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Tech Musings
Don Lancasters
Tech Musings
May, 2000
Pendulum accelerometers
Servo and feedback books
Log file 404 error extractor
How to listen in on bat nav
Using synchros and selsyns
Copyright c 2000 by Don Lancaster and Synergetics (520) 428-4073 www.tinaja.com All commercial rights and all electronic media rights fully reserved. Reposting is expressly forbidden.
147.1
Fig. 1 AN OLDER MILITARY ACCELEROMETER that makes use of
a pendulum coupled to a synchro transmitter.
I
just picked up some interesting
1970s
military accelerometers.
They consist of a fist-sized mag
damped pendulum coupled to a
syncro transmitter. These do seem to
offer sub-g sensitivities.
Yeah, solid state accelerometers
from
Analog Devices
(check out their
ADXL05
in particular) or
Motorola
or
others are getting fairly cheap, stable,
and quite sensitive. But these older
mechanical beasties might still lead
you to lots of interesting robot nav,
physics demos, racing car apps, or
even mil collectibles.
With mods, they might also make
dandy inclinometers or levels. Figure
one shows us a typical photo. More
details on these are now posted to my
www.tinaja.com/bargms01.html
All of which reminded me that it is
way past time for us to look at
Servos, Selsyns, and Friends
A lot of the stuff we routinely do
with cheap electronics these days at
one time had to be handled by special
and expensive mechanical devices. A
servo is any system that uses position
or other feedback to gain accuracy.
The feedback of most closed loop
systems improves precision but does
so with price and stability problems.
Usually, some position or velocity or
whatever is measured and compared
against your current goal, generating
an error signal. The servo then seeks
to minimize (or null out) errors.
A servomechanism is a servo that
involves mechanical motion of some
sort. A servomotor is just any motor
or mechanical actuator which can be
run in either direction and can also be
safely stalled in any position.
A selsyn is a system that consists
of two or more motor-like devices
that are intended to control or sense
shaft positions. These might be true
servo closed loop devices. Or they
may simply work open loop.
Selsyns played a major role in all
WWII
ships and aircraft for position
control. They still do see use for ham
antenna sensors, for rugged industrial
needs, or to measure wind speed and
direction. Useful sources for selsyn
parts are
Fair Radio
,
Servo Systems

or
C & H
Sales
.
Figure two shows a typical selsyn
setup. A synchro transmitter consists
of a three phase rotary transformer.
The single phase rotor is driven from
a
400
cycle (aircraft) or a
60
cycle
(ship or land) source.
Three sensing windings physically
spaced
120
degrees around the stator
output three sinewaves whose values
are related to the shaft angle.
A second device can be connected
backwards across your three outputs.
Its shaft will automatically follow the
input shaft position. Thus a pair of
synchros could send any mechanical
position or speed from one location
to another distant one.
Do note that this type of selsyn is
really open loop. The input shaft has
to do enough work to overcome all
friction, electrical losses, and output
shaft load inertia. So, your input
shaft is loaded somewhat. Normally
when you are only powering a meter
pointer, this is no big deal. Other
types of output selsyns will apply
feedback for flap, rudder, or other
serious tail twisting capabilities.
Related devices include control
transmitters (to pick up improved
accuracy), differential transmitters
(to add or subtract two shaft angles),
or receivers (handy for shaft position
displays). A synchro resolver does
the calculations needed to find an
angles X and Y components.
Fancier selsynn combinations can
do amazingly sophisticated analog
trig calculations or similar tasks. Copyright c 2000 by Don Lancaster and Synergetics (520) 428-4073 www.tinaja.com All commercial rights and all electronic media rights fully reserved. Reposting is expressly forbidden.
147.2
Tech Musings
Fig. 2 A SELSYN SYSTEM consists of a synchro transmitter and a synchro
receiver. The output shaft position follows the input.
ac source
input
shaft
synchro
transmitter
ac source
output
shaft
synchro
receiver
Fig. 3 SYNCHRO TRANSMITTER
used to sense shaft position.
Fig. 4 A BATS CHIRP uses this sophisticated Fourier Transform scheme to
dramatically improve its nav resolution.
Linear time delay
- vs - frequency
network
linear fm chirp
high resolution pulse
A useful synchro application guide
tutorial is available at
www.litton-

ps.com/DataSheetList.htm
Figure three shows us a standard
synchro transmitter. Older sizes are
usually measured in tenths of an
inch. Thus a "size eight" selsyn is
0.8
inches in diameter while the "size
fourteen" safely clears an inch and a
half space.
Let me know if you need any of
these to play with.
Dealing With 400 Hertz
Military and other aircraft selected
400
Hertz as an operating frequency
because the size and weight (and thus
the horsepower per pound) are much
better than the usual
60
Hertz utility
power. The short transmission range
of higher frequencies is not a factor
in most airborne apps.
Working
400
cycle power used to
be a pain, but these days a
PIC
, some
coils, and a few power semis easily
generates it. Mechanical "dynamotor"
60
Hertz to
400
Hertz converters are
readily available as military surplus
at
www.tinaja.com/barg01.html
.
And this is a perfect way to apply
my new magic sinewave techniques
you will find described in detail at
m y





www.tinaja.com/magsn01.html
Can you run a
400
cycle device at
60
Hertz? If you one-on-one try this,
smoke and fire are certain to result.
The lower winding inductance draws
much higher current at a set voltage,
saturates the iron, and burns up.
Do not ever plug any
400
cycle
device into the same voltage
60
cycle
power line!
But there is a sneaky workaround
that few folks have picked up on. Uh,
flux density is the name of the game.
A
400
cycle device will usually run
just fine on
60 Hertz
if you lower the
voltage to
60/400
ths or
0.15
. The
same current will produce the same
magnetic flux, and the beast should
remain happy.
Naturally, the power and speed of
a motor will be miniscule if you try
this. But possibly still useful.
400
Hertz iron is often cheap.
Thankfully, in the case of a selsyn
transmitter, these days you can use
higher impedance electronic loads
you route to a
PIC
or whatever. A
12

or
15
volt
60
Hertz ac supply should
work just fine.
Thus, selsyn synchro transmitters
may still be able to hold their own
against encoders or whatever for
some experimental uses. Especially
wind speed and position or for ham
antenna sensing. And just may offer
cost and reliability advantages.
Servo and Feeback Books
Selsyns may seem dated, but servo
and feedback concepts definitely are
not. With this in mind, Ive gathered Copyright c 2000 by Don Lancaster and Synergetics (520) 428-4073 www.tinaja.com All commercial rights and all electronic media rights fully reserved. Reposting is expressly forbidden.
147.3
May, 2000
Fig. 5 POSTSCRIPT CODE to extract 404 log error messages.
% POSTSCRIPT "404" ERROR EXTRACTOR/REPORTER
% ==========================
% Copyright c 2000 by Don Lancaster and Syenrgetics, Box 809, Thatcher, AZ, 85552
% (520) 428-4073
don@tinaja.com
Web site:
http://www.tinaja.com
% Consulting services available per
http://www.tinaja.com/info01.html

% All commercial rights and all electronic media rights fully reserved.
% Personal use permitted provided header and entire file remains intact.
% Linking welcome. Reposting expressly forbidden.
% This PostScript-as-language routine reads an ISP log file and extracts all
% "404" or FILE NOT FOUND entries into a separate analysis file.
% To use this program, enter the full path sourcefile and target file names
% and keyphrase below and resave. Then distill the file.
% Note that a NO FILE PRODUCED message is normal and expected.
% IMPORTANT: Be sure to use "\\" when you mean "\" in any PostScript string!
/sourcefilename (C:\\medocs\\logfile.txt) def
% sourcefile
/targetfilename (C:\\medocs\\e404file.txt) def
% output file
/searchphrase (- 404) def
% term to filter on
/workstring 2000 string def
% string holds analysis line
/ws {writefile exch writestring} def
% create output file object
/addtooutfile {ws (\n) ws} def
% add line to output file
/endoutfile {(\n\n) ws
% close file when done
writefile closefile} def
% checkline tests to see if a dash followed by a space and a 404 is present...
/checkline {dup searchphrase search {pop pop pop addtooutfile}{pop pop} ifelse} def
% /startoutfile creates an output file object...
/startoutfile {targetfilename (w+) file /writefile exch def(\n\nLines containing ) ws
searchphrase ws ( in document ) ws sourcefilename ws (:\n\n) ws} def
% Main loop reads one logfile line at a time for processing...
/grabphrase {sourcefilename (r) file /workfile exch def startoutfile
{mark workfile workstring readline {checkline}{exit} ifelse
cleartomark} loop endoutfile pop} def
grabphrase
% This actually does it
%% EOF
together some of the better books on