[amsat-bb] Re: inquiry about homebrew az-el systems

Gus 8p6sm at anjo.com
Sat Mar 2 14:10:03 PST 2013

On 03/02/2013 06:55 AM, Robert C. Campbell wrote:
> My thought would be to count the pulses instead of a pot. I think we 
> will be stuck with gear motors if the goal is to go small and have 
> lots of torque.

Counting pulses will only be practical if there is a direct correlation 
between pulse count and angular displacement.  That means you are 
pulsing a stepper and counting those pulses, or you are activating an 
ordinary motor and the pulse train is coming back from some sort of 
shaft encoder.  But I prefer this approach (digital) to reading the 
voltage on a pot (linear/analog).  I am mentally working on a 
straightforward, easy-to-build shaft-encoder. But how accurate do we 
need to be, if we're building a system for small antennas with 
reasonable wide beam-widths?

I am coming to the conclusion that for a globally ubiquitous supply of 
second hand motors, we will have to turn to the automobile junk yard.  
Windshield wiper motors and window wider motors spring instantly to 
mind.   Newer cars have motors everywhere. Repositioning seats, 
adjusting rear view mirrors, opening and closing tailgates.  Perhaps 
central locking mechanisms can be adopted to lock the rotors in position 
between moves to prevent weather-cocking.  I'm fairly sure that 
virtually anywhere in the world you could get your hands on two wiper 
motors from a junk yard without breaking the bank.
> I would like to move away from stops and find another way to establish 
> 0~360az and 0~90/180ex indication so as to enable continuous rotation 
> to do  continuous sky scan or search and rescue with out having to 
> wind up and damage my coax.

Continuous rotation probably requires some sort of coaxial slip-ring 
system.  Google "coax rotary joint."  They look expensive, and I can't 
help worrying about insertion loss.  But perhaps limit switch is the 
wrong word.  But with a cam on your shaft and a micro-switch, you have 
hard position-detection of two places on the circle:  when the switch 
goes ON and when the switch goes OFF.  (In theory, enough switches and 
cams and you could detect any number of places on the circle.)  I think 
it's useful, e3specially with a pulse counting system, to be able to 
confirm your position at least a couple points around your circle.  This 
would allow your controller to calibrate itself when ever it needed to, 
and possibly to double-check itself during normal operation.  Whether or 
not you choose to wire a hard-shutoff based on the output of these 
switches depends on whether you have a continuous-rotation coax joint or 

> I am not sure what battle ship radar uses for constant rotation but 
> that would be the ticket.
> Bob Campbell

73, de Gus 8P6SM
Barbados, the easternmost isle.

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