29 Mar
2013
29 Mar
'13
4:44 p.m.
If you want something precise and slow, you probably want to go with a
high gear ratio and a stepper motor. The load on this system should be
fairly uniform (unless you accidentally start yanking on it, of
course), so if it's well designed you shouldn't see too much trouble
with steps getting skipped. You could always zero out the positioning
using endstops.
A good alternative would be to use a DC motor with an optical encoder.
If you use a worm gear with a 360:1 gear reduction, and you attach a
black/white crosshatch on the fast moving worm, you can measure the
position of the slow moving worm wheel to within 1/4 of a degree,
which is plenty for your purpose.
Patrik
On Fri, Mar 29, 2013 at 3:53 PM, Steve Berl <steveberl(a)gmail.com> wrote:
Just to clarify MY requirements a bit, the DirecTV
radio telescope
(http://www.gb.nrao.edu/epo/ambassadors/ibtmanualshort.pdf) has a beamwidth
of about 3-5 degrees, so getting to within 1 degree would be great. I have
thought that the pan/tilt mounts used for larger traffic and surveillance
cameras would be perfect for this.
It also doesn't need to be fast. The first application is tracking the sun.
It needs to move at about 15 degrees/hour during the day. Overnight it has
to be able to swing back from the sunset position to the sunrise position,
but it's got all night to get there. Of course later on, or at night, we
might want to look at other things, and move from one celestial object to
another much more quickly.
Optical telescopes (and big radio telescopes) generally have much smaller
beamwidths/field of view, so pointing accuracy and smooth motion (to track
the apparent move across the sky for long exposure photography,etc) are
critical. For more info on telescope drive mechanisms check out
http://www.dfmengineering.com/news_telescope_gearing.html. Theres a lot of
good info there about precise pointing control of big heavy things. These
are the guys who made the control system for Nellie, the 36" reflector
telescope at Chabot Space & Science Center (www.chabotspace.org).
-steve
On Fri, Mar 29, 2013 at 3:13 PM, Scott Garrison
<scottrobertgarrison(a)gmail.com> wrote:
I'd have to disagree on this one. Servos have an encoder that allows the
motor to know it's exact position. Stepper motors will miss steps and won't
be able to catch itself. I guess this is an accuracy vs precision argument
as at least the stepper motor is stable in it's adjusted position.
If you are getting oscillations or wobbles in your position with a servo
then most of the time you are sending it a corrupted signal. This happens
with microcontroller code whose servo libraries are not interrupt driven as
the PWM signal gets corrupted by other processes. Or with unshielded cables
that are experiencing crosstalk from other wiring.
As with all things the hardware has to be matched with the job though. If
your servo gears have lots of slop or aren't up to the job of positioning
something heavy they will struggle. It's possible to use a potentiometer as
a secondary encoder of position to help if your gearing is introducing extra
error.
-=[Scott]=-
On Fri, Mar 29, 2013 at 2:37 PM, David Rorex <drorex(a)gmail.com> wrote:
--
-steve
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If you want ultra precise control, you need stepper motors. Or maybe
really expensive servos would work, but the cheap servo's I've used are only
accurate to a couple degrees and can wobble a little.
On Thu, Mar 28, 2013 at 3:13 PM, Scott Garrison
<scottrobertgarrison(a)gmail.com> wrote:
I'd be interested in doing something similar for controlling a camera.
I also have an extra house satellite dish that I would love to put to use.
I have experience controlling servos with microcontrollers.
-Scott
On Wed, Mar 27, 2013 at 3:07 PM, Steve Berl <steveberl(a)gmail.com> wrote:
>
> I notice on the etherpad that someone is interested in a 2 axis control
> for a radio dish. I'm interested in that also. In particular I want to mount
> a house satellite TV dish sized antenna on a 2 axis mount for radio
> astronomy use. It requires smooth and precise, but slow movement.
>
> Who is it that is interested in this project? Can you get in contact
> with me?
>
> -steve
>
>
> On Sun, Mar 24, 2013 at 5:43 PM, <hol(a)gaskill.com> wrote:
>>
>> Howdy folks,
>>
>> We are two days away from the beginning of the second microcontroller
>> project hacking night at sudo room. As someone suggested, I created a
>> facebookpage for the event. Please invite people you think might be
>> interested! I previously ordered 10 ATMega328p chips and crystals, and will
>> have them available at cost ($5) for those just starting out who want a
>> basic controller.
>>
>> Event Link:
>> https://www.facebook.com/events/502675869769339/
>>
>> Also, there is an etherpad to jot down projects you intend to work on
>> in case anyone out there is interested in collaborating:
>> https://pad.riseup.net/p/microbotics
>> This will be an ongoing document, and is intended to provide a
>> subsystem-level view of project elements in order to encourage people to
>> join together to work on individual elements or principles where interests
>> overlap in a way that permits a limited scope of collaboration, without
>> having to commit to recurring work on an entire project.
>>
>> We had some interesting projects last time and I expect even more this
>> time, hopefully with a few people from the last event returning to show what
>> they've done with their projects since then. Personally I will try to go
>> back and forth between 50% working on 2-3 projects, 30% getting newbies
>> started on their controller builds, and 20% snacking/shooting the shit.
>> Looking forward to seeing y'all!
>>
>> Cheers,
>> Hol
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>
>
>
>
> --
> -steve
>
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