Those motors are just closed-loop steppers. I'd say they're inappropriate for robot arms because they can only apply their specified torque when the angle happens to be in proximity to a full step [0].
Proper AC servo motors like the A3 series from Delta are - as far as I understand - able to apply their specified torque with "infinite" resolution, in practice limited primarily by the encoder.
In robotics though, BLDC motors are often used. You might want to look into ODrive [1].
Make sure you're choosing an appropriate gearbox too. A planetary gearbox has some amount of backlash associated with it, often around 10 arcmin from what I've seen in data sheets. If that's inappropriate for you, look into harmonic drive. Much more expensive but for a reason.
Proper servos with high torque are quite expensive.
I wanted to keep the project under a certain amount to scale it.
With the springs to compensate the static loads (gravity) I can use weaker servos but I didn’t yet completed the “sensitivity table” of the generated angular momentums.
Yes indeed, the ~10arcmin should represent something like 5mm at about 1m distance.
Plus the ~30mm of the 1.8deg/step again at 1m or 3mm with 10:1 gearbox.
I thought I could tackle it by using more expensive parts (but weaker) at the end part of the arm for instance and write a software to orchestrate all the 6motors together.
I didn’t know about the harmonic drive I’ll look into it.
The proper industrial stuff can be quite expensive for sure. IIRC I paid around $400 per motor in total. You might be able to get a better deal on used motors/drives though.
But steppers won't necessarily save you either, because that torque specification only applies to full steps.
Harmonic drives are great but unfortunately quite expensive.
What you might want to do is go cheap on most of the components and add a decent-ish encoder and use software to compensate for the imprecision. CUI's AMT series of encoders is widely available and comes with up to 14-bit resolution. I believe this can be a pain in the arse to get right but it sounds like you're comfortable with maths so it might work well for you.
Proper AC servo motors like the A3 series from Delta are - as far as I understand - able to apply their specified torque with "infinite" resolution, in practice limited primarily by the encoder.
In robotics though, BLDC motors are often used. You might want to look into ODrive [1].
Make sure you're choosing an appropriate gearbox too. A planetary gearbox has some amount of backlash associated with it, often around 10 arcmin from what I've seen in data sheets. If that's inappropriate for you, look into harmonic drive. Much more expensive but for a reason.
[0]: https://www.machinedesign.com/archive/article/21812154/micro...
[1]: https://odriverobotics.com/