Optical sensor

Abstract

To provide an optical sensor that is less susceptible to disturbance light while inhibiting increase in cost.SOLUTION: An optical sensor comprises: a light projecting element projecting detection light; a light receiving element receiving the detection light reflected on an object; a housing storing the light projecting element and the light receiving element; a transmission window provided on one surface of the housing, and transmitting at least part of the detection light travelling toward the light receiving element; and a reflection member having wavelength selectivity of reflecting, toward the light receiving element, light of a detection wavelength band including at least part of a wavelength band of the detection light, of light incident from the transmission window while not reflecting at least one of a wavelength band in a higher frequency side than the detection wavelength band and a wavelength band in a lower frequency side than the detection wavelength band.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an optical sensor.

Background Art

[0002] An optical sensor is known that projects detection light onto a measurement object and receives the reflected light with a light receiving element to measure the distance to the object or the displacement of the object (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In view of the fact that such an optical sensor is used in various environments, the light receiving element may receive disturbing light other than the light projected by the light projecting element. In order to remove the disturbing light, for example, it is conceivable to give wavelength selectivity to a plate glass which is a light receiving window installed in a housing, and to configure it to mainly transmit a wavelength band including the wavelength of the light projected by the light projecting element. However, the light receiving window is often designed to be relatively large, and it requires a great deal of cost for forming a thin film for giving wavelength selectivity. In addition, other functional coatings such as an antifouling coating may be applied to the light receiving window forming one surface of the housing, and it is difficult to impart effective wavelength selectivity.

[0005] The present invention has been made to solve such problems, and an object thereof is to provide an optical sensor that is less affected by disturbing light while suppressing an increase in cost.

Means for Solving the Problems

[0006] In one aspect of the present invention, an optical sensor includes a light projecting element that projects detection light, a light receiving element that receives the detection light reflected by an object, a housing that houses the light projecting element and the light receiving element, a transmission window provided on one surface of the housing that transmits at least a part of the detection light directed toward the light receiving element, and a reflection member that reflects light in a detection wavelength band including at least a part of the wavelength band of the detection light among the light incident from the transmission window in the direction of the light receiving element and has wavelength selectivity not to reflect at least one of a wavelength band on the higher frequency side than the detection wavelength band and a wavelength band on the lower frequency side than the detection wavelength band.

[0007] By interposing a reflection member having such wavelength selectivity, it is not necessary to provide the transmission window with the function of a band-pass filter, so that cost can be reduced. In addition, it becomes easier to apply a coating with other functions to the transmission window. Also, it becomes easier to make the transmission window common among models with different sizes of condenser lenses and types of light receiving elements. Furthermore, by providing the reflection member, the degree of freedom in optical path design increases, which in turn contributes to miniaturization of the housing.

[0008] In the above optical sensor, the transmission window may transmit light in the detection wavelength band among the incident light in the direction of the reflection member, and one of the transmission window and the reflection member functions as a high-pass filter and the other functions as a low-pass filter, so that a band-pass filter is constituted by the transmission window and the reflection member. Since the transmission window can be relatively easily provided with wavelength selectivity as long as it functions as a high-pass filter or a low-pass filter, it can share the role with the reflection member. By configuring in this way, it is possible to cope with differences between models by adjusting the wavelength selectivity of the reflection member while making the transmission window common among different models.

[0009] At this time, the transmission window may be a red transparent plate. For example, a red acrylic plate is a widely used member, so it is advantageous in terms of availability and cost.

[0010] In addition, in the above-described optical sensor, a condenser lens for condensing the detection light reflected by the object onto the light receiving element may be provided between the transmission window and the reflecting member. According to such a configuration, since the reflecting member is arranged on the light receiving element side with respect to the condenser lens, the reflecting surface of the reflecting member can be made smaller, wavelength selectivity can be imparted at low cost, and it also contributes to miniaturization of the housing.

Effects of the Invention

[0011] According to the present invention, it is possible to provide an optical sensor that is less susceptible to the influence of ambient light while suppressing an increase in cost.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

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Modes for Carrying Out the Invention

[0013] Hereinafter, the present invention will be described through embodiments of the invention, but the invention according to the claims is not limited to the following embodiments. Also, not all of the configurations described in the embodiments are essential as means for solving the problems.

[0014] FIG. 1 is a perspective view schematically showing the usage state of the optical sensor 100. The optical sensor 100 according to the present embodiment is a sensor that measures the distance to the workpiece W as an object, the displacement of the workpiece W, or detects the presence or absence of the workpiece W, and is installed and used, for example, on a manufacturing line in a factory. The optical sensor 100 projects detection light L1 from a light projecting element, which is a laser diode for example, toward the workpiece W as a detection target, and receives the detection light L2 reflected by the workpiece W and returning with a light receiving element, which is a CMOS sensor for example, and measures the distance to the workpiece W and the displacement of the workpiece W based on the light receiving position. Further, the presence or absence of the workpiece W can also be detected based on the presence or absence of the detection light L2.

[0015] The detection light L1 passes through the light projecting window 113 provided on one surface of the housing and is projected onto the workpiece W. When the detection light L1 reaches and is reflected by the workpiece W, a part of it returns toward the light receiving window 114 as the detection light L2. The detection light L2 enters the light receiving window 114 at an angle corresponding to the distance of the workpiece W. The light receiving element receives the detection light L2 and outputs a detection signal corresponding to the distance to the workpiece W.

[0016] The detection signal is transmitted to the amplifier unit via the cable 112. The amplifier unit converts the received detection signal into a numerical value and displays it on the display unit, or outputs it to an external device such as a PLC or a PC. Note that the optical sensor 100 may incorporate the function of the amplifier unit. In that case, the housing 111 includes a display unit that displays numerical values and the like and a communication unit that communicates with an external device. Also, as shown in the figure, the x-axis, y-axis, and z-axis are defined. In the following drawings, the same coordinate axes as in FIG. 1 are also shown together, so as to indicate the orientation of the components represented by each drawing.

[0017] FIG. 2 is a cross-sectional view of the optical sensor 100. Specifically, it is a cross-sectional view cut along the xz plane including the detection light L1. Inside the housing 111, a base frame 120 is accommodated, and the light-emitting element 121, the light-emitting lens 122, the light-receiving lens 123, the mirror 124, and the light-receiving element 125 are directly or indirectly fixed to the base frame 120. The exterior material constituting the housing 111 is also fixed to the base frame 120 via screws.

[0018] The light-emitting element 121 and the light-emitting lens 122 are arranged on the base frame 120 such that the detection light L1 emitted from the light-emitting element 121 is adjusted by the light-emitting lens 122 to be, for example, parallel light rays and is projected toward the work W through substantially the center of the light-emitting window 113. The light-emitting window 113 is, for example, a plate glass of a parallel plate, and an antifouling coating is applied to the outer surface.

[0019] The light-receiving lens 123 condenses the detection light L2 that has entered the housing through the light-receiving window 114 and forms an image on the light-receiving surface of the light-receiving element 125. The mirror 124 is arranged on the optical path of the detection light L2 condensed by the light-receiving lens 123 and reflects the detection light L2 to guide it to the light-receiving surface of the light-receiving element 125. The imaging position of the detection light L2 changes along one direction (baseline direction) according to the distance to the work W. The pixels of the light-receiving element 125 are arranged in a plurality along the baseline direction and output a detection signal indicating the intensity distribution according to the imaging position of the detection light L2. The light-receiving lens 123, the mirror 124, and the light-receiving element 125 are arranged on the base frame 120 so as to satisfy such an optical relationship.

[0020] Now, generally, optical sensors are assumed to be used in various environments, and the light-receiving element may receive light incident from the light-receiving window under certain conditions, not limited to the detection light reflected by the workpiece W. The presence of such disturbing light causes misdetection. Laser light, which is often used as detection light in optical sensors, has a narrower wavelength band (spectral width) and higher intensity compared to general ambient light. FIG. 3 is a diagram schematically showing the intensity characteristics of the detection light (L1, L2) output by the laser diode employed as the light-projecting element 121 of the optical sensor 100 according to the present embodiment. The horizontal axis represents the wavelength, and the vertical axis represents the intensity with the peak being 100%. As shown in the figure, the detection light has a wavelength spectrum limited to a narrow band of approximately 660 nm to 700 nm.

[0021] If such laser light is used, in order to remove disturbing light, a band-pass filter that cuts off wavelength bands other than the wavelength band of the laser light may be interposed in the optical path of the detection light. For example, it is conceivable to impart such wavelength selectivity by forming a thin film on the light-receiving window. However, since the light-receiving window is designed to be relatively large, forming a thin film to provide wavelength selectivity as a band-pass filter requires a great deal of cost. In addition, the light-receiving window forming one surface of the housing needs to be provided with other functional coatings such as an antifouling coating, and it is difficult to impart effective wavelength selectivity.

[0022] Therefore, in the optical sensor 100 according to the present embodiment, wavelength selectivity is imparted to the reflection by the mirror 124. Specifically, it is realized by alternately depositing dielectric thin films of high refractive index and low refractive index on the surface of the mirror 124 by vapor deposition to form a reflection film. Specifically, as the high refractive index material, TiO 2 , Ta203, etc., and as the low refractive index material, SiO 2 , MgF 2By using the like and laminating them alternately with each having a film thickness of about a quarter wavelength of the design wavelength (660 to 700 nm), the reflected light from each layer interface becomes in phase with the surface reflected light, and a reflective film can be formed. It is possible to impart wavelength selectivity as a band-pass filter to the mirror 124, but it can be realized relatively inexpensively by imparting wavelength selectivity as a high-pass filter or a low-pass filter to the light receiving window 114. Further, when imparting wavelength selectivity as a high-pass filter or a low-pass filter to the mirror 124, the number of layers of the thin film formed on the surface can be reduced compared to the case of imparting wavelength selectivity as a band-pass filter, so it is also advantageous in terms of manufacturing cost. Therefore, in the present embodiment, by imparting wavelength selectivity as a high-pass filter to the light receiving window 114 and imparting wavelength selectivity as a low-pass filter to the mirror 124, the wavelength selectivity of the band-pass filter is realized together.

[0023] FIG. 4 is a diagram schematically showing the transmittance wavelength characteristics of the light receiving window 114. The horizontal axis represents the wavelength, and the vertical axis represents the transmittance. The light receiving window 114 is based on a transparent resin plate or a glass plate, and the wavelength selectivity shown in the figure is obtained by applying a multilayer coating to its surface.

[0024] According to the example of the figure, the light receiving window 114 has wavelength selectivity such that in the range of practical incident angles (for example, 0° to 30°), the transmittance is less than 5% in the band less than 630 nm and 90% or more in the band 660 nm or more. According to the light receiving window 114 having such wavelength selectivity, almost all the light amount of the detection light L2 is transmitted, and the disturbance light less than 630 nm is almost blocked.

[0025] If such a coating is applied to the inner surface of the light-receiving window 114 facing the inside of the housing, other functional coatings such as an antifouling coating or an antireflection coating can be applied to the outer surface of the housing. Not limited to the antifouling coating, a coating for wavelength selection may be applied to the outer surface of the housing, and other functional coatings may be applied to the inner surface of the housing. Also, not limited to the case where wavelength selectivity is imparted by a multilayer film coating, for example, even if a red transparent plate is used as the light-receiving window 114, wavelength selectivity similar to that in FIG. 4 can be provided. For example, a red transparent plate such as a red acrylic plate is inexpensive, so the adoption of a red transparent plate is preferable from the viewpoint of manufacturing cost.

[0026] FIG. 5 is a diagram schematically showing the reflectance wavelength characteristics of the mirror 124. The horizontal axis represents the wavelength, and the vertical axis represents the reflectance. The mirror 124 is, for example, a dichroic mirror that reflects light in a specific wavelength band and transmits light in other wavelength bands.

[0027] According to the example of the figure, the mirror 124 has wavelength selectivity such that in the range of practical incident angles (for example, 20° to 60°), the reflectance is 85% or more in the band from 550 nm to 680 nm and less than 5% in the band of 720 nm or more. According to the mirror 124 having such wavelength selectivity, almost all the detection light L2 is reflected, and the disturbance light of 720 nm or more is substantially blocked.

[0028] Particularly in the optical sensor 100, since the light-receiving lens 123 as a condensing lens for condensing the detection light L2 onto the light-receiving element 125 is disposed between the light-receiving window 114 and the mirror 124, the reflecting surface of the mirror 124 can be made smaller by the amount of the detection light L2 being condensed. Therefore, not only can wavelength selectivity be realized at low cost, but it also contributes to the miniaturization of the housing 111.

[0029] FIG. 6 is a diagram schematically showing the wavelength characteristics of the reach rate reaching the light receiving element 125 by synthesizing the characteristics of the light receiving window 114 and the mirror 124. The horizontal axis represents the wavelength, and the vertical axis represents the reach rate reaching the light receiving element 125. Specifically, it represents the ratio for each wavelength of the light incident on the light receiving window 114 that reaches the light receiving element 125. Substantially, it is obtained by multiplying the graphs of FIGS. 4 and 5 for each wavelength, and it can be seen that it functions as a band-pass filter as a whole. Note that, in order to make it a band-pass filter as a whole, it is not limited to the case of imparting the wavelength selectivity of a high-pass filter to the light receiving window 114 and a low-pass filter to the mirror 124 as in the present embodiment, and the reverse combination may also be used.

[0030] Also, such wavelength selectivity as a band-pass filter can be realized by the mirror 124 alone. In this case, it is not necessary to apply a coating for wavelength selection to the light receiving window 114. If the mirror 124 alone is given the function of a band-pass filter, for example, between different models, while making the light receiving window 114 common, by adjusting the wavelength selectivity of the mirror 124, it is possible to cope with differences between models. For example, between models that employ laser diodes with different intensity characteristics, the wavelength selectivity of the mirror 124 may be adjusted so as to be a band-pass filter that matches each intensity characteristic.

[0031] In the optical sensor 100 described above, the light projecting window 113 and the light receiving window 114 are separate bodies, but these may be integrally formed as one transmission window. Also, the optical sensor 100 employs the mirror 124 as a reflecting member, but the reflecting member capable of imparting wavelength selectivity is not limited to this. A dichroic prism or the like may be used. It is more desirable to impart wavelength selectivity to the reflecting member as in the optical sensor 100 according to the present embodiment than to impart wavelength selectivity to the transmission member from the viewpoint of effectively utilizing the internal space of the housing 111. That is, by interposing the reflecting member, the degree of freedom in the layout of the light receiving element 125 increases, which is advantageous from the viewpoint of miniaturizing the housing 111.

[0032] [Appendix] A light projecting element (121) that projects detection light (L1), A light receiving element (125) that receives the detection light (L2) reflected by an object (W), A housing (111) that houses the light projecting element (121) and the light receiving element (125), A transmission window (114) provided on one surface of the housing (111) that transmits at least a part of the detection light (L2) heading toward the light receiving element (125), A reflection member (124) that reflects light in a detection wavelength band including at least a part of the wavelength band of the detection light (L2) among the light incident from the transmission window (114) in the direction of the light receiving element (125), and has wavelength selectivity not to reflect at least one of a wavelength band on the higher frequency side than the detection wavelength band and a wavelength band on the lower frequency side than the detection wavelength band An optical sensor (100) comprising the above.

Explanation of Reference Numerals

[0033] 100... Optical sensor, 111... Housing, 112... Cable, 113... Light projection window, 114... Light reception window, 120... Base frame, 121... Light projecting element, 122... Light projection lens, 123... Light reception lens, 124... Mirror, 125... Light receiving element

Claims

1. A light projecting element that projects detection light, A light receiving element that receives the detection light reflected by an object, A housing that houses the light projecting element and the light receiving element, A transmission window provided on one surface of the housing that transmits at least a part of the detection light directed toward the light receiving element, A reflection member having wavelength selectivity that reflects light in a detection wavelength band including at least a part of the wavelength band of the detection light among the light incident from the transmission window in the direction of the light receiving element, and does not reflect at least one of the wavelength band on the higher frequency side than the detection wavelength band and the wavelength band on the lower frequency side than the detection wavelength band and comprising the transmission window transmits light in the detection wavelength band among the incident light in the direction of the reflection member, An optical sensor that constitutes a band-pass filter with the transmission window and the reflection member by one of the transmission window and the reflection member functioning as a high-pass filter and the other functioning as a low-pass filter.

2. The optical sensor according to claim 1, wherein the transmission window is a red transparent plate.

3. The optical sensor according to claim 1 or 2, further comprising a condenser lens that condenses the detection light reflected by an object onto the light receiving element between the transmission window and the reflection member.

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