Dear all,
from my memory I would like to support the view of Andy concerning a
linear system. On one occasion around 1975 I also had to design an
automatic tuning system for a transmitter tank circuit. Both motors were
linear DC motors driven by a four transistor bridge circuit including a
feedback loop. When the motor was passing through the correct tuning
position and was beginning to have some overshoot the motor worked like
dynamo engine and fed current back into the feedback loop, therefore the
motor immediately turned back to its correct tuning position. Each
feedback loop also had an CR unit, but with different time constants for
the tune and the load capacitor (with equal time constants the tune and
load capacitors would never find a common end position. But this is not
Stefans problem here).
73 Ha-Jo, DJ1ZB
"Stefan Schäfer" <[email protected]> schrieb:
> Hi Andy,
>
> Thanks for the hint.
>
> I intended to use a hard drive since the motor does not rotate when
> supplied with < 4 V DC. So it would always stop before the resonance is
> reached. That problem may be reduced by increasing the gain of the input
> stage but then i expect oscillation problems and small currents running
> through the motor all the time...
>
> 73, Stefan
>
> Am 23.12.2011 20:24, schrieb Andy Talbot:
> > Stefan..
> >
> > An idea. You have implemented a bang-bang driver with a dead zone.
> > Switched hard on in either direction, or stopped close to the middle.
> >
> > Change the circuit so you linearly amplify the output from the phase
> > detector, and make youself a bidirectional bridge driver, using say an
> > op-amp and NPN-PNP pair - as in audio amplifiers. Or it may even be
> > possible to use an audio amplifier chip if the gain can be brought
> > down low enough..
> >
> > Now, the motor will automatically slow as the phase error is reduced,
> > and you 'shouldn't' have to include any dead zone.
> > You may have to compensate the loop to prevent overshoot or high
> > frequency instabilities if there is too much mechanical lag in the
> > system or inertia, but that is something that can be done pure with CR
> > networks.
> >
> > Andy
> > www.g4jnt.com
> >
> >
> > 2011/12/23 Stefan Schäfer<[email protected]>:
> >
> >> Jim,
> >>
> >> Another thought:
> >>
> >> Am 23.12.2011 12:17, schrieb James Moritz:
> >>
> >>> [...] I guess the inertia of the motor and gearbox will produce some
> >>> hysteresis, moving the variometer to a very slightly over-adjusted
> >>> position
> >>> after the motor drive is removed, which will also help to prevent the
> >>> system
> >>> continuously searching around the correct tuning point.[...]
> >>>
> >>
> >> This effect can be compensated by choosing the dead band treshold value.
> >> Then the motor gets stopped a bit before the resonance point and will
> >> exactly land on it. As good as having an infinitesimal small dead band ;-)
> >> Anyway we are talking about phase angles which are very very small, much
> >> below that what could become critical for a PA.
> >>
> >> 73, Stefan/DK7FC
> >>
> >>
> >
>
>
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