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Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
This thread will document the progress on my current project, a Bipolar, Mains driven, 'Complex waveform' coilgun. I'll explain exactly what all that means in a bit. At this point, a vast majority of the project beyond some basic proof of concept experiments is on paper, and many elements of it are still largely theoretical. I'm starting this thread to document things until such time as I'm far enough along to start a proper project thread.
This project is (I hope) the culmination of nearly 8 years of frequent thought experiments, and occasional practical developments, largely stemming from work I did in high school on a 6 stage, 4.7KJ, SCR switched capacitive coilgun. This system demonstrated many of the flawed practices and theories surrounding many amateur coilguns, and inspired me to try and find a means of significantly improving efficiency. I also hoped to find a means of decreasing the price per joule of kinetic energy.
The concept went through a slow, steady evolution over time. It started as an IGBT switched, capacitively powered system that used exotic materials for a projectile (Permalloy, ultra high silicon steel, several others), and careful measurements to keep the magnetic field as close to saturation point as possible without exceeding it. Through a bit of math, it became apparent that those field levels could be reasonably achieved with far less current than expected, about 200A, which made Mains a potential power source.
This lead me to a number of different versions. Until recently, the final 'build' version was a 30-40 stage, 100-150A, IGBT switched traditional reluctance coilgun. Each stage would be as simple as possible, and would use my inductive trigger design to determine on and off time. During a brain storming session with Chris Russell over the winter, we came up with the first version of this new idea, and the whole design process has started over from there, but with some shared goals and hypotheses to test.
The new system is complex. It's also radically different from any other coilgun design I've found as yet, but others have at least tickled the notion. I will do my best here to describe the system, but I'm slowly working on generating some graphical representations of what I'm trying to do. All of this will be subject to modification and evolution. As an example, in the few minutes since I started typing this post, a new switching scheme has entered my head that I'll be investigating.
The basic principle is not dissimilar from a traditional reluctance coilgun. Instead of a steel projectile, the system utilizes N50 grade Neodymium magnets. The coils will function at a low voltage and very low current compared to a typical design. Right now I'm planning on using 150VDC (Rectified 110v mains) on a dedicated 30A breaker. The coils will be designed to keep the current draw at or below 30A. The coils will be driven in a bipolar configuration, with multiple polarity reversals with each shot. As the projectile approaches the coil, the coil will energize and draw the magnet toward it. My tinkering has suggested that the coil and magnet, being equal length, will have a fairly strong influence on each other at a distance approximately equal to the length of one of them.
This is where the graphical representations will come in handy. Just as the leading edge of the projectile reaches the edge of the solenoid, it will turn off, allowing the projectile to coast into the coil. When the projectile is approximately 1/3 of the way into the coil, the coil will fire again, this time in opposition to the projectile. The placement of the projectile in relation to the coil will result in the magnetic poles looking a bit like Np===={Sc==Sp Nc} where Np and Sp are the projectile's poles, and Sc and Nc are the coil's poles. This configuration results in a powerful attraction of the projectile toward the center of the coil based on the combined repulsion of the similar South poles and the attraction of the dissimilar N and S poles.
Once the projectile and coil become completely aligned, the coil is de-energized again, allowing the projectile to coast approximately 1/3 of the way out of the coil. The polarity is then swapped again, pushing the projectile out of the coil under force. Lastly, the coil is again turned off just before the projectile completely exits the coil, allowing it to coast completely out. When the leading edge of the coil lines up with the trailing edge of the projectile, the polarity is swapped once again, pushing the projectile away from the coil until it is approximately 1 projectile-length away, when the coil is shut off for the final time. While this final pulse in opposition is occurring, the projectile will already be within range of the next stage, and that stage will fire, attracting the projectile at the same time. Unlike a standard coilgun where the projectile coasts between stages, this should result in the projectile actually experiencing it's greatest acceleration between coils. It will also mean that the projectile is only without the influence of a coil during the brief inter-pulse periods inside the coils.
This same pattern is repeated in every stage.
The first version will use 1/2 inch diameter magnets and probably have 5 stages or so. From there I will decide whether to move up to a larger diameter projectile, or stay with the 1/2 inch diameter. The projectile is approximately 1oz per inch of length, and I'm currently planning to use a projectile about 2 inches long. The coils will be the same length as the projectile. Unlike a traditional coilgun, the idea that bigger magnet wire is better doesn't apply here. It is quite possible to ruin neodymium magnets by applying an opposing magnetic field that is considerably stronger than the magnet itself. The poles of the magnet are prone to flipping, resulting in an off-center and much weaker magnetic field. Thus the field strength of every coil must be carefully managed to keep it close to the ~1 Tesla field strength of the Nd magnets. This will also allow for maximum bang for your amp. I'm currently running simulations to determine what size magnet wire provides the best balance of field strength, resistance, physical size, and thermal mass (the less mass the better for cooling reasons).
The coils will be switched using 40N60 or similar FETs in an H bridge configuration most likely, but the possibility of a Half Bridge, push-pull type setup just became a possibility, so it will be investigated.
The triggering is currently the hang up. My original plan on the traditional 30 stage reluctance system was to use my inductive trigger to sense projectile position with no need to cut holes or slots in the barrel, and allow for sensing directly beneath the propulsion coils. However, with the more complex waveform this new design employs, I will need a number of data points for every stage, potentially as many as 4. This could result in more than 140 trigger circuits on the barrel, encompassing nearly as much cost as almost everything else combined. It would also be a huge amount of labor to set up.
Other possibilities are being investigated, including ultrasonic detection, fiber optics or some other clever use of optical beams, and an extremely clever system devised by Bjorn. He's come up with a means of detecting marks made on a kevlar thread (.014" diameter, most likely) towed behind the projectile. This would allow me to implement almost any timing scheme simply by altering the placement of black marks on a yellow thread.
I'm sure I'll come up with more information to include over the next day or two, but for the moment this is what I can think of to include.
Registered Member #2648
Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291
Problems I see: Switching/pulse control: Achieving the fast switch required will be an issue with the inductance of the coil if the coil is big enough to limit the current to 30 amps. Also the rectified current will not be pure DC so that would cause yet move problems.
Switching Timing: If the coil is reversed at 1/3 of the front of the projectile in, it the projectile would be propelled backwards. Also the allowing of coasting between will mean less time for the field to act on the projectile when the will be in greatest coupling with the electromagnetic field meaning efficiency loss.
wrote ... Once the projectile and coil become completely aligned, the coil is de-energized again, allowing the projectile to coast approximately 1/3 of the way out of the coil. The polarity is then swapped again, pushing the projectile out of the coil under force. Lastly, the coil is again turned off just before the projectile completely exits the coil, allowing it to coast completely out. When the leading edge of the coil lines up with the trailing edge of the projectile, the polarity is swapped once again, pushing the projectile away from the coil until it is approximately 1 projectile-length away, when the coil is shut off for the final time. While this final pulse in opposition is occurring, the projectile will already be within range of the next stage, and that stage will fire, attracting the projectile at the same time. Unlike a standard coilgun where the projectile coasts between stages, this should result in the projectile actually experiencing it's greatest acceleration between coils. It will also mean that the projectile is only without the influence of a coil during the brief inter-pulse periods inside the coils.
I'm not sure what you mean here. So the south is past the middle so the coil goes in reverse current to repel projectile then it turned of and repelled again?!
I think your thinking of each pole as an individual magnet. Not the thing as 1 magnet.
Suggestion: Forward current the coil, to attract the coil until the projectile reaches half way (equilibrium) where not more force can be applied to the projectile in the desired direction ( out of the barrel/to the next stage) Then reverse current (and polarize) the coil to repel the projectile on wards until such time as found efficient.
I've done some simulation but I have run out of time (need to do my C++ coarse >.<) so I only gotthe first to parts for your idea 2, here they are:
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Would it be an idea to try and use the induced EMF from the coils to operate the triggering at the right time?
I'm just thinking along the lines of how we drive PMSM motors at work, once we're up to speed (using either position sensors or sensorless) we drop into 'vector' control which is based on back EMF. Similar principals could apply here possibly?!
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
GhostNull wrote ...
Problems I see: Switching/pulse control: Achieving the fast switch required will be an issue with the inductance of the coil if the coil is big enough to limit the current to 30 amps. Also the rectified current will not be pure DC so that would cause yet move problems.
Interestingly, this issue has already been largely addressed by the SSTC community. They drive coils with far higher inductance at far higher frequency than what I'm using, and my switching scheme will be very similar in many regards to the H bridge topologies frequently used in SSTCs. Granted, this is a potential issue, but it can be partially mitigated by finding a balance between field strength and wire resistance, using smaller wire to achieve a more compact coil and a higher resistance. This will also help keep thermal mass low and make cooling easier. The mains will be augmented with a small capacitor bank that will help mitigate the potentially long rise time associated with mains power. The statement that it won't be 'pure DC' is just silly. It will have ripple, but no more or less so than any full-wave rectified DC power supply. The capacitors mentioned will also provide some smoothing to reduce the ripple.
GhostNull wrote ...
Switching Timing: If the coil is reversed at 1/3 of the front of the projectile in, it the projectile would be propelled backwards. Also the allowing of coasting between will mean less time for the field to act on the projectile when the will be in greatest coupling with the electromagnetic field meaning efficiency loss.
The interaction of the projectile and the solenoid have already been carefully evaluated with an actual single stage mock-up. Everything I described is more or less proven accurate, though I still have to tinker around with exact placement of the projectile when pulses begin for maximum energy transfer. The projectile will not be drawn backwards into the coil during the third pulse, so long as south pole of the projectile has exited the solenoid at least to some extent. This would only occur if the polarity was not reversed between the second and third pulse. The small periods of coasting are only a tiny fraction of the total time that the projectile interfaces with the coil, and there is no feasible way to utilize these brief periods constructively due to the location of the projectile's poles relative to the solenoid. Regardless, this method is very nearly a 100% improvement over the physical distance that the coil can interface with the projectile compared to a standard reluctance coilgun.
GhostNull wrote ...
I think your thinking of each pole as an individual magnet. Not the thing as 1 magnet.
Suggestion: Forward current the coil, to attract the coil until the projectile reaches half way (equilibrium) where not more force can be applied to the projectile in the desired direction ( out of the barrel/to the next stage) Then reverse current (and polarize) the coil to repel the projectile on wards until such time as found efficient.
I'm not exactly sure what you're referring to there, but that isn't at all how a permanent magnet projectile and a solenoid with a fixed magnetic field will interact. If you energize the solenoid, the magnet will absolutely not enter the coil half way as you suggest. It will enter the coil approximately 1/8" and stop unless the coil is shut off.
Avalanche wrote ...
Posted: Tue Aug 24 2010, 09:24AM Would it be an idea to try and use the induced EMF from the coils to operate the triggering at the right time?
I'm just thinking along the lines of how we drive PMSM motors at work, once we're up to speed (using either position sensors or sensorless) we drop into 'vector' control which is based on back EMF. Similar principals could apply here possibly?!
That only works when the motor reaches a steady velocity though, right? In a system where the armature is constantly accelerating, would it even be possible to pull of something like that?
Registered Member #2648
Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291
@Dave: Ah, okay then that make sense. And when I said pure DC i meant like not sine wave ripply not pure pure ... >.<
I'm still skeptical about your theories about the solenoid, magnet, field interaction though. I can't see why the projectile would come into coil and stop 1/8" into the coil. As you can see in the first simulation picture the flux goes all the way through. The projectile would still go all the way down till it reaches the center of the solenoid. Also if you look at the second simulation picture (when projectile is 1/3 of the way in and current is reversed) you can see the flux lines diverging/ repelling from the outward part of the solenoid (South) and linking/connecting with the backwards part of the solenoid (North). This means the projectile will be pushed backwards.
Maybe your thinking of the interactions only as basic north & south magnets not as in the actual magnetic flux =/
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
I'm confused as to what you're thinking will happen. I interpretted it one of two ways. Either you're thinking that with the coil energized the projectile will travel completely into the coil until their center points align, or you think that the leading edge of the projectile will travel into the coil until one pole of the projectile aligns with the center of the solenoid. Remember it's a permanent magnet projectile with a field nearly as intense as the solenoid. Neither of those scenarios fit with how magnets will behave around each other.
You can try it with a pair of bar magnets and see what I'm describing, but I'll try and get the graphical bits together tonight to eliminate any confusion.
EDIT: Added a graphic of how things will work. Note that the coil polarity is displayed, and changes with each pulse. The coil is energized between the start and end points of each pulse. It is not energized between pulses, and for the most part the distance the projectile coasts is listed. Between the third and fourth pulse, my test results on how far it needs to coast were a bit ambiguous. The experiment needs to be repeated.
Registered Member #2648
Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291
I still don't think thats right. I definitely think, incorrectly, your thinking of the solenoid like a magnet now. Solenoids shouldn't even even labeled as north and south as they have no north or south. In an electromagnetic field (ie. in a solenoid) the field/magnetic flux travels around the current flow. there is no north or south. I think your thinking of the solenoid like a hollow, tube magnet. Which it is not like.
Can I get someone better at writing to explain the difference between a solenoid and a magnet =/
In all your theory is all wrong and the more I look at it the more I see its wrong. If you don't believe me take a coil, put some current through it, put your magnet in and watch it get sucked into the center.
Edit: I tried it out my self and you are definitely wrong =/
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
The reason I was comparing it to a motor was because in say, a permanent magnet synchronous motor torque can be generated by 2 means - reluctance torque, and flux line cutting. In the extreme cases, 'switched reluctance' motors operate with no permanent magnets, just iron - and therefore all the torque is reluctance torque. This would be similar to a traditional coilgun with a steel or iron projectile which has permeability. In PMSM motors we try to get as much torque as possible from flux field line cutting, although there is always a small amount of reluctance torque due to the iron in the motor.
I think as soon as you replace the steel projectile with a magnet (which being a neo has a permeability of unity) you will lose the effect of reluctance completely, and all your 'torque' will be generated by another means (flux line cutting). However if you operate it in a 'synchronous' fashion, and match the EMF waveform with a similarly shaped current waveform, you will be able to drive the magnet through the coil in the same way a PMSM motor drives its rotor round.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I agree with the guys that are saying the middle pulse of current will not work (sorry Dave).
The best way to determine what will happen is to use the physicist's friend energy. See how the total energy stored in the space changes as the position of the magnet changes, the force is then dE/dx.
To compute the energy, use an FEMM, adding the H field from the magnet to the H field from the coils, and solve for multiple magnet positions.
Registered Member #2648
Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291
Thank you Dr. Slack =')
Quickfield might be a better sim, since it can calculate the force on an object. However, Quickfield as a none user setable mesh (in free version) and you have to go through some registations and stuff.
Whether FEMM or Quickfield I would not trust both for accurate values. I use sims more to visualise flux lines and proportions
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Mains Driven Bipolar Coilgun
