JP2674666B2 - Optical position measurement circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] The present invention relates to an optical position measuring circuit used in a height inspecting device or the like for obtaining a value corresponding to a ratio of a pair of analog signal values output from a photodetector as an optical spot position. A first analog signal converting a first analog signal value into a first digital value for the purpose of reducing a calculation error due to quantization even if a ratio of a maximum value and a minimum value of a sum of a pair of analog signal values is large.
/ D converter, a second A / D converter for converting a second analog signal value into a second digital value, and an optical position calculation circuit for outputting a value according to the ratio of the first digital value and the second digital value And the first
And a reference voltage generation circuit for applying a voltage value corresponding to the larger value of the analog signal value and the second analog signal value to the reference voltage terminals of the first A / D converter and the second A / D converter. To do.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical position measuring circuit used in a height inspection device or the like and for obtaining a value corresponding to a ratio of a pair of analog signal values output from a photodetector as a position of a light spot.

[Prior Art] FIG. 8 shows a height measuring circuit which is a kind of an optical position measuring circuit used in a visual inspection apparatus for a printed wiring board and the like.

The light spot formed on the printed wiring board is imaged on a light position detector (PSD) 35, and a signal corresponding to the image position is output from a pair of terminals of the light position detector 35.
These signals are amplified by amplifiers 37 and 39, respectively, and then digitally converted by A / D converters 42 and 44.

When the output values of the A / D converters 42 and 44 are A and B, respectively, the height position of the light spot on the printed wiring board is (A
-B) / (A + B). This operation is an adder circuit,
Although it can be executed using a subtraction circuit and a division circuit, since the configuration becomes complicated, the output values A and B of the A / D converters 42 and 44 are usually supplied to the operation table memory 46 for addressing. , The height data stored in advance is output. Assuming that the number of quantized bits of the output of the operation table memory 46 is m, the address (A, B) has a value H expressed by the following equation.
Is stored.

H = (A−B) / (A + B) × (2 ^m / 2) (2 ^m / 2) (1) The range of (A−B) / (A + B) is −1 to 1, and H
The value range of is 0 to 2 ^m .

[Problems to be Solved by the Invention] The maximum value Δ _max of the height calculation error based on the quantization error of the A / D converters 42 and 44 and the calculation table memory 46 is the A / D converter.
When the number of output bits of 42 and 44 is n, it is approximately expressed by the following equation.

δ _max = (DR / 2 ^{n + 1} ) + (1/2 ^{m + 1} ) (2) Here, the dynamic range DR is DR = max (A + B) / min (A + B) (3) Yes, max indicates the maximum value, and min indicates the minimum value. That is, the maximum value of the height calculation error increases almost in proportion to the ratio of the maximum value and the minimum value of (A + B). In the inspection of a printed wiring board or the like, since this ratio is extremely large, there is a problem that the height calculation error becomes extremely large especially in a region where the value of (A + B) is small, that is, a region where the reflectance is small. .

In view of the above problems, an object of the present invention is to reduce a calculation error associated with quantization even if the ratio of the maximum value and the minimum value of the sum of a pair of analog signal values output from the photodetector is large. Another object of the present invention is to provide an optical position measuring circuit capable of performing the above.

[Means for Solving the Problems] FIG. 1 is a block diagram showing the principle configuration of the present invention.

In the figure, 1 is a first A / D converter, which converts a first analog signal value into a first digital value.

2 is a second A / D converter, which outputs the second analog signal value to the second
Convert to digital value.

Reference numeral 3 denotes a light position calculation circuit, which outputs a value according to the ratio of the first digital value and the second digital value. The optical position calculation circuit 3 may use a table memory in which the calculation result is stored in advance, or a calculator.

Reference numeral 4 is a reference voltage generation circuit, and the voltage value corresponding to the larger value of the first analog signal value and the second analog signal value is used as the reference voltage of the first A / D converter and the second A / D converter. Apply to the terminal. If it is known in advance whether the larger value is one of the analog signal values, it is not necessary to use the comparison circuit.

[Operation] For example, a pair of output signal values of the optical position detector (PSD),
Alternatively, if the output signal values of the pair of photodetectors that detect the intensities of the reflected light and the transmitted light split by the splitter as shown in the following examples are A and B, the position of the light spot is (A- B) / (A + B) = {1- (B / A)} / {1+
(B / A)}. That is, the position of the light spot is determined by the ratio of A and B.

In such a case, even if the reference voltages of the A / D converters 1 and 2 are changed, if the reference voltages are the same, the ratio between the analog signal values A and B and the digitally converted values C and D The ratio is the same ignoring the quantization error.

On the other hand, when the value of (A + B) is small, that is,
When the reflectance is small and the light intensity is small, the difference (A-
Even if B) slightly changes, (A−B) / (A + B) changes greatly, so that the calculation error increases. Especially in the digital calculation, the quantization error has an influence, and this calculation error becomes remarkable.

However, in the present invention, since the voltage value corresponding to the larger value of A and B is applied to the reference voltage terminals of the A / D converters 1 and 2, the value of (C + D) becomes large. Therefore,
The change in (C−D) / (C + D) with respect to the change in the difference (C−D) becomes small, in other words, the ratio between the maximum value and the minimum value of (C + D) becomes small, and the calculation error is smaller than in the past. Get smaller.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

(1) First Embodiment FIG. 1 shows a main configuration of a height inspection device for a ceramic sheet 10.

As shown in FIG. 4 (A), the soft ceramic sheet 10 placed on the XY table 12 before firing has a large number of through holes (for example, tens of thousands of holes are formed in one ceramic sheet of 200 mm square). ) Is provided, and each through hole is filled with a paste-like conductor 11 in which a metal cloth and a solvent are mixed. This conductor 11 is for electrically connecting the laminated ceramic sheets 10. If even one of them has an insufficient filling defect, the entire ceramic multilayer substrate after firing becomes a defective product. Must be accurately measured to reliably detect underfill defects.

In FIG. 2, the laser light emitted from the laser 14 is expanded and collimated through the beam expander 16, reflected by the plane mirror 18 and the polygon mirror 20, and the fθ lens 22.
Is reflected by the plane mirror 24 and is convergently irradiated onto the ceramic sheet 10 almost vertically to form a light spot P ₁ . When the polygon mirror 20 is rotated by the motor 26, the light spot P ₁ linearly scans the ceramic sheet 10 to form a light cutting line 28.

The light spot P ₁ is imaged on the splitter 34 via the imaging lens 30 and the plane mirror 32. The intensity of the light reflected by the splitter 34 is detected by the photodetector 36, and the splitter 34
The intensity of the light transmitted through is detected by the photodetector 38.
These photodetectors 36 and 38 are, for example, photomultiplier tubes.

As shown in FIG. 3, the splitter 34 is formed by coating a reflection film 34b on the surface half of a rectangular glass plate 34a. In the splitter 34, when the ceramic sheet 10 is an ideal plane, the center of the image P ₂ of the light spot P ₁ is the reflection film 34.
It is arranged so as to scan on the boundary line 34c which is the long side of b. In reality, the locus of the center of the image P ₂ is, for example, locus 40. The deviation of the center of the image P ₂ from the boundary line 34c is (A−
B) / (A + B).

The value range of (A−B) / (A + B) is −1 to 1.
When the value of (A + B) is small, that is, when the reflectance is small and the light intensity is small, (AB) / (A + B) is large even if the difference (AB) changes slightly. Since it changes, the calculation error increases. Especially in the digital calculation, the quantization error has an influence, and this calculation error becomes remarkable.

These output voltages A and B are supplied to A / D converters 42 and 44 and converted into digital values C and D, respectively, as shown in FIG. The arithmetic table memory 46 is addressed by a value (C, D) and outputs its prestored contents as height data.

On the other hand, the output voltages A and B are supplied to the comparator 48 for comparison, and the output of the comparator 48 causes a changeover switch 50.
Is controlled, and the output voltage A, B is selectively changed over.
It is supplied to the gain adjusting circuit 52 via 50. That is, A
When> B, the output voltage A is supplied to the gain adjusting circuit 52, and when A <B, the output voltage B is supplied to the gain adjusting circuit 52. The gain adjustment circuit 52 uses the A / D converter 4 with a voltage proportional to this input voltage, for example, a voltage 1.2 times the input voltage.
Applied to 2, 44 reference voltage input terminals.

Next, the operation of the first embodiment constructed as described above will be explained based on FIG.

When the light spot P ₁ scans along the cross section shown in FIG. 4A, the output voltages A and B of the photodetectors 36 and 38 change as shown in FIG. That is, since the reflectance of the ceramic sheet 10 is higher than the reflectance of the conductor 11, the ratio of the maximum value and the minimum value of the sum (A + B) is large.

Therefore, when it is necessary to accurately measure the height of the conductor 11 with respect to the ceramic sheet 10, when the reference voltage of the A / D converters 42 and 44 is fixed as in the conventional case, the conductor 11 is The height calculation error increases.

However, in the present embodiment, since the reference voltage is set to a value proportional to the larger value of A and B, the A / D converter 4
The output values C and D of 2 and 44 respectively change as shown in FIG. Therefore, the value of the sum (C + D) also becomes large on the conductor 11, and the error in the height data output from the operation table memory 46 becomes smaller than that in the conventional case.

(2) Second Embodiment FIG. 5 shows a height measuring circuit according to the second embodiment.

In this example, the output of the gain adjustment circuit 52 is the A / D converters 42, 4
It shows the case where the limit range of the reference voltage of 4 may be exceeded.
Is applied to the reference voltage terminals of the A / D converters 42 and 44 via the voltage limiting circuit 54. The voltage limiting circuit 54 is generally a base clipper, but a peak clipper or a limiter is used depending on the gain of the gain adjusting circuit 52.

 The other points are the same as in the first embodiment.

(3) Third Embodiment FIG. 6 shows a height measuring circuit of the third embodiment.

Some of the A / D converters 42, 44 require a certain offset in the input voltage or the reference voltage depending on the type. For example, when the reference voltage is 5V, the input voltage range is set to 3 to 5V. In such a case,
The output voltages of the photodetectors 36 and 38 are respectively fed to the A / D converters 42 and 4 via voltage shift circuits 56 and 58 which apply a constant voltage to the output voltages.
Supply to 4. Further, a voltage shift circuit 60 is connected to the subsequent stage of the voltage limiting circuit 54, and a voltage obtained by adding a constant voltage to the output voltage of the voltage limiting circuit 54 is applied to the reference voltage terminals of the A / D converters 42 and 44.

 The other points are the same as in the second embodiment.

(4) Fourth Embodiment FIG. 7 shows a height measuring circuit of the fourth embodiment.

If the digital conversion speed is slower than the signal change speed, the photodetector 3 may be activated during digital conversion by the A / D converters 42 and 44.
The output voltages A and B of 6 and 38 fluctuate. In such a case, the sample and hold circuits 62 and 64 are connected in front of the A / D converters 42 and 44 and the comparator 48 so that the input voltage and the reference voltage do not change during the digital conversion.

 The other points are the same as the second embodiment.

[Effects of the Invention] As described above, according to the optical position measuring circuit of the present invention, the voltage value corresponding to the larger value of the pair of analog signal values is used as the reference voltage for both A / D converters. Since it is applied to the terminal, the ratio of the maximum value and the minimum value of the sum of digital conversion values becomes small, and even if the ratio of the maximum value and the minimum value of the sum of a pair of analog signal values is large, the calculation error due to the quantization It has an excellent effect of being able to reduce the height, and contributes greatly to improvement of measurement accuracy such as height.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the principle configuration of the present invention. 2 to 4 relate to the first embodiment of the present invention, FIG. 2 is a block diagram of a main portion of a height inspection device to which an optical position measuring circuit is applied, and FIG. 3 is a height of a light spot P ₁ . FIG. 4A is a partial cross-sectional view of the ceramic sheet along the light cutting line, and FIG. 4B is a change in the pair of outputs of the photodetector. FIG. 6C is a diagram showing changes in the output of the A / D converter. 5 to 7 are height measuring circuit diagrams according to the second to fourth embodiments of the present invention, respectively. FIG. 8 is a conventional height measuring circuit diagram. In the figure, 36 and 38 are photodetectors 42 and 44 are A / D converters 46 are calculation table memories 48 are comparators 50 are changeover switches 52 are gain adjusting circuits 54 are voltage limiting circuits 56, 58, 60 are voltage shifting circuits 62 and 64 are sample hold circuits

Claims (1)

(57) [Claims] 1. A first A / D converter (1) for converting a first analog signal value into a first digital value, and a second A / D converter for converting a second analog signal value into a second digital value.
In a light position measuring circuit having a converter (2) and a light position calculating circuit (3) for outputting a value according to a ratio of a first digital value and a second digital value, a first analog signal value and a second analog signal value The voltage value corresponding to the larger one of the analog signal values is used as the first A / D converter (1) and the first
2) An optical position measurement circuit characterized in that a reference voltage generation circuit (4) applied to the reference voltage terminal of the A / D converter (2) is attached.