Reversal of pneumatic cylinder

Hi folks
i am more accustomed to permanent magnet (12 VDC ,24 VDC ETC )electric linear actuators.They are very fascinating to try hands on especially for the newbies.
I am trying my hands at a 10'' pneumatic double acting cylinder. I need a little guidance on how to reverse a double acting cylinder with the afore said specs. -
wiring
accessories

Cheerio
kwame

Hi,

What do you need help with? A pneumatic cylinder works on air pressure not electronics. The air valves are most likely electronic, but then you just open one and close the other or possibly have to open another valve to vent.
Maybe explain what you are doing a little more?

For instance if i want to make the cylinder static midway ,is it possible to use something like a rocker switch to achieve this end?

Generally, opening and closing a pneumatic cylinder is done with a 4-way valve. The 4-way valve will have 4 ports. 2 exit and 2 cylinder.

You can stop the cylinder by closing the exit port at the right time.

Here is an odd valve that I haven't used before: https://www.reidsupply.com/detail.aspx?itm=MFP-433&gclid=CO65_4StyrMCFYKK4AodKz8AZQ

With a pneumatic cylinder air pressure is apllied to one side through the port and the cylinder moves in that direction, applying pressure to the port on the other side reverses the motion (beware that most cylinders have twice the force in one direction than the other, unless it has a rod each side).
To slow down the motion you restrict air flow on the opposite side of the cylinder using a restrictor.
You can you as you say use the cylinder as a positional actuator but its more complicated, usually such an application applies constant pressure to the annular side of the cylinder (side where the rod sticks out), and then you vary the pressure on the full bore side, usually by using an i to p pressure regulator, this is a device that produces a pressre in proportion to an input current.
Positional feedback can be done through reedswitches fastened on the side of the cyl, or sometimes its done by a linear position sensor or lvdt.
A cheaper way instead of using an i to p is to use a high speed air valve and drive it with pwm, however both ways are fairly pricey.

if the cyl is pushing something sideways rather than lifting then you might get away with using lots of restriction and firing the valve just long enough to get the position you want.

Pneumatic cylinders work fine if you intend to fully stroke them from one end to the other. However if you want to use them for positioning duty, you have to take into account that the air is compressible (unlike the fluid in a hydraulic cylinder). So, if you reach the desired position, and then simply close all the valves, the pressure differential between opposite sides of the cylinder will cause it to continue to move until the pressures on each side are equal (or friction wins out). Otherwise you'll need a well tuned control system to keep things where they belong.

On one project, we managed to get around this, by restricting the air flow rate to slow the piston movement, and then when it reached the desired position, the air supply was quickly shut off, and both sides of the cylinder were vented to atmosphere, so that there was no unbalanced pressure, and hence nothing to make it creep. However, this worked because it was operating a device that had no back-forces, and would happily stay in its last position.

I've seen an application where a pneumatic cylinder was piggy backed with a hydraulic cylinder, there was oil on one side of the cyl and a spring on the other, its purpose was to damp the motion of the pneumatic cylinder, as this was part of a circular saw.

kwame said:

For instance if i want to make the cylinder static midway ,is it possible to use something like a rocker switch to achieve this end?

Click to expand...

Hello again,


Oh so you are looking for position control, which means you need feedback.

This would be something like a servo system, but instead of a rotating sensor you would use a linear sensor that is attached to your piston shaft. The sensor itself may be a rotational device connected to a drive arm but either way you get feedback and that goes to the control system which compares the signal to a reference and determines which side of the piston is to be driven. There's going to be some dynamics to consider as the air is compressible and also the dynamics of the system the piston is connected to (it is driving) has to be taken into account also in order to obtain stability and the desired response.

If you have ever worked with a servo system you should have an idea how this works.

Tying to do it without feedback may be very difficult as you would have to calibrate the whole system and then with wear and also alterations over time due to lubrication variability it would probably have to be calibrated too often to be useful. A possibility might be to overdrive the cylinder so that the pressure on both sides is always much higher than needed, but im not sure how this would work out in practice as you'd have to calibrate that anyway. Using feedback you never have to worry because the system is constantly checking itself
The question of how much accuracy you really need comes up too.

BobW said:

On one project, we managed to get around this, by restricting the air flow rate to slow the piston movement, and then when it reached the desired position, the air supply was quickly shut off, and both sides of the cylinder were vented to atmosphere, so that there was no unbalanced pressure, and hence nothing to make it creep. However, this worked because it was operating a device that had no back-forces, and would happily stay in its last position.

Click to expand...

I suppose you could move the cylinder where you wanted it to go and then hit both sides with pressure to equalize and hold that posititon. Much easier to use Linear Actuator if you're looking for position control. Most have a built in pot for feedback control.

Thanks folks, i am really grateful.This is without a doubt, the best technology blog on the planet.

There is another option which avoids the pneumatic creep due to unbalanced pressure. It's called an "air over oil" system. The cylinder is filled with oil (or other liquid such as water or glycol), and the lines on either side rise up to small fluid reservoirs, with pressurized air above. The solenoid valves are below the liquid level, so that when they close, the cylinder locks in position. Some additional valving is also required above the liquid level in the air supply, because it is still the air pressure that operates the cylinder. Though a bit more complicated than pure pneumatic, it's still much simpler than a totally hydraulic system.

BobW said:

There is another option which avoids the pneumatic creep due to unbalanced pressure. It's called an "air over oil" system. The cylinder is filled with oil (or other liquid such as water or glycol), and the lines on either side rise up to small fluid reservoirs, with pressurized air above. The solenoid valves are below the liquid level, so that when they close, the cylinder locks in position. Some additional valving is also required above the liquid level in the air supply, because it is still the air pressure that operates the cylinder. Though a bit more complicated than pure pneumatic, it's still much simpler than a totally hydraulic system.

Click to expand...

This type of system would add a whole 'nother layer of complexity. Number one, you would have to monitor the level of the hydraulic fluid in the reservoir. Number two, you'd have to fill the reservoir with fluid at a higher pressure than system pressure to ensure it goes in. Number three, you'd have to have a vented reservoir that the return fluid could return to.

This is not unlike the hydraulic system on our nuclear submarines. We had three hydraulic accumulators, one pressurized, one vented, and one spare. The pressurized accumulator was full of fluid and had air pressure applied to the top. This accumulator supplied pressurized fluid to the hydraulic system. The vented accumulator was (initially) empty, and allowed displaced fluid to return to it. At some point (assuming no leaks), the pressurized accumulator would (nearly) empty of fluid, as the vented accumulator filled, and then they would switch rolls, the empty accumulator vented and the full accumulator would be pressurized, and their roles reversed. This encompassed a vast and complex array of piping and control systems, even above the normal acuator valves associated with hydraulic systems. The major advantage of using a system like this was you wouldn't lose hydraulics if you had a power failure, as the hydraulics worked on air pressure and we had a large reserve of air banks at 4500 psi.