09 May 2016

2.77 PUPDate: Manufacturing, Assembly, System Integration, Testing

THE PARTS ARRIVED!

And I have plenty of MDF. 

I made the mockup on the underside of a wall we had for an old climbing robot we had in lab. The climber, funny enough, is to perform the outside fastener drilling, reaming, countersinking, and placement for the 787. It seems fitting to have a mockup on the inside of the climbing wall!

I waterjet the Shear Tie and Stringer out of some UHMW Polyethylene we had lying around in lab. The coefficient of friction between these parts and the urethane rubber I purchased for the clamp axes felt quite high, and was 0.5 from looking up tests online. I bent the components using a heat gun to heat a perforated line I waterjetted into each component, then clamped them to big pieces of Aluminum we had in lab. This worked fairly well, and while the components are not nearly as stiff as the composites used in the 787, they will work well to hold the FASBot and  demonstrate the concept to the Boeing visitors!

Here's my bigger Master's project, the Triple Scissor Extender, with an 8020 end effector attached to the top plate, moving up against the fuselage facade. 


Here it is matching the angle with the fuselage facade. This will work nicely!

I can't wait to talk all about this big guy to you and the rest of the Machine Design Qual committee this IAP! I hope you like the work I've done!

In a desperate attempt to put together a quick kinematic coupling in a situation where accuracy is not important, but repeatability would be useful, I put together this crazy thing out of 8020 adjustable-angle bracket bottoms that actually worked! It's three grooves for the FASBot-TSE Kinematic Coupling, which will have a set of permanent magnets in the center for preloading the FASBot. 


Off to lasercutting! First I cut a set of components where the fine hole tolerances didn't matter as much. Because the lasercutter cuts ON the line and does not compensate for kerf, I know through experience that all of the slots and tabs will fit nicely. 
For the parts that do require nice fits, such as for the ACME nuts and the leadscrew and linear slide bushings, I cut a nominal test piece out, measured the actual size of the holes and of the piece, and then tested the fits. They all fit too loosely, so I made the holes a little smaller in the CAD so the cut parts would come out exact. 

Et Voila! Lots and LOTS of parts! Time to assemble it all!

They fit together well enough, but there were issues with tapering, like you'd see on a waterjet, but something I didn't predict would happen with the lasercutter. 

The taper preloaded the bushings radially and actually made it difficult to slide the linear rail through!

I took a reamer to every hole that mattered, and it seemed to fix the issue. 

Axis 1 and 2 assembled! I made a 3D printed holder for the DC motor powering the nut driver, that couples to the planetary gearbox housing using the same U-shaped pin from the electric screwdriver from the past Seek and Geek.The structure also serves to clamp the vertical axis together. 

Here are the twin clamp axes! 

I cut the ACME screws to length, and then turned down the ends to be able to slide into the double bearings, the shaft collar and conical spring for linear preload and constraint, the belt pulleys (for the clamps), and the shaft couplings for the driven axes. On the other end was also a shaft end for the opposite radial bushing. These operations took quite a while, and I got well acquainted with the Makerworks lathe's autofeed features to ensure my final passes were very smooth. 


To put the leadscrew onto each axis was tricky, because I wanted to eliminate backlash and preload the nuts outward. I threaded one end in, and kept threading until it touched the second nut. Then, I back it off a bit, preloaded both nuts together with a small clamp, and then threaded the acme screw back again through both. After releasing the clamp, play and backlash were eliminated, and the carriage had the same stiffness as the twin preload springs, making this a Type 1 antibacklash mechanism (same stiffness in both directions). This technique for assembly also worked for the other axes. 

Here is the assembled double-bearing block at the driven end of a clamp axis. Note the pair of conical springs between the bushing and the shaft collar. (I decided on two series springs to achieve the preload force I wanted. One spring was too much/not enough force resolution). 

And one assembled axis! The linear rails are constrained from translating only by the walls of the end blocks, which are constrained by the overall structure. Lots of stacked components!



Both clamp axes assembled into both halves, with the backplate holding the drive stepper ready to couple to the leader clamp. Note the pulleys connecting the leader clamp to the follower clamp. 

At this point, I attached the stepper and wanted to see the axes moving. This video was taken before I figured out how to increase the current limit. This is probably at about 10% current, and you can hear the poor motor beggine for more juice. BUT IT MOVES! MY AXIS IS HAPPY! 

Testing my settings for motion/revolution, after changing those grbl settings and increasing the current limit! SO EXCITING! 

Now it's time to put the belt between the two! I ordered two belts, one that was JUUUUST undersize, shown here, which is way too short, and one that was oversize. These are what McMaster had on hand. 

An idler pulley was made using a pair of flanged bearings and by drilling out the center of a pulley. 

Oh no! My calculations for how long the belt was going to be for the tensioning mechanism were off!

I ended up drilling a bunch of holes until I found one that was sufficiently far enough to properly tension the belt. This works!

Now both axes move! WOW!


I put together most of the FASBot at this point, and gave the Arduino an XBee Wireless Adapter shield in order to create a wireless serial bridge between my computer interface and the now-unteathered battery-driven FASBot! I can't believe it worked so easily!

Remember that horrible excuse for a Kinematic Coupling? Well, it works!

Here is the chunk of steel on the FASBot and a couple of rare earth magnets clamped to the center of the three grooves. 

After the FASBot surviving a couple of accidental 3-foot drop tests, I eventually switched to much more powerful Neodymium magnets. 
 

I ended up changing the front plates to be one bigger plate in order to increase rigidity as the horizontal axis moved. 

Nut Driver Axis pair closeup. 
  
Clamp Pair closeup. 


Look how happy it is! 

Professor Asada seems pleased!




Here is the Triple Scissor Extender coming by to pick up the FASBot!
  
"Two Robots: A Story of Trust, Courage and Friendship In A Dungeon Laboratory"
A Master's Thesis by Daniel J. Gonzalez






Time to make all of the wiring pretty! I used a drill to twist up the stepper cables. 

You can see the wiring from the spindle and the vertical axis going to a adhesive ziptie holder in the dcenter of a plate under the top plate. Adhesive hook-and-loop fasteners attach the yellow battery to the bottom surface in the electronics box. No shaking components while this thing goes inverted! 
Here at the side, a pair of ziptie holders keep the wiring from the horizontal axis neat. 

All excess wiring is then folded up and ziptied to a holder. 



Wiring and electronics area closeup. 

Here is the TSE with FASBot in the lowest position. 

After upgrading the magnets, the full-weight FASBot can mount to the TSE without worry of falling!

The final kinematic coupling with the nicer magnets. 

And here is the entire demo we showed to Boeing of the TSE and the FASBot working in conjunction! 

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