[time-nuts] beaglebones, time, web services

[Bob Camp]

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.

Bob

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