Passive intrusion detection system

An intrusion detection system in which crystal oscillators are used to provide a frequency which varies as a function of fluctuations of a particular environmental property of the atmosphere, e.g., humidity, in the protected volume. The system is based on the discovery that the frequency of an oscillator whose crystal is humidity sensitive, varies at a frequency or rate which is within a known frequency band, due to the entry of an intruder into the protected volume. The variable frequency is converted into a voltage which is then filtered by a filtering arrangement which permits only voltage variations at frequencies within the known frequency band to activate an alarm while inhibiting the alarm activation when the voltage frequency is below or above the known frequency band.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to an intrusion detection system 
and, more particularly, to a passive intrusion detection system for 
indicating entry of an intruder into a protected volume. 
2. Description of the Prior Art 
Quartz crystal microbalance (QCM) instruments have been proposed to measure 
different phenomena such as wind velocity, direction, temperature as well 
as air humidity, contaminants in gas systems as well as in biological 
studies. Basically, in these studies quartz crystals are coated with 
different materials which affect the crystals' sensitivities to the 
phenomena to be measured. These crystals affect the frequencies of 
oscillators, with the changes in frequencies being measured to indicate 
the changes in the phenomena being measured. 
There are many situations in which it is desired to detect the intrusion of 
an unauthorized person or animal into a protected volume, containing 
sensitive and/or dangerous materials, such as radioactive material or 
classified documents. Presently available infrared light and ultrasonic 
beam intrusion protection systems are quite inadequate since they can be 
perceived by an intruder, tampered by him and thus defeated. Also, 
presently existing systems produce an excessive number of false alarms and 
often require sensitive adjustments whenever the content in the protected 
volume is changed. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is a primary object to provide a new intrusion detection system. 
Another object of the invention is to provide an intrusion detection system 
for an enclosed volume which produces relatively few false alarms. 
A further object of the present invention is to provide an essentially 
tamperproof intrusion detection system for an enclosed volume which is 
relatively inexpensive, easily adjustable for the protected enclosed 
volume so as to produce relatively few false alarms. 
These and other objects of the invention are achieved in one embodiment in 
which a quartz crystal microbalance system is used. The system includes at 
least one nude quartz crystal used as the frequency determining element of 
at least one oscillating circuit. The crystal is sensitive to changes of a 
specific environmental property in the atmosphere in the enclosed volume, 
e.g., humidity, thereby changing the frequency as a result of humidity 
changes. A filtering arrangement is incorporated to distinguish between 
the rate of change of the frequency due to changes in humidity as a result 
of the entry of an intruder and the rates of change in frequency due to 
humidity changes as a result of other causes. The function of the 
filtering arrangement is to minimize the production of false alarms. 
The novel features of the invention are set forth with particularity in the 
appended claims. The invention will best be understood from the following 
description when read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Attention is directed to FIG. 1 wherein A designates a first frequency 
oscillator, whose frequency is controlled by a quartz crystal Q.sub.A. A 
similar oscillator B has its frequency controlled by another quartz 
crystal Q.sub.B. The frequency outputs of the two oscillators are mixed by 
a mixer 12 whose output frequency .DELTA.f is the difference between the 
frequencies of oscillators A and B. 
The crystal Q.sub.A is sensitive to changes in humidity, for example, 
however, if desired, it may be coated with known materials which can 
enhance its sensitivity to humidity changes in the protected volume. 
Q.sub.B is protected from exposure to the atmosphere and therefore the 
frequency output of oscillator B is held constant. Thus, any change in 
humidity in the protected volume affects Q.sub.A and therefore the 
frequency of oscillator A while that of oscillator B remains constant. 
Consequently when humidity in the atmosphere changes .DELTA.f changes. A 
larger change in humidity results in a larger change in the frequency 
difference .DELTA.f from mixer 12. 
The frequency difference .DELTA.f from mixer 12 is converted into a voltage 
V by a frequency to voltage converter 14. The voltage V is amplified by a 
bandpass amplifier 15, whose output is rectified by a rectifier 16. When 
the output of the latter exceeds a selected amplitude an output device 17 
is activated. The output device may be an alarm, a light indicator, a 
recorder or any other similar device. In practice the output device may 
include a latching circuit, so that once the output device is activated, 
it remains activated until it is manually turned off by an operator. For 
explanatory purposes hereafter the output device 17 will be referred to as 
the alarm 17. 
It has been discovered that occasional electrical storms or power line 
"bumps" produce spurious responses evidenced by very rapid, short-duration 
oscillator frequency changes. Also, slowly varying changes in oscillator 
frequency often occur due to diurnal variation in relative humidity or 
temperature or by the usual hunting of an air conditioner system. In one 
example, it was discovered that the rate of change of oscillator frequency 
due to an intruder is within a bandpass of 0.5 to 5 seconds. Thus, in the 
particular example, the bandpass of amplifier 15 is chosen to be limited 
to between 0.5 second and 5 seconds or between 0.2 Hz and 2.0 Hz. By 
limiting the bandpass of amplifier 15 the changes in frequency at rates 
other than those caused by the entrance of an intruder do not cause the 
amplifier to trigger the alarm 17 and thereby produce false alarms. 
However, changes in frequency occurring at rates which are characteristic 
of the entry of an intruder will cause the amplifier to produce an output 
which will trigger the alarm 17 to produce a valid indication. 
Preferably the humidity in the protected volume, e.g., a room, should be 
different than that outside the room. Consequently when an intruder enters 
the room either through a window or a door a significant change occurs in 
the humidity in the protected room. In addition the entrance of the 
intruder into the protected room results in a change in the humidity 
content of the room and an increase in the aerosol content of the room. 
Movement of the intruder creates turbulence in air flow with 
characteristic fluctuation of local humidity. 
Herebefore the system was described in connection with the two oscillators 
A and B and their crystals Q.sub.A and Q.sub.B, where only Q.sub.A is 
sensitive to humidity changes. While Q.sub.B, for simplicity may be left 
in its sealed environmentally protected case, it is also possible to 
enhance the system sensitivity to intrusion by using a nude crystal whose 
temperature coefficient, i.e., frequency vs temperature, is opposite that 
of Q.sub.A. Heat of adsorbtion of a specific crystal B coating is another 
method of increasing the specificity of the system response. Such an 
arrangement is believed to be preferable. It permits one to use high 
frequency oscillators, e.g., 1 mHz to 30 mHz, and thereby increase the 
system's sensitivity, while the frequency difference .DELTA.f, which is a 
low frequency, is converted into a varying amplitude voltage. However, the 
invention is not intended to be limited to the two oscillators embodiment. 
If desired oscillator B with its crystal Q.sub.B and the mixer 12 may be 
eliminated. In such a case the output frequency of oscillator A is 
supplied directly to converter 14 as shown in FIG. 2. 
The passband of 0.5 second to 5 seconds for amplifier 15 is presented for 
explanatory purposes only, and is based on one experimental system in 
which the rate of change of frequency due to other than an intruder was 
greater than 5 seconds while the rate of change of frequency due to 
unusual electrical storms or the like was less than 0.5 second. In any 
practical application the bandpass will have to be determined 
experimentally. From experiments conducted to date it seems clear that the 
rate of change of oscillator frequency due to electrical storms or very 
sudden changes in humidity is considerably higher than the rate of change 
of oscillator frequency due to humidity changes produced by an intruder. 
Thus, the upper bandpass limit can be easily established. As to the lower 
limit the rate of change of oscillator frequency due to diurnal humidity 
and temperature changes and/or air conditioner system hunting is typically 
(600--1200 seconds) lower than that produced by an intruder. Thus, the 
lower band limit can be established depending on the actual rate of change 
of oscillator frequency produced due to other than the intruder's entry 
into the protected room. 
The invention is not intended to be limited only to sensing the humidity in 
the atmosphere. Any other environmental parameter or property of the 
atmosphere may be sensed. As used herein the term an environmental 
property of the atmosphere intends to refer to the typical characteristics 
of or content in the atmosphere. Examples of environmental properties of 
the atmosphere, in addition to humidity, include atmosphere temperature, 
dust content in the atmosphere, ozone content in the atmosphere, humanoid 
aerosols or the like. The term is not intended to include 
specially-induced phenomena, such as infra-red light, ultrasonic waves or 
the like. 
The primary requirement is to employ a crystal such as Q.sub.A which is 
very sensitive to the particular environmental property of the atmosphere 
which is to be sensed, while the other crystal Q.sub.B is either less 
sensitive to the atmospheric property which is being sensed, completely 
protected therefrom, or sensitive to a different parameter with a 
frequency sensitivity of the opposite polarity. It is clear however that 
only those atmosphere properties which change in the protected room as a 
result of the entrance of an intruder should be sensed in accordance with 
the present invention. 
The system, except for the alarm, can be fabricated to be very small so as 
to be unnoticeable. For example, it can be installed in the return duct of 
the ventilation system of an air conditioning system so that its presence 
in the room is not detectable, thereby preventing tampering with its 
operation. 
The previously described embodiments are essentially of the analog type. If 
desired some digital circuitry may be employed. For example, the frequency 
from oscillator A when one oscillator is used or the frequency difference 
.DELTA.f from mixer 12 may be supplied to a counter which provides a 
digital count of the frequency. The counter can be operated to reset at 
the end of a selected period, e.g., 0.1 second, and start a new count. 
Just prior to being reset the entire count in the counter or at least 
several of its least significant digits may be supplied to a digital to 
analog converter which would convert the count into an analog signal, 
e.g., a voltage. The output voltage can then be filtered by a passband 
filter, amplified and rectified to activate the alarm 17 only when the 
frequency changes at a rate which is due to the entry of an intruder into 
the protected room. 
From the foregoing it should thus be appreciated that the present invention 
comprises an intrusion detection system. It is based on sensing changes in 
an environmental property of the air or atmosphere in the protected 
volume. The environmental property is one which is affected by the entry 
of an intruder. The sensed changes in the property are indicated by 
changes in the frequency of an oscillator. The frequency is used to 
provide a first signal, such as the voltage from converter 14, that 
changes at a rate related to the rate of change of the sensed property. 
The rate of change of the sensed property is within a definable range, 
e.g., 0.5 second-5 seconds, when an intruder enters the protected volume. 
The system includes circuitry to provide an output signal which activates 
an output device, e.g., an alarm, only when the first signal changes at a 
rate within the definable range. By limiting the production of the output 
signal to occur only when the rate of change of the sensed atmosphere 
property is within the definable range, few if any false indications, due 
to changes of the sensed property take place. 
Although particular embodiments of the invention have been described and 
illustrated herein, it is recognized that modifications and variations may 
readily occur to those skilled in the art and consequently, it is intended 
that the claims be interpreted to cover such modifications and equivalents 
.