What is PLC?
Programmable
Logic Controller
- · Ladder Diagram (LD)
- · Sequential Function Charts (SFC)
- ·
Function Block Diagram (FBD)
- ·
Structured Text (ST)
- ·
Instruction List (IL)
- Ladder Logic (LD)
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"Ladder"
diagrams
Ladder diagrams
are specialized schematics commonly used to document industrial control logic
systems. They are called "ladder" diagrams because they resemble a
ladder, with two vertical rails (supply power) and as many "rungs"
(horizontal lines) as there are control circuits to represent. If we wanted to
draw a simple ladder diagram showing a lamp that is controlled by a hand
switch, it would look like this:
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The "L1"
and "L2" designations refer to the two poles of a 120 VAC
supply, unless otherwise noted. L1 is the "hot" conductor,
and L2 is the grounded ("neutral") conductor. These
designations have nothing to do with inductors, just to make things confusing.
The actual transformer or generator supplying power to this circuit is omitted
for simplicity. In reality, the circuit looks something like this:
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Typically in
industrial relay logic circuits, but not always, the operating voltage for the
switch contacts and relay coils will be 120 volts AC. Lower voltage AC and even
DC systems are sometimes built and documented according to "ladder"
diagrams:
So long as the
switch contacts and relay coils are all adequately rated, it really doesn't
matter what level of voltage is chosen for the system to operate with.
Note the number
"1" on the wire between the switch and the lamp. In the real world,
that wire would be labeled with that number, using heat-shrink or adhesive
tags, wherever it was convenient to identify. Wires leading to the switch would
be labeled "L1" and "1," respectively. Wires
leading to the lamp would be labeled "1" and "L2,"
respectively.
These wire
numbers make assembly and maintenance very easy. Each conductor has its own
unique wire number for the control system that it's used in. Wire numbers do
not change at any junction or node, even if wire size, color, or length changes
going into or out of a connection point. Of course, it is preferable to
maintain consistent wire colors, but this is not always practical. What matters
is that any one, electrically continuous point in a control circuit possesses
the same wire number. Take this circuit section, for example, with wire #25 as
a single, electrically continuous point threading to many different devices:
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In ladder
diagrams, the load device (lamp, relay coil, solenoid coil, etc.) is almost
always drawn at the right-hand side of the rung. While it doesn't matter
electrically where the relay coil is located within the rung, it does matter which end of the ladder's
power supply is grounded, for reliable operation.
Take for
instance this circuit:
Here, the lamp
(load) is located on the right-hand side of the rung, and so is the ground
connection for the power source. This is no accident or coincidence; rather, it
is a purposeful element of good design practice. Suppose that wire #1 were to
accidently come in contact with ground, the insulation of that wire having been
rubbed off so that the bare conductor came in contact with grounded, metal
conduit. Our circuit would now function like this:
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With both sides
of the lamp connected to ground, the lamp will be "shorted out" and
unable to receive power to light up. If the switch were to close, there would
be a short- circuit, immediately blowing the fuse.
However, consider what would happen to the circuit with the same
fault (wire #1 coming in contact with ground), except this time we'll swap the
positions of switch and fuse (L2 is still grounded):
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This time the
accidental grounding of wire #1 will force power to the lamp while the switch
will have no effect. It is much safer to have a system that blows a fuse in the
event of a ground fault than to have a system that uncontrollably energizes
lamps, relays, or solenoids in the event of the same fault. For this reason,
the load(s) must always be located nearest the grounded power conductor in the
ladder diagram.
·
REVIEW:
·
Ladder diagrams (sometimes called "ladder
logic") are a type of electrical notation and symbology frequently used to
illustrate how electromechanical switches and relays are interconnected.
·
The two vertical lines are called "rails"
and attach to opposite poles of a power supply, usually 120 volts AC. L1
designates the "hot" AC wire and L2 the
"neutral" (grounded) conductor.
·
Horizontal lines in a ladder diagram are called "rungs,"
each one representing a unique parallel circuit branch between the poles of the
power supply.
·
Typically, wires in control systems are marked with
numbers and/or letters for identification. The rule is, all permanently
connected (electrically common) points must bear the same label.
Digital
logic functions
We can construct
simply logic functions for our hypothetical lamp circuit, using multiple
contacts, and document these circuits quite easily and understandably with additional
rungs to our original "ladder." If we use standard binary notation
for the status of the switches and lamp (0 for un-actuated or de-energized; 1
for actuated or energized), a truth table can be made to show how the logic
works:
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Now, the lamp will
come on if either contact A or contact B is actuated, because all it takes for
the lamp to be energized is to have at least one path for current from wire L1
to wire 1. What we have is a simple OR logic function, implemented with nothing
more than contacts and a lamp.
We can mimic the
AND logic function by wiring the two contacts in series instead of parallel:
Now, the lamp
energizes only if contact A and contact
B are simultaneously actuated. A path exists for current from wire L1
to the lamp (wire 2) if and only if both switch
contacts are closed.
The logical
inversion, or NOT, function can be performed on a contact input simply by using
a normally-closed contact instead of a normally-open contact:
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Now, the lamp
energizes if the contact is not actuated,
and de-energizes when the contact
is actuated.
If we take our
OR function and invert each "input" through the use of
normally-closed contacts, we will end up with a NAND function. In a special
branch of mathematics known as Boolean
algebra, this effect of gate function identity changing with the inversion
of input signals is described by DeMorgan's
Theorem, a subject to be explored in more detail in a later chapter.
The lamp will be
energized if either contact is
un-actuated. It will go out only if both
contacts are
actuated simultaneously.
Likewise, if we
take our AND function and invert each "input" through the use of
normally-closed contacts, we will end up with a NOR function:
A pattern quickly
reveals itself when ladder circuits are compared with their logic gate
counterparts:
·
Parallel contacts are equivalent to an OR gate.
·
Series contacts are equivalent to an AND gate.
·
Normally-closed contacts are equivalent to a NOT
gate (inverter).
We can build
combinational logic functions by grouping contacts in series-parallel
arrangements, as well. In the following example, we have an Exclusive-OR
function built from a combination of AND, OR, and inverter (NOT) gates:
The top rung (NC
contact A in series with NO contact B) is the equivalent of the top NOT/AND
gate combination. The bottom rung (NO contact A in series with NC contact
B)
is the equivalent of the bottom NOT/AND gate
combination. The parallel connection between the two rungs at wire number 2
forms the equivalent of the OR gate, in allowing either rung 1 or rung 2 to energize the lamp.
To make the
Exclusive-OR function, we had to use two contacts per input: one for direct
input and the other for "inverted" input. The two "A"
contacts are physically actuated by the same mechanism, as are the two
"B" contacts. The common association between contacts is denoted by
the label of the contact. There is no limit to how many contacts per switch can
be represented in a ladder diagram, as each new contact on any switch or relay
(either normally-open or normally-closed) used in the diagram is simply marked
with the same label.
Sometimes,
multiple contacts on a single switch (or relay) are designated by a compound
labels, such as "A-1" and "A-2" instead of two
"A" labels. This may be especially useful if you want to specifically
designate which set of contacts on each switch or relay is being used for which
part of a circuit. For simplicity's sake, I'll refrain from such elaborate
labeling in this lesson. If you see a common label for multiple contacts, you
know those contacts are all actuated by the same mechanism.
If we wish to invert
the output of any switch-generated
logic function, we must use a relay with a normally-closed contact. For
instance, if we want to energize a load based on the inverse, or NOT, of a
normally-open contact, we could do this:
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We will call the
relay, "control relay 1," or CR1. When the coil of CR1
(symbolized with the pair of parentheses on the first rung) is energized, the
contact on the second rung opens,
thus de-energizing the lamp. From switch A to the coil of CR1, the
logic function is non-inverted. The normally-closed contact actuated by relay
coil CR1 provides a logical inverter function to drive the lamp
opposite that of the switch's actuation status.
Applying this
inversion strategy to one of our inverted-input functions created earlier, such
as the OR-to-NAND, we can invert the output with a relay to create a
non-inverted function:
From the switches to the coil of CR1, the logical
function is that of a NAND gate. CR1's normally-closed contact
provides one final inversion to turn the NAND function into an AND function.
·
REVIEW:
·
Parallel contacts are logically equivalent to an OR gate.
·
Series contacts are logically equivalent to an AND gate.
·
Normally closed (N.C.) contacts are logically
equivalent to a NOT gate.
·
A relay must be used to invert the output of a logic gate function, while
simple normally-closed switch contacts are sufficient to represent inverted
gate inputs.
