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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The development of bubble sextants
From: Hanno Ix
Date: 2009 Aug 14, 10:08 -0700
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From: Hanno Ix
Date: 2009 Aug 14, 10:08 -0700
George: I beg to disagree in a particular point you brought up: Actually, a pendulum IS a good model for the classic spirit level: You have a system that has one degree of freedom and suspended such that its center of gravity is below the suspension point. If the friction is not too bad you can actually make it oscillate! Answering another question of yours: With "more stable" I mean the behavior of a "low pass filter" as used, for instance, in stereo amplifiers, car suspensions, water distribution systems, etc. This might not be obvious but the math is quite similar. Also, the physical size of the thought model does not speak for or against its correctness. Should the model be correct and useful we could find methods to adapt it to practice. I say, the resonance frequency of the classic spirit level seems too high and, therefore, it follows external disturbances too quickly. Therefore, I am searching for a method to control its resonance frequency. Also, I am going to choose the losses in the system such that oscillations decay "just right": not too much overshooting but not too slow a response either. However, the sensitivity of the spirit level can still be maintained as is - if so needed! So far, though, sensitivity has not been my concern. I do not know if such considerations were used in the design of bubble sextants. I once used one, and the bubble seemed to have been very quirky. In passing let me observe: Accelerometers are being used in inertia navigation systems to model a system that resembles, in part, the behavior of the bubble. There, the math is implemented by electronic integrators, amplifiers, etc. The difference is you can give the electronic model nearly ideal properties that cannot implement in a spirit level. A gyro compass implements a system with a very low pass-band frequency. That is why it has been built. And, strange as it may sound, the surface of water in a big tank could be used as a spirit level of sorts if it acts as a low pass filter! None of all this, though, proves my original proposal. Regards --- On Fri, 8/14/09, George Huxtable <george@hux.me.uk> wrote:
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