Servo + Cycloidal or Harmonic gears are usually the way to go, but to get them backlash-free is hard (or expensive, if you're buying). Once you got that down challenges include:
* How rigid are the links between my joints? Plastic will wobble, metal is better
* How heavy is my arm and how does that limit its movement? If you go for stiff metal castings, you add weight you need to move. The lever arm relative to the base can get really long
* Motors are heavy! Ideally you can mount them towards the base, but then you need drive shafts or belts, which again add flex. (See KUKA arms which have motors 4, 5 and 6 on the elbow often)
* How much payload do you need to move? 5kg is already challenging in ~1m arms and if you need to move it fast the problem gets even bigger.
* Where do you run your cables? Internal is tricky to build, external can get you tangled.
And so on. When approaching this you get a totally new appreciation for biological arms which arae insane in most aspects except for repeatability. And on the software side you can enjoy inverse kinematics :)
Do you think there is a way to take out backlash with sensors and software? Something like how additive manufacturing systems can use accelerometers to smooth artifacts from motor movement. [0] Let's say two cheaper cycloidal geared motors running in opposition with a load cell between them to maintain the materially compatible force.
I don't want to say no, however it seems very hard to do. You get feedback about motor position via an encoder, which is usually located on the motors axis and not the output element. Since the motor axis spins a lot more, you get more resolution. Backlash happens on the output, so you could add a second encoder there (but now you've got more complexity + cost). An oldschool CNC solution is to add brakes to lock an axis out, but this makes your system less flexible and doesn't prevent backlash during motion. A more modern solution might be to factor backlash in to your motion software so that you tell the kinematic solver what compliance is acceptable in some direction.
> Let's say two cheaper cycloidal geared motors running in opposition with a load cell between them to maintain the materially compatible force.
This might work, but now you have twice the amount of motors.
The problem with backlash comes into play when the direction of force on an axis changes. If you are applying force in one direction and all the backlash has been taken up, everything is fine -- any force you apply or movement you make will be transmitted to the tool like you'd expect. However, if you have to decelerate, or you've gone over-center, or the tool/load pulls harder than you're pushing, now you have to apply force in the other direction, which you can't do until you take up the backlash.
If your axis has high enough friction, then nothing will move when your actuator is in the decoupled backlash region, so you can compensate by adding the backlash amount to your target position whenever you switch directions. But that means you need more friction than tool force, with bigger motors and drivetrain to compensate. It's often easier just to build a system with zero backlash, then you can focus on tuning for system rigidity/resonance (as shown in your link).
That was why OP suggested to have 2 motors on each joint, going in opposite direction. The problem with this is that you now have twice the amount of motors.
Oof, I appreciate you pointing that out because somehow I got the first part and skipped that one. Yeah, I could see that working, but it sounds inefficient.
* How rigid are the links between my joints? Plastic will wobble, metal is better
* How heavy is my arm and how does that limit its movement? If you go for stiff metal castings, you add weight you need to move. The lever arm relative to the base can get really long
* Motors are heavy! Ideally you can mount them towards the base, but then you need drive shafts or belts, which again add flex. (See KUKA arms which have motors 4, 5 and 6 on the elbow often)
* How much payload do you need to move? 5kg is already challenging in ~1m arms and if you need to move it fast the problem gets even bigger.
* Where do you run your cables? Internal is tricky to build, external can get you tangled.
And so on. When approaching this you get a totally new appreciation for biological arms which arae insane in most aspects except for repeatability. And on the software side you can enjoy inverse kinematics :)