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(meteorobs) Re: rotating shutters
Hi all,
I was interested to hear some discussions on rotating shutters going on. I have recently prepared
a small article on rotating shutters for WGN (still to appear) that does not address all issues
mentioned on this list. Therefore, I add some remarks below.
In particular there were the descriptions of George and Lew of using an unregulated rotating
shutter and measuring the true speed whenever a meteor appears. This is of course a very
practical solution: it is easy to get it going and any indication of the angular velocity of the meteor
is better than nothing! On the long run, however, it should be worthwile to look for a more reliable
solution (which also works whenever a fireball appears during a nap :-) ). You can look then in two
directions:
1. use an AC synchronous motor if you have the 110V/60 Hz mains voltage available. A good
example of such a motor is the bicycle dynamo (see the photographic handbook and the WGN
article). Note that many AC motors are not synchronous motors!
2. use a DC motor with a servo control loop. It is not enough to apply a constant voltage to a DC
motor since this only sets the dissipated power of the motor which does not have a stable, 1:1
relation to the speed of the motor. Variable factors are temparature, air pressure, wind speed,
dust in the motor, shape of the shutter blade, etc. For a control loop you need some means to
count the actual number of revolutions of the shutter, convert this number electronically to a
voltage and then compare this voltage to a stable reference voltage. The difference can be used
in an electronic feedback loop.
Unfortunately I have no standard design for a servo control loop available, but I know it requires
rather a lot of electronic circuitry. Within the Dutch Meteor Society, Hans Betlem has developed
such a system, but it relies on a specific type of motor that is not readily available. To Marc Neyts:
you mentioned the floppy drive motor. Does this have such a servo control loop??? Are there
maybe other affordable commercial systems available?
See you,
Marc
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Marc de Lignie
Prins Hendrikplein 42 Tel. +31 (70) 320 5673
NL-2264 SN Leidschendam Fax. +31 (70) 332 6477
The Netherlands E-mail: mcdelign@pidot net
Predicted progress always fails. Real progress is all the small
steps you realize to have made when looking back in time.
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