PSN-L Email List Message

Subject: Re: A Free Mass seismometer
From: "David H. Youden" dyouden@.........
Date: Sun, 06 Apr 2003 06:30:30 -0400


I guess complexity depends upon personal viewpoint to some degree. What you have described seems complex to me. I have built systems similar to what you describe and the complexity springs from having to prevent rotation.

My stack of tubes originates from the challenge of making a relatively accurate prismatic shape without resorting to machine tools, which most of us don't have. Commercial tubing is pretty repeatable stuff, although not especially accurate.

At any rate, it doesn't matter, make what is easy for you.

I have built a commercial system, with a moving mass of about 10 Kg. (It cost about $800,000) with 0.3 nanometer laser position feedback. I have never thought of looking for seismic effects on this device, but I'll bet that they are there. That particular device is used for metrology, and has an overall position accuracy of less than 10 nanometers.

The noise from air bearings is low enough so that I have been able to machine 10 Angstrom RMS surface finishes in aluminum on these bearings.

As for web references, I don't know of any at the moment, but I'll look. There is not much published on the topic, but I'll try to generate a reading list in the next couple of days.

At 03:34 PM 4/5/03 -0500, you wrote:
In a message dated 05/04/03, dyouden@......... writes:

I have been thinking about a seismometer that has no natural frequency. Perhaps (probably) this idea is not new, but never-the-less, here it is:

Hi Dave,

Imagine, if you will, a stack of, say 1" diameter aluminum tubes about 10 inches long. There are 9 tubes, two on the bottom, three in the next layer up, and four in the top layer........

      I would rather not thank you! Why make it extremely complicated, when you can make it dead simple?

      Take a circular piece of plate glass and drill a hole in the middle for the air supply. Support the outside edges on a circular metal structure with three levelling screws. Use another, but smaller diameter sheet of plate glass or of Al etc. more or less centrally placed. This will need to have the bottom surface modified, possibly by sticking on foil or by etching, to give either a circular or a clover leaf shaped air bearing. Fit the top of the plate with two pairs of Maxwell coil + magnet force actuators, at right angles and a shadow type capacitative distance transducer and you might be 'in business'! See
      You might need to lap polish the two glass plates together with fine carborundum and rouge, rather like you do when lapping optical flats, to get a near perfect fit. It is usually inviting trouble to put unscreened magnets on a seismic mass, but you could probably get away with small opposed magnet pairs. You might need an additional pair at some point on the periphery of the disk, to prevent rotation.

      In operation, the top disk is floated using the air supply at the centre of the base plate. There is no wiring or tube connections needed for the armature disk. The distance transducer measures the deflections in the X and Y directions and feeds back PID signals to the coil pairs to keep the plate centralised. You need maybe 1 to 2 lb mass total to keep down the inherent noise.

The surface of the aluminum disks which face the glass is recessed about .001" leaving a smooth, lapped rim about 0.100" wide touching the glass. In the centre of the recess is a small hole, say about .062" which is cross drilled to a small air fitting, I think the common size is 10-32 (Look in an aquarium shop for this.) This hose is also connected to an aquarium air pump. When the pump is turned on - Voila! The free mass will rise slightly on an air film which will form between the
plates, and the mass will float freely, with zero static coefficient of friction.

      Can you give us references for the design of air bearings, please? I know that they are designed as overdamped pneumatic LCR circuits, but I don't have any formulae, etc. They will need very careful design. While air bearings have zero static friction, they may have small dynamic forces.
      We used to use a glass plate surrounded by spring wires and heavy metal 'pucks' to demonstrate the laws of mechanics. Each puck had a gas tight chamber which was filled with dry ice. This slowly sublimated and provided the gas flow for the bearing underneath. However, there was always a very slight 'dither' on them. I do not know if this was due to varying gas pressure, to inadequate sublimation control, to poor bearing design, to variations in the surface of the glass plate, or to turbulent flow.   

There are enhancements possible, including an air receiver and filter to reduce air pressure pulsations and dirt (although the pressure pulsations are far above our frequency of interest, and the mass can't follow them anyway) and a temperature control system. (Always include a temperature control system. Always.)

      You will need a very stable and clean air supply. The air bearings will need to be run with laminar flow, or turbulence noise and random forces will be generated. If the rig could be totally enclosed, the air could be filtered and circulated. The pumping pressure fluctuations will need a considerable amount of smoothing. The mass will tend to follow pressure fluctuations, whatever the frequency - it depends directly on the gas pressure to elevate it! I would be concerned about possible transitions from laminar to turbulent gas flow in the bearings. You might need to use a multiple tube peristaltic pump.

      One reason why professional systems do not often use temperature control, is that they are usually buried / installed below ground level. Once you get below about 1 m depth in most rock formations, the daily temperature change is reduced to milli degree levels. It is very difficult indeed = expensive to get anywhere near this sort of stability using active thermostatic control.        

I hereby declare the season open.

      It would be an interesting experiment to try. You might initially use simple differential optical sensors. A pair of VTD34 7.4 sq mm photodiodes and an OPA2134 opamp can give about 20 nm resolution at 10 Hz. If this works, you can invest in capacitative transducers which give better than 1 nm resolution.


      Chris Chapman

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