In the world of logistics, fully automatic stacker cranes ensure that warehouses are utilised to their full capacity at all times. All of the parts that go to make up the system have to work reliably and offer high levels of overall availability. The objective is to keep increasing the container throughput within the same time period. Lenze, in partnership with storage and retrieval specialist Beewen, have applied intelligent servo drives which optimise the travel and lifting movements of stacker cranes whilst at the same time lowering energy consumption.
Beewen, based in Germany to the south of the Ruhr area, can be seen wherever small goods items have to pass quickly through the logistics chain. Beewen’s customers include big mail order houses, and nationwide office equipment and textile merchants that also operate internet retail outlets. “Order today – receive tomorrow”, is the retailers’ motto – which is why their logistics centres have to work around the clock. Smooth operation depends on technology with a high level of availability.
That is why Beewen chooses Lenze’s intelligent and highly reliable drive technology, which is the ideal way to regulate the movements of their stacker cranes. Up to 14 metres in height, the cranes are made of aluminium and are incredibly light, for which reason the drives only have to accelerate low masses and require less power. This reduces energy consumption. Typically a stacker crane moves horizontally at up to 6m/s, lifts up to 2m/s and handles loads up to 200kg.
Beewen’s systems, called Quickstore, move along the aisles between the shelves on two rails. Their twin rail design has a number of advantages: the system is more stable, forces are more evenly distributed, and the floor of the warehouse is subjected to lower loads. The design of the system wastes very little space: shelves that are just 35 centimetres above the floor can still be accessed, which allows the storage space to be used as efficiently as possible.
A travel path is calculated for every single order. The stacker crane moves along the rails horizontally, the lifting unit vertically. The carrier attached to the latter either inserts the goods into the shelf, or takes them out. An industrial PC optimises the route. Using the stored coordinates of the origin and target compartments, it calculates and compares the theoretical times for lifting and moving horizontally, so that the crane and lifting unit reach their target coordinates accurately and simultaneously. Errors are in the order of just one or two millimetres. Lenze’s MCA series of asynchronous servo motors, combined with GKS series helical bevel gearboxes, are responsible for the stacker cranes’ fast and precise movement. The units are driven by 9300 series Lenze servo controllers with power ratings up to 20kW. The linear transmission of power during motion is via Lenze toothed belts and an Omega drive. The principle is simple and accords with Beewen’s philosophy, which is to make assembly and subsequent servicing as quick and easy as possible. “Logistics have to keep running – something we consider in every detail of our designs, however small”, points out the company’s director Udo Beewen.
The servo drives receive their setpoints via the standard CAN system bus from the position controller integrated into an industrial PC, and they move along what are known as S-ramps (sin2-shaped curves). Instead of positioning abruptly, these gentle acceleration profiles protect the structure against resonance, and enable it to be adapted to varying masses and mechanical conditions. Compared to synchronising both drives and driving at full speed, the gentle drive management lengthens the service life of the motors and wearing parts, while using less energy. “This drive and control concept works quickly and precisely, and offers tangible benefits in economic and ecological terms”, sums up Beewen, who founded the company in 1978.
The experts at Beewen go a step further with their power management. Because several stacker cranes often work simultaneously in small-part warehouses, there are often peaks in energy demand. High starting currents load the power supply system, and lead to an altogether higher rating in the power supply. To prevent this, their power management system permanently monitors the power consumption of all the system’s components and prevents different stacker cranes from starting up at the same time – but without detracting from the overall performance.
Energy costs are a significant part of the Life Cycle Costs (LCC) of a plant. The LCC is divided into four phases. In the acquisition phase there are engineering, training and commissioning costs as well as the obvious purchase cost. The operation phase includes the energy costs but also other daily costs such as people, production and plant overheads. The maintenance phase covers both planned and unplanned maintenance plus spares holding. Finally the disposal phase includes disassembly and disposal. In a logistics centre, drive technology represents about 10-15% of the capital costs of the plant – a relatively small part of the Life Cycle Costs. The largest and most important element is the energy costs which can be as high as 40% of the total due to the high operating times of the plant. Therefore LCC can be strongly influenced by saving part of this 40% energy cost, a concept that becomes all the more significant as energy costs rise.
The partnership of Beewen and Lenze has created a tailor-made package of drive and automation technology for logistics warehouses that outperforms systems that are simply based on individual components. This saves energy in two ways. The drive components are selected to match the requirements, not exceed them, with low size, weight and inertia – a process Lenze call “Rightsizing”. Secondly the operation and motions of the stacker cranes are optimised, both individually and as a system. The result pays back with increased throughput and savings in electricity costs. Udo Beewen summaries “The drive and control concept works quickly and precisely, and yet has benefits that can be measured economically and ecologically”.
Lenze
Tel: 01234?321321
Email: sales@lenze.co.uk
www.lenze.co.uk
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