Hydraulics II: The Pressure Is On

I present an upgrade of the poor person's first hydraulic system in which I have solved some of its major problems and achieved a meaningful performance boost. This demo has now advanced from "barely goes through the motions" to "demonstrates practical functionality." Hydraulic system Version 2 was able to lift about 16 g (0.5 oz.) of coins on the end of a lever, and to repeat this motion over many pressurize/depressurize cycles.

The basic architecture of the system was identical to that of Version 1, but I traded out both the pump and the actuator.

The Pump

I replaced the syringe pump I built last year with my newly completed peristaltic pump. I've already showcased the pumps in a previous article. The main benefits of switching to the peristaltic pump were increased flow rate and improved regularity of flow. The syringe pump was powerful but miserably slow. The high friction of the rubber seal on the syringe plunger made it prone to stalling the motor if the speed was not set with caution. And since the syringe pump has very distinct intake/exhaust cycles, it has a pulsating output flow that becomes very pronounced at low speeds. (As in, if you don't have a big enough pressure reservoir to soak up that variation, you have to wait for the exhaust cycle before your actuator will even move.) The syringe pump also incorporated a pair of check valves, which were very bad about trapping air bubbles. These bubbles wasted some of the pump's work by expanding and compressing as it cycled, making room for water that *should* have gone into the pressurized volume to stay inside the pump. And lastly ... the syringe pump leaked. The seal in the syringe clearly wasn't perfect, and water would leak out the back at times. I can only imagine that this would have gotten worse as the pump aged.

The peristaltic pump trades much of the syringe pump's high maximum pressure for a more reasonable flow rate. Its output pulsates slightly, since the water moves through the tubing in "pockets" captured between each pair of rollers, but it's much more regular than the syringe pump. And it has no seals; there is no direct contact between the fluid and the pump mechanicals.

For a stronger comparison, I used the same stepper motor and gear assembly (unknown model, powered by 12 V) to drive both pumps.

The Actuator

In hydraulic system Version 1, a second syringe functioned as a linear actuator. At the pressure level where my system was operating, friction losses turned out to be a major problem. I couldn't put a load on the syringe because the pump was only equal to overcoming the resistance of the sliding seal; any extra load prevented it from extending the syringe. So the demo consisted of the syringe filling (very slowly) as water was pumped into it during the pressurize cycle, and being manually emptied by me during the depressurize cycle (since there was no load to passively push it closed again).

Rather than try to find better syringes, I thought to move away from them entirely. Those friction losses seemed like such a waste. But what other sort of hydraulic actuator could I use? The option I came up with was an inflateable bladder. What I had in mind was something very simple: literally an inflateable bag that, when pumped full of fluid, would change shape so as to move a joint or other mechanical element. It would have little to no friction losses to waste the effort of the pump. And it would have no sliding rubber seals to stiffen, shrink, or wear out over time.

As interesting as the idea seemed, I once again found it difficult to purchase what I wanted. There are such things as "lifting bags" or "air bag jacks," but they're designed for much larger applications. I think AliExpress had a few inflatable bags on the order of a few inches per side; still too large. I concluded that I would have to make my new actuators myself. I hit on the idea of cutting and heat-sealing plastic bags to create tiny bladders in any size or shape I wanted. Since I did not have a dedicated heat sealer, I heat-sealed my bladders by sandwiching the layers of plastic between two layers of aluminum foil, and running my soldering iron across the top of the foil.

In addition to sealing the edges to form the shape of the bladder, I also needed a way to include a valve stem - so I could connect the bladder to a tube and get fluid in and out. At first I was hoping I could scavenge some plastic tubing from my collection of mechanical pencils, cut it up into stems, and heat-seal the bladder walls directly to those stems. This never worked. The tubes and the plastic bags I was working with were likely different materials, and distinct types of plastic resist adhering to each other. I also tried sealing one edge of the bladder with silicone sealant and inserting the stem between the two layers of plastic, through the silicone. These seals always leaked. The silicone seemed to adhere to both the bag and the stem well enough, but didn't fill the gaps perfectly.

What eventually worked out was a 3d-printed valve stem with a circular flange, inserted not through an edge of the bladder but through one of its flat sides. The flange was one piece with the stem, and was attached to the inside of the bladder; the stem poked out through a hole in the plastic sheet. The second piece of the assembly was a plastic ring that fitted around the stem and attached to the outside of the bladder. I applied a ring of cyanoacrylate adhesive (Super Glue) between the flange or ring and each side of the plastic sheet. This finally gave me a watertight seal around the valve stem.

I still had problems with the bladders leaking at the edges, from holes next to the seal. This led me to experiment with some different kinds of plastic. All my plastic films are salvaged, and I don't know enough about package manufacturing to be sure what any of them are. They're likely all different variants of polyethylene or polypropylene. My first attempts used a smooth, soft, transparent plastic from packing air pillows (which felt very similar to the plastic from bread bags, for instance). It was inclined to melt into swiss cheese if I applied just a little too much heat when trying to seal it. Ramen noodle packets are made from a stiffer, more crinkly plastic. This heat-sealed reasonably well. But the real winner was the tough, thick, pewter-gray plastic from the bags that Thriftbooks sends my used paperbacks in. (I expect to have a never-ending supply of this.) Not only is this plastic very tear-resistant, but when I melt two layers of it together, it really fuses - such that the seal can't be peeled apart without ripping the plastic next to it. I think this is important for the durability of the bladders. For typical plastic packaging, heat seals that peel apart when the end user wants to open it are often a positive - but I don't want the water pressure to slowly work my bladders open over repeated cycles.

Even when using this heavier plastic, I had to be very careful that my aluminum foil sandwich didn't conduct heat to the wrong part of it and melt a hole next to the seal. Now that I've proven the concept, I think I'm going to buy myself a real heat-sealer.

After many trials, I produced a small bladder that was water-tight. I gave it a simple machine to actuate, in the form of a wooden lever attached to a piece of cardboard with a sloppy pivot. I mounted two pennies and two quarters on the end of the lever as a load.

The Results

This is the demo I wanted last year. To start with, the fact that it can actually raise a load is a major improvement. The speed of operation is also much more reasonable. I am now ready to move past this toy demo and think about final pump designs and real applications.

One issue that remains to be solved, which you may notice in the video, is that the lever experiences (relatively) small oscillations of position, which are likely following the pulsating outflow of the pump. In a real system with control electronics guiding the actuator, this would interfere with precise positioning of the lever. I think this could be mitigated by a larger pressurized reservoir (the demo system has none to speak of - the tubing makes up the entire pressurized volume) and use of a pair of two-way valves instead of a single three-way valve, which would allow the bladder to be closed off from the pump's influence once inflated to the desired size.

The one part I haven't tried to improve on yet is the cheapo pressure relief valve. For the demo, I basically just closed it all the way. The peristaltic pump has a little more "give" than the syringe pump, and seems to be able to drive against its maximum pressure without stalling. If I want a better valve, I may end up having to build one myself. We'll see.

For now, I'm very pleased with how far I've come, and hope to be showing you a hydraulic robot or two, someday.

Until the next cycle,
Jenny

Hydraulic Pump Parade

Ever since I got the proof-of-concept mini-hydraulic system off the ground last year, I've been working to refine the elements, starting with the pump. As a brief recap of my previous findings: the traditional water pump I bought had a high flow rate, but not enough power to push open a syringe, even when over-volted. And since the motor is contained inside the sealed pump housing, there's no way to gear it down or otherwise modify it. The syringe pump I built for the system had adequate power, but was very slow (trying to run it too fast stalled the motor), wasted considerable energy overcoming its own internal friction, required check valves that reduced efficiency by trapping air bubbles, and sometimes leaked from the back of the syringe.

The syringe pump set up for maximum pressure testing. The small-diameter syringe is shown loaded into the pump; the large-diameter syringe and its cradle are beside it.

I decided I wanted to get some PSI measurements to better characterize my two pumps, and the difference between two variants of the syringe pump with different syringe diameters. I also wanted to build and test a third pump design: a peristaltic pump. This and the syringe pump both belong to the class of positive-displacement pumps, meaning that they only permit fluid to move one direction (inlet to outlet) and guarantee that a fixed volume of fluid is moved on each cycle - assuming, of course, that the motor does not stall and there are no other malfunctions. A third subtype of positive-displacement pump is the gear pump. I haven't tackled this one, mainly because the pump mechanicals contact the fluid, so the housing has to be sealed. I didn't feel like bothering with that yet.

Pump Design

A peristaltic pump moves fluid by squeezing it through a flexible tube. The tube is curled around the inside of the housing, and the motor connects to a rotary element that spins at the housing's center. This rotor has three or more rollers which contact the tube. Pockets of fluid are sealed between each pair of rollers and pushed along the tube's length as the rotor spins. Since the fluid remains contained in the tubing, there's no need to seal any part of the pump. The flow pulsates slightly, following the distinct "pockets" of fluid as they reach the outlet, but is more continuous than that of the syringe pump with its distinct intake/expulsion cycles.

Peristaltic pump version 1

There are existing 3d-printed peristaltic pump designs, some even open-source ... but I made my own so I could have modifiable design files in my preferred CAD program. (DesignSpark Mechanical/Spaceclaim doesn't seem to be widely popular among the 3d printing community, I'm afraid.) That way I can freely adjust the dimensions and motor mount design, add integrated bearings, etc. I designed my first peristaltic pump for standard aquarium tubing (6 mm outer diameter, 4 mm inner diameter) and the same salvaged stepper motor and gear assembly I'd used in the syringe pump. I figured I would get a better comparison by powering them both the same way

I went through two major design iterations to get a pump that worked well. The first version ended up not being quite tall enough for the 4 mm tubing - the tubing expanded so much when flattened that it tended to escape the rollers. So I made the second version deeper, but also reduced the diameter to decrease the size of each fluid pocket and the total length of tubing that must be filled during self-priming. I added guides to help hold the tubing down in the track, even when the front half of the housing wasn't on the pump (this feature isn't strictly necessary, but allows a view of the pump interior and fluid movement during testing) and integrated bearings for the drive shaft, since I notice it was sometimes binding against the housing in Version 1. I followed this bearing design [https://www.thingiverse.com/thing:4547652] (available in many variants around the web), but used 4.5 mm airsoft BBs instead of 6 mm. The outer half of each bearing is part of the pump housing, and the inner half is locked into place by inserting the BBs after printing. (I also tried a design with fully captive BBs that are inserted during a pause mid-print, but there wasn't enough clearance between the printer nozzle and the BBs, so they stuck to the nozzle and were dragged out of the race.)

The final major change between peristaltic pumps V1 and V2 was the type of tubing. I swapped the silicone aquarium tubing for latex tubing, which turned out to be much softer and easier to compress. It's available in a variety of diameters at relatively low cost, and it seems to reduce the rotational resistance of the pump considerably. I experimented with a smaller diameter of tubing, and it was easier to get fluid flow going with this size, but only at a reduced flow rate. I went ahead and optimized for the 4 mm tubing that was my original plan.

Peristaltic pump version 2, with geometry correction shim (black)

The last tweak that was necessary to get pump V2 to self-prime and move water with the 4 mm tubing was a correction of the pump geometry. I mistakenly set the bottom arc of the pump to match the outer diameter of the tube track (inner walls of housing), instead of the inner diameter of the tube track (inner walls of housing plus width of squashed tubing). This prevented the rollers in contact with the tubing from properly sealing it closed, because the tubing's resistance would instead push the rotor off-center, into the extra space created by the bottom arc's slightly too-large diameter. Instead of re-printing the pump housing, I corrected this issue with a shim. This prevented me from putting the pump's lid on; fortunately, because of the tubing guides I'd added, I didn't have to.

Tuning the roller diameter is also important. Too big and the motor stalls because the rotary resistance of the compressed tubing is too high. Too small, and no fluid can move because the rollers do not compress the tubing enough to create a seal. The range of workable diameters seems to be quite small; I had to print several sets of rollers to get their size dialed in.

Testing Methodology

I tested all my pumps by dropping a long piece of aquarium tubing from my upstairs window to the back patio, and measuring how far the pump could raise water up the tube. The maximum height to which a pump can lift fluid, measured from the top of the fluid in the reservoir to the top of the column raised by the pump, is called the head. This can be converted to pressure via the following formula:

pressure (PSI) = 0.433 * head (feet) * SG

SG is the specific gravity of the fluid, in this case water, whose SG is 1.

Pump testing on the back patio

I didn't attempt to measure flow rate. I only made the general observation that the submersible water pump is very fast, reaching its maximum head within a few seconds; the syringe pump, operating at a step rate that avoided stalling before maximum head could be reached, was agonizingly slow; and the peristaltic pump, operating at the maximum step rate of the motor (which produced both the best flow rate and the greatest head) was somewhere in between.

Data

Adafruit 3V submersible water pump

Syringe pump (9 mm inner diameter syringe), unknown stepper motor

Syringe pump (5 mm inner diameter syringe), unknown stepper motor

Peristaltic pump, 4 mm ID latex tube, 3 rollers, unknown stepper motor, step frequency 0.5 Hz

Conclusions

I like the peristaltic pump's balance of pressure and flow rate, and will probably try to use it in my next iteration of a hydraulic system. The large syringe pump excels at slow high-pressure operation and performs better at low voltages, however. It might be valuable in certain applications, if I could figure out how to avoid leakage.

I still want to experiment with different numbers of rollers in the peristaltic pump.

Until the next cycle,
Jenny

Poor Person's First Hydraulic System

I've been taking tentative steps into the field of hydraulics, and I'm probably done with those for the year, so I want to write them up. My starting goal was pretty simple: get a minimum viable system together and make it cycle a cylinder. And I got there, mostly.

Why am I doing hydraulics now?

The robots I've showcased here so far have all been pretty simple. The eyeball has two motors; ACE in theory has at least eight. But in the back of my mind there's always a lurking desire for something with a *ton* of degrees of freedom. And once you add an actuator for every possible direction of movement around a joint, the weight of all those actuators starts to become prohibitive. Long-time readers of this blog will recall that I experimented with artificial muscles, specifically the ones made out of nylon. They disappointed me. My home-made fishing line muscles were wondrously cheap and, yes, lightweight ... but slow, power-hungry, and worst of all, not durable. So I'm beginning to explore other options.

The attraction of a fluid-driven actuation system is that you can have *one* motor - in the pump or compressor - which you locate near your robot's center of mass, and a panoply of lighter and smaller actuators that control fluid flow to the appendages. The full power of the central motor can be applied in any part of the robot; instead of having some smaller motors as useless dead weight when their joints aren't currently moving, you can use all the available motive force for whatever the current motion is. And with the right design, the system doesn't need to expend any energy to hold a joint in position either - contained fluid pressure will just do that. A choice remains between pneumatics (gas as working fluid) and hydraulics (liquid as working fluid).

Issues with pneumatics:

Noisy
Compressed air is a potential safety hazard
Motion tends to be fast and jerky - precision control is difficult
Leaks are harder to locate

Issues with hydraulics:

Messy (leaks can cause damage or contaminate environment)
Working fluid adds weight
Motion tends to be slow
System must be primed to remove air

Pneumatics are somewhat common in FRC robots, so I have experience with them from mentoring one of the local FRC teams. We tended to use them for binary motions, like gripper operation (the gripper can be either open, or closed as far as the held object will allow). See video of one of these robots. You can tell when the pneumatics are operating by the hissing sounds. Another year, we used pneumatics to power a throwing arm, which was expected to move quickly through its full range. We stuck to motors for applications that called for positional control.

And that, I think, was the main reason why I decided to go with hydraulics for my own recent experiments. I want that option of precision control. Concerns about the safety of compressed air were also a big factor. I want the freedom to work with unrated home-made or repurposed parts, without worrying about whether my system might explode and skewer me with shrapnel.

The parts

Now for the first hurdle: miniature hydraulics barely seem to be in use among other robotics hobbyists. In my research, I discovered that there's a small market for them among people who build working scale models of earthmoving equipment. Huh. (I took a look at some of their stuff - it felt too expensive for a first try.) But the maker community at large doesn't seem to have an interest, and that makes parts hard to find. The best prior project example I uncovered was this little hydraulic quadruped, which doesn't seem to be documented anywhere.

Fortunately, there is basically no issue with repurposing a lot of pneumatic equipment for hydraulics. I got a pair of solenoid "air" valves from Adafruit, and a bag of cheapo check and regulator valves meant for use with aquarium air pumps. My tubing is all aquarium tubing also.

My "cylinders" are Monoject medical syringes, which I already had a collection of because I save them whenever they come to me (usually with cat medicine). They're a bit of a problem. There is so much friction between the seal and the inside of the cylinder that moving the plunger takes serious effort, even without a load on the cylinder. (If you've never noticed, it's because you don't know your own strength, you big muscley human, you.) And lubrication doesn't help all that much. So I may need to find another solution for the next phase of this project, but I don't know what yet.

The pump was the biggest issue. It's easy enough to get a tiny water pump - aquarium suppliers and CPU cooling system suppliers are both good places to look. But these pumps are built for *moving* water, not pressurizing it, and I suspect they're all built with a poor speed-to-force tradeoff. They don't even advertise maximum pressure in their specs; the closest you'll usually get is a measure of the pump's "head," which is the height to which it can raise water in a vertical pipe. (This is not a pressure measurement, but you can convert it to one.) I included a little 3V water pump in my Adafruit order, hoping it would be enough ... and no way could that poor thing push water into one of my syringes. I'll have to use it for a decorative fountain or something.

So I built my own pump, following the well-known "syringe pump" design. A linkage converts the rotary motion of the pump motor to linear motion that draws a syringe plunger back and forth. Above and below the intake of the syringe are two one-way valves (check valves). When the volume inside the syringe is expanding, water is drawn in through the upper valve; when the volume inside the syringe is shrinking, water is expelled through the lower valve. Control over the connection between the motor and the pump mechanicals means I can theoretically use any gear ratio I want.

For this first attempt, I grabbed a random stepper motor and gear assembly out of my pile of salvage, hooked it to one of my driver boards and got it running under control from the Arduino, whipped up a 3d model of a compatible coupler and syringe cradle, and somehow made the whole thing go. It'll draw the pump syringe back and forth if powered with 6V, and run the hydraulic system well enough to push another syringe at 10V. (I can't seem to find specs for this motor, so I have no idea what voltage it's supposed to run at.)

The system

Diagram of an extremely basic hydraulic system

A basic hydraulic system is a closed loop in which fluid flows from the reservoir (not pressurized) into the pressure chamber, and back to the reservoir again. Valves admit fluid from the pressure chamber to the cylinders, and permit fluid to drain from the cylinders back into the reservoir. A relief valve or bleed valve allows fluid to move directly from the pressure chamber to the reservoir if the pressure in the chamber exceeds the relief valve's limit (this keeps the pump from stalling if the pressure is not being "used up" to drive the cylinders at the moment).

I was going for a single three-way valve that would connect my cylinder to either the pressure chamber or the reservoir - with a passive load on the cylinder, so that it would automatically drain back into the reservoir if connected thereto. (The pump could not help pull the cylinder closed because my reservoir was open to the air.) That's what the rubber band on the end of the syringe is for - I could wrap that around the plunger to create elastic tension that would empty the unpressurized syringe. In practice, my system was using up so much power just overcoming the friction in both syringes that there was virtually none left to drive a load. So I let it fill my syringe and then applied a dynamic load (with my fingers) to empty it. If you watch the video you will also notice that it is waaayyy toooo sloooow. I need more power and more fluid flow all around. So ... okay for a proof of concept, but this has a long way to go before I put it in a robot and it does anything useful.

Future work and thoughts

I already mentioned that the friction losses in the system are far too high. I probably need something that is better designed for easy motion than these syringes are. And despite how high-friction the plunger seal is, my pump syringe eventually started leaking out the back.

The check valves work well enough, but as I mentioned they are not designed for use with liquids. They tend to retain some air inside their housing; getting it all out when priming the system is difficult. And then that air soaks up some of the pump power. A part of its strength during each cycle goes into temporarily compressing the retained air bubbles instead of driving the cylinder.

The silly little aquarium air regulator valves are pretty imprecise. But I looked and looked for an actual relief valve (hydraulic or pneumatic) without finding one that could operate in an adjustable low psi range, for a reasonable amount of money.

So in principle this "works," but I need better parts all around. I wanted to demonstrate a poor man's hydraulic system and I'm reasonably satisfied that I have done so, but now I have to trade out all the pieces in ways that get the performance up.

Until the next cycle,
Jenny

Artificial Muscle Brain Dump

Hello readers!  I've finally taken everything that I know (or think I know) about nylon monofilament artificial muscles and rolled it up into one page that I hope can serve as a fairly comprehensive how-to and a handy list of related resources.  If I run across any new discoveries, I'll be adding them there as time goes on.  You can read the page here: http://writerofminds.blogspot.com/p/nylon-fishing-line-artificial-muscle.html  It will also be perpetually available under the "Special Treasures" heading in the right-hand sidebar.

Happy coiling!
Jenny

Artificial Muscles Actuating Things

They're still not terribly fast, and they don't move terribly far, but I've arrived at the crucial step of getting artificial muscles to actuate something more than just a weight hanging from a string. First up, we have a heterochiral muscle (the type that expand when heated) flexing a piece of paper. Each coil of the muscle is sewn to the paper on one side with a loop of thread, so the coils expand on one side and are constrained on the other, causing the paper to bend. This is almost a no-load movement, and strikes me as being most useful for something decorative, such as an artificial plant. You might notice that the cooling cycle is almost as quick as the heating cycle. I attribute that to chilly ambient temperatures in the upstairs laboratory.

Next we have a homochiral (contracting) muscle, rotating a piece of cardboard on a hinge. The opposing force of the rubber band on the opposite side pulls the cardboard back into its original position during the cooling cycle. The homochiral muscle featured in the video has been annealed with some space between the coils when at rest, so it doesn't have to be put under tension in order to have working room.

Both of these muscles are drawing about 1 A of current. They are made of Trilene Big Game fishing line, test strength 50 lb., diameter 711 um, with a heating element of 10/46 copper litz wire. The homochiral muscle has two strands wired in parallel. The heterochiral muscle was coiled on a rod of 3/16” diameter, while the homochiral muscle was coiled on a 1/8” rod.

Annealing muscles with built-in coil spacing

I determined in some previousexperiments that trying to spread the coils of a homochiral muscle when one first coils it on the rod is a bad idea. At this point there is a great deal of tension on the line – it wants to uncoil itself – and the coils don't cooperate very well. Coiling the muscle on a threaded rod is a possibility that I haven't tried yet; the spacing of the threads would limit the coil spacings one could achieve.

I started looking at ways to adjust the coil spacing after the initial annealing. First, I tried putting the muscle under tension (with no rod in the center) and running a high current through the heating wire, hoping to anneal it into its new shape. This method gives the most even coil spacing one could ask for, but the amount of heat applied to the muscle was only sufficient to “soft-set” it. I noticed that as it sat around for a few days, the coils slowly returned to their original close-packed configuration. When I tried annealing a muscle under tension at full heat in the oven, without a supporting rod in the center, the coils just went flat.

In the end, the best method I found was to put the muscle through its first annealing phase, manually spread the coils on the rod, then anneal it a second time. It's a little tedious – friction holds the coils against the rod, so you have to slide each one into the right position with your fingernail to get the spacing even – but it seems to work.

A close-up photo of the homochiral muscle with spread coils

An aside about plastic springs

In addition to artificial muscles, you can make simple passive springs by coiling nylon monofilament around a rod and annealing it (without primary twisting or a heating wire). The spring constant is determined by the thickness of the filament (larger diameters yield larger constants) and the size of the rod (smaller diameters yield larger constants). I got rather excited about this a few months ago, thinking I'd never need to buy a spring again. The problem is, these plastic springs don't necessarily hold up well.

If you follow any of my social media feeds, you might remember when I posted this spider leg video. There's a plastic spring at each joint, made from the 533 um Zebcom Omniflex line, and I'm actuating them by pulling the tendons with my fingers:

I took that video the day I finished building the leg. A few days later, the leg was in sorry shape, merely because I had allowed my house to heat up in the afternoons. This was sufficient to make the springs relax a good deal, so that I had to shorten them to get the same degree of tension I had before.

Naturally, this leaves me in some concern about the muscles as well. How might they be affected by high ambient temperatures? I haven't done any tests in which I compared a muscle's performance across many sessions of operation, with temperature spikes in between.

Blog news

Comments now require moderator approval, because spammers have been really junking up the place. Sorry.

In the new year, I think I'm going to try to build an “artificial muscle summary” page featuring everything I've learned, for the benefit of others who want to experiment. Once that's done, I may set muscles aside for a while. There are soooo many other things I want to work on.

Have a most excellent New Year!
-- Jenny

DIY Nylon Muscles VII

IT'S A NEW MUSCLE POST, EVERYONE! Sorry, I got distracted by other projects and life in general. One note before I get into my most recent work: I am now collecting other blogs/sites that deal with nylon muscles in the right-hand sidebar. If you run or know of a site that isn't listed, please tip me off so I can include it.

There were several things I got very tired of while experimenting on these muscles, and one of them was trying to measure deflections on the order of millimeters by squinting at a ruler mounted beside the muscle. Dangling everything from the edge of the dining table or ottoman was a bit awkward too. So I built myself a muscle test rig out of scrap wood. It provides a place to suspend an actuator and a weight and converts the linear motion of the actuator into the angular motion of a long needle. Small movements at one end of the needle are amplified at the other end, making them easier to see and measure.

The first version of the test rig just had a needle which pivoted on a piece of stiff wire driven through the board behind it. The pivot point was located very close to one end of the needle, and on the short end there were too loops of wire attached to the needle: one to connect the muscle, and the other to connect the weight. This arrangement left some things to be desired. For one thing, I could never get the wire perfectly straight, or constrain the needle so that it would lie flush with the graduated backdrop. That meant the needle's rotation was not planar, or it would stick as it turned, etc. On top of that, the motion of the short end of the needle didn't leave the muscle free to move straight up and down. Near the needle's zero point, the motion of the muscle is approximately vertical … but as the needle continues to rotate, its end begins moving more and more in a horizontal direction, changing the mechanical advantage the muscle has and introducing complications that I would rather not deal with.

 Left: Muscle suspending weight on deflection test rig, version 1.  Right: close-up of the reel on version 2.

Wanting something better, I replaced this lever system with a reel. A screw through the center of the reel provides it with a rotary axle. Two strings are tied to holes in the reel and wrap around it so that unwinding one string winds the other. When both strings are put under tension, they remain perpendicular to the side of the reel, regardless of the reel's angular position. I connect one string to the muscle and hang the weight from the other string, and the muscle is forced to lift the weight when it contracts. The needle is a piece of thin aluminum tubing inserted into a hole in one side of the reel; I can “zero” it by adjusting the length of string between the muscle and the reel. (I knotted multiple loops in the string, and a piece of flexible wire between the muscle end and one of the loops is helpful for getting things just right.)

For testing muscles that only contract over a short distance, this thing is amazing. No more staring at the muscle and thinking, Huh, is it doing something? I'm not quite sure. When the muscle starts moving, I get an obvious deflection out of the needle. The biggest remaining issue is that there's enough friction and/or elasticity in the system that there isn't a well-defined zero point for the needle. For a given muscle-and-weight setup hanging passively (muscle is turned off), there's a fairly wide angular range within which I can position the needle and have it remain stable. When I take a muscle through a heat-cool cycle, the needle generally doesn't return to its original position at the end of the cool cycle. How much of this is due to the muscle stretching out and how much is just the equipment, I unfortunately can't say.

The other little quality-of-life improvement I attempted for this round of muscle experiments has to do with ease of manufacturing. I was sick of going through the effort of making a muscle, only to have the fragile heating wire snap at the last minute. When that happens, the nylon can't be returned to its pristine state, and now the wire is too short – so often I would be forced to throw everything away and start over. Putting extra slack in the wire could result in bunching and loose wire coils, promoting uneven and inefficient heating of the muscle. So I tried a couple of different methods to relieve strain on the wire and keep it unified with the nylon.

Coiling a muscle with tape tags.

 For Method 1, I tried attaching the wire to the nylon at intervals of a couple inches, using little tags of adhesive tape. These can be removed after annealing by sliding a straight pin in next to the nylon and pulling outward to separate the two sides of the tape tag. Method 2 was a little more wild: I tacked the wire to the nylon by coating both with a thin layer of silicone caulk. Messy as it sounds, I found that the best way to apply this was to stroke it on with my fingers. It cleans up just fine with some mineral spirits (paint thinner).

In the end, I'm not sure if either trick helped a lot. For each method, I made four muscles and lost one out of the four (due to snapped wire). That's not horrible, but certainly not great either. I did seem to get nice even coiling of the wire around the nylon.

Besides trying out these manufacturing tricks, the principal experiment for this month involved rod-coiled muscles with spread coils. I had previously noted that the muscles with a smaller coil diameter could lift more weight, but had difficulty achieving a good contraction distance because their coils were already so tightly packed. I thought that coiling them around the rod with some spacing between the coils might improve that situation.

Actually doing this turned out to be harder than I expected. Homochiral muscles naturally form close-packed secondary windings. You have to fight the muscle to get it to lie on the rod any other way – and the small-diameter ones fight pretty hard. What you see in the photo is the best I could do. The mandrel diameter used for all of these is ~1 mm (large size paperclip wire). One of each type has a silicone coating, and one doesn't. The close-packed “controls” are on the top, and the ones with spread coils are on the bottom.

Yeeccch. Those look terrible. But I decided to see if they would work anyway. I used a current of ~220 mA and ran a bunch of tests with different weights. All the muscles were allowed to heat for at least 4 minutes and cool for at least 9 minutes, with the idea that this would be sufficient time for them to reach “steady state.” Results are given in terms of the needle displacement in degrees, and represent the maximum distance the needle moved from whatever its initial position was. An entry of “failure” in the table means that the muscle started to stretch under the load when heated, i.e. the needle displacement was negative. None of these muscles went flat or limp and became permanently unusable. For all the muscles, the lightest-weight test was the last one performed.

Muscle lifting @ 220 mA	71.6 g	60.0 g	50.0 g	25.0 g
Muscle 1: Packed, no silicone	6.5º	10º	13º	26º
Muscle 2: Spread, no silicone	Failure	Failure	2º	18º
Muscle 3: Packed, silicone	Failure	Failure	1º	16º
Muscle 4: Spread, silicone	Failure	2º	4º	12º

Thanks to the new test rig, I think these are more reliable than results I've posted previously – but you should still take them with a grain of salt, because running the tests spanned a hot summer afternoon, and I can't hold ambient temperature in the house constant. I wish I could repeat all of these many times and take an average, but I really don't have the time right now. So I'm putting up what results I have.

With that disclaimer out of the way – the plain old close-packed muscle without silicone is the best performer by a good margin. I wanted to see if the silicone coating would have any detrimental effects on the properties of the muscle, and it appears that it did … so even if it does help cut down on manufacturing failures, it's probably not a good choice. My awkward attempt to spread the coils on the annealing rod doesn't appear to have panned out well either. But that doesn't mean spread-coil muscles are entirely out of the question.

Perhaps one could wrap the muscle around the rod with close-packed coils – as it is naturally inclined to configure itself – and anneal it that way, achieving nice, even coils. Then the muscle could be removed from the rod, put under load and stretched a fixed distance, and annealed again by running an especially high current through the heating element. I suspect this would achieve much nicer results.

Lots of failure in this post, in that none of my little “improvements” really worked out for the better – but maybe someone else can avoid the same dead ends.

Until the next cycle,

Jenny

Nylon Fishing Line Artificial Muscles VI

I've finally done some characterization of these muscles. My primary goal was to investigate the relationship between the diameter of the secondary coils and various muscle properties/behaviors.

The Test Muscles

I made three rod-coiled test muscles, of different internal diameters determined by the size of the rods on which they were coiled. I attempted to keep other variables constant, including the following:

• Materials: All muscles were made from Trilene Big Game “Solar Collector” fishing line (50 lb. test, 711 um in diameter), with a single heating element of 10/46 litz wire twisted in.
• Secondary Coils: I tried to cut the filament to a length that would produce the same number of secondary coils in each muscle when finished (ensuring all muscles would have the same length when at rest). Due to a lack of preciseness in my manufacturing methods, I could not get them quite the same, but corrected for this by reporting relative contraction distance instead of absolute contraction distance in the weight lifting results.
• Primary Coils: I measured the coiling time required, going at the speed of the drill I was using, before a length of filament would begin to spontaneously form secondary coils. I then used this time as a guide for coiling subsequent muscles, scaling it by the length of filament used. Ideally, this would ensure that each muscle had almost enough tension to begin self-coiling, but not quite. In practice, again, the results were less than precise; for instance, two of the muscles developed a few secondary coils at the top. (I treated these as part of the muscle's “tail”, so they did not affect measurements. However, they show that these muscles ended up with a higher primary coiling tension than the remaining one.)
• Tension: I used the same 2 lb. weight as a load for each muscle during the coiling process. Unfortunately this does not entirely guarantee a consistent tension, because some of the nylon filaments were too long to suspend the weight for all or part of the coiling process – meaning it was acting as more of an anchor, and the force on the line would depend on the weight's friction with the floor as well.
• Secondary Coil Spacing: When coiling each muscle around its rod, I packed all of the coils as tightly as possible, so that the muscle would be fully contracted when at rest.
• Annealing: All muscles were annealed at a temperature of 300°F for 20 minutes.
• Chirality: All muscles were homochiral.

Experimental Setup

The passive spring constant test involved suspending various weights from a muscle and measuring the amount of deformation (stretching) produced. The muscles were not powered for this test.

The weight lifting test was intended to measure each muscle's ability to lift weight under power. One muscle was tested at a time. It received its current through a power MOSFET switched by an ATTiny85 microcontroller. A multimeter was included in the circuit to measure current. I had programmed the ATTiny to read an analog voltage, which I could adjust by turning the dial on a potentiometer. The ATTiny sent a PWM signal to the gate of the transistor, varying the duty cycle based on the value of the analog voltage. I hand-adjusted the value of the potentiometer to get the same (effective) current value for each muscle. The distance through which the weight was lifted was measured at steady-state, after the load had risen as far as was possible for that current.

Results

Passive Spring Constant Tests

Each data set has been fitted with a cubic curve.  A spring constant can be obtained for each muscle by taking the slope of the linear portion of the curve.

Weight Lifting Tests

Each data set has been fitted with a parabolic curve.  The (0,0) data point is assumed.

Summary

Muscle	Coil Diameter	Spring Constant	Optimum Load*	Max. Possible Contraction**
1	4.763 mm	8.63 N/m	32.4 g	22.7%
2	3.175 mm	40.12 N/m	60.3 g	14.3%
3	1 mm	237.62 N/m	82.8 g	11.1%

*Averaged over all currents tested on this muscle
**At highest current tested on this muscle, computed from curve

Conclusions

I am going to emphasize again that I'm not working with the greatest equipment here. Distances moved by all of these muscles were on the order of millimeters, and difficult to measure. Muscles can be reshaped by the heat, making the repeatability of the measurements imperfect. Couple that with the possibility of varying ambient temperatures in the house between test sessions, and you have a recipe for a lot of error. Nonetheless, I hope the data is useful in a rough, qualitative way. I'll posit the following:

• All else being equal, muscles with a smaller secondary coil diameter have a larger spring constant.
• Every muscle has an optimum load, a “sweet spot” on the curve, which allows for maximum lift distance when the muscle is powered. Too little weight doesn't stretch the coils far enough apart to provide a working distance, and too much begins to exceed the muscle's strength. I'm sure the optimum loads found for these muscles are dependent on the fact that they all have close-packed secondary coils. It would be interesting to re-try the experiment using muscles that are not fully contracted at rest, and thus have less need to be stretched by the weight.
• The size of the optimum load increases as the secondary coil diameter decreases. I don't know whether the optimum load is related to the amount of current as well, or is the same regardless of current; the data is too irregular to tell.
• At a given current, muscles with a large secondary coil diameter, lifting their optimum load, will lift higher than muscles with a small secondary coil diameter, lifting their optimum load. However, note that large-diameter muscles may have access to a smaller range of currents due to the risk of “going flat” (see yesterday's post).

I now have better answers to a couple of questions that have been asked previously:

Q: Once you've made a muscle contract, can you continue to run current through it and hold it in position?

A: Yes! Just be careful not to overheat the muscle. Current values that are tolerated in brief bursts may be enough to make the muscle flatten or go limp if they are applied for too long.

Q: What is the advantage of rod-coiled muscles over self-coiled muscles, or vice versa? What is the best secondary coil diameter?

A: The best diameter depends on your application. As I suspected, muscles with smaller diameters (including the self-coiled ones, which have the smallest diameter possible) can manage heavier weights, but can't lift them as far as large-diameter muscles can lift their optimal lighter weights (though I do wonder how the more tightly coiled muscles would perform if they were annealed with their coils spread out). The new revelation here is that diameter plays in to current requirements as well – large muscles working at their optimum point need less current than small muscles working at theirs.

Until the next cycle,
Jenny

Homemade Nylon Artificial Muscles V

It's high time to talk about artificial muscles again. I have the long-awaited data from the Weight Lifting Test, but to keep this blog post from getting monstrous, I think I'm going to save that for tomorrow. Today's post will be devoted to a couple of side issues.

It's Not Just about Fishing Line Any More

So far, I've only heard of these muscles being made out of two kinds of nylon monofilament: 1) fishing line and 2) conductive thread with a nylon core. Trimmer line – for weed-eaters – is made of nylon too. And it's thick nylon. Special lines for deep-sea fishing are the only other filaments I've seen that are this stout. So I decided to try it. Behold the super-muscle:

Nylon artificial muscle: the giant edition.

It's made of RINO-TUFF Universal Trimmer Line, manufactured by Jarden Applied Materials. The cross-section of this trimmer line is round (there are no ridges around the outside, as some trimmer lines have).  The diameter is 2.0 mm, which puts the appropriate coiling load at about 5.45 kg (12.0 lb.). After initial twisting, I wound it around a 6.35 mm (0.25 in.) rod and baked it in the toaster oven as usual, though I let it go almost half an hour to make sure it was fully set, since the line is so thick. The heating element is a single enameled copper wire, I would guess 22 gauge.

During my first quick test, the super-muscle lifted 0.454 kg (1 lb.) of dry beans about 1.5 mm. That may not sound like a very impressive distance given the size of the muscle, but it's more than I would expect my other muscles to do with that much weight. It's also possible that the heating element I used was a hindrance; single-stranded copper wire at that thickness has a noticeable springiness of its own that gives the muscle an additional resistance to motion. A more flexible multi-stranded heating element would be preferable, but I haven't tried to make better versions yet; I mainly wanted to see if the trimmer line would respond. Since it seems to be a viable muscle material, I hope to make more of these big guys, and will of course let you all know how that goes.

The Flattening Problem

When rod-coiled muscles are overheated, well before the line melts through they will usually “go flat.” The coils tilt into the plane along which tension is being applied, ruining the muscle's effectiveness as a spring. Something I have been noticing during recent tests is that certain muscles seem to be more prone to going flat than others. Take as an example the one on the right below, which was originally going to be the largest diameter muscle in the Weight Lifting Test … until it spontaneously went flat, while hanging vertically without any load on it, when my apartment got particularly hot one summer afternoon. The other muscles hanging up beside it were not similarly affected.

A healthy rod-coiled muscle (left) compared to a ruined flat one (right).

The muscle that went flat was the largest member of the set, so I thought that perhaps rod-coiled muscles with a larger diameter were more prone to flattening. However, when I started working with the remaining muscles in the Weight Lifting Test, it turned out that things weren't that simple. Of the three muscles, the 1/8” muscle seemed the most prone to flattening (which is why, in the data coming tomorrow, you'll see me using  no currents larger than 40 mA for this muscle … it could barely take that much). It was at least as sensitive to high temperatures as the larger 3/16” muscle, if not more so.

Muscles that “go flat” can be re-annealed to get them back to their correct shape. While this restores their effectiveness (at least temporarily), it does not seem to take away their inherent propensity for going flat. Therefore I wonder if there is some variable in the manufacturing method that makes some muscles more likely to flatten than others. I made efforts to keep the muscles that I used for the Weight Lifting Test identical in all but diameter, but my methods aren't exactly precise, so perhaps some differences crept in. Here are my best thoughts of some things to look at:

1) Amount of tension applied via primary twisting before the muscle is coiled on the rod. I tried to keep this the same between muscles by using the same load and twisting each one for an amount of time proportional to the length of the line. Nonetheless, it's quite likely that they have small differences. I sometimes had to stop early because the line started forming secondary coils before time was up, or appeared about to.

2) Ambient temperature in the environment during primary twisting. I didn't make all the muscles on the same day, so there could be some differences here depending on what the weather was doing.

3) Amount of tension applied during secondary coiling, i.e. initial tightness of coils around the annealing rod.

Until the next cycle,

Jenny

Homemade Artificial Muscles IV

Okay, I've been busy, but it's finally time for another artificial muscle update! I've achieved some of my best results yet, and I think I'm about ready to try using these in an actual application.

Oven Calibration and Annealing

I wanted the ability to test different annealing temperatures. However, the helpful KD5ZXG, who has been leaving a lot of comments on the first post in this series, warned me that temperatures in my toaster oven might not match what's shown on the dial. To get an idea of the actual temperature, I calibrated my oven with sugar and found out that it runs about 50°F hotter than the dial indicates when I'm trying to set the oven in the 300°F neighborhood.

Once I had a better idea of what the oven was actually doing, I baked a bunch of small nylon loops at different temperatures and for different lengths of time. These weren't muscles, just short pieces of nylon wound into circles, with their ends pinned in place by an alligator clip. Each time I turned up the heat, I allowed the oven and the metal pan to pre-heat for at least ten minutes before introducing any nylon samples. This is more time than it actually seems to need to get up to temperature, but I wanted to be safe – the oven can swing to even higher temperatures during its pre-heat cycle, so it's best to let it gain stability before annealing muscles. All the temperatures given in the conclusions below take the 50°F offset of the dial into account.

My green 711 um line (Trilene Big Game) is fine with being annealed for up to 30 minutes at 300°F, but starts melting at 350°F. The other two brands I'm working with (Cousin Clear Monofilament 381 um and Zebco Omniflex 533 um) do just fine at 350°F and below. I previously saw some of the Cousin stuff melt at 350°F, but I wonder if that was because I didn't pre-heat the oven properly before putting it in.

At 200°F, 10 minutes in the oven won't do much to set the loops of nylon … they want to unwind again after being unclipped. However, 20 minutes or more at this temperature will do the trick. At 250°F, 10 minutes still isn't enough, though the loops set better than at 200°F. At 300°F, 10 minutes is sufficient to set a simple loop, though for tightly coiled muscles you might want to go longer. This is the recommended annealing temperature, and since I've confirmed that it doesn't melt any of the lines (assuming I set my oven dial so I actually get 300°F!) it's the temperature I'll be using in the future. I've been annealing my most recent muscles for 20 minutes at 300°F.

Heating Elements

In my last post, I mentioned that tinsel wire seemed like the best heating element out of the ones I tried. My supply was very limited, though, since I pulled it out of an old pair of headphones – and try as I might, I could not find any place to buy it! The only tinsel wire that I found for sale in small quantities was intended for repairing speakers, and its diameter was far too large. So instead, I bought some 10/46 litz wire from this Ebay seller. Litz wire is made from many small copper strands coated with a film of insulation, much like tinsel wire, but it lacks the fiber core. It's so flexible that I suspect you could use it like conductive thread as well. I also bought some 30 AWG nichrome wire from this seller.

Conclusion: the litz wire is my favorite. I might buy a slightly thicker version next time, since it seems a little delicate; it's not nearly as bad as the single-strand magnet wire, but I've had it snap on me a couple of times. As part of a muscle, it heats fast, cools fast, and generally provides great performance.

As for the nichrome, it's reasonably good, but due to its high resistivity, it doesn't heat up nearly as fast as the litz wire. If your muscle is very short, your system has enough voltage to drive a reasonable current through the nichrome, or you just don't need your muscle to respond very fast, it's probably fine – but I would say that it's not as versatile as the litz wire. (You can always reduce voltage if you want less current through your muscle. Stepping it up is harder.) It's also not insulated, so if adjacent coils short together when the muscle is fully contracted, interesting things might happen. The best thing about it is its strength. I never had problems with the nichrome wanting to break during coiling.

But nichrome is MADE for heating! Why are you saying it doesn't work as well as copper?!

Nichrome is used in places like your toaster mainly because it can get very hot without melting or oxidizing. But I don't need or want the heating elements in my artificial muscles to be red-hot. I would like to be able to use them in low-voltage systems, say 5-6V. The amount of resistive heat you can get out of a wire is proportional to the amount of power you pump into it. And since P = V²/R, increasing resistance when voltage is fixed will decrease your power. All else being equal (same power supply, same length and diameter of wire, same environment), a copper wire will produce more heat, faster, than a nichrome wire. Copper also has better thermal conductivity than nichrome, meaning it will cool more quickly after the power is turned off.

Demonstration

Here's a video of my latest muscle. This one has two litz wires wrapped around the nylon in opposite directions to provide better coverage, for faster and more even heating. It is lifting a bag of pennies weighing 35g (slightly more than one ounce). You might notice that the coils near the top are going flat. I got greedy and overheated it.  I'm powering it with a 3V wall wart, and it is drawing 1.17 A.

Until the next cycle,

Jenny

DIY Fishing Line Artificial Muscles III

Most of what I've done since the last blog post has involved testing different heating elements. In the process, I've built some muscles that I thought were good enough to film. They're slower than I would like (not just in the cooling phase, but also in the heating phase), but getting a proper driver circuit built might fix that. Blowing air on them helps them cool off faster, but I purposely avoided doing that so that you can see how long it takes them to relax in still air.

First of all, I'm about ready to give up on the copper magnet wire. It's simply too easy to snap – even if I could make it work, I don't want a delicate, frustrating manufacturing experience, especially since I might end up making a lot of these.

The first alternative I tried out was Beadalon 7-strand beading wire, which is made of stainless steel with a thin nylon sheath for insulation. It's highly flexible, and has a much higher resistance than the copper wire: about .30 Ω/cm, yielding a total of 14 Ω at the length I used. The same length of my copper magnet wire has a resistance of only 3.5 Ω (0.075 Ω/cm).  (Don't set too much store by those numbers. My multimeter doesn't seem to be very good at reliably measuring resistances this small.) The total diameter of the wire and insulation is 0.46 mm. I made a homochiral (contracting) rod-coiled muscle using this wire and some of the 50 lb. test (711 um) nylon line, and got what I would call my first really good muscle. I was able to cycle it many times without damaging it and making the coils go flat (a problem I had previously with the muscles that used the copper magnet wire). It contracted enough to lift a suspended ferrite core about 1 cm. The main disadvantage of this wire, as compared with the copper, is that it heats up much more slowly, and I think it cools more slowly as well. This could be due to its higher resistance, which would have reduced the current flowing through the wire (I've been using about the same voltage for all my tests). Or it could be mainly due to the insulation limiting the rate of heat transfer from wire to muscle.

I also tried some tinsel wire – multiple strands of enameled copper wrapped around a fiber core – which I scavenged from the cord of an old pair of headphones. This is my favorite heating element so far. It doesn't break easily, it heats quickly, and it's less bulky than the Beadalon wire, because it's insulated with enamel rather than a nylon sheath. When it's put under tension and wound up into a muscle, the strands shift and flatten so that the wire takes up very little vertical space between the coils, and it spreads out a little to touch a greater surface area of the nylon. I made a heterochiral (expanding) rod-coiled muscle with this stuff.

In the comments of a previous blog post, bluesmokelounge suggested using a heating wire without any enamel, presumably to get quicker heat transfer. I'm a little wary of doing that. Loops of wire on adjacent muscle coils can contact each other when the muscle is fully contracted, and would short together without insulation, throwing off any calculations based on the length of the wire and possibly causing uneven heating. If the muscle were cycling continuously, this condition would be momentary, but it could be more of an issue if one wanted to hold the muscle in a contracted state for a while.

Last of all, I tried electrically heating some self-coiled muscles. Even though I like the tinsel wire best, I don't have much of it right now, so I used the stainless steel beading wire instead. My first attempt to heat a self-coiled muscle with this was a flop. I don't know if that was because the wire was too long (hence high-resistance) and I couldn't get it hot enough with the battery pack I was using, or if the large diameter of the beading wire was preventing the muscle from contracting by taking up too much space between the coils. I suspect the latter, because the very top of the muscle (which wasn't coiled as well) did try to move. So instead of coiling the wire up with the nylon monofilament, I tried wrapping a shorter length of wire around an already-formed self-coiled muscle. This wire got hotter and didn't interfere with the movement of the coils, and I was able to see some results. Using a self-coiled muscle made from my 25 lb. test (533 um) line in this configuration, I was able to lift a weight of about 55 g a millimeter or two … but it's more dramatic to watch this same muscle flex a piece of paper:

Until the next cycle,

Jenny