Laser surface velocimeter used for online billet measurement

Laser surface velocimeter has been well applied in medium and thick plate factories. The laser measuring instrument is based on the measurement principle of the Doppler spectrum and 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 online billets, avoiding the correction rate caused by short-length billets and improving the efficiency of the enterprise. It is a non-contact measurement with the advantages of fast dynamic response, high spatial resolution, and large measurement range. It has broad application prospects in the field of measurement.

1. Measurement principles of laser surface velocimeter


The laser Doppler measuring instrument uses a non-contact sensor to measure the speed and length of the steel billet. It emits two intersecting laser beams to the surface of the billet, causing light interference and forming the Doppler effect. The moving speed of the interference fringes is proportional to the speed of the object passing through the intersection of the two laser beams. Most of the devices used do not contain a reference light but focus and intersect two beams of equal intensity light at the measurement point. By receiving the spectrum and recording the moving speed of the interference fringes, the moving speed of the billet can be accurately determined. The accuracy of laser measurement reaches ±0.02%, and the measurement accuracy reaches ±0.02mm/min when the speed is 0.1m/min.

The operation of the laser surface velocimeter laser sensor uses the basic principle of Doppler laser differentiation. The beam of the Doppler laser diode is divided into two separate beams and hits the surface of the measured object, forming a three-dimensional space (measurement volume) separated by black and bright stripes equidistantly spaced ΔS. When a particle passes through the stripe pattern, the brightness returned by it is signal modulated.


(1) Stripe interval


The stripe spacing ΔS is a system constant, which depends on the wavelength λ of the laser and the angle 2φ between the two laser beams.



The photodetector on the sensor generates an AC signal. The ratio of the Doppler frequency fD to the velocity component VP in the measurement direction is:


fD—-Doppler frequency; VP—-velocity component in the measurement direction; ΔS—-stripe spacing within the measurement volume


The fringe interval ΔS represents the etalon for speed and length measurement (it has two parallel mirrors that can be adjusted). It accurately measures each sensor head and prints it on an identification label. The stripe spacing is stored in flash memory as a standard factor when configuring the LSV controller, so it is a basis for calculating measured values. If the sensor head is replaced, the new stripe spacing must be reset via the LSV software.


(2) Calculation of offset


The LDV operates in heterodyne mode, where a laser beam is moved by an offset of 40 MHZ. The fringes in the measurement volume are therefore shifted to the offset frequency fB with corresponding velocities. It is thus possible to detect the direction of motion of an object and start measuring from zero velocity. In the heterodyne method, the resultant modulation frequency Fmod is as follows:


Fmod=fB+ VP/ΔS


The modulation frequency in the LDV controller is calculated by Fourier transform and converted into the measured value of the velocity component VP. Another function of the controller is that it can use the integral principle for length measurement. LDV is generally used in process control systems as an intelligent sensor for speed and length measurement and can implement user-defined interfaces.


(3) Optical configuration of Laser surface velocimeter sensor


The stable wavelength beam emitted from the laser diode is divided into two beams of equal intensity through the Bragg unit, and the optical frequency of one of them is shifted to 40MHZ. A beam splitter separates the two beams into two spaces. Three prisms are used to adjust the passage of two beams of light within a certain distance. The light scattered back from the measurement object passes through the receiving optics and focusing distance to the optical detector, from where the detection signal is transmitted to the controller for measurement.


2. Online billet measurement and analysis of common problems


1) Measurement examples


The measured length of the billet can be displayed on the main screen (accurate to 10-4m). Through the correlation and comparison of speed and length, you 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 measure for manual measurement, and it must be done when the billet is stopped. Manual measurement can be done when the billet is cold, but if it is hot material, manual measurement cannot be done close to it. The steel tape measure generally used in industry is Class II and only reaches 10-2m. The laser Doppler non-contact measuring instrument has obvious advantages in terms of accuracy level and adaptability to fast production rhythm.


2) Frequently asked questions and solutions


(1) There is only a special cable connected between the signal processor of the measuring instrument and the sensor probe. This cable is a multi-core cable that transmits detection signals and supplies power to the probe. Therefore, the requirements for anti-interference from the outside world are particularly strict. The power supply must be reliably grounded to avoid strong current interference in the surroundings.

(2) The sensor probe is prone to heat when used for a long time. Therefore, the effects of water cooling and air purging must be ensured. 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 an online, continuous, non-contact measurement. Thus, the steel billets transported on the roller conveyor must have a certain gap, at least 2mm, to ensure that the system can distinguish between different billet measurements.


3. Determine the plan for laser surface velocimeter installation location


Laser Doppler velocimeter is widely used in steel rolling mills. Generally speaking, a Doppler velocimeter can be installed above, below or on both sides of the steel billet. If the instrument is installed above the billet, the heat radiation from the wide surface of the billet will be too strong and the cooling pressure will be high.

Once the speedometer is installed and fixed above, it will affect the production of the billet. There is also a problem of heat radiation when the instrument is installed under the steel billet. There are also waste materials falling and contaminating the lens. Cleaning the lens will also delay production. The speedometer is installed on the side of the steel billet, and the lens is installed on a mobile and controllable acceleration platform. It can move with the change of the width of the steel billet, thus avoiding high-temperature radiation, reducing the installation cost of the instrument and reducing the impact on production.

There are several factors to consider when determining the installation location of laser measurement equipment. First of all, it should be considered that the outlet of the equipment shell is in a harsh environment with high temperatures. The measuring equipment must have good protection measures and can withstand this harsh environment. Secondly, it should be considered that the installation of the measuring equipment cannot affect the operation of other equipment. Therefore, it is recommended that the laser measurement equipment should be installed outside the roller table.


4. Conclusion


The Laser surface velocimeter system has no contact with the billet. The maintenance rate of the measuring instrument is low and has no impact on the work of the billet. It can measure the length and speed of the slab. The laser measurement system has high measurement sensitivity and solves the problem of inaccurate billet length. At the same time, the system has interface components with high compatibility functions.

Multiple independent interface components can be selected simultaneously or at will according to actual usage conditions to transmit measured values in real time through the internal data and address bus. Users can realize interface communication and data sharing with the enterprise’s automatic control system. The system is simple to install, easy to maintain, and has high detection accuracy, which improves the yield rate of rolled steel plates. It can save 2 million yuan in loss of judgment every year and has good promotion value in the same industry.

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