JPH0453361B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to an arithmetic circuit, and more particularly to an arithmetic circuit for an automatic focusing distance measuring circuit used in a camera.

<Conventional technology> In recent cameras, PSD is used as a position detection element.
Automatic focusing circuits using (Position Sensitive Diodes) are becoming mainstream, but this method
This is an active type ranging method based on the principle of triangulation.

In other words, when photographing, an IRED (infrared light emitting diode) is momentarily emitted to illuminate the subject with a highly directional lens, and the reflected light is placed at a location perpendicular to the optical axis by a certain baseline length from the IRED.
The distance is measured by projecting onto the PSD using a lens, and detecting the ratio by utilizing the fact that the ratio of the output current flowing between the two electrodes of the PSD changes depending on the projection position.

In the distance measurement method described above, the intensity of the reflected light changes depending on the reflectance of the subject, so it is necessary to detect the ratio value. To do this, first obtain a voltage by logarithmically compressing each current, and then create a voltage by calculating it. A distance signal is obtained by simultaneously comparing and discriminating this using multiple comparators.

FIG. 2 shows a circuit conventionally used to create a distance signal from such two input signals.

I ₁ and I ₂ are IRED taken out from each electrode of PSD
This is the signal current of reflected light. The above signal currents I ₁ , I ₂
flows into the logarithmic compression diodes D ₁ and D ₂ and is applied to the non-inverting inputs of the first and second operational amplifiers 1 and 2, respectively. A resistor R 1 is connected to the inverting input of the first operational amplifier ₁ to the reference voltage Vref ₁ , and a resistor R ₂ is connected between the first operational amplifier 1 and the output. Furthermore, a resistor R 3 is connected between the inverting input of the second operational amplifier 2 and the output of the first operational amplifier 1, and a resistor R ₄ is connected between the output and the inverting input _of the second operational amplifier 2.
is connected. The output of the second operational amplifier 2 is given to one input of a plurality of comparators _C1 to _C4 . A reference voltage for comparison according to the distance division is given to the other inputs of the comparators _C1 to _C4 .

In the above circuit, when input currents I ₁ and I ₂ are applied, the forward voltages of diodes D ₁ and D ₂ are as follows.

D ₁ :V _F (D ₁ )=(K・T/q)・ln(I ₁ /I ₀ ) D ₂ :V _F (D ₂ )=(K・T/q)・ln(I ₂ /I ₀ ) Here, K is Boltzmann's constant, q is the electron charge, T is the absolute temperature, and I ₀ is the reverse saturation current.

Now, assuming that the value of each resistor is R ₁ = R ₂ = R ₃ = R ₄ ,
The output Vout (OP2) of the second operational amplifier 2 is as follows.

Vout (OP2) = 2 (V _F (D ₂ ) − V _F (D ₁ )) + Vref ₁ = 2・
(K・T/q)・{ln(I ₂ /I ₀ )−ln(I ₁ /I ₀ )}+V _r
ef1 = 2・(K・T/q)・ln(I ₂ /I ₁ ) + V _ref1 ……(1) From now on, the absolute values of the signal currents I ₁ and I ₂ have no relation to the output determined only by the ratio. I know that it will happen.

Next, let's consider what value the ratio of signal currents I ₁ and I ₂ takes depending on distance.

In autofocus using the zone method, although the width of each zone is constant within each zone and the focus point is constant, the size of the maximum circle of confusion caused by the width of the zone is Ideally, they should be determined to be equal. In such a case, when the first, second, ..., n-th zones are set from the infinity side, the difference in the amount of lens extension for each zone is close to a constant value, so
For ease of design, it is customary to make the feeding amount constant for each zone.

Assuming that the focal length of the photographing lens is _f1 , the relationship between the extension amount n·a (n=1, 2, . . . ) and the focus position L is as follows.

1/L+1/f ₁ +na=1/f ₁ 1/L=1/f ₁ -1/f ₁ +na = na/f ₁ (f ₁ +na) f ₁ 〓If approximated as na, the above L becomes L= f ₁ ² /na.

From this, for example, when f ₁ = 40 mm and a = 0.4 mm, L = 4000/nmm, that is, the focus position will change from the infinite position to 4 m, 2 m, 1.33 m, 1 m as you extend the lens in 0.4 mm increments. Recognize.

On the other hand, the imaging position x of the reflected image of IRED on the PSD is
Subject distance is L, baseline length (IRED light emitting part and PSD
(optical axis distance of each lens) is l,
If the focal length of the PSD imaging lens is _f2 , it can be expressed as follows, with the imaging position at infinity as the origin.

From l/L=x/f ₂ , x=l・f ₂ /L Here, if we set L=f ₁ ² /na, x=n・a/f ₁ ²・l・f ₂ Therefore, for example, f ₁ =40mm, _f2 =20mm, a=0.4mm,
Under the condition that l=40 mm, the imaging position x has the following value.

x = 0.2n・mm If the total length of the effective light receiving part of the PSD is 2mm, and the optical system is set so that the image is centered at infinity, then
0.2mm from the center for 4m, 0.4mm for 2m, 1.333m
For example, when the focus position is 4 m/n, the image formation position on the PSD is 0.2·n·mm. At this time, the ratio of the currents I ₁ :I ₂ taken out from the two electrodes of the PSD is 1:1 at infinity,
The ratio is 1+0.2n:1-0.2n, such as 1.2:0.8 for 4m and 1.4:0.6 for 2m.

When the ratio of the above currents I ₁ and I ₂ is set as K·T/q=26 mV and substituted into equation (1), the output Vout (OP2) becomes as follows depending on the value of n.

Vout(OP2)=26×2×ln(1+0.2n)/(1-0.2n)
+V _ref1 n = 1 21.08 (mV) + V _ref1 n = 2 44.06 (mV) + V _ref1 n = 3 72.09 (mV) + V _ref1 n = 4 114.26 (mV) + V _ref1 Therefore, each comparator in the circuit of Fig. 2
The comparison voltage of C ₁ to _{C 4} , comparator C ₁ is
21.08mV, comparator C ₂ is 44.06mV, comparator C ₃ is 72.09mV, comparator C ₄ is 114.26mV
The reference voltage V _{ref2 ,} reference voltage V _ref3 , dividing resistance value R ₅ ,
If R ₆ and R ₇ are determined, the comparator C ₁ to C ₄ outputs will all be "low" for the measured distance ∞ to 4 m.
For 4-2m, comparator C ₁ is "high", 2
For ~1.33m, comparators C ₁ and C ₂ are “high”;
Comparators C ₁ , C ₂ , C ₃ for 1.33 to 1 m
is “high” and for less than 1m, comparator C ₁ ~
All _C4 's are set to ``high'' and distance measurement is possible.

<Problems to be Solved by the Invention> In the conventional distance measuring circuit described above, since calculation is performed based on the ratio of 1 resistance, highly accurate resistances (R ₁ to _{R 4} ) are required.

2 The circuit is complicated because two OP amplifiers are required for calculation.

3 Because the difference in comparison voltage is uneven, the dividing resistor R ₅ ~
As R ₇ , it has to be set to a halfway value, making it difficult to increase accuracy.

4. If the optical system is misaligned, the ratio will also change, so precision is required in adjusting the optical system.

There were other drawbacks.

<Means for Solving the Problems> The present invention has been made for the purpose of eliminating the drawbacks of the above-mentioned conventional circuits and providing a simple and easily adjustable arithmetic circuit for distance measurement.

For PSD, which detects reflected light from the subject,
The two detected currents from the PSD are logarithmically compressed using the logarithmic compression characteristics of each transistor or diode, and the two logarithmically compressed outputs each include a set of transistors directly or via a follower amplifier in the base. The signal is inputted to a differential amplifier circuit to form an arithmetic circuit, and the output of the differential amplifier circuit is applied to a plurality of comparison circuits provided according to distance division to form a signal based on distance measurement.

<Operation> The signal currents from the PSD are input to the bases of the respective transistors, and the calculation results of the two detection currents are obtained as the output of the differential amplifier.

<Example> In FIG. 1, two detection currents I ₃ and I ₄ output from a photodetector (PSD, not shown) for position detection are applied to diodes D ₃ and D ₄ for logarithmic compression, respectively. At the same time, via the follower amplifiers 4 and 5
Provided to the bases of NPN transistors TR ₁ and TR ₂ . Note that the cathodes of the logarithmic compression diodes D ₃ and D ₄ are connected to the reference voltage _VO .

The emitters of both transistors TR ₁ and TR ₂ are commonly connected to a constant current source I ₅ . transistor
The collector of TR ₁ is connected to the power supply +V _CC , and the transistor
The collector of TR ₂ is connected to the power supply +V _CC via resistor R ₈ . The output led to the collector of the transistor _TR2 is given to each input terminal of one of the plurality of comparators _C5 to _C8 . the comparator
The other input terminals of C ₅ to _{C 8} are given voltages for comparison obtained by dividing the voltage between the power supply +V _CC and the constant current source I ₆ using dividing resistors R ₉ to _{R 12} , respectively. A distance signal is formed by comparison with the voltage level applied to the input terminal.

In the circuit with the above configuration, diodes D ₃ and D ₄
As described above, a detection current having a ratio of 1+0.2n:1-0.2n flows through the PSD as in the conventional circuit. Therefore, the voltage difference V _F between both diodes D ₃ and D ₄ can be expressed as follows.

V _F (D ₄ )−V _F (D ₃ )=(K・T/q)・ln{(1−0.2n
)/(1+0.2n)} Also, the collector current of transistor TR ₁ is I _C
(TR ₁ ), the collector current of transistor TR ₂ is I _C
(TR ₂ ), the relationship between the base voltages V _BE (TR ₁ ) and V _BE (TR ₂ ) of both transistors TR ₁ and TR ₂ is as follows: V _BE (TR ₂ )−V _BE (TR ₁ )=(K・T/q)・ln{Ic
(TR ₂ )/Ic (TR ₁ )}.

The voltage of the diode D ₃ is applied to the base of the transistor TR ₁ via the follower amplifier 4, and the voltage of the diode D ₄ is also applied to the base of the transistor TR ₂ , so that (K・T/q)・ln{(1-0.2n)/(1+
0.2n)}=(K・T/q)・ln{(Ic(TR ₂ )/Ic(TR ₁
)}, Ic(TR ₂ )/Ic(TR ₁ )=1−0.2n/1+0.2n.

Here, between the constant current I ₅ and I _C (TR ₂ ) + I _C (TR ₁ ) =
Since there is a relationship α・I ₅ , if α≒1 is assumed, the following relationship can be obtained from the above equation.

I _C (TR ₂ ) = I ₅ × (1-0.2n) / (1-0.2n) + (1 + 0
.2n) = I ₅ × 1-0.2n/2 Therefore, if the + power supply voltage is V _CC , the positive input of the comparator will be R ₈ · I ₅ · 1-0.2n/2.

At this time, for example, I ₅ = 2・I ₆ R ₉ = R ₁₀ = R ₁₁ =
If you choose R ₁₂ = 0.2・R ₈ , all comparator outputs will be “low” when the distance is from infinity to 4 m like in the conventional circuit.
From 4m to 2m, only comparator _C5 is "high", and from 2m to
At 1.333m, comparators C ₅ and C ₆ are “high” 1.333~
At 1 m, the comparators C ₅ , C ₆ , and C ₇ are "high." When the distance is 1 m or less, the comparators C ₅ to _{C 8} are "high," and the distance measurement results are obtained as the outputs of the comparators C ₅ to C ₈ .

In the circuit with the above configuration, even if there is a slight deviation in the optical system, this will cause the two output currents to be 1-
0.2n: No shift is given to 1+0.2n. That is,
Since it appears in the form 1-0.2n+K:1+0.2n-K, the output voltage, R ₈・I ₅ 1-0.2n/2, will eventually become R ₈・I ₅ 1-0.2n+K/2, which will eventually become R. Since it can be expressed as _{8.I 5} 1-0.2n/2+K', it is just a shift, and can be adjusted by, for example, changing the value of the resistor _R9 .

Further, since the dividing resistors R ₁₀ to _{R 12} have the same value, the configuration is easy and accuracy can be easily achieved.

<Effects of the Invention> According to the present invention, a circuit can be configured using easily adjustable resistance values, the burden on design can be reduced, and calculation accuracy can also be improved. Furthermore, the overall circuit configuration is simplified, and an arithmetic circuit that is easy to integrate and has excellent practicality can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a calculation circuit diagram for camera distance measurement according to an embodiment of the present invention, and FIG. 2 is a conventional distance measurement circuit diagram. I ₃ , I ₄ ... PSD output current, D ₃ , D ₄ ... Logarithmic compression diode, 4, 5 ... Follower amplifier, TR ₁ ,
TR ₂ ...Transistor, _C5 to _C8 ...Comparator, _R8 to _R12 ...Resistor, _I5 , _I6 ...Constant current source.

Claims (1)

[Scope of Claims] 1. A position detection element that outputs two detection currents to its two output terminals based on incident light having a ratio according to the position of the incident light; and A logarithmic compression means consisting of a single stage diode or a transistor which logarithmically compresses the detected current, and a pair of logarithmic compression means to which the two output voltages logarithmically compressed by the logarithm compression means are respectively applied directly or through a follower amplifier to their bases. and a differential amplifier that includes a constant current source common to the pair of transistors and outputs a voltage based on the collector current of one of the transistors, and detects two of the position detection elements. A calculation circuit for distance measurement, characterized in that a current calculation result is taken out as an output voltage of the differential amplifier.