FR2605400A1 - Proximity detector - Google Patents

Abstract

The invention relates to a proximity detector of the inductive type, made from at least one conducting strip 7 deposited on the face of a dielectric wafer 6, this strip being wound around one of its ends C, in order to define a flat winding of circular, elliptical or rectangular cross-section. For application to systems for detecting presence and positioning.

Description

PROXIMITY SENSOR
The present invention relates to a new embodiment of a proximity sensor, according to the technology of printed circuits.

Proximity detectors are used mainly in two cases firstly in systems requiring presence detection, such as for example coin counting systems or filling measurement and then the response of the detectors must be binary "in all or nothing ", and in a second case, in robots requiring positioning in which case their response must be proportional.

Currently, there are essentially three types of proximity detectors apart from the probes allowing a detection of precision but whose inertia increases the response time. The most common contactless proximity sensors are either photoelectric, capacitive, or inductive. Photo-electric or opto-electronic detectors allow distance measurements from zero to a few meters but are expensive and sensitive to the environment, requiring frequent cleaning.

In addition, their implementation is delicate for uses in robotics. Capacitive sensors are often used for filling or level control, whatever the material. Their drawback comes from their sensitivity to the environment, humidity for example can modify their response. Finally, the inductive sensors are of simple design and little contuse, and possible use in environments polluted by oil, mud ... These sensors consist of a coil of copper wire around a core of ferrite as shown in Figure 1 which will be described later. The main disadvantage of these detectors remains the weight, especially when you want a range of r order of the decimeter.

The present invention relates to a proximity detector of the low-weight inductive type, of simple construction and inexpensive, which can very well be adapted to the end of a robot arm.

For this, object of the invention is a proximity detector of the inductive type, produced from a conductive tape deposited on a wafer.
of dielectric material, this ribbon wrapping around one of its
ends to draw a flat coil of elliptical circular section,
or rectangular.

Other characteristics and advantages of the invention will appear during the description which follows, illustrated by the following figures which represent: - Figure 1: a dry section of a detector
proximity according to the prior art - Figures 2a and 2b: a top view and a sectional view
transverse respectively of a proximity detector according to the invention; - Figures 3a and 3b: two exemplary embodiments of a coil of the
detector according to the invention - Figure 4: the plot of the field lines omitted by the
detector according to the invention - Figure 5: the response curves of a detector according to the invention - Figure 6: the plot of the field lines omitted by a
shielded detector according to the invention.

An inductive detector produced according to the prior art comprises a coil 1 of copper wire wound around a ferrite core 2, as shown in the schematic cross section of FIG. 1. The coil 1 is associated with an oscillator device 3, the oscillation frequency of which is proportional to the value of the inductance of the coil 1. When an alternating current flows in the coil, it creates a magnetic field, and when the detector thus constituted approaches a target 4 in conductive material at a distance such that the field lines 5 of the coil 1 are cut, there is creation of eddy currents in the target 4. These currents themselves generate a magnetic field which is opposed to that of the coil, thus modifying the value of the inductance
of the coil therefore the oscillation frequency of oscillator 3. In
detecting after amplification the value of this oscillation frequency, we
can know the distance between the target 4 in question and the detector
proximity. As has been said, the main drawback of this
detector is too heavy.

As shown in Figures 2a and 2b, the proximity detector according to the invention consists of a plate 6 of m dielectric material of small thickness, on which is deposited, a thin layer of material
conductor in which etching by conventional photogravure for example, a ribbon 7. This ribbon 7 is wound around a central point C to form a flat coil. In Figure 2, the coil has the shape of a spiral, but according to other exemplary embodiments, it could have an elliptical or rectangular section (Figures 3a and 3b).

The plate 6 may be made of epoxy and the ribbon 7 of copper, and the technology for producing this detector is that of printed circuits.

Thus, this new embodiment of a proximity detector offers the advantage of being easily reproduced in large numbers at very low costs.

FIG. 4 represents the topology of the magnetic field lines radiating from the coil. Only a half cross-section view of the detector is shown. The topology of the field lines 8 is similar to that of a conventional coil. They carry flat winding, produced by the conductive tape 7 deposited on the dielectric plate 6, and form closed loops. A metallic surface, for example a target whose distance from the detector is to be measured, will cut the field lines when the detector approaches it.

The dimensions of the detector are calculated as a function of the characteristics desired for the latter, in particular as a function of the desired detection distance and of the value of the desired inductance.

où: - R i est le rayqn moyen de la cible spire - p est le rayon du conducteur constituant cette spire - J1 (k) et J2 (k) sont les intégrales de Legendre avec

ou - Ri et Rj sont les rayons moyens de la iième et de la jième spires - 2 (k') et j 2 (k') sont les intégrales de Legendre avec

In the case of a detector rda3ised from a conductive spiral, r inductance is calculated as follows. We consider that the spiral is equivalent to a series of concentric turns for each of which we calculate the magnetic field. We then use the superposition method and we calculate the own inductances and the mutual inductances for each turn. After summation, the overall inductance of the coil is obtained. The expression of the coefficients Lii and LÜ of the inductance matrix is obtained by assimilating the rectangular section of the turns, etched on the dielectric plate, To a circle of radius p such that the surface 5 p2 is identical to the real surface lxL of the section of the coil. This approximation has a negligible impact since it concerns a part of the calculation occurring for less than 0.5 fi in the final result. The coefficients Lii and LÜ are given by the following expressions

where: - R i is the mean rayqn of the target turn - p is the radius of the conductor constituting this turn - J1 (k) and J2 (k) are the Legendre integrals with

or - Ri and Rj are the mean radii of the i-th and j-th turns - 2 (k ') and j 2 (k') are the Legendre integrals with

The detector being associated with an oscillator whose oscillation frequency is proportional to the value of the detector's inductance, itself a function of the distance d between the latter and the target, response curves similar to those of the figure are obtained. 5.

In FIG. 5, the distance d between the detector and the target is shown on the abscissa and the frequency of the oscillator associated with the detector on the ordinate. In the example chosen, the detector is produced from
of a coil with an outside diameter close to 10 mm and it can be seen that the
detector Plays its role very well between 14 mm and 1 m m. Curve C
corresponds to a dural turn and the curve C2 to an alloy turn
soft magnetic m ent.

 Generally, the electronic control and processing circuits of the detector will be placed on the face 9 of the dielectric plate 6 opposite that 10 on which the conductive tape is deposited. To remove the magnetic chain at this location, which can create parasites, a shield 11 is produced on this face 9 by depositing, for example, a layer of soft magnetic material, for example ferrite (FIG. 6).

According to a variant. of the detector according to the invention, the flat coil can be deposited on a flexible dielectric support, such as
Kapton, which allows for example to bend the baleen coil to decrease or increase the solid angle and thus adjust the response of the detector to a given application.

Finally, as previously indicated, the coil of the detector according to the invention may have an elliptical or rectangular section to favor a direction of detection.

 This sensor can of course be used for conventional applications such as counting parts or filling measurement where proximity sensors of the coil type are then used.

 The invention is obviously not limited to the embodiments described and shown. It also includes all the technical equivalents of the means described and their combinations carried out in the spirit of the invention and implemented in the context of the claims which follow.

Claims (3)

R EV E N DIC A TIO NS 1 / Inductive type proximity sensor, characterized in that it is  made from at least one conductive tape (7) deposited on a  face of a dielectric substrate board (6), this ribbon winding  around one of its ends serving as a center (C) to draw a  flat winding of circular, elliptical or rectangular section. 2 / Detector according to claim 1, characterized in that it comprises  a shield (11) produced by a layer of soft magnetic material  deposited on the second face (9) of the dielectric plate (6)  opposite the face (10) on which the conductive tape (7) is deposited.  conductive tape (7) is made of copper.  3 / Detector according to claims 1 or 2, characterized in that the  from Kapton.  dielectric plate (6) is made of epoxy or flexible material such as  4J Detector according to claims 1, 2 or 3, characterized in that the