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Pneumatic Cylinder Control

01/28/2007 2:44 AM

Can the piston of pneumatic cylinder can be positioned in its intermediate position rather than the usual extremities? What type of valves can be used? The postioning should be done while lifting load. Is that possible?

What can be done for smooth operation of the pneumatic cylinder like hydraulic cylinder?

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Guru
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#1

Re: pneumatics cylinder control

01/28/2007 7:45 AM

Yes, the piston can be positioned in any intermediate position. You need to use a linear potentiometer to sense the linear position and use a 5/3 way solenoid valve for control. Some manufacturers even sell these units as a set. Check out SMC Pneumatics. I believe Festo also sells these. The units accept a 4 to 20 mA signal as the positioning signal.

To smoothen out the operation, you might need to use pneumatic speed controllers. These basically slow down the movement of the cylinder for better control.

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#32
In reply to #1

Re: pneumatics cylinder control

01/31/2007 8:40 AM

Can we use flow control valve for pneumatic speed control.what can be used to reduce the preassure of inlet air to the pneumatic cylinder.

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#34
In reply to #32

Re: pneumatics cylinder control

02/01/2007 1:35 AM

The pneumatic speed control valves are actually two devices in one. The first device is a check valve which allows the air to flow almost unrestricted into the cylinder. When the air flow is in the other direction (coming out of the cylinder), the air passes through a variable restrictor.

A simple flow control valve does not have the check valve so it restricts in both directions. The speed controller allows you to individually control the rate of extension or retraction of the cylinder. A flow controller will control both. So which one you use, depends on your application.

In the application made by the original poster, the vertical load will affect the speed of the cylinder making it fast in one direction and slow in the other. If you want to equalize the speed, you need the speed controller.

I'm not sure if others use the same name for the speed controller. There may be other names for it but that's the one we've used for as far back as I can remember.

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#2

Re: pneumatics cylinder control

01/28/2007 10:48 AM

Vulcan's got it summed up, here.

The only thing I would add is to point out the different names for the two types of control system. A cylinder running from end to end (or anything else, for that matter, only being stable in one of two states) is a "bang-bang" control system. The other sort, where you can control its position between the two limits, is a "Proportional" control system. There are probably other names about for the two systems, but these describe them quite well.

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Associate

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#4
In reply to #2

Re: pneumatics cylinder control

01/29/2007 1:28 AM

Yes Possible.

1. Provide a Limit Switch at the desired position. When the Power Cylinder reaches the desired position the Limit Switch actuates. The Limit Switch contact can be suitably wired to the Solenoid Valve that controls the pnumatic air supply to the Power Cylinder and such air supply is cut off to stop the Power Cylinder Power Cylinder at the desired position .

OR

2. Provide a Positioner to the Power Cylinder. With positioner in service the Power Cylinder will be in the minimum position or fully open or any other any other position corresponding to input signal (i.e) 3 to 15 psig. Now find out the proportional signal corresponding to the desired position. Inject that signal from a signal generator to the Positioner and that will smoothly position the Power Cylinder at the desired position.

R.Thiyagarajan

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#3

Re: Pneumatic Cylinder Control

01/29/2007 1:21 AM

If the cylinder is required to stop on the same intermediate stop then a positive stop coud be added actuated by a small cylinder and the main cylinder will stop at that intermediate stop.

If the middle possition is changing on every stroke then an intermediate oil reservoir should be addae on both the inlet and out let of the cylinder making the system hydropnumatic and adding a flow controle valve in the the return line along with a direction controle valve to stop the cylinder at any intermediate stop but a sensor and feed back electronics are required to complete the loop.

A hydraulic check is usualy added to a main pnumatic cylinder to reduce the feed of the pnumatic cylinder the hudraulic check also house a flow controle valve and is adjustable, it could also adjusted to provide quick traverse in the start of the stroke and then engaged to preform the feed.

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#5

Re: Pneumatic Cylinder Control

01/29/2007 2:17 AM

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RGDS

PRAKAASH

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#6

Re: Pneumatic Cylinder Control

01/29/2007 4:40 AM

It all depends on what you want to achieve: force and precision or only force with "a" stopping point.

Pneumatics are not suitable for precision, use hydraulics for that application.

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Anonymous Poster
#7
In reply to #6

Re: Pneumatic Cylinder Control

01/29/2007 5:13 AM

CAN THE PNEUMATIC CYLINDER BE STOPPED AT ITS INTERMEDIATE POSITION WHILE LIFTING LOAD NOT ACCURATELY.BUT AT SOME PREDETERMINED POSTION BY SOME EXTERNAL MECHANICAL RATHER THAN BY THE PNEUMATIC CYLINDER ITSELF.

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Anonymous Poster
#22
In reply to #6

Re: Pneumatic Cylinder Control

01/30/2007 11:46 AM

Force with "a" stopping point in intermidiate position.

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Anonymous Poster
#26
In reply to #6

Re: Pneumatic Cylinder Control

01/30/2007 12:49 PM

How a pneumatic cylinder can be locked in case of power or rupture of a hose will they hold on the sudden surge of load,.

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#8

Re: Pneumatic Cylinder Control

01/29/2007 5:58 AM

As everyone has said, you need a 'positioner' if you want to do it automatically - this will adjust pressure either side of the piston to make it sit at the chosen point.

Unfortunately air is very spongy and if the piston can be moved by the external load then it will bounce about and become a never ending battle for the 'positioner' to keep up. Better to us hydraulics that will 'lock up' solid when at the chosen point.

But if you want a simple manual control then you need two pistons connected in series (in line). One piston will have it's port at each end connected together with a pipe via a stop valve (it could be a solenoid or manual).

This piston will be filled with oil. You will see that with the oil shut-off valve open, the oil will move from one side of the piston to the other, depending on the direction the piston is being pushed by the external force. When the oil valve is shut the piston will be locked in place.

The other piston under pneumatic control provides the power to move the load in either direction depending on your choice. You simply open the oil valve to allow movement and then you shut it to stop movement at the desired point.

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#9

Re: Pneumatic Cylinder Control

01/29/2007 6:23 AM

Air, being compressible will cause "sagging" if not stopped against a mechanical stop. As already said, if precision is not a critical issue, than it might work just fine. But if it is, than a hydraulic cylinder might be the better choice. In both cases a linear potentiometer can be used to automate the system.

Wangito

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Anonymous Poster
#10

Re: Pneumatic Cylinder Control

01/29/2007 6:44 AM

Festo sells pneumatic servo systems. This is exactly what you want. Pick the salesman's brain for all the infor you require!

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Anonymous Poster
#11

Re: Pneumatic Cylinder Control

01/29/2007 8:47 AM

The part of the circuit that is exhausting the actuator, that will be stopping it - make it as short as possible. Air is 'spongy' so make as small a volume of it as possible.

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#12

Re: Pneumatic Cylinder Control

01/29/2007 9:04 AM

Forgot to mention about the lifting part.

Depending on the weight of your load, pneumatic cylinders may work fine. If your system is mounted on a vibrating platform, you may have problems with bouncing (the proper term is "hunting"). That's when the positioner consistently over-corrects. "Hunting" for the desired position. If vibration is not a problem, then pneumatic cylinders will work. Slow and easy is the key.

We use pneumatic cylinders to lift 200kg weights and they do it smoothly. We don't position them in intermediate positions, however. We use positioners for dampers, not for lifting.

Your friendly neighborhood pneumatic salesman can give you more details.

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#13
In reply to #12

Re: Pneumatic Cylinder Control

01/29/2007 9:08 AM

Oops! Forgot one other thing. Someone mentioned a stop. Good idea.

Some cylinder models have a pneumatic lock on them. This is integral with the cylinder. You could use this to prevent the cylinder from moving once it's in position. It's an extra signal but might be useful for your application.

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Anonymous Poster
#28
In reply to #13

Re: Pneumatic Cylinder Control

01/30/2007 1:57 PM

what can be used to prevent the sudden failure of pneumatic cylinder as a safety measure which will activate automatically when hose pipe fails.

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Anonymous Poster
#14

Re: Pneumatic Cylinder Control

01/29/2007 11:46 AM

Can somebody in this discussion can tell me ,why pneumatic cylinder can"t be positioned

its intermediate position like hydraulic cylinder using an ordinary 3-position valve with 5

ports.Using its neutral position to stop the cylinder in its intermediate position .

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#17
In reply to #14

Re: Pneumatic Cylinder Control

01/30/2007 1:19 AM

What are you saying? Nobody said it can't be done. In fact everybody said it can be done!

In post #1, I mentioned that you need to use a 5/3 way solenoid valve which is exactly what you mentioned. It's a 5 ported, 3 position valve (that uses two solenoids).

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#15

Re: Pneumatic Cylinder Control

01/29/2007 3:11 PM

There are numerous ways to position a pneumatic cylinder. It is staight forward to lift and hold. Assuming the rod is up, apply pressure to the cap end of the cylinder that is adequate to lift the load. When you reach the desired position block the pressure.

A pilot operated check valve or similar valve will block and hold for an intermediate time. To start smoothly use a soft start valve in the supply line. Standard flow control valves will lower most loads smoothly.

Consider Air over Oil, where an oil reservoir is pressurized with air and drives oil into the cylinder to move the piston. This can be blocked to hold the load as with air. The non-compressible nature of oil will make your load stable. It will not sink if weight or downward force is added and will not forge ahead if the load is reduced.

As always with either air or oil, people and property should not be in harms way in the event of a failure that might allow the load to fall.

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#16

Re: Pneumatic Cylinder Control

01/29/2007 8:37 PM

Simply Cut off further air into the lifting cylinder.

It will hold the lifted load there for a few seconds/minutes--till piston rings leak from high pressure side.

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Anonymous Poster
#19
In reply to #16

Re: Pneumatic Cylinder Control

01/30/2007 8:26 AM

since air is compressible does"nt it move from the fixed position he required

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#21
In reply to #19

Re: Pneumatic Cylinder Control

01/30/2007 11:17 AM

You stopped further air entry at the point of balance of forces.

So everything will rest there.

If you pull down with some more force , it will tend to move down-but as soon as you let go,the last balance position is where load will return to. And hence piston too-to the last balance position.

You don't need any POSITIONER!

KISS (Keep it simple stupid!)

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Anonymous Poster
#23
In reply to #21

Re: Pneumatic Cylinder Control

01/30/2007 12:20 PM

dear sir, but hunting occurs when you stop the loaded pneumatic cylinder in its intermediate position with a 5/3 valve and can you advice me on at what preassure the air will be compressible if we want to lift a load of 2500N .

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#24
In reply to #23

Re: Pneumatic Cylinder Control

01/30/2007 12:25 PM

We need to know what size cylinder you are using

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#25
In reply to #23

Re: Pneumatic Cylinder Control

01/30/2007 12:35 PM

what is a 5/3 valve?

What is area under pressure(sq.meter)?=A

{2000 divided by A} will be the pressure(above atmospheric) in Pascals needed.

Hunting-never!

Damped -out oscillations(overshoot) may be seen--2-3 rapidly dying cycles. Let it!

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#31
In reply to #25

Re: Pneumatic Cylinder Control

01/31/2007 12:13 AM

A 5/3 valve or 5 by 3 way valve is a type of directional valve with 5 ports. This is have two solenoids for controlling what position you want it to be in.

One port is where the air supply comes in, two ports are for exhaust, and the other two connect to each side of the cylinder.

The 3 positions are: position 1 = air to extend; position 2 = air to retract; position 3 = all ports closed.

When all ports are closed, no air comes in or out. It's locked so the cylinder should remain in its last position.

What do you mean by "hunting-never"? Hunting is not possible?

I've been working in installation/commissioning for 9 years and maintenance for 17 years. I've seen cylinders "hunt" due to poor system design or adjustments. It's definitely possible.

Whether you allow damped-out oscillations depends on your application. If your requirements don't allow it, don't let it.

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#20
In reply to #16

Re: Pneumatic Cylinder Control

01/30/2007 8:54 AM

If the piston leaks, replace it .

Seriously, if you buy a good quality pneumatic cylinder, you won't have problems with leaks. Even if it does eventually leak, the positioner will try to restore it to the required position. What happens is that the cylinder will hunt or oscillate up and down since this is a vertically loaded application. The same thing will happen if the solenoid valve is leaking.

The stopper or lock that I mentioned can be used to prevent this. When the shaft is in the desired position, the lock is engaged and keeps the shaft from moving.

Even hydraulic cylinders will not work properly if the piston leaks. In fact, hydraulic cylinders have a natural leak around the piston. The rate of leak depends on the pressure and is usually specified in milliliters per minute.

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Anonymous Poster
#27
In reply to #20

Re: Pneumatic Cylinder Control

01/30/2007 12:58 PM

what can be used to prevent sudden fall of load due rupture of hose?.will pneumatics will hold on the safety circuits used for hydraulics.

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#29
In reply to #27

Re: Pneumatic Cylinder Control

01/30/2007 3:16 PM

In post # 15 I suggested.

1. A pilot operated check valve which should be connected directly to the cylinder port with a metal pipe nipple. The check function holds the fluid air or oil in the cylinder if the hose breaks or a power failure interrupts the source of pressure.

2.Air over Oil will tame any tendency of the cylinder to hunt, rise or sink when the oil is trapped within the cylinder by the pilot operated check valve.

To position:

  • Manually - A person could stop the cylinder with a manual valve at any time.
  • Sensors - Limit valve, limit switch, electric eye, proximity sensor etc. can be adjusted to the desired position and stop the cylinder with a solinoid or pilot operated valve. Multiple positions can be controled with a PLC or manually.
  • Positioner - A Linear Displacement Transducer, Temposonic or other electronic feed back device including one that measures the amount of oil that has been forced into the cylinder. Most of these are analog positioning techniques where the transducer sends more or less electric current or voltage to a PLC.
  • Mechanically - One or more mechanical stops. These can be moved in and out so that multiple positions may be pre-selected.

If you tell your local pneumatic/hydraulic/automation/controls supplier what you want to do and how you want to do it they can provide an answer.

Congratulation on generating plenty of interest.

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#30
In reply to #29

Re: Pneumatic Cylinder Control

01/30/2007 8:57 PM

This is the best and most rational summary.

We should move on henceforth -to another Topic.

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Anonymous Poster
#18

Re: Pneumatic Cylinder Control

01/30/2007 7:57 AM

1. use sensor for position.

2.use hydraulic oil media and air-hydraulic valve

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#33

Re: Pneumatic Cylinder Control

02/01/2007 12:32 AM

There are several ways to position a pneumatic cylinder in an intermediate position. The selection will depend on the characteristics of the system, for ex., weight of the load, constant or variable intensity of the load, constant or variable speed of travel, desired precision of positioning, cost and so on.

Some of the solutions were described above.

The best you can do is to contact a manufacturer as Parker, Festo, MSC etc and ask them a solution. I'm sure they will easily solve your problem.

P.S. It's also possible to use the hydro-pneumatic system which is not expensive and have very good precision (and the changes in your present equipment will be insignificant or even unnecessary).

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