[time-nuts] beaglebones, time, web services

[Jim Lux]

On 7/5/15 8:43 AM, Bob Camp wrote:
> Hi
>
>> On Jul 5, 2015, at 8:46 AM, Jim Lux <jimlux at earthlink.net> wrote:
>>
>> On 7/4/15 7:53 PM, Hal Murray wrote:
>>>
>>> jimlux at earthlink.net said:
>>>> Exactly... I've got an array of mirrors on az/el mounts (two servos
>>>> stacked) and the reflection from the mirrors on the wall forms the display.
>>>
>>> How many pixels in that display?  Or what is the unit of quality measurement?
>>>
>>> What sort of ADEV are you aiming for?  If your goal is solar time rather than
>>> TAI or UTC, you should be able to get pretty good.
>>>
>>
>>
>> Prototype is 6 pixels to demonstrate concept and work out the bugs. Long term, probably several dozen.
>>
>> Time Accuracy? better than a second
>>
>> Turns out, having done some experimenting, the real issue is angular accuracy. RC servos aren't all that great, and have significant jitter (probably not an issue in their design application which tends to have good mechanical low pass filtering).  They're cheap and easy to use (as in, I had a bunch in the garage I could cannibalize out of another project).
>>
>> But if you have 3x3 inch mirrors (call it 7.5 cm), and want to create a picture on the wall that's, say, 10 meters away, you really need angular pointing of 0.007 radians.. that's about 1/2 degree.  An RC servo has roughly 270 degree rotation corresponding to 256 steps of PWM (in the Arduino implementation).
>
> Probably a good place to use the “drive a stepper as a selsyn trick. Steppers are dirt cheap these days and you can either program the drive yourself or get chips that will do it for you. You have essentially zero load and zero acceleration. There is no need for anything big.
>
Indeed, microstepping might be the way to go in a production system.

But steppers don't have convenient mechanical mounting stuff like RC 
servos do. I could assemble my prototype with zip ties, double sided 
foam tape and a few screws.   For a stepper scheme I'd need to design 
and build (e.g. fabricate) bracketry.  It's also more complex than just 
plugging a servo into a pin on the Arduino; that's pretty easy.

And then you also get into the "do you really want to use an arduino, 
why not program a X microcontroller  on a custom board you've designed 
for the purpose with all the driver components, etc."

If I were building up a full scale system, that's probably what I'd do. 
  BUT, in the mean time, my 6 RC servo az/el thingys are good to fool 
with and get a feel for various configurations and what the design 
issues on a larger system would be.


The virtue of the BBB and Arduino scheme is mostly that it can be 
cobbled together without much work. And you can leverage large consumer 
equipment volumes for the actuators, servos are <$10 each in any sort of 
quantity; it would be hard to find a packaged motor/gear train with a 
feedback pot for that much (leaving aside surplus).

I used to have a box of small 200 step/rev motors (floppy drive 
positioners), but they had a weird sized shaft, so we're back to the 
fabrication of mounts: the servo has a nice splined nylon shaft that 
mates with cheap other injection molded stuff.