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> So if you wanted to describe the motion of a complex arrangement of rigid
> components (e.g., a car gearbox), you'd use kinematics?
Yes, for analysis of how fast each bit spins, but that part is actually very
simple and well known about gearboxes. Kinematics would be more useful to
design the windscreen wiper mechanism for instance, where it is assumed that
the motor can generate enough torque to maintain a constant speed. What the
designer must consider is how the constant angular velocity of the electric
motor is converted to that movement you see the wiper blades making.
> So... the resonant modes of a effectively 1D system (a string, a
> gas-filled pipe, etc.) would just be harmonics of the main resonant
> frequency?
Haha I should have known this had something to do with organ pipes! Yes
pretty much.
> Heh. So you know how an ideal gas is different from a real gas then? ;-)
From what I recall, ideal gasses are ones that follow the PV=nRT equation
and a host of other equations. Which works pretty well for most "normal"
gasses at "normal" temperature and pressure. IIRC it doesn't work for steam
though, which is why we needed hideous fold-out tables and charts.
> If I take a piece of paper and hold it horisontal, it flops under its own
> weight. But if I fold it down the middle, now it *can* stand up under its
> own weight. (But only if you hold it the right way.)
You can do that without folding it too, along the edge that you are holding
it, just push down in the middle with your thumb above the paper with two
fingers underneath either side. By introducing that slight curvature in the
paper you are making it very difficult to bend downwards without stretching
or ripping the paper, so of course then the paper does not have enough
weight to do that by itself. I don't know what category this would come
under, dynamics or thin bodies or something ;-)
> At the same time, a straight metal rod is very strong, but once bent it
> becomes drastically weaker,
Actually it usually becomes stronger after the 1st bend because you have
work hardened it at that point.
> and it seems that nothing will restore it to its original condition.
Because when you try to bend it back, it just wants to bend at a different
point rather than where the original bend was, because that point is now
stronger! Eventually of course if you repeatedly bend it enough fatigue
will set in and it will break.
> Wait - there's a way to *solve* differential equations?? o_O
Use Laplace transforms, makes things way easier for non-trivial differential
equations.
> OMG, the first time I watched this on TV, I killed myself laughing. All
> those hours to build, and it ****ed itself to pieces in seconds! :-D
Hehe yeh I love that program. Our robot almost did the same, when I
disconnected the control interface to the PC while the motor that lifted the
arm was still running. Of course it was in software that the motor is
stopped when it hits the switch at the end of the arm's travel, so with no
more control input the motor continued to wind up and start bending the
metal arm before I could pull the power :-) The mechanics guys weren't
pleased that they had to rebuilt it.
> By the way... how much of the stuff you learnt do you actually *use* now
> anyway? ;-)
Around 10% probably, but I would imagine every job would have a slightly
different 10% so I certainly don't regret doing such a wide range of
subjects within Engineering.
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Tom Austin wrote:
>> There were also two modules taught by Mr Apathy. Mr Apathy was tasked
>> with teaching us about computer hardware, and later about operating
>> systems. In Mr Apathy's opinion, knowing about binary is "pointless"
>> because "the computer will do it all for you anyway". He believed that
>> "20 years ago it might have been necessary to know this stuff, but in
>> the modern world you're really never going to need this information.
>> But it *is* in the exam, so I have to teach it to you." I cannot tell
>> you what an inspirational motivation for learning this was.
>
> To a point he is right - programming in upper level languages does not
> require a knowledge of the very low level happenings. That's part of
> why they are there - you can program faster without having to worry
> about as much as it is taken care of for you (e.g. garbage collection)
>
> In a sense it would be like teaching you about electron flow in a diode
> and FET so that you can type the word 'print' better.
>
> At level do you stop digressing?
Well now, it all depends on what you're trying to do.
How many times have you seen a program that behaves strangely given a
large enough number as input? Many people seem confused that large
positive numbers come out negative. But this is a simple and obvious
consequence of 2s complement arithmetic. I don't care what language
you're programming in, you need to have a basic high-level understanding
of this stuff.
(As to whether you need to know exactly how many bits are assigned to
the mantissa of an IEEE double-precision float, or the exact bit
patterns for demonals... er, no, most people will never actually need to
know that.)
The stuff about how hardware interrupts work is fairly irrelevant to
most programming exercises, but if you were doing something slightly
more specialised it would become highly relevant.
In the end, what it boils down to is that low-level hardware details
interest me, whereas Mr Apathy did has absolute level best to discourage
anybody from even *attempting* to put any effort into learning this stuff.
(FWIW, this is the same guy who told us that the rebuilding of Colossus
was "pointless" because "it will never be the same machine as the
original".)
> Computer Science is not about math, it's about laying down code, user
> interface, and bringing it together.
> The thought is that if you need math, someone will show what they need.
Actually, "Information Technology" is about applying computers to solve
real-world problems. "Computer Science" is the abstract study of
theoretical models of computation, which results are actually
computable, computer algorithms, and so forth.
> It sounds like you think very technically. What is you interest level
> in electronics. There's plenty of heavy lifting math there ;-)
My *interest level* is moderately high. My *knowledge level* is very low.
(As in, I know how it's *supposed* to work. It just doesn't work that
way when *I* do it.)
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scott wrote:
>> So if you wanted to describe the motion of a complex arrangement of
>> rigid components (e.g., a car gearbox), you'd use kinematics?
>
> Yes, for analysis of how fast each bit spins, but that part is actually
> very simple and well known about gearboxes. Kinematics would be more
> useful to design the windscreen wiper mechanism for instance, where it
> is assumed that the motor can generate enough torque to maintain a
> constant speed. What the designer must consider is how the constant
> angular velocity of the electric motor is converted to that movement you
> see the wiper blades making.
So things like cams and eccentrics then?
>> So... the resonant modes of a effectively 1D system (a string, a
>> gas-filled pipe, etc.) would just be harmonics of the main resonant
>> frequency?
>
> Haha I should have known this had something to do with organ pipes! Yes
> pretty much.
Well, it was a simple example of something I've already looked at a fair
bit.
Where I actually heard it mentioned was in relation to "the resonant
modes of [the air in] this room".
Presumably a 2D surface like a drum head would have quite a lot of
possible resonant modes?
How is all this related to standing waves?
>> Heh. So you know how an ideal gas is different from a real gas then? ;-)
>
> From what I recall, ideal gasses are ones that follow the PV=nRT
> equation and a host of other equations. Which works pretty well for
> most "normal" gasses at "normal" temperature and pressure. IIRC it
> doesn't work for steam though, which is why we needed hideous fold-out
> tables and charts.
...because steam can condense? (Or at least is likely to under typical
conditions, whereas air isn't.)
>> If I take a piece of paper and hold it horisontal, it flops under its
>> own weight. But if I fold it down the middle, now it *can* stand up
>> under its own weight. (But only if you hold it the right way.)
>
> You can do that without folding it too, along the edge that you are
> holding it, just push down in the middle with your thumb above the paper
> with two fingers underneath either side. By introducing that slight
> curvature in the paper you are making it very difficult to bend
> downwards without stretching or ripping the paper, so of course then the
> paper does not have enough weight to do that by itself. I don't know
> what category this would come under, dynamics or thin bodies or
> something ;-)
Ah yes - just curving the paper makes it behave quite differently. (See
corrigated sheet metal.) But why, I wonder?
Similarly, a hollow tube responds differently to a solid rod. And again,
a tear tends to propogate along a sheet, but if you put a hole in the
sheet, it actually stops the tear. WTF?
>> At the same time, a straight metal rod is very strong, but once bent
>> it becomes drastically weaker,
>
> Actually it usually becomes stronger after the 1st bend because you have
> work hardened it at that point.
>
>> and it seems that nothing will restore it to its original condition.
>
> Because when you try to bend it back, it just wants to bend at a
> different point rather than where the original bend was, because that
> point is now stronger! Eventually of course if you repeatedly bend it
> enough fatigue will set in and it will break.
Really? OK, well that's even weirder!
Presumably there's some molecular-level *reason* for all of this?
>> Wait - there's a way to *solve* differential equations?? o_O
>
> Use Laplace transforms, makes things way easier for non-trivial
> differential equations.
...because the Laplace transform turns differential equations into
algebraic equations?
Presumably there are still equations which can't be "solved" in this way
though? (It seems quite easy to come up with a set of equations which
yield absurdly complicated behaviour...)
>> OMG, the first time I watched this on TV, I killed myself laughing.
>> All those hours to build, and it ****ed itself to pieces in seconds! :-D
>
> Hehe yeh I love that program.
Did you see Scrappy Races, where the same guy tried to "tune" a 6 L
diesel engine, and the govener fell off? (I don't know what a govener
is, but it sounds important...)
> Our robot almost did the same, when I
> disconnected the control interface to the PC while the motor that lifted
> the arm was still running. Of course it was in software that the motor
> is stopped when it hits the switch at the end of the arm's travel, so
> with no more control input the motor continued to wind up and start
> bending the metal arm before I could pull the power :-) The mechanics
> guys weren't pleased that they had to rebuilt it.
Niiiice! :-D
>> By the way... how much of the stuff you learnt do you actually *use*
>> now anyway? ;-)
>
> Around 10% probably, but I would imagine every job would have a slightly
> different 10% so I certainly don't regret doing such a wide range of
> subjects within Engineering.
Mmm, true...
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scott wrote:
> The best part was the robot design project. We were put into teams of 6
> and told to make a robot that drove around a track marked by white lines
> on a black board and picked up containers. The containers would either
> be empty or full, and we had to move the containers to the appropriate
> bin. In most teams 2 people actually made the robot mechanics, 2 did
> the electronics (motor drive control, interface to PC, etc) and 2 did
> the software. Was really good fun, especially as the quicker your robot
> completed the task the more points you got.
We did a "software development project". This is where a team of (IIRC)
6 of us got together to plan, design, build and test a "large" piece of
software, and exhibit it at a show.
Of course, of the 6 people in the group, I was the only one who knew how
to program. So this "project" consisted of the following:
- We all sat round a table, and we're like "OK, so what are we gonna make?"
- After a while, we decide what to build, and generate a few design
ideas. "Do you think you can do that, Andrew?" "Yeah, probably." "OK
mate, sounds like a plan."
- I sit in a corner by myself, building and testing the program. Every
now and then another team member will casually ask me how it's going. I
tell them it's going OK. They smile and nodd, and wander off.
Fortunately, for this particular module, most of the marks are for
producing all the pretty GANT charts and all the project-management
garbage, and not for actually producing a working product.
[The urge to insert something about software consultants here is
overwhelming!]
Which is just as well - my "product" was a trivial job searching engine
that would have taken about 3 seconds if I had used a real database
engine rather than manually full-scanning flat files held entirely in
RAM. >_<
Can you spell "not scalable", "lost update problem" and "security"?
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> So things like cams and eccentrics then?
Yep, or just plain linkages of rods and stuff, some of them can get quite
complex. On my old Peugeot 206 it had some pretty funky wiper blade
mechanism to that it would cover much more of the window than a simple arc.
> How is all this related to standing waves?
A standing wave is just a wave that stays in position, ie it's frequency and
spatial phase remain constant. If you see a resonant mode by itself, it
will look like a standing wave and you can easily identify the stationary
points that remain at rest.
> Presumably a 2D surface like a drum head would have quite a lot of
> possible resonant modes?
Yep, see these photos that show the stationary points, try to imagine how
the non-stationary parts would bend up and down.
http://www.meta-synthesis.com/webbook/34_qn/2d_waves.jpg
> Ah yes - just curving the paper makes it behave quite differently. (See
> corrigated sheet metal.) But why, I wonder?
To curve a flat sheet of paper all you need to do physically is to stretch
the top half of the thickness by a tiny amount and compress the bottom half
by an equally tiny amount. Once you start curving it in other directions as
well the amount of stretching and squashing you need to perform becomes
orders of magnitudes higher. You can see this just by looking at the
geometry of a curved piece of paper and thinking about how it must deform in
order to curve it in the other direction.
> Similarly, a hollow tube responds differently to a solid rod.
For a hollow tube and a rod of the same weight, the tube will always be
stronger and stiffer because it will have a higher 2nd moment of area.
Actually the I-beam is pretty much the best useful shape you can have, which
is why they are so common (tubes are not so easy to join together and funny
angles).
> And again, a tear tends to propogate along a sheet, but if you put a hole
> in the sheet, it actually stops the tear. WTF?
It's to do with the radius at the tip of the tear (very small) compared to
the radius of the hole (much bigger). It's why the windows in aeroplanes
are round and not square (they had some square ones to start with but they
got cracks at the corners once the pressure difference went up).
> Presumably there's some molecular-level *reason* for all of this?
Yeh, something to do with dislocations in the crystal lattice from what I
remember.
> ...because the Laplace transform turns differential equations into
> algebraic equations?
Yup.
> Did you see Scrappy Races, where the same guy tried to "tune" a 6 L diesel
> engine, and the govener fell off? (I don't know what a govener is, but it
> sounds important...)
No I didn't see that one, the governer stops the engine going too fast if
there is no load. Diesel engines are quite capable of self-destruction if
you try to run them flat out with no load.
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scott wrote:
>> How is all this related to standing waves?
>
> A standing wave is just a wave that stays in position, ie it's frequency
> and spatial phase remain constant. If you see a resonant mode by
> itself, it will look like a standing wave and you can easily identify
> the stationary points that remain at rest.
I see... So the resonant modes of something are all possible standing
waves that can be created in that space?
>> Presumably a 2D surface like a drum head would have quite a lot of
>> possible resonant modes?
>
> Yep, see these photos that show the stationary points, try to imagine
> how the non-stationary parts would bend up and down.
>
> http://www.meta-synthesis.com/webbook/34_qn/2d_waves.jpg
Ooo... so there are both concentric and radial modes, and combinations
thereof?
Presumably for a square surface it would be different though.
>> Ah yes - just curving the paper makes it behave quite differently.
>> (See corrigated sheet metal.) But why, I wonder?
>
> To curve a flat sheet of paper all you need to do physically is to
> stretch the top half of the thickness by a tiny amount and compress the
> bottom half by an equally tiny amount. Once you start curving it in
> other directions as well the amount of stretching and squashing you need
> to perform becomes orders of magnitudes higher. You can see this just
> by looking at the geometry of a curved piece of paper and thinking about
> how it must deform in order to curve it in the other direction.
So it happens due to the infintesimal but finite thickness of the sheet?
Interesting....
>> Similarly, a hollow tube responds differently to a solid rod.
>
> For a hollow tube and a rod of the same weight, the tube will always be
> stronger and stiffer because it will have a higher 2nd moment of area.
> Actually the I-beam is pretty much the best useful shape you can have,
> which is why they are so common (tubes are not so easy to join together
> and funny angles).
"2nd moment of area"? ._.
Well anyway, it strikes me that a hollow tube of the same weight would
be very much larger, and hence obviously stronger.
The reason I mention this is that apparently trees that have rotted
hollow tend to survive storms better. Thus, the fungi that rot trees
actually *prolong* their life, not shorten it. (It's amazing what you
can find out from wildlife books!)
>> And again, a tear tends to propogate along a sheet, but if you put a
>> hole in the sheet, it actually stops the tear. WTF?
>
> It's to do with the radius at the tip of the tear (very small) compared
> to the radius of the hole (much bigger). It's why the windows in
> aeroplanes are round and not square (they had some square ones to start
> with but they got cracks at the corners once the pressure difference
> went up).
How interesting...
>> Presumably there's some molecular-level *reason* for all of this?
>
> Yeh, something to do with dislocations in the crystal lattice from what
> I remember.
Ouch. "Doctor, I've got a dislocated crystal in my lattice. It hurts
when I bend over..."
>> ...because the Laplace transform turns differential equations into
>> algebraic equations?
>
> Yup.
I see...
>> Did you see Scrappy Races, where the same guy tried to "tune" a 6 L
>> diesel engine, and the govener fell off? (I don't know what a govener
>> is, but it sounds important...)
>
> No I didn't see that one, the governer stops the engine going too fast
> if there is no load. Diesel engines are quite capable of
> self-destruction if you try to run them flat out with no load.
Guy was tinkering with this fuel-injected diesel engine. (The engine is
about the size of a small bath tub.) Obviously the camera crew weren't
paying too much attention. (This was during build time.) Suddenly we got
a shot of the machine in the distance with ****-off black Clouds Of Doom
spewing out of the exhaust pipes, and the team of hill-billies running
for their lives in every direction like startled rats.
Apparently the governer just fell off, and the engine is now running at
full tilt in neutral with the fuel injection system at non-default settings.
A few moments later the team run back to the machine. One of them is
trying to stuff his shirt into the air intake. Another is trying to
physically rip the leads off the battery. (Uh, how does that help?)
Another is trying to squeeze the fuel lines with his hands. I don't know
quite what happened, but after a few moments the machine fell silent.
Cut to a scene 20 minutes later, and they've taken the head block off
the engine, and the team captin is holding up bits of pistol rods, and
other items which look like they really ought to be *attached* to
something, but clearly aren't...
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> I see... So the resonant modes of something are all possible standing
> waves that can be created in that space?
Yes I guess so.
> "2nd moment of area"? ._.
Determines the geometric stiffness of a cross-section. Basically the more
"material" further away from the axis of bending the stiffer it will be. So
if you have an area budget of 10cm^2, better to put it all as far as
possible away from the centre (ie a circle), or if it only needs to bend in
one direction, then two parallel plates (usually connected though, so that
will be an I-beam then).
> Apparently the governer just fell off, and the engine is now running at
> full tilt in neutral with the fuel injection system at non-default
> settings.
I had a similar thing happen to my radio controlled car, the throttle fell
off, literally the bit that regulates the air-flow into the engine fell out
of the air intake. Of course then the car is going full throttle in a
street full of parked cars and all I can control is the steering! Finally I
think it got wedged underneath a real car and not too much damage. I found
the throttle too and put it back in, with some loctite this time!
> Cut to a scene 20 minutes later, and they've taken the head block off the
> engine, and the team captin is holding up bits of pistol rods, and other
> items which look like they really ought to be *attached* to something, but
> clearly aren't...
Try running a lawn mower with no oil, part way through cutting the grass it
stopped, not like the running out fuel stopped, but instant stop from 200rpm
or whatever to silence. Took the head off, a big bit of wood and a big
hammer freed up the piston from the side of the cylinder, then a good dose
of oil and it was working again!
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Invisible wrote:
> - I sit in a corner by myself, building and testing the program. Every
> now and then another team member will casually ask me how it's going. I
> tell them it's going OK. They smile and nodd, and wander off.
I had one of those. The funny part was when the partners came back three
weeks later for the next project and said "You wanna work together
again?" Noooo...
--
Darren New / San Diego, CA, USA (PST)
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Mike Raiford wrote:
> This one, not so much, I'm basically basically n-tier hell.
That can be fun on a sufficiently large project. You know, like the
stuff google does. :-)
--
Darren New / San Diego, CA, USA (PST)
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Invisible wrote:
>> It sounds like you think very technically. What is you interest level
>> in electronics. There's plenty of heavy lifting math there ;-)
>
> My *interest level* is moderately high. My *knowledge level* is very low.
>
> (As in, I know how it's *supposed* to work. It just doesn't work that
> way when *I* do it.)
I bet that if you dedicate yourself to learning electronics the same way
that you dedicated yourself to playing the church organ and uploading
the file to youtube you could likely have something working.
Parts of digital electronics can be very simple - join wire a to wire b.
Some of it can get tricky, but most of the work has been done for you
and is packaged into nice chips.
Analog can get a bit more tricky. You have to calculate resistor values
and capacitor values to make things work. Then the parts you can
actually use have a tolerance of +-10% - but i calculated I needed a
2045 ohm resistor.....
Take a look at
http://www.amazon.com/Cmos-Cookbook-Donald-E-Lancaster/dp/0750699434
It's a pretty good book that gives some basics and provides enough
information to get started hooking thing up. I own it myself.
If you want to *program* something, then you need a microcontroller or
the like. BasicStamp is a good place to start that does not cost too
much to get started. You could look at their SX lineup that involves
raw assembly programming.
You can hook all of these up on a breadboard pretty easily - no
soldering needed. Start small - don't try to build a antonymous robot
the first time you light up a circuit.
There are a bunch of guys in here in Pr.O.T that know how this stuff
works - you can always ask questions.
Tom
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