Looking for info on improving the stability of hydraulic actuators

Hello

I'm writing a thesis in industrial engeneering and I need info/documentation on ways to improve stability in a hydraulic actuator. Stuff like working at higher pressure or improving the control system.

I can't seem to be able to find much info on the net, or in my local library for that matter, much less technical documentation.

Any pointers will be appreciated.

You came to the right place

http://www.deltacompsys.com/applications/index.php

I'm writing a thesis in industrial engeneering and I need info/documentation on ways to improve stability in a hydraulic actuator.

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Why just improve stability and why does stability need to be improved? Hydraulic systems can hold position very accurately and even move accurately.

Stuff like working at higher pressure or improving the control system.

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It really does seem like you are fishing. Higher system pressures provide more force and more linearity but not stability.

Control systems make a big difference. That is what we sell.

I can't seem to be able to find much info on the net, or in my local library for that matter, much less technical documentation.

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There is plenty of documentation.

If you have specific questions then ask. I can probably point you in the right direction. The main things you should be interested in is the phsyics of hydraulics. This includes things like calculating the natrual freqency. How pressure changes when compressed or fluid is added. How servo valves work.

There are some hydraulic books you can buy written by Jack Johnson or George Keller that are very good. Vickers also makes some good hydraulic manuals. I would visit this site:
Hydraulics and Pneumatics.
http://www.hydraulicspneumatics.com/
They sell the books and also publish a magazine called 'Hydraulics and Pneumatics'. You should find a lot of interesting articles in the article archive.

I am mostly interested in hydraulic motion controllers. You can get the set up software and on-line manuals for our newest hydraulic motion controller here:
http://www.deltacompsys.com/dloads/downloads.php?category=rmc70&mode=navigation
The manual is only 684 pages long but if you skim through it you will see all the things and applications that we have considered. Fortunately one normally only need to read about 5% to get any one application done. It is just that every one has a different 5% and all must be covered.

The page that is most often downloaded off our site is the 'DOs and Don'ts' article on this page.
http://www.deltacompsys.com/applications/technical.php

Here is a directory of movies of hydraulic systems.
ftp://ftp.deltacompsys.com/public/movies
The FTP site is slow so have patience.

I think you have enough to digest for a while.

why higher pressure doesn't provide stability?

hi, first i want to thank you for the quick answers and good directions you gave me.
but i didn't understand why you told me that higher pressure doesn't provide higher stability. if you give more pressure to oil inside the actuator, the oil become more rigid so that the less compressibility let the piston become more controllable, isn't it? it should be something like a spring less elastic, may be the frequency improves but with less bandwidht. please correct me if i'm in error but i studied industrial engineerig and i'm not so goog in mechanics

s_masiero said:

hi, first i want to thank you for the quick answers and good directions you gave me.
but i didn't understand why you told me that higher pressure doesn't provide higher stability.

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Pressure does increase the bulk modulus of oil effect is so small to be negligible. Other factors are more important like temperature and the amount of air that gets into the oil. In any case the controller can be tuned to accommodate the bulk modulus of oil. It is the changes in the bulk modulus of oil that causes problems and it is air and temperature that causes the bulk modulus of oil to change.

if you give more pressure to oil inside the actuator, the oil become more rigid so that the less compressibility let the piston become more controllable, isn't it?

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I like to think it makes the 'spring' on either side of the piston stiffer. Again the controller can be adjusted to compensate for the bulk modulus of oil. By your thinking it would be better if the bulk modulus of oil is much higher but that would make controlling pressure almost impossible.

Where:
p is the pressure
v is the volume
B is the bulk modulus of oil

What you are suggesting is making B, the bulk modulus of oil, bigger will make the system more controllable. It just means that to change the pressure / force by a given amount requires less oil which means the valves must be much better at controlling the flow of small amounts of oil and still be able to flow the large amounts to actually make the piston move. In other words, increasing the bulk modulus of oil just moves the control problem from the cylinder to the valve. I think the bulk modulus of oil is just fine as it it.

it should be something like a spring less elastic, may be the frequency improves but with less bandwidht. please correct me if i'm in error but i studied industrial engineerig and i'm not so goog in mechanics

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This doesn't make sense. If you improve the natural frequency you will increase the open loop bandwidth. See the spread sheets on this web pages.

http://www.patchn.com/hydrauli.htm

Stability is controlled by the controller and where poles and zeros are placed. The location of the poles and zeros is specified by the tuning parameter.

If you look at the equation for natural frequency of a cylinder you will find that pressure is not part of the equation.

ok, but now i feel a littel confused...! let me explein the whole problem:
I have to write a chapter of my thesis and talk about the STABILITY on a hydrulic actuator. i shoud write:
-why stiff hydrulic are better
-which are the causes of instability in a hydraulic system
(and in this section i would talk about stiffness of oil, stiffness of pipes, entrained air into oil)
-ways to improve stability in a hydraulic system
(in this section i would talk about:-working at high pressure
-improve the control system)
-what happens if i work at high pressure
(in this section i wolud talk about:-design of the pipes
ways to increase forces)
-what happens if i act on the control system
-why it could be the better way to improve stability
-use of robust control
-what happens if control is not enough robust


do you think these sections are enough for the chapter? For my work i need some help on develo these arguments. do you think you can help me?

s_masiero said:

ok, but now i feel a littel confused...! let me explein the whole problem:
I have to write a chapter of my thesis and talk about the STABILITY on a hydrulic actuator. i shoud write:
-why stiff hydrulic are better

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If you are positioning a mass by push or pulling a spring connected to the mass then would you want to have the spring stiff or weak? You want it stiffer. Systems with a higher natural frequency have 'stiffer springs'.

Do you know what a spring constant is?
Do you know what the natual frequency of a mass and spring is?
Do you know how to calculate the natural frequency?

ftp://ftp.deltacompsys.com/public/PDF/Mathcad - Natural Frequency.pdf

-which are the causes of instability in a hydraulic system
(and in this section i would talk about stiffness of oil, stiffness of pipes, entrained air into oil)

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What is your definition of stability? Mine is the tradition control theory version where pole move to the right hand plane or the system as a gain of one with 180 degrees of phase lag.

These are control issues.

[/quote]
-ways to improve stability in a hydraulic system
(in this section i would talk about:-working at high pressure
[/quote]
Again I don't thingk stability is the right word. I think you are look for natural frequency or stiffness.

-improve the control system)

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Improving the controls will make a big difference but it is hard to compensate for a poorly designed hydraulic system.

-what happens if i work at high pressure
(in this section i wolud talk about:-design of the pipes
ways to increase forces)

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Higher pressures do not increase the stiffness or natural frequency. Just the overall gain. Which equation below indicates that?

-what happens if i act on the control system

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??? I don't know what you are asking here.

-why it could be the better way to improve stability

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The motion controller does contribute to what I call stability by placing the poles of the system as close as possible to the negative real axis. This results in a critically damped or underdamped response.

-use of robust control

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Buzz words. Not practical yet. At least not for commerial purposes.

-what happens if control is not enough robust

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The world begins to shake.

do you think these sections are enough for the chapter? For my work i need some help on develo these arguments. do you think you can help me?

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You are trying to take years of info an condense it into a chapter? Will I get the master's degree?

What is the VCCM formula?
What is the formula for natural frequency?
What is the formula for oil flowing through a valve?
What is the formula for the change in pressure as a function of compressing or adding or removing more oil?
What is the operationnal envelope of a hydraulic system?
What is the forumla for a general closed loop transfer function?
When do systems become unstable?
What is a gain margin?
What is a phase margin?
What are poles and zeros?

It seems to me you are not prepared to answer these qestions. You must understand these equations. I have pointed you in the right direction. Now it is time for you to do some research and simulations.

If you insist on saying that raising the pressure will make the system more stable then you will get a poor grade.

hi! thanks for your answers, give me a few days to study the things you've pointed to me and then i will make you more accurate questions. during university i never studied hydraulics so deeply and i want to improve my knowledge with this work. but don't worry, you are giving me a lot of help and you will be one of i will dedicate my thesis to!