From: "Geoffrey" gmvoeth@...........

Date: Mon, 9 Oct 2006 07:36:59 -0700

What is the possibility of using one turn of a heavy guage wire as a single loop and sharpening the ends to a point then sensing the voltages at the points I would think that changing the cross sectional area would increase the voltages by concentrating electrons or if you are faster in your thought holes sort of like having a resistor in the middle but no power loss. This is the way you change impedances in antennas or so i understand and what is a velocity sensor but an odd kind of antenna. ----- Original Message ----- From:To: Sent: Sunday, October 08, 2006 5:13 PM Subject: Re: Shunt damping > In a message dated 2006/10/08, Bobhelenmcclure writes: > >> > A wire size of #38 or less will allow this number of turns to fit >> > comfortably within the gap field cross-section. The coil will have a >> resistance low >> > enough to permit resistive shunt damping of a pendulum weighing up to a >> > kilogram, in my opinion. My sensors have a pendulum mass of about 0.1Kg, >> and >> > critical damping is achieved at about 30 kOhms. Since the coil resistance >> is only >> > 340 Ohms, the shunt damping imposes negligible loss on output sensitivity. >> >> > Even a kilogram mass would require only about 10% loss of output using >> shunt >> > damping. >> >> The damping force required also depends on the set period. What >> period are you using? >> What effect does this damping current have on the input noise in >> practice? Can it be significant? >> >> Hi Chris, >> I bought cheap low temperature enameled magnet wire from Alltronics. I >> have both #38 (3.97 thou OD) and #40 gauge (3.14 thou OD). I never tried to >> use the #40 wire, as it is difficult for me to see, let alone handle. I strip >> by burning off the enamel. My friend Victor frowns on that, and recommends >> fine emery paper. > > Hi Bob, > > Magnet wire is available from Alltronics in 1/4 lb reels, but it is Kynar > insulated > http://www.alltronics.com/cgi-bin/category.cgi?&category=MW&start=0 > > If you buy the polyurethane insulated wire like Beldsol, you don't > have to strip it. If you put a hot iron and solder on it, the insulation just > melts - no problems - but I have only found this wire down to 36 AWG - 5 thou OD. > > With the Kynar insulated wire, you have to strip it first before you > can solder it. I usually use the edge of a wax candle flame (or a match) to > first burn off this insulation. (You have to be very careful with a butane > lighter not to melt the wire) Then I pull it fairly gently several times through two > small pads of the very fine wire wool. This both cleans it effectively and > hardens it a bit. > > I once only wound two coils with 40,000 turns of 44 AWG copper wire > with 1 thou paper interleaving to measure paramagnetic susceptability - it took > me two whole days.... > >> My horizontal sensors are easily shunt damped. On one of them set to 12 >> second period, I measured a Q of 1.1 with a shunt resistance (including the >> amplifier) of 66 kOhms. My formula for Q is Q = K * R / P. The value for K is >> therefore 0.2. The mass of the coil and solder weight is about 100 grams. The >> coil resistance is 340 Ohms, the number of turns is 1100, the field strength >> is ~0.8T, the field length per turn is 0.1m, and the sensor output is 85 >> v-s/m. If you know your own sensor's output, pendulum mass, and period, you can >> work out your own value for K from the above information, and determine what >> shunt damping resistance you would need. However, remember that the coil >> resistance, in series with the shunt resistance (in parallel with the amplifier >> input resistance) is the damping resistance. > > What is P please? > >> Volts= B*L*(dx/dt) >> Force=B*L*I >> I= Volts/R = B*L*(dx/dt)/R >> Force= B*L*B*L*(dx/dt)/R >> Force / (dx/dt)= (B*L)^2 / R >> >> I have not checked yet to see if the above equation is consistent with my >> observed damping versus resistance. > > In the force equation, isn't the force proportional to the number of > turns, whereas the inductance is proportional to the square of the number of turns > and depends on the magnetic return path? > > How does the length of the pendulum, the set period and the mass > factor into these equations, please? > >> Shunt damping makes it easy for me to check my sensors. I measure their >> natural period by disconnecting the shunt, and discharging a small capacitor >> across the sensor-amplifier terminals. The decay of oscillation amplitude >> lets me make sure that the partially undamped oscillation has the expected Q >> value, and the time between zero crossings gives me the natural period. I would >> abandon the sport of seismometry if I could not control damping this way. > > I prefer to have my damping and sensors on separate fittings and their > setup independant, but I can see the attraction of variable resistive damping > if you choose very powerful sensor magnets and a low to moderate seismic > mass. > > I tend to use rather smaller / thinner magnets for sensing and they do > not have a serious diamagnetic repulsion problem, although I have observed > this type of effect. > I use wide Cu plate for the damping, so that the arm hits the side > stops before the outer edge of the damping plate gets close to the outside edge > of a magnet. I try to 'design out' problems when possible. > > If you are using feedback sensors with electronically extended > periods, Cu plate damping is a lot quieter than velocity feedback from a > differentiated position signal. > > Have you any comparisons of the input noise due to shunt damping vs > plate damping - or the noise when undamped and when damped? Won't the induced > current generate additional noise directly? > > Regards, > > Chris Chapman > __________________________________________________________ Public Seismic Network Mailing List (PSN-L)