[AMRadio] Motorized Variables

Jim candela jcandela at prodigy.net
Sun Jul 2 10:27:21 EDT 2006


 I am no guru, but maybe I can help. See below.

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Rick Brashear
Sent: Friday, June 30, 2006 11:01 AM
To: AMRadio at mailman.qth.net
Subject: [AMRadio] Motorized Variables

Calling all Vacuum Variable Gurus...

I am in need of education on vacuum variables and how they are motorized:

>Is it necessary to use "stepper" type motors or will a high gear
>reduction type configuration work as well?

Stepper motors need a point of reference such as a switch or optical flag
somewhere in the midst of travel, such as at one extreme. They are dumb
since they are given a set amount of pulses to move a set amount of degrees
rotationally per pulse. If the torque load is too high they will slip, and
then the position is unknown. Using a stepper motor is an option, and I've
seen kits in electronic stores (Ramsey kits??)for a PCB based stepper motor

>How are the limit switches accomplished?

At both ends of travel, either a mechanical limit switch or a optical sensor
with a flag will work. The idea is to limit further motor movement in that
direction only, and still allow for travel the other way.

>Is a friction type clutch safe

maybe if the speed is not too high when hitting the stop. This should not be
your first choice.

>enough or are the capacitors too delicate for that?

I don't like ramming a servo into the mechanical stop. I'd rather have the
slip clutch (or motor current limit control) and mechanical limit for
situations when things are out of control.

>What is the best speed (RPM) to use when turning the capacitor?

I think that depends on the tuning rate, circuit 'Q', etc. You clearly want
to be able to pick a tuning peak or null without going too fast, and passing
it. Going too slow, and you might burn up a tube while looking for the plate
current dip.

>Should I be concerned with interference if I use 115 vac motors?

No not really, but if you look at 115 vac motors, look for ones with a 2
phase winding, and an external phase shift capacitor, and a gear reduction
gearbox. These can be had with high torque at low speed, and with some
switching of the phases, be reversed in direction. This type of motor is
best for a move CW or CCW (while pushing a button) until you are where you
want to be. A CW or CCW switch, and push button, and limit switches will
work here. This will be the simplest approach to take, but might be
unpleasant to operate. Some switches, relays, and relay logic to deal with
the limit switches are all that is needed.  No ic chips, power supplies,
etc. This approach uses 1930's technology quite well.

I would personally prefer to use a DC motor, and a gear box with a output
shaft position pot. Then at the control end use a operator tune pot (use a 3
turn precision pot), and a servo feedback amplifier where the two pots are
part of a resistance bridge (is that a weinbridge circuit?). You turn the
pot, and the motor tunes automatically in the correct direction until the
bridge is nulled. The hard part will be getting the position sensing pot to
track the capacitor position over it's full range. Consider a linear pot
instead of a rotating pot. You can do this all analog, go switching,
stepper, or even with a microprocessor embedded controller with an encoder
to sense position.

All said, I have never tried any of this on a vacuum variable capacitor....

Now maybe a real guru will step in, and set me straight.. How would you do

For those familiar with B-29 gun turret control systems, you could use
vacuum tubes, mag amps, and a 3 phase ac resolver!! That technology was
state of the art, and even by today's standards are very good indeed. This
was position control by wire of the gun angle, and turret rotation position.
My 2006 Mazda 3 has throttle by wire, whereas the B-29 had this in the early
1940's for the gun turrets. Absolutely amazing. When I was in tech-school in
the mid 1970's we had a fully operational gun turret in the lab. We had to
analyze every circuit, and were tested on how it worked. This thing would
take your head off easily if you were standing in the way of the gun
barrels. The servo control, precision, backlash, response, overshoot
specifications were all excellent, and would be a challenge to achieve today
with modern components.


As you can tell, I'm dumb as a stick about these things.  Any advice and
help would very much appreciated.  I'm sorry for the duplicate posts to
those who are on multiple lists.


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