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Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Using FEMM in "magnetostatic" mode (F=10KHz eddy current problem) i estimated the variation of voltage drop accross the coil (100 Turns 0.1A current, 5cm diameter 40 ohms approx) if a small (5mm diameter 5cm long object travels accross it) alluminium object is placed near (2cm off-center) and at center of the coil. THat small difference is divided by the "permanence time" of the projectile at 200m/s simulating an opamp differentiator with R = 100K and C=1uF obtaining a 6V pulse, enough to trigger TTL circuitry or a comparator with a reasonable treshould. Seems feasible using two of these coils to determine the speed of very fast objects (railguns, AirRifles, 0.22 Rifles etc), an application where optical sensors will fail, or work improperly (small and very fast projectiles). The tradeoff is compexity of course but this system should work ok on every conditions of lighting.
The system should be composed by two rf oscillators with 5V amplitude, two 40ohm coils, (distant, to minimize coupling) , two differentiator circuits, a dual comparator and a microcontroller to estimate the pulse delay between the two outputs
What do you think about it? It is simply an idea, supported only with some simulation and a bit of basic calculus.
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Theoretical study on an induction-detector speedtrap
