Sqaure Parabolic Dish Calculations?
Jim richards
shaped parabolic dishes? I have calculations for round shaped dishes
and all models from these calculations work fine, but using the same
calculations for a square shaped dish and no go. Does someone know
what I'm missing?
TIA
Jim
David Williams
-> shaped parabolic dishes? I have calculations for round shaped dishes
-> and all models from these calculations work fine, but using the same
-> calculations for a square shaped dish and no go. Does someone know
-> what I'm missing?
-> TIA
-> Jim
It would help if you showed us what calculations you are doing, and
what seems to be wrong when you use them for square dishes, as opposed
to round ones.
Also, what do you mean by a "square" dish? It has four corners, I
assume, but what defines the line beween adjacent corners? Is it
straight when projected onto a plane perpendicular to the axis of the
paraboloid, or does it follow the shortest path between the corners
along the paraboloidal surface, or what?
dow
Jim richards
On Thu, 26 Jun 2008 22:23:39 -0500, david.w...@bayman.org (David
Williams) wrote:
Thanks for your response David,
>-> Is there a difference between the calculations for round and square
>-> shaped parabolic dishes? I have calculations for round shaped dishes
>-> and all models from these calculations work fine, but using the same
>-> calculations for a square shaped dish and no go. Does someone know
>-> what I'm missing?
>
>-> TIA
>
>-> Jim
>
>It would help if you showed us what calculations you are doing,
Linear Diam. 20.49
Diameter 20.00
Depth 02.00
Focal Length 12.50
x -10.00 y 2.00
x -5.00 y 0.50
x 0.00 y 0.00
x 5.00 y 0.50
x 10.00 y 2.00
>and what seems to be wrong when you use them for square dishes, as opposed
>to round ones.
>
>Also, what do you mean by a "square" dish? It has four corners, I
>assume, but what defines the line beween adjacent corners? Is it
>straight when projected onto a plane perpendicular to the axis of the
>paraboloid, or does it follow the shortest path between the corners
>along the paraboloidal surface, or what?
>
> dow
While pondering your question to my question, it may have sparked to
answer to my original question. If there is no difference in
calculations between a round and square shaped parabola then in
essence a square shaped parabola is just a round parabola with it
edges trimmed square?
That would seem to make sense, since the calculation is prompting for
diameter.
Jim
Duane C. Johnson
Jim richards <jim.ric...@yahoo.com> wrote:
> Is there a difference between the calculations for
> round and square shaped parabolic dishes?
The basic and exact answer is no.
We usually think that a round dish is a paraboloid caused
by rotating a parabola around the central radix.
Technically the square dish is the same thing.
Now the cool part:
If you take that same parabola and test it at all other
locations it "Exactly" matches the paraboloid there also.
Of course this requires that the radix of the test
parabola is parallel to the paraboloid radix.
Here is an example of a square dish:
http://www.redrok.com/electron.htm#vdish1
The ribs used in this dish are exactly the same
curve except they are moved in translation
instead of rotation.
http://www.redrok.com/electron.htm#vdish1ribs
Another example:
http://web.mit.edu/newsoffice/2008/solar-dish-0618.html
This is Doug Wood's design.
This picture shows the ribs, which all have exactly
the same shape, and assembled in 2 directions.
http://web.mit.edu/newsoffice/2008/s-dish-1-enlarged.jpg
Here is his patent:
US6485152
And a couple of pictures lifted from the patent:
http://www.redrok.com/images/US6485152a.gif
http://www.redrok.com/images/US6485152b.gif
> I have calculations for round shaped dishes and all
> models from these calculations work fine, but using
> the same calculations for a square shaped dish and
> no go. Does someone know what I'm missing?
Is it possible that you are not aligning the radix's
of the all the parabolas parallel?
> TIA
> Jim
Hope this helps!
Duane
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Steve
news:f6m8641m7g94frac3...@4ax.com...
[snip]
> While pondering your question to my question, it may have sparked to
> answer to my original question. If there is no difference in
> calculations between a round and square shaped parabola then in
> essence a square shaped parabola is just a round parabola with it
> edges trimmed square?
Exactly.
When speaking of a parabola, it is in reference to a parabolic cross
section.
The basic formula for a parabola is:
y = (x^2) / 4c, where c is the focal length.
There are simpler ways of constructing parabolic curves than plotting
points.
When I build a cylindrical parabolic reflector I used the technique
described in "Direct use of the sun's energy" by Farrington Daniels. The
technique is to draw a parabolic curve on a piece of plywyood using a
straight edge and a nail, and a rectangle (for my dish I used a carpenter
square).
The straght edge is held along a "base line" that is the bottom of the
curve, and a nail is driven at the focus. Initially the square is placed
against the base line and the nail. Then the square is moved such that the
corner of the square is still against the base line and the nail forming an
angular gap between the base line and the straight edge. A line is drawn
along the base of the square. The square is moved a little more, and
another line drawn. As this is repeated the interior of those lines form a
parabolic curve.
Its easier to follow in a diagram, but I don't have a place to post one.
There is probably a diagram already available on the internet somewhere.
Regards,
SteveD
Jim richards
Thanks for the reply Steve,
What can't be done with a framing square? I saw a similar concept that
involved a piece of plywood, a nail, and (what looked like) a
T-Square. My problem is that neither technique works well in Auto-Cad.
Being an old computer guy it's cheaper for me to waste bits than
materials. Since I'm not just talking about this, I plan to build one
so I need blueprints.
I think David pointed to my problem, I was trying apply parabolic
calculations to a square instead of making round parabolic then
trimming it to a square shape. It's 2:45 now and I'm toast, I'll try
David's idea tomorrow.
Thanks again for the help.
Jim
Jim richards
<red...@redrok.com> wrote:
Thanks for your reply Duane too,
I've run into your site many times although I don't know how I missed
the MIT link Thanks.
Jim
Duane C. Johnson
Jim richards <jim.ric...@yahoo.com> wrote:
> What can't be done with a framing square? I saw a similar
> concept that involved a piece of plywood, a nail, and
> (what looked like) a T-Square.
Here are some links to the technique you mention:
http://www.redrok.com/main.htm#parabola
> My problem is that neither technique works well in
> Auto-Cad. Being an old computer guy it's cheaper for me
> to waste bits than materials. Since I'm not just talking
> about this, I plan to build one so I need blueprints.
Here is a program to generate these data points
by Jeremiah Chace:
http://www.redrok.com/main.htm#chace
> I think David pointed to my problem, I was trying apply
> parabolic calculations to a square instead of making
> round parabolic then trimming it to a square shape.
Yes, now you have it.
> It's 2:45 now and I'm toast, I'll try
> David's idea tomorrow.
> Thanks again for the help.
> Jim
Duane
David Williams
-> answer to my original question. If there is no difference in
-> calculations between a round and square shaped parabola then in
-> essence a square shaped parabola is just a round parabola with it
-> edges trimmed square?
-> That would seem to make sense, since the calculation is prompting for
-> diameter.
-> Jim
A paraboloid is really an infinitely-large shape. Usually, dishes just
consist of a small bit of the paraboloid centred on its vertex (or
"point"). Theoretically, any other part of the paraboloid could be
used, and this is done in a few situations, but not normally.
So I think we can assume that your software assumes the dish to be
centred on the vertex. If the dish is circular, then it's just a slice
off the end of the paraboloid, with the vertex in the middle and a
circular rim. A "square" dish is presumably assumed to have its corners
symmetrically arranged around the vertex, so it is, as you say, just
like a circular dish but with some material trimmed off the edges.
But this still doesn't answer the question as to what its exact shape
is. A two-dimensional square has straight sides, but a straight line,
in three dimensions, can't be drawn on the surface of a paraboloid.
Imagine you are a long way from the dish, and it is pointing right at
you. If you were to take a photograph of it, what shape would it have
on the photo? Would it be a two-dimensional square, or would the sides
curve outward or inward? Obviously, this would affect the amount of
sunlight the dish would receive, since the sun would "see" it from the
same direction as you do when you take the photo.
dow
David Williams
-> points.
There is a really cool method. Make a shallow, circular dish, mounted
on a horizontal turntable. Put some liquid epoxy resin or wet concrete
into the dish, and rotate it at a constant speed until the liquid has
solidified. Obviously, centrifugal force will push the liquid outward
so the final shape will have a depression in the middle. The
interesting thing is that its shape is exactly a paraboloid! Obviously,
you can remove the solid paraboloid from the turntable, which can be
used over and over again. This method is actually sometimes used to
make parabolic dishes.
dow
J. Clarke
The catch is that if there's any vibration you get waves. For some
purposes that's a problem, for others it's not.
--
--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)
Jim richards
<red...@redrok.com> wrote:
Duane,
I will be using software to position the dish (4.3 Tracking
conclusions "C Christopher Newton") but I was wondering if the
circuitry on one of your trackers could be modified to feedback the
relative cloud cover (brightness) to the software for performance
monitoring purposes?
Jim
David Williams
-> > mounted
-> > on a horizontal turntable. Put some liquid epoxy resin or wet
-> > concrete
-> > into the dish, and rotate it at a constant speed until the liquid
-> > has
-> > solidified. Obviously, centrifugal force will push the liquid
-> > outward
-> > so the final shape will have a depression in the middle. The
-> > interesting thing is that its shape is exactly a paraboloid!
-> > Obviously, you can remove the solid paraboloid from the turntable,
-> > which can be used over and over again. This method is actually
-> > sometimes used to make parabolic dishes.
-> The catch is that if there's any vibration you get waves. For some
-> purposes that's a problem, for others it's not.
Sure. The rotation has to be as smooth as possible, without vibration,
at constant speed, and so on.
The focal length of the paraboloid depends on the speed of rotation and
nothing else (except the acceleration due to gravity, which is beyond
control in most situations). The relationship is:
2f(w^2) = g
f is the focal length, g is the acceleration due to gravity, and w is
the angular speed of rotation in radians per second. 30 RPM equals pi
radians per second, so 1 radian per second is about 9.55 RPM. f and g
have to be in compatible units, of course, metres and metres per
second-squared, or feet and feet per second-squared, for example.
dow
Jim richards
<jim.ric...@yahoo.com> wrote:
Thanks to all who helped with this thread, after a lot of head
scratching I finally got the CAD program to play nice. There are no
short cuts or the horizontal and vertical frame members won't align
properly. But when the calculations are correct, the CAD is visual
confirmation.
See attachment. (if this group allows attachments?)
Now that the parabolic calculations are confirmed it's time to
concentrate on the over-all design.
Jim