Hello, /diy/, I have a question for you.
Recently I've been reading about tendon-driven manipulators, and I want to build one for fun and learning. I think I got the idea for the arm and ropes itself, my question is about motors. What should I use? Servos, stepper motors, dc motors? Or maybe long-ass linear actuators? My goal would be to create something strong enough that it could beat an average human in arm wrestling. Assuming that whole thing does not break, how can I achieve required torque/force as cheaply as possible? I'm a student living in Poland, so delivery fees and tight budget kinda screw me over.
The strongest thing you could realistically get is hydraulic servos. Naturally, these are complex and expensive. But ANYTHING you can use to pull the tendons will work. Cheapest practical option is probably going to be a geared stepper motor.
That being said, unless you kind of cheat a little (and use different kinematics than the human arm) or greatly limit the capabilities of your machine such that all it can really do is arm wrestle, you are never going to beat a human in a strength contest on a budget. Animals are insanely strong for their size.
On another note, I would suggest UHMW (often referred to as Spectra/Dyneema) for the tendons. Cheap, low-stretch, and very strong.
Thanks for the UHMW tip, definitely going to try that. Hydraulics are probably out of my reach, will look into stepper motors. As I was poking around the Internet some more I found this video
Guy uses one drill motor set on high torque/low rpm to lift himself off the ground on a rope. And poking some more I found that quite a lot of people are selling old drills cheaply. Usually something about them is broken, but I guess I could try to repair it and salvage the motors? Of course that would need addition of encoders for precision, but could it work?
>could it work
It should. Your issue, however, is that those usually have cheaply-made, multi-stage gearboxes. They have a ton of lash, which is anathema to robotics work since it fucks up your positioning. Might not be an issue for you, though, since tendon bots don't have very good positioning anyway (at least not compared to typical robotic arms).
Although it would significantly lower the overall cost, if you could find a few for dirt cheap/free. Odds are most of your budget is going to go to motors, so big savings there translates to big savings total. Downside there is that you'd need encoders, which you wouldn't with steppers. Low-count encoders could be had quite cheap to take car of that issue. They could be fit before the gearbox and give you high resolution, but low precision since you couldn't reliably account for the lash of the gearbox. On the other hand, high-count encoders would be significantly more expensive, but could be fitted after the gearbox to help with the lash issue and still provide useful resolution. Best deal I could find for relatively high-count encoders was no-name optical A/B encoders for like $14USD. Search "encoder 600", otherwise you're never gonna find them.
>Recently I've been reading about tendon-driven manipulators, and I want to build one for fun and learning. I think I got the idea for the arm and ropes itself, my question is about motors. What should I use? Servos, stepper motors, dc motors? Or maybe long-ass linear actuators?
stepper motors are the only practical electrical option
Industrial type servos would be REALLY expensive..... A nema23 stepper motor costs around $30 USD, a Chinese servo motor + driver that size costs around $150 to $200, and an industrial-grade (name brand) servo & drive that size would cost over $1000-$1500.
linear actuators have have a lot of power, but the disadvantage is that they tend to work rather slow. For one that has a 10cm range, it may push or pull REALLY hard but it might only move 1 cm per second.
dude, you are in fucking poland, with all the shit industrial countries around you, there should be no problem with this
they might have a BST section for trade or tell you where you can get it
they also have lots of robot shit