Laser surface velocimeter
A laser surface velocimeter (LSV) is a non-contact optical speed sensor measuring velocity and length on moving surfaces. Laser surface velocimeters use the laser Doppler principle to evaluate the laser light scattered back from a moving object. They are widely used for process and quality control in industrial production processes.
The Doppler effect (or Doppler shift) is the change in frequency of a wave for an observer moving relative to the source of the wave. The wave has a frequency f and propagates at a speed c When the observer moves at a velocity of v relative to the source, he receives a different frequency f' according to
The above analysis is an approximation for small velocities in comparison to the speed of light which is fulfilled very well for practically all technically relevant velocities.
To make a measurement on moving objects, which can in principle be of any length, requires a measurement design with an observation axis for the sensor which is at a right angle to the direction of movement of the object under investigation.
Laser surface velocimeters work according to the socalled Difference Doppler Technique. Here, 2 laser beams which are each incident to the optical axis at an angle φ, are superimposed on the surface of the object. For a point P, which moves at 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. At the point P of the object which is moving at the velocity v, the following frequencies therefore occur:
The point P now emits scatter waves in the direction of the detector. As P is moving with the object, the scattered radiation in the direction of the detector is also Doppler shifted. Thus for the frequency of the scatter waves in the direction of the detector, it can be said:
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