Measuring principle of ultra-precise laser surface velocimetry
Precise and continuous determination length and speed is a key condition of cost and process optimization measures in the production of sheet goods. In industrial manufacturing, laser surface velocimeters (LSV) are among the most precise non-contact measuring systems for acquiring these data and are highly popular on account of their reliability.
The velocimeters calculate the length and speed of steel, wire and cable products using an optical measuring method which directs two lasers to the moving product surface. The results can be used to derive reliable statement on the quality of production and rapidly identify faulty production processes.
The differential Doppler measuring process
The laser surface velocimeter uses the principle of the laser Doppler process. The observation axis of the sensor is at a right angle to the direction of movement of the object under investigation. Two laser beams are directed at a selected measuring field on the production line at an angle φ so that they are superimposed on the surface of the workpiece that has just been produced.
For a point P, which moves at a velocity v through the intersection point of the two laser beams, the frequencies of the two laser beams are Doppler shifted in accordance with the above formula. This generates an interference pattern of bright and dark fringes. The velocimeter can modulate the intensity of the light scattered back from the movement of the surface along the projected fringe pattern.
The sensor’s photo receiver determines the modulation frequency, which behaves proportionally to the speed. In contrast to other non-contact methods which exclusively evaluate speed, manufacturers can also measured a change in direction and even the standstill of the object using the LSV 2000 from Zumbach.
The measuring accuracy is always irrespective of the speed, thus also allowing precise detection of very small movements.
Detailed explanation of the differential Doppler measuring process
The two laser beams are superimposed in the measurement volume and in this spatial area generate an interference pattern of light and dark fringes. The fringe spacing Δs is a system constant which depends on the laser wavelength λ and the angle between the laser beams 2φ:
Δs = λ/(2 sin φ)
If a particle moves through the fringe pattern, the intensity of the light it scatters back is modulated.
As a result of this, a photo receive in the sensor head generates an AC signal, the frequency fD of which is directly proportional to the velocity component of the surface in measurement direction vp and it can be said that:
fD = vp/Δs = (2v/λ) sin φ
• fD = Doppler frequency
• vp = Velocity component in the direction of measurement
• Δs = Fringe spacing in the measurement volume
The value λ/sinφ forms the material measure for the speed and length measurement. It is precisely measured for each sensor and is printed on the identification sticker.
Zumbach LSV 1000/2000
This sophisticated non-contact measuring method results in ideal monitoring and control options in continuous production processes, for example in the wire and cable industry, the plastics and rubber industry and the steel and metal industry.