JP5824342B2 - Linear encoder - Google Patents

Description

  The present invention relates to a linear encoder that can be used for position measurement of a machine tool such as a milling machine or a semiconductor manufacturing apparatus, and more particularly to a gap detection technique in a signal processing unit inside the linear encoder.

  A conventional linear encoder will be described with reference to FIG. FIG. 3 is a perspective view illustrating an example of a signal detection unit of a general optical linear encoder. A linear encoder used as a position detection device for a moving axis of a machine tool or the like includes a scale 10 having a periodic main grid scale 16 and a slider 15 that moves linearly relative to the scale 10. . The slider 15 includes a light-transmitting index scale 13 in which four periodic sub-lattices corresponding to the main grid scale 16 are formed in a square shape, and four light-receiving elements 14 corresponding to the four sub-lattices. And a collimator lens 11 that collimates the light emitted from the light emitting element 12. Then, the light quantity change caused by the relative movement of the slider 15 in the longitudinal direction of the scale 10 is photoelectrically converted by the light receiving element 14, thereby obtaining four signals a1, a2, b1, b2 having the same period and different phases. .

These four signals a1, a2, b1, b2 are connected to Sig. Assuming L, the position POS is derived by the following calculation. First, a signal A having no offset and a signal B whose phase is advanced by 90 degrees with respect to the signal A are obtained by calculation of A = a1-a2 and B = b1-b2. Subsequently, the position POS is derived by an interpolation operation of POS = P × tan ⁻¹ (A / B) / 2π. P is the pitch of the main grid scale 16.

  Next, a method for detecting a gap between the main grid scale 16 of the scale 10 and the index scale 13 of the slider 15 in the above-described linear encoder will be described. In a general linear encoder, the optical system component of the slider 15 has a mechanism that comes into contact with the scale 10, and the gap is determined by this contact mechanism. In this case, it is not necessary to detect the gap. However, in this case, a bearing for relative displacement is required for the contact mechanism of the slider 15 and the scale 10, and wear of this bearing, lack of grease, and the like have deteriorated long-term reliability.

  Therefore, conventionally, there has been proposed a configuration in which the scale 10 and the slider 15 are completely non-contact and the gap of the linear encoder is determined by the mounting surface on the machine side on the machine. In the case of such a technique, the scale 10 and the slider 15 need to be attached with great care, and it is necessary to inspect the gap after the installation operation on the machine to confirm that there is no problem in the installation dimensions. For this reason, in the prior art, the gap is detected using the signal amplitude of the signal used for position detection.

  Hereinafter, gap detection in this conventional technique, for example, the conventional technique disclosed in Patent Document 1 will be described. FIG. 7 is a graph showing the change in the amount of received light due to the relative displacement in the longitudinal direction of the index scale 13 (slider 15) with respect to the main grid scale 16 (scale 10). In FIG. 7, the dark line indicates the change in the amount of received light when the gap is small, and the light line indicates the change in the amount of received light when the gap is large. As is apparent from FIG. 7, the amplitude of the change in the amount of received light becomes smaller as the gap is larger.

The characteristics of the signal amplitude with respect to the gap can be expressed by the pitch P of the main grid scale 16, the wavelength λ of the light emitting unit, and the gap, and the amplitude changes with a period G = 2P ² / λ. FIG. 5 is a graph showing an example of the relationship between the gap and the amplitude. In the example of FIG. 5, there is a range in which the amplitude value is negative because the phase information is included because of the calculation method, but in reality, all the amplitudes are positive. Therefore, in the example of FIG. 5, the amplitude when the gap standard value is 1 is 1. As shown in FIG. 5, the amplitude changes sinusoidally according to the gap between the gratings. In the example of FIG. 5, when the gap standard value is 0.5, the amplitude is almost zero.

  Specifically, when the wavelength of the light emitting element is 0.88 μm and the lattice spacing of the main lattice scale 16 is 80 μm, the period G = 14.5 mm. Consider the case where the scale 10 and the slider 15 designed under these optical conditions are used, the standard mounting state of the gap is 1 mm, and the gap changes from 0.7 mm to 1.3 mm. Since the signal amplitude changes like a sine wave with a period G = 14.5 mm so as to take 1 when the gap is 0 mm, the amplitude L at the time of the gap g is L = sin (π / 2 + 2π × g / 14 .5) = cos (2π × g / 14.5). When the gap g is 0.7 mm to 1.3 mm, the signal amplitude is 0.95 to 0.84 from the above calculation. That is, the gap can be detected based on the signal amplitude.

A conventional gap detection technique will be described with reference to FIG. 4 includes four signals a1, a2, b1, b2 (Sig.L) as a result of photoelectric conversion by the light receiving element 14 of the light amount change caused by the relative movement of the slider 15 in the longitudinal direction of the scale 10. ) Is detected. In the amplitude calculation unit 22, Sig. From L, the calculation of A = a1-a2 and B = b1-b2 is performed to obtain A and B signals without offset. Further, a signal amplitude AMP_L = (A ² + B ² ) ^1/2 is obtained from the A and B signals.

  The gap determination unit 23 has an upper limit and a lower limit of a gap target value (signal amplitude) as initial fixed values in advance, and performs comparison determination with the signal amplitude AMP_L with the target value. When the signal amplitude AMP is smaller than the gap target value lower limit (gap target value lower limit value> AMP_L), the state that the gap is wide is displayed, and the signal amplitude AMP is larger than the gap target value upper limit (gap target value upper limit value). At the time of <AMP_L), the state that the gap is narrow is transmitted to the host control device 50 outside the slider.

  When the scale slider is attached, the attachment is confirmed by displaying the gap judgment value by the host controller 50. Or it is used as an adjustment value for mounting correction. As a result, an interference accident between the slider and the scale can be prevented. In addition, since the degree of contamination from the environment around the scale is represented by the magnitude of the signal amplitude after the completion of the mounting, the accuracy of the alarm determination can be improved. Also, these effects can be realized without the need for any additional sensors or signals.

Japanese Patent Laid-Open No. 4-86513

  However, a linear encoder that employs the above-described gap detection technique has the following problems. It is known that the signal amplitude AMP_L is also affected by the temperature characteristics of the light emitting element 12. Specifically, when the temperature is high, the light amount of the light emitting element decreases and the signal amplitude also decreases. When the temperature is low, the amount of light increases and the signal amplitude also increases. For this reason, when the temperature is high, there is a possibility that it is erroneously determined that the gap is wide, and when the temperature is low, there is a possibility that it is erroneously determined that the gap is narrow. In detail, the temperature characteristics of the light output of the light emitting element 12 are as shown in FIG. 6, and the light output is changed by the change of the ambient temperature, and the photoelectrically converted signal amplitude is also changed. As a result, it is difficult to distinguish the temperature factor and the gap factor from the signal amplitude.

  As described above, the method of detecting the gap from the amplitude of the amount of received light has a simple detection principle, but in principle, it is easily influenced by a temperature change, and it has been difficult to perform accurate mounting adjustment work. In addition, in order to prevent inaccurate mounting adjustment work, there has been a problem that a defective mounting position cannot be detected when the range between the upper limit and the lower limit of the gap target value is increased in advance.

  An object of the present invention has been made in view of the above problems, and an object of the present invention is to provide a linear encoder that can improve the reliability of gap detection.

The linear encoder of the present invention receives a light source that emits parallel light, a fixed scale, an index scale that is relatively displaced with respect to the fixed scale, and light that is transmitted or reflected by the parallel light through the fixed scale and the index scale. Then, in a linear encoder comprising a photoelectric conversion means for converting into an electric signal, the first grid scale and the second grid scale having different pitches provided on the fixed scale, and transmitting or passing through the first grid scale. Means for obtaining the amplitude of the signal obtained by photoelectric conversion of the reflected light as the first signal amplitude, and the amplitude of the signal obtained by photoelectric conversion of the light transmitted through or reflected by the second grid scale as the second signal amplitude Means for obtaining the temperature influence amount based on the second signal amplitude, and the temperature from the first signal amplitude. Influence quantities to get the correct signal amplitude was removed, the correction signal and means for calculating the gap amount between the index scale and the fixed scale based on the amplitude, comprising a grating pitch of the second grating graduation Is at least twice the lattice pitch of the first lattice scale .

  According to the present invention, the gap can be detected more accurately, and mounting on the machine can be performed more accurately. In addition, after installation adjustment is completed, a decrease in the signal amplitude is detected to detect when the scale is contaminated. However, by adjusting the installation accurately, the margin to the alarm level generated when the signal drops is increased. Therefore, the accuracy of alarm determination can be increased.

It is a figure which shows the structure of the linear encoder which is embodiment of this invention. It is a block diagram which shows the structure of a gap detection part. It is a figure which shows the structure of the conventional linear encoder. It is a block diagram which shows the structure of the gap detection part of the conventional linear encoder. It is a graph which shows the relationship between gap amount and signal amplitude. It is a graph which shows the relationship between LED light quantity and temperature. It is a graph which shows the change of the light quantity with respect to the displacement of a slider.

  Embodiments of the present invention will be described below with reference to the drawings. In the following description, the description of the same parts as those in the prior art in the configuration of the linear encoder of this embodiment will be omitted. FIG. 1 is a perspective view illustrating an example of a signal detection unit of an optical linear encoder according to an embodiment of the present invention. In addition to the main grid scale 16, the scale 10 of the present embodiment is provided with an auxiliary scale grid 17 having a pitch twice or more that of the main grid scale 16. As is apparent from FIG. 1, the auxiliary scale grid 17 and the main grid scale 16 are arranged vertically (in the minor axis direction of the scale 10) in the plane of the scale 10.

  Further, the index scale 13 a is provided with eight sub-lattices to correspond to the main scale grid 16 and the auxiliary scale grid 17. The light receiving element is also provided with eight light receiving elements 14a so as to correspond to the sub-lattice.

  The position calculation method when the position is read from the slider 15 is the same as that in the prior art, and the Sig. L (a1, b1, a2, b2) signal positions and the Sig. Detection is performed by combining the positions obtained from the H signal.

  The main grid scale 16 described above is configured so that the pitch of the grid scale is sensitive to the gap, and the auxiliary grid scale 17 has a scale pitch that is twice or more that of the main grid scale 16 and substantially affects the gap. Configure so that there is no.

  For example, consider the case where the pitch of the main grid scale 16 is 80 μm and the pitch of the auxiliary grid scale 17 is 2 mm. In this case, in the main grid scale 16 of 80 μm, the light quantity change period due to the gap is about 14.5 mm. On the other hand, in the auxiliary grid scale 17 having a pitch of 2 mm, the period of amplitude change due to the gap is 9090 mm, which is a very large period. As a result, the signal amplitude on the auxiliary grid scale 17 side is hardly affected by the gap. On the other hand, since the influence of the ambient temperature on the light quantity of the light emitting element is independent of the pitch of the grid scale, the influence on the light quantity or signal amplitude on the main grid scale 16 side and the auxiliary grid scale 17 side is equivalent. It is. With this configuration, the change in signal amplitude on the 2 mm grating side is a result of temperature influence and can be regarded as a temperature coefficient. As a result, by multiplying the signal amplitude of the 80 μm grating by this temperature coefficient, a signal representing the gap is obtained by removing the influence of temperature.

  FIG. 2 is a diagram showing the configuration of the signal calculation unit in the embodiment of the present invention. Unlike the conventional example, the signal Sig. The amplitude calculation unit 25 for calculating the amplitude of H, the temperature coefficient calculation unit 26 for calculating the temperature coefficient from the obtained amplitude, and the signal Sig. A gap calculation unit 27 for newly obtaining a gap from the amplitude obtained from L and the temperature coefficient obtained from the temperature coefficient calculation unit 26 is newly added.

Specifically, the signal detection unit 31 in FIG. 2 detects four signals Ha1, Ha2, Hb1, and Hb2 (Sig.H) as a result of photoelectrically converting the change in the amount of light transmitted through the auxiliary grid scale 17 by the light receiving element 14a. This Sig. From H, the amplitude calculation unit 25 obtains HA and HB signals without offset by HA = Ha1-Ha2 and HB = Hb1-Hb2, and further calculates the signal amplitude AMP_H by the calculation of AMP_H = (HA ² + HB ² ) ^1/2 . Ask.

  AMP_H obtains a temperature coefficient K = AMP_HF / AMP_H by comparison with the amplitude AMP_HF at the reference temperature. The amplitude at the reference temperature is a numerical value determined by the light amount adjustment of the light emitting element 12 performed at the reference temperature with the scale slider set, and is a fixed value.

  Next, in the gap calculation unit 27, Sig. The correction signal amplitude AMP_LK is obtained by multiplying AMP_L obtained from L by the temperature coefficient K obtained above (AMP_LK = AMP_L × K). From the correction signal amplitude AMP_LK obtained by this calculation, the gap determination unit 23 similar to the prior art detects the suitability of the gap.

  In the above, the case where the pitch of the main grid scale 16 is 80 μm and the pitch of the auxiliary grid scale 17 is 2 mm has been described. However, this value is an example, and the grid pitch of the auxiliary grid scale 17 is as calculated from the principle. If it is more than twice the main grid scale 16, the effect on the signal amplitude due to the gap of the auxiliary grid scale 17 is sufficiently low, so that a sufficient effect can be obtained.

  In the present embodiment, the timing for performing the gap calculation and determination is not particularly specified, and it has been described that it always operates when the encoder is operating. However, the gap calculation and determination is actually performed on the machine. The gap calculation and determination may be performed only at the time of adjustment, and only at the time of this mounting adjustment.

  Further, in the present embodiment, the case of a light transmission type encoder is illustrated, but the same principle can be realized even with a light reflection type encoder, and an equivalent effect can be obtained. .

  10 scale, 11 collimator lens, 12 light emitting element, 13, 13a index scale, 14, 14a light receiving element, 15 slider, 16 main grid scale, 17 auxiliary grid scale, 21, 31 signal detection section, 22 amplitude calculation section, 23 gap Determination unit, 26 temperature coefficient calculation unit, 50 control device.

Claims (1)

A light source that emits parallel light, a fixed scale, an index scale that is displaced relative to the fixed scale, and light that is transmitted or reflected by the parallel light through the fixed scale and the index scale is received and converted into an electrical signal. In a linear encoder comprising a photoelectric conversion means,
A first grid scale and a second grid scale provided on the fixed scale and having different pitches;
Means for obtaining, as the first signal amplitude, the amplitude of a signal obtained by photoelectrically converting light transmitted or reflected through the first grid scale;
Means for obtaining, as a second signal amplitude, the amplitude of a signal obtained by photoelectrically converting light transmitted or reflected through the second grid scale;
Means for obtaining an influence amount of temperature based on the second signal amplitude;
Means for obtaining a correction signal amplitude obtained by removing an influence amount of the temperature from the first signal amplitude, and calculating a gap amount between the fixed scale and the index scale based on the correction signal amplitude;
Equipped with a,
The lattice pitch of the second lattice scale is at least twice the lattice pitch of the first lattice scale.
A linear encoder characterized by that.

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