Electronic apparatus for detecting stray variations in an electrical voltage as a function of time

An electronic apparatus is proposed for detecting variations of significance in an electrical voltage as a function of time. The apparatus includes a tracker (2) of these variations, a comparator (6) comparing the voltage with a reference voltage V.sub.2, and a counting circuit (10, 15, 20). The tracker (2) monitors variations in the voltage supplied to a filtering and rectification system (4), which eliminates the continuous component of the signal and within the high frequency range greatly diminishes the levels of the parasitic signals having an oscillatory character. The comparator (6) furnishes a calibrated pulse (25) each time the voltage exceeds the threshold V.sub.2, and the counting circuit (10, 15, 20) counts the number of these calibrated pulses within a given time period T and furnishes a logic signal (30) if this number exceeds a predetermined quantity.

FIELD OF THE INVENTION 
The present invention relates to the examination of electrical signals 
which are variable with time, and more particularly to detecting stray, 
localized variations in an electrical voltage which is variable with time. 
BACKGROUND OF THE INVENTION 
In particular, but in a non-limiting manner, the invention relates to 
monitoring signals emitted by certain tracers used in robot systems for 
checking the dimensions of metal parts of various shapes. It relates 
especially and quite directly to zero-contact-pressure tracers or feelers, 
that is, those in which there is no physical contact with the part to be 
checked, such as the tracer for which French patent application No. 80 
25457 was filed on Dec. 1, 1981 by the Commissariat a l'Energie Atomique 
[French Atomic Energy Commission]. 
The electrical signal emitted by a tracer of this type is a direct current 
or alternating current voltage which at certain instants can vary rapidly 
about its nominal value and can exhibit significant phenomena which need 
to be reliably detected in the presence of the influence of stray 
electricity which may be present. 
In particular, the electrical voltage established in robot systems between 
a tracer and the part to be tested undergoes rapid high frequency 
variations when the tracer approaches quite near the position of contact 
with the part. These variations, in turn, translate into major variations 
in the electrical field between the tracer and the part, which are 
characteristic of the approach toward the part; it is these variations 
that need to be precisely and reliably detected by isolating them, 
especially from any background or stray noise. 
SUMMARY OF THE INVENTION 
Accordingly, the subject of the present invention is an electronic 
apparatus for detecting stray variations in an electrical voltage as a 
function of time, which is especially adapted for solving the 
above-addressed problem. 
This apparatus is characterized in that it includes the following: 
a tracker of the variations in this voltage supplying a filtering and 
rectification system which eliminates the continuous component of the 
signal and greatly diminishes the levels, in the high frequency range, of 
the stray signals having an oscillatory nature; 
a comparator of this voltage, which has thus been filtered and rectified, 
with a reference step voltage V.sub.2, the comparator furnishing a 
calibrated pulse each time this voltage exceeds the threshold voltage 
V.sub.2 ; and 
a circuit which counts the number of these calibrated pulsed within a given 
time period T, the counting circuit furnishing a logic signal if this 
number exceeds a predetermined quantity. 
According to a particularly important embodiment of the present invention, 
the electronic detection apparatus includes a counting circuit which 
comprises an AND gate, which at its two inputs receives the signal of the 
comparator output and the signal constituting a measurement window of time 
duration T, respectively, and the output of which is applied to a counter 
which, upon reaching a count of N, outputs a signal to a bistable 
multivibrator, which changes its state when the electrical voltage to be 
monitored exhibits stray variations. 
The invention will be better understood from the ensuing description of a 
non-limiting exemplary embodiment of the invention, taken in conjunction 
with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, the input 1 is shown to which the direct current or alternating 
current electrical voltage is applied, this voltage varying as a function 
of time, and of which the rapid variations that may occur about its 
nominal value are to be monitored. In accordance with the invention, the 
apparatus includes a tracker 2 of the variations in this voltage, which in 
the present example comprises an operational amplifier of high input 
impedance arranged as a tracker. The tracker 2 then sends the signal via 
the line 3 to a device 4 which includes a filter and a rectifier, the 
output 5 of which is applied to the first input of a comparator 6. The 
second input 7 of the comparator is supplied by a source 8 reference 
voltage V.sub.2. The operation of this first portion of the electronic 
apparatus which is the subject of the invention is as follows: The circuit 
4 includes a bandpass filter, which suppresses the continuous component of 
the signal and rectifies it in accordance with either the positive or the 
negative alternation of the signal. The comparator 6, via its output 9, 
furnishes calibrated pulses each time the voltage on the line 5 exceeds 
the voltage threshold V.sub.2. 
The remainder of the diagram in FIG. 1 relates to the circuit for counting 
the number of these calibrated pulses which are located within a 
measurement time window of a given duration. To this end, it includes an 
AND gate 10 which at its input 11 receives the output signal of the 
comparator 6 which is present on the line 9, and at its input 12 receives 
the signal from a timing circuit 13 which constitutes the measurement time 
window having a duration T. The output 14 of the gate 10 is applied to a 
count-to-N counter 15, the output 16 of which is applied in turn to a 
bistable multivibrator 17, the output of which is identified by reference 
numeral 18. 
A coupling line 19 joins the input 11 of the AND gate 10 to the circuit 13; 
two control lines for resetting the counter and the multivibrator to zero 
are also shown at 20 and 21. 
The operation of this counting circuit and of the electronic detection 
apparatus which is the subject of the invention will now be explained, 
referring to FIG. 2. 
In FIG. 2a, the input signal voltage V.sub.0 is shown, as it is introduced 
via the input 1 of the tracker 2. This signal includes a signal 31 having 
a low-frequency mean value, which includes noise 22, parasitic oscillation 
signals 22a and significant phenomena 23 comprising a packet of high 
frequency oscillations which are to be identified. 
At the output 5 of the filter and rectifier 4, the signal V.sub.1 takes the 
course shown in FIG. 2b, where the mean low-frequency component 31 is 
reduced to a continuous amplitude and where the noise 22 and the 
significant phenomena 23 are rectified such that they are all located on 
the same side of the low-frequency component 31. Also shown in FIG. 2b, at 
24, is the voltage threshold V.sub.2 corresponding to the reference 
voltage above which the irregularities of the signal to be monitored are 
taken into consideration. At the output 9 of the comparator 6, the signals 
of FIG. 2c are present, for which calibrated pulses, such as 25, 
correspond to each instance when the threshold V.sub.2 is exceeded by the 
input voltage that is to be monitored. In the example described herein, 
the significant phenomenon 23 is split up into seven peaks, which in the 
signal of FIG. 2c correspond to seven of these signals 25, which are 
labeled 1-7 in FIG. 2c. 
Each time a pulse 25 appears, the timing circuit 13, which receives this 
information via the line 19, sends a window, in the form of a voltage step 
of duration T such as those seen at 26, 27 and 28 in FIG. 2d, to the input 
of the AND gate 10 via the line 12. The count-to-N counter 15, which in 
the present example is conceived of as being set such that it provides an 
output signal pulse 29 whenever N attains or exceeds the number of 6, 
releases a pulse 29 via the line 16 whenever one of the windows 26, 27 or 
28 contains more than six calibrated pulses. This is precisely the case 
for the window 28, and the counter 15 then furnishes a pulse 29 over the 
line 16 which reveals the appearance of one of the significant phenomena 
which are to be monitored. To complete the apparatus, a multivibrator 17 
then converts this pulse 29 into a change of voltage state 30 which is 
present at its output on the line 18 and specifically characterizes the 
variation in the signal was to be detected. 
The electronic apparatus which has been described above makes it possible 
to detect the presence of a danger signal by monitoring the validity of 
this signal within an entity which may include various stray electrical 
signals. The apparatus eliminates slow drifts, background noise on the 
line and parasitic electricity in the signal that is to be studied. 
As noted at the outset herein, an electronic apparatus of this kind is 
particularly advantageously applicable for detecting contact between an 
electrode and a surface in the field of robot welding. Nevertheless it 
will be understood that this application is not limiting, and that the 
present invention is also applicable to other fields in which there is a 
need to process alarms or danger signals.