Since getting my custom pump to work well earlier this year, I've been pushing ahead on hydraulic actuator designs. There isn't a lot of miniature hydraulic equipment available for hobbyists, at least not at a good price, so I'm trying to make my own. I've settled on inflatable bladders as a fruitful direction to investigate - they don't have the same friction and seal wear issues as cylinders (syringes especially), and they can be customized in all manner of ways. In parallel, I've been working on moving parts designed to contain the bladders and be actuated by them.
Bladder Geometry
In the hydraulic demo from last year, I showcased my very first working bladder, which was a simple "pillow" - two rectangles of plastic film, with a valve stem inserted through the center of one rectangle, sealed together at their four edges. The problem with a bladder like this is that you can't get much range of motion out of it. When filled with fluid, it bulges a little - maybe a centimeter or so, at the small scales where I'm working. That wasn't enough for everything I wanted to do. I needed bladders with more complex geometries that could expand further, while still collapsing flat when fully deflated.
First of all I bought a heat-sealer, so I could melt plastic films together the professional way. (My first bladders were sealed with a soldering iron. The thought of going back to that method gives me the horrors.) This greatly improved the ease of making "pillows." Then I started trying for three-dimensional pouches with accordion folds. Getting all the little pieces of plastic lined up in the heat-sealer before they were attached to each other was too difficult, so I hit on the idea of sewing them together before sealing the seams. I even bought some plastic thread so that it could melt and be incorporated into the seal.
In short, I gave it a good try, but it was such a struggle. Sewing the little bladders together was a lot of work, and I never got one that was leak-free. It was particularly hard to get good seals where three seams met at a corner; no matter how much I might practice and refine the process, trying to make that on the heat-sealer was just plain awkward. Given how little success I had making wedge-shaped bladders with just one fold, I shuddered to think about my dreams of piston drivers with five or more. Manufacturing the bladders this way simply wasn't realistic.
So I found a better way. You can make accordion folds by sealing the sides of two pillows together at the center, cutting a tiny hole in the middle of the seal, and then sealing the edges of both pillows. This avoids a lot of fiddly cutting and sewing, and more importantly, there are zero three-seam corners. All the seals are two-dimensional even if the bladder as a whole is not!
Methods of creating central seals
I made my first center seals by folding the plastic around a strip of cardboard and putting it in the end of the heat-sealer, once on each side. This was non-ideal; it created a very small sealed area, the crease where the film bent around the cardboard got melted too much and could develop holes, etc. A better method is to shield all but the circular region you want to want to seal together, then blast that area with a heat gun. You might think I should be using an insulator (like cardboard) for the shielding, but from my experience so far, metallic objects that will soak up the heat actually work better! So you can fuse your two pillows in the middle, then snip the very center of the sealed area with scissors to allow fluid through.
Bladder Materials
Throughout the process of figuring this out, I had lots of general quality problems. I seemed to need a double seal at each edge (seal once, fold the edge over, seal again) to have decent chances of a working bladder, and they still sometimes leaked. Well ... I think the printed layer on one side of my film was interfering with a good seal. You may recall that I was using plastic cut from salvaged Thriftbooks poly mailers. I started noticing that the green side with the company logos didn't seal as well. In fact, when I'm doing center seals with the heat gun, those must be made silver side to silver side; the printed sides simply won't fuse (though I can get them to fuse, somewhat poorly, on the heat sealer). And then the pillow edges have to be sealed green side to green side, which doesn't work so well! I also lost at least one bladder because a tiny flaw or strain in the plastic popped open the first time I put it under pressure. These flaws are present because the plastic has already been wrapped around a book and beaten up in transport.
So I finally bit the bullet and got some pristine plastic films to try out. When you buy materials, you have more control over what you're getting, so I sampled two different thicknesses: a 2 mil painter's dropcloth (transparent) and a 4 mil plant bed cover (black). Neither has any coating or printing on it, and they both heat-seal wonderfully. Now I can get good edge seals on the first try, without folding the edge over and doubling the seal.
Bladder Inlets
The last element I need to talk about is the attachment of the valve stems. After some early experiments with silicone gel that didn't have much success, I used cyanoacrylate (Super Glue) for my first working bladders. Eventually I also tried a polyurethane sealer intended for use on nylon tents and such. The brand I got is Gear Aid Seam Grip+WP. The cyanoacrylate works okay, but in my tests (sample size = 3 each), the polyurethane proved more likely to make a good seal. It is necessary to respect its long cure time; I actually consider that a positive, since I cannot seem to use Super Glue without getting it on my fingers and possibly other things nearby. One thing I have NOT tried yet is two-part epoxy.
So, okay, the glues are tolerable. But for real quality and durability, I would love to melt the valve stem and the plastic film together. So far all my attempts to heat-seal this part have been unsuccessful; they are two different materials and don't want to adhere. The option that remains is some sort of chemical weld. But it's hard to find anything that is safe and available for in-home use that will dissolve HDPE/LDPE film. I tried acetone on the off chance that it might work. It did smooth the surface of my PLA valve stems, but it never seemed to get them to a tacky or gooey state, and it did absolutely nothing to the films. So this may be a non-starter, unless I hear tell of a magical solvent that can do it.
Hydraulic Actuators
So how can we use all these bladders, exactly? After pondering how I might get one of them to operate my existing quadruped joint designs, I decided it would make more sense to come up with a new joint that was optimized to hold a bladder. Meet my hydraulic spade joint. A wedge-shaped bladder fits into one side of the backshell and pushes the spade upward (or forward, depending on the joint's orientation) when inflated. There are attachment holes for a tension element to return the spade to its original position when the bladder is depressurized. The spade's edge is constrained to remain in contact with the center of the backshell by a string tied through a hole in the spade and the backshell.
The other actuator I tried out was this simple cylinder. To produce its relatively large linear motion, I made a bladder with five accordion folds, my most ambitious one to date. I hadn't worked out the quality problems yet at that time, so this bladder leaked in multiple places ... but if I inflated it quickly enough, I could still practice driving the cylinder. So it worked as a proof-of-concept.
Conclusion
I think I have all the components I need now: the pump, the valves, and the actuators are working well enough that it's about time to see how things play in an integrated system. I might be ready to start the process of designing and budgeting full projects that use these parts. Look out for more of my hydraulics work next year (if not sooner).
Until the next cycle,
Jenny
