PLCs take the work out of minor ingredients

A Smart Cart minor ingredients weighing system at Berwin Polymer Processing Group in Manchester, is being expanded to double the number of feeder stations from 20 to 40.

A Smart Cart minor ingredients weighing system at Berwin Polymer Processing Group in Manchester, is being expanded to double the number of feeder stations from 20 to 40.

A start/finish station is also being added, where bags will be formed prior to filling and subsequently sealed and dispatched.

The system was originally developed to overcome the constraints of conventional minor ingredients systems, which typically are labour intensive and therefore costly to run and prone to error.

It consists of static feeders arranged around a monorail track circuit, around which travel intelligent mobile weigh carriages or Smart Carts.

In use, a bag is formed and clamped onto a carriage and a set of instructions (otherwise known as a recipe) is fed from the main control system to the carriage's onboard controller.

This initiates the carriage's journey, around the track to the required feeder stations, where specified quantities of each ingredient are collected in accordance with the recipe.

At each feeder station a weight tolerance check is automatically carried out after filling and before the carriage is allowed advance to all the next station.

At the final feeder station a double check ensures that the recipe has been accurately fulfilled, after which the carriage completes the circuit back to the start/finish station for sealing and unloading station.

Once unloaded, a new bag is formed and clamped onto the carriage ready for a new set of instructions.

The Smart Cart system was developed and installed by John P Waterhouse Co of Nottingham, which is now undertaking the expansion.

The control system is based on hardware from Mitsubishi Electric, with communication involving two separate CC-Link fieldbus networks.

In the original installation the twin network architecture and Mitsubishi hardware were attractive because of the simplicity and connectivity, and these attributes paid dividends when it was decided to expand the Smart Cart system because reconfiguration and expansion was completely straightforward.

At the new start/finish station, bags are made automatically from a roll of tubular low-melt material.

In essence the leading edge of the material is gripped and sealed to form the base of the bag.

This is then pulled forward a specified distance and cut to form a tall open mouthed bag.

This station is controlled by a powerful Mitsubishi Q series PLC (programmable logic controller), which is fitted with a stepper motor control card for operating the various axes of the bag making machine.

A CC-Link card allows the PLC to communicate with other stations in the system and the system's central controller.

Once formed, the bag is swung into position on the carriage where clamps hold it in position.

The clamps consist of twin sets of jaws which operate independently to hold the bag's mouth open for filling then close it for sealing, while retaining a positive grip at all times.

The carriage (of which there were originally six on the Berwin system) then travels freely around the track, operating independently via its own onboard Mitsubishi FX1N PLC to collect the ingredients required.

The recipe is held in the FX PLC's memory, and determines at which stations the carriage stops to collect particular ingredients.

The quantity of each ingredient to be collected is specified by the recipe and controlled by the FX.

A serial comms module allows the FX PLC to monitor highly accurate onboard beam scales so that exactly the right quantity of each ingredient is collected.

A CC-Link module allows each carriage to "talk" to the weighing station at which it docks, so that it can activate and stop ingredient flow etc.

Starting and stopping of the carriage is also controlled by the FX1N, operating in conjunction with an onboard Mitsubishi inverter and induction motor.

Considerable processing capability is required here because not only does each carriage have to go to the right filling stations, it also has to know if it is coming up behind a stationary carriage and act accordingly.

A busbar runs parallel to the track, with three connections for power and three more for signals, so that the carriages are always in communication with the system's central control cabinet.

This houses a second Q-series PLC which is fitted with a massive 06H CPU (central processor unit).

It communicates, via the Internet, with a personal computer running Mitsubishi's MX4 graphics overview software and Waterhouse's KS2000 recipe packager software.

The central Q-series PLC also communicates via the second CC-Link network with 10 local control panels, each of which controls four feeder stations operating feeder mechanisms, slide gates, etc and monitors local storage hoppers to ensure that there are always sufficient ingredients available.

When all the ingredients of the recipe have been collected the carriage completes the track's circuit and returns to the start/finish station.

The bag's mouth is closed, flattened, and heat sealed across its top edge.

The bag is then tipped off the carriage and onto an accumulation conveyor where it resides until manually removed and stacked.

All these later stages are controlled by a third Q series PLC, this one being configured identically to the first.

Once unloaded, the carriage immediately moves forward the short distance to the bag forming area where it collects a new recipe so that it can go around the loop again.

Other than initial start up at the beginning and occasional oversight, the Smart Cart system is completely automatic and will run unmanned until stopped.

The recipes are assembled with precision accuracy and there is no cross contamination between recipes.

Currently Berwin is operating at a rate of two bags a minute, a level that would require 10-12 personnel to maintain over an eight hour shift, and the system's inherent expandability means production rates can be increased if and when required by adding more carriages and/or more filling stations.

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