Using Hydraulic Systems for precise postioning

[zipfactor]

Hi All,

First time poster, long time lurker.

I have an application where I need to have a high force generating motion control system, and after a bit of research I've landed on using hydraulics for the application. I've had some design experience with fluid power, albeit mostly in a university setting classroom/lab.

The application is moving a platen up to a fixed plate for a pressing application. The fixed plate will have a load cell which is used as feedback for whether more force needs to be applied, or less. I need to generate around 24,000 pounds of force, with positional control of at least .005" or better over a 8" stroke. I looked at using a hydraulic cylinder, but the size required would be greater than a 4" bore. This lead me to look at hydraulic motors. It looks as though I could attach the motor to a leadscrew, and the leadscrew would be connected to the platen. Ok, so now I potentially have a hydraulic power source selected....

Now onto the control of the hydraulic motor, this is where I am in unfamiliar territory. I've seen that electro-hydraulic servo valves can be used in conjunction with a feedback mechanism (LVDT, MDT/LDT) and a motion controller. Is it really this simple to get high accuracy for my application? Am I overlooking a less complex alternative?

Thanks in advance!

EDIT: I should add that after the pressing of the part occurs, the platen needs to return to it's original position within .005" or better.

EDIT #2: I'm seeing that a leadscrew is not a very suitable choice as the life span would be very short. Assuming this, a hydraulic cylinder may be a better choice. Are my positioning requirements feasible with motion control of a hydraulic cylinder?

 

[xtal01]

I am definitely no expert in hydraulics (that is obvious from my posts) but I would think using hydraulics for positioning would be difficult. I worked on the electronics portion of a system that used pneumatic cylinders for positioning and it just never worked correctly. Most of it was attributed to the compressibility of air. This may not be a problem with hydraulics since for "practical" purposes, I think we can consider hydraulic oil as incompressible.

On the other hand, I have used linear actuators for this purpose.

I just did a quick search and the first hit I got seems like it might be suitable for your application.

Here is the link:

http://www.edriveactuators.com/catalogs/eliminator_web.pdf

From a quick skim over the catalog, it looks like you can get a force of 25,000 lbs from a "stock" unit.

Stock accuracy is .002 in/ft with an option for .0005 in/ft.

Stock backlash is .003 with zero backlash an option.

Repeatable position accuracy is .0005

The B10 life at 24,000 lbs looks like 8 million inches

Hope this helps ..... Mike

 

[zipfactor]

I've read on a few sites that use of a servo valve with a hydraulic cylinder allows for positioning as fine as .0005". I'm interested in the actuator as that may work as well, dependening on the cost of course.

Thanks for the input!

 

[PNachtwey]

A hydraulic cylinder would be perfect for this unless there is some reason a larger bore cylinder can't be used. Hydraulic motors have lots of friction and leakage which makes them more challenging to control with precision. A larger diameter cylinder would be relatively easy to control. Use a Balluff or Temposonic MDT rod for position feed back. 5 micron resolution is standard but 2 and 1 micron rods are available. The load cell must have a fast amplifier in the 4KHz to 8KHz range. The valve shouldn't be to big. Just big enough to meet any speed requirements. Other than that a small valve with zero overlap make it easy to control the force.

Do a search for my company and add the word press. You will see we have lots of experience in pressure force control. This application should be easy.

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[Compositepro]

A load cell is a waste of money unless you need very precise force control. The advantage of hydraulic cylinders is that the oil pressure controls what the force is. In fact, there are things called hydraulic load cells. You do need a pressure sensor.

 

[PNachtwey]

Compositepro, I agree a load cell is not required if only position control is required. If the hydraulic system can exert enough force the position control will press until the desired position is achieved. However, sometimes force feed back is used in pressing applications even if it isn't used for closed loop control. In powdered metal presses one often presses to position but monitors the force required. If the final force is not in the final range then the part density is either too high or too low.

zipfactor should tell use what his application is. I/we have lots of experience in many different industries with position/force control or limit.
One thing the OP should be aware of is that one can control force or control position but both can't be controlled at the same time. However, one can limit position or force at the same time.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[zipfactor]

Thanks for the feedback guys, great info! Using a large bore cylinder will be no problem as I am in the system design phase, so any cylinder really can be used. The only issue I have with the hydraulic system is the potential for leaks. What would be a reasonable expectation for leak free operation without maintenance? Also, could the pump unit be located in an enclosure to reduce the noise from the pump?

The application is pressing composite layers together for an automotive company. I would say that position control is the priority for the hydraulic system, and force could be managed by the load cell. Does this arrangement sound feasible?

Thanks again!

 

[zipfactor]

One quick additional question....what rough ballpark am I looking at for a hydraulic solution like this? $10k?

 

[PNachtwey]

What leaks? There may be leaks when first starting up the system but after that the smooth motion of a motion controller minimizes pressure spikes that will cause leaks in the future. Yes, the HPU can be enclosed. Our HPU is enclosed in a wooded box with egg cartons stapled to the inside of the enclosure to absorb sound. Our hydraulic systems are made to be quiet and for research and training.
The hydraulic system and controller can probably be bought for 10K. The programming and installation would probably cost more unless you are going to do it.

Your system seems feasible but why not tell us what you are trying to do. We have been in business controlling hydraulic systems for decades and about 100K axes of control. There is little we haven't seen before.



Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[andyhyd]

$10K sounds a little on the light side price wise for the control and accuracy you require. If you plan on using servo valves and LVDT positional feedback on the cylinder your $10K will spent there alone before you have purchased the HPU.
If you know your cylinder sizes and the required force and speeds you can work back from these to calculate your power requirements. Rule of thumb I allow ~ $1000/Kw to build the required power unit on a one off installation.