Subject:
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Re: pneumatic cylinder: why not hydraulic ??
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Newsgroups:
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lugnet.technic
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Date:
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Sat, 10 Apr 2004 15:39:02 GMT
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Viewed:
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3357 times
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In lugnet.technic, Kevin L. Clague wrote:
> In lugnet.technic, Mark Bellis wrote:
> > I get the impression you've complicated things by putting different switches at
> > different lever angles. My reverser idea puts the switch lever pivots 16mm
> > apart, all in a line with 2-beams between: BSBSBSBSB along a beam of length 17
> > holes, so an 8-beam and a 10-beam with plates above and below should suffice.
> > The levers have toggle joints on top, with a 14-beam connecting them. This beam
> > can be pushed by 2 cylinders if required. All the switches move together over
> > their full movement range.
>
> I'd love to see a picture of your reversor, as verbal explanations by themselves
> are often much harder to comprehend.
>
> >
> > This reverser deliberately sends air from the unpowered side of the cylinder
> > back to the unpowered input hose, with no leaks to the atmosphere. It is just
> > as important to release air correctly as it is to not release it incorrectly!
> > These reversers can be daisy-chained and so a single switch can provide the
> > first stage of reversal, forming an exclusive OR gate. This is useful because
> > the single switch can be mounted on the end of a robot arm, whereas the 4
> > switches are too heavy and need to be on the base. On my pick and place robot
> > they sit on the base, pushed by cylinders in parallel with the grab, so they
> > form a counterweight. The grab is an easier load, so it always moves first.
>
> Again, I'd like to see this. My pneumatic mux that can act as a polarity
> reversor, can just as easily be used for an XOR gate with one piston and four
> switches.
>
> Are you claiming you can do XOR with one switch and one piston? If this is
> true, you've got my attention. Single phase or double (i.e. do you have outputs
> from the gate to both expand *and* contract the next piston?)
>
> I'm very curious about your reversors. LEGO pneumatics are a very strong
> interest of mine.
>
> >
> > Of course the new switches can be closer together, so only a 12 beam is required
> > for 10 holes, and 1 14-beam for 12 holes to mount the switches.
> >
> > The only place I've used offsets between switch lever angles is in pneumatic
> > steering, when I want to get cylinders to stop half way. My other gates have
> > the switch levers all at the same angle, 'cos you know that all switches will
> > block or pass air at the same time.
>
> Again, I'd love to see pictures of reversor, AND, OR and XOR gates. I'm always
> interested in alternate solutions, better, worse or the same.
>
> I can perform AND/XOR with one piston and seven switches. Mark Terabain can do
> full adder much better than I can. I have not taken the time yet to figure out
> how his design works, but I can testify that it indeed does.
>
> >
> > Mark
>
> Kevin
A XOR gate has two inputs, one with 1 switch and one with 4 switches. The
4-switch one is a standard reverser and the 1-switch one has the air supply on
the middle and the two outer ports go to the input ports of the reverser (the
middles of switches 1 and 2 from the earlier description).
An AND/OR/NAND/NOR gate has 2 banks of 3 switches. As I don't have pictures
yet, please try this from text.
Create the two banks on beams with the levers connected together (with 7-hole
beams) so that they're all at the middle position at the same time and travel
fully both ways together.
Make the banks of switches have their nozzles facing each other. Number the
switches A1 (left bank, furthest from you), A2 and A3 (nearest) and B1 (right
bank, furthest from you), B2 and B3 (nearest), with nozzles U, M and L for Upper
(furthest away) Middle and Lower (nearest).
Connect A1U to A2U and B1U to B2U
Connect A1M and B1M to a T junction. This goes to the top of the output
cylinder.
Connect A1L to B3U and B1L to A3U
Connect A2M and B2M to a T-junction. This goes to the air supply.
Connect A2L to B3M and B2L to A3M
Connect A3L and B3L to a T-junction. This goes to the bottom of the output
cylinder.
Set both banks of switches to the lower position, pump some air in and the
cylinder should be contract.
Move only one bank to the upper position and the cylinder should remain
contracted.
Set both banks to the upper position and the cylinder should extend. This is
now an AND gate.
To get a NAND gate, swap the connections to the output cylinder.
To get a NOR gate, turn both banks of switches round or swap connections to the
cylinders that drive them.
To get an OR gate do both the above swaps.
This gate has the same air leakage properties as a single switch i.e. it leaks
only the air of the side of the cylinder that is meant to be expelling air. If
you have both banks of the AND gate in the lower position, with the cylinder
contracted, and it is lifting something, and you then move one bank to the upper
position, the amount of leakage from the cylinder is only the amount in the
pipes. Therefore I suggest keeping the pipes short and/or using flex tubing as
much as possible. That way the cylinder won't move much when only one bank of
switches is moved. If it becomes a problem, drive a single switch with the
output cylinder of the gate and let this switch control the cylinder that does
the work. This is effectively a buffer.
To control the AND gate, you might need two cylinders per bank of switches.
Looking at your article on pneumatics, I think this gate is what you need for
your D and E problem. I would suggest using the 3x5 bracket to hold the
cylinders that move the switches, and putting "1" joints on the top of the
switches, rather than using 3x3 brackets and longer levers on axle extenders.
These new switches mount lower down than the old ones.
I have covered approximately half the work in your article. Where my work
diverged from yours is the point where the article seems to show that
non-leaking cylinders are added on if necessary, which may save on switches in
the short term but will cost you in the complexity of adding in leak proofing
later. I have always worked with the fundamental principle of no leaks in
steady-state, i.e. if a cylinder is not required to move the other way, pressure
is continuously applied to keep it where it is. Therefore all my logic building
blocks have been developed this way. For robots that have to apply permanent
forces to their leg cylinders, I would suggest a blanket no-leaks policy at the
start.
To add extra leak proofing to the AND gate, isolate the air supply by connecting
both the supply and the hose from the gate supply input to the middles of two
switches. Connect the right nozzles together and block both the left nozzles.
Only open the isolator (switches in the left position) when you are adding air
to the AND gate and isolate it when you don't want air to leak out if the output
cylinder is under load.
A half adder would use AND and XOR gates in electronic logic. Making a full
adder (two half adders and an OR gate) in pneumatics would use a lot of
switches. You would need two XOR gates, two AND gates and an OR gate, a total
of 28 switches with a leak-proof system. However, being a no-load system, you
don't need leak proofing, so you can save on the number of switches.
My octopus arm logic has two banks of 4 switches, one bank of 3 and one bank of
9! The first diagram had even more till I optimised the number of switches by
deciding which ones could handle two functions together without parasitic leaks.
In my electronics degree I did a module on finite state machines. The tutors
stressed the importance of a system being able to get into a defined state from
any possible state, even if that state was not used in the normal working of the
system. That's another principle I stick to when designing pneumatic systems.
Mark
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Message has 1 Reply:
 | | Re: pneumatic cylinder: why not hydraulic ??
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| (...) Hmmmm.... When talking about pneumatic gates in the past, I came to realize that different people defined gate inputs and outputs differently. Mark Tarrabain's single switch AND gate has two individual pressure inputs; one forces the piston to (...) (21 years ago, 11-Apr-04, to lugnet.technic)
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Message is in Reply To:
| | Re: pneumatic cylinder: why not hydraulic ??
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| (...) I'd love to see a picture of your reversor, as verbal explanations by themselves are often much harder to comprehend. (...) Again, I'd like to see this. My pneumatic mux that can act as a polarity reversor, can just as easily be used for an (...) (21 years ago, 8-Apr-04, to lugnet.technic)
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