Checkweighing Conveyor

Overview

A checkweighing conveyor weighs every package as it moves along a line and removes any package that falls outside its allowed weight band. It does this without stopping the product: the package is carried across a short weighing belt for a fraction of a second, its weight is captured, compared to set limits, and a reject device removes it if it is too light or too heavy. Because it checks every unit rather than a sample, a checkweigher catches missing items, short fills, double fills, and missing components, and it lets a filler run as close to the target weight as the law and the recipe allow, which reduces the costly overfill known as giveaway.

The machine is built from three conveyors in a line plus the weighing and control hardware. The Infeed Conveyor spaces and steadies incoming packages, the Weigh Section takes the actual weight, the Outfeed Conveyor carries packages on, the Reject System removes the bad ones, the Weigh Controller and HMI runs the logic and the screen, and the whole thing sits on a stainless Stainless Frame with Guarding and Sensors.

Taking a weight in motion

The hard part is weighing accurately while the package is moving and only on the belt for a moment. The Weigh Section uses a short, light Weigh Belt mounted on an isolated Weigh Frame so that nothing but the package and that one belt rests on the cell. Under the belt is the Load Cell, a machined Flexure Beam beam fitted with four Strain Gauge elements in a bridge. As the package crosses, the beam bends a tiny, repeatable amount and the gauges change resistance in proportion to weight. A Cell Amplifier mounted at the cell amplifies and digitizes the signal before line noise can corrupt it. An Overload Stop limits travel so a dropped load cannot bend the flexure past its elastic range.

Settling, filtering, and timing

A weight signal taken mid-flight is full of vibration from the belt, the motor, and the package itself bouncing. The Main Board runs a digital filter that settles this into a single stable weight in the brief window the package is fully supported on the weigh belt. To know that window, the Photoeye sensors mark when the package enters and leaves, and an Line Encoder tracks belt position so the captured weight is tied to a specific package. The Weigh Belt Motor drives the weigh belt at a speed matched to the Infeed Motor so the package does not lurch as it transfers. A Draft Shield blocks drafts that would otherwise jitter the reading on light products.

Classifying and rejecting

Once a stable weight is in hand, the controller compares it to the limits for the running product. Packages inside the band ride the Outfeed Conveyor on untouched. Packages outside the band are flagged, and the Line Encoder tracks them to the reject point. There the Reject System fires: a Air Valve triggers the Air Cylinder and Pusher Paddle that sweeps the bad package into the Reject Bin. A Confirm Sensor checks the reject actually left the belt and raises an alarm if a bad package slips through, which matters where the checkweigher is a control point in a food-safety or legal-weight program.

Control, data, and feedback

The Weigh Controller and HMI does more than pass or fail. On its LCD Panel touchscreen it shows running average weight, standard deviation, giveaway, and reject counts per product. Many installations feed the average weight back to the upstream filler so it can trim the fill toward target, which is where a checkweigher pays for itself. The I/O Board handles the photoeyes, the reject valve, alarm beacons, and line stop signals, and a Emergency Stop cuts belt power at once. In legal-for-trade modes the controller logs weights and locks calibration so the results stand up to inspection.

Variants and use

Checkweighers scale from small pharmaceutical units weighing blister packs to a fraction of a gram, up to case checkweighers handling several kilograms at the end of a packing line. Light, fast lines use a short Weigh Belt and an air Pusher Paddle; heavier cases use a pusher arm or a pop-up diverter instead. Many are paired with a metal detector or an x-ray unit so a single station both weighs and inspects, and some add a label or coder check. The frame is built for washdown because most run in food and beverage plants, so the Stainless Frame is stainless with sloped surfaces and the drives are sealed. Accuracy depends on a level, vibration-free install, a weigh belt sized to the product, and a clean Load Cell, so siting and maintenance matter as much as the electronics.