JPH0454579A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To satisfactorily read the symbols after the light reflected regularly from the symbol surface is made incident on a detection element by resetting a peak holding level by means of the reset signals which are produced for each symbol reading action in the prescribed frequency. CONSTITUTION:A peak holding circuit 20 is connected with a reset circuit 30 which resets the peak holding level with the reset signals produced for each symbol reading action in the prescribed frequency. The output of a synchronizing signal generating circuit 40 which produces the synchronizing signal in response to the start of an electrical scanning operation of an optical image is applied to the circuit 30. Then the peak holding level of the circuit 20 is reset in response to the synchronizing signal. Thus the output signals of a detecting element increase temporarily owing to the regular reflected light and the gain of a variable gain amplifier 10 is extremely reduced. Even under such conditions, the gain of the amplifier 10 can be quickly returned to the value corresponding to the quantity of light received by the detecting element by a resetting action. As a result, the symbols can be satisfactorily read.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> 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 method that scans a symbol surface on which a symbol such as a bar code or character is formed using a laser beam, and detects the reflected light 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 or the signal is level-discriminated at an appropriate slice level and [ 1) or to a binarization circuit (not shown) that binarizes it into rOJ. The output signal of the variable gain amplifier circuit is also applied to a peak hold circuit 2, and the output of the peak hold circuit 2 is fed back to the variable gain amplifier circuit 1 as a control signal for controlling its gain.

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

With such a configuration, the amount of reflected light is large, so the output signal of the detection element becomes thick (when this happens, the gain of the variable gain amplifier circuit l is reduced, and the amount of reflected light is small, so the output signal of the detection element becomes small). When this happens, the gain of the variable gain amplifier circuit is increased.Therefore, the output of the variable gain amplifier circuit 1 remains almost constant regardless of the amount of light reflected from the symbol surface. By absorbing variations in the amount of light received by the detection element due to variations in distance, it is possible to accurately read symbols over a range of reading distances.

<Problem to be solved by the invention> FIG. 4 is a waveform diagram showing changes in signals at each part of FIG. 3, and FIG. Figure 4 (bl shows the change in the output signal of the peak hold circuit 2, and Figure 4 (C) shows the change in the output signal of the variable gain amplifier circuit 1.
shows the output signal of

During the 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 l fluctuates in accordance with the input signal level, so as shown in FIG. 4(C). As shown in FIG. 1, the output signal of this variable gain amplification circuit l 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 misaligned, 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 of times 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 11. 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 l has a relatively large time constant as described above, even after the output signal of the detection element becomes low, as shown by reference numeral 13, the output signal remains at a large value. The gain of the variable gain amplifier circuit l continues to be extremely small. For this reason, as shown by reference numeral 14 in FIG. 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, when 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, for example, it is possible to input the output of the detection element to the variable gain amplifier circuit l via a limiter circuit, but since the output of the detection element fluctuates greatly in response to changes in the amount of light received, However, 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 whose gain varies in response to an external control signal; and a peak of the output signal of the second variable gain amplifier circuit. and a peak hold circuit that detects and holds the level, and by feeding back a control signal corresponding to the held peak hold level to the variable gain amplifier circuit, the amplitude of the output signal of the variable gain amplifier circuit is kept almost constant. The automatic gain control circuit is provided with a reset circuit that resets the peak hold level in the peak hold circuit by a reset signal generated every predetermined number of symbol reading operations.

It is preferable that the predetermined number of times is one.

<Operation> According to the above configuration, the peak hold level in the peak hold circuit is reset every predetermined number of symbol reading operations (for example, once), so the output of the detection element is reduced due to the specularly reflected light from the symbol surface. Even when the signal increases temporarily and the gain of the variable gain amplifier circuit becomes extremely small due to the control signal from the peak hold circuit, the above reset operation quickly adjusts the gain of the variable gain amplifier circuit to the amount of light received by the detection element. It is possible to restore the value to the corresponding value.

<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) and converted into a binary signal of "1" or "0".

The output signal of the variable gain amplifier circuit IO is also given to a peak hold circuit 20, and the output signal of this peak hold circuit 20 is inputted to the variable gain amplifier circuit 10 as a control signal for controlling the gain of the variable gain amplifier circuit IO. be done.

A reset circuit 30 is connected to the peak hold circuit 20 to reset the peak hold level.

The reset circuit 30 is supplied with the output of a synchronization signal generation circuit 40 that generates a synchronization signal in response to, for example, the start of symbol scanning by a laser beam or the start of electrical scanning of an optical image in an image sensor. The peak hold level of the peak hold circuit 20 is reset in response to the synchronization signal.

The variable gain amplifier circuit 10 has an operational amplifier 11, and the operational amplifier 11. A signal from the detection element is inputted to an inverting input terminal of the detector via a resistor 12, and a reference voltage v1 is applied to its non-inverting input terminal. Also,
The output thereof is fed back to the inverting input terminal via the field effect transistor 13. The control signal from the peak hold circuit 20 is input to the gate of the transistor 13, and as the gate potential changes, the electric resistance between its source and drain changes, thereby changing the gain of the variable gain amplifier circuit IO. .

The peak hold circuit 20 includes a capacitor 22 that is quickly charged via a diode 21 by the current from the variable gain amplifier circuit IO, and a capacitor 22 that is quickly charged by a current from the variable gain amplifier circuit IO, and the charge accumulated in the capacitor 22 is It is configured to include a time constant circuit including a resistor 23 for discharging 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.

A line 25 that connects the diode 2I and the operational amplifier 24 and is also connected to the capacitor 22, etc.
The collector of an NPN transistor 31 constituting the reset circuit 30 is connected.

This NPN) transistor 31 is connected to the synchronizing signal generating circuit 40.
conducts in response to a synchronization signal from the line 25, grounding the line 25 and quickly discharging the capacitor 22.

The synchronization signal generation circuit 40 has a detector 41 that detects the start timing of scanning a symbol plane by the laser beam, for example in a laser barcode reader. This detector 41 can be realized, for example, in a so-called hand belt type barcode reader, by a photodetector provided in a housing near one end of a window from which laser light is emitted.

The output signal of this detector 41 is amplified by an amplifier 42 and input to a comparator 43. This comparator 43
By comparing the reference voltage V2 and the output signal of the amplifier 42, a detection pulse is derived in accordance with the scan start timing. This detection pulse is used as a synchronization signal in the reset circuit 3.
The signal is input to the zero transistor 31 via the resistor 32.

FIG. 2 is a waveform diagram showing the signals derived from each part of FIG. 1, and FIG. 2 (C) shows a control signal given from the peak hold circuit 20 to the transistor 13 of the variable gain amplifier circuit 10, and FIG. It shows an output signal derived from lO 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. By being received by the element, the input signal exhibits a peak as shown by the reference symbol /11 in FIG. is the reference number /12 in FIG. 2(b).
Stand up quickly as shown. As a result, the gain of the variable gain amplifier circuit 10 becomes extremely small, and the output signal becomes a so-called collapsed state as shown by reference numeral f13 at the peak in FIG.

Since the charge accumulated in the capacitor 22 is discharged via the resistor 23 with a relatively large time constant, the output signal of the peak hold circuit 20 does not immediately fall to a low level (see reference) even after the input signal falls. It is maintained at a high state as indicated by the symbol I!14.

If this state remains as it is, the gain of the variable gain amplifier circuit IO will be in a small state or continue for a long time, so it will be impossible to read symbols for a while, but in this embodiment,
At time t12 in FIG. 2, the reset circuit 30 sets the peak hold level of the peak hold circuit 20 with reference symbol I! by the detection pulse pl output from the synchronization signal generation circuit 40. Since the control signal is reset as shown at 15, in the subsequent period, the control signal from the peak hold circuit 20 changes again following the change in the output signal of the detection element. As a result, after the detection pulse pl is applied to the reset circuit 30, the state returns to a state where symbols can be read accurately.

Since the synchronization signal generation circuit 40 outputs a detection pulse every time the laser beam is scanned, even if the detection element receives specularly reflected light from the symbol surface within the - scan period, the next scan will be performed. During this period, the gain of the variable gain amplifier circuit IO will be controlled so that the symbol can be read accurately.

Note that the present invention is not limited to the above-described embodiments. For example, in the above embodiment, the synchronization signal generation circuit 40 generates the synchronization signal at the timing of the start of scanning by laser light, but in a configuration in which an optical image is electrically scanned using an image sensor, the A synchronization signal may be generated at the start of target scanning. Also,
A synchronization signal may be generated in response to the end of a scan. Further, in the above embodiment, the peak hold level is reset every time a scan is performed, but the peak hold level may be reset once for each scan, for example. . 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 gain of the variable gain amplifier circuit becomes extremely small as a result of the specularly reflected light from the symbol plane being incident on the detection element, Even though
By resetting the peak hold level of the peak hold circuit, this state can be quickly released and the symbol can be read properly.

[Brief explanation of the drawing]

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. A block diagram showing the configuration, and FIG. 4 are waveform diagrams showing signals derived to each part of FIG. 3. DESCRIPTION OF SYMBOLS 10... Variable gain amplifier circuit, 20... Peak hold circuit, 30... Reset circuit, 40... Synchronization signal generation circuit Fig. 1 2,

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 whose gain fluctuates in response to an external control signal, and a peak hold circuit which detects and holds the peak level of the output signal of this variable gain amplifier circuit. 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 set to a predetermined value. An automatic gain control circuit characterized by comprising a reset circuit that is reset by a reset signal generated every number of symbol reading operations.