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Author Topic: Help with a 2 DOF (I think..) telescope base?  (Read 2053 times)

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Offline sundialsTopic starter

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Help with a 2 DOF (I think..) telescope base?
« on: May 05, 2008, 09:08:28 PM »
Hello everyone:

I am interested in building a robotic mount for a telescope and was hoping to find some insight here.  I would not even qualify as 'new to robot building' yet, as my only knowledge of the subject comes from digging through your website + forum and some college-level physics..  I have tried to gather sufficient information prior to posting, and I apologize if this is too basic or vague to work with.  Please bear with me.

My plan is to make a mount for a small telescope that has a rotating base (the degrees of rotation for the base is not critical at this point) and one 'joint' where the telescope will be mounted.  I have some estimates for the dimensions and weight of this amputated arm - would anyone out there be so kind as to help me brainstorm this project from the ground up?

I picture the mount looking as follows-
                              
                    O          <-- Telescope - 7in. long, ~45oz.
                   |  |
                   |  |
                   |  |   
               |  Base  |

The "O" in the diagram above will be the joint where the telescope is mounted.  Ideally, the telescope, parallel to the ground in the diagram, will be able to rotate about 100degrees up from the parallel position and about 60degrees down from the parallel position.  From what I can tell, the 45 ounce mass of the telescope itself is roughly centered, so I do not expect to need extreme amounts of torque in that joint.  If my torque calculations are correct, then it appears that an average servo could accomplish this lifting.  I am concerned, however, by the need to be able to freeze this joint.. one of my many questions.  Is there an alternative to a servo that could be better-suited to this purpose?

I have searched through the forum and the website for help on this - lots of bits and pieces, but still no clear image.  Could someone please help me with some early-phase planning, primarily for the top joint in my diagram?  What is the best way to accomplish that ~160degree range of motion and still be able to freeze the joint in place?

If it looks like this project might get off the ground, I would also love some input on controlling and powering the mount.. but that can wait.  Thanks very much in advance!

Offline AndrewM

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Re: Help with a 2 DOF (I think..) telescope base?
« Reply #1 on: May 07, 2008, 12:46:13 PM »
I don't know what your telescope weighs, but it would seem to me that the average home telescope would be heavier than what a servo can handle.  Perhaps you can post your telescopes weight and the torque calculations you created for this.

Without seeing the weight/torque data, I am inclined to recommend stepper motors for this project.
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Offline sundialsTopic starter

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Re: Help with a 2 DOF (I think..) telescope base?
« Reply #2 on: May 07, 2008, 05:03:24 PM »
This is much lighter than the average home telescope.. as I indicated in my explanation and diagram above, the length is roughly 7 inches with about 45 ounces distributed pretty evenly across the length.  My plan is to mount the telescope in such a way that the 45 ounce weight is roughly centered on the joint to take advantage of gravity pulling down on one side of the 7 inches as the mechanical part tries to pull up on the other.

I guess this is not so much trying to do a dead lift of the entire weight of the telescope as it is trying to balance the weight across 7 inches and tilt it slightly with some help from a servo.  If this still does not make sense, I can try to draw another sketch.. if you understand what I am trying to do and think that it is impossible, I am all ears.  Thanks for any input.

Offline AndrewM

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Re: Help with a 2 DOF (I think..) telescope base?
« Reply #3 on: May 07, 2008, 09:22:19 PM »
I'm just blind  :P  There are servos that will cover 180 deg of movement, but they are few and far between.  Also my understanding is that outside of a certain range they become less accurate, for instance from -45 to +45 accuracy is good, over that and it starts degrading.  Plus I don't think a servo will be strong enough despite the "balancing act" (see below)

I'm still thinking stepper motor would be best for your project.  Better overall accuracy than a servo, much more precise movements, and more torque.  Granted, you are going for something very well balanced, but you will still need something with decent torque to hold it in place when it is in an out of balance position.  The further out of parallel with the base, the more holding force you will need to keep it in place.  Just think of a see-saw at a playground, if the wooden beam is stopped parallel to the ground it sits there, the further away from parallel the more inclined the beam is to slam to the ground one on side.  If it is not perfectly balanced and not in a parallel position, gravity is going to try to pull it to a perpendicular position.
« Last Edit: May 07, 2008, 09:23:41 PM by AndrewM »
blog: www.iamwhen.com
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Offline airman00

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Re: Help with a 2 DOF (I think..) telescope base?
« Reply #4 on: May 08, 2008, 02:30:26 PM »
Or an encoder on a DC motor  instead of a stepper( easier and cheaper , but maybe not that precise)

many beginners have trouble controlling steppers
Check out the Roboduino, Arduino-compatible board!


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Offline sundialsTopic starter

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Re: Help with a 2 DOF (I think..) telescope base?
« Reply #5 on: May 10, 2008, 07:23:03 AM »
Well that makes a lot of sense - I will do some digging on both of those suggestions.  What is an average range of motion for these two types of motors?  And can they both be 'locked' in place with reasonable stability in order to stay on a target?

Thanks very much for all of your input.

Offline Admin

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Re: Help with a 2 DOF (I think..) telescope base?
« Reply #6 on: May 17, 2008, 04:30:12 PM »
Do you require computer control, or will you be fine pushing a few buttons to control the telescope?

Do research into these three things:
motor driver
DC motor control
worm gear

The worm gear will give you high torque slow smooth motion, and is best for lifting things that don't move much.

http://www.societyofrobots.com/mechanics_gears.shtml

 


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