Savings that stack up
February 1st 2009 Lenze has applied intelligent servo drives that it claims
optimise the travel and lifting movements of stacker
cranes while lowering energy consumption
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. Lenze, in partnership with storage
and retrieval specialist Beewen, has applied
intelligent servo drives which optimise the
travel and lifting movements of stacker cranes
while 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. 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 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 (sinÇ-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 economic and ecological benefits",
says company founder, Udo Beewen.
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 per cent 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 per cent of the total due to the
high operating times of the plant. Therefore
LCC can be strongly influenced by saving part
of this 40 per cent energy cost.
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,
claims Lenze. 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
calls '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. More articles from Lenze Ltd: | 
