this is slick! have you considered routing the cables through the actuator arm itself?

As others have mentioned, one or two PTFE or Rulon J linear sleeve bearings would be lighter and probably stiffer than 6 6001RS steel bearings. You could also swap to acetal plastic ball bearings instead of steel to lighten them up and reduce wear. Other than that you could replace the push and pull strings with a single string and have a tensionning spring at one end to remove backlash. You could also have the pulleys be tensioned against each other to acheve the same result but that sounds clunkier.

14:20 use a ceramic eye from a fishing rode for strung position 😉

u can use the bike/bicycle brake tubes those are quite tough instead of the ptfe

I would put bearings on the 3 rods, back end of the extending arm, just for a bit more stabilization.

I was thinking the same but with Braided monofilament as used in fishing, for the same reasons.

UHMWPE does have problems with creep, though. So you'd want to be sure that it's not stored under tension when not in use. Otherwise slack will creep in, and you'd need to re-tighten everything before you restart

Use SK99 for the lowest stretch and creep.

Thanks for the kind words! Re. PTFE ideas: a big consideration for these actuators in this project is that they need to be able to move around fairly freely, and I'm worried that bike brake sheaths would be too stiff, and I can't visualise how a printed support structure would be both flexible (in bending) and stiff (in compression). I would love to know if you have any suggestions on that front! FWIW, a few people have suggested trying brake cables and I'll be testing that (and a few other) ideas soon. I'm sure we can eventually find the perfect tube! (or completely get rid of them, as Eric McGraw suggested in another comment. So many options!)

@@harrisonlow Maybe Jagwire Elite Link can do the job.

I was thinking about this while falling asleep last night, and I realized that simple beads-on-a-string will provide resistance against compression, but still allow flexibility while not under compression. Anything that allows the tube to freely pass through it, with a rounded edge to allow flexibility, would probably work. However, if it needs to remain flexible _while under compression_ it may be more of a challenge.

@@James02876 "beads-on-a-string" is basically the idea behind Elite Link and similar products. They allow smaller radius than conventional bowden tubes and at least one of such (of which I just can not remember the name) claims to be flexible under load… so further investigation may be worth it. I never used such myself because on all of my vehicles, classic bowden works just fine.

Of you go down the bicycle cable option. Try with gear cable and gear cable housing rather than brake cable (thinner diameter cable and come with far more housing and connector options)

Damn, that's a good trick

You can also use pressurized air and it will blow it right though. My wife has an embroidery machine and that's how she loads the threads.

If two of the bearings that holds the shaft straight had a channel in it to guide the string this could be done easily.

exactly what i was thinking

@@nemesis1588 Maybe turn the whole system inside out, where the actuator is a large tube on the outside, and the bearings are on the inside mounted on a smaller rod. Hide everything. Meh ... may make it too heavy.

Wow! Thanks for the great feedback!

I agree! My notepad is rapidly filling up with suggestions. So good!

Cheers!

Cheers for the kind words! I agree that it's remarkable that this mechanism doesn't seem to exist on the internet anywhere else (at least not that I could find). Hopefully anyone else in the future who needs to build themselves a speedy actuator can now save themselves the ~1.5 yrs of development that I had to go through to get these working!

Thanks for the support!

Interesting idea! I think this might have the same issue as what Charlie Wynn was suggesting in another comment - the central tube should be perfectly centred in the actuator for what I'm using it for. Precision will be very important for Jugglebot and I want to try and keep everything as concentric as possible. Thankfully the spacers/guides are really easy to swap out and once you find a size that works, there's no need to change them again

@@harrisonlow if you’re worried about accuracy, I would adjust your attachment points on the rod for your strings. They look like they are at an angle in your video. Combined with the fact you’re only measuring distance on your motor, you’ve introduced potential backlash in the system, AND you’re measuring the change of the hypotenuse of the triangle you create with the string instead of the actual linear, parallel distance. It is probably not a lot at all, but in belt driven systems that need accuracy, the small angle can definitely throw off measurements.

​@@harrisonlow as long as the system is stiff enough, can't you compensate for the lack of concentricity through software?

@@JustTryGambling Good thinking! Something I didn't explicitly say in this video is that these actuators (there will be 6 of them) are being used to power a Stewart Platform, so the actuators need to move around quite a bit. The PTFE/string attachment points are angled to (hopefully) lessen the degree to which the PTFE inhibits the platform from moving around

@@LimitedWard Yeah, probably! Though I want to limit the amount of hard-coding of this sort of physical "defect" that I need to do

Thanks for the kind words 😊

You'd still want cable guides (shifter cable housing seems like the best bet) into the bottom block but yes, using the structural tubing as part of the string guide is a great idea.

Cheers! Routing the strings through the CF tubes would probably work well for many applications, but I'm using these actuators (there will be 6 of them) to power a Stewart Platform and they need to move around quite a bit. Having the strings routed through the PTFE tubes means that the strings don't get in the way of the actuator moving around

@@harrisonlow i had the same idea, why not routing the strings through the CF tubes. I don't exactly undestand why would the strings get in way of the actuator?

Good thinking! The main reason I haven't done that is that these actuators (there will be 6 of them) will be powering a Stewart Platform, so the actuators need to move around quite a bit, meaning it's tricky to route the string through the central tube since the central tube moves so much. I love the idea though, and it'd certainly make the design a lot cleaner. Cheers!

I wouldn’t recommend standard bearing or bushings on CF. You could go with a plastic bushing like the IGUS ones though and they seem to work okay, but there’s a bit of friction that needs to be overcome to get them moving. They also have to be preloaded quite heavy for tolerances to be good unless you go for their bushings that are already preloaded with a metal housing, which will add a bit more weight.

Linear bearing is a top idea, I would recommend a Teflon bushing though as it will work well with CF

Sure, igus are great, i used them on one of my 3D printers (Prusa, got rid of that damn bed slinger). I didn't notice the friction, though - but maybe that was because they were on steel rods, not CF tubes, and had only a small contact surface at the top. But yeah, anything other than those axial bearings, or: any kind of a linear bearing, would be the perfect application for this - because that's what they're made for! 🥳

@@jangrewe Great ideas! Some thoughts: 1) I love the idea of heating up the CF tubes. I wish I saw this comment before I built the remaining actuators; some of them developed cracks from my... enthusiastic... hammering 😅Doesn't seem like anything structural was damaged, but heating would certainly be better. Cheers! 2) I previously tried a linear bearing instead of the 6 radial bearings, but the carbon fiber tubes that I'm using aren't specced for that level of tolerance; one manufacturers tubes had a loose fit, another didn't fit at all. Linear bearings would be awesome (smaller, lighter, more elegant) but I don't have the cash to get high tolerance, light tubes... yet 😉 3) Good idea for PTFE with larger wall thickness. Very obvious but never came to mind! I'll see if I can find any. Cheers for the ideas!

@@harrisonlow cheers, 2 out of 3! ;-) I'll let you know if i can think of anything else...

Thanks! Hmm, I'm having difficulty visualising what you're describing. Do you know of any real-world systems that use something similar? I do quite like the sound of a tighter transmission 😁

@@harrisonlow Skyentific has shown a mechanism using pulleys co-axial with arm rotation, but for the purpose of actuating the arm itself. kzfaq.info/get/bejne/q9p0lMqg2t6xZYk.html The idea I'm suggesting simply uses such pulleys to guide the cable onto and back off of the rotating body. I will try drawing the idea a little later and sending a picture if you would find that helpful.

​@@ericcmcgraw Very interesting! I just watched that video and I think I have a better idea of what you're describing. I have a few thoughts: 1) I'm quite averse to shifting the linear axis away from the centre of rotation as that makes the inverse kinematics for the stewart platform (what these actuators will be used for) significantly more difficult. I don't know exactly *how* much more difficult, but I started working through it a while ago and after a while decided to just make the joint axes coincident with the linear axis 😅 2) This design looks like it'd require quite a few parts, and at the scale I'm working at, they'd have to be fairly small. I'm trying to cut down on complexity as much as possible (especially where it concerns manufacturing). 3) As a slight (maybe) spoiler, I'm actually thinking I'll change the lower universal joint for a magnetic joint, in a similar fashion to the upper joints (this video if you haven't seen it already: kzfaq.info/get/bejne/d6mZl8SlxKu1Y4U.html ). I'm not sure if this would affect the design you're describing, but it seems relevant to mention. Again, thanks for the idea! I'm interested to know what you think about the above points 😊

@@ericcmcgraw Grr, I think KZfaq may have removed your last comment. I can see some of it in my notification, but nothing past a "small error" re. shifting the linear axis. Did you include any links? KZfaq seems to dislike links to external sites 😡

@@harrisonlow Yep, I tried obfuscating the link by spelling it out, but still no luck. Here's the comment with the link removed: 1) I thought of that too, so I played around in Geogebra and found that you could still assume the linear axis rotates through a single point and you'd have relatively small error compared to reality. Assuming ±45 degree range of rotation, and 10mm diameter pulleys, the actual length error would only be ±0.1 mm throughout the range of motion. Here's the Geogebra file if you want to play around with it: (redacted) There do exist solutions which would cancel this error exactly, but the complexity is higher still. Let me know if you're interested in them. 2) I've found some design possibilities which add only a few additional pulleys to the part count you already have. Complexity is a little higher, yes, but the benefits are: no sliding friction in bowden, no risk of bowden buckling, higher tension and higher stiffness capability. It's up to you whether it's worth it. 3) Yes I saw those ball joints, it's a pretty elegant solution and I like how it makes the actuators detachable and modular. Of course the only drawback, as you mention, is the pulling force capability. As you start to push the limits of acceleration (how many balls can it juggle at a time, etc.) you may want to be able to apply high forces in both directions without worrying about the joints falling apart. Again, it's up to your own judgement (and level of ambition 😁)

Haha I remember a question in high school math on this exact topic! Something about a toilet roll being unwound and dealing with the changing radius. I would be very interested to know how big of an effect that would have; my intuition says not much, but I've been wrong before 🤔

Ben you are a smart thinker.

You can use a badge reel that winds and unwinds really small wire/string to measure linear travel without the same issue of the radius changing that you have using that CF spring. A commercially available sensor product that does this is a string potentiometer.

That's a very interesting idea! One potential issue I can see with that is that it'd make straightening the extending rod a little tricky. I didn't explicitly mention this in the video but I've been keeping each "set" of spacers (ie. top three or bottom three) the same size so that the central rod is kept centred. Having the spacers be eccentric might make this a little difficult. Very cool idea though and I'll keep that in mind if I need to do anything similar in the future. Cheers!

@@harrisonlow ooh, very good point about wanting to put the same spacers around so it stays centered!

Cheers!

Great idea! Never heard of that and I've got a can of compressed air on hand. Cheers!

I like the idea of bearings on the main push-rod! I hadn't thought of that before! Might be tricky for a few reasons: 1) While the CF tubes are stiff, having the central tube pushing against the outer tubes (via the bearing) might cause the outer tubes to flex a little 2) I think you've hit the nail on the head with respect to excess weight: if the actuator's moving at > 2m/s, I want to keep that as light as possible. Cheers for the suggestions, and I hope to see some more of these ideas in future designs!

Haha yep, colour differentiation helps so much. Got the idea from the adapters that came with the encoders to fit differently sized shafts. Such a simple solution! Cheers for the kind words 😊

Cheers!

Cheers!

How would it hold up long term? When used in bikes those cables don’t need to move an awful lot compared to what they would in this application.

.. so you like puppets ?

@@boonjabby 😁 only those that have pulley systems... Not just strings!

I am making a bipedal robot with screws and brls motors.

Oh man I've been dreaming of having some better tools; SLS printer being pretty darn high on that list 🤤

Interesting! I'm not sure I 100% understand what you're suggesting, but I have a few thoughts related to this: 1) I'm interested by the idea of flex shafts / push-pull cables (are these the same things?). I've seen people suggest them on the Wintergatan project but haven't had the time to look into them much. Are they particularly flexible? 2) For my application (Stewart Platform) it's somewhat useful to have the motor be physically separate from the actuator itself, as the actuator will be moving around and the motor would get in the way and just be more mass to move around. 3) An unintended boon to the way I have the strings routed now is that the end caps on the central tube are only ever pulled in towards the tube. This is rather nice as it means that they'll never get pulled off if the string pulls too fast/hard, since they're only connected via a not-super-strong interference fit. I could of course make that fit better, but that would be more effort! Re. backlash removal: Yes! I agree with your idea of the spring, and this is something that I will absolutely be adding if backlash becomes an issue for whatever reason. Cheers for the ideas!

Haha the potential isn't exhausted yet! I've been working on a improved version that is better in many ways. Will hopefully finish collecting data and putting the video together soon 😁

Cheers for the suggestions! That's interesting re. kevlar strings. Thanks for sharing that! Thankfully I'm pretty sure I have both brake and shifter sheathes lying around that came with cables for my bike. I'm planning on making a testing rig to test a bunch of the ideas that everyone has suggested, and I'll absolutely be trying shifter sheathes. Cheers!