The invention concerns an alarm system to prevent the entry of intruders
inside an enclosed space.
Volumetric alarm systems signal the presence of intruders inside
enclosed spaces by detecting the forms of disturbance made by said intruders.
Systems based on ultrasonic waves use the Doppler effect by means of which an ultrasonic
signal undergoes an alteration of frequency and of amplitude when it encounters
a moving object.
The signal is made to circulate in the space to be protected by means
of a transmitting station and a receiving station formed of transducers that can
operate a nominal frequency.
The volume to be protected becomes filled with stationary waves that
form a complex tangle due to their reflection against the various surfaces within
When the variations in frequency and amplitude of the return signal
caused by an intruder reach the receiving station, they are amplified, filtered
and used to set off the alarm.
The signal picked up by the receiving station is constituted of the
reflections of the signal sent out by the transmitting station.
If therefore the signal has a fixed frequency, environmental variations
due to thermal dilation or to other causes can make the system unreliable.
It may happen, especially in systems with a constant frequency, that
,due to particular circumstances,the signal picked up by the receiving station
is equal to zero thus simulating an action of sabotage which in actual fact has
not taken place.
To avoid this situation, therefore, the position of the receiving
or transmitting station must be changed, or else the frequency of the transmitting
station must be changed, while remaining within the sphere of the transducer's
nominal frequency which could wary because of the tolerance of the component and
the temperature change.
To pilot the transmitting transducer, the patents US-A 4 661 936
and US-A 4 499 564 adopt a frequency generated by an internal generator that corresponds
to the transducer's nominal frequency without making any allowance for all the
possible environmental variations referred to above.
The patent US-A 4 661 936 uses a computer which, by means of a motor
controls the position of the receiving station or else determines commutation of
various transducers in different positions (Figs. 1, 1a, 5 and 6).
Clearly, being in a fixed position, said transducers (12, 37, 38,
39) are subject to the same faults as a simple single transducer system since in
all said stations there might always be a return of a signal equal to zero in
spite of their different position.
There must be a minimum of three fixed transducers and the function
of signal movement is realized by phase variation among the signals of the transmitting
station (fig.7) Particularly in the circuits illustrated in Fig .8 of this USA
patent, serious irregularities can arise in the way they function as, on both transducers
57 and 58 a return signal equal to zero,and therefore a circuit 60, would be entirely
useless since there would be no signal to verify.
The principle underlying the patent US-A-4 499 564 consists in applying
to the transmitting station constant frequency pulses at a particular time (Fig.
2 c) and not optimizing said frequency, moment by moment, according to the characteristics
of the transducers.
The circuit is very complex and clearly the signal (Fig. 2 d), found
by analyzing the environment, is the result of all the echoes of the signal transmitted.
These echoes may have small amplitudes and it would be easy to cover
them with an interference as shown in Figs. 6b, 7b and 9d. In circumstances such
as those the system cuts itself out of action as seen in Fig. 7e.
The invention subject of the present application according to Claim
1 eliminates or lessens these serious drawbacks, improving reliability and dependability
of the system as will be explained hereafter.
Subject of the invention is a volumetric alarm system based on ultrasonic
waves in which the signal picked up by the receiving station is retransmitted in
amplified form by an electronic circuit.
On returning to the receiving station, the amplified signal becomes
further amplified, and so on by continuous and cyclic repetition of the phenomena
and by creation of resonance.
Amplification is preferably attained in three stages.
Since, when passing through the enclosed space, the frequencies forming
the signal are varyingly attenuated in relation to a prevailing frequency, resonance
occurs on said prevailing frequency which is selected automatically from among
the component frequencies.
If the prevailing frequency becomes attenuated by environmental variations,
thermal expansion or other causes, it will be automatically replaced in the electronic
circuit by another which will become the new prevailing frequency.
The signal issuing from the circuit that extracts modulation in amplitude,
is filtered to eliminate unwanted signals, such as very slow or very rapid variations.
Modulation of amplitude, provoked by the presence of an intruder in the enclosed
space, is compared in another circuit for this purpose having a reference threshold
which can be regulated by the user.
The pulse generated by the presence of an intruder determines the
charge of a certain capacitor in the circuit.
A fixed reference threshold is exceeded and the alarm is set off
when said capacitor is charged with at least three pulses within a restricted time.
By means of the system described above, the presence of a broad signal
is assured at the receiving station irrespective of the position of the stations.
Reliability and efficiency of the system are considerably improved
since practically all uncertainties and false alarms connected with the systems
at present known are eliminated.
If the signal at the receiving station is zero, any simultaneous
lack of resonance signal in the circuit would show that non-functioning of the
signal was due to malicious causes, different therefore from accidental causes.
A description, accompanied by an electronic diagram, explains one
example of execution of the invention.
- Fig. 1:
- Electronic wiring of the system subject of the invention.
As shown by the symbols and connecting lines, the circuit in Fig.
1 comprises block A with three amplifiers 12, 13, 14 that amplify the signal picked
up by receiving station 10.
Said amplified signal, retransmitted by the emitting station 11,
returns to the receiving station 10 and is further amplified and retransmitted,
repetition of said phenomena being continued ad infinitum and a phenomenon of resonance
being thereby created.
As the signal,emitted by the transmitting station 11, is formed of
frequencies which, when crossing the enclosed space, are subject to varying degrees
of attenuation in relation to a prevailing frequency whose amplitude prevails over
that of the others, the resonance of the whole system will be established on said
The signal received and progressively amplified by the receiving
station 10, passes over to block B that extracts the change in amplitude of said
signal in accordance with the movements made by objects within the enclosed space.
This signal is filtered in block C for elimination of any false signals
such as the very slow variations due to movement of air or the very rapid ones
due to disturbance on the feed line.
The signal passes from the filtering block C to block D. where it
is compared to a reference which can be regulated by the user.
If said reference is exceeded, at the output of block D a zero voltage
peak will occur which is passed on to mono-stable block E.
For each pulse received, said circuit E supplies a positive pulse
at its output equivalent to a time pre-set by components 15 and 16.
Said pulses go to charge a capacitor 17 in block F.
In circuit G the charge value of this capacitor is compared with
the fixed reference threshold determined by components 18 and 19.
If this threshold is exceeded after three pulses the alarm is set
Detection of possible sabotage is made by evaluating the amplitude
of the signal at the output of block D. If said signal falls below the level of
a pre-established reference with simultaneous lack of the phenomenon of resonance,
the circuit gives the alarm.