"His name was Hysteresis" is a short illiteracy training, provided by
IH for those, who still think that hysteresis is a foreign guy's name.
One part of this article should be taken seriously, while the other
should NOT!
Through my hydraulic troubleshooting and technical
assistance practice I've come to learn, appreciate and apply
certain "psychological" approaches to problem solving. Those of
you, who already have assistance practice, know that every technical
assistance call or troubleshooting situation has two sides - technical
and psychological. Technical side is nothing more than bolts and parts
behind the problem, and normally doesn't constitute that much of a
problem for a skilled and properly equipped technician. The
psychological or human-related part, however, is the one that can give
the most headache, as behind every hydraulic problem there is a always
a person (the owner of the equipment, the operator, mechanic, engineer,
the-one-who-is-in-charge, etc...) or a group of persons, whom you will
have to "sell your opinion to", convincing him/them that a certain
course of action must be taken in order to fix the problem. No matter
how good a technician you are, if you are not a good "convincer" -
there is a chance that the client will ask for a second opinion
somewhere else.
You might ask me - what the hell does all this have
to do with the above mentioned HYSTERESIS matter - be patient - I am
getting there...
OK, the so very interesting matter of human relation
skills every troubleshooter must master (in any area, not just in oil
hydraulics) is a topic that can be extended into a separate article, or
even a book, and surely is not going to be discussed here and now. But
one thing is for sure - one of the most important things a technician
must do in order to convince a client that his technical diagnosis is undoubtfully correct, and that his
solution of the problem is the best, is to gain client's faith in his
qualification - in other words - to look confidently competent no
matter what.
That's right - confidence is the main clue to
looking like a skilled professional whichever might be your area of
expertise. Make a doubtful face - and you are dead. That
is why an experienced troubleshooter develops a series of tactics
in order to gain a client's trust at first sight.
Many techniques, particularly suitable for
hydraulic technicians, can be named, among them classics, like
presenting sophisticated testing gear with a superior look on your face
(digital pressure-gauges work the best, especially in remote lightly
populated agricultural areas...), throwing a tool in the air and
catching it after it has flipped a couple of times (requires training
and works best if you catch it behind your back, but has a drawback of
producing exactly the opposite impact when you catch a hammer with your
unprotected head), and, my personal favorite and the classiest of them
all - the confident use of expensive
words, by which I mean, of course, calling things by their names. After
all - there are correct scientific names for certain technical
phenomenae and, when you know what you are talking about, using
these terms is no shame at all. Words like "hydrostatic",
"volumetric","delta P", "resonance", "regenerative circuits", "load
sensing", "reservoir", "carryover" and what not may sound dull and
everyday for you, but there's a good chance that they will be news for
your client, and will contribute to your credibility status. With one
of the sneakiest words to throw at people, instantly making them
surrender to your technical opinion, being the aforementioned HYSTERESIS.
Please understand, that although I defend the use of
correct technical vocabulary without simplifications, I am absolutely
against the use of special terms without understanding or out of the
context. The only positive thing that can come out of this is a solid
basis for popular anecdotes and cartoons like the Simpsons. Why
"hysteresis" you say? Well, it's because I, more often than not, see
stumbled faces of many mechanics out there, when confronted with this
term, and yet the phenomenon of hysteresis plays an important role in
hydraulics, especially in the function of pump and motor controls. This
is the main reason why I, perhaps in a slightly satiric manner,
allowed myself to make the following attempt to enhance the technical
vocabulary of those, who imagine a bearded foreign man's face whenever
they hear the word hysteresis.
A little bit of history behind the term - the word
HYSTERISIS is derived form an Acient Greek word that means "lagging
behind", and was introduced by a Scottish physicist Sir James Alfred
Ewing in the late nineteenth century. He applied the term at the time
to describe certain magnetic properties of metals, but later on this
term began to be used to describe "lagging behind" of any system
responding to a certain input. The "book" definition of the term would
be "the lagging of the effect behind its cause".
Let me clarify this with a hydraulics related example. Say
you have a pump equipped with an electro-hydraulic proportional
1000ma control. Increasing the input electric signal form 0 to 500ma
will result in a displacement, for example 50 cu cm, however if you
increase the current to the maximum value of 1000ma, and then decrease it to the same 500ma, you will see that the displacement is now 52 cu cm. The same input signal resulted in two different outputs
- 50 and 52 cu cm. If the pump's full displacement were 100 cu cm, you
would say that the displacement control has hysteresis of
(52-50)/100=2% of the full displacement.
ANY hydraulic control - whether it is pressure,
flow, position, torque, displacement, or anything else related -
has hysteresis, and will result in two different outcomes from the same input, depending
on whether the input level was increasing or decreasing. Even the
simplest relief valve will crack at one pressure (pressure increasing)
and re-seat at a lower one (pressure decreasing). However, it is
important not only to understand the notion of the hysteresis,
but also to know when it really becomes important to take it under
consideration.
With hydraulic controls things are simple - the best
hysteresis is no hysteresis. Why? Because what we want from them
controls is predictable results.
In general, hysteresis presented by standard hydraulic controls is
tolerable for most industrial applications. Still, normal wear of
control mechanisms can increase the hysteresis to unacceptable levels,
that is why it is a good practice to evaluate hysteresis when testing
overhauled pumps and motors equipped with used control modules.
There are applications, when especially precise
control levels are required. Take, for example, hydraulic generator
drive systems, which must produce fixed flow to maintain the current
frequency stable. One possible solution for such a system is a simple
load sensing controlled pump and a fixed orifice. There are brands
which manufacture special pumps for this purpose, and these babies are
equipped with LS controls that have extremely low hysteresis. So,
whenever overhauling such a pump, an extra care should be taken to
evaluate hysteresis of the control during testing phase, to check
whether it is still suitable for the application.
Also, detecting high hysteresis of a hydraulic
control during tests gives you an idea of what to expect when servicing
or adjusting it on the actual machine in the future. In
some cases, knowing that a control has confirmed high hysteresis, can
be a key factor to explain certain phenomenae in a system and help to
find a "plan B" solution.
It should be noted that in the case of
hydraulic controls the hysteresis level is rate-dependent, which means
that it will change (increase usually) with the rate of the input
signal change.
Main causes for hysteresis in mechanical control systems are
freeplay/backlash (feedback links), friction, and wear of spool/sleeve
assemblies. There are other causes, but from practical point of view
the first two are the only ones that can be improved.
Let us recapitulate:
- In control systems a certain result is expected from a certain input, and the same input produces two different
results, depending on whether the input level was increasing or
decreasing. This difference is the hysteresis. A smart Scottish
scientist invented the word. Scotland rules!
- In hydraulics hysteresis is everywhere, because hydraulics is all about controls. Hydraulics rules!
- It is important to evaluate hysteresis of
hydraulic controls because for some applications certain levels of
hysteresis can not be tolerated, and even in case of standard
applications you will know what to expect.
- Hysteresis level is rate-dependent.
- Best hysteresis is no hysteresis (at least in
hydraulic controls). Normal wear of parts causes the increase of
hysterysis.
- Hysteresis has nothing to do with Hysterical
- Next time you hear the phrase like "Her cheeks flushed as he pulled out his huge hysteresis..."
you will know that what is meant here is a conversation between two
colleagues, who work in a hydraulics related company, and the female
colleague got very upset with the pump testing results, her male
colleague just showed to her, because the results proved high
hysteresis levels of the pump's control unit....
