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Industrial Automation



Herald and Weekly Times West Gate Park Victoria

Algorithms

Variable Speed Drives

Truck Unloading

Crane Loading

Other Details

This installation features a single overhead vacuum crane, with three vacuum heads, which lift three reels of paper from one of two shuttles on a single track. The shuttles receive reels from two conveyors which are fed from a single truck unloading system. These reels are stacked on the warehouse floor in columns of up to eight metres in height. Later the crane would pick up a set of three reels from the warehouse and deliver thise back to the shuttle conveyors. The shuttles then deliver those reels to the 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 three reels of the same type, with reels from other trucks, down one of two lanes, being careful not to create traffic snarls.

Additionally, the selection of which lane to unload and which shuttle to use had to be made carefully, so as to alleviate traffic snarls.

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.

There were also complex algorithms involved in deciding where to place the reels on the warehouse floor, and which set of reels to collect for stripping. This project had a false start, where we (Coromont) were given the task of programming the crane, while a Norwegian company called NDC was given the task of writing the computer software. What put a stop on that approach was that it took the NDC-programmed computer system 10 minutes to decide upon which set of reels to retrieve. I remember the programmer saying, “There's no alternative to reading the database for each reel to find the reel that has the appropriate code and oldest date.” I advised that you could reduce that time by a factor of five by just checking the base reel, knowing that all reels above it are of a later date, and that you could further reduce the time by a factor of three by just checking one reel in a set, as the set is guaranteed to be homogenous. When it came my turn to program the computer system, I used the database summaries that I had developed for Mobil Oil in Wellington New Zealand, to instantly know where the oldest cell of reels of each type was located. This approach involved linked lists that would organise the reels by type and sequence them by date.

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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 added to the impetus to develop the “Conveyor PLC Code Generator”.

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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 two lanes, where they are assembled into groups of three like reels to await a shuttle.

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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 three 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.

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Other Details

The conveyor system incorporates shuttles, truck levellers, inflating skates, barcode scanners, sizing, position measurement, down-enders, light curtains, turntables, and simultaneous transfers.

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.

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