Subject:
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Re: Brickfest Pneumatic Master and new Pneumatic Gate Circuits
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Newsgroups:
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lugnet.technic
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Date:
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Wed, 24 Aug 2005 19:37:51 GMT
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Viewed:
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4509 times
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In lugnet.technic, Kevin L. Clague wrote:
> In lugnet.technic, Mark Bellis wrote:
>
> > [SNIP]
> I figured that if you're going to write an article for BrickJournal on pneumatic
> basics up through sequencers, you would probably want to talk about the simplest
> gates, the single output gates. If not, I'll write up a followup article that
> talks about them.
I'm looking into alternative gate designs and I'd like to have a good library of
them on my Brickshelf to then use as a resource for article pictures. This
includes N-input gates of each type, as I intend to draw the module that can be
continually added to a gate to add another input. I have found this for the AND
gate, based on the sketches you posted the other day.
My AND gate uses 3+3++4 switches and yours uses 2+4++4, so the number of
switches is equal. The only difference is that for more than 3 switches you
need 2 cylinders and it's also awkward to put more than 2 on a limb.
> Well, any gate design that leaks the source pressure is a broken. It seems like
> you use leak to sometimes mean pressurized. For example here: In my mind, all
> leaks are bad.
>
> > one output or the other is always
> > pressurised and there is never a state where both outputs leak.
>
> Do you mean both outputs are pressurized?
No, I omitted to say that there is also never a state where both outputs are
pressurised. There is also never a state (except during switch transitions)
where either output to he cylinder is stoppered (i.e. leads only to a tube with
an antenna in the end).
[SNIP]
> My four switch AND-gate minimizes pressure loss by having the minimum number of
> hoses inherint in the gate.
You are using the input hose pressures as the object that has a state (B and
!B). You have to include the switches that derive those pressures from either
the air supply or the atmosphere. I think that requires two B switches in
addition to the four A switches, one to take the supply input to the middle
port, one to send the B signal to the bank of 4 and a link between the same port
of the two switches. Therefore the total is 6 switches, the same as my 3+3 AND
gate.
One thing I was trying to do when designed it was to make A and B have the same
number of switches, which is why I got 3+3 instead of 2+4.
> On this aspect of gate design, can you compare and contrast your AND gate and
> mine? Should I be concernted about minimal pressure loss? How do your gates
> minimizing this?
In gate design it's all about density and tubes. Denser gates mean shorter
tubes, which makes for less air wastage. More flex and less rubber reduces air
storage in hoses and reduces air wastage. Therefore you will need less
compressor power for a model, or get better performance out of the compressor
you have.
Pressure loss is a bigger concern if you're driving load-bearing cylinders with
it. If so, you either need a buffer (1 cylinder + 1 switch to make a no-leak
version of the gate output, as with your Quad 242 pages) or flex tubes and as
little rubber hose as possible. I've almost declared war on the balloon effect
as it wastes compressor power by leaking more air when you move switches.
> > Another thing was to avoid any gate being transparent. This means, similarly to
> > electronic flip-flops, that the input can't go through to the output without a
> > proper change of state. In a transparent electronic D-type flip-flop, if the D
> > input changed, the Q and !Q outputs would change with it. In a non-transparent
> > flip-flop the change of Q and !Q has to wait for the data at D to be clocked
> > into the flip-flop, such that it changes state. This avoids logic races in
> > electronic flip-flops and it was also something I wanted to avoid in pneumatics,
> > in order to be sure of the next state of the system at all times and prevent
> > movement into an undesirable or lock-up state.
>
> I understand transparent latches, but I'm not sure I understand how that concept
> relates to here.
>
> Can you tell me on my four switch, two output AND gate, how it might allow for
> undesirable transparency.
If B (the pressure) changes, the output will change instantaneously, without a
propagation delay of moving a switch. If A changes, the switches have to move.
In fact if you include the switches that derive B and !B there is no
transparency, since B must move its switches too.
>
> > Another featurge of my designs is that the input need not necesarily be a
> > pneumatic function. All that is required is something that moves a set of valve
> > switches, so it could be a motor or hand input. I've not yet tried adding
> > machines but I could make one on the same basis as a binary full adder from an
> > electronics book, using and AND gate and an XOR gate for each of two half adders
> > and and OR gate for the summation.
>
> This is a given in my AND, and XOR gate design, and very doable with my OR gate
> design.
>
> I have made a half adder based pneumatic/electronic adding machine, and a full
> adder based pneumatic/elecronic adding machine. This was was published in
> "Mindstorm Masterpieces", a book I coauthored. I used a clock mechanism to save
> the carry from bit column to bit column (I pressurized the input switch that
> drove the carry piston, and then depressurized it before I changed the inputs.)
>
> I'm sure my most recent gate designs would work even better as a full adder.
> Smaller part count, means much cheaper when talking pneumatics.
There's a thing - I'd like to see whether synchronous logic could be done, but
it'll need a lot of switches! A synchronous system will clock in the results of
the current state in order to calculate its next state at a regular time
interval. The interval must be longer than the maximum propagation delay of the
combinational logic in the system. Elecronics designers prefer synchronous
systems as they have fewer race conditions. We haven't quite got as far as
parallel combinational logic paths causing a race condition yet, but it would
happen when we have enough switches!
> >
> > Don't blame me, I'm just an electronics engineer :-)
>
> I'm a computer engineer, so I have similar understanding. I'm just trying to
> understand why you feel six switches are neccessary to make a "differential
> output" AND gate.
Surely your B and !B are derived from switches too? That makes it more than 4.
> For the most part, the muscle pistons are also timing pistons. Typically my
> circuit is made of only single switch/single output AND gates. There are lock
> up problems when a piston does not complete its transition due to outside
> forces.
Ah, that is what I sought to avoid by using differential systems.
> > On the first of your gates, are you feeding pressure A into the cylinder and
> > also through a B switch to the AB port?
>
> This A pressure could just as easily be directly from the pump. Pressure never
> goes through that switch unless there is pressure at the A input.
Yes, you're gating the incoming pressure, not the position of a switch. I think
it's dangerous to feed a pressure to a gate input and also to a cylinder. This
is because movement of the gate will reduce cylinder pressure, possibly causing
parasitic movement, or cylinder movement could affect the gate input and hence
its output, causing it and the gate output cylinder (two different functions) to
be run in parallel. This is like electronic gates feeding into each other with
an LED hung off the connection - a big no-no in electronics!
> > Also, you seem to be addressing hose pressures as the objects that have a state.
> > I have always used valve positions as the objects that have "0" and "1" states.
>
> Do you want to only look at gate outputs, or are you willing to also look at the
> nets that come out of them.
>
> The switch state alone cannot tell you the pressure at either differential
> output. You must know the inputs to be able to truly tell.
I'm assuming the input to a gate is only ever pressure derived from the air
supply ("1") or a leak to the atmosphere ("0"). The gate output is the same,
expressed as a differential pair, never an intermediate state.
The input pressures don't matter since all of them are derived from switches,
the states of which are known. You must include the switches that derive the
pressures from the air supply or atmospheric leak, even if the're half way down
a robot's leg!!!
It seems these different ways of looking at it are the cause of cross-purpose
conversation. I wasn't on about glitches earlier!
> This is fun!
>
> Kev
Mark
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