Seismometers and Pickup Coils

It would seem that a speaker would work; connect the moving load to the cone near the voice coil, or at/near the center of the dome in the middle (center).

There is a lot to be said for optical pickup - you can easily give yourself a long 'lever' arm, and these days a small laser will provide a very convenient light source. You also don't have to worry about the damping effects of a magnetic pickup - (in fact, you will probably want to add some damping)

Dave

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There is a lot to be said for optical pickup - you can easily give yourself a long 'lever' arm, and these days a small laser will provide a very convenient light source. You also don't have to worry about the damping effects of a magnetic pickup - (in fact, you will probably want to add some damping)

Dave

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I don't know much about optical pickups except that I'd decided building an interferometer would be a project in itself. Is it easier than that?

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If you are sure the vibration is 50Hz or thereabouts, you only need to get the resonant frequency of the proof mass suspension a way below that frequency. 1/10Hz is a major engineering exercise.

The loudspeaker suggestion could be used directly. Pick a drive unit with resonant frequency well below the vibration frequency you expect, and fix a small mirror to the cone near the middle. Fix a laser pointer to the frame, reflect the beam off the mirror onto a convenient wall, and use a ruler to measure the amplitude of the beam movement. Simple geometry let's you work back to the amplitude at the loudspeaker cone, and that is the amplitude of ground motion you are getting.

There are plenty of woofers out there with resonances below 20Hz, you can tweak the damping using the drive coil and a resistor. Just putting the driver resting vertically on a bench is good enough.

DON'T look into the laser. DO make sure there are no highly reflective surfaces anywhere near the beam path.

Regards Ian

I've been discovering the joys of 1/10 Hz. It seems so easy when you plug in w=sqrt(k/m), but then you discover the spring reaches its maximum extension before it can support the required m. I'm going to brush up on my differential equations and try revising my design goal to the measured amplitude being 90% of the actual amplitude.

I'm not sure what role the loudspeaker plays in that case.

"Gregory L. Hansen" a écrit dans le message de news:cuod3p$ic4$ snipped-for-privacy@rainier.uits.indiana.edu...

Sure.

w=sqrt(k/m)

but also for a spring, L-L0 = g m / k

so you have L-L0 = g /(w^2)

If you want 0.1Hz resonance frequency that gives you about 25 meters spring displacement. A pretty nice spring :-)

If you really want 0.1Hz, you'll have to go for the right k/m ratio, then compensate for the mass weight by some *constant force* mean (either a short solenoid with a long core, or a short core in a long solenoid).

You'll probably have to add some rest position servoing with a low corner frequency.

Another possibility is, with the same setup, to servo your sensing mass position up to the highest frequency of interest. Then the solenoid current is proportionnal to the mass acceleration.

Google on 'folded pendulum'. That should bring up a wealth of ways to suspend a weight with low resonant frequencies.

Jeroen Belleman

You aren't also known as "NEWS 2020", are you? He could feel things that nobody else could feel too.

Jim

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Nice. Looks like that could be a compact design.

On a related note, I was thinking of using knife edges for pivot points. That's nice and scientific, right? But as I thought about it, I couldn't think of any reason not to use strips of rubber sheet, fabric, or even string for hinges. String hinges would be strong enough, easy to construct, little friction, no stiction, cheap... what's not to like?

I was thinking of this design, but I think I like the folded pendulum better, looks like it might be lighter and more compact, easier to carry.

But if you're measuring vertical vibrations I think the engineering would be a little more challenging.

Usually the hinges are (very) thin hard metal strips. You want something that bends easily, but doesn't stretch or creep. Strings or rubber sheets don't qualify. Naturally, the resulting pedulum has only one degree of freedom.

Jeroen Belleman

What does it matter if the hinges creep? If their length changes by 1%, the overall structure might change by 0.1% or 0.01% if the booms, platforms, etc., aren't creeping with it. Probably nothing else in the apparatus or the measurement matches that precision anyway.

I didn't think the knife edges were meant to bend, just to have a very small contact surface. Torque is force times length, so making the length as small as possible reduces the frictional effect on the pivot.

Well, a folded pendulum gets it long period from balancing a stable pendulum against an unstable one. For long periods, this balance gets increasingly delicate.

If the hinges change their properties for whatever reason, the period would drift strongly and the pendulum might even 'collapse'. (Meaning it no longer seeks a central position, like a normal pendulum.)

However, I agree that if you do not push things too close to the edge, it doesn't matter.

Jeroern Belleman

A suspended mass ground motion sensor acts as a 2nd order high pass filter to the ground motion. Above the resonant frequency the amplitude of the proof mass approaches zero quite quickly if you get the damping right, and the frame (obviously) will still have the full ground motion. The difference is what you are trying to measure.

What resonant frequency do you need, to be able to measure what you want? Let's say you can get hold of a speaker drive unit with a resonant frequency of 16Hz. If you tweak the Q to a little more than

A loudspeaker driver gives you a proof mass, a way of controlling the damping, support spring and structures to constrain the motion of the proof mass to be axial. It is also cheap and readily available. It is also easy to rotate the axis of the driver to let you check for any horizontal motion as well.

Low resonant frequency mechanical structures are a significant problem, in terms of keeping them stable and avoiding multiple response modes and resonances. The pivots are critical, stability of the springs is a nightmare. A very common mistake is to think that a large proof mass is needed - it isn't, even for sensing very small ground motion, many orders of magnitude below what you can perceive.

The next issue is what to use for a sensor, and how to calibrate the whole system. A simple optical lever, geometry and a ruler will give you this, and will let you see if you need to go any further (if the measured amplitude of motion is negligible, you are done, if not then ask again).

Regards Ian

I'd like to go to about a Hz.

Loud speakers make me nervous because I don't know the stiffness, voltage versus velocity, the sensitivity, etc.

I don't understand the role of the mirror. A simple displacement won't mean anything, it doesn't help unless the mirror rotates. But my vibrations are too small to make water ripple, whatever amplitude that might be.

Just side note, I heard that scientists saw where the north pole had moved an inch after the Asian Tsunami.

Another compact method, probably quite a bit easier to build than the folded pendulum, is a superspring. You hang a real mass/spring combination from the centre of a small loudspeaker, sense the mass's position using e.g. a laser and split photodiode, and apply positive feedback to the speaker using a dc amplifier. Overall loop gain should be 0.99 or thereabouts. This reduces the effective spring constant by a factor of 100, leading to a 10x lower resonant frequency for the same physical size.

The SNR isn't improved by this idea, but the frequency response is--it's a mechanical version of the old idea of driving a coaxial shield from an emitter follower on the sensor.

Cheers,

Phil Hobbs

I was not suggesting using the speaker unit as a speaker, or using the voice coil to sense the motion, just to control the damping with an external resistor. Using it just as a suspended mass on a spring above the resonant frequency means the stiffness, mass, Bl product have absolutely no effect on the output, other than on the damping. The worst aspect is sensitivity to air movements, but you are going to need a pretty airtight box whatever you do.

The role of the mirror and laser pointer are to provide an optical lever to sense the relative displacement of the cone and frame. You're right about needing rotation.

Regards Ian