Introduction and Purpose

In the fall semester, the engineering projects involved the construction of models that helped to integrate the various portions of the Foundation Coalition course. The catapults were useful for the demonstration of kinematics; the bungee jump reinforced the value of Newton's Laws. The projects also asked you to combine mathematical principles and English composition skills with engineering design concepts. But the models themselves were really just toys - sophisticated toys - but toys just the same. This semester, you have already produced something more significant - instruments that can measure some physical phenomenon. Scientists and engineers, of course, design and build structures, systems, products, and countless other items - but they also build and use tools and instruments. And this project involves the construction and use of another interesting instrument.

In this project your team will be expected to build a seismometer. This project will once again integrate the physics and calculus that you are currently learning with engineering modeling and design components. The seismometer, in essence, employs mechanical oscillatory motion (a pendulum) to measure other wave phenomena (seismic waves traveling through the earth) and electromagnetics to convert the mechanical motion to electrical energy which can be amplified and examined on the classroom PCs.

The goal of this project is to design and construct a sensitive seismometer that will allow your team to examine mechanical vibrations transmitted through the earth. These vibrations are produced both by natural and man-made processes. A seismometer should be able to detect minute ripples in the earth produced by someone walking across a room as well as the substantial motion of the earth's crust caused by earthquakes.

The Seismometer

The word seismometer is derived from two Greek words - seismos for earthquake and metros for measure - and was first invented in 1841 by J.D.Forbes for "measuring earthquake shocks and othe concussions." The essential feature of any seismometer to examine the motion of the "ringing earth" is that some point or line within it remains at rest during the complicated movements of the ground. Various methods of obtaining such steady points have been propsed, but instruments in general use use various forms of pendulums. The motion of the pendulum can be observed and recorded with mechanical means (such as a pen tracing the movement on smooth paper), optical means (such as a beam of light reflecting from a mirror attached to the pendulum, then striking photographic film), or electromagnetic means (to be explained later).

You are probably somewhat familiar with the operation of a geologist's seismometer - perhaps you've seen the strip chart seismograph traces shown on TV news reports whenever California shakes substantially. You probably also think that a seismometer is a formidable piece of instrumentation. It is true that the world's best seismometers have massive frames anchored to piers sunk in bedrock and moving arms pivoting on sapphire bushings and sophisticated motors turning precision drums of paper for recording the pen movements. However, your team can build a sensitive seismometer using:

• Frames and moving parts from the Erector Sets
• Amplification of the electrical signals with op-amp circuits
• Investigation of the measured vibrations with the MPLI equipment

But you will have to add some additional parts to complete the construction.

Several issues of Scientific American from the 50's and 60's had articles in the Amateur Scientist section describing the construction of seismometers suitable for measurement of earthquakes, "a distant storm center, the birth of a volcano, or merely the tell-tale trace that follows your wife as she walks from the kitchen to the living room." These articles showed figures of the amateur seismometers which are reproduced here, and will also be distributed in class. We suggest that your seismometer observe the construction details shown in these figures.

According to one of these articles, most earth vibrations range in frequency from 10 Hz to 0.0001Hz. This range is substantially below the minimum frequency detectable by the human ear. The small amplitude tremors are called microseisms, and they are caused by "tornados, hurricanes, collapse of of small caves, rock slides, and the impact of meteors." Earthquakes, which occur globally on the average of 10 times per day, have fascinating vibration spectra because they consist of so many kinds of waves:

• Pressure waves (P waves), which are longitudinal
• Shake waves (S waves), which are transverse in nature
• Surface waves (L waves), which undulate like a water wave and travel around the surface of the earth

What you are to model and design

• The structure
• The detector
• The amplifier
• The predicted operation
• The observed operation