Deleted post?

A few days ago I posted looking for advice on building a robotic arm. My post seems to have disappeared.
If there was something in it that offended someone or violated a rule of
the group, I am completely unaware of it. Should I re-post it?
-j
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The voice of "Jay Silver" drifted in on the cyber-winds, from the sea of virtual chaos...

It's still there...

No, but you could drop the 78 line SIG...
Wait a second, that's the questions. OK, try not using SIG delimiters for question separators. }:8)
--
The Tech Zero, Maxwell C.G. Pollare

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I see your post on Jan. 23 asking about weight considerations for a robotic arm.. No replies though :( You might have your newsreader set up to ignore posts you have read, and since there were no replies, you no longer see the original message. If you are using outlook express, look under TOOLS then OPTIONS and in the MAINTENANCE tab there is an option called "Delete read message bodies in newsgroups" Make sure it is not checked.
Anyways, other than that, I don't think I have any useful advice for your original inquiry other than to go to your local library and look up books on traditional machines (like the windlass, inclined plane/screw, levers, block and tackle, flywheels,etc...) as these will give you great ideas for machine physics principles to apply to your design. Any good physics book (even my crappy physics textbook) will be able to give you the formulae for torque and potential energy/center of gravity. These are your friends when determining the fulcrum point and required amount of force.
BTW, intuition (and a very crappy college physics course) tells me that your counter-weight would be best placed at the top of the shoulder, so that it ends up in the correct position to "counter" when the shoulder is moved. Otherwise, it will help in one direction, but hinder in the other. It should be balanced so that it will assist the joint. Also to properly balance with a counter-weight, you will either need to have it at an equal distance to the fulcrum (pivot point) or have a substantially heavier counter weight (IOW: much bigger) if it must be closer.
Perhaps a counterwieght is not such a good idea afterall: I do not know the technical term for this, but intuition again tells me that it is not possible to use a counterweight without causing additional work. In maintaining the position of the arm, it will need to work against all forces applied. Although the counter-weight would be placed so that it logically assists the motor, between pole reversals of the armatures of the motor, the weight will be free-wheeling and the motor will need to catch it and hold it (is this called backlash?). Something similiar to a flywheel or pendulum in the gearing might be more efficient. I know I probably didn't explain that right, so I am hoping someone else might offer a better explanation.
Perhaps (not something I have tried) the proper approach would be to build the arm without the gearing and use a spring scale (or torque wrench) to turn it and measure the amount of torque required to move the particular joint. This will help you in choosing your gearing and motor, of course alot of this will also have to do with what you are using for a power source. All things are related...torque to work to watts to amps...etc..
There are so many variables that unless you already have the mechanisms designed and have a pretty good idea about the length of the joints, size and weights of objects expected to be "effected", and power availible, it will be hard to give any direct advice. Perhaps this is why there was no reply.
Anyways, I am not an expert on this in any way, but good luck.
Scott McDonnell
P.S. Also googling for "mechanical advantage" should come up with some good sources. Here is a link that may help you get started: http://www.comfsm.fm/~dleeling/physics/torque.html

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