A belt 'scale' that is easy to install and maintain

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January 2007 Sensors & Transducers

The new LM50SBR conveyor belt scanner from Laser Measurement uses completely different measuring principles from conventional belt weighing instruments. Instead of detecting the physical load on a conveyor belt (a value that is subject to variations due to changes in belt tension and wear) the non-contact, laser technology of the LM50SBR directly measures the volume of material flowing along the belt.

The volume flow (indicated in cubic metres per minute) is derived from the product of the average cross-sectional area of the material and the belt speed. The LM50SBR is able to measure both of these parameters without making contact with either the material or the belt giving the instrument immunity to spillage and other mechanical damage. The LM50SBR has four 4-20 mA outputs. They indicate the material flow, belt speed, cross-sectional area and cumulative total (totaliser).

The non-contact measuring principle of the LM50SBR simplifies the installation procedure. The instrument is mounted 1 m directly above the centre of the conveyor belt looking down onto the material. The laser scanner and its associated belt speed sensor make all the necessary measurements from this safe location. The instrument is completely self-contained, requiring only a 24 V d.c. power source. There is no external control box or complicated field wiring. This makes it possible to commission and configure the device without stopping the conveyor belt.

The LM50SBR has two external switch inputs. The first is an automatic 'belt zeroing' facility. This makes it easy to reset or recalibrate the zero load condition on the conveyor belt. The switch can be activated at any time while the belt is running and empty. The zeroing sequence takes about one minute. The second switch input resets the totaliser. Both switches can be activated locally, remotely or both options can be used simultaneously.

Whilst conventional belt scales can be used for both process control and accurate weighing for dispatch purposes, the LM50SBR is intended as a retro-fit instrument for basic control and inventory management. Its accuracy is limited to about ±3% on large belts but can worsen to ±10% on smaller belts. Also, the measurement taken is of material volume flow not mass. Multiplying the flow by the average density gives the mass.

For material storage and management it is often more useful to measure the flow (and hence the volume) of the material because this is not affected by moisture content. It is therefore possible to fill storage bins to their volume rated capacity rather than relying upon their approximate mass storage capacity which is directly affected by the material density.

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Sensors & Transducers

A belt 'scale' that is easy to install and maintain

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