NavList:
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 18, 15:30 -0700
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From: Hanno Ix
Date: 2009 Aug 18, 15:30 -0700
George: You wrote below: But now, with the shunting engine, start moving the flatcar backwards and forwards. The pivoted mirror will respond to the acceleration forces, and it will start swinging. To reduce that effect, you could add a sort of dumbell arrangement, like a see-saw, with its centre of gravity aligned with the pivot. tThat would act like a trapeze artist's balance pole, to increase the moment of inertia. Now you have a system with a long period. Its response to short-period perturbations will be reduced by the enhanced inertia. You can make the period as long as you like by increasing the dumbell weights, or decreasing the weight and spacing of the ballast. And you are totally right - so far. However: you cannot accelerate forever in the same direction, with the same amount. Pretty soon you will reach top speed and then there will be no more acceleration - other than gravity! The mirror will dampen its oscillations and go back to equilibrium - i.e the horizontal position - while the train keeps going with constant speed from then on. Until, of course, the train slows down with neg. acceleration - causing the opposite movement of the mirror. The trick is to find out what in the particular application the biggest acceleration is, and how long it can possibly last. I bet shipbuilders can give us rather good estimations of the roll/pitch frequencies of vessels from size boat to size oil-tanker. I guess, as I said before, we are talking about 1/10 Hz to 1 Hz with exceptions on both sides. If the cut-off frequency of the lowpass is low enough, say 1/10 of the roll/pitch frequency, the acceleration will barely have caused the mirror to move before accelerations in the opposite direction occurs, thereby cancelling almost the first one. Howver, there is one more problem: In our design the mirror will be deflected while the acceleration goes on. If the engine not simply accelerates, but it does so differently with each expansion of the steam, you will see the jerks in the mirror movement. Solution: put another lowpass filter in front of the first one. Ugly, but easy to understand: let the pivot of your pendulum slide softly on a rail that is attached to the car and oriented in the direction of the accelarations, here front to back. Connect the pivot with a soft spring to the point where is was fastened to before. Make sure this secondary system is sufficently damped. Now, short jerks cannot be transfered to the pendulum anymore, only those slower accelerations will be transferred that really affect the overall speed of the train. The mirror will move much slower and in a mode commensurable with the train movement, and it will, in average, maintain now its pointer much closer to the gravity vector. I think I correctly described the line of thought that leads from the original spirit level to the Bush patent. Please, forgive my lapses of precision in my explanations. I hope you can follow the argument nevertheless. Regards H --- On Tue, 8/18/09, George Huxtable <george@hux.me.uk> wrote:
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