Automatic gain control circuit

Abstract: An automatic gain control (AGC) circuit for use in an optical code reader which eliminates error caused when regular reflecting light is directly incident on the detector element of the optical code reader. The AGC has a variable gain amplifier (30) which receives a signal from the detection element and outputs a signal to a peak-holding circuit (40). The output from the peak-holding circuit is used to control the gain of the variable gain amplifier. A reset circuit (60) resets the output of the peak-holding circuit either when the output reaches a predetermined level, or at regular intervals during the operation of the optical code reader. The output of the variable gain amplifier is fed to a binary circuit.

Description: The present invention relates to an automatic gain control circuit for use in an optical code reader for optically reading bar codes or characters according to the preamble part of claim 1. Such a gave control circuit is known for example from US-A-4528444. Optical code readers such as bar code readers and optical character readers (OCRs), are widely used to read a variety of codes and symbols through an optical system to be input into computers. There are two types of optical code readers, the laser type scanner and the CCD (Charge Coupled Device) type scanner. The laser type scanners scan a surface for bar codes or characters by using a laser beam and detecting the light reflected from the scanned surface. The CCD type scanners employ an image sensor on the surface of which the optical images are formed by the light reflected from the code surface, and the optical images are scanned electronically. With such an optical code reader as usually carried by the hand of an operator, in order to ensure that variations in the reading distance between the code display surface and the optical code reader, and resulting variations in the quantity of light reflected by the code display surface, do not degrade the accuracy of the optical code reader, an automatic gain control (AGC) circuit is used. The AGC circuit includes a variable gain amplifier circuit which converts the output of a detection element, such as a photodiode or an image sensor, into a signal with a predetermined amplitude. Fig. 1 is a block diagram of a general AGC circuit arrangement. A detection element generates an electric signal corresponding to the quantity of light reflected from a code display surface. This signal is input to a variable gain amplifier circuit 10. The output of the variable gain amplifier circuit 10 is supplied to a binary circuit (not shown) which converts the output into a value of [1] or [0] based on a suitable slice level. The output signal of the variable gain amplifier circuit 10 is also fed back to the variable gain amplifier circuit 10 through a peak-holding circuit 12 to control the gain of the variable gain amplifier circuit 10. The output of the peak-holding circuit 12 quickly increases as the output of the variable gain amplifier 10 increases, whereas when the output of the variable gain amplifier decreases, the output of the peak-holding circuit 12 decreases slowly according to a relatively large time constant. This stabilises the circuit operation from the turbulent variations of the input signal. Fig. 2 is a schematic diagram of a variable gain amplifier circuit 10. The variable gain amplifier circuit 10 includes an operational amplifier 14. The inverting input terminal of the operational amplifier 14 receives the output signal from the detection element via a resistor 16. The output signal of the operational amplifier 14 is fed back to the inverting terminal via a field effect transistor 18. The control signal from the peak-holding circuit 12 is input to the gate of the transistor 18. With this arrangement, a reverse-bias voltage is applied between the gate and the source of the transistor 18 creating a variable electric resistance between the drain and the source of the transistor 18 which varies with the gate potential. The gain of the amplifier circuit 10 is thus controlled by the peak-holding circuit 12 which sets the drain to source resistance of the transistor 18. A large quantity of reflected light causes the output signal of the detection element to increase which causes the gain of the variable gain amplifier circuit 10 to decrease, whereas a small quantity of reflected light causes the output signal of the detection element to decrease which causes the gain of the variable gain amplifier circuit 10 to increase. As a result, the output of the variable gain amplifier circuit 10 remains substantially constant, irrespective of the quantity of light thus reflected from the code display surface. Therefore, variations in the quantity of light received by the detection element, caused by variations in the reading distance, do not degrade the accuracy of the optical code reader. Fig. 3 is a waveform diagram showing how the circuits of Figs. 1 and 2 process a signal. Fig. 3(a) shows the input to the variable gain amplifier circuit 10; Fig. 3(b) shows the output from the peak-holding circuit 12; and Fig. 3(c) shows the output from the AGC circuit. The output signal of the peak-holding circuit 12 follows the variations of the input signal with a period T. As the gain of the variable gain amplifier circuit 10 varies in proportion to the input signal level, the output signal of the variable gain amplifier circuit 10 oscillates corresponding to a code with a substantially constant amplitude as shown in Fig. 3(c). It is impossible for a conventional optical code reader, as described above, to function when the laser beam is incident on the code display surface in a direction substantially opposite to the reflected light. In this case, the regular reflection light, which is of substantially greater intensity than scattered reflected light, is directly incident on the detection element. The regular reflection light has an intensity from tens to hundreds of times greater than that of the scattered reflected light and consequently the input signal from the detection element to the variable gain amplifier circuit 10 indicates a great peak as shown by the reference number 20. When a great peak occurs in the input signal from the detection element, the output of the peak-holding circuit 12 sharply rises as shown by the reference number 22 and consequently the gain of the variable gain amplifier circuit 10 becomes extremely small. Because the peak-holding circuit 12 has a relatively large time constant, the output signal from the peak-holding circuit 12 is kept at a large value even after the output signal of the detection element has decreased and the gain of the variable gain amplifier circuit 10 remains extremely small. Therefore, the amplitude of the output from the variable gain amplifier circuit 10 decreases as shown by the reference numeral 24 in Fig. 5(c) and it is impossible for the optical code reader to properly read bar codes or optical characters. To fix this problem, it may be possible to supply the output of the detection element to the variable gain amplifier circuit 10 via a limiter circuit. However, because the output of the detection element drastically varies with the variation in the quantity of light received, this could increase reading error by the optical code reader by simply cutting off the input signal at a given level. From US-A-3949233 a linear direct current amplifier with an automatic gain control system for regulating the peak to peak amplitude of the signal output that is acquired by scanning a coded label with a hand-held photo optical scanner is known. The gain of the amplifier is set primarily by the white background on the coded label and is further adjusted at the black bar as the pen passes over the label surface on which the codes is imprinted. A control circuit feedback compensates for input amplitude and contrast variations. The object of the present invention is to provide an automatic gain control circuit for using an optical code reader which provides excellent reading properties and is free of reading errors. This object is solved by an automatic gain control circuit having the features of claim 1. Preferred embodiments are subject to various dependent claims. These and other advantages of the present invention will become more clear upon consideration of the following detailed description of the preferred embodiment together with the drawings in which: Fig. 1 is a block diagram of a conventional optical code reader. Fig. 2 is a schematic diagram of a conventional variable gain amplifier circuit. Fig. 3 is a waveform diagram showing how the circuits of Figures 1 and 2 process a signal. Fig. 4 is a schematic diagram of a first embodiment of the present invention. Fig. 5 is a waveform diagram showing how the circuit of Fig. 4 processes a signal. Fig. 6 is a schematic diagram of an example of a conventional AGC-circuit. Fig. 7 is a wave form diagram showing how the circuit of Fig. 6 processes a signal. Fig. 4 is a schematic diagram of an embodiment, an automatic gain control (AGC) circuit according to the present invention. The AGC circuit is used in an optical code reader which illuminates the surfaces on which bar codes or characters are formed and detects the reflected light. The output signal of a detection element (not shown), such as a photodiode or a CCD image sensor, is applied to the input terminal IN of a variable gain amplifier circuit 30. The output signal of the variable gain amplifier circuit 30 is applied from the output terminal OUT to a binary circuit (not shown) and the signal is converted to a binary signal of [1] or [0]. The output signal of the variable gain amplifier circuit 30 is also applied to a peak-holding circuit 40 and the output signal of the peak-holding circuit 40 is applied to the variable gain amplifier circuit 30 as a control signal for controlling the gain of the variable gain amplifier circuit 30. The peak-holding circuit 40 is reset by a reset circuit 60. The variable gain amplifier circuit 30 has an operational amplifier 32. A signal from the detection element is applied via a resistor 34 to the inverting input terminal of the operational amplifier 32, and a reference voltage V1 is applied to the noninverting input terminal of the operational amplifier 32. The output of the variable gain amplifier circuit 30 is fed back via an N-channel field effect transistor 36 to the inverting input terminal. The control signal output from the peak-holding circuit 40 is applied to the gate of the transistor 36 to create a variable electric resistance between the source and drain of the transistor 36 which varies with the gate potential. Thereby, the gain of the variable gain amplifier circuit 30 varies with the source to drain resistance of the transistor 36. For the transistor 36 to function as a variable resistor, a reverse-bias voltage has to be applied between the gate and the source such that the gate voltage VG is less than the source voltage VS (V = V1). The peak-holding circuit 40 comprises a capacitor 44 which is charged quickly by the current from the variable gain amplifier circuit 30 via a diode 42, and which slowly discharges through a resistor 46 due to a relatively large time constant (e.g., the time required for scanning once with a laser beam). The voltage across the capacitor 44 is subjected to positive-phase amplification by an operational amplifier 48 before being applied to the gate of the transistor 36. Because a reference voltage V2 is applied via a resistor 52 to the inverting input terminal of the operational amplifier 48, the output signal of the operational amplifier 48, the control signal for the transistor 36, is rendered lower than the source voltage VS (= V1) as required for the drain to source of the transistor 36 to operate as a variable resistor. In the embodiment of the present invention, the collector of an NPN transistor 62 of a reset circuit 60 is connected to a line 50 to which the diode 42, the operational amplifier 48 and the capacitor 44 are connected. The NPN transistor 62 is caused to conduct/break by the output of a comparator 64 which compares the gate voltage VG of the transistor 36 in the variable gain amplifier circuit 30 with the reference voltage V1 given to the noninverting input of the operational amplifier 32. Because the voltages of the two terminals of the operational amplifier 32 are equal, the source voltage VS of the transistor 36 is equal to the reference voltage V1. Therefore, the comparator 64 compares the gate voltage VG with the source voltage VS. The comparator 64 supplies current via a resistor 66 to the base of the NPN transistor 62 when the gate voltage VG is higher than the reference voltage V1, thus holding the NPN transistor 62 in conduction. Because this grounds the line 50, the capacitor 44 is quickly discharged and the peak level in the peak-holding circuit 40 is reset. Therefore, the reset circuit 60 resets the peak level when the control signal becomes higher than the reference voltage V1. The peak-holding circuit is reset when the gate voltage VG of the transistor 36 becomes higher than the source voltage VS (= V1) in this embodiment because the transistor 36 is no longer able to function as a variable resistor because the gate to source of the transistor 36 is forward biased. Fig. 5 is a waveform diagram showing how the circuit of Fig. 4 processes a signal. Fig. 5(a) shows the input to the variable gain amplifier 30; Fig. 5(b) shows the output of the peak-holding circuit 40 which is applied to the transistor 36 of the variable gain amplifier circuit 30; and Fig. 5(c) shows the output signal from AGC circuit. As the output signal of the peak-holding circuit 40 varies with the input signal over a period T1, the gain of the variable gain amplifier circuit 30 varies with the level of the control signal from the peak-holding circuit 40. The resulting output signal at the OUT terminal oscillates with a predetermined amplitude. The binary circuit connected to the OUT terminal changes the signal into a binary signal by using a predetermined slice level. When the laser is directed at the code display surface in a direction substantially opposite to the reflected light during a period starting with t1, the detection element receives the regular reflected light causing a peak in the input to the variable gain amplifier circuit 30 as shown by the reference number 66 in Fig. 5(a). Consequently, the capacitor 44 in the peak-holding circuit 40 instantly charges and the control signal output of the peak-holding circuit 40 quickly rises as shown by the reference number 68 in Fig. 5(b). When the control signal from the peak-holding circuit 40 reaches the reference voltage V1, the output of the comparator 32 is inverted from the low to the high level causing the transistor 62 to conduct thereby discharging the capacitor 44 to reset the peak-holding circuit 40 over a period ▴. As shown by a reference character 70, the control signal also falls when the output signal of the detection element falls and thereby the transistor 62 is interrupted, whereas the peak-holding circuit 40 is restored out of the reset state. In a period following the period ▴, the control signal from the peak-holding circuit 40 again varies with the output of the detection element. The output of the variable gain amplifier circuit 30 again oscillates with a constant amplitude. Therefore when the detection element detects regular reflected light, the peak level in the peak-holding circuit 40 is reset to allow the optical code reader to continuously and smoothly read codes. The embodiment of the present invention is not limited to the above description. Although the N-channel field effect transistor 36 is used in the variable gain amplifier circuit 30 in this first embodiment, a P-channel field effect transistor may readily be used. However, it will be needed to invert the polarity of the control signal from the peak-holding circuit 40. Additionally, although the reset circuit 60 includes the comparator 64 and the transistor 62, the transistor 62 is unnecessary if an open collector output comparator is used. However, the two inputs to the comparator should be interchanged. Finally, although the peak hold level is reset when the control signal output from the peak-holding circuit 40 exceeds the reference voltage V1, the source voltage Vs of the transistor 36 in the first embodiment, the reference voltage for the reset circuit 60 may be different from the source voltage of the transistor 36. Fig. 6 is a schematic diagram of a conventional AGC-circuit. Like elements that function similarly to those in the embodiment of the present invention, as shown in Fig. 4, have like reference numerals. Here, the reset circuit 60 is connected to the peak-holding circuit 40 to reset the peak level. The output of a synchronizing signal generator circuit 80, which generates a synchronizing signal in response to the commencement of a scanning cycle, is supplied to the reset circuit 60. The peak level in the peak-holding circuit 40 is reset in response to the synchronizing signal. The collector of an NPN transistor 74 of the reset circuit 60 is connected to the line 50 to which the diode 42, the operational amplifier 48 and the capacitor 44 are connected. The NPN transistor 74 conducts in response to the synchronizing signal from the synchronizing signal generator circuit 80 and causes the capacitor 44 to quickly discharge by grounding the line 50. The synchronizing signal generator circuit 80 has a detector 82 which detects the start of a scanning operation by the laser type scanner. The detector 82 can be an optical detector provided in a casing near one end of a window through which the laser beam is emitted in a so-called hand-held type bar code reader. The output signal of the detector 82 is amplified by an amplifier 84 and is input to a comparator 86. The comparator 86 compares the output signal of the amplifier 84 and a reference voltage V2 and introduces a detection pulse corresponding to the start of a scanning operation. The detection pulse is input to the transistor 74 of the reset circuit 60 via a resistor 76 as the synchronizing signal. Fig. 7 illustrates how the circuit of Fig. 6 processes a signal. Fig. 7(a) shows the output signal of the detection element which is input to the IN terminal; Fig. 7(b) shows the synchronizing signal output from the synchronizing signal generator circuit 80; Fig. 7(c) shows the control signal output from the peak-holding circuit 40; and Fig. 7(d) shows the output from the AGC circuit. As described above with reference to the embodiment of the present invention, when the laser is directed at the code display surface in a direction substantially opposite to the reflected light during a period starting with t1, the detection element receives the regular reflected light causing a peak in the input to the variable gain amplifier circuit 30 as shown by the reference number 90 in Fig. 7(a). The capacitor 44 in the peak-holding circuit 40 instantly charges and the control signal output of the peak-holding circuit 40 quickly rises as shown by the reference number 92 in Fig. 7(c). The gain of the variable gain amplifier circuit 30 thereby becomes extremely small and the output of the AGC circuit appears to be ruined as shown by the reference number 94 of Fig. 7(d). The detection pulse P1 applied by the synchronizing signal generator circuit 80 at time t3 causes the reset circuit 60 reset the peak level in the peak-holding circuit 40 as shown by the reference number 96, so that the control signal from the peak-holding circuit 40 again varies with the output of the detection element. After the detection pulse P1 is applied to the reset circuit 60, the optical code reader can accurately read codes again. Because the synchronizing signal generator circuit 80 outputs a detection pulse each time that scanning is commenced, the optical code reader can accurately read codes during a period of scanning directly following a period in which the detection element receives regular reflection light from the code display surface. Although the synchronizing signal generator circuit 80 generates a synchronizing signal at the start of a scanning operation of a laser type scanner, the synchronizing signal may be generated at the start of a scanning operation in a CCD type scanner. Moreover, the synchronizing signal may be generated in response to the termination of scanning. Additionally, although the peak level is reset each time scanning operation, the peak level may be reset once every two or more scanning operations.

Claims: An automatic gain control circuit for use in an optical code reader, comprising: a variable gain amplifier circuit (30) adapted to receive an input from a detection element of said optical code reader; a peak-holding circuit (40) which detects and holds a peak output of said variable gain amplifier circuit (30) and which controls a gain of said variable gain amplifier circuit by feeding the peak output back to said variable gain amplifier circuit as a control signal; and a reset circuit (60) which resets said peak-holding circuit, characterized in that said reset circuit (60) resets the peak-holding circuit (40) when the control signal exceeds a predetermined level. The automatic gain control circuit according to claim 1 wherein the predetermined level is a reference voltage (V1) applied to said variable gain amplifier circuit (30). The automatic gain control circuit according to claim 1 or 2 wherein the variable gain amplifier circuit (30) comprises: an operational amplifier (32); a resistor (34) having a first end attached to an inverting input of said operational amplifier (32) and having a second end coupled so as to receive the input from said detection element; a reference voltage source (V1) connected to a noninverting terminal of said operational amplifier (32); and a field effect transistor (36) having a source connected to the inverting input of said operational amplifier (32), a drain connected to an output terminal of said operational amplifier and a gate coupled so as to receive the control signal from said peak-holding circuit (40). The automatic gain control circuit according to claim 3 wherein said field effect transistor (36) is a N-channel device. The automatic gain control circuit according to claim 3 wherein said field effect transistor (36) is a P-channel device. The automatic gain control circuit according to at least one of claims 1 to 5 wherein said peak-holding circuit (40) comprises: a diode (42) adapted to receive the output signal of said variable gain amplifier circuit (30); a capacitor (44) connected in parallel to a first resistor (46), both being connected between said diode (42) and ground; a reference voltage source (V2); second and third resistors (52); and an operational amplifier (48) having a noninverting input coupled to said diode (42), said first resistor (46) and said capacitor (44), said second resistor (52) being connected between an inverting terminal and said reference voltage source (V2), said third resistor connected between an output of said operational amplifier (48) and the inverting input, the output being the control signal. The automatic gain control circuit according to at least one of claims 1 to 6 wherein said reset circuit (60) comprises: an NPN transistor (62) having an emitter grounded and a collector connected to said peak-holding circuit (40); a reference voltage source (V1); a resistor (66); and a comparator (64) having a noninverting input coupled so as to receive the control signal from said peak-holding circuit (40), an inverting terminal connected to said reference voltage source (V1), the output terminal connected to the base of said NPN transistor (62) through said resistor (66). The automatic gain control circuit according to claim 7 wherein said reference voltage source (V1) connected to the inverting terminal of said comparator (64) is the same reference voltage source (V1) used in said variable gain amplifier circuit (30). The automatic gain control circuit according to at least one of claims 1 to 6 wherein said reset circuit comprises: a reference voltage source (V1); and an open collector output comparator having a noninverting input connected to the reference voltage source, an inverting terminal coupled so as to receive the control signal from said peak-holding circuit (40), the open collector connected to said peak-holding circuit.

Inventors: TAKENAKA SHINYA, TAKENAKA, SHINYA, TAKENAKA, SHINYA OSAKA WORKS SUMITOMO ELECTR.IND.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: G06K7
IPC codes: G06K7, H03G3