After a year of very successful operation of my Lehman seismograph, even with various upgrades, I'm aware of some limitations:

  • It is bulky and taking up a lot of space in my garage.
  • It is difficult to protect from air currents and insects.
  • The gain seems to vary a lot depending upon the exact position of the magnet relative to the coil and it is very difficult to set this up reproducibly, partly because the arm moves when I stand next to it.
  • The damping is hard to adjust reproducibly.
  • The period cannot be made more than about 15 seconds and the frequency response is unknown, but probably poor.

The answer to many of these issues is to use a force-balance system. Such a device forces the inertial mass to move with the frame when it is shaken and the force required to do that provides the seismic signal. It is possible to add a force-feedback mechanism on to a Lehman seismograph, but to achieve correct operation the sensor must be a position sensor rather than an inductive coil type.

A very interesting force-balance design suitable for amateur construction is the Shackleford-Gunderson seismometer. It comprises a short vertical pendulum with a relatively low inertial mass. An unusual electromagnetic position sensor is used to drive a simple coil-plus-magnet actuator that holds the pendulum in the resting position even when the base is shaking.

I plan to build such a device. However, I intend to start by experimenting with a simpler CdS photocell position sensor. The known non-linearity of such cells should not matter when movement is constrained.

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