FR2461233A1 - FOUCAULT CURRENT DISTANCE MEASURING APPARATUS USING A POSITIVE REACTION AMPLIFIER - Google Patents

Abstract

DEVICE FOR NON-CONTACT MEASUREMENT OF THE DISTANCE BETWEEN A DETECTION COIL AND A METAL BODY FACING FACE TO FACE. A CO CONDENSER IS MOUNTED IN PARALLEL WITH COIL 3 TO FORM A RESONANT CIRCUIT; COIL 3 IS MOUNTED IN THE REACTION CIRCUIT OF A POSITIVE REACTION AMPLIFIER 4; THE AMPLIFIER RECEIVES ON ONE OF ITS INPUTS THE ALTERNATIVE OUTPUT SIGNAL OF AN OSCILLATOR AT FREQUENCY REGULATED BY VARIATION IN VOLTAGE V CO5; A PHASE 6 COMPARATOR CONTROLS THE OSCILLATOR OSCILLATION FREQUENCY BASED ON THE PHASE DIFFERENCE BETWEEN THE OSCILLATOR AND AMPLIFIER OUTPUT SIGNALS. THE FREQUENCY OF THE OSCILLATOR OUTPUT SIGNAL VARIES WITH THE IMPEDANCE OF THE DETECTION COIL SO AS TO FOLLOW THE RESONANCE FREQUENCY OF THE RESONANT CIRCUIT. APPLICATION IN INSTRUMENTATION.

Description

The invention relates to apparatus for measuring

  distance from an analyzed metal body, using the effect of eddy currents induced in this metal body. In particular, it relates to improvements made to such apparatus,

  for automatic compensation of the effect of

  temperature measurements on the measurement characteristic of

the distance from the device.

  An apparatus is known in which a reference signal with fixed frequency and amplitude is applied on

  the inverting input terminal of a different amplifier

  the output signal is applied to its boundary

  non-inverting entry via a cir-

  baked positive reaction, a sense coil being excited by the feedback signal, and the distance since

  a metal body placed in front of the coil being

  surée according to the variation of the impedance of this

  coil. This type of distance measuring device is

  so that the post-reaction amplification factor

  tif of a positive feedback amplifier is controlled

  depending on the impedance value of a coil of de-

  tection, the output voltage of the amplifier being

  to detect, as a distance measurement value, an impedance variation of the detection coil, which

  is the distance between the metal body and this

the detection coil.

  In this known apparatus, no device is provided to correct the effects of impedance variations

  of the sensing coil due to temperature variations

  ture, and there is a need for sophisticated equipment

  conceived in such a way that the effect of these temperature variations on the characteristic of measuring the distance

  metal object and coil of detection is compensated

  with great precision.

  The rate of the variations of the impedance of the

  of detection according to the variations in temperature

  varies according to the material used for the car-

  breakage of the detection coil, the shape of the

  bine, the winding mode of this coil, etc ... Furthermore, in the case where the detection coil comprises a core of ferromagnetic material such as ferrite, the variation of the permeability of the ferrite due to temperature variations follows a hysteresis curve, which makes the desired temperature compensation even more difficult when the sensing coil

has a ferrite core.

  The object of the invention is to provide an apparatus

  improved distance measurement in which are automati-

  compensated for temperature-dependent variations in the "distance" output characteristic

  of the coil used to measure this distance.

  The invention also aims at an apparatus

  measuring distance in which, in addition to the compensation

  tion, eliminates the phase variation in the feedback circuit of a positive feedback amplifier, in order to obtain a more stable apparatus and

with a simpler adjustment.

  The invention also aims at an apparatus

  distance measurement that does not require the use of

  use of a very high stability oscillator

such as a quartz oscillator.

  According to the invention, an apparatus

  an improved eddy current distance measurement grating of the type in which the alternating signal of an oscillator is applied to a detection coil

  through the feedback circuit of an amplifier

  positive feedback, the amplification factor

  post-reactive amplifier being controlled according to

  the impedance value of the sensing coil, so that the amplifier provides an output signal corresponding to the distance between the sensing coil and a metal object, the improvements provided consisting of a resonant circuit formed by connecting a capacitor in parallel with the detection coil, the oscillator of the variable frequency type, or of which

  the frequency is regulated by voltage variation,

  at the resonance frequency of the resonant circuit connected to the feedback circuit of the amplifier

  positive reaction so as to control the factor of am-

  plication after reaction, and a phase comparator

  forming an output voltage corresponding to the difference

  phase difference between the oscilla-

  transmitter and the output signal of the

  to control the oscillation frequency of the bone

  cillateur. More precisely, the oscillator is of the type

  frequency adjusted by voltage variation, and the

  phase counter with this oscillator can be replaced

  placed by a phase-locked loop integrated circuit. The positive feedback amplifier has two input terminals, an inverting terminal and a non-inverting terminal, and an output terminal, and the alternating signal provided by the variable frequency oscillator

  is applied to the inverting input terminal. A resistance

  reaction is mounted between the input terminal of

  non-inversion and the output terminal so as to constitute

  kill a positive feedback loop, and the feedback circuit

  sound is connected to the non-inverting input terminal.

  With the correct distance measuring equipment

  to the invention, even if the inductance of the coil of de-

  tection varies with temperature, frequency of reso-

  the resonant circuit is modified in response to the change in the inductance of the sense coil, and the coil drive frequency always follows the changes in the resonant frequency. In this way,

  the reaction factor of the reaction amplifier

  sitive is maintained at a constant value determined by the composite impedance of the resonant circuit at resonance,

  and any variation of the output voltage due to a varia-

  temperature is eliminated, making it possible to establish an automatic temperature compensation

  temperature. Moreover, since its frequency of

  nance is always provided to the resonant circuit of

  so that its complex impedance is maintained at a

  representing only the actual component without

  imaginary health, no phase variation takes place in the positive feedback circuit, which has the result of improving the stability of operation and rendering

  the setting is simpler and easier. Also, the os-

  used oscillator is an oscillator whose frequency

  is regulated by voltage variation, and it is not

  use a high stability oscillator such as a crystal oscillator. This oscillator with a voltage regulated frequency and the comparator of

  phase can be replaced by an integrated circuit with

  phasing key to simplify the structure of the circuit. With the apparatus according to the invention, by measuring the output voltage of the phase comparator, it

  It is possible to detect continuously the values of

  riation of the inductance of the detection coil.

  The rest of the description refers to the drawings

  appended which represent: FIG. 1, the block diagram of a known apparatus, FIG. 2, the block diagram of an exemplary embodiment according to the invention,

  - Figure 3, the curve of the variations L / L of the inductan-

  this (in ordinate) of the coil of detection according to its temperature T (in abscissa), - figure 4, the curve of the variations A Vo / Vo of the output voltage of the amplifier (in ordinate) according to the temperature T of the detection coil (in abscissa)

  FIG. 5, the curve of the output voltage Vo (in

  data) of the amplifier of the measuring equipment

  distance according to the invention according to the dis-

Tance D (as abscissa) to be measured.

  The apparatus illustrated in FIG.

  distance measuring reel of known type.

  There is shown in 1 a metal body, in

  2 a reference oscillator, in 3 a detection coil

  at 4 an amplifier, and at Zs a feedback impedance. Oscillator 2 provides a fixed frequency and fixed amplitude alternating signal which is applied to

  the amplifier 4. The output signal of the amplifier

  4 is transmitted to the detection coil 3 by means of

  intermediate of the reaction impedance Zs, so that when an alternating magnetic flux created by the coil

  3 passes through the metal body 1,

  Foucault rays are induced in the metallic body,

  and, by reaction, the impedance Z of the detection coil

  is modified, which modifies the reaction factor A p = Z / (Z + Zs). As a result, the output voltage Vo

  amplifier 4 varies with the distance between the amplifier

  3 and the metal body 1. Thus, by measuring the output voltage Vo, it is possible to measure "without contact" the distance between the coil of

  detection 3 and the metal body 1.

  However, the distance measuring apparatus

  by eddy currents, of the reaction amplifier type,

  tion and as shown in Figure 1, has certain disadvantages

  vantages. More explicitly, the impedance of a coil goes

  generally more or less with the temperature of the

  coil, and it was impossible to compensate until then-

  with a very high precision the effect of the variations of

  temperature on the output characteristic which represents

  the distance between the detection coil 3 and the

  metal body 1. The rate of change with temperature

  the impedance of the coil of detection 3 differs

  depending on the material of the coil carcass, according to

  mounts of this carcass, depending on the winding mode of the coil, etc.

  Thus, in the case of a carcass using, as is known, a ferrite core which is a ferromagnetic material, the permeability of the ferrite varies with the temperature, and this variation in .DTD: temperature function follows a curve of hysteresis.

  The use of a ferrite core makes it even more difficult

  the temperature compensation of the characteris-

exit tick.

  The abovementioned disadvantages are eliminated in the apparatus according to the invention, in which the oscillation frequency of a reference oscillator is varied as a function of the inductance of a detection coil, and a resistor is used. the place of

  the reaction impedance, so that the im-

  is eliminated, which maintains the reaction factor

  P 'p at a constant value irrespective of the

  variations in the temperature of the sensing coil.

  Reference will now be made to FIGS. 2 to 5 to describe a recommended exemplary embodiment, in accordance with

to the invention.

  In FIG. 2, there is referenced in 1 a body

  metal, 3 a detection coil and 4 amps

  These components are similar to those

  have the same references in Figure 1. We have

  presented in Rp a resistance of reaction, in Co a con-

  resonance densifier in parallel, in 5 an oscilla-

  whose frequency of oscillation is regulated by

  of a continuous control voltage, and at 6 a

phase comparator.

  The detection coil 3 and the capacitor Co form a parallel resonant circuit whose composite impedance Zo at the resonance frequency fo is given by the following equation (1) 2 7rfo.L Zo = YY = Y + Q.2 7rfo .LQ = Wo L / Y (1) Y is the resistance of the sensing coil, L is the inductance of the sensing coil, and Wo

  (2 MFO) is the angular frequency.

  It can be seen that the composite impedance Zo of the circuit

  resonant cooked parallel to the resonance frequency fo

  has only one real component. The circuit resonates

  parallel and the reaction resistance Rp form the positive feedback circuit of the amplifier 4,

  and to this amplifier is provided the oscillating voltage

  tion of the oscillator 5 whose oscillation frequency fm is substantially equal to the resonant frequency fo of the parallel resonant circuit. The aforementioned oscillation voltage is first amplified by the amplifier 4, then it is transmitted to the phase comparator 6. The output voltage of the oscillator 5 is also transmitted to the

  phase comparator 6, so that the phase difference

  between the applied voltages is detected, the

  phase generator 6 providing a continuous voltage cor-

  corresponding to the phase difference. The aforesaid DC voltage is then transmitted to the oscillator 5 as a

  adjusting voltage, so that the frequency of oscillation

  This emitter becomes equal to the resonance frequency of the parallel resonance circuit, the phase of the output voltage of the oscillator 5 being also blocked. Assuming that the inductance of the sense coil 3 is changed due to a variation

  the temperature of this coil, the frequency of

  f is also variable, and the rate of change is given by the following equation (2): t L / L = (fo / fo ') 2 - 1 (2) With L = inductance of the sensing coil AL = variation inductance as a function of temperature

  fo = resonant frequency of the resonant circuit

  adjusts the temperature variation

  fo '= resonance frequency of the resonant circuit

  Raises after the temperature change.

  As a result, the composite impedance Zo of the circuit

  parallel resonant cooked is automatically controlled so that even though the value of inductance L of

  the detection coil 3 increases, the frequency of detection

  its fo loss decreases, and the composite impedance Zo is now

  constant holding. If the composite impedance Zo is now

  constant, the reaction factor 6 p = (Zo / (Rp + Zo) is kept constant, as is the output voltage

Vo of the amplifier 4.

  Figure 3 shows the variations in

  ductance L of the detection coil 3 with the temperature.

  FIG. 4 illustrates the variations of the voltage of

  of amplifier 4 according to the variations of the

  temperature of the sensing coil 3. The line between

  The broken line represents these variations for the circuit of FIG. 1, whereas the solid line represents these variations for the circuit of FIG.

  figure 5 the distance measurements obtained with the apparatus

of Figure 2.

  The apparatus according to the invention which has just been described has the following advantages:

  (a) The capacitor Co being connected to the

  detection circuit 3 to form a resonant circuit

  whose frequency of resonance varies with the inductive

  detection coil even if this inductance

  varies with temperature-, the reaction factor is now

  kept constant, which makes possible an automatic compensation

  the effect of temperature changes.

  (b) Since the oscillation frequency

  Oscillator 5 is equal to the frequency of

  sound of the resonant circuit, and that the impedance

  site of this resonant circuit contains only a

  health, improves the stability of the equipment

  measuring distance and simplifying the adjustment.

  (c) It is not necessary for the oscillator used to be a very high stability oscillator

than a quartz oscillator.

  (d) Circuits closed by the international line

  2 of Figure 2 may be replaced by a phase-locked loop integrated circuit, which

  simplifies the structure of these circuits.

  (e) By measuring the output voltage of the

  phase 6, it is possible to detect

  manence the inductance variations of the coil of de-

tection 3.

Claims (2)

  1) A distance measuring apparatus comprising a positive feedback amplifier whose feedback circuit is connected to a detection coil placed in front of a metal object, the object-coil distance being to be measured, and an oscillator providing a alternating signal to the amplifier for exciting the sense coil, the amplifying factor of the positive feedback amplifier being controlled according to the impedance of the sense coil which varies with the distance between the metal body and   this detection coil, so that the distance   cited is measured in accordance with the amplitude of the output signal of the amplifier, characterized in that a capacitor (Co) is connected in parallel with the detection coil (3) to form a resonant circuit, the resonant circuit being connected to the reaction circuit   (Rp) of the positive feedback amplifier (14) of   to control the post-reaction amplification factor   tif of this amplifier, the oscillator (5) being of the variable frequency type, or of the frequency-controlled type, so that it always oscillates on   an oscillation frequency equal to the frequency of   sound of the resonant circuit, a phase comparator (6) being coupled to the oscillator (5) so as to provide an output voltage which corresponds to the phase difference between the output signal of the oscillator and the output signal of the positive feedback amplifier and allow control of the oscillation frequency of this oscillator.   2) Apparatus according to claim 1, characterized   in that the positive feedback amplifier (4)   has an inverting input terminal, a terminal of   non-inverting beam and an output terminal, the output of the oscillator (5) being connected to the input terminal 1 1   inversion, and the resonant circuit (3, Co) being connected   strung to the non-inverting input terminal, the feedback circuit of the amplifier being constituted by a resistor (Rp) connected between the output terminal and the non-inverting input terminal.