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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 a

by adolph 3y ago

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. https://www.klipper3d.org/Measuring_Resonances.html https://www.klipper3d.org/Measuring_Resonances.html

Gracana 3y ago
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).
- mathiasgredal 3y ago
  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.
  - Gracana 3y ago
    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.
mglz 3y ago
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.

by adolph 3y ago

Gracana 3y ago

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).

mathiasgredal 3y ago
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.
- Gracana 3y ago
  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.

mglz 3y ago

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.