Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Wed Mar 04 2015, 09:01AM

Ok, lets be constructive. (...and state the obvious again..)

The principle would be that it we can manage to make the discharge duration equal to the time the moving part will take to move from one end to the other of L

Ok. Basic multistage design.

Lets think about the mechanics:

using a ferromagnetic moving part to short the circuit

Thats describing a mechanical switch with the Projectuile as the closing mechanism.
The plasma and arcing of such a rubbing switch leads to rapid degeneration of the device equal to the Railgun-Problem. It also causes some friction. The flexibility of the whipers make the projectile additional sussceptable to being asymetric inside the coil wich causes additional friction (in massive proportions) and may even make the switch unrelieable. (When the projectile is forced to one side of the barrel, you need to make sure the other side still connects). To circumvent this, you could make the contact wipers tighter, thus increasing friction and wear&tear again.

Would it be practical to design such a circuit? my opinion is yes, with L and C in a PFN design

See above. The Switch is allready a big ESR. Its therefore not only a transmission line, but a lossy transmission line. (comparable to an oscilloscope probe cable). Math gets more complicated. So lets avoid it ^_^ Also you state yourself that you give bonus to everyone who could slow down the Pulse enough. And still you say "YES, its practical", with the knowledge that this problem is extremely hard to solve. This allone speaks against "practical".

Would it be able to produce a sufficient current during all the discharge? my opinion is no, but a sufficient current might be achieved with high C and clever design (thus coaxial)

Coaxial is not the best formfactor for high energy density (that would be Multilayer). You want high C and tend to something that inherently gives a quite low C per volume and weight? You wanted to be constructive! Please explain.

Would it be applicable to a coilgun design? in my opinion, a coaxial L and C with tubular design is close enough of a barrel that it would be realistic

As i understand you would wrap the coax around the barrel as we do it with copper to form the coil? If you speak of PFN, you would know that you might be confronted with contradicting goals here... the geometry of the Coax forms the pulse, so you cant be allways be effectively close to the barrel, since you need to vary in thickness while the optimal thicknes for couppling would be 0.
Next thing is that Coax guides a EM-Wave INSIDE the coax. there is no external magnetic nor electric field to attract the projectile. The external magnetic field would depend on the ESR, which you need to make big to have an outside effect. Yet you actually want 0 ESR. Hmmh.

Anyone wants to discuss this constructively?

Sry dude, constructively is different from "say what i want to hear". It IS productive to smash the idea.

Added bonus if you can form the pulse to propagate in the L C coaxial line at the projectile speed ;)

Formulas for this are on wikipedia. Present your ballpark calculations and lets discuss them! You said allready such a circuit would be practiacal. Sure this opinion is based on something solid?


See i was nice. You please be nice too and solve the problems above with reasonable answers WITHOUT guessing. Maybe if i got the Coax-Coil deisgn wrong, explain it further along the solutions for the mechanical switch problems and capacitor design reasons.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Wed Mar 04 2015, 11:44AM

I was not mentioning multistage design

I was also refering to the inductor being coaxial (in mechanical terms) to the capacitor, so that the magnetic field would be generated by the inductor

for the capacitor lets imagine two cylindrical electrodes of 5 and 3 centimeters in diameter and 20 centimeters in length which are coaxial mechanically speaking... that makes 1.7nF according to http://www.ajdesigner.com/phpcapacitor/cylindrical_capacitor_equation.php with a permitivity of 710 according to http://www.physics.usyd.edu.au/teach_res/db/d0006c.htm

that is, for me, a practical high voltage capacitor with high C

for the inductor I took a diameter of 2.5cm and a length of 20cm with 50 turns of 3mm diameter copper wire which according to http://hamwaves.com/antennas/inductance.html gives an inductance value of 6.8µH approximately (and 8.9*10^-5 ohms R)

that makes for a decent inductor which can be placed inside the inner capacitor electrode in a coaxial position


now that capacitor, at 50kV, will store approximately 2 joules in an ideal world, so lets say 1.6 joules for the 80% storage which is more realistic

discharge that in a 6.8µH inductor which I simulated here :

internal resistance of the capacitor is derived from a flat bar of width equal to half the cylinder cross sectional perimeter (as if it was flattened) to ease calculation, and I will take 1mm width for the material

outer = 15cm perimeter so 7.5cm wide and 2mm width which gives 0.0000222 Ohms
inner = 8.8cm perimeter so 4.4cm wide and 2mm width which gives 0.0000379 Ohms
total is 0.0000601 Ohms (source for bar resistance is http://www.eeweb.com/toolbox/trace-resistance which applies nicely)

so far I'm trying to simulate the capacitor discharge in ltspice but I have strange behavior if I set .IC V(capacitor node) 50k as it gives me a damped oscillation at the resonant frequency of gigavolts, so it will have to wait for further reasoning ;)

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Wed Mar 04 2015, 12:03PM

I compare it to a multistage design, because its also the conept of a moving magnetic field.

Back to Topic: first of all, your C=1.7nF and L = 6.8uH gives you a resonant frequency of about 1.5MHz.
Please use
and put in 1500 kHz and press calculate. Then redesign your material and reevaluate your resistances.

For LTSpuice: make sure you use "uic" in the Run command. you need no voltage sources.
your cap capacity entry should be "1.7n IC=50k".
Please also notice that LTSpice assumes a default ESR in Caps and Inductors of 1mOhm if you leave this field empty. Replace this with the values you recalculated using the equivalent skin depth.

Also notice that your capacitor wont work as expected. since the coil is at the potential of at least one plates/zylinders the shpae you use to calculate is wrong. The other plate is effectively only as far away as the coils diameter is away from the plate.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Wed Mar 04 2015, 01:05PM


how would you reevaluate that resistance based on which redesign criterias? do you mean a 0.1mm wire would be the one that matches the R I have in mind?

If I recalculate the inductor for 1.5Mhz wihch I missed first I have a reactance of approx. 60 ohms and an equivalent resistance of approx. 0.3 ohm

Can you please explain the effect it has on the discharge?


I'm OK with the default values ltspice will give and will change that, thanks

with those parameters I have a 800A current through L, but I still have ringing which, I think, would not occur if the mobile short was progressing inside the inductor

Do you have any idea of the way I could decrease L in time according to the moving short in a practical way?


I don't get what you mean there

I'm totally open to have my idea crushed if it's in a constructive way, as you can see, and will use this to progress ;)

Re: Coaxial LC design for pulse compression using a moving short
Dédé!, Wed Mar 04 2015, 02:28PM

Shrad, why would you use such a large distance between the inner layer and the outer layer of the water capacitor? distilled water has a dielectric strength of 65-70kv/mm so a few millimeters in difference should do it. If you also decrease the size of the inner tube and use multiple layers for the capacitor, you would get a way higher capacitance.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Wed Mar 04 2015, 03:10PM

that would also do the trick, I must admit I chose the values totally arbitrarily... and also I think my mind did the shortcut of a single layer capacitor due to size and weight constraints of a barrel, but a couple layers wouldn't hurt...

what is not practical though is that your endcaps have to be of the same dielectric strength (maybe PTFE insulator material would suit this use, but not standard machine grade)

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Wed Mar 04 2015, 04:01PM

Skineffect or better the skindepth gives you an equivalent thicknes of the conducting layer. No matter how thick your copper is, if you have 1.5MHz the resistance of a round wire is then the same as a holow cylinder with a wall thickness of the skindepth. (for a round conductor of course.
On a PCB-ground plane for example if the frequency is high enough there is no point in calculating trance resistance based on the full 35µm copper.. its the skindepth that counts. Same with your 3mm wire. Its more like a 3mm cylinder with a quite thin wall.
For the wire in the coil you could counteract that by making many parallel (isloated) wires. But you dont get the copper fill that way so your resistance is at least 130% of that what you expect.- practically its maybe 200%.
But it has huge impact on your capacitor ESR. There you cant have many parallel plates in your coaxial design which makes it way more resistive.
in other word:
"outer = 15cm perimeter so 7.5cm wide and 2mm width which gives 0.0000222 Ohms"
that 2mm assumption is nothing more than a wish! use the skin depth.

Your capacitor is calculated wrong, because the coil in beween the plates will have a potential too! The effective capacitance is then not caluclated by the distance of the 2 plates, its caluclated by the distance of one plate to the nearest inductor outline.
If you still dont understand: think about it this way: you have a conducting stuff inside the dielectric. That must do something weird to your capacitor and cant be ignored therefore.

Now lets think about the concept a little bit more: youve got 2 Joules stored and your half wafe is 0.3us. for a 20cm length coil. That makes 10cm acceleration distance (neglet the moving short, thats a bonus). That proposes a constant acceleration (if the world would be ideal ) of 1.05e6 m/s² and an end velocity of 0.315m/s. LOL
if you have a 100% conversion efficiency you would launch a projectile that is as heavy as 40kg.

Of course thats only with the most perfect ridicules assumtions which are far off the practical world. But as you can see: the ballpark figues are completely ridicules.

The problem is here, tha you start completely wrong.
You should define a Projectile and an energy specification first. Then work out which coil shape would be the best for the projectile and the required energy transder and THEN think about how to apply your concept. However i think the last step wont be so successfull. The stuff you can realize in your setup is by it own far away from that whats known to be efficient.

Re: Coaxial LC design for pulse compression using a moving short
Andy, Thu Mar 05 2015, 01:22AM

Shrad
This might work for the timing, the outer coils are open circuit when the inner coil fires they build up voltage triggering the spark cap, feeding power back into the discharge.

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Thu Mar 05 2015, 02:21AM

A spark gap is the same as a SCR. thats now the common multistage design. The only difference is that the sparkgap lossier, louder and degenerating rapidly.
Thats a completely unecessary step as long as you havent done the ballpark calculations stating that you cant achieve your goals with 6.5kV SCRs and REALLY need more than this.

Sry, to throw in practical considerations again.. starting to be the ass for the second time -.- . Proofe me wrong before complaining please

Re: Coaxial LC design for pulse compression using a moving short
Andy, Thu Mar 05 2015, 02:59AM

Ill argue sideways ;) the sparkcap can be replaced with any switch, im more constraing on the inductor value change be open and closing the secondary coils, the other parts were a cheap way to recylce some of the lost energy, depending if you close it quickly more time, but low eff, if you slowly increase the resisance high eff to a point, but quicker discharge.

The idea is pretty much the you change the L of a coil when the energy is stored in the field.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Thu Mar 05 2015, 01:09PM


I thought about designing the thing so that the discharge would be over before one period of oscillation of the equivalent LC so didn't account for AC properties, that's the reason I mentioned to create a design to shape the pulse as so the pulse duration matches the moving short travel duration


I was thinking that my spiral coil not being between the two plates of my capacitor, it would not reduce the capacitance between the two tubes


This is out of my reach as is, thus me wanting to discuss in order to better understand the implications and flaws of the idea


The problem is, from my point of view, that as everyone expects a projectile to be a cylinder of ferromagnetic material which is essentially a steel rod, or maybe a bead for some originality, everyone will design it the classical way and follow the same path again

I don't tell anyone that it's the way to do it efficiently or anything like that, just that as a coaxial capacitor is practical for high power pulses and multiple stages induce losses and timing constraints, it may (I insist, may) be interesting to follow simpler approaches

this is just to nourish my humble mind and the pleasure to discuss technical topics

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Thu Mar 05 2015, 04:19PM

I thought about designing the thing so that the discharge would be over before one period of oscillation of the equivalent LC so didn't account for AC properties, that's the reason I mentioned to create a design to shape the pulse as so the pulse duration matches the moving short travel duration

So you argue that the effective time is even less which increases frequency even more. So AC-Properties are even more important.....

I was thinking that my spiral coil not being between the two plates of my capacitor, it would not reduce the capacitance between the two tubes

Right, i double checked, my Fault

The problem is, from my point of view, that as everyone expects a projectile to be a cylinder of ferromagnetic material which is essentially a steel rod, or maybe a bead for some originality, everyone will design it the classical way and follow the same path again

The problem is, that your approach right now leads you to completely ridicules technical specifications. At least iterate over it. Make changes, make ballpark calculations.. mace changes.. calculate again. Just be practical.
Determining the projectile specification will NOT lead you to the same path all the time. You still get the oppertunity to use your apporach of delivering the energy into the projectile, however you at least should know what orders of magnitude you try to achieve. You design some kind of frequency thats compeltey ridicules. Actually the frequency MUST be caluclated in order to fit to the projectile, there is no other way around.

If an initial specification of what you want to achieve can make you idea impractical or not worth persuing, thats at least an answer to everything in this discussion. If its immeadiately obvious that you cant achieve ~20J into a ~40g projectile... then.... ? Its impractical.
If it still works.. then fine! You design a new method of energy delivery, nothing else. I dont see how this could lead to the usual paths.

You argue right now, that not knowing the practicality of the idea for the sake of the idea is a good point of view to estimate if its practical. I disagree. Sorry.

Re: Coaxial LC design for pulse compression using a moving short
hen918, Thu Mar 05 2015, 06:32PM


I think what DerAlbi is saying is, you shouldn't take lots of new concepts and put them all together. Think of one concept, then design outwards, finding and solving the problems from the middle, If you find a problem impossible / impractical to overcome, find a new, or modify the original concept.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Thu Mar 05 2015, 09:43PM


the thing is I have a work which eats my time from 7:00AM to 6:30:PM and then I have to get things done in the house and spend the half hour I can with my son before he goes to bed... so I have not had time so far to refine and iterate

all I can do is spend ten minutes now and then to read here and there and google a bit


if you take into consideration the time the capacitor takes to discharge and that the projectile has reached the end of the solenoid during that time, do you really care about AC and resonant frequency? I would think that it is the equivalent of a decreasing DC, no? again, maybe that is totally unpractical, I have not made calculations to check if the projectile would have reached that distance when the pulse has ended and not after several periods...

if there is AC, wouldn't the field even go back and forth in direction? the projectile would then just vibrate, no?

Re: Coaxial LC design for pulse compression using a moving short
Signification, Fri Mar 06 2015, 01:24AM

Shrad, you wrote:
That is valid if the circuit used to make the discharge time long enough for the projectile to move to the end of L keeps a somewhat constant energy flowing during the discharge
---------------------------------------- ---------------------------------
Do you mean a constant energy maintained during virtually the entire discharge OR constant DECREASE (to zero) in energy during the shorting interval?

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Fri Mar 06 2015, 04:11AM

a decreasing DC

Thats called a Oxymoron.
DC is by definition constant. Thats it. DC. If it decreases it inherently has a bandwidth and therefore frequency content ("AC").

Aa fourier analysis of the current waveform would show you the frequency characteristic. That would be mainly a peak at the resonant frequency which is a little widened and flatened out due to the exponential decay.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Fri Mar 06 2015, 08:13AM

I mean that the time constant for the C discharging would coincide with the time taken by the projectile to travel the barrel

but maybe I'm totally off

if you discharge a cap in an inductor, it will ring, OK this is AC, but if you disconnect the inductor at the moment C is discharged (positive part of one period of the AC frequency of the LC), do you really have to take care of all this?

I have to simulate this with a vswitch but I can't figure the hysteresis value for now

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Fri Mar 06 2015, 11:01AM

Every change in voltage or current implies a bandwidth. It doesnt mater how small or short the change is. Think about it, as you would buy an oscilloscope: to see the waveform on the oscilloscope you would no doubt agree that you need a decent speed scope with a certain Bandwidth. Because you know that you cant follow the waveform with your multimeter with the DC-Voltage/Amps range.
Why would you need a fast Oscilloscope, if it was DC?
And no matter what, a certain frequency will behave according to the laws of electrodynamics. Therefore Skineffect is alsways present and it will increase the ESR for the higher frequencies.

For inductors thats not a problem, because you can use parallel wires. The equivalent would be a multy laver capacitor with multiple plates. But then you are back to the usual design again.

But the actual problem is here, that you chose wrong components. Just reduce the freuqency.. and everything is all right. you cant accelerate a practical projectile that fast anyway. (To make is light will reduce size and therefore the effect µr changes dramatically at small scales,) Thats why you shouldnt design someting vice verca. Start with specifications, then look for solutions how you can meet them!

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Fri Mar 06 2015, 01:26PM

I'll simulate a vswitch driven by a pulse of the same duration as the first positive part of the oscillation and see where it goes

I'm totally OK with you DerAlbi, but as I stated I have no experience with coilguns and don't even want to build one, that was for the sole reason of discussing the idea

if we take out the surplus things in the concept, I would keep in mind the fact that a moving short would displace the magnetic center and there would be no suckback... what is your opinion on this?

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Fri Mar 06 2015, 01:50PM

I have realized that the time constant of a RC circuit having ten times the capacity and the same equivalent resistance would be under the picosecond discharge time, so you are totally right by saying my idea is impractical

do you see any mean of changing that time constant to a more practical value while still being able to transfer a decent amount of energy to the coil?

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Fri Mar 06 2015, 09:30PM

Sry, i lost the context.
RC-Constant in a LC-Circuit? Increased Capacitance, but decreased discharge time? Nothing here fits together.
Attach your simulation file.

To the moving short: what can i say. a traveling magnetic field is allready a known concept prooven working in the transrapid train. Of course thats a good thing, but there are many ways to realize this.

If you have only so short time to spare to think about stuff and do things, maybe you should resort to stuff that are practical for you and will source real projects.. this was a global was of time for everyone who was forced to react to get this content clean. there is a backside to sharing irrelevant ideas. imho.

Re: Coaxial LC design for pulse compression using a moving short
Andy, Sat Mar 07 2015, 12:43AM

Sorry bad day :(

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Mon Mar 09 2015, 09:08AM

Doesn't the RC constant of the capacitor and path to the inductor account for the discharge of the capacitor into the coil?

I would at least appreciate to be explained... or if this is such a waste of time for you to discuss on a conversation that you encouraged (developing my idea and confront it to your knowledge) then so be it, I won't cry you know... anyway, don't take offense but I was at least expecting you to educate my reasoning to a point of understanding my errors, which seemed to be a claim of yours

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Mon Mar 09 2015, 11:27AM

I do everything i can :)
I understand now what you mean by RC-Constant. You describe your Capacitor modely only and its ESR*C is ~10e-12s. Are you sure you used R TIMES C and not R over C ?
But i dont see why this concludes the setup as impractical. The circuit alone would still work. just assume practical values for R, L and C..

There are others things here that make it impractucal. Only if you would specify a projectile and then do the calculations you can know it. If there values emerge so that you need a RLC-Circuit with unrealistic quality factor you know for sure its bogus.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Mon Mar 09 2015, 12:26PM

then assume an iron projectile of 3mm diameter for 2.5cm of length

I assume that if the projectile acts as a switch, current will flow only during the time it is shorting the circuit

my questions is then how to design the circuit so that the projectile launch occurs only in the positive part of the first period of oscillation of the RLC equivalent, to take advantage of having only one direction of a magnetic flux

I'm almost sure you know what I mean, but I just have no clue where to start regarding practical values for coilguns so I won't be able to deliver correct ballparks

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Mon Mar 09 2015, 02:39PM

"correct ballparks" Oxymoron again.
What equations are your considering and where is your exact problem? I cant come up with values if you dont say what you need.

Using the projectile as a switch wont work. As long as there is any current flowing it will arc as hell. But if you wish to use the whole positiv halfwave of the current, replace the switch with an SCR.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Mon Mar 09 2015, 03:32PM

I don't really care about arcing, but I wouldn't use an SCR

I can reduce arcing later on and want to keep with a pure LCR design like I explained

to start on values I'd say 10µH for the inductor and 10nF for the capacitor are what I would consider practical if I was to build simple components

what is the achieved speed for a usual coilgun projectile? you'll say that depends and I must give values

I want to slow down the discharge into L so that its duration is low enough that a projectile would be able to travel a distance of 20cm or 30cm during the whole positive halfwave of the first period of the current, while keeping enough of that current so that the magnetic flux is strong enough to project the unpractical projectile in a practical way

is this practically practical?

joke apart, my simulation shows a current of 640 amps at max with a 50 ohm current limitation resistor... that makes a discharge of 1.6µs

I'd be curious about your advise, without arguing about the unfeasability of the thing ;)

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Mon Mar 09 2015, 05:50PM

Use to get an overview what people achieve. You can estimate Speed and Efficiency of the common designs there.
What you try to do is called critical dampened oscillation. Thats done before. See for theoretical background. I suggest you read the whole site at least twice.
The question is: do you really want to proceed from this point? You struggle with the most basic things.
Basically you ask how you could increase the resonant frequency of an LC-Oscillator with a fixed C... soo.. increase the L ? Why do you come up with 10uH anyways.. What pulls the projectile is Amp*Turns. Having low inductance brings nothing but a short pulse and high current..
I really dont understand why you consider this as practical when you allready see that your discharge time is in the mirosecond range.

Sry, i can not make the research for you. I am not paid for this and its going nowhere anyways. This whole situation is not the most promising situation for something "new" that actually works even after one does the math.
And i am getting bored to discurage you and beeing the ass for it just because people stuggle to accept their limitations. Sry again.

Re: Coaxial LC design for pulse compression using a moving short
Signification, Tue Mar 10 2015, 05:05AM

There are actually four RLC system configurations of a series RLC circuit (each configuration is determined by the RLC values. The series circuit remains the same). I have found that each can have their place for particular needs of projectile accelerators. They are: un-damped, under-damped, critically-damped, and over-damped. Each is described by a differential equation. The oscillatory systems (un- and under-damped) occur when the characteristic equation involves complex roots (but cancellation leaves only real solutions). The 'over-damped' systems occur when the equations yield only real numbers in the solutions. The interesting, critically-damped, system occurs in the transition from the under-damped (where R=0) to the un-damped system. I am in the final stages of working out in detail, application-friendly versions specifically for coilguns. It would be inconvenient to try to write the equations with corresponding solutions here.

On another note: it is incorrect to write "dampened"--the word is "damped". The former always makes me think "inexperienced" user. To increase the frequency of an LC circuit, DECREASE L and/or C.

Shrad: those are some excellent-looking current and voltage waveforms--no ringing! I really need to look into obtaining that "LTspice"? simulator.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Tue Mar 10 2015, 07:57AM

@Signification

this is a free tool made available by linear technology, with an extensive library of their switching and analog devices and a good support of general SPICE language which beats anything for the price

I'd like to simulate a decrease of L proportional to the displacement of the projectile, but I still lack some knowledge in coilguns as DerAlbi said

@DerAlbi

I'm not struggling with the most basic things like you say, I know my basics like any guy who has done an EE/System graduation and has done electronics since his young ages, so I guess the problem comes mostly from cultural/linguistic issues in passing ideas

I agree and thank you to point me to those sites as I admit I have not searched those routes yet, so I guess the read will be interesting

I hope that as the projectile shorts turn after turn of the inductor the current slope would get more linear and the amps per turns would keep a better acceleration than with a fixed inductor, but I still cannot consolidate my reasoning

Re: Coaxial LC design for pulse compression using a moving short
Signification, Tue Mar 10 2015, 01:51PM

Thanks (so that's what LT stands for). I will look into this. I forgot to mention that the frequency ALSO changes with R. I have not gone far into this yet but remember taking the advantage of the fact that in a coil, it's R is approximately equal to its H. I first had to determine, somewhat as you, how the reactance of the inductor, XL, dynamically changes with projectile position.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Tue Mar 10 2015, 02:49PM

XL=1/2pifL if I recall well enough

I assume that R is the main factor for keeping the amps per turn constant with a moving short

I'll read about practical projectile velocities and think a bit

Re: Coaxial LC design for pulse compression using a moving short
Signification, Tue Mar 10 2015, 03:04PM


I think it is actually XL=2pifL = wL since an inductor fights higher frequencies, and Xc=1/2pifC = 1/wC, since Xc decreases with frequency.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Tue Mar 10 2015, 07:51PM

you're right, I inverted the two formulas

Re: Coaxial LC design for pulse compression using a moving short
Signification, Tue Mar 10 2015, 08:30PM

I think there is something that will help provide answers we both need--If you know, or another member, know please post it--I will keep searching. It is what--I guess would be called--"INSTANTANEOUS REACTANCE" of a coil. L. I remember wondering about this in physics years ago--but now, during coilgun research this knowledge would be vital. When dealing with inductive reactance (XL=wL) and capaitance reactance (Xc=1/wC) all conditions I have seen involve persisting sine waves such as i(t)=sin(wt) continuously over time. However, what is the reactance when we are dealing with just the first quarter of a sine pulse?? As is the case in coilguns. The college physics textbook formulas only give this reactance for a steady continuous sine wave. Since I am looking for the wL of an inductor as just a fraction (0 to pi/2) of the current waveform goes through it, I would guess it has something to do with the instantaneous rate of change of current (di/dt) along the pulse. So, I guess the answer wouldn't be just a constant number of ohms. Perhaps, since XL=wl and I=V/R, then the answer is proportional to: R=V/Iwt??

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Tue Mar 10 2015, 08:53PM

What you search is called fourier transformation. A quater sine pulse is not possible without involving higher frequencies.

In general its not very descriptive at all to think about a Coil or Capacitor as it is a imaginary resistance. The only thing that really discribes its behavior is the components differential equation.
But you should know that since you elaborated about that just some posts ago.
Thx by the way, i am indeed a inexperiences english user, i hope i can keep up with andy at least

Re: Coaxial LC design for pulse compression using a moving short
Signification, Tue Mar 10 2015, 09:33PM

Pretty much -nothing- other than a pure sine wave is mathematically describable without a Fourier description. Which is the basis of the guess I made of R=V/Iwt. I think for a certain squarewave it was R=16V/piwt or something. My next step was to review "impulse response" in my old Fourier Analysis book. I am not concerned with imaginary resistance, just another way to look at the behavior due to this coil current pulse.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Tue Mar 10 2015, 09:40PM

in LTSpice you can right click on a graph an show the FFT

for the LC circuit above without a switch to limit the discharge time to the first positive half wave the FFT showed a nice peak at fres

when critically damped like this, I assume there would be another peak related to the EMI generated (as would tell the negative megavolt peak in the simulation)

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Wed Mar 11 2015, 12:10AM

X=V/(Iwt) only applies to sine wave.
on an Inductor the only thing that counts is dI=(U/L)*dt.
Or in you notation style dI = U/Ldt. hahaha
as said before, differential equations is the only thing that is important to Coilguns. there is no way to get around calculus. Coils arent some resistance and inductances doesnt change in presence of a projectile... its motor theory. Please get rid of your thought with Reactance stuff. it will help no understanding.

Re: Coaxial LC design for pulse compression using a moving short
Signification, Wed Mar 11 2015, 01:25AM

DerAlbi, you previously posted:
===================================
Really: i am offering my help by questioning your fears. First show me a basic calculation that shows the relevance and thats it.
For basic ball park figures its often enough to know
Resistance R = U/I;
Inductance I = U*dt/L
Capacitance: U = I*dt/C
=====================================
This is an improper format for differential notation--you cannot have a differential notation of the independent variable (dt) with NO corrosponding differential component of the dep. variable in the same equation. I only bring this up because you seem to be laughing at a notation, DI=U/Ldt, which I agree is improper, but you called it my notation. If I actually wrote this differential eq. verbatim: ""DI=U/Ldt"" I would like you to point it out--I don't usually make this error.

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Wed Mar 11 2015, 01:58AM

...........really? duuuude go buy a bigger car.

Re: Coaxial LC design for pulse compression using a moving short
Uspring, Wed Mar 11 2015, 10:02AM

Shrad wrote:

if there is AC, wouldn't the field even go back and forth in direction? the projectile would then just vibrate, no?

No, it wouldn't, unless the projectile is a permanent magnet. See e.g. Significations remark:

F(b) = (k) * d/dx(B^2) joules; where k is a constant.

Force is dependent on B^2, thus doesn't change under sign change of B. The force depends on a gradient of B. The projectile is pulled into regions of larger B.

There are other issues with the design:
a) Energy: You have 2 Joules, which can accelerate an iron projectile 3mm diameter and 2.5cm length (mass = 1.4g) to at most about 50 m/s. The distance it will travel during a say 1us pulse time is just 50um.

b) Pulse time: The velocity increase is F * tp / m. To accelerate a projectile of 1.4g mass to 50m/s within a time tp of 1us you need a force of 7 tons. That looks unlikely.

In order to get by using lower forces you need much longer pulse times.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Wed Mar 11 2015, 12:36PM

to get back to our critically damped oscillator

with formulas this time,

For L=10µH and C=10nF

half period is 1/2*2pi*sqrt(LC) = 1µs

resistor for critically damped behavior = 2*sqrt(L/C)=63.25ohm (take 64 as nearest R value)

I'm desperately trying to specify the L value as a decreasing function of time but LTSpice is throwing up errors

do anyone here know how to correctly set a dynamic value component in LTSpice?

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Wed Mar 11 2015, 06:36PM

Use arbitary behavioral source to implement the differential equation. Component parameters cant change during a simulation run
You could also use the flux-extension of the Spice-coils....

Re: Coaxial LC design for pulse compression using a moving short
Signification, Wed Mar 11 2015, 07:51PM


I really think there is a way--it exists and is a simple concept...

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Wed Mar 11 2015, 07:52PM

I have been able to include a .func directive in a .subckt block with a value decreasing from 10µH to 0µH in a set period of time, but it lacked the rest of the models

I don't want to implement a differential equation at this time, just to approximate an inductance which decreases over time

component parameters can change during a simulation by specifying a custom model or package, but this is not that easy to find documentation on how to do it

Re: Coaxial LC design for pulse compression using a moving short
Signification, Wed Mar 11 2015, 10:39PM


FWIW, I understand EXACTLY what you want to do and why.
Similarly, I just want to implement a specific current shape (through an inductor) which increases over time. I also realize this will be different than the standard differential equation approach.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Thu Mar 12 2015, 08:47AM

I'm glad someone has seen where I want to go ;)

Do your current shape need to sharpen over time? If so you can achieve pulse compression with a succession of LC in a coaxial ladder design : http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-5432.pdf

Ideally I would like the decrease of L to be a direct function of the I and time values but I can't figure what would better approximate this

I'm first trying to simulate a constant speed (constant decrease of L) and once done I'll try to simulate a constant acceleration... would a square function of time suffice? I still need to read about it

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Thu Mar 12 2015, 10:00AM

I gave the solution above. You dont wanted it. Now you keed stuggeling. WTF is wrong with you guys ?

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Thu Mar 12 2015, 10:11AM

...have fun. its a shame you dont want this solution..

]justignoretheunknown.zip[/file]

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Thu Mar 12 2015, 10:41AM

we don't even know the actual differential equation you are talking about, and you did not provide information which would drive some search for someone who doesn't know

I find it logical that I want to approximate first, dig math later

if you know the business that much it wouldn't pose a problem to explain why the complex math is needed for what appears to be a simple problem to novice eyes

to my eyes, it appears as a voltage which decreases over time to a value of zero, and I don't really care of the rules which determine the discharge curve... the fact that the current has a maximum at a certain value and reaches zero seems to be satisfactory enough, whatever the imaginary and real parts or differential equation

if the complex math is needed to simulate the decrease of L to zero over the span of time the current and voltage reach zero, please explain, or I see no goal of saying over and over that the solution relies on complex equations that we are not aware of

if you're not open to explaining this from start to a point where we are able to find things, then please let us progress at our own rate and don't complain

Re: Coaxial LC design for pulse compression using a moving short
Uspring, Thu Mar 12 2015, 11:29AM

DerAlbi wrote:

..have fun. its a shame you dont want this solution..

justignoretheunknown.zip

AFAIK this is incorrect. For time varying inductances you have

V = L * dI/dt + I * dL/dt

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Thu Mar 12 2015, 12:47PM

Uspring: AFAIK you are correct
I didnt want to make it more complicated and easier to understand. Small formula is better than a big one and considering the Simulation there is not so much difference since he got 50R in series. Also the inductance doesnt change so much, as long as the motor is moving..
Your formula still lacks the term for BackEMF which should have a bigger influence. (V = -F*v/I)


Shrad: the equations were mentioned multiple times within this threads alone (last by Signification), in wikipedia about indurctances and i really dont see how one can actually miss them if one doesnt ignore them on purpose.
There is no way to explain the necessity of math or equations besides the fact that this is the way how the system is understood and described. I keep telling this over and over and stay ignored just to be blamed that i dont explain them... wtf. The real problem here is that you dont want to hear about how complicated stuff is. You want to live in an easy world with rainbows everywhere and where you find a breaking new concept which all the bright people out there didnt find for no reason.
If you "don't really care of the rules which determine the discharge curve" then take a piece of paper and daw your current curve as you want it to be, i am sure physics will bend to your will. they you concept will work for sure and.. heeey next goal is to extract vacuum energy, right?

Re: Coaxial LC design for pulse compression using a moving short
Signification, Thu Mar 12 2015, 12:59PM

I can't seem to open DerAlbi's zip file for some reason! From Uspring's statement it looks like DerAlbi may be treating L as a constant thus still missing our point of view???

I have not had time to think about it yet, so the following is a partial start:

@Shrad,
Let Lf be final inductance (=zero), Li, initial inductance (max inductance). Similarly, let Nf final 'number of effective turns (zero), and Ni, initial number of turns all turns). Now looking at the following version of the equation for the inductance of a single layer inductor (solenoid):

L=Uo*pi*(r^2)*(N/L)*N,

where r=radius of inductor, N/L=turns per unit length, N = number of (effective) turns, we see here that the only variable for L as the projectile moves down the barrel is N. Thus we have L as a function of N or L=f(N) We can simply write this as a liner equation with initial and final coordinate pairs: (Ni,Li) and (Nf,Lf) with slope=(Lf-Li)/(NF-Ni).

Next, for the N variable changing as a function of time, a 'velocity' may be represented as v(t)=dN/dt; with initial condition N(0)=Ni. And for L: v(t)=dL/dt; initial condition: L(0)=Li.

This is my first thought written live. Before continuing, and dealing with time (t), current (I), etc, I have some ideas, but would rather further review the entire thread (I think there is another one). I admit I have not read it 'at least twice', and will further try to retrieve DerAlbi's zip file.

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Thu Mar 12 2015, 02:30PM

Try again... then the raw ASC-File... I did not miss the point, i just simplified it for the first impression. It does not take into acount that the field energy is preserved while inductance changes. It seems useless to be correct anyways. todays motto seems to be "if theres a peak and a zero crossing, its enough".

]renamefileextensionto_asc.zip[/file]


But thanks for the doubt.

Re: Coaxial LC design for pulse compression using a moving short
Signification, Thu Mar 12 2015, 04:17PM


NO WAY!!

99% of the time, people really interested will listen if you present it in a proper manner--ask (don't tell them) what they need, likewise listen to the reply, and then work up (together) from there.

Your information has been very helpful, but 'picked out' of your negative responses--you don't leave options to receive questions.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Thu Mar 12 2015, 06:59PM

@DerAlbi

I do not deny that there is AC and that it influences the behavior, and I do not deny that you can calculate the complex math... I know that there is a trigonometric relation between I and U and that one doesn't go without the other

simply, I have not had extensive math during my studies and that was nearly a decade ago, so I have forgotten almost all the sine and cosine things as well as integer and imaginary terms

if you deliver the complete equations at first I simply cannot understand them and my brain just refuses to connect

I'm totally receptive to successive approximation and my brain will accept to iterate through the increasing complexity to a certain level

what Signification explains for reducing the differential relation to a simple function of the turn ratio is just perfect, it explains how to approximate and why to approximate that way, and it lets me understand better

now, I can apprehend the problem a bit deeper and will surely meet another aspect of the problem which will require to get a step further into math

btw I shouldn't have expressed that I wanted to modify the discharge current curve to my needs, but rather that the moving short would have improved the acceleration

and I know for sure that it has been done before in hypervelocity experiments these last decades

for what it's worth, my last response doesn't seem to have been posted, and I was thanking you for the .asc file which made things clearer

Re: Coaxial LC design for pulse compression using a moving short
Uspring, Thu Mar 12 2015, 07:29PM

DerAlbi wrote:

Your formula still lacks the term for BackEMF which should have a bigger influence.

I have some reservations about that statement. Think of a superconducting loop of wire carrying a current and a piece of steel near it. When the steel is sucked into the loop, its inductance will increase but the flux inside the loop won't change since there is no voltage around the loop. The flux is given by L*I. Since L increases, I will decrease by the same factor. The energy, being 0.5*L*I², will decrease. The difference might be just the energy, that is spent sucking the steel into the loop. Back EMF is, as you write V = -F*v/I, is also a statement of energy conservation.
This isn't a rigorous proof and might be faulty. Just a thought.

Re: Coaxial LC design for pulse compression using a moving short
DerAlbi, Thu Mar 12 2015, 11:28PM

If thats just a thought.... its a good one. you might be right, that just one of both "attachments" are at work. they could both be the same. havent checked that.

that would mean:
I * dL/dt = F*v/I
dL = integral(F*v/I²) since F ~ I² that would give dL = integeal(someFactor*v) sice the higher v makes shorter integration time dL is a constant within 2 integration-bounds. Yep. makes sense. cool! thats actually new to me. I knew both attachments to the formula, but i discarded one of both depending on the simulation i run (mechanical vs electrical).

That makes the most powerfull coil shape the shape that changes the inductance most in presense of the projectile. Which would be a single layer coil (best coupling factor) which has a bad L/R... aaawww i am sensing a true optimum there

Re: Coaxial LC design for pulse compression using a moving short
Uspring, Fri Mar 13 2015, 10:02AM

I * dL/dt = F*v/I

Yes, that's, what I suggested. It certainly doesn't hold, though, in the case of magnets moving near a coil such as in a motor. It might be true for the case of a projectile, which is not magnetized itself, i.e. only by the surrounding field.

Re: Coaxial LC design for pulse compression using a moving short
Uspring, Tue Mar 17 2015, 09:04AM

I've found some sort of a proof for that the term I * dL/dt contains the complete backEMF. Therefore we have for the coil voltage:

V = L * dI/dt + I * dL/dt

and for the force:

F = 1/2 * dL/dx * I^2

For e.g. constant coil current 50% of the energy suppiled through backEMF goes into mechanical energy and 50% into the magnetic field energy. If L(x) is known a full simulation could be made inside spice.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Tue Mar 17 2015, 10:15AM

thanks guys for the formulas, that makes it all clearer

in the case of a projectile being a moving short, it would be magnetized AND produce a field...

would that field be orthogonal as I think? what would be the interaction with the field of the solenoid? Somehow I can't figure this clearly (I have a sinusitis actually, which could play a role ;) )

Re: Coaxial LC design for pulse compression using a moving short
Signification, Tue Mar 17 2015, 04:13PM


Is the proof you found on the web? If so could you reference the link,
thanks

Re: Coaxial LC design for pulse compression using a moving short
Uspring, Tue Mar 17 2015, 04:36PM

@Sig: No, painful own calculation.

@Shrad:

would that field be orthogonal as I think? what would be the interaction with the field of the solenoid?

Think of a simple loop of wire carrying a current. It will experience a force trying to widen it. This is a special case of the general rule, that the forces try to increase the inductance. Something similar happens to a solenoid. It will try to widen and also to become shorter. Both of these also would increase inductance. Same thing with a piece of iron near the coil. It will be sucked into it and therefore also increases inductance.

If you have a piece of iron, which at the same time will change the number of turns, if moved, two forces appear:
One force will suck the piece of iron into larger B fields, i.e. into the coil. The other force will try to increase inductance by lengthening the coil. That will be a force in the opposite direction. The forces won't necessarily cancel, but the larger force will be weakened.

Re: Coaxial LC design for pulse compression using a moving short
Shrad, Sat Mar 21 2015, 05:10PM

as I understand it then the moving short would also reduce the coil mechanical stresses, right?