Exactly, as shown in the bottom trace. There is a gradual reduction from one amp to zero when power is shut off. Wonder why the electrical experts can't see that?
Datatek
Well-Known Member
You polish your helmet?Good job it's Remembrance Sunday, I was up early polishing my helmet.
dirty bugger
Datatek
Well-Known Member
There is no gradual delay, you're talking about milli seconds, it switches off and that's that, nothing is instant when switching inductors and you can get some very interesting wave forms. If you look at the voltage waveform, it goes off in nanoseconds.
It seems to me you read something somewhere and translate that into an immutable fact even though you actually have no actual hands on knowledge of what is happening, maybe your name is actually Thor.
I have the beginings of a complete working EAS system in the workshop using air rams to simulate the airbags, when it's finished I will be able to run it as if it was in a car to test parts.
I've been looking at replacing the driver pack, I could just make a copy of the LR one, apart from being unethical, a new version could be made using uptodate components making it much simpler.
Consideration has to be given to the electrically noisy vehicle environment, especially on the petrol, wouldn't do to have it play up to to ignition interferance for example.
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Keith the graph clearly shows the amperage falling from one amp to zero in a comparatively gradual manner after power is cut. That would be enough to cushion the seal onto the seat. Or is it a pure act of chance that possibly the most used item in the entire valve block never needs changing. Who mentioned gradual delay?
Datatek
Well-Known Member
Look at the time scale, 0.060 seconds is the rise time of the pick pulse, the fall is a hundreth if that, in mechanical terms it's nothing, as the hold current is one third of the pull current, the armature will drop very early on in the decay process. Look also at the leading edge of the pick pulse, it's not vertical, that's because it's switching an inductor, but in mechanical terms the armature appears to move instantly, same applies to the switch off.
I say again, look at the voltage trace, it switches off in nanoseconds, ie instantly to all intents and purposes.
O rings never need changing? since when?
As I said, air pressure may slow the closure, but electronics does not.
Who mentioned O'rings? The solenoid end seals are not O'rings.
tomcat59alan
Well-Known Member
You polish your helmet?
Well it took long enough for the penny to drop for somebody.
IIRC the springs aren't that stiff so there could be a mechanical cushioning effect from air pressure. Testing is the way to know for sure. I wouldn't be surprised if no-one really knows. How would you check in a pressurised system with almost no room? Tiny camara? Make some sort of electrical contact when the valve drops into place?
Be interesting to see Datatek's test rig when it is all up and running.
Be interesting to see Datatek's test rig when it is all up and running.
They are stiff enough to hold 60 to 70 or so pounds per square inch of air pressure back.
Datatek
Well-Known Member
I see you are ignoring the time frame of the decay waveform which in mechanical terms is nothing.
The 3 amp lift pulse lasts 0.060 of a second the decay from one amp to zero takes approximately three times as long. Any residual magnetism in the coil after switch off is enough to cushion the seal onto the seat.
ukadamwest
Well-Known Member
You guys are damaging my waistline
Hi,
I'm by no means an electronics expert and I have no idea one way or the other, of whether the solenoid valves in the p38 EAS close slowly or quickly, but I have to disagree that you cannot soften the closing of the valve by a gradual reduction of the voltage across the coil. You have the spring pushing one way and the magnetic field of the coil pulling the other way. If you were to lower the voltage to where the spring force only just overcomes the pull of the coil, the valve would certainly close with less force than if you just cut the power in an instant. Quite simply, the force with which the valve slams shut would be reduced by the same amount as the force acting from the coil on the needle (or stem or whatever moves up and down inside it). I'm sure it would still close quickly, as the power needed to hold the valve open is much lower than what's needed to get it moving from closed position, but in physical terms, a magnetic field pulling the opposite way of the spring will slow it down.
To the original question of the thread: I would recommend a tank, and a pressure switch to operate the compressor as needed, then build your own manually controlled manifold with 12V dc solenoid valves which are available from a number of sources. A less home-brewed solution is an Air Lift system, running RR bags, as already suggested by someone else.
Good luck with it!
Cheers,
Henrik
Datatek
Well-Known Member
The decay to near zero current (20 to 30 thousandths of an amp) takes about 6 to 10 milli seconds, that's quicker than the armature mass pushed by the spring can respond, the long tail you keep refering to has zero volts and about 20 milli amps descending, far too little to have any noticable effect. You will see a similar waveform if you scope a relay. Release time is defined as the point at which the magnetic field has decayed to the point where the spring will overcome it, at which point movement is pretty rapid, and any slowing effect of the residual magnetism is so small as to be insignificant.
However, the armature is pushing against 60 or so psi, so like a foot pump connected to an inflated tyre, one would suppose that air pressure would have a cushioning effect.
I should know for certain tomorrow.
Datatek
Well-Known Member
You forget that the armature will remain in the energised state until a critical point is reached in the decay, at which point the magnetic field has insufficient influence to hold the armature against the spring pressure, even if you leave a residual voltage into the coil, the slowing effect is marginal, that's why modulation techniques are used to close solenoid valves slowly. Don't forget also, the movement of the armature will generate a reverse polarity voltage in the coil which is why supression is needed, unsupressed the back EMF can reach many hundreds of volts as in an ignition coil.
kurtjohnson10
Well-Known Member
:nothingtoadd:
So we have now progresses from no slowing effect whatsoever to a marginal slowing effect?
while we are on the topic of complicating things, what about the bounce effects of the rubber end on the plunger, and the natural oscillations of the spring when it reaches the maximum position as the plunger closes ?
Pete
One would wonder just how the four corner valves maybe the most used and stressed items in the valve block never need changing. Maybe they are made from Land rover super rubber.
