Linear encoder

Abstract

To provide a linear encoder capable of obtaining a temperature of a glass scale.SOLUTION: A linear encoder comprises: a scale 110 on which a plurality of tracks 111, 112 is formed along a prescribed measurement direction; a light-emitting element and a light receiving element arranged facing one another with the scale 110 interposed therebetween; and a sensor unit receiving light emitted from the light-emitting element and passing through the scale 110 by the light receiving element for light source conversion. One or more thin films 113 including thermochromic material are disposed on the scale 110.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a linear encoder mounted on a machine tool or the like, and more particularly to a linear encoder that optically detects a scale made of a glass plate. [Background technology]

[0002] A schematic configuration of a linear encoder and a numerical control device mounted on a conventional machine tool will be described with reference to the drawings. FIG. 9 is a schematic configuration diagram showing an example of a conventional linear encoder and a numerical control device. A scale unit 901a of a linear encoder 901 is fixed to a table 905 to be controlled, while a sensor unit 901b of the linear encoder 901 is fixed to a bed (not shown), detects the position of the scale unit 901a, and outputs the position data to a numerical control device 902. A ball screw nut 908 is fixed to the table 905 and is in a non-rotating state. The ball screw nut 908 can move in the axial direction with the rotation of a ball screw 907. Both ends of the ball screw 907 are supported by tack bearings (not shown), and are connected to the output shaft of a motor 904 via a coupling 906. A rotary encoder 903 is provided on the load shaft side of the motor 904, detects the rotation position of the motor 904, and outputs the position data to a numerical control device 902. The numerical control device 902 supplies a control current to a motor 904 to rotate the motor 904 in order to move a controlled object (table 905) to a desired position based on position data from a rotary encoder 903 and position data from a sensor unit 901b.

[0003] Now, the significance of the existence of the linear encoder 901 will be explained. In the example of FIG. 9, if the linear encoder 901 is not installed, the position of the table 905 to be controlled is indirectly controlled by the rotational position output by the rotary encoder 903. Since the position is detected indirectly, the table 905 cannot be controlled to the intended position. That is, the weight of the table 905 and the weight of the workpiece mounted on the table 905 (not shown) cause frictional force to be generated on a guide surface (not shown), which in turn causes twisting in the ball screw 907. This twisting cannot be detected by the rotational position of the rotary encoder 903. As a result, the table 905 cannot be controlled to the intended position. However, if the linear encoder 901 is installed, the position of the table 905 can be directly detected, and therefore the significance of its existence is established.

[0004] The linear encoder 901 also has a reason to exist against thermal displacement caused by the temperature environment. When the ball screw 907 rotates, heat is generated between the ball screw nut 908, which is constrained to not rotate. Heat is also generated at the guide surface between the table 905 and a bed (not shown). If a workpiece mounted on the table 905 is processed by spraying coolant liquid, this can cool it. The bed and table 905 are also cooled in an ambient environment using air conditioning equipment. In such cases, if the linear encoder 901 is installed, the effects of thermal displacement can be reduced to a certain degree.

[0005] In order to further reduce the influence of thermal displacement, a widely known technique is to embed a temperature sensor such as a thermistor in the table 905 or a bed (not shown) and correct the amount of thermal displacement. Since the table 905 and bed, which are generally made of cast steel, and the glass material constituting the scale unit 901a of the linear encoder 901 have different linear expansion coefficients, the linear expansion difference according to the linear expansion coefficient of the linear encoder 901 is also taken into account in the stage of correcting the amount of thermal displacement.

[0006] Even so, the temperature sensor of the table 905 and the glass scale in the scale unit 901a of the linear encoder 901 are far apart, and the actual temperature of the glass scale cannot be grasped. In addition, the scale unit 901a is generally protected by sealing compressed air in order to protect it from dust and water from the outside. When this compressed air is released into the scale unit 901a, the gas temperature drops according to Boyle's law. As a result, the ambient temperature inside the scale unit 901a tends to be lower than the ambient temperature outside. Therefore, the temperature of the glass scale often becomes lower than the temperature of the table 905 in which the temperature sensor is embedded. In addition, compressed air is generally introduced from either the left end or the right end of the scale unit 901a, and the glass scale on the side where the compressed air is introduced, either the left or right, is cooled to a lower temperature. As a result, there will be a temperature difference between the temperature detected by the temperature sensor embedded in the table 905 and the temperature of the glass scale in the scale unit 901a of the linear encoder 901, causing an error in the correction of the amount of thermal displacement.

[0007] Of course, it is theoretically possible to fix a plurality of temperature sensors directly to the glass scale. FIG. 10 is a cross-sectional view showing an example of a cross section of a conventional linear encoder 901. The scale unit 101a has a glass scale 110 on which a lattice pattern made of a thin chrome film is laid. The glass scale 110 is placed in a groove of an aluminum cover 114 and partially pressed down by a round rubber band 115. The center part of the glass scale 110 in the longitudinal direction is firmly fixed by an adhesive (not shown). The opening of the aluminum cover 114 is sealed by lip seals 116a and 116b. The sensor unit 101b has a light-emitting unit 117 made of an LED and a light-receiving unit 118 made of a photodiode. The light-emitting unit 117 and the light-receiving unit 118 are placed on the upper part of a frame housing 119 so as to sandwich the glass scale 110. FIG. 11 shows an example of a front view of a conventional glass scale 110. The hatched areas are light-shielding films formed by vapor-depositing a thin film of chromium, and the other areas are light-transmitting. A fine digit track 111 repeating in a regular pattern is laid on the lower side of the glass scale 110, and an absolute position digit track 112 in an irregular pattern using an M-sequence code is laid on the upper side.

[0008] The light emitted from the light emitting unit 117 passes through an area of ​​the glass scale 110 that is not blocked by the chrome light-shielding film, and reaches the light receiving unit 118. The light that passes through the fine digit track 111 becomes a moire fringe pattern when it passes through an index scale (not shown) installed in the light receiving unit 118, and is photoelectrically converted by a photodiode for the fine digit to become a two-phase sinusoidal electric signal that is perpendicular to each other. The light that passes through the absolute position digit track 112 is converted into an electric signal by an image sensor unit made of a photodiode installed in the light receiving unit 118. Although not shown in FIG. 10, a processing circuit board is built in the lower part of the frame housing of the sensor unit 101b. An electric signal from the light receiving unit 118 is input to this processing circuit board. The two-phase sinusoidal electric signal derived from the light that passes through the fine digit track 111 is subjected to an interpolation calculation process. From the result of this interpolation calculation process, the electric angle within one pitch length of the fine digit track 111 is calculated. The signal derived from the light transmitted through the absolute position digit track 112 is converted into a coarse absolute position signal. This coarse absolute position signal is combined with the electrical angle within one pitch length described above to calculate fine absolute position data. In addition to the arithmetic circuits that perform calculations on this position data, the processing circuit board also includes a communication circuit for communicating with the numerical control device.

[0009] In this structure, if one were to directly attach a new temperature sensor such as a thermistor to the glass scale, it would be necessary to change the distance from the LED light source of the light-emitting unit 117 to the light-receiving unit 118, which would make space extremely tight. Furthermore, when the layout of the wiring for connecting the temperature sensor is also taken into consideration, the space becomes even more limited.

[0010] Therefore, Patent Document 1 discloses a technology that can grasp the temperature of the glass scale itself without installing a temperature sensor directly on the scale. However, in Patent Document 1, two scale sections with different linear expansion coefficients must be prepared. Since the scale sections require precision, they tend to be the most expensive component of the scale unit. Preparing twice as many such scale sections as usual was a negative factor in terms of cost. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] JP 2005-195367 A Summary of the Invention [Problem to be solved by the invention]

[0012] As described above, a linear encoder mounted on a machine tool or the like is affected by thermal displacement to a certain extent. Therefore, there is a method of embedding a temperature sensor in the table 905 or bed of the machine tool to measure the temperature and perform thermal displacement correction. However, because the temperature sensor and the glass scale are distant from each other and because of the influence of compressed air supplied to the glass scale, a difference is likely to occur between the temperature detected by the temperature sensor and the temperature of the glass scale. As a result, an error occurs in the correction of the thermal displacement amount. Therefore, it is possible to directly attach a temperature sensor to the glass scale 110, but as shown in the example of FIG. 10, there is not enough space in the scale unit 101a. Even if the temperature sensor could be attached, the distance from the LED light source of the light-emitting unit 117 to the light-receiving unit 118 would have to be changed, which could impair the requirements for optical detection. In addition, when the handling of the wiring material for connecting the temperature sensor is also taken into consideration, it can be seen that the space becomes even more limited. In addition, in the technology of Patent Document 1, two scale parts with different linear expansion coefficients must be prepared, which increases costs.

[0013] In consideration of the above-mentioned problems, this specification discloses a linear encoder that can grasp the temperature of a glass scale without changing the size of the linear encoder, without compromising the optical detection requirements, and without significantly increasing costs. [Means for solving the problem]

[0014] The linear encoder disclosed in this specification comprises a scale on which a plurality of tracks are formed along a specified measurement direction, and a sensor unit including a light-emitting element and a light-receiving element arranged opposite each other on either side of the scale, in which light from the light-emitting element that has passed through the tracks is received by the light-receiving element and converted into a light source, and is characterized in that the scale is further provided with one or more thin films made of a thermochromic material.

[0015] In this case, the one or more thin films may include a plurality of thin films having different thermochromic properties from each other.

[0016] In this case, the plurality of thin films may be laid side by side in a direction perpendicular to the measurement direction.

[0017] The device may further include a processing circuit, which calculates position information based on a signal obtained by photoelectrically converting light that has passed through the track, and calculates temperature information of the scale based on a signal obtained by photoelectrically converting light that has passed through the thin film.

[0018] In this case, the processing circuit may output the position information and the temperature information in association with each other.

[0019] The processing circuit may also calculate an amount of linear expansion of the scale based on the temperature information, and correct the position information based on the amount of linear expansion. Effect of the Invention

[0020] According to the linear encoder described above, it is possible to grasp the temperature of the glass scale itself, which is the object to be detected for position detection, without increasing the size of the encoder or significantly increasing the cost. [Brief description of the drawings]

[0021] [Figure 1] FIG. 2 is a front view showing an example of a glass scale of a linear encoder. [Diagram 2] 4 is a front view showing an example of a light receiving element portion of the light receiving unit; FIG. [Diagram 3] FIG. 11 is a front view showing another example of the glass scale of the linear encoder. [Figure 4] FIG. 4 is a diagram showing an example of a serial communication data frame output from a linear encoder. [Diagram 5] FIG. 13 is a diagram showing an example of a table in which absolute positions and temperature data are stored in association with each other. [Figure 6] FIG. 6 is a graph showing the relationship between the absolute position and temperature data in FIG. 5. [Figure 7] FIG. 13 is a graph showing the amount of expansion / contraction of all sections. [Figure 8] 13 is a graph showing an amount of error calculated from an amount of expansion / contraction. [Figure 9] FIG. 1 is a schematic configuration diagram showing an example of a conventional linear encoder and a conventional numerical control device. [Figure 10] FIG. 1 is a cross-sectional view showing an example of a cross section of a conventional linear encoder. [Figure 11] FIG. 1 is a diagram showing an example of a front view of a conventional glass scale. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The configuration of a linear encoder will be described below with reference to the drawings. FIG. 1 is a front view showing an example of a glass scale constituting a linear encoder, and the same components as those in FIG. 11 are given the same reference numerals. Also, explanations of the same processes and structures as those in FIG. 10 and FIG. 11 will be omitted. As shown in FIG. 1, a glass scale 110 has a thin film 113 made of a thermochromic material laid between a fine digit track 111 and an absolute position digit track 112. This thin film 113 made of a thermochromic material is in close contact with the glass scale 110, is at substantially the same temperature as the glass scale 110, and exhibits a color change reaction according to the temperature.

[0023] FIG. 2 is a front view showing an example of a light receiving element part of a light receiving unit of a linear encoder. As shown in the conventional example of FIG. 10, the light receiving element part 201 of the light receiving unit is arranged to face the glass scale 110 with a gap therebetween. The light receiving element 202a receives the moire diffracted light transmitted through the fine digit track 111 and the index scale (not shown), and outputs a signal obtained by photoelectrically converting the received light as a cosine positive phase signal. The light receiving element 202b is arranged in a phase shifted by a predetermined phase difference Δ1 from the light receiving element 202a in the measurement axis direction (the direction in which the pitch of the lattice of the fine digit track 111 is arranged). This phase difference Δ1 is Δ1=(n×λ+λ / 2), where λ is the length of one pitch of the fine digit track 111. The light receiving element 202b receives the moire diffracted light that has passed through the fine track 111 and the index scale (not shown), and outputs a signal obtained by photoelectrically converting the received light as a cosine negative phase signal. The light receiving element 202c is arranged in a phase shifted by a predetermined phase difference Δ2=(n×λ+λ / 4) from the light receiving element 202a in the measurement axis direction. The light receiving element 202c receives the moire diffracted light that has passed through the fine track 111 and the index scale (not shown), and outputs a signal obtained by photoelectrically converting the received light as a sine positive phase signal. The light receiving element 202d is arranged in a phase shifted by Δ3=(n×λ+3×λ / 4) from the light receiving element 202a in the measurement axis direction. The light receiving element 202d receives the moire diffracted light transmitted through the fine digit track 111 and the index scale (not shown), and outputs a signal obtained by photoelectrically converting the received light as a sine negative phase signal. The positive cosine signal and the negative cosine signal are differentially amplified by a processing circuit board (not shown) to become a cosine signal. The positive sine signal and the negative sine signal are similarly differentially amplified by a processing circuit board (not shown) to become a sine signal. The mutually orthogonal two-phase signals thus obtained are subjected to an interpolation operation as described above in the background art.

[0024] The light receiving element 203 performs spectroelectric conversion of a predetermined number of bits on the light transmitted through the absolute position digit track 112, and outputs an absolute position signal. The subsequent processing is the same as that described above in the background art, and a description thereof will be omitted.

[0025] The light receiving element 204 receives the light filtered by the discoloration state of the thin film 113 made of a thermochromic material, and outputs a signal obtained by photoelectrically converting the received light as a temperature signal. The temperature signal is converted into temperature data by a processing circuit board (not shown). The conversion from the temperature signal to temperature data will be described. It is necessary to select a material composition for the thin film 113 made of a thermochromic material that has a discoloration characteristic in the operating temperature range of the linear encoder. In addition, the wavelength of the light on the LED side (not shown) is selected so that the transmittance of the light is easily changed due to the discoloration characteristic of the thin film 113 made of a thermochromic material described above, and the composition of the photodiode constituting the light receiving element unit 201 (for example, a Si photodiode or an InGaAsPIN photodiode) is selected so that the composition has sufficient photoelectric conversion characteristics for the wavelength of the light on the LED side. As the temperature of the glass scale 110 changes, the thin film 113 made of a thermochromic material changes color, and this color change changes the transmittance of the light emitted from the LED when it passes through the thin film 113. A voltage value corresponding to this transmittance, that is, a temperature signal, is obtained by photoelectric conversion in the light receiving element 204. The processing circuit board converts the obtained temperature signal into temperature data. For this conversion, the processing circuit board may store, for example, a comparison table between the actual temperature and the temperature signal, or a temperature calculation formula using the temperature signal as a variable, in a memory or the like. The voltage value (temperature data) output from the light receiving element 204 is amplified to an appropriate level by an amplifier and converted into digital information by an analog-to-digital converter.

[0026] In this way, the selected combination is converted into temperature data. However, there is a possibility that errors may occur due to variations in the coating thickness of the thin film 113, individual variations in the amount of light emitted by the LED, individual variations in the sensitivity of the light receiving element 204, etc. Therefore, at the manufacturing stage of the scale unit and the sensor unit, each combination may be installed in equipment that can change the environmental temperature, such as a low-temperature thermostatic bath, and the relationship between the actual temperature environment and the temperature signal level of the light receiving element 204 may be measured, and the result may be set in the aforementioned comparison table of actual temperature and temperature signal.

[0027] Next, another example of a linear encoder will be described. FIG. 3 is a front view showing a glass scale of another linear encoder. In FIG. 3, the same components as those in FIG. 1 are given the same reference numerals. In this glass scale 110, three layers of thin films 113a, 113b, and 113c made of a thermochromic material are laid between a fine digit track 111 and an absolute position digit track 112. The thin film 113a laid in the upper layer has a color change characteristic in which the thin film 113a maintains a predetermined color at 5° C. or less, gradually fades when the temperature exceeds 5° C., becomes an almost intermediate color at 10° C., and is completely faded at 15° C., and maintains the faded state at 15° C. or more. The thin film 113b made of a thermochromic material laid in the middle layer has a color change characteristic in which the thin film 113b maintains a color at 15° C. or less, gradually fades when the temperature exceeds 15° C., becomes an almost intermediate color at 20° C., and is completely faded at 25° C., and maintains the faded state at 25° C. or more. The thin film 113c made of a thermochromic material laid in the lower layer has a coloring property in which it develops color at 25° C. or less, gradually fades above 25° C., becomes almost an intermediate color at 30° C., completely fades at 35° C., and maintains the faded state above 35° C. In this way, by laying three types of thin films 113a, 113b, and 113c with different coloring properties in three layers, the temperature change of the glass scale 110 in the range of 5° C. to 35° C. is expressed as a gradation-like change in color of the three layers of thin films 113a, 113b, and 113c. In other words, by laying three types of thin films 113a, 113b, and 113c with different thermochromic properties in three layers, the difference in color change can be expressed in a wider temperature range.

[0028] The light transmitted through the three stages of thin films 113a, 113b, and 113c is photoelectrically converted by the light receiving element 204 to become a temperature signal, as in the examples of Figs. 1 and 2. The process of converting the temperature signal into temperature data may be the same as described above. The number of stages of the thin films 113a, 113b, and 113c to be laid may be changed as appropriate. Similarly, the color change characteristics of the thin films 113a, 113b, and 113c and how they are laid may also be changed as appropriate.

[0029] Next, an example of a serial communication data frame output from a linear encoder will be described. FIG. 4 is a diagram showing an example of a serial communication data frame output from a linear encoder. The communication data frame output from a linear encoder is configured in the following order from the beginning: a sync code (SYNC), a start flag (SF), a destination address (DA), a source address (SA), position data (P), a position detection status (ST), a first extension data (EX1), a second extension data (EX2), a cyclic redundancy check (CRC) code, and an end flag (EF). The start flag (SF) is a specific code indicating that the linear encoder starts communication. The destination address (DA) is a net address of a destination to which the linear encoder transmits communication, and in most cases, is a net address assigned to a higher-level numerical control device. The source address (SA) is a net address of the linear encoder itself. As described above, the position data (P) is position data obtained by processing signals from the fine digit track and the absolute position digit track in the processing circuit board. The position detection status (ST) displays the detection status and communication status of the linear encoder, such as warning level and alarm level. The first extension data (EX1) and the second extension data (EX2) are data areas that can be freely set when using the linear encoder. For example, the amplitude of each signal is set in the first extension data (EX1) and the second extension data (EX2), or an internal variable in the microcomputer mounted on the processing circuit board is set. The cyclic redundancy check (CRC) code is a literal code, and an explanation is omitted. The end flag (EF) is a specific code that indicates that the linear encoder has ended communication.

[0030] Here, the temperature data obtained from the temperature signal photoelectrically converted by the light receiving element 204 is set in either the first extension data (EX1) or the second extension data (EX2), or in both. The need to set the temperature data in both the first extension data (EX1) and the second extension data (EX2) is, for example, when the data size of the temperature data does not match the data size of any of the extension data (EX1, EX2). In this way, by setting the temperature data in either the first extension data (EX1) or the second extension data (EX2), or in both, the position data (P) and the temperature data are included in the same communication data frame. In other words, the absolute position of the linear encoder and the temperature at that absolute position are output in association with each other. Therefore, the numerical control device, which is the upper unit of the linear encoder, can grasp the absolute position of the linear encoder and the temperature distribution at that position, and can perform thermal displacement correction taking this into consideration. How to perform thermal displacement correction using the temperature associated with the absolute position of the linear encoder may be freely set as appropriate.

[0031] Next, an example of the use of temperature information obtained by a linear encoder will be described. As described above, in the linear encoder disclosed in this specification, the absolute position of the linear encoder can be correlated with temperature data. The correlation between the absolute position and the temperature data is stored as a table in a memory in a processing circuit board. FIG. 5 is a diagram showing an example of a table in which the absolute position and the temperature data are correlated and stored in a linear encoder. The absolute position has a width of 50 mm, and the latest temperature data when the absolute position of the linear encoder exists within that range is continuously updated. FIG. 6 is a graph showing the relationship between the absolute position and the temperature data in FIG. 5. As described in the background art, compressed air is usually introduced from either the left end or the right end of the scale unit. In the example of FIG. 6, compressed air is introduced from the right end side, that is, the plus stroke end side. Therefore, it can be seen that the temperature on the plus stroke side is relatively lower than the temperature on the minus stroke side.

[0032] As explained in the prior art of FIG. 10, the glass scale 110 is partially pressed in the groove of the aluminum cover 114 by a round rubber band 115. The central portion of the glass scale 110 in the longitudinal direction is firmly fixed with an adhesive (not shown). The linear expansion coefficients of the aluminum cover 114 and the glass scale 110 are significantly different. Therefore, if the two are firmly fixed over the entire longitudinal area of ​​the glass scale 110, the glass scale 110 may crack and break. Therefore, the central portion of the glass scale 110 is firmly fixed, and the linear expansion difference is a structure that allows the glass scale 110 to freely expand and contract to the left and right. In the case of this example, the point at the absolute position of 500 mm is firmly fixed, and this becomes the base point for the expansion and contraction of the glass scale 110. Next, the linear expansion coefficient of the glass material of the glass scale 110 is 8.0×10 ―6 The expansion / contraction amount of each absolute position is calculated using the standard temperature of 20°C and an absolute position of 500 mm as the base point. Note that the calculations described below are performed by an arithmetic calculator such as a microprocessor mounted on the processing circuit board.

[0033] The amount of expansion and contraction ΔL from the base point to a point a distance L away is expressed as ΔL = α × ΔT × L, where α is the linear expansion coefficient of the material of the glass scale 110, and ΔT is the value obtained by subtracting the reference temperature from the temperature of the measurement point. In the example of FIG. 5, the linear expansion coefficient α of the material of the glass scale 110 is 8.0 × 10 ―6 m / °C, and the reference temperature is 20°C.

[0034] Based on this, first, the amount of expansion / contraction at the absolute position of 500 mm is considered. The absolute position 500 mm is the base point, and since L=0, the amount of expansion / contraction ΔL at the absolute position 500 mm is 0 μm. Next, the amount of expansion / contraction from the absolute position 500 mm to the absolute position range 500-550 mm is considered. In this case, the amount of expansion / contraction at 525 mm, which is the center position of the absolute position range 500-550 mm, is calculated. Therefore, in this case, L=525-500=25 mm. Furthermore, in the example of FIG. 5, the temperature difference ΔT in the absolute position range 500-550 mm is ΔT=23.0°C-20°C=+3°C. Therefore, the amount of expansion / contraction ΔL from the absolute position 500 mm to the absolute position range 500-550 mm is ΔL=(8.0×10 ―6 m / ℃×3℃×25×10 ―3 m) = 0.6 × 10 ―6 m = 0.6 μm.

[0035] Next, consider the amount of expansion and contraction from the absolute position range of 500-550 mm to the absolute position range of 550-600 mm. In this case, the distance L between both ranges is L = 575 - 525 = 50 mm. In addition, in the example of FIG. 5, the difference ΔT between the temperature in the absolute position range of 550-600 mm and the reference temperature is ΔT = 23°C - 20°C = +3°C. Therefore, the amount of expansion and contraction ΔL from the absolute position range of 500-550 mm to the absolute position range of 550-600 mm is ΔL = (8.0 x 10 ―6 m / ℃×3℃×50×10 ―3 m) = 1.2 × 10 ―6 And the amount of expansion and contraction from the absolute position of 500 mm to the absolute position range of 550 to 600 mm is 1.2 μm + 0.6 μm = 1.8 μm.

[0036] A similar calculation can be performed for the glass scale on the negative side from the base point, and the position of 475 mm, the center position of the 450 to 500 mm absolute position range, is an extension of 0.54 μm from the base point, and the position of 425 mm, the center position of the 400 to 450 mm absolute position range, is an extension of 1.54 μm from the base point.

[0037] FIG. 7 shows a graph in which the amount of expansion and contraction in all sections is calculated in this manner. Since the temperature is higher than the reference temperature of 20°C in the range of absolute positions 0 to 850 mm, the amount of expansion and contraction of the glass scale 110 on both the negative and positive sides is a positive value, that is, it is expanded to both sides, as viewed from the base point 500 mm. If this is converted into an error amount as the absolute position of the linear encoder associated with the amount of expansion and contraction of the glass scale 110, it becomes as shown in the graph in FIG. 8. The processing circuit board built into the linear encoder generates and outputs position data to correct this error amount. Note that the graph in FIG. 8 only shows the error amount at the base point 500 mm position and the absolute positions at 50 mm intervals, but for absolute positions in between, the error amount can be obtained by linear interpolation. For example, at the absolute position 305 mm, the error amount at the absolute position 275 mm is -4.38 μm and the error amount at the absolute position 325 mm is -3.54 μm, so interpolation is performed as shown in formula (1).

[0038]

number

[0039] In addition, since the error amount cannot be linearly interpolated in the absolute position range of 0 to 25 mm and the absolute position range of 975 to 1000 mm, the error amount of -7.54 μm at the absolute position of 25 mm may be used as it is in the absolute position range of 0 to 25 mm, and the error amount of 4.44 μm at the absolute position of 975 mm may be used as it is in the absolute position range of 975 to 1000 mm. Alternatively, it may be obtained by extrapolation. In this way, the linear expansion from the base point of the glass scale 110 is calculated based on the temperature signal output from the light receiving element 204, and the position data is corrected, thereby realizing the linear encoder according to the fourth claim of the present application.

[0040] It should be noted that the linear encoder may freely select whether to output position data in association with temperature information as shown in Fig. 4, or to correct position data based on temperature information as shown in Fig. 8. For example, the specification to which the linear encoder corresponds may be set in the initialization protocol at the time of initialization start-up between the linear encoder and the numerical control device, which is the upper unit to which the linear encoder is connected.

[0041] Moreover, all of the configurations described so far are merely examples, and other configurations may be changed as appropriate as long as the configuration of claim 1 is included. For example, the composition, shape, number, etc. of the material of the thin film 113 made of a thermochromic material may be changed as appropriate. Also, the wavelength of the light of the LED may be changed as appropriate. In the example shown in FIG. 5, the temperature data is acquired at intervals of 50 mm, but this interval may also be changed as appropriate. Therefore, this interval may be narrowed to make more precise correction. Furthermore, although the linear encoder with a stroke length of 1000 mm has been described as an example, the stroke length of the linear encoder may also be changed as appropriate. In any case, as long as the thin film 113 made of a thermochromic material is laid on the glass scale 110 to detect the temperature of the glass scale 110 itself, other configurations may be changed. [Explanation of symbols]

[0042] 101a, 901a scale unit, 101b, 901b sensor unit, 110 glass scale, 111 fine digit track, 112 absolute position digit track, 113, 113a, 113b, 113c thin film, 114 aluminum cover, 115 round rubber band, 116a, 116b lip seal, 117 light emitting unit, 118 light receiving unit, 119 frame housing, 201 light receiving element section, 202a, 202b, 202c, 202d, 203, 204 light receiving element, 902 numerical control device, 903 rotary encoder, 904 motor, 905 table, 906 coupling, 907 ball screw, 908 ball screw nut.

Claims

1. a scale having a plurality of tracks formed along a specified measurement direction; a sensor unit including a light emitting element and a light receiving element disposed opposite to each other with the scale interposed therebetween, the light receiving element receiving light emitted from the light emitting element and transmitted through the track, and converting the light into a light source; Equipped with The scale further includes one or more thin films of a thermochromic material. A linear encoder comprising:

2. 2. The linear encoder of claim 1, The linear encoder according to claim 1, wherein the one or more thin films include a plurality of thin films having different thermochromic properties from one another.

3. 3. The linear encoder according to claim 2, A linear encoder, characterized in that the plurality of thin films are laid side by side in a direction perpendicular to the measurement direction.

4. 2. The linear encoder of claim 1, further comprising: A processing circuit is provided, the processing circuit calculates position information based on a signal obtained by photoelectrically converting the light transmitted through the track, and calculates temperature information of the scale based on a signal obtained by photoelectrically converting the light transmitted through the thin film. A linear encoder comprising:

5. 5. A linear encoder according to claim 4, The linear encoder according to claim 1, wherein the processing circuit outputs the position information and the temperature information in association with each other.

6. 6. A linear encoder according to claim 4, The linear encoder according to claim 1, wherein the processing circuit calculates an amount of linear expansion of the scale based on the temperature information, and corrects the position information based on the amount of linear expansion.

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