(Oops saw other comment linked, but I’ll leave this comment in case summary is helpful)
Check out the SO100 arm, being supported by Huggingface and others. Only 4 axis, but cheap AF (<$250/arm) and using ML to make it more capable than 4 axis would seem. Also using identical arms for mirror teleportation/training.
4 is still though fundamentally limited: you need a minimum six DOF to position all three translational and rotational freedoms. The human arm has 7, and I think a case can be made the smart software would reduce the problems from having only exactly as many as required. ... but not eliminate: the extra degree of freedom lets you "get out of your own way" when moving objects that are not zero size trough multiple positions :)
Perhaps 4 is enough for any specific application, but then again perhaps 3 is or 2. :P
I've never programmed a robot arm, but I've spent a fair amount of time using a seven axis faro arm (a coordinate measuring device, sort of the opposite of a robot arm) and it certainly takes some practice to avoid "cant move there from here without reorienting everything", it's easy to take for granted what our brains do automatically for us. :)
The point is that you can achieve a lot without being able to access every position and the human arm having a lot of degrees of freedom doesn't necessarily mean it's more capable than a different arm with fewer.
Sure but why not, say, an XYZ or XYZT gantry instead?
There are a lot of downsides in 'arms' -- stacking all the axis hurts their speed, stiffnes, accuracy, increase complexity (e.g. in pathing, limiting speed, restricting to safe areas), limited reach.
I've always thought of them primarily as beneficial for flexibility at considerable cost, but if it's not 6dof then the flexibility isn't so great, then why not some other geometry?
in simpler term: an object has position and rotation, and we're in 3D, so we need minimum 6 linearly independent parameters to be able to both point at a direction and be at a location at the same time.
Drone and helicopters has 4, and they are able to control max 4 of 6 parameters. Usually 3 positional + 1 rotational, and the rotatinal axis go first.
It uses cheap hobby servos which is, to put it mildly, not the same as steppers. You will never get 0.2mm precision with them. They will were out quickly and even quicker under load.
BTW, looks like it doesn't have closed loop control. "0.2mm repetability" (should be repeAtability) is only 'under certain conditions', no load.
You'd be surprised what you can do with servos modified with encoders, setup with an industrial grade cascading control loop. First demo video below demonstrates threading a mechanical pencil lead in and out through the writing tip. Second video demonstrates a 187gram weight at the end of a 470mm long rod. Third video shows that the modification is now a quick fitting of some 3D printed parts to the servo motor axle and servo housing.
It has closed loop control, these [0] are a bit more proper serial servos with digital absolute magnetic encoders. Each one can be sent positional, velocity and even acceleration targets. I doubt anyone is looking for 0.2mm precision for $300 total. Or even 1mm precision.
The PAROL6 BOM doesn't provide any cost estimate, but it looks quite a bit more expensive.
I really never understood why hobby servos are so shitty. Their protocol uses only a very narrow range of PWM values and not the full 0-100% scale and that alone completely wrecks their precision beyond what their motors can achieve.
Also they do measure position to achieve their feedback, might as well just output that on a 4th wire.
It's because the default "analog output" PWM mode of a microcontroller will only give a rough approximation of the signal that the servo actually requires. For a servo, the duty cycle is (almost) irrelevant, the 0-100% scale has no meaning here. What matters is the actual length of the control pulses in milliseconds - the gaps between them can be arbitrarily long within a certain range.
If you think about it, it actually makes a tiny bit of sense. First, it is failsafe: Breaking the control line or shorting it to ground will not move the servo to 0%, shorting it to signal level will not move it to 100% - it just doesn't move at all and stops applying force. Any sentient being within the movement range will definitely prefer it that way instead of random movements. Second, it can actually be pretty precise: The driver circuit can be completely analog, it doesn't have to be limited by arbitrary digital quantization steps. All it needs to do is check if the current encoder value is above or below the target and apply power to the motor accordingly.
They've been around for a really long time. I suspect that back in the analog radio days of yore, the control pulses transmitted by your Futaba were recieved as a pulse train - 6 channel radio, six pulses and a big enough gap between frames to reset the index. At the RX all you have to do is feed those pulses through to successive servos one after the other, which is easy and cheap to do with basic logic ICs.
So when you use hobby servos for robots, you're taking a low precision actuator meant to make a flap or throttle or other control surface go relatively up, down, in, or out, that was designed in like, the 70s, and asking it to do modern robotics stuff
My guess would be they use variable resistors for encoders. I haven't done servos for a while, but the industrial stuff all used optical encoders with Gray code patterns.
There are bit more expensive servos with proper encoding. The thing is hobby airplanes don't need high precision as pilot controls looking at the whole plane movement. Now that hobby is killed by regulations. Robotics is coming..
This is my favorite DIY robot arm at the moment. The parts seem to be fairly high quality, and it has a leader / follower arm option if you build a second one:
When looking at low cost arms I check how it connects to the mount. If it hangs on just a servo it will be very wobbly. Most mentioned in this thread are. For this price or bit more you can get a way better arm on ebay with big ball bearing on the base for stability. I got a kit for $60 or $80 recently. No servos, full metal. It's easy to add servos and put together, for now I have other arms assembled ;) No steppers because I want them to be cheap and imprecise for ML project.
Just got for $40 on amazon cheap arm with servos and bearing at the base. "Robot Arm Kits for Arduino Coding". Not that I need it, just like putting things together. Arduino not included, but I have it already. Let's see if I can use Raspberry Pico instead.
If you ever wonder what you could actually do with a nice arm… well, I had one for a while, and the following is the best I could come up with at the time. A fun little video:
Heh, the video is a little deceptive. The arm had no vision capability, so I faked it. The dominos only look like they are sitting in a random pile. Actually, they are sitting in very precise positions, so the arm would find them correctly when it ran a program I built by pointing the arm at the locations using the joystick.
How does this cost around 1000euros to diy? It looks like some steppers and some printed parts and a control board… the kind of parts you find on a 3d printer you can buy for $200 - what am i missing? Genuine question!
In what way? It’s using nema 17 motors, the same as most printers. They sell completed units for ~3000eur and that totally makes sense - but if I want to do the labour and parts sourcing myself what is driving the estimated cost so high? $250 for the custom controller board, $20*6 for the motors, $60 of plastic. That’s well under $500
The problem with these robots is the accuracy. You can't really use them for precision work, like populating a board with SMD components. This robot also seems quite slow.
It's probably relatively safe for humans though (be careful with your pets), the robot is 5kg, it move quite slowly and motors are nema 17 (the same than in a 3d printer) with a max gearbox ratio x20. In case of problem press the e-stop button if you have chosen to have one.
As far as I understand there are no encoder on the joints so it can't be back-driven, it's not a compliant robot.
TL;DR: Cost looks to be about $2K, they sell the 3D printed parts for 200Euro, the control board for 220Euro, and a kit for 2,000Euro. Looks very cool, but it's out of my "build it and see what I can do with it" budget, I'd need a real reason to build it.
The control board is listed for me at price €262,80 EUR (with tax included).
The control board version with the 6 additional 10€ stepper driver (with the link they provide in their BOM, is listed at €456,00 EUR, (at this price I can plug stepper driver all day long).
Rant mode activated : Those prices are just excessive. Before 2020 we could buy some 5-axis 8-bits RAMPS 1.4 board with stepper for less than 50€. Then there was some transition to use some more powerful 32-bits microcontroller (which are cheaper on the BOM), but no standard has emerged, 32-bits chips were impacted by the shortage, so everyone and their dog are creating their own 3d printer control boards, and selling them for more than €100, and they are less customizable, harder to upload. (The best replacement for RAMPS 1.4, is probably BTT SKR V1.4, which sells for €43 with silent drivers) (but you are on your own for the non-standard software and development environment).
Those control boards are just connector boards, they are mainly used to replace some wires, it's just to connect the microcontroller to the stepper driver. You can have your own pcb connector board made for less than €10 for 10. You just have to solder the header yourself.
Is your rant more towards RAMPS, or is it actually has more to do with lack of a 32-bit Mega/Due successor? Because RAMPS is still around and there isn't much wrong about it, it just works okay.
Kind of towards the 3d printing industry and greed in general. The RAMPS guys didn't offer a successor when they were in the perfect position to do so after having built a large diy community of RepRappers and opensource 3d-printing.
The 32-bits transition is better and cheaper on the BOM compared to the Mega2560. The speed boost is really useful when you want to add more advance control algorithm for your motors for robotics application.
Creality CR-10 quality was just better and it was the beginning of the transition away from open standards. A slow but sure erosion of what was once a great flexible community solution, where what was built by open-source is being privatized.
Chinese clones replicated CR-10 model each with their own control board. But when your machine is repairable, you can't sell it for a lot more than the sum of individual parts, so the whole economics of how much a control board that run marlin is got delirious.
This trend continue today with the emergence of companies like BambuLabs.
The whole arc makes me sad, as 3d-printing is the entry point for many makers around the world and these initial core values of really owning your hardware stack are important. For building robots, building your own 3d printer was a great introduction. Being able to replace a fried motor driver because your robot took an unexpected fall for cheap is kind of essential. If every time a part break you need to pay more than €100 and more than to 2 weeks of delivery time, you can't test and push your machines. Whereas if you are using standard parts, you'll have a stock of them, and it will be €5 and 5 minutes and you are good to go, and the mistake is forgotten, and the lesson learned.
A BTT octopus w/ replaceable 6xTMC2209 and plenty of I/O (including CAN support) costs <70 eur; this is light-years from a RAMPS, and still very cheap. the prices of the controller boards of this robotic arm is just... ridiculous. A money grab.
I mean, if those improvement deltas are all in the processor part, would a hypothetical Mega 32-bit, or a standardized 32-bit Arduino platform(that isn't Uno R4 - no way a Renesas part can be an industry standard with their typical "JTC" style of business) not solve the issues?
Check out the SO100 arm, being supported by Huggingface and others. Only 4 axis, but cheap AF (<$250/arm) and using ML to make it more capable than 4 axis would seem. Also using identical arms for mirror teleportation/training.
https://github.com/TheRobotStudio/SO-ARM100
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