Motion detector suitable for detecting earthquakes and the like

A motion detector for detecting earthquakes or the like provides a warning signal when vibrations having a frequency of the order of the natural frequency of an earthquake tremor are detected. A preferred embodiment of the device employs a vertical spring barb member which is mounted on a suitable support on one end thereof. A coupler member is supported on the other end of the barb member. This coupler member is connected through a coil spring to an inertial mass which is vertically positioned generally in external concentricity with the barb and the coupler, the spring being either compressed or extended to provide a resilient coupling between the coupler member and the inertial mass. The spring and mass elements are chosen so as to have a natural resonant frequency at the frequency of an earthquake tremor (0.7-3 Hz), or other disturbance to be detected. An electrical switching circuit is provided so that when the disturbance to be detected occurs, motion of the inertial mass will cause the electrical switch to close thereby activating a suitable alarm device for providing a warning signal.

This invention relates to motion detectors and more particularly, to such a 
device suitable for detecting disturbances such as earthquakes. 
It has been found that earthquakes have particular distinguishing vibration 
characteristics which identify them from other random vibrations which are 
normally encountered. Thus, an earthquake tremor involves sudden transient 
motions or a series of motions of the ground which spread from the point 
of origin in all directions. Primary or "P" waves which are principally in 
a vertical direction precede the arrival of secondary, "S" waves which are 
principally horizontal (i.e. at right angles to the motions of the "P" 
waves). The "S" waves are generally larger in amplitude than the "P" waves 
and are potentially the waves that cause the principal destruction to 
structures. This is in view of the fact that most structures are designed 
to counteract vertical vibrations, being considerably more vulnerable to 
shear or lateral forces. 
It has been found that "P" waves travel about twice as fast as "S" waves. 
Therefore, depending upon the distances from the epicenter of an 
earthquake, "P" waves are received a long enough period before the "S" 
waves arrive to make their detection significant in providing a warning 
(about 20 seconds at 70 miles distance). It is well recognized as 
indicated in prior art U.S. Pat. Nos. 3,813,505 to Shoji, issued 5/28/74 
and 4,364,033 issued 12/14/82 to Tasy that earthquakes have both vertical 
and horizontal components and that there is some horizontal vibration and 
vertical vibration in both "P" and "S" waves. As indicated above, the "P" 
waves are principally vertical and the "S" waves principally horizontal. 
Therefore, earthquake detectors should be sensitive to both vertical and 
horizontal tremors. 
Several problems exist, in providing sufficient sensitivity, particularly 
to the initial "P" wave vibrations at a moderate distance from the 
epicenter without going to rather costly detection equipment. Further, it 
is essential to avoid false alarms that the instrument be sensitive only 
to earthquake tremors and not to other extraneous vibration signals that 
appear from time to time. 
The device of the present invention overcomes the aforementioned problems 
in a simple and highly economical instrument. This end result is achieved 
in the present invention by employing a resonant vibration system which is 
tuned to the frequency of earthquake tremors (typically the order of 0.7-3 
Hz). This resonant circuit may be vibratorily sensitive to both vertical 
and lateral vibrational forces or may be sensitive only to vertical 
forces. The preferred embodiment of the present invention is implemented 
by employing a spring barb member which is anchored at one end thereof. An 
inertial mass is connected to the spring barb by means of a spring which 
may be in the form of a coil spring which is either a compression or 
extension spring. The compliance of the spring barb and the coil spring, 
and the total mass interconnected therewith, including that of the 
inertial mass member, and the coupler member, are chosen so as to 
resonantly vibrate at the frequency to be anticipated from an earthquake 
tremor in the particular geographical area of operation (typicaly 0.7-3 
Hz.). For high sensitivity, a resonant vibration system having low 
friction (i.e. high "Q") is to be desired and the system of the present 
invention satisfies these requirements. When an earthquake tremor occurs, 
the instrument of the present invention is set into resonant vibration and 
the mass thereof is displaced sufficiently to cause an electrical switch 
to be actuated, this switch setting off an alarm signal. 
It is helpful to the comprehension of this invention to make an analogy 
between a mechanical resonant circuit and an electrical resonant circuit. 
This type of analogy is well known to those skilled in the art and is 
described, for example, in Chapter 2 of "Sonics" by Hueter and Bolt, 
published in 1955 by John Wiley and Sons. In making such an analogy, force 
F is equated with electrical voltage E, velocity of vibration u is equated 
with electrical current i, mechanical compliance Cm is equated with 
capacitance C, mass M is equated with electrical inductance L, mechanical 
resistance (friction) Rm is equated with electrical resistance R, and 
mechanical impedence Zm is equated with electrical impedance Z. Thus, it 
can be shown that if a member is elastically vibrated by sinusoidal force, 
Fo sin .omega.t, being equal to 2 times the frequency of vibration, that 
##EQU1## 
Where .omega.M is equal to (1/.omega.Cm), a resonant condition exists, and 
the effective mechanical impedance Zm is equal to the mechanical 
resistance Rm, the reactive impedance components .omega.M and 
(1/.omega.Cm) cancelling each other out. Under such a resonant condition, 
the velocity of vibration u is at a maximum, effective power factor is 
unity, and energy is most efficiently delivered to the object being 
vibrated. It is such a high efficiency resonant condition in the elastic 
system being driven that is preferably utilized in the device of this 
invention to achieve the desired end results. 
Just as the sharpness of resonance of an electrical circuit is defined as 
the "Q" thereof, and is indicative of the ratio of energy stored to the 
energy used in each cycle, so also the Q of a mechanical resonant circuit 
has the same significance and is equal to the ratio between .omega.M and 
Rm. Thus, high efficiency and considerably cyclic motion can be achieved 
by designing the mechanical resonant circuit for high Q. The Q can be 
varied to give broader or narrower frequency response of the mechanical 
resonant circuit by choice of materials for the circuit and the damping of 
such circuit. 
It is therefore an object of this invention to provide a motion detector of 
relatively simple and economic construction suitable for providing a 
warning signal of an earthquake. 
It is a further object of this invention to provide an improved earthquake 
detection system which employs resonant operation to provide high 
sensitivity and selectivity to earthquake tremors.

Referring now to FIG. 1, a first embodiment of the invention is 
illustrated. Spring barb member 11 is fabricated of a relatively durable, 
electrically conductive resilient material such as spring stainless steel 
or beryllium copper and is anchored on the base 12 of the device in 
support member 14 to which one end of the barb is fixedly attached and 
which, in turn, is fixedly attached to the base. Base 12 is fabricated of 
an electrically insulative material such as a suitable plastic. Suspended 
on the opposite end of spring barb 11 is a conical coupler member 16, the 
apex of the cone being seated on the end of the barb. Mounted in external 
concentricity with conical coupler member 16 and not attached thereto is a 
metal cylindrical sleeve member 20. Fixedly attached to the bottom end of 
sleeve member 20 as by threadable attachment is ring-shaped inertial mass 
member 21. An end plug 22 is threadably attached to the top end of 
cylindrical sleeve member 20 and has an inner conical surface 22a which 
matches the outer conical surface 16a of coupler member 16. A pair of 
spaced apart ring-shaped contacts 24a and 24b extend outwardly from the 
wall of cyclindrical member 20 directly above inertial mass 21. 
Ring-shaped metal contact plate 27 is mounted in external concentricity 
with sleeve member 20 on base 12 by means of screws 31 which fit through 
sleeves 28 and threadably engage the base, there being a space between 
contacts 24a and 24b and plate 27 and between the wall of cylindrical 
sleeve member 20 and this contact plate when the instrument is in its rest 
condition as shown in FIG. 1. 
The device is shown in its unactuated at rest condition in FIG. 1. A coil 
spring 29 is installed in compression between ledge portion 16b of coupler 
member 16 and the inner wall of plug member 22 such as to resiliently urge 
member 16 away from plug member 22. The compliances of springs 11 and 29 
and the combined total mass of the inertial mass 21 cylindrical member 20, 
coupler member 16 are chosen to resonate both laterally and vertically at 
the typical frequency of an earthquake tremor. This frequency will vary in 
the 0.7-3 Hz. range depending upon the particular geographical operating 
location. It has been found, for example, that for the Los Angeles area, 
that this frequency is approximately 1.4 Hz, for a predicted earthquake of 
sizeable magnitude. Thus, the system will only resonantly vibrate in 
response to tremors at substantially this frequency and will be relatively 
insensitive to vibrations at other frequencies. When the resonant 
vibration of an earthquake "P" wave is received, it will initiate 
principally vertical, but also horizontal vibration of the system which 
will effect resonant vibration of the system, thereby greatly amplifying 
the sensed vibrations. This will cause sufficient motion of the 
cyclindrical assembly 20 to cause contact 24a or 24b to come into contact 
with contact plate 27 and/or contact plate 27 to come into contact with 
the wall of metallic sleeve member 20. This will provide an electrical 
switching signal to alarm device 30 to close a switching contact in this 
device, thereby activating alarm device 30 which may comprise a buzzer, 
warning light, solenoid, etc. 
Referring now to FIG. 2, a second embodiment of the invention is 
illustrated. This embodiment is similar to the first in its operation 
except that it employs an extension spring rather than a compression 
spring, and has a somewhat different physical configuration. As for the 
first embodiment, spring barb member 11 is fixedly supported on one end 
thereof by means of a support member 14 which is fixedly attached to 
electrically insulative base 12. Mounted on base 12 is a cylindrical metal 
frame 32 which has a pair of spaced apart rings 32a and 32b. Suspended on 
the top end of barb member 11 is conical coupler member 16. Mounted over 
coupler member 16 and barb member 11 in external concentricity therewith 
and not attached thereto is cylindrical sleeve and inertial mass member 
20. Extension spring 29 is attached at one end thereof to threaded insert 
41 which is attached to the base of cylindrical member 20 by means of 
threaded coupler 42. The spring is attached at the other end thereof to 
coupler member 16 such that cylindrical member 20 is resiliently coupled 
to barb member 11. A contact ring 37 extends outwardly from the wall of 
cylindrical member 20, this ring being located between contact rings 32a 
and 32b. As for the previous embodiment, the spring and mass components 
are chosen so that they form a resonant vibration system for both lateral 
and vertical vibration at the typical frequency of an earthquake tremor 
for the geographical area of operation. When the system is set into 
resonant vibration, contact member 37 will come into contact with one or 
both of contact members 32a and 32b and/or the inner wall of frame 32 
which provides a switching signal for alarm, device 30 so as to activate 
this alarm device. 
Referring now to FIGS. 3-5, a further embodiment of the invention is 
illustrated. This embodiment operates in the same general fashion as the 
embodiment of FIG. 2, employing an extension coil spring which is extended 
by a spring barb, these spring members forming a resonant vibration 
circuit with a mass member which is suspended on the barb and which forms 
a sleeve which surrounds the barb and coil spring. The structural 
configuration of this embodiment, however, is somewhat different from that 
of the prior embodiment. Base portion 35 which may be fabricated of 
plastic has a removable cover 40 mounted thereon which may be of a 
transparent plastic. An electrically insulative board 38 is mounted on 
base 35 by means of screws 39. A base support 14 for spring barb 11 is 
fixedly attached to board 38. The spring barb is similar in configuration 
to that described for the previous embodiments. Coil spring 29 is attached 
at one end thereof to threaded insert member 40 which abuts against the 
base of inertial mass member 20. Mass member 20 has a hollow interior 20a 
which forms a sleeve surrounding coil spring 29 and spring barb 11. The 
upper end of coil spring 29 is attached to coupler member 16 which has a 
hollow interior 16a. The upper end of spring barb 11 fits within the 
hollow interior portion 16a of coupler 16 and thus spring 29 is extended 
within sleeve 20a such that inertial mass member 20 is resiliently 
supported above board 38 by means of coil spring 29 and spring barb 11. 
The resonant vibration frequency of the mass and springs and coupler, in 
combination, are chosen as in the previous embodiments for resonance at 
the frequency of the tremors to be detected. 
Inertial mass 20 has an upper tapering conical portion 20b and a 
cylindrical base portion 20c, there being a shoulder 20d formed between 
the upper and base portions. 
A pair of electrically conductive plates 32a and 32b are mounted in spaced 
apart opposing relationship on base member 35 and insulative board 38 by 
means of screws 39 which have conductive sleeves 43 which act as spacers 
between plates 32a and 32b. A circular aperture 32c is formed in upper 
plate 32a and the upper conical portion 20b of the inertial mass is fitted 
through this aperture with shoulder portion 20d in opposing relationship 
with plate 32a. The bottom surface of base portion 20c of the inertial 
mass is spaced from and in opposing relationship with plate 32b. Leveling 
screws 37 are used to level the base 35 to center inertial mass 20 within 
aperture 32c. Further, the mass and springs are chosen so that in its 
resting position the mass is vertically centered between plates 32a and 
32b with plate 32a spaced from shoulder 20d and the bottom surface of the 
mass spaced from plate 32b. Thus, in its at rest condition, electrically 
conductive mass 20 is electrically insulated from plates 32a and 32b. 
Operation of the device is as for the prior embodiments. In response to 
tremors at the resonant frequency of the system, the mass will vibrate 
vertically and/or horizontally so that it will come into contact with 
plate 32a and/or plate 32b, such electrical contact completing a circuit 
to a switch which will cause an alarm or other warning device to be 
energized. 
The horizontal and vertical sensitivity of the device will vary as a 
function of the diameter of aperture 32c and the length of conductive 
spacer 43. Various values for the diameter and length will give different 
distances between aperture 32c and inertial mass member 20 and between 
plates 32a and 32b and the shoulder 20d and the bottom surface of the 
inertial mass member. 
Referring now to FIG. 6, a further embodiment of the invention is 
illustrated. This embodiment is sensitive only to earthquake tremors along 
the vertical axis thereof and is substantially insensitive to horizontal 
components of such tremors. Construction is generally similar to that of 
the embodiment of FIGS. 3-5. In this embodiment, however, a rigid post 49 
is substituted for barb 11. Threaded sleeve 40 is slidably mounted on post 
49, the base of this post being fixedly supported on board 38. The spring 
29 is extended between threaded portion 49a on the top of the post and the 
threaded portion of sleeve 40. As for the previous embodiment, mass 20 is 
resiliently supported for vertical motion by means of spring 29 by virtue 
of the abutment of the base of the mass against sleeve 40. The device thus 
is sensitive principally to vertical disturbances and is resonantly tuned 
to respond to the vertical components of earthquake tremors of interest. 
The leveling screws are eliminated in this embodiment. 
While the invention has been described and illustrated in detail, it is to 
be clearly understood that this is intended by way of illustration and 
example only and is not to be taken by way of limitation, the spirit and 
scope of this invention being limited only by the terms of the following 
claims.