When Parceline needed to upgrade the capacity of four sorting lines at
its Birmingham 'Superhub', drive technology was the key to increased
throughput. The choice of decentralised inverter drives without holding
brakes led to both performance gains and cost reduction
Parceline, part of a European group owned by GeoPost, which also includes
Interlink Express, handles over 250,000 parcels each night. More than
half of those pass through four steel belt sorter lines at its Birmingham
'Superhub'. Each night the operation starts at 6 00pm and aims to
complete the sorting task by 2.30 am, releasing trucks to 38 depots
around the UK. Increasing levels of business, together with more demand
for next-day delivery, had meant that the capacity of the 20 year-old
installation was reaching its limit. A decision was made to upgrade the
four sorting lines.
Each line comprises a 150m long stainless steel belt conveyor and 41
diverters - basically 'flipper' arms that push a parcel across the belt
into a specific destination tube. These arms were driven via a crank
system by a brake motor, ten motor revolutions taking the arm through a
single cycle. The role of the electromagnetic brake was to stop and hold
the diverter in the parking position. It proved impossible to reduce the
time for this ten-revolution cycle, which would have allowed a faster
belt speed. Limiting factors included the inertia of the diverter arm and
drive mechanism, and the capacity of the electromagnetic brake.
Furthermore, inaccuracies in the parking position caused by the braking
system resulted in some parcels being struck rather than guided by the
diverter arm, causing damage to parcel contents. A final factor was the
need for regular maintenance on the brakes.
The task of upgrading the system fell to Redditch based System
integrator, Automation Software Engineering (ASE), which had previously
been in contact with Lenze on the subject of decentralised drives.
Together, the two companies came up with a solution that involved fitting
each diverter with a new geared motor and integrated frequency inverter.
Indeed, once the decision to use inverters was made, a decentralised
solution became essential. With each line having diverters along 150m of
belt, the cost of panel-mounted inverters would have been unacceptable.
As well as panel space, each would require shielded cables between the
motor and the inverter. Instead, the Lenze motec terminal box-mounted
inverters allowed ASE to use an 'energy bus' with short spur connections
to each motor. Shielded cables were not required as the cable connections
between inverter and motor are short.
Another important decision was to use motors without electromagnetic
brakes. Previously, spring applied brakes had been viewed as essential,
not just for stopping, but also for holding the diverter arms under
conditions of vibration. However, electromagnetic spring-applied brakes
require finite times to release and to re-engage, typically between 50
and 75ms on units of this size. While these times can be reduced by using
fast excitation, the additional complexity and cost was undesirable.
Lenze's solution was to use the standard braking features built in to the
inverter for dynamic braking and holding. Control of this dual braking
action is entirely within the inverter. The result is reduced cycle times
of below 600ms and an increased belt speed of up to 168m/min.
Low cost and ease of installation were good reasons to go with the AS-i
fieldbus, and two AS-i networks are used for each sorter line. In
operation, the inverters are given a high signal to start the arm
movement, while an existing proximity sensor (already hard-wired to a PLC
in the control cabinet) detects the position at which braking should
start. The AS-i output subsequently goes low and the inverter stops the
arm, followed by the holding torque. The