LDV Laser und Lichtsysteme GmbH

LASER DOPPLER VELOCIMETER (LDV)
Laser Doppler velocimetry is the method which is most frequently used to measure the speed of solid surfaces without contact. A laser beam is split into two beams of equal intensity using a beam splitter. These two laser beams are processed using optical components, and at a measurement distance, they are placed at the intersection via an adjustable mirror. When two laser beams intersect at an angle, a striped pattern of light and dark strips arises in the measurement volume. The photo receiver sees this pattern of strips and recognises the Doppler frequency in the light which is scattered back, i.e. the frequency caused by the movement of material.

The way a LDV system works is easy to understand if we first consider a single particle, which runs through the striped pattern. In this case, it always transmits a bright pulse of a light when it passes through a light coloured field. The photo receiver records this flash frequency and converts into an electrical frequency in the evaluation unit. This original Doppler frequency is made available to our pulse output from the evaluation unit. Although solid surfaces do not have individual particles, the surface structure behaves exactly the same.

Additionally to the well established Velemeter 500 series a new compact sensor is available. This model is called Velemeter C - it is designed for use in or attached to C-frames of thickness gauges. It has already an integrated angle optic.

Nowadays, it is possible to control the mill gauge in a way that variations in the thickness of the material at the entry side are eliminated in the mill by using the material speed and the measured thickness for mass flow control. However, the new drive technology and the possibilities of gauge control are also responsible for the need of new techniques for speed measurement. The old existing mechanical detectors are no longer state-of-the-art.

The main applications for mass flow control are in the aluminium and stainless steel industry. For the speed measuring device itself, it makes no difference. In all applications VELEMETER® 500 is used.

The gauge head is typically integrated in the extractor hood at the entry and exit side where also the thickness gauges are installed. In some applications, the gauge head is equipped with an angle optic, so that it is possible to mount the gauge head at the same c-frame as the thickness gauge.

Since nearly all new rolling mills nowadays have built-in lasers, the users of old mills know that it is absolutely essential to re-equip their old mills with new measuring and control technology.

For cooling purposes the gauge head is equipped with a liquid-cooled housing which can be supplied from existing cooling facilities or with an own temperature control device or chiller. The VELEMETER® 500 is also able to use the emulsion which is used at the mill for cooling. It is also essential that the laser is able to see the material under all rolling conditions. Therefore, an air purging device which is directly mounted at the laser gauge is used.

As the mass flow control itself is typically supplied by the contractors for the electri-cal part of the mill, it is necessary to con-nect the VELEMETER Model 500 to a host computer. For this reason, the system has a standard pulse output with a resolution of 10 µm.

The Velemeter® C provides highly accurate speed and length measurements and all the advantages of non-contact laser based measuring equipment. The sensors are used in mass flow application as well as for elongation measurement.
As a supplementary sensor to the well established Velemeter® 500 series, the Velemeter® C is the more compact version but also makes use of the advantage of a He/Ne Laser tube.

The Velemeter® C is designed for use in or attached to C-frames of thickness gauges and has an integrated angle optic. The sensor head is mounted to the direction of movement of the material, usually at a standard stand-off distance of 450 mm. We can also provide every stand-off distance between 300 and 1500 mm to allow easy mounting at the mill construction.

The digital signal converter box (DAB) has to be mounted in a cubicle by the user. As basic information we supply a pulse output. The pulse frequency is equal the Doppler frequency and can be supplied with 2 or 4 channels according to the RS422 standard. This output is compatible to mechanical encoders.
Based on this, it is possible to do an upgrade from mechanical encoders to non-contact sensors and still use the same mill control system.