[sudo-discuss] Fwd: Re: robotics: 2-axis pointing
hol at gaskill.com
hol at gaskill.com
Sun Mar 31 09:55:30 PDT 2013
didn't get much of a rise out of sudo-doscuss...
steve i have a 2-axis compass chip that is earmarked for rover use. excellent point about the cloudy day problem - i gave derek my paired arduino/processing utility for passing servo instructions from PC to microcontroller and my understanding is that he is going to port the PC side to python using the GPIO pins of a raspberry pi for serial connection with arduino to do the low-level actuation since it runs at 5V like servos and some ICs, with the end goal of pointing a webcam. I will take a look at your code when i get back to town and try to find a schematic for driving those HVAC servos. we can fabricate whatever mount we need, but for starters it looks like we could get away with some 1/2" or 5/8" square tube through the servo, with holes drilled for attaching plywood to which we could duct tape some small panels (i have 2 5-watt car charging panels for testing purposes). any interest in sensing the battery voltage and having the computer enter sleep mode during the night? Not sure how to do that on the rpi but there's some documentation for the arduino. The whole rpi could be turned off and on with a single power transistor, as could individual transmitters
------------Forwarded message------------
From: steveberl at gmail.com
Date: Mar 31, 2013 09:29:23 AM
Subject: Re: robotics: 2-axis pointing
To: hol at gaskill.com
Cc: patrikd at gmail.com, sudo-doscuss at lists.sudoroom.org
On Sun, Mar 31, 2013 at 9:07 AM, Steve Berl steveberl at gmail.com> wrote:
I've already got the "calculating the daytime paths" part. I have python code using pyephem that given a latitude, longitude, and time, gives back the altitude and azimuth of the sun. For a fixed platform, that you can align once, that should be all you need. For a portable or moving, rotating platform, you need a way to know the orientation of the platform, which might be a magentometer, or even better, a 9DOF IMU like https://www.sparkfun.com/products/11486
Dithering around the sun would probably be OK for solar panels, but on cloudy days, it might spend a lot of time hunting for a peak that doesn't exist. For radio astronomy, it would add noise to the system which you would need to figure out how to filter out somehow.
-steve
On Sat, Mar 30, 2013 at 9:13 PM, hol at gaskill.com> wrote:
+1 on using a geared DC motor with encoder. we will basically wind up with a custom servo. i have some IR sensors that i got with the intention of making a photo interrupt type encoder - just need to print some dartboard-esqe patterns on transparency sheet and center it on the shaft or a coupler. steve i am very interested in building a sun-tracking mount for solar panels and those servos we have at sudo room would work for that application...i think there are also a few boxes that have geared stepper motors too, from the same batch of parts which i think was an overflow from AMT. two main options fro pointing as i see it are calculating the daytime paths geometrically, or using closed loop control to dither the panels around the sun like the thermopile on a sidewinder missile. also would be interested in one where the yaw axis of the panels is actuated by virtue of being mounted on a rover chassis (roomba???) with 2-axis magnetometer as feedback on pointing.
Mar 29, 2013 04:44:09 PM, patrikd at gmail.com wrote:
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 at 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 at 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 at gmail.com> wrote:
>>>>
>>>> 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 at 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 at 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 at 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
>>>>>>> _______________________________________________
>>>>>>> sudo-announce mailing list
>>>>>>> sudo-announce at lists.sudoroom.org
>>>>>>> http://lists.sudoroom.org/listinfo/sudo-announce
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> -steve
>>>>>>
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>>>>>
>>>>>
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>>>>
>>>
>>
>>
>>
>> --
>> -steve
>>
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--
-steve
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-steve
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