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Associated
Engineering is not tied to any one vendor or group of vendors. Equipment selection
and system architecture is always based on client needs and preference. Experience
with systems based on PLCs, DCSs, micro and mini computers and SCADA RTUs contributes
to assurance of a successful project. Some of these systems use a combination
of these technologies to provide the required functionality. The architecture
is developed based on the size and scope of the control system. A small system
may utilize a single processor with a simple panel operator interface. Multi-processor
systems are often used to reduce wiring costs, ease maintenance and increase reliability
through distributed control. Larger systems also utilize remote I/O and smart
instrumentation for further functionality and cost reduction. Associated Engineering's
designs employ control system architectures that are hierarchical where each layer
in the hierarchy is built on the layer below it as shown in the figure. This greatly
simplifies maintenance, upgrades and expansion. Also, if a component fails, all
the components in the layers below the failed component still operate as a system.
The number of layers varies depending on the project needs.
| Control
System Levels | | enterprise
network | | human -
machine interface network | | human
- machine interface | | process
controller network | | process
controller | | device
communication highway or local loops | | instruments
and control devices | | local
manual monitoring and control | | plant
equipment |
The equipment is selected based on a
variety of factors. Increasingly, raw performance is less important as most manufacturers
produce processors that are fast enough to stay on top of the process. Rather,
selection is based more on economics, support and compatibility. Over the
past few years, the hardware costs have fallen so much that they are not usually
a major cost. The savings from competitive bidding on the hardware can be more
than offset by other factors such as: - how cost effectively can the system
be programmed?
- what spares are locally in stock?
- how good is the local
supplier at providing parts and support?
- how easy is the system to maintain?
- what training do the client's maintenance people have?
- how easy is the
system to expand or modify?
Equipment is also selected considering compatibility
with products that are likely to be required and with systems that it will or
may eventually be tied in with. An open architecture is developed where appropriate.
Client preferences are given first consideration.
Associated Engineering has experience
with the following standard telemetry methods:
- Direct connection
- Wide-area
network (WAN)
- Dial-up
- Leased line
- Fixed frequency radio
- Spread
spectrum radio
These methods are selected on a case by case basis to provide
the most cost effective and reliable system. Many times the final architecture
uses a combination of methods. Where reliability is critical, different methods
may be implemented in parallel to provide multiple communication paths.
There are many protocols in use
in the industry, particularly at the process controller level. As Associated Engineering
is not tied to any one vendor, the company has developed experience in implementation
of numerous protocols including:
- TCP/IP
- Modbus
- Modbus Plus
- Data
Highway
- Data Highway Plus
- TI-Way
- Genius LAN
- ASCII
- MAP
Selection of the protocol or protocols to be used depends on the equipment to
be supplied and the type of telemetry. Expertise in connecting systems together
that use different protocols helps the most cost effective use of existing equipment
in new systems.
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Instrumentation 
Architectures
