JP2007141762A - Proximity sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a proximity sensor capable of preventing reduction in detection sensitivity even if a thick surface is to be detected and the sensor is placed by being embedded in a mounting metal member formed of a magnetic metal, such as iron or the like. <P>SOLUTION: The magnetic flux FL1 from a detection coil 6 to a side part 24 is induced in the direction toward the mounting metal member, so that a part of the magnetic flux FL1 reaches the mounting metal member as the magnetic flux FL3. Eddy current is generated by the magnetic flux FL1 in the side part 24; the magnetic flux FL2 is generated in the direction toward a detection coil 6 by the eddy current, and the magnetic flux FL4 is generated toward the mounting metal member 51. The magnetic fluxes FL2 and FL4 cause loss due to the eddy current in the mounting metal member 51. Since the magnetic flux passes easily in the side part 24, the loss due to the eddy current decreases in the side part 24, but it increases in the mounting metal member 51. By adjusting oscillation frequency ω, the loss due to the leak magnetic flux is adjusted so that it does not change irrespective of existence of the mounting metal member 51. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a proximity sensor that determines the approach of a detected object by detecting the loss of a magnetic field due to the approach of the detected object made of metal. In particular, the present invention relates to a proximity sensor in which variation in detection distance due to the influence of a surrounding metal body to which the sensor is attached is reduced.

  The proximity sensor (or proximity switch) is a general term for sensors that detect the movement or presence of an object to be detected and output the detection result as an electrical signal. There are various types of proximity sensor detection methods. For example, there is a method that uses eddy currents generated in a metal object to be detected by electromagnetic induction.

  A proximity sensor using electromagnetic induction generally includes a high-frequency oscillation circuit having a coil. Such a proximity sensor is in an oscillating state when there is no object to be detected, and has a method in which conductance increases in response to arrival of the object to be detected and oscillation is weakened (or stopped), and oscillation stops when there is no object to be detected. There is a method in which the conductance decreases and the oscillation starts according to the arrival of the detected object, and the amplitude (or frequency) of the conductance change according to the presence or absence of the detected object is detected by the circuit processing in the subsequent stage. Thus, the arrival of the detected object can be detected.

  Conventionally, the detection surface and the side surface of the case of the proximity sensor are made of a metal of SUS (Stainless Used Steel) material. Since SUS is a non-magnetic material and has low conductivity, the magnetic flux can pass through the detection surface by setting the oscillation frequency of the oscillation circuit to be relatively low. Therefore, the proximity sensor can detect the detected object (metal workpiece) existing in front of the detection surface.

  The oscillation frequency that maximizes the detection sensitivity (that is, lengthens the detection distance) differs depending on the outer diameter of the case, that is, the diameter of the detection surface. The detection sensitivity is maximized at a higher oscillation frequency as the diameter of the detection surface is smaller. Conversely, the detection sensitivity is maximized at a lower oscillation frequency as the diameter of the detection surface is larger.

  Further, if the thickness of the detection surface is increased in order to improve the case strength of the proximity sensor, the detection sensitivity is reduced, that is, the detection distance is reduced. The reason is that by increasing the thickness of the detection surface, it becomes difficult for the magnetic flux to pass through the detection surface of the case.

FIG. 20 is a diagram illustrating an example of an installation state of the proximity sensor.
Referring to FIG. 20, a screw thread is provided on the side surface of proximity sensor 101. The proximity sensor 101 is attached in a state of being embedded in the attachment metal 151 by screw tightening. Further, the proximity sensor 101 is attached so that the detection surface 121 and the main surface of the attachment metal 151 are in the same plane. In many cases, iron is generally used as the material of the mounting metal 151 in terms of strength and cost.

FIG. 21 is a diagram illustrating another example of the installation state of the proximity sensor.
Referring to FIG. 21, it differs from FIG. 20 in that detection surface 121 protrudes from one main surface of mounting metal 151. The proximity sensor 101 is fixed by attaching a hexagon nut 152 to a portion protruding from the attachment metal 151.

FIG. 22 is a diagram showing still another example of the proximity sensor installation state.
Referring to FIG. 22, similarly to FIG. 20, the proximity sensor 101 is provided so that the detection surface 121 is the same as one main surface of the mounting metal 151. However, in FIG. 22, a hexagon nut 152 for fixing the proximity sensor 101 to the side opposite to the detection surface 121 with respect to the mounting metal 151 is provided.

FIG. 23 is a diagram illustrating still another example of the proximity sensor installation state.
Referring to FIG. 23, the thickness of mounting metal 151 is thinner than that of mounting metal 151 shown in FIG. The proximity sensor 101 is fixed by hexagon nuts 152 and 153 provided with the attachment metal 151 interposed therebetween.

  As described above, by selecting a relatively low oscillation frequency, the magnetic flux easily passes through the detection surface made of a non-magnetic metal. That is, the case made of SUS material has a weak effect of shielding magnetic flux. For this reason, leakage magnetic flux is generated from the side of the case. A phenomenon that the detection distance of the proximity sensor changes occurs due to the magnetic flux acting on the mounting metal by the leakage magnetic flux.

  The magnitude of the influence on the detection distance depends on the material of the mounting metal. For example, if the mounting metal is a non-magnetic material such as brass or aluminum and has a high conductivity, the detection distance is greatly reduced.

  When the mounting metal is aluminum or brass, a magnetic flux repelling the leakage magnetic flux is generated by an eddy current generated according to the leakage magnetic flux. As a result, since the magnetic flux interlinking the case side surface is reduced, the loss on the case side surface is reduced. As the loss on the side of the case is reduced, the loss of the detection coil is also reduced. Therefore, the output (oscillation amplitude) of the oscillation circuit is increased. In this case, if the detected object does not sufficiently approach the proximity sensor, the oscillation amplitude does not decrease to a predetermined value, so that the detection distance is greatly decreased.

  On the other hand, if the mounting metal is a magnetic metal such as iron, a decrease in the detection distance can be suppressed. In this case, the leakage flux is induced in the direction of the mounting metal. Even if the loss on the side of the case is reduced, the loss is supplemented by the magnetic flux generated by the mounting metal, so that fluctuations in oscillation amplitude can be suppressed. As a result, a decrease in detection distance can be suppressed.

  However, the material of the detected object is often iron. Therefore, when iron is used as the material of the mounting metal, there arises a problem that the sensor reacts as if the detected object does not actually exist. A conventional method for preventing such a problem will be described with reference to the drawings.

FIG. 24 is a diagram illustrating an example of a conventional method that makes it possible to prevent a decrease in the detection distance.
A cross section of the proximity sensor 101 is shown with reference to FIG. Inside the case 102, a core 108 around which the detection coil 106 is wound and a shielding plate 154 surrounding the core 108 are provided. The shielding plate 154 is made of a metal having high conductivity. By preventing the leakage magnetic flux from the detection coil 106 to the case 102 by the shielding plate 154, it is possible to suppress the influence on the detection distance due to the attached metal, thereby preventing a decrease in the detection distance.

  FIG. 25 is a diagram showing another example of a conventional method that makes it possible to prevent a decrease in the detection distance.

  Referring to FIG. 25, grooves are formed on the inner surface of case 102 in a lattice shape. Thereby, since the eddy current in the case 102 can be reduced, a decrease in the detection distance can be prevented.

  FIG. 26 is a diagram showing still another example of a conventional method capable of preventing a decrease in detection distance.

  Referring to FIG. 26, around the core 108, a ring 155 made of a material having a high magnetic permeability such as ferrite and a small loss is provided. The magnetic flux on the side of the core 108 is guided to the ring 155 by the ring 155. As a result, leakage magnetic flux to the mounting metal 151 can be prevented, so that a decrease in detection distance can be prevented.

  FIG. 27 is a diagram showing still another example of a conventional method capable of preventing a decrease in detection distance.

  Referring to FIG. 27, auxiliary coil 156 is provided on the side of core 108. A reverse magnetic flux generated by the auxiliary coil 156 induces a leakage magnetic flux generated by the detection coil 106 in a direction away from the attachment metal, thereby preventing a decrease in detection distance.

  For example, Japanese Patent No. 3023311 (Patent Document 1) discloses a proximity sensor including a case made of stainless steel and an attachment member made of iron.

As an application example of the technique of FIG. 27, for example, Japanese Patent No. 2603628 (Patent Document 2) discloses a proximity sensor in which auxiliary coils are provided on the outer periphery of both ends of a detection core provided with a sensor coil.
Japanese Patent No. 3023311 Japanese Patent No. 2603628

  As described above, when the thickness of the detection surface of the case is increased, the detection distance is shortened. That is, the thickness of the detection surface and the detection distance are in a trade-off relationship. By selecting an oscillation frequency in the vicinity of the maximum sensitivity of the sensor, a detection distance of a certain length can be secured. However, when the mounting metal material is iron, the effect of the mounting metal increases as the sensitivity of the sensor increases. Therefore, depending on the oscillation frequency to be selected, the detection distance becomes extremely short or extremely large.

  As shown in FIGS. 21 and 23, the influence of the mounting metal can be reduced by mounting the proximity sensor with the detection surface protruding from the mounting metal. However, in order to make the detection surface protrude from the mounting metal, it is necessary to secure a certain amount of space. If the space cannot be secured, the proximity sensor cannot be attached.

  Further, since the trajectory is unstable when the detected object moves, the detected object may move up and down. Therefore, when the detection surface of the proximity sensor protrudes from the mounting metal, the detected object comes into contact with the detection surface of the proximity sensor or collides with the side surface of the case. This makes it easier for the proximity sensor to fail.

  In the case of the prior art shown in FIGS. 24 to 27, the cost is a problem. For example, in the case of the prior art shown in FIGS. 24, 26 and 27, cost increases due to the addition of parts. In the case of the prior art shown in FIG. 25, a cost increase accompanying the design and processing of the inner surface of the case occurs.

  Furthermore, in the case of the prior art shown in FIG. 27, adjustment between individuals becomes difficult. Further, since the core diameter is reduced, the detection distance is shortened. In a proximity sensor with a small detection surface diameter, it is difficult to insert an auxiliary coil in terms of space.

  An object of the present invention is to provide a proximity sensor that can increase the thickness of the detection surface and prevent a decrease in detection sensitivity even if it is embedded in a mounting metal made of a magnetic metal such as iron. It is to be.

  In summary, the present invention is a proximity sensor that detects the approach of a detected object made of metal, and includes a coil, an oscillation circuit connected to the coil, and a change in conductance of the coil during oscillation operation of the oscillation circuit. Thus, a detection circuit for detecting that the detected object has approached the coil, and a case made of a nonmagnetic metal and containing at least the coil are provided. The oscillation frequency of the oscillation circuit is such that the difference in the rate of change in conductance with respect to the change in the distance between the object to be detected and the coil is within a predetermined range between when the magnetic metal is present around the case and when no magnetic metal is present. Set to fit.

Preferably, the case includes a detection surface provided between the detected object and the coil. The detection surface is made of a metal having a relative permeability of substantially 1 and a volume resistivity of 72 × 10 ⁻⁸ (Ω · m) or more.

More preferably, the material of the detection surface is stainless steel.
More preferably, the case further includes a side surface provided to surround at least the detection surface and the coil. The side surface portion is made of a metal having a relative permeability of substantially 1 and a volume resistivity of 72 × 10 ⁻⁸ (Ω · m) or more.

More preferably, the material of the side part is stainless steel.
Preferably, the rate of change in conductance is represented by the following formula:
(G _110% -g _100% ) / g _100% x100
However, g _100% is a conductance value when the detection distance between the object to be detected and the proximity sensor is a predetermined distance, and g _110% is a conductance when the detection distance is 1.1 times the predetermined distance. Is the value of

More preferably, the value of g _{100% is} a value between 10 microsiemens and 150 microsiemens.

More preferably, the value of g _{100% is} a value between 30 microsiemens and 100 microsiemens.

  More preferably, the width and height of the coil are determined such that the product of the ratio of the coil height to the coil width and the predetermined distance is equal to or greater than a predetermined value.

  More preferably, the oscillation frequency is set to be lower than the frequency at which the absolute value of the change rate of conductance according to the above equation is maximized.

  According to the proximity sensor of the present invention, even when the attachment metal is a magnetic material such as iron, the detection surface can be attached to be the same as the main surface of the attachment metal without reducing the detection distance.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same reference numerals indicate the same or corresponding parts.

FIG. 1 is an external view of a proximity sensor according to the present embodiment.
Referring to FIG. 1, the proximity sensor 1 includes a case 2 that houses the main body and a signal cable 5. As will be described later, a detection coil is housed on the detection surface 21 side in the case 2. Further, a screw thread 23 is formed on the outer periphery of the case 2 for installation on a mounting metal. The signal cable 5 is provided for outputting a detection result of the proximity sensor 1, that is, an electric signal.

  FIG. 2 is a schematic diagram showing a cross section of the proximity sensor 1 of FIG. The cross-sectional direction is parallel to the paper surface of FIG.

  Referring to FIG. 2, case 2 is fixed to mounting metal 51 by screwing. The attachment metal 51 is a magnetic metal such as iron. Inside the case 2, a detection coil 6, a core 8, and a circuit board 10 are provided. The circuit board 10 may be provided outside the case 2 as long as at least the detection coil 6 is provided inside the case 2.

  The case 2 includes a detection surface 21 provided to face the detection coil 6 and a side surface portion 24 provided so as to surround at least the detection surface 21 and the detection coil.

  Both the detection surface 21 and the side surface portion 24 are made of a nonmagnetic metal. Since the detection surface 21 is made of a nonmagnetic metal and has a large volume resistivity, the magnetic flux from the detection coil 6 can easily pass through the detection surface 21, so that the sensitivity of the proximity sensor 1 can be increased. Moreover, the side part 24 is comprised with a nonmagnetic metal, and when the attachment metal 51 is magnetic metals, such as iron, the sensitivity fall of the proximity sensor 1 can be prevented.

Note that a nonmagnetic material generally has a relative permeability μ (ratio of a material permeability and a vacuum permeability at the same magnetic field strength) μ (for example, the relative permeability of SUS303 is about 1.03). However, the volume resistivity varies depending on the material. In the present embodiment, the volume resistivity of the nonmagnetic metal used for the detection surface 21 is preferably 72 × 10 ⁻⁸ (Ω · m) or more. Examples of the material satisfying such characteristics include nonmagnetic stainless steel such as titanium and SUS303. However, the detection surface 21 is preferably made of stainless steel in terms of strength and cost.

Moreover, it is preferable that the metal used for the side surface portion 24 also has a relative permeability μ of substantially 1 and a volume resistivity of 72 × 10 ⁻⁸ (Ω · m) or more. That is, the material of the side surface 24 is preferably the same as the material of the detection surface 21. By using the same material for the detection surface 21 and the side surface portion 24, it is possible to prevent a decrease in the detection distance, and it is possible to reduce the cost. In the present embodiment, SUS303 is used as the material of the detection surface 21 and the side surface portion 24.

  The detection coil 6 is wound around the core 8. The core 8 is, for example, an E-shaped ferrite core, and guides the magnetic flux generated in the detection coil 6 forward (detection surface 21 side).

FIG. 3 is a diagram showing a configuration of the circuit board 10 of FIG.
Referring to FIG. 3, an oscillation circuit 31, a detection circuit 32, and an output circuit (OUT) 33 are mounted on the circuit board 10. The detection circuit 32 includes a detection circuit 34 and a comparison circuit 35.

  The oscillation circuit 31 is connected to the detection coil 6. The oscillation circuit 31 oscillates in a state where the object to be detected does not arrive. The detection circuit 34 converts the oscillation signal of the oscillation circuit 31 into a DC signal. The comparison circuit 35 compares the DC signal output from the detection circuit 34 with a preset reference level.

  When the detected object A shown in FIG. 2 approaches the detection surface 21 (and the detection coil 6) during the oscillation operation of the oscillation circuit 31, the magnetic flux generated by the detection coil 6 is affected by the eddy current generated by the detected object A. For this reason, the oscillation amplitude of the oscillation circuit 31 is reduced. Therefore, the output level of the detection circuit 34 is below the reference level. In this state, the output signal of the comparison circuit 35 is output to the outside via the output circuit 33, whereby the approach of the detected object is detected. The distance between the detection surface 21 and the detected object A at this time is shown as a detection distance D in FIG.

  In an actual product, the detection circuit 32 detects a change in oscillation amplitude as a change in conductance g of the detection coil 6. That is, the detection circuit 32 detects that the detected object A has approached the detection coil 6 due to a change in conductance of the detection coil 6. The value of conductance g is expressed by the following equation (1).

g = R / {R ² + (ωL) ² } (1)
Here, R is the resistance value of the detection coil 6, L is the inductance value of the detection coil 6, and ω is the oscillation frequency of the oscillation circuit 31.

FIG. 4 is a diagram illustrating the magnetic flux generated in the proximity sensor 1 of FIG.
Referring to FIG. 4, a magnetic flux FL0 is generated from detection coil 6 toward detected object A during oscillation of oscillation circuit 31 in FIG. At this time, the skin effect is generated in the detected object A, and the detection distance D is equal to the skin depth. Here, the skin depth is the depth at which the magnetic flux penetrates into the metal body and the eddy current flows when the magnetic flux generated from the detection coil interlinks with the metal body. Therefore, the skin depth S is expressed by the following equation (2). As shown.

S = {2 / (ω × μ × σ)} ^1/2 (2)
Here, σ is the conductivity of the detection surface 21, and μ is the relative permeability of the detection surface 21. Therefore, in order to increase the skin depth S (that is, to increase the detection distance D), the oscillation frequency ω of the oscillation circuit 31 is decreased, the relative permeability μ is decreased, or the conductivity σ is decreased (that is, the volume). It is effective to increase the resistivity). In the present embodiment, in particular, the oscillation frequency ω is lowered and the volume resistivity is raised.

  Magnetic flux FL1 (that is, leakage magnetic flux) from the detection coil 6 to the side surface portion 24 is induced in the direction of the mounting metal 51, and a part of the magnetic flux FL1 reaches the mounting metal 51 as magnetic flux FL3. Further, an eddy current is generated in the side surface portion 24 by the magnetic flux FL 1, and the magnetic flux FL 2 is generated in the direction of the detection coil 6 due to the eddy current, and the magnetic flux FL 4 is generated toward the attachment metal 51. Due to the magnetic fluxes FL2 and FL4, a loss due to eddy current occurs in the mounting metal 51. Since the magnetic flux easily passes through the side surface portion 24, the loss due to the eddy current at the side surface portion 24 decreases, but the loss increases in the mounting metal 51. By adjusting the oscillation frequency ω, the loss due to the leakage magnetic flux can be adjusted so as not to change regardless of the presence or absence of the attachment metal 51. Therefore, the proximity sensor 1 can prevent the influence of the attachment metal 51.

  As described above, the proximity sensor 1 has a detection sensitivity even if it is mounted in a state where the thickness of the detection surface 21 is thick and it is embedded in a mounting metal made of a magnetic metal by setting the oscillation frequency appropriately. Can be prevented. Hereinafter, the characteristics of the proximity sensor 1 will be described in more detail.

FIG. 5 is a diagram showing a magnetic field analysis model of the proximity sensor 1 of FIG.
Referring to FIG. 5, a magnetic field analysis model in which the proximity sensor 1 of FIG. 1 is cut by ¼ along its central axis is shown. In this analysis model, the detection surface 21, the side surface portion 24, the detection coil 6, and the core 8 are shown. For convenience of analysis, the attachment metal 51 is broken down into attachment metals 51A and 51B in FIG.

  Next, the results of CAE (Computer Aided Engineering) analysis using the magnetic field analysis model of FIG. 5 will be described with reference to FIGS. 6 to 9, the indicators METAL1, TARGET, COIL, CORE, KANAGU2, and KANAGU1 in the graph are the attachment metal 51A, the detected object A, the detection coil 6, the core 8, the side surface portion 24, and the detection of FIG. The surface 21 is shown. 6 to 9, the detected object A (TARGET) is set to iron, the detection surface 21 (KANAGU1) and the side surface 24 (KANAGU2) are set to stainless steel, and the mounting metal 51B (METAL2) is set to air. The protruding distance of the detection surface 21 from the mounting metal 51A is set to 0 mm. Also, the size standard (outer size) of the proximity sensor in this model is M18 size.

  FIG. 6 is a diagram showing a change in active power when the mounting metal 51A (METAL1) is iron.

  Referring to FIG. 6, it is shown how the loss generated by the components constituting the proximity sensor due to the magnetic flux from the detection coil 6 varies depending on the oscillation frequency. The loss caused by the linkage of the magnetic flux of the detection coil 6 to the component parts is proportional to the active power (the power actually consumed). For this reason, a change in loss can be regarded as a change in active power.

  As shown in FIG. 6, when the mounting metal 51A is iron (magnetic metal), the effective power of the mounting metal 51A, the case 2 (the detection surface 21 and the side surface portion 24), and the detected object A is in a region where the oscillation frequency is low. Will increase. That is, the detection distance increases. On the other hand, these active powers decrease in the region where the oscillation frequency is high. That is, the detection distance decreases.

  The reason why the active power changes in this way is considered as follows. That is, since the leakage magnetic flux from the detection coil 6 is less likely to be induced by iron at a high oscillation frequency, the loss generated in the side surface portion 24 increases. The eddy current generated by interlinking with the mounting metal 51A generates a counter magnetic flux that cancels the magnetic flux interlinking with the side surface portion 24, but the antimagnetic flux also increases. Therefore, the loss of the case 2 is reduced by canceling out the magnetic flux and the anti-magnetic flux interlinking with the side surface portion 24.

  FIG. 7 is a diagram showing a change in active power when the mounting metal 51A (METAL1) is aluminum.

  Referring to FIG. 7, as the oscillation frequency is lower, the active power at detection surface 21 and side surface portion 24 (KANAGU1, KANAGU2) increases in the negative direction. In other words, the lower the oscillation frequency, the lower the loss and the shorter the detection distance. This is because the aluminum has a high electrical conductivity, so that the antimagnetic flux due to the eddy current generated in the mounting metal 51A acts in a direction to cancel the loss generated in the side surface portion 24, and as a result, the effective power decreases. The lower the frequency, the more the magnetic flux passes through the side surface portion 24, and the eddy current generated in the mounting metal 51A becomes larger, so the effect becomes larger.

  6 and 7, when the mounting metal 51A is iron (magnetic material), the oscillation frequency at which the change in loss due to the presence or absence of the mounting metal can be set to 0 or near 0 exists in the range of 5 kHz to 100 kHz. I understand that These results show that if the oscillation frequency is set in the range of 5 kHz to 100 kHz, the detection distance can be prevented from being lowered (that is, the detection sensitivity of the sensor is reduced) even if the sensor is installed on the mounting metal (iron).

  FIG. 8 is a diagram showing a change in loss when the detected object approaches in the case where the attachment metal 51A is not present. The “case where the attachment metal 51A does not exist” is a case where the attachment metal 51A (METAL1) is set to air in the model of FIG.

  Referring to FIG. 8, the sensitivity of the proximity sensor is maximized when the change in loss is maximized before and when the detected object A (TARGET) approaches. As shown in FIG. 8, at the oscillation frequency of 20 kHz or less, the effective power of the detected object A increases and the effective power of the case 2 increases as the detected object A approaches. At this time, the active power of the detection coil 6 itself decreases (increases in the negative direction). As already mentioned, an increase in active power means an increase in loss. In addition, the oscillation frequency when the change in the overall loss is maximum is equal to the oscillation frequency when the sensitivity of the sensor is maximum.

FIG. 9 is a diagram showing a change in the total effective power with respect to the oscillation frequency.
Referring to FIG. 9, the total active power of each component in FIG. 8 is shown in a graph. It can be seen from FIG. 9 that the total active power becomes maximum at an oscillation frequency of about 10 kHz. That is, when the size of the outer shape is M18, the oscillation frequency when the sensitivity of the sensor is maximum is about 10 kHz.

  FIG. 10 is a diagram illustrating a rate of change in conductance g of the detection coil 6 as the detected object A approaches.

  As described above, in an actual proximity sensor, a change in active power is regarded as a change in conductance g of the detection coil 6, and a change in oscillation amplitude of the coil is detected by circuit processing. Therefore, changes in parameters such as active power obtained by CAE analysis can be replaced with changes in conductance g.

  Referring to FIG. 10, the detection coil 6 when the detected object A (iron) is moved away from the rated detection distance of the proximity sensor 1 (“predetermined distance” in the present invention) by 10% of the rated detection distance. The rate of change of conductance g is shown.

  Here, the “rated detection distance” indicates a detection distance defined by a product standard, for example. Further, in the present embodiment, the change rate (unit:%) of conductance when the detection distance D shown in FIG. 2 is increased by 10% of the rated detection distance from the rated detection distance is denoted by Δg. Δg is defined by the following equation (3).

Δg = (g _110% −g _100% ) / g _100% × 100 (3)
In Expression (3), g _100% is a conductance value when the detection distance D is the rated detection distance, and g _110% is a conductance value when the detection distance D is 1.1 times the rated detection distance.

  FIG. 10 shows changes in Δg with respect to the oscillation frequency when the thickness t of the detection surface 21 is 0.2 mm, 0.5 mm, and 0.8 mm. There are two curves when the thickness of the detection surface 21 is 0.8 mm (shown as t = 0.8 and t = 0.8C in FIG. 10). The change of (DELTA) g when it mutually differs is shown.

  At a certain oscillation frequency (for example, 30 kHz), Δg tends to decrease as the thickness of the detection surface 21 increases. This is because the thicker the detection surface 21, the harder the magnetic flux passes.

  The oscillation frequency corresponding to the black dot on each curve indicates an oscillation frequency at which the absolute value of Δg is maximum (detection sensitivity is maximum). The oscillation frequency at which the detection sensitivity is maximized decreases as the thickness of the detection surface 21 increases. Further, it can be seen from the curves at t = 0.8 and t = 0.8C that the detection sensitivity is hardly affected when the wire diameter of the coil is wound in the same volume.

FIG. 11 is a diagram showing a change in conductance depending on the presence or absence of the attachment metal 51.
Referring to FIG. 11, the change rate of the conductance g obtained when the proximity sensor 1 is attached to the attachment metal 51 with respect to the conductance g obtained when the proximity sensor 1 is not attached to the attachment metal 51. Show.

  In FIG. 11, the thickness of the detection surface 21 is set to 0.2 mm, 0.5 mm, and 0.8 mm, respectively. The oscillation frequency corresponding to the black circles shown on each curve is the oscillation frequency when the change in conductance becomes 0 (the change in conductance is minimal), and the detection distance is reduced even if the proximity sensor is provided on the mounting metal. Not oscillating frequency.

  From FIG. 11, it can be seen that as the thickness of the detection surface 21 increases, the oscillation frequency at which the conductance change becomes zero decreases. If the oscillation frequency is lowered, the conductance change increases in the positive direction, and if the oscillation frequency is lowered, the conductance change increases in the negative direction. This tendency is common regardless of the thickness of the detection surface 21.

  Increasing and decreasing the conductance change rate means increasing and decreasing the detection distance, respectively. Therefore, with reference to the oscillation frequency indicated by the black dot in FIG. 11, the case where the magnetic metal (mounting metal) is present around the case 2 and the case where the magnetic metal is not present correspond to the change in the distance between the detected object and the coil. By setting the oscillation frequency range so that the change in conductance is within a predetermined range (for example, 0 ± 0.5%), even if the proximity sensor is attached to a magnetic metal, the detection distance can be prevented from decreasing or the detection distance can be decreased. Within the product standard.

  As described above, in the present embodiment, the detection surface 21 needs to be made of a nonmagnetic material (for example, SUS material) having a high volume resistivity. Hereinafter, the reason will be supplementarily described with reference to FIGS.

  FIG. 12 is a diagram showing the rate of change of the circuit constant according to the presence or absence of an attachment metal when the detection surface 21 is a SUS material.

  With reference to FIG. 12, the change rate with respect to the oscillation frequency of resistance value R, inductance value L, and conductance g is shown, respectively. In FIG. 12, the change rate of the resistance value R is a ratio indicating how much the resistance value when the proximity sensor is installed on the mounting metal is changed with respect to the resistance value when the proximity sensor is not installed on the mounting metal. . The same applies to the rate of change of each of the inductance value L and the conductance g.

  When the detection surface 21 is a SUS material, an oscillation frequency at which the change rate of the conductance g is 0% exists between 20 kHz and 40 kHz. This tendency is the same as in FIG.

  FIG. 13 is a diagram showing the rate of change of the circuit constant depending on the presence or absence of the mounting metal when the detection surface is resin. Referring to FIG. 13, the oscillation frequency at which the change rate of conductance g is 0% does not exist in the range of 0 to 100 kHz.

  From FIG. 12 and FIG. 13, it is necessary to use a metal that is a non-magnetic material and has a high volume resistivity for the detection surface in order to prevent the influence of the attachment metal on the detection sensitivity in this embodiment. Specifically shown.

  FIG. 14 is a diagram showing an optimum range of conductance g in the proximity sensor of the present embodiment.

  Referring to FIG. 14, the relationship between the value of conductance g and the oscillation frequency in the state where the detected object A is located at the rated detection distance is shown. The graph of FIG. 14 shows changes in conductance g when the thickness of the detection surface 21 is 0.2 mm, 0.5 mm, and 0.8 mm. There are two curves when the thickness of the detection surface 21 is 0.8 mm (t = 0.8, t = 0.8C). These curves have different wire diameters of the detection coil 6 as in FIG. The change in conductance is shown.

The value of conductance g at the rated detection distance (that is, the value of g _100% ) is an important value for adjusting the detection distance of the proximity sensor 1. In order to suppress current consumption, it is desirable that the conductance g itself is small. In the present embodiment, the range of conductance g is set between 10 μS and 150 μS. The range of conductance g is more preferably between 30 μS and 100 μS. Here, the unit μS represents “micro Siemens”.

  As shown in FIG. 14, the value of conductance g depends on the oscillation frequency, and the influence of the thickness of the detection surface 21 is small. The oscillation frequency can be determined by changing the wire diameter of the detection coil 6 and adjusting the conductance. The reason is that changing the wire diameter of the detection coil 6 changes the number of turns of the detection coil 6 and changes the inductance value. The conductance g can be reduced by reducing the wire diameter of the coil, and the conductance g can be increased by increasing the wire diameter.

  As described above, by selecting an oscillation frequency (for example, a frequency in the range of 10 to 50 kHz) that satisfies the detection sensitivity shown in FIG. 10, the influence of the mounting metal shown in FIG. 11, and the conductance g shown in FIG. The proximity sensor of the present embodiment can realize the maximum detection distance, minimization of the influence of the attachment metal (iron material) (not affected by attachment to the iron material), and reduction of current consumption. The characteristics of the proximity sensor created using the parameters obtained by the above analysis will be described.

FIG. 15 is a diagram showing measured values of the proximity sensor actually created.
Referring to FIG. 15, the value of Δg (“Δg” in FIG. 15) is used as a measurement value of a proximity sensor using a SUS case (the thickness of the case detection surface is 0.8 mm) whose outer size is M18. ), The difference in Δg depending on the presence or absence of the mounting metal (iron) (shown as “g change rate at 0 mm” in FIG. 15), and the value of conductance g.

  In this proximity sensor, the optimum oscillation frequency is set to about 12 kHz. The reason is that the detection sensitivity, that is, the absolute value of Δg (approximately 1.0%) is a value that can be detected by the detection circuit (more than the minimum detectable sensitivity), and the conductance change (approximately This is because 0.5%) is close to the minimum value of 0%, and the value of conductance g at the rated detection distance (about 70 μS) falls within the desired range (30 to 100 μS).

FIG. 16 is a diagram showing the results of actual measurement of the influence of the mounting metal.
Referring to FIG. 16, when the protruding distance of detection surface 21 from the mounting metal is 0 mm, the change rate of the detection distance by the mounting metal is as small as about + 3%. Further, when the detection surface 21 is protruded by 4 mm from the attachment metal 51 and a SUS303 hexagon nut is attached to the protrusion (in the attachment state shown in FIG. 21), the change in the detection distance is 0%. Thus, by setting the oscillation frequency optimally, the proximity sensor 1 can reduce the influence on the detection distance due to the mounting metal.

  FIG. 17 is a diagram showing the conductance change rate according to the protruding distance of the detection surface 21 from the mounting metal when the oscillation frequency is changed from the optimum value. Here, the optimum value is the oscillation frequency when the conductance change is minimum when the protrusion distance is 0 mm, the conductance value is within a predetermined range, and the detection sensitivity (Δg) is within the detectable range.

  Referring to FIG. 17, when the oscillation frequency becomes lower than the optimum value (about 12 kHz in this example), conductance g increases and the detection distance increases. On the other hand, when the oscillation frequency increases, the conductance g decreases and the detection distance decreases. Thus, it can be said that the result of FIG. 17 is equal to the tendency of the CAE analysis result shown in FIG. From FIG. 17, if the oscillation frequency is set lower than the oscillation frequency when the absolute value of Δg is maximum (in this example, about 15 kHz), the proximity sensor is installed on the mounting metal (iron). Since it is possible to suppress the change within a predetermined range with respect to the protruding distance of the detection surface 21 from the mounting metal, a certain amount of detection distance can be secured even if the detection distance decreases.

  A specific example of the design of the detection coil 6 is shown. In the present embodiment, the width and height of the detection coil are determined according to the following equation (4).

(Rated detection distance) × (height of detection coil 6) / (width of detection coil 6) ≧ (predetermined value) (4)
That is, the width and height of the detection coil 6 are determined so that the product of the detection distance and the ratio of the height of the detection coil 6 to the width of the detection coil 6 is equal to or greater than a predetermined value. The “width” and “height” of the detection coil 6 indicate the dimensions a and b of the detection coil 6 in FIG. That is, Expression (4) is expressed as D × (b / a) ≧ (predetermined value).

FIG. 18 is a table showing the design values of the detection coil 6 as a list.
Referring to FIG. 18, for each case outline size (size standard), detection distance (A), detection coil 6 height (B), detection coil 6 width (C), and equation (4) The obtained results are shown. FIG. 18 shows that the result obtained by equation (4) is greater than 7. However, this predetermined value may be a value other than 7, and can be appropriately set according to the performance required for the proximity sensor.

  The proximity sensor according to the present embodiment is not limited to the one having the configuration shown in FIG. The proximity sensor of the present embodiment may have the following configuration, for example.

FIG. 19 is a cross-sectional view showing another configuration example of the proximity sensor of the present embodiment.
Referring to FIG. 19, the proximity sensor 1 includes a detection surface 21 and a side surface portion 24 as a single member.

  As described above, according to the present embodiment, the oscillation frequency of the oscillation circuit is determined based on the distance between the object to be detected and the coil depending on whether or not the mounting metal made of magnetic metal is present around the case. The difference in conductance change rate with respect to the change is selected so as to be within a predetermined range. As a result, even if the proximity sensor is installed on the mounting metal, a decrease in the sensitivity of the sensor can be suppressed, so that the detection surface of the sensor can be arranged on the same surface as the main surface of the mounting metal, When approaching from the side, collision with the proximity sensor can be prevented. Therefore, the replacement frequency of the sensor due to the failure of the proximity sensor can be greatly reduced.

Moreover, according to this Embodiment, the side surface and detection surface of a proximity sensor are comprised with a nonmagnetic metal. As the nonmagnetic metal, a metal having a relative permeability of substantially 1 and a volume resistivity of 72 × 10 ⁻⁸ (Ω · m) or more is used. Thereby, since it becomes easy for magnetic flux to pass through a detection surface, the fall of detection distance (decrease in detection sensitivity) can be prevented. In this embodiment, by using stainless steel as such a nonmagnetic metal, the strength of the case can be secured and the cost can be reduced.

  Further, according to the present embodiment, the conductance g is set between 10 μS and 150 μS (more preferably between 30 μS and 100 μS). Thereby, power consumption can be reduced. In addition, according to the present embodiment, by setting the conductance change depending on the presence or absence of the attachment metal to a predetermined range (for example, 0 ± 0.5%), it is possible to realize a proximity sensor with stable performance, The production yield of the proximity sensor can be improved.

  Further, according to the present embodiment, the above-described oscillation frequency is determined to be lower than the oscillation frequency when the absolute value of Δg is maximized. Thereby, even if the proximity sensor is installed on the mounting metal and the detection distance is reduced, a certain detection distance can be secured.

  According to the present embodiment, the width and height of the detection coil are determined such that the product of the ratio of the coil height to the coil width and the detection distance is equal to or greater than a predetermined value. Thereby, the above-described proximity sensor can be specifically realized.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is an external view of the proximity sensor of this Embodiment. It is a schematic diagram which shows the cross section of the proximity sensor 1 of FIG. It is a figure which shows the structure of the circuit board 10 of FIG. It is a figure explaining the magnetic flux which generate | occur | produces in the proximity sensor 1 of FIG. It is a figure which shows the magnetic field analysis model of the proximity sensor 1 of FIG. It is a figure which shows the change of active power in case the attachment metal 51A (METAL1) is iron. It is a figure which shows the change of active power in case the attachment metal 51A (METAL1) is aluminum. It is a figure which shows the change of a loss when a to-be-detected object approaches when the attachment metal 51A does not exist. It is a figure which shows the change of the whole active power with respect to an oscillation frequency. It is a figure which shows the change rate of the conductance g of the detection coil 6 accompanying the approach of the to-be-detected object A. FIG. It is a figure which shows the conductance change by the presence or absence of the attachment metal 51. FIG. It is a figure which shows the change rate of the circuit constant according to the presence or absence of an attachment metal in case the detection surface 21 is a SUS material. It is a figure which shows the change rate of the circuit constant according to the presence or absence of an attachment metal in case the detection surface is resin. It is a figure which shows the range of the optimal conductance in the proximity sensor of this Embodiment. It is a figure which shows the measured value of the proximity sensor actually produced. It is a figure which shows the result of having actually measured the influence of the attachment metal. It is a figure which shows the conductance change rate by the protrusion distance of the detection surface 21 from an attachment metal when changing an oscillation frequency from the optimal value. It is a table | surface which shows the design value of the detection coil 6 with a list. It is sectional drawing which shows another structural example of the proximity sensor of this Embodiment. It is a figure which shows an example of the installation state of a proximity sensor. It is a figure which shows another example of the installation state of a proximity sensor. It is a figure which shows another example of the installation state of a proximity sensor. It is a figure which shows another example of the installation state of a proximity sensor. It is a figure which shows an example of the conventional method which makes it possible to prevent the fall of a detection distance. It is a figure which shows another example of the conventional method which makes it possible to prevent the fall of a detection distance. It is a figure which shows another example of the conventional method which can prevent the fall of a detection distance. It is a figure which shows another example of the conventional method which can prevent the fall of a detection distance.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1,101 Proximity sensor, 2,102 Case, 5 Signal cable, 6,106 Detection coil, 8,108 Core, 10 Circuit board, 21,121 Detection surface, 23 Thread, 24 Side surface part, 31 Oscillation circuit, 32 Detection circuit, 33 Output circuit, 34 Detection circuit, 35 Comparison circuit, 51, 51A, 51B, 151 Mounting metal, 152, 153 Hex nut, 154 Shield plate, 155 ring, 156 Auxiliary coil, A Detected object, FL0 to FL4 Magnetic flux.

Claims (10)

A proximity sensor for detecting the approach of a detected object made of metal,
Coils,
An oscillation circuit connected to the coil;
A detection circuit for detecting that the detected object has approached the coil by a change in conductance of the coil during the oscillation operation of the oscillation circuit;
A case made of a non-magnetic metal and including at least the case for storing the coil;
The oscillation frequency of the oscillation circuit is the difference in the change rate of the conductance with respect to the change in the distance between the object to be detected and the coil when the magnetic metal is present around the case and when the magnetic metal is not present. Is a proximity sensor that is set to fall within a predetermined range. The case is
Including a detection surface provided between the detected object and the coil;
2. The proximity sensor according to claim 1, wherein the detection surface is made of a metal having a relative permeability of substantially 1 and a volume resistivity of 72 × 10 ⁻⁸ (Ω · m) or more.   The proximity sensor according to claim 2, wherein a material of the detection surface is stainless steel. The case is
It further includes a side surface provided to surround at least the detection surface and the coil,
4. The proximity sensor according to claim 2, wherein the side surface part is made of a metal having a relative permeability of substantially 1 and a volume resistivity of 72 × 10 ⁻⁸ (Ω · m) or more. .   The proximity sensor according to claim 4, wherein a material of the side surface portion is stainless steel. The proximity sensor according to claim 1, wherein the change rate of the conductance is represented by the following equation:
(G _110% -g _100% ) / g _100% x100
However, g _100% is a value of the conductance when a detection distance between the detected object and the proximity sensor is a predetermined distance, and g _110% is 1.1 of the predetermined distance. It is the value of the conductance when it is doubled. The proximity sensor according to claim 6, wherein the value of g _{100% is} a value between 10 microsiemens and 150 microsiemens. The proximity sensor according to claim 7, wherein the value of g _{100% is} a value between 30 microsiemens and 100 microsiemens.   The proximity sensor according to claim 6, wherein the width and height of the coil are determined such that a product of a ratio of the height of the coil to the width of the coil and the predetermined distance is equal to or greater than a predetermined value.   The proximity sensor according to claim 6, wherein the oscillation frequency is set to be lower than a frequency when the absolute value of the change rate of the conductance according to the equation becomes maximum.

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