The PLC is activated by
command inputs
such as inputs from pushbutton switches, selector switches
and digital switches located at the operating panel, and by
sensor inputs
, such as inputs from limit
switches, proximity switches and photoelectric switches, which detect the status of the system, in order
to control
drive loads
such as solenoid valves, motors and electromagnetic clutches, and
indication
loads
such as pilot lamps and digital indicators.
The behaviors of output signals corresponding to the input signals are determined by the
contents of programs provided to the PLC.
Light loads such as small solenoid valves and pilot lamps can be directly driven by a PLC, but loads
such as 3-phase motors and large solenoid valves must be driven through contactors and intermediate
relays.
As well as PLCs, contactors, intermediate relays and circuit breakers for the power supply are installed
in the control box.
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+
+
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Device to instruct
when and how
the system should
be activated
Pushbutton
switch
Devices to detect the
status of the system
Selector
switch
Digital
switch
Digital
indicator
Devices to indicate
the status of the
system
Devices to judge the
status of the system
and to provide instructions
Pilot lamp
PLC
Input
Solenoid
valve
Proximity
switch
Limit
switch
Actuator section
Devices to drive
the system
Electromagnetic
clutch
Motor
Output
Control section
Contactor
Power supply
Sensor section
Circuit
breaker
Operation section
indicator section
The Definition of a PLC
A programmable Logic Controller (PLC) is referred to as a Programmable Controller
(PC) or Sequence Controller (SC).
A PLC is defined as "an electronic device which controls many types of systems through
its I/O ports and incorporates a memory to store programmable instructions."
Actual Usages
PLCs are broadly used as core components for FA (Factory Automation) and as electronic
application products essential for saving labor costs and improving automation.
PLCs can be used for many types of applications such as systematical applications
which supply control all over a factory or as standalone applications to control an
independent machine.
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