Why does the altimeter use a coarse and fine synchro ?

[flame_bringer]

why do we have 2 synchros ( course and fine ) in altimeters that are fed from digital air data computers ?
i tried researching in google but i coulnt find much of helpful info , all i could find is that the course synchro corrects for misalignment errors but i really dont understand it .
please someone explain cuz im really desperate

[nodrama]

I don't pretend to understand it, but maybe this will help......

"A DC signal representative of the rotary position of a fine synchro altimeter is subtracted from a DC signal representative of the angular position of a coarse synchro altimeter so as to provide a coarse staircase, the step positions of which are accurately aligned with the fine altimeter transition from maximum to minimum altitude indication. This staircase is used to select corresponding voltages, stepwise fashion, which are precisely generated so as to generate a staircase in which the voltage levels, as well as the positioning of the steps, are extremely precise. The fine altimeter voltage is added to the precise staircase so as to generate a precise DC signal of altitude. The voltage outputs of the fine synchro altimeter are resolved to a smooth, linear voltage as the function of altitude by means of electronic resolution equipment employing an RC/CR bridge, the input signals of which are reversed for a range of shaft angles displaced from 0° and 360° rotation; angular and voltage biases are also provided in this signal conditioning. "

Also a link for synchro theory:

CHAPTER-10-B (http://www.eugeneleeslover.com/USNAVY/CHAPTER-10-B.html)

[flame_bringer]

i tried researching using '' coarse ''
i understood a few bits of the system is that when there is a disgreement between the altimeter reading and the ADC the coarse mode takes over , and that a coarse mode provides more of an altitude increment when its synchro rotor is spun through 360 degrees than the fine mode .
but nothing much i could find about it, there is a lack of information in google about this subject .
i tried going back to my EASA 66 notes to check it up but the notes werent very enlightening in this topic,
its really confusing i just want someone to explain to me this system in a brief manner without using big words like '' coarse staircase '' so i can comprehend.
i know how a control synchro works i know how the ADC works and i know the principle of an altimeter its just this little part is creating a confusion to me .

[forget]

Does this help? The preamble from US Patent 3789391.

http://i21.photobucket.com/albums/b270/cumpas/altimeters.jpg

[doppleganger]

In very basic terms, the coarse synchro is for high rates of climb, the fine synchro is for fine tuning when the aircraft levels off. Really more applicable to fighters which are likely to climb at very high rates, faster than a single synchro could cope.

This page should explain it.

DUAL-SPEED SYNCHRO SYSTEM (http://www.tpub.com/content/neets/14187/css/14187_55.htm)

[mono]

Don't try to get to complicated. There are 2 reasons for having a fine and coarse synchro.

1. Accuracy - exactly the same reason you have hours and minutes hands on your watch. Think of hours as coarse and minutes as fine.

2. Ambiguity resolution - if there should be a power interrupt and the a/c changes altitude by more than 5000ft (one revolution of the fine synchro equates to 5000ft) the coarse synchro takes priority and drives the pointers to the approximate altitude, then the fine synchro drives the pointers to the exact altitude. (this is also the case for very high climb/descent rates)

:ok:

[punkalouver]

Does this help? The preamble from US Patent 3789391.

http://i21.photobucket.com/albums/b270/cumpas/altimeters.jpg
I had never heard of a Coarse-Fine Altimeter until a few minutes ago while looking at cockpit drawing key for…….

…….a Focke-Wolfe 200C Condor. Not sure if the date of this patent for an invention but it would be interesting to know the patent date.

Perhaps the Germans also invented this.