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- uses_tools commented on uses_tools's instructable Coil Gun Simulation With Femm4.2
- uses_tools commented on uses_tools's instructable Coil Gun Simulation With Femm4.2View Instructable »
Glad you can work with this. The intent was to provide a free tool set that allowsthe design and optimization of a linear accelerator. Maybe YOU can improve on the pathetic energy transfer from capacitor to slug. I wish you lots of success

- uses_tools commented on uses_tools's instructable Coil Gun Simulation With Femm4.2View Instructable »
Mach10? Build it before anything changes! :-)More seriously, I could imagine the units in the setup playing tricks ...Are there older versions of femm 4.2 that yield more realistic results when you do a simulation run?

- uses_tools commented on DavidSkeptic's instructable Making & Using a Shooting Board and Mitre BoardView Instructable »
This shooting board combo checks all the boxes:slanted plane with reference to the plane so that all of the blade edge is used,easy slide due to slick plastic surface for the plane.I also like the combination of shooting and mitre board working together - this saves at least a bit of storage space compared to two separate setups.I like it.

- uses_tools commented on holol's instructable How to Make Rubber Band Gun Which Fires Multiple Rubber BandsView Instructable »
Looks a lot like a nice simplification of one of oggcraft's release mechanisms:http://oggcraft.jp/eng/eng_p307.html

Hi mmdalix,interesting question.The way I understand it, the inductance change is assumed to be with the position of the slug - slug out, lower inductance, slug in coil -> higher inductance.Within an iteration step, a time dependency of L is not taken into account.femm 4.2 calculates the force, which is the starting point for acceleration, speed and position,by dealing with the change in stored energy in the coil-slug system based on how deep the slug is in the coil, I think it is called "virtual work" or something like that:The result is F=i^2 * dL/dx. The dL/dx is calculated inside of femm4.2, independent of the iteration of the program.A good explanation I found in the femm manual. Please look there.For your formula V=L * di/dt, the inductance is constant during a step n, …

see more »Hi mmdalix,interesting question.The way I understand it, the inductance change is assumed to be with the position of the slug - slug out, lower inductance, slug in coil -> higher inductance.Within an iteration step, a time dependency of L is not taken into account.femm 4.2 calculates the force, which is the starting point for acceleration, speed and position,by dealing with the change in stored energy in the coil-slug system based on how deep the slug is in the coil, I think it is called "virtual work" or something like that:The result is F=i^2 * dL/dx. The dL/dx is calculated inside of femm4.2, independent of the iteration of the program.A good explanation I found in the femm manual. Please look there.For your formula V=L * di/dt, the inductance is constant during a step n, the di/dt is replaced by ( i(n)-i(n-1) )/timestep.So even the current is constant during the step.The magic is in the small timesteps :-)I hope this helps a bit.Regardsuses_tools