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Buses for valves, actuators and sensors
reduce need for proprietary interfaces to connect incompatible systems
The first reason that most chemical process engineers investigate the
use of a bus-structured system to connect all of the valves, actuators
and sensors as well as instruments and analytical devices used
throughout a process plant, is to cut wiring costs. But it’s a digital
bus’s many other advantages that may prove to be the most important in
the long run. A trend report.
The conventional practice of individually wiring every field device back
to a central process controller is not efficient. By comparison, the use
of a “multi-drop” or “tree-shaped” bus structure — one that emanates
from a controller communications card, and connects to a wide range of
field devices — makes point-to-point wiring unnecessary. This cuts the
installed cost of such devices by 40 % or more. Then, new devices can be
added by simply attaching them to the bus.
But it’s a digital bus’s many other advantages that may prove to be the
most important in the long run. For example, a fieldbus-based
control-system architecture allows for drastic reductions in wiring,
input-output (I/O) subsystems and controllers in the field, and it
simplifies enterprise, control and remote I/O networking at the host
level. The use of a fieldbus-based control architecture greatly
simplifies complex, multi-level control and plant networks, and —
perhaps most important — it greatly reduces the need for proprietary
interfaces to connect incompatible systems and devices from different
manufacturers.
A bus-compatible field device needs to be specifically designed for a
particular bus, and it must incorporate a microchip-based communications
package. As a result, bus-connected field devices generally cost more
than conventional analog and discrete analytical and diagnostic devices,
monitors and instruments. But their increased functionality and
interconnectability often helps to justify this price differential.
Also, while the installed cost of the first bus-connected device is
almost always higher than comparable, traditionally wired point-to-point
devices, as the number of devices grows, the incremental costs
associated with adding additional bus-connected devices falls.
Even in the most bus-centric control system, a certain number of
traditional, hardwired analog and discrete devices are usually still
used. Devices that do not have a bus communications capability, and some
extremely fast-response equipment, such as gas valves and
emergency-safety-shutdown (ESS) systems, are a couple of examples.
Development of fieldbus technology
In the beginning, the development of digital-fieldbus technology was
complicated by a number of initiatives that seemed to evolve in
parallel, and all claimed to be “the industry standard.” Eventually it
became clear that each bus format offered particular strengths and
weaknesses, and no single fieldbus protocol could meet all of the
requirements. Now that most advanced automation platforms support
several buses running simultaneously in the same controller
architecture, competition between the protocols seems to be decreasing.
Following is a brief description of each of the major bus protocols that
are available for instrumentation. They are presented in order from
simple to complex, and are divided into actuator/sensor, device,
fieldbus and control-network categories. Factors to consider when
evaluating different buses include such items as cost; impact on
existing plant controls; environmental safety; data-transmission
distances; response speed; conduit runs and wall seals; and
electromagnetic interference (EMI).
Actuator/Sensor-interface
AS-i (Actuator/Sensor-interface) is a two-wire, master-slave network
in which power and electronic signals are carried on a single pair of
wires. Originally introduced in Europe for factory automation, AS-i has
also gained popularity in the U.S. to connect discrete devices to the
process control system. For simple on/off service — such as that
provided by on/off valves, switches, solenoids, starters and
push-buttons, for example — and installations with short distances
between the controller and the devices, AS-i offers a rugged, fast,
noise-resistant, and low-cost bus that allows several devices to be
attached to a single node.However, the deficiencies of the AS-i bus
include relatively small message size and relatively simple diagnostics.
And, although the AS-i bus power exceeds intrinsically safe
requirements, several manufacturers of discrete valve communication
packages must be certified for hazard classified areas.
Device buses
The next step up the ladder of bus proficiency and costs are open,
byte-level serial communications buses. In chemical process operations,
the most common ones are DeviceNet and Profibus DP. Both are supported
by numerous hardware and instrument vendors, and both support greater
levels of process automation, more complex transmitters and valve
actuators.
A number of other open device-level buses available, but they are
largely used for factory automation. These include Interbus, FIP,
Seriplex, Modbus, LonWorks, and Sercos. The Modbus protocol, however, is
used by many chemical process operations to provide serial links at the
process-control level.
Both DeviceNet and Profibus DP are cyclical, bi-directional networks.
Power and electronic signals are carried on separate wire pairs for the
DeviceNet bus, and independent power is required for Profibus DP. But
unlike the bit-level AS-i bus, these buses transmit larger
(small-to-midsize) packets of information for data and basic analog
functions. Therefore, they should be wired with special cables that are
manufactured specifically for these buses. In addition to
sensor/actuator duty, the DeviceNet and Profibus DP buses are widely
used for variable-frequency-drive (VFD) control; current and voltage
monitoring; barcode reading; device-level diagnostics; local-panel
displays; and intelligent motor control centers (MCC), to name a few.
Applicable transmitters include those used to monitor pressure, level,
flow and temperature data.
In a plant environment, connecting various field devices using a bus can
be very helpful by allowing an operator to interface with the process
without having to walk to the control room. An intelligent MCC —
especially one with VFD starters — that has been factory-wired and
connected with a byte-level bus can provide a large amount analog,
status, health and diagnostic information on its display.
DeviceNet is the most popular byte-level bus in North America. Because
of the high volume of DeviceNet devices that are available, its
communications chips are relatively inexpensive. DeviceNet offers both
peer-to-peer and master/slave data exchange, and a device may behave as
a client, a server, or both. Its communication is good. Rather than the
addressing performed by most networks, prioritized messages are
broadcast to all nodes and the nodes individually determine whether
messages pertain to them, using embedded identifier functionality.
For the highest data-transmission reliability, the bus has several types
of error detection and fault confinement — including Cyclic Redundancy
Checking (CRC) and automatic retries — to prevent a faulty node from
disrupting communication across the network.
DeviceNet supplies power that exceeds the regulatory restrictions for
intrinsic safety. And, a relatively large number of DeviceNet products
are available for Hazard Classified areas.
Different dominance
While DeviceNet currently dominates the market in North America,
Profibus DP is the most widely specified, open byte-level bus in Europe.
It is one of several offshoots of the basic Profibus protocol; another
is Profibus PA, which is discussed later. Profibus DP is a master/slave
bus that can support more than one master. The bus also permits the
addition and removal of devices and step-by-step commissioning, without
disturbing other devices; and expansions have no effect on devices
already in operation —assuming that the host supports such
functionality.
Profibus DP diagnostics are extensive, with messages assigned by device,
module, and channel for quick location of faults. Unusual for a
device-level bus, Profibus DP includes optional extended functions that
permit acyclic parallel transmission of alarms and read/write functions.
This allows statuses to be read and slave parameters to be optimized,
without disturbing control. Isolators and I/O multiplexers are available
to make Profibus DP intrinsically safe. Further, numerous classified
devices are available for areas with a Class 1/Division 2 hazard
classification.
Fieldbuses
The most proficient open digital-instrumentation buses are the
block-level types that carry large packets of information. Falling into
this category are Foundation Fieldbus and Profibus PA. Fieldbuses are
bi-directional and primarily intended to communicate with intelligent
field devices. Unlike sensor- and device-level buses, fieldbus segments
carry both signals and device power on the same wires. Devices can also
be externally powered.
The primary objectives for developing fieldbuses were: (1) to permit
control algorithms to be performed in field instruments, as well as in
the central controller, (2) to permit remote calibration, commissioning,
diagnostics, and maintenance of field devices; and (3) to provide true
device interoperability.
Unlike sensor- and device-level buses, fieldbuses are optimized to
continuously transmit messages containing multiple, floating-point
process variables. These variables are sampled at the same time, and
contain signals relationg to respective status. And being digital,
fieldbuses eliminate drift in conveying analog signals.
The older Highway Addressable Remote Transducer protocol is sometimes
thought of as a fieldbus, but in practice, it isn’t. Although it can
transmit messages just fine, such capability is achieved by
superimposing a digital data signal on top of a conventional, 4–20 mA
process signal. HART must therefore follow the restrictions of any 4–20
mA, point-to-point wiring scheme, unless the multi-drop capability is
used. Not many hosts support the multi-drop capability, as the
transferring rate can be too slow for anything other than monitoring
applications.
Fieldbuses are slower
In general, fieldbuses tend to be slower than sensor- and
device-level buses. This is due to the fact that process control in
chemical process operations requires more-intense data communications
compared to factory automation. This results from the larger amounts of
data that needs to be continuously transmitted, the requirement for
intrinsic safety, and the fact that power and electronic data are
transmitted over the same wire pair. Both Foundation Fieldbus and
Profibus PA operate at 31.25 kilobits/second (kbps) and currently, not
many of these systems are actually installed or in use in the field.
Fieldbuses were designed from the start to support intrinsically safe
(IS) connections, although the number of devices permitted on an IS bus
segment must be sharply reduced, compared to a segment serving a non-IS
area. Fortunately, manufacturers of power supplies have recognized this
and solutions are becoming increasingly available.
Foundation Fieldbus may be the most versatile of the block-level buses,
and was developed from the start as a fieldbus. The application layer
defines function blocks, which encapsulate basic process-control
configurations or functions (such as PID control, analog input, and so
on) and the data traditionally found in a distributed-control system
(such as PID and ratio control). This user layer also carries device
descriptions, capability files and system management, and allows data
acquisition and control functions to be performed across the bus between
devices of different manufacturers. Thus it represents a truly
interoperable bus architecture.
In addition, Foundation Fieldbus procides so-called peer-to-peer
protocol. Devices can communicate with each other without a host, and
they can initiate communications without a specific host command. For
example, if a Foundation Fieldbus device experiences a problem, it can
send an alarm. This peer-to-peer communication also enables Foundation
Fieldbus devices with the appropriate functionality to execute control
in the field, independent of the host system.
Profibus PA is an intrinsically safe add-on to the Profibus DP bus, and
is primarily used in Europe. The upstream Profibus DP portion, and the
2-wire downstream Profibus PA portion, are connected by either an
intelligent link or a segment coupler, depending on the Profibus DP
speed that is required. The fact that Profibus PA is an add-on to a
device-level bus, and does not incorporate a user layer in its
communications stack, limits its versatility compared to Foundation
Fieldbus.
Like Profibus DP, Profibus PA permits the addition and removal of
devices — and step-by-step commissioning — without disturbing other
devices.
Profibus PA is a master-slave protocol. A field device is a slave that
can only respond to a command from a master. That means if a Profibus PA
device experiences a problem, it cannot report the problem unless the
host specifically asks.
Control networks
Ethernet as a fieldbus? Interest in using Ethernet to network
field-level devices comes from the desire to combine high-performance
connectivity with low cost. For discrete manufacturing, this idea has
merit. However, for process automation, the issue becomes more complex.
A process-automation fieldbus has requirements that are very different
from those of an office-automation network, including:
§ Extreme environmental conditions
§ The need for intrinsic safety
§ Power and signal transmitted over the same wires (for two-wire
devices)
Compatibility
Commercial, off-the-shelf Ethernet cannot meet these requirements.
Industrial Ethernet — with environmentally hardened components,
different memory requirements, and greater robustness — comes closer.
But, the cost of adding those capabilities reduces the economic
advantage of Ethernet. And industrial Ethernet does not provide
intrinsic safety, power and signal over the same wires, or compatibility
with standard instrument wiring. Ongoing work with industrial Ethernet
is aimed at resolving these issues.
However, until these problems are resolved, the best approach is to use
each technology where it is most appropriate: Profibus or Foundation
Fieldbus for process automation, and Ethernet, with appropriate
extensions, as an automation-system backbone and a link to business
systems. Chemical-process companies that wait for an Ethernet-based
fieldbus will miss the reduced project costs and increased operational
benefits that are available today with Foundation Fieldbus or Profibus.
For new plants or plant managers, a combination of the following control
system components offers a realistic balance of simplicity and
capability:
Foundation Fieldbus or Profibus for basic and advanced regulatory
control, and for discrete control associated with regulatory control:
§ One type of device or sensor bus for motor control and machine control
§ An Ethernet-based, automation-system backbone, such as Foundation
Fieldbus High-Speed Ethernet (HSE)
§ A switch or gateway to the Ethernet business network
In addition, existing plants may have to include other networks for
legacy equipment. But in general, it is best to avoid buying devices or
systems that require different or proprietary buses. <<
Source: Trend Report Dechema
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