The purpose of a PLC was to directly replace and substitute electromechanical relays as logic elements, substituting instead a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks.
A PLC has many "input" terminals, through which it explain"high" and "low" logical states
from sensors and switches. It also has many output terminals, through which it outputs "high"
and "low" signals to power lights, contactors,solenoids, relays, small motors, and other devices lending
themselves to on/off control. In an effort to make PLCs easy to program, their programming
language was designed to resemble ladder logic diagrams. Thus, an industrial electrician or electrical
engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC
to perform the same control functions.
PLCs are industrial computers, and as such their input and output signals are typically 120 volts
AC, just like the electromechanical control relays they were designed to replace. Although some
PLCs have the ability to input and output low-level DC voltage signals of the magnitude used in
logic gate circuits, this is the exception and not the rule.
Signal connection and programming standards vary somewhat between different models of PLC,
but they are similar enough to allow a "generic" introduction to PLC programming here. The
following illustration shows a simple PLC, as it might appear from a front view. Two screw terminals
provide connection to 120 volts AC for powering the PLC's internal circuitry, labeled L1 and L2. Six
screw terminals on the left-hand side provide connection to input devices, each terminal representing
a different input "channel" with its own "X" label. The lower-left screw terminal is a "Common"
connection, which is generally connected to L2 (neutral) of the 120 VAC power source.
Inside the PLC housing, connected between each input terminal and the Common terminal, is an
op-to-isolator device (Light-Emitting Diode) that provides an electrically isolated "high" logic signal
to the computer's circuitry (a photo-transistor interprets the LED's light) when there is 120 VAC
power applied between the respective input terminal and the Common terminal. An indicating LED
on the front panel of the PLC gives visual indication of an "energized" input.
Output signals are generated by the PLC's computer circuitry activating a switching device
(transistor, TRIAC, or even an electromechanical relay), connecting the "Source" terminal to any
of the "Y-" labeled output terminals. The "Source" terminal, correspondingly, is usually connected
to the L1 side of the 120 VAC power source. As with each input, an indicating LED on the front
panel of the PLC gives visual indication of an "energized" output.
The actual logic of the control system is established inside the PLC by means of a computer pro-
gram. This program dictates which output gets energized under which input conditions. Although
the program itself appears to be a ladder logic diagram, with switch and relay symbols, there are
no actual switch contacts or relay coils operating inside the PLC to create the logical relationships
between input and output. These are imaginary contacts and coils, if you will. The program is
entered and viewed via a personal computer connected to the PLC's programming port.
Equally important to understand is that the personal computer used to display and edit the
PLC's program is not necessary for the PLC's continued operation. Once a program has been
loaded to the PLC from the personal computer, the personal computer may be unplugged from
the PLC, and the PLC will continue to follow the programmed commands. I include the personal
computer display in these illustrations for your sake only, in aiding to understand the relationship
between real-life conditions (switch closure and lamp status) and the program's status ("power"
through virtual contacts and virtual coils).
The true power and versatility of a PLC is revealed when we want to alter the behavior of a
control system. Since the PLC is a programmable device, we can alter its behavior by changing the
commands we give it, without having to recon figure the electrical components connected to it. For
example, suppose we wanted to make this switch-and-lamp circuit function in an inverted fashion:
push the button to make the lamp turn off, and release it to make it turn on. The "hardware"
solution would require that a normally-closed pushbutton switch be substituted for the normally open switch currently in place. The "software" solution is much easier: just alter the program so
that contact X1 is normally-closed rather than normally open.
A PLC has many "input" terminals, through which it explain"high" and "low" logical states
from sensors and switches. It also has many output terminals, through which it outputs "high"
and "low" signals to power lights, contactors,solenoids, relays, small motors, and other devices lending
themselves to on/off control. In an effort to make PLCs easy to program, their programming
language was designed to resemble ladder logic diagrams. Thus, an industrial electrician or electrical
engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC
to perform the same control functions.
AC, just like the electromechanical control relays they were designed to replace. Although some
PLCs have the ability to input and output low-level DC voltage signals of the magnitude used in
logic gate circuits, this is the exception and not the rule.
Signal connection and programming standards vary somewhat between different models of PLC,
but they are similar enough to allow a "generic" introduction to PLC programming here. The
following illustration shows a simple PLC, as it might appear from a front view. Two screw terminals
provide connection to 120 volts AC for powering the PLC's internal circuitry, labeled L1 and L2. Six
screw terminals on the left-hand side provide connection to input devices, each terminal representing
a different input "channel" with its own "X" label. The lower-left screw terminal is a "Common"
connection, which is generally connected to L2 (neutral) of the 120 VAC power source.
Inside the PLC housing, connected between each input terminal and the Common terminal, is an
op-to-isolator device (Light-Emitting Diode) that provides an electrically isolated "high" logic signal
to the computer's circuitry (a photo-transistor interprets the LED's light) when there is 120 VAC
power applied between the respective input terminal and the Common terminal. An indicating LED
on the front panel of the PLC gives visual indication of an "energized" input.
Output signals are generated by the PLC's computer circuitry activating a switching device
(transistor, TRIAC, or even an electromechanical relay), connecting the "Source" terminal to any
of the "Y-" labeled output terminals. The "Source" terminal, correspondingly, is usually connected
to the L1 side of the 120 VAC power source. As with each input, an indicating LED on the front
panel of the PLC gives visual indication of an "energized" output.
The actual logic of the control system is established inside the PLC by means of a computer pro-
gram. This program dictates which output gets energized under which input conditions. Although
the program itself appears to be a ladder logic diagram, with switch and relay symbols, there are
no actual switch contacts or relay coils operating inside the PLC to create the logical relationships
between input and output. These are imaginary contacts and coils, if you will. The program is
entered and viewed via a personal computer connected to the PLC's programming port.
Equally important to understand is that the personal computer used to display and edit the
PLC's program is not necessary for the PLC's continued operation. Once a program has been
loaded to the PLC from the personal computer, the personal computer may be unplugged from
the PLC, and the PLC will continue to follow the programmed commands. I include the personal
computer display in these illustrations for your sake only, in aiding to understand the relationship
between real-life conditions (switch closure and lamp status) and the program's status ("power"
through virtual contacts and virtual coils).
The true power and versatility of a PLC is revealed when we want to alter the behavior of a
control system. Since the PLC is a programmable device, we can alter its behavior by changing the
commands we give it, without having to recon figure the electrical components connected to it. For
example, suppose we wanted to make this switch-and-lamp circuit function in an inverted fashion:
push the button to make the lamp turn off, and release it to make it turn on. The "hardware"
solution would require that a normally-closed pushbutton switch be substituted for the normally open switch currently in place. The "software" solution is much easier: just alter the program so
that contact X1 is normally-closed rather than normally open.
No comments:
Post a Comment