Create a Versatile Pneumatic Oscillating Cylinder: Expert Help Needed

[justinreeves]

Im not sure if this is the correct forum for this question if not let me know and Ill post it to the right one. Anyway I need to creat a small scale pneumatic oscillating cylinder, I have a couple ideas but could use the help of someone witha background or knowledge in pneumatics.

I need the cylinder to have a adjustable throw length and hopefully have a adjustable throw strength.

I have seen similar systems where a fan driven device is used with a cam connected to the center drive, the cam giving it a oscillating output, and the amont of air controlling the speed. but I am trying to use a cylinder system.

I know there is a difference in air pressure and speed but I am not sure how
what does what, Is it better to control air pressure on this type of setup or to control air speed.

I really need this system to be as simple as possible due to fabrication costs.

If you want to email me directly jreeves16@msn.com I can send you some images of what I've bben working on so far and tell me if it will work.

 

[brewnog]

This should be the right forum!

I'm not quite sure what you're trying to do.

You don't put "pressure" into a cylinder, you put air in. The pressure at which the air will reach depends on the load applied to the cylinder, - if there is no load, very little pressure (and hence, not much air) will be needed to actuate the cylinder.

If you want the cylinder to oscillate by itself, you will need a double acting setup (supply air to the bottom to extend, supply air to the top to retract).

You will also need some way of controlling your cylinder, but you haven't been specific about your application to discuss that. Typically, you'll have a compressor, some valves (with some kind of control system), and some pipes!

As for adjusting the "throw length", I assume you mean the stroke? In which case, you just put enough air into extend the piston by the distance you need, and then stop it (often using an electrical 'limit' switch to determine at which point the cylinder stops being filled. Again, I don't know what you mean by 'throw strength", but the force the cylinder exerts will depend on the load applied, and the pressure which can be supplied to the cylinder.

I don't know what you mean by the 'fan driven device', possibly because I don't know what you're trying to do!


Pictures might be useful, but I think sharing your application would be better!

 

[Integral]

In normal practice, the speed of a pneumatic cylinder is controlled by restricting the air flow on the exhaust line. For a double action cylinder this is done with a flow control valve that has a check valve for free in one direction combined with a needle valve for restricting the flow around the reverse direction of the check valve.

As Brewnog suggests you will want to use limit switch's to reverse the direction. I am not sure how you can achieve a controlled but variable distance of throw, that is not a normal mode of operation for a pneumatic cylinder.

 

[faust9]

My company used pneumatics as described by Justin. Justin, you will need to make a servo and this is no easy task when dealing with pneumatics. You might be better served looking into motor controlled linear actuators because the controls needed to give variable height and rate control for a pneumatic device would be daunting to say the least. You could give thomas registry or globalspec a try though.

Good luck.

 

[FredGarvin]

When you say adjustable throw length, do you mean that it needs to be adjustable on the fly (each stroke a different length) or just adjustable to accommodate various operating conditions?

Also, what frequency would you require this cylinder to oscillate? Usually offset cam set ups, like you mentioned with the fan set up, are used to produce higher frequency oscillations easily.

The force of the cylinder is going to be a function of the air source pressure and the buckling limit of the cylinder shaft itself. Controlling the pressure is not a big deal. HOWEVER, if you are also looking at the velocity of the stroke then you will run into problems because then you will need to control on the flow. You can control pressure or you can control flow. You can't have it both ways. So you will have to choose between strength or speed. Something to think about.

I echo the desire for a more indepth description of what you are trying to achieve.

 

[Danger]

For sure more information would be pretty important before a practical set-up can be discussed. When you say 'small-scale', it could mean almost anything dependent upon what you're comparing it to. For personal purposes in regard to my Hallowe'en costumes or goofing-off projects, I find that plastic hypodermic syringes joined with aquarium tubing or automotive brake-line tubing make pretty nifty little pneumatic systems. The most force that I ever tried to get out of a 50cc unit was about 50 kg, and it handled it fine. As for variable, positive stoppage of the stroke, you can also use a solid physical bumper system mounted on a lead-screw adjustment. A pneumatic cylinder can also be used with a vacuum system rather than positive pressure, so there's a lot of variety available in what you can do.

 

[justinreeves]

Ok Ill give you more specifics and hope this doesn't blow up in my face.
The system is I am trying to design is for a Pneumatic tattoo machine, there is one already on the market (Neuma2)but it runs on a fan/cam system.
This design has no adjustments for the needles throw length or oscillation strength.It adjusts its speed through not a pressure regulator but a foot pedal that allows varied air output through the foot pedal. Having a on/off full foot pedal would be better, with a control valve up closer to the table.

Also another advantage to a pneumatic machine is that it is Autoclavable, a electr-mag. is not.

You have to have a machine capable of adjusting the length of oscilalation out put, they call it throw length, it needs to be able to adjust from the width of a dime to the width of a nickel in some cases a little larger than a nickel.They actually use these coins to adjust the machines.This doesn't need to be done on the fly but each machine needs to be capable of having a varible adjustment.
The machine also must have a stop point for the downward stroke , so as to be able to tell the absolute depth of the needle, ie it cannot fluctuate depending on pressure, so if you used a spring for the return stroke on the cylinder then you need to make sure that the down force on the spring compression stroke always ran the full length to a stop point,
You must have variable speed and stregth, you could get away with the same strength always but you must have variable speed at least,wich in a standard electromagnetic machine you adjust the speed and strength with just a single adjustment, where increasing speed increases also the strength of magnetic pull. You would optimally want the speed and the strength to be sepereate, to compensate for this they change the strength of the elec-coils and the strength of the return springs to vary the stregnth and speed of the machine.

There is also a power supply that has dual adjustments, one for oscillation speed and oone for voltage. The osciillation is controlled by a couple 555's and a pot and a FET that then outs put the voltage set by the voltage pot. The tat machine is then resetup not to operate like a tank oscillation system, its just wired straight to the coils, not through the mechanics of the machine.

SO a liner machine has a small (dime) stroke length, a fast oscillation speed ( to minimize vibration, and to draw quickly) and usually the down stroke strength is harder comparable to a shader.

A shader has a larger stroke length(Nickel+), slower oscillation speed, and the down stroke strength is softer.

The patent on the Pneumatic fan driven machine is simplified enough that a cylinder driven machine can be patented over it .So no problems there.

Im not sure the range of speed the machine must do, I've heard 50hz+ but I am not sure.
If I still had my O'scope I could tell you.

I know these are small/tight tolerences for pneumatics, and I am sure this is a quick cycle for them also, but I have torn apart a air saw and a air file to check out the setups, they have been fairly complex for me to try to fit into something say 1"+ diameter* 3" height or so.
Ive seen a medical device similar , I couldn't tell what it was used for but its size was acceptable so I know it can be done.
It would be nice if a air brush compressor could power the unit, because of the size and quietness of these compressors, I am sure they would be fine with this small of a system.

One thing I was thinking was that the systems initial chamber would have a down stroke with a spring for resistance, the spring could either be changable for different resistances or be mounted on a screw to vary the strength witha stop in place also.

I also thought one way to adjust throw length would be to have a rubber stop at the top of the cylinder chamber that was scrw mounted and adjustable so that on the up stroke the cylinder would stop at varied hieghts,
and it would need to be on the up stroke because like I said you want the down stroke to stop at the same position every time, If not say you insert your needle, adjust your grip-tube-tip asembly to where the needle is barely inside the tip, then once you started to tattoo and wanted to vary the stroke length you would have to re-adjust the tube assemblies hieght which you can't do during a tatto do to contamination reasons.

 

[justinreeves]

Ive never used pneumatic solenoids,simple enough though, do you think a solenoid connected to a single action/spring return cylinder would work at these high of speeds. If so this would simplify things extremly. I am not sure if air pressure coming though 5' min. of 1/4" hose would be able to pressurize fast enough, I don't even know what the solenoid speeds are capable of.

 

[brewnog]

Ok, thanks for the description. I'm sure you'll understand that this isn't exactly a frequently asked question, but we'll do our best.

Can I just check that when you say "width" of those coins, you mean thickness, so the stroke of your needle is to the order of 1mm?

To me, this seems very small for a pneumatic system, but I have virtually no experience with pneumatics and (other than tolerances) I can't see any reason why it wouln't be possible.

When reading your second post, I was starting to think that you could use a spring return setup, such that you'd only have to use air to power the piston on the 'down' stroke, before the air is exhausted and the spring causes the needle/piston to return. I doubt that you'd even need to vary the strength of the spring, its sole purpose is to return the piston/needle to the top of the stroke. You'd also (as you say) be able to have a simple mechanical adjuster to vary the degree to which it retracts, too.

In my head, you just feed air into the cylinder until the preset stroke is reached, when the air just escapes through a valve. This assembly would be adjustable mechanically along the axis of the cylinder, so that no limit switches would be needed. It would almost be like a steam engine. You'd be able to alter the speed of oscillation by varying the air flow going into your machine.

As I say, I have virtually no experience with pneumatics, but it was an interesting question so I thought I'd have a stab.

 

[justinreeves]

Thanks, I was just checking out solenoids and I am used to using solenoids in car audio for door actuators and trunk pops, they are capable of high speeds, Anyone know if the pneumatic ones are the same. Also I would think that the fastest through put, would be to have a fairly large exhaust port at the bottom of the down stroke, to allow the air out as fast as possible for the next pressurization, If anyone can tell me if they think the above mentioned tubing size and length can pressurize at 50cycles or so Is the last question. I would really like to use standard airbrush hoses at 10' but 5' would be a minimum.

Do you think I could vary the stroke stength by using different spring strengths on the up force or by using a adjustable spring setup.
I would figure that if you lower the pressure and have a soft spring it would reach max down stroke softer, and that higher spring strength and higher air pressure you would have a harder down stroke.

This is actually seeming like it might work.without having to pressurize both sides of the cylinder, which is what I was hoping for, simplifies thing extremly.

 

[justinreeves]

Sorry yes its like 1-2.5 mm

 

[brewnog]

I might be talking out of my depth here, but here goes.

The only limitation I see on the speed of a pneumatic actuator is being able to get enough air in, at a high enough pressure, as quickly as possible, without breaking anything. I suppose you've got seals to worry about, hopefully someone else will be able to comment on rapidly oscillating actuators.

The pressure you want to run at depends on the size of the cylinder you want to use, which in turn depend on the amount of force you want to exert. I have absolutely no idea what kind of force is needed to tattoo someone (is tattoo a verb?), so wouldn't even like to toy with numbers, but I can't see it being a great deal of force. If this is the case, a very small cylinder is likely to be adequate (in which case you can use low volumes, and get the air in and out quickly, more easily), or use a larger cylinder with lower pressures (and possibly exploit your airbrush hose idea).


Anyway, I'll be interested to see what Fred has to say (particularly regarding high-frequency use of actuators). I suspect that there's a very good reason why the fan/cam arrangement is used; if I was designing such a product from scratch this (fan/cam type) would definitely not be the obvious choice at first glance, and I dare say that if your (seemingly simpler) arrangement would work, it would already have been done. Hopefully not though, eh!

 

[justinreeves]

Most tattoo artist don't like to change, the power supply I mentioned earlier that can control speed and strength and gets rid of capacitors and minimizes electrical wear and maintenance, but people don't want to use it or try it, there set in there ways or the ways they are taught and don't want to try newer technologies, and most of the time they arent engineer related people doing tats, its usually artists.
PLus a pneumatic machine means you have to have a air compressor in the shop making noise, but they don't understand that pneumatic machines last far longer than electrical, and that a pneumatic machine is Autoclavable.

They have had rotary motor machines out forever, but the problem is that a rotary machine, similar to the pneumatic fan machine do not have a adjustment for the softness or hardness of the down throw, so no one uses a rotary machine except Permenant Cosmetic people because they don't vibrate as much, but then again there not doing a art piece there just doing a couple lines or solid colored in area.
Ive done both tattoos and PC and used both machines and not having the soft/hard adjustment sucks. If your machine hits to hard and fast it will leave lines and needle dots all over, if its soft and slow you can't tell it was done with a tat machine, it looks airbrushed. but it has to be fast and semi hard to outline with. you need a minimum of 2 machines.
And actually the fan/cam arrangment is far simpler, it is exactly the same as a elec/rotary machine except the speed fluctuates with the air/foot pedal.
You don't need a pneumatic-solenoid which is going to have to have some type of electric oscillator circuit and a pot controll for speed. and a electric (on/off)foot pedal to turn the oscillator on.

but if this works youll gain stroke length adjustment, speed adjustment, stroke strength adjustment, everything a elec/mechanical machine has but possibly on the fly adjustments.and patentability. All in a Autoclavable machine.

 

[Danger]

I've had enough beer that I couldn't even properly read those last couple of posts, but I'm curious as to whether or not you could use an air turbine, such as a dental drill, channelled through a transmission of some sort to give reciprocal motion. I believe that current dental equipment runs at about 200,000 rpm, but that can be down-tuned. FredGarvin was discussing in another thread the use of 50,000 rpm+ gear trains. He tends to deal with larger scales than this, but can probably advise you as to whether or not it's practical.

 

[brewnog]

I've had enough beer that I couldn't even properly read those last couple of posts, but I'm curious as to whether or not you could use an air turbine, such as a dental drill, channelled through a transmission of some sort to give reciprocal motion. I believe that current dental equipment runs at about 200,000 rpm, but that can be down-tuned. FredGarvin was discussing in another thread the use of 50,000 rpm+ gear trains. He tends to deal with larger scales than this, but can probably advise you as to whether or not it's practical.

I believe this is the existing design the OP described, good job the beer's working!
It sounded to me like it involved an air motor with a kind of swash plate (or, thinking about it, a cam could indeed work too). In any case, a mechanical mechanism to convert rotary to oscillatory linear motion is not a difficult thing to design.

 

[Danger]

I believe this is the existing design the OP described, good job the beer's working!

Not well enough, apparently, if you could still understand me.
Knowing my propensity for doing things bass-ackwards, I'm sure you won't be surprised to hear that something else crossed my mind about this critter. Would it maybe be easier to adjust the stroke, force, and speed by using a spring for the extension half of the stroke and the air for the retraction part? Just a thought, since you would then not have to vary the air pressure for control purposes.

 

[brewnog]

Knowing my propensity for doing things bass-ackwards, I'm sure you won't be surprised to hear that something else crossed my mind about this critter. Would it maybe be easier to adjust the stroke, force, and speed by using a spring for the extension half of the stroke and the air for the retraction part? Just a thought, since you would then not have to vary the air pressure for control purposes.

Not as stupid as you look eh?

I like that idea actually. I remembered today about seeing a 'continous reciprocating' type cylinder in a Parker catalogue once. Requiring just an air supply, it controls its open/close valves itself. Speed control for extend/retract strokes can be altered independently by adjusting the flow control valves.

Anyway, I'm still uncertain as to whether you can use a pneumatic cylinder to reciprocate this quickly over such a small stroke; the more I think about it the more I fear that friction in the seals would be a big problem in terms of heat generation.

Where are the pros anyway?

 

[Danger]

Not as stupid as you look eh?

Would that even be possible?

Anyway, I'm still uncertain as to whether you can use a pneumatic cylinder to reciprocate this quickly over such a small stroke; the more I think about it the more I fear that friction in the seals would be a big problem in terms of heat generation.

Heat for sure would be one problem. Maybe Teflon piston rings instead of the neoprene or whatever they usually use? Most normal pneumatic cylinders or tools such as an impact gun have oil injected into the airstream for lubrication. That could end up clogging the ports on something this small. The main thing that bugs me is the compressibility of air. Getting a positive stop just with valving isn't possible. There's always a bit of 'squish'. Hydraulic cylinders stop right now, but pneumatics are 'softer'. Even if it doesn't cause too much overage of stroke, it will still restrict the cyclic rate.

Where are the pros anyway?

No one but cons here, dude.

 

[FredGarvin]

I have never looked into a dentist's drill, but I think that the way to go would still be a basic copy of it. The one thing I would do is to have the air turbine to get the reciprocating speed you need without having a difficult valving/exhaust scheme. The fan could be connected to something along the lines of a variable wobbler plate. Controlling the degree of tilt on the wabler plate could control the depth of stroke and the air flow could be controlled with an inline valve. I would think that you might have to have it spring loaded to simplify the mechanism to worry about only moving the cylinder in one direction and letting the spring take care of the other direction.

Getting a positive stop just with valving isn't possible. There's always a bit of 'squish'. Hydraulic cylinders stop right now, but pneumatics are 'softer'. Even if it doesn't cause too much overage of stroke, it will still restrict the cyclic rate.

Do tatoo artists really need the tool to stop that quickly? Can't they just pull it away from the work?

 

[Danger]

Do tatoo artists really need the tool to stop that quickly? Can't they just pull it away from the work?

I know virtually nothing about the technicalities of tattooing, but I've heard the needles in operation and 30-50 strokes per second is probably about right. It's comparable to electric hair clippers, anyhow. As for the depth, I think that they might require a physical adjustment rather than 'pulling away' because of it being a dynamic surface that they're working on. I'm sure that flinching is common amongst tattoo recipients, which would make manual control very unpredictable. If you can set it mechanically and then just apply firm pressure, it eliminates that problem.

 

[justinreeves]

The turbine system has been done and patented, except that they did not incorporate a adjustment for oscillation depth/height, also because the needle bar has been attached solid to the cam which is attached to the turbine motor you can't adjust softness, unless you look at it as the speed and softness are in the same adjustment.
via a speed control foot valve.

 

[justinreeves]

I found a highspeed long life micro solenoid and a micro cylinder, but after thinking about it I really don't want any electrical component, then its not autoclavable.

http://www.ascovalve.com/smartcat/sc_app/sc_filter.asp?famID=2&code=0,0,0,0&uom=en&StateVar=1%3DAM%7C

http://www.metalwork.it/eng/img/prodotti/cilindri/03%20Cil%20Cartuccia.pdf

 

[justinreeves]

I think I might be a little lost as to the "positive stop" and "does it need to stop that fast,why can't they just pull away" but this might help hopefully, different people put needle to skin differently, some people set the needles to extend out of the tip say 1mm, then they push the tip on your skin just like writing, no need to vary the pressure of there hand as long as the tip is touching the patient your in 1mm. Other people like to set the needle out farther say 2+mm and watch the needles enter the skin, ( you usually need a large oscillation for this, so as to get ink on the tip of the needle)knowing the needle depth into skin by eye, this method is more acurate due to the tip being much larger than the actual needles by 75% min., also some inks, good inks are thick and as the get aggitated in the tip they get a thicker viscosity and reduce the machines speed, you then need to turn up the speed control to keep it at its past speed, or you need to rinse out the tip. So if you have a machine that varies its needle depth along with its speed control a person who pushes tip to skin will not be able to use it, there needle depth out of the tip would always vary.



The reverse option sounds promising, you could control the spring better than the air, on the down stroke. It sounds like no matter what, it is going to need some type of air transmission, to control the incoming air on/off, that solenoid would work but I don't want electric comp.

The pneumatic air saw I looked at had a return spring but also was a 2 way system, because of it being a saw it needs air pressure strength on both strokes, but its oscillation stroke length was adjustable via a valve, It had a transmission connected to the cylinder as the cylinder + pressurized it closed the air valve to itself opening another valve to the other side, there was one strange thing though that I didnt take the time to look at because the whole transmission was to big, but it also had a air inlet right in the middle of the 2 chambers and it opened and shut and strange intervals.
I think it might have been there to finish the whole stroke after the main inlet was closed, didnt really check it out to far.

 

[justinreeves]

Also the air saw was a aluminum head with some type of lightweight high temp non-metal material I've been looking at a few different ones.

 

[Danger]

Also the air saw was a aluminum head with some type of lightweight high temp non-metal material I've been looking at a few different ones.

Aluminum, or ceramics, or plastic or such-like makes very good sense because they will have lower inertial than something denser. That will allow for higher reciprocating speed.
If it helps any, I just thought of a way to double the needle frequency relative to the piston strokes. Haven't really worked out the details, but it might solve some of the other problems. I'm going to send about 10% of my mind off to a corner to work on it while I go make life miserable for the folks in GD. (I'm into the beer again, so it might take a few hours.)

 

[FredGarvin]

I was looking at the Asco valves you linked to. I use Asco quite a bit in test set ups. I haven't heard of anything that has an actuating frequency or duty cycle in the range you are looking at (20-30 Hz IIRC). That doesn't mean that they don't make them, but I haven't seen them. Also, the valves you linked to, the AM series, have very small Cv values (.015 - .07). Do you have a rough idea what the air consumption would be for this kind of application? What do the dentist drills use? With a Cv that small, I would be worried about getting enough air to the part. I haven't looked at any numbers, I'm just going by feel right now.

Just poking around, I came across this site:
http://www.teknocraft.com/products/proportional/

I have never used them, but their claims seem pretty darned good for solenoid/proportioning valves, especially in the frequency response category.

Perhaps the PWM approach may be your best bet. I haven't used PWM outside of hydraulic set ups, but the application should be the same.

 

[justinreeves]

I don't know anything about pneumatics so I am not sure how much air would be needed, one of the valves they has had a bout a 1.4mm port, I thought that would be enough, the saw I looked at had 2.5mm lines in the transmission.

 

[justinreeves]

Plus like I said i don't want to use elec. comp. , i want it all pneu. for Autoclave purposes. and i don't want 2 lines coming off the machine(1 air 1 elec.) it makes it harder to manuever. I am just trying to come up with the easiest transmission I can think of.

 

[justinreeves]

http://www.fpef.org/teacher_resources/Curriculum2/IntroductionCurriculum2.htm

Air saw uses a 4 way valve. too large, Ill need to use a 2 way.with spring.

 

[Jmomaly]

I just noticed this forum and had my 2 cents to put in. It seems that none of the posters here have any idea of what is required.
I have developed this idea for almost 20 years and actually have a working prototype.
you can see it at jamcaddesigns.com/pneu2u

 

[Jmomaly]

pneu2u

I just noticed this forum and had my 2 cents to put in. It seems that none of the posters here have any idea of what is required.
I have developed this idea for almost 20 years and actually have a working prototype.
you can see it at jamcaddesigns.com/pneu2u