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Weighing In at the End of the Line

In a field where the use of custom components is the norm, one company solved two problems by going off-the-shelf all the way. A few years go, Richard Fallas, VP of Technology for Fallas Automation, Inc., (FAI), Waco, TX, was presented with the challenge of designing a check weigher to weigh RSC cases at the end of a packaging line.

In a field where the use of custom components is the norm, one company solved two problems by going off-the-shelf all the way.

A few years go, Richard Fallas, VP of Technology for Fallas Automation, Inc., (FAI), Waco, TX, was presented with the challenge of designing a check weigher to weigh RSC cases at the end of a packaging line. Other check weighers available were designed using a transporting conveyor that attached to one or more load cells. As the case traveled over the conveyor, the weight of the conveyor was subtracted or "Tared" from the total weight to obtain the weight of the case. Having the conveyor be part of the load being weighed meant that the conveyor components selected were restricted to those that would not add a substantial amount of weight to the conveyor. As a result, industrial design was compromised. A second consideration for his new design was to eliminate the higher cost of maintenance and repair due to the use of proprietary components such as control boards, custom designed load cells and signal conditioners commonly used in Check Weigher designs.

In response to these design limitations, FAI developed the CSW weigher with its now patented "Lifting Weigh Platform." The CSW is a compact (38.75x36.06x20 to 44) end of line check weigher that accurately weighs cases, trays, cartons, and other containers to ensure correct product weight before shipping. The patented "Lifting Weigh Platform" allows the product to be weighed without having to weigh the entire conveyor. This protects the load cells from possible damage caused by excessive weight or force being applied to the conveyor, and produces highly accurate results.


Check Weigher Operation

Product is transported to the CSW weigher from an upstream conveyor. A Next Case HoldBack Mechanism assures that only one case is on the weigh platform at a time. In the past, designers used a separate sequencing conveyor for this purpose, but those designs added considerable overall length to the line. Once the back edge of the product is detected by a photocell, the platform lifts the product off the conveyor after a preset delay. At full up position, the weigh platform enters a 0.6-second delay to allow the load cell signal to settle. The advantage of this sequence is that the CSW Check Weigher weighs only the product, not the conveyor. Weight accuracy is measured to within grams and maximum speed is up to 30 cycles/minute.

The product is either accepted or rejected after the weight is measured and compared to an over/under weight which is part of the product recipe entered through the touchscreen operator interface. If the product is within weight tolerance, it is allowed to proceed down the production line; if it is out of weight tolerance it is automatically removed from the packaging line by the reject mechanism which is built on the side of the CSW Check Weigher conveyor. Again, the Check Weigher saves plant floor space by rejecting off the same conveyor that is used for weighing. Both right hand and left-hand reject mechanisms are available.


Standard Component Design

As mentioned above, another advantage to the Lifting Weigh Platform is that industrial conveyor components can be used without regard to their weight. The unit uses standard, off-the-shelf components, such as AC Induction motors, 56c gearboxes, and 1" diameter drive shaft, all of which are common industrial conveyor components. According to Mr. Fallas, FAI also used industry standard control components such as Allen-Bradley PLCs, and a standard touchscreen operator interface with pre-programmed recipes, alarm reporting, and production reporting capabilities. The weigher stands on a stainless steel frame and legs, and is provided with a Nema 4 electrical enclosure.

Not only are the control components standard, but FAI selected off the shelf load cells and signal conditioners from HBM. The load cells are key components in the CSW design, since they are the weighing elements selected. The PWS series load cells from HBM were selected for their C3 class accuracy rating, tolerance to off center loading, rugged stainless steel construction, and off-the-shelf availability. Along with the HBM 60 kg and 15 kg capacity load cells, FAI uses the company's AE301S6 signal conditioner.

This signal conditioner was selected for several reasons. Most packaging plants are noise intensive and the AE301S6 was one of the few devices on the market that used an excitation voltage that was placed on a 600 Hz carrier frequency. This made the amplifier extremely noise immune when compared with devices that ran off a DC voltage. The AE301S6 is also highly linear and has proven to be extremely reliable, providing the CSW Check Weigher with a long life cycle.

When it came to design assistance, HBM was very helpful in the preliminary design process, as well. Although the CSW was designed around standard components, HBM engineers suggested a slight modification be made to the load cell-mounting bracket. They noticed that the minimal alteration would help to minimize off center loading error, which in turn increased the accuracy of the Check Weigher.

One of the main design principles behind the CSW Check Weigher was to stay away from proprietary components and select devices that could be acquired from a manufacturer's stock. As a machine manufacturer, FAI focuses on designing machines that are industrial, compact and easy to operate. FAI relies on the expertise of its vendors to assist in the selection of the right components, while maintaining creative design control based on FAI's engineering knowledge of the industry.

The high accuracy, off-the-shelf PWS load cell series, along with corresponding signal conditioners from HBM eliminated the design difficulties as well as the fulfillment problems associated with incorporating custom components into the CSW Check Weigher. Overall, the system is not only a one-of-a-kind design, but is built totally from standard components. The uniqueness of the system is in the machine design and concept rather than in custom-designed components.