|
Algorithms
Variable Speed Drives
Truck Unloading
Crane Loading
Other Details
This installation features a single overhead vacuum crane, with five vacuum
heads, which lift five reels of paper from one of three shuttles on a single
track. The shuttles receive reels from four conveyors which are fed from
a single truck unloading system. The shuttles deliver reels to two conveyors
feeding stripping stations, before they are taken via automatic guided vehicles
to the presses.
There are several interesting features of this installation: complex
algorithms, the use of variable speed drives for all conveyor motors, the
method of unloading the trucks, and the method of getting the reels to and from
the crane.
Algorithms
The number of reels delivered at a time was usually 12, 20, or 22, and of
a single type. These had to be grouped in lots of five reels of the same type,
with reels from other trucks, down one of four lanes, being careful not to
create traffic snarls. This problem was further exacerbated by the rear conveyor
of each lane only holding four reels.
Additionally, the selection of which lane to unload and
which shuttle to use had to be made carefully, so as to alleviate traffic
snarls, and to allow the other shuttles to be accessed by the crane, which
could only take reels from the shuttles at two locations.
In order to make the decisions, the algorithm would make use of the already
written emulation of the conveyor system, running it without timing to
see how the current arrangement of reels would coalesce.
Back to top
Variable Speed Drives
There were a number of reasons for the use of variable speed drives
throughout the installation, but chiefly it was because there was only just
enough room on each conveyor to accomodate the intended number of loads. So as
the loads approached the end of a conveyor, if it could not continue on would
need to slow down at the first photocell then stop dead on the stop photocell.
Also as individual loads are added to a group, the source conveyor would need
to run slow until contact is made then both conveyors would then run together.
At the end of these transfers, the conveyor would need to stop with its last
reel at the precise end of the conveyor.
The resulting algorithms doubled the extent of the PLC programming for the
heavy-unit load conveyor system and thereby provided the impetus to develop
the
“Conveyor PLC Code Generator”.
Back to top
Truck Unloading
The trucks back into the conveyor, where the truck leveller automatically
raises the truck so that its floor is level with the conveyor system. The
conveyor system then extends its skates into the truck to the prescribed
distance for this truck type, then inflates the skates, withdraws them with the
reels, and then deflates the skates lowering the reels to the conveyors.
The reels then advance, in the columns of two as loaded on the truck, then
are separated into two rows, scanned (the barcode is matched to an entry in a
manifest previously downloaded from the host computer system), and directed
down one of the four lanes, where they are assembled into groups of five like
reels to await a shuttle trolley.
Back to top
Crane Loading
At the end of the conveyor, the offset of each reel from the centre of the
conveyor is measured and the shuttle loading position is suitably adjusted to
load each reel.
The reels are then loaded onto the shuttle in a balanced configuration,
should there be less than five reels to load. The shuttle then takes the reels
to one of the crane positions.
The crane raises the reels by around six inches and waits to see if they are
going to fall. Given that they don't, the shuttle moves out of the way. Once
out of the way the reels are raised to the top and are located with like reels
of a similar age, and according to their movement class.
Back to top
Other Details
The conveyor system incorporates shuttles, truck levellers, inflating skates,
barcode scanners, sizing, position measurement, down-enders, light curtains,
and turntables.
Communication between the
“Movement Controller”and the crane is via an infra-red serial link.
Communication between the
“Movement Controller”and the host computer system is via FTP/TCP-IP/Ethernet.
All PLCs are Allen-Bradley.
Back to top
| 
