Check weigher

A checkweigher is an automatic or manual machine for checking the weight of packaged commodities. It is normally found at the offgoing end of a production process and is used to ensure that the weight of a pack of the commodity is within specified limits. Any packs that are outside the tolerance are taken out of line automatically.

A checkweigher can weigh in excess of 500 items per minute (depending on carton size and accuracy requirements). Checkweighers can be used with metal detectors and X-ray machines to enable other attributes of the pack to be checked and acted upon accordingly.

An automatic checkweigher incorporates a series of conveyor belts. These checkweighers are known also as belt weighers, in-motion scales, conveyor scales, dynamic scales, and in-line scales. In filler applications, they are known as check scales. Typically, there are three belts or chain beds:

For high-speed precision scales, a load cell using electromagnetic force restoration (EMFR) is appropriate. This kind of system charges an inductive coil, effectively floating the weigh bed in an electromagnetic field. When the weight is added, the movement of a ferrous material through that coil causes a fluctuation in the coil current proportional to the weight of the object. Other technologies used include strain gauges and vibrating wire load cells.

It is usual for a built-in computer to take many weight readings from the transducer over the time that the package is on the weigh bed to ensure an accurate weight reading.

Calibration is critical. A lab scale, which usually is in an isolated chamber pressurized with dry nitrogen(pressurized at sea level) can weigh an object within plus or minus 100th of a gram, but ambient air pressure is a factor. This is straightforward when there is no motion, but in motion there is a factor that is not obvious-noise from the motion of a weigh belt, vibration, air-conditioning or refrigeration which can cause drafts. Torque on a load cell causes erratic readings.

A dynamic, in-motion checkweigher takes samples, and analyzes them to form an accurate weight over a given time period. In most cases, there is a trigger from an optical(or ultrasonic) device to signal the passing of a package. Once the trigger fires, there is a delay set to allow the package to move to the "sweet spot" (center) of the weigh bed to sample the weight. The weight is sampled for a given duration. If either of these times are wrong, the weight will be wrong. There seems to be no scientific method to predict these timings. Some systems have a "graphing" feature to do this, but it is generally more of an empirical method that works best.

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