Subject:
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Re: Homing with the IR Tower
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Newsgroups:
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lugnet.robotics
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Date:
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Sun, 25 Jul 1999 16:26:38 GMT
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Viewed:
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1031 times
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In lugnet.robotics, Hao-yang Wang writes:
> > My latest design uses a six-wheel drive setup with three wheels on each side
> > which are direct-geared to spin as one. I have found this to be much easier
> > to work with than the tractor treads so far, and it grips so well, I half
> > expect it to climb up a wall while I'm not looking!
>
> Funny I tried this approach before. My problem is that when the robot turns,
> the tyres are kind of reluctant to go sideways. I think it depends on the kind
> of tyres you use. Which kind of tyres are you using?
I am using the small "balloon tires" that are marked "43.2 x 28 S", and which,
I believe are the ones used in all of the Mars Rover designs. These tires have
nice, rounded edges that seem to make them slide sideways a bit easier than the
larger tires that have square corners. I don't have a digital camera or a
scanner so I can't post any pictures yet, but here's the basic idea: I'm using
three wheels and five 24-tooth gears on each side of the platform. The
24-tooth gears are mounted in a row, and the wheels are connected to the 1st,
3rd, and 5th gear axles. I have one motor on each side with a 16-tooth gear
directly meshing with any one of the five 24-tooth gears to drive all three
wheels together. The front and back axles pass through the farthest-separated
holes in a 14-stud Technics beam.
The base platform is built up out of Technics beams that are "bolted" together
using 2 or 3-stud friction pins, resulting in a flat platform that is 16 studs
wide. The drive gears rise slightly above (and below) this platform, leaving a
10-stud wide flat area in between which is ideal for mounting the RCX and the
two drive motors.
The "corner" wheels definitely have a little bit of sideways drag when turning,
but I've found that this design turns in a very nice in-place circle even
though my motors are not perfectly matched in terms of RPM or power. The
corner wheels tend to work their way loose from the axles over time unless you
put collars on like hub-caps to increase their holding force. (Hint: Instead
of using a single 1-stud collar, use a pair of 1/2 stud pulleys; they have
better holding power than a single part.)
As I said in my previous post, I am currently running the encoders directly off
of the drive gears. I expect that this will introduce errors when turning,
since there is likely to be some wheel slippage in a turn. But I'm hoping that
perhaps the middle wheel on each side will not slip much, being
somewhat balanced by the opposing slippage of its two neighbors. My current
prototype has the encoders driven off of an 8-tooth gear in direct contact with
the top of one of the drive gears.
All told, this is by far the most stable robotic platform I've built in the few
months since I came out of my dark ages. It has much better traction and less
vibration than any of my other designs. The lack of traction and the wheel
slippage caused when a platform "hops" during a sudden maneuver have been the
worst navigational challenges that I've encountered so far, and this design is
my best attempt yet to minimize these effects.
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Message is in Reply To:
 | | Re: Homing with the IR Tower
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| (...) So far I have been using the tachometers mainly to find the mid-point of the signal band, so they don't have to be too accurate. The need of accuracy will arise when I use them for soda can detection. Let's see. I have posted my current design (...) (25 years ago, 24-Jul-99, to lugnet.robotics)
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