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Yesterday I told, that I haven't found an another way how to improve electromagnets,
It was partly true - I found one way, but I thought that it was impossible due to some reasons. Then I realized, that I might know a solution! And it seems possible to me now.
In previous logs was the idea - closer you place wire to a core, more efficiently it use current to induce magnetic field into core. So how we can maximize surface area of a core to place as much coil on the first layer as we can?
Solution for that was something similar to radiators which dissipate heat. However there was one problem - on the different sides of wire you have field with opposite direction, it weakens nearby electromagnets.
And today epic idea came to my head - it's right for straight wires, but around curved ones much more dense field is on the internal side of a curve, while outside it's kind of a weaker field!
Hello everyone! This log gonna be long-term one (not long)
Why? Well, of course there would be an update about new version of control circuit, which should arrive .. in some period of time (says "Export of international mail" since 20th of May), but periodic logs about how things going are gonna be less periodic, because almost everything what can be done to make first version of an actuator a reality was done, really:
- control circuit is fully tested, it retains energy well and controls everything what needed, as beta-version it works amazingly well and I hope it gonna work even greater with a new PCB : )
- technology of stripe manufacturing is fully developed, reliable and easy, all problems were fixed, I like current design of stripes too, as it provides great pulling force
^ this picture represents duel with fluid dynamics and glorious victory (thanx, corn syrup!)
Of course, I've measured a pulling force, now I have a nice equipment for that!
Everything there is measured with 20kg magnets on each side and 10x10x4mm inside:
- stripe can reduce it's length at least on 60%. more? possible, but with different proportions
- stripe provides from 500 to 660 grams of pulling force on it's own, I found that pretty persistent!
- attraction force between big magnets (or electromagnets) increases pulling force - 1kg to 1.5kg
Only one thing is not ready - electromagnets. Of course, I've done one and showed it in previous log, but it still can't provide significant attraction forces - it has flaws. About them:
First of all we need to have a look at coil itself. Wire here (along with current) works as a source of electromagnetic field, shown on the left side, and further we place core from wire - weaker field becomes. We need this field to orient magnetic field of our core's material atoms, which works as an "amplifier".
That means: less layers of wire we have in our magnet - more efficient use of current we get.
Also, material of core really matters, parameter called permeability is significant - so we need to get best material we can. That's why I want to use hydrogen chamber, to make Fe annealed in it. As I told in previous logs - such a material has one of greatest characteristics which is possible to get, so it's quite helpful in that project, but requires some time and additional equipment.
Well, everyone knows what syrup is. Sweet thing!
Probably it isn't on your kitchen, wide-spread in cakes.
As you may remember from previous update, I faced with capillary effect problems another time - oil just runs away from epoxy. One approach was to change mold some way or to pour more oil. But you know - more oil isn't good for health of an actuator, so I decided to find a way how to prevent runaway with same mold and same amount of oil.
And I gracefully present you this -> !!!I thought that problem was in viscosity... And it surely was!
I bought a bottle of magnificent corn syrup and water-thinned it (10 syrup per 1 water) to get epoxy-like viscosity. It's dirt cheap and yes, it has pretty high viscosity, without water-thinning it outperforms epoxy drastically.
Main thing: as a syrup it's just a high-concentration solute, so reducing concentration by water-thinning is a reasonable process. And as you may guess - it's possible to dissolve it in water after hardening of epoxy!
How great! Results are incomparable to results with oil, stripes are bending perfection : )
P.S. I love that it doesn't requires any noxious solvent.
Secondary part about electromagnet, as I've one completed:
Of course it still has somewhat illogical design, but as experimental equipment it should work. One thing with which I faced - compromise between voltage and current, to reduce heat dissipation on control circuit's semiconductors.
This electromagnet has a resistance of 2.2 ohms, well... :D
Quite low, but it is required from my perspective to have small powerful coil. As you know - inductance rules and induction heater still being delivered, so that's why I can't make cores with high permeability now, sadly
However I made pretty pictures from oscilloscope and first version of control circuit with that magnet!
You can see current on coil (reminder: it represents energy of magnetic field), and how boundaries of energy oscillations can be changed with magic of this circuit - here I move lower bound, different runs are coloured differently.
Log about fabricated stripes and accidental capillary effect!
What's wrong? As soon as I realized, that not much oil is not the case, I've imagined this picture:
- On top you can see previous structure. Oil stays in place, because it is pushed by epoxy from two sides
- Bottom is a new version, there is big air gap. And come on! Epoxy pushes oil somewhere!
How queer! But looks reasonable after all
Now main target is to find a way how to prevent it, because new structure looks promising and bends not so bad even with this major flaw in design. I made a wonderful GIMPed picture of that:
Of course I had only one stripe which bends well, so I mirrored it on this picture, just to show how it would be then contracted. I don't know why, but I like how "geometrically" it looks : )
Also I found a core for electromagnet, and designed a coilformer for test purposes:
This update is going to be quite clumsy, but... It's a pretty important one!
Why clumsy and also important? Because during yesterday's midnight design troubleshooting (also chill out) I made everything simpler and better. I guess so, because discovered things are pretty neat.
//And a-also-o, showing them is a hard task, so I would try to describe it without pictures.
Let's start from some questionable stuff :D
Bird here shows me yesterday, thinking about bending. Bending is quite important part of construction, as it is semi-soft and it's being squeezed along with bending of stripes...
And yes - bending is a tough guy.
Why so? Fabrication process is amazing, but still had hardcore part - fighting with capillary effect. Epoxy loves to ruin stripes bending-abilities in all sort of places. We used oil to prevent that stuff and it works. However, it requires some skill.
Either way, even if you managed to fight back epoxy, you still have small gaps between stiff (containing magnets) sections, it also affects bending. On the other hand, to make small angles between elements possible, we also used a lot of epoxy on sides of magnet, and this is no good... So!
So there it is. After some too complicated solutions I've got an idea, what we can just leave central sections untouched. It showed as a question marks on that strange picture above.
And there fun things started!
- we use bigger magnets, one big instead of two medium-sized
- by that we reduce epoxy/magnets ratio, good
- magnetic field near magnets is pretty dense
- so we get greater pivoting torque and pulling force, as magnets are bigger and closer
- distribution of magnets mass is more reasonable now, that is very good
- yep, it's softer now and can withstand small twists, great
- also, doesn't require precise pouring of oil, because it wouldn't change much
- so, easier to fabricate overall, use of epoxy also lower
I see there only positive sides. As control circuit worked pretty well, structure with drawbacks annoyed me pretty much, and it makes me happy to know that they were solved at the end!
So there is new version of mold! I want to 3d-print it tomorrow:
As almost everything was done with electronics,
(v1.2 control circuit PCB's are on their way)
it's time to think about structural questions.
And first one - electromagnets.
If we even can control them efficiently, it doesn't mean, that we solved problem of energy density, which defines how much force we can get from the electromagnet within some weight and size.
There are some things which I discovered recently:
- according to precious Coil64 (big thanx to developers, very useful software), with limited voltage best way to improve pulling force - is to use wire diameter as big as possible, however, too much current would generate big energy loss on semiconductors, so it's a questionable decision. Control circuit has a way to regulate amount of energy being pushed into coil during a cycle, so it might be a solution. Also, smaller capacity on C3 means significant rise of voltage (comparing to a supply voltage)
- cores with great permeability are wild! They can increase inductance (read as "top energy of magnetic field for a specific current") very much. I don't know where to find Metglas cores with needed dimensions, seems to be impossible now, but I've bought induction heater to produce "very-pure" iron, annealed in hydrogen. Permeability of such iron is very close to Metglas and it's possible to fabricate for yourself (of course - you need to be careful)
Combination of a great core material and control method should do the job, maybe I'll come up with additional ideas about increasing energy density later
A concept how it might look, with coilformers, cores e.t.c ->
Hooray! I've managed to simplify circuit even more!
Choose of electric components is more reasonable now - main MOSFET in D2PAK package and stabilizer for 5V logic voltage in small DPAK package, as logic gates never consume too much.
Here is how new schematics looks:
- one transistor instead of two
- low-pass filters on inputs, it can use PWM output from a MCU
- as well, filters are helpful to suppress high-frequency noise
Still, it has all experimenting stuff - V_TRG isn't connected to BAT+, so it's possible to play with it (or place wire between them) and still there is a possibility to try another approaches to measuring current with help of jumper JP1.
PCBAround 5x5cm now, with mounting holes
Now it's time to fabricate it and check everything, but I'm pretty sure what it should work, because previous version worked as needed:
Last two days I've spent in nature's lap, now I'm ready to think about ongoing things to do.
First of all I measured energy retaining rate more precisely:
- blue channel is voltage on gates of FETs
- yellow channel represents current on electromagnet, as always
This time I fixed problems from log(7.5) about PWMed output of an Arduino, I added two voltage dividers as an temporary alternative, now they work as a reference voltage source:
And yes, it seems that everything works now!
First of all, about changing energy oscillation boundaries. There is three pictures, each is made with the same settings of energy oscillation, but with different voltages:
15V, there you can see absolutely fine shape due charging process and adequate timings
18V, everything goes even faster, it's logical, because with greater voltage you can achieve target energy (current on coil) quicker
You can change oscillations of energy by changing reference voltages also
Well, what about conservation of energy now?
Here you can see current on a coil. I think it's legitimate to say, that there we see about 67% duty cycle
Here it is. Resistor connected to ground directly this time!
First of all I noticed stronger magnetic field, as well - low-frequency oscillations. I was happy about that and that is what I've seen on gates of MOSFETS:
196Hz square wave, Nice!
As predicted, everything was great on I_COIL input too:
Then I felt very frustrated, I realized what reference voltages (which should affect energy boundaries) unable to change behaviour of board much. And then I discovered that: