AC or DC Pneumatic Valves

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Peter Nachtwey:
Quote from: "rsdoran"


Peter offered some xls file that calculates the natural frequency of a cylinder which I think can also be applied to pneumatics.
http://www.patchn.com/hydrauli.htm

I was hoping Bud or Peter would drop in and give some info.


I thought there was nothing more to be said after you mention the AC zero cross over times.   We have used a lot of  MAC poppet valves engergized by a DC supply.   Our pneumatic applications are extremely fast.

However, you can't use the natural frequency calculator for computing air UNTIL you change the bulk modulus of oil to the bulk modulus of air.   The will GREATLY affect your natural frequencies.   The Bulk modulus of air is proprtional to the air pressure

Bulk Modulus of Air = 1.4 air pressure.

You can see this is a lot different from the 200,000 or 220,000 thousound value for the bulk modulus of oil.   At 100 psi the bulk modulus is about 140 psi.   Now take the ratio of 200,000/140  and take the square root of that.   The natural frequency of a cylinder with air at 100 psi will be about 38 times less than the natural frequency of the same cylinder filled with oil.   38 times.   That is a BIG difference.    What this means is that most air cylinders have a natural frequency around 1 or less.

This article covers bulk modulus of oil and air but it is metric.   Hey guys, tough it out and learn the metric system.  

ftp://ftp.deltacompsys.com/public/PDF/SpringEffectEffBulkModl.pdf

I have lots of good info on my FTP site.

Bob:
Randy

I agree that the valve/spool has a response time but I am "assuming" that it is the same valve/spool for both an AC and DC model. With that assumtion made, I think that the response time comes down to the coil.

Time for coil + Time for valve/spool = Total Time

Ron and Peter

I will have a look more at what you posted this weekend.

This valve is being used for a brief burst of air direct at a plastic lid to flip it. The guys are rebuilding some equipment that has a mixture of AC and DC valves installed and I am looking to standardize it.

Thanks,
Bob O.

PS Ron I have tried to log in from work but have not been able to. Any ideas what I should check?

Peter Nachtwey:
BoB O.  We used our Mac poppet valves to push a small piston that had a knife on the other end.  The knife cut freedom fries as they went by at 330 fpm.   Actually, we were trying to gut the rot and skin of the fat fries.   We applied TWICE the normal voltage for only about 3 milliseconds.   We could adjust the length of the pulse by microseconds if we needed to.  The pulse would not hurt the valve because its duty cycle was so low.   Our test show that the pulse was over before the air built up enough pressure to move the piston/knife.   A complete cycle was about 16 milliseconds.  That is so fast that one couldn't film it with a normal camera.   We had to get a 1000 fps camera.

You can't control AC at micro second intervals.   What happens when you need to blow th lid and the AC is at zero?

Randy:
Check out this site http://www.ascovalve.com/products_detail.asp?detail=engineering.  There is a pdf file of Engineering data that you can download but here is an excerpt from it on the coil response times:

"Response time from fully closed to fully open or vice
versa depends on the valve size and operating mode,
electrical service, fluids, temperature, inlet pressure,
and pressure drop. The response time for AC valves on
air service, under average conditions, can be
generalized as follows:
● Small direct acting valves: 5 to 10 milliseconds.
● Large direct acting valves: 20 to 40 milliseconds.
● Internal pilot operated valves:
1. Small diaphragm types: 15 to 50 milliseconds.
2. Large diaphragm types: 50 to 75 milliseconds.
3. Small piston types: 75 to 100 milliseconds.
4. Large piston types: 100 to 150 milliseconds.
Generally speaking, operation on liquids has relatively
little effect on small direct acting valves; however,
response time of large direct acting and internally
piloted valves will slow by 50% to 100%.

Response time of DC valves will be 50% slower than
equivalent AC valves. For specific response time on any
critical-timing applications, response time can be
reduced to meet specific requirements."

Of course we all know that ASCO is not the only valve company nor are they the last word on response times, but this is their data.  Kinda makes ya think!

rsdoran:
Bob O usually login problems are because of cookies, may want to go into the temp internet folder and delete cookies pertaining to this site then try loggin in. If possible clear cache and cookies but if you do that then you may lose auto login at other sites.

The other factor about using low voltage DC (or AC) is that there are no requirements for using PPE with it. The use of safety gloves etc is being pushed when 120vac is used (technically it is required)...can you imagine using "hot gloves" when working with a 120vac sensor/switch?

I knew DC was more applicable in high speed situations but Peter has a better ability to explain why.

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