21 Jun
2012
21 Jun
'12
6:30 p.m.
[...]
ll no longer need to do this.It is interesting=2C how
it works though. Inst=
ead of a DC servo motor=2Cit uses an AC synchro motor. It chops the error s=
ignal and uses thatto drive the motor. A clever way to avoid DC offset with=
That was not at all uncommon in ploters (and chart recorders) using
valve amplifiers (a 'chart recorder' here is a device with one
votlage-cotnroled axis (moving the pen across the papar) and one time
axis (moving the paper at a constant speed using a synchronos motor and
gearbox).
WHt was often done in such unts was to take the DC error voltage optained
by the differnece between the input nad the slidewire position. THis was
then choppend by a vibrating reed device eneerginsed from the heater
winding (normlally). The resulting signal conld then be amplifieed in a
normal AC-coupled valve amplifier. The amplitude of the output was then
the magnitude of the eorror (and thus couple be used to cotnrol the speed
of the motor), wehter it was in phase or anitphase iwth the heater
winding gave the direction. The output of said amplifier drove one
winding of a 2 phase motor, the other widing being mains-supplied (maybe
from a transformer winging).
Certainly that Houston Instruments unit I mentioned last night works like
this. So do the (well-known over here) Honeywell-Brown 'continuos
balance' chart recorders.
One minor problem for running thse units now. THe votlage acorss the
sliderwire needs to be constant And may need to be flpating wrt to
ground. In smome units, this was provided by a mercury cell. Or course
those are now unobtainalbe. An alkaline AA cell will do for testing, but
don't expect the thing to hold its calibration (if the voltage drops, the
thing gets _more_ sensitive). It's not hard to make up a PSU though.
-tony