information
directly to humans. Redundancy
is usually available for these nodes.
Operator interface node. This node
presents information to the process operator
and accepts commands from the
process operator. In the ideal world, no
other functions are implemented in an
operator interface node, but in practice,
other functions are sometimes
“piggy-backed” onto the operator interface
node. Usually the easiest way
to address failure issues is to provide
multiple operator interface nodes.
Special functions node. A common
special function is data collection
(historian). An individual node can be
dedicated to a special function, or one
or more special functions can be implemented
in a general purpose computer.
Failure is usually not an issue, as most
processes can be operated (perhaps not
very well) without these nodes.
In Figure 1, the various nodes are
connected by a communications network.
This raises the specter of a
rather nasty failure situation. A total
failure of the communications network
results in the dreaded “loss of
view” — each node continues to function,
but the operator interface nodes
cannot acquire data from or send data
to the regulatory control nodes. There
are numerous consequences, most of
which are bad. This must be addressed
using options such as redundancy for
the communications network, multiple
communications networks with the
operator interface nodes spread over
two or more networks, or otherwise.
As the computer industry evolved,
Ethernet became the default standard
for networking. For control applications,
concerns about collisions and predictability
were raised. But what can
compete with a 1 GHz Ethernet whose
components are readily available and
very cost competitive? As always, you
will be amazed at our flexibility when
we see the price. Apparently a few collisions
aren’t that bad after all.
The initial installations of both smart
transmitters and smart valves relied
on the traditional 4–20-mA current
loop to interface to the control system.
Configuration was via a hand-held programmer
that either plugged directly
into the field device or transmitted via
a signal superimposed on the 4–20-mA
current loop. This approach permitted
the advantages of both smart transmitters
and smart valves to be realized,
but networking the field devices would
make much more possible.
Such a network became known as
Fieldbus and is included in the configuration
in Figure 1. In process control,
the competition appears to have narrowed
to two:
Foundation Fieldbus. This technology
is more common in the U.S.
Profibus. This technology is more
common in Europe.
These are well on the way to displacing
the traditional current loop
interfaces.
DCS architectures
DCSs date from the 1980s, with the
initial products designed primarily
for petroleum refining. However, they
quickly expanded to other industries,
including specialty batch.
The early products tended to be focused
in one of the following directions:
Continuous control. An extensive
set of algorithms complemented the
PID control function to provide whatever
was required to automate a continuous
process.
Sequence control. Although these
also provided the PID control function,
the extensions were mainly features
CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2011 31