Newtons Cradle Compressor

A (possibly) new kind of gas compressor, similar to a blade compressor.

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The latest article in Low Tech Magazine is on compressed air energy storage.

Reading around led me to Blade compressors.

There's a fairly clear video here, starting 60 seconds in.

Essentially the cylinder is a hollow doughnut/toroid as the cylinder with a piston moving around inside. 2 holes in the cylinder, an input and output are separated by a disc which has a slot in it to let the cylinder through (watch the video).

This got me thinking. its a nice design but pretty impractical for a home builder to cobble together. The 'blade' needs to match up perfectly with the piston, and the cylinder has a slot cut into it to attach the piston to.

To avoid the slot the entire thing could be turned into a linear motor. In my head there would be a ball bearing for the piston, copper coils around the cylinder to propel the piston.  The problem then is syncing up the blade to open when the piston is approaching and then I thought what about a second ball bearing? one ball bearing goes around the loop compressing the gas, once it gets to the ball bearing 'cylinder head' it hits it, the cylinder head gets knocked out of position becoming the new cylinder (like a Newtons cradle, thus the name) and the old cylinder stops becomes the new cylinder head. the new cylinder does it's loop, before hitting and replacing the current cylinder head and the cycle repeats.

Much googling hasn't turned up any alternatives, and I can't think of any show stopping issues.

  • getting closer to a workable plan

    bholroyd05/27/2018 at 22:19 0 comments

    I think a plan for building a prototype is starting to crystallise

    stator coils will be placed in pairs (to balance the force acting on the piston) around the loop.  Hall effect sensors will precede each stator pair and the stators will be connected to a H bridge so that polarity can be reversed.

    I foresee the density of stator coils increasing the further round the track. As the pressure increases,  more torque(?) will be needed to move the piston around. Although increasing power to the later coils my work as well.


    Most of the information I'm coming across is for x pole rotating motors. this 'motor' is going to be different in a few respects.

    1. Normal motors  tend to have little distance between the stator coils. Obviously a bigger gap would indicate the motor could be smaller, so you never come across ones with larger gaps. This means I don't really have a good idea what the optimum distance between stator coils might be.  My motor has the advantage that I can tune the distance between the coils but I'm no really sure whether I'll need 2, 20 or 200?

    2. in a normal motor, relatively few hall effect sensors (HES) will give you a good idea of where the motor is in it's cycle. In this motor, the piston is going to be pushing against a rapidly increasing pressure all the time, not only that but at some point the exhaust valve is going to open and at that point the pressure will essentially stop increasing so we can't really predict where the piston is going to be.

    The three solutions that I can think of are:

    A. Add a pressure sensor at the exhaust valve. from this we can calculate how fast the piston will travel and potentially get away without any HES (although we would need them at start up?) the issue is you still need to know where the pistons are when you turn the motor on, and if anything were to go wrong, the piston/cylinder head would get out of sync and you wouldn't know about it, and there would be no way to recover the situation.

    B. Use HES is pairs to calculate the current speed, this could be combined with a pressure sensor so you can work out when pressure stops increasing and work out how fast the piston is travelling and how fast it is likely to continue travelling.

    C. Use one HES for each stator pair. this is sort of my preferred option but not knowing how many stators I'll need gives me pause.

    3. the whole Newtons cradle thing. This video gives be some confidence that the whole newtons cradle thing will work with just magnetic force. there is some 'bounce' but relatively little. somehow I need to get it to work reliably with a cylinder head that is being held strongly enough to withstand a high pressure but then be pushed out of the way when hit by the cylinder. I have the option of  turning off the stator holding the cylinder head in place, but then there is nothing to stop the old cylinder flying off.. I could add stators in the run up to the cylinder head to slow down/speed up the approaching piston to the right speed.

    Ideally the cylinder head stator would just be permanent magnets that is just not quite powerful enough to hold the cylinder head in place against the max air pressure + the piston hitting it. But I can't see it being that simple. With issues 1 and 2 above I'm not sure I'm going to be able to read and control the the speed of the income piston well enough, and I don't know how much wiggle room I'll have with that speed.  

  • Research

    bholroyd05/26/2018 at 22:19 0 comments

    2 possibly relevant technologies are coil guns and tubular linear motors.

    Coil guns:

    Pros: Simplicity, and seem to be quite popular with the maker community. I managed to find numerous example circuits etc.

    Cons: very inefficient? down to 1%, but that might not be measuring what I think its measuring.

    Tubular linear motor:

    Cons: as can be seen by comparing the 2 Wikipedia pages above, TLMs have much less info available online. it isn't so bad though, TLMs are just a particular type of low speed linear motor, and is basically a flattened permanent magnet dc motor.

    pros: like dc motors could reach high efficiencies (90%+). Less need for high voltage. easier to control?


    my preference at the moment would be to go with a TLM. my main concern at the moment is stopping/starting the cylinder/cylinder head. coil guns and coil gun research seems focused on moving the projectile as fast as possible. Not so much about stopping it. So I'm not too sure how many hoops I would have to jump through to get that.

    Only downside is I can't use ball bearings any more :(

    clear tubing has been ordered, as have neodymium magnets.

  • Todo

    bholroyd05/25/2018 at 23:17 0 comments

    1. Read up on relevant stuff. Linear motors seem most relevant so far.

    2. find a good cylinder, Plastic hose? needs to be correct diameter. 3d print? withstand the pressure? tolerances?

  • Issues

    bholroyd05/25/2018 at 23:11 0 comments


    1. There's a reason I can't find anyone else doing this :-s

    more practically

    2. The cylinder head will need to be kept in place firmly enough that it won't be shifted by the increasing pressure of the compressed gas in the cylinder, yet will still reliably knocked out of the way by the cylinder, potentially the spacing of the exhaust valve could be such that the last X mm of cylinder travel is past the exhaust, 'super' compressing the resulting  air cavity, slowing the cylinder and softening the impact with the cylinder head? .

    2b. Active management of those coils might be needed to allow the cylinder head to be knocked out of the way.

    3. the cylinder need to reliably stop in the cylinder head position. perhaps the air cavity mentioned in point 2 could help?

    4. how close would the intake/exhaust ports need to be? intuitively I feel there should be as little dead air around the cylinder head as possible, but thinking about it some more. As long as the cylinder head is close enough to the intake so the the departing cylinder doesn't create a vacuum before reaching it, it should be ok? also air trapped past the exhaust is wasted? (or does it help launch the cylinder head)

    5. will a ball bearing actually work?

    5a. Shape. 'maximum' efficiency would need a flat face so the isn't any dead air? although muskets/bb guns show a ball bearing is at least workable.

    5b. Material. obviously needs to be ferrous. Linear motors that I've seen have magnets on them would the (bad terminology alert) 'magnicity' be strong enough without the magnets? or am I misunderstanding the issue???

    6. Coil Timing. the cylinder needs to be tracked and the coils 'firing' at the correct time. need to read up but wouldn't the ball bearing induce a current in the coil which could be detected and reversed? I'm guessing this needs a micro controller.

    7. compression effects.

    as air pressure increases the cylinder is going to slow down. but pressure will vary through the 'orbit' and the pressure of the canister, is this going to create problems, certainly timings won't be able to be hard coded.

    8. valve or valveless

    it is quite possible to get away without having a no return valve on the exhaust port. Advantages: one less loss of efficiency. easier to run backwards as a generator.

    Disadvantages: cylinder fighting against high pressure for the entire stroke.

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