JP2534824B2 - Proximity switch - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a proximity switch for detecting the approach of an object by passing a high frequency current through a detection coil.

[0002]

2. Description of the Related Art This type of switch is constructed by attaching a detection coil wound around a ferrite core to a printed circuit board on which a detection circuit is mounted with an adhesive or the like. When approaching, the switching operation is performed based on the fact that the coil loss increases and the oscillation amplitude decreases. However, since the detection coil occupies a large area, a large noise component is induced in the detection coil, which causes a malfunction and a decrease in detection accuracy.

Therefore, in the prior art, for example, the actual development Sho 61-179.
As disclosed in Japanese Patent No. 638, there has been provided a structure in which a printed circuit board is provided with a conductor pattern for shielding, and a detection coil is attached thereon to electrically shield the detection coil.

[0004]

However, when the shield pattern is provided on the back surface of the detection coil as described above, eddy current loss is unavoidable. Since the magnetic flux density is inversely proportional to the distance from the coil, when a shield pattern is provided in the immediate vicinity of the detection coil, the leakage magnetic flux from the detection coil causes at least a large loss, and the oscillation amplitude tends to be greatly reduced.

Here, if the loss in the shield pattern similarly occurs in any situation, it simply causes a level change in parallel, so that no problem occurs, but in reality, the distance between the detection coil and the object to be detected is small. Is larger, the loss ratio in the shield pattern is large, and as the distance is shorter, the loss ratio in the detected object increases and the loss ratio in the shield pattern decreases. Specifically, the distance between the object to be detected and the detection coil is plotted on the abscissa and the oscillation amplitude is plotted on the ordinate, and the change in the oscillation amplitude with and without the shield pattern is shown in FIG. When the shield pattern is provided, the change in the oscillation amplitude with respect to a minute change in distance becomes small. This means that the switching operation accompanying the proximity of the detected object cannot be performed at a farther distance. Also,
In order to secure the same oscillation amplitude as when there is no shield pattern, it is necessary to increase the coil current in order to compensate for the reduction in oscillation amplitude due to eddy current loss, which increases the current consumption of the proximity switch. Invite.

Recently, in this type of proximity switch,
It is desired to increase the detection distance, and for this reason, it is considered that a so-called T-shaped core having a high degree of openness is used as the core of the detection coil instead of the E-shaped core having a small leakage flux. However, the higher the degree of openness of the core, the more the leakage flux increases, so that the loss in the shield pattern increases, and the above-mentioned problem tends to become more serious.

The present invention has been made in view of the above circumstances. Therefore, while ensuring the electric shield of the detection coil, the electromagnetic loss caused by the shield conductor layer is suppressed as small as possible, and the high detection accuracy is achieved. It is an object of the present invention to provide a proximity switch that can obtain the power and save power.

[0008]

In the proximity switch according to the present invention, a shield conductor layer for electrically shielding the detection coil is provided on a substrate holding the detection coil, and the shield conductor layer is provided with the shield conductor layer in a plane direction. The feature is that a slit for dividing is formed. More preferably, the slit is formed so as to extend in the radial direction from a position corresponding to the center of the detection coil in the shield conductor layer.

[0009]

The detection coil is electrically shielded by the shield conductor layer located on the substrate, and the noise resistance is enhanced.
Since a slit is formed in the shield conductor layer so as to divide the shield conductor layer in the surface direction, the eddy current loss is reduced even if the magnetic flux from the detection coil is linked to the shield conductor layer. Loss is reduced. Further, in the shield conductor layer in which the slit is formed so as to extend in the radial direction from the position corresponding to the central portion of the detection coil, the eddy current is generated in a situation where the eddy current is generated concentrically with the detection coil. You will be able to effectively kill.

[0010]

As described above, according to the proximity switch of the present invention, the shield conductor layer can surely electrically shield the detection coil, and the magnetic flux from the detection coil is linked to the shield conductor layer. Since it is possible to suppress the occurrence of eddy current loss as much as possible, it is possible to improve detection accuracy and save power.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to FIGS. The proximity switch of this embodiment is formed by molding the printed circuit board 11 with resin 12. The circuit board 11 is, for example, a ceramic double-sided board, and the detection coil 13 having one surface on which the detection coil 13 is mounted and the other surface on which an IC or the like forming a large part of the detection circuit is mounted are, for example, A magnet wire is wound around a ferrite core 14 a large number of times. The core 14 has a winding core projection 14b protruding from the center of a circular substrate 14a, and the section along the axial direction is T-shaped. The T-shaped core has a relatively large leakage flux to enable long-distance detection of an object to be detected.

Now, on the surface of the circuit board 11 on which the detection coil 13 is mounted, a conductor pattern as shown in an enlarged view in FIG. 3 is formed by printing firing of silver-palladium paste, for example. To do. The upper half of the circuit board 11 in the drawing is a mounting area of the detection coil 13, and the shield conductor layer 15 is formed almost all over this area. The shield conductor layer 15 is provided with a large number of slits 16 extending in a radial direction from a position corresponding to the center of the detection coil 13, and the shield conductor layer 15 is intermittently divided in the circumferential direction while leaving the center thereof. Has become. A connecting portion 15a extends from the shield conductor layer 15 toward the outside of the mounting area of the detection coil 13, and a ground side land 17 is formed at the tip thereof. Furthermore, the detection coil 1
A signal side land 18 is formed next to the ground side land 17 on the outside of the mounting region of No. 3, and the shield conductor layer 15 and the signal side land 18 are formed with through holes 19,
It is electrically connected to the detection circuit on the back surface via 20.

Circuit board 11 having the above-mentioned conductor pattern
On the other hand, the detection coil 13 has a circular substrate 14 of its core 14.
a is fixed to the central portion of the shield conductor layer 15 with an adhesive, for example, and the signal side lead wire 13a of the detection coil 13 is soldered to the signal side land 18, and the ground side lead wire 13b is grounded. Soldered to 17. Reference numeral 21 shown in FIGS. 1 and 2 denotes a cable soldered to a predetermined terminal portion of the circuit board 11.

According to the proximity switch having the above structure, a high frequency current is caused to flow through the detection coil 13 by the oscillation circuit included in the detection circuit, and a high frequency magnetic field is generated around the detection coil 13. When an object to be detected made of metal approaches the detection coil 13, the loss in the detection coil 13 increases due to the eddy current loss generated there, and the oscillation amplitude decreases,
Based on this, the switching operation is performed in the detection circuit. Here, even if an external electric field acts on the detection coil 13,
Since the shield conductor layer 15 connected to the ground side is formed on the back surface of the detection coil 13, the noise component induced in the detection coil 13 becomes extremely small, and malfunction due to electric field noise can be prevented. The shield conductor layer 1
Although a large number of slits 16 are formed in 5, the width of the slits 16 is narrow, so that the shielding effect is hardly affected and excellent noise resistance can be exhibited.

On the other hand, since the high frequency magnetic flux generated by the detection coil 13 is linked to the shield conductor layer 15, an eddy current is generated in the shield conductor layer 15. However, since a large number of slits 16 are formed in the shield conductor layer 15 to divide the current path, the path through which the eddy current flows becomes long and the current value can be kept low. Moreover, in this embodiment,
Since the slits 16 are formed to radiate from the position corresponding to the center of the detection coil 13, the eddy current tends to flow concentrically around the detection coil 13 under the circumstances. The current path is effectively divided, and the eddy current can be dramatically suppressed. As a result, the loss in the detection coil 13 is suppressed to a low level, so that the approach of the detected object can be detected with high accuracy, and power saving can be achieved.

By the way, the loss of the detection coil 13 due to the proximity switch of the present embodiment structure is actually measured by comparing the loss of the detection coil 13 with that without the shield conductor layer and that with the shield conductor layer without slits. Shown in. In this measurement example, the core 14 is made of ferrite, the circular substrate 14a has a diameter of 11.3 mm, a height of 3.3 mm, a magnet wire diameter of 0.13 mm, a number of turns of 55, 0.2 Vrms, 300 kHz.
A high-frequency current is flowing at z. The detected object is 30
The loss of the detection coil 13 was measured according to the distance between the object to be detected and the detection coil 13 using an iron plate having a square size of 1 mm and a thickness of 1 mm. Those without a shield conductor layer are shown by solid lines,
The one provided with a shield conductor layer having no slit (so-called "solid") is shown by a one-dot chain line, and the one provided with the shield conductor layer 15 of the structure of this embodiment is shown by a two-dot chain line. As is clear from this experiment, when a shield conductor layer having no slit is provided, a large loss occurs. However, the shield conductor layer 15 having the structure of the present embodiment has the same low loss as the shield conductor layer having no shield conductor layer. It will be a loss.

As described above, according to the present embodiment, while the shield conductor layer 15 provides excellent resistance to electric field noise, since the slit 16 is formed here, the eddy current loss is reduced and the loss in the detection coil 13 is increased. Can be reduced. This makes it possible to detect the detected object with high accuracy and reduce power consumption.

The present invention is not limited to the above-mentioned embodiments, but can be implemented with various modifications within the scope not departing from the gist of the invention, for example.

(B) The radial slits are not necessarily formed in the shield conductor layer, and for example, as shown in FIG. 4 as a second embodiment, the shield conductor layer 30 provided in the detection coil disposition region is not shown. The slit 31 may be formed in a meandering shape, and the slit 31 of this form is also included in the concept of “a slit for reducing eddy current loss” in the claims of the present application.

(B) In the above embodiment, the shield conductor layer 15
Was formed by printing and firing a conductive paste, but the invention is not limited to this, and the shield conductor layer is formed by, for example, metal plating, adhesion of a metal foil to a circuit board, vapor deposition of metal, or etching of a metal foil previously provided on the circuit board. Any conductive layer that may be formed by any other forming method including these, as long as it functions as a “shield conductive layer that electrically shields the detection coil on the substrate”, It is included in the concept of "shield conductor layer" in the range of.

(C) In the above embodiment, the T-shaped core having a large leakage flux is used in order to increase the detectable distance of the object to be detected, but the present invention is not limited to this, and the magnetic material is annularly formed on the outer peripheral side of the coil. May be used, or a coreless detection coil may be used, and these are all included in the concept of "detection coil" referred to in the claims of the present application.

Besides, the present invention is not limited to the above description and is not limited to the drawings, and various modifications can be made without departing from the scope of the invention.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a proximity switch according to a first embodiment of the present invention.

[Fig. 2] Similarly, a vertical sectional view

FIG. 3 is a plan view of the same circuit board.

FIG. 4 is a plan view of a circuit board according to a second embodiment of the present invention.

FIG. 5 is a graph in which the loss of the detection coil in the structure of the first embodiment is measured.

FIG. 6 is a graph showing an oscillation amplitude for comparison between a switch without a shield and a proximity switch provided with a conventional shield pattern.

[Explanation of symbols]

 11 ... Printed circuit board (board) 13 ... Detection coil 14 ... Core 15 ... Shield conductor layer 16 ... Slit 30 ... Shield conductor layer 31 ... Slit

Claims (2)

(57) [Claims] 1. A shield conductor for electrically shielding the detection coil on the substrate, wherein a high frequency current is passed through the detection coil provided on the substrate to detect an approach of an object to the detection coil to perform a switching operation. A proximity switch characterized in that a slit for reducing the eddy current loss is formed in the shield conductor layer while the layer is provided. 2. The proximity switch, wherein the slit is formed so as to extend in a radial direction from a position corresponding to the center of the detection coil in the shield conductor layer.