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Rotator for heavy object around 400kg

  1. Feb 12, 2017 #1

    I want to create rotating platform for my satellite dish equipment which weigh around 400kg. I'm wondering what's the best rotator for this? any OTC device, or DIY?

    this is the gear box used for that thread
    Fredini-Turntable2.0-29_preview_featured.JPG
    but pretty sure it wont be able to handle 400kg.
    this will be used to rotate on regular interval (once every 15-20 mins), and rotate about 3-5 degrees per rotation. if possible, the rotation is accurate to 0.1 degree, and it also needs to have a "locking" mechanism, meaning, once the rotation is complete, it should be immovable (affected by wind blowing, etc). This will be placed on 4th floor, where wind can be quite strong, especially during rainy seasons.
    what's the best recommendation when considering the following factors (sorted on priority):
    1. least energy consumption since this is used regularly
    2. durability (wear and tear)
    3. price

    low RPM is fine, as low as 1/6 RPM is fine.

    thanks
     
    Last edited: Feb 12, 2017
  2. jcsd
  3. Feb 12, 2017 #2
    You have many things to specify & calculate.
    Required torque to turn your 400kg object is Torque = (mass moment of inertia of load) x (angular acceleration).
    (angular acceleration) is approximated by (delta angular velocity / delta time to accerlate to that speed).
    Add in all the other rotational inertias, friction loads, etc.
    This has been discussed many times and is fundamental dynamics. Once you have the torque, then you can start looking for motors, bearings, mechanical transmission components that will be designed & specified to give you the motion performance you require.
     
  4. Feb 12, 2017 #3
    ok, but what about the best mechanism (worm gear, pneumatic, etc) when considering the best overall value of:
    1. durability (wear and tear)
    2. energy consumption cost
    3. price
     
  5. Feb 12, 2017 #4

    Baluncore

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    What diameter of the dish at what wavelength do you have that has such a narrow beam that it needs to be so accurately controlled? How will you control elevation in that case?

    Radio astronomy observatories do better than 0.1° with much bigger antennas. You will need some way to measure platform azimuth. The platform will probably move against the lock by more than 0.1° as wind forces change, so you will need dynamic control in the form of two slightly opposed servo motors.
     
  6. Feb 12, 2017 #5
    yes, for now, we're still working on azimuth. will follow the model applied in azimuth for the elevation. 0.1 degree precision is best case scenario. if possible, we need no more than 1 degree error. so, if the azimuth is supposed to be 15 degree, then the azimuth ideally falls in the range of 14 - 16 degree
     
  7. Feb 12, 2017 #6

    Baluncore

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    The construction of a tracking system needs to be optimised for the mode of operation. Will your system be following astronomical objects, geostationary satellites, tracking fast low orbit satellites, or Earth based targets ?
     
  8. Feb 12, 2017 #7
    for now, it's earth based target. so, to align between one dish in building A to dish in building B. maybe will use satellite in the future, but for now, it's a simple dish-to-dish alignment.
    elevation setting is for later, when we need to account for difference in altitude from sea level of dish A and dish B
     
    Last edited: Feb 13, 2017
  9. Feb 13, 2017 #8

    Baluncore

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    AZ-EL will work for terrestrial, but will be useless for tracking low orbit satellites or RA due to axis velocity passing the zenith.

    1. durability (wear and tear).
    That will depend on environment. Keep electronics and position sensors out of the sun, rain, bird and insect attack. Use small DC electric motors, avoid hydraulic or pneumatic. Gears on slightly opposed AZ motors should be able to be driven backwards against each other through the driven circumference.

    2. energy consumption cost.
    Minimum operating costs will be with two small electric motors independently driving the AZ circumference. A reduction gear driven, self locking, screw drive will do best for restricted terrestrial EL. Do not use a ball screw for EL or you will have to keep fighting sag due to mass balance or wind pressure.

    3. price.
    Start using 600 steps per turn shaft encoders for axis angles. They are cheap at $15 from China. Search; 600P/R Photoelectric Incremental Rotary Encoder. Try to avoid driving a mass through supporting shafts that pass through bearings. Instead, dedicate support pins or shafts to measure direction without drive torque, drive the mass through large radius circle circumference by using peripheral belts or friction drive to save on gearbox costs.
     
  10. Feb 13, 2017 #9
    what about worm drive? is it resistant to wear and tear (teeth breaking, worm getting dull, grease, friction, etc)?
    for DC motor, any recommendations?
     
  11. Feb 13, 2017 #10

    Baluncore

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    For AZ, a motor driven threaded rod could run on the periphery of a large wooden disk. Over time the thread will press a pattern into the wood. But it would be higher friction and not have sufficient sensitivity to counter wind forces.

    Worm gears are expensive, have small diameter output shafts and tend to be heavy or inefficient. You need a drive system that will not be broken by a wind gust.

    Your skills at fabrication and the availability of parts will decide your choice of the solution. All I can do is give you a guide to a geometry that will get it going. You can always improve it later to optimise the design.
     
  12. Feb 13, 2017 #11
    ok, will look for more info for specs, etc. will come back once I got more to ask. but feel free to drop any suggestions
    thanks.
     
  13. Feb 13, 2017 #12
    lets say the moment inertia of the disk plus load is 3600 kgm2, and I only need angular accel of 0.5 degrees/s2. does this mean a 32 Nm torque motor will get the job done? is torque output the only thing I need to look out for when buying the motor?
    thanks
     
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