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Cylinder position

Cylinder position

Cylinder position

(OP)
I am trying to get ideas on how to sense when a equal acting hydraulic cylinder bottoms out on extend and retract. Do you have any ideas? I wanted to do this without limit switches.

RE: Cylinder position

What kind of indication do you want?  Visual?  Audible?

Ted

RE: Cylinder position

A MDT rod will tell you where the piston is at all times.
The MDT rod is mounted inside the cylinder.

RE: Cylinder position

Depending on the system you could sense the pressure spike. ISZ

RE: Cylinder position

Quote:


Depending on the system you could sense the pressure spike. ISZ
Yes but then the problem shifts to how does one differentiate the pressure reading.  This can easily be done with a controller.

Lets do this again.

Quote:


I am trying to get ideas on how to sense when a equal acting hydraulic cylinder bottoms out on extend and retract. Do you have any ideas? I wanted to do this without limit switches
Why? The answer makes a big differents the answers.
1. are there physical limitations?  My thoughts.
2. is fun4sparkie too cheap for limit switches.  In this case a MDT rod will not do.  Neither will a controller to differentiate the pressure.
3. Are there safety hazzards where the one can't have electricity in the operating enveironment?

This should have been provided by fun4sparkie to keep this from being a guessing game.

 

RE: Cylinder position

(OP)
First, thank you for the responses. Second, sorry if I didn't give enough info.

To try and answer all questions, we have a special cylinder already ordered, we can not have a MDT rod put in at this point (we asked, they said not on this cylinder because of the lugs), we are not cheap, we don't have the alot of space to use limit switches, possibly absolutely no space on one side. Our cylinder not only moves in a linear path, but it is able to twist up to 3 degrees at anytime during its stroke. We want the machine to stop/machine operator to know when the cylinder has bottomed out on either ext/ret. positions because we could overturn at that point. I was just trying to come up with something hydraulic to control this, but if it isn't possible, then I will work something else out.

Thanks!
fun4sparkie

RE: Cylinder position

Check out cylinder position sensors. Turck makes one that mounts in the head end or cap end of the cylinder.

http://www.turck-usa.com/assets/B1005_F07-F08.pdf

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

RE: Cylinder position

Can you use a linear cable rotary encoder?  Mount it on one end and read zero at one bottomed position and full stroke at the other bottomed position.

http://www.encoder.com/modellce.html

Peak hydraulic pressure could not differentiate bottoming or stalled load.  The same would be true with a flow indicator.

Ted

RE: Cylinder position

I did not say that pressure transducers with a controller had to be used. My intension was to use pressure switches. Here is but one supplier for reference.
http://www.ifmefector.com/ifmus/web/padv02_20_10_10.htm

For this to be feasible though
1) The working pressure must be lower than relief pressure so that the switches are not tripped at mid stroke.  
2) In a similar manner there must not be high dynamic loads that would hit relief pressure.
3) This would not work for static state, as the pressure will probably decay enough for the switch to reopen.
4) This would not tell you when you are at 98% stroke.
5) ...others things I couldn't think of at the moment...smile

ISZ

RE: Cylinder position

You can install a valve in the piston to dump pressure to the other side when bottoming out. This is used in many dozer blade cylinders.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

RE: Cylinder position

Go the encoder

RE: Cylinder position

Ed's post made me think, why do you want to know if the cylinder is bottoming out?  Do you just want to sense it or do you want to avoid it.  I think this is important information and affects the answers you will get.

RE: Cylinder position

(OP)
I want to sense it. I need to know when it happens so that the machine operator will not keep torquing the machine because it will cause the cylinder to fail or something else on the machine. I don't care if it bottoms out, I just need to know when.

Thank you all for your help!

RE: Cylinder position

To make the Build-Up or Loss of Pressure to indicate when a cylinder has bottomed out Fail Safe you can drill a small port in the cylinder tube that will be pressurized as the Piston passes it at the end of stroke. Sense the passing with another Pressure Switch and use an "AND" circuit to show loss of pressure on one side and positive pressure on the opposite side of the Piston.

This would work well with a cylinder that has Piston Ring Seals but may not be workable with Resilient Seals. Your cylinder manufacturer could advise.

These type ports are sometime installed as Air Bleed Ports in critical cylinder applications and are an option on most cylinder manufacturers price list.

Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

RE: Cylinder position

Port relief valves will limit over pressurization of the seals and limit structural failure. The piston valve will stop the cylinder from bottoming out at either end. If you are trying to signal the operator when the end of stroke is reached a limit switch and light will be the simplest most reliable and least expensive. If you want to collect data about operation that is a different subject.

If bottoming out the cylinder will cause cylinder failure you need a different cylinder vendor.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

RE: Cylinder position


In driving some cars with power steering the relief valve does a lot of squealing at lock (maximun travel). Most people who want their cars to last will immediately stop forcing the steering wheel at that point.
Purchase the loudest relief valve you can.

RE: Cylinder position

Peter's Q I think is key: Does it matter? Is there a relief? If this is a pressure compensated pump, who cares if it bottoms and stays stalled? The structure and cylinder should be designed for that force, won't hurt the cylinder or hydraulics. One big advantage of hydr, it can be stalled without damage.  Now, the electric actuators, smoke comes out. Cylinder, just stall it all day.
   but if there are other reasons, I'd go with limit switch externally, prox switch internally, or Temposonics installed in parallel to the motion somewhere outside of the cylinder (if you are looking for indication, not trying for closed loop control, a bit of slack motion won't matter)

kcj

RE: Cylinder position

You don't need to sense bottoming the cylinder.  Once the cylinder bottoms it can no longer deliver force to the machine.  It stops, it's done, it's stalled.  All that is left is a pressurized system, a squealing relief, and a cylinder with no remaining travel.

Ted

RE: Cylinder position

Hydtools is right. The end of the cylinder is now opposing all force so no more can be applied to the load so is a sensor necessary?  Not to keep the machine from damaging anything else.  The damage to the end of the cylinders/piston/rod has already been when the actuator bottomed out.  That is the moment where there is the most stress on the cylinder/rod/piston.

RE: Cylinder position

I too have a need to know when one of my eight cylinders has reached the end of their travel. This is required for the operator to begin the next step.

I would use a pressure switch set near the supply pressure on the cylinder extend/retract lines. When the light is off (pressure low/switch open) the cylinder is moving, when light is on (pressure high/switch closed) the cylinder is at the end of its travel. Note that pressure low is relative and is < system pressure.

My actual system is a little more sophisticated in that I have a separate switch for each line, and I use a double pole pressure switch wired to a red light for transit and a green light for complete. In addition a pressure gauge may be observed by the operator who looks for a dip.

RE: Cylinder position

horsefeather, is it possible in your system for the cylinder to be stalled mid-stroke?  This would give a false pressure indication that the cylinder reached full stroke.

If stalling mid-stroke is not possible, then a pressure switch will work for you.

Ted

RE: Cylinder position

Anything is possible.

My particular application is 3000 psi with a 28" travel and is used to crimp and shear oilwell pipelines.

Watching for the pressure dip on a gauge is how the operators determine closure now. The system is visually inspected (exercised) for correct operation before operations begin and the pressure drops are noted and signed on a chart recorder. This provides a reasonable assumption that the system will work in the vent of an emergency.

If absolute certainty is required that a piece of iron must be at a particular position, I agree with previous posters that a position sensing sensor (micro-switch, LVDT, etc) be used.

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