The following troubleshooting episode is a good example of how modern
semiconductor electronics can easily become seriously affected by
things that good old bullet-proof relay based circuitry used to
consider a mere annoyance.
This summer I was asked to take a look at a
malfunctioning scrap-metal press, which had the nasty come-and-go type
failure - according to the owner, the main
pressing cylinder occasionally wouldn't extend - sometimes it would
work fine for weeks, and sometimes would malfunction almost every hour.
The technicians were able to track the malfunction down to a pressure
switch, connected to the pressing cylinder bottom port. It appeared
that the pressure switch would stay in the "on" position all the time,
giving the faulty signal and not allowing the machine's logics to
extend the cylinder after it's retracted. The client came to the
conclusion that the problem was being caused by pressure trapped inside
the manifold the switch was mounted on. The pressure switch had been
replaced several times and the malfunction remained.
The pressing cycle was driven by a small 24 VDC
Siemens PLC (programmable logic controller), and the pressure switch
was supposed to give the PLC a signal when the set pressure level was
reached - a very simple on/off arrangement, no proportional signals or
pressure sensors.
Pressure reading showed that there was no "trapped
pressure" problem, and verifying the pressure switch function proved
that it was functioning perfectly. The wiring seemed intact and the
electrician confirmed there was no circuit interruptions or any extra
connections in between the pressure switch and the PLC. The owner of
the machine was puzzled on where could the signal come from...
It turned out that the faulty "on" signal came
from the wiring itself! If you look at the pictures, you will see that
the wiring wasn't protected from the elements and, to
top it up, the connectors used were of a very poor quality, allowing
rain and condensation water along with other contaminants to enter inside the plug and
the wires. Modern PLCs, as a rule, have high impedance digital inputs
(this one had the impedance of around 5 kOm), and generally consider any voltage above
half of the supply voltage a "1", or "ON" signal, so the small leakage
current (around 3 ma), caused by the liquid contamination that entered
the wires, was enough to signal the PLC and cause the faulty operation
of the press.
Normally, the rain water isn't conductive, but
the rig was a scrap metal press, which meant that all sorts of acid,
alkaline and other liquids and solids could get in contact with the
poor wires, situated right next to the pressing tunnel. When it rained
- the machine failed more. When sunny weather settled - the wiring
would dry-out and the malfunction would miraculously disappear.
Instant and cheap solution was keeping the wiring (I
did change the plug with a more protected one, though) and applying a
relay next to the PLC so that the pressure switch triggered the relay,
which in its turn gave the 24V "ON" signal to the PLC input. The relay
required 50 ma to trigger, almost 20 times as much as the PLC,
which made the leakage current of the wiring irrelevant. Furthermore, a
protective metal structure was created over the pressure switch and the
wires to protect them from the harsh environment.
This type of malfunction is not uncommon. Nowadays
the use of PLCs to control automated hydraulic machinery is becoming a
"must", and the fact that these gadgets require low current signals to
trigger the input state can make the small currents, caused by
conducting liquid contamination of the wiring (the most common liquids
are sea water and contaminated rain water), a serious problem, that can
jeopardize normal machine operation. Therefore special care must be
taken when installing or checking the PLC signal wiring in order to
make it as much insulated and protected as possible.
This also shows that troubleshooting of modern hydraulic systems is impossible without the understanding of basic electric circuits.
