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Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
In my thinking, it is partly due to overcoming static friction which is always greater than kinetic friction. Other than that, the statement is mostly based on experimental results from many different people.
Also, double posting, especially to get attention, is against the rules.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Turkey9 wrote ...
In my thinking, it is partly due to overcoming static friction which is always greater than kinetic friction. Other than that, the statement is mostly based on experimental results from many different people.
Also, double posting, especially to get attention, is against the rules.
I'm bumping this thread for good reason, I've got a theoretical answer to this question.
First I want to discount the friction theory. Barrel friction should be negligible compared to the forces exerted by the coils. I've got experimental evidence for this. My three stage coilgun gets 3.5, 8.3, and 10% efficiency on respective stages. If i gently budge the projectile into the breech (static initial conditions), the coilgun accelerates it to 27m/s. If I use my finger to knock it pretty hard (at least a few m/s) into the breech, still 27m/s, no change. This was done multiple times with just one stage and with all 3 stages- always the injection velocity made no consistent difference on exit velocity. So from that I conclude that the difference in performance is not due to static friction. Furthermore, I suggest that injection velocity does not directly improve efficiency.
Here's why (I theorize) the later stages are more efficient: not because of initial velocity, but because of initial projectile magnetic field. Consider the projectiles point of view. The first stage imposes the greatest change, from zero flux through the projectile to a very strong magnetizing field- this rapidly changing magnetizing field induces eddy currents in the projectile, which along with hysteresis are responsible for the additional losses. As the projectile exits the first coil and enters the second, it's still magnetized because current in the first coil is still ramping down as current ramps up in the second coil. As the projectile moves faster and faster, time between stages reduces, and ramp-up and ramp-down overlap more and more. The less the magnetizing field changes between stages, the less eddy current and hysteresis losses will be experienced in the projectile. Eventually the projectile sees a constant magnetizing field, and no eddy currents or hysteresis occurs at all.
What do you think? This is based on my beginners knowledge of magnetics and may have some holes, but I'm convinced static vs sliding friction is not the explanation. Please critique!
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Do the currents in the coils really overlap? I would think that current would have to be at zero by the time the projectile reached the center of the coil to prevent suck back. Magnetic field is after all proportional to the current. It doesn't have a phase change like between voltage and current. So by this thought, there would never be any overlap in the current ramp down and ramp up between the coils.
Not saying that the magnetizing of the previous coil doesn't occur. I'd be interested to see a trace of coil current as the stages fire that can relate them all on the same timeline. Same with magnetic field. I might do this using hall sensors on my current multistage design.
An interesting experiment: Have a coil that has no purpose except to magnetize the projectile. Have it so that while on, the projectile is in the initial position for the accelerating coil and doesn't move at all. Then turn in off just before the first coil fires. Compare the velocity with the a shot without the magnetizing coil on and see if there is a difference.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Turkey9 wrote ...
Do the currents in the coils really overlap? I would think that current would have to be at zero by the time the projectile reached the center of the coil to prevent suck back. Magnetic field is after all proportional to the current. It doesn't have a phase change like between voltage and current. So by this thought, there would never be any overlap in the current ramp down and ramp up between the coils.
I'm thinking of the case where the coil is the same length as the projectile. So if you open the switch when the center of the projectile is at the center of the coil, the nose of the projectile is already about to exit the first coil and enter and trigger the next coil. If the current in the first coil takes long enough to quench and if the projectile is moving fast enough and if the coils are close together, there will be some overlap. I guess you could also test this theory by spacing the coils far apart so as to ensure that the projectile isn't magnetized in between.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
I really want to understand what is causing the efficiency increase in a multi-coil design. There are many different ideas that seem to make sense to me, but they are still just theories. I think I'm going to try and make a projectile that has an accelerometer attached to it to get a good idea of the forces over time while accelerating in a coil. The projectile will push a light thin plastic rod with the accelerometer board attached to it. This of course will only work on at most 2 stages, but would be interesting to see. Once I get my project done, I'm going to experiment with different injection velocities as well. The hobby community needs a good source of data to answer common questions that no one has a solid answer to.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Turkey9 wrote ...
I really want to understand what is causing the efficiency increase in a multi-coil design. There are many different ideas that seem to make sense to me, but they are still just theories. I think I'm going to try and make a projectile that has an accelerometer attached to it to get a good idea of the forces over time while accelerating in a coil. The projectile will push a light thin plastic rod with the accelerometer board attached to it. This of course will only work on at most 2 stages, but would be interesting to see. Once I get my project done, I'm going to experiment with different injection velocities as well. The hobby community needs a good source of data to answer common questions that no one has a solid answer to.
Great! I'm surprised that this hasn't been done yet. Please keep us updated on your progress and findings because this is something I'm really interested in. You might also consider testing magnetic vs. spring injection at the same velocity to determine if it's a kinetic or EM effect
On another note, this experiment casts some doubt on my theory of overlapping current. He gets an efficiency boost from 4% to 14% from the first to second coil, and the current has plenty of time to fall to zero in between. Does magnetization take some time to wear off? I need to do more reading to understand this fully. I'll let you know if I find anything interesting and relevant to your research.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
I don't think an accelerometer would be useful unless it could measure 100's of g's (example calc: 0 to 20m/s in a 2cm length -> ~1000g)
Couldn't any residual magnetization be found from the BH curves of a material?
If the projectile material does follow the BH curves and has a residual magnetization, then successive coils wound in the same direction as the first would cause the projectile to be attracted to the coils more strongly i imagine. If that is the case, then the first coil is acting on an initially unmagnetized projectile and so would have less attraction. This, along with the law of inertia and to a lesser extent the static>kinetic friction would decrease the efficiency of the first coil. I suppose you could test the residual magnetization idea by placing a permanent magnet outside of the coil such that a non magnetized projectile would be attracted to it, but a projectile magnetized in the direction of the coils B field would be repelled. Another method would be to reverse the current direction on a 2 stage gun's 2nd stage. If the projectile is magnetized when it reaches the second coil, it will be slowed down compared to when the coil current was in the other direction.
Residual current in the coil would be a bad thing after the projectile passes through the center because it would experience a force in the opposite direction of its motion (suckback) Another interesting thought: If the residual magnetization of the project does occur (to an appreciable extent) then that would seem to suggest that the negative current spike of an underdamped coil would cause an additional repulsive force if it occurred as the projectile left the coil.
any thoughts? I'm a bit hesitant to believe anything i said, despite it initially making sense.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Forty, the residual magnetization thing is kind of what I had in mind when I was thinking that the first coil does most of the magnetization of the projectile. I think there is some validity to that and it would be interesting to experiment with magnetic and non magnetic injection techniques.
However after some more reading I've found that, like most questions about coilguns, the efficiency question has already been answered by James Paul and he points to a different cause.
He states that the cause of poor efficiency in linear motors is a low motion induced voltage. He explains the principles and derives some basic equations for the performance of a linear motor here. See page 4 for the results. I’ll sum them up here:
With a typical clamped inductive load (coil with no projectile), 100% of the supply energy becomes thermal energy, dissipated in the coil’s resistance. When you introduce the projectile, some of the supply energy is converted into kinetic energy of the projectile. From the supply’s point of view, this energy loss appears as an induced voltage of opposite polarity created by the moving magnetized projectile as it passes through the coil. Also, the induced voltage impedes current flow through the coil, which reduces the thermal energy wasted in the coil’s resistance. This answers the question as to why the later stages are more efficient. Induced voltage is proportional to velocity- so as the projectile moves faster, it induces greater voltage in the coils, causing less energy to heat the coil and more energy to transfer to the projectile.
Here we see the effect illustrated clearly, thanks again to James Paul (in the first graph). You can see what James calls a “knee†in the slope of the leading edge of the current pulses for the second and third coil. These knees indicate the point where the projectile enters the coils, and the induced voltage causes a reduction in coil current, and thus a reduction in coil heat dissipation. No such knee is present in the first coil’s current plot because the projectile is initially at rest, with nearly 100% of the energy going to heat since there is no induced voltage until the projectile gets moving.
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Some advices for my first coilgun
