Thursday, February 20, 2014

TOTT Bot: Chassis Design

Requirements | Design 1 | Design 2 | Chassis ]


At long last I finished the Trash Bot's chassis. You can build your own Take Out The Trash roBot (TOTT Bot). Just read on to see the design and find out what Actobotics parts I used...

I built an initial version of the chassis a few weeks ago and have since refined it. I've also settled on a better way to grab and tow the trash cans. So here goes.

Towing Design

Thanks to brilliant input from clever readers, the new design involves towing rather than lifting the trash cans. It seems simpler and quicker to implement. Lifting or grabbing the handle may come later but are too involved given time constraints.

Please forgive my crummy drawings :)
The robot will grab onto a handle that I'll bolt onto the trash can just above the axle. The robot will grab this handle with a mechanism located just above the robot's axle. This arrangement means more motor torque can be applied before the robot and/or can begin to rotate around their axles.

The handle is just two Actobotics channels bolted together back to back and bolted onto the can. The grabbers are also channel. I'll post up more on the grabber mechanism next time.

That's engineering for you. Evaluate the options and sometimes realize they all suck, throw them out, and come up with a better option. Find the optimum solution, adapt to change.

Change? Yeah. My requirements changed, midstream. Time-to-implement became more important than my requirement against modifying the trash cans. I think the new solution will still be trash-collector-friendly but quicker to implement (heck, I'm almost done, in fact).

Fasteners

  • (2) 1/4" 6-32 Socket Head Machine Screw, 25-pack (632106)
  • (3) 5/16" 6-32 Socket Head Machine Screw, 25-pack (632108)
  • (1) 3/8"  6-32 Socket Head Machine Screw, 25-pack (632110)
  • (1) 7/16" 6-32 Socket Head Machine Screw, 25-pack (632112)
  • (1) 5/8" 6-32 Socket Head Machine Screw, 25-pack (632118)
  • (1) 9/16" 6-32 Socket Head Machine Screw, 25-pack (632116)
Since they're cheap and you need a bunch, stock up on the above, get at least the recommended amount. You'll want to double-check my counting, too.

Frame

  • (1) 15" Aluminum Channel Product Page (585458)
  • (1) 18" Aluminum Channel Product Page (585462)
  • (2) 12" Aluminum Channel Product Page (585454)
  • (2) 9" Aluminum Channel Product Page (585450)
  • (6) Side Mount Bracket Pair Product Page (585470)
    • for each pair, use (4) of the 1/4" Machine Screws
    • for each pair, use (4) of the 5/16" Machine Screws
  • (2) Quad-D Hub Mount  Product Page (545324)
    • Attach to channel with (4) of the 5/16" Machine Screws
  • (2) 0.5" Tube Clamp Hub, 0.5" bore Product Page (545340)
    • Attach to the Quad-D Hub Mount with (4) of the 3/8" Machine Screws
  • (2) 0.25" Clamping Hub, 0.5" bore Product Page (545600)
    • Attach to channel with (4) of the 3/8" Machine Screws
  • (2) 12" x 0.5" hollow tubing Product Page (635262)
The basic chassis frame is simply made of channel attached with Side Mount Brackets. There's two 12" pieces on the side, a single 15" cross member (offset toward the middle for drive chain clearance) and an 18" cross member across the front. Two 9" vertical channels are affixed to the 12" side channels at the back.


Side Mount Brackets on an Aluminum Channel
The vertical 9" channel may need to be shorter or longer depending on the remainder of the design. The motor is mounted above the axle some distance. Closer means you can get away with less chain. I think I can use 3 packs of the 12" delrin 0.250" chain. I'll cover the parts required to mount the motor and axle later in the article.

The wider 18" cross member at the front was chosen to support caster wheel tubes. The tubes run through the 1/2" holes in that channel (below right). The tubes are clamped up front with a 0.25" tube clamp (below right). A Quad-D Hub Mount is mounted into the side channel as a rear support for the tubes (below left). A 0.5" tube clamp clamps onto the tube and bolts to the Quad-D Hub Mount (below left).

Side view of front where tubes are mounted.
This may be overkill but it's pretty darned strong. You can probably just cut the tubes down, skip the Quad-D Hub Mount, and use a 0.25" tube clamp bolted to the other side of the 18" channel.

Caster Wheels

  • (2) Channel Bracket B Product Page (585482)
  • (2) D-Shaft 0.25" x 2.5" Product Page (634074)
  • (4) 0.25" Collars Product Page (6432K12)
  • (1) 0.25" Shaft Spacers, Pack Product Page (633104)
  • (2) 0.770", 0.25" Set Screw Hub Product Page (545548)
  • (2) 4" Heavy Duty Wheel, 0.770" pattern Product Page (595410)
  • (2) 90 degree Hub Hub Mount Product Page (545400)
  • (2) 0.5" Tube Clamp Hub, 0.5" bore Product Page (545340)
    • Attach to the 90 degree Hub Hub Mount with (4) 5/8" Machine Screws
  • (2) Swivel Hub Product Page (545364)
    • Attach to the 90 degree Hub Hub Mount with (4) 1/4" Machine Screws
    • Attach to the Channel Bracket B with (4) 1/4" Machine Screws
  • (4) Flanged Ball Bearing 0.25" ID, 0.5" OD Product Page (535044)
Originally the robot was to straddle the trash can. Unsupported channels flexed too much with caster wheels so I went with tubes. They are unnecessary for the new design but look cool so I kept them. Here are some detail shots.


The Swivel Hub mounts onto the 90 degree Hub Hub Bracket and Channel Bracket B with 1/4" machine screws. The Tube Clamp Hub mounts to the other part of the Hub Hub Bracket with 5/8" machine screws. Don't tighten those screws yet. 

The Tube of course goes into the Tube Clamp and then you screw down the clamp screws. Once those are tight, then finish by tightening the Tube Clamp to the Hub Hub Bracket. For better grip, sand the tube with 220 grit sandpaper.


The wheels mount to the Channel Bracket B as follows. Use the 0.25" D-Shaft and onto that you use a pair of Collars (or add another, inside the Channel Bracket B if you want to make the assembly more secure), a Bearing each side, a single 0.770" set screw hub adapter, and the 4" heavy duty wheel. I put plastic shaft spacers between the bearings and collar though that isn't strictly necessary.

This is one side of the caster axle assembly
This is the other side of the caster axle assembly

Drivetrain

  • (4) Bearing Mount - Pillow Block Round (1" Bore) Product Page (535170)
    • Attach each to 9" Channel with (2) 5/16" Machine Screws
  • (4) Clamping Hub 1" Bore, Threaded Product Page (545352)
  • (2) Aluminum Set Screw Collar, 1" Bore Product Page (9946K19)
  • (2) 6" Heavy Duty Wheel Product Page (595610)
    • Attach each Clamping Hub to each Wheel with (4) 9/16" Machine Screws
  • (2) 48T Sprocket, 1" Bore, 0.25" pitch Product Page (615126)
    • Attach each Clamping Hub to each Sprocket with (4) 5/16" Machine Screws
  • (2) 16T Sprocket, Hub Mount, 0.25" pitch Product Page (615102)
  • (2) Set Screw Hub, 6mm bore Product Page (545576)
    • Attach to 16T Sprocket with (4) 7/16" Machine Screws
  • (2) 4" x 1" Stainless Steel Tubing* Product Page (635138)
  • (2) 90 RPM Precision Gear Motor** Product Page (638238)
  • (2) Aluminum Clamping Motor Mount Product Page (555116)
    • Place motor in mount, clamp, attach to 9" Channel with (2) 7/16" Machine Screws
  • (3) 12" lengths of 0.250" Plastic Chain Product Page (615150)
  • (4) Shaft Spacer, 1" Product Page (633116)
* I had to do some sanding on the OD to clear the bearings
** I may need to select more powerful gear motors. We'll see.

I selected a chain dive configuration as it was easiest to assemble. The gear motor provides gear reduction already but I added 1:3 reduction with 16T and 48T sprockets, thereby tripling torque and reducing speed by one third.

Robot is rotated 90 degrees in this picture. Front is at the top of the shot.
Mount the pillow blocks on to the lowest position of the 9" vertical Channel, with the bearing flanges on the outside, like in the picture below.



Mount the Wheel onto the Clamping Hub, loosely, insert the 1" axle tube flush with the wheel, clamp down the hub, then tighten it to the wheel. Next, slide on the collar and clamp it in place. Finally, slide on a Spacer. Slide the free end of the axle tube into the pillow blocks.

Slide a Spacer onto the other end of the Tube followed by a Clamping Hub. Attach the 48T sprocket onto the clamping hub loosely, then slide this assembly onto the Tube. When clamping down the Hub, leave just enough side play to ensure low friction rotation. You may have to play with it a bit. Clamp down the hub, tighten the sprocket to the hub. 




Attach the Motor Mount onto the 9" Channel. Insert the 90 RPM Motor and clamp down.  Attach the 16T sprocket onto the 6mm Clamping Hub. Slide onto the Motor shaft. Slide the sprocket along the shaft until it is co-planar with the 48T sprocket below. Tighten the set screw.

Connect two 12" Chain lengths together, measure it around both sprockets, remove unneeded links. Stretch the loose ends and attach (this is hard). A little slack in the chain is ok, as long as it can't easily jump on the sprocket.

Conclusion

That's it for now. Next time I'll write about the towing mechanism on the robot. After that I'll be working on electronics.



Next: Towing

1 comment:

  1. As always, enjoy following your work on this project. We have decided to build a miniature version first, posting our progress here:

    http://www.societyofrobots.com/robotforum/index.php?topic=17610.new;topicseen#new

    We are thinking that a forklift approach with side bars to prevent from falling side to side or forward may work. We'll put a wireless camera on it, so to watch progress from our computer.

    Look forward to seeing your progress!!

    ReplyDelete