JPH0439019B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides two different numbers of cord plates in the circumferential direction of the code plate.
In addition to providing a pair of slit rows, the movement of the slit rows is detected by a detection means such as a photodiode array, and the absolute rotation angle is determined from the phase difference between the two output signals and the phase of one of the output signals. The present invention relates to a slit number determining method for determining the optimum number of slits in order to obtain high detection accuracy in an absolute encoder.

[Conventional technology]

Generally, when designing an encoder, the number of slits provided on the code plate is a major factor that affects the detection accuracy (resolution) of the rotation angle. For example, in an incremental encoder, the higher the number of slits, the higher the resolution.
Detection accuracy is improved.

[Problem that the invention seeks to solve]

However, in the absolute type encoder as described above, the number of slits must be determined not only in consideration of the target accuracy but also in consideration of the accuracy in reading the phase of the output signal. That is, if the number of slits is increased, the final resolution will improve, but it will be necessary to read the phase difference signal formed from the two output signals with high precision, making signal processing difficult.

For this reason, conventionally, no clear method for determining the number of slits has been established, and the number of slits considered to be appropriate has been sought through repeated trial production.

The present invention eliminates the drawbacks of the conventional method as described above, and provides a method for easily determining the optimum number of slits that can increase the error allowed for phase detection in order to obtain the target accuracy. It is intended to provide a method for making decisions.

[Means for solving problems]

The method for determining the number of slits in an absolute encoder of the present invention is to provide two sets of slit rows with different numbers in the circumferential direction of a code plate, detect the movement of the slit rows with a detection means such as a photodiode array, In an absolute encoder that calculates the absolute rotation angle from the phase difference of the output signal, it is possible to achieve the target accuracy by ±
If 1/N, then the optimum number of slits n ₀ is determined from the formula n ₀ =√/2, and the number of slits is set to an integer value close to this optimum number of slits n ₀ .

[Effect]

In this way, by expressing the optimum number of slits using a mathematical formula, the number of slits can be easily determined according to the target accuracy. Also,
The number of slits determined by this formula is the number of slits that can most alleviate the tolerance of the detection means, and is therefore easy to implement.

〔Example〕

Hereinafter, a method for determining the number of slits in an absolute encoder according to the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an absolute encoder to which the method of determining the number of slits of the present invention is applied. In the figure, 1 is a rotating shaft, 2 is a code plate fixed to the rotating shaft 1, 2 ₁ and 2 ₂ are two slit rows provided on the code plate 2, 3 is a light source that irradiates light to the code plate 2, 4 is a slit row 2 ₁ , 2 ₂
This is a photodiode array that receives the light (slit image) that has passed through the slit. Here, slit row 2 _1
, 2 ₂ are provided with 1, n and (n-1) slits, respectively.

FIG. 2 is an enlarged view of the slit rows 2 ₁ and 2 ₂ and the photodiode array 4 portion.

In the absolute encoder configured in this way, the brightness and darkness of the light depending on the position of the slit is determined by electrically scanning the photodiode array 4 at an angular frequency ω, thereby changing the brightness and darkness of the light according to the position of the slit, respectively, using alternating current phase signals Va and A. Converted to Vb. Further, the phases of the output signals Va and Vb of the photodiode array 4 change by 2π when the code plate 2 rotates by one pitch of the slit. Therefore, code plate 2
When rotates once, the phases of output signals Va and Vb change φa,
φb are {2π×n} and {2π×(n-1)}, respectively.
only changes. Therefore, the two phase differences (φa − φb)
The absolute value θ of the rotation angle can be found from . Expressing the above relationship in formulas, Va=sin(ωt+φa) (1) Vb=sin(ωt+φb) (2) Also, since φa=nθ and φb=(n-1)θ,
The phase difference φab between Va and Vb is φab=φa−φb=θ (3).

Figure 3 shows the rotation angle θ using the output signals Va and Vb.
The calculation method is shown below. As shown in the figure, the phase φa of the output signal Va is 0 per revolution of the code plate 2.
to 2π n times, and the phase difference φab between the output signals Va and Vb changes once. Therefore, if the interval i of the output signal Va is determined by the phase difference φab and the interval is interpolated and measured by the output signal Va (φa), θ=2πi/n+φa/n (4), and the rotation angle θ can be measured with an accuracy equal to φa.

Now, in the above, in order to correctly determine the interval i, the error εab that occurs when measuring φab must satisfy the following condition.

εab=εa+εb=2εa＜2π/2n (5) ∴εa＜2π/4n (6) Here, εa is the measurement error of φa (=εb), that is,
Since φab is the phase difference between φa and φb, the error εab
is the sum of each error, and this must not cause a half-interval of φa.

Also, the error of the measured value θ in equation (4) is εa/n
Therefore, in order to achieve the target accuracy of ±1/N, the error εa of φa needs to satisfy the following equation.

εa/n<2π/N (7) ∴εa<2πn/N (8) In this way, when the target accuracy ±1/N is given, the number of slits n is determined by the tolerance εa of the detection means.
Therefore, the number n of slits must be determined so that the tolerance εa required for the detection means is a value that can be achieved.

FIG. 4 shows the range of the allowable error εa shown in equations (6) and (8), and the shaded area in the figure is the region where both equations are satisfied. As is clear from the figure, the intersection of the two graphs (n ₀ ,
εa ₀ ) is the point that maximizes the tolerance εa,
Choosing the number of slits to this value n ₀ is the most preferable choice in terms of device design.

Therefore, the optimum number of slits n ₀ is determined from FIG. 4 as follows: 2πn ₀ /N=2π/4·n ₀ ∴n ₀ =√/2 (9). Note that since the value of n ₀ determined by this formula is generally not an integer, an integer value around n ₀ is adopted as the number of slits. Also, in this case, the value of tolerance εa decreases as the number of slits changes, but
There is no problem as long as the tolerance εa after the change is an achievable value.

Here, an actual design example is shown. For example, when the target accuracy is ±1/4000, n ₀ becomes n ₀ =31.622... from equation (9). Therefore, 32 may be adopted as the number of slits n, but in this case, considering the ease of manufacturing the code board, it is preferable to adopt n=36 so that the pitch of the slit rows is 10°. Further, the tolerance εa required for the detection means at this time is ±5°, which is a fully achievable value.

In this way, when the number n ₀ of slits is determined using equation (9), the goal of determining the number of slits is clear, and the number of slits can be easily determined according to the desired resolution. Furthermore, the number of slits n ₀ determined by this equation is the number of slits that can most alleviate the tolerance εa of the detection means, so that the detection means can be easily realized.

In addition, in the above explanation, the case where the optimal number of slits that can maximize the permissible error of the detection means is determined from the target resolution is illustrated, but if the approximate accuracy (tolerance) of the detection means is given in advance, Based on this, the number of slits that can obtain the highest resolution can be determined.

〔Effect of the invention〕

As explained above, in the method for determining the number of slits in an absolute encoder of the present invention, two sets of slit rows with different numbers are provided in the circumferential direction of the code plate, and the movement of the slit rows is detected by a photodiode array or the like. In an absolute encoder that detects the absolute rotation angle from the phase difference between two output signals and the phase of one of the output signals, if the target accuracy is ±1/N, then The optimal number of slits n ₀ is determined from the formula n ₀ = √/2, and the number of slits is set to an integer value close to this optimal number of slits n ₀ , so the goal for determining the number of slits is clear and the goal can be easily met. It is possible to provide a method for determining the number of slits that easily determines the optimum number of slits that can increase the tolerance required for the detection means in obtaining the accuracy of the detection means.

[Brief explanation of drawings]

1 to 3 are block diagrams showing an embodiment of an absolute encoder to which the method of determining the number of slits in an absolute encoder of the present invention is applied, and an explanatory diagram of its operation. FIG. 2 is an explanatory diagram showing the concept of a method for determining the number of slits in a utility encoder. 1...Rotating shaft, 2...Code plate, 2 ₁ , 2 ₂ ...
Slit row, 3... light source, 4... photodiode array (detection means).

Claims (1)

[Claims] 1. Two sets of slit rows with different numbers are provided in the circumferential direction of the code plate, and the movement of the slit rows is detected by a detection means such as a photodiode array, and the phase difference between the two output signals is detected. In an absolute encoder that calculates the absolute rotation angle from the phase of one output signal, if the target accuracy is ±1/N, then the optimal number of slits n can be calculated from the formula n ₀ =√/2. A method for determining the number of slits in an absolute encoder, in which the optimum number of slits _{n is set to an integer value close to 0 .}