Principle analysis and application of the hottest

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Brief analysis of the principle and application of Doppler laser measuring instrument

I. brief introduction

lsv series Doppler laser measuring instrument is the product of German Polytec company, which is a composite part of the mechanical arm with loose structure designed by engineers of NASA, and has been well applied in Jigang medium and heavy plate plant. According to the measurement principle of Doppler spectrum, the laser measuring instrument has the characteristics of high measurement accuracy and simple maintenance. According to the actual situation on site, it is installed on the side of the roller table to detect the speed and length of the billet, so as to avoid the correction rate caused by the short billet and improve the efficiency of the enterprise. It belongs to non-contact measurement and has the advantages of fast dynamic response, high spatial resolution and large measurement range. It has broad application prospects in the field of measurement

II. System composition

1. Hardware composition

Doppler laser speed and length measurement system is mainly composed of the following components: a set of optical sensor components, a set of LSV system controller, a set of sensor probe and mobile frame, a data transmission line, a host computer, an electro-pneumatic control cabinet, and a set of water cooling and air purging components

2. Software architecture

2.1 software composition

the upper computer operating system of this system is Windows 2000; The programming software is visual c++ Net, the IDE integrated development environment of c++ language, which has the same basic database types and user-defined types, classes and interfaces, realizes the interaction of different languages, greatly simplifies application development and improves programming efficiency; The database is designed with SQL Server 2000, which has rich graphical management tools, dynamic automatic management and optimization functions, rich programming interface tools, good scalability, reliability and simple management mode

2.2 human computer interaction interface hmi

start the PC and enter the windows 2000 interface. There are two shortcut keys on the HMI desktop. Lsv6200 is used to set the built-in parameters of the laser measuring instrument and monitor the status diagram of the measured data; Lsvsetup is used to run HMI measurement length display and alarm information screen, and has data information storage function at the same time

(1) main screen: display the measured length and billet simulation diagram. The alarm information in the upper right corner: green is normal and red is alarm, indicating that the temperature of the laser probe is too high or the laser itself has problems (voltage fluctuation or electromagnetic interference, etc.). In case of alarm, the laser will no longer measure

(2) parameter configuration: you can enter steel plate information, such as shift, steel type, standard length, batch number, etc

(3) history browsing: you can view historical data

III. analysis of measurement principle

Doppler laser measuring instrument is an instrument that uses non-contact sensors to measure the speed and length of billet. It emits two intersecting lasers to the surface of slab, producing light interference phenomenon and forming Doppler effect. The moving speed of interference fringes is directly proportional to the speed of the object passing through the intersection of two lasers. Most devices used do not contain reference light, but focus and intersect two beams of equal intensity light at the measuring point. By receiving the spectrum and recording the moving speed of the interference fringe, the moving speed of the billet can be accurately determined. The accuracy of laser measurement is ± 0.02%, and the measurement accuracy at the speed of 0.1m/min is ± 0.02mm/min. (Figure 1)

Figure 1 LSV laser measurement principle

the basic principle of Doppler laser differentiation is used in the work of LSV laser sensor. The beam of Doppler laser diode is divided into two separate beams, which hit the surface of the measured object, forming an equidistant Δ The three-dimensional space (measurement volume) between the black and bright stripes of S. When a particle passes through the fringe pattern, it will signal modulate the brightness it returns

1, stripe interval

stripe interval Δ S is a system constant, which depends on the wavelength of the laser λ And the included angle of two laser beams 2 φ。

the photoelectric detector on the sensor generates an AC signal, and the proportion formula of Doppler frequency FD and velocity component VP in the measurement direction is:

fd ---- Doppler frequency; VP ---- velocity component in the measurement direction; Δ S---- fringe interval in the measurement volume

fringe interval Δ S represents the standard tool for measuring speed and length (with two adjustable parallel mirrors on it). It can accurately measure each sensor head and print it on the identification label. When four LSV controllers are configured to establish branches in Yucheng, the fringe spacing is stored in flash memory as a standard factor, so it is a benchmark for calculating the measured value. If the sensor head is replaced, the new fringe spacing must be reset through LSV software

2. Calculation of offset

lsv operates in heterodyne mode, that is, a laser beam is moved by an offset of 40MHz. Therefore, the fringe in the measurement volume is shifted to the offset frequency FB corresponding to the corresponding speed. In this way, it is possible to detect the direction of motion of the object and measure it from zero speed. In the heterodyne method, the resultant modulation frequency Fmod is as follows:

the modulation frequency in the LSV controller is calculated by Fourier transform and converted into the measured value of the velocity component VP. Another function of the controller is to use the integral principle to measure the length. LSV is generally used in process control system. As an intelligent sensor for speed and length measurement, it can realize user-defined interface

3. Optical configuration of LSV sensor

Figure 2 LSV sensor composition

as shown in the above figure, the beam with stable wavelength emitted from the laser diode is divided into two beams with equal intensity through the Bragg unit, and one of the optical frequencies is shifted to 40MHz. The beam splitter separates the two beams into two spaces. Use three prisms to adjust the two beams of light through a certain distance. The light scattered from the measuring object passes through the receiving optical device and the focusing distance to the optical detector, from which the detection signal is transmitted to the controller for measurement

IV. billet measurement and common problem analysis

4.1 measurement example

Figure 3 main screen of measuring instrument HMI

Figure 4 billet measurement speed and length curve display

the measured length of billet can be accurately displayed on the main screen (Figure 3), accurate to M. Figure 4 above shows the continuous measurement waveform of the billet with a measurement length of about 3 meters [2]. Through the correlation and comparison of speed and length, we can clearly know the current measurement trend of the billet and the corresponding roller speed. You can also use different cursors to view the distance and speed deviation between each two points on the curve. The general measurement method is to use a steel tape for manual measurement, which must be carried out when the billet is stopped. It is OK when the material is cold. If it is hot, manual measurement cannot be carried out. Generally, the steel tape used in industry is class II, with an accuracy of only M. The Doppler laser non-contact measuring instrument has obvious advantages in terms of accuracy level and adapting to the fast production rhythm

4.2 common problems and solutions

(1) the connection between the signal processor of the measuring instrument and the sensor probe is only a special cable, which is a multi-core cable to transmit the detection signal and supply power to the probe. Therefore, the anti-interference requirements for the outside world are particularly strict. The power supply should be reliably grounded to avoid strong current interference around

(2) the sensor probe is easy to generate heat after being used for a long time. Therefore, the effect of water cooling and air purging must be guaranteed. Generally, the probe temperature should be ≤ 35 ℃, otherwise the system will alarm and automatically turn off the laser

(3) the Doppler laser measuring instrument is a continuous and non-contact measurement, so the billet transported on the roller table must have a certain gap of at least 2mm to ensure that the system can distinguish different billet measurements

v. scheme determination of equipment installation position

Doppler laser velocimeter is widely used in rolling mills. Generally speaking, laser velocimeter can be installed above, below or on both sides of billet. If the instrument is installed above the billet, the wide surface heat radiation of the billet is too strong and the cooling pressure is large. Once the tachometer is installed above and fixed, the production of the billet will be affected. There are also thermal radiation problems when the instrument is installed under the billet, as well as the falling of waste materials, polluting the lens, and cleaning the lens will also delay production. The tachometer is installed on the side of the billet, and the lens is installed on a mobile and controllable acceleration platform. It can move with the width of the billet, so as to avoid the radiation of high temperature in the international architecture exhibition held in Las Vegas, reduce the installation cost of the instrument and reduce the impact on production

several factors should be considered in determining the installation position of laser measuring equipment. First of all, it is necessary to consider that the outlet of the equipment shell is in the harsh environment of high temperature, and the measuring equipment must have good protective measures to withstand this harsh environment; Secondly, the installation of measuring equipment should not affect the operation of other equipment. Therefore, it is recommended that the laser measuring equipment should be installed outside the roller table

VI. conclusion

the Doppler laser measuring system has no contact with the billet, the maintenance rate of the measuring instrument is low, and it has no impact on the work of the billet. It can measure the length and speed of the slab, and the laser measuring system has high sensitivity, which solves the problem of inaccurate billet length. At the same time, the system has highly compatible interface components, which can select a variety of independent interface components at the same time or at will according to the actual use situation, and transmit the measured values in real time through the internal data and address bus. It can be real and low-cost with the enterprise's automatic control system. Carbon fiber will eventually be sold in the manufacturing commodity market of carbon nexus factory for interface communication and data sharing. The system has the advantages of simple installation, convenient maintenance and high detection accuracy, which improves the yield of rolled steel plates, and can save 2million yuan per year. It has good promotion value in the same industry. (end)

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