Author Topic: mechanics, a-frame on lazy-suzan  (Read 1168 times)

ben

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mechanics, a-frame on lazy-suzan
« on: September 06, 2015, 05:30:18 AM »
Hi All,

Here is my preliminary design.
Thread is locked until I get more of the design up online.
I welcome your comments (when thread is unlocked).
Thanks for your interest.

Ben.

« Last Edit: September 07, 2015, 03:01:43 AM by ben »


ben

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Re: mechanics, a-frame on lazy-suzan
« Reply #1 on: September 06, 2015, 05:46:02 AM »
As shown in the images below:

Four planks make up a base, this base is ideally square.
Each side (L & R) will have an a-frame arrangement that supports the mirror-frame.

The mirror frame is made of four planks of wood, so that the mirror fits inside the frame snugly.
Quad will be used to secure the mirror within the frame, so that the mirror is equally spaced from the front to the back.

The mirror frame will be hung onto the a-frame base so that the mirror is balanced.
Either hinges or shaft/coupling will be used.

A disk will be mounted to one side of the mirror-frame to control the rotation of the mirror.

A stepper motor will be mounted to a leg of an a-frame (shown in red) with a pulley going between the stepper motor and the disk.

The stepper motor will control the rotation (elevation) of the mirror,.

This design considered:
- wanting the mirror to be suspended and placed at any angle without difficulty - ie minimal strain on the stepper motor.
- using a stepper motor and pulley as I prefer this to the threadded rod alternative.
- wanting the reflected beam not to interfere/collide with the a-frame-base.
- wanting a large range of motion, ie mirror normal to achieve +45 to -45 degrees. - i want the mirror to park overnight at -45.


ben

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Re: mechanics, a-frame on lazy-suzan
« Reply #2 on: September 07, 2015, 02:57:08 AM »
My calculations were:

From http://www.sunearthtools.com/dp/tools/pos_sun.php?lang=en
the maximum sun elevation is +73 in summer and +30 in winter.

i want the reflected beam to go to +45 and to -45.

so i need the normal angle of the mirror:
- max elevation = (73 + +45) /2 = +59
- min elevation = (30 + -45) /2 = -7.5

i want to park the mirror at night to about -45

=> so, required elevation movement is up to 60 deg and down to -45.

60 + 45 = 105deg.
basically just over one quarter of a revolution.

ben

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Re: mechanics, a-frame on lazy-suzan
« Reply #3 on: September 07, 2015, 02:58:48 AM »
I will probably get 1.8 deg stepper motors.
i assume these will have a cog approx 10mm diameter.
The circular disk mounted to the mirror frame will probably be 100mm diameter.
A notched belt will be secured to the disk, go around the stepper-cog and then secured back onto the disk.

My calculations are a bit sketchy here, but will assume a 10:1 ratio.

This means I should be able to rotate the mirror frame 0.18deg per stepper full-step.

I assume the sun changes elevation approx 2 deg max in 10 mins.
so max movement is 0.2deg per minute.

if i use 1/4 stepping mode, I will need to adjust by about "four" 1/4 steps per minute.

Can somebody comment on my logic here?
Because I was expecting much greater resulution...

Gabriel

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Re: mechanics, a-frame on lazy-suzan
« Reply #4 on: September 10, 2015, 02:21:35 PM »
Hi Ben,

I like the simplicity of your design. I once tried to build one that was sort of similar. I will mention that this sort of design is a bit of
a pain to align, unless it is on a reasonably level surface. Then it's not such a big deal.

I tend to look at this calculation a little bit differently.
1.8 deg per step stepper motors take 200 steps to do a full revolution. A driver board with 1/4 stepping mode will increase
that 200 steps to 800 steps through some electronic trickery. Sense you have a 10:1 ratio set up, the stepper motor
would need to take 8000 steps to turn the altitude axis 360 degrees.

This gives a resolution of 360 / 8000 = 0.045, so each single step of the stepper motor will move the mirror
just 0.045 of a degree. This is pretty decent resolution and should be fine a homemade heliostat.

Does that make sense?

Gabriel


ben

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Re: mechanics, a-frame on lazy-suzan
« Reply #5 on: September 13, 2015, 05:42:48 AM »
Hi Gabriel,

Yes I have a level surface, and understand that the heliostat does need to know where level is, IE its no good with the azimuth axis of rotation not being anything but vertical.

thanks for your calculations regards the stepper motors. and for confirmation that a 10:1 ratio will achieve decent resolution.

i am not fixed on using the belt-pulley design with 10:1 ratio, and may increase this to 25:1 because i think the pulley/belt would then have larger holding torque, as well as greater resolution.

The design places the mirror centrally on both the axis, this is a design consideration.
This should avoid the error caused by mirror not being placed on the rotational axis as discussed in this thread.

The mirror is also supported the maximum distance away from the center, this should give less wobble in the wind.

I also want to be able to make a single design that can accommodate different length mirrors, simply needing to use different lengths of wood along the bottom.


ben

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Re: mechanics, a-frame on lazy-suzan
« Reply #6 on: September 26, 2015, 02:49:41 AM »
Below is my design for the base of the heliostat. This performs Azimuth movement.

Basically two pieces of wood, seperated by rollers.
- the RoundMountBoard and SquareBaseBoard are laminated smooth both sides, 12mm thick,
- the lowerPulley is made from the same benchtop, to be made perfectly circular.
- support rollers and lasySusan support the weight.
- 6x or 8x support rollers, placed furtherest distance from center of rotation,
- lasy suzan supporting weight at the center

A Lasy Susan is basically two steel rings, seperated by ball or roller bearings.
Pictures are shown below.
These are either sourced from a tabletop lazysusan, or swivel stools.

The final height of the base should only be 50mm or so.

Although I could attain 360 degree rotation, I will probably fix (in two places) a length of belt to the lower pulley, and have a stepper motor and gear assembly located on the square base board.

emimina

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Re: mechanics, a-frame on lazy-suzan
« Reply #7 on: February 27, 2017, 01:56:23 PM »
Did the idea work?
looks like the simplest design so far, however i am concerned on how will the mirror stand still in wind knowing that step motors are loose when its off