Home build: a High power Electronically regulated Air pistol

[Finn]

Analog hall effect sensor could be used for "ram" position measurement, only needs a small magnet and it would be a solid state measurement. I don't remember the model I used but it was TO-92 package and analog 0-5 v output, very cheap part. Using arduino is not ideal if high precision timing is required, problem is the language itself, of course it is possible to get close with clever use of interrupts and inline ASM, but I guess you already know all this.

[MJP]

Make the valve NC with a spring / pressure and all you need to control is open time. No need to over complicate the system for no reason.
Coil energizing time is the only variable then with pressure.
You dont need to control or monitor the closing, what there is to gain for it?

Marko

[Scotchmo]

Analog hall effect sensor could be used for "ram" position measurement, only needs a small magnet and it would be a solid state measurement. I don't remember the model I used but it was TO-92 package and analog 0-5 v output, very cheap part. Using arduino is not ideal if high precision timing is required, problem is the language itself, of course it is possible to get close with clever use of interrupts and inline ASM, but I guess you already know all this.

What Marko said, so...

No need to sense the position. If the time is adjustable and can be programmed, that's enough. The design has a pressure sensor. The time should be based on the pressure available. The time needed for any given pressure can be determined experimentally after the build is complete. That will be the tuning process. That time-vs-pressure curve gets mapped into the processor. As an example: 1st shot at 4500psi might require 0.0010s. 2nd shot at 4000psi might require 0.0015s. 3rd shot at 3500psi might require 0.0020s. As the pressure falls, velocity stays relatively constant for the three shots.

[Finn]

Yes I know it is useless to sense the position, I thought it would just be interesting to see whats happening.

[WobblyHand]

Well, if you guys like to run blind, that's ok with me.  Yeah, it isn't necessary, you could always do it the way you always have.  That's not a way to progress, in my opinion. 

One could, with adequate internal sensors have the peak air efficiency for a projectile, by measuring what's going on.  Or dial in the exact FPE you want.  The computer would adapt the dwell time to the existing air pressure to maintain consistent shots.  Whatever. 

Just because you kill the pulse to the solenoid, doesn't mean you know it's position.  Postulating what happens is not the same as measuring and knowing what happens.  My two cents.  Spend them as you may  

[Scotchmo]

Well, if you guys like to run blind, that's ok with me.  Yeah, it isn't necessary, you could always do it the way you always have.  That's not a way to progress, in my opinion. 

One could, with adequate internal sensors have the peak air efficiency for a projectile, by measuring what's going on.  Or dial in the exact FPE you want.  The computer would adapt the dwell time to the existing air pressure to maintain consistent shots.  Whatever. 

Just because you kill the pulse to the solenoid, doesn't mean you know it's position.  Postulating what happens is not the same as measuring and knowing what happens.  My two cents.  Spend them as you may  

I agree that even if the final model does not need position sensing, position sensing could serve as confirmation during development. But it would complicate the development prototype.

"...run blind"? - hardly,  position can be calculated:
https://www.gatewaytoairguns.org/GTA/index.php?topic=170568.msg155946628#msg155946628

This thread should probably be in the "Engineering Research & Development" section as it is not "Simple...".

[darkcharisma]

i would vote for some type of pressure sensor in the exhaust area between the pellet and the exhaust. if anyone know of such a sensor please post it here. it would be very helpful to any build

[rsterne]

I must be missing something, I still don't understand what closes the pilot valve to allow repressurization of the main valve to close it....

Bob

[Scotchmo]

I must be missing something, I still don't understand what closes the pilot valve to allow repressurization of the main valve to close it....

Bob

I'm guessing the armature return spring (spring force would be = 1/2 solenoid force). My feeling is that the difficulty will be to get a small enough solenoid that can operate the pilot valve.

[rsterne]

In the drawing the "purple" piece that switches the flow is not shown attached to the armature of the solenoid, but "drawn" towards it because it is made of magnetic material.... I'm hoping the OP will explain, rather than us guessing....

Bob

[Scotchmo]

In the drawing the "purple" piece that switches the flow is not shown attached to the armature of the solenoid, but "drawn" towards it because it is made of magnetic material.... I'm hoping the OP will explain, rather than us guessing....

Bob

I missed that part - "magnetic material". I'm not familiar with them but that could be a bistable latching solenoid with two opposing latched positions. Apply a pulse and it latches in 1st position. Apply an opposite polarity pulse and it latches in the opposite position.

[MJP]

Looking at the picture, that slide valve for the pilot will never work!
There is no way and no how to move the slide against the full reservoir pressure, when the pressure is pushing it sideways against the port wall where it is sealing the port, if you get it to seal in the first place.
You have pressure in the end of the magnetic piece and at the side.
With an air gap to the coil. No bueno.

Let's assume 4mm sealing solenoid plunger port and at 300bar you are seeing 376N of force on the sealing surface. So basically you are trying to move a 37kg with a tiny solenoid. Given my rough calculations are correct.

Marko

[Scotchmo]

Looking at the picture, that slide valve for the pilot will never work!
There is no way and no how to move the slide against the full reservoir pressure, when the pressure is pushing it sideways against the port wall where it is sealing the port, if you get it to seal in the first place.
You have pressure in the end of the magnetic piece and at the side.
With an air gap to the coil. No bueno.

Let's assume 4mm sealing solenoid plunger port and at 300bar you are seeing 376N of force on the sealing surface. So basically you are trying to move a 37kg with a tiny solenoid. Given my rough calculations are correct.

Marko

I also have some reservations about the valve but it's not as bad as you think. That 4mm in your example could be closer to 1mm.

[rsterne]

We're all just guessing.... 

Bob

[Jay_sparky]

I was going to start with very short on times until the valve starts opening ....(the mass of the actuator and the back of the ram vent times will be reasonably long) then plot the muzzle velocity vs on time until the curve starts flattening off  and this will be the when the bullet has left the barrel do this at 3000 or so PSI and this will tell me the max on time then see how long I need it on to get the same velocity at max pressure

I think my biggest problems will be 1: having enough energy to move the actuator off the vent port   and 2: being able to close the ram fast enough (for efficiency)

So 1:  I want to use 5v (very low I know) and 4amps will let me lift about 800grams with a m8 bolt more amps doesnt seem to mean more "lift" just more heat.  Iv found .5mm wire the best so far.
It seems like to get more "lift" I need more iron m12 bolt will lift way more for the same number of turns&amps    (more of an electronics question I know)   but does anyone know if "800grams" of force will move my 5mm˛ actuator off the vent port (4mm˛)

2: when the ram is open and the actuator closes the vent it will take time for the back of the ram to pressurize and I think it will close "slowly" will i need to but a partial block in the air tank between the ram port and the actuator port? which should help with a pressure difference when air is flowing

[Jay_sparky]

Last reply I did not realize there was another page of posts.... attached is a more detailed picture of pilot valve I was just going to scratch the wall of the valve to let air to the back of the actuator

And I KNOW I will not be able to make the actuator seal! so I was going to put the "safety" after the actuator which will seal it.
So when you're ready to fire it'll start a slow leak, but will only do so when the safety is off.

As far as actuator material...I was thinking about having a magnetic actuator slightly repelled by the solenoid core (also magnetic) When you fire you pulse the core and pull the actuator in and then you reverse the coil and push it closed again and the hysteresis should keep it slightly repelled... but I'm keeping it simple.

As far as the timing in the arduino goes I was hoping an interrupt would do it.

The pressure sensor I was going to use is very small - 12mmDx6mmH  I could put one in the port but also I'm trying to keep it small and simple.

[MJP]

Your valve closing force at 300 bar is something like 147N or 14kg with 2.5mm port or ~5mm2 surface area. Not accounting surface friction in to that.
Your 800g force is too little by a large margin!
Not trying to put you down, but do some calculations before you make something out of metal.

Marko

[Jay_sparky]

Thanks Marko that is what I am here for Your knowledge!      that is sideways force not the force to slide it?  but still to much force     

So here is another design that I am not happy with as I do not think it will seal   

So the ram is completely balanced and contains a magnet that holds to the electromagnet keeping it shut...  when fired the electromagnet is turned on and repels the ram....
then reversed and pulled shut again

But.. Iv spent a bit of time thinking about the next design..

[Jay_sparky]

So here it is!   I think it looks good and I am keen to hear what you think

So you hit the pin with a hammer (not necessarily electronic    )   this vents the back of the ram through the tube with the sleeve covering the ports..... the ram moves back opening the H.P.A. to the barrel, then it reaches the back the travel in the screws runs out and pules the sleeve back opening the ports  by now the hammer has backed off the pin resealing the tube and the air from the ports flow through the tube into the back of the ram, closing the ram with the ports open until nearly closed then the ram pushes the sleeve over the ports..... with only leakage keeping the pressure on the back

tuning would be done by adjusting the screws that allow travel of the sleeve or by adjusting the expanding air past the pin

[Finn]

If you are asking for opinions on a particular design I think it would be much better to draw it correctly first. Put all the seals and correct dimensions in there, often the devil is in the detail, it is easy to come up with craziest of ideas and only when you start making parts you realize that something does not fit or is too difficult to make.