Angle detection structure, polarizing encoder, and use of phase shift element having spatial distribution of optical axis in polarizing encoder

Abstract

To provide technology that makes it possible to heighten the resolution of a polarizing encoder.SOLUTION: A polarizing encoder 10 comprises: a light source 1; a polarizing plate A2 for receiving incidence of light from the light source 1; a phase shift element 3 having the spatial distribution of an optical axis capable of freely converting the polarization direction of incident light from the polarizing plate A2; a polarizing plate B4 for receiving incident light from the phase shift element 3; a sensor 5 for detecting incident light from the polarizing plate B4; and signal processing means 6 for processing detected light by the sensor 5 and generating angle data. Either the polarizing plate A2 or the polarizing plate B4 is constituted so as to change the polarization direction of incident light by rotation.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an angle detection structure, a polarization type encoder, and the use of a retarder having a spatial distribution of optical axes in a polarization type encoder, and more particularly to a technique for improving the resolution of a polarization type encoder. [Background technology]

[0002] AC servo motors, which are widely used in the industrial equipment industry, including industrial robots and semiconductor / liquid crystal manufacturing equipment, are a field in which Japan boasts a global advantage, and their performance is improving day by day. Ten years ago, the response speed was at most a few hundred Hz, but now servo systems with response speeds approaching 5 kHz, which is about 10 times faster, are commercially available and have become widespread. This performance improvement is largely due to both the improvement of dedicated CPUs and FPGAs that realize high-speed sampling control, and the improvement of the resolution of angle detectors (encoders) that obtain angle information from the servo motor shaft. Of these, the former is currently improving smoothly, and as for the latter, the encoder, by increasing its angle resolution, we can expect to improve the controllability and efficiency of servo motors.

[0003] Encoders include so-called "resolvers" that are made of windings and electromagnetic steel plates, and "optical encoders" that detect the rotation angle by the transmission or non-transmission of light. Resolvers have excellent environmental resistance to temperature, humidity, vibration, etc., and are therefore used in high-temperature, high-vibration engines, space equipment in cryogenic environments, and robots used under high temperatures, high humidity, and high vibration. On the other hand, optical encoders that use disks engraved with patterns are mainly used for applications such as industrial robots that require high-speed operation, processing equipment that requires high precision, and elevators that require controllability for a comfortable ride, as higher resolution is required.

[0004] Various developments have been made to increase the resolution of such conventional optical encoders, and while ten years ago the resolution was 17 bits / turn, today encoders with a resolution of 27 bits / turn are commercially available. However, there is a structural limit to the increase in resolution of conventional optical encoders that use slits and gratings, and the barrier of 27 bits (approximately 1 / 130 million) has not been overcome in recent years. Even if we continue to pursue higher resolution, we are already reaching our limit. Therefore, it is certain that this will lead to a deterioration in the position accuracy within the control period of high-speed sampling control.

[0005] Therefore, as an alternative to the conventional method, a polarized encoder has already been developed that applies the principle of generating a shutter effect by placing a grating with a polarizing effect between a disk with a linear polarizing effect and a light receiving element, thereby obtaining a sine wave-like waveform (JP Patent Publication No. 2016-53562, JP Patent Publication No. 2018-128276, etc.). With this method, angle information can be obtained by processing the obtained two-phase sine wave.

[0006] In addition to the above-mentioned documents, patent applications and the like related to such a polarization type encoder have been filed. For example, Patent Document 1, which will be described later, discloses a configuration in which an optical encoder that keeps the light intensity of a light source constant includes an optical scale whose polarization direction changes with rotation, a detection unit that detects incident light via the optical scale, and a control unit that controls the light emission of the light source, the detection unit including a first detection unit that detects the intensity of light separated in a first polarization direction, a second detection unit that detects the intensity of light separated in a second polarization direction, a third detection unit that detects the intensity of light separated in a third polarization direction, and a fourth detection unit that detects the intensity of the incident light, and the control unit feedback controls the light intensity of the light source so that the intensity of the incident light detected by the fourth detection unit is within a preset range of light intensity. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2019-7753 A "Optical sensor and optical encoder" Summary of the Invention [Problem to be solved by the invention]

[0008] FIG. 10 is an explanatory diagram showing the configuration and operation of a conventional polarization type encoder. That is, in a conventional polarization type encoder 710, the light passing through the polarizing slit of the rotator (polarizing plate) 72 is polarized in a uniform direction, and the rotation angle is calculated by measuring two periods of sinusoidal light intensity according to the angle of the rotator 72 by the arrangement angle of the polarizing light shielding plate 74 on the photodetector element (analyzer) 75. With this method, it is impossible to detect the absolute angle during one rotation of the rotator 72. On the contrary, since it is not possible to obtain a change in light intensity of two or more periods per rotation, the only way to improve the resolution at present is to upgrade the photodetector element 75 and the signal processing circuit 76. However, this inevitably increases the cost.

[0009] Furthermore, to improve the resolution of an optical encoder, it is generally necessary to narrow the slit width of the disk or analyzer. However, the dimensional accuracy, positional relationship, and angle detection accuracy of each part have almost reached their limits, and from the perspective of mass production, it is extremely difficult to improve the accuracy of processing and assembly.

[0010] The problem that the present invention aims to solve is to provide a new method and technology that enables polarization-based encoders to achieve higher resolution than ever before, in order to overcome the limitations of the conventional technology. [Means for solving the problem]

[0011] As a result of studying the above problems, the inventor of the present application came to the idea that the problems can be solved by using a retarder having a spatial distribution of an optical axis capable of freely changing the polarization direction of light, which the inventor of the present application has already invented. In other words, by disposing a retarder having a spatial distribution of an optical axis capable of freely changing the polarization direction between a rotator and an analyzer disposed in front of a light receiving element, it is possible to increase or decrease the light intensity received by a light detecting element per one rotation as a sinusoidal wave of one or many periods, or linearly. In addition, by using a sensor arranged in an array such as an image sensor as the light detecting element to extract a larger number of signals, it is possible to improve the resolution. Based on this idea, the present invention has been completed. That is, the invention claimed in this application as a means for solving the above problems, or at least the invention disclosed therein, is as follows.

[0012] [1] An angle detection structure comprising a polarizing plate A which receives light from a light source, a retarder having a spatial distribution of optical axes capable of freely converting the polarization orientation of the incident light from said polarizing plate A, and a polarizing plate B which receives the incident light from said retarder, wherein at least one of said polarizing plate A or said polarizing plate B changes the polarization orientation of the incident light by rotation. [2] A polarization type encoder comprising a light source, a polarizing plate A which receives light from the light source, a retarder having a spatial distribution of optical axes which can freely convert the polarization orientation of the incident light from the polarizing plate A, a polarizing plate B which receives the incident light from the retarder, a sensor which detects the incident light from the polarizing plate B, and a signal processing means which processes the detected light by the sensor to generate angle data, wherein at least one of the polarizing plate A or the polarizing plate B changes the polarization orientation of the incident light by rotation. [3] In a polarization type encoder comprising a light source, a polarizing plate A which receives light from the light source, a polarizing plate B which detects the incident light from the polarizing plate A side, a sensor which detects the incident light from the polarizing plate B, and a signal processing means which processes the detected light by the sensor to generate angle data, a retarder having a spatial distribution of optical axes is used as a conversion means which freely converts the polarization state of the light incident from the polarizing plate A and outputs it to the polarizing plate B. [4] The polarization type encoder according to [2], characterized in that a Q-Plate is used as the retarder. Effect of the Invention

[0013] Since the angle detection structure, the polarization type encoder, and the use of the retarder having a spatial distribution of the optical axis in the polarization type encoder of the present invention are configured as described above, the resolution of the polarization type encoder can be dramatically improved compared to the conventional one. That is, the function and performance of the encoder can be dramatically improved, such as the detection of an absolute angle that was impossible with the conventional polarization type encoder, the improvement of the resolution by making it multi-period, and the simplification of the encoder by obtaining a linear change in light intensity. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is an explanatory diagram showing a basic configuration of a polarization type encoder according to the present invention. [Diagram 2] 1 is an explanatory diagram showing an example of use of a polarization type encoder according to the present invention; [Diagram 3] 1 is an explanatory diagram showing the operating principle of a polarization type encoder of the present invention using an example of a retarder; [Figure 4] FIG. 13 is an explanatory diagram showing another example of use of the polarization type encoder of the present invention. [Diagram 5] FIG. 1 is an explanatory diagram showing an experimental setup of an embodiment of the present invention. [Figure 6] FIG. 11 is an explanatory diagram showing an angle calculation method by image processing in the embodiment. [Figure 7] FIG. 7 is a diagram showing an angle calculation result by the calculation method shown in FIG. 6. [Figure 8] FIG. 11 is an explanatory diagram showing an angle calculation method based on pixel luminance information in the embodiment. [Figure 9] FIG. 9 is a diagram showing the results of angle calculation by the calculation method shown in FIG. 8. [Figure 10] 1 is an explanatory diagram showing the configuration and operation of a conventional polarization type encoder. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention will now be described in detail with reference to the drawings. Fig. 1 is an explanatory diagram showing the basic configuration of a polarization type encoder of the present invention. As shown in the figure, a deflection type encoder 10 of the present invention is composed of a light source 1, a polarizing plate A2 that receives light from the light source 1, a retarder 3 having a spatial distribution of optical axes that can freely change the polarization direction of the incident light from the polarizing plate A2, a polarizing plate B4 that receives the incident light from the retarder 3, a sensor (not shown) that detects the incident light from the polarizing plate B4, and a signal processing means 6 (not shown) that processes the detected light by the sensor to generate angle data, and the main configuration is that at least one of the polarizing plate A2 and the polarizing plate B4 changes the polarization direction of the incident light by rotation.

[0016] With this configuration, in the polarization type encoder 10 of the present invention, light from the light source 1 enters the polarizing plate A2, and this light is polarized by the polarizing plate A2. The light emitted from the polarizing plate A2 enters a retarder (hereinafter simply referred to as a "retarder") 3 having a spatial distribution of the optical axis, and its polarization orientation undergoes an extremely diverse conversion based on the fact that the retarder 3 has a cylindrically symmetric polarization spatial distribution. The light emitted from the retarder 3 enters the polarizing plate B4, and the emitted light polarized by the polarizing plate B4 is detected by a sensor. The light detected by the sensor is processed by a signal processing means to generate angle data.

[0017] In addition, at least one of the polarizing plate A2 arranged in front of the phase shifter 3 and the polarizing plate B4 arranged behind the phase shifter 3 has a rotator function, so that the polarization direction of the incident light is changed by rotation in at least one of them.

[0018] Also within the scope of the present invention is an angle detection structure 8 that is composed of a polarizing plate A2, a retarder 3, and a polarizing plate B4, and in which at least one of polarizing plate A and polarizing plate B changes the polarization direction of incident light by rotation. The angle detection structure 8 is a structure that constitutes the main part of the modified encoder 10 of the present invention.

[0019] Fig. 2 is an explanatory diagram showing an example of the use of the polarization type encoder of the present invention shown in Fig. 1. That is, by using the retarder 3, it is possible to freely change the polarization direction of the incident light, and by disposing it between two polarizing plates -- polarizing plate A2 and polarizing plate B4 -- it is possible to arbitrarily set the period of the sine wave or set it to change linearly, unlike the conventional method in which the light intensity received by the sensor 5 during rotation can only receive two periods of a sine wave per rotation. Therefore, it is possible to achieve high resolution and detect absolute angles.

[0020] The Q-Plate in the inventor's previous invention can be suitably used as the retarder 3 having the spatial distribution of the optical axis, which is a characteristic configuration of the deflection type encoder 10 of the present invention. The operating principle of the polarization type encoder of the present invention using such a retarder will be described.

[0021] 3 is an explanatory diagram showing the operation principle of the polarization type encoder of the present invention using an example of a retarder. As shown in the figure, the polarization type encoder 310 is composed of a light source 31 such as an infrared light source, a polarizing plate A (polarizer) 32 that receives light from the light source 31, a retarder 33, i.e., a Q-Plate, having a spatial distribution of optical axes that can freely convert the polarization direction of the incident light from the polarizing plate A (polarizer) 32, a polarizing plate B (analyzer) 34 that receives the incident light from the retarder 33, a sensor (not shown) such as an image sensor that detects the incident light from the polarizing plate B (analyzer) 34, and a signal processing means (not shown) that processes the detected light by the sensor to generate angle data. A magnifying lens system 30 is disposed between the light source 31 and the polarizing plate A (polarizer) 32.

[0022] In the configuration of the polarization type encoder 310, polarizer A32 is used as a rotator, and polarizer B34, which has similar transmission characteristics, is used as an analyzer to generate light and dark of light intensity that is linearly dependent on the rotation angle. The Q-Plate, which is a geometric phase element or retarder 33 arranged between the polarizers A32 and B34, converts the cylindrically symmetric polarization spatial distribution, thereby creating a sector-shaped light and dark image that rotates within a narrow range.

[0023] In conventional encoders, the angle information obtained was one-dimensional position information in a line, but in the present invention, the rotation angle information can be obtained from this sector-shaped light and dark image in a plane, that is, two-dimensionally, resulting in high resolution. Therefore, it is expected that a resolution comparable to that of expensive angle sensors can be achieved with simple assembly and at a low price.

[0024] 4 is an explanatory diagram showing another example of use of the polarization type encoder of the present invention. The polarization type encoder 410 of this example uses a sensor array such as an image sensor in which two or more sensors are arranged as the sensor 45. This makes it possible to obtain multiple sine waves and to detect rotation by detecting the rotation of the image through image processing.

[0025] It is within the scope of the present invention to use the retarder 3 etc. having the spatial distribution of the optical axis as described above as a conversion means for freely converting the polarization state of light incident from polarizing plate A2 etc. and outputting it to polarizing plate B4 etc. in a polarization type encoder 10 etc. consisting of a light source 1 etc., polarizing plate A2 etc. that receives light from the light source 1 etc., polarizing plate B4 etc. that detects incident light from the polarizing plate A2 etc. side, a sensor 5 etc. that detects incident light from polarizing plate B4 etc., and a signal processing means 6 etc. that processes the detected light by the sensor etc. to generate angle data. EXAMPLES

[0026] Examples of the present invention will be described below, but the present invention is not limited thereto. Note that the examples will be described by outlining some of the experiments conducted in the process of completing the present invention. 5 is an explanatory diagram showing an experimental setup of an embodiment of the present invention. As shown in the figure, in this experiment, a polarization type encoder 510 was configured using a Q-Plate as a retarder 53, a polarizing plate 52 was rotated at a constant speed by an A / C servo motor, and a sector-shaped bright and dark image was acquired by an image sensor 55. Furthermore, software using OpenCV and a Raspberry Pi were used as signal processing means 56, whereby the acquired sector-shaped bright and dark image image was processed and the rotation angle was calculated.

[0027] The following two methods were used to calculate the rotation angle. First, Fig. 6 is an explanatory diagram showing an angle calculation method by image processing in the embodiment. Also, Fig. 7 is a diagram showing the angle calculation result by the calculation method shown in Fig. 6. That is, the obtained image is binarized, and then the angle is calculated from the center of gravity calculation of OpenCV. As a result, the desired resolution was obtained.

[0028] Next, Fig. 8 is an explanatory diagram showing an angle calculation method using pixel brightness information in the embodiment. Also, Fig. 9 is a diagram showing the result of angle calculation using the calculation method shown in Fig. 8. The image obtained by angle calculation using pixel brightness information has the same degree of polarization on radial lines from the center of rotation, and changes sinusoidally with respect to the angle. This brightness change is calculated by calculating the brightness on two straight lines with a phase of 2 / πrad, and angle calculation is performed. As a result, the desired resolution was obtained. [Industrial Applicability]

[0029] The angle detection structure, the polarization type encoder, and the use of the retarder having a spatial distribution of the optical axis in the polarization type encoder of the present invention can dramatically improve the resolution of the polarization type encoder compared to the conventional one. Therefore, this invention has high industrial applicability in the fields of encoder manufacturing and use and all related fields. [Explanation of symbols]

[0030] 1, 31, 41, 51…light source 2, 32, 42, 52...Polarizing plate A 3, 33, 43, 53...Retarder with spatial distribution of optical axis 4, 34, 44, 54...Polarizing plate B 5, 45, 55…sensor 6, 46, 56...Signal processing means 8...Angle detection structure 10, 310, 410, 510... Deflection type encoder 30, 50...Magnifying lens system 580...Servo motor 71...Light source 72...Rotor (polarizing plate) 74...Polarizing light shielding plate 75...Photodetector (analyzer) 76...Signal processing circuit 710... Polarization Encoder

Claims

1. A polarizing plate A that receives light from a light source; a retarder having a spatial distribution of an optical axis capable of freely converting the polarization direction of the incident light from the polarizing plate A; a polarizing plate B for receiving the incident light from the retarder, At least one of the polarizing plates A and B changes the polarization direction of the incident light by rotation. The angle detection structure is characterized in that

2. A light source; a polarizing plate A that receives light from the light source; a retarder having a spatial distribution of an optical axis capable of freely converting the polarization direction of the incident light from the polarizing plate A; a polarizing plate B for receiving incident light from the retarder; A sensor for detecting incident light from the polarizing plate B; a signal processing means for processing the light detected by the sensor to generate angle data; At least one of the polarizing plates A and B changes the polarization direction of the incident light by rotation. A polarization type encoder comprising:

3. A light source; a polarizing plate A that receives light from the light source; a polarizing plate B for detecting incident light from the polarizing plate A side; A sensor for detecting incident light from the polarizing plate B; and a signal processing means for processing the detected light by the sensor to generate angle data. A retarder having a spatial distribution of optical axes is used as a conversion means for freely converting the polarization state of light incident from the polarizing plate A and outputting the light to the polarizing plate B.

4. 3. The polarization type encoder according to claim 2, wherein a Q-Plate is used as the retarder.

Images