Movement detector

The invention relates to a movement detector, more particularly a sensor comprising a container partially filled with a liquid into which three electrodes extend. An input alternating signal is applied to two of the electrodes and an output signal amplitude modulated in accordance with movement of the liquid in the container, is detected from another two of the electrodes by means of a sensing circuit which includes a demodulator for detecting the amplitude modulation and a circuit arrangement which provides a container movement signal in response to successive peaks in the demodulated signal which are in excess of a given threshold value being spaced apart in time from one another by a time dependent upon the peak magnitudes of the demodulated signal.

The present invention concerns a movement detector. 
Movement detectors are known which include a sensor comprising a container 
closed in a fluid-tight manner partially filled with an electrically 
conductive liquid and three electrodes electrically insulated from one 
another and extending substantially within the container in such a manner 
as to be in contact with the liquid, a voltage generator connected between 
a first and a second electrode and a measuring circuit connected between 
the second and the third electrodes. Movement detectors of this type are 
known from the following prior publications: U.S. Pat. Nos. 3,042,888 and 
3,164,023; an article by C. A. RATCLIFFE entitled "A Seismometer with a 
Water Wall as a Sensing Element" which appeared in the IEEE Transactions 
on geoscience electronics, vol. GE-10 No. 2, April 1972; German patent 
application (Federal Republic) DOS 2,332,124; and French Pat. No. 
2,311,310. 
It is an object of the present invention to provide an improved movement 
detector of this general type and from one aspect the invention provides a 
movement detector comprising a closed container partially filled with 
liquid, three electrodes extending into the liquid in such a manner as to 
always be in contact with the liquid whatever the orientation of the 
container may be, drive means adapted to apply to a first two of said 
electrodes an alternating input voltage, a sensing circuit responsive to 
an output voltage established across another two of said electrodes in 
response to said input voltage, said sensing circuit including a 
demodulator adapted to provide a demodulated signal indicative of the 
amplitude modulation of said output signal which modulation is caused by 
movement of the liquid in the container, means adapted to provide a 
container movement signal in response to successive peaks of the 
demodulated signal being of a magnitude greater than a predetermined 
threshold magnitude and occurring within a particular time range from one 
another, and said detector being only responsive to movement of the liquid 
occurring at a frequency within a particular range thereof. 
The invention furthermore provides a movement detector comprising a 
container closed in a fluid tight manner and partially filled with a 
liquid, three electrodes electrically insulated from one another and 
extending within the container in such a manner as to be in contact with 
the liquid, drive means connected to a first and a second of said 
electrodes and arranged to apply an alternating voltage thereto, and a 
sensing circuit connected to said second and the third of the electrodes, 
wherein said drive means includes a source of d.c. voltage, an oscillator 
driven from said voltage and means for maintaining the peak amplitude of 
the alternating signal produced by the oscillator substantially constant 
notwithstanding fluctuations in said d.c. voltage, said electrodes extend 
into the container in such a manner as to be constantly in contact with 
the liquid whatever the orientation of the container may be, the viscosity 
of the liquid is so selected that the detector is only sensitive to 
movements of the container which have a frequency within a predetermined 
frequency range, capacitors are interposed between the drive means and the 
first electrode and between the sensing circuit and the third electrode to 
prevent a direct current passing through the liquid, said sensing circuit 
includes a demodulator for providing a demodulated signal indicative of 
movements of the liquid in the container, and said sensing circuit 
includes means adapted to provide a container movement signal in response 
to successive peaks in the signal from the demodulating means having an 
amplitude greater than a predetermined level and within a particular time 
range from one another. 
A preferred embodiment of movement detector in accordance with the present 
invention has the following features and advantages, namely the drive 
means is adapted to produce an input alternating voltage at a frequency of 
at least ten times that of the movements it is desired to detect, the 
drive means being supplied from an independent source of d.c. voltage and 
being provided with means for maintaining the amplitude of the alternating 
signal which it provides, constant, whatever the fluctuations in the d.c. 
voltage provided by the source may be; the sensing circuit comprises a 
demodulation circuit in which the signal collected between the second and 
third electrodes is amplitude demodulated to form a demodulated signal 
varying continuously as a function of displacements and changes in 
orientation of the sensor; the container is compact in form and that each 
electrode extends substantially within the container in such a manner as 
to be constantly in contact with the liquid whatever the orientation of 
the sensor with respect to the vertical may be, so that the detector may 
function for any orientation of the container; the viscosity of the liquid 
is chosen so that the sensor is only sensitive to movements at frequencies 
included within a particular band of frequencies, termed a pass band; 
electrically isolating capacitive means are interposed between the drive 
means and the first electrode, so as to prevent a d.c. current from 
circulating in the liquid through the electrodes and from producing 
electrolysis which would release gas, which might cause the fluid-tight 
container to explode; the demodulated signal is processed in a circuit 
providing a signal which has an amplitude indicative of the peak to peak 
periodicity of the demodulated signal, which amplitude is compared in a 
comparator with a threshold value, the comparison information provided by 
the comparator determining the operation of a signalling device supplied 
from a separate source of d.c. voltage, by means of which the presence or 
absence of the emission of a signal by the signalling device may provide a 
remote indication of the presence or absence of movements of the sensor, 
the said movements having particular characteristics defined especially by 
the band pass of the sensor and by the threshold value. 
The signalling device is constituted, for example, by a radio signal or an 
audio signal or a light source. 
By virtue of this combination of characteristics, the preferred movement 
detector may be completely self-contained whilst being supplied from a 
cell or a battery; this detector may be mounted on any kind of member 
sensitive to the most varied movements since it functions in any position; 
by virtue of its independent signalling device, this detector provides 
remote indications of the presence or absence of movements with 
pre-selected characteristics. Furthermore, the sensor may be minaturized 
without losing its sensitivity nor its long life since the only element 
sensitive to movement which it comprises, is the liquid which it contains 
and the capacitive isolating means prevent electrolysis of the liquid by 
the electrodes. 
By virtue of means permitting the peak value of the input signal to be 
maintained constant, it is possible to supply the drive means and to 
supply the signalling device from one and the same independent source of 
d.c. voltage such as a battery. The result is a still greater 
simplification and miniturisation of the detector. 
A detector according to the invention can be used to selectively detect 
normal movements of a person; for this it is sufficient to adapt the 
detector so that it only detects movements, the frequency of which is 
between 0.1 and 20 Hz inclusive and preferably between 0.5 and 3 Hz. This 
filtering is obtained either by adjusting the viscosity of the liquid in 
the container and/or by using a band pass filter circuit.

The movement detector illustrated in the drawings comprises a sensor and an 
associated electronic circuit. 
According to the embodiment illustrated in FIG. 1, the sensor 10 comprises 
a container 1 of electrically insulating material containing a liquid 2 
slightly conductive to electricity. The container 1 has a compact shape 
and it is closed in a fluid-tight manner by a plug 3 and the liquid 2 does 
not completely fill the interior of the said container 1 so that a volume 
4 filled with air, with a gas or empty, exists at the upper part of the 
said interior volume. 
Three electrodes 5a to 5c passing through the plug 3 in a fluid-tight 
manner are immersed in the liquid 2; these electrodes are formed and 
disposed in the container 1 so that all three are constantly in contact 
with the liquid 2 whatever the inclination of the said container may be 
with respect to the vertical. 
In the example illustrated in FIG. 1, each electrode 5a to 5c is bent at 50 
in a central zone of its length and these electrodes are each disposed so 
that their bend 50 constitutes the point at which they are nearest to the 
other electrodes. The maximum level of the liquid 2 in the container 1 is 
arranged slightly above the bends 50 in the vertical position of the axis 
of the said container. Once closed by the plug 3, the container 1 is 
immersed in a block of resin 6 constituting a protection for the sensor 
against shock. Advantageously, this resin is of the epoxy type. 
As the diagram of FIG. 2 shows, one of the electrodes 5a is connected to 
earth. A second electrode 5b is connected through an isolating capacitor 7 
to a generator 8 of alternating voltage at high frequency, typically 
between some hundreds of Hertz and some tens of Kilo-Hertz inclusive. The 
generator 8 is supplied from an independent supply source (+V) and it 
comprises a zener diode 8a mounted in parallel with the output from the 
said generator 8. This diode ensures that the amplitude of the signal 
produced by the generator 8 remains constant whatever fluctuations there 
may be in the supply voltage +v from the source (+V). The capacitor 7, is 
of the non-electro-chemical type, that is to say a non-leakage type; the 
value of which is typically between 10 and 1000 nF inclusive, permits the 
prevention of a direct current causing the production of electrolysis 
effects in the sensor 10. 
The alternating voltage taken from the third electrode 5c is demodulated in 
a circuit 9 which, in the illustrated example, is a diode pumping circuit, 
an isolating capacitor 11 also of the non-electro-chemical type, being 
interposed between the elements 5c and 9 for the purpose of preventing 
possible electrolysis effects within the sensor 10. The capacitors 7 and 
11 must provide a very low impedance with respect to that of the sensor 
10; advantageously, these capacitors are of the metallised film type, for 
example with a paper or plastics dielectric. 
The variations in the output voltage from the circuit 9 are amplified by an 
amplifier circuit 12 and filtered by a band pass filter. 
For these purposes, the circuit 12 comprises a differential amplifier 12a 
the positive input to which is connected to the output from the circuit 9; 
a capacitor 12b and a resistor 12c are connected in parallel between the 
negative input to the amplifier 12a and the output from the latter. A 
resistance bridge 12e, supplied by the voltage source (+V), ensures the 
correct polarity at the positive input to the amplifier 12a. The circuit 9 
comprises a capacitor 9a and a resistor 9b mounted in parallel between 
earth and the cathode of the downstream diode 9d, as well as an output 
series capacitor 9c. The elements 12b and 12c provide a low-pass filter 
whilst the capacitor 9c in co-operation with the resistance bridge 12e of 
the circuit 12 provide a high-pass filter. The value of these components 
12b, 12c, 9c and 12e are calculated so as to control the band width of the 
filter to frequency values corresponding to the type of movement which it 
is desired to detect by means of the sensor 10. A resistor 12d connecting 
the negative input to the amplifier 12a to earth, enables the gain of the 
amplifier circuit 12 to be determined. 
The output signal from the circuit 12 is rectified in a circuit 13; then, 
the value of the signal produced by the circuit 13 is compared in a 
comparator circuit 14 with an adjustable reference voltage produced by a 
voltage divider 15 supplied by the source (+V). The comparator 14 
comprises an operational amplifier 14a connected as an open loop amplifier 
and supplied by a source (+V). At its inverting input, the amplifier 14a 
receives the signal produced by the circuit 13. Thus, at its output 14b, 
it provides a zero voltage if the output voltage from the circuit 13 is 
higher than the reference voltage, and a constant voltage equal to the 
value +v of the independent voltage source (+V) appears at the output 14a 
if the output voltage from the circuit 13 is lower than the reference 
voltage. 
Through the capacitor 7, the generator 8 applies to the electrode 5b, an 
alternating voltage signal of square waveform A, the diagram of which is 
represented in FIG. 3. 
This voltage signal A is amplitude modulated by the movements of the liquid 
2 within the sensor 10, such that a voltage signal B, which is represented 
by the waveform shown in FIG. 4, is applied to the electrode 5c. 
The signal B is rectified and demodulated in the circuit 9 and this 
demodulated signal is filtered by the high-pass filter formed by the 
elements 9c and 12e described above. This signal is also filtered whilst 
being amplified by the amplifier 12a, 12b, 12c which acts as a low-pass 
filter only amplifying the components of the input signal, the frequency 
of which is lower than a cutoff frequency determined by the value of the 
components 12b and 12c. FIG. 5 illustrates the output signal C from the 
circuit 12. It can be seen that the components B.sub.1 and B.sub.2 of the 
envelope E of the modulated signal B have not been amplified at all, 
although the component B.sub.3 and the component B.sub.4, the sizes of 
which are clearly greater than those of the components B.sub.1 and 
B.sub.2, have been amplified considerably by the circuit 12. 
The circuit 13 comprises a diode pumping circuit, the capacitor 13a of 
which is associated with a resistor 13b discharging the said capacitor to 
earth. 
A capacitor 13c is connected in series at the input to this diode pumping 
circuit. This capacitor 13c eliminates the minimum value Ca from the 
signal C. The circuit 13 is of similar design to a circuit providing at 
its output, a peak to peak measurement of an input voltage signal. 
However, the resistor 13b is chosen sufficiently low for the output signal 
D from the circuit 13, as represented in FIG. 6, to decrease substantially 
between the occurrence of two consecutive components C.sub.1 and C.sub.2 
of the signal C. This rate of decrease is chosen at a value permitting 
initiation of the triggering action of the comparator at the end of the 
period of absence of the component of the signal C, a period of which the 
duration is longer the greater is the amplitude of the last detected 
component of the said signal C. 
The circuit 16 is a relaxation oscillator with a cyclic ratio differing 
from "one"; to this end, it comprises an operational amplifier 16a to 
which are connected positive feedback resistors 16b, a resistor 16c 
connecting the positive input to the amplifier 16a to the source (+V). A 
capacitor 16d connects the negative input to the amplifier 16a to earth. 
Two diodes 16e and 16f are connected in parallel in opposite senses and in 
reverse feedback to the amplifier 16a and an adjustable resistor 16g and 
16h is mounted in series with each of the diodes 16e and 16f respectively. 
This arrangement of the elements 16e to 16h enables an independent control 
of the duration of the high and of the duration of the low output signal 
from the circuit 16 to be effected (only being capable of receiving the 
values +v or zero volts) 
When the output from the circuit 14 is at the high level as defined above, 
a current is injected through the diode 18 into the negating input to the 
amplifier 16a and, due to this, forces the circuit 16 to the low level, 
preventing it from oscillating. 
Thus, when the liquid 2 is in motion, the alternating voltage collected by 
the third electrode 5c varies in amplitude, which gives rise to a signal 
after demodulation in the circuit 9 and amplification in the circuit 12; 
the latter circuit only amplifies those portions of the signal the 
frequency of which is to be found within a particular frequency band 
defined at the lower end by the cut-off frequency of the high-pass filter 
9c, 12e and at the upper end by the cut-off frequency of the low-pass 
filter 12b, 12c. 
The voltage of the signal is measured crest to crest in the circuit 13 and 
this measurement is compared in the circuit 14 with a reference the 
threshold of which can be regulated in accordance with the type of motion 
it is desired to detect. Thus the output of the comparator circuit 14 
comprises a container movement signal indicative of movement of the liquid 
in the container, which occurs within a particular frequency range, the 
signal more particularly being indicative of whether successive peaks of 
the demodulated signal exceed a threshold level and lie within a 
particular time range from one another. 
The resonant frequency of the sensor 10 depends on the dimensions of the 
container 1, 3; its attenuation depends on the viscosity of the liquid 2. 
Thus, it is possible to provide a sensor which responds to the needs of a 
paarticular application. 
Thus, the sensor 10 is of the type providing a signal varying continuously 
as a function of the change in position of the container 1,3. The circuits 
9 and 12 analyse the movements of the sensor 10 and only retain those 
which are provided at frequencies within the band pass defined above. 
Thus, a signal is present at the output 14a from the circuit 14 when the 
sensor 10 is subject to movements satisfying determined characteristics of 
frequency and amplitude. 
The output from the circuit 14 is connected to the control input to a 
cyclic generator 16 supplied from an independent source (+V), the 
generator periodically producing a pulse. These periodic pulses control 
the operation of a radio transmitter 17. These pulses are inhibited when 
the comparator 14 indicates an absence of movement. It has been noted that 
the essence of normal movements of a person gives rise to a signal TBF the 
frequency of which is between 0.1 and 20 Hz inclusive and preferably 
between 0.5 and 5 Hz so that, in the case where the detector is used for 
the supervision of persons by radio, the band pass of the apparatus is 
advantageously controlled to that frequency band by a suitable choice of 
components 9c, 12e, 12b and 12c. 
The circuits 8, 12, 14 and 16 each make use of an operational amplifier. 
These four operational amplifiers may be located in one and the same 
integrated circuit (for example a circuit known under the commercial name 
LM 324). 
According to a non-limiting example, the liquid 2 which is used is a 
mixture of water and of ethanol containing a salt in solution such as 
copper sulphate in a ratio of 1/1000 by weight. The addition of a glucide 
such as glycerol, saccharose, etc., or of a similar substance enables the 
viscosity of the liquid to be increased as desired so as to widen the band 
pass of the sensor 10 whilst attenuating it. As a salt, copper 
sulphate--or another copper salt--is chosen in the case where the 
electrodes 5a, 5b and 5c are of copper because, in this case, an 
electrolysis does not produce liberation of gas risking causing explosion 
of the container 1,3. 
Typically, the resistivity of the liquid 2 is between 1K and 100K.OMEGA./cm 
inclusive, its viscosity is between 10.sup.-3 and 1000 poises inclusive 
and the volume of the interior of the container 1,3 is of the order of 0.1 
to 200 cm3. 
Apart from its application to the supervision of persons, the detector 
which has just been described may also be used to detect movements of the 
ground for example in seismology, soil mechanics, etc. It may also be used 
in anti-theft devices. 
In the case of application to the supervision of a person, the detector is 
carried by the person and may thus detect the movements of the said 
person. 
An anti-gel of some kind in solution in water, could be used instead of 
ethanol. 
The radio transmitter 17 may be replaced by an audio signal generator such 
as a siren or by a luminous signal emitter such as an electric lamp.