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Laser Surface Velocimetry Principle of Operation
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Laser Doppler Velocimeters are non-contact measurement systems used to make velocity and length measurements on moving surfaces, such as steel sheeting,films, paper, textiles and other strip goods. The non-contact optical measurement process allows very high accuracy. It can be applied in complex measurementtasks, where touch sensors can not make a measurement or only with great difficulty, such as making measurements on red-hot objects. |
Thus in continuous casting systems, Laser Doppler Velocimeters replace the measurement rollers traditionally used for measuring casting lengths and velocities. Thanks to the non-contact measurement process, slippage, scaling deposits or damage to bearings no longer affect the results of the measurement as they did when using measuring wheels.
Laser Surface Velocimeters work on the Laser Doppler Principle and evaluate the laser light scattered back from a moving object. Polytec's LSVs are based on the sophisticated heterodyne detection method. Unlike conventional non-contact methods which measure only the absolute value of the velocity, Polytec's velocimeters are able to detect changes in direction and even standstill conditions. The measurement precision is fine enough that minute motions can be accurately measured.
Inside the LSV-065 and LSV-026 sensors, a Bragg Cell splits light from a laser diode into two beams and introduces a frequency shift of 40 MHz to one of the beams.
The sensor head incorporating a frequency stabilized laser diode (German patent), highly sensitive APD (Avalanche Photo Diode) detector and an optical frequency shifter (Bragg cell).
The two beams intersect at an angle j on the moving surface where they form a pattern of equally spaced bright and dark fringes. Light scattered from a material moving through this fringe pattern experiences an intensity modulation with a frequency proportional to the speed of the material. A portion of the scattered light is collected by the receiver lens, converted by a photo detector to an electrical signal. The frequency offset (40 MHz in this case) is particularly important because it acts as an FM (frequency modulation) carrier for the scattered light.
Fringe pattern
The 40 MHz carrier allows the LSV to measure the direction of movement, as well as absolute zero-speed (i.e. the LSV-6200 controller recognizes that the surface has stopped when the detected signal exactly equals 40 MHz). Because the spacing of the projected fringe pattern depends only upon the laser beam intersection angle and the wavelength of the stabilized diode laser (European patent 0152916), the measurement accuracy is not affected by environmental influences. The calibrated spacing between the fringes is preserved even if the measurement object moves closer or farther (within the limits of the optical depth of field).
The detector signal of an LSV is characterized by bursts, i.e. the signal is not timecontinuous but merely consists of short sections with few coherent vibration periods, (the so-called bursts). An exact determination of the instantaneous frequency is therefore only possible within the duration of a burst. Because the burst duration is unknown and also the signal amplitude is subject to strong variations, the realizing of reliable measurement methods requires advanced signal conditioning algorithms.
Polytec developed the Automatic Surface Adaptation (ASA) algorithm to provide a signal with a continuous amplitude, independent of the scattering characteristic of the material.
Polytec`s Fast Burst Detector (FBD) circuit identifies the Doppler frequency ( i.e. speed) and immediately adjust the A/D for maximum information of the Doppler burst and for optimum accuracy.
At zero velocity the peak in the frequency spectrum is located at 40 MHz. This frequency shift makes it possible to measure velocities down to zero and negative velocities. Frequencies less than 40 MHz correspond to a negative velocity and frequencies higher than 40 MHz correspond to a positive velocity. The maximum allowed Doppler frequency is ±4.8 MHz.
Frequency / Velocity Diagram
After the accurate measurement of the frequency the controller calculates velocity with the following equation:
vp = fD * Δs, Δs = fringe spacing
The integration of the velocity over time provides the length. These values are displayed on the front panel and are available as system outputs for data acquisition and control.
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