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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Tue, 27 Jul 1999 13:51:44 GMT
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Viewed:
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994 times
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In lugnet.robotics, Chris Phillips writes:
> In lugnet.robotics, Hao-yang Wang writes:
> > A while ago I was intrigued by the soda can retrieval challenge.
>
> I'm glad to hear that I'm not the only person who's still trying to solve
> Joel's challenge. It sounds like you've made some great progress!
I'm intrigued too by Joel's challenge. This was the reason for I started my
experiments with odometry months ago. The problem is always the same: much
more projects in mind than time to carry them out :-)
<snip!>
> > The robot is a standard two-track design. It has two motors, each drives a
> > track with a 1:3 gear reduction. It uses two rotation sensors as tachometers
> > to maintain its heading. The rotation sensors are not connected to the • motors,
> > but to two unpowered wheels. This way the rotation sensors still detect the
> > actual movement of the robot even when the tracks are slipping. The wheels • are
> > suspended in a way that maintains their contacts to the ground even on an
> > uneven terrain.
>
> I had considered switching to a tracked design also, and thought of using this
> approach of having non-driven encoder wheels in contact with the ground. I
> had trouble getting the wheels to maintain solid contact so I put it on hold.
> I was trying to spring-load my wheels to hold them on the ground, but it
> sounds like you're having better luck just using old-fashioned gravity?
>
> Do you find that you are able to get fairly accurate odometry using this
> approach? I think the entire Mindstorms community could benefit from somebody
> figuring out how to track linear motion and turns reliably.
>
> 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! For now, my encoders are
> driven directly off the wheels, but I plan to eventually incorporate some kind
> of free-floating odometry wheels if I can get them to track properly.
I believe that tracks (or wheels coupled to behave like tracks) are a poor
choice to get good results from odometry. I mean, as this mechanical
arrangement relies on slippage to turn, it's unprecise for its own nature.
IMO a differential drive is much more suitable to get the best from odometry.
> > 1) Can detection. I have an interesting idea of detecting soda cans using
> > Dennis Clark's IRPD: The robot wanders around. When it detects something in
> > the IRPD, it does a bit of wall-following. (See my earlier post
> > <http://www.lugnet.com/robotics/?n=4527>.) From the readings of its two
> > tachometers, the robot can deduce the shape of the "wall". If the "wall" is a
> > cylinder with a diameter in a certain range, then it may be a can.
>
> This is similar to my latest approach. I'm using an IR-radar "ping" to locate
> the can. When I'm close to (and directly facing) an object, I turn a few
> degrees left and right, comparing the ping values to either side of the peak
> reading. I'm still trying to tune the heuristic, but the idea is that if the
> ping is much lower on _both_ sides, it's probably a can. If _either_ reading
> is comparable in magnitude to the center peak, it's probably a wall. (Since
> my odometry isn't anywhere near perfect yet, I'm trying to minimize any
> movement during this process.)
I have another idea about using the Dennis Clark's IRPD to detect cans: we can
turn it 90 degrees so the left and right detectors are on the same vertical
line and become "bottom" and "top" detectors. Placing the sensor at the proper
height on the bot, we should be able to tell if an obstacle is a wall (bottom
detection only) or a can (bottom and top detection at the same time).
Obviously I assume that the walls are higher than the cans :-)
Hope this may help your project. I'll start mine sooner or later :-)
Mario
http://www.geocities.com/~marioferrari
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Message has 2 Replies:
 | | Re: Homing with the IR Tower
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| (...) My first thought: If you don't connect the tachometers directly to the tracks, but to the separate, unpowered wheels, you don't have to care if the tracks slip or not. My second thought: When the robot turns, there is exactly one "fixed point" (...) (25 years ago, 27-Jul-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: Homing with the IR Tower
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| (...) I'm glad to hear that I'm not the only person who's still trying to solve Joel's challenge. It sounds like you've made some great progress! (...) Congratulations! Another Lego Fan is born! (...) Same here! <snip!> (...) I had considered (...) (25 years ago, 23-Jul-99, to lugnet.robotics)
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