Upgrade Wheels
Hi All...
Just assembled my Boe Bot last night and it's pretty cool. I was just wondering if there are any wheels available that don't need those rubber bands?
Also, one of my reasons for building the "critter" is to haul around a small (~175 gm) wireless, color video cam. Would this be an acceptable load for the Bot?
Thanks for any advice!
Amanda
Just assembled my Boe Bot last night and it's pretty cool. I was just wondering if there are any wheels available that don't need those rubber bands?
Also, one of my reasons for building the "critter" is to haul around a small (~175 gm) wireless, color video cam. Would this be an acceptable load for the Bot?
Thanks for any advice!
Amanda
Comments
Your 175 gram camera should not overload the BoeBot; a D cell weighs ~150 g. I wouldn't go much heavier. Keep it as light as possible for maximum battery life.
Yeah... it is kinda the look. Thinking a bout chucking them into my drill and carve out a v-groove for a silicone o-ring.
Also thinking of going to tank treads... I like that look. (Johnnie-5 dontcha know!) :->
Still =playing= with it and testing on my kitchen floor so traction isn't a big issue so far.
Looks like the Pololu wheels would work. Same diameter and the spline is "supposed" to be the same.
Thanks for y'alls feedback! :-)
Amanda
Amanda: There's a digital encoder kit add-on for BoeBot that is useful. It is designed to use the stock wheels, so if you change those you might not be able to use that kit, FYI. http://www.parallax.com/Store/Robots/RoboticAccessories/tabid/145/ProductID/80/List/0/Default.aspx?SortField=ProductName,ProductName
And there's also a walker kit if you want to go really nuts... http://www.parallax.com/Store/Robots/RoboticAccessories/tabid/145/ProductID/314/List/0/Default.aspx?SortField=ProductName,ProductName
Interesting. From what I've found, the Pololu wheels have an encoder pattern moulded in. Is this not compatible? (Not really sure what I would use the encoder thingy for, but I'm always wanting to learn!!!)
Amanda
I am sure you brought a tear to erco's eye when you asked about the encoder thingy. erco is kinda the encoder king. I will let him tell you why you will have to have encoders someday!
Lol... I have a rough idea of how encoders might be used, but I'm always open to new concepts. Something like that is a bit down the road tho'. Bob is still getting used to my rusty programming with the occasional headlong rush into a door frame or driving off the desk when I forget to turn the servos off! ;-)
Amanda
http://solarbotics.com/products/sw/
These are Futaba compatible splines.
Interesting site worth bookmarking and poking around; they have some offbeat parts the best prices around on many sensors: http://www.junun.org/MarkIII/Store.jsp
You might want to take a look at the wheels from Budget Robotics, http://www.budgetrobotics.com/category/Wheels-and-Casters-67. They don't use rubber bands and have Futaba-compatible splines.
As the author of the ArdBot series in SERVO Magazine, and chief bottle washer of Budget Robotics, makers of numerous robot bases and components that use expanded PVC, I wanted to address some of the common misunderstandings regarding PVCX.
Let’s start first by mentioning there is no single “ideal” construction material for building robots. If there were, we’d all be using it.
With that out of the way, despite its common “foam” moniker, there is nothing foamy or spongy about expanded PVC. In fact its compression strength rivals wood and many plastics commonly used to build bots.
PVCX is a solid, rigid plastic (little or no phthalate plasticizers added), engineered to have less density than the same bulk of unexpanded PVC sheet. The manufacturing process involves extruding the molten PVC through a shaped orifice while injecting nitrogen or other non-flammable gas into the plastic. The gas expands the plastic - hence “expanded” PVC.
The cross-section of PVCX has a cell-like structure that some people say looks like foam. But it is not at all like “foamed polystyrene” (blue/pink foam) which is used as insulation or sub floor material. PVCX is hard and solid and (contrary to what you may read elsewhere) is not porous.
PVCX exhibits far superior elongation at break than most any other readily-available plastic: some 20-40%, as opposed to less than 5% for polystyrene. This is among the chief reasons I prefer using it. This make it better for any application that’s subjected to repetitive stress. (PVCX has about 1.5X the fatigue strength as polystyrene, and maybe 2X or so that of acrylic.)
Over the years I’ve built numerous robots using all kinds of wood, various metals, and several dozen types of plastics. And over those years - and given the right job - I’ve learned to favor PVCX for its low-cost, but also its ease of workability, especially for younger learners where hand tools are more appropriate for cutting and drilling.
For desktop robots - those under 5 or 10 pounds - it’s especially important to match the construction material to the job. I favor PVCX for smaller bots because it adds less weight than that of most other materials. Added weight consumes battery life, so it’s never a bad idea to put your bots on a diet.
Regarding the comment about the use of PVCX for small wheels: I think there must be some confusion as to the compressive strength of PVCX versus other plastics. Its compressive strength puts it on par with an equal mass of plastics used to produce similar wheels (typically polystyrene and ABS), using either injection molding or fused deposition. Regardless, the limiting factor for all such wheels is the radial loading on the motor shaft, and all these wheels perform well.
Downsides to PVCX: It has poor impact resistance, though depending on its cross section no worse than polystyrene. Its melting point is quite low, and it releases corrosive gas when cut with a laser cutter. The difficulty in cutting it is why most people don’t offer PVCX for robot bases. It's a PITA to machine. We use a CNC router, which is more labor intensive and very, very messy.
BTW, I love all your robot books. Your insight & experience in building bots is legendary. If you say PVCX is good for robots, this court will not dispute it!
http://www.youtube.com/watch?v=9VHr7Fi6tCg&feature=related
He made that video back when he was selling the CBA robot kit. Like Gordon's ArdBot, it used expanded PVC for the wheels and chassis. So I imagine making the ArdBot looks similar (and makes a similar mess).
I liked Gordon's books as well. At some point I plan to build PropBot from my propeller platform kit and Gordon might be getting an order from me.
For those who prefer to use their material of choice, in a few weeks I'll be posting CAD (DXF) files of the ArdBot pattern files, plus to-size drilling and cutting templates (in PDF form).
FWIW Dept: Though called the ArdBot, it's really microcontroller-agnostic. SERVO was interested in the Arduino angle first - they have a regular column in N&V on the Propeller but nothing on the Arduino at the moment. One of the reasons I dropped by was to get more immersed in the Propeller. Anyone here know anything about that controller?...
So by all means, have a peek at http://www.budgetrobotics.com . Mention my name for a 20% price INCREASE!
Rich H
Which do you think would be a more fun project. A tankbot of some sort, or the ArdBot chassis? Also assume the need to determine position which implies encoders of some sort. Additional IR would be used in either case to avoid running off edges or ramming walls.
The ArdBot would be a snap to add on quadrature encoders like Wheel Watchers. The downside is that it's similar to other servo driven differential drive robots, but with a propeller chip.
A tankbot would likely be a difficult fit for wheel encoders. But I have the crazy idea of using the mouse sensor a smidge above the floor to track position. So the upside would be going off in a different direction.
I would also likely want to be able to add on a three axis manipulator of some sort to either bot.
@W9GFO, that's a really nice looking robot arm. How did you attach the both sides of the joint to the servo?
Thanks, the servos are glued in place and each joint opposite the servo output has a disk type hinge made out of the same material as the rest of the arm.
Rich H
I know the side to side shimmy well because it's an issue with all the Lego arms I've built. Add enough structure to make them stiff and they're too heavy.
Treads absorb a lot of power even driving straight. Turning is even worse; somethings always dragging, and the center of rotation changes a lot based on the surface you're turning on, so odometry is much less accurate. Indoors on a smooth floor, wheels seem to have every advantage in power, maneuverability & repeatability and lend themselves to encoders and accurate odometry (and all that wonderful trigonometry you've been doing recently!). Outdoors on really rough terrain, treads are definitely best for getting brute force power to the ground. But getting even a small rock caught between the tread and bogie can lock up a tread system. If I ever built an all-terrain outdoor bot, my first choice would be 4 or 6 fat rubber tires, all wheel drive.
Fun fact from http://www.treehugger.com/files/2008/09/us-military-combat-vehicles-fuel-efficiency-economy-gas-mileage.php?page=2 :
Fuel Economy of the M1 Abrams Battle Tank: It is powered by a 1,500 hp turbine and gets about 0.6 miles per gallon. It will burn 12 gallons per hour when idle.
John Abshier
-Phil
With you on this one. For outdoor or uneven terrain I prefer 4+WD, through for the most part tracked robots are cheaper to build. Maybe that's why they so popular.
For 4/6WD it can be difficult finding suitable wheels, though if you want to allow it to run indoors and out, and assuming you don't want to pay a fortune.
Years back I lucked onto a shipment of some inexpensive hard rubber wheels imported from China (for some toy long lost), They had nubs on the the tire treads, which effectively reduced the contact area when over a hard surface, and acted as cleats for softer terrain. So, a vehicle like this one:
http://www.budgetrobotics.com/shop/?itemid=233
could steer over Formica, carpet, linoleum, hardwood floor, dirt, grass, and sand equally well -- that is, no chatter when over a hard surface, and good traction over a soft surface. Alas, the supply of those wheels dried up. (I went to *every* Dollar Tree in Southern California to buy up their stock of the toy that used this wheel!)