The measurement of workpiece size plays an important role in the industrial production, processing and manufacturing process. It is not only related to the production cost and quality of products, but also related to the use safety of products. As a typical representative of geometric measurement, thickness measurement has attracted much attention in the industry, and the automatic measurement of light transmitting and absorbing materials such as glass and black rubber plate is one of the difficulties in the industry.
The common thickness measurement methods can be divided into contact type and non-contact type. Although the precision of contact measurement is high, its efficiency is low, and it can not realize real-time online and extreme environment (such as high temperature) measurement, which is prone to error and wear of the probe and the surface of the measured object, and can not meet the needs of modern production. Therefore, this paper mainly introduces the non-contact thickness measurement scheme.
1. Electrical method
Electrical measurement methods mainly include eddy current method, magnetic flux leakage method and capacitance method, which are relatively mature. Taking the capacitance method as an example, the basic principle is to calculate the thickness between two electrode plates by measuring the change of capacitance before and after. The sensitivity and stability are high, but the measurement range is limited, which is not conducive to online measurement (the electrode plate is fixed, and the dielectric change system needs to be adjusted accordingly).
2. Ray method
The ray method is mainly based on the principle of absorption, reflection and scattering of rays. Before and after the ray penetrates the object, there is a certain attenuation change in the intensity. From this, the thickness of the object to be measured can be inferred by the degree of change. The measurement accuracy is high and the stability is good, but there are defects such as the ray tube is easy to age, the equipment is expensive and harmful to human health.
3. Ultrasonic method
The ultrasonic method uses the ultrasonic transmitter to transmit pulses through the measured object to reach the material interface, and reflect them to the receiver. The thickness of the measured object is calculated by measuring the propagation time. The method is mature, but it is easily affected by the surrounding environment, and the sound pressure amplitude gradually attenuates with the extension of the transmission distance, resulting in low signal-to-noise ratio, which affects the measurement accuracy.
In addition to the advantages of conventional non-contact measurement scheme, the optical thickness measurement scheme also has the advantages of simple structure, fast response, flexible use, harmless to personnel, and can be extended to the field of three-dimensional topography measurement, which will be introduced below.
1. The laser triangulation method is used to measure the thickness of most conventional materials (such as steel plate, battery pole piece, etc.), such as opaque and strong light absorbing materials. Based on the principle of triangulation, the system is mainly composed of two linear laser measuring instruments; The upper and lower laser emission systems measure the real-time position of the body to the upper and lower surfaces of the steel plate respectively, and the instantaneous thickness of the steel plate can be obtained by calculating the difference between the positions of the upper and lower surfaces in combination with the inherent parameters.
Although the measurement method based on two devices can achieve high precision measurement (partial elimination of system error), there may also be optical axis mismatch, system vibration and other impacts, which requires high installation and site environment. However, when the single side position is fixed relative to the reference plane, a single sensor can be similarly used for measurement, which can avoid the influence of optical axis mismatch.
2. For transparent materials (such as glass), spectral confocal method is difficult to measure with conventional optical scheme, which is exactly the advantage of spectral confocal method; It can be used to measure the thickness of various plates and the micro displacement, and has nothing to do with the surface properties of the measured materials. Compared with the laser triangulation method, it has almost no requirement for the reflection angle. It has the characteristics of strong adaptability to the environment and insensitivity to external factors such as temperature.
The system consists of a broad spectrum light source, a spectroscope, a dispersive lens, a confocal aperture and a spectrometer. The light emitted by the light source is incident to the dispersive lens through the spectroscope to generate spectral dispersion, and a continuous wavelength distribution is generated on the outgoing optical axis: that is, light of different wavelengths will focus on different positions on the optical axis to generate position chromatic aberration. There is a focus corresponding to the wavelength on the upper and lower surfaces of the corresponding object to be measured, and light of different wavelengths focused on the upper and lower surfaces of the object to be measured will be reflected back to the spectroscope, and the light of other wavelengths will be filtered through the confocal small hole frequency selection, The spectrometer collects and processes the accurate wave peak wavelength information to obtain the upper and lower surface position information, and then the thickness value of the object to be measured can be obtained.
3. Considering the strong light absorption characteristics of some materials (such as black rubber), the light transmission method can be used. Two sensors can be installed at both ends of the object to be measured. The thickness can be measured by detecting the edge position to avoid the influence of color and material. For example, Kienz TM-X5000 series online projection image measuring instrument uses telecentric optical system to illuminate green LED parallel light from the transmitter, and the receiver uses CMOS to image the shadow, and measures the thickness and other dimensions according to the imaging information.
Similarly, high frequency micrometers can also be used for thickness measurement in vibration, tilt measurement and other scenarios. This kind of light transmission scheme has the advantages of high accuracy and almost no material limitation, but the measurement range is small (within the light transmission range), which is only applicable to the thickness measurement of small workpieces.
This article mainly outlines the common optical thickness measurement schemes. In fact, with the development of modern industry, high-precision geometric measurement of displacement, thickness, etc. is no longer difficult in principle, but the errors caused by such detailed problems as coaxiality of dual laser displacement sensors, measuring inclination and temperature drift of various sensors still need attention.