Marchal, Taking into mind what Jim H. has pointed out, resistive damping is a much simpler approach than mechanical eddy-current damping in that it does not require any additional construction on your seismometer. But, as Jim points out, it does require a strongly coupled coil-magnet arrangement, and it also requires that the resistance of the coil be low enough to pass the damping current. Some long-period seismometers were equipped with 100 000 ohm coils to produce lots of output voltage, but had to be provided with separate 500 ohm coils for damping. So how do you use resistive damping? You can plug parameters into the formulas, or you can experiment. The damping resistor is simply connected across the output terminals of the signal coil. If you use a speaker coil/magnet with a 4 ohm coil, the damping resistor has to be connected right at the coil, since the external wiring resistance will be more than the 4 ohms needed to damp the seis. To see if your coil/magnet is strong enough to do the job, simply short it at the output terminal. Introduce a small offset and watch the mass move: it should be very sluggish, almost like it was dragging. Then you can replace the short at the output with trial resistors and again observe the effect. I would start with a resistor about equal in value to the resistance of the coil, which you should measure. If the coil is 500 ohms, I would try values from 300 to 1500 ohms in about 100 to 200 ohm steps, or use a variable resistor. With proper damping, the mass should at first move quickly about half the distance it will eventually travel, then slow down and stop and move backwards just ever so slightly. You can observe this with the seismometer pointer or in the output of your digitizer. A very slight (about 3%) overshoot will indicate critical damping, which provides the most output but a very narrow-band response. Less overshoot will be slightly overdamped, with a broader response (a 15 second seis will more easily record 60 second waves), but at a reduced output, which should be no problem with additional amplifier gain.. If you cannot get critical damping with even very low resistance, like 100 ohms, either the coil resistance is too high from using too much very fine wire, or the winding doesn't adequately fill the air gap of the magnet. The air gap must be predominately filled with turns of the coil and not the form for the coil or lots of clearance. The coil must be wound slowly turn-beside-turn, not scramble wound, to get the maximum number of turns inside the gap. And any form used must take up a minimum of space and fit closely to the magnet, preferably with clearances of less than 1 mm. I will later describe a method I use to wind a very compact coil on a temporary form that allows only windings of the coil to fill the magnet gap. Regards, Sean-Thomas _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>