Subject:
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(Un)steerable Shrimp
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 1 Oct 2001 22:26:43 GMT
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Original-From:
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Elijah Meeker <EMEEKER@AUSTIN.RR.stopspammersCOM>
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Viewed:
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658 times
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Hi all,
I have finally gotten my steerable shrimp to it's next blocking point.
The "Un" part of my subject comes about as I am still lacking a Barnes
Motor Amplifier to handle the 5 minimum motor controls and two micro
motors for the steering mechanism. As I have been noticeably lacking
employment for the past few months I think completion will be predicated
upon my success with the southern Wisconsin job market.
However, there were a few things I wanted to discuss and see if anyone
had ideas or suggestions.
http://dlbk.com/elijah/full_right.jpg
http://dlbk.com/elijah/top.jpg
The geometry of this shrimp is greatly influenced by:
http://www.bluebotics.com/products/shrimp/
but at about 3/4th size. I arrived at 3/4 because the motorcycle wheels
are ~ .75 of the bluebotic shrimp wheel and it seemed to me that wheel
height was a defining aspect of the shrimp design. This also means that
at the moment it can only climb 3/4 sized stairs, of which I have found
few, but it does climb those very well.
Steering issues:
http://dlbk.com/elijah/rear_close.jpg
I settled on a two wheel design initially in the pursuit of ground
clearance. If I was going to run this over uneven terrain there seemed a
big risk of the gear box catching on things. I tried rotating the
gearbox all the way up, but that required the power train to take a
right angle which added a lot of brittleness to the design. Putting the
gear box between two wheels greatly lowers the chance of getting stuck.
After adopting this front and back I decided that the worm gear in the
drive was giving more power than a single wheel could convert to
traction so I added wheels to the bogeys as well. As soon as I did this
turning was greatly impaired so, for the moment at least, I put both
wheels to the outside, losing the clearance but keeping the traction and
allowing a reasonable turning radius.
Another nice thing has been that double wheels allows the shrimp to
balance on just front and back wheels, which has allowed it to navigate
a few things it otherwise might not have.
http://dlbk.com/elijah/steer_mech.jpg
shows a close-up of the steering mechanism. I had tried connecting front
and back steering mechanically so as to simplify the motor and sensor
requirements but couldn't get an aesthetic and robust mechanism for
crossing from the front wheel to the body. I don't have any micromotors
but I am hoping they will have suitable power for turning the turntable
via a worm gear. If anyone is sure they won't I would greatly appreciate
a heads up.
Going to separate motors for front and back wheel raises the issue of
sensors for each. I currently have a rotational sensor, but am thinking
that I will likely wind up using a light sensor with an encoder strip.
Again, any suggestions would be welcome.
Bogey Issues:
I am not sure that the powered side wheels technically are bogeys, but
it's an easy distinction given shrimp design. Sorry for all the black, I
will try to get some more blue in the bogeys so you can see what is
going on.
The main issue with bogeys is lateral strength and stability. In this
design I tried to address both by putting the rocker arms flush against
the shrimp body and trying to eliminate places the arms would catch.
This worked marginally well, but I have since headed down the path of
putting a a single pulley as a pivot bushing and using 1/2 width lift
arms as runners farther out.
It is really hard to make the pivot points strong without having a lot
of places for the arms to catch against the body. I addressed this two
ways, first, with apologies to the anti-grinders, by filing down a few
6587s (3 axle with stud)so they would be flush. Then, as that wouldn't
work for the top I found that shoving a bit of flex system hose in a
regular black pin (some pins allow this, others don't) I could make a
very strong, and flush, pivot point.
I think that in theory there is an absolute height that a shrimp can't
climb, that being above 1.5 times the wheel height, however it can
usually do a bit better. Still, I headed down the path of putting
sensors on each bogey (which will be followed by one on the front wheel,
for sensing when a given wheel assembly will likely fail to climb it's
bit of the obstacle. However, as I need three of these at a minimum (and
would like to know whether the problem is on the right, left or center)
I am going to run out of sensor inputs given the need for them with
steering. Soooo, there will be more purchasing opportunities before I
get to a shrimp that navigates an unknown environment.
For programming I am heading down the Augmented Finite State Machine
path, which may be a bit of overkill for simple navigation in a rolling
platform but I want to learn more about AFSMs and hope to make this
project a lot more complex (learning etc.). I figure I will stick to NQC
but if any solution to my various shrimp issues can be found lower down
I would happily take that on. Again, any suggestions are greatly
appreciated.
Elijah
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Message has 1 Reply:
 | | Re: (Un)steerable Shrimp
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| (...) It seems so from the literature, but I wonder if rocker length also plays a part. There probably is a rocker arm length to wheel diameter ratio beyond which increasing the rocker length doesn't do much good. The seminal idea was to climb the (...) (23 years ago, 2-Oct-01, to lugnet.robotics)
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