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Speed and phase servo control of motor (was Cliff's notes)


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From:  Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent:  Monday, January 26, 1998 9:27 AM
To:  Tesla List
Subject:  Re: Speed and phase servo control of motor (was Cliff's notes)


> From:  FutureT [SMTP:FutureT-at-aol-dot-com]
> Sent:  Sunday, January 25, 1998 10:02 AM
> To:  tesla-at-pupman-dot-com
> Subject:  Re: Speed and phase servo control of motor (was Cliff's notes)
> 
> In a message dated 98-01-25 00:36:04 EST, you write:
> 
> << Thanks for the info, John. It's snowing again in Ohio and time to make
> > things in the lab! I probably used a wrong term by saying dwell. what I
> > am trying to construct is a 20 degree adjustment on the fixed gaps on
> > the outer ring. (Adjustable along the radius of the rotor.)
>   >>
> 
> Cliff,
> 
> By coincidence, it's snowing here too, but I'm in a lazy coiling mood.
> Suppose you need more than a 20 degree range?  The caps may
> achieve a peak charge about 90 degrees offset from the input
> voltage.  Since you're adjusting the speed of your motor
> electronically, perhaps you can adjust the phase electronically also?
> (like the head drum in a VCR).  This would make any mechanical 
> adjustments unnecessary.  This feature was what first stirred my
> interest in the electronic method...no mechanical phase adj. 
> capability needed, if electronic phase control is used.  makes it
> easy to adjust the phase while the coil is running without getting
> zapped.  
> 
> If your speed control circuit uses some sort of feedback to adjust
> the speed, then by variably delaying that feedback you should be
> able to adjust the phase.
> 
> John Freau
> 

I don't know what kind of servo control mechanism Cliff will be using,
but the standard ones won't do you much good for phase control.
Phase control (and speed control at the same time) CAN be accomplished fairly
easily, however. Your sensor can be as simple as a photo pickup
that detects the beginning of each revolution, or a setup with a 
photo pickup and SEVERAL slits (equal to the number of spark gap
presentations per revolution).

A variable speed pulse source is used as the 'reference'. This tells the
circuitry how often you WANT a feedback pulse to occur. The pickup
produces your actual feedback pulse. When everything is in sync, the 
edge of the reference pulse and the edge of the feedback pulse are aligned.
Phasing is accomplished by delaying the feedback pulse. In actual
practice this is implemented by having the pickup signal trigger a
one-shot circuit that has a variable pulse width. The TRAILING edge of the
one-shot signal is then used to produce the feedback pulse. A problem
will occur if the one-shot pulse is stretched too far. If the stretch
extends into the NEXT pickup pulse, then both sync and speed control
are lost. With a single pickup pulse per rev you have almost a full
360 degrees of phase control. With four pickup points you have a range
up to 90 degrees. The degrees mentioned are relative to total SHAFT
rotation. Note that the pickup should be from the main rotary shaft,
and NOT from the motor shaft (if a pulley arrangement is used).

There are other methodologies possible for the phase control, such as
DIGITAL feedback and digital phase control via hard-wired logic or
a small microcontroller such as a PIC. In this case phase control can
be made so that it is based on a percentage basis. It is always best
if the speed and phasing functions can be kept separate, and this
is easier in the digital design. Note that speed and phase could be
entered digitally via thumbwheel switches.

Hope this helps.
Fr. Tom McGahee