Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Pew Matics - High Drawl Icks and synchronized lifting. 3

Status
Not open for further replies.
itsmoked

itsmoked

Electrical
Feb 18, 2005
19,114
So, I'm faced with lifting an object(lid) using either air or water-based hydraulics.
Do you guys have any ideas how you'd lift this thing -evenly- with 3 to 5 gas/water cylinders? (Approximately 2" per sec.)

Are there any companies that have O.T.Shelf servo controllers for this type thing or should I gab a PLC and try to roll my own? I'm sure Gunnar will suggest an "Arduno". LOL



Keith Cress
kcress -
 
Replies continue below

Recommended for you

Sort by date Sort by votes
An 'ard un? Why not? [2thumbsup]

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
Upvote 0 Downvote
I would definitely not use air. Stick-slip, jamming and such baddities. Water much easier, especially if the load on the cylinders isn't perfectly evenly distributed.

I could use servos but, if a slight on/off effect is tolerable, I would have a set of small valves that add 10% each to the flow and then set the slave cylinders at 90% of the master. That way, you will have 90, 100 and 110% speed on the slaves and it is then an easy (and cheap) matter to make the slaves follow the master.

To control four or five cylinders that are connected to the same load (perhaps also rigidly connected) will give you endless problems with interaction between them if you use servos. The simple master/slave configuration with compensation is much easier and safer in the long run. Cheaper also.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
Upvote 0 Downvote
Hi Gunnar,
I'm not sure I'm following what you mean with 'slave' and 'master' in this context. Let's say four cylinders. I'd use 4 'big' valves and 4 smaller parallel valves? I'm not seeing how I get 90% 100% and 110% with just two valves.

Keith Cress
kcress -
 
Upvote 0 Downvote
Let me draw an electric diagram. Stand by!

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
Upvote 0 Downvote
Unless you're doing this repeatedly, I don't see the ROI on programming something from scratch. It's not like you'd have gobs of time to test and tune it, is it?

TTFN
faq731-376
7ofakss

Need help writing a question or understanding a reply? forum1529
 
Upvote 0 Downvote
Sorry, in this case it seems that words say more than a thousand pictures. So:

1. All cylinders receive the same pressure. But you choke the separate cylinders differently. The Master shall get 10% more flow (less choked) than the Slaves.
2. There are two small extra valves for each Slave and these valves open for one 10% flow each.
3. For each Slave, there is a simple mechanism that checks to see how the Slave is doing w/ regard to the Master. This could be a sophisticated linear transducer (Tempsonic, Balluff etc) or a photocell or a mechanical contraption that actuates two switches.
4. Start with one +10% valve activated. All cylinders will then receive 100% flow and should move synchronously. But we know that they won't.
5. Now, one cylinder moves faster than the Master. The secret sensor (see item 3) then switches the corresponding +10% valve off so that cylinder moves at a reduced speed until it is level again.
6. If one cylinder moves too slowly, same thing but different takes the cylinder home level (activates the other +10% valve, I didn't have to write that - agree?).
7. Don't forget to stop at end-of-travel (didn't have to write that either, but wanted to).

There are a few things to think about. First, is the somewhat jerky movement accepted - think about the violent oscillating movement that could be the alternative! Second, the contraption and the secret sensor may not have been invented yet. The 10% can probably be reduced to 5% or less. That will make the already small jerkiness even smaller.

Good luck! I think that I would like to build one myself. Sounds like an interesting thing to do. But, if I could have it my way, I would use ball screws and synchronized stepper motors. Much simpler and everything COTS.


Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
Upvote 0 Downvote
Electric 'cyinders' as Gunnar says.

Second choice, one gear pump per cylinder (they can be purchased 'ganged'), all driven by one electric motor.



Mike Halloran
Pembroke Pines, FL, USA
 
Upvote 0 Downvote
Gunnar; That did the trick. I totally get it now. Thanks for that nice "picture".

IR; I'm not actually sure yet whether this is a single research job or expected to be a product. But I don't see any alternative to 'rolling my own' because I'm working with an enormous pneumatics/hydraulics company and they're asking me to help them provide the controls. i.e. they don't know of anyone that does this.

I sure wish this was four stepper run screws!! Unfortunately the environment demands pure water or inert gas. I think any screws would need some sort of verboten grease.

Mike; I will look into your 'ganged gear pumps'. That sounds almost control-free.

Keith Cress
kcress -
 
Upvote 0 Downvote
"Second choice, one gear pump per cylinder (they can be purchased 'ganged'), all driven by one electric motor."

Why not one stepper motor per pump, Mike? How would you compensate for one motor having a bit more volume per revolution than the others?
 
Upvote 0 Downvote
Hydraulic flow dividers which ensure equal flow to multiple cylinders are one option.

There are some smart guys in forum1083: some are old-school and some are knowledgable about high-performance servos. Even moderate performance servo systems turn complex and expensive pretty quickly, so unless you have a lot of money to throw at this I think I'd stick with pure hydraulic. If you want performance then Moog are probably the market leaders with a price to match, and yes, it was the same guy who made the synthesiser.
 
Upvote 0 Downvote
Ready for another word picture?

OK. If the cylinders had the same volume on the rod and rod-less end, you could simply put them all in series. But they have not. So they will not move synchronously if you do that.

But there's a trick. (You need to think out of the box to follow this).

Connect the first cylinder as usual. Then connect outlet from cylinder A to outlet of cylinder B. The water volume trapped in rod end of A then acts on rod end on B. The result is that they both move practically the same way. Continue connecting the "wrong" way and you will have a number of cylinders that move in synch. Water doesn't compress, so it works quite well. With a gas, it wouldn't work at all.

You will have to turn half of them upside down or use a rod to transmit the movement from the downwards pointing cylinders to the lid.

You should also make sure that you can open a small by-pass when the lid is closed. Doing that will synchronize them in their home position.

Not much programming. And not many valves. Sorry.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
Upvote 0 Downvote
The specification of water as a hydraulic fluid suggests that the object being lifted is enormous. There is some prior art, as in grandstands that rise into projection spheres, at Disney World and such.

A hydraulic flow divider is just a bunch of gear motor/pumps coupled by one shaft. I think it's simpler to just power the shaft directly, rather than adding one big pump to drive the divider. The same motor is required.

Not known at this time, and probably significant to choosing a solution, is the allowed phasing error.

Initial phasing error can be dealt with by dump valves at the cylinders, to bleed the system just before lifting. That assumes mechanical stops top and bottom.

Differences in pump displacement and internal leakage can be dealt with by means of giant anti-roll bars. Same problem exists with flow dividers. If the bars are big enough and stiff enough, the flow dividers are not required anyway.

Also not known is the budget. Keith's involvement suggests that it's not large, and that they don't want any sort of mechanical solution, is as in flow dividers or giant torque bars.

Which leaves some sort of linear transducer and control valve per cylinder, with a microcontroller or similar watching the transducers and twiddling the control valves to keep the transducers in phase.






Mike Halloran
Pembroke Pines, FL, USA
 
Upvote 0 Downvote
Doesn't need to be "enormous". The water (or inert gas) was specified for Ex or environmental reasons. Smoked even thinks that grease for a ball screw cannot be used.

How enormous is this thing, Keith? Is it the lid of a nuclear reactor?

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
Upvote 0 Downvote
Good stuff here guys thanks for your thoughts one-and-all!

It's not toooOOOOo big. It's a lid to a reaction chamber. It weighs many hundreds of pounds. Couple of feet across (meter). With regards to your fingers, it would be enormous, but otherwise it's not really enormous, it's the atmosphere or rather lack of allowable contaminants in the atmosphere that dictates the likelihood of water.

Speed; 2" (50mm) per second. Aiming for 1/8" (3~5mm)level maintenance during motion.

Keith Cress
kcress -
 
Upvote 0 Downvote
Simplest thing, if you don't have to control the trajectory, is a triple chain sling under an engine hoist, converted to use water. Just the one single acting cylinder, no complications.

If you do have to control the trajectory, then Gunnar's idea of series connected cylinders should work nicely. ... provided that you add enough little valves in the right places to fill and bleed them.



Mike Halloran
Pembroke Pines, FL, USA
 
Upvote 0 Downvote
Following the KISS principle I would try to utilize one large lifting cylinder. Can you use a davit arrangement where the lid is lifted a few inches and then swung out of the way? (A variation on Mike's engine hoist suggestion.)

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Upvote 0 Downvote
Mike and Bill; Neither of those would work here. The chain thing is just too wobbly and non-centering and the rotate out-of-the-way is too space problematic. Thanks for suggestions though. :)

Keith Cress
kcress -
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

VLFit
  • Locked
  • Question
Low PD High Voltage Transformer Design
Replies
3
Views
113
Gr8blu
Gr8blu
freetown
  • Question
Regadring thermocouple sensor and calibration 2
Replies
8
Views
2K
waross
waross
SilverfoxUK
  • Locked
  • Question
Pre ATEX and Exe
Replies
0
Views
57
SilverfoxUK
SilverfoxUK
v6racer
  • Locked
  • Question
Arc Flash and Short Circuit Studies 1
Replies
8
Views
139
Electic
Electic
A
  • Question
Difference between Final temperature and Fusion temperature of copper in earthing calculations
Replies
2
Views
1K
jghrist
jghrist

Part and Inventory Search

Sponsor

Back
Top Bottom