PLC (Programmable Logic Circuit)

PROGRAMMABLE LOGIC
CONTROLLER (PLC)
OR PROGRAMMABLE
CONTROLLER

Definition:
A programmable microprocessor-based
device that is used in discrete
manufacturing to control assembly lines and
machinery on the shop floor as well as
many other types of mechanical, electrical
and electronic equipment in a plant.

 Early PLCs were designed to replace relay
logic systems. These PLCs were
programmed in “ladder logic", which
strongly resembles a schematic diagram of
relay logic. This program notation was
chosen to reduce training demands for the
existing technicians. Other early PLCs used
a form of instruction list programming,
based on a stack-based logic solver.

 Many early PLCs did not have accompanying
programming terminals that were capable of
graphical representation of the logic, and so the
logic was instead represented as a series of logic
expressions in some version of Boolean Format,
similar to Boolean Algebra. As programming
terminals evolved, it became more common for
ladder logic to be used, for the aforementioned
reasons and because it was a familiar format used
for electromechanical control panels.

PROGRAMMABLE LOGIC CONTROLLER : EH-150 :
MODULE LINE-UP

 Features:
 The main difference from other computers is that
PLCs are armored for severe conditions (such as
dust, moisture, heat, cold) and have the facility for
extensive input/output (I/O) arrangements. These
connect the PLC to sensors and actuators. PLCs
read limit switches, analog process variables (such
as temperature and pressure), and the positions of
complex positioning systems. Some use machine
vision.

 On the actuator side, PLCs operate electric
motors, pneumatic or hydraulic cylinders,
magnetic relays, solenoids, or analog outputs. The
input/output arrangements may be built into a
simple PLC, or the PLC may have external I/O
modules attached to a computer network that plugs
into the PLC.

SCAN TIME
 A PLC program is generally executed repeatedly as
long as the controlled system is running. The status
of physical input points is copied to an area of
memory accessible to the processor, sometimes
called the "I/O Image Table". The program is then
run from its first instruction rung down to the last
rung. It takes some time for the processor of the
PLC to evaluate all the rungs and update the I/O
image table with the status of outputs

 This scan time may be a few milliseconds for a
small program or on a fast processor, but older
PLCs running very large programs could take much
longer (say, up to 100 ms) to execute the program.
If the scan time was too long, the response of the
PLC to process conditions would be too slow to be
useful.

 As PLCs became more advanced, methods were
developed to change the sequence of ladder
execution, and subroutines were
implemented. This simplified programming and
could also be used to save scan time for high-
speed processes; for example, parts of the program
used only for setting up the machine could be
segregated from those parts required to operate at
higher speed.

 Special-purpose I/O modules, such as timer
modules or counter modules, can be used where
the scan time of the processor is too long to reliably
pick up, for example, counting pulses and
interpreting quadrature from a shaft encoder. The
relatively slow PLC can still interpret the counted
values to control a machine, but the accumulation
of pulses is done by a dedicated module that is
unaffected by the speed of the program execution.

SYSTEM SCALE
 A small PLC will have a fixed number of
connections built in for inputs and outputs. Typically,
expansions are available if the base model has
insufficient I/O.
 Modular PLCs have a chassis (also called a rack)
into which are placed modules with different
functions. The processor and selection of I/O
modules are customized for the particular
application. Several racks can be administered by a
single processor, and may have thousands of
inputs and outputs. A special high speed serial I/O
link is used so that racks can be distributed away
from the processor, reducing the wiring costs for
large plants.

USER INTERFACE
 PLCs may need to interact with people for the
purpose of configuration, alarm reporting or
everyday control. A human machine interface(HMI)
is employed for this purpose. HMIs are also
referred to as man-machine interfaces (MMIs) and
graphical user interface (GUIs). A simple system
may use buttons and lights to interact with the user.
Text displays are available as well as graphical
touch screens. More complex systems use
programming and monitoring software installed on
a computer, with the PLC connected via a
communication interface.

COMMUNICATIONS
 PLCs have built in communications ports, usually 9-
pin RS-232, but optionally EIA-485or
Ethernet. Modbus, BACnte orDF1 is usually
included as one of the communication protocol.
Other options include various fieldbuses such
as DeviceNet or Profibus. Other communications
protocols that may be used are listed in the List of
automation protocols.

 Most modern PLCs can communicate over a
network to some other system, such as a computer
running a SCADA (Supervisory Control And Data
Acquisition) system or web browser.
 PLCs used in larger I/O systems may have peer-to-
peer (P2P) communication between processors.
This allows separate parts of a complex process to
have individual control while allowing the
subsystems to co-ordinate over the communication
link. These communication links are also often used
for HMI devices such as keypads or PC-type
workstations.

PROGRAMMING
 PLC programs are typically written in a special
application on a personal computer, then downloaded by
a direct-connection cable or over a network to the PLC.
The program is stored in the PLC either in battery-
backed-up RAM or some other non-volatile flash
memory. Often, a single PLC can be programmed to
replace thousands of relays.
 Under the IEC 61131-3 standard, PLCs can be
programmed using standards-based programming
languages. A graphical programming notation
called Sequential Function Charts is available on certain
programmable controllers. Initially most PLCs utilized
Ladder Logic Diagram Programming, a model which
emulated electromechanical control panel devices (such
as the contact and coils of relays) which PLCs replaced.
This model remains common today.

PLC COMPARED WITH OTHER CONTROL SYSTEMS
 PLCs are well adapted to a range
of automation tasks. These are typically industrial
processes in manufacturing where the cost of
developing and maintaining the automation system
is high relative to the total cost of the automation,
and where changes to the system would be
expected during its operational life. PLCs contain
input and output devices compatible with industrial
pilot devices and controls; little electrical design is
required, and the design problem centers on
expressing the desired sequence of operations.

 PLC applications are typically highly customized
systems, so the cost of a packaged PLC is low
compared to the cost of a specific custom-built
controller design. On the other hand, in the case of
mass-produced goods, customized control systems
are economical. This is due to the lower cost of the
components, which can be optimally chosen
instead of a "generic" solution, and where the non-
recurring engineering charges are spread over
thousands or millions of units.

 A microcontroller-based design would be
appropriate where hundreds or thousands of units
will be produced and so the development cost
(design of power supplies, input/output hardware
and necessary testing and certification) can be
spread over many sales, and where the end-user
would not need to alter the control. Automotive
applications are an example; millions of units are
built each year, and very few end-users alter the
programming of these controllers. However, some
specialty vehicles such as transit buses
economically use PLCs instead of custom-designed
controls, because the volumes are low and the
development cost would be uneconomical

 PLCs have similar functionality as Remote Terminal
Unit. An RTU, however, usually does not support
control algorithms or control loops. As hardware
rapidly becomes more powerful and
cheaper, RTUs, PLCs and DCSs are increasingly
beginning to overlap in responsibilities, and many
vendors sell RTUs with PLC-like features and vice
versa. The industry has standardized on the IEC
631131-3 functional block language for creating
programs to run on RTUs and PLCs, although
nearly all vendors also offer proprietary alternatives
and associated development environments.

DIGITAL AND ANALOG SIGNALS
 Digital or discrete signals behave as binary
switches, yielding simply an On or Off signal (1 or
0, True or False, respectively). Push buttons, limit
switches, and photoelectric sensors are examples
of devices providing a discrete signal. Discrete
signals are sent using either voltage or current,
where a specific range is designated as On and
another as Off. For example, a PLC might use 24 V
DC I/O, with values above 22 V DC
representing On, values below 2VDC
representing Off, and intermediate values
undefined. Initially, PLCs had only discrete I/O.

 Analog signals are like volume controls, with a
range of values between zero and full-scale. These
are typically interpreted as integer values (counts)
by the PLC, with various ranges of accuracy
depending on the device and the number of bits
available to store the data. As PLCs typically use
16-bit signed binary processors, the integer values
are limited between -32,768 and +32,767.
Pressure, temperature, flow, and weight are often
represented by analog signals. Analog signals can
use voltage or current with a magnitude
proportional to the value of the process signal. For
example, an analog 0 - 10 V input would
be converted into an integer value of 0 - 32767.

PLC (Programmable Logic Circuit)

More Related Content

What's hot (20)

Viewers also liked (19)

Similar to PLC (Programmable Logic Circuit) (20)

More from Maria Romina Angustia (20)

Recently uploaded (20)

PLC (Programmable Logic Circuit)