From: "steve hammond" shammon1@.............

Date: Wed, 16 Feb 2000 22:03:50 -0800

Sean-Thomas, BTW, I take great interest in your formulas, I find them very informative. But I keep coming up with more questions as I think more about the problem. For example, where is the end of the boom? In my case, the boom is 38 1/4-in long tip to tip. I use a 1/4-copper plate for damping and it hang on a L-bracket at the at the end. At the 32-inch point, the upper wire intersects the boom. The coil and magnet are mounted at the 24 3/4-inch point. So in your equation, L=which point? The end of the boom or the intersection point where the guide wire is mounted? Regards, Steve Hammond PSN Aptos, California > >Regarding Steve's question about the angles: > >For simplicity the horizontal pendulum has always been designed so >that the boom and the hinge axis (either upper/lower pivots or more >continuous flexures) are at right angles. This then makes the angle >of the boom with respect to the horizontal the same as the angle of >the axis of rotation with respect to the vertical, so only one variable >angle is involved, which greatly simplifies the mathematical description >of the pendulum. The angle of the boom and therefore the period can then >be adjusted by tilting the base or frame along the axis of the boom, which >is usually an external adjustment so the cover does not have to be removed. >(The boom is often also designed to be parallel to the base or frame, which >facilitates the alignment of magnets, coils, and transducers.) > >However, Since the boom or axis angle is small, sine(i) = i (radians), >and the natural period Tn = 2*pi*sqrt(L/(g*i)). As i gets very small, >the period gets very large. Actually, there is usually a small restoring >moment from the hinges or pivot wires, so the boom may stay at equilibrium >even if perfectly level. Obviously, if the mass end is raised up >so that the boom slopes down toward the lower pivot, it will flop over >to one side. At the other extreme, if the boom is vertical and hangs >straight down from the hinges, it is a simple pendulum, as in the SG design. > >Regards, >Sean-Thomas > >_____________________________________________________________________ > >Public Seismic Network Mailing List (PSN-L) > >To leave this list email listserver@.............. with the body of the >message: leave PSN-L _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)

Larry Cochrane <cochrane@..............>