[time-nuts] Re Danjon Astrolabe

[Dr Bruce Griffiths]

Tom Van Baak wrote:
>> The scheme probably needs three photocells to be sure that the one
>> in the middle is darker than the others. Might be able to mask it
>> with a slit and use a fine wire gnomon, in a coarse/fine servo.
>> Could use a variable frequency motor and precision reduction, like
>> a phonograph turntable only much slower.
>>     
>
> Not sure about needing three. This is what I had in
> mind -- given there's a stepper/servo on the sundial
> base, I suggest a detection method not unlike the
> way a 5061A stays on the cesium resonance peak.
> (see http://www.leapsecond.com/pages/cspeak/)
>
> Namely, you continuously sweep the base a small
> amount, perhaps a fraction of a degree, at a couple of
> Hz rate, back and forth across the minimum gnomon
> shadow to determine just where the "zero crossing" is.
> The two photodiode *differential* is all you care about.
>
> Using a sampling ADC (free inside most uC these
> days) you can then infer where the center is. When
> the virtual sweep center is off by more than one full
> physical step of the geared stepper, you advance
> the sundial base one step.
>
> I've not done the math, but I suspect you can get quite
> accurate this way. Not only do you have the digital
> step count as a function of time of day, but you also
> have the digital/analog interpolation of the steps; so
> the resolution is quite good.
>
> Also if the photodiodes are sensitive beyond visible
> this digital servo'ed sundial might work in cloudy
> weather too -- which, being here in Seattle, is one
> reason I came up with the idea.
>
> /tvb
> http://www.LeapSecond.com/time-nuts.htm
>
>
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>   
Whilst the resolution may be good, the accuracy of an open loop 
microstepped stepper motor isn't that great.
Its usually worse than when not using microstepping.
Variations in friction torque on the motor will also dramatically affect 
its positioning accuracy.
A high resolution position encoder mounted on the sundial base is 
essential if you need to accurately determine its direction.
Servomotors with encoder feedback will achieve a much higher performance 
than a stepper motor.
If gears or rollers are used then backlash in gears or microcreep in 
rollers will reduce the positioning accuracy.
The sundial base bearing runout can also affect positioning accuracy.

The equivalent time error is not likely to be much  smaller than a few 
seconds at best


Bruce