Subject:
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Some help or advice requested
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Newsgroups:
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lugnet.technic
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Date:
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Tue, 19 Jan 2010 08:55:36 GMT
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Viewed:
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21694 times
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Hi all.
I am currently working on a project (to be revealed in due course!) and I've
come across something of a stumbling block. I have a tray that is constrained to
move in one dimension, and a series of short liftarms under the tray, all
rotating in phase with each other, and placed such that once per rotation, they
connect with the tray (ok, so the tray can move vertically as well, but only a
little bit!) and move it along a specified distance. So far so good. What I need
is for the motor to change direction when the tray has reached the limit of its
travel, and to have such a mechanism at each end of the track, so the tray moves
back and forth for as long as power is supplied. Make sense?
Now, this would be trivial to do using a couple of sensors and a computer (RCX
or NXT or similar), but aside from the fact that I don't own any sensors for my
RCX, I have committed to using only electromechanical principles. Until now, I
have been trying to achieve this using a pole reverser switch, but have run into
the problem where the switch turns the motor off at the top of travel, and the
tray doesn't have enough momentum to finish the job itself. I have experimented
with using rubber bands/shock absorbers to make the natural state of the switch
either Forward or Reverse (kind of like the setup in the 8480 Space Shuttle,
where the natural state of the switch is Off), but I've found that this doesn't
actually help much - there is still enough dead space at the top of the switch
to cause problems.
So, my first question is can I use two switches, linked together such that
switch A is Forward when switch B is Off, and switch A is Off when switch B is
Reverse, similar to what the pneumatic fanatics do to eliminate dead space at
the top of a pneumatic switch (see
http://www.brickshelf.com/gallery/kclague/Computing/p6040061.jpg for an
example), or will I get into trouble with short circuiting stuff and blow
something up?
If that's a no-go, can anybody else suggest some mechanism to do what I need it
to do? Bearing in mind that the way the tray travels means that whatever trips
the switch is going to have to be very sensitive, because the tray is not going
to be able to push very hard against anything. The mechanism also needs to be
self-resetting. It would also be acceptable to have the motor running the same
direction all the time, but have the output diverted to a different shaft to
move the tray in the other direction.
Thanks in advance for any suggestions.
Owen.
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Message has 7 Replies:
 | | Re: Some help or advice requested
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| (...) SNIP (...) I have been asked for some pictures of what I am working on (very much prototypes at the present - don't expect anything elegant). (URL) is the track for the tray, and the drive mechanism. As the axle at the front (bottom) of the (...) (15 years ago, 22-Jan-10, to lugnet.technic)
| | | Re: Some help or advice requested
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| (...) SNIP I don't think there is a 'save' way to do that. In fact thats the reason reversal switches have dead space, because you will short circuit stuff otherwise (might be only for a millisecond though). I don't know if you ever worked with some (...) (15 years ago, 22-Jan-10, to lugnet.technic)
| | | Re: Some help or advice requested
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| (...) <snip> Hi Owen, As you say, you want a mechanism where the switch wants to be either on or off, but not in the middle. Had you thought about adding a pivot point at the top of the switch handle, with an axle going up and a weight at the top. (...) (15 years ago, 22-Jan-10, to lugnet.technic)
| | | Re: Some help or advice requested
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| (...) Last year I built a pneumatic excavator module for a GBC display exactly for this reason: doing it with an RCX would have been like cheating when it could be done mechanically :D As for your 'problem', I've been turning this over in my mind (...) (15 years ago, 23-Jan-10, to lugnet.technic)
| | | Re: Some help or advice requested
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| Here is yet another possible solution that I have found effective for something very similar (a vehicle that reverses its direction when it hits an obstacle). This one is purely mechanical (no electrical switch needed). (2 URLs) The mechanism relies (...) (15 years ago, 31-Jan-10, to lugnet.technic)
| | | Re: Some help or advice requested
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| (...) SNIP (...) So I bet you had all been waiting with bated breath for the revelation of my project, and then I bet you had all thought that I had given up, when there was no revelation forthcoming. In actual fact, I had been working on this until (...) (14 years ago, 27-Mar-11, to lugnet.technic) !
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