Research on Photoelectric Glass Thickness Measurement Method Based on Linear CCD

At present, most domestic glass manufacturers use traditional manual measurement of glass thickness. It is difficult to achieve high precision and real-time monitoring for some high-demand glass products. There are few non-contact measurement methods that can achieve real-time detection by using laser measurement combined with photoelectric detection technology, and most of them are expensive. With the development of photoelectric detection technology, various optical measurement methods continue to emerge, and the theory and equipment for measuring glass thickness using optical methods are slowly emerging. This project uses CCD sensors as photoelectric detection devices for various glass products to study the method of laser detection of glass thickness.

1 Linear CCD photoelectric sensor
Charge coupled device (CCD) is developed on the basis of MOS integrated circuit technology. It has functions such as photoelectric conversion, information storage and transmission, and has the advantages of high integration, low power consumption, simple structure, long life and stable performance. A complete CCD device consists of a photosensitive source, a transfer gate, a shift register, etc. When the CCD is working, within the set integration time, the photosensitive element samples the light signal and converts the light intensity into the charge amount of each photosensitive element. After sampling, the charge of each photosensitive element is transferred to the corresponding unit of the shift register inside the CCD under the drive of the transfer gate signal
. Under the action of the driving clock, the shift register sequentially transfers the signal charge to the output end for the signal processing device to complete the signal processing.

2 Photoelectric glass thickness measurement method
2.1 Transmission measurement method
A typical measurement method using the laser transmission principle is shown in Figure 3. It uses the method of light refraction during the transmission of glass to measure the thickness. This method is to place the object to be measured between the light source and the photoelectric element. When the light emitted by the constant light source passes through the object to be measured, the photoelectric element receives the light signal and performs photoelectric conversion, and then processes it to obtain the thickness information of the measured glass.

In Figure 3, when the light passes through a flat glass with a thickness of d and a refractive index of n, two refractions will occur. As a result of the refraction, the direction of the light does not change, but it moves sideways for a distance. If the refractive index of the flat glass is n, the beam offset is x, and the beam incident angle is θ, the relationship between the incident angle and the refractive angle can be calculated. The formula for the thickness d of the flat glass is:

Where: θ is the incident angle of the laser beam; n is the refractive index of the measured glass.
After calibrating θ and n before measurement, as long as the beam offset x is measured, the glass thickness d can be obtained. In order to accurately measure the beam offset, the system uses a linear array CCD to receive the reference beam and the offset beam position. Since the linear array CCD has self-scanning capability, it can convert the light intensity distribution signal in one-dimensional space into a time series electrical signal. After the electrical signal is processed by the subsequent circuit, the pulse width corresponding to the beam offset is obtained. By measuring the pulse width, the beam offset can be obtained. Through the above formula, the refractive index of the measured flat glass can be obtained. This method obtains the thickness information of the glass through the displacement information obtained by the CCD.
Method features: 1) The transmission method for measuring glass thickness is suitable for installation and measurement of glass production lines with space on both sides, but not suitable for situations where the space on one side is small; 2) During the measurement process, the laser must be incident at a certain angle, so the angle requirement is relatively high, and the angle error has a greater impact on the measurement accuracy. [page]

2.2 Single laser reflection thickness measurement method
The principle of single laser reflection thickness measurement method is shown in Figure 4: it mainly uses optical triangulation theory combined with CCD detection technology. The optical triangulation method uses a beam of light to illuminate the surface of the object to be measured, and then observes the characteristic parameters of the reflected light through an imaging observation mirror to obtain the measured information.

When the laser light P0 emitted by the laser is directed at the glass plate at a certain angle, a small part of it will directly reflect the light P1 on the upper surface of the glass plate to the CCD photosensitive surface. Its reflection angle A1 is equal to the incident angle A0. Most of the laser light refracts through the air-glass interface to produce light P2. After total reflection on the lower surface, a large part of P2 refracts at the glass-air interface to produce light P3 to the CCD photosensitive surface. The other part reflects at the glass 2 air interface to produce light P4. Similarly, light P4 refracts through the glass-air interface to produce light P5. It is easy to conclude that light P1, light P3, and light P5 are parallel, and the mutual distance between the three light rays is linearly related to the thickness of the glass plate. In practice, the actual thickness of the glass plate is measured by calculating the vertical distance of the three light rays through linear array CCD detection.
Assuming that the refractive indexes of air and glass are n0 and n respectively, the incident angle of the light is T0, and the refraction angle inside the glass is T2, then we can get formula (2):

The distance D between light P1 and light P5 detected on the CCD is linearly proportional to the actual thickness H of the glass plate when the incident angle is fixed. If U0=1.00, the relationship between the two is expressed by formula (3):

During measurement, the value of D is obtained by processing the CCD data, and the glass thickness H can be obtained. Formula (3) can be simplified as:
H=a·D (4)
In practical applications, the value of a is obtained through online calibration. Record the peak spacing value D (the distance between light P1 and light P5) at a certain moment, and obtain the glass thickness H at that time after the glass is formed to obtain the calibration value a.
Features of the method: 1) Affected by the phase difference of the optical system, the size of the light spot, the inherent position of the detector, the dark current of the detector and external stray light. 2) Affected by the jitter of the detected glass. 3) Affected by the main factors such as the measurement accuracy and noise of the detector detection circuit, and the circuit temperature drift system.
2.3 Dual laser reflection thickness measurement method
Based on the shortcomings of the single laser reflection measurement method, a reflection measurement method using dual lasers combined with optical triangulation is proposed as shown in Figure 5: a laser is placed on each side of the glass plate to be measured, and two linear array CCD sensors are used to form upper and lower triangulation measurement systems to measure the displacement of the upper and lower surfaces of the glass strip at the same time. According to the displacement values ​​of the two linear array CCD sensors, the thickness of the glass strip is calculated. If you want to increase the light intensity of the CCD and improve the CCD resolution, you only need to install the two sets of linear array CCD sensors at a suitable angle. Because the light is also incident at an angle due to the tilted installation of the sensor, the collected voltage output value has an angle relationship with the actual distance.

由图5中有公式(5)：
d=x-x1cosα1-x2cosα2         (5)
对(5)式中的x-x1cosα1、x2cosα2以及x测定。实际应用中因玻璃是透明体，为了避免传感器所发射激光束相互干扰，两个线阵CCD传感器在垂直方向装配时需错开几毫米。
方法特点：1)可有效克服传感器位置偏移、玻璃抖动等因素造成的对玻璃厚度测量的影响。2)受外界环境温度影响较位严重，在退火炉热端进行测量时，测厚系统必须有冷却装置才能正常工作。

3 结束语
文中分析了透射式测厚法、单激光反射式测厚法、双激光反射式测厚法的原理及性能特征。利用线阵CCD传感器作为光电转换器件结合光三角法，采用激光透射式，单激光反射式、双激光反射式测量玻璃厚度，具有高速度、小型化、高精度、非接触等优点。其与传统玻璃测厚方法比较：稳定度、灵敏度及准确度等方面都有了显著的提高，具有一定应用价值。