Design of non-contact online detector based on CCD technology

1. Introduction

Detection technology is a widely used technology. In many fields, various processed parts and various moving objects are tested to ensure the qualified rate of products and the safety of production and life. Traditional detection methods include manual detection and detection by mechanical, optical or electromagnetic detection instruments. In particular, manual detection depends entirely on actual work experience. If the structure of the components is complex, it will not only increase the labor intensity of the workers, but also reduce the accuracy and efficiency. It can neither complete non-contact detection nor realize online detection, and it also increases the danger to the workers during detection. Therefore, with the rapid development of science and technology, there must be a more perfect detection technology, that is, CCD technology. Here, we will discuss CCD technology with an example based on CCD online non-contact detection-"Online Detector for Diameter of Lifting Wire Rope".

2. System composition

The entire detection system consists of an illumination system, a workpiece system, an imaging lens, a CCD photoelectric detection system, and a computer measurement and control system (8031 single-chip microcomputer and 8279 keyboard/display chip, etc.). The voltage-stabilized and current-stabilized dimming power supply provides stable illumination light for the telecentric illumination system. The illuminated workpiece is imaged on the photosensitive array surface of the linear array CCD through the imaging lens. Since the workpiece is opaque, the image of the workpiece forms a dark band in the middle and bright bands on both sides. The width of the dark band is the size of the image formed by the size of the workpiece. The linear array CCD completes the photoelectric conversion under the action of the driving pulse and generates a video signal. The system schematic diagram is shown in Figure 1.

3. CCD photoelectric detection system

In CCD photoelectric detection system, the selection of CCD is very important. There are many types of CCD, and they all have their own characteristics and different applications, so the correctness of CCD model will directly affect the correctness of the measured information.

The measurement range required in this system is 15mm~45mm, and the measurement accuracy and relative requirements are relatively high, so as long as a linear array CCD with more than 1000 pixels is selected, the accuracy requirements of this measurement system can be met. Therefore, this system should use TCD1206UD, which has 2160 effective pixels, a pixel size of 0.014×0.014mm, and a pixel center distance of 0.014mm, which is sufficient to meet the requirements of this measurement system.

The main technical indicators of the device:

Number of pixel units: 2160 Total pixel length: 30.24 mm

Pixel center distance 14μm Drive frequency 1MHz

Line cycle 2.5ms Sensitivity 45 V/lx·s

When working at 1MHz data rate, the effective pixel output time is 2.16ms, and the wire rope diameter signal is generated during 2.16ms. The dark level of the output signal can be controlled at about 1.0V. The high level can be close to 10V, which is a big difference. When the optical system is adjusted well, the signal at the edge of the image is steeper and the measurement error is smaller. The working principle and driving circuit of TCD1206UD are as follows:

(1) Working principle. TCD1206UD works under the driving pulse shown in Figure 2. When the high level of the ΦSH pulse arrives, a deep potential well is formed under the positive Φ1 electrode. At the same time, the high level of ΦSH makes the deep potential well under the Φ1 electrode communicate with the MOS capacitor storage potential well.

As shown in Figure 3, the signal charge packet in the MOS capacitor is transferred to the potential well under the Φ1 electrode of the analog shift register through the transfer gate. When ΦSH changes from high to low, the shallow potential well formed by the low level of ΦSH isolates the potential well under the storage gate from the potential well under the Φ1 electrode. The storage gate potential well enters the light integration state, and the analog shift register drives the signal charge transferred to the potential well under the Φ1 electrode to shift to the left under the action of the Φ1 and Φ2 pulses, and outputs it from the OS electrode through the output circuit. Due to the structural arrangement, the OS end first outputs 13 dummy unit signals, then 51 dark signals, and then continuously outputs the effective pixel unit signals from S1 to S2160. After the S2160 signal is output, 9 dark signals are output, and then 2 parity detection signals are output, and then it is empty drive. The number of empty drives can be arbitrary. Since the device transmits in two columns in parallel, divided into odd and even, there must be at least 1118 Φ1 pulses in one ΦSH cycle, that is, TSH>1118T1. ΦR is the reset pulse of the reset level, and one signal is output for each reset.

(2) Driving circuit. The driving circuit of TCD1206UD is shown in Figure 4:

The pulse signal source composed of a crystal oscillator generates the master clock ΦM. The ΦM pulse generates four drive pulses ΦSH, Φ1, Φ2, and ΦR through the programming logic device ISPLSI. Under the action of these four drive pulses, TCD1206UD outputs OS and DOS signals. The two output signals are sent to the positive and negative input lines of the differential amplifier LF357 for differential amplification, suppressing the interference caused by the common mode ΦR, and obtaining the signal waveform shown in Figure 3. SP and ΦC are control pulses provided for users. SP is synchronized with the pixel photoelectric signal output by the CCD and can be used as a sampling and holding control signal. The rising edge of ΦC corresponds to the first effective pixel unit S1 of the CCD, so it can be used as a line synchronization. Of course, ΦSH can also be used for line synchronization, but since the CCD prefers to output 64 dummy unit signals, it is better to use ΦC than ΦSH. The selection of the illumination source of the lighting system and the imaging lens of the imaging system in this detection system is also very important. In this case, it is just a general inspection of the wire rope, so only an incandescent lamp or a halogen lamp is needed as the illumination source; and the imaging lens, here, will be selected with a magnification of β=1, f/=130mm, D/f/=0.5, and 2ω=140. [page]

4. Binary data acquisition and single chip microcomputer interface

This system is used to detect the size of workpieces in motion. Since the workpiece will change position due to swinging during the motion, the amplitude of the video signal output by the CCD will fluctuate, and the change in light source intensity will also cause the CCD video signal to fluctuate. If the floating threshold method is used, when the CCD video signal fluctuates due to the above reasons, the fluctuation of the light source or the fluctuation of the CCD video signal can be fed back to the threshold through the circuit, so that the threshold potential changes accordingly, so that the width of the square wave pulse generated by the CCD video signal after the binarization circuit remains basically unchanged. Therefore, the binarization processing method of the floating threshold method is selected.

In addition to generating various driving pulses required by CCD, CCD driver also generates line synchronization pulse ΦC and input pulse Φt used for binary counting. It is required that ΦC and SH have the same cycle, and the rising edge of ΦC corresponds to the first effective pixel unit of CCD output signal. It is required that the pulse frequency of Φt is an integer multiple of the frequency of reset pulse ΦR. Set the GATE position of mode register TMOD of timer T0 to 1. Here, timer T0 is controlled by the input level of external pin, that is, INT0 controls the operation of T0. The square wave pulse signal output by the binary circuit and the line synchronization pulse ΦC are input into the "AND" gate together. Their output signals are connected to the INT0 pin of the microcontroller, and INT0 controls the start of the microcontroller timer T0. At the same time, the reset pulse ΦR is connected to the P3.4 pin of the microcontroller. When both the line sync pulse ΦC and the binary square wave pulse signal are at high level, the AND gate outputs a high level as well. This high level is used to start the timer T0 of the microcontroller to count the reset pulse ΦR. When one of the line sync pulse ΦC and the binary square wave pulse signal is at low level, the Y output of the AND gate is also at low level, and the timer T0 stops counting. Here, the number recorded by the timer T0 is the number of reset pulses. Since the reset pulse ΦR has the same cycle as the CCD pixel, the number recorded by the timer T0 is the number of pixels covered by the high level of the binary square wave pulse signal. In this way, the processing of the CCD output signal is completed, and the data related to the workpiece size, that is, the number of CCD pixels, is recorded in the timer T0. The magnification in the optical system is 1, so the number of pixels measured is multiplied by the center distance of the CCD pixel, and the result is the true value of the working size within the allowable error range.

5. Program flow chart

The program flow chart can be used to complete the compilation of the assembly program, which will not be described in detail here.

6. Conclusion

Since the manufacturing technology of mechanical, optical and electromagnetic measuring instruments is very mature, they still occupy an important position in the current detection field. However, it is believed that with the improvement of manufacturing technology, the production cost of CCD image sensing elements will be reduced, while its accuracy will be further improved. By then, CCD technology will be widely used in both the field of precision detection and in general detection systems, and CCD technology will become the dominant technology in the detection field in the future.