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Summary: I need to build a linear actuator that has a very fast actuating speed but fits in a very small space.

My Question/Idea: Electric gearmotors, c02, and air pressure, are all ideas that come to mind for this but they all have drawbacks. Gearmotors can only be so small before the power output to rpm is too low. C02 is dangerously high pressure and the small canisters are not very suitable as regulators and fittings seem to be scarce for their use in non purpose built applications. Air compression requires the compressor and air tank which are too large for my application. This leads me to wonder is it possible to drive a pneumatic cylinder using an ignited hydrogen oxygen mixture created using a very small electrolysis hydrogen fuel cell? When considering this I think of how a combustion engine works much the same way. Igniting a gas to drive a cylinder. I'm not sure whether pressure is maintained after ignition within the cylinder like in standard pneumatics with an air compressor. I would expect not. But if using a spring return cylinder would it not be possible to drive that cylinder forward with a very small amount of ignited hydrogen and let the spring return it to its original position? If this works we get a powerful and very small linear actuator with a regenerative gas supply that only requires water and a battery as inputs as well as electrolyte at a less frequent rate. I am interested in doing some experiments to test this idea but before I do I would like to hear from the engineering community. Do you think this feasible? Is there something I have not considered?

Specification Update: I am building a working version of the mantis blades from the cyberpunk 2077 game as referenced in the first image. The following two images are of my prototype so far. The linear drive is what I am trying to solve currently. My current configuration does not produce enough torque and is not fast enough. I can't find any alternatives that exceed this motor in power/rpm to size ratio which is why I'm exploring alternative linear drive options. The replacement needs to fit in the available space of the right and left side panels of the assembly. The dimension of space in each panel is 40mm Wide, 40mm High, 200mm Length. The base joint and upper arm joint are connected by .25inch roller chain(not shown in model) to allow blade to stay parallel with the base as it is raised and extended. I measured the linear force required to lift the upper arm assembly. This was done by pulling the rack assembly rearward with a fish scale in place of a pull force gauge so this is a rough estimate but I measured 19lbs or 84.5N. Most parts are currently 3D printed but will be aluminum so the solution will need to exceed the current state. I can calculate that later.

Motor: 24V, 3kg cm torque, 600rpm no load, 37mm diameter x 64mm length (more torque needed, not fast enough)

Lead screw: 6mm, 1.33mm pitch, 6 start, 8mm travel per rotation.

I plan to power the system with a lipo battery. I can specify further on microcontroller control and electronics if needed.

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  • $\begingroup$ Define "fast" 10m/s, 100m/s, 1000m/s? $\endgroup$
    – Solar Mike
    Feb 1, 2023 at 16:39
  • $\begingroup$ @SolarMike, .01 m/s. Ideally I want 40mm travel in .25 seconds. $\endgroup$
    – dutenh
    Feb 1, 2023 at 16:56
  • $\begingroup$ a solenoid from a car starter motor - even has a return spring already. $\endgroup$
    – Solar Mike
    Feb 1, 2023 at 17:00
  • $\begingroup$ 0.01 m/s is not fast. You’ve also not specified the force required. $\endgroup$
    – Eric S
    Feb 1, 2023 at 17:23
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    $\begingroup$ Your thing to make hydrogen is not a fuel cell it is an electrolyser. $\endgroup$ Feb 1, 2023 at 20:54

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I asked our chemical engineer at work about this and she was able to source a NASA research project that tested and achieved almost exactly what I'm trying to do. They built an electrolysis generated hydroxy gas combustion piston. Therefore my question is answered and I will begin development of my own much smaller version. Here is an excerpt from the document and I will include a link as well.

Extremely powerful, reusable actuators can be constructed by generating hydrogen and oxygen through electrolysis and then igniting the gas mixture when actuation is desired. The amount and rate of gas generated is simply a function of electrolysis current and time; the more Coulombs delivered, the more gas will be generated, and the more power will be available. - NASA/CP-2010-216272

https://ntrs.nasa.gov/api/citations/20100021938/downloads/20100021938.pdf

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Can you do it? Probably, assuming you have a NASA-grade funding system.

Should you do it? Probably not. Hydrogen and oxygen is fuel, messing this up is an explosion. You're talking about creating an engine that isn't an engine; everything you do will be experimental. Keep your life insurance up to date if anyone depends on you, I doubt you can even buy property and liability insurance for development of such things, the people doing such things (NASA, Toyota) will be self-insuring.

I would expect a team trying to do this to already have an expert in robotics so that they understand the current state of the art that you're trying to take to new places. The team then needs expertise in combustion so that we can start with at least a hydrogen engine that we try to convert into linear power.

We haven't even touched on hydrogen generation, storage, and delivery. Note that fuel cells consume hydrogen, not produce it. On my submarine, the nickname for the oxygen generator (electrolysis machine) was The Bomb.

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  • $\begingroup$ Thanks for voicing your concern. I am aware of the dangerous aspects of this type of system. However, I'm not a novice. I've built a few variations of hydrogen/oxygen elecrolyzers and a pretty massive tesla coil. Both could've killed me if I didn't know what I was doing. To produce 100 psi I'll only be dealing with very small amounts of hydroxy gas and the design will include over pressure relief. You don't need corporate support or a team of experts to create something great. Just an idea, time, and willpower. Doing the same thing we've always done won't take us anywhere new. $\endgroup$
    – dutenh
    Feb 3, 2023 at 17:41

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