Laser surface velocimeter

A laser surface velocimeter (LSV, English) is a non-contact optical measuring instrument for the determination of velocity ( a velocimeter ) and, derived from, the length of moving surfaces ( engl. surface). LSVs operate on the laser Doppler principle and the value judgments of a moving object backscattered laser light. They are widely used for production control in industrial processes.

Principle of operation

The difference Doppler method

, The Doppler effect describes how the state of motion of the observer or the receiver affects the result of a frequency measurement on a light wave propagating in the stationary reference system with the velocity c of a wavelength λ and a frequency f, when the observer or receiver with the velocity v with respect to the rest frame moves. The permissible for small velocities non- relativistic analysis reveals that the observer has a frequency f ' measures that v is in the following context with its speed:

The foregoing analysis is an approximation for small compared to the speed of light speed, which is practically very well fulfilled for all technically relevant speeds.

The measurement task to the moving objects, which can be of any length, in principle, requires a measurement setup with an observation axis of the sensor perpendicular to the direction of movement of the measurement object.

LSVs operate on the differential Doppler method as above. In this case, two laser beams are incident at an angle φ to the optical axis, is superimposed on the surface of the measurement object. For a point P which is moving at the speed V by the intersection of the two laser beams, the frequencies of the two laser beams according to the above formula are Doppler shifted. At the point P of the measurement object, which moves at the velocity v, the following frequencies thus occur:

The point P is now scattered waves emitted toward the detector. As P moves with the object to be measured, the radiation emitted in the direction of the detector scattered radiation is also Doppler-shifted. Thus applies for the frequency of the scattered waves in the direction of the detector:

The scattered waves are superimposed on the detector. By interference of the scattered waves from the two laser beams occurs in the superposition for the various frequency components. Is Metrologically evaluated the low-frequency beat frequency of the superimposed scattered radiation corresponding to the Doppler frequency fD. This is obtained at the same frequency (same wavelength ) between the two incident laser beams as the difference of fe2 and fe1 to:

For vertical movement of the point P with respect to the optical axis and at the same incident angle φ where:

And

Thus one finally obtains:

The Doppler shift is therefore directly proportional to the speed. A clear explanation that leads to the same result, is the following:

Tableau

The two laser beams overlap in the measurement volume, and in this spatial area, an interference pattern of bright and dark stripes.

The fringe spacing? S devices is a constant that depends λ of the laser wavelength and the angle between the measurement beam 2φ:

A particle moving through the stripe pattern, the light backscattered by it is modulated in its intensity.

A photodetector in the sensor thus generates an alternating current signal, whose frequency f is directly proportional to the velocity component of the surface in the measuring direction, VP, and we have:

The heterodyne method

LS velocimeter work in the so -called heterodyne mode, that is, the frequency of the measurement beam is shifted by an offset of for example, 40 MHz. The strips in the measuring volume travel at a speed corresponding to the offset frequency fB. This makes it possible to detect the direction of movement of the measurement object, and measuring at the speed of zero. The resulting modulation frequency fmod on the photoreceptor is in the heterodyne mode:

The modulation frequency is obtained by Fourier transformation in the controller and converted in the measuring value of the speed Vp. The length is measured by integration of the velocity signal.

Areas of application

LSVs are used in the cutting control in hot and cold area in the control of flying saws, in the cargo length measurement on plasterboard, cardboard, wood, or form sheet metal, as well as in the measurement of roll length of cable, wire, textile, paper, paperboard or film. Velocity measurements with LSVs serve the process control in hot and cold rolling mills, for example, to stretch coefficient measurement with the aid of differential speed provisions for the measurement of elongation and skin pass or mass flow control. Furthermore, it is also a synchronization of speeds possible eg for slip measurement and compensation or for lamination.