Subject:
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Re: RCX 2.0 -- ideas.
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 15 Jan 1999 05:59:30 GMT
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Reply-To:
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BRETT@SR.HP.COMavoidspam
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Viewed:
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1373 times
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Ian Sinclair wrote:
> I am most interested in knowing how you used a RCX motor to shift
> the motors into the two different drive trains. Does it require
> any sensors?
I replied to these questions from an e-mail Ian sent, but since it's
here as a post also, I'll (partially) reply here too.
I don't have any pictures, but I am working on a web site that will
document all my custom input and output devices. I do have at least a
rough draft of the text that will go with the pictures. I've attached
it to the end of this message.
It's pretty useless without the pictures (sorry) but it's the best I can
do for now. I'm afraid that it'll be a while before the site is ready.
I don't want to do one of those "look at my site" when there's almost
nothing there yet. So I plan to have it pretty well filled out before
any public anouncement.
I cut and paste the text in so I hope it looks OK (but I'll bet the line
lengths will be messed up somehow). Anyway, here goes...
--
Brett Carver
brett@sr.hp.com
(707) 577-4344
--------------------------------------------------------------------
With the introduction of a third motor to the parts supply, the focus
turned to: how to drive more than three things from three motors.
Typical design thus far have required two motors to handle mobility
(movement forward and backward, turning right and left). This leaves
only one motor for doing work once the robot reaches it's goal. We can
do better.
The picture shows the first prototype of a method that allows motors to
drive more than one drive train. This design used one motor to drive a
transmission that can "shift" the other two motors between (in this
case) one of two different drive trains. In their "A" position the
motors drive the left and right treads allowing independent control of
each tread for a full range of movement and direction control. When
shifted to the "B" position, the motors can each drive a second
independent drive train to do other work.
Limitations in space of this prototype allowed for only the left motor
to have a second drive train. And limitations in gears didn't allow for
anything to be actually hooked up to the second drive train. But that's
OK, the purpose was to demonstrate the feasibility of the design. A
future implementation could be based on a wider vehicle design allowing
space not only for a second drive train on the other motor, but
potentially allowing for a third or fourth drive train on both motors.
The problem of inaccurate motor control is a very real problem in this
design. Since motors are controled by power/duration (not revolutions)
getting repeatable movement is very hard. Because of this, cumulative
error can result in the drive gears not being placed in the correct
locations. To solve this problem the sliding arm was build such that
when it is moved fully into the "A" position it's guide rod is free from
the guide holes. This means that the motor can keep running past the
end point and the gear simply slips. No damage is done, and a known
starting point can always be guaranteed. If every drive train shift
(assuming three or four positions) is proceeded by a reset to the "A"
position before moving to the requested location cumulative errors are
eliminated. Any minor error in the movement from the "A" position to
the position of choice is hopefully small enough that the width of the
gears will compensate for it (the gear teeth will engage even if a bit
off-center).
This design does use one very specialized piece: a flex connector from
the 8412 NightHawk helocopter. This allowed for a connection between
the motor movement and the drive gears that had a lot of give
(flexible). This reduced the stress on the system and lowered the
posibility of bricks being forced apart while in operation. But with a
little work a design could be created that would use more standard
pieces. Perhaps replacing the flex connector with a rod that has some
give in the connection at each end. Or, perhaps by reversing the
orientation of the entire motor mount making a more direct connection
between the two moving rods.
The other critical area was in the software to perform the shifting
operation. An obvious problem is that if the gears aren't perfectly
lined up during a shift, they can bind rather than engage causing a
failure of the entire sub-system. The solution was to create a task
with the specific purpose of simply osolating the two drive motors back
and forth a little bit so that and the gears came into contact they
would quickly engage before the stress had a change to build to
dangerous levels.
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Message has 1 Reply:
 | | Re: RCX 2.0 -- ideas.
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| (...) Another way to do it might be ( - I have not built this, but been thinking about it for a while, as I want to use it with a (manual, and possibly later also motor-driven) car transmission of the common sliding-axle type - ) to use rubber (...) (26 years ago, 15-Jan-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: RCX 2.0 -- ideas.
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| (...) Brett, I have been working on a couple different transmission designs with multiple gear ratios. I am most interested in knowing how you used a RCX motor to shift the motors into the two different drive trains. Does it require any sensors? Any (...) (26 years ago, 14-Jan-99, to lugnet.robotics)
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