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Alt azimuth mount for heliostat design

  1. Oct 25, 2014 #1
    Hi Folks! I'm new here and need your help.
    I`m in the process of designing an alt azimuth mount for a Heliostat. For those of you who don't know a Heliostat is a mechanism that allows a mirror to rotate around the vertical axis(horizon) and horizontal axis(altitude). It is(usually) placed outdoors and lets you control the direction in which the sunlight is reflected by the mirror by tracking the position of the sun in the sky. Here is a picture of such a device:

    case-study-teardrop-park-additional-4.jpg

    This particular design would be ideal for me but I don`t have the tools to make it. This is a personal project and I've decided to build the whole thing myself, therefore my budget is quite limited. The thing is that I want to build three heliostats, each one will support a round glass mirror that will be about 1,2 meters in diameter. I have estimated the weight of the mirrors to be around 8 kilograms each.
    My problem is the following: The starting point of the heliostat design is deciding what motor and gear configuration to use. I would like to use the smallest stepper motor posible, I'm looking at these because they are cheap and easy to control. They are pretty standard CNC machine steppers.
    Now comes the second problem: Speed and Power. Since heliostats are placed outdoors the mirrors are subject to the forces of the wind and it's important that they hold their position steadily.
    Another factor to consider is that since the sun moves slowly across the sky I don`t need the heliostat to rotate quickly. Instead I need the Heliostats to be very precise since the target they will point the sun's reflection to will be located at about 200 meters away.

    The heliostat design that I've come up with(given the tools at my disposal) involves two metal brackets. The bottom bracket will rotate at a 360º around the vertical axis. The top bracket will be mounted on the bottom bracket and will rotate at a 180º around the horizontal axis. Like this:

    esquemas.jpg

    The mirror will be mounted on top of the top bracket.

    With all that in mind I think I have to gear choices to move these things: Timing belts or worm gears made out of Technyl plastic(Polyamide)
    Worm Drive
    PROS:
    - Very precise control of rotation with high torque and smooth movement.
    - Simpler than timing belts since only one per axis is required
    - Strong
    - Holds its position: I can have the worm gear machined in such a way to be self-locking. That way even if the heliostat is subject to strong winds it will not be able to force the stepper motor out of its position.
    CONS:
    - Friction: Im not sure if the friction needed to rotate the worm gear will be low enough for the steppers I intend to use. (Even if its made of Technyl)
    - More expensive than timing belts

    Timing Belts
    PROS:
    - Cheap: I can 3d print all the pulleys.
    - Can be made very precise
    - Very little friction
    - Lighter
    CONS:
    - More complicated to setup since they will require more pulleys and bearings in order to be as precise as a worm drive.
    - Not as strong as worm drive
    - Not self locking

    Given the context that I just described, Which gear choice would you recommend? Am I using the correct stepper motor? I would like to use the worm drive but im not sure the friction needed for its rotation would cause the motor to overheat.

    Thanks a lot!

    pd: Geez I ended up writing a lot more than I expected, Sorry! I just want to make the problem as clear as possible so you guys can help me.
    Disclaimer: Im not an engineer. Im actually an industrial designer with a passion for machines :D
     
  2. jcsd
  3. Oct 26, 2014 #2

    billy_joule

    User Avatar
    Science Advisor

    Have you designed the structure yet?
    What are the static loads? (self weight & ??)
    What are the dynamic loads? (wind & ??, There are wind load calculators online iirc)
    What angular resolution do you need? quite fine for a target 200m away...
    What gear ratio will get you that resolution? What gear types are compatible with that ratio?
    What loads will the gear/motor system take at that ratio? What holding torque is required?
    Why consider belts at all? are there constraints on where the motors can be placed?
     
  4. Oct 27, 2014 #3
    Thanks a lot for your questions, they helped me clear the picture.
    I haven`t designed the structure yet(besides the schematics I showed above). The only constraint regarding motor placement is that I think it would be better to have both motors near the center of the machine rather than on the sides. That way it is easier for me to build a housing for them which will be required since they will be placed outdoors.
    I have approximated the weight of each complete heliostat to be around 15kg with structure, stepper motors and mirror included.
    The angular resolution I need is pretty high, actually. The model tells me I need to control the rotation of the mirrors with a 0.1º of accuracy. Considering stepper motors usually have a 1.8º step angle(200 steps per revolution with +-5% accuracy) the required ratio would be 1:18.

    I realized the size of the mirrors was probably exaggerated. Will use a round 85cm diameter mirror instead(5.67 square meter). Considering anual average wind speeds of 10km/h in my area the resulting side load is 1,6kg. I'm going to assume 2kg side load just to add a generous safety margin(is that generous enough? idk) So the required holding torque for the motor would be the 2kg side load x 18(given the 1:18 ratio) = 36kg? The steppers are rated 3.6 kg.cm what does that exactly mean? Am I thinking this correctly? Thanks!
     
  5. Oct 28, 2014 #4
    I have been working on a Heliostat mount design in Inventor. Its still a draft. I recalculated the necessary accuracy and realized the best angular resolution would be 0.01º so worm gears would be the most robust and compact gear solution at a 1:180 ratio. Another cool thing about this type of gear is that I can divide the actual gear(not the worm) into sections of 40 degrees(20 teeth) and 3D print each section. Here is the design, hope you guys can give me any feedback before I continue with this project. Would be greatly appreciated!
    HeliostatDesign-01.jpg
     
  6. Oct 7, 2016 #5

    BDM

    User Avatar

    Frikiflax, I came across this discussion and wondered how the project is coming along? Perhaps I can add some hard lessons learned from participating in a MakerSpace build of an alt-azimuth mount for a telescope. Like you we chose the 40-degree sections of 3D printed gear. The challenge was in the assembly. Try getting all of those "puzzle pieces" lined up in a perfect arc! It is not trivial. You may find that you need a spring on the stepper motor/worm gear assembly in order to compensate for any wobble. If you don't you may end up with slippage and ultimately stripped gears. Just my two cents. Please let me know how it goes!
     
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