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EXCERPT
#1
Automated
Drill
Press
The best way to learn how to
troubleshoot is to look at a
real world example with an
automation controller.
Let’s say you get a phone
call telling you that a machine
is stopped. It seems to stop
intermittently for no apparent
reason. This machine is part of
a production line; if it is not
running, the whole line is
stopped.
However, before you can do
any troubleshooting, you must
have a clear understanding of
how the machine is supposed to
work. This furniture
manufacturer has a machine that
drills a 3/8” hole in a certain
spot on a piece of wood. The
entire process is
automated.
The main conveyor transports
the part into the machine where
the part meets a pneumatically
actuated stop gate. At that
time, another pneumatic
cylinder actuates a clamp that
pushes the part back against
the conveyor wall. This holds
the part in place during the
drilling process. Photocells
verify that the part is in
position; the spindle lowers
and proceeds to drill a hole in
the part. After the hole has
been drilled and the spindle
has retracted to its home
position, the clamp releases,
the stop gate raises and the
part exits.
Troubleshooting the
Logic
As we go
through these rungs, make sure
that you understand how each
rung works. If you do not
understand the purpose of the
individual rungs, you will not
be able to troubleshoot the
system.
We are now online with the
processor, so let’s take a look
at the first rung.
We should first look at the
output and notice that it is
not true, or off.
The instructions that are
highlighted in green are true.
In order for the OTE
instruction (System Running
B3:0/0) to be true, the must be
a path of logic leading to the
output that must be true.
Written in pure Boolean
form, we can say that:
If the emergency stop is
cleared and the guard is in
place and there is not a system
fault and the start button is
pressed (or if the start button
is not pressed, the system is
running) and the stop button is
not pressed the system running
bit is on.
To make sure we understand,
let’s look at each instruction
in detail.
EmergStop Cleared
CR1 I:1/4
This is a hardwired input, as
indicated by the “I” in the
address. It comes from relay
CR1 (CR is an electrical
abbreviation for Control
Relay).
E-Stop pushbuttons are wired
so that when the button is
pushed, an emergency stop is
initiated. You will see these
at any gas station. It is much
faster to push a button than to
pull it; the operator can just
“hit” the e-stop to stop the
system. This is, obviously, the
quickest way to stop a system
in an emergency.
Most e-stop pushbuttons are
2-position, maintained
switches. You can pull on the
switch to place it in the first
position, or push it in to
place it in the second
position. It will “maintain”
the position in which it was
last placed.
Sometimes e-stop switches
are used in conjunction with a
relay, so that other switches
or sensors may be placed in
series with it. These
components are then wired to
the coil of a relay. Wiring the
circuit in this way provides
the most fail-safe method of
assuring that the e-stop
circuit will work when it needs
to.
We may not have electrical
prints for the machine, but we
can assume that if the E-Stop
pushbutton is pulled out, it is
“cleared” and will allow the
machine to run. The e-stop
instruction is highlighted with
green, so we see that the
instruction is true, and it is
not keeping the machine from
running.
Guard in Place LS26
I:2/4
Most
industrial machinery has a
guard to keep people from
getting hurt by being inside
the machine when it is running.
A guard can be anything from a
personnel gate to a Plexiglas
window. In our case, it is a
Plexiglas shield that swings
down in front of the
spindle.
LS26 is a limit switch that
is hardwired into the PLC as an
input. It is highlighted green,
so that is not our problem
System Fault
B3:0/12
The “B” in
the address tells us that this
is an internal bit. Also,
because the instruction is
normally closed and it is
false, we know that bit B3:0/12
is on. Something is turning on
the System Fault bit,
somewhere, and it is keeping us
from running. This is where we
will begin tracing the logic,
but first let’s complete our
understanding of the rung.
Start System PB2
I:1/1
This is a hardwired input
coming from the “Start”
pushbutton. Typically, a start
button is a momentary switch;
it only makes contact if it is
being pushed. That is why there
is a branch around it. With
everything else in the rung
being true, when the button is
pushed, the “System Running”
bit B3:0/0 will come on. The
XIC instruction in the branch
will be true and the “Start”
button can be released. The
system will continue to
run.
Stop System PB3
I:1/2
This pushbutton is wired to a
hardwired input in a fail-safe
manner. That is why the
instruction is an XIC, or
normally open. If the wire to
the input is removed, or cut,
the system would not run.
You probably will
troubleshoot systems that have
rungs that are much more
complex than this rung. As you
become more adept at
troubleshooting, you will be
able to quickly scan a rung
from left to right and look for
bits that are not true (not
highlighted). In this case, the
“System Fault” bit really jumps
out.
RSLogix 500 allows a user
to customize colors. Green is
the default color to indicate
an instruction is true.
However, you may, at some
point, pick up someone else’s
laptop and the colors might be
modified.
We can clearly see that the
emergency stop button, the
machine guard or the “Stop”
button instructions are not
keeping the machine from
running. No one is pressing the
“Start” button, so we would not
expect that to be true.
The culprit is the “System
Fault” bit (B3:0/12).
The “B” in the “System
Fault” address tells us that it
is binary bit, also known as an
“internal” bit. Let’s track
down this bit.
Searching the
Program
Right-click on the instruction
and choose “Find All”.
The search results window
appears.
It tells us that the bit
B3:0/12 appears in File 2 on
Rung 33 as an OTE, or
output.
It also tells us that the
bit appears in Rung 0, which is
the rung we are already viewing
(searches in RSLogix return all
occurrences of the bit).
Click on the first line of
the search results window and
Rung 33 appears.
First, take note of the fact
the output instruction is true.
This explains why the XIO, or
normally closed, System Fault
instruction in Rung 0 is
false.
EXCERPT
#2
Modifying A Program To Trap
Faults
Let’s
add some logic to latch these
faults. Close the search
results window and double-click
on the rung number for Rung 19.
You will see that a copy of the
rung is created for you to
edit. The original rung is
still shown below. Until you
edit the copy and accept it,
the program will continue to
run the logic in the original
rung.
Right-click on the branch and
select “Add Branch Level”.
The screen looks like this.
Click on the “Examine On”
button in the tool bar and drag
it to the new branch you have
created.
Double-click on the tag name
field (currently occupied by
the question mark) above the
instruction. Use the drop-down
menu to navigate to the tag
“ValveAVQRFault.DN”.
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