CHECKWEIGHING: Precision at High Speeds

Checkweighing, where the weight of products is inspected before shipment, needs to comply with high-speed production lines that have become increasingly faster. For small-sized products, a checkweigher can measure up to 400 to 500 packs every minute.

There are many factors that adversely affect weighing accuracy when checking objects moving on conveyors at high speed. At the same time, manufactures require high accuracy in checkweighing in order to guarantee that the amount said to contain in a package is reliable and meets customer expectations.

Causes of inaccuracy and remedies

• Short weighing time due to high speeds
As a product passes through weighing conveyors at speeds of 400 packs per minute, checkweighers have to stabilize weighing data in a short time in order to weigh the object accurately with minimum noise.

• Floor vibrations
There are various vibrations that humans normally can’t feel, for example, from machines operating in factory, tracks passing through the factory, and operators walking around checkweigher. Even such very slight vibrations can have a bad effect on load cells where the minimum graduation is normally 0.1gram.

• The effects of air resistance
Airflow against moving objects and acceleration, and the phenomena of lift and drag act on the object, depending on its shape or operation speed. This causes a deviation in weighing, which results as the net weight is measured statically.

• Resistance caused by product configuration
As tall products (large height to width ratios) are subjected to swing on the conveyor, while vibrating products such as beans in bottles and milk in cartons shake inside a package, these are not stabilized when measured on a conveyor.

These external factors influence weighing accuracy and cause deviation between the products’ actual weight and the weight measured by the checkweigher. For improved weighing accuracy, it is important that a checkweigher can effectively minimize the effects from such factors.

A digital filter technology, for instance, can process weight that is changing every second by a digital signal in a short time. This technology enables the checkweigher to select an optimum weighing time or filter time, depending on product configurations and weighing conditions, allowing the product to be weighed more accurately than conventional analog filters with only one fixed weighing time. It also minimizes noise levels.

A high sensitivity load cell (AFV cell) can be used to detect excessive floor vibrations. The AFV cell detects and cancels out external vibrations from the floor, allowing for more stable checkweighing. A dynamic calibration system also helps to take care of deviations caused by airlift or drag. Such a system can learn and adjust to conditions on the floor by calculating the deviation after weighing a test object on a moving conveyor several times. Correcting this deviation during the measurement of moving objects ensures even greater weighing accuracy.

In order to infeed easy-to-swing products stably, it is necessary to use a conveyor that best suits the product size or shape. In addition to the minimum graduation that a checkweigher can indicate, it is important that a checkweigher is effective in reducing external factors acting on the product due to high-speeds, and measures products at minimum deviations from the net weight measured statically.

X-ray functions
Product weights are typically measured with a checkweigher. However, products packed in series—such as chain-bagged powdered soups and liquid soups for instant noodles—cannot be placed on the weighing stage of a weight checker. For such products, checking the weight of each individual bag is physically impossible.

The X-ray weight estimation function estimates a product weight from a transparent X-ray image captured by the system. Specifically, the system calculates the volume of the product from the brightness of the image.

The X-ray inspection system learns the relation between the weight of an item and the brightness of its X-ray image when a non-defective inspection object passes through the system 10 times. The system calibrates the optimal system settings automatically. In this way, the weight of individual bags in a long series of packages is checked, and defective products are detected.

This system also allows operators to check the weights of individual items in a package. For example, checking accurate weights is possible for each item in a package of six individually packed hamburger patties, which makes it possible to detect a defective item even when the total weight is within a specified range.

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