High force solenoid

[i_am_fubar]

I want to try and compress a syringe plunger to give a jet of air by gluing a neodymium magnet to the plunger and discharging a cap bank through a coreless coil. Basically, a speaker coil. But on a higher energy scale. Does anyone know of a 'thing' that already uses this mechanism to produce an air jet that I can get and take apart?

Ta

[SeanB]

Not going to work too well, you will degauss the magnet well before you make the air jet. how about using a motor and a gearbox to drive the syringe via a crank, or just use a peristaltic pump like inkjets use. Depends on how much air you need volume wise, how much pressure and what flow rate you want, but you are better off using a lower loss thing like a diaphragm or a collapsible bellows instead of a high friction rubber bung.

[T3sl4co1l]

I mean, without pole pieces, it's just disgusting.  But even with, you're limited to a pressure of the Maxwell Stress, B^2 / (2 mu_0).  A 1T magnet is equivalent to about 4 atm, if the field is entirely on one face and zero on the other.

Much better off using a servo, gearing and crank.  Needs far less magnet (low torque motor), the magnet goes through many cycles as the plunger is pushed, you get all the mechanical advantage of the gear train and (about) as much power as the motor is capable of, without quirky or custom magnetic designs.

(Even if direct drive is stipulated, it's still better to use a transverse-field linear motor over a copper rod: the entire surface area of the rod can produce a shear pressure, rather than just the cross-sectional area of a magnet the same size.  Well, fine, it works the same with a stack of properly magnetized ring magnets on a steel shaft, doesn't need to be an induction machine.  But yeah, area or speed is the key to power.)

Tim

[Zero999]

A syringe will have a lot of friction.

How often is the air jet needed and at what pressure?

You might be better off using a small compressor, a reservoir and solenoid valve.

[i_am_fubar]

It's for a BB gun mechanism. I know, I know, they use a rack and pinion. Have designed a few of the conventional ones. This is just a 'I wonder...'

Diagram is definitely a better idea.

Will look into the magnet physics. Degaussing is a concept I knew about, but now I actually understand why it's the limiting factor in magnet Vs electromagnet.

Am I right in thinking that electromagnet to electromagnet has no theoretical upper limit? They just need to be big and powerful enough?

[tautech]

It's for a BB gun mechanism. I know, I know, they use a rack and pinion. Have designed a few of the conventional ones. This is just a 'I wonder...'

Linear actuators are another thought I had but too slow for this application.

You might have better luck controlling an already compressed air charge.

[T3sl4co1l]

Oh well what the hell, use a rotating system. All the work is already done, and optimized a century over!

The best you'll get in a BB gun sized package is probably one of those portable car tire pumps, 12V powered.  Pitiful flow, maybe not even good for 100 PSI, but that should be well within the domain of a BB gun I think?

Am I right in thinking that electromagnet to electromagnet has no theoretical upper limit? They just need to be big and powerful enough?

Eventually, you'll melt or ionize one or both coils, but there is indeed a pretty wide way to go until that happens.  Quarter shrinkers blow up the coil, but that happens in some hundreds of microseconds, in which time the peak flux density is, what... a lot?

On a related note, this is in part why I'm always amused by "gauss guns" -- they pull a ferrous armature, terribly inefficient.  A series of synchronous pusher coils, and a copper slug, can deliver real pressure -- if still with pitiful overall efficiency and power density.

Tim

[i_am_fubar]

Gauss guns are just that though, an amusement. There are some pretty high powered ones out there. But as you saw, efficiency sucks.

With regard to this project, I've got an electric tack / staple gun that puts out a reasonable force. Pretty sure that uses a cap driven solenoid. But also sure that direct kinetic impact is more efficient that pneumatics.

[Berni]

Yep using a solenoid for such high power pulse application wont go well. Its the exact same thing as a coil gun, i have built one of those and it could send nails straight trough things like tuna cans, but it used a big pile of capacitors to power it (totaling about 800J of energy)

Mechanical power storage is not so great. If you use inertial mass storage (like a flywheel or moving hammer) you will generate lots of outside forces due to newtons law of equal and opposite reaction, so the whole machine will want to jump or move when its operated. Using a spring instead solves this issue, but then you need a very strong spring and so because of it a strong mechanism to put energy into it and release it.

Pneumatic solves both of those problems. Gas is very light so no heavy moving mass to throw the whole contraption around and force generated by pressure can be controlled by controlling its surface area. This allows a small pump to generate a huge pressure by simply using a small surface area piston. Controlling the pressure is also easy because the valve can be designed to have a small surface area too and trough clever design the pressure itself can be used to actuate the valve. This makes it easy to control the large pressure with a small external force or electrical signal. Then where you want the large force you simply give it a large piston to push against.

But yes for generating the highest peak forces mechanical impact is the best. This is why jackhamers (be it electric or pneumatic powered) involve a heavy piston inside that repetitively smacks the back of the chisel. This is because the piston has a few milliseconds to get up to speed and then stops in under a millisecond on the chisel creating a huge peak force (many tons) but the energy of that impact is only a few joules to tens of joules. If this energy was around 1000 joules it would be the equivalent of pointing a full automatic machine gun at the concrete firing constantly at about 1000 rounds per minute. If it was around 4000 joules it would be the equivalent of fireing a large shotgun shell at the same fire rate (Likely resulting in the user being thrown backward like a too powerful firehose)

[Neomys Sapiens]

A better component than a syringe to start with would be a pneumatic cylinder. You could actuate it with a spring and use a small motor to tension it. Then look for so-called 'pressure boosters' respectively the principle they are build upon. They use a larger piston driven by air rigidly connected to a smaller piston in a separate chamber, which results in a pulse of air with a lower volume, but higher pressure. Such devices are commercially available (for example, to charge a 10 bar accumulator from a 6 bar airflow), but their flow requirements are rather high. Maybe if you could build your own, miniature version?

[i_am_fubar]

The brain fart that lead me to this thought was that it mitigates the need for mechanical parts other than a single linear member. As has been mentioned, this isn't going to be easy and while possible, will be very, very far from efficient.

The thread has given me some nice thoughts for diaphragms, inductive motors and repulsion as well as some pointers to magnet physics that I've not previously considered. So brain fart success.

With respect to compressors, air tanks and spring driven racks. These are all well understood principles to me, which makes them kinda boring. I'll use them when I need to, but for Hits & Giggles, naaa 

[joeqsmith]

You could skip the solenoid and just dump the cap right into a section of copper wire.   

[Berni]

You could skip the solenoid and just dump the cap right into a section of copper wire.   

Yep that also has a name of ETG (ElectroThermal Gun). To make it easier to reset and more effective the copper can be replaced by two contacts and a little bit of water (While using a few kilovolts cap bank). The water serves as a easily conductive medium to establish the arc, also later on flashes into steam to provide even more propulsive gas than just expanding hot air. This method scales better than coilguns into higher powers(tens of kJ input energy) tho is still very far from the efficiency of an electric motor.

This method is also used in oil exploration to generate a repeating explosion under the ship as a way of generating loud sound waves that penetrate the sea bottom and echo back with valuable info what is down there.