Unauthorized personnel detection system

A detection system for a doorway discriminates between persons who are authorized to go through the doorway and those who are not. An oscillator with a resonant L-C circuit includes metallic capacitor plates on opposite sides of the doorway to reduce the oscillator frequency when a person walks through and to provide a transient frequency increase if the person wears or carries a small metal object. A discriminating circuit is connected to the oscillator output to discriminate between persons in the doorway with and without such a metal object.

SUMMARY OF THE INVENTION 
This invention relates to a detection system for use in a passageway, such 
as a doorway, to discriminate between persons who are authorized to pass 
through the doorway and persons who are not, such as patients in a mental 
institution who are not supposed to leave a designated area on one side of 
the doorway unless escorted by a nurse or other staff member. 
In accordance with the present invention, metal plates on opposite sides of 
the doorway are the plates of a capacitor in a tuned-circuit oscillator. 
The frequency of this oscillator is relatively high as long as no one is 
between the capacitor plates because air has a low dielectric constant. A 
person's body has a much higher dielectric constant so the oscillator 
frequency drops while a peson is passing through the doorway. If the 
person is wearing or carrying an electrically conductive article, such as 
a small metal object, this produces a transient increase of the oscillator 
frequency while that object is between the capacitor plates. 
In the preferred embodiment of the invention, the oscillator output is 
connected by two separate circuit paths to the non-inverting and inverting 
inputs of an operational amplifier. One circuit path includes a buffer 
amplifier, a low pass filter and a potentiometer. The other circuit path 
includes a buffer amplifier, a variable high pass filter, and a low pass 
filter. The cutoff frequency of the high pass filter is varied by a 
frequency-to-voltage conveter connected to a field effect transistor which 
is part of the resistive impedance in the high pass filter, so that the 
cutoff frequency follows changes in the oscillator frequency. 
The potentiometer is adjusted so that the ouput of the operational 
amplifier is substantially at its mid-point when a person with no metal 
object is between the capacitor plates. The time constants of the two 
sepaarate circuit paths (to the non-inverting input and the inverting 
input, respectively, of the operational amplifier) are different enough 
that the presence of a metal object on the person in the doorway causes 
the operational amplifier to produce a transient output signal, which may 
be used to operate an alarm signalling arrangement. 
A principal object of this invention is to provide a novel detection system 
for use at a passageway, such as a doorway, to discriminate between 
authorized and unauthorized persons there. 
Another object of this invention is to provide such a detection system 
which differentiates between a person in the passageway who is wearing or 
carrying an electrically conductive article, such as a metal object, and a 
person who is not. 
Another object of this invention is to provide such a system in which metal 
plates on opposite sides of the doorway or other passageway are the plates 
of a capacitor which controls the frequency of an oscillator. 
Further objects and advantages of this invention will be apparent from the 
following detailed description of a presently preferred embodiment which 
is illustrated schematically in the accompanying drawings.

Before explaining the disclosed embodiment of the present invention in 
detail it is to be understood that the invention is not limited in its 
application to the details of the particular arrangement shown since the 
invention is capable of other embodiments. Also, the terminology used 
herein is for the purpose of description and not of limitation. 
DETAILED DESCRIPTION 
Referring to FIG. 1, in accordance with the present invention a pair of 
capacitor plates 10 and 11 are mounted on the opposite sides of a doorway 
12 leading into a room or multiroom area inside a building which only 
certain authorized persons are supposed to enter and leave freely. Other 
persons, such as mental patients, are not supposed to pass through this 
doorway without initiating an alarm or a notifying signal to staff 
personnel. Thus, if an unauthorized person, such as a patient, goes 
through the doorway that fact will be signalled to person whose job it is 
to keep track of the whereabouts of the patients. 
As shown in FIG. 2, the capacitor plates 10 and 11 and whatever occupies 
the space between them (normally air) constitute a capacitor which is 
connected in parallel with an inductance coil 13 to provide a parallel L-C 
resonant circuit. This L-C circuit is part of an oscillator which includes 
a PNP transistor 14 having a grounded collector 15, and an emitter 16 
connected to the oscillator output terminal 17 and to the juncture 18 
between capacitor plate 11 and the lower terminal of coil 13. The base of 
19 of transistor 14 is connected to an intermediate tap 20 on coil 13. A 
positive power supply terminal 21 is connected to the upper end of coil 
13. 
In FIG. 1 all the components of this oscillator except the capacitor plates 
10 and 11 are enclosed in a housing H on the building wall near the 
doorway 12 being monitored. 
The frequency of oscillation depends upon the reactance values of coil 13 
and the capaitor which has plate 10 and 11. The reactance of this 
capacitor depends upon the dielectric constant of what is between its 
plates. 
When the doorway 12 is empty and air fills the space between capacitor 
plates 10 and 11,the dielectric constant is low and the oscillator 
frequently is relatively high. 
When a person's body is in the doorway between the capacitor plates 10 and 
11,the dielectric constant of the capacitor is much higher than it is for 
air and it remains at that high level as long as any substantial part of 
the person's body is still between the capacitor plates. Consequenly, the 
oscillator frequently at this time is much lower than it is when only air 
is between the capacitor plates. 
When the person in the doorway is wearing or carrying a small metallic 
object, such as a bracelet on the wrist or the ankle or a metallic name 
tag on a chain around the neck on the person's clothing, the passage of 
that small metallic object between the capacitor plates will reduce the 
dielectric constant of the capacitor abruptly, and the oscillator 
frequently will increase as long as that object is between the capacitor 
plates 10 and 11. 
FIG. 3 shows a circuit for detecting whether or not a person going through 
the doorway is wearing such a metal object. The oscillator output terminal 
17 in FIG. 2 is the input terminal if the FIG. 3 circuit. 
This circuit has an operational amplifier 22 with a non-inverting input 
terminal 23 and an inverting input terminal 24. Between terminals 17 and 
23 the circuit has in succession, in series, a buffer ampliflier 25, a 
rectifier 26, a potentiometer 27 and a resistor 28. A resistor 29 is 
connected between the output terminal of ampliflier 25 and ground. This 
terminating resistance for the ampliflier has a low value, such as 100 
ohms. A capacitor 30 is connected between the output terminal (cathode) of 
rectifier 26 and ground. The complete resistor which provides 
potentiometer 27 is connected in parallel with capacitor 30, i.e., between 
the output side of rectifier 26 and ground. Capacitor 30 acts as a low 
pass filter which bypasses to ground any signals of frequencies above a 
certain cutoff value, which is determined by the capacitance of capacitor 
30. 
Between terminals 17 and 24 the circuit of FIG. 3 has in succession, in 
series, a buffer amplifier 31, a capcitor 32, a rectifier 33 and a 
resistor 34. A resistor 35 is connected between the output terminal of 
amplifier 31 and ground. This terminating resistance for the amplifier has 
a low ohmic value, such as 100 ohms. Two resistors 36 and 37 are connected 
in sreies between the input terminal (anode) of rectifier 33 and ground. A 
capacitor 38 is connected between the output terminal (cathode) of 
rectifier 33 and ground. Capacitor 38 acts as a low pass filter. 
The circuit of FIG. 3 also has a frequency-to-voltage converter 39 of known 
design connected between terminal 17 and the base of a field effect 
transistor 40 whose output terminals are connected across resistor 37 
i.e., one output terminal of FET 40 is connected to the jucture 41 between 
resistors 36 and 37 and the other output terminal of FET 40 is grounded. 
The frequency-to-voltage converter 39 produces an output signal whose 
instantaneous voltage amplified is proportional to the instantaneous 
frequentcy of the input signal it recieves at terminal 17 from the 
oscillator of FIG. 2. 
Capacitor 32, resistors 36 and 37, and FET 40 constitute a high pass 
filter. Converter 39 and FET 40 control the cut-off frequency of this high 
pass filter. 
The operational amplifier 22 has a feedback loop with an adjustable 
resistor 42 to control the amplifier gain. 
A resistor 43 is the terminating impedance for the circuit of FIG. 3. The 
output of the operational amplifier 22 is connected through a flip flop 44 
of known design to an output terminal 45, which may be connected to an 
audible or visual alarm signalling device (not shown). 
In the operation of this system, as long as the doorway 12 is empty the 
oscillator frequency will be so high that the low pass filters 30 and 38 
prevent the oscillator output signal from being applied to either input 
terminal of the operational amplifier 22. 
When a person enters the doorway, his or her body capacitance reduces the 
oscillator freuency. This reduction in the oscillator frequency reduces 
the output voltage of the frequency-to-voltage converter 39 and through 
FET 40 this causes the cutoff frequency of the high pass filter 32, 36, 
37, 40 to be lowered, i.e., to follow the now-reduced oscillator 
frequency. The adjustable tap on potentiometer 27 will have been set so 
that when a person's body (without any metal object) is in the doorway the 
output of the operational amplifier 22 is at its midpoint (substantially 
zero volts). 
However, a small metal object worn or carried by the person will cause the 
oscillator frequency to increase because that object reduces the effective 
dielectric constant between the capacitor plates 10 and 11 on opposite 
sides of the doorway. The path from the oscillator output 17 to the 
non-inverting input 23 of the operational amplifier 22 has a different 
time constant than the path from the oscillator output 17 to the inverting 
input 24 of the op-amp. As a result of this time constant difference the 
signal applies to the inverting input 24 of the operatinal amplifier 22 
goes positive with respect to the signal applies to its the non-inverting 
input 23. This causes a transient negative output signal from the 
operational amplifer which trips flip-flop 44, which now provides a signal 
for actuating an alarm device of any suitable type, audible, visual or 
otherwise. 
Thus, the present system discriminates between a person who is carrying or 
wearing a small metal object and a person who is not. 
Preferably, the alarm signalling equipment which is turned on by the 
flip-flop 44, as already described, includes a radio transmitter 46 (FIG. 
4) which broadcasts a signal from an antenna 47 which turns on a "beeper" 
or other sound-producing device 48 worn by a person or persons. If that 
person is a patient who has just passed through the doorway, other persons 
close enough to the patient to hear the beeper will be alerted to the fact 
that the patient is outside the premises where he or she is supposed to 
be. Alternatively, if the beeper is worn by nurses and other hospital 
attendants, the beeper will notify all of them that a pateient has just 
gone through the doorway and those who are nearby can immediately go 
toward it for the purpose of intercepting the patient. 
As shown schematically in FIG. 5, the beeper 48 includes a radio receiver 
49 powered by a battery 50 and tuned to the frequency of transmitter 46. 
The radio receiver 49, in response to the reception of a signal from 
transmitter 46, turns on an audible alarm device 51 which may be reset to 
an "off" condition by any suitable reset arrangment 52, which may be 
operated manually by a nurse, security guard or other authorized person.