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Geared motors key in new seismometer design

Geared motors key in new seismometer design

Unbelievable precision is being achieved with the latest high performance seismometers from Swiss company Streckeisen. Micro geared motors make an important contribution to making these highly sensitive sensors carry out their work reliably.

When we think about seismology, most often it is the dramatic effects of earth quakes that come to mind. And yet the science of seismology is about so much more than that, helping to explore the interior of the planet by researching the vibrations from a multitude of different sources. This might include, for example, the micro seismatics of the washing of the ocean waves.

While an earthquake that can be readily felt without the need for instruments is relatively easy to measure because the vibrations are intense, micro seismatics are much more difficult. This is where Swiss specialist Streckeisen comes in, with its broadband seismometer.

The traditional seismometer essentially consists of a weight – the pendulum – which is suspended from a spring. A pin is attached onto the weight which draws curves onto a continuous roll of paper when the vibrations occur. The deflection indicates the strength of the vibrations – but only as far as edges of the paper roll. At the other end, the sensitivity is limited by the thickness of the pin: minimal deflection of less than the line width is no longer clearly recognisable. This corresponds to dynamics of approximately 60dB.

The only thing that the Streckeisen broadband seismometer has in common with its classic predecessor is the moving pendulum, which reacts to outside forces. A pin is no longer needed, particularly since a straight line would always be drawn. The trick with which the Streckeisen seismometer achieves its extremely large dyna-mics lies in the tracking: an electro-magnetic feedback system ensures that the pendulum always remains in the established, balanced equilibrium position. The correction signal that is needed for tracking is also the output signal. The more powerful the movement acting upon the device the greater the signal, and the pendulum therefore always remains in the middle. A dynamic range of approximately 145dB can be achieved using a measuring principle such as this, which corresponds to a sheet of paper that would be 18km wide.

In order to record the movements of the earth in all three dimensions, each Streckeisen seismometer has three pendulums. They are in a slightly tilted position, arranged in a circle, each offset by 120 degrees. They react differently depending on the direction of the force, and the three-dimensional image of the spacial variations can be calculated from the differences.

Once installed, the high-precision electromechanics work automatically and can go without human intervention for a long period. Exact alignment of the seismometer and balancing of the pendulum are crucial before starting operation. Traditionally, seismometers have always been aligned eastwards. In order to balance the pendulums, a moving mass on the pendulums is moved until they are perfectly balanced. The mass is a toothed ring, which is moved to and fro by rotating on an axis. It is driven by a screw that is positioned at right angles to it. There is a small amount of play in the toothed ring and the screw. When perfect equilibrium is reached, the tooth of the adjusting weight remains between the tooth flanks of the screw, such that the pendulum can be moved freely.

This process, which is known as ‘centring’, is carried out in the Streckeisen seismometer by an AM0820 stepper motor with 16:1 planetary gearheads from Faulhaber, whose products are available in the UK from EMS. The geared motor was selected for its small dimensions and current consumption, precise movement and insensitivity to low temperatures – particularly important with Streckeisen devices used everywhere from Northern Alaska to the South Pole. But the most important consideration was long term reliability: it might be ten years before a pendulum is to be rebalanced, and then the motor must perform with guaranteed precision. 

Not all devices are used for long-term measurements. In so-called array measurements, a large number of seismometers are arranged in a grid formation in a certain area in order to record the special features of the subsoil in this location. Once the measurements are complete after several months or a few years, the grid is moved and the devices travel to their next location. For example, the states of the USA are measured one after the other in this way. However, the unavoidable movement that takes place during transport is not good for the sensitive sensors. In order to prevent damage, the moving parts are therefore secured with a transportation lock. This is carried out by the fourth motor in the Streckeisen seismometer, a Faulhaber 0816 DC-micromotor. 

The latest seismometers from Streckeisen are smaller and easier to set up than the first generation. A new variant with a tubular housing can be lowered into boreholes. The installation is still carried out by means of delicate manual work. It cannot be checked whether the components interact with the necessary accuracy until right at the end. Every sensor is therefore subjected to extensive testing in an air-raid shelter for insensitivity to air pressure fluctuations, amongst other things. A ventilator blows air into the chamber, which is surrounded by thick concrete, and ‘inflates’ it. It might be hard to believe, but the increased air pressure actually alters the room, even though it is only by a few nanometres. The Streckeisen devices can detect this change. If a device was not sealed, the result would be a deviating signal. Using tests such as this, Streckeisen ensures that only perfectly functioning seismometers are delivered that precisely record what is going on in the earth’s interior.

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