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Suicidal.Banana
01-12-2013, 06:44 PM
So i have some questions about arm designs, and seeing how we have an Robotic Arms forum i figured i'd post it here, its not really robot arms like WidowX or PhantomX though, so if it doesnt belong here please dont hesitate to move it elsewhere.

Ive been walking around with an arm design i would love to use, but cant really feasibly achieve with the ax12's i have now, below is a video and some pictures of an ax12 version i build a few years ago;

http://www.youtube.com/watch?v=SnRn9QT3Uik



http://forums.trossenrobotics.com/gallery/files/3/5/9/9/200910041605_145_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1843&catid=member&imageuser=3599)
http://forums.trossenrobotics.com/gallery/files/3/5/9/9/200910041604_144_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1842&catid=member&imageuser=3599)
http://forums.trossenrobotics.com/gallery/files/3/5/9/9/200910041334_143_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1841&catid=member&imageuser=3599)
http://forums.trossenrobotics.com/gallery/files/3/5/9/9/200910041329_142_thumb.jpg (http://forums.trossenrobotics.com/gallery/showimage.php?i=1840&catid=member&imageuser=3599)



The problem with it was that the arm is just too heavy for the shoulder servos, an animation like in the video works, but when i tried to make it pick up anything with a little weight or hold its arms in an horizontal position for a few seconds the servos would just shut themselves down to prevent damage.

Now my first question is, if i were to build this arm design out of more capable servos, what would you recommend i go for? i dont really have the budget to build the entire arm out of RX-64's so i was thinking of replacing the blue servos as shown below with more powerfull ones, leaving the elbow/wrist rotation and gripper action to AX-12's;

4454

I may have to include the elbow rotation servo (the one between the two blue ones) though, not sure about that.
I was looking at some RX-24F's in garage sale but Inxfergy pointed out they wouldn't be very suitable as shoulder joints due to their heavy load and suboptimal heat dissipation. I guess this graph (http://www.trossenrobotics.com/images/gpimages/dynaChart.jpg) is also a good pointer since id mostly be looking for strength instead of speed. Any tips on (dynamixel) servo combinations highly appreciated. RX-28's maybe? and if so how many/where in the arm? the 3 i colored are in all honesty pretty much just guesses of where the load would be the heaviest.

My second question; got some some cool arm designs or ideas? ive been set on this idea, but realize its flaw (heavy) and would like to see some of the arm designs 'out there', personally im looking for as much degree of freedom as possible, but feel free to post anything, not limited to servo's either for the sake of a more interesting thread.

tician
01-12-2013, 08:16 PM
MX-series servos would probably be a better choice than RX-series, since they can be used with the AX-series without any additional hardware. They also have higher precision position sensing and full PID control. Not sure the MX-28T would be sufficient for a lot of heavy lifting, but they work well enough to make a DARwIn-OP do quite a bit of walking and even a back flip/roll over its head (http://www.youtube.com/watch?v=cFvxUqffLSc&feature=youtu.be&t=30s) to right itself after falling to the ground. The MX-28 also have roughly the same body size as AX-12, but larger mounting flanges on the end plates (not compatible with AX plastic frames, except mounting to the horn). Also note that repeated high speed movements at little load can cause them to overheat (arms waving everywhere at high speed with a little plastic sword in its grip), and the motors can fail/seize if repeatedly overheated without disconnecting them from the power supply to cool off sufficiently (not just turning off torque).

One possible way to cut down on the weight of the actual arm is moving the servos to the chest and using cables to enable them to actuate the joints. The fingers of the DLR arm comes to mind (http://youtu.be/nw_PRZeiNs8?t=1m7s) (the arm's wrist and forearm rotation mechanisms are a bit more complex - got the IROS papers). It uses two motors and two strands of dyneema per finger joint. One motor controls position of the joint by rotating a horn/roller to which one end of each strands secured, and the other ends are connected to mount points on each joint. The strands of dyneema pass through a small rotating mechanism that can adjust the tension in the strands without affecting position control, which allows you to adjust the stiffness of the joint to accommodate the situation (grip an egg without breaking it or grip a bowling ball).

Also really like the little DLR (http://www.youtube.com/watch?v=TqvW_6PmBIA) crawler (http://www.youtube.com/watch?v=T2DrDCylsUk).

Suicidal.Banana
01-13-2013, 10:24 AM
Hmm, thanks for that info/those ideas, for starters i didnt think DARwIn-OP would use MX-28T's, (i thought it was build completely from RX-64's) but your absolutely right.

About your other remark regarding dyneema (type of rope for anybody unaware) idk, it certainly would be awesome and come with a bunch of added extras (safely holding fragile things like a egg, like you point out) but i dont really have enough faith in my building skills to make 2 arms powered from a array of servos in the chest, besides im wondering if it would even be possible, considering the rotations in the arm (the example you gave me were only 'hinges' instead of rotation axis)

And about the MX series (or other series for that matter) having different attachment points, thats ok really, ive reached the point where im ok with dropping/redoing half the robot (including brackets) to get it to actually succeed this time around.

Do you think replacing only the blue ones (for MX-28T's or '1 higher') would suffice? or should i also look at replacing the other servos?

And yea, idk, once again the idea with ropes and servos in the chest does sound like a awesome way to improve most issues, but at the same time i doubt ill ever really want it to hold a egg, i guess id rather keep it a arm of servos (servos being in the arm i mean) also because the space in the torso (that will be bigger then the default one used in the pics) will prolly house the controller and/or batterys.

Oh and btw, lil bit offtopic but whatevers; do you have a thread about your arduino with ax mx and rx support? I guess my best option atm is a CM700 to combine the different series, but yea, anxious to see how you go about combining them.

Thanks for the input!

tician
01-13-2013, 04:35 PM
I mentioned the variable stiffness stuff just because I think it is a giant bag of awesome to be able to control how much force a joint applies to maintain a position without affecting position control significantly (instead of making the position motor control torque as well). The DLR hand arm system is extremely complex and expensive with a butt load of processing power (lots of CPLDs and FPGAs and motor drivers and magnetic encoders linked via gigabit ethernet). There is the much less expensive, but very bulky, open arm (http://www.hizook.com/blog/2011/04/27/highly-anticipated-icra-2011-talks-low-cost-arms-helicopter-manipulation-and-ball-ca) that uses lasercut plywood and make-shift linear elastic actuators made from rubber tubing inserted between rotating plates/linkages.

I've never owned or used any RX-series servos. The early RX-28M (later MX-28, and finally MX-28T) in the DARwIn-OP were my first encounter with any non-AX-12/18 dynamixels. It is definitely possible to control both the 'TTL' bus and RS-485 bus simultaneously with a single UART, but it requires a hardware interface for both. The RS-485 transceiver itself acts as a full-duplex to half-duplex converter (can only transmit or receive unless you add a second transceiver and its own pair of data wires) while the TTL bus would need a full-duplex to half-duplex conversion circuit made from as few as one IC (if you choose the right one). If you post on the robotsource CM-900 forum (http://www.robotsource.org/xe/Circle_CM9_Developer_World) describing your project and requesting one, you probably will get a CM-900.

Alan brad
09-05-2013, 05:45 AM
Wow Super Arm designs.How do you make this Arm..? What program do you have write for the Arm...