A High-Density Ternary Barcode Detection System with a Dual-Bias Differential Method ()
1. Introduction
The real-time identification of barcodes (BCs) containing much information is needed for applications such as goods management on production lines in which highspeed detection is required. Though the use of highdensity two-dimensional binary BCs or color two-dimensional BCs for identification systems has been considered [1-3], the maximum scanning speed with these techniques is limited to nearly 50 scans/sec because of the complicated image processing and focus adjustment by the CCD cameras [4,5]. This low-scanning speed makes the high-speed sorting of goods problematic. It also has the problem that an auxiliary light must be provided.
The authors devised a ternary barcode detection system (TBDS) using a dual-threshold detection method and a laser, in which a ternary barcode with much information is employed, to resolve the above low-speed scanning and auxiliary light problems [6]. Because this system is capable of providing an unbent average signal with a non-hyperbola-shaped enveloped line through the averaging method developed to lessen hyperbolic shading caused by different scanning distances from the barcode to the surface of the scanner mirror, it has the features of providing a long detection range and a high scanning speed, and its effectiveness was confirmed through a prototype [6-8]. However, the detection range was limited to 0.4 mm, because of distortion of the detection signals caused by uneven signal changes depending on the bar width.
To resolve this problem for high-speed high-density BC detection, the author developed a TBDS that employs an envelope-differential composite method, featuring the subtraction of an attenuated and slightly enveloped line of the detection signal from the original signal and its differential [9,10]. Because this system detects a ternary barcode with narrow bars while eliminating signal distortion using the slightly enveloped method and emphasizing the transitions of the BC signal by applying the differential method, it can reliably detect high-density BCs. The effectiveness of this technique in providing high BC densification to a bar-width level of 0.3 mm was demonstrated using a single-line ternary BC. However, the detection limitation of this system was a nearly 0.3 mm detectable minimum bar width because of the distortion of the delayed mixed code signal through the counting period-latch timing instability and the fluctuation of the differentiated signal through the noise contained in the average signal.
To resolve this problem and provide a longer detection range for high-density barcodes with a higher scanning speed, the author’s recently developed TBDS employed an envelope-differential fixed-period delay (EDFPD) method, which combined a fixed-period delay method and an envelope-differential detection technique containing nonlinear filtering [11]. In this system, the detection range of nearly 5 cm is inadequate even for a practical W = 0.25 mm and needs to be extended because of the distortion of gray code signals through the decrease in the differential signal and the phase delay caused by the high averaged bias level of small gray signals.
In this paper, a TBDS with a dual-bias differential (DBD) method [12] is proposed to provide a longer detection range at high speeds for high density BCs while being able to handle a great amount of information. In Section 2, the DBD method is described, in which gray signals are processed optimally with a bias control using a clamping circuit and black signals is processed optimally with the EDFPD technique. In Section 3, the clamping and differential circuits are introduced. In Section 4, the prototype TBDS with this DBD scheme is demonstrated and its scheme effectiveness is confirmed experimentally. In Section 5, by estimating gray and white code widths through the dynamic simulation of the differential circuit using SPICE, it is confirmed that the dynamic simulation is effective for appropriate estimation of an optimum clamp bias voltage.
2. Dual-Bias Differential TBDS
Figure 1 shows an outline of the TBDS using the DBD method. Its operation principle waveforms are shown in Figure 2. In the system, a black code signal is obtained by comparing the average signal of the detected barcode signal with a black level comparator threshold VT1. An adjusted black code signal is obtained through sampling/gating the black code signal by the delayed mixed code signal 1 obtained from the conventional differential delay circuit 1 and the nonlinear filter, based on the EDFPD detection technique containing nonlinear filter processing. In the nonlinear filter processing, the nonlinear filter is used to suppress the sharp-edge noise contained in the average signal to ensure stable detection [11]. In the EDFPD detection technique, bias control of the average signal is not performed. An optimal gray code signal is obtained through gating a selection pulse and a delayed mixed code signal 2 from a differential delay circuit 2 with bias control for lowering the aver aged level of the gray signals. The differential delay circuit 2 consists of a clamping circuit for the bias control of the average signal, a differential circuit for differentiating its clamped average signal, level comparators with two thresholds VT2 and VT3 for processing differentiated signals, a preset-reset circuit for changing comparator outputs to a gray-black mixed code signal 2 and a shift-register delay circuit 2 for the delay of the grayblack mixed code signal 2 by a delay time td. The substantial difference between the conventional EDFPD method and the new DBD method is shown in Figure 3. In the conventional EDFPD method, an average level of the gray signal is located above the bias level of the average signal (nearly 0 V). It means that the gray signal is differentiated near the upper saturation level of the differential circuit. This provides a small differentiated signal and thus a large phase-delay at the beginning of the gray signal, which causes an extension of the white code signal and a contraction of the gray code signal. On the contrary, in the DBD method, the bias level of gray sig-
Figure 1. Schematic outline of the DBD TBDS.
Figure 2. Operation principle waveforms.
nals is adjusted to nearly 0 V by lowering its averaged level using the clamping circuit. As a result of this adjustment, appropriate differentiation of the gray signals is carried out, creating a large differentiated signal, and consequently a small phase-delay for the gray signals is obtained. Though the bias level for black signals is changed to a higher level than the averaged level of the average signal by the clamp operation, there is hardly any phase delay because the black signals are large. Therefore, a gray-black mixed code signal with a nearly normal code width by the DBD method should be obtained.
3. Clamping and Differential Circuits
Figure 4 shows a configuration of clamping and differential circuits. The clamping circuit used consists of a coupling capacitor C, a resistor R for providing a ground bias level, a clamp diode D and a bias power supply Vbias for providing the clamp bias voltage Vbias [13]. In this circuit, higher level signals than Vbias + Vj (a junction potential of clamp diode: ~0.6 V) are clamped to a level below Vbias + Vj. Therefore, the bias level of the average signal is adjusted to an optimum level through the adjustment of Vbias. The adjusted average signal is differentiated by the differential circuit, which amplifies the