How to apply air pressure gradually to a pneumatic cylinder 1

[sammcc]

Hi All,

I have a load test machine where I set a regulator to a certain air pressure which translates to a specific force on a pneumatic cylinder.
For example, 10psi on the regulator would equal 100lbf on the cylinder
So, if I want to carry out a load test on a component I set the regulator to the value I require and test the component.
The problem I would like to address is that at the moment the machine instantly goes to the test force as opposed to pulling the load on slowly.
I know that I can use a speed controller to control the speed of the cylinder when the load is applied but how can I control the rate at which the force is applied?

Hopefully this makes some sense.

 

[MintJulep]

The requirements are rather poorly defined by the OP.

Buy my imagination of the problem matches handleman's.

Controlling speed of motion and force simultaneously with a pneumatic cylinder is damn near impossible.

Depending on the magnitudes of the load required, and the accuracy required pneumatic would not likely be my first choice for solving the problem as I imagine it.

Maybe hydraulic. More likely a force-controlled linear actuator.

 

[SwinnyGG]

common, and erroneous, impression that speed control valves can be used to throttle to inlet flow to an air cylinder

Uh... they can. I do a LOT of pneumatics, and I've done it before in situations like this that warranted such an arrangement.

SMC makes flow control valves specifically designed for this scenario.

 

[EngineerTex]

After the cylinder rod is in contact with the part being pressed, it is now a statically-indeterminate problem wherein the loads cannot be accurately predicted, since minute displacements could be caused by significant changes in force brought about by stick-slip of the seal. There is the other issue of whether or not this cylinder is double-acting or single-acting, which we do not know, noting that it is completely impossible to regulate outflow on many single-acting cylinders. As mentioned by @CompositePro, a rolling diaphragm could address the problem of seal sticktion in a pneumatic cylinder and this would fix the problem.

However, if you want to use the pneumatic cylinder, place a spring of moderate stiffness between the cylinder rod and the part being pressed. This will make the cylinder move a certain amount for every additional amount of force applied to the test piece. It is now a determinable problem, since you know the spring rate.

So with the addition of a spring, if you regulate the flow into the cylinder, then using a flow control valve or an *orifice will work just fine.

*As for using an orifice, if you are dealing with air of questionable quality, your orifice can get clogged. However, in the refrigeration industry, there is tubing called "capillary tube" which has a much larger ID than you would use with an orifice, but by coiling up 5 - 10 feet of it, you will have something that works like an orifice, but will be much more tolerant to contamination in the system.

Engineering is not the science behind building. It is the science behind not building.

 

[PNachtwey]

Relief valve are not control valve.
Those that say the pressure on the opposite side of the piston must be taken into account are correct.
EngineerTex is correct about the problem with stick slip. The cylinder must high quality.
I/we can control pneumatic pressures and forces accurately but the solution is not cheap.
We can control both force or position but not at the same time.
We can simultaneously limit force or position which ever is limited first.

http://deltamotion.com/peter/Videos/PneuMove.mp4

You can see that the cylinder has rough spots at certain positions where the control output must compensate.
The controlling the rate of applied force is not a problem.
Force feed back can be from a load cell or to pressure sensors on either end of the cylinder.
The pressure sensors on either end of the cylinder are a must for good motion control but the load cell will be more accurate for measuring forces because it doesn't care about friction.

This application used pneumatics.

http://deltamotion.com/applications/appnotes/documents/prosthesis.pdf

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/