JPH0454580A - automatic gain control circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to an automatic gain control circuit used in an optical symbol reading device that optically reads symbols such as bar codes and characters.

<Conventional technology> Conventionally, barcode reading devices and optical character reading devices (
Optical symbol reading devices such as OCR (OCR) are widely used as information input devices for computers, for example. Such optical symbol reading devices include, for example, a laser system that scans a symbol surface on which symbols such as barcodes and characters are formed with laser light, and detects the light reflected from the symbol surface during this scanning. There are also scanner-type devices that illuminate the symbol surface, receive the reflected light with an image sensor, and electrically scan the optical image of the symbol formed on the light-receiving surface.

In order for the above-mentioned optical symbol reading device to be able to accurately read symbols even when the distance (reading distance) between the symbol surface and the device is varied within a certain range,
It is necessary to absorb fluctuations in the amount of reflected light from the symbol surface due to fluctuations in reading distance using a circuit. For this reason, automatic gain control circuits (AGC circuits) that include variable gain amplification circuits have traditionally been used to convert the output of detection elements such as photodiodes and image sensors into signals with approximately constant amplitude regardless of fluctuations in the amount of light. This technology is commonly applied.

- The configuration of the AGC circuit for boat fishing is shown in FIG. The electrical signal from the detection element corresponding to the amount of reflected light from the symbol surface is input to the variable gain amplifier circuit 1, and the output of the variable gain amplifier circuit 1 is used to level-discriminate the signal at an appropriate slice level and output the signal to NJ. Alternatively, it is applied to a binarization circuit (not shown) which binarizes it to "0". The output signal of the variable gain amplifier circuit 1 is also given to a peak hold circuit 2, and the output of this peak hold circuit 2 is fed back to the variable gain amplifier circuit 1 as a control signal for controlling its gain.

The peak hold circuit 2 rapidly increases its output signal when the input signal level increases, and decreases the output signal with a relatively large time constant when the input signal level decreases. be. This stabilizes the circuit operation without being affected by severe fluctuations in the input signal.

The variable gain amplifier circuit 1 is realized by the configuration shown in FIG. 4, for example. That is, this variable gain amplification circuit l includes an operational amplifier 4 to which an output signal from a detection element is applied to its inverting input terminal via a resistor 3. It was designed so that they could return home. A control signal from the peak hold circuit 2 is input to the gate of the transistor 5. With this configuration, by applying a reverse bias voltage between the gate and the source of the transistor and changing the gate potential, the source and source of the transistor 5 can be changed.
By changing the electrical resistance between the drains, this transistor 5
By operating as a variable resistor controlled by the output signal of the peak hold circuit 2, the amplifier circuit l
gain is controlled.

With the AGC circuit as described above, when the amount of reflected light is large and therefore the output signal of the detection element becomes large, the gain of the variable gain amplifier circuit 1 is reduced, and when the amount of reflected light is small and the output signal of the detection element becomes small. When this happens, the gain of the variable gain amplifier circuit l is increased. Therefore, the output of the variable gain amplifier circuit 1 remains approximately constant regardless of the amount of light reflected from the symbol surface. This results in
It is possible to absorb fluctuations in the amount of light received by the detection element due to fluctuations in reading distance, and to read symbols accurately over a range of reading distances.

<Problems to be Solved by the Invention> FIG. 5 is a waveform diagram showing changes in signals at each part in FIG. 3, and FIG. 5(a) shows changes in the input signal to the variable gain amplifier circuit 1, FIG. 5(b) shows the change in the output signal of the peak hold circuit 2, and FIG. 5(C) shows the change in the output signal of the peak hold circuit 2.
shows the output signal of

During period TI, the output signal of the peak hold circuit 2 fluctuates following the fluctuation of the input signal, and therefore the gain of the variable gain control circuit 1 fluctuates corresponding to the input signal level. As shown in FIG. 1, the output signal of the variable gain amplifier circuit 1 exhibits oscillations with approximately constant amplitude and corresponding to the symbol.

By the way, when the direction of incidence of light such as a laser beam that is incident on the symbol surface and the direction of reflection of the reflected light that is reflected by the symbol surface and detected by the detection element are almost opposite directions, high-intensity The specularly reflected light will be incident on the detection element. In other words, when the direction of incidence of the laser beam and the direction of reflection of the reflected light detected by the detection element are different, the detection element detects the scattered light from the symbol surface;
In the above case, the specularly reflected light is received by the light receiving element.

This specularly reflected light is several tens to several tens of times larger than the scattered reflected light.
Therefore, the input signal from the detection element to the variable gain amplifier circuit l exhibits a large peak as indicated by reference numeral fl. Since the peak hold circuit 2 follows this, its output rises significantly as shown by reference numeral 12, and therefore the gain of the variable gain amplifier circuit 1 becomes extremely small.

However, since this peak hold circuit 1 has a relatively large time constant as described above, even after the output signal of the detection element becomes low, the output signal remains at a large value as indicated by reference numeral 13. The gain of the variable gain amplifier circuit 1 remains as it is, and the gain of the variable gain amplifier circuit 1 continues to be extremely small. Therefore, in FIG. 5(C), reference symbol I! As shown in 4,
The amplitude of the output signal of the variable gain amplifier circuit 1 becomes small, making it impossible to read symbols.

In this way, in the above-mentioned AGC circuit, once the specularly reflected light from the symbol surface is received, symbol reading cannot be performed for a while thereafter.

In order to solve this problem, it is conceivable to input the output of the detection element to a variable gain amplifier circuit l via a limiter circuit, for example, but the output of the detection element fluctuates greatly in response to changes in the amount of light received. Therefore, if the input signal is cut at a certain value, there is a risk that the reading error will increase.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned technical problems, and enables the subsequent symbol reading operation to be performed satisfactorily even when specularly reflected light from the symbol surface is incident on the detection element in an optical symbol reading device. The purpose of the present invention is to provide an automatic gain control circuit that enables automatic gain control.

Means for Solving the Problems> To achieve the above object, the automatic gain control circuit of the present invention illuminates a symbol surface on which a symbol is formed and detects the reflected light with a detection element, thereby detecting the symbol. a variable gain amplifier circuit used in an optical symbol reading device, into which the output signal of the detection element is input, and which varies the gain in response to an external control signal; and a peak level of the output signal of the variable gain amplifier circuit. A peak hold circuit detects and holds the peak hold level, and a control signal corresponding to the held peak hold level is fed back to the variable gain amplifier circuit to keep the amplitude of the output signal of the variable gain amplifier circuit almost constant. The automatic gain control circuit is provided with a reset circuit that resets the peak hold level in the peak hold circuit when the peak hold level in the peak hold circuit exceeds a predetermined reference value.

According to the above configuration, when the peak hold level in the peak hold circuit exceeds the reference value, the peak hold level is reset, so the output signal of the detection element is reduced due to the specularly reflected light from the symbol surface. Even when the gain of the variable gain amplifier circuit temporarily increases, the state in which the gain of the variable gain amplifier circuit becomes extremely small does not continue. As a result, even when the detection element receives specularly reflected light from the symbol surface,
It is possible to quickly return to a normal reading state.

<Example> Embodiments will be described in detail below with reference to the accompanying drawings showing embodiments.

FIG. 1 shows an automatic gain control circuit (hereinafter r) of an embodiment of the present invention.
1 is an electric circuit diagram showing the basic configuration of an AGC circuit. This AGC circuit is compatible with laser-based or scanner-based barcode readers and optical character readers (OCR).
It is used in an optical symbol reading device that illuminates a symbol surface on which symbols such as barcodes and characters are formed, and reads the symbols by detecting the reflected light.

An output signal from a detection element (not shown) including a photodiode, an image sensor, etc. that detects the reflected light is input to the variable gain amplifier circuit 10 from an input terminal IN. The output signal of this variable gain amplifier circuit 10 is output from the output terminal OUT.
The signal is then applied to a binarization circuit (not shown), etc., and the signal is converted into a binary signal of rlJ or "0".

The output signal of the variable gain amplifier circuit 10 is also given to a peak hold circuit 20.
The output signal is input to the variable gain amplifier circuit 10 as a control signal for controlling the gain of the variable gain amplifier circuit 10. A reset circuit 30 for resetting the peak hold level is connected to the peak hold circuit 20.

The variable gain amplifier circuit IO has an operational term and a width amplifier 11. The signal from the detection element is inputted to the inverting input terminal of the operational amplifier 11 via the resistor 12, and the signal is input to the non-inverting input terminal of the variable gain amplifier circuit IO. is given a reference voltage V1. Also,
The output thereof is fed back to the inverting input terminal via an N-channel field effect transistor 13, for example. The control signal from the peak hold circuit 20 is transmitted to the transistor 1
The voltage is input to the gate of the variable gain amplifier circuit 10, and the electric resistance between the source and drain changes as the gate potential changes, thereby changing the gain of the variable gain amplifier circuit 10. In this way, the variable gain amplifier circuit O uses the field effect transistor 13 as a variable resistor to vary its gain.

In order to change the resistance between the source and drain of the transistor 13 by controlling the gate voltage, it is necessary to apply a reverse bias voltage between the gate and the source. In other words, since the transistor 13 is an N-channel one,
Gate voltage ■6 and source voltage V, (=V l ) are VG < va... (1)
If the output signal of the peak hold circuit 20 is set so that the transistor 13 can be operated as a variable resistor.

The peak hold circuit 20 includes a capacitor 22 that is quickly charged by a current from the variable gain amplifier circuit 10 via a diode 21, and a capacitor 22 that is quickly charged with a current from the variable gain amplifier circuit 10, and a capacitor 22 that uses a relatively large time constant (for example, a laser It is configured to include a time constant circuit including a resistor 23 that discharges the discharge at a rate approximately equal to the scanning time. The voltage across the capacitor 22 is amplified in positive phase by the operational amplifier 24 and applied to the gate of the transistor 13 described above. The reference voltage V2 is applied to the inverting input terminal of the operational amplifier 24 via the resistor 26, so that in a normal state, the output signal of the operational amplifier 24, which is a control signal, is the source voltage V of the transistor 13, ( = V1) to satisfy the condition of the above-mentioned formula (11).

A line 25 connects the diode 21 and the operational amplifier 24, and also connects the capacitor 22, etc.
The collector of the NPN transistor 31 of the reset circuit 30 is connected. This NPN) transistor 31 is connected to the gate voltage V of the transistor 13 of the variable gain amplifier circuit 10.
6 and the reference voltage v1 applied to the non-inverting input terminal of the operational amplifier 11. The conduction/cutoff control is performed by the output of the comparator 32 which compares the reference voltage v1 applied to the non-inverting input terminal of the operational amplifier 11. Since the voltages between the two input terminals of operational amplifier 110 are equal, the reference voltage Vl is
3, and therefore the comparator 32 is equal to the gate voltage V. and the source voltage V.

This comparator 32 has a gate voltage V. is higher than the reference voltage Vl (at this time, the gate of the transistor 13 -
The sources will be forward biased. ) to N
A current is applied to the base of the PN) transistor 31 through the resistor 33, thereby making the NPN) transistor 31 conductive. As a result, the line 25 is grounded, so that the charge accumulated in the capacitor 22 is quickly discharged, and the peak hold level in the peak hold circuit 20 is reset. In this way, the control signal is set to the reference voltage Vl
In other words, when the peak hold circuit 2
When the peak hold level of 0 exceeds a predetermined reference value corresponding to the reference voltage v1, the reset circuit 30 resets the peak hold level.

FIG. 2 is a waveform diagram showing the signals derived from each part of FIG. 1. From the circuit 20 to the transistor 13 of the variable gain amplifier circuit IO
FIG. 2(C) shows an output signal derived from the variable gain amplifier circuit 10 to the output terminal OUT.

During the period Tll, the output signal of the peak hold circuit 20 fluctuates following the fluctuation of the input signal, so that the gain of the variable gain amplifier circuit 10 becomes low when the control signal is high, and high when the control signal is low. This allows the output terminal O
The output signal delivered to the UT is a signal that oscillates with a substantially constant amplitude. Therefore, the binarization circuit connected to the output terminal OUT can binarize the signal in a good manner by, for example, level-discriminating the signal at a constant slice level.

On the other hand, during the period from time tll, for example, if the direction of incidence of the laser beam on the symbol surface and the direction of reflection of the reflected light received by the detection element are almost exactly opposite, the specularly reflected light from the symbol surface will be detected. When the light is received by the element, the input signal shows a peak as shown by the reference symbol All in FIG. In FIG. 2(b), reference numeral f12
Stand up quickly as shown.

However, when the control signal from the peak hold circuit 20 rises and reaches the reference voltage Vl, the output of the comparator 32 is inverted from low level to high level, so that the transistor 31 becomes conductive and the capacitor 22 is discharged. Ru. In this way, during the period Δ in which the control signal is about to exceed the reference voltage v1, the transistor 31 is turned on and the peak hold level in the peak hold circuit 20 is reset. When the output signal of the detection element falls, the control signal also falls as indicated by reference numeral l113, thereby blocking the transistor 31 and causing the peak hold circuit 20 to fall.
returns from the reset state.

In this manner, during the period after the period Δ has elapsed, the control signal from the peak hold circuit 20 changes again following the change in the output signal of the detection element, and as a result, the output of the variable gain amplifier circuit 10 is kept constant in its amplitude. In this manner, when the specularly reflected light is received by the detection element, the peak hold level of the peak hold circuit 20 is reset, so that symbols can be read successfully continuously.

Note that in this embodiment, the peak hold level is reset when the gate voltage v0 of the transistor 13 becomes higher than its source voltage Vs (=V l), because in this case, the peak hold level is reset. This is because the transistor 13 can no longer operate as a variable resistor because the voltage between the transistor 1 and the source is biased in the forward direction, and therefore, in this case, it is considered that an abnormal operation has occurred.

Note that the present invention is not limited to the above-described embodiments. For example, in the above embodiment, an N-channel field effect transistor is used as the field effect transistor used in the variable gain amplifier circuit 10, but a configuration using a P-channel field effect transistor can also be easily realized. However, in this case, the polarity of the control signal from the peak hold circuit should be
It is necessary to reverse the case shown in the figure.

Furthermore, in the configuration shown in FIG. 1, the reset circuit 30 includes a comparator 32 and a transistor 31, but if an oven collector output comparator is applied,
No output transistor is required. However, since the logic is reversed at this time, it is necessary to replace the inverting input and non-inverting input of the comparator with those shown in FIG.

Furthermore, in the above embodiment, the control signal outputted by the peak hold circuit 20 is the source voltage V of the transistor 13,
Although the peak hold level is reset when the reference voltage v1, which is equal to , is exceeded, the reference voltage for operating the reset circuit 30 may have a value different from the source voltage of the transistor I3.

Various other design changes can be made without changing the gist of the present invention.

<Effects of the Invention> As described above, according to the automatic gain control circuit of the present invention, when the specularly reflected light from the symbol surface is incident on the detection element, the peak hold level of the peak hold circuit is reset, so that the subsequent Symbol reading continues to be good.

[Brief explanation of drawings]

FIG. 1 is an electric circuit diagram showing the basic configuration of an automatic gain control circuit according to an embodiment of the present invention, FIG. 2 is a waveform diagram showing signals derived from each part of FIG. 1, and FIG. FIG. 4 is a block diagram showing the configuration, FIG. 4 is an electric circuit diagram showing an example of a variable gain amplifier circuit, and FIG. 5 is a waveform diagram showing signals derived to each part of FIG. 3. 10... Variable gain amplifier circuit, 20... Peak hold circuit, 30... Reset circuit, 32... Comparator,
vl...Reference voltage

Claims (1)

[Claims] 1. Used in an optical symbol reading device that reads the symbol by illuminating a symbol surface on which a symbol is formed and detecting the reflected light with a detection element, the output signal of the detection element being input. a variable gain amplifier circuit that varies the gain in response to an external control signal, and a peak hold circuit that detects and holds the peak level of the output signal of the variable gain amplifier circuit, and In an automatic gain control circuit that keeps the amplitude of the output signal of the variable gain amplifier circuit substantially constant by feeding back a control signal corresponding to the hold level to the variable gain amplifier circuit, the peak hold level in the peak hold circuit is predetermined. An automatic gain control circuit comprising: a reset circuit that resets a peak hold level in the peak hold circuit when the peak hold level exceeds a reference value.