Rugged Sensor Technology for Harsh Operating Conditions

Sand and gravel are important resources for the modern building and construction industry, which could not exist without them. Coming in different particle sizes of less than a millimetre to several centimetres, the resource is fundamental to buildings, roads, bridges or custom-designed backyards.

A widespread gravel pit can be found on the Lower Rhine, where sediments from Taunus, Hunsrück, Sauerland and Eifel came to rest. At this very place, Hülskens digs up to 800 tonnes of sand and gravel out of the Rhine’s floodplains and inward areas every hour.

After digging the sediments out of the Lower Rhine plain with huge swimming multi-bucket excavators, the unsorted pieces of rock are transported to the processing facility via conveyor belts. Here, the sediments are stored temporarily on a crude gravel stockpile. Afterwards, the sediments are washed, drained and sorted in seven steps according to their particle size. The different grain sizes are subsequently transported to huge silos that feed a subterranean mixing facility.

Here, just below earth’s surface, individually ordered gravel compositions are produced – whether construction gravel for new buildings or special compositions for concrete goods such as paving stones. “We produce compositions for the entire construction industry in Germany and numerous customers in the Low Countries,” says Hermann Kerkenpass, Electrical Engineering Master Technician with Hülskens. “Depending on the customers’ demands, we are able to produce various compositions with grain sizes between 0.05 and 32 mm.”

One cornerstone for the automated processing facility is a mile long transport system that enables Hülskens to operate the facility with only 22 employees working in two shift operation. Sand and gravel pass each and every processing station on so called collating conveyors – from the actual gravel pit to the crude gravel stockpile, from the stockpile to the washing and draining facility, from there to a total of twelve silos and finally to transport ships standing by.

This transport system and each and every processing station have to be monitored at all times, because a single conveyor breakdown might disrupt the entire production process. Hülskens uses Turck’s sensor solutions to guarantee the plant’s availability even under high mechanical stress induced by stone impacts, dampness or frosted conveyors.

A total of approximately 150 uprox sensors monitor all the mechanical plant components’ end positions – at the silos’ feed units, the subterranean feed stations and the drives’ hydraulic system. Turck’s factor-1 sensors do not have a ferrite core and thus provide the same switching distance for all types of metal – making them perfectly suited for use on butterfly valves. Here, the sensors guarantee that the different grain sizes are transported to their corresponding silos. Ultimately, they even ensure a precise mixing process at the subterranean feed stations where sand and gravel have to be added to the conveyor in just the right amounts.