JPH05266239A - Digitizer signal processing circuit for bar code - Google Patents

Abstract

PURPOSE: To read out bar code symbols by accurately detecting signal transition points by a differentiating circuit. CONSTITUTION: A scan is made across a bar code symbol and a light spot for scanning, which is reflected by the bar code symbol, is detected to generate an analog signal representing it. Then, the primary derivative of the analog signal is found and compared with a primary derivative signal which is delayed by a delay circuit 18 to detect the transition of the signal. A false transition gating circuit means 22 which changes a state only when a transition signal larger than a given threshold value is detected changes the state of an output signal only when changing the state by discriminating the false transition signal.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a laser scanning system for reading bar code symbols, specifically at the time when a scanning light spot moves from space to bar code symbol or from bar code symbol to space. The present invention relates to a novel digitizer processing circuit for detecting a time point at which the digitizer is processed.

[0002]

BACKGROUND OF THE INVENTION The increasing use of bar code symbols to identify products, especially in the distribution industry, has led to the development of various bar code reading systems. Many users of bar code readers demand portable handheld scanners with an emphasis on small size, metering, and low power consumption. One such system is the US Patent
The laser scanning bar code reading system described in 4,496,831.

The laser scanning system described in US Pat. No. 4,496,831 can be implemented in a variety of shapes, but is preferably a portable handheld scanning head having a lightweight plastic housing in the shape of a gun. Is included. The handle and barrel portion house the various components of the scanning head therein. Inside the barrel, a small light source, a small optical train that includes a focusing lens and a scanning system that guides the light from the light source to the bar code symbol, and a small detector that detects the reflected light from the bar code symbol during scanning. And means are attached.

The compact light source may comprise a laser tube such as a coaxial helium neon laser tube, but preferably comprises a semiconductor laser diode. Since semiconductor laser diodes are much smaller and lighter than laser tubes, they reduce the required size and weight of the scan head and also make the scan head easier to handle and operate. The light generated by the light source passes through an optical train that directs the beam onto a scanning system mounted in the barrel portion of the operating head.
The scanning system includes at least one scanning motor that sweeps the laser beam longitudinally across the bar code symbol. The scanning system can also include two electric motors, with the second electric motor sweeping the beam across the symbol widthwise. Mirror-like light-reflecting means are mounted on the motor shaft and direct the beam through the exit hole to the symbol. The detection circuit that detects and processes the light reflected from the symbol generally consists of a photodetection element such as a semiconductor photodiode. When the user positions the handheld device so that the scan pattern traverses the symbol to be read, the photodetector element produces a serial electrical signal to be processed to identify the bar code. A circuit, such as a digitizer signal processing circuit according to the present invention for a bar code, produces a signal which is directed to a bar pattern decoding circuit which decodes the bar pattern. The reader should be able to activate the light beam and detector circuits when the reader is aimed at the symbol to be read, and to save battery power if the device has a built-in power supply. A movable trigger can be used. The lightweight plastic housing contains the laser light source, detector, optics, signal processing circuit, CPU and battery. The reading device is designed to aim at the symbol without the user moving the reading device away from the bar code symbol, i.e. without touching or across the symbol. Typically, this type of handheld bar code reader is designated to scan within a distance of perhaps a few inches.

Although the present invention will be described in the context of linear or single line bar codes, the present invention is not limited to these examples but includes more complex scanning patterns such as Code 49 and similar symbologies. It is also applicable to stacked or two-dimensional barcodes. The present invention is also applicable to applications where information is used with various machine visions derived from other types of indicia, such as letters, or from surface characteristics of the article being scanned, or optical character recognition applications. is there.

Of course, it is desirable to detect when the center of a moving scanning laser spot moves on the bar and again when it leaves the bar, where it is located at the edge of the bar. It is well known that the analog signal has an inflection point corresponding to this time point. In bar code scanners, various shapes of signal processing methods have been used to detect these inflection points.

The most common technical way to achieve this is to use a zero-crossing detector that tracks when the second derivative of the analog signal crosses zero. This is because this zero crossing temporally coincides with the inflection point of the analog signal. One problem with this method is that some zero crossings of the second derivative do not coincide with the inflection points of the analog signal. Therefore, the desired zero crossing and false zero
Various technical methods have been developed to distinguish between crossings. Most of these methods rely on information obtained either from the analog signal or the first derivative of the analog signal. Examples of these technical methods are described in Spectraphysics Patents 4,000,397 and 4,749,879. Another known method is to offset the analog signal by half its amplitude so that it is centered at 0 and then use a voltage comparator to compare it directly to the second derivative. Another common feature is a circuit that ensures noise-free margins (white areas on either side of the bar code) by setting a noise threshold based on the average of the noise over time. Signals below this average noise level are not digitized.

[0008]

SUMMARY OF THE INVENTION The main object of the present invention is to read a bar code symbol by detecting when the scanning light spot moves from space to bar code symbol or from bar code symbol to space. New,
And to provide an improved signal processor digitizer circuit. This novel signal processor digitizer circuit takes all of the information from the first derivative of the analog signal generated by the photodetector that detects the scanning light spot, digitizes it, eliminates false transitions, and cleans the margins. To do. Primitive analog signals and second derivative signals are not used, which offers some advantages as described below.

Since no primitive analog signal is required, the signal differentiating circuit can be placed very early in the amplifier train. An additional amplification stage can be used to amplify the differentiated signal until it is large enough to be digitized. This has two benefits. The differentiating circuit effectively filters the effect of ambient light. This will improve the ambient light performance, since the ambient light would saturate the final amplification stage if not filtered. It also provides another form of high pass filtering or AC coupling transient response normally required to improve ambient light characteristics or to eliminate the accumulation of large offset voltages associated with DC coupled amplifiers. Exclude. The prior art typically required a large power supply voltage to eliminate the possibility of amplifier clipping.

In the circuit of the present invention, the digitizer and its associated amplifier are designed to work at low supply voltages, such as a single +5 volt supply. This is a voltage that can be almost always used in the digital decoding circuit of the scanner. This eliminates the need for expensive DC converters or other high voltage sources. The present invention provides a signal processor digitizer circuit that detects when a scanning light spot moves from a space onto a bar code symbol or from a bar code symbol into space. The signal processor digitizer circuit includes a photodetector that scans across a bar code symbol to detect a scanning light spot reflected by the bar code symbol and generate an analog signal representative thereof. The differentiating circuit receives the analog signal as an input, generates a first derivative signal of the analog signal, and supplies it to a delay circuit which produces a delayed first derivative signal. The peak finding comparator circuit receives the first derivative signal as a first input and the delayed first derivative signal as a second input and detects transitions of both signals. The false transition gating circuit discriminates the false transition signal and changes the state only when the transition signal above a given threshold is detected. The output circuit receives a first input from the peak detect comparator circuit and a second input from the false transition gating circuit, and the false transition gating circuit is false after the peak detect comparator circuit last changed state. The state of the output signal is changed only when the transition signal is discriminated and the state is changed.

Specifically, the output circuit also includes a marginal threshold detection circuit which receives the first derivative signal as an input and acts as a retriggerable one-shot circuit. This extra threshold detection circuit does not time out as long as the first derivative signal contains a series of pulses that exceeds the threshold of the extra threshold detection circuit, but when it does, the output circuit provides the output signal. Stop doing. The level above which a false transition gating circuit exceeds a certain threshold is determined by the level of hysteresis provided by its positive feedback. In addition, an amplifier having lowpass filtering characteristics is coupled to the output of the differentiating circuit to provide first derivative amplification and lowpass filtering.

The above objects and advantages of the present invention relating to a digitizer signal processing circuit for bar codes will be readily understood from the following detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings. In the drawings, the same elements are given the same numbers.

[0013]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. The light from the scanning light spot reflected from the barcode pattern is detected by the photodiode 10 which produces a current proportional thereto. This current is processed by the current to voltage converter circuit 12 which produces a voltage proportional to the input current. This voltage signal is introduced as an input to the differentiating circuit 14, and the differentiating circuit 1
4 generates a time-dependent first derivative signal (B of FIG. 2) of the analog voltage input signal (A of FIG. 2). Differentiating circuit 14
The first derivative signal output from is amplified and filtered by an amplifier 16 having a low pass filtering characteristic.

After amplifying the differentiated signal in amplifier 16, the circuit functions as follows. The differentiated signal is supplied to the delay circuit 18 that delays the signal, and
The signal shown in is obtained. The peak detection comparator 20 compares the differentiated signal with the delayed output signal of the delay circuit 18. From FIG. 2B, it can be seen that these two signals intersect approximately one delay time after the peak. Further, it can be seen from C in FIG. 2 that the output of the peak detection comparator 20 is transited by this. If the output of the false transition gating (comparator) circuit 22 has changed state since the peak detect comparator 20 last changed state (D in FIG. 2), the latch comparator circuit 24 changes state. FIG. 2E). In other words, the latch comparator 24 includes the false transition gating circuit 22.
The state is changed only when the peak detection comparator 20 makes the first transition after the transition. This also causes any noise that could cause false transitions in the output of the peak detect comparator 20 to cause false transitions in the output of the latch comparator 24 unless the noise is large enough to invert the false transition gating circuit 22. There is nothing to do.

The level at which the false transition gating circuit 22 inverts is determined by the amount of hysteresis to ensure maximum digitization accuracy. Marginal threshold circuit 2
The marginal threshold of 6 is set to some fixed DC voltage below 0. The marginal threshold circuit 26 receives a pulse from the first derivative signal as an input voltage. These amplitudes are determined by the voltage divider consisting of resistors 28 and 30. The marginal threshold circuit 26 operates as a retriggerable one shot and will not time out as long as successive pulse trains in the first derivative signal exceed the threshold. If the pulse is stopped long enough to time out the circuit, the digitized bar output is returned to the white (space) state. Until the boundary area is encountered, the output of the marginal threshold circuit 26 is high (see F in FIG. 2) charging the capacitor 32 so that the inverting transistor 34 is turned on and the latch comparator 24 is unable to provide the output. .. When the bar code symbol begins to be scanned, the output of the marginal threshold circuit 26 goes low (see F in FIG. 2) to discharge the capacitor 32, causing the inverting transistor 34 to turn off and the latch comparator 24 to provide the output. Come to do.

The output of the latch comparator 24 is supplied to the bar pattern decoding circuit 36. In some embodiments, decoding circuit 36 may be driven directly by latch comparator 24. As a modification, when the latch comparator 24 is connected to the bar pattern decoding circuit 36 through an output cable that adds capacitance to the circuit as shown in FIG. It can be connected to the circuit 36.

A circuit including a constant voltage diode 40 is also shown in FIG. The circuit containing the voltage regulator diode 40 can be combined such that the signal level in the circuit described above is centered around 2.5 volts, and the voltage regulator diode 40 circuit can be considered to be part of the power supply circuit. it can. The circuit of the present invention performs the same function as the circuit of the prior art, but its scheme is different and simpler. For example, all prior art circuits use either undifferentiated analog signals (for window or ambient light rejection) or second derivative signals (for zero crossings). The circuit of the present invention, except for inducing a first derivative signal,
Neither of these signals is used for signal processing. The circuit of the present invention only uses a known single comparator (false transition gating circuit 22) that determines the window. The high-to-low transition of gating comparator 22 allows latch comparator 24 to respond to the first and only low-to-high transition of peak-finding comparator 20, and also to low-to-high transition of gating comparator 22. The transition allows the latch comparator 24 to respond to the first and only high to low transitions of the peak hunt comparator 20. In contrast, other prior art circuits are from real bar to space,
Or we are making a narrow window centered around the expected transition from space to bar.

The circuit of the present invention uses a separate marginal comparator 2 as described above, using only known circuits of the type described above, to improve marginal noise performance.
6 and uses only known circuits without diode-based peak detectors that limit the small signals that can be processed and make it difficult to use a single low voltage power supply. ..

FIG. 3 shows a highly simplified embodiment of one type of bar code reader which can be used incorporating the digitizer signal processing circuit of the present invention. The reader 100 can be implemented as a handheld scanner as shown, or as a desktop workstation or stationary scanner. In the preferred embodiment, the arrangement is implemented in a housing 155 that includes an exit aperture 156 through which the output laser light beam 151 impinges upon and scans across the symbol 170 located outside the housing. ..

The handheld device shown in FIG. 1 is generally of the style described in Swartz et al., US Pat. No. 4,760,248, or US Pat. No. 4,896,026 assigned to Symbol Technologies, Inc. Part number LS8100 or LS 2000 is similar to the barcode reader configuration. As an alternative
Alternatively, or additionally, the features of US Pat. No. 4,387,297 to Swarts et al. Or US Pat. These 4,760,248, 4,896,026 and 4,409,4
As will be understood with reference to the '70 patent, a general design overview of these devices is provided.

As is apparent from FIG. 3, the output light beam 15
A reading device 1 is usually a laser diode or the like.
Generated in 00 and directed to impinge on a bar code symbol located on the target several inches away from the front of the reader 100. The output beam 151 is scanned in a scan pattern and the user positions the handheld device so that the scan pattern traverses the symbol to be read.
The reflected light 152 from the symbol is the photosensitive device 1 in the reading device.
Detected by 58, converted into a serial electrical signal to be processed and decoded to reproduce the data represented by the bar code.

In the preferred embodiment, reading device 100.
Is a gun-type device having a pistol grip-type handle 153, and a user can move the movable trigger 1 aiming at a symbol to be read.
54 can be subtracted to activate the light beam 151 and the detection circuit. The lightweight plastic housing 155 includes a laser light source, a detector 158, an optical and signal processing circuit, a CP.
It contains U140 and a power supply or battery 162. The output light beam 151 exits through the translucent window 156 at the front end of the housing 155, and the incoming reflected light 152 can enter. The reader 100 is designed to aim at a bar code symbol away from the symbol, i.e. without touching or across the symbol. Typically, handheld bar code readers of this type are specified to operate at distances of perhaps several inches.

Further, as shown in FIG. 3, the beam to be scanned can be focused to a bar code symbol on a suitable reference plane using a suitable lens 157 (or multiple lens system). A light source 146, such as a semiconductor laser diode, is positioned to direct the light beam to the axis of the lens 157, the beam being semitransparent mirror 147, and optionally other lenses or beam shaping structures, and trigger 154 when triggered. It passes through a vibrating mirror 159 attached to the scanning electric motor 160 to be activated. If the light generated by the light source 146 is invisible, a visible light spot, fixed by the aiming light or scanned just like the laser beam, can be generated, if desired. The user uses this visible light to aim at the symbol with the reader before pulling the trigger 154.

While some embodiments and variations of the present invention have been described with respect to digitizer signal processing circuits for bar codes, those skilled in the art will appreciate that the above description of the invention suggests many variations. Will be understood.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of a digitizer signal processing circuit for a bar code designed according to the present invention.

2 is a set of waveform diagrams useful for explaining the operation of the circuit of FIG.

FIG. 3 shows a very simplified embodiment of one type of bar code reader that can incorporate and use the circuitry of the present invention.

[Explanation of symbols]

 10 Photodiode 12 Current / voltage converter circuit 14 Differentiation circuit 16 Amplifier 18 Delay circuit 20 Peak detection comparator 22 False transition gating circuit 24 Latch comparator 26 Marginal threshold circuit 34 Transistor 36 Bar pattern decoding circuit 40 Constant voltage diode 100 bar Code reading device 140 CPU 146 Light source 147 Semi-transparent mirror 151, 152 Light beam 153 Handle 154 Movable trigger 155 Housing 157 Lens 158 Photosensitive device (detector) 159 Vibration mirror 160 Scanning motor 162 Battery 170 Bar code symbol

Claims (6)

[Claims] 1. A signal processor digitizer circuit for detecting when a scanning light spot moves from a space onto a bar code symbol or from a bar code symbol to a space, comprising: a) across a bar code symbol. A photodetector means for scanning to detect the scanning light spot reflected by the bar code symbol and producing an analog signal representative thereof; b) receiving this analog signal as an input and producing a first derivative signal of the analog signal. Differentiating circuit means for: c) delay circuit means for receiving this first derivative signal as an input and producing a delayed first derivative signal; d) receiving the first derivative signal as a first input and delaying it A peak finding comparator circuit that receives a first derivative signal as a second input and detects transitions of the first derivative signal and the delayed first derivative signal. A) a false transition gating circuit means for discriminating false transition signals and changing states only when a transition signal above a given threshold is detected; and f) a peak detection comparator circuit means 1 and a second input from the false transition gating circuit means, the peak transition comparator circuit finally changes the state, and then the false transition gating circuit discriminates the false transition signal to change the state. And an output circuit means for changing the state of the output signal only when the signal processor digitizer circuit is used. 2. The output circuit means also includes a marginal threshold detection circuit, the marginal threshold detection circuit functioning as a retriggerable one-shot circuit that receives the first derivative signal as an input, the first derivative signal. 2. The signal of claim 1 which does not time out as long as it contains a series of pulses that exceeds the threshold of the marginal threshold detection circuit, but which in turn prevents the output circuit means from providing an output signal. Processor digitizer circuit. 3. The signal processor digitizer circuit of claim 2, wherein the level at which the false transition gating circuit exceeds a threshold is determined by the level of hysteresis introduced by positive feedback in the circuit. 4. The signal processor digitizer circuit of claim 3, further comprising an amplifier coupled to the output of the differentiator circuit, the amplifier having lowpass filtering characteristics for amplifying and lowpass filtering the first derivative signal. 5. The signal processor digitizer circuit of claim 1, wherein the level at which the false transition gating circuit exceeds a threshold is determined by the level of hysteresis introduced by positive feedback in the circuit. 6. The signal processor digitizer circuit of claim 1, further including an amplifier coupled to the output of the differentiating circuit, the amplifier having lowpass filtering characteristics for amplifying and lowpass filtering the first derivative signal.