Subject:
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pneumatics revisited
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 5 Aug 1999 22:24:01 GMT
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Viewed:
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1002 times
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Here I list out some of the benefits of pneumatics. These benefits, by
themselves, are not as essential as the ones mentioned earlier in this forum.
Still, they are cool features.
First, pneumatics provide you linear actuation directly. With motors you have
to convert its rotationary motion into linear motion through some mechanisms
(most commonly through a worm gear).
Pistons are sturdy, in particular the larger one. With motors, you can convert
its rotationary motion into linear motion through a worm gear, then if you
need more power, gear it down even more or add a second motor. Still, under
heavy load you will find the axles twisted and bent. Pistons are sturdier.
Pistons have built-in motion limits. With motors, you have to use touch
sensors, software timers, or slippage mechanisms to avoid stalling the motors
or even stressing the parts.
Pneumatics are not reversible, so the inputs are isolated from the outputs. If
you use motors to move up something then turn off the power, it may slide back
to its original place. The standard way to prevent this is to use a worm gear,
which has lots of friction. You don't have this problem with pneumatics.
There are some tricks you can play with pneumatics. For example, if you
connect several pistons in parallel then apply air pressure to them, the
piston with the lightest load moves first. After it reaches its limit, then
the next one moves, etc. This way you can perform several actions in sequence.
(From the instructions, it seems to me the alternative model of 8868 uses this
method to grab then lift its claw. Alas <http://www.kl.net/scans/8000/8868/>
does not have the instructions for the alternative model.)
(You can play the same trick using differential gears. If you connect two
actions to a differential gear then apply power to the differential gear, the
action with a lighter load moves first. One of the alternative models in the
bar-code set uses this trick to turn the wheel then back up the car. Still I
think the pneumatic version is simpler.)
(There are several elegant designs of grab-then-lift claws from Lego that uses
neither of the tricks mentioned above, but plainly linkage (pneumatic or not).
I don't know how to generalize from these designs and use them in something
other than grab-then-lift claws. Actually I am having trouble mastering this
wizardry of linkage design. Any good books on this topic?)
(If the first action you want to perform does not have the lightest load, you
can artifitially increase the load of other actions. The Search Sub uses a
shock absorber for this purpose.)
(Speaking of 8868, it has an air compressor without an air tank. What is the
point of this?)
Cheers,
Hao-yang Wang
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