Research on Dimension Measurement Based on Linear CCD

Charge Coupled Device (CCD) is a new type of semiconductor integrated optoelectronic device developed in the early 1970s. It has the characteristics of high sensitivity, large dynamic range, and high pixel division accuracy. CCD uses charge as a signal, and converts the light signal of the scene in the visible range into a charge signal through the photosensitive element, and then outputs a video signal after storage, transmission and detection, and then displays an image that can be seen by the eye. CCD is divided into linear array CCD and area array CCD. Linear array CCD is widely used in the measurement of size and displacement in the industrial field due to its simple drive and relatively easy signal processing, while area array CCD is mainly used for the transmission of graphics and text.
This measurement system uses 89C2051 to control TCD1206UD to measure micro-size, and after the lighting system, signal conversion, data processing and other processes, it is finally displayed through LED. It has the characteristics of stability, reliability, high measurement accuracy, and is suitable for various high-sensitivity and high-precision detection.

1 System working principle
The system principle is shown in Figure 1. This system is based on 89C2051 as the core, TCD1206UD, ICL7135 and other devices.

The lighting system provides stable lighting. The illuminated object is imaged on the photosensitive array of the linear array CCD through the imaging objective lens. The length of the dark band reflects the length of the object being measured. After the CCD video signal is processed by the binarization circuit, the binary signal is converted into a digital signal through the A/D converter ICL7135. The signal is then processed by 89C2051 and displayed through the LED.

2 System Hardware Design
2.1 Optical System Design
Due to the certain spacing between the photosensitive units of the CCD itself, and the changes in the light source, diffraction and external interference, the image of the object on the CCD cannot be directly converted from dark to bright, but there is a slow transition zone. In order to have a good imaging effect, this puts forward higher requirements on the illumination of the object. A good optical system can improve the measurement accuracy.

As shown in Figure 2, this lighting system uses a high-power LED as the lighting source. The light-emitting diode is used as the lighting source. Due to its advantages of small size, light weight, good monochromaticity of the light source, high brightness, high luminous efficiency, and easy brightness adjustment, it is currently widely used in digital instrument display and CCD application technology. The light emitted by the light-emitting diode LED is converged to a point F through a double-cemented lens L1. Point F is exactly the object focus of the lens L2, and is expanded into the required parallel light, which is irradiated onto the device to be tested. It is imaged on the CCD through the imaging system (composed of the imaging objective lens L3 and the aperture), forming a shadow. [page]

2.2 CCD drive timing requirements and implementation
2.2.1 TCD1206UD timing requirements
Generally speaking, different CCD devices can measure different sizes, and accordingly, the measurement accuracy is also different. Here we choose TCD1206UD linear array CCD, which has 2160 effective photosensitive pixels, the total length of the photosensitive pixel array is 30.24 mm, the center distance of the pixel is 14 μm, the driving frequency is 1 MHz, the line period is 2.5 ms, and the photoelectric sensitivity is 45 V/lx·s.

Figure 3 is the driving timing diagram of TCD1206UD. SH is the transfer pulse, and its period is the optical signal integration time. OS is the output signal, and its output period is at least the output period of 2236 pixels; the clock frequency of φ1 and φ2 is 0.5 MHz; RS is the reset pulse, and its clock frequency is 1 MHz, and the duty cycle is 1:3.
2.2.2 Driving pulse design of TCD1206UD
Figure 4 is the driving pulse circuit diagram of TCD1206UD.

The pulse is drawn from the ALE port of the microcontroller. One D flip-flop can divide the pulse by 2, and two can divide the pulse by 4. The frequency drawn from the ALE port of the 89C2051 is 2 MHz. After passing through two D flip-flops, a 0.5 MHz pulse that matches the frequency of φ1 is obtained. Adding an inverter to φ2 can obtain a pulse that completely matches φ1 and φ2. The reset pulse is a pulse with a frequency of 1 MHz and a duty cycle of 1:3. Here, a pulse with a frequency of 1 MHz and 0.5 MHz is used with an AND gate to obtain a reset pulse that meets the requirements.

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The transfer pulse of TCD1206UD is led out from P1.2 port. There are at least 1180 pulses in one cycle of CCD sensor. The frequency of clock pulse is 0.5 MHz, so the cycle of transfer pulse should be 590 μs. The transfer pulse that meets the requirements can be obtained through simple procedures.
2.2.3 TCD1206UD driving circuit
The driving circuit diagram of TCD1206UD is shown in Figure 5.

Under the action of the previous φR, φ1, φSH, 4-way driving pulses, TCD1206UD outputs OS signal and DOS signal. These two output signals are sent to the positive and negative input terminals 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 5.

SP and φC in Figure 6 are control pulses provided to users. SP and the pixel photoelectric signal output by CCD are synchronized and can be used as sampling and holding control signals. The rising edge of φC corresponds to the first effective pixel unit S1 of CCD, so it can be used as line synchronization. Of course, φSH can also be used for line synchronization, but since CCD first outputs 64 dummy unit (including dark current signal) signals, it is better to use φC than φSH.

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2.3 Signal Binarization Processing Circuit The
fixed threshold method is the simplest binarization processing method. The video signal output by the CCD is sent to the non-inverting input terminal of the voltage comparator, and the inverting terminal of the voltage comparator is added with an adjustable level to form a fixed threshold binarization circuit.

When the amplitude of the CCD video signal is greater than the threshold voltage, the output is a binary square wave signal. By adjusting the threshold voltage, the leading and trailing edges of the square wave pulse will move, and the pulse width will change.

3 System software flow chart
The system software flow chart is shown in Figure 8.

4 Conclusion
The illumination light source plays a vital role in the linear array CCD. It is one of the keys to meet the accuracy requirements of the measurement system. This illumination system is cost-effective and does not require the purchase of expensive parallel light generators. However, the parallel light that can be obtained is uniform in brightness and strong in light, which fully meets the illumination requirements of this measurement system. The fixed binarization method is a simple binarization method. This measurement system ensures the stability of the light source and the stability of the threshold voltage, ensuring the accuracy of the binarization. This measurement system collects 10 data, and the software removes the maximum and minimum values ​​and calculates the average value to improve the accuracy of the measurement system.