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Thursday, September 8, 2011

Back at the MIT + DragonBot Update

As the title mentions, I'm back on campus!  I've been very busy with my fraternity's rush, so not much work has gotten done, but I have at least made it through my first day of class!  Hopefully I'll have time to go into Pappelardo to make the rest of DragonBot before the end of the week.  Since the grabber is done I just need to make a four bar mechanism and a base with wheels and aluminum box extrusion uprights.  Oh, and I also need to figure out how to lift the massive grabber.  To cut back on weight the four-bar will be made out of steel sheet metal which I'll bend to make more rigid.  I'm hoping this will be strong and very lightweight.  The base will be made out of ABS and will have standard 2.007 continuous rotation servos and wheels for front wheel drive, with a caster wheel or two in the back.  To stabilize the tall aluminum box extrusions I plan on bolting on aluminum brackets to attach it to the base.  Here's what it will probably look like:
At this point I decided to do some calculations in order to get an idea of how much force I'd need to lift the giant arm.  To get the mass of each part I applied material properties to each model in Solidworks.  The four-bar is 16 inches long (0.41 meters) and should weight about 0.94 pounds (0.43 kg) and the dragon wing pieces weight about 0.12 pounds (0.05 kg) together.  The grabber should weigh about 0.77 pounds (0.35 kg) and is another 2.5 (0.06 m) inches out.  Simplified, this is 0.34kg at 0.41m and 0.35kg at 0.47m.  This means that all together I'll need to apply at least 0.3 kg*m of torque at the top of the box extrusions.  I was planning to use a Vigor VS-11 servo which applies about 0.16 kg*m of torque at 5V, so I'll need to at least get a mechanical advantage of 2x; 4x would probably be ideal.  If I use a 1/4" diameter pulley attached directly to the VS-11, I'll be able to pull with a force of 25.2N.  To get 4*0.16= 0.64 kg*m of torque I'll need to extend the four bar mechanism to stick out 1 inch the other way and attach it to the pulley with a string.

Well, that's about it for now (I am le tired).  And don't think I forgot about my other not-so-secret project, I promise I will reveal the details in my next post!

1 comment:

  1. Actually, considering it will need to able to actually lift things I'll probably need to give it some more torque. If I extend the pulley attachment out 2" instead of 1" it will have over 1.2kg*m of torque, which should allow it to lift a 1kg weight easily. I did a stress analysis on the four-bar and it should be able to handle that without much deflection.