Actuator Stroke Timing Calculation

There are too many variables to give any simple answer.

Some relevant formulas are F = ma, F = PA, v = at, x = vt; but those are only the simple cases where the independent parameter does not change. If it does also change, then calculus is required.

Thanks for your reply.

we have to include some critical parameters (inputs) such as individual accessories combined CVs, Actuator Area, Piston area, Volume, Spring Thrust etc.,. do you have any calculus theory reg this task?

Measure it. In theory, that would work.

No festivity on New Year? Just throw a google search and wallah! Actuator Guide

Huh!!! Cool.. happy New year. In India, we don't have leave for new year...We have office on today...

Wow, but anyhow, God bless and happy new year to you.

wallah Voila

You can start by doing your own research.

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Hi Lyn, Actually i tried with emersion pnuematic softwares. we have to find these things through basic. for those things, your links are very useful.. thank you.

For a pneumatic actuator this computation is quite complex since the air flow type depends on the pressure drop at the control area level, the seals friction depends on the pressure sum at the piston level and of the pressure at the piston rod seal level.

The flow law is a square root if pressure ratios is reduced under a critical value and it stays contant if the pressure ratio is over the threshold.

The seal friction depends on the assembly preload (which ever the type) and of the air pressure.

For the rest the given counsels are OK, the differential equation you have to integrate is classical in form but the variable coefficients depending on the variable value make the problem complex.

The best is to use a simulation tool as Modelica where the different aspects are included.

As Tornado has said, and others, there are too many variables to take into account for an exact answer. Any calculated result will have to be qualified with numerous provisos.

If it really matters you will have to build a rig and test it to arrive at an actual speed. - that will then only be repeated if used in an identical set-up.

However, with the appropriate pressure and flow to start with, in general practice you will probably find that you will need adjustable flow restrictors in th line to slow the actuator down to the speed you want - which incidently is usually achieved by regulating the speed that exhaust air is dumped.

Regulate the outlet is not an incident, it is justified by the fact that if the 2 sides of the piston are under pressure the system stiffness is higher since the 2 volumes add their spring constants.

Total stiffness is piston position dependent and minimal around the stroke middle. The higher the stiffness the lower the risk for stick-slip. In pneumatic linear actuators because of a low stiffness (air is very compliant) seal friction is a major problem in the stick slip generation. This explain also the different design and materials for those seals.

Same problem for the guide rings which must have a low low speed friction and as much as possible an increasing trend versus speed.

If you are using an well-known brand of cylinder eg Festo, Martonair etc.., they will probably have published the standard travel times for their cylinders as per diameter of cylinders, supply air pressure etc..

If not, check out their web-sites and use the size closest to yours for comparison.

Next time please tell us what the service is. Municipal garbage trucks also use pneumatic pistons as do food processing facilities (such as bottling plants ). It would be nice to know whether your cylinder travel is 6feet or 6mm.

We are using this actuator in control valves. we have some stringent case like 1sec open and 1sec close timing with 25sq.inch actuator. at that time we are struggling to find the stroke timing through theory. these cases, we have to wait until the valve fully assembled stage or actuator assembled stage. if we have any theory that will be very helpful to find in prestage of assembly. i am working towards on it. i need some initial idea to ignite this case. so that i asked some ideas. can you please?

OK let us put a bit of theory together although it will not be easy.

First you should consider ONLY massflow since due to compressibility volumetric air flow is not the right way to master the process.

Second if the ratio between pressures is higher than about 2 the flow is chocked and the air velocity through valve control area is constant. For ratios under this threshold the flow is proportional to square root of pressure difference.

So that you should integrate an equation for the air mass in the cylinder working volume and compute back the pressure in order to obtain the pressure difference and make the choice of the flow law. The flow can be considered as adiabatic since the time is short and, in the cylinder, even if turbulence is high the time to heat in contact with the wall is very short so that the temperature in the cylinder is the one obtained by the adiabatic expansion.

Discharge coefficient of the valve depends in fact of the valve opening if it is low then you can neglect the other pressure losses and consider that whole pressure difference is concentrated at the opening. If not you must consider the pressure losses in the connections which is flow proportional and depends on the Reynolds number. There are equations for it.

If the valve opens in a very short time from zero area to a value then you can consider the section as constant over time if not you should take the opening law into consideration since it is a factor in the flow calculation.

Now you have all input to make your equations and integrate them. You can use a spread sheet as EXCEL and I suggest to use the Runge Kutta-4 algorithm which is self starting. Your integration step should be less 1/100 (in your case 1 ms or less) of the full time in order to obtain a not too far from reality results.

Do not forget to take into consideration the masses and the seal friction. The last has 2 terms:one is constant related to the seal design and preload at assembly, the second is more or less proportional to the sliding speed and other tribological conditions.

To simplify, since the problem could be too complex if you use all parameters, you could accept that:

- pressure difference is constant at about half of the extreme values

- friction is constant for mean velocity

Any way the simulation is ONLY an indication and will NEVER give the true value as long as you do not combine theory and practice i.e. measuring the true discharge coefficients and friction.

As I mentioned in a previous comment there are softwares for simulation of such systems and I gave the name of a FREE soft which is easy to use even if you have not much experience with simulations, as it seams.

In fact all what you ask for is in your hand if you knowledge of physics is correct.

If you have other questions do not hesitate to ask. For your information such simulation works are done in general by experts (and not for free) since the major error is to believe in the soft results and not have a critical regard on them.

I APOLOGISE.the reference to "valve" the first time around, and considered only a pneumatic piston. I don't see any necessity for calculating from first principles here...the supplier of the valves must have done that before quoting. All you need do is go the the supplied valve manual. The stroke speed can be measured while using the stroking procedure, if your maintainance guys know what they are doing.

I have years of experience with such valves. Your query should be sent to the manufacturer, as the user doesn't normally have the expertise to revert to first principles of valve design, where the configuration of the trim needs to be considered, as well as the operating conditions. That is why the manufacturer is in business, after all.

obviously here we are trying to drive equations theoretically from the basics to find out the pneumatic actuator stroke time. i don't think so, that valve design has play the major role for stoking time. the actuator has been sized and provided by valve major configuration like packing, trims which is mainly affect the actuator performance. I guess accessories CVs will be playing the major role for actuator stroking timing which we used in that valve.

"How Can I calculate actuator Stoke time for Pneumatic actuator? i physically tested timing with required accessories and learned. but i have to know about these things from basics. so Theoretically i have to learn and strong. can you any one guide me???"Was it what you asked for or not ? Did you get the guide lines or not ? That is the question!Your last comment is not easy to understand.

What I still haven't got from this thread is whether you want to calculate a possible actuation time, and impose that on the valve motion, or whether you need to calculate the speed for process control purposes. eg: if you want to know the speed because you have a slow reaction time in the process after you have signalled the valve to open/close, then this is process-related and could well be as a result of incorrect trim in the valve. As for the calculation you wish to do, unless you are studying theoretical control, I think you are wasting your time (and possibly ours as well). You can actuate the valve manually and time it on your wrist watch, after all!

Whoever gave you the task of calculating it, tell him/her to state their exact reason/s, or alternativey, to get a life~!

Since bench-stroke and loop tuning have not been mentioned, I am of the opinion that you require help with a homework assignment. Such requests don't go down well on this forum...

"I guess accessories CVs will be playing the major role for actuator stroking timing which we used in that valve."

Exactly! -and the external displacement forces acting on the valve that have to be accommodated - the closed loop timing - and the stability and quality of the compressed air supply itself.

When you refer to 'valve' and 'stroke' I imaging a piston/vane type action. Either spring return or double-acting. The speed of the 'forward' stroke is one thing, but overall operation is governed by the speed of the 'back' stroke as well - and in both strokes - the time it takes for the initial signal and the feed-back signal to operate the valve in both directions.

The internal characteristics of your valve affecting the speed of stroke are but a very small part of the overall operation.

Nick-Name # 15 has gone to a great deal of trouble to spell out the detail. Then to make life easier for you he has said "To simplify, since the problem could be too complex if you use all parameters....."

This good advice means in effect that you omit (or assume) parameters that (unknown to you) could have a marked effect on the result. The work you need to do to establish these parameters is probably a lot more than the work in building an actual rig that gives you the exact answer.

Because of all these 'unknowns' (and they are likely to fluctuate by the minute) the well tried and trusted route that I would take is to ensure the compressed air supply (pressure, temperature, flow and quality)(and suitable sized pipework, valves and fittings) is enough to easily operate the valve at a speed is in excess of the maximum required.

Then with regulators (to stabilise the pressure) and adjustable restrictors - you adjust them to give the actual speed required.

This construction technique ensures the finished product can be adjusted as part of a routine maintenance programme to accommodate local conditions and variations in the air supply.

To get the correct values for the parameters is not as complicated as you think, a lot of values are available as well in general literature about seals as in other sources about discharge coefficients. When I though about complexity I meant the complexity to write (for him) the program on the spread sheet since it has a set of 2 differential equations which are interconnected and to write it for the RK4 algoritm it is not so easy.

This is the reason I mentioned the soft which has all those mathematical-numerical methods inside and reachable with a quite simple "human" interface. It is not always needed to make a bench to measure all, this is the reason I mentioned the necessary expertise.

As for final adjustment it is a must since even if the parameters are correct they present dispersions and require an adjustment.

To Nick-Name.

Great help. I am out of touch with today's pneumatic devices and modern software and never very good with Excel anyway, and used so infrequently, that it would take too long, as did my earlier efforts with programmes in BASIC back in the 80's - memories linger and old habits die hard.

So if it mattered it would be quicker for me to 'knock something up' on the bench and try it out. But then for me I had access to 'bits-and-pieces' to do this.

If the OP likes the idea of the mathematical approach and can easily obtain the parameters needed to use in the formulas and software programmes now available - then why not.

It is the way I would probably go if I had my time over again - and if I got bogged down - I cold always fall back on practical experience.

I hope the OP takes note of what we are saying.

I hope too.

In fact I consider the mathematics only as an engineering tool not a goal in itself.

The advantage is that if you build a mathematical - physical model and work with it you get the possibility to - at least - estimate the influences of different parameters and be able to decrease the number of practical tests. It is a way to decrease as well cost as time to finalize a project.

I shall give you an example. The traditional time to develop a valve was about 6-8 month before life tests and several prototypes. When I was confronted to the development of a proportional valve, via modeling - and at this time the soft ware was not yet available so that I was obliged to use Fortran-4 programs (very similar to the Basic I used later), my team and I were able to do it in only 2 month. Every thing was optimized on "paper" and when we build the prototype we were not far from what we wanted. We had only one prototype to build and had to make minor adjustments most of them related to production.

Fortran 4 ! Now there's a last from the past. I wouldn't have a clue now though...

It was soo many years back that the Fortran 4 was the updated programming language for mini computers.

As you see computer simulation is already since many years a possibility! Today it became much more easy to use. I had my first dynamic simulation soft in DOS and my first version of a FEA soft working as well under DOS. One of my first simulations was a pressure cycle in a high pressure plunger pump (700 bar ≈ 10,000 psi) the goal being optimization of the geometry for a low leak and avoid cavitation noise. Between my results on paper and the values obtained on the bench the differences were under 5%. The simulation was done with a hand calculator!

In my career I had to deal with Fortran, Basic, Assembler and a lot of other soft packs based on java or Pyton.

My practical view on your question is that there are so many parameters affecting the speed that you could avoid them by building the system first to save time and guesswork on math models - that would require you to build the system to find out if your calculations are right - so why bother!

I assumed here that you had one system to attend to - and that's it - job done!. But on reflection, if you have more than one system. or more than one valve, or a requirement for different speeds etc, then my direct-hit practical solution would not help you 'learn' much about the finer points of pneumatic control and compressed air - which you might want to know.

Getting hold of a 'modern' computer progamme (Nick-Name might help here) that does the arithmetic for you, and feeding in all the parameters - or most importantly - varying the parameters to see what effect they have on the result - would be invaluable - and the way I would recommend to incoming engineers today.

I would then feed-back actual results from the built system - and by tweaking the programme -I would build up database of parameters and results for quick future reference. Albeit for me it is academic, I am old-school and have no future in pneumatics today.

But even with a supercomputer to do the modelling, you will only get the actual 'speed' you want with flow and pressure controllers adjusted on site in actual working conditions.