英语翻译Process control applications PLCs can also control conti

英语翻译
Process control
applications
PLCs can also control continuous
processes that use analog I/O.For
example,a temperature sensor may
provide a variable signal,such as 0-10
volts,based on the measurement of an
actual temperature.The PLC program
monitors the sensed values continuously
and operates devices that may also be
analog in nature.This could include
setting the position of a valve between
0-100% open,or controlling the speed of
a motor.Continuous applications are so
called because they typically have no
defined start or end once they are initiated;
they maintain a process in a “steady”
operating state.
Today’s PLC
As PLC technology has advanced,so have
programming languages and communications
capabilities,along with many other
important features.Today's PLCs offer
faster scan times,space efficient high-
density input/output systems,and special
interfaces to allow non-traditional devices
to be attached directly to the PLC.Not only
can they communicate with other control
systems,they can also perform reporting
functions and diagnose their own failures,
as well as the failure of a machine or
process.
Size is typically used to categorize today’s
PLC,and is often an indication of the
features and types of applications it will
accommodate.Small,non-modular PLCs
(also known as fixed I/O PLCs) generally
have less memory and accommodate a
small number of inputs and outputs in
fixed configurations.Modular PLCs have
bases or racks that allow installation of
multiple I/O modules,and will accommodate
more complex applications.
Which PLC is right
for you?
Choosing the most effective PLC for your
application depends on a number of
factors.To begin the selection process,a
drawing of the machine or process is a
good start.This can help identify field
devices and physical requirements for
hardware locations.From the drawing,you
can determine how many analog and/or
discrete devices you will have.
Once the field device requirements and
hardware locations are defined,you can
review PLCs that will meet your requirements.
See the PLC Selection Worksheet in
this section that will help you work through
the considerations for determining the type
of PLC you will need,regardless of which
manufacturers you are evaluating.

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