My Field Target Project

[Motorhead]

So question begs to be asked HOW STABLE the cap voltage is while guns cocked and sitting there in a charged up state ready to fire ?

Secondly as battery Voltage & Current diminish from a full charge on down .... How do you control or alter the caps voltage set point to cycle the solenoid the same independent of batteries state of charge ?   

[michaelthomas]

Well..... cap voltage will always be what the dc to dc boost converter is putting out.

The boost converter is regulated, and should maintain consistent output as long as the input stays between 3v and 34v.  I plan on using a plain old 9v battery.

So, the voltage set point should not change until the 9v is really, really dead.  As long as the voltage to charge the cap is consistent.....the amperage from the charged cap will be consistent, too.

I'm sure there will be some tolerance.....but it's hard to say (for me anyway) until it's actually in a complete unit and able to evaluate.

I conservatively came up with a shot count for the 9v battery......and it's a ridiculous 10,000 shots.   That seems like a lot to me, but I cannot find the hole in my math if there is one.

I don't know.......time will tell.

For now, I'm happy with the big bang.

Mike

[jessered]

Oh man! Just waiting to see what the rig will do with back pressure on it. Keeping my fingers crossed and hoping you are really on to something here Mike.

[michaelthomas]

I tried it out at 2000 psi today,  just a bigger bang.

Screwing in the endcap actually produce a bigger bang (shortening travel) than unscrewing......not what I would have originally guessed.  I'm sure it has to do with the solenoid being more powerful with the plunger deeper to start with.  Screwed in fully has 1/4" of travel to valve stem.....fully out is 1/2".

Here is s a pic of the ultra sophisticated (lol) test circuit.  Just a 9v battery going to the dc to dc booster......then splitting off the spst center off switch as the charge side and the discharge side.  Charge has a 2.2k resistor in series, and discharge has a big diode to protect the boost convertor after the collapse of the solenoid coil.  2 200 microfarad photo caps in parallel.  It seems to do just fine on 300 mf, too.

Takes about 2 seconds to charge with this resistor.

I rested a pellet on top of the transfer port pointing up......and it shot through the drywall ceiling in my kitchen......oops.

My wife didn't like that. 

I suspect that the back pressure of the barrel will extend the opening of the valve at any given setting.

Mike

[michaelthomas]

I forgot to mention that I tried a new 9v( read 8.99)  vs a substantially weaker 9v (7.86).  Both output 60.1v on the booster. 

Mike

[rsterne]

For a given hammer strike energy, as you increase the backpressure the valve lift will increase, and therefore the time required to complete the round trip (the dwell) also increases.... I recently measured the lift on a .30 cal running on 3000 psi, and with no pellet, the lift was 0.06", with 46 gr. pellet it was 0.07", and with the barrel blocked about 95% (call it a VERY heavy pellet) it was 0.08".... Since the pressure inside the valve is the same, and the valve spring is the same, in all cases, the difference must be due to a higher pressure in the exhaust port section of the valve (ie downstream of the seat) because of the pellet.... This means a smaller DIFFERENTIAL in pressure across the head pf the poppet, and hence less closing force during the shot.... It is that reduction in closing force that resullts in increased lift and dwell.... Since the third case, although interesting, is not a "real world" situation, the difference between the first two is the significant one.... and that is about 17% more lift and dwell with a pellet compared to open bore.... call it 15-20% depending on pellet weight....

You should note that once you open a poppet valve 1/4 of the throat diameter, the flow rate can no longer increase as the "curtain area" (circumferential area above the valve seat) equals the throat area at that lift.... and at higher lift, the throat area becomes the limiting factor.... That seems to happen at the high pressure end of the shot string in many unregulated PCPs, and as the pressure drops, the lift and dwell increase, becoming roughly double at the bottom end of the string.... However, only the dwell increase is significant, the additional lift sees any flow rate increase "clipped" by the throat restriction....

Bob

[michaelthomas]

Thanks for that info, Bob.  Nice to have all of your testing information.

I went to find a 004 o ring today to put over my valve stem so I could get an idea of how far it was opening, but didn't have any luck with that itty bitty size locally.  I guess I'll have to order some......and pay 500 times the cost of them to be shipped.

I'm excited to get a barrel on this thing and get it in front of the chrony. 

Mike

[rsterne]

Smallest I have is an 006 or I'd just mail you one....

Bob

[michaelthomas]

Thanks for the offer, Bob......006 is the smallest available around here, too.

I'm just going to poke a hole in some rubber sheet and shove it over the stem and see what happens.  If I can make it snug, it should work.  My valve stem has only a few tenths clearance, so I'm hoping there won't be enough air coming past the stem to artificially move the sheet.

Thanks,

Mike

[michaelthomas]

I tried out the rubber sheet dealio.  I drilled a hole in the rubber then used a paper punch to make a donut around it.

It's difficult for me to measure very precisely, but it's between .025" to .030".  I really don't know what that means to me right now.....other than the valve is not opening greater than 1/4 of my .140" throat.

Mike

[rsterne]

Gaps that small are easiest to measure using a set of feeler gauges.... Your lift measurements sound feasible for a valve that small.... and incidently efficiency should be good as a results.... The amount of FPE generated will be the question, as that's not a lot of "area under the (lift) curve"....

Bob

[michaelthomas]

I had read in quite a few places that a .100" transfer port was plenty of area for a 20fpe .177.  That is what I based my .100" transfer port, and .140" throat with .078" valve stem.  It will be .100" all the way to the barrel.

I'll be satisfied with less than 20fpe.....but not less than 12fpe. 

I don't know.....hopefully I will sometime soon, though.

Mike

[rsterne]

At 0.035" or more lift (to maximumize the flow of the 0.140" throat) I would think you should be OK for your 20 FPE goal.... At your measured 0.025" lift I'm not so sure, that was all I meant.... However, getting more lift is just a matter of more hammer strike....

Bob

[michaelthomas]

At 0.035" or more lift (to maximumize the flow of the 0.140" throat) I would think you should be OK for your 20 FPE goal.... At your measured 0.025" lift I'm not so sure, that was all I meant.... However, getting more lift is just a matter of more hammer strike....

Bob

........maybe the 24" barrel will get me closer.  This test was done with 2000 psi in the reservoir.  I wasn't planning on regulating any higher than that.

I have a boost convertor that is capable of up to 100v on the way.  Right now, 60v is all I have with the current one.  That will allow me some more striking power to play with. 

I suppose I will use the 100v, even if the 60 is adequate for power.  I will be able to use less capacitance with the higher voltage to get the same kick.......and get a quicker lock time.

I cycled the cap through the solenoid 2500 times today on the original 9v battery.  It's still charging the cap to 60.1v.  I'll try to see if I can wear the battery out tomorrow.   Resting voltage of the "9v" is now 7.6v.  I checked the cap voltage throughout, and never found it to be anything but 60.1v......so that's a good sign.

Mike

[michaelthomas]

I did some more clicking this morning while drinking my coffee.......and at 3500 shots, there are still no signs of any fluctuation of the cap charge voltage (still 60.1).  Battery is still over 7 volts.  This is an enercell from Radio Shack......which seems to be rated about 310mah.  I'm betting I could still get another 1000 shots from it......but I honestly don't care anymore.  3500 is plenty for me. 

I had considered a 11.1v 850mah 3s lipoly.  That would easily get me well over 10,000 shots based on what I am seeing so far.  Pack weight is 54grams compared to the 45 grams of the standard 9v.  Footprint is slightly larger.  Doesn't really seem worth worrying about at this point, as it will probably take me months to wear out a cheap 9v.

Mike

[michaelthomas]

I got some more done this past weekend.  Scope rail, swing breech, barrel block and other little things. 

I'm really pleased with the way the swing breech feels......it's very tight and smooth.  I have to make a handle for it , now.

I should be ready to go when (if) the barrels from LW show up......aside from machining them. 

Maybe this week?

Mike

[QVTom]

Looks gorgeous! And fantastic progress too!

Tom

[michaelthomas]

Thanks Tom.......thankfully it's just a little less complicated than your project.

Mike

[michaelthomas]

Still no barrels yet......big surprise, there.

I spent some time today (actually most of the evening) trying to get the stock in 3d so I can cut it. 

I think it's a good start, anyway.  I really struggle with freeform drawing.  I think someone proficient could have knocked it out pretty quick.  Still have to put the cheek rest in, too.

Mike

[SciGuy]

You're doing some great work.  Thanks for letting us tag along.