JPS6316221A - Displacement converter - Google Patents

Abstract

PURPOSE:To reduce a nonlinear error by setting the number of photodetecting elements provided within one pitch of light transmission slits and the ratio of light transmission slits in the one pitch at the optimized relation. CONSTITUTION:Light is projected on the plural light transmission slits 11 formed in a code plate 1 at specific pitch and light passed through the slits 11 is detected by plural photodetecting elements 3 to find the displacement of the code plate 1 with the detection light. In this case, when the number of elements 3 within one pitch of the array of the slits 11 is odd, the ratio of slits 11 in one pitch is set to 50% and when even, the ratio is set closer to 50% as the number of elements increases. For example, when a duty ratio is varied while the number of the elements 3 is held constant, the nonlinear error caused at the time of the interpolation of one pitch is minimum at the time of a 40% duty ratio. Further, an axis of symmetry is set to 50% and then the nonlinear error becomes minimum even at a position of 60$% symmetrical with the position of 40%.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a displacement transducer that detects mechanical displacement using light. For more details on the history, t, light is projected onto a plurality of transparent slits formed on a code board and arranged in predetermined bits, the light passing through the transparent slits is detected by a light receiving element, and this detected light is This relates to a displacement converter that determines the displacement of a code plate based on the displacement of the code plate.

[Prior Art] As a type of such a displacement transducer, there is a displacement transducer disclosed in Japanese Patent Application No. 13951/1983 filed by the present applicant. This displacement converter arranges a plurality of light-receiving elements within one pitch of a light-transmitting slit, and electrically moves one pitch of the light-transmitting slit inward and outward (for example, 100 to 100 degrees), and has high resolution. This allows displacement to be detected.

[Problems to be Solved by the Invention] However, in such a displacement converter, as shown in FIG.
It includes a harmonic component as the fundamental wave, and due to the influence of the high A wave component, a detection error as shown in FIG. 8 occurs. Hereinafter, this detection error will be referred to as a non-linear difference.

The present invention was made in order to solve the above-mentioned problems, and the present invention is based on the number of light receiving elements provided within one pitch of the light-transmitting slits and the ratio of the light-transmitting slits within this one pitch (hereinafter referred to as duty ratio). The purpose is to reduce nonlinear errors by setting the ratio (ratio) to an optimal relationship.

Means for Solving Problems L] The present invention is formed on a code board and arranged at a predetermined pitch. !
In the displacement converter that projects light onto a 2ai light transmitting slit, detects the light passing through the light transmitting slit with a plurality of light receiving elements, and determines the displacement of the code plate based on the detected light, If there is an odd number of light-receiving elements within one pitch of the array of light-transmitting slits, the ratio occupied by the light-transmitting slits within one pitch is set to 50%, and if there is an even number, the ratio is set to 50%. This displacement transducer is characterized in that it is set so that it approaches 50% as it increases.

[Example] The present invention will be explained below using the drawings.

FIG. 1 is a configuration diagram of an embodiment of a displacement converting device according to the present invention.

In the figure, 1 is a code plate, and 11 is a plurality of translucent slits arranged circumferentially at a predetermined pitch on this code plate.

2 is a light source, and 21 is a lens for converting the light beam from the light source 2 into a parallel beam.

3 is an image sensor that receives the light (slit image) from the light source 2 that has passed through the transparent slit 11;
This is a photodiode array in which 31 to 3 photodiodes are arranged in an array.

SW1 to SW8 are photodiodes 3. This is a switch that sequentially extracts signals from 38 to 38 at a fixed timing.

4 is an OP amplifier, which amplifies the signal applied via each switch SW1 to SW8.

5 is a band pass filter, which extracts the fundamental wave component of the output signal of the OP amplifier 4.

In such a displacement transducer, if the duty ratio is changed while keeping the number of photodiodes constant, the nonlinear error caused by interpolating one pitch changes as shown in FIG.

As shown in the figure, the non-linear error is minimized when the duty ratio is 40%. Also, if 50% is the axis of symmetry, then 4
The nonlinear error is also minimized at a position symmetrical to the 0% position, that is, at the 60% position. The arrangement relationship between the light-transmitting slit 11 and the photodiode 3 at this time is shown in FIG. As shown in this figure, eight photodiodes 31 to 3e are provided within one pitch P of the light-transmitting slits 11, and the ratio of the light-transmitting slits in one pitch P is 40%.

When nine photodiodes 3 are provided, the relationship between the duty ratio and the nonlinear error is as shown in FIG. In this case, the nonlinearity error is minimized when the duty ratio is 50%.

The relationship between the number N of photodiodes and the duty ratio that minimizes the nonlinear error is as shown in FIG. The graph showing the change in nonlinear error at a duty ratio of 50% to 100% is 0 to 100% on the vertical axis when the duty ratio is 50%.
This graph is symmetrical to the 50% graph. For this reason, the fifth
Even with the duty ratio shown within 0 in the figure, the nonlinear error takes the minimum value.

Such a result is obtained as follows.

In the above-mentioned Fig. 7, the duty ratio of the transparent slit is W
/P (W is the width of the transparent slit).

When the light intensity function of the light projected onto the transparent slit is expressed as a 7-Lier series, the light intensity function is given by the following equation.

'11 motion LtL? If such a light intensity function is represented by a graph, it will be as shown in FIG. In FIG. 6, the vertical axis represents the light intensity function, and the horizontal axis represents the spatial frequency. Further, the figure shows a case where there are eight light receiving elements (N=8), where the solid line is a signal component and the broken line is a folding noise component. As shown in the figure, the component of the signal waveform that is folded back when it hits the vertical axis becomes the noise waveform.

Therefore, the seventh-order noise component a7' and the ninth-order noise component a9' are superimposed on the first-order signal component (fundamental wave component a+).

Since the fundamental wave is extracted and used as a photodetection signal, the larger the signal component of the fundamental wave is compared to the 7th and 9th order noise components, the more the S/
The N ratio becomes better and non-linear errors become smaller.

Here, the number N of light receiving elements and the coefficient I of the Fourier series! 1
I will explain about the person in charge.

When the duty ratio of the code plate is D, the coefficient a of the Fourier series representing the light intensity function is expressed as follows.

(24 - Twill 5) ■ξ bar (Ez o, 1.λ---) Here, if the number of elements N is an odd number, the signal component of the fundamental wave is N±1〉 Next, that is, even-order harmonic noise components are superimposed.

The coefficient of the even-order noise component is the formula ■, and since order 1 is an even number, if D = 0.5, it becomes 1πD-Eπ (where E is an integer), and the coefficient of the even-order haze component is Becomes O. Therefore, when the number of elements N is an odd number, the optimal duty ratio is set to 5.
Set it to 0%.

When the number N of elements is an even number, unlike when the number N is odd, the optimum duty ratio cannot be easily determined. Therefore, the optimum duty ratio is determined by simulation.

An example of the optimum duty ratio obtained in this manner is shown in FIG. In Fig. 5, when the number of elements N is an odd number, the optimal duty ratio is 50%. Also, when the number of elements N is an even number, the duty ratio becomes 50% as the number of elements increases. Near (= 1) tends to increase.

Also, as shown in Figures 2 and 4, 40% of Figure 2,
There are duty ratios other than 50% shown in FIG. 4 that reduce the error. For example, in FIG. 2, they are 26% and 13%. However, as the duty ratio moves away from 50%, a large error occurs due to the error in the duty ratio.
'＋t. For example, at 13% in FIG. 2, the non-linear error when a 1% error occurs in this duty ratio is much larger than the 1% error at a duty ratio of 40%. Therefore, in practice, it is optimal to take a duty ratio close to 50%.

In the embodiment, the case where the displacement converter is a rotary encoder has been described, but the present invention is not limited to this, and the displacement converter may be of a linear displacement type.

Further, as the light receiving element, something other than a photodiode may be used.

[Effects] According to the present invention, the light-transmitting slit is set at a duty ratio that minimizes the nonlinearity error with respect to the number of light-receiving elements, so the detection accuracy of the displacement converter can be improved.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram of an embodiment of a displacement transducer according to the present invention, Figs. 2 and 4 are diagrams showing an example of the relationship between duty ratio and nonlinear error, and Fig. 3 is a diagram showing an example of the relationship between the duty ratio and nonlinear error. Figure 5 is a diagram showing an example of the arrangement of photodiodes, and Figure 5 is a diagram showing the relationship between the number of photodiodes and the optimum duty ratio.
Figure 6 shows the light intensity function of the light thrown into the transparent slit, Figure 7 shows the relationship between the intensity of the light received by the photodiode and the position of the code plate. FIG. 8 is a diagram showing the distribution of nonlinear errors occurring in a conventional displacement transducer. 1... Code plate, 11... Translucent slit, 2...-
Light source, 3... Light receiving element, 4... Amplifier, 5... Band pass filter, SW+ to SW e... Switch. )・− Patent attorney Shinsuke Ozawa: \, 1)
, N
Kusakaki field moat harvesting 9 525 figure 7 shi 4 set figure 6

Claims (1)

[Claims] Light is projected onto a plurality of light-transmitting slits formed on a code board and arranged at a predetermined pitch, and the light passing through the light-transmitting slits is detected by a plurality of light-receiving elements. In a displacement converter that calculates the displacement of the code plate based on the detected light, if there is an odd number of light receiving elements within one pitch of the arrangement of the transparent slits, the ratio occupied by the transparent slits within one pitch. is set to 50%, and in the case of an even number of elements, the ratio is set to approach 50% as the number of elements increases.