Multihead weighers use a number of different weigh heads to generate precise measurements of products by calculating the weight in each weigh head. Each weigh head has its own precision load cell. The perfect algorithm for an accurate target weight is typically 4 weigh heads. The weighing process begins when product is fed into the top of the Multihead Weigher.
A dispersion table distributes the product into a series of small linear vibratory feeders which individually deliver product to each weigh head. The dispersion table is normally equipped with a load cell which controls the feed of product to the multihead weigher. Optimising the flow of product to the dispersion table is critical to maximise the accuracy of the multihead weighing process. The typical issues encountered with poor flow of product are overfeeding and underfeeding.
The multihead weigher will never be accurate if the dispersion table is overfed. In this case the target weight becomes much more difficult to achieve because too much product is supplied to each weigh head. Production lines will stop in some cases as the multihead weigher shuts down if certain levels are surpassed. Operators occasionally overfeed the multihead weigher under the impression that this helps. This is not true, as it disables the system’s ability to release product when programmed weight tolerances are exceeded.
When the dispersion table is underfed the production line slows down. Not providing enough product into the weigh buckets slows down the operation of the multihead weigher and makes it more difficult to achieve the target weight. This can also increase the amount of rejected products which are below the required threshold.
Two feed systems into the dispersion table are currently industry standard, vibratory and belt conveyors. There are several issues which arise from feeding a multihead weigher directly with a belt conveyor.
Responsiveness of Belt Conveyors to the Feedback of the Multihead Dispersion Table
Belt conveyors are not responsive enough to stop accurately when the dispersion table is full. Due to the action of the belt conveyor and the way it conveys product, when the multihead weigher gives the signal to stop, excess product continues to fall onto the dispersion table. This leads to overfeeding of the multihead weigher, which hinders the ability of the machine to produce an accurate weighment and often leads to significant amounts of giveaway of product.
Biased Feed of the Multihead
Due to the belt conveyor being a straight edge discharge, product feed tends to fall primarily on one side of the multihead weigher. Buckets on one side are overfilled and on the opposite side are underfilled. This makes it more difficult for the multihead to produce an accurate weighment and often leads to significant product giveaway.
Product Dispersion on the Belt Conveyor
A belt conveyor does not disperse product across its width during conveying. This leads to a feast and famine effect at the outfeed where product is not evenly fed onto the dispersion table.
Improved production efficiency can be achieved through the use of a vibratory cross head feeder. Through extensive trials over the last 25 years we have proven that by feeding a multihead weigher with our peripheral (circular discharge), which is fully powered to get the perfect flushing effect, allows the dispersion table on the top of the multihead to be evenly fed. This in turn allows the software to run the perfect algorithm which is typically 4 weigh heads. Our unique ‘Cox & Plant Synchronisation’ system harmonises the infeed speed to the multihead weigher by tracking the run times which then automatically adjusts the speed of the conveyor. Please see data below gathered from trials we carried out in collaboration with Ishida.
Results from In-Field Trials:
7 different product sectors were trialed to achieve 100gm or 500gm bag weight accuracy for two different typical bag weights. The following results were consistently achieved
|Feed Mechanism||Synchronisation||% Accuracy of Target Weight|
|Vibratory Straight Edged Discharge||YES||93%*|
|Cox & Plant Spigot Cross Head Feeder||YES||98%*|
The % of bags that were within target range
*Verified by a check weigher