A proximity detector comprises a parallel oscillatory circuit the inductance of which is influenced by the metallic element of which it is desired to ascertain the distance, this oscillatory circuit forming part of an oscillator. The oscillator is fed with constant current; current is injected into the oscillatory circuit through a negligibly small resistance; and the output signal is taken off at the output terminals of the oscillator; the device is used to provide an analog signal which is a linear function of distance, in order to classify articles or to measure their speed of approach.

BACKGROUND OF THE INVENTION 
The invention relates to proximity detectors of the analog inductive type 
comprising a parallel oscillating circuit the coefficient of quality 
factor and the inherent impedance of which vary as a function of the 
distance of a metal article which influences the coil and an amplifying 
means, the circuit being arranged to inject the output current of the 
amplifying means into said oscillating circuit through a resistance, said 
oscillating circuit forming part of a feedback circuit between the output 
and the input of the amplifying means. 
These analog proximity detectors are capable of many industrial uses, such 
as the classifying of articles by their dimensions or by their position, 
measuring the speed of their approach, the indication of their direction 
of displacement, and others. 
Such applications are suitable for distances which generally cannot exceed 
several centimeters and it would be desirable, at this scale of distances, 
to generate an analog signal which is as linear a function as possible of 
the distance. 
The known circuits do not obtain a satisfactory linearity, in particular 
from the fact that they are fed at constant voltage and that the output 
signal is derived from the high frequency oscillation itself, which 
introduces a disturbance in the operation of the oscillatory circuit by 
applying an impedance at its terminals. 
OBJECT OF THE INVENTION 
The object of the present invention is to provide a circuit which is free 
from these inconveniences, and which is principally characterised in that 
the oscillator is fed with current which is substantially constant, and 
that the said resistance has a sufficiently low value to permit the 
maintaining of the oscillations as soon as the article to be detected 
ceases to be in the immediate proximity of the elements of the oscillatory 
circuit, and that the analog output signal is taken off between the input 
feed terminal of the oscillator and the ground terminal of the oscillatory 
circuit. 
The various advantages, as well as other features of the invention, will 
appear clearly from the following description.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 there is shown an oscillatory circuit constituted by two 
inductances 1 and 2 mounted in a ferrite pot and mutually coupled. The 
inductance 1 of value L.sub.1 forms, with a condenser of value C.sub.1 
connected in parallel, a resonant circuit in which is developed a high 
frequency oscillation, provided that the losses of energy are compensated 
by the injection of a current, through a resistance 4 of value R.sub.E. 
This current is provided by a transistor 5, for example of the NPN type, 
fed by a constant current generator 6. The terminal for feeding of the 
circuit with a d-c voltage +V has been shown at 61. 
A resistance 7 of value R3 is mounted between the base and the collector of 
the transistor 5, whilst a condenser 8 is mounted, in series with the 
inductance 2 (of value L.sub.2), between the base and the common point of 
the resistance 4 and the resonant circuit. A condenser 9 is mounted in 
parallel with the terminals of the oscillator thus constituted, that is to 
say between the collector of the transistor 5 and the ground point of the 
resonant circuit. 
##EQU1## 
impedance of the circuit oscillating at the inherent frequency .omega., 
Q.sub.1 being its quality factor in the absence of an adjacent metallic 
element. In the presence of a metallic element at the distance D from the 
pot, Q.sub.1 will assume a value Q(D) and Z.sub.1 will assume a value 
Z(D). 
It can be shown that the limit condition for maintenance of the 
oscillations is given by the relationship: R.sub.E .perspectiveto.Z(D)/2N, 
N being the ratio, relatively high, of the number of turns of the windings 
1 and 2 (which are wound such that the current re-injected into 2 shall be 
in phase with the output HF signal of the transistor). 
In practice, there is given to R.sub.E a very small value, so that the 
condition of maintenance of the oscillations is achieved when the values 
Q(D) or Z(D) are themselves very small. 
In fact this happens when the metal element which is to be detected is 
itself at a very small distance from the pot. When the element is very 
remote, Q(D) or Z(D) achieve their inherent values Q.sub.1 and Z.sub.1 
respectively, which are respectively greater than Q.sub.(0) and Z.sub.(0) 
; in short, the oscillation is maintained no matter what D is. 
The peak voltage Vc of the HF signal is equal to 
##EQU2## 
I being the constant current provided by the generator 6. 
The d-c voltage Vosc at the terminals of the oscillator, filtered through 
the condenser 9, is little different from V.sub.C (the transistor being 
practically saturated, its voltage between collector and emitter is 
practically nil when the value of R.sub.E is very small). 
Finally it results that the d-c voltage Vosc is a quasi-linear function of 
the distance D, in a range of distances less than 50 mm. for example. 
This voltage is amplified by a transistor 10, for example of the NPN type, 
the emitter of which is coupled to ground through a resistance 11 of value 
R. The current Is which passes through this transistor, measured by an 
amperemeter 12, is substantially equal to the ratio V.sub.R /R between the 
voltage at the terminals of the resistance 11 and the value R of this 
resistance. 
Thus, one finally has: 
##EQU3## 
V.sub.B E being the voltage between base and emitter of the transistor 10. 
(This latter being with high gain, its base current is negligible). The 
current I.sub.s is thus a quasi-linear function of the distance D. 
If it is desired to have an analogous signal in voltage, it is only 
necessary to replace the amperemeter 12 by a resistance 120 of value 
R.sub.s shown in broken line. The voltage V.sub.s at the terminals of this 
resistance will then be substantially equal to 
##EQU4## 
that is to say a quasi-linear function of D. 
In reality, in the circuit which has just been described, the curve 
Vosc=f(D) presents the two defects of linearity illustrated in FIG. 2. 
The defect of linearity in the region of D=0 is advantageously corrected by 
replacing the constant current generator 6 by a resistance 60 (shown in 
broken line in FIG. 1), of value R.sub.60 large as compared with the 
variable impedance Zosc between the terminals of the oscillator circuit. 
One thus has 
##EQU5## 
In other words Vosc is then a hyperbolic function of Zosc which tends 
asymptotically towards the feed voltage V when Zosc increases and presents 
a curvature, in the region of Z=0, inverse to that of the curve of FIG. 2. 
One can thus, by a suitable choice of R.sub.60, correct the defect of 
linearity of Vosc as a function of D in the region of D=0. This correction 
has on the other hand the effect of worsening the defect of linearity for 
the greater values required for D. This latter defect of linearity will be 
advantageously corrected by disposing, as shown in FIG. 1, in parallel 
with the resistance 11, a Zener diode 13 (or other non-linear threshold 
device) in series with a resistance 14 of value R.sub.14. 
For small values of D, the Zener diode remains blocked and I.sub.s is not 
modified by its presence. For the relatively high values of D, the value R 
is replaced by 
##EQU6## 
which increases the slope of increase of I as a function of distance. One 
can thus correct the defect of linearity which is present for the 
relatively high values of D. 
It will be noted that the oscillator is not influenced by the amplifier 
stage 5, which has a high input impedance. The output signal is taken off 
between the constant current feed terminal of the oscillator, and ground, 
and not from the high frequency signal. 
As the losses of the resonant circuit increase with temperature and, as a 
result, Vosc has a tendency to decrease when the temperature increases, 
but the amplifier 10 has on the contrary a voltage V.sub.BE which reduces 
and a gain which increases when the temperature increases, a compensation 
between the two opposed influences of temperature on the oscillator stage 
and on the amplifier stage can be obtained by a suitable choice of the 
parameters which define them. 
In FIG. 3, there are to be seen the same components as in FIG. 1, 
designated by the same references. It will be seen that the emitter of the 
transistor 5 is coupled directly to the inductance 1, that is to say 
R.sub.E =0. The condenser 3 is coupled to the live end of the winding 2 
instead of being coupled, as in FIG. 1, to the live point of the winding 
1. The feed V+ takes place across a diode 15 intended to prevent reversing 
of polarity in the case of error of connecting up, and the return to 
ground takes place across the resistance 60 and a ballast constituted by a 
PNP transistor the base voltage of which is fixed by a Zener diode 17, and 
of which the base is connected to the collector by a resistance 18. 
The voltage Vosc is amplified by a "Darlington" circuit constituted by two 
transistors 19 and 20 of the PNP type; the point common to the emitter of 
the transistor 19 and the base of the transistor 20 is coupled to the feed 
line by a resistance 21 choked by a condenser 22. The resistance 21 
permits draining away of the leakage currents which may exist when the 
transistor 19 is blocked. The current I.sub.s passes through the 
transistor 20 and a resistance 11 in series. The Zener diode 13 and the 
resistance 14, in parallel with the resistance 11, play the same part as 
the components having the same references in FIG. 1. 
It will be apparent that various modifications of detail could be made in 
the circuits described and shown, without exceeding the scope of the 
invention.