A Folded Pendulum seismometer
David H. Youden
dyouden at ipass dot net
February 21, 2003
(Details of Electronics)
During the winter of 2001-2002 I completed the construction of a folded pendulum seismometer based, in part, on work done at the University of Western Australia. The device, which is shown in figure 1, is approximately 10.27 high, 8.46 wide, and 3.74 deep. It is constructed mostly of aluminum and glass.
There is a tower on the left side of the assembly that provides a mounting point for a 4.33 (110 mm) long conventional pendulum, which is designed so that its center of percussion is located at the lower hinge where the pendulum attaches to the carriage. The flexures, which attach the pendulum to the tower and to the carriage are 1.12 long pieces of .001 thick stainless steel shim stock .500 wide. There is one flexure at each pivot point of the pendulum
The inverted pendulum is located at the right side of the assembly. This pendulum is also 4.33 (110 mm) long, and is constructed of aluminum and glass so that it can serve to pass an electrical signal to the force feedback coil. There are four flexures on this pendulum, two each at the bottom and near the top. This pendulum is also designed so that its center of percussion is located at the moving flexures. The flexures on this pendulum are identical to those on the conventional pendulum.
There is a force feedback coil beneath the carriage on the right hand side, and a differential capacitance sensor beneath the carriage at about its center. A raised platform under the carriage supports the stationary plates of the capacitance sensor, and the rare-earth magnet that completes the force feedback motor. A moveable counterweight is mounted on top of the carriage.
The base plate is a piece of .25 aluminum plate 8.46 long and 3.74 wide. The exact dimensions are not important. I scavenged the piece from a junk pile and trimmed it to size. There are two Ό-20 threaded holes in the corners at the left-hand end, and an 11/32 diameter hole centered at the right-hand end. The threaded holes accept screws that form two of the three feet and the larger hole accepts a New Focus 9311 Ό-80 adjusting screw. This screw is one of the few purchased parts in this assembly ($18.00). Other methods of precision leveling could be used if a purchased part is deemed too expensive.
The mounting points for the inverted pendulum are located near the right-hand end of the base plate. A piece of Ύ square aluminum tube 3.74 long is cemented to the base plate with aquarium cement. In this, and all other cemented joints, it is crucial to use a minimum of cement and strong clamping force in order to assure a thin bond line and a strong joint. Centered on the top of the tube is a Ύ wide, 2.36 long piece of window glass .090 thick. This glass forms an insulator so that the two flexure mounts that go on top of it are electrically isolated from ground, and from each other. The two flexure mounts are pieces of ½ square aluminum bar stock 1.00 long with a 6-32 threaded hole centered in one end and a second 6-32 threaded hole in one side near the opposite end. These parts are located 1½ apart, centered on the glass, and with the end with the hole overhanging the glass and the square tube by Ό. The second threaded hole should be in the top surface. It is important that the overhanging ends of these two parts be in line and square to the base plate.
The tower that supports the conventional pendulum is constructed of aluminum. The sides are 2.00 wide by 1/8 thick and are 10.04 long (high). There are two plates on the back side of the tower, one at the top, and another at the bottom. These plates are each 2.00 wide, 1/8 thick and 2.50 long. A pair of 2.00 long pieces of Ύ aluminum angle are located inside of the side plates with their top surfaces 2.75 from the bottom of the assembly. These plates have 6-32 threaded holes where the metrology plate will attach to the assembly. At the top of the tower, on the inside faces of the side plates, are two small plates .50 wide, 1/8 thick and 2.00 long. These are located so that their upper edges are 9.41 from the bottom of the tower. A 2.00 inch wide, 2.25 long piece of glass is located on the top of the two small plates. Two flexure mounts are cemented in position on top of the glass plate. These mounts are identical to the two mounts on the inverted pendulum lower flexure mount. These, too, are positioned so as to overhang the glass plate by Ό. At the bottom of the tower, on either side of the two side plates, are 1.00 long pieces of Ό brass angle stock that was obtained from a model airplane shop. The entire tower assembly is cemented in place on the base plate so that it is centered on the 3.74 dimension, and is flush with the left hand end.
This platform supports the stationary parts of the capacitance gauge and the rare-earth magnet for the force feedback motor. The platform itself is a piece of 2.00 wide, 1/8 thick aluminum 6.50 long. It is screwed to the top surfaces of the two angle plates inside of the tower assembly, and is supported near its right end by a second piece of aluminum 2.00 wide, 1/8 thick and 2.75 long. A 2.00 long piece of aluminum angle stock is cemented to the top of the 2.75 long leg and the metrology platform is screwed to the horizontal surface thus formed. At the bottom of the leg an aluminum plate Ύ wide, 1/8 thick and 2.00 long spaces the leg away from the Ύ square tube. The parts of the capacitance gauge and force feedback motor that mount on the metrology plate are best located after the device has been assembled.
The conventional pendulum hangs from the top of the tower and supports the left-hand end of the carriage. It also serves to ground the carriage (Although it is insulated to provide design flexibility.). The pendulum consists of three parts, the main plate, and two weighting plates, which are positioned to adjust the center of percussion. The main plate is 2.00 wide, 1/8 thick and 6.46 long. It has a .788 square hole located so that its lower edge is 1.46 above the bottom end of the pendulum. This hole is centered on the width of the plate. 6-32 threaded holes are located Ό below the top edge of the plate, and Ό above the top edge of the square hole. The first weighting plate is .39 wide, 1/8 thick and 2.00 long. It is located so that its lower edge is .31 above the lower edge of the pendulum. The upper weighting plate is .98 wide, 1/8 thick, and 2.00 long. It is located so that its lower edge is 2.75 above the lower edge of the pendulum.
The inverted pendulum is built up of two pieces of 1/8 thick aluminum and four .090 thick pieces of glass. The aluminum pieces are mirror images of one another. They are 6.46 long and 7/8 wide. Each piece has two .788 wide notches .59 deep in its outer edge. The lower notch has its lower edge ½ above the bottom end of the plate, and the upper notch has its lower edge 4.16 above the bottom end of the plate. The glass parts are all 1.00 wide and 2.00 long. Two of them sandwich the two aluminum parts at their upper ends, and the remaining two sandwich the aluminum just above the lower notches. There are 6-32 threaded holes Ό below the lower notches and Ό above the upper notches.
The carriage connects the free ends of the two pendulums. It is a Ό aluminum plate 2.36 by 4.41. A piece of glass 1.00 by 2.36 is cemented to the bottom surface at each end. The flexure mounts are three 1.00 long pieces of steel key stock threaded identically to the other three flexure mounts. They are mounted on the glass plates, two on the right-hand side, and one on the left, each extending Ό beyond the end of the plate.
There are seven identical flexures they are made from ½ wide .001 stainless steel shim stock. Shim stock can be purchased from industrial supply houses or from Small Parts Inc in Miami Lakes, FL (www.smallparts.com). Small Parts Inc is a good source for much of the material used here, although local hardware stores may be a less expensive source for what they can supply. The flexures are 1 1/8 long. I made mine shaped like the letter H by cutting slots in from each end, being careful not to bend or kink the material. The slots are about 5/32 wide and about 3/8 deep. They must be able to accept a 6-32 screw. I originally tried flexures with round holes for the screws, but I found them to be difficult to install and adjust.
The 12 flexure clamps are 1/8slices from a ½ square bar of aluminum. After slicing them off and sanding them flat, I drilled a 5/32 hole thru the center of each one, then sanded them flat again. You could also make these by cutting up left over 1/8 aluminum plate.
The counterweight is used to get the center of gravity of the carriage in the right place. My solution is only one way to accomplish this. I found a round piece of steel with a Ό-20 threaded hole through it. I mounted it on a screw and cut two pieces of 1½ aluminum angle to fit on the top of the carriage (I cut one leg of the angle short.). These angle plates are screwed to the carriage with 6-32 screws.
The bracket that holds the permanent magnet for the force feedback motor is a 2 long piece of 1½ aluminum angle with one of the legs cut to Ύ long. There are two clear holes for 6-32 screws drilled thru the short leg of the angle. The magnet that I used is rectangular. It is 1 by 11/8 and is Ό thick and is glued to the bracket so that it faces the coil and coil bracket.
The bracket that holds the coil for the force feedback motor is a 2 long piece of 1½ aluminum angle with one of the legs cut to Ύ long. There are two clear holes for 6-32 screws drilled thru the short leg of the angle. The coil is wound on a ½ diameter nylon spacer. The coil is Ό thick and 1½ OD. I used #30 enamel wire and wound the coil between two Teflon washers, coating the wire with epoxy as I wound it. After the epoxy has cured, the Teflon washers can be removed and reused, but the nylon spacer becomes a part of the coil. The coil is glued to the bracket.
This assembly is made up of two parts. The angle bracket is a 2.00 long piece of Ύ square tubing cut away to leave an angle with one leg .625 long and the other leg .218 long. The plate is a 2.00 square piece of 1/8 aluminum plate. The plate is screwed to the angle with two 4-40 screws into the short leg of the angle. The assembly is screwed to the carriage with two 6-32 screws that pass through the long leg of the angle.
There are two identical stationary capacitor plates. Each plate is made up of three parts, an angle bracket, an insulator, and the plate itself. The angle bracket is another 2.00 piece of 1½ angle plate with one leg cut to Ύ long. Most of the long leg is cut away leaving only two Ό wide prongs at each end. This is done to minimize the capacitance between the plate and the bracket. The insulator is a 1½ by 2.00 piece of glass that is glued to the two prongs of the angle bracket. The capacitor plate is a 1½ by 2.00 piece of 1/8 aluminum that has two 6-32 threaded holes located along one of the 2 sides, Ό in from the corners. The plate is glued to the glass so that it extends above the angle plate by ½ with the threaded holes in the area that extends above the glass.
I mounted the parts of the capacitor by clamping all three parts together with .010 thick shim stock on each side of the center plate. I then blocked the assembled pendulums in their center position with both pendulums vertical. I placed the capacitor package in position and transferred the positions of the mounting holes to the carriage and the metrology plate. Next I took everything apart and drilled and threaded the screw holes. While I had things apart, I connected the coil leads to the two flexure mounts on the right side of the carriage and connected the carriage itself to the flexure mount on the left side. Finally, I reassembled everything, and removed the clamps and the shims.
Nearly all of the materials came from local hardware stores. 2 wide aluminum is sold in 36 long pieces, as is 1½ angle and Ύ square tubing. Window glass comes most any size you want. You have to learn how to cut it up, but it really isnt hard. I had good luck with the aquarium cement for gluing glass to aluminum, but I used some epoxy as well. Ό aluminum can be had from Small Parts Inc, but a junkyard might be a better source. I bought the 80 pitch leveling screw, and a piece of ½ square aluminum. Shim stock might be available through an automotive supply store.