Measuring arrangement for capacitive security fences

A plurality of transmitters operating at respective frequencies is connected to at least some electrodes of a capacitive security fence. Each electrode is provided with a transformer whose primary winding is connected between the transmitter and ground. The secondary of each transformer is connected to a plurality of bandpass filters having adjustable center frequencies which correspond to the transmitter frequencies. The voltage available at the outputs of the bandpass filters are fed to a measuring device and supplied to a microcomputer. The interelectrode capacitances and the self-capacitances of the electrodes of the system are identified from the measured voltages which are proportional to the capacitances.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a measuring arrangement for capacitive 
security fences having a plurality of electrodes for identifying the 
individual sub-capacities between two respective electrodes and the 
self-capacitance sub-capacity between a respective electrode and ground, 
in which alarm or interference criteria are derived in a central 
evaluation device from the identified capacitance values and alarm or 
interference reports are displayed. 
2. Description of the Prior Art 
In order to reliably secure an object, the surroundings are frequently 
secured, for example, with a security fence, in addition to building 
security. Unauthorized penetration is to be prevented, or at least made 
more difficult. In order to perceive unauthorized penetration, capacitive 
security fences are constructed which trigger an alarm given the approach 
or given the penetration of an intruder. Objects particularly in need of 
surveillance and subject to jeopardy, for example, nuclear power plants or 
military installations, require an extremely reliable ambient security 
system. 
Such ambient security systems, particularly open-air systems, are subject 
to certain disruptive influences so that the susceptibility to disruption 
is relatively high and false alarms are frequently triggered. Ambient 
security systems must therefore be constructed such that the effects to be 
indicated reliably lead to an alarm, but disruptions do not. Usually, the 
interelectrode capacitances between transmission and reception electrodes 
are measured and the resulting difference of the interelectrode 
capacitance is evaluated by way of a differential bridge (for example as 
disclosed in the German Letters Pat. No. 1,220,298, fully incorporated 
herein by this reference). Symmetrically occurring environmental 
influences can thereby be eliminated. An alarm is derived from the 
resulting differential capacitance. An alarm is therefore triggered either 
due to a discontinuous change of capacitance of a specific size or due to 
a steady change of capacitance having a defined rate of change. These 
relatively simple alarm criteria, however, also produce low protection 
against interference. In order to be able to reliably perceive an 
intruder, the response sensitivity of such a system can be increased. 
This, however, also generally means that the susceptibility to 
interference also becomes greater, i.e. the immunity from interference is 
reduced. 
It is likewise been proposed to respectively measure the individual 
interelectrode capacitances between the electrodes and/or the 
self-capacitances between an electrode and ground, to store and evaluate 
the time curves of the individual capacitance changes and to derive an 
alarm criterion therefrom, as disclosed in the German patent application 
No. P 31 10 352.9. Such a method, however, has the disadvantage that a 
multitude of involved transfer devices must be provided in the proximity 
of the electrodes and that relatively high potentials, for example, 100 
volts, must be constantly connected to and disconnected from the 
individual electrodes. Moreover, only one interelectrode capacitance or, 
respectively, self-capacitance, can be measured at a time. This means a 
longer time until all capacitances are measured and are available for 
evaluation. Since the constant transfer causes certain transient problems, 
this time is further increased. 
SUMMARY OF THE INVENTION 
The object of the present invention, therefore, is to provide a measuring 
arrangement for identifying the individual interelectrode capacitances 
and/or self-capacitances at a capacitive security fence with which all 
interelectrode capacitances of an electrode system can be measured and the 
respective self-capacitance can be identified by way of a sum measurement. 
Thereby, all interelectrode capacitances should be capable of being 
measured simultaneously. The self-capacitance of the respective electrode 
should be derivable from the measured values of the interelectrode 
capacitances. 
The above object is achieved, and a system of the type generally set forth 
above, in that a respective transmitter operating at a respective 
difference frequency is connected to the sum of the electrodes, in that 
each electrode has a transformer primary connected between the transmitter 
and ground, in that each transformer secondary is connected to a plurality 
of band-pass filters having adjustable center frequencies corresponding to 
the frequencies of the plurality of transmitters, in that all band-pass 
filters are connected to a measuring device with which the voltages for 
all center frequencies at all band-pass filters are measured, and in that 
all interelectrode capacitances and self-capacitances are identified from 
the measured voltages proportional to the capacitances, these being 
identified with a microcomputer following the band-pass filters. 
The measuring arrangement, according to the present invention, therefore 
comprises a transmitter for each electrode and a transformer. Each 
transmitter operates at a different frequency. The secondary of each 
transformer is connected to as many band-pass filters as there are 
transmitters, their center frequencies corresponding to the transmitter 
frequencies. Given n electrodes and n transmitters, therefore, n times n 
band-filters are required. The band-pass filters are connected to a 
measuring device which measures the voltage available at the output of the 
respective band-pass filter. These voltages are proportional to the 
interelectrode capacitances or, respectively, to the sum of the 
interelectrode capacitances and self-capacitances of the electrode system. 
Given this arrangement, the transfer devices at the electrodes are 
eliminated, these being necessary when a respective transmitter is 
connected to all electrodes once and the remaining electrodes are 
connected as receiving electrodes. The high expense for filters is 
justified given a measuring arrangement constructed in accordance with the 
invention, since all capacitances can be simultaneously measured or, 
respectively, identified. The filters, further, need not be disposed in 
the direct proximity of the electrodes. They are advantageously disposed 
at the evaluation device. It is therefore advantageous to design the 
measuring arrangement as a part of a microcomputer in which the 
interelectrode capacitances or, respectively, self-capacitances and 
interelectrode capacitances can be stored for further processing. 
Since all interelectrode capacitances and self-capacitances need not be 
measured and evaluated in many instances in order to recognize an 
intruder, fewer transmitters than electrodes can be provided. The number 
of filters is thereby likewise reduced. 
In accordance with an advantageous feature of the invention, the measuring 
arrangement operates in a multiplex mode. A first switching network is 
provided between the transformers and the band-pass filters; the plurality 
of band-pass filters is thereby reduced by the factor n. Therefore, the 
number of required band-pass filters is reduced to the number of connected 
transmitters. Given, for example, four electrodes with four assigned 
transmitters, the four transformers are connectible to the four band-pass 
filters with the first switching network. Given four band-pass filters, 
four switching steps are required in order to connect each transformer 
once to each filter. Their respective voltages are thereby measured and 
the corresponding capacitance values are identified in the microcomputer. 
A second switching network can be similarly provided between the band-pass 
filters and analog/digital converters receiving the computer in order to 
therefore reduce the number of analog/digital converters of the filters to 
n in number. 
A crystal-controlled master generator is thereby advantageously provided 
for generating the various transmission frequencies, the master generator 
being controlled by way of a divider, for example, a Siemens AG SAB 8253, 
preceding the computer. The i band-pass filters will likewise be realized 
via a phase-sensitive rectifier circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The electrodes E1--En are schematically illustrated in FIG. 1 for a 
security fence. Each electrode is connected to a transmitter SEN1--SENi, 
so that i=n. Each transmitter generates a frequency which differs from the 
frequency of the other transmitters (F1--Fi). A respective transformer 
U1--Ui is connected between a respective transmitter and ground. 
Respective band-pass filters BF1--BFi are connected in parallel to the 
secondaries of the transformers. The center frequencies F01--F0i 
correspond to the respective transmitter frequencies F1--Fi. The 
respective measuring voltage (U11--Uni) is identified at each band-pass 
filter (BF1--BFi) with a measuring device ME. These voltages are 
proportional to the capacitances of the electrode system. The capacitances 
are identified in a microcomputer MC which is connected to the measuring 
device ME. 
Given such a measuring arrangement, all electrodes are transmission 
electrodes. In the voltage measurement after the band-pass filters, 
however, only the electrode, for example the electrode E1, to which a 
band-pass filter whose center frequency is identical to the transmission 
frequency of the electrode assigned thereto is effective as a transmission 
electrode. In this case, the transmitter SEN1 is connected to the 
electrode E1 so that the voltage U11 at the output of the band-pass filter 
BF1 is in a specific relationship to the current I which flows to the 
electrode E1. The measuring voltage U11 is proportional to the 
capacitances C.sub.E1 +.sub.r.sup.n =2.multidot.C.sub.1i, the voltage U12 
is proportional to the capacitance C.sub.12 and the voltage U1i is 
proportional to the capacitance C.sub.1n. 
Given sufficiently low output impedances of the transmitters SEN and 
sufficiently low impedances of the transformers U, the electrodes act as 
though grounded and, therefore, act as receiving electrodes. With this 
measuring arrangement, each electrode is a transmission electrode once and 
the remaining electrodes are receiving electrodes without their being a 
requirement for a constant transfer of the electrodes from one condition 
to the other. Moreover, all interelectrode capacitances and 
self-capacitances can be determined in this manner from the 
simultaneously-measured voltages. 
For the sake of simplicity, it will be assumed that the measuring voltages 
and the proportional capacitances are such that there are three electrodes 
n=3 and three transmitters i=3 as illustrated in FIG. 2. Respectively 
three band-pass filters BF1--BF3 are assigned to each transformer U1--U3, 
i.e. a total of nine band-pass filters BF11--BF33. Nine voltages U11--U33 
are accordingly measured. The voltage U11 measured at the band-pass filter 
BF11 with the center frequency F01 is proportional to the sum of the self 
capacitance CE.sub.1 plus the interelectrode capacitance C.sub.12 of the 
electrode E1 relative to the electrode E2 plus the interelectrode 
capacitance C.sub.13 of the electrode E1 relative to the electrode E3. The 
voltage U12 at the output of the band-pass filter BF12 is proportional to 
the interelectrode capacitance C.sub.21 between the electrodes E2 and E1 
and the voltage U13 available at the output of the band-pass filter BF13 
is proportional to the interelectrode capacitance C.sub.31 between the 
electrodes E3 and E1. 
The analogous case applies to the voltages at the other band-pass filters, 
as can be seen from FIG. 2. Therefore, for example, the voltage U21 at the 
output of the band-pass filter BF21 is proportional to the interelectrode 
capacitance C.sub.12. As the result of differential measuring paths, the 
interelectrode capacitance C.sub.12 and the interelectrode capacitance 
C.sub.21 can be determined in this manner, these necessarily being of the 
same size (C.sub.21 =C.sub.12) An additional control criterion for 
evaluating the changes of capacitance at the capacitive security fence 
derives therefrom. 
A measuring arrangement which employs two switching networks is illustrated 
in block form in FIG. 3. As viewed from bottom to top, the electrodes 
E1--En are illustrated at the bottom, these being connected to the 
transmitters SEN1--SENi having the respective frequencies F1--Fi. The 
transformers U1--Ui have their primaries connected between the 
transmitters and ground. The secondaries of the transformers U1--Ui are 
connected to band-pass filters BF1--BFi by way of a first switching 
network SNW1, the band-pass filters being provided only once for all 
electrodes. The band-pass filters BF1--BFi are connected by way of a 
second switching network SNW2 to an analog/digital converter ADW, which 
includes a plurality of analog/digital converters, which is connected to a 
microcomputer MC. Given, for example, four electrodes E1--E4 with four 
transmitters SEN1--SEN4, the switching network SNW1 connects the four 
transformers U1--U4 to the four band-pass filters BF1--BF4 in a first 
switching step so that the transformer U1 is connected to the band-pass 
filter BF1, the transformer U2 is connected to the band-pass filter BF2, 
etc. In a second switching step, the transformer U1 is connected to the 
band-pass filter BF4, the transformer U2 is connected to the band-pass 
filter BF1, the transformer U3 is connected to the band-pass filter BF2 
and the transformer U4 is connected to the band-pass filter BF3. Given 
four transmission electrodes, each transformer is connected to each 
band-pass filter once in four switching steps. 
A similar switching network SNW2 is necessary when the number of 
analog/digital converters provided between the band-pass filters BF1--BFi 
and the microcomputer MC is not greater than the number of connected 
transmitters. The plurality of analog/digital converters is also reduced 
by the factor n here in a multiplex mode. 
In FIG. 4, the electrodes E1--Ei are operated as transmitting electrodes, 
i.e. they are respectively connected to a transmitter SEN1--SENi and, over 
the primary of a respective transformer U1--Ui, are connected to ground. 
The remaining electrodes E(i+1)--En are connected directly to ground over 
a respective primary winding of the transformers U(i+1)--Un. The 
secondaries of all transformers are connected to the band-pass filters 
BF11--BFni or, given a multiplex mode, are connected via a switching 
network SNW1 to the band-pass filters BF11--BFi. The outputs of the 
bandpass filters BF are connected to the measuring device ME and to the 
microcomputer MC. The band-pass filters BF, the measuring device ME and 
the microcomputer MC in FIG. 4 are illustrated as a processing unit VE. 
Although I have described my invention by reference to particular 
illustrative embodiments thereof, many changes and modifications of the 
invention may become apparent to those skilled in the art without 
departing from the spirit and scope of the invention. I therefore intend 
to include within the patent warranted hereon all such changes and 
modifications as may reasonably and properly be included within the scope 
of my contribution to the art.