LaserSpeed Pro® Gauges: Non-Contact Speed and Length Measurement

A laser surface Doppler, sometimes referred to a Laser Doppler Velocimeter (LDV), is an instrument used for measuring the velocity of a moving surface or object using a non-contact method. The instrument is also able to determine length as this is calculated by integrating the velocity over time. The device emits two laser beams of the same wavelength that converge to form a single spot at the surface. When two laser beams cross interference pattern is formed known as a fringe pattern (see diagram). The fringe separation or distance is a constant and a function of laser wavelength and beam angle. By measuring the frequency of scattered light emitted from the object as the surface particles pass through the fringes the velocity of the surface can be calculated. 

The fringe separation or distance is a function of laser wavelength and beam angle. 

Velocity is distance over time. 

Period is the inverse of frequency. 

Velocity is integrated to determine length 

The LaserSpeed Pro uses the principle of Laser Doppler Velocimeter (LDV) to provide non-contact measurement of velocity of a moving surface or object. Since length can be calculated by integrating the velocity over time it is also possible for the LaserSpeed to provide both measurement of velocity and length simultaneously. 

Velocity is integrated to determine length. 

LaserSpeed Pro gauges can be used for any application where it is important to have accurate speed and length measurements. They are capable of measuring any type of material, metallic or non–metallic, in  any colour or surface finish and any temperature including red-hot metals. They are widely used to measure the length of a product during the manufacturing process. Many products are sold by length and supplied in spools, reels or coils formats. It is important that the manufacturer ensures that each spool produced is supplied to comply with the length specification. Examples of length measurement applications are cables, wire, paper, foil, steel strip coils, aluminum strip coils, film reels, various web products, laminates to name a few. Another common application is where a continuous length of product is manufactured and needs to be cut to shorter lengths to be sold to the customer. This will be done using a cut-to-length line where a shear or saw cuts the moving product to shorter lengths. The LaserSpeed Pro can be used to accurately control this operation. Other manufacturing processes require real-time speed measurement where the LaserSpeed Pro will provide very precise speed measurement even on rapidly accelerating or decelerating processes. 

The longer stand off (1.5, 2.0, 2.5 & 3.0m) versions of LaserSpeed Pro LS8500 or LS9500 E or X series gauges are designed to measure both hot or cold materials out of the box. Making them suitable for all applications found within the steel manufacturing process from continuous casting to final processing of cold product. LaserSpeed Pro gauges are ideal for non-contact speed and length measurement on steel billet or slab casters and hot rolling mills with product temperatures up to 1350°C. The E or X series have air- or water-cooling facility built into the housing to protect the gauges from excessive heat radiation. 

There is a wide range of LaserSpeed Pro models which are suitable for any non-contact speed or length measurement application found within industry. The key considerations for selecting the right model are as follows: – 

 Stand-off Distance (SO):  

This is the mounting distance from the object to be measured and the aperture window on the LaserSpeed Pro. It is defined as the distance to the centre of the depth of field (DOF). When measuring very hot materials (red hot steel for example) it is an advantage to have a longer stand-off distance to keep the LaserSpeed Pro at a safe distance. With other applications, for example on cold materials, there may be limited space so having a shorter stand-off will be an advantage. The stand-off distance does not affect the measurement performance or accuracy. 

Options available (model dependent) 

100mm 

300mm 

600mm 

1000mm 

1500mm 

200mm 

2500mm 

3000mm 

Depth of Field (DOF) 

The material being measured must be within the depth of field (DOF) for correct measurement.  The DOF increases as the stand-off increases (see table below). It might be an advantage to choose a model with a longer stand-off to ensure that the gauge has a sufficient DOF for the application in mind. 

Zero speed and very slow measurement 

The LaserSpeed Pro LS4500 & LS800 models are not able to measure very slow speed (for example below 0.4 m/min for the LS4500-403 model). Therefore, on processes that are creeping very slowly, inching back and forth or moving in both directions the LaserSpeed LS9500 model should be selected. This model can measure zero speed and track direction.  

Harsh environments 

For applications with harsh environments, such as elevated ambient temperature, steam or dust contamination, it is important to select a model that is designed to operate reliably in this type of situation. These are most common in steel mills where the LaserSpeed Pro is used to measure red hot steel in a hot mill or continuous caster and the gauge needs to be cooled due to the high ambient temperatures and the optic needs to be kept clean with an air purge. For these applications, either the LaserSpeed Pro E Series or X Series should be selected. 

The LaserSpeed Pro gauges simultaneously measure both speed and length. The speed and length data is available on all the digital outputs at a user selectable update rate. The update rate range is preset between 1-256ms.