# Length And Speed Measurement

## LENGTH AND SPEED MEASUREMENT

#### Polytec

Measurement principle
The Laser-Doppler Velocimetry
Laser surface velocimeters work according to the Laser Doppler Principle and evaluate the laser light scattered back from a moving object or surface. Polytec’s LSVs are based on the very challenging and sophisticated heterodyne detection method. Unlike conventional non-contact methods which measure only the absolute value of the velocity, Polytec’s LSVs can also detect changes in the object’s direction and even standstill conditions. The measurement precision is not dependent on the velocity, so even very small movements can be precisely recorded.

The Differencial Doppler Technique
To perform a speed measurement on moving objects, which can in principle be of any length, you require a measurement setup with a sensor which is mounted perpendicular to the surface it is measuring on. Polytec velocimeters work according to the so-called Differential Doppler Technique. Here, two laser beams – which are each incident to the optical axis at an angle φ – are superimposed on the surface of the sample. 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.

Both the laser beams are superimposed in the measurement volume and, in this spatial area, generate an interference pattern of bright and dark fringes. The fringe space Δs is a system constant that 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, then the intensity of the light it scatters back is modulated. As a result of this, a photo receiver 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 = (2vp/λ) sin φ
• fD = Doppler frequency
• vp = Velocity component in the direction of measurement
• Δs = Fringe space in the measurement volume
The value λ/sin φ makes up the material measure for the velocity and length measurement.

The Heterodyne Technique
Laser surface velocimeters that work in the so-called heterodyne mode show a frequency offset of 40 MHz of one of the laser beams. This makes the fringes in the measurement volume travel with a velocity corresponding to the offset frequency fB. This then makes it possible to identify the sample’s direction of movement and to measure at the velocity zero. The resulting modulation frequency fmod at the photo receiver in heterodyne mode is:

fmod = fb + vp / Δs = fb + (2vp/λ) sin φ

The modulation frequency is determined in the controller using Fourier transformation and converted into the measurement value for the velocity vp. The length measurement is performed by integrating the velocity signal.

Polytec laser surface velocimeters, known as LSVs for short, provide you with support when you’re monitoring continuous material and piece goods in the production process. They can be integrated in a very flexible way, are convenient to set up and yet are extremely easy to use and dependable in operation. It doesn’t matter whether you’re producing belts, pipes or profiles with a shiny or a matt surface – a Polytec LSV adapts to any task and measures the surface velocity and cut lengths of the items with zero contact and from a safe distance.
The LSV product line includes compact sensors, modular systems and models certified according to MID.

#### ProSpeed LSV 2100 – NEW Laser surface velocimeter

MEASURE SPEED AND LENGTH WHILE DETECTING STANDSTILL AND DIRECTION
• Easy process integration with long stand-off distances up to 3m
• Transparency thanks to enhanced connectivity with touch screen, web interface, Ethernet, fieldbus protocols
• Multi-user access (3 via Ethernet + 1 serial)
• No recalibration required
• Determines direction of motion and standstill
• Visible laser for easy alignment
• Robust sensor technology (IP 66, 67)
• Application-optimized accessories such as thermo-protective housing, air wipe and air amplifier, cooling plate, beam deflection unit and more

#### LSV 2000 – Laser surface velocimeter

MEASURE SPEED AND LENGTH WHILE DETECTING STANDSTILL AND DIRECTION
• Attractive price / performance ratio and excellent cost-effectiveness
• All-in-one system, simple process integration
• Heterodyne detection method for highly accurate length and velocity measurements
• Detects forward and backward movements as well standstill conditions
• Robust sensor technology for reliable operation even under harsh conditions; housing protection classes: IP66 and IP67
• Reliable material detection (“Material Present” function)
• Two trigger inputs available for additional light barriers, or optical switches for high-precision edge detection and offset length compensation
• Fast signal processor with powerful command set for short response times
• Flexible interface concept
• Application-optimized accessories such as optional cooling, air purge and thermo-/ protective housing for use in challenging production environments

#### LSV 1000 – Laser surface velocimeter

• Attractive price / performance ratio and excellent cost-effectiveness
• All-in-one system, simple process integration
• Robust sensor technology for reliable operation even under harsh conditions; housing protection classes: IP66 and IP67
• Reliable material detection (“Material Present” function)
• Two trigger inputs available for additional light barriers, or optical switches for high-precision edge detection and offset length compensation
• Fast signal processor with powerful command set for short response times
• Flexible interface concept
• Application-optimized accessories such as optional cooling, air purge and thermo-/ protective housing for use in challenging production environments

#### LSV Logger

Lsv Logger was developer by SSK company to provide a clear view of real-time measurement results and archiving. Archived data can be used for statistics purposes. This software provides a clear overview of the current measurement data as well as the basic data of the Polytec measuring head. LSV Logger offers language localization in all languages according to the customer’s requests. There is an option to switch different measurement units.

Main features:
• On-line displaying
• Archiving of measured data
• Basic diagnostics of the Polytec
• Material cutting mode
• Ethernet communications
Software offers 30-day TRIAL VERSION and FULL VERSION, which is available from a local distributor of Polytec. The licence is bound to the seriál number of the Polytec measuring head.

## Contact person

technician, sales
Phone: +420 731 653 633
E-mail: haltof@ssktrinec.cz