JP5506503B2 - Proximity switch - Google Patents

Description

  The present invention relates to a proximity switch that determines the approach of a detected object by detecting a change in impedance of a coil due to the proximity of the detected object such as metal.

  The proximity switch is formed by housing a coil on one end side in the casing and injecting a filler such as resin from the other end side. Conventionally, the detection surface of the case is made of plastic and the side is made of brass. However, in recent years, the case is made of non-magnetic stainless steel for the purpose of preventing destruction and deterioration due to the collision of the detected object to the proximity switch. There are cases (see, for example, Patent Document 1). The proximity switch configured as described above forms a magnetic field in front of the casing detection surface as the coil oscillates. When the detected object enters this magnetic field, eddy current loss occurs in the detected object, and the impedance of the coil changes accordingly. This change in impedance can be detected by an attached circuit to determine the proximity of the detected object.

In proximity switches, the distance from the detected object to the detection surface (hereinafter referred to as the detection distance) that is determined to be detected depends on the temperature because the impedance of the coil changes in response to temperature changes. Shows the characteristics. FIG. 5 is a graph showing the temperature characteristics of the proximity switch of the stainless steel casing, in which the vertical axis represents the resonance impedance Zosc [kΩ] of the coil and the horizontal axis represents the detection distance [mm]. This resonance impedance is a value unique to the coil that is approximately expressed by Zosc = ω ² L ² / R. Here, L is the inductance of the coil, R is the resistance value of the coil, and ω is the oscillation frequency. For example, assuming that the detection / non-detection of the detected object is determined based on whether or not the resonance impedance Zosc of the coil exceeds 6.3 kΩ, the detection distance is 7 mm at an ambient temperature of 25 ° C., but 6 mm at −25 ° C. When it reaches 70 ° C, it becomes 8 mm. For this reason, a separate temperature compensation circuit is required to realize a constant detection distance in a wide temperature range regardless of the difference in temperature conditions (see, for example, Patent Document 2).

  FIG. 5 shows the temperature characteristics when the gap between the detection surface and the coil is fixed, but the detection distance also changes due to the fluctuation of the gap. FIG. 6 is a graph showing the relationship between the gap of the proximity switch of the stainless steel case and the detection distance, where the vertical axis shows the detection distance [mm] and the horizontal axis shows the gap [mm]. In the case of a stainless steel housing, the detection distance tends to increase as the gap between the detection surface and the coil decreases.

JP 2007-141762 A JP 2003-298403 A

  Since the conventional proximity switch is configured as described above, it is necessary to use a temperature compensation circuit as in Patent Document 2 in order to compensate for the temperature characteristics of the coil, leading to high costs.

  Further, since the temperature characteristic of the detection distance in the case of the stainless steel case is the reverse temperature characteristic as in the case of the conventional plastic case, the compensation circuit of Patent Document 2 cannot be used as it is, and a high-accuracy temperature is obtained. There was a problem that compensation would be difficult. In order to divert, it is necessary to replace the NTC thermistor (resistive element having a negative temperature coefficient) included in the compensation circuit of Patent Document 2 with a PTC thermistor (having a positive temperature coefficient). However, since there are few types of this PTC thermistor, highly accurate temperature compensation becomes difficult.

  Furthermore, if the filler that seals the coil in the housing expands and contracts according to the temperature change, the gap between the coil and the housing detection surface changes. There was also a problem of becoming.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a proximity switch that performs temperature compensation without providing a circuit that compensates for the temperature characteristics of the coil.

Proximity switch according to this inventions comprises a housing of non-magnetic metal, it is accommodated in the housing, a coil unit for detecting an approach to the housing detection surface of the object to be detected, and a holding portion for holding the coil portion A holding portion that is partially fixed at a predetermined position on the inner wall of the housing, and the holding portion can be expanded and contracted with the position as a base point . The fixed part has a temperature characteristic in which the detection distance fluctuates, and the amount of change in the gap between the inner wall of the casing detection surface and the coil part that changes due to the expansion and contraction of the casing and the holding part according to the temperature change is It is provided at a position that cancels out the temperature characteristics.

  According to the present invention, the holding portion is partially fixed to the inner wall of the nonmagnetic metal housing, and the holding portion can be expanded and contracted with the position as a base point. Therefore, it is possible to provide a proximity switch capable of performing temperature compensation without providing a circuit or the like for compensating the temperature characteristics of the coil portion.

It is sectional drawing which shows the structure of the proximity switch which concerns on Embodiment 1 of this invention. It is a graph which shows the temperature dependence characteristic of the detection distance of the proximity switch shown in FIG. FIG. 6 is a cross-sectional view showing a modification of the proximity switch according to the first embodiment. FIG. 6 is a cross-sectional view showing a modification of the proximity switch according to the first embodiment. It is a graph which shows the temperature characteristic of the coil of the proximity switch of a stainless steel housing | casing. It is a graph which shows the relationship between the clearance gap between a detection surface and a coil, and a detection distance of the proximity switch of a stainless steel housing | casing. It is a perspective view which shows the structural example of the housing | casing of the proximity switch shown in FIG. It is a perspective view which shows the structural example of the housing | casing of the proximity switch shown in FIG.

Embodiment 1 FIG.
FIG. 1 is a cross-sectional view showing the configuration of the proximity switch 1 according to the first embodiment. In FIG. 1, a coil 2 is wound around a bobbin 3 and housed in an annular groove of a ferrite core 4, and a substrate 5 on which various electronic components are mounted is connected to the back surface of the ferrite core 4 to form a coil portion. 6 is constituted. The coil part 6 is accommodated in a bottomed cylindrical cap 7, and the coil part 6 is accommodated together with the cap 7 in a bottomed cylindrical casing 9 to which a release material 8 is applied, and a filler (holding part). 10 is filled. A cable holder 11 is attached to the opening of the housing 9, and the cable 12 is pulled out from the hole of the cable holder 11.

  FIG. 2 is a graph illustrating the temperature dependence characteristics of the detection distance of the proximity switch 1, where the vertical axis indicates the change rate [%] of the detection distance and the horizontal axis indicates the ambient temperature [° C.]. In the figure, the □ plot represents the temperature characteristics of the coil section 6 when the gap A does not vary as a detection distance. ◇ The plot represents the characteristic of the detection distance according to the variation of the gap A. When the housing 9 is made of stainless steel and the filler 10 is a thermosetting resin, the linear expansion coefficient of the stainless steel and the resin is different. Therefore, as the temperature rises, the detection surface inner wall 9a of the housing 9 and the bottom surface of the cap 7 The gap A tends to be small and the detection distance tends to be large (the ◇ plot in FIG. 2). On the other hand, due to the temperature characteristics of the coil 2, the detection distance tends to decrease as the temperature increases (□ plot in FIG. 2). Thus, the fluctuation of the gap A and the temperature characteristic of the coil 2 tend to compensate each other for the temperature dependence characteristic of the detection distance. Therefore, in the first embodiment, the temperature characteristic of the coil 2 is compensated by using the fluctuation of the gap A due to the expansion and contraction of the casing 9 and the filler 10, and the detection distance change rate of the proximity switch 1 is set to the temperature. Regardless of whether it is constant or not (Δ plot in FIG. 2).

  More specifically, a fixing portion 20 is provided at a position where the amount of change in the gap A cancels the temperature characteristics of the coil 2, and a part of the filler 10 is partially fixed to the housing 9. The position of the fixing portion 20 is a base point for expansion and contraction of the contents including the coil portion 6 and the filler 10. Even if the casing 9 and the filler 10 on the detection surface inner wall 9a side from the base point expand and contract, and the gap A changes (the ◇ plot in FIG. 2), the temperature characteristics of the coil 2 cancel each other (see FIG. 2). 2 □ plot), the temperature dependence characteristics of the detection distance can be compensated (Δ plot in FIG. 2).

Next, a method for determining the position of the base point will be described.
Here, it is assumed that the proximity switch 1 has the characteristics shown in the graphs of FIGS. According to the graph shown in FIG. 5, the detection distance increases by about 1 mm with respect to the temperature change from 25 ° C. to 70 ° C. Furthermore, according to the graph shown in FIG. 6, the fluctuation of the gap A corresponding to the fluctuation of the detection distance of about 1 mm is about −0.05 mm.

Here, the distance from the bottom surface of the cap 7 to the base point is B [mm], and the difference between the linear expansion coefficient of the filler 10 that seals the coil portion 6 and the cap 7 and the linear expansion coefficient of the housing 9 is 5 ×. ^Assuming 10 ⁻⁵ mm / mm ° C., the fluctuation amount C [mm] of the gap A accompanying expansion and contraction is expressed by the following formula (1). However, assuming that the linear expansion coefficient of the filler 10 is 6.8 × 10 ⁻⁵ and the casing 9 is SUS (Stainless Used Steel) 303, the linear expansion coefficient 1.8 × 10 ⁻⁵ is used.
C = 5 × 10 ⁻⁵ × (70−25) × B (1)

  5 and 6, when the above equation (1) is solved with the variation amount C of the gap A being −0.05 mm, B = 22.22 mm, and the fixing portion 20 is fixed with the base point of about 22 mm from the bottom surface of the cap 7. Provide.

Next, a configuration example of the fixing unit 20 will be described.
In the configuration example shown in FIG. 1, a groove 21 is formed at the base position on the inner peripheral surface of the housing 9, and the convex portion 22 of the filler 10 is engaged with the groove 21 to form the fixed portion 20. Since a layer of the release material 8 is formed between the inner peripheral surface of the housing 9 and the outer peripheral surfaces of the coil portion 6 and the filler 10, the convex portion 22 of the filler 10 is fixed to the coil portion 6. The side becomes an open end, expands and contracts in the direction of the detection surface inner wall 9a, and compensates for the temperature characteristics of the coil 2.
Note that the groove 21 may be formed in the filler 10 and the convex portion 22 may be formed in the housing 9.

Further, the fixing unit 20 may be configured as shown in FIG. In the configuration example of the proximity switch 1 a shown in FIG. 3, a concave step 23 is formed at the base position on the inner peripheral surface of the housing 9, and the convex step 24 of the filler 10 is engaged with the step 23 to thereby fix the fixed portion. 20a. The mold release member 8 is provided only on the detection surface inner wall 9a side from the base point, and the concave step 23 and the convex step 24 are fixed.
The concave step 23 may be formed on the filler 10 and the convex step 24 may be formed on the housing 9. Further, the opposing surfaces of the concave step 23 and the convex step 24 may be screwed and fixed with screws.

  Further, the fixing unit 20 may be configured as shown in FIG. In the configuration example of the proximity switch 1 b shown in FIG. 4, a flange 25 is formed on the opening periphery of the cap 7, and this flange 25 is engaged with a concave step 23 formed at the base point position of the inner peripheral surface of the housing 9. Let it be a fixed portion 20b. In the case of this configuration, since the gap A is determined according to the difference in expansion and contraction between the housing 9 and the cap 7 (corresponding to the holding portion), the above formula ( The distance B of the base point is determined from 1).

  As described above, according to the first embodiment, the casing 9 made of nonmagnetic stainless steel, the coil unit 6 that is housed in the casing 9 and detects the proximity of the detection target to the detection surface, and the coil In the proximity switch 1 including the filler 10 that holds the part 6, the amount of change in the gap A between the detection surface inner wall 9 a and the coil part 6 that changes due to the expansion and contraction of the housing 9 and the filler 10 according to the temperature change. In a configuration in which the filler 10 is partially fixed to the inner wall of the housing 9 at a position where the temperature characteristics of the coil portion 6 are offset, and the fixing portion 20 is provided so that the filler 10 can be expanded and contracted from this position. did. For this reason, the temperature dependence characteristic of the detection distance of the coil unit 6 can be compensated by the change of the gap A according to the temperature change, and the temperature compensation can be performed without providing a conventional temperature compensation circuit or the like. .

In the first embodiment, the coil unit 6 is configured by the coil 2, the bobbin 3, the ferrite core 4, and the substrate 5. However, the coil unit 6 includes at least the coil 2 for generating a magnetic field. What is necessary is just to exist and you may abbreviate | omit another member as needed.
Moreover, although the ferrite core 4 is a product made from a ferrite, you may use the core comprised with materials other than this.
Furthermore, although the coil part 6 was accommodated in the insulating cap 7, this cap 7 may be omitted.

In the first embodiment, SUS303 is particularly used as an example of the material of the housing 9, but the material is not limited to this, and any nonmagnetic stainless steel may be used. Or, the relative magnetic permeability (ratio of the magnetic permeability of the substance at the same electric field strength to the vacuum magnetic permeability), which is a characteristic of general nonmagnetic stainless steel, is substantially 1, and the volume resistance at 20 ° C. A metal other than stainless steel (such as titanium) having a rate of 45 × 10 ⁻⁸ [Ωm] or more may be used.

  Moreover, you may make it provide the fixing | fixed part 20, 20a, 20b over the perimeter of the inner wall of the housing | casing 9, and it is formed by providing two or more for every predetermined angle among the perimeters of an inner wall, Also good. That is, the fixing portions 20, 20 a, and 20 b can appropriately fix the filler 10, and the angle (generally parallel) formed by the detection surface inner wall 9 a and the bottom surface of the cap 7 facing it is Any shape can be used as long as the shape can be kept constant with an acceptable degree of accuracy regardless of the expansion and contraction of the filler 10 accompanying the temperature change.

  Specific examples are shown in FIGS. 7 and 8 correspond to the perspective view of the housing 9 shown in FIG. 1, and by engaging a convex portion 22 of the filler 10 (not shown) with a groove 21 provided on the inner wall of the housing 9. The fixing portion 20 is not shown. In the example of FIG. 7, the groove 21 is provided over the entire circumference of the inner wall of the housing 9 at the base point position indicated by the alternate long and short dash line, whereas in the example of FIG. Three grooves 21 are provided.

1, 1a, 1b Proximity switch 2 Coil 3 Bobbin 4 Ferrite core 5 Substrate 6 Coil part 7 Cap (holding part)
8 Release material 9 Housing 9a Detection surface inner wall 10 Filler (holding part)
11 Cable holder 12 Cable 20, 20a, 20b Fixed part 21 Groove 22 Convex part

Claims (1)

In a proximity switch comprising a non-magnetic metal housing, a coil portion that is housed in the housing and detects the proximity of the detected object to the housing detection surface, and a holding portion that holds the coil portion,
The holding part is partially fixed at a predetermined position on the inner wall of the housing , and includes a fixing part that allows the holding part to expand and contract based on the position .
The coil section has a temperature characteristic in which a detection distance of the detection object varies according to a temperature change,
In the fixing portion, the amount of change in the gap between the inner wall of the housing detection surface and the coil portion that changes due to expansion and contraction of the housing and the holding portion according to temperature change cancels out the temperature characteristics of the coil portion. Proximity switch characterized by being provided at a position .

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