Insane Hydraulics

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Hysteresis of Hydraulic Controls

"Hysteresis of Hydraulic Controls" was written for those who still think that "hysteresis" is a popular boy name from the last century. Just kidding - it's for every hydraulic tech who heard of this term but is not entirely sure what it means. Please note that one part of this article should be taken seriously, while the other should not!

Over the years of my 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 field experience, know that every assistance call or a troubleshooting scenario has two sides to it - the technical and the psychological. The technical side is nothing more than "bolts and parts" behind the problem and normally doesn't constitute that much of a challenge for a skilled and properly equipped technician. However, the psychological or human-related side can be most problematic, because behind every hydraulic problem there's always an individual (the owner, the operator, the service mechanic or engineer, the-guy-who-is-in-charge, etc...) or a group of individuals, whom you will have to "sell your opinion to", convincing him/them that a certain course of action must be taken in order to address the problem. No matter how good a technician you are, if you are a poor "convincer" - the client will look for a second opinion somewhere else.

You may rightfully ask me here: "But what the hell does all this have to do with the hysteresis?!" Bear with me for another minute, please, - I am getting there...

Now, this very interesting matter of human relation skills that every hydraulic troubleshooter must master is a topic that can be extended into a separate and lengthy article, or even a book, and is not going to be discussed in this post, but one thing is certain - in order to gain the client's unconditional trust, and convince everyone that the presented diagnosis, opinion or solution is correct, what every technician must do is to look confidently competent no matter what!

That's right - confidence is the key to looking like a skilled professional whichever might be your area of expertise. Make a doubtful face - and you are as good as dead! This is why every experienced troubleshooter subconsciously develops a series of psychological tactics aimed at looking ACAP (As Confident As Possible) in the client's eyes.

Many ACAP techniques, particularly suitable for hydraulic technicians, can be named, among them the classics, like presenting sophisticated test gear with superior look on your face (digital pressure-gauges and data-loggers are the best, especially in remote and lightly populated rural areas...), or throwing a tool in the air and catching it after it has flipped a couple of times (this technique requires training and works wonders if you can catch the tool behind your back, but can also be quite ineffective when the "object thrown" is a hammer and you catch it 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 phenomena 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", "carry-over" may sound dull and everyday to you, but there's a good chance that they're new to your client, and thus can potentially skyrocket your credibility and 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 still against the use of special terms without understanding or out of the context. But - why the "hysteresis"? Well, it's because this one seems to be one of the hardest ones, for some reason. I confronted many mechanics with this term in the past, and more often than not got stumbled faces and silent replies, you know, like - "What did you just call me?" And yet, understanding the phenomenon of hysteresis is very important, especially when you work with hydraulic pump and motor controls. This is the main reason why I, perhaps in a slightly satiric manner, allowed myself to make this attempt to enhance the technical vocabulary of those, who picture a bearded face whenever they hear the word "hysteresis".

A little bit of history now - the term hysteresis is derived from an Ancient 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 to describe certain magnetic properties of metals, but later on, this term began to be used to describe "lagging behind" of any system's response to an input. The "book" definition of the term would be "the lagging of the effect behind its cause".

Allow me to illustrate with a hydraulic example. Imagine a 100 cc pump equipped with an electro-hydraulic proportional displacement control. By increasing the solenoid current you will get to the half displacement - 50 cc - at, say 400 mA, however, if you increase the current to the max. displacement, and then dial it back to the 400 mA, you will see that the displacement of the pump is now 53 cc and not 50 cc. The same input - 400 mA - resulted in two different outputs - 50 and 53 cc! That's hysteresis for you! And in this particular case, you could say that this displacement control has hysteresis of (53-50)/100=3%.

Any hydraulic control - whether it is pressure, flow, position, torque, displacement, or anything else - has hysteresis, and will produce two different outcomes from the same input value, depending on whether the input is 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 becomes really important to take it under consideration.

With hydraulic controls things are simple - the best hysteresis is no hysteresis at all. Why? Because we want our controls to be predictable. But since the ideal zero-hysteresis controls only live in fairy tales, we want our controls to be predictable enough for whatever it is that they are supposed to be doing. In general, the hysteresis of modern hydraulic controls is perfectly suitable for virtually any application you can think of. Still, normal wear of parts can increase the hysteresis to unacceptable levels, which is why it is a good practice to evaluate it when testing overhauled pumps and motors equipped with "aged" control modules, and then use common sense to decide whether it's acceptable or not.

Take, for example, a hydraulic generator drive that must provide a stable flow to the hydraulic motor to maintain the current frequency stable. One possible solution for such a system would be an open center load-sensing pump and a fixed orifice. After overhauling such a pump, extra care should be taken to evaluate the hysteresis of the control during the testing phase to make sure that it is still suitable for the application. However for a human-controlled hydraulic system, like a forest crane, a slightly higher hysteresis of the control would hardly be noticed, and therefore would be perfectly acceptable.

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 phenomena in a system and help find a "plan B" to make things work.

It should be noted that in the case of hydraulic controls the hysteresis level is rate-dependent, which means that it will change with the rate of the change of the input signal. I can't think of a system that had such a dynamic input coupled to a high precision output requirement that this would matter (not from my service hystory anyway) but still - it's an interesting fact to be aware of.

The main causes for hysteresis in mechanical control systems are free-play/backlash (e.g. in feedback links), friction, and wear of spool/sleeve assemblies. There are other causes, but from a practical point of view, the first two are the only ones that you can do something about.

Let us recapitulate: