Strain wave gearing scales to small dog robot size reasonably(EG:boston dynamics spot) thanks to square cube law but can’t manage human sized robots without pretty horrible tradeoffs(IE:ASIMO and the new Tesla robots walk slowly and have very much sub-human agility).
What? No. You can make larger strain wave gears, they’re just expensive & sometimes not made in the right size & often less efficient than planetary + cycloidal gears.
hydraulic actuators fed from a single high pressure fluid rail using throttling valves
That’s older technology.
Rotary actuators are preferred over linear hydraulic cylinders
No. There’s a reason excavators use cylinders instead of rotary vane actuators.
For roller screws the ring/planet contact patch operates this way with zero friction for a zero sized contact patch.
No. Without sliding, screws do not produce translational movement.
What? No. You can make larger strain wave gears, they’re just expensive & sometimes not made in the right size & often less efficient than planetary + cycloidal gears.
Not in the sense of you can’t make them bigger but square cube means greater torque density is required for larger robots. Hydraulic motors and cylinders have pretty absurd specific force/torque values.
hydraulic actuators fed from a single high pressure fluid rail using throttling valves
That’s older technology.
Yes you can use servomotors+fixed displacement pumps or a single prime mover + ganged variable displacement pumps but this has downsides. Abysmal efficiency of the a naive (single force step actuator+throttling) can be improved by using ≥2 actuating cavities and increasing actuator force in increments (see:US10808736B2:Rotary hydraulic valve)
The other advantage is plumbing, You can run a single set of high/low pressure lines throughout the robot. Current construction machinery using a single rail system are worst of both worlds since they use a central valve block (two hoses per cylinder) and have abysmal efficiency. Rotary hydraulic couplings make things worse still.
Consider a saner world where equipment was built with solenoid valves integrated into cylinders. Switching to ganged variable displacement pumps then has a much higher cost since each joint now requires running 2 additional lines.
No. There’s a reason excavators use cylinders instead of rotary vane actuators.
Agreed in that a hydraulic cylinder is the best structural shape to use for an actuator. My guess is when building limbs, integration concerns trumped this. (Bearings+Rotary vane actuator+control valve+valve motor) can be a single very dense package. That and not needing a big reservoir to handle volume change meant the extra steel/titanium was worth it.
No. Without sliding, screws do not produce translational movement.
This is true, the sun and planet screws have relative axial motion at their point of contact, Circumferential velocities are matched though so friction is much less than in a leadscrew. Consider two leadscrews with the same pitch (axial distance traveled per turn). One screw has twice the diameter of the first. The larger screw will have a similar normal force and so similar friction, but sliding at the threads will be roughly twice that of the smaller screw. Put another way, fine pitch screws have lower efficiency.
For a leadscrew, the motion vectors for a screw/nut contact patch are mismatched axially (the screw moves axially as it turns) and circumferentially (the screw thread surface slides circumferentially past the nut thread surface). In a roller screw only the axial motion component is mismatched and the circumferential components are more or less completely matched. The size of the contact patches is not zero of course but they are small enough that circumferential/radial relative motion across the patch is quite small (similar to the ball bearing case).
Consider what would happen if you locked the planet screws in place. it still works as a screw although the effective pitch might change a bit but now the contact between sun and planet screw involves a lot more sliding.
square cube means greater torque density is required for larger robots. Hydraulic motors and cylinders have pretty absurd specific force/torque values
No. Strain wave gears are lighter than using hydraulics.
Circumferential velocities are matched though so friction is much less than in a leadscrew
No. Screw translational movement is proportional to the amount of sliding.
No. Strain wave gears are lighter than using hydraulics.
Note:I’m taking the outside view here and assuming Boston dynamics went with hydraulics out of necessity.
I’d imagine the problem isn’t just the gearing but the gearing + a servomotor for each joint. Hydraulics still retain an advantage so long as the integrated hydraulic joint is lighter than an equivalent electric one.
Maybe in the longer term absurd reduction ratios can fix this to cut motor mass? Still, there’s plenty of room to scale hydraulics to higher pressures.
The small electric dog sized robots can jump. The human sized robots and exoskeletons (EG:sarcos Guardian XO) aren’t doing that. improved motor power density could help there but I suspect the benefits of having all power from a single pump available to distribute to joint motors at need is substantial.
Also, there’s no power cost to static force. Atlas can stand in place all day (assuming it’s passively stable and not disturbed) an equivalent robot with electric motor powered joints pays for every Nm of torque when static.
Sorry, I should have clarified I meant robots with per joint electric motors + reduction gearing. almost all of Atlas’ joints aside from a few near the wrists are hydraulic which I suspect is key to agility at human scale.
A lot of people look at progress in robotics in terms like “humanoid robots getting better over time” but a robotic arm using modern electric motors and strain wave gears is, in terms of technological progress, a lot closer to Boston Dynamics’s Atlas robot than an early humanoid robot.
I would argue that the current Atlas robot looks a lot more like the earlier hardiman robots than it does a modern factory robot arm. The hydraulic actuators are more sophisticated (efficient) and the control system actually works but that’s it.
Contrast the six axis arm which has a servomotor+gearing per axis. Aside from using a BLDC motor to drive the pump, and small ones for the control valves, Atlas is almost purely hydraulic. If the Hardiman Engineers were around today Atlas seems like a logical successor.
Perhaps you think Atlas is using one motor per joint (It would be hard to fit 24 in the torso) or ganged variable displacement pumps in which case there would be more similarities. IMO there aren’t enough hydraulic lines for that. Still of the 28 joints in atlas only 4 are what you’d find in a conventional robot arm (the ones closest to the wrist)
Again, strain wave gearing (as an approach, including electric motors with high specific power) is lighter than using hydraulics, overall. The same is true for planetary roller screws. This is true regardless of scale. Hydraulics are used for other reasons than maximum performance physically achievable.
Boston Dynamics decreased the weight of their hydraulics system by 3d printing hydraulic channels in the skeleton. That’s expensive, and planetary roller screws are still better if done properly.
Take an existing screw design, double the diameter without changing the pitch. The threads now slide about twice as far (linear distance around the screw) per turn for the same amount of travel. The efficiency is now around half it’s previous value.
There was a neat DIY linear drive system I saw many years back where an oversized nut was placed inside a ball bearing so it was free to rotate. The nut had the same thread pitch as the driving screw. The screw was held off center so the screw and nut threads were in rolling contact. Each turn of the screw caused <1 turn of the nut resulting in some axial movement.
Consider the same thing but with a nut of pitch zero (IE:machined v grooves instead of threads). This has the same effect as a conventional lead screw nut but the contact is mostly rolling. If the “nut” is then fixed in place you get sliding contact with much more friction.
a roller screws advantage is having the efficiency of a multi-start optimal lead-screw but with much higher reduction.
A lead-screw with an optimal pitch and a high helix angle (EG: multi-start lead-screw with helix angles in the 30°-45° range) will have just as high an efficiency as a good roller screw (EG:80-90%). The downside is much lower reduction ratio of turns/distance.
We might be talking past each other since I interpreted “a planetary roller screw also must have as much sliding as a lead-screw” to mean an equivalent lead-screw with the same pitch.
If you paid attention to the animation, you should see that, while it has rolling movement, a planetary roller screw also must have as much sliding as a leadscrew, since threads rolling around without sliding don’t produce net movement. Rather, the improved efficiency comes from a reduced coefficient of friction, which is caused by the rolling movement putting oil between the threads, producing hydrodynamic lubrication. This requires very smooth surfaces, and machining to those fine tolerances makes planetary roller screws relatively expensive.
Just to clarify, my above suggestion that roller screws and optimal low reduction lead-screws are the equivalent (lubrication concerns aside) is correct or incorrect?
Are you saying a roller screw with high reduction gets its efficiency from better lubrication only and would otherwise be equivalent to a lead screw with the same effective pitch/turn? If that’s the case I’d disagree. And this was my reason for raising that point initially.
Are you saying a roller screw with high reduction gets its efficiency from better lubrication only and would otherwise be equivalent to a lead screw with the same effective pitch/turn? If that’s the case I’d disagree.
Yes. OK, we disagree about that. Glad we could get to the point.
What? No. You can make larger strain wave gears, they’re just expensive & sometimes not made in the right size & often less efficient than planetary + cycloidal gears.
That’s older technology.
No. There’s a reason excavators use cylinders instead of rotary vane actuators.
No. Without sliding, screws do not produce translational movement.
Not in the sense of you can’t make them bigger but square cube means greater torque density is required for larger robots. Hydraulic motors and cylinders have pretty absurd specific force/torque values.
Yes you can use servomotors+fixed displacement pumps or a single prime mover + ganged variable displacement pumps but this has downsides. Abysmal efficiency of the a naive (single force step actuator+throttling) can be improved by using ≥2 actuating cavities and increasing actuator force in increments (see:US10808736B2:Rotary hydraulic valve)
The other advantage is plumbing, You can run a single set of high/low pressure lines throughout the robot. Current construction machinery using a single rail system are worst of both worlds since they use a central valve block (two hoses per cylinder) and have abysmal efficiency. Rotary hydraulic couplings make things worse still.
Consider a saner world where equipment was built with solenoid valves integrated into cylinders. Switching to ganged variable displacement pumps then has a much higher cost since each joint now requires running 2 additional lines.
Agreed in that a hydraulic cylinder is the best structural shape to use for an actuator. My guess is when building limbs, integration concerns trumped this. (Bearings+Rotary vane actuator+control valve+valve motor) can be a single very dense package. That and not needing a big reservoir to handle volume change meant the extra steel/titanium was worth it.
This is true, the sun and planet screws have relative axial motion at their point of contact, Circumferential velocities are matched though so friction is much less than in a leadscrew. Consider two leadscrews with the same pitch (axial distance traveled per turn). One screw has twice the diameter of the first. The larger screw will have a similar normal force and so similar friction, but sliding at the threads will be roughly twice that of the smaller screw. Put another way, fine pitch screws have lower efficiency.
For a leadscrew, the motion vectors for a screw/nut contact patch are mismatched axially (the screw moves axially as it turns) and circumferentially (the screw thread surface slides circumferentially past the nut thread surface). In a roller screw only the axial motion component is mismatched and the circumferential components are more or less completely matched. The size of the contact patches is not zero of course but they are small enough that circumferential/radial relative motion across the patch is quite small (similar to the ball bearing case).
Consider what would happen if you locked the planet screws in place. it still works as a screw although the effective pitch might change a bit but now the contact between sun and planet screw involves a lot more sliding.
No. Strain wave gears are lighter than using hydraulics.
No. Screw translational movement is proportional to the amount of sliding.
Note:I’m taking the outside view here and assuming Boston dynamics went with hydraulics out of necessity.
I’d imagine the problem isn’t just the gearing but the gearing + a servomotor for each joint. Hydraulics still retain an advantage so long as the integrated hydraulic joint is lighter than an equivalent electric one.
Maybe in the longer term absurd reduction ratios can fix this to cut motor mass? Still, there’s plenty of room to scale hydraulics to higher pressures.
The small electric dog sized robots can jump. The human sized robots and exoskeletons (EG:sarcos Guardian XO) aren’t doing that. improved motor power density could help there but I suspect the benefits of having all power from a single pump available to distribute to joint motors at need is substantial.
Also, there’s no power cost to static force. Atlas can stand in place all day (assuming it’s passively stable and not disturbed) an equivalent robot with electric motor powered joints pays for every Nm of torque when static.
jumping: https://www.youtube.com/watch?v=tF4DML7FIWk
Sorry, I should have clarified I meant robots with per joint electric motors + reduction gearing. almost all of Atlas’ joints aside from a few near the wrists are hydraulic which I suspect is key to agility at human scale.
Inside the lab: How does Atlas work?(T=120s)
Here’s the knee joint springing a leak. Note the two jets of fluid. Strong suspicion this indicates small fluid reservoir size.
I’m getting the impression you didn’t read what I wrote.
Hopefully it helps to get back to the source material Articulated Robot Progress
I apologize if I’m missing anything.
I would argue that the current Atlas robot looks a lot more like the earlier hardiman robots than it does a modern factory robot arm. The hydraulic actuators are more sophisticated (efficient) and the control system actually works but that’s it.
Contrast the six axis arm which has a servomotor+gearing per axis. Aside from using a BLDC motor to drive the pump, and small ones for the control valves, Atlas is almost purely hydraulic. If the Hardiman Engineers were around today Atlas seems like a logical successor.
Perhaps you think Atlas is using one motor per joint (It would be hard to fit 24 in the torso) or ganged variable displacement pumps in which case there would be more similarities. IMO there aren’t enough hydraulic lines for that. Still of the 28 joints in atlas only 4 are what you’d find in a conventional robot arm (the ones closest to the wrist)
Predictively Adjustable Hydraulic Pressure Rails
The patents coming out of BDI suggest they’re not doing that and this is closer to Hardiman than it is a modern factory robot arm.
Again, strain wave gearing (as an approach, including electric motors with high specific power) is lighter than using hydraulics, overall. The same is true for planetary roller screws. This is true regardless of scale. Hydraulics are used for other reasons than maximum performance physically achievable.
Boston Dynamics decreased the weight of their hydraulics system by 3d printing hydraulic channels in the skeleton. That’s expensive, and planetary roller screws are still better if done properly.
Take an existing screw design, double the diameter without changing the pitch. The threads now slide about twice as far (linear distance around the screw) per turn for the same amount of travel. The efficiency is now around half it’s previous value.
https://www.pbclinear.com/Blog/2018/February/What-is-Lead-Screw-Efficiency-in-Linear-Motion
There was a neat DIY linear drive system I saw many years back where an oversized nut was placed inside a ball bearing so it was free to rotate. The nut had the same thread pitch as the driving screw. The screw was held off center so the screw and nut threads were in rolling contact. Each turn of the screw caused <1 turn of the nut resulting in some axial movement.
Consider the same thing but with a nut of pitch zero (IE:machined v grooves instead of threads). This has the same effect as a conventional lead screw nut but the contact is mostly rolling. If the “nut” is then fixed in place you get sliding contact with much more friction.
Perhaps we don’t disagree at all.
a roller screws advantage is having the efficiency of a multi-start optimal lead-screw but with much higher reduction.
A lead-screw with an optimal pitch and a high helix angle (EG: multi-start lead-screw with helix angles in the 30°-45° range) will have just as high an efficiency as a good roller screw (EG:80-90%). The downside is much lower reduction ratio of turns/distance.
We might be talking past each other since I interpreted “a planetary roller screw also must have as much sliding as a lead-screw” to mean an equivalent lead-screw with the same pitch.
No. Did you read my post?
Just to clarify, my above suggestion that roller screws and optimal low reduction lead-screws are the equivalent (lubrication concerns aside) is correct or incorrect?
Are you saying a roller screw with high reduction gets its efficiency from better lubrication only and would otherwise be equivalent to a lead screw with the same effective pitch/turn? If that’s the case I’d disagree. And this was my reason for raising that point initially.
Yes. OK, we disagree about that. Glad we could get to the point.