PDA

View Full Version : How do they do it???



ooops
01-09-2009, 08:07 AM
I was looking at the iRobot Engineer kit (See picture below) and am wondering what they are using for motors in their arm. PDF link here (http://www.irobot.com/filelibrary/pdfs/gi/robots/iRobot_PackBot510_Engineer.pdf)

They are claiming the ability to lift 10 - 30lbs depending on the position of the arm. 10lbs at 80"



Elbow joints enable the robot


to lift up to 30 lbs (13.6 kg) with arm in a compact position
or 10 lbs (4.5 kg) while extended.


Manipulator Specification
Extension: 80 (203.2 cm)
Lifting capacity:
10 lbs (4.5 kg) at full extension
30 lbs (13.6 kg) at close-in position




That is impressive, especially when you look at the size of the motors in the joints of the arm.
Just to put it in perspective, my arm (on my body ... not a robot arm) is aprx 24" long, and I can lift 10lbs easily at full extension, but put that same 10 lbs on the end of a five foot stick and lift it with my arm at full extension it gets heavy quick!


Soooo, what are they using; small steppers, gearhead motors, or other?

ooops
01-09-2009, 08:11 AM
Sorry, picture didn't attach.

Connor
01-09-2009, 09:02 AM
Worm Gear Possibly?

lnxfergy
01-09-2009, 10:19 AM
I was looking at the iRobot Engineer kit (See picture below) and am wondering what they are using for motors in their arm. PDF link here (http://www.irobot.com/filelibrary/pdfs/gi/robots/iRobot_PackBot510_Engineer.pdf)

They are claiming the ability to lift 10 - 30lbs depending on the position of the arm. 10lbs at 80"



That is impressive, especially when you look at the size of the motors in the joints of the arm.
Just to put it in perspective, my arm (on my body ... not a robot arm) is aprx 24" long, and I can lift 10lbs easily at full extension, but put that same 10 lbs on the end of a five foot stick and lift it with my arm at full extension it gets heavy quick!


Soooo, what are they using; small steppers, gearhead motors, or other?



The key here is: at what speed? You can buy those gear boxes for servos (from servo city and others) that will give you 10x the torque - at the cost of 1/10th the speed. You could probably easily get to a similar size arm using some of the higher-end servos and those boxes, but it would take a half minute to raise itself, use gobs of power to stay up, and cost >$500, probably more like a grand.

-Fergs

ooops
01-09-2009, 12:13 PM
The key here is: at what speed? You can buy those gear boxes for servos (from servo city and others) that will give you 10x the torque - at the cost of 1/10th the speed. You could probably easily get to a similar size arm using some of the higher-end servos and those boxes, but it would take a half minute to raise itself, use gobs of power to stay up, and cost >$500, probably more like a grand.

-Fergs

Well, since these are bought by "government agencies" I expect price isn't so much an issue. I have not seen a video of these running, so I can't speculate on the speed. My interest is what are they using for the actuators.

Adrenalynn
01-09-2009, 12:14 PM
I would say linear actuators, screw-drive. It's not tough to find 200-1200lb linear actuators

ooops
01-09-2009, 01:24 PM
I would say linear actuators, screw-drive. It's not tough to find 200-1200lb linear actuators

You know that could easily be the case and the round "motor housing looking thingy" on each end of the arms are actually housing the fulcrums that the actuator pushes. That being the case those fulcrums must be made from some good stuff!
Worm gear ... maybe the more likely, it would fit the joints better, but that would mean really small motors housed in the arm.
How ever they do it, it is still impressive.

Connor
01-09-2009, 01:58 PM
You know that could easily be the case and the round "motor housing looking thingy" on each end of the arms are actually housing the fulcrums that the actuator pushes. That being the case those fulcrums must be made from some good stuff!

Could be a drum and cables. The screew would then pull rather than push. fulcrums wouldn't be that big of a issue then.

Adrenalynn
01-09-2009, 02:06 PM
Actually, those arms look pretty large in tube ID. They could put a good sized motor inside the tube and drive a worm-gear at 90deg in the shoulders. That may be more practical - similar to the way that my telescope works.

ooops
01-09-2009, 02:31 PM
Could be a drum and cables. The screw would then pull rather than push. fulcrums wouldn't be that big of a issue then.

I think I have seen other arms done that way in the past ... a very good possibility.


Actually, those arms look pretty large in tube ID. They could put a good sized motor inside the tube and drive a worm-gear at 90deg in the shoulders. That may be more practical - similar to the way that my telescope works.

I see where that would work as well.

Thanks for the theory's:)
It is always good (and fun) to try and figure out how the "big boys" do things.

robologist
01-09-2009, 11:45 PM
I believe the arm motors in the Packbot make use of harmonic drive gearing (http://www.harmonicdrive.net/reference/operatingprinciples/), a type of system where a flexible "cup gear is camed against a ring gear and has a tooth or several less than the ring. The net effect is a gear reduction of 100 or 150 to 1 in a small space with no real backlash. I found a Youtube video of the arm moving a little, before climbing some stairs at a Robonexus conference where I'd seen it. I can't say I remember asking about the arm, but would tend to think it was a harmonic drives since I'm pretty sure they use them in the flippers too.

ooops
01-10-2009, 09:09 AM
+rep:)
I do believe you have answered the question. I highly recommend checking out the web sight linked above. That looks like a "if you have to ask the price you won't be using it in your hobby project".
Thank you Robologist:)