Tuesday, February 19, 2019

LEGO Technic Seismometer Prototype



Well, in its barest concept, a seismometer (device for measuring Earth's surface movement, namely quakes) is a rather simple thing. A large weight, suspended to move freely, attached to something that can note its movements - and that's it. And while assembling a rough one from parts scavenged from an old rusty car at the dump may seem just as simple, building one from LEGO Technic brings its own set of challenges along.

Although a group of dedicated weight bricks (such as 73090a) would have been a "purer" solution and possibly serve its purpose better, I went for the more pragmatic approach and connected a set of several battery packs together - thus indirectly using batteries as weights. Altogether, something the size of a large coffee mug ended up weighing well over one kilogram - more than enough to have the concept proven.

This weight is suspended, hanging on two rubber bands. I agree that, in theory, going for a more complex setup featuring six or eight bands (one for each corner) would have helped with force distribution and rotation, but would have been a nightmare to tune and set up. Two rubber bands allow for sufficient freedom of motion, however, and have a better chance of retaining the seismographer's sanity.

Furthermore, the weight is attached to two independent pieces of thread, one in vertical and another in horizonal plane, both forming a closed loop, which is led through a system of pulleys to translate the weight's movements into proportional movements of the threads. Finally, attaching pencils to the threads and letting a writeable surface slide perpendicularly underneath the pencils, completes the essential seismometer.

Hence, each pencil notes the weight movement in its own direction over time - one vertical, and other horizontal. This is a common approach with real life seismometers as well, because horizontal and vertical movements of the Earth surface lead to different effects and are measured separately. True, this weight could have included a third thread loop measuring the weight's movements in the third direction (sort of like X-Y movement) but that would add lots of complexity while bringing only marginal functionality, and likely go beyond the 48x48 baseplate I decided to use as a caliber for this project.

The writing surface, which is in this case a cascade of white Technic panels, is driven across the direction of the pencils via a rack and pinion system with variable speeds, controlled with a gearbox, allowing for 10 possible settings, balancing between precision in timing and total duration of a single "plate". With only minor adjustments, one could convert it so that a standard roll of paper (e.g. like those used in shop counter printers) can be used.

Finally, apart from dozens of accurately perpendicularly tuned pulleys (perpendicular pulleys retain linearity better), both threads pass through a simple, manually controlled tension mechanism employing a large 40T gear and a worm gear, allowing the seismographer to adjust the correct tension level finely once the seismometer has been set up. The aim is to keep it tense enough so that all weight movements are precisely translated to the movements of pencils running across the "writable" white panels, yet loose enough to avoid friction that would overly hamper the movement of the weights. With some experimentation, I've settled for the tension of about 1.5 Newton, roughly the tension you would get if you suspended your mobile phone off a thread.

The only active component in the entire mechanism is the motor driving the writable panels, mounted on springs to further isolate it along with its vibrations from the rest of the mechanism (though I admit it's not as bad with fixed mounting either). But this could even be done manually if one wanted - the motor at least assures that the graph movement speed is at least somewhat constant. The range of possible speeds, of course, increases if one uses regulated voltage at the input, like I did with the classic 9V train controller.

So, an obvious next question is - how does it behave? The good news is that it proves the concept indeed: having been set upon the table, it is sensitive enough to measure, at least with some consistency, average table bumps, and would surely have no problem measuring a mild earthquake. But it is, on the other hand, far too insensitive to actually measure micro-movements of the Earth surface which are typically imperceivable for people, and which real life seismometers are built to measure. Let alone even finer things like steps of people in the building, opening and closing faucets, bass kicks from someone listening to music somewhere, etc.

I don't say that the latter group could not theoretically be measured with LEGO, but that would require a different design, utilizing more specialized parts, and would likely be far too sensitive for actual quakes. It would probably revolve around a multi-reflection design, featuring a mirror on the weight, with a laser pointer pointed at it, and furthermore with the ray being reflected multiple times to artificially create distance and thus increase resolution. Finally, the laser spot's movements would be monitored by a camera, or by a cascade of Mindstorms' light detectors, or something similar. Such a setup's sensitivity can in theory be increased virtually infinitely, but at the cost of maximum measurable extents' window reducing. Id est, it may well measure the vibrations of the roof being shaken by the wind five stories above, and someone sorting out cutlery in the kitchen in the other wing of the house, yet go completely off scale if breathed into.

Finally, a bit of historical trivia: although commonly thought to be of modern origin, the first seismometers actually got built as early as 2nd century AD, in China. It was a rather simpler approach, with a precariously ballanced ball at the top of an inverted bowl. The fact that the ball dropped from the bowl indicated that there was an earthquake, while the direction of the ball's fall gave a rough idea about where did the earthquake originate from - letting the owners know in which direction should the help and rescue teams be sent.

Built for BrickStory 2019.

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