JPH0143243B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an encoder for measuring length and angle, and more particularly to an encoder that achieves high measurement accuracy and high resolution. Conventionally, linear encoders and rotary encoders are known as encoders. Linear encoders are used to measure length, and rotary encoders are used to measure angles, but these two are essentially the same, and the encoder arrangement is The only difference is whether it is on a straight line or on a circumference. There are also types of encoders, such as a photoelectric type that uses light, and one that uses magnetism or electromagnetic induction. Therefore, when reading and measuring lengths and angles, there is a manufacturing limit to the subdivision of the scale, and therefore the minimum measurement limit is determined by the limit of the subdivision of the scale. In order to measure even smaller amounts beyond this limit, it is necessary to interpolate between the minimum scales, and various methods have been considered for this purpose. For example, in the case of an optical encoder, two sensors are installed in order to read out information on the optical grating or magnetic grating corresponding to the scale of the code plate, and each sensor detects the 1/4 pitch, or phase angle, of the code plate. Read out information with a phase difference of 90°,
There is a method of interpolating between the minimum graduations of the grid pattern of the code plate by processing these two signals. The following three methods are known as methods of interpolation. The first method is called the zero-point reference interpolation method, in which the point where the zero potential of two sinusoidal signals with different phases read from the code plate is cut is used as a reference point to form a square pulse, and the square pulse of the two pulse sequences is This method interpolates one cycle by counting the rising and falling edges of . The second method is called the amplitude-based interpolation method, which is also obtained from the encoder.
Interpolation for one period is achieved by combining the amplitudes of output signals having a phase difference of 90° with a fixed ratio difference. The third method is called the phase-based interpolation method, which modulates two signals with a 90° phase difference using a carrier wave with a constant frequency, and detects the phase difference between these combined waves and the carrier wave. and interpolates between one period. However, in the case of the above-mentioned interpolation method, the accuracy obtained is only up to 1/4 pitch. Additionally, in order to improve this, it has been considered to use signals with a phase difference other than 90 degrees, but this would complicate the signal processing circuit, and it is not easy to obtain such high accuracy. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a highly accurate and high-resolution encoder that employs an effective interpolation method based on the absolute method. Another object of the present invention is to provide an encoder that can easily achieve the required high accuracy without major changes to the configuration. The present invention will be described in detail below with reference to the drawings. First, the principle of the present invention will be described using FIGS. 1 to 9, taking an optical encoder as an example.
In Figure 1a, 1 is an optical grating (code plate);
Portions with different light transmittances are arranged periodically. Here, the total length of this code plate 1 is L, 1/
Let the length of 2 pitches be W. Further, in FIG. 1b, 2 is a linear sensor consisting of N+1 photoelectric conversion elements, the total length of which is L, and the pitch P between each element is P=WN/(N+1). Furthermore, these N+
Each photoelectric conversion element is adapted to output a signal independently. That is, in the present invention, a code plate 1 and a linear sensor 2 are provided, a total of N transparent and opaque slits are provided between the entire length L of the optical grating 1 which is the code plate, and N+1 slits are provided in the linear sensor 2 of one length in one direction. The optical conversion elements are arranged so that when the code plate is irradiated with light, the light that passes through the code plate is received by these N+1 photoelectric conversion elements. Therefore, in the present invention, the linear sensor 2 is used as a so-called vernier scale, and this is a feature of the present invention. In addition, in this specification, transmittance "1"
Both an optical grating with a transmittance of "0" and an optical grating with a transmittance of "0" are called a slit. Next, consider the relative movement between the code plate 1 and the linear sensor 2 with reference to FIG. Figure 2a shows the code plate and Figures 2b-g show the linear sensor. Then, one vertical side of one slit of the code plate 1 is taken as a reference, and this reference line A is aligned with one end of the linear sensor 2 (Fig. 2b)
Therefore, it is assumed that the linear sensor 2 sequentially moves to the right. Here, regarding the movement amount S, let W be the slit width of the grating 1, and consider S ₀ =W/(N+1) as one unit of length. Then, as shown in Figure 2 c, d, and e, the amount of movement increases to S = S ₀ , 2S ₀ , 3S ₀ , and in the state of Figure 2 f, S = (N + 1) S ₀ = W, that is, the code plate 1 This means that the code plate 1 has moved 1/2 period, and furthermore, the code plate 1 has moved exactly one period in the state shown in Fig. 2g. Now let us assume that the code plate 1 is a regular grid with complete and incomplete transmission of light, and N=10. Therefore, the prime component of the linear sensor 2 is 11 (=N+1). The parallel light beams passing through this code plate are received by the linear sensor 2, and each element of the linear sensor 2 receives an electrical signal proportional to the amount of light irradiated. Table 1 shows the relationship between the relative value of the electric signal amount and the value of the movement amount S.

【table】

[Table] The values in this table are relative values, where the signal when the entire surface of one element of the linear sensor 2 is completely irradiated with light is 10, and the signal when no light is incident is 0. FIGS. 3 a to 3 v show the peak values of the signals when the signals from each element of the linear sensor 2 are read out in time series. Figures 4 a to d are Figures 3 a to v.
In the diagram corresponding to a, b, l, and m, the signals obtained from the odd-numbered elements of the linear sensor 2 are the first series, and the signals obtained from the even-numbered elements are the first series. As the second series, changes in the first and second series as a result of movement are shown. In these figures, the point where the signal levels of the first series and the second series are the same is the amount of movement S.
It can be seen that it changes with the change in. That is, for example, when S=0, the intersection exists at the element number 6 position, and when S=S ₀ , the intersection exists at the element number 7 position. Here, the intersection points in FIGS. 4a to 4d correspond to the phase inversion points in FIGS. 3a to 3v. Therefore, by measuring the amount of movement of the intersection (phase inversion point) with reference to when the intersection (phase inversion point) is at the position of element number 6, for example,
The amount of relative movement between the grating 1 and the linear sensor 2 can be measured with high precision. In addition, when finding the intersection point, the peak value of the pulse signal from the linear sensor 2 may be discriminated at a level of, for example, 50±5%, as shown in FIG. 5, and the position of the pulse at that peak value may be found. good. The above is the measurement principle of the present invention, but since each light-receiving element of an actual linear sensor is divided, a dead zone of light exists. That is, Fig. 6a,
As shown in b, when the slit width of the code plate 1 is W and the width of each element of the linear sensor 2 in a vernier relationship with the slit width is P, the width l of the width P becomes a light dead zone. A case where there is such a dead zone will be briefly explained. Now l is 1 unit of length S ₀ =
Consider it as a multiple of W/(N+1), that is, l=αS ₀ . Here, if N=10 and α=3, then P=
Since WN/(N+1), l=0.3P, and 30
% have no sensitivity to light. When this linear sensor receives light transmitted through the code plate, the signals are as shown in FIGS. 7a to 7f. Figures 7 a to f show examples only for S = 0, S ₀ , 2S ₀ , 11S ₀ , 12S ₀ , and 13S ₀ , but as can be seen from these figures, phase reversal is detected in this case as well. Similarly to the measurement principle described above, interpolation with high precision and high resolution can be performed using the amount of movement of this phase inversion point. Next, the width P of each element of linear sensor 2 is WN/
The case of (N-1) and the case of WN/(N+2) will be described below. First, P=WN/(N-1),
The signals of the linear sensor in the case (N=10, α=0) are as shown in FIG. 8 a to j. However, Figure 8 a~
j is an example of the case where S=0, S ₀ , . . . , 9S ₀ . As is clear from these figures, phase inversion is detected in this case as well, and it can be seen that this phase inversion point moves in accordance with the amount of S. For example, when S=0, the phase inversion point is at element number 5, when S=S ₀ it is 4, when S=2S ₀ it is 3, and so on. As you can see here, P=
Movement amount S of phase inversion point in case of WN/(N-1)
is in the opposite direction. However, this fact does not deny the measurement principle in any way. Then P=
Consider the case of WN/(N+2), (N=10, α=2). The output signals of the linear sensor 2 in this case are as shown in FIGS. 9a to 9l. As these figures show, phase inversion appears in two places in this case.
In other words, the phase inversion when S=0 is element number 3.5
and appears at position 9.5. (Actually, there are no elements called 3.5 or 9.5, but if you detect that the outputs of element numbers 3 and 4 are the same, or that the outputs of element numbers 9 and 10 are the same, the phase will be reversed at the intermediate position.) (It can be determined as a point.) Therefore, if the average position of these two positions or one position is detected, this phase and the relative movement amount S correspond one-to-one, and the scale of the code plate can be determined as a point. It can be inserted. The above description of the measurement principle of the present invention is based on the case where the code plates are arranged linearly, but even when a circular code plate is provided with an arc-shaped or linear linear sensor as a vernier, there is no problem. It is easy to understand that they are similar. By arranging a circular code plate and arc-shaped or linear linear sensors as verniers on the circular code plate, it is possible to measure the rotation angle with high precision. Next, an embodiment of the optical encoder of the present invention using the above measurement principle will be described with reference to FIGS. 10a and 10b and FIGS. 11a to 11e. 1 in Figure 10a
is a code plate, which is fixed to a part of the member whose displacement is to be measured, such as a moving table of machine tools, a rotary table, or a fine movement stage of a projector or tool microscope. Further, 2 is a linear sensor, which is installed in parallel to the code board 1. Here, the linear sensor 2 is stationary and the code plate 1 moves with the movement of the member, so that the two move relative to each other. Then, the code plate 1 is irradiated with the light from the light source 3 through the lens 4 as parallel light. The light transmitted through the code plate 1 becomes light with a pattern of light intensity corresponding to the pattern on the code plate 1, and passes through the lens 5 to the code plate 1.
is projected onto the light-receiving surface of the CCD linear sensor 2 located behind. By changing the magnification of the lens 5, the vernier relationship between the code plate and the linear sensor can be set arbitrarily. Now, as shown in FIG. 10b, the optical grating of the code plate 1 is assumed to be an array of black and white slits with a width of 10 μm at equal intervals, and the linear sensor 2, which is in a vernier relationship with the code plate 1, is attached to the code plate 1. Assume that 11 photoelectric conversion elements are arranged for a length of 100 μm. That is, it is assumed that 11 (N+1) elements are arranged within 5 pitches (N=10) of the code plate 1. The code plate 1 and the linear sensor 2 are shown in Fig. 10b.
The output of the linear sensor 2 in the positional relationship shown in FIG. 11a is as shown in FIG. 11a. This waveform is determined by the relative phase between the code plate 1 and the linear sensor 2. Then, while the code plate moves for one period (from N=1 to N=10), the waveform is as shown in Figure 11a.
The waveform shown in Figure 11a changes sequentially according to the amount of movement, and when the code plate moves one cycle,
The waveform will look like the one shown below, and this process will be repeated one after another. Next, a method of measuring the amount of relative movement using the output signal of the linear sensor 2 as described above will be explained. According to the apparatus of the present invention shown in FIG. 10a, "coarse reading" is performed by the incremental method, and "fine reading" is achieved by the absolute method. That is, rough reading is first performed depending on how many slits on the code plate have moved from a certain point in time, and fine reading is performed at the time when the code plate is stationary based on the above-mentioned interpolation principle. First, I will explain the rough reading method. As is clear from the previous explanation, if we focus only on the output of one specific element on the linear sensor 2, the output for each linear sensor scan of that element is a sine whose movement amount is 2W as one cycle. The output is wavy. This is clear even when focusing on any one of the 11 elements shown in Table 1 and looking at the value of the element output as a function of the relative movement amount S. Therefore, in the present invention, as shown in FIG. Samples and holds are repeated every time the sensor scans, and the output is envelope-detected by an integrator 8 having an appropriate time constant depending on the sampling interval to obtain the sinusoidal signal. Here, the determination of the point in time to be sampled and held by the sample-and-hold circuit 7 (that is, the determination of the element whose output is to be sampled and held) is made by the sampling signal sent from the drive circuit 9. That is, the drive circuit 9 uses a counter to count clock pulses for reading the contents of the linear sensor 2, for example, and compares the counted number with a predetermined value stored in a register using a comparator, and uses the matching signal as a sampling signal. It's like getting.
Therefore, by storing, for example, a value of 3 in the register, that value will be sampled and held every time the contents of the third element of the linear sensor are read. The output of the integrator 8 is then applied to a Schmitt trigger circuit 10 to set the appropriate slice level (e.g.
50% level) into a square wave, and the counter 11 counts the rising and falling edges of this square wave. In other words, since the period from the rise to the fall or from the fall to the rise of this square wave corresponds one-to-one to the slit width W of the code plate 1, from the count value of the counter 11, the slit width W is taken as the minimum unit. It becomes possible to measure the amount of movement. Counter 1
1 continues counting while the code plate 1 is moving, and when the code plate 1 stops, the counting of the counter 11 also stops. The count value obtained by the counter 11 is then added to a conversion circuit 12 to determine the actual amount of movement. Note that the determination of the counting direction of the counter 11 at the time of rough reading, that is, the determination of whether to add or subtract the rising and falling edges, is performed in such a way as to cause a phase shift with respect to the output of one element of interest. This can be easily done by checking the output of any element together and checking whether the phases of both are leading or lagging. The above is about the method of rough reading, and as is clear from the above explanation, the minimum unit of measurement in rough reading is the slit width W. Therefore, in order to read with higher precision, interpolation is performed according to the principle described above. This method of fine reading will be explained below. In FIG. 10a, the output of amplifier 6 is applied to signal splitter 13. In FIG. This signal distributor 13 is a circuit for distributing signals of even-numbered elements and signals of odd-numbered elements of the linear sensor 2, and is constituted by a simple gate circuit. The two pulse sequence signals distributed by the divider 13 are respectively supplied to envelope detectors 14 and 15 and subjected to envelope detection. As a result, the output signal of the linear sensor 2 shown in FIG. 11a becomes as shown in FIGS. 11b and 11c. These signals are then compared by comparator 16 and a comparison output is generated at the point where the two signals are at the same potential. This comparison output is then added to the counter 17, and the counter 17 outputs the number of clock pulses (count number) that it has been counting up to that point to the conversion circuit 12. FIG. 11d shows a state in which the outputs of the envelope detectors 14 and 15 are compared by the comparator 16,
Figure e shows the counting state of the counter 17. 11th
Figure e shows an example where a comparison output is generated when six clock pulses are counted. Fine reading is thus performed by reading the inside of the slit width W of the code plate with a resolution of 1/(N+1) (N=10). The fine reading result obtained in this way is converted into an actual movement amount by the conversion circuit 12 and displayed on the display device 18 in the same way as the rough reading result described above. An example of the configuration of the envelope detectors 14 and 15 is shown in FIG. 16. These consist of a sample-and-hold circuit 23 and an integrator 24, and the sample-and-hold circuit 23 is supplied with every odd clock pulse (in the case of the detector 14) or every even clock pulse (in the case of the detector 15). Ru. As explained in detail above, in the above embodiment, an optical grating in which slits having a certain ratio of light transmittance are arranged periodically, and a photoelectric conversion element having a vernier relationship with the optical grating, are connected to the optical grating. and photoelectric conversion devices arranged in parallel with each other, and the electrical signal obtained from the photoelectric conversion device when the optical grating is irradiated with light is used to interpolate and read within the slit width of the optical grating. It is something. In this case, since N slit images are projected onto (N+1) photoelectric conversion elements, highly accurate interpolation of 1/N of the sensor pitch is possible. FIG. 12 is a diagram showing another embodiment of the present invention. However, in this embodiment, only a circuit for fine reading is shown. In this embodiment as well, the light emitted from the light source 3 is converted into parallel light through the lens 4, and the code plate 1 is irradiated with this parallel light. and code board 1
The image is formed on the linear sensor 2 by the lens 5. By imaging, a vernier relationship between the pattern of the code plate 1 and the linear sensor element structure is generated. It is assumed that the pattern of the code plate 1 and the structure of the linear sensor 2 are the same as those shown in FIG. 10b. In this embodiment, the output of the amplifier 6 becomes a pulse train as shown in FIG. 13a. This signal is supplied to the envelope detector 19 to produce a signal as shown in FIG. 13b. This signal is then applied to the phase inversion discriminator 20 to detect the phase inversion point P of the signal shown in FIG. 13b. The counter 17 counts clock pulses from a predetermined starting point, and stops counting when the phase inversion point P is detected by the phase inversion discriminator 20. This situation can be seen in Chapter 13.
Shown in Figure c. The counting result of the counter 17 is then supplied to the conversion circuit 12 to determine the actual amount of movement. The output of conversion circuit 12 is displayed by display device 18. Note that the envelope detector 19 is the first one.
The one shown in Figure 6 can be used. FIG. 14 is a diagram showing still another embodiment of the present invention. However, this embodiment also shows only a circuit for fine reading. In this embodiment, the output of amplifier 6 (FIG. 15a) is applied to pulse shaper 21 and shaped into a signal as shown in FIG. 15b. This signal is then applied to a pulse height analyzer 22 to detect the time point at which a signal having a pulse height value of 50%±Δ arrives, and a counter 17 counts the number of clock pulses up to that point. In addition, the peak value of the signal shown in Fig. 15b is discriminated at a 50% level, and converted into a binary pulse code with 50% or more as a "1" level and below as a "0" level. If the pulse height analyzer in FIG. 14 is configured so that the signal in FIG. 15b becomes a pulse sequence of "1" and "0" as shown in FIG. 15c, the phase inversion can be obtained from this signal. Points can be easily detected. FIG. 17 shows an example of the configuration of the conversion circuit 12 used in each of the above embodiments. If the counted value by the counter 11 is A and the counted value by the counter 17 is B, then A
is the counting result obtained by coarse reading, and B is the counting result obtained by fine reading. A and B are registers 121 and 1, respectively.
It is set to 22. Next, the value B of register 122
The corresponding interpolated value is read from the ROM 123 using as an address signal and set in the register 124. The ROM 123 stores a correspondence table as shown in Table 2.

[Table] In Table 2, for convenience of explanation, both address B and the contents of that address are expressed in decimal numbers. However, the count value B indicates the position of the phase change point from the left end of the linear sensor 2. This correspondence table shows the relationship between the count value B (that is, the position of the phase change point) and the interpolated value C. For example, if the count value B is 7, this means that the phase change point is at the 7th bit as shown in _FIG. There is. Therefore, the interpolated value is 1/11 of the slit width W of the code plate 1. Next, the contents C of register 124 and register 121
is added to the content A by the adder 125.
This addition result is supplied to multiplier 126. Multiplier 126 connects register 127 to the output of adder 125.
Convert to position or angle data by multiplying by the contents of . The register 127 stores in advance length data or angle data corresponding to the slit width W of the code plate 1. For example, in the case shown in FIG. 10b, W is 10 μm. Multiplying the output of adder 125 by this value yields the actual position. This multiplication result is sent to the display device 18 and displayed. As explained in detail above, this invention has “1”
It is characterized in that the sensor that reads the lattice pattern of the code plate has a vernier relationship with respect to the code plate that has a lattice pattern in which "0" information is periodically arranged. This enables highly accurate interpolation that could not be achieved with conventional methods. Further, arbitrary accuracy can be easily achieved by simply changing the correspondence between the number of slits in the grid pattern and the number of elements in the sensor. The present invention is not limited to the above embodiments, and can be implemented with various modifications as shown below. (1) The code plate and sensor are not optical as in the embodiment, but may use magnetism or electromagnetic induction. In particular, encoders that utilize magnetism have the disadvantage of being used for special purposes and have lower accuracy than optical encoders, but by applying the present invention, the accuracy can be sufficiently improved. (2) In the above embodiment, the position or angle was measured when the code plate and sensor were moved relatively in a straight line or in an arc, but the code plate and sensor were fixed and the code plate was If the image is reflected by a displaceable object, such as a reflecting mirror, and then projected onto the sensor, the amount of displacement of the object can be read with high accuracy. (3) The optical encoder can be configured to project the image reflected by the code plate onto the sensor without using the transmitted image of the code plate. (4) Since the present invention is characterized by interpolation reading by fine reading, a coarse reading method consisting of an incremental method or an absolute method, which is a conventionally known method of measuring up to one pitch of a code plate, is used. They may be used in combination. It goes without saying that various circuit configurations other than the above-described embodiments can also be used for the signal processing system that processes the output signal from the linear sensor.

[Brief explanation of drawings]

FIGS. 1 to 9 a to l are diagrams for explaining the principle of the present invention, FIG. 10 a is a diagram showing an embodiment of the present invention, and FIG. 10 b is a code used in FIG. 10 a. A diagram for explaining the arrangement relationship between the plate and the linear sensor, FIGS. 11a to 11e are signal waveform diagrams of various parts of the embodiment shown in FIG. 10a, and FIG. 12 is a diagram showing another embodiment of the present invention. , FIGS. 13a to 13c are signal waveform diagrams of various parts of the embodiment shown in FIG. 12, FIG. 14 is a diagram showing still another embodiment of the present invention, and FIGS. 15a to c are shown in FIG. Signal waveform diagrams of each part of the example, Part 1
6 and 17 are diagrams showing specific configuration examples of a part of an embodiment of the present invention. 1... Code board, 2... Linear sensor, 3...
...Light source, 4, 5...Lens, 6...Amplifier, 9...
... Drive circuit, 12 ... Conversion circuit, 13 ... Signal distributor, 14, 15 ... Envelope detector, 16 ... Comparator, 17 ... Counter, 18 ... Display device.

Claims (1)

[Scope of Claims] 1. A code having a carrier row pattern in which carriers having information of “1” and carriers having information of “0” each having the same width are alternately arranged in series at a predetermined pitch. In an apparatus for measuring length and angle using a plate and a sensor for reading the carrier row pattern of the code plate; the sensor is arranged parallel to the carrier row pattern;
Consisting of a large number of sensor elements arranged substantially continuously; configured with a slight difference between the 1/2 pitch (W) of the carrier and the pitch (P) of the sensor elements; A device for detecting the position of a phase inversion point of a vibration waveform created from an output signal level read from a column, and a device for converting the position of the detected phase inversion point into a displacement within one pitch. An encoder that does this. 2. The means for performing the interpolation includes means for sequentially extracting electrical signals obtained by each element of the sensor using clock pulses; and means for detecting a phase inversion point of the signal extracted by this means. and; counting means for counting the clock pulses; the counting means counts the clock pulses from a predetermined starting point until the phase reversal point is detected or from the phase reversal point to a predetermined end point. An encoder according to claim 1, characterized in that: 3. The means for detecting the phase reversal point includes means for waveform-shaping the signal taken out from the sensor by the clock pulse; and a signal having a maximum peak value and a minimum waveform among the signals shaped by this means. and means for detecting a signal that takes a peak value that is approximately midway between a signal that takes a high value; and a time when the signal that takes a peak value that is approximately midway is determined as a phase inversion point. Encoder according to range 2. 4. The carrier of the code plate is composed of a group of slits having a certain ratio of light transmittance, and the sensor is composed of a group of photoelectric conversion elements, and the slit group has a 1/2 pitch (W). 4. The encoder according to claim 1, wherein the encoder is configured to be projected to have a width different from a pitch (P) of the photoelectric conversion element group.